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Department of Electrical and Computer Engineering
Queen's University
ELEC-486 Fiber-Optic Communication Systems
Laboratory 2. Optical Spectrum Analysis
NOTE: Please read the lab instructions before coming to the lab.
This laboratory uses a modern optical spectrum analyzer to characterize important properties of
Fabry-Perot lasers, distributed feedback (DFB) lasers, and erbium doped fiber amplifiers
(EDFAs). Many fiber optic communication systems use the intensity modulation of an optical
carrier to encode information. The frequency (or wavelength) of the carrier must be compatible
with the attenuation and dispersion properties of an optical fiber. Wavelength and frequency are
used interchangeably, e.g., a carrier frequency of 193.1 THz corresponds to a carrier wavelength
of 1552.52 nm (c = fλ where c is the speed of light, f is the frequency and λ is the wavelength).
Two wavelength bands are examined: one around 1310 nm and one from 1530 to 1565 nm, the
“C-band”. A schematic of the experimental setup is shown in Fig. 1.
ILX
Controller
Fabry-Perot
Laser
2x2
Coupler
DFB
Laser
1
2
ILX
Controller
Input VOA
ILX
Controller
EDFA
Output VOA
3
1x2
Coupler
3x1
Optical
Switch
Optical
Spectrum
Analyzer
Optical
Filter
Fig. 1. Block diagram of experimental setup.
Lab 2 – Optical Spectrum Analysis
ELEC-486 – Winter 2012
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This laboratory involves the measurement of (i) a Fabry-Perot laser with a peak emission
wavelength of around 1310 nm, (ii) a DFB laser with a peak emission wavelength in the C-band,
(iii) an EDFA which operates in the C-band, and (iv) a filtered optically amplified signal.
Depending on which laser is activated and the state of the 3×1 optical switch, there are 4 main
components to the laboratory.
Do not change any of the optical connections.
For the 2×2 optical coupler, each input port is coupled to both output ports.
The test setup is connected to the OSA though a 3x1 optical switch. The configuration of the 3×1
optical switch is as follows.
Optical Connection
1
2
3
Red Pin
5V
0V
0V
White Pin
0V
0V
5V
Throughout the lab you will change the optical connection to the OSA by varying the voltages on
the power supplies labeled “Red Pin” and “White Pin”.
The lasers can be damaged by an excessively large bias current; be careful to apply
currents as instructed.
A single-mode optical fiber is a dielectric waveguide that is 125 μm in diameter and confines the
propagating optical signal to a central core region about 9 μm in diameter. It is imperative that
the fibers be handled with care.
Procedure
1. Ensure optical connection 1 is selected by adjusting the “red pin” and “white pin” power
supplies appropriately. Before applying a DC bias current to a laser, the temperature
control circuit must be activated using the ILX Lightwave laser diode controller.
Under TEC MODE, the OUTPUT key is used to activate the temperature control circuit. The
ON indicator must be illuminated.
Under LASER MODE, the OUTPUT key is used to activate the laser current source. To
adjust the DC bias current applied to the laser, under ADJUST, the LAS key must be
activated. The rotary dial is used to adjust the DC bias current.
Fabry-Perot Laser
2. The optical power emitted by a semiconductor laser exhibits a threshold behaviour as a
function of the DC bias current. The threshold current marks the transition from spontaneous
emission (a small increase in the optical power with increasing current) to stimulated
emission (a large increase in the optical power with increasing current).
Lab 2 – Optical Spectrum Analysis
ELEC-486 – Winter 2012
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Before applying a DC bias current to a laser, the temperature control circuit must be
activated - this may have already be done prior to you starting the lab. With a DC bias
current of 20 mA applied to the Fabry-Perot laser from the ILX Lightwave laser diode
controller and the optical switch in position 1, measure the optical spectrum. The DC bias
current applied to the DFB laser from the ILX Lightwave laser diode controller should be set
to 0 mA. To ensure that the optical spectrum analyzer is properly aligned use:
CAL
AUTO-ALIGN (soft-key)
After the instrument indicates that auto alignment is complete, enable repeated sweeps by
hitting the Repeat hardkey under Measure.
Familiarize yourself with the measurement menus of the optical spectrum analyzer. In
particular, examine the effect of the SPAN and RESOLUTION (0.01 to 0.5 nm) on the
measurement. Use LEVEL SCALE to adjust the vertical axis in dB/division. The reference
level can be conveniently adjusted using:
REF LEVEL
PEAK→REF LEVEL (soft-key)
You can use the cursor by selecting “ON/OFF” under the cursor wheel. The λ1, λ2, L1, L2
cursor buttons allow you to adjust markers on the horizontal and vertical axes.
The Fabry-Perot laser is a multi-mode laser. Each discrete spectral line corresponds to an
integer number of half-wavelengths fitting within the length of the laser cavity (~300 μm).
Note the sweep-to-sweep fluctuations in the peak values of the discrete spectral lines. This is
known as mode competition. The Fabry-Perot laser emits radiation over a very broad range
of wavelengths or frequencies.
3. Using a suitable span and resolution, measure the optical spectrum for DC bias currents in
the range of 0 to 20 mA. The span can be in either wavelength or frequency; use both for
some of your measurements. To change from one to the other, use:
SPAN
WAVE/FREQ (soft-key)
Results can be printed to obtain a hardcopy. Select Device under Data Out, the select the
select output softkey, and then the EXTERNAL PRINTER softkey. Then select the COPY
key under DATA OUT , and:
DEVICE
printer (soft-key)
external printer (soft-key)
MODE: GRAY (soft-key)
COMMAND: HP PCL (soft-key)
4. Set the DC bias current to 20 mA. To deal with the sweep-to-sweep fluctuations in the peak
values of the discrete spectral lines, successive traces can be averaged (in this case 8 traces)
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ELEC-486 – Winter 2012
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and stored in memory:
AVG
8
ENTER
After the averaging is completed, save the waveform:
SAVE
SAVE MEAS 1 (soft-key)
RECALL
RECALL MEAS 1 (soft-key)
To measure the –20 dB spectral width:
ON/OFF under the CURSOR menu
NORMAL (soft-key)
L1, adjust the cursor to the peak of the spectrum
L2, adjust the cursor to 20 dB below the peak
λ1, adjust the cursor to the –20 dB wavelength below the peak
λ2, adjust the cursor to the –20 dB wavelength above the peak
The –20 dB spectral width is given by λ2-λ1.
5. The total power emitted by the laser can measured as follows:
Set the SPAN to include the entire spectrum
ON/OFF under the CURSOR MENU
POWER (soft-key)
λ1 is now active, adjust λ1 to a wavelength below the spectrum
λ2, adjust λ2 to a wavelength above the spectrum
The total power is displayed as ΣL
Measure the total power for bias currents in the range of 0 to 20 mA. Plot your results to
obtain the L-I curve for the Fabry-Perot laser (light output in mW versus DC bias current in
mA) and the threshold current.
DFB Laser
6. Before applying a current to the laser, the temperature control circuit must be
activated. With a DC bias current of 80 mA applied to the DFB laser from the ILX
Lightwave laser diode controller and the optical switch in position 1, measure the optical
spectrum. The DC bias current applied to the Fabry-Perot laser from the ILX Lightwave
laser diode controller should be set to 0 mA. Examine the effect of the SPAN and
RESOLUTION (0.01 – 0.5 nm) on the measurement.
The DFB laser is a single-mode laser that has much better spectral purity than a Fabry-Perot
laser. Note the asymmetry in the spectrum about the main spectral wavelength. The region
of lowest output power that occurs immediately beside the main spectral wavelength (it can
occur on either side) is referred to as the stop-band. DFB lasers are used in wavelength
division multiplexing (i.e., frequency division multiplexing in the optical domain) with
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channel spacings of 100 GHz – multiples and sub-multiples are also used.
7. Using a suitable SPAN and RESOLUTION, measure the optical spectrum for DC bias
currents in the range of 0 to 100 mA. Measure the total power for DC bias currents in the
range of 0 to 100 mA. Plot your results to obtain the L-I curve and threshold current for the
DFB laser.
8. The side mode suppression ratio (SMSR) is an important measure of the spectral purity of a
DFB laser. It is defined as the difference (in dB) between the peak value of the main mode
and the strongest of the weak side modes. Using the cursors L1 and L2, measure the SMSR
for DC bias currents in the range of ~10 mA to 100 mA. The smallest value of the current is
determined by the lasing mode being clearly distinguishable.
Erbium Doped Fiber Amplifier
9. An erbium doped fiber (EDF) can act as a gain medium for signals in the C-band. The fiber
is similar to that used for transmission except that the core region has been doped with
erbium ions. When the signal from a high power pump laser with a wavelength of 980 nm or
1480 nm is absorbed by the EDF, the conditions exist for the amplification of an input signal
with a wavelength in the C-band. See Fig. 2. EDFAs can simultaneously amplify all the
signals in a wavelength division multiplex.
10. The variable optical attenuator (VOA) before the EDFA is used to control the strength of the
input signal and the VOA after the EDFA is used to keep the input signal to the OSA below
the maximum permissible value (23 dBm). The VOAs have an insertion loss of 1.8 dB.
λ= 980 nm or 1480 nm
EDF
Pump Laser
Input
Signal
Tap
Optical Isolator
Output
Signal
WDM
Tap
Optical Isolator
Fig. 2. Single stage erbium doped fiber amplifier.
Before applying a current to the laser, the temperature control circuit must be
activated. Set the DC bias currents applied to the Fabry-Perot laser and DFB laser from the
ILX Lightwave laser diode controllers to 0 mA, the attenuation of the input VOA to 0 dB,
and the attenuation of the output VOA to 0 dB. With a DC bias current of 150 mA applied to
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the EDFA pump laser from the ILX Lightwave laser diode controller and the optical switch
in position 2, measure the noise spectrum of the EDFA. Without an input signal, the
amplified spontaneous emission (ASE) noise generated by the optical amplifier is measured.
Examine the effect of the SPAN and RESOLUTION (0.01 - 0.5 nm) on the measurement.
11. Using a suitable span and resolution, measure the noise spectrum for pump laser bias currents
in the range of 15 to 150 mA. Determine the total ASE noise power as a function of the DC
bias current for the pump laser.
12. Set the attenuation of the input VOA to 30 dB, the attenuation of the output VOA to 5 dB,
the DC bias current applied to the DFB laser from the ILX Lightwave laser diode controller
to 80 mA, and the DC bias current applied to the pump laser from the ILX Lightwave laser
diode controller to 150 mA. Using a suitable resolution, measure the optical spectrum of the
amplified signal for a large span (to include the ASE noise spectrum). Increase the
attenuation of the input VOA to 50 dB and then decrease the attenuation to 5 dB. As the
attenuation is changed,
REF LEVEL
PEAK→REF LEVEL (soft-key)
will conveniently adjust the reference level. Note the changes in the amplified signal and
ASE noise spectrum. For a weak input signal, the EDFA is linear with a constant gain. For a
strong input signal, the EDFA saturates and the gain is reduced.
13. Set the RESOLUTION to 0.1 nm and the SPAN to 5 nm, centered about the amplified input
signal. Vary the attenuation of the input VOA from 5 dB to 50 dB and use the cursors L1 and
L2 to measure the peak value of the amplified signal and the value of the ASE noise power at
the signal wavelength, respectively. This data can be used to determine the output power
versus input power, taking into consideration the attenuation of the output VOA (5 dB), the
insertion losses of the VOAs (1.8 dB each), and the insertion loss of the 1×2 coupler (3 dB),
and the output power from the DFB laser (measured with the 3×1 switch in position 2).
From this, the gain versus input power can be obtained. The data can also be used to estimate
the optical signal-to-noise-ratio (OSNR) of the output signal as a function of the strength of
the input signal using the difference between the measurements for L1 and L2. Note that to
properly interrupt the OSNR the RESOLUTION must be specified.
14. For an attenuation of 30 dB for the input VOA, measure the total power (amplified signal and
ASE noise) and the total ASE noise power (set the DC bias current applied to the DFB laser
to 0 mA). Since the ASE noise is broadband (~30 nm), the total noise power is large. Optical
filters are used to remove out-of-band noise.
Filtering of an Amplified Optical Signal
15. With the optical switch in position 3, the tunable optical filter is inserted into the path.
Before applying a DC bias current to the laser, the temperature control circuit must be
activated. Set the DC bias currents applied to the Fabry-Perot laser and DFB laser from the
ILX Lightwave laser diode controllers to 0 mA. With a DC bias current of 150 mA applied to
the EDFA pump laser from the ILX Lightwave laser diode controller and the optical switch
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ELEC-486 – Winter 2012
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in position 3, measure the filtered ASE noise spectrum of the EDFA. Use a suitable SPAN
and RESOLUTION. Set the center frequency of the tunable filter to the peak emission
wavelength of the DFB laser. Measure the filtered ASE noise spectrum as the center
frequency of the tunable filter is adjusted across the ASE noise spectrum.
16. Set the DC bias current applied to the DFB laser from the ILX Lightwave laser controller to
80 mA. For an attenuation of 30 dB for the input VOA, measure the total power (amplified
signal and filtered ASE noise) and the filtered ASE noise power (set the DC bias current
applied to the DFB laser to 0 mA).
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