Lab #6 – Erbium Doped Fiber Amplifiers and

Lab #6 – Erbium Doped Fiber Amplifiers (EDFAs) and

Wavelength Division Multiplexing (WDM)

ECE 483/583, Spring 2008

Goals:
1. Learn to operate the HP Optical Spectrum Analyzer (OSA) to take optical spectra.

2. Gain a basic working knowledge of the EDFA and the concepts of WDM.

3. Develop a basic understanding of optical amplification.

4. Measure some basic EDFA characteristics such as saturation power and gain.

5. Learn to use TE coolers, DFB telecom lasers, SM fiber connectors, higher power pump laser, and the Newport tunable diode laser.

Reading:
Palais, 5^th ed.: pp. 200-211 (DFB Lasers, Tunable Laser Diodes, Optical Amplifiers)
Other Resources:
Emmanuel Desurvire, Erbium-Doped Fiber Amplifiers, Wiley and Sons, 1994

Derickson, Fiber Optic Test and Measurement, Prentice Hall, 1998

Preparatory Questions:

1. 
   From your reading in the course text, what two properties of a data pulse ultimately limit fiber optic systems?
   
   1. 
      
      
   2. 
      
      
2. 
   What are the advantages of an EDFA over a semiconductor optical amplifier?
   
3. 
   Explain “excited state absorption”.
   
4. 
   What are the two most efficient pumping bands for EDFAs?
   
5. 
   Why do we want our fiber systems to operate at 1550 nm?
   
6. 
   Draw a simple schematic for an EDFA design and label the important parts.
   

Laboratory Experiment:
This lab examines two properties of the EDFA: 1) Amplified Spontaneous Emission (ASE) and 2) Gain. The EDFA you will be using was built in the lab and uses very fragile UltraSplice temporary splices. So, use extra care when handling it. Just be gentle with connectors, splices, etc. and provide strain relief for the splices.
Your characterization of the EDFA will involve looking at the LI curve for the input laser, at ASE as a function of pump power (drive current), and at gain as a function of input signal power. Finally, you will look at more advanced phenomena.

What are we measuring in this lab?
1. Use the OSA to measure the spectral output [power vs. wavelength] and linewidths of the AT&T-DFB (communication) laser and the Newport tunable laser (1520 -1570 nm) at 60 mA drive.
2. Measure and sketch the amplified spontaneous emission (ASE) of the Er-doped fiber vs. the pump laser (980 nm) drive current (hence, proportional to power) for both directions: co-propagating and counter-propagating pump/ASE.

3a. Co-propagating:  EDFA gain @ 1540 nm DFB λ_o vs. laser input power [ -35 dBm to 0 dBm by 7 dBm increments, ie, about 5-6 points] Note: Get close to these dBm values; it is not necessary to hit them exactly.
3b. Tunable laser (partly saturated) gain vs. wavelength for fixed (150 mA) pump laser level from 1520 nm to 1570 nm.
4. ASE gain and tunable laser gain @ 1560 nm vs 2^nd laser signal (AT&T DFB) @ 1540 drive current level.

Procedure:
(Always set laser powers (currents) back to 0mA (voltage to 0.1V) then disconnect one lead before turning power on or off.)
Make sure to first familiarize yourself with the setup. Trace out the fiber leads with reference to the diagram on the board. Work with the HP OSA to learn how it works and how to read spectral data, etc.

Part I – DFB

1. 
   Enable TE cooler power supply (should be set to 0.80 V and 0.80 A so thermistor reads > 14 kohms). Turn on communication [AT&T DFB] laser by slowly increasing the current limit to 60mA on the Fluke series ammeter.
   
2. 
   Connect SM fiber FC connector to the OSA (optical spectrum analyzer) and record the wavelength and the linewidth (∆l). [Push orange “automeasure” button to acquire signal. Push “amplitude” button on left of screen, then push “ln/log” button or “linear” button to look at the laser spectrum both ways. Push “peak search”, record wavelength. Push button on left of screen that corresponds to “marker”. Use dial to move marker.
   
3. 
   Record peak power level into OSA in dBm.
   
4. 
   Now connect the laser back to the attenuator. Turn the attenuator screw counter-clockwise until the laser output power is down to -35dBm. [What is this in mW?] We will come back to use this source in a bit.
   

Part II – ASE (Amplified Spontaneous Emission) Spectrum

First we will look at “co-propagating” (pump and input are on the same end of the Er fiber; output will be on the opposite end) ASE

1. 
   Make sure the TE cooler supply for the pump is set at 0.24 V and is on. Increase current to 0.150 A. The multimeter for the thermistor should read between 13~16 kohms. Make sure it reads above 11 kohms. [This means the laser T is below room T.]
   
2. 
   (Make sure the pump laser supply is set to 0 V, “current limit” is set to minimum level, and one lead of the laser is disconnected before you turn on the output. Then connect the lead and slowly increase V and I limit) Increase the pump laser voltage until the pump laser current is 75mA on the mA meter. Look at the Er ASE spectrum on the OSA. Draw this spectrum in your notebook. Repeat this for pump laser currents of 100mA, 125mA, and 150mA. What is happening here?
   
3. 
   Next, switch the OSA to the same side as the pump laser (“counter-propagating” orientation). Set pump laser current to 25mA. Repeat step 2 for currents of 50mA, 75mA, and 100mA.
   
4. 
   Switch the connection back to the “co-propagating” configuration and set the 980 nm pump laser current at 150 mA for the next parts.
   

Part III a. – EDFA Gain

1. 
   Plug the DFB laser output (set at -35 dBm from Part 1) into the EDFA input (co-propagating).
   
2. 
   Do you see the signal on the OSA? Draw what you see.
   
3. 
   Measure the output power using the “marker” on the OSA. Calculate the gain. [take the difference in dB between the amplified DFB laser signal and the un-amplified input signal.]
   
4. 
   Unplug the DFB laser output from the EDFA. Unplug the EDFA output from the OSA.
   
5. 
   Plug the attenuated DFB laser output into the OSA.
   
6. 
   Increase the DFB laser output (by turning the attenuator screw clockwise) by 7dBm.
   
7. 
   Plug the DFB laser output with attenuator back into the EDFA input. Plug the EDFA output back into the OSA.
   
8. 
   Again measure the signal output power from the EDFA and calculate the gain.
   
9. 
   Repeat steps 4-8 until the attenuator is at its maximum position. (DO NOT FORCE ATTENUATOR SCREW PAST ITS LIMIT!)
   
10. 
    Plot the gain as a function of input power (dB-dB scale).
    

Part III b.-Gain Spectrum

The tunable laser is very expensive so please be careful. If you are not sure how to operate it read the manual and/or ask your instructor. If not already on, turn on the key switch in the back of the black laser drive unit. Wait about 10 minutes for the TE cooler to stabilize.

1. 
   Turn on the tunable laser with the front panel on/off button. [There is a few second delay after pushing the buttons until things happen – be patient.] Wait for ~5 minutes to let it stabilize (the TE cooler is cooling the laser inside).
   
2. 
   Turn down the drive current to the DFB laser to ~8 mA since we won’t need it for a bit now. Unplug the DFB laser from the EDFA input. Unplug the output of the EDFA from the OSA.
   
3. 
   Plug the output of the tunable laser into the OSA.
   
4. 
   Push the “wavelength” button on the tunable laser control panel. Tune the wavelength to 1550 nm by gently turning the knob on the back side of the laser unit.
   
5. 
   Push the current button. (You should see “IOP 0.0mA” on the panel. If you hold the button too long, it might go into the “current limit set” mode instead. If it does, just push the button again. Make sure to not change the current limit from 80 mA.) Turn the current (at a moderate rate) up to 60mA.
   
6. 
   Measure the laser wavelength, bandwidth, and peak power on the OSA.
   
7. 
   Change the tunable laser wavelength to 1520 nm and measure the peak laser power on the OSA each 10 nm from 1520 nm to 1570 nm.
   
8. 
   Disconnect the output of the tunable laser from the OSA and connect it to the input of the EDFA (co-propagating). Plug the output of the EDFA into the OSA.
   
9. 
   Now, tuning the laser from 1520-1570 nm, record the signal power at 10 nm intervals. Calculate the gain at each of these points. What is the gain doing across the ASE spectrum? Do you notice anything interesting?
   

Part IV – Effects of two sources (WDM) – the AT&T DFB laser and Tunable Laser

1. 
   Connect the DFB laser, through the attenuator, to the OSA and increase its drive current to 60 mA. Turn the attenuator screw counter-clockwise to set the laser power at -35 dBm again.
   
2. 
   Connect the attenuated DFB laser to the blue fiber on the 3dB coupler. Connect the tunable laser to the other 3 dB coupler input.
   
3. 
   Set the tunable laser to ~1560 nm and make sure pump laser current is set at 150 mA. Draw the spectrum you see on the OSA.
   
4. 
   Now, gradually increase the DFB laser signal by turning the attenuator screw clockwise. Do this until the DFB laser signal is at its maximum. Describe what is happening to both laser signals and the ASE gain spectrum.
   

Questions:

1. 
   What is P_sat (out) for the EDFA you used? Hint: Look at I_sat (saturation intensity) in “Laser Engineering” by Kelin Kuhn or any laser text such as Yariv, Verdeyen, etc.
   
2. 
   Explain the gain characteristics of the EDFA for both co-propagating and counter propagating pump ASE.
   
3. 
   Draw a schematic of the EDFA and show where a designer would want to put an optical isolator.
   
4. 
   How could we turn an EDFA into a fiber laser? [We will do this next week in a continuing extension of this week’s lab.]
   
5. 
   Draw a plot of the Gain vs. Pump power (you did not measure this) and label the point where the gain is 1. This is called the transparency point.
   
6. 
   Draw your design for a two stage EDFA and label the components. Give an explanation as to why you use the components you chose in the design.
   
7. 
   Describe what was different about the two ASE spectra that you saw in Part II of the lab (co-propagating and counter-propagating). Try to explain why they are different.
   

Written by Jimson Lounsbury, modified by tkp, 5/5/08