Instrumentation in photonics

INSTRUMENTATION IN PHOTONICS
Karolinekersin .E
Assistant Professor

INTRODUCTION
• The common feature of spectroscopic measurements is that they all
measure some spectroscopic properties that are related to the composition
and structure of biochemical species in the sample of interest.
• There are several types of spectroscopic measurements: absorption,
scattering (elastic and inelastic), and emission.
• A typical spectroscopic experiment that allows us to analyze complex
biological systems is conceptually simple.
• Light at a certain wavelength λ (or frequency ν = c/λ) is used to irradiate a
sample of interest. This process is called excitation.

CONTD..
• Properties of the light that then emerges from the sample are measured and
analyzed.
• Some properties deal with the fraction of the incident radiation absorbed by the
sample: the techniques involved are collectively called absorption spectroscopy
(e.g., ultraviolet [UV], visible, and infrared (IR) absorption techniques).
• Other properties are related to the incident radiation reflected back from the
samples (elastic scattering [ES] techniques).
• Alternatively, one can measure the light emitted or scattered by the sample,
involving processes that occur at wavelengths different from the excitation
wavelength; the techniques involved are fluorescence, phosphorescence, and
inelastic scattering (Raman scattering).

CONTD..
• Other specialized techniques can be used to detect specific properties of the
emitted light, such as its degree of polarization and decay times.
• The range of wavelengths used in various types of molecular spectroscopy
to study biological molecules is quite extensive.

COMPONENTS OF BASIC SPECTROPHOTOMETER
1. An excitation light source
2. A dispersive device (optical filters, monochromators, or polychromators)
3. A sample to be analyzed (usually in a compartment having a sample holder)
4. A photometric detector (equipped with a readout device)

• The recorded spectra (absorption, reflection,scattering, emission, or excitation)
represent the photon emission rate or power recorded at each wavelength over a
wavelength interval determined by the slit widths and dispersion of the
monochromator.
• There are a large variety of manufacturers of spectrometers, each offering several
models with different performance characteristics and each offering different options.
• Basic instrumentation components are also commercially available. For special
applications, an investigator may assemble off-the-shelf components for his or her
particular applications.
• The basic components can be adapted to design the instrument for each type of
spectroscopic measurements.

ABSORPTION MEASUREMENTS
• The collimated output of a light source is focused on the entrance slit of an
excitation monochromator for wavelength scanning.
• The output of the excitation monochromator is directed to the sample
inside the sample compartment.
• The light transmitted by the sample is collected through appropriate optics
and focused onto a detector.

.
• This simple instrumental setup is often used in a single-beam absorption
spectrometer.
• Double-beam instruments include a reference beam, which is used to
automatically correct intensity fluctuations in the light source in order to
reduce electronic drift and lamp warm-up periods

SCATTERING MEASUREMENTS
• The ES technique involves detection of the backscattering of a broadband
light source irradiating the sample of interest
• A spectrometer records the backscattered light at various wavelengths and
produces a spectrum that is dependent on sample structure, as well as
chromophore constituents.
• In general, the sample is illuminated with the excitation light, which is
selected with a dispersive element and then directed to a specific point
location (e.g., via an optical fiber) of the sample

CONTD..
• The scattered light is measured at the same wavelength as the excitation
wavelength.
• With inelastic scattering measurements, one measures the scattered light
from the sample in a spectral region different from the excitation
wavelength.
• In this case, the basic setup is similar to the ES setup but has an
additional dispersive element to analyze the scattered emission from the
samples

ELASTIC SCATTERING
MEASUREMENTS

INELASTIC SCATTERING
MEASUREMENTS

EMISSION MEASUREMENTS
• The excitation light source is usually a laser or high-intensity xenon
arc lamp.
• The collimated output of the light source is focused on the entrance
slit of an excitation monochromator.
• The output of the excitation monochromator is directed to the
sample. When a laser is used as the excitation source, the excitation
monochromator is not required.

• The emission from the sample is collected through appropriate optics
and focused onto the entrance slit of an emission monochromator.
• The excitation beam and the emission beam are usually focused at
right angles for minimum interference from scattered light.

EXCITATION LIGHT SOURCES
• UV light is generally used for excitation in many spectroscopic
measurements.
• These UV sources may be classified into two categories, namely, line
or continuum type, and can be used in a continuous wave (CW) mode
or in a pulsed mode.
• The line sources provide sharp spectral lines, whereas continuum
sources exhibit a broadband emission.

HIGH-PRESSURE ARC LAMPS
• High-pressure arc lamps are the most commonly used radiation sources.
• These lamps produce an intense quasi continuum radiation ranging from
the UV (1000 nm) with only a few broadbands at approximately 450–500
nm.
• The lamps consist of two tungsten electrodes in a quartz envelope
containing gases under high pressure, for example, xenon (Xe), mercury, or
a Xe– mercury mixture.
• Lamps of this type are commercially available in a wide range of input
power from a few watts to several kilowatts.

CONTD..
• The high-pressure mercury arc lamp is similar to the high-pressure xenon
lamp in appearance and performance.
• The spectral output of mercury lamps is of a line type, whereas that of the
xenon lamps is of a continuum type.
• If excitation can be carried out at only one wavelength or a few fixed
wavelengths of the mercury emission lines, the mercury lamp is probably
the most effective radiation source.
• The Xe lamp, however, is more commonly used, because it provides a
smoother spectral profile that is more suitable for conducting excitation
spectra measurements.

CONTD..
• The Xe arc lamp is the most versatile light
source for steady-state spectrometers and
has found widespread use.
• This lamp provides a relatively continuous
light output from 250 to 700 nm.
• Xe arc lamps emit a continuum of light as
a result of the recombination of electrons
with the ionized Xe atoms.
• Complete separation of the electrons from
the atoms yields the continuous emission.

CONTD..
• Xe lamps are available with an ellipsoidal reflector as part of the lamp
itself.
• Parabolic reflectors in some commercially available Xe lamps collect a
large solid angle of light and provide a collimated output.
• The operation of high-pressure arc lamps requires special care and
handling, such as reduction of the excitation stray light with a good
monochromator, use of a highly regulated direct current (dc) power
supply and removal of the heat generated by the lamp output in the
IR range.

CONTD..
• A warm-up period is also necessary to minimize arc wandering,
because there is always some tendency for the arc to change its
location inside the lamp envelope during the first half hour of
operation.
• This arc wandering effect may cause sudden variations in the
observed intensity, especially when the image of the arc is focused
into a small slit aperture

CONTD..
• Extreme care should be exercised when inspecting high-pressure arc
lamps.
• These lamps may explode when dropped or bumped because they are
filled with gases at high pressures (~5 atm at ambient temperature
and 20–30 atm at operating temperature conditions).
• It is recommended that special leather gloves, safety glasses, and
protective headgear be used whenever the lamp housing is opened.
• One should not look directly at an operating Xe lamp.

CONTD..
• The extreme brightness will damage the retina, and the UV light can
damage the cornea.
• With some older lamps, proper ventilation or use of deozonators is
required to remove the ozone produced by the UV radiation of the
lamp.
• Recently, many available Xe lamps are considered ozone-free since
their operation does not generate ozone in the surrounding
environment.

LOW-PRESSURE VAPOR LAMPS
• Low- (or medium-) pressure mercury vapor lamps are often used as
line sources.
• They are simple to use, require little power, and offer intense UV
radiation concentrated in a few lines (e.g., 253.7 nm,
365.0/265.5/366.3 nm multiplet).
• The mercury vapor lamps are widely used in simple filter-type
spectrometers because of their low cost, intense emission
characteristics, and good stability.
• The lamps do not need a complex power supply system and provide
excellent reference light sources for calibration of spectrometers.

INCANDESCENT LAMPS
• The tungsten filament incandescent lamp is the simplest continuum
source.
• This type of incandescent lamp exhibits a smooth, continuous spectral
profile determined by the blackbody radiation characteristics given by
Planck’s equation:
• where Sλ is the spectral radiance (W cm−2 sr−1 nm−1)
• λ is the wavelength (nm)
• T is the temperature (K)
• Eλ is the spectral emissivity of the filament material (dimensionless)

CONTD..
• Since incandescent lamps usually have low UV output, they are seldom
used as excitation sources, especially for luminescence measurements
where samples absorb the UV.
• Their smooth spectral profile, however, makes them very suitable for
intensity calibration procedures.
• Standard incandescent lamps with calibration data provided by the
National Institute of Standards and Technology (NIST) are readily available
commercially.
• Intensity calibration data are available for the spectral range from 250 nm
to 2.5 μm.

SOLID-STATE LIGHT SOURCES
• Light-emitting diodes (LEDs) are solid-state light sources, which provide output
over a wide range of wavelengths.
• These devices require little power and generate little heat. One can use a few LEDs
to cover a spectral range from 400 to 700 nm.
• LEDs are practical light sources for many low-power photonic applications. LEDs
can be amplitude modulated up to hundreds of megahertz.
• Another type of solid state light source is the solid-state laser, which is described in
the next section.

LASERS
• Although primarily conventional light sources have been used for absorption
analyses, lasers are increasingly used in luminescence and Raman measurements.
• The advantages offered by lasers as excitation sources include
1. Monochromaticity
2. High degree of collimation
3. High intensity
4. Phase coherence
5. Short pulse duration (with pulsed lasers)
6. Polarized radiation

• Selection of laser excitation sources is determined by the wavelengths that
can be matched to the absorption band of the compounds to be analyzed in
order to take advantage of maximum absorption.
• If time-resolved measurements are performed, the pulse width of the laser
is an important factor to consider.
• The intensity of a laser is very high at (or even near) the laser emission
lines and, therefore, often interferes with the lower intensity of the emission
or scattering signal being measured.
• A number of devices, such as a spike filter or a single monochromator, may
be used to reject the Rayleigh scattered light. Notch filters, which consist of
crystalline arrays of polystyrene spheres, exhibit very high-efficiency
rejection of laser lines.

OPTICAL FILTERS
• Optical filters are passive devices that
allow the transmission of a specific
wavelength or set of wavelengths of
light.
There are two classes of optical
filters that have different
mechanisms of operation:
Absorptive filters
Dichroic filters.

ABSORPTIVE FILTERS
• Absorptive filters have a coating of different organic and inorganic materials
that absorb certain wavelengths of light, thus allowing the desired
wavelengths to pass through.
• Since they absorb light energy, the temperature of these filters increases
during operation.
• They are simple filters and can be added to plastics to make less costly
filters than their glass-based counterparts.
• The operation of these filters does not depend on the angle of the incident
light but on the properties of the material that makes up the filters.
• As a result, they are good filters to use when reflected light of the
unwanted wavelength can cause noise in optical signal.

DICHROIC FILTERS
• Dichroic filters are more complicated in their operation.
• They consist of a series of optical coatings with precise thicknesses that are
designed to reflect unwanted wavelengths and transmit the desired
wavelength range.
• This is achieved by causing the desired wavelengths to interfere
constructively on the transmission side of the filter, while other
wavelengths interfere constructively on the reflection side of the filter

TYPES OF OPTICAL FILTERS
shortpass
filters
longpass
filters
bandpass
filters

SHORTPASS FILTER
A shortpass filter allows
shorter wavelengths than the
cut-off wavelength to pass
through, while it attenuates
longer wavelengths

LONGPASS FILTER
A longpass filter transmits
longer wavelengths than the
cut-on wavelength while it
blocks shorter wavelengths

BANDPASS FILTER
• A bandpass filter is a filter that lets a
particular range, or “band”, of
wavelengths to go through, but
attenuates all wavelengths around
the band.
• A monochromatic filter is an extreme
case of a bandpass filter, which
transmits only a very narrow range
of wavelengths

IMPORTANCE OF OPTICAL
FILTERS
• Optical filters are very important in laser experiment applications.
• Laser safety glasses are optical filters you can wear to protect your eyes from laser
radiation.
• They typically filter out a small range of wavelengths to allow you to see your
surroundings while working on an optical experiment while a laser is turned on.
• It is very important to wear safety glasses that are rated for the laser wavelength
being used.

IMPORTANCE OF OPTICAL
FILTERS
• Optical filters are also important as optical components in laser experiments. For
example, when measuring the photoluminescence (PL) of a material, all the light
being emitted from the spot that is being excited by a laser is coupled into an
optical fiber and measured in a spectrometer.
• The light of the laser is very intense, and depending on what value the wavelength
of photoluminescence is, the laser light could overcome that signal.
• Applying a filter for the laser wavelength somewhere between the film and the lens
that collects the light coming from the film reduces or eliminates the laser peak,
allowing us to see the photoluminescence peak clearly.

MONOCHROMATORS
• Continuous wavelength selection is performed with monochromators, which are
used to disperse polychromatic or white light into the various colors.
• The performance specifications of a monochromator are characterized by the
spectral dispersion, the efficiency, and the stray light levels.
• The spectral dispersion is usually expressed in nanometers per millimeter, where the
slit width is expressed in millimeters.
• Low stray light and high efficiency are desired qualities in selecting a
monochromator.
• There are two types of monochromators: prism and grating monochromators.

PRISM MONOCHROMATORS
• In prism monochromators, light dispersion is due to the change of the refractive
index of the prism material with the wavelength of the incident light. The angular
dispersion D is given by
where θ is the angular deviation n is the refractive index of the prism materials λ is
the wavelength of the light source

Prism monochromators usually produce less stray light than grating devices and are
free from overlap from multiple orders, but they are less convenient to use than
grating monochromators due to their nonlinear scanning dispersion.

GRATING MONOCHROMATORS
• Most spectrometers are now equipped with monochromators having diffraction
gratings.
• Gratings comprise a large number of lines, or grooves, ruled on a highly polished
surface.
• The density and shape of its grooves determine the characteristics of a grating.
• Energy throughput and resolution increase with increasing number of grooves per
millimeter.
• The width of a groove should be approximately equal to the wavelength of the light
to be dispersed.

• The shape of the groove should be such that the maximum amount of light at a
given wavelength is concentrated at only one specific angle for each order.
• The design and construction of the grating also determine the other properties of
the monochromator, for example, reflectivity (or radiance throughout) and stray
light rejection

The general diffraction grating formula is given by
where
θ′ is the angle of incidence
n is the number of grooves per unit length
d is the groove spacing
k is the dispersion order
Most gratings used in modern spectrometers are of the reflection type (θ = θ′). In this case, the
observation is in the direction of illumination (Littrow configuration). The grating formula then
becomes

OPTICAL FIBERS
• A widely used component that provides an optical link between a spectroscopic
instrument and a remotely located sample is the optical fiber.
• The rapid growth of fiber-optic sensing has paralleled the commercial availability of
low-attenuation optical fibers.
• In many applications, the optical fibers comprise a core made with an optically
transparent material (e.g., glass, quartz, or polymer) with a certain refractive index,
n1, surrounded by a cladding made with another material (e.g., quartz or plastic)
having another refractive index, n2.

COMPONENTS OF
OPTICAL CABLE
Core - Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core
that reflects the light back into the core
Buffer coating - Plastic coating that protects the fiber from
damage and moisture
optical cables -Hundreds or thousands of these optical
fibers are arranged in bundles in optical cables.
Jacket -The bundles are protected by the cable's outer
covering

Optical fibers can be used to
transmit the excitation light to a
sample and transmit the signal
(reflected or scattered light) from the
sample to the detector.

OPTICAL FIBER CONFIGURATIONS
Single-fiber system
Bifurcated-fiber system
Dualfiber system

SINGLE FIBER SYSTEM
• A single fiber is used to
transmit the excitation beam
from a light source to the
sample and transmit the
emission from the sample to
the detector.
• A dichroic filter is used to
transmit the light at the
excitation wavelength and
reflect light at the emission
wavelength.

BIFURCATED FIBER
Bifurcated fiber is used, one end
transmitting the excitation light to the
sample and the other end transmitting the
sample emission light to the detector

DUAL FIBER
• Two separate parallel fibers are used, one fiber
transmitting excitation light and the other fiber
transmitting the emission light.
• Two perpendicular fibers are used. The angle
between the fibers can be varied and optimized in
order to optimize the overlap of the excitation
and detection volumes and to minimize scattered
light.

TYPES OF OPTICAL FIBERS
The types of
optical fibers
depend on
the refractive
index,
materials
used, and
mode of
propagation
Step Index
Fibers: It
consists of a
core
surrounded by
the cladding,
which has a
single uniform
index of
refraction.
Graded Index
Fibers: The
refractive index of
the optical fiber
decreases as the
radial distance
from the fiber axis
increases.

TYPES OF OPTICAL
FIBRES
Based on the materials
Plastic Optical Fibers: The
polymethylmethacrylate is used as a core
material for the transmission of the light.
Glass Fibers: It consists of extremely fine
glass fibers.

The classification based on the mode of propagation of light is as follows:
Single-Mode Fibers: These fibers are used for long-distance transmission of signals.
Multimode Fibers: These fibers are used for short-distance transmission of signals.

TYPES OF FIBERS
The mode of propagation and refractive
index of the core is used to form four
combination types of optic fibers as follows:
Step index-single
mode fibers
Graded index-
Single mode fibers
Step index-
Multimode fibers
Graded index-
Multimode fibers

OPTICAL DETECTORS
• Detectors Selection of a suitable detector is one of the most critical steps in the
development of a spectrometer.
• Detectors for electromagnetic radiation can be classified into photoemissive,
semiconductor, and thermal types. Photoemissive detectors are generally used in
optical measurements.
• These devices include PM tubes, photodiodes (PDs), and imaging tubes.
• The PM tubes are the most commonly used because they are the most sensitive
detectors for the visible and near-UV regions.

There are two types of detectors, single-channel detectors and multichannel.
Multichannel detectors include 1D and 2D detector arrays.
Traditionally, spectroscopy has involved using a scanning monochromator and a
single-element detector (e.g., PD, PM. Detectors that permit the recording of the
entire spectrum simultaneously, thus providing the multiplex advantage, are known as
multichannel detectors.
With multichannel systems, a complete spectrum can be recorded in the same time it
takes to record one wavelength point with a scanning system

SINGLE-CHANNEL DETECTORS
Photomultiplier
tube
Photodiode and
avalanche diode
Hybrid
detectors

MULTICHANNEL DETECTORS
Vidicons
Photodiode
Array
Charge-
Coupled Device

• A photomultiplier tube, useful for light detection of very weak signals, is a
photoemissive device in which the absorption of a photon results in the emission of
an electron.
• These detectors work by amplifying the electrons generated by a photocathode
exposed to a photon flux

• Photomultipliers acquire light through a glass or quartz window that covers a
photosensitive surface, called a photocathode, which then releases electrons that
are multiplied by electrodes known as metal channel dynodes.
• At the end of the dynode chain is an anode or collection electrode.
• Over a very large range, the current flowing from the anode to ground is directly
proportional to the photoelectron flux generated by the photocathode.

• The spectral response, quantum efficiency, sensitivity, and dark current of a
photomultiplier tube are determined by the composition of the photocathode.
• The best photocathodes capable of responding to visible light are less than 30
percent quantum efficient, meaning that 70 percent of the photons impacting on
the photocathode do not produce a photoelectron and are therefore not detected.
• Photocathode thickness is an important variable that must be monitored to ensure
the proper response from absorbed photons.
• If the photocathode is too thick, more photons will be absorbed but fewer electrons
will be emitted from the back surface, but if it is too thin, too many photons will
pass through without being absorbed.

• Electrons emitted by the photocathode are
accelerated toward the dynode chain, which may
contain up to 14 elements.
• Focusing electrodes are usually present to ensure
that photoelectrons emitted near the edges of the
photocathode will be likely to land on the first
dynode.
• Upon impacting the first dynode, a photoelectron will
invoke the release of additional electron that are
accelerated toward the next dynode, and so on.

• The surface composition and geometry of the dynodes determines their ability to
serve as electron multipliers
• Because gain varies with the voltage across the dynodes and the total number of
dynodes, electron gains of 10 million are possible if 12-14 dynode stages are
employed.
• Photomultipliers produce a signal even in the absence of light due to dark current
arising from thermal emissions of electrons from the photocathode, leakage current
between dynodes, as well as stray high-energy radiation. Electronic noise also
contributes to the dark current and is often included in the dark-current value.

PHOTO DIODE
• A special type of PN junction device
that generates current when exposed
to light is known as Photodiode.
• It is also known as photodetector or
photosensor.
• It operates in reverse biased mode
and converts light energy into
electrical energy.

PRINCIPLE OF PHOTODIODE
• It works on the principle of
Photoelectric effect.
• The operating principle of the
photodiode is such that when the
junction of this two-terminal
semiconductor device is illuminated
then the electric current starts flowing
through it.
• Only minority current flows through
the device when the certain reverse
potential is applied to it.

CONSTRUCTION OF PHOTODIODE
• The PN junction of the device placed inside a glass material.
• This is done to order to allow the light energy to pass through it. As only the
junction is exposed to radiation, thus, the other portion of the glass material is
painted black or is metallised.
• The overall unit is of very small dimension nearly about 2.5 mm.
• It is noteworthy that the current flowing through the device is in micro-ampere and
is measured through an ammeter.

OPERATIONAL MODES OF PHOTODIODE
Photodiode basically operates in two modes:
Photovoltaic mode: It is also known as zero-bias mode because no external reverse
potential is provided to the device. However, the flow of minority carrier will take
place when the device is exposed to light.
Photoconductive mode: When a certain reverse potential is applied to the device
then it behaves as a photoconductive device. Here, an increase in depletion width is
seen with the corresponding change in reverse voltage.

• In the photodiode, a very small reverse current flows through the device that is
termed as dark current.
• It is called so because this current is totally the result of the flow of minority carriers
and is thus flows when the device is not exposed to radiation.

• The electrons present in the p side and holes present in n side are the minority
carriers.
• When a certain reverse-biased voltage is applied then minority carrier, holes from n-
side experiences repulsive force from the positive potential of the battery.
• Similarly, the electrons present in the p side experience repulsion from the negative
potential of the battery.
• Due to this movement electron and hole recombine at the junction resultantly
generating depletion region at the junction.

• Due to this movement, a very small reverse current flows through the device known as dark
current.
• The combination of electron and hole at the junction generates neutral atom at the
depletion. Due to which any further flow of current is restricted.
• Now, the junction of the device is illuminated with light. As the light falls on the surface of
the junction, then the temperature of the junction gets increased. This causes the electron
and hole to get separated from each other.
• At the two gets separated then electrons from n side gets attracted towards the positive
potential of the battery. Similarly, holes present in the p side get attracted to the negative
potential of the battery.

• This movement then generates high reverse current through the device.
• With the rise in the light intensity, more charge carriers are generated and flow
through the device. Thereby, producing a large electric current through the device.
• This current is then used to drive other circuits of the system

• The intensity of light energy is directly proportional to the current through the
device.
• Only positive biased potential can put the device in no current condition in case of
the photodiode

• Here, the vertical line represents the reverse
current flowing through the device and the
horizontal line represents the reverse-biased
potential.
• The first curve represents the dark current
that generates due to minority carriers in the
absence of light.
• all the curve shows almost equal spacing in
between them. This is so because current
proportionally increases with the luminous
flux.

ILLUMINATION VERSUS
CURRENT CURVE

ADVANTAGES OF
PHOTODIODE
It shows a quick response when
exposed to light.
Photodiode offers high operational
speed.
It provides a linear response.
It is a low-cost device.

DISADVANTAGES OF PHOTODIODE
It is a temperature-dependent device. And shows poor
temperature stability.
When low illumination is provided, then amplification is necessary.

APPLICATIONS OF PHOTODIODE
Photodiodes
majorly find its use
in counters and
switching circuits.
Photodiodes are
extensively used in
an optical
communication
system.
Logic circuits and
encoders also
make use of
photodiode.
It is widely used in
burglar alarm
systems.

HYBRID DETECTORS
The hybrid device combines
photomultiplier and
semiconductor-photodiode
technologies.

HYBRID DETECTORS
• While its structure is similar to a conventional photomultiplier, the hybrid
photodetector (HPD) also has differences.
• Like PMTs, the hybrid is a vacuum tube with a photocathode that reacts to light, an
electron multiplier that multiplies electrons, and an output terminal that outputs an
electrical signal.
• But whereas PMTs use multiple dynodes as electron multipliers, the HPD uses a
silicon avalanche diode (AD) instead.
• This diode is composed of semiconductor layers: a thin layer of heavily doped p-
region that faces the photocathode and is connected to the output terminal, a much
thicker silicon substrate in the middle, and a p-n junction connected to a bias
terminal

• The HPD and PMT have different methods of multiplication.
• In a PMT, photoelectrons from the photocathode are accelerated by a voltage
difference towards the dynodes, where secondary electrons are generated from
dynode to dynode, yielding a signal gain of about 106.
• In an HPD, photoelectrons from the photocathode are accelerated toward the AD by
a larger voltage difference (about 8 kV).
• These photoelectrons are then multiplied in the AD in two steps: electron-
bombardment gain, followed by avalanche gain.

ELECTRON-BOMBARDMENT GAIN
• In electron-bombardment gain, each photoelectron deposits its kinetic energy in the
AD and produces many electron-hole pairs in the silicon substrate.
• The gain generated in this step depends on the voltage applied to the
photocathode—a voltage of–8 kV results in an electron-bombardment gain of about
1600.2
• The electrons then drift toward the p-n junction, and avalanche gain occurs.

AVALANCHE GAIN
• In avalanche gain, the electrons collide with the crystal lattice of the silicon and
create new electron-hole pairs that create more electron-hole pairs in a series of
chain reactions.
• Depending on the reverse-bias voltage applied to the AD, the avalanche gain can
range from 10 to 100.
• The total gain of the HPD is the product of the electron-bombardment and
avalanche gains and can be greater than 105.

VIDICONS
• Vidicon camera is a television camera which converts the light energy into electrical
energy.
• It functions on the principle of photo conductivity, where the resistance of target
material decreases when exposed to light.

CONSTRUCTION
• The Vidicon consists of a glass envelope with an optically flat face plate
• A photosensitive, target plate is available on the inner side of the face plate. The
target plate has two layers.
• To the front, facing the face plate, is a thin layer of tin oxide.
• This is transparent to light but electrically conductive.
• The other side of the target plate is coated with a semiconductor, photosensitive
antimony trisulphide. The tin oxide layer is connected to a power supply of 50V.

Grid-1 is the electron gun, consisting a cathode and a control grid.
The emitted electrons are accelerated by Grid-2.
The accelerated electrons are focussed on the photo conductive layer by Grid-
3.
Vertical and Horizontal deflecting coils, placed around the tube are used to
deflect the electron beam for scanning the target.

WORKING
• The light from a scene is focussed on the target.
• Light passes through the face plate and tin oxide, incident on the photo conductive
layer.
• Due to the variations in the light intensity of the scene, the resistance of the photo
conductive layer varies.
• The emitted electrons from antimony trisulphide reach the positive tin oxide layer.

CONTD..
• So, each point on the photo conductive layer acquires positive charge.
• Hence, a charge image that corresponds to the incident optical image is produced.
• As the electron beam from the gun is incident on the charge image, drop in voltage
takes place.
• As a result, a varying current is produced. This current produces the video-signal
output of the camera

PHOTODIODE ARRAY
Photodiode arrays (semiconductor devices) are used in the detection unit. A DAD
detects the absorption in UV to VIS region. While a UV-VIS detector has only one
sample-side light-receiving section, a DAD has multiple (1024 for L-2455/2455U)
photodiode arrays to obtain information over a wide range of wavelengths at one
time, which is a merit of the DAD.

CHARGE COUPLED DEVICE
• Charge coupled device (CCD) is an integrated circuit etched onto a silicon surface
forming light sensitive elements called pixels.
• Photons incident on this surface generate charge that can be read by electronics
and turned into a digital copy of the light patterns falling on the device.
• CCDs come in a wide variety of sizes and types and are used in many applications
from cell phone cameras to high-end scientific applications

CONTD..
• CCDs are 1D or 2D arrays of silicon PDs with metal–oxide semiconductor architectures.
• The detector arrays consist of individual detector elements, called pixels, which are defined by
capacitors, calledgates.
• Electrons generated by the light impinging onto the CCD charge these capacitors.
• Silicon exhibits an energy gap of 1.14 eV.
• Incoming photons with energy greater than this can excite valence electrons into the conduction
band, thus creating e–h pairs. The average lifetime for these carriers is 100 μs.

CONTD..
• After this time, the e–h pair will recombine. Photons with energy from 1.14 to 5 eV
generate single e–h pairs.
• Photons with energy of >5 eV produce multiple pairs.
• A 10 eV photon will produce 3 e–h pairs, on aver-age, for every incident photon.
• Soft x-ray photons can generate thousands of signal electrons, making it possible
for a CCD to detect single photons.
• For use as an IR imager, a CCD must be made of anothermaterial like germanium
(band gap 0.55 eV).

CONTD..
• The current sources (e–h pairs) produced are localized in small areas, an array of
capacitors, called pixels.
• Common 2D CCD chips have 512 × 512 or 1024 × 1024 pixels. The charge
accumulates in proportion to the light intensity impinging onto the pixel.
• A CCD sensor provides only one serial output, the readout register through which
each capacitor can be discharged (each pixel can be read).
• A differential voltage is applied across each gate to perform charge transfer.

CONTD..
• The photogenerated charge is moved to the readout register by a series of parallel shifts,
sequentially transferring charge from one pixel to the next within a column, until the charge
finally collects in the readout register.
• The charge from each row of pixels can be binned before readout to improve the S/N value.
• Furthermore, the dark count of CCDs is very low, especially when the detector is cooled.
• A CCD array can accumulate charge generated by photoelectrons almost noiselessly.
However, CCD noise is produced in the act of commutating the charge out to a charge
detector.
• Readout noise also tends to increase with increasing readout speeds; typically, the best CCD
camera systems currently available give around five electrons of readout noise per pixel.

CONTD..
• CCDs have several structures, including front-illuminated, back-illuminated, and
open-electrode structures .
• In front-illuminated CCDs, incident photons have to penetrate a polysilicon
electrode before reaching the depletion region. In back-illuminated or back-thinned
CCDs, the sub-strate is polished and thinned to remove most of the bulk silicon
substrate.
• Since illumination occurs from the back, the polysilicon on the front does not affect
the QE of the detector.
• CCDs are currently the detectors with the highest QEs.
• Typically, a CCD array has a QE of around 40%, but back-thinning can increase the
QE to around 80% at 600 nm.

CONTD..
• Back-thinned CCDs are usually coated with an antireflection material for enhanced
response in either the UV or the NIR region.
• Often, reflections from boundaries of the back-thinned devices form constructive
and destructive interference patterns, often referred to as the etaloning effect.
• The etaloning effect often causes an undesired oscillation superimposed on the
spectrum at wavelengths longer than 650 nm.

FRONT AND BACK ILLUMINATED CCD

CONTD..
• The transmittance of the electrode depends on its thickness. Since the polysilicon
electrode material does not transmit below 400 nm, some versions of front-
illuminated UV CCDs have the detector coated with a phosphor, which converts UV
radiation to green light.
• These UV CCDs can provide a 10%–15% QE response in the 120–450 nm spectral
range.
• The coating is selected so that it does not degrade the visible and NIR response of
the detector.
• In open-electrode CCDs, the central area of the electrode is etched to expose the
underlying photosensitive silicon.
• These types of CCDs exhibit QEs of 30% or greater in the UV.

APPLICATIONS
Digital still and video cameras.
Astronomical telescopes, scanners, and bar code
readers.
Machine vision for robots, in optical character recognition
(OCR), in the processing of satellite photographs, and in the
enhancement of radar images, especially in meteorology.

Instrumentation in photonics

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