FACULT Y OF MANAGERIAL AND TECHNOLOGIC AL ENGINEERING DOM AIN: MECHATRONICS AND ROBOTICS MASTER OF SCIENCE PROGRAMME: ADVANCED MECHATRONICS SYSTEMS… [607824]

UNIVERSIT Y OF ORADEA
FACULT Y OF MANAGERIAL AND TECHNOLOGIC AL
ENGINEERING
DOM AIN: MECHATRONICS AND ROBOTICS
MASTER OF SCIENCE PROGRAMME: ADVANCED
MECHATRONICS SYSTEMS
FORM OF EDUCATION : Full time learning

DESIGN OF AN OPTICAL
SENSOR USED FOR REAL -TIM E
ALCOHOL CONCENTRATION
MEASUREMENTS

SCIENTIFIC COORDINATOR
Associate Professor PhD SORIN MARCEL PATER

GRADUATE
DAN IOAN ȚARCĂ

ORADEA
2017

DISERTATION THESIS
2

DISERTATION THESIS
3

DISERTATION THESIS
4

DISERTATION THESIS
5 1. ABSTRACT
Optical sensors have recently become extremely important and useful, especially in the
field of chemical detection, because of their substantial advantages compared to conventional
electrical sensors: the ir immunity to electromagnetic interference, their small siz e, very high
sensitivity, m ultiplexing, reduction of fire risk, very short response time, etc.
Fiber -optic chemical sensors use in particular the absorption phenomenon to determine
the concentrati on of soluble solutions (such as ethanol) and also to detect a large number of
volatile gaseous compounds (methane, carbon oxides, nitrogen, hydrogen, water vapor,
ammonia, volatile organic compounds, etc.). In order to obtain the most efficient results, t he
fiber optics are treated by covering them with with various materials (organic, anorganic,
nanomaterials, biological, etc.), which induce modifications of the analyzed optical properties
and which are subsequently interrogated at set wavelengths, result ing in obtaining the
experimental data accurately in a short time and at very low concentrations of the products
analyzed.
Studies in this field mainly relate to the development and perfection of VOC detection
techniques on fiber optic sensors, increasing the duration of use of the sensors and the
possibilities of their multiplexing.

DISERTATION THESIS
6 CONTENTS
1. Abstract ________________________________ ________________________ 5
2. Optical Sensors ________________________________ __________________ 7
2.1. Radiometry ________________________________ _________________ 8
2.2. Photometry ________________________________ _________________ 14
2.3. Windows ________________________________ __________________ 17
2.4. Mirrors ________________________________ ____________________ 19
2.5. Lenses ________________________________ ____________________ 22
2.6. Fresnel Lenses ________________________________ ______________ 23
2.7. Fiber Optics and Waveguides ________________________________ __ 26
2.7.1. Theory ________________________________ __________________ 26
2.7.2. Working Principle ________________________________ _________ 32
2.7.3. Mitigation Mechanisms ________________________________ _____ 34
2.7.4. Light Scattering ________________________________ ___________ 35
2.7.5. UV-VIS-IR Absorption ________________________________ _____ 36
2.7.6. Fiber Optic Fabrication ________________________________ _____ 37
2.8. Concentrators ________________________________ _______________ 38
2.9. Coatings for Thermal Absorp tion _______________________________ 39
2.10. Nano -optics ________________________________ ________________ 41
3. Ethanol Sensors on Optic Fibers ________________________________ ____ 42
3.1. Introduction ________________________________ ________________ 42
3.2. Experimental Procedure and Se nsor _____________________________ 44
3.2.1. Experimental Procedure ________________________________ _____ 44
3.2.2. Sensor ________________________________ ___________________ 46
4. Conclusions ________________________________ ____________________ 50
5. References ________________________________ _____________________ 51

DISERTATION THESIS
7 2. OPTICAL SENS ORS
Light phenomena such as reflection, refraction, absorption, interference, polarization
and velocity are used in the structure of optical sensors that manipulate light in several ways
[1] [2] [3].
Light can be manipulated in many ways, using o ptical components . In the followings ,
optical components will be analyzed from the point of the geometrical optics, so that properties
of light, described by cuantum mechanics and quantum electrodynamics will not be taken into
account and same will be made for wave properties. Light will be considered as a moving front
or a ray, which is normal to the front. In these assumptions no optical elements smaller than
wavelength will be discussed . It means that, for example, a glass window is impregnated with
submicron sized particles, these particles will be ignored for geometric calculations from the
near infrared to longer wavelengths. Also, diffractive grating cannot be described by the
method of geometrical optics, so that it will not be considered .
In this part of the thesis only those optic elements will be considered which are most
applicable for sensor design. More information regarding the geometrical optics can be found
in [1] [4].
To manipulate light, first it must be generated. There are several natural ways to
generate light, such as celestial objects: sun, moon, stars. Also, natural heat generating sources
are light generators in t he mid and far infrared spectral range, depending on their temperature.
They include fire, exothermal chemical reactions, living organisms and so on, which have
different temperatures from their surroundings. Other sources including filaments in electric
bulbs, light emitting diodes, lasers, heaters and so on, are human-made light sources, and some
of them can be incorporated into measurement devices.
After being generated, light can be manipulated in many ways. Several examples of
light manipulation in se nsors are presented in Fig.2.1. Some of them changes the direction of
the light, while some of them block selectively certain wavelength, which is called filtering
(Fig.2.1a). Direction of the light can be changed through refraction, using lenses, prisms,
chemical solutions, crystals and so on . Also, light direction can be changed through reflection,
sing mirrors, optical wave guides or diffractive gratings. When passi ng through these objects,
light properties may be modified using different stimuli. These stimuli can usually be generated
by electrical means/signals. They can alter the intensity, polarization, spectral contents and

DISERTATION THESIS
8 direction of propagation of light beam . Also speed of light and the phase of its wave can be
modified.
Two aspects are involved in developing light sensors: radiometry which deals with the
light power and its modifications and photometry in which illumination is subjected to
modifications.

Fig. 2.1 Examples of optical system which use refraction (a) and reflection (a, b, c)
2.1. Radiometr y
When a ray of light passes through a material made from multiple layer s of different
substances , called media , changes in its trajectory occurs . In Fig. 2.2 is show n what happens to
a ray of light, passing from the first medium into a flat plate of a second medium, and then to
a third medium. Examples of the media are air, glass, and liquid. Part of the incident light is
reflected from a planar boundary between the first and second media according to the law of

DISERTATION THESIS
9 reflection, which historically is attributed to Heron of Alexandria (first century AD) who
notic ed that angle of incidence equals angle of reflection :
'
1 1
(2.1)

Fig. 2.2 Light passing through materials with different refractive indices
Equation 2.1 is equivalent to say that reflected light takes the shortest path or the
shortest time to travel between two points. The latter can be derived from the Fermat’s
principle. This mi rror-like reflection is called a specular reflection. Refl ection not necessarily
should be specular as defined by ( 2.1).
When light reaches a granular boundary between two media, it bounces off in all
direction s due to the microscopic irregularities of the interface , which is called diffuse
reflection. The exact form of reflection depends on the structure of the surface.
Part of light flux enters the second medium at a different angle. The new angle , θ2 is
governed by the refraction law, which was discovered in 1621 by Willebrord Snell (1580 –
1626) and is known as Snell’s law:
2 2 1 1 sin sin   n n
(2.2)
where n 1 and n 2 are the indices of refraction of t wo media.
In any medium, light moves slower than in vacuum. Index of refraction is a ratio of
velocity of light in vacuum, c 0, in respect to that in a specific medium, c

DISERTATION THESIS
10
ccn0 (2.3)
Because c<c 0, the refractive index of a ny medium is always grater than unity. The
velocity of light in a medium directly relates to a dielectric constant εr of that medium, which
subsequently determines the refractive index:
r n
(2.4)
where n is function of a wavele ngth. A wavelength dependence with index of refraction
can be seen in a pris m, property which was used by Sir Isaac Newton in his experiments with
the light spectrum. In the visible range, the index of refraction n is usually specified at a
wavelength of 0 ,58756 μm, which corresponds to the yellow -orange helium line.
A refractive index dependence of wavelengths is called dispersion. The change in
refraction index with the wavelength is usually gradual, unless the wavelength approaches a
region where the material is not transparent. Fig. 2.3 shows transparency curves of some optical
materials employed in various optical sensors.
A portion of light flux reflected from a boundary at angle
'
1 depends on light velocities
in two adjacent media. The a mount of reflected flux Φρ relates to incident flux Φ0 through the
coefficient of reflection ρ, which can be expressed using refractive indices
2
2 12 1
0



nnnn
(2.5)
Equation (2.5) indicate that both the reflection and the absorption ( or emissivity) depend
only on n for a certain wavelength.
If the light flux enters from air into an object having refractive index n , equation (2.5)
can be simplified as
2
11


nn
(2.6)

DISERTATION THESIS
11 Before light exits the second medium (Fig. 2.2) and enters the third medium having
refractive index n 3, another part o f the flux is reflected internally from the second boundary
between the n 2 and n 3 media at angle θ2.

Fig. 2.3 Transparency characteristics for various optical materials
The remaining portion of light exits at angle θ3, also governed by the Snell ’s law. For
the case when media 1 and 3 are the same (for instance, air ) at both sides of the plate, then
n1=n3 and θ1=θ3. This case is illustrated in Fig. 2.4. From equation (2.5) results that coefficients
of reflection are the same for light which strikes a boundary from either direction.
A combined coefficient of two reflections from both surfaces of a plate can be found
using the following simplified formula
1 1 2 2
(2.7)
Where ρ1 is the reflective coefficient from one surface. As a fact , the light reflected
from the second boundary is reflected again on the first boundary , back to the second boundary,
and so on. Thus, assuming no absorption occurs in the material, the total refl ective loss within
the plate can be calculated through the refractive index of the material :
1212 2nn
(2.8)
When increasing the differences in refractive indices , the reflection will also incre ase.
For example , if visible light travels without absorption from air through a heavy flint glass

DISERTATION THESIS
12 plate, two reflections will result in loss of about 11%, while for the air -germanium -air interfaces
(in the far infrared spectral range) the reflective loss is about 59%.

Fig. 2.4 Light passing through an optical plate
To reduce losses, optical mater ials are often given antireflective coatings (ARC), which
have refractive indices and thickness geared to specific wavelengths.
The radiant energy balance will account , for two reflections in an optical material:
12
(2.9)
where α is a coefficient of absorption and γ is a coefficient of transmittance. In a
transparency region, α≈0, so that transmittance can be expressed as
1212nn
z
(2.10)
The above equation specifies th e maximum theoretically possible transmittance of an
optical plate.
Considering the above example, transmittance of a glass plate is 88.6% ( in the visible
spectrum ), while transmittance of a germanium plate is 41% ( in the far IR range ). In the visible
range, germanium transmittance is zero, which means that all of the light is reflected and

DISERTATION THESIS
13 absorbed. Figure 2.5 shows reflectance and transmittance variation of a thin plate as functions
of refractive indices. In th is assumptions , a plate means any optical device ( such as a window
or a lens) operating within its useful spectral range, that is, where its absorptive loss is small .
Figure 2.6 shows a light energy distribution within an optical pl ate when incident light
flux F 0 strikes its surface. A part of incident flux Φρ is reflected, another part Φα is absorbed by
the material, and the third part Φγ is transmitted through the plate . The absorbed portion of light
is converted into heat, a portion of which ΔP is lost to supporting structure and surroundings
by the mean o f thermal conduction and convection.

Fig.2.5 Reflectance and transmittance of a thin plate as functions of a refractive index

Fig.2.6 Radiant energy distribut ion for optical plate
The rest of the absorbed light raises temperature of the material. The tempera ture
increase may be a real problem when the material is used as a window in a powerful laser.
Another application where temperature increase may cause problem s occurs in far –
infrared detectors. The problem is associated with the flux
P , which is radiated

DISERTATION THESIS
14 by the material due to its temperature change. This phenomenon is called secondary radiation.
Naturally, a radiated spectrum relates to a temperature of the material and is situated in the far
infrared region of the spectrum. The spectral distribution of the secondary radiation
corresponds to the absorption distribution of the material because absorptivity and emissivity
are the same.
For ma terials with low absorption, the absorption coefficient can be determined with
the equation :
0 212TdtdT
dtdT
nn mcL g





(2.11)
where m and c are the mass and the specific heat of the optical material, T g and T L are
the slopes of the rising and lowering parts of the temperature curve of the material, respectively,
at test temperature T 0. Strictly speaking, light in the material is lost not only due to absorp tion
but to scattering as well.
A combined loss within mate rial depends on its thickness and can be expressed through
the so -called attenuation coefficient g and the thickness of the sample h. The transmission
coefficient can be determined from ( 2.10), which is modified to take into consideration the
attenuation:
ghe2 1
(2.12)
The value of attenuation (or extinction) coefficient g is specified by optical materials
manu facturers .
2.2. Photometry
For the case of using light -sensitive devices (photodetectors), it is mandatory to
consider both the sensor and light sources. In some applications, light might come from
independent sources, while in others the light source is part of the m easurements system. In
any case, the photometric characteristics of the optical system should be taken into account .
These characteristics include light, luminance, emittance, brightness, etc.
Special units have been devised in order to measure radiant intensity and brightness .
The r adiant flux situated in the visible portion of the spectrum, is referred to as luminous flux,

DISERTATION THESIS
15 due to the inability of the human eye to respond in the same manner to all levels of visible
wavele ngths.
For example, red blue light s having the same intensity will be perceived in different
manners : the red will be notices as being much brighter. This is why , when comparing lights
of different colors, the watt is not suitable for measur ing brightness , so that a special unit called
lumen was introduced . Lumen is defin ed considering a standard radiation source made from
molten platinum formed in a shape of a blackbody and visible through a specified aperture
within a solid angle of one steradian. In a spherical geometry a solid angle is defined as [5]
2rA
(2.13)
where r is the spherical radius and A is the spherical surface of interest. For the
particular case of A=r, the unit is called a spherical radian or steradian.
Illumi nance is given as [5]
dAdFE
(2.14)
that is, an infinite small amount of luminous flux (F) over a n infinite small area (A).
Luminance is most often expressed in l umen s/.
The luminous intensity specifies flux over solid angle:
ddFIL
(2.15)
is expressed usually in lumens per steradian or candela. When the luminous intensity is
constant with respect t o the angle of emission, the above equation becomes
FIL
(2.16)
When the wavelength of the radiation modifies , but the light flux is kept constant, it
was found that the radiative p ower in watts is changing , so that it is mandatory to specify a
relationship between illumination and r adiative power for a particular frequency. The point of

DISERTATION THESIS
16 specification was found to be at a wavelength of 0 ,555 nm, which is the peak of the spectral
response for the human eye. At this wavelength, 1 W of radiative power is equivalent to 680
lumens.
Table 2.1. shows some useful terminol ogy [5]
Table 2.1 Radiometric and photometric terminology
Description Radiometric Photometric
Total flux Radiant flux (F) – W Luminous flux (F) – lumens
Emitted flux density at a source
surface Radiant emittance (W) – W/cm2 Luminous flux (F) – lumens
lumens/cm2 (lamberts) or
lumens/ft2 (foot -lamberts)
Source intensity (point source) Radiant intensity (Ir) – W/sr Luminous intensity (IL) in
lumens/sr (candela)
Source intensity (area source) Radiance (B r) – W/sr/cm2 Luminance (B L) in lumens/sr/cm2
(lambert)
Flux dens ity incident on a surface Irradiance (H) – W/cm2 Illuminance (E) – lumens/cm2 or
(candle)
lumens/ft2 (foot -candle )

The main concern i n the selection of electro -optical sensors must be the design
considerations of li ght sources . A light source may appear as either a point source or an area
source . This is a function based on the size of the source versus the distance betwe en the source
and the detector.
A point sources is defined as a source whose diameter is less than 10% of the distance
between the source and the detector. Usually is desirable that a photodetector should be aligned
in such manner that its detecting area is tangent to the sphere with the point source at its center,
but it can be possible that the surface of the detector is sloped from the tangent plane. Under
this condition, the incident flux density (irradiance) is proportional to the cosine of the
inclination angle φ:
cosrIH
(2.17)
The illuminance is:
cos2rIEL
(2.18)
The area source is defined as th at whose diameter is greater than 10% of the distance
between the source and receiver . For the case when radius R of the light source is much larger
than the distan ce r to the sensor , a special case occurs . Under this condition

DISERTATION THESIS
17
2 2 2RAB
RrABHsr sr (2.19)
where A s is the light source area and B r is the radiance.
Because the area of the source is
2R As ,the irradiance equation can be written
W B Hr
(2.20)
which m eans that the densities of emitted and incident flux are equa l. When the detector
area is the same as the source area and R>>r, the total incident energy is almost as great as the
total radiated energy, which shows that unity coupling exists between the source and the
detector.
For the case of an optical system which consists of channeling, collimating, or focusing
components, its e fficiency and coupling coefficient must be considered . Important relationships
for point light source and area light source are given in Tables 2.2 and 2.3. [5]

Table 2.2 Point source relationships
Description Radiometric Photometric
Photometric Ir, W/sr IL, lumens/sr
Incident flux density Irradiance, H =I r /r2, W/m2 illuminance, E =I L/r2, lum ens/m2
Total flux output of a point source P =4πI r, watts F =4πI L, lumens

Table 2.3 Area source relationships
2.3. Windows
The main purpose of windows is to separate and protect in ner part of sensors and
detectors from environment. A good window must transmit , with minimal distortions, light rays
in a specific wavelength range. Therefore, windows must have appropriate characteristics for
each particular application.
For an optical detector to operate under water, it must have a window which should
have the following properties: mechanical strength enough to withstand water pressure, low Description Radiometric Photometric
Point source intensity Br, W/(cm2 sr) BL, lumens/(cm2 sr)
Emitted flux density W =πB r, W/cm2 L =πB L, lumens/cm2
Incident flux output of a point
source density
2 2R rABHsr
 W/cm2
2 2R rABHsL
 , lumens/cm2

DISERTATION THESIS
18 water absorp tion, a transmission band in accordance to the light rays wavelength and a
refractive index close to that of wa ter.
The best shape f or a window to withstand high pressures , is spherical , as shown in Fig.
2.7. For a spherical window , in order to minimize optical distortions , two limitations must be
applied : the window’s s pherical radius R 1 should be greater than the aperture D, and thickness
of the window (d) should be uniform and much smaller than radius R 1, otherwise the window
becomes a concentric spherical lens.
Surface reflectivity of a window must be taken into accou nt when assessing its overall
performance. Antireflecting coatings (ARC) can be applied on the sides of a window t o
minimize reflective loss es. These coatings give shades of blue and amber to photographic
lenses and filters. Due to refraction in the window (as presented in Fig. 2.4), a passing ray is
shifted with a distance L, which for small angles Θ1 may be computed as follow s:
nndL1
(2.21)
where n is the index of refraction for the considered material.

Fig. 2.7 Spherical window Fig. 2.8 Spectral tra nsmittance of a silicon window.

DISERTATION THESIS
19 Sensors operating in the mid and far infrared ranges require special windows that are
opaque in the visible and ultraviolet (UV) ranges while transparent in the wave length of
interest.
Spectral transmittances of some materials are shown in Fig. 2.3. When selecting material
for a mid and far-infrared window, the refractive index must be taken into account because it
determin es the coefficient of absorptivity, reflectivity, and in some cases, transmittance.
Figure 2.8 shows spectral transmittances for two silicon windows with different
thicknesses. Total radiation at the window is divided into three zones : Reflected ( which is about
50% o f the entire spectral range), absorptive ( it varies at different wavelengths) and transmitted,
which is the remaining after the reflection and absorption processes.
Since all windows have specific spe ctral transmissions, often they are called filters.
2.4. Mirrors
Mirror s are the oldest optical instrument s known in human history . Each time light
trverses different medium s, some reflection occurs . A single or multilayer reflecting coating is
applied on eithe r the front surface or the rear surface of a plane -parallel plate for enhanc ing an
object’s reflectivity .
The most accurate mirrors are those using the first surface. For the case of the secon d
surface mirror, light travels a plate with a different index o f refr action than the outside medium.,
thus a second surface mirror is a com bination of mirror and window.
For a second surface mirror , there are s everal effects which must be taken into
consideration. Firstly, because of the index of refracti on of a plate n, a reflective surface appears
closer (Fig. 2.9). For th e case of smaller angles Θ 1, a virtual thickness d of the carrier may be
depicted using the following formula [5]:
nLd
(2.22)
An interesting fact is that the front side of the second surface mirror may also reflect a
substantial amount of light , thus creating the so -called ghost reflection, such as, for example, a
glass plate which typically reflects about 4% of visible light. Further more , a carrier material
may have a s ubstantial absorption in the analyzed wavelength.

DISERTATION THESIS
20
Fig. 2.9 Second surface mirror
Considering a mirror which operates in far or mid -infrared spectral range, it should have
either first surface or second surface metalized with a substrate fabricated of ZnSe or other long
spectral range transparent materials. Because of their strong reflectivity, m aterials such as Si
or Ge are not suitable for covering the second surface of the mirrors.

Fig. 2.10 Spectral reflectance of some mirror coatings
Using an appropriate coating, any desired value, from nearly 0 to almost 1, can be
achieved for the reflectance (Fig. 2.10). Silver, al uminum, chromium, and rhodium can be used
as reflecting coatings for the visible and near -infrared range, while gold is suitable for the mid
and far -infrared spectral range devices. [5]
For broadband purposes, mirrors with pure metallic layers are used , deposited on glass
by vacuum or electrolytically means , fused silica, or metal substrates. To get a leveling effect,

DISERTATION THESIS
21 a mirror may be covered with an undercoat of copper, zirconium -copper or molybdenum,
before the reflective layer deposition.
Another useful reflector can be a prism in which is used the effect of total internal
reflection (TIR) . It may serve as a second surface mirror without the need for reflective
coatings . Its angle of total internal reflection is a function of a refractive index:


n1arcsin0
(2.23)
The total internal reflectors (TIR) have the greatest effici ency in the visible and near
infrared spectral ranges as the ir reflectivity coefficient is close t o unity. The TIR principle is
the main principle for the operation of optical fibers.
A reflective surface may have any shape to change the direction of light. It can be
mentioned that i n optical systems, curved mirrors produce similar effects to that of l enses. The ir
advantages consists of:
a. higher transmission, especially in the longer wavelength spectral range , where
lenses become less efficient due to higher absorption and reflectance loss,
b. absence of distortions generated by refractin g surfaces due to dispersion , and
c. lower size and weight .
For applications in which light has to be collected and focused, s pherical mirrors are
suitable to be used. In any case , spherical mirrors are suitable only for parallel or near parallel
beams of light which land on a mirror close to normal. These mirrors suffer from aberrations
[5].
Figure 2.11a shows a spherical mirror having the center of curvature in C. A focal point
is located at half of t he distance between the center of curvature and the mirror surface. The
spherical mirrors are astigmatic, which means that the off -axis rays are focused away from their
focal point.
For focusing light off -axis, parabolic mirrors are used. They allow comple te access to
the focal region without shadowing, as shown in Fig. 2.11b.

DISERTATION THESIS
22
Fig. 2.11 Spherical (a) and parabolic (b) first surface mirrors
2.5. Lenses
Lenses are useful i n sensors and detectors to divert the direction of light rays and
arrange them in a desirable fashion .
In Fig. 2.12 a plano -convex lens is shown; it has one spherical surface , while the other
is flat. The lens h as two focal points, one at each side: F and F ’, equally distanced from the
lens.

Fig. 2.12 Geometry of a plano -convex lens
When light rays from object G pass through the lens, their directions change , accordi ng
to Snell’s law.
It is convenient, t o determine the size and the position of an image created by the lens,
to draw two rays that have special properties. One of them is parallel to the optical axis and
after exiting the lens that ray goes thro ugh focus F’. The other ray initially passes through focus
F and after exiting the lens, travels parallel to the optical axis. The focal distance f of a thin lens
which have the radius of curvature much larger than the thickness of lens calculated with the
equation [5]:

DISERTATION THESIS
23




2 11111
rrnf (2.24)
where r1 and r 2 are radii of the lens curvatures. Image G ’ is inverted and positioned at a
distance b from the lens. That distance can be calcul ated using the thin lens equation :
baf111
(2.25)
When lens thickness t is comparable with the radii of curvature (thick lenses) , the focal
distance may be found using formula
1 12 121
ntrrn nrnrf
(2.26)
If needed, l enses can be combined into a more complex system. For example, two lenses
situated at a distance d from each other , will have a focal length calculated from equation [5]
dfffff
2 121
(2.27)
2.6. Fresnel Lenses
When is not required a high quality of focusing in sensors and detectors, the main
concern being the energy of light, Fresnel lenses can be used, Th ey are optical elements having
surfaces in shape of steps . Usual applications includ e light condensers, magnifiers, and
focusing eleme nt in occupancy detectors. Fresnel lenses can be made of glass, acrylic (visible
and near infrared range), or polyethylen e (mid and far -infrared ranges).
The history of Fresnel lenses began in 1748, when Count Buffon proposed grinding out
a solid piece of glass lens in steps of the concentric zones in order to reduce the thickness of
the lens to a minimum and to lower energ y loss. He realized that only the surface of a lens is
needed to refract light, because once the light is inside the lens, it propagates in a straight line.
His idea was modified in 1822 by Augustin Fresnel (1788 –1827), who made a lens in which
the centers of curvature of the different rings receded from the axis according to their distances
from the center, so as to practically eliminate spherical aberration. [5]

DISERTATION THESIS
24 The concept of that lens is illustrated in Fig. 2.13, where a regular plano convex lens is
depicted. The lens is sliced into several concentric rings. After slicing, all rings remain lenses,
which refract incident rays into a common focus defined by (2.24).
A change in an angle occurs when a ray exits a curved surface, not inside the lens;
hence, the section of a ring marked by the letter x does not contribute to the focusing properties.
If all such sections are removed, the lens wi ll look like as it is shown in Fig. 2.13b and will
fully retain its ability to focus light rays. Now, all of the rings may be shifted on one another to
align their flat surfaces (Fig. 2.13c).
A resulting near -flat but grooved lens is called Fresnel, which has nearly the same
focusing properties as the original planoconvex lens.
A Fresnel lens is an optical device which have a series of concentric prismatic grooves
designed to cooperatively direct incident light rays into a common focus. It has several
advantages over conventional lens, such as low weight, thin size, ability to be curved (for a
plastic lens) to any desirable shape and, most importantly, a lower absorpti on loss of the light
flux.
This is the main reason why this type of a lens is almost has exclusively been used in
lighthouses to form parallel beams of light (Fig. 2.14).

Fig. 2.13 Concept of a Fresnel lens.

Fig. 2.14 Fresnel lens for a lighthouse

DISERTATION THESIS
25

Fig. 2.15 Grooves of the Fresnel lens (a); computation of the groove angle ( b)
A lower absorption loss is very important for fabrication of the mid and far infrared
lenses , where absorption in the material may be significant. This is the reason why low -cost
polymer Fresnel is used almost exclusively in the far -infrared motion dete ctors.
When fabricating a Fresnel lens, it is difficult to maintain a curved surface of each small
groove; hence, the profile of a groove is approximated by a flat surface (Fig. 2.15a). This
demands that the ste ps be positioned close to each other. In fact, the closer the steps, the more
accurate the lens.
The limiting factor is the ability to tool and fabricate such closely positioned grooves.
There are several ways of designing grooves of the lens. The most co mmon is the so -called
constant step where all groves have the same pitch, that is, the distance between the neighboring
grooves.
A computation of the lens is essentially the computation of a groove angle, depending
on its number [5]. It is assumed that a monochromatic parallel beam is incident normally from
the left onto a flat surface of the lens.
The refraction takes place only at the grooved side. Applying Snell’s law of refraction
of a ray passing through the center of the groove, we arrive at
m m mn  sin sin
(2.28)
where n is the material’s refractive index for the desired wavelength, and the angles are
defined according to Fig. 2.15b.

DISERTATION THESIS
26 Considering ym be the distance from the optical axis to the mth groove, then for that
particular groove





tf tf yny
mm
m2 21tan
(2.29)
where f denominates the focal length and t represents the mean l ens thickness. This
equation can be rewritten in a dimensionless form, considering
ftt andfyym
m  ' '
(2.30)
and finally one can find the basic formula for computing a Fresnel lens:





'1 '1'tan
2 2'
1
t t yny
mm
m
(2.31)
The angles βm of the refracting prisms are fixed such that all the central rays of a
particular wavelength have a common focus. Refractind indexes may be found in tables.
For the mid and far -infrared ranges, low -density polyethylene (LDPE) has refractive
index 2.510, while the high -density polyethylene (HDP) has n= 1.540.
A Fresnel lens may be slightly bent if it is required for a sensor design. However, a bend
changes the positions of focal points. If a lens is bent with it s groves inside the curvature, all
angles βm change depending on the radius of curvature. A new focal distance can be found
from inverting (2.29) and solving it .
2.7. Fiber Optics and Waveguides
2.7.1. Theory
Optical fibers are widely used in telecommunication, wh ere they allow long -range
transmissions and wavelengths larger than wire cables. Fiber is also used for illumination and
is packaged in bundles, so they can be used to carry images, allowing viewing in closed spaces.
Specially designed optical fibers are u sed for a variety of other applications, including laser
sensors and fibers [6] [7].

DISERTATION THESIS
27 Fiber optics typically include a transparent core surrounded by a material with a lower
refractive index. Light is preserved in the core by total internal reflection. This makes the fiber
act as a waveguide. Fiberboards that support multiple propagation paths or cross -ways are
called multimodal fibers (MMFs), and those that support a single mode are single -mode fiber s
(SMFs). Multimodal fibers generally have a larger core diameter and are used for short
communication distances and applications where high-power information is transmitted [8] [7].
Fiber optics ha ve many uses in remote sensing [9] [10]. In some applications, the sensor
itself is an optical fiber, but in other cases the fiber is used to interface a non -fiber optical sensor
with the measuremen t system.
The use of fiber optics has a number of advantages: small size, no electrical energy at
the remote location, multiple sensors using different wavelengths of light can be assembled on
the length of the fiber, or allow detection of delay of light t ransmission, Along the fiber through
each sensor.
Sensors that vary light intensity are the simplest because they only use a simple light
source and a detector. A particularly useful feature of these optical fiber sensors is that they can
measure on distan ces of up to one meter.
A useful propery of extrinsic sensors is their ability to reach inaccessible places. An
example is the measurement of the temperature inside the aircraft engine. The extrinsic sensors
can be used to measure the internal temperature of electric transformers, where the extreme
electromagnetic fields make other measurement techniques impossible, or can be used to
measure vibration, rotation, displacement, velocity, acceleration, twist, etc. [11] [12] [13].
Optical fiber is also used to obtain optical images. A coherent fiber package is used ,
sometimes along with lenses, to create an imaging device called an endoscope that is used to
view objects throug h a small hole. Medical endoscopes are used for minimally invasive surgical
or exploratory procedures. Industrial endoscopes are used to inspect hard -to-reach targets, such
as jet engine interiors.
In spectroscopy, the optical fiber transmits light from a spectrometer to a substance that
can not be placed inside the spectrometer to analyze its composition. A spectrometer analyzes
the substances by which light falls on and through them. By using fiber, a spectrometer can be
used to study material at a distan ce [14] [15].

DISERTATION THESIS
28 An optical fiber can be doped with certain chemical elements (such as erbium, used as
an optical amplifier) having applications in detecting substances, concentrations, chemical
reacti on rates, etc. [15] [16].
Although light does not go around the corner, it can be channeled along complex paths
by the use of waveguides. For guides t o operate in the visible and near infrared spect ral ranges ,
they may be fabricated from glass or polymer fibers . For the mid and far-infrared spectral
ranges, the waveguides are made of special materials or as hollow tubes with highly reflective
inner surfaces.
The tubular waveguide operates on the pri nciple of reflection where light beams travel
in a zigzag pattern. A fiber can be used to transmit light energy in the otherwise inaccessible
areas without any transport of heat from the light source.
The surface s and ends of a round or other cross -section fiber are polished. An outside
cladding may be added . When glass is hot, the fibers can be bent to curvature radii of 20 –50
times their section diameter and after cooling, to 200–300 diameters. Plastic fibers fabricated
of polymethyl methacrylate may be be nt at much smaller radii than glass fibers. A typical
attenuation for a 0.25 -mm polymer fiber is in the range of 0.5 dB/m of length.
Light propagates through a fiber by means of a total internal reflection, as shown in Fig.
2.16b. It follows from ( 2.23) that light passing to air from a medium that have the refractive
index n is limit ed to propagate in air by the angle of total internal reflection [5].
In a more general form, light may pass to another medium (cladding) having refractive
index n 1, then, ( 2.23) becomes


nn1
0arcsin
(2.33)
Figure 2.16a shows a profile of the index of refraction for a single fiber with the
cladding where the cladding must have a lower index of refraction to assure a total internal
reflection at the boundary. For exam ple, a silica -clad fiber may have compositions set so that
the core (fiber) material has an index of refraction of 1.5, and the clad has an index of refraction
of 1.485.

DISERTATION THESIS
29 To protect the clad fiber , it is typically enclosed in some protective rubber or plas tic
jacket. This type of the fiber is called a “step index multimode” fiber, which refers to the profile
of the index of refraction.
When light enters the fiber, it is important to determine the maximum angle of entry
which will result in total internal re flections (Fig. 2.16b).

Fig. 2.16 Optical fibers: A step -index multiple fiber (a) ; determination of the maximum angle of entry (b)
If considering that minimum angl e of an internal reflection Θ 0=Θ3, then the maximum
angle Θ2 can be found from Snell’s law:



nnn2
12
max2 arcsin
(2.34)
Applying Snell’s law again and considering that for air n =1, one can obtain
 max2 1 max sin sin  nin
(2.35)
Through the combination of (2.34) and ( 2.35), one can obtain the largest angle with the
normal to the fiber end for which the total internal reflection will occur in the core:
2
12
max arcsin nnin  
(2.36)
Light rays which enter the fiber at angles greater than Θin(max) will pass through to the
jacket and will be lost. For data transmission, this is an undesirable event. However, in a
specially designed fiber -optic sensor, the maximum entry angle can be a useful phenomenon
for modulating light intensity.

DISERTATION THESIS
30 Sometim es, the value Θin (max) is called the numerical aperture of the fiber. Due to
variations in the fiber properties, bends, and skewed paths, the light intensity does not drop to
zero abruptly but rather gradually diminishes to zero while approaching Θin (max ).
In practice, the numerical aperture is defined as the angle at which light intensity drops
by some arbitrary number (e.g . 10 dB of the maximum value).
One of the useful properties of fiber -optic sensors is that they can be formed into a
variety of geom etrical shapes , depending on the desired application. They are very useful for
the design of miniature optical sensors which are sensitive to stimuli like pressure, temperature,
chemical concentration, and so forth.
The basic idea for the use of fiber opt ics in sensing is to modulate one or several
characteris tics of light in a fiber and, subsequently, to optically demodulate the information by
conventional methods.
A stimulus may act on a fiber either directly or it can be applied to a component attached
to the fiber’s outer surface or the polished end to produce an optically detectable signal. To
make a fiber chemical sensor, a special solid phase of a reagent may be formed in the optical
path coupled to the fiber.
The reagent interacts with the analyte to produce an optically detectable effect (e.g.,
modulating the index of refraction or coefficient of absorption). A cladding on a fiber may be
created from a chemical substance whose refractive index may be changed in the presence of
some fluids [3].
When the angle of total internal reflection changes, the light intensity varies.
Optical fibers may be used in two modes. In the first mode (Fig. 2.17a), the same fiber
is used to transmit t he excitation signal and to collect and conduct an optical response back to
the processing device.

Fig. 2.17 Single (a) and dual (b) fiber -optic sensors.
In the second mode, two or more fibers are employed whe re excitation (illumination)
function and collection function are carried out by separate fibers (Fig. 2.17b). The most

DISERTATION THESIS
31 commonly used type of fiber optic sensor is an intensity sensor, where light intensity is
modulated by an external stimulus [17].

Fig. 2.18 Fiber -optic displacement sensor utilizes the modulation of reflected light intensity
Figure 2.18 shows a displacement sensor where a single -fiber waveguide emits light
toward the reflective surface. Light travels along the fiber and exits in a conical profile toward
the reflector. If the reflector is close to the fiber end (distance d), most of the light is reflected
into the fiber and propagates back to the light detector at the other end of the fiber. If the
reflector moves away, some of the rays are reflected outside of the fiber end, and fewer photons
are returned.
Due to a conical profile o f the emitted light, a quasilinear relationship between the
distance d and the intensity of the returned light can be achieved over a limited range.

Fig. 2.19 Fiber -optic microbend strain gauge (a) and a waveg uide for the far infrared radiation (b)
The so -called microbend strain gauge can be designed with an optical fiber, which is
squeezed between two deformers, as shown in Fig. 2.19a. The external force applied to the
upper body bends the fiber, changing the shape of an internal reflective surface. In this case , a
light beam, which normally would be reflected in x direction, impacts the lower part of the
fiber at an angle which is less than Θ0, the angle of total internal reflection ( 2.33). Thus, instead
of being reflected, light is refracted thus changing its path in y direction, through the fiber wall.

DISERTATION THESIS
32 The greater the force , the more light goes astray and the less light is transmitted along
the fiber.
For operation in the spectral range where loss in fibers is too great (mid and far infrared
spectral ranges), hollow tubes are used for light channeling (Fig. 2.19b).
The tubes are highly polished inside and coated with reflective metals. For instance, to
channel thermal radiation, a tube may be fabricated of brass and coated inside by two layers:
Nickel as a leveling underlayer and the optical quality gold having t hickness in the range 500 –
1,000 Å. Hollow waveguides may be bent to radii of 20 or more of their diameters [5].
Although fiber optics use the effect of the total internal reflection, tubular waveguides
use a first surfac e mirror reflection, which is always less than 100%. As a result, loss in a hollow
waveguide is a function of a number of reflections; that is, loss is higher for the smaller diameter
and the longer length of a tube.
At length/ diameter ratios more than 2 0, hollow waveguides become quite inefficient
and fiber optic devices should be considered; for example, AMTIR .
2.7.2. Working P rinciple
An optical fiber is a dielectric cylinder that transmits light along its axis through the
total internal reflection process. The fiber is made of a core surrounded by a skeleton layer,
both of which are made of dielectric materials. In order to keep the optical signal in the core,
the refractive index of the core must be greater than that of the sheath.
Refractive index is a way to measure the speed of light in a material and is a feature
commonly used in optical sensors.
To keep light in the core of optical fibers , total internal reflection is used . Light passes
through the core of the fiber, falls back and forth on the boundary between the core and the
sheath. Since light must strike the border at an angle greater than the critical angle, only light
entering the fiber between certain angles can pass the fiber accordingly. This series of angles
is called fiber acceptance cone, and its size depends on the difference in refractive index
between fibers and skeletons.
2.7.2.1. Optical Multi-Mode Fiber
Optical fibers with a core diameter greater than 10 micrometers are called multimodal
fibers from electromagnetic analysis (Fig. 2.20 and 2.21). Light rays are guided on the fiber

DISERTATION THESIS
33 depending on the total internal reflection. Rays that meet the boundary of the core at a larger
angle (measured relative to a norma l line at the limit) than the critical angle for this limit , are
fully reflected.

Fig. 2.20. Propagation of light through a multi -mode fiber. [18]

Fig. 2.21. Total internal light reflection in multi -mode optical fiber [18]
The critical angle (the minimum angle for total internal reflection) is determined by the
difference in refractive index between the base and the plating material. Rays that meet the
boundary at a small angle are refracted from the core in the sheath and cannot transmit light,
so there is no information along the fiber. The critical angle determines the angle of acceptance
of the fiber, of ten reported as a numerical aperture. A large numeric aperture allows the light
to propagate into the fiber in radii both close to the axis and at different angles, allowing
efficient light coupling into the fibers. However, this large numerical aperture i ncreases the
amount of dispersion.

DISERTATION THESIS
34 2.7.2.2. Optical Single -Mode Fiber
Fiber with a small core diameter can propagate light along it in one or a limited number
of transverse modes and is referred to as mono filament or monofilament (Fig. 2.22).
The most common type of single -mode optical fiber has a core diameter of 8 to 10
micrometers and is designed for use near the infrared range. The structure of the fiber depends
on the wavelength of the light used, so that this fiber actually carries a small number of
additional modes at visible wavelengths.
2.7.2.3. Special Fiber
Some special optical fibers are constructed with a non -cylindrical base and / or skeleton
layer, usually with an elliptical or rectangular cross section

Fig. 2.22. Typical structure for single -mode fiber
1. base: 8 µm dia; 2. sheath: 125 μm dia; 3. buffer: 250 μm dia; 4. jacket: 400 µm dia. [18]
The fiber uses diffraction effects instead of or in addition to the total internal reflection
and can be adapted to a wide variety of applications [19].
2.7.3. Mitigation M echanisms
Fiber optic mitigation is the reduction in intensity of the passing beam (or signal)
moving through th e transmission medium. Fiber optic mitigation coefficien ts typically use
dB/km units due to the relatively high transparency quality of the optical transmission medium.
Medium is usually a silicon glass fiber, which limits the incident light beam within th e fiber.
Attenuation is an important factor, mainly due to scattering and absorption, and scientific

DISERTATION THESIS
35 research focuses on limiting, attenuating, and maximizing optic signal amp lification (Fig. 2.23)
[18].

Figur e 2.23. Mitigation of light through ZBLAN fibers and silicon dioxide
2.7.4. Light Scattering
Propagation of light inside the core of an optical fiber is based on the total internal
reflectio n of light. Irregular and tough surfaces, even at the molecular level, can cause light
rays to be reflected in random directions. This is called diffuse reflection and is usually
characterized by a wide variety of angles of reflection.
The scattering of li ght depends on the scattered light wavelength. Thus, the boundaries
of the spatial visibility scales appear, depending on the frequency of incident light and the
physical size of the spreading center (Fig. 2.24 and 2.25) [18][19].

Figure 2.24. Specular reflection

DISERTATION THESIS
36
Figure 2.25. Diffuse reflection
Thus , mitigation results from incoherent scattering of light from internal surfaces and
interfaces. When the scattering center size decreases below the scattered light wavelength, the
dispersion no longer appears to be significant. This phenomenon turned the a ttention toward
the production of transparent ceramic materials. Similarly, the scattering of light in quality glass
fiber glass is caused by molecular irregularities (compositional fluctuations) in the glass
structure. Scattering can also be caused by non linear fiber optic processes when high optical
powers are involved [20].
2.7.5. UV-VIS-IR Absorption
In addition to light scattering, the attenuation or loss of the signal may occur due to
selective absorption of specific wavelengths a nd the appearance of color. They are determined
by both electrons and molecules as follows:
1) If the electron orbitals are spaced , they can absorb a light (or photons) of a certain
wavelength or frequency from ultraviolet (UV) or visible, giving rise to c olor.
2) At atomic or molecular level, depending on atomic, molecular or chemical bond
vibrations, the material can transmit higher infrared (IR) wavelengths causing color.
Selective infrared absorption (IR) of a particular material occurs because the sele cted
wavelength of light matches the frequency (or a multiple frequency multiple) at which the
particles of that material vibrate. Since different atoms or molecules have different natural
vibration frequencies, they will selectively absorb different (or i nfrared) (IR) spectrum
frequencies.
Because light wave frequencies do not match the natural resonance frequencies of
object vibrations , reflection and transmission of light waves occur . When the IR light of these
frequencies hits an object, the energy is e ither reflected or transmitted [21] [22] [23] [24].

DISERTATION THESIS
37 2.7.6. Fiber O ptic Fabrication
2.7.6.1. Materials
Optical glass fibers are almost always made of silicon, b ut other materials can be used,
such as fluorosurconate, fluoroaluminate as well as crystalline sapphire materials.
Optical plastic fibers (POF) typically have attenuation coefficients higher than glass
fibers, and this high attenuation limits POF -based se nsor systems [25].
Silicon dioxide offers a fairly good optical transmission over a wide range of
wavelengths. It can be pulled into fibers at fairly high temperatures and has a relatively efficient
cutting capacity. It also ha s high mechanical strength, is relatively inert chemically and is not
hygroscopic (absorbs water) [26] [27].
Silicon glass can be doped with various materials. One of the doping purposes is to raise
the refractive index, for example with GeO 2 or Al 2O3 or to lower it (for instance with B2O3).
Silicon fiber also has a high threshold for optical damage. Because of these properties,
silicon fibers are used in many optical applications, such as communicat ions, laser fibers, fiber
amplifiers and optical fiber sensors [28] [29].
Fluoride and phosphates are also used as fiber optic materials.

Fig.2.26. P4O10 Cage like structure — base block for glass phosphate .
In order to obtain glass phosphate, phosphorus pentoxide (P 2O5) is used instead of SiO 4,
which crystallizes in at least four different forms. Th e most popular polymorph (Fig. 2.26)
comprises P 4O10 molecules. A mixture of glass fluoride and phosphate glass is glass
fluorophosphates [30] [31].

DISERTATION THESIS
38 2.7.6.2. Coatings
Light is guided into the fiber core by an optical she ath with a lower refractive index that
keeps light in the core of the fiber through total internal reflection. The skeleton is covered with
a buffer that protects the fiber from moisture and physical damage. The coatings protect very
delicate fiber optics and allow them to maintain manufacturing rigors, test evidence, wiring,
and installation. The optical fiber may be multiple coated: an inner layer acting as a shock
absorber and a secondary outer layer that protects the main layer against mechanical damage
and acts as a barrier against lateral forces. Sometimes a metal armor layer is also added for
increased fiber protection.
These coatings are applied by two methods : moist on dry and moist on wet. Fiber optic
coatings surround the core in concentric layers to avoid damage to the fiber and to protect glass
fibers from scratches that could lead to resistance degradation. The combination of moisture
and scratches accelerates the aging and damage of the fiber [18].
2.8. Concentrators
There is an important issue of increasing density of the photon flux impinging on the
sensor’s surface. In many cases, when only the energy factors are of importance, and a focusing
or imaging is not required, special optical devices can be used quite effecti vely. These are the
so-called non imaging collectors, or concentrators [32] [5]. They have some properties of the
waveguides and some properties of the imaging optics (like lenses and curved mirrors ).
The most important characteristic of a concentrator is the ratio of the area of the input
aperture versus the area of the output aperture, which is the concentration ratio C. Its value is
always more than unity. That is, the concentrator collects light from a larger area and directs it
to a smaller area (Fig. 2.27a) where the sensing element is positioned.
There is a theoretical maximum for C:
iC2 maxsin1
(2.37)
where Θi is the maximum input half-angle. Under these conditions, the light rays
emerge at all angles up to π/2 from the normal to the exit face.

DISERTATION THESIS
39
Fig. 2.27 Nonimaging concentrator: General schem atic (a), concentrator having a parabolic profile (b)
and Winston cone attached to a pyroelectric sensor
This means that th e exit aperture diameter is smaller by sin Θi times the input aperture.
This gives an advantage in the sensor design as its linear dim ensions can be reduced by that
number while maintaining a near equal efficiency. The input rays entering at angle Θ will
emerge within the output cone with angles dependent of point of entry.
The concentrators can be fabricated with reflective surfaces (mi rrors) or refractive
bodies (e.g., Fresnel lenses), or as combinations of both. A practical shape of the reflective
parabolic concentrator is shown in Fig. 2.27b and its connection to a sensor in Fig. 2.27c.
An interesting observation is that cone light receptors in the retina of a human eye have
a similar shape to that shown in Fig. 2.27b [33].
The tilted parabolic concentrators may have very high efficiency; they can collect and
concentrate well over 90% of the incoming radiation. If a lesser efficiency is acceptable, a
conical rather than paraboloid concentrator can be employed. Some of the in coming rays will
be turned back after several reflections inside the cone, however, its overall efficiency is still
near 80%. Clearly, the cones are easier to fabricate than the paraboloids of revolution.
2.9. Coatings for Thermal Absorption
All thermal radiati on sensors rely on absorption or emission of the electromagnetic
waves in the mid and far -infrared spectral ranges. According to Kirchhoff’s discovery,
absorptivity a and emissivity e is the same thing.
Their value for the efficient sensor’s operation must be maximized, i.e., it should be
made as close to unity as possible. This can be achieved by either processing the surface of a

DISERTATION THESIS
40 sensor to make it highly emissive or covering it with a special coating having a high emissivity.
Any such coating should have a good thermal conductivity and a very small thermal capacity,
which means that it must be thin.
Several methods are known to give a surface the emissive (absorptiv e) pro perties. Some
of them are deposition of thin metal films (like nichrome) having reasona bly good emissivity,
galvanic deposition of porous platinum black [34] and evaporation of metal in atmosphere of
low-pressure nitrogen [5].
The most effective way to create a highly absorptive (emis sive) material is to form it
with a porous surface [35]. In all cases particles having sizes much smaller than the wavelength
absorb and diffract light. High emissivity of a porous surface covers a broad spectral range;
however , it decreases with the increased wavelength. A film of goldblack with a thickness
corresponding to 500 mg/cm2 has an emissivity over 0.99 in the near, mid -, and far -infrared
spectral ranges.
To form porous platinum black, the following electroplating reci pe can be used [36]:
Platinum chloride H2PTCl 6 aq 2 g
Lead acetate Pb(OOCCH 3)2∙3H 2O 16 mg
Water: H2O 58 g

Out of this galvanic bath, the films were grown at room temperature on silicon wafers
with a gold underlayer film. A c urrent density was 30 mA/cm2. To achieve absorption better
than 0.95 , a film of 1.5 g/cm2 is needed [5].
To form a goldblack by evaporation, the process is conducted in a thermal evaporation
reactor in a nitrogen atmosphere of 100 Pa pressure. The gas is injected via a microvalve, and
the gold source is evaporated from a tungsten wire electrically heated, from a distance of about
6 cm [5].
Due to collisions of evaporated gold with nitrogen, the gold atoms lose their kinetic
energy and are slowed down to thermal speed. When they reach the surface, their energy is too
low to allow surface mobility, and they stick to the surface on the first touch event. Gold atoms
form a surface structure in the form o f needles with linear dimensions of about 25 nm. The
structure resembles a surgical cotton wool. For the best results, goldblack should have thickness
in the range from 250 to 500 mg/cm2.

DISERTATION THESIS
41 Another popular method used to enhance emissivity is t o oxidize a su rface metal film
thus creating metal oxide, which is highly emissive. This can be done by a metal deposition in
a partial vacuum [5].
Another method forimproving the surface emissivity is to coat a surface with an organic
paint (the color of the paint is not important ). These paints have far-infrared emissivity from
0.92 to 0.97; however, the organic materials have low thermal conductivity and this is why they
cannot be effectively deposited with thicknesses less than 10 μm, which may significantly slow
the sensor’s speed response. For micromachined sensors, the top surface may be passivated ,
which not only provides an environmental protection, but has emissivity of about 0.95 in the
far infrared spectral range.
2.10. Nano -optics
Nano -optics deals with the interaction of light with particles or substances, at deeply
subwavelength scales. Nano -structure based optics, or nano -optics, is a class of optical devices
based on finely patterned materials with critical dimensions several times smaller than the
wavelength of light at which they are applied . By combining material and structural properties,
nano -optics allow optical devices that are thin, offer high performance, and are highly reliable.
Because nano -optic devices are produce d using semiconductor -like manufacturing methods,
they are readily integrated in arrays or multilayer structures, or with other optical and electronic
components. This enables optical circuit designers to simplify optical circuits by combining
optical func tions or to increase functional capability by using tunable devices [5].

DISERTATION THESIS
42 3. ETHANOL SENSORS ON OPTIC FIBERS
3.1. Introduction
One of the most important substances having application in food industry,
biotechnologies, fermenters , pharmaceutical and medicine fields and also in new fields, like
bioethanol , for its application in fuel technology , is ethanol . For any application it is necessary
to detect and quantify the ethanol with high accuracy, but all of these applications have differ ent
requirements, including sensitivities, detection limit and assay time. In order to determine the
concentration of ethanol many methods can be used, such as electrochemical, Raman and mass
spectroscopies, polarography chromatography, and enzymatic assay , etc [37]. However, these
methods are generally either time consuming and require the use of expensive instrumentation.
These disadvantages can be overcome using enzymatic methods. In such enzymatic methods,
most enzyme -catalyzed r eactions can be followed by simple, widely available spectroscopic or
electrochemical methods. Optical methods require light devices, such as spectrophotometer s.
Moreover, such methods cannot be applied to turbid samples, such as blood or food samples.
Furthermore, for carrying out the enzymatic reaction, a troublesome procedure involving
dissolution of various reagents and distribution of the resulting solutions into reaction vessels
in amounts specified beforehand is required, and the reagent solutions, s uch as prepared NAD+
solution have poor storage stability. For these and other reasons, those methods are not suitable
for general use [37].
Sensors involving light measurements associated with fiber -optics are particularly
attract ive. The different configurations for fiber optic based sensors, their characteristics and
their potential applications have been recently reviewed [38] [39]. In most cases the fiber optic
function is usually nade from a fluorescent dye or a colorimetric indicator , secured at one end
of the fiber optic . For biochemical analysis, substrates or enzymes can be immobilized,
allowing the determination of enzyme activities [37].
The demand for faster and more sensitive methods of analysis in the medical,
environmental and industrial fields has generated in recent years the development of optical
devices based on an enzymatically catalyzed light emission [40].
The standard approach uses an enzyme assay coupled with an electrode or optical
detection technique.

DISERTATION THESIS
43 An ethanol enzyme assay may use either alcohol oxidase (AOD) or alcohol
dehydrogenase (ADH).
One of the main factors which affect the performance o f biosensor s is the enzyme
immobil ization procedure . Until now it seems that no fully satisfactory method for wide range
ethanol concentrations quantification is yet available. The previously mentioned methods are
not only limited to a specific range of et hanol concentration, but they also have difficulties
mentioned above. Furthermore, some of them are expensive. [41]
In the reaction involving alcohol dehydrogenase (ADH) ethanol is converted to
acetaldehyde with the reduction of th e nicotinamide adenine dinucleotide cofactor (NAD +) to
NADH. [37]
Et-OH + NAD+ + ADH → acetaldehyde + NADH+H+ (3.1)
This reaction offers the advantage over the more commonly used alcohol oxidase
reaction of not requiring oxygen as a reacta nt. Using the enzyme reaction 3.1, ethanol
concentration can be determined by measuring the absorption of NADH at a wavelength of 340
nm.
Ethanol sensing analysis methods are essentially similar to spectrophotometric
analytical methods, but much more elaborate. The most common method used is the detection
of spectral transmission analysis. This can be achieved in particular in two main reg ions of the
optical spectrum:
a. The 250 – 500 nm region ( i.e. UV – visible) is generally used to detect absorption or
emission lines that result from electronic transitions i n atoms and molecules
concerned . The absorption process involves an increase in electr on energy and a
decrease in emissions. This region of the spectrum is very useful for remote sensing,
which can occur in the atoms or molecules of a large number of species.
b. the wavelength region also covers a second middle IR range for liquid species but
in this absorption region and fiber optic silicon and therefore this area is generally
avoided.
Most methods used for concentration of liquids (in this case ethanol) sensors were based
on simple absorption, although occasionally Raman mechanisms were used as a detection
method [42]. The main limitation imposed on the use of optical fibers for these sensors is

DISERTATION THESIS
44 associated primarily with the restrictions imposed by light transmission through fiber. The use
of conventional silicon f iber optic fibers mainly uses NIR absorption lines [43] [44].
Laser sources significantly facilitate constraints caused by the detection system, but
they can introduce so -called "modal noise", a noi se intensity resulting from modal mixing.
Detection systems often operate with a very low absorption level, a slight change in intensity
can cause serious noise or systematic errors .
3.2. Experimental Procedure and S ensor
In this chapter is presented the develo pment of a novel absorption fiber optic biosensor
for ethanol, based on the use of reaction ( 3.1) in order to determine the alcoholic concentration.
The optical fiber biosensor consists of the enzyme alcohol de hydrogenase entrapped with
NAD+ in a sol -gel TEOS/PEOS die.
Changes in alcohol concentration are reflected in the amount of NADH produced, thus
resulting in a change of absorbance of the laser wavelength at 340 nm [37].
3.2.1. Experimenta l Procedure
3.2.1.1. Instrumentation
The following instruments/devices were used for measurements:
a. Portable laser spectrophotometer, an Ocean Optics Jazz, with two channels, one
channel of "master" type for UV -VIS range (1025 -200 nm) and one "slave" channel
for the VIS-NIR (360-1100nm). The spectrophotometer has a built -in
microprocessor and OLED display used when independent applications are needed
(without PC) . It has 2-channel and source excitation deuterium -halogen -tungsten
7W laser diode, in the range of 210 -1100 nm, with lithium -ion battery integrated
power supply , included in the platform.
b. Bifurcated Optical Fiber – Southwest Bible Institute for the 210 -1100 nm, consisting
of 2 fibers inside an external coating , one for light-emission purposes , and the other
for reading, fiber length 2 m, with SMA 905 connectors .
c. SMA 905 Multimode fiber connector, p repared for enzyme coating : the cut end of
the optical fiber has been polished and a portion of 30 -40 mm from the end of the
cladding has been removed for the future steps of enzyme applying.

DISERTATION THESIS
45 d. Spectrasuite Software for data acquisition and real time analysis, as well as for data
processing.

Fig. 3.1 Experimental setup used to measure the spectral response of the interrogat or system
The experimental setup used to measure the spectral response of the interrogator system
used for determinations is presented in Fig. 3.1 and it consists of:
a. Portable laser spectrophotometer, Jazz from Ocean Optics;
b. PC that runs software Spectrasuite for acquisition and real time analysis of data ;
c. Optical fiber bifurcated;
d. Optical fiber biosensor .
The instrumentation setup realised to interrogate the sensing probe performed the
illumination on the tip of the optical fiber while collected the reflected beam.
3.2.1.2. Materials
All materials used for determinations were purchased from S igma -Aldrich Company
(Germany):
a. β-nicotinamide -adeninedinucleotide
(NAD+) solution 0 .063mM
b. ADH (EC 1.1.1.1) Alcohol
dehydrogenase enzyme , 60 U/ml,
c. High purity ethanol (99.985%) used in
preparing standard ethanol samples. d. Tetraethoxysilane 98% (TEOS) –
e. Methyltriethoxysilane 90% (PEOS) –
f. Solution HCl 1N
g. Solution, NH 4OH 1N
h. Phosphate buffer solution pH = 7
i. Ethanol solutions (4 -18%)

3.2.1.3. Procedure
Working principle is based on a classic colorimetric approach (Fig. 3.2). The
interrogator light is scattere d and absorbed by the sensing matrix that include ADH and NAD.
The formation of NADH in biochemical reaction induce a change of the optical spectrum of
the outgoing light collected by the fiber. Analisys of the spectrum for this optical signal offers
the c oncentration of the ethanol [41].

Fig. 3.2. Schematic diagram o f an optical fiber detector
Sol-gel enzyme immobilization technique under mild conditions of temperature and
pressure wit hout altering the biological activity has been used .
3.2.2. Sensor
The developed sensor is based on plastic optical fiber with the core diameter of 1μm
terminated with a sensing element. The fiber end was cut and polished while a portion of 30 –
40 mm of the cladding from one end was removed for the next enzyme applying steps. The
sensing matrix is made from PEOS/ TEOS hybrids obtained using sol-gel technique.
In a glass bottle a certain amount of TEOS/PEOS in the ratio 1:1 with solution HCl 1N
has been mixed by magnetic stirring and the mixture was homogenized for 2 -3 hours. After
getting the homo genous solution the pH was adjusted to 6 using 25 % ammonium hydro xide
solution. NAD+ and ADH solution were added . The fiber optic was immersed into solution at
the moment w hen the mixture had a proper consistence stirred it with a certain speed , assuring
an uniform film matrix deposition onto optical fiber. After this t he layer was dried at room
temperature for 24 hours prior to use and kept immersed in phosphate buffer having pH = 7
[41]. All the measurements were performed at room temperature.

DISERTATION THESIS
47 In order to evaluate the performance of the developed sensor, the spectral properties of
the sensor have been calibrated using software Spectrasuite for acquisition in real -time analysis
of data and compared with spectral analysis gathered form the UV -VIS spectrophotometer
Specord 210 plus (Analytik Jena, Germany).
In order to test the optical sensor and to determine the alcohol content it was realized
the standard curve using ethanol solutions from 0 to 18 %, as presented in Fig. 3.3.
The sensor was immersed in calibrated ethanol solutions whi le reading the optical
density at 340 nm.

Fig. 3.3 Calibrating curve
Fig. 3.3 shows a linear relationship, which can be used in quantification of ethanol
concentration , the sensor equation being y = 0.1511x + 0.1673
Data used to draw the calibrating curve are presented in table 3.1
Table 3.1 Standard curve
Crt. no. Ethanol concetration (%) Absorbance (OD)
1. 2 0.318
2. 4 0.489
3. 6 0.632
4. 8 0.751
5. 10 0.909
6. 12 1.077
7. 14 1.204
8. 16 1.371
9. 18 1.556

DISERTATION THESIS
48 A typical spectr um in real time raised for one of the samples is presented in Fig. 3.4

Fig. 3.4 Absorbance spectr um of sample 1
In order to compare the precision of the calibrated fi ber optic sensor similar tests were
conducted for measure ADH activity on both Jazz spectrometer using the designed sensor and
UV-VIS spectrometer SPECORD 210 Plus from Analytic Jena, Germany.
ADH activity measured on Jazz spectrophotometer using optical f iber sensor in real
time is presented in Fig. 3.5 and ADH activity measured on SPECORD spectrophotometer is
presented in Fig. 3.6.
All th e spectra show a well visibl e maximum at the waveleng th at about 340 nm. The
results were compared with classical spectrophotomet ric determinations and indicated a good
correlation between the ethanol concentration amount.

DISERTATION THESIS
49
Fig. 3.5 ADH activity measured by optical fiber sensor in real time

Fig. 3.6 ADH activity measured by SPECORD spectrophotometer

DISERTATION THESIS
50 4. CONCLUSIONS
Optical fiber biosensors are suitable to measure the ethanol concentration in the range
of 2-18% ethanol and allows experimental determinations in real time. During experiments a
good correlation between ethanol concentration measured with optical fiber sensor and
classical spectrophotometer method has been obtained .
Despite the fact that the technology of manufacturing these sensors is relative simple, a
great amount of care must be accorded, both on fabrication process and during preservation
until measuring, due to the fact that sensor implies biologic elements. This is why sensors must
be preserved in solutions having pH=7.
The main advantage of using these sensors is, among their low -cost procedures, the fact
that they can be used on -site (usually in cellars), and due to the Jazz spectrophotometers which
are portable and can funct ion independently from computers, the fermentation process of wine
can be supervized in real time.
One of the disadvantages, but not a major one, is that each of the sensor must be
independently calibrated and from time to time, a recalibration must be con ducted.

DISERTATION THESIS
51 5. REFERENCES

[1] B. Begunov, N. Zakaznov, S. Kiryushin and V. Kuzichev, Optical
instrumentation. Theory and Design, Moscow: Mir Publishers, 1988.
[2] Motorola, Applications of Phototransistors in Electro -optic Systems,
1988.
[3] J. Giuliani, "Optical waveguide chemical sensors," in Chemical sensors
and microinstrumentation. Chapter 24 , Washington, American Chemical Society,
1989.
[4] L. M. Johnson, "Optical modulators for fiber -optic sensors," in Fiber
Optic Sensors: In troduction for Engineers and Scientists , New York, JohnWiley
& Sons, 1991.
[5] J. Fraden, Handbook of Modern Sensors Physics, Designs, and
Applications, New York, Berlin, Heidelberg: Springer -Verlag, 2004.
[6] P. S. Archibald and H. E. Bennett, Scatt ering from infrared missile domes,
1978.
[7] C. Kurkjian, P. Simpkins and D. Inniss, "Strength, Degradation, and
Coating of Silica Lightguides," Journal of the American Ceramic Society, vol. 76,
no. 5, p. 1106, 1993.
[8] G. S. Glasesmenn, "Advancemen ts in Mechanical Strength and Reliability
of Optical Fibers," in Proceedings of SPIE CR73 , 1999.
[9] K. Thyagarajan and A. K. Ghatak, Fiber Optic Essentials, Wiley –
Interscience, 2007.
[10] J. Hecht, City of Light, The Story of Fiber Optics, New York: Oxford
University Press, 1999.
[11] A. Skontorp, "Nonlinear mechanical properties of silica -based optical
fibers," in Proceedings of SPIE. Fifth European Conference on Smart Structures
and Materials , 2000.

DISERTATION THESIS
52 [12] M. Karabulut, E. Melnik, R. Stefan, G. K. Marasinghe, C. S. Ray, C. R.
Kurkjian and D. E. Day, "Mechanical and structural properties of phosphate
glasses," Journal of Non -Crystalline Solids, no. doi:10.1016/S0022 –
3093(01)00615 -9, p. 288, 2001.
[13] Wikipedia, "Fiber optic sensor," Wikipedia , [Online]. Available:
https://en.wikipedia.org/wiki/Fiber_optic_sensor. [Accessed 3 05 2017].
[14] C. Kurkjian, "Mechanical properties of phosphate glasses," Journal of
Non-Crystalline Solids, no. doi:10.1016/S0022 -3093(99)00637 -7, p. 263 –264,
2000.
[15] J. Gowar, Optical communication systems, Hempstead: Prentice -Hall,
1993.
[16] J. Hecht, Understanding Fiber Optics, Prentice Hall, 2002.
[17] G. Mitchell, "Intensity -based and Fabry -Perot interferometer sensors," in
Fiber optic sensors: an intro duction for engineers and scientists. Chapter 6. , New
York, Wiley, 1991.
[18] Wikipedia, "Optical fiber," Wikipedia, [Online]. Available:
https://en.wikipedia.org/wiki/Optical_fiber. [Accessed 21 04 2017].
[19] Wikipedia, "Single -mode optical fiber," Wikipedia, [Online]. Available:
https://en.wikipedia.org/wiki/Single -mode_optical_fib. [Accessed 21 04 2017].
[20] B. Samson, A. Carter and K. Tankala, "Rare -earth fibres power up,"
Nufern, [Online]. Available: http://www.nufern.com/library/item/id/383/.
[Accessed 12 04 2017].
[21] A. M. M. F. J. M. &. F. L. P. Azevedo, "Ethanol biosensors based on
alcohol oxidase," Biosensors and Bioelectronics, vol. 21, pp. 235 -247, 2005.
[22] J. M.Lee, Biochemical engineering, Washington: Pullman.
[23] M. A. D. C. P. Luzzana, "Enzymatic reaction for the determination of
sugars in food samples using the differential pH technique," Journal of Analyst,
vol. 126, pp. 2149 -2152, 2001.

DISERTATION THESIS
53 [24] M. Luzzana, G. Dossi, A. Mosca, A. Granelli, D. Berger, M. Rovida, M.
Ripam onti, A. Musetti and L. Rossi -Bernardi, "Measurement of glucose in plasma
by a differential pH technique," Clinical Chemistry, vol. 29, no. 1, pp. 80 -85,
1983.
[25] N. &. B. I. Majki, "Spectrophometric determination of ethanol by an
enzymatic method wit h 2,2 –azino -di-(3-ethylbenz -thiazoline, 6 -sulfonate),"
Anal.Chim.Acta, vol. 115, pp. 401 -405, 1980.
[26] J. Canning, "Fiber Gratings and Devices for Sensors and Lasers," Lasers
and Photonics Reviews, vol. 2, pp. 275 -289, 2008.
[27] R. e. a. Aagard, "Development of a Selective Natural Gas Detector," in
Proc 1986 International Gas Research Conf. , Toronto, 1986.
[28] G. orrest, "Simple Diode Laser Spectroscopy from 6 to 10 microns,"
Spectrochimica Acta, vol. 42, pp. 281 -284, 1986.
[29] J. S. A. Si lver, "Airborne Measurements of humidity using a Single -Mode
Pb-Salt Diode Laser," Appl. Optics, vol. 26, pp. 2558 -2566, 1987.
[30] D. Cassidy, "Trace Gas Detection using 1.3µm InGaAsP Diode Laser
Transmitter Modules," Applied Optics, vol. 27, pp. 6120 -6140, 1988.
[31] L. R. H. C. C. G. T. Wang, "Comparison of Approaches to Modulation
Spectroscopy with GaAlAs Semiconductor Lasers:Water Vapour," Appl. Optics,
vol. 27, pp. 2071 -2077, 1988.
[32] W. T. W. R. Welford, High Collection Nonimaging Optics, San Diego:
Academic, 1989.
[33] R. Winston and J. M. Enoch, "Retinal cone receptor as an ideal light
collector," J. Opt. Soc. Am., vol. 61, pp. 1120 -1121, 1971.
[34] G. von Hevisy and T. Somiya, "Über platinschwarz," Zeitschr. Phys.
Chemie A, vol. 41 , p. 171, 1934.
[35] M. Persky, "Review of black surfaces for space -borne infrared systems,"
Rev. Sci. Instrum., vol. 70, no. 5, pp. 2193 -2217, 1999.

DISERTATION THESIS
54 [36] W. Lang, K. Kühl and H. Sandmaier, "Absorption layers for thermal
infrared," in Transducers’91. International Conference on Solid -state Sensors ,
New York, 1991.
[37] R. C. Țarcă, A. M. Cărăban, S. Bota, I. C. Țarcă, A. Dergez and A. C.
Cozma, "A New Optic Fiber Sensor for Measuring the Concentration of Ethanol
in Wine," Revista de chimie, vol. 65, no. 10, 2014.
[38] R. Verger, "Enzyme kinetics of lipolysis," Methods Enzymol., vol. 64, pp.
340-392, 1980.
[39] e. a. Skoog, Principles of Instrumental Analysis, Thomson Brooks/Cole,
2007.
[40] N. &. B. I. Majki, "Spectrophometric determination of ethanol by an
enzymatic method with 2,2 –azino -di-(3-ethylbenz -thiazol ine, 6 -sulfonate),"
Anal.Chim.Acta, vol. 115, pp. 401 -405, 1980.
[41] A. Cărăban, I. Țarcă, R. Țarcă, S. Bota, A. Dergez, S. Filip, M. Toderaș,
E. Macocian and A. Cozma, "STUDIES ABOUT WINE FERMENTATION
USING OPTICAL FIBRE BIOSENSOR," Analele Universității din Oradea,
Fascicula Protecția Mediului, vol. XXIV, 2015.
[42] K. I. H. I. H. Chan, "An Optical Fiber -Based Gas Sensor for Remote
Absorption Measurements of Low -Level Methane Gas in the Near -Infrared
Region," J. Lightwave Tech., pp. 234 -237, 1984.
[43] S. e. a. Stueflotten, "An Infrared Fibre Optic Gas Detection Syst em," in
Proc "OFS '84" Int. Conf. , Stuttgart, 1984.
[44] A. e. a. Kersey, "Single -Mode Fibre Fourier Transform Spectrometer,"
Electron. Letts. , vol. 21, pp. 463 -464, 1985.
[45] W. Lang, K. Kuhl and H. Sandmaier, "Absorption layers for thermal
infrared detectors," in Transducers’91. International Conference on Solid -State
Sensors and Actuators. Digest of technical papers , IEEE, 1991.

DISERTATION THESIS
55 [46] N. M. Mohammed Al -Mhanna and H. Huebner, "Quantification of Full
Range Ethanol Concen trations by Using pH," International Journal of Chemistry,
vol. 3, no. ISSN 1916 -9693, 2011.
[47] R. Bates, Optical Switching and Networking Handbook, New York:
McGraw -Hill, 2001.

DISERTATION THESIS
56

OPIS

The Dissertation Thesis contains:

✓ Number of pages: _______________________ 56
✓ Number of figure s: ______________________ 33
✓ Number of tables: ________________________ 4
✓ Number of equations: ____________________ 37
✓ Number of Annexes : ______________________ 2