Photonic Devices I Purpose of the Lab

Photonic Devices I
Purpose of the Lab
The purpose of this lab is to understand the operating principles and to study the major
characteristics of the Fabry-Perot laser, Distributed Feedback laser, Fiber Ring laser and
the Light Emitting Diode.
It’s up to the students to search any complementary information to complete their lab
reports.
Theory
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Light Emitting Diode (LED)
Almost all light sources used in telecommunication today are made from semiconductors,
which can be classified into two categories: Light Emitting Diodes (LED) and laser diode
(LD). LED is simpler than LD, therefore they are discussed first.
Figure1. Electrical Potentials across a p-n Junction
Figure 1 shows a p-n junction with an electrical potential across it. When the field is
applied in one direction the device conducts electricity (called the forward direction), but
when the field is applied in the opposite direction almost no current can flow.
In forward bias, on the n-type side free electrons are repelled from the contact and pushed
towards the junction. On the p-type side holes are repelled from the positively charged
contact towards the junction. At the junction electrons will cross from the n-type side to
the p-type side and holes will cross form the p-type side to the n-type side.
As soon as they cross most holes and electrons will re-combine and eliminate each other.
When this happens the free electrons may lose a quantum of energy to fill the available
hole. This quantum of energy is radiated as electromagnetic energy with the wavelength
depending on the size of the energy “gap” that the free electron crosses when it fills the
hole. This phenomenon is called Injection Luminescence. If you choose your materials
correctly this emits light at designed wavelength and you have built an LED.
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As mentioned above in its most basic form, an LED is just a forward biased p-n junction.
When free electrons from the “conduction band” recombine with holes, they fall down to
the (lower energy) “valence band” and light is emitted. The wavelength of light emitted
by the LED is inversely proportional to the bandgap energy. The higher the energy the
shorter the wavelength. The formula relating electron energy to wavelength is given
below:
λ = 1.24 / εph (µm)
Where εph is the photon energy in eV, and the photon energy of 1µm wavelength is 1.24
eV.
This means that the materials of which the LED is made determine the wavelength of
light emitted. The following table shows energies and wavelengths for commonly used
materials in semiconductor LEDs and lasers:
Every time an electron recombines with a hole one photon is emitted. This means that the
amount of optical energy (power) produced is proportional to the number of electrons
that recombine multiplied by the energy of the bandgap. The output power is directly
proportional to the drive current.
Useful Definitions:
3 dB Bandwidth: spectral width of the LED based on the separation of the corresponding
wavelengths to the half-power points.
Mean wavelength (3 dB): The wavelength that is average to the two half-power points’
wavelengths.
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Fabry-Perot (FP) Laser Diode
The Fabry-Perot laser diode has a pair of end mirrors.Usually carriers (electrons and
holes) are confined in a small region (i.e., active layer) using a specially designed
structure. The mirrors are needed to create the right condition for lasing to occur. The FP
laser gets its name from the fact that its cavity acts as a FP resonator.
Figure2. Fabry-Perot Cavity
To understand the operation principle of the FP laser it’s necessary to understand the FP
cavity. When you put two mirrors opposite one another they form a resonant cavity. Light
will bounce between the two mirrors. When the distance between the mirrors is an
integral multiple of half wavelengths, the light will reinforce itself. Wavelengths that are
not resonant undergo destructive interference. (Question: What happens to these
wavelengths in the FP laser diode? )
This principle also applies in the FP laser although the light is emitted within the cavity
itself rather than arriving from outside.
In some sense every laser cavity is a Fabry-Perot cavity. But when the cavity is very long
compared to the wavelength involved we get a very large number of resonant
wavelengths all of which are very close together. So the important filtering characteristics
of the FP cavity are lost.
We consider a laser to be “Fabry-Perot” when it has a relatively short cavity (in relation
to the wavelength of the light produced). Wavelengths produced are related to the
distance between the mirrors by the following formula:
Cavity length = λ*m / (2*n)
Where λ = the wavelength, m = arbitrary integer and n = effective refractive index of
active medium.
In practice, we can’t make the laser so short that we restrict it to only one wavelength.
We need some space for stimulated emission to amplify the signal and we are limited by
the density of the power we can deliver to a small area. Typically the cavity length is
between 100 and 200 microns.
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Thus an FP laser can produce a range of wavelengths. Each wavelength has to be able to
resonate within the cavity and it must be within the gain window of the medium.
The laser output of each possible lasing mode is called longitudinal mode. A simple gain
guided FP laser produces a number of longitudinal modes over a range of wavelengths
called the “spectral width”.
Useful Definitions:
FWHM (Full Width at Half Maximum): Describes the spectral width at half-power points
of the FP laser, assuming a continuous, Gaussian power distribution.
Mode Spacing: The average wavelength spacing between the individual spectral
components of the FP laser.
Peak Amplitude: the power level of the peak spectral component.
Peak Wavelength: The corresponding wavelength.
•
Distributed Feedback (DFB) Laser
When we want to use lasers for long distance communication we find that standard FP
lasers have significant problems as they produce many wavelengths over a spectral width
of between 5 and 8 nm. In Wavelength Division Multiplexed (WDM) systems we want to
carry many multiplexed optical signals on the same fiber. To do this it is important for
each signal to have as narrow a spectral band as possible and to be as stable as possible.
Figure3. DFB Laser - Schematics
Distributed feedback lasers are one answer to this problem. The idea is that you put a
Bragg grating into the laser cavity of an index-guided FP laser. This is just a periodic
variation in the refractive index of the gain region along its length. The presence of the
grating causes small reflections to occur at each refractive index change (corrugation).
When the period of the corrugations is a multiple of the wavelength of the incident light,
constructive interference between reflections occurs and a portion of the light is reflected.
Other wavelengths destructively interfere and therefore cannot be reflected. The effect is
strongest when the period of the Bragg grating is equal to the wavelength of light used
(first order grating). However, the device will work at any (small) integer multiple of the
wavelength. Thus only one mode (the one that conforms to the wavelength of the grating)
can lase.
Early devices using this principle had the grating within the active region and were found
to have too much attenuation. As a result the grating was moved to a waveguide layer
immediately adjacent (below) the cavity. The evernescent field accompanying the light
wave in the cavity extends into the adjacent layer and interacts with the grating to
produce the desired effect.
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In principle a DFB laser doesn’t need end mirrors. The grating can be made strong
enough to produce sufficient feedback (reflection) for lasing to take place. However, in a
perfect DFB laser there are actually two lines produced (one at each side of the Bragg
wavelength). We only want one line. A way of achieving this and improving the
efficiency of the device is to place a high reflectance end mirror at one end of the cavity
and either an anti-reflective coating or just a cleaved facet at the output end. In this case
the grating doesn’t need to be very strong – just sufficient to ensure that a single mode
dominates. The added reflections (from the end mirrors) act to make the device
asymmetric and suppress one of the two spectral lines. Unfortunately they also act to
increase the linewidth.
Useful Definitions:
Peak Wavelength: The wavelength at which the main spectral component of the DFB
laser occurs.
Side Mode Suppression Ratio (SMSR): The amplitude difference between the main
spectral component and the largest side mode.
Mode offset: Wavelength separation (in nm) between the main spectral component and
the side mode.
Peak Amplitude: The power level of the main spectral component of the DFB laser.
Stopband: Wavelength between the upper and lower sides mode adjacent to the main
mode.
Center Offset: Indicates how well the main mode is centered in the stopband. This value
equals the wavelength of the main spectral component minus the mean of the upper and
lower stopband-component wavelength.
Linewidth: Measure the bandwidth at half-power points of the main spectral component
of the DFB laser spectrum.
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Tuneable Fiber Ring Laser
A fiber ring structure can be used to make a very narrow linewidth laser. The conceptual
structure, as shown in figure 4, is very similar to that of a fiber ring resonator.
Figure4. Tunable Fiber-Ring Laser
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Ring resonators are also comparable in principle to Fabry-Perot resonators. However,
unlike the FP resonator, resonant wavelengths are passed through rather than reflected
back to the source. Optical power re-circulates on the ring if (and only if) the ring length
is an integral multiple of the wavelength.
However in the case of a Tuneable Fiber Ring Laser the wavelength is controlled by a
tunable FP filter and not by the length of the fiber loop. (Long fiber loops of this kind
have resonances spaced very close together and without the FP filter would produce
multiple lines.) An isolator is necessary to prevent counter-propagating lasing mode
being generated.
•
How to use the Optical Spectrum Analyzer (OSA)
Here go some important functions of the OSA:
- To scan the spectrum: SWEEP → AUTO → (wait…) → STOP.
- To check the peak power: PEAK SEARCH → PEAK SEARCH.
- Set the resolution: SETUP → RESOLUTION.
- To change the span: SPAN → SPAN → Use knob or arrow buttons to set the span.
- Markers for the linewidth measurement: MARKER → SET MARKER 1/2 → LINE
MARKER 1 → Use knob or arrow buttons to adjust the markers.
- 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.
•
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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
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)
-
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)
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Experiment
NOTE: The ends of all optical fibers must be cleaned with acetone and a lint free cloth
every time before coupling with any of the instruments
•
Characterization of the Distributed Feedback Laser
1) Connect the DFB laser to the OSA.
2) The settings of the OSA may need to be changed to make the graph look nicer. It is
recommended that the number of sampling points be set to 250. This can be done by
pressing SETUP, followed by SAMPLING POINTS and then typing in 250 and
pressing nm/ENTER. Try to measure as broad of a range as possible but remember
that the OSA can only read between 350-1750nm.
3) Determine the peak wavelength, peak amplitude, side mode suppression ratio, mode
offset, stop band, center offset and linewidth.
4) Print or save your graphs to a floppy whenever needed.
5) Use the EXFO optical power meter to measure the output power of the laser. Make
sure to set the power meter to the peak wavelength:
- To set the readings unit press: dBm/Watt.
- Press the λ key to view the available wavelengths.
- To add a new wavelength: SETUP → arrow “→” button → LAMBDA →
arrow “↑” button → Add → (set the wavelength) → ENTER.
6) Note any discrepancy between the power meter and the OSA’s peak power
measurements. The power meter readings can be considered exact.
7) Connect the DFB laser to the attenuator and then to the wavemeter. There are two
optical fibers that go to the other side of the room, to be used to connect to the
attenuator. The connections with tape are the same fiber.
8) Measure the exact wavelength of the light and compare this to the OSA’s
measurement of peak wavelength. Be sure the wavemeter is measuring in nm by
pressing the UNITS button. Caution: there is laser light emitted from the aperature of
the wavemeter so do not look in to it!
9) Use the Matlab simulations of the DFB transmission spectrum and compare it to the
graph generated by the OSA.
10) From the simulations, evaluate the cavity length of the laser diode.
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Characterization of the Ring Laser
1)
2)
3)
4)
Connect the ring laser to the OSA.
Set the laser’s peak wavelength to 1550nm.
Determine the peak wavelength, output power, SMSR and linewidth.
Connect the ring laser to the EXFO power meter and measure the peak power and
compare this to the OSA’s reading.
5) Connect the ring laser to the wavemeter (it does not need to be attenuated) and
compare the peak wavelength to the OSA’s measurement.
6) Compare the results with those of DFB LD and comment on the difference.
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7) Assume the cavity length is 20m, the refractive index of the fiber used in the fiber
laser is 1.5, calculate the mode spacing
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Characterization of the Fabry-Perot Laser Diode (FPLD)
1) Observe the laser diode through a microscope. Describe the connection of the diode
and estimate its size. Sketch the FPLD. Be careful when handling the FPLD as it is
very fragile.
2) Couple the FPLD beam into a fiber. The FPLD should be connected to the current
source with the positive side connected to the back of the diode and the negative side
connected to the tab on the side. Connect the fiber to the OSA.
3) Determine the peak amplitude, peak wavelength, mode spacing and the spectral width
at FWHM.
4) Use the power meter set at 1550 nm to measure the output power of the FPLD. Note
that the power meter should be put as close as possible to the LD)
5) Evaluate the driving current dependence of the output power.
6) Compare the results with those of DFB LD, and comment on the difference.
1)
2)
3)
4)
• Characterization of the LED
Caution: Do not exceed 90 mA driving current when using the LED.
Couple the LED light into a fiber, using a lens to focus the beam. Connect the fiber to
the OSA.
Determine the 3 dB bandwidth, peak wavelength and peak amplitude.
Connect the fiber to the power meter. Report any discrepancy between the peak
power measurements of the OSA and the power meter.
Compare the results with those of DFB LD and FP LD and comment on the
difference.
•
Polarization angle of the lasing wavelength
1) Collimate the DFB laser beam from the fiber into the polarizer.
2) Measure power using a power meter. (Don’t forget setting the power meter to the
peak wavelength)
3) Evaluate the polarization angle dependence of the power. Evaluate the extinction ratio
of the beam in function of the polarization angle.
4) Repeat the steps 1) to 3) for the ring laser and FPLD.
Comment on the differences of the polarization of the lasing mode of the DFB, FP and
ring lasers.
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DFB laser spectrum simulation
1) Open and run the Matlab simulation DFB_spectrum.m.
2) Specify the grating period within the DFB laser cavity (take values from 238.2 to 240
nm).
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3) Estimate the grating period dependence of the peak power and corresponding
wavelength of the lasing mode and the second largest mode.
4) Evaluate the grating period dependence of the stop band.
5) Discuss your observations using the physics of a DFB laser.
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FPLD simulation
1) Open and run the Matlab file FPLD_simulation.m.
2) Specify the length of the laser diode (use values from 50 to 700µm).
3) Estimate the length of the laser diode dependence of:
a. The power and center wavelength of the two largest modes.
b. The width of the lasing mode.
c. The number of available modes.
4) Discuss your observations using the physics of an FPLD laser.
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]
G. Morthier, P. Vankwikelberge, Handbook of Distributed Feedback Laser
Diodes, Artech House Inc., Norwood MA, 1997.
[6]
Peter W. Milonni, J. H. Eberly, Lasers, John Wiley & Sons, Toronto, ON, 1988.
[7]
E. Kapon, editor, Semiconductor Lasers 1, Fundamentals, Academic Press,
Toronto ON, 1999.
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