Download Photonic Devices II Purpose of the Lab

Photonic Devices II
Purpose of the Lab
The purpose of this lab is to introduce the characteristics of the Optical Spectrum
Analyzer, filter, switch and Erbium Doped Fiber Amplifier and illustrate their
applications in the optical communications industry.
It’s up to the students to search any complementary information to complete their lab
reports.
Theory
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Understanding Diffraction Gratings
A diffraction grating is a device that reflects or refracts light by an amount varying
according to the wavelength. For example, if sunlight falls on a diffraction grating (at the
correct angle) then the sunlight will be broken up into its component colors to form a
rainbow. This function (diffraction) is the same as that of a prism. The device is
performing a Fourier Transform and separating a waveform in the time domain into a
number of waveforms in the frequency domain.
Figure 1: Grating Principle of Operation
Planar Diffraction Grating: Gratings work in both transmission (where the light passes
through a material with a grating written on its surface) and in reflection. In optical
communication only reflective planar gratings have a widespread use and so the
description is restricted to these.
A reflective diffraction grating consists of a very closely spaced set of parallel lines or
grooves made in a mirror surface of a solid material. A grating can be formed in almost
any material where we vary the optical properties (such as refractive index) in a regular
way with a period close to the wavelength (actually the grating can be up to few hundred
times the wavelength). Reflective gratings are wavelength-selective filters. In optical
communications they are used for splitting and/or combining optical signals in
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Wavelength Division Multiplexing systems (where many independent optical channels
are sent on a single fiber) and as reflectors in external cavity of Distributed Bragg
Reflector lasers.
The basic grating equation is given by: m*λ = gs*(sin θ + sin φm)
Where: gs = groove spacing, m = order of the refracted ray, λ = free space wavelength of
the incident ray, θ = angle of incidence and φm = angle of refraction.
A schematic of a reflective grating is shown in figure 1. An incident ray (at an angle of θ
to the normal) is projected onto the grating. A number of the reflected and refracted are
produced corresponding to different orders (values of m = 0, 1, 2, 3…). When m = 0 we
get the ordinary reflection sin θ = sin φ0 which is exactly the same as for any mirror.
When m = 1 we get a ray produced at a different angle.
What is happening is that parts of the ray (or beam) are reflected from different lines in
the grating. Interference effects prevent reflections that are not in-phase with each other
from propagating. Thus we get resultant rays at a series of angles that correspond to
points of constructive interference (reinforcement) between the reflections. The number
of orders of refracted rays produced depends on the relationship between the groove
spacing and the wavelength. We can design the grating to ensure that only the 0 and 1 are
produced by making the groove spacing smaller than the wavelength.
Figure 2: Grating Profiles
The shape of the grooves has no effect on the angles at which different wavelength are
diffracted. However, grove profile determines the relative strength of diffracted orders
produced. This enables us to control the distribution of power into the different orders.
For example we will want to transfer as much power as possible into the first order
refracted beam.
Some different groove profiles are illustrated in figure 2. Profile (b) in the figure is a socalled “blazed” grating. This is perhaps the most popular groove profile because allows a
very high proportion of power to be transferred into the first order mode. However, a
particular blazed grating will operate efficiently over only a very restricted range of
wavelengths.
Figure 3: Wavelength Division Multiplexing with a Reflective Grating
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Characterization of the Optical Filter
In optical communication systems, filters are seldom used or needed. They are sometimes
used in front of an LED to narrow the linewidth before transmission, but in few other
roles. In the Wavelength Division Multiplexing (WDM) networks, filters are very
important for many uses:
- A filter placed in front of an incoherent receiver can be used to select a particular
signal from many arriving signals.
- WDM networks are proposed which use filters to control which path through a
network a signal will take.
There are many filtering principles proposed and many different types of devices have
been built in laboratories. The result is that there are many kinds of active, tunable filters
available which will be important in WDM networks.
It’s important to note that gratings are filters. Indeed Fiber Bragg Gratings (FBG) is
probably one of the most important optical filters in the communication world.
Figure 4: Transmission Characteristics of Ideal Filters
When discussing filters there are a number of concepts that need to be understood. In an
ideal world filters might have characteristics similar to the ones shown in figure 4.
However there’s no ideal world. Practical filters almost always are quite different from
the ideal. The profile of a practical Fabry-Perot filter is shown in figure 5:
Figure 5: Transmission Characteristics of a Fabry-Perot Filter
Polarization is also an important factor here. Both the center wavelength and the
bandwidth are often polarization dependent. Sometimes the center wavelength and the
bandwidth are quoted as maxima and minima indicating the range of variation possible
with changing polarization.
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The Fabry-Perot Filter (Etalon): One of the simplest filters in principle is based on the
Fabry-Perot interferometer. It consists of a cavity bounded on each end by a partiallysilvered mirror. If the mirrors can be moved in relation to each other the device is called
an “interferometer”. If the mirrors are fixed in relation to each other then it is called an
“Etalon”.
Figure 6: Fabry-Perot Filter
Operation is as follows:
1. Light is directed onto the outside of one of the mirrors.
2. Most is reflected and some enters the cavity.
3. When it reaches the opposite mirror some (small proportion) passes out but most is
reflected back.
4. At the opposite mirror the same process repeats.
5. This continues to happen with the new light entering the cavity at the same rate as light
leaves it.
If you arrange the cavity to be exactly the right size, interference patterns develop which
cause unwanted wavelength to undergo destructive interference. Only one wavelength (or
narrow band) passes out and all others are strongly attenuated.
Tunable Fabry-Perot Filter: The device can be tuned by attaching one of the mirrors to a
piezoelectric crystal and changing the voltage across the crystal. Such a crystal can be
controlled to the point that you can get accuracy of movement down to less than the
diameter of an atom!
Figure 7: Tunable Fabry-Perot filter
Figure 7 shows an ingenious variation of the Fabry-Perot filter. Two pieces of fiber are
used with their ends polished and silvered. The ends are places precisely opposite one
another with a measured gap (this is the hard part). This avoids the cost of getting the
light into and out of a “regular” FP filter – because it arrives and leaves on its own fiber.
The device shown is mounted on two piezo-electric crystals. By applying a voltage across
the crystals we can change the distance between the ends of the fibers and hence the
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resonance wavelength. As mentioned above, piezo-electric crystals can be controlled
such that the resulting movement is comparable with the diameter of an atom!
If you want to tune a filter then of course there are two alternative approaches. You can
physically move the mirrors such that the size of the gap changes or perhaps you could
change the Refractive Index of the material inside the cavity!
Tunable FP filters can be built by putting a liquid crystal material into the gap. The
refractive index of the liquid crystal material can be changed very quickly by passing a
current through the liquid. Reported tuning times for this type of filter are around 10μsec.
Tuning range is about 30-40 nm. Such filters are expected to be low in cost and require a
very low power.
The important characteristics of filters are:
- Center Wavelength: This is the mean wavelength between the two band edges. It
is usually quoted without qualification but sometimes it may be necessary to
quote the distance below the peak at which the centre is measured.
- Peak Wavelength: The wavelength at which the filter attenuation is least.
- Bandwidth: It is easy to see from the shape of the filter that the bandwidth of the
filter is going to depend a lot on just where you measure it. Bandwidth is the
distance between the filter edges (in nm) at a particular designated distance from
the peak. The distance is always quoted in dB. It is common to talk about the 3 dB
bandwidth or the half peak power points separation.
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Characterization of the Optical Spectrum Analyzer (OSA)
There are many occasions where we want to look at the wavelength spectrum of the
signal(s) on the fiber. One such occasion would be to examine the wavelength spectrum
of a Wavelength Division Multiplexing (WDM) system to help understand system
operation and to diagnose faults. A spectrum analyzer scans across a range of
wavelengths and provides a display showing the signal power at each wavelength.
Figure 8: Simplified OSA Block Diagram
A simplified OSA block diagram is shown in figure 8 . The incoming light passes
through a wavelength-tunable optical filter which resolves the individual spectral
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components. The photo-detector then converts the optical signal to an electrical current
proportional to the incident optical power.
The most common OSAs for fiber optic applications use diffraction gratings as the basis
for a tunable filter. Figure 9 shows what a diffraction grating based OSA might look like.
Figure 9: Concept of Diffraction-Grating Based OSA
In the monochromator, a diffraction grating (a mirror with finely spaced corrugated lines
on the surface) separates the different wavelengths of light. The diffracted light might
comes off at an angle proportional to the wavelength. The result is similar to the rainbow
by visible light passing through a prism. In the infrared range, prisms do not work very
well because the dispersion (in other words, change of refractive index versus
wavelength) of the glass in the 1 to 2 μm wavelength is small. Diffraction gratings are
used instead. They provide a greater wavelength separation allowing for better
wavelength resolution. A diffraction grating is made up of an array of equidistant parallel
slits (in the case of a transmission grating) or reflectors (in the case of a reflective
grating).
The spacing of slits or reflectors is in the range of the wavelength of the light used. The
grating separates the different wavelengths because the grating lines cause the reflected
light to undergo constructive interference only in very specific directions. Only the
wavelength that passes through the aperture reaches the photo-detector to be measured.
The angle of the grating determines the wavelength to which the OSA is tuned. The size
of the input and output apertures together with the size of the beam on the diffraction
grating determine the spectral width of the optical filter.
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Characterization of the Optical Switch and Modulator
Many of the optical sources that we would like to use are either impossible to modulate
or have unwanted bad characteristics when we do modulate them. Most lasers cannot be
modulated by turning them on and off more than few kilohertz. But theses kinds of lasers
are very good fiber light sources. The answer is to have the laser produce a constant beam
of light and to modulate the light beam after it leaves the source.
The job of a modulator is to replicate variations in an electronic signal onto an optical
one. The light intensity should vary with some characteristic of the electrical one (voltage
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or current). For most applications we need digital modulation so we need the light to be
switched ON and OFF and we don’t care about the states in between.
Modulators consist of a material that changes its optical properties under the influence of
an electric or magnetic field. Because modulators (in general) turn the signal on and off
they make excellent switches. Indeed a digital modulator is just a very fast switch.
Often we need switches to direct their output to one of two different paths, whereas with
a modulator we usually only want to control the light intensity on one particular path. In
almost any network where communications channels are routed or switched within nodes
along the path you need a way of switching a single channel from one path to another.
Fortunately there are number of ways to build quite efficient components to do just this.
Figure 10: Digital Optical Switch
Figure 10 shows one configuration. A Y-coupler is modified by the addition of electrodes
which can impose an electric field on the waveguide material. The material changes its
refractive index under the influence of an electric field. This element is called the digital
Optical Switch.
When an electric field is applied the refractive index of the waveguide material is
increased in one arm of the coupler and decreased in the other arm. This action routes the
input light (from Port 1) to either one of the output ports depending on the refractive
index (RI) of the material in the path (and hence on the electric field direction). Light
travels on the path which has the higher RI. When the electric field is reversed the RI
changes in the opposite direction and the output light is now directed out of the other port
of the coupler.
This is quite a low loss operation (1 or 2 dB) and large multi-way switches have been
constructed with switching elements such as these as their central switch fabric.
The switch used in this lab is a mechanical switch. It runs on a stepping motor that is
controlled by a microprocessor. In this case the fiber is actually moved in the switch and
focused in to the output channel by a collimating lens. This is a slower system than the
one discussed above and it should be noted that there is usually a delay whenever the
output channel is changed.
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Characterization of the Erbium Doped Fiber Amplifier
An optical amplifier is a device which amplifies the optical signal without ever changing
it to electricity. The light itself is amplified.
The most important type of amplifier is the Erbium Doped Fiber Amplifier (EDFA)
because it is low in cost (relatively), highly efficient and low in noise.
An Erbium Doped Fiber Amplifier consists of a section of fiber which has a small
controlled amount of the rare earth element erbium added to the glass in the form of an
ion (Er3+). This is illustrated in figure 11.
Figure 11: Erbium Doped Fiber Amplifier
The principle involved here is similar to the principle of a laser and is very simple.
Erbium ions are able to exist in several energy states (these relate to the alternative orbits
which electrons may have around the nucleus). When an erbium ion is in a high-energy
state, a photon of light will stimulate it to give up some of its energy (also in the form of
light) and return to a lower-energy (more stable) state. This is called “stimulated
emission”.
To make the principle work, you need a way of getting the erbium atoms up to the
excited state. The laser diode in the diagram generates a high-powered (between 10 and
200 milli-watts) beam of light at a wavelength such that the erbium ions will absorb it
and jump to their excited state. (Light at either 980 or 1480 nanometer wavelength will
do this quite nicely.)
The basic principle of the EDFA as illustrated in figure 11:
1. A (relatively) high powered beam of light is mixed with the input signal using a
wavelength selective coupler. (The input signal and the excitation light must of course be
at significantly different wavelengths.)
2. The mixed light is guided into a section of fiber with erbium ions included on the core.
3. This high powered light beam excites the erbium ions to their higher energy state.
4. When the photons belonging to the signal (at a different wavelength from the pump
light) meet the excited erbium atoms, the erbium atoms give up some of their energy to
the signal and return to the lower energy state.
This doesn’t happen for all wavelength of signal light. There is a range of approximately
35 nm wide that is amplified.
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5. A significant point is that the erbium gives up its energy in the form of additional
photons are exactly the same phase and direction as the signal being amplified. So the
signal is amplified along its direction of travel.
6. There is usually an isolator placed at the output to prevent back reflections.
Wherever there’s gain in a system there is also noise. The predominant source of noise in
EDFAs is the Amplified Spontaneous Emission (ASE). What happens is that some of the
excited erbium decays to the ground state (undergoes spontaneous emission) before it has
time to meet with an incoming signal photon. The photons created by this spontaneous
decay will then be indistinguishable from the signal (from the point of view of the
amplifier) and be amplified. Also the ASE is produced over a range of wavelength
exactly corresponding to the gain spectrum of the amplifier.
The important characteristics of an optical amplifier are:
Figure 12: Gain curve of a Typical EDFA
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Gain (amplifier): This is the ratio in decibels of input power to output power.
Bandwidth: This is the range of wavelength over which the amplifier will operate.
Gain saturation: This is the point where an increase in gain ceases to result in an
increase in output power. All of the pump power is used up already and no more
power is available. When an EDFA saturates the overall gain of the amplifier is
lessened but there’s no distortion of the signal.
Noise: EDFAs add noise to a signal mainly as a result of Amplified Spontaneous
Emission (ASE). The noise figure of an amplifier is expressed in decibels and is
defined as the ratio of the signal-to-noise ratio (SNR) at the input to the SNR at
the output.
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Introduction to LabVIEW
LabVIEW is short for Laboratory Virtual Instrument Engineering Workbench. It’s an
instrumentation and analysis software development application. Its programs are called
Virtual Instruments, or VIs for short. LabVIEW is different from text-based
programming languages in that it uses a graphical programming language to create
programs relying on graphic symbols to describe programming actions.
LabVIEW communicates with and controls instruments (such as an optical switch or
power meter) using GPIB (General Purpose Interface Bus). In this lab the student will use
labVIEW to control a switch and plot a power meter readings.
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The following tips may help make the information you get from LabVIEW more useful.
If the scale of the graph needs to be resized click on the hand tool in the upper left corner
of the tools palette and left click on the scale to be changed. Type in the new value and
press enter. Grid lines can also be added to the graph to help determine times. To do
this, right click on the scale to add grid lines to and select formatting. Click the leftmost
box in grid options and choose the style of grid to add; you can also change the colors to
the right. When you are finished, click OK. Different plots may also be useful as well.
To change the plot style click on the box that corresponds to the plot you would like to
change and select common plot and then select the style of plot you would like. It may
be useful to count data points knowing that there are 16 points per cycle, 8 in the first half
and 8 in the second half.
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Some Basic Concepts
-
Refractive Index (n): The refractive index of a material is the ratio of the speed of
light in a vacuum over the speed of light in the material: n = Cfreespace/Cmaterial
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Extinction ratio (ER): ER is a concept related to digital signals where 1 and 0 are
represented by different signal levels. The ER is simply the ratio of the power
level representing a 1 bit to the power level representing the 0 bit:
ER = Power1 bit/Power0 bit
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Optical Decibels: Usually a dB is the amount of loss or gain of signal power. In
the case of a component that attenuates a signal, the attenuation in dB is given by:
Attenuation = 10*log10(Output Power/Input Power)
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dBm: As noted before, dB is a ratio of signal powers. Sometimes it’s convenient
to quote a power level in dB but if you do that it must be in relation to some fixed
power level. A dBm is the signal power level in relation to one milliwatt:
Power level (dBm) = 10*log10(Signal Power/1 milliwatt)
Experiment
Note: The ends of all optical fibers must be with acetone and a lint free cloth every time
before coupling the fiber with any instruments.
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Calibration of the OSA
1) Couple the He-Ne laser beam directly into the optical fiber, using the XYZ stage.
Connect the fiber to the OSA.
2) Scan the spectrum of the laser and determine the peak power and the corresponding
wavelength:
- To scan the spectrum: SWEEP → AUTO → (wait…) → STOP.
- To check the peak power: PEAK SEARCH → PEAK SEARCH.
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-
Set the resolution: SETUP → RESOLUTION.
3) Record any discrepancy between the peak wavelength and the expected lasing
wavelength of the He-Ne laser 632.8 nm. This can be considered as the wavelength
offset on the part of the OSA.
4) Save the graph to a floppy disk as a text file or a Bitmap image.
- To format the floppy: insert a blank floppy → FLOPPY → DISK
INITIALIZE → YES.
- Select the format of the copied file:
Text file: FLOPPY → TRACE RD/WRT.
Bitmap image: FLOPPY → DATA GRPH RD/WRT.
- Name the graph: FLOPPY → WRITE → FILENAME → DONE.
- Save the graph: FLOPPY → EXECUTE.
5) Answer the following questions in your report:
- Why can the He-Ne be used as a wavelength reference?
- What’s the reason for the OSA offset from the expected 632.8 nm?
- How can the OSA’s calibration accuracy be enhanced?
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Slit width dependence of the peak power
1) Use the JDS Uniphase optical power meter to record the peak power of the ring laser:
- To set the readings unit press: POW(dBm) or REL(dB).
- To set the wavelength press the mode button and then use the WL button
to scroll through the available wavelengths
2) Connect the ring laser to the OSA.
3) Measure the peak power and linewidths at resolutions of: 0.05, 0.5, 1.0 and 5.0 nm.
To Adjust the resolution/slit width of the OSA:
- Set the resolution: SETUP → RESOLUTION.
- Use the arrow buttons or knob to select the resolution or type in the
resolution with the keypad and press nm/ENTER
To use markers to measure linewidth:
- MARKER→SET MARKER 1/2→LINE MARKER 1→use knob or arrow
buttons to adjust the markers
To scan the spectrum press:
- SWEEP→SINGLE or
- SWEEP→REPEAT (wait) STOP
4) Again measure peak power and linewidth with the OSA, this time, varying the span
at: 1, 5, 10 and 15 nm. To change the span press SPAN→SPAN, type in the new
span and press nm/ENTER
5) Compare the power meter and the OSA’s peak power readings.
6) Evaluate the slit width and span dependence of the measured peak power and
comment on the results.
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The tunable filter
1) Measure the spectrum of the broadband light source (e.g., ASE from the EDFA) using
an OSA. To operate the EDFA the switch must be on and the key must be turned to
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2)
3)
4)
5)
6)
7)
enable, a green light will come on when it is operating. The pump current of the
EDFA should not be set higher than 140mA for this and all other parts of the
experiment.
Add a tunable filter between the light source and the OSA.
Save the graphs and evaluate the bandwidth, the transmission spectrum and
transmittance of the filter.
Determine the wavelength dependence of the insertion loss of the tunable filter.
Measure the output power of the ring laser using the power meter. Add a filter (set at
1550 nm) and record the power reading.
Determine the insertion loss of the filter at different wavelengths using the OSA.
If the fiber is based on the diffraction gratings, answer the following questions:
a) How to reduce the insertion loss? b) How to reduce the bandwidth?
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The Optical switch
1) Measure the insertion loss of the switch using the ring laser and the power meter.
2) Open LabVIEW and select open VI. Open switch + PM. Complete vi and set the
frequency and duty cycle of the driving signal for the switch.
3) Turn the switch and the power meter on.
4) Choose the modulator option.
5) Run switch + PM. Complete vi.
6) Determine the time response of the switch.
7) Repeat the procedure for different frequencies, duty cycles and switch outputs. Try to
use a few frequencies in the following ranges: 0.2-1, 3-4, 7-10 and 20-30.
8) Choose the switch option in the VI and set the frequency and duty cycle of the driving
signal of the switch.
9) Repeat the procedure for different frequencies, duty cycles and switch outputs.
10) Discuss the following points:
- The frequency and duty cycle dependence of the response delay.
- The channel uniformity of the switch.
- How can we enhance the response time?
- Can this switch be used effectively as a modulator?
- What are the sources for any time delays?
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The EDFA
1) Use an OSA to evaluate the bandwidth of the ASE.
2) Measure the output power Pin of the ring laser.
3) Connect the ring laser to the attenuator then to the EDFA. Join the output of the
EDFA to a filter then to a power meter. Be sure the attenuator is reading dBm by
pressing the ATT/PWR button. Also be sure the infinity LED is off. It can be turned
off by pressing the infinity button.
4) Connect the output of the EDFA to a filter then to a power meter.
5) Record the amplified spontaneous emission (ASE) of the EDFA.
6) Determine the pump current and wavelength dependence of the EDFA gain.
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7) Design a transmission system. Assume that all the devices required in the system are
available. The power launched into the fiber at the transmitter is 0dBm, fiber loss is
0.4dB/km, the insertion loss of the modulator is 6dB, and receiver sensitivity is 30dBm. Estimate the maximum link length without repeater. How long distance is it
if an EDFA with a gain of 20dB is used soon after the modulator? Assume that the
required safety margin is 6 dB.
• The Attenuator
1) Connect the ring laser to the power meter and determine the output power of both
sides of the output panel (the two sides have a different wavelengths).
2) Connect the ring laser to the attenuator and then to the power meter.
3) Measure the power loss at different attenuations.
4) Repeat for the other set of outputs of the ring laser.
5) Connect the ring laser directly to the OSA.
6) Connect the ring laser to the attenuator and then to the OSA, making graphs at
different attenuations.
7) Repeat for the other set of outputs of the ring laser.
8) Discuss the following points:
- How accurate is the attenuator?
- Attenuator dependency on wavelength?
- How does the attenuator modify the signal?
References
[1]
Harry J. R. Dutton, Understanding Optical Communication,
[2]
Dennis Derickson, Fibre Optic Test and Measurement, Prentice Hall, Inc, Upper
Saddle River, New Jersey, 1998.
[3]
Andreas Orthonos, Kyriacos Kalli, Fibre Bragg Gratings, Artech House, Boston,
1999.
[4]
Frank L. Pedrottic, Leno S. Pedrotti, Introduction to Optics, 2nd ed., Prentice Hall
Inc., Englewood Cliffs, New Jersey, 1987.
[5]
Gerd Keiser, Optical fiber communications, 3rd ed., McGraw-Hill, 2000.
[6]
John Wilson and John Hawkes, Optoelectronics, an introduction, 3rd ed., Prentice
Hall, 1998.
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