OP  301 / E01 spring 2002
Fiber Optic Communications
Professor: Weiler
Submitted By:
Andrew Buettner
Lab #2: Basic Optical Communications Link
Lab #3: Analog Optical Communications Link
Lab #4: Digital Optical Communication Link
Table Of Contents
1) Cover Page 1
2) Table of Contents 2
3) Objective 4
4) Components Used 4
5) Procedures 4
A) Basic Optical Communications 4
B) Analog Optical Communications 5
C) Digital Optical Communications 6
6)
Lab Data / Results 6
1) Table 1 6
2) Diagram 1 7
3) Table 2 7
4) Diagram 2 8
5) Table 3 8
6) Table 4 8
7) Table 5 9
8) Table 6 9
9) Table 7 9
10) Table 8 10
11) Diagram 3 10
12) Table 9 10
13) Table 10 11
14) Table 11 11
15) Table 12 11
16) Diagram 4 12
17) Diagram 5 12
18) Diagram 6 13
19) Diagram 7 13
20) Diagram 8 14
21) Diagram 9 14
22) Diagram 10 15
Table Of Contents (Continued)
23) Diagram 11 15
7) Answers to Lab Questions 15
8) Conclusions 21
9) Attachments 22
Objective
The objective of this lab is to assemble a simple communications link to measure the various parameters involved in that communications link. Using the link, measurements of the voltampere relationship of the transmitter LED, the optical efficiency of the source, the quantum efficiency of the receiver, the responsivity of the detector, and the coupling loss of the link will be made. Then, an analog signal will be applied to the circuit and the resulting 3dB bandwidth of the signal will be measured, and calculated to form an rc network associated with the receiving circuit. Also, the tradeoff between signal gain and bandwidth will be observed. The dark current for the photodetector, the transfer function for the overall system, the electrical bandwidth, and the photodetector rise time will also be calculated here. Lastly, the system will be reconfigured to handle a digital signal. The system's performance will be measured through rise times: The rise time will be defined as the time to transition form 10% power to 90% power. The calculated rise and fall times will be then compared against observed ones, using a square wave function generator.
Components Used
1) Analog trainer #1982
2) Multimeter #1376
3) NOYES MLP 42 multimode light pack
4) Function generator #0200004
5) Oscilloscope #0100045
6) Gain – phase meter #0900002
7) Decade box #0500009
8) Adjustable voltage source #1022
9) Various components from the OP301 lab kit
Procedures
A: Basic Optical Communications
1) Assemble the following circuit:
2) Connect the fiber from the LED to the optical power meter.
3) Adjust R_{1} so that the voltage at the emitter is 4V.
4) Measure the voltage across the LED.
5) Measure the optical power from the LED.
6) Calculate the current through the LED.
7) Repeat steps 4 through 6 for emitter voltages of 3.7V, 3.4V, 3.1V, 2.8V, 2.5V, 2.2V, 1.9V, 1.6V, 1.3V, 1.0V, .7V, .4V, and .1V.
8) Connect the fiber optic cable to the remote side.
9) Set the emitter voltage back to 4V.
10) Adjust R_{L} so that the voltage across it is at least 2V.
11) Measure the voltage across R_{L}.
12) Calculate the current through R_{L}.
13) Repeat steps 11 and 12 for emitter voltages of 3.7V, 3.4V, 3.1V, 2.8V, 2.5V, 2.2V, 1.9V, 1.6V, 1.3V, 1.0V, .7V, .4V, and .1V.
B: Analog Optical Communications
1) Assemble the following circuit:
2) Power the detector circuit.
4) Measure, and calculate the current through R_{L}, the dark current.
5) Remove power from the detector and power the transmitter side.
6) Saturate the transistor to allow the diode to emit maximum power.
7) Calculate the current through the LED assuming a voltage drop of 1.5V.
8) Calculate the power dissipated by R_{E}, determine if it is beyond the rated power dissipation of ½ watt.
9) Calculate the minimum resistor value necessary to meet the ½ watt specification and determine if that value will allow saturation of the LED.
10) Power both the receiver and transmitter.
11) Adjust R_{1} so the current through the LED is between 10ma and 15ma.
12) Apply a 1V_{pp}, 1KHz sine wave to the input of the transmitter.
13) Observe the output of the receiver to ensure that it is an optimized sine wave, adjust R_{L} if it is not.
14) Using the gain – phase meter, observe the signal loss through the link.
15) Repeat step 14 using sine waves of 2KHz, 5KHz, 10KHz, 20KHz, 50KHz, 100KHz, 200KHz, and 500KHz.
16) Make additional measurements to find the 3dB point as necessary.
17) Repeat steps 14 through 16 using double R_{L}, triple R_{L}, and quadruple R_{L}.
C: Digital Optical Communications
1) Assemble the following circuit:
2) Starting with R_{L} of approximately 500W, and using a decade box to set R_{1} to 20KW, power the transmitter to 2V.
3) Power the receiver and adjust R_{L} until the voltage across it is 3V.
4) Find the lowest transmitter voltage that produces a voltage drop across R_{L}.
5) Set the transmitter voltage to 4V.
6) Record voltage across R_{1}, base voltage, voltage across R_{c}, the voltage across the LED, and the voltage across R_{L}.
7) Calculate the current through the decade box, the 820W resistor, base current, collector current, and current through R_{L}.
8) Repeat step 6 and 7 for transmitter voltages of 3.5V, 3V, 2.5V, 2V, 1.5V, 1V, and .5V.
9) Remove the decade box from the circuit and connect the function generator.
10) Set the function generator to generate a 1KHz 4V_{pp} square wave.
11) Ensure that there is a signal visible across the load resistor, if not, increase the function generator's output voltage.
12) Measure the rise and fall time for the circuit.
13) Calculate the bit rate of the circuit.
14) Measure the average base voltage, the voltage across R_{c}, and the voltage across the LED.
15) Measure the voltage across R_{L}.
16) Repeat steps 13 through 15 for frequencies of 1KHz, 2KHz, 4KHz, 8KHz, 16KHz, 32KHz, 64KHz, and 128KHz.
17) Using a double R_{L}, triple R_{L} and quadruple R_{L}, repeat the R_{L} measurement for the frequencies given in step 16.
Lab Data / Results
1) Table 1: Transmitter Threshold
R1 
RL 
Threshold Voltage: 
Load Voltage: 

640W 
10.54KW 
1.343V 
.2V 
2) Diagram 1: Schematic 1 enlarged view
3) Table 2: LED Power vs. Optical Power vs. Receiver Current:
V_{E} 
V_{LED} 
Optical Power 
LED Current 
Load Voltage 
Load Current 

4.0V 
1.31V 
14.41dBm 
18.2mA 
2.29V 
3.95mA 
3.7V 
1.30V 
14.86dBm 
16.8mA 
2.12V 
3.66mA 
3.4V 
1.29V 
15.20dBm 
15.5mA 
1.950V 
3.36mA 
3.1V 
1.29V 
15.81dBm 
14.1mA 
1.773V 
3.06mA 
2.8V 
1.28V 
16.32dBm 
12.7mA 
1.593V 
2.75mA 
2.5V 
1.27V 
16.60dBm 
11.4mA 
1.414V 
2.44mA 
2.2V 
1.26V 
17.80dBm 
10.0mA 
1.233V 
2.13mA 
1.9V 
1.25V 
17.82dBm 
8.64mA 
1.053V 
1.82mA 
1.6V 
1.24V 
18.63dBm 
7.27mA 
.860V 
1.48mA 
1.3V 
1.23V 
19.56dBm 
5.91mA 
.687V 
1.18mA 
1.0V 
1.22V 
20.96dBm 
4.55mA 
.507V 
0.87mA 
.70V 
1.20V 
22.68dBm 
3.18mA 
.334V 
0.58mA 
.40V 
1.17V 
25.72dBm 
1.82mA 
.1711V 
0.30mA 
.10V 
1.11V 
33.82dBm 
0.46mA 
.0281V 
48.4mA 
4) Diagram 2: Schematic 2 enlarged view:
5) Table 3: Saturation Voltage:
Voltage: 
Current: 
Resistance: 
Power: 
Will it Saturate? 

1.5 
20.45mA 
220W 
92.0mW 
No 
1.5 
333mA 
40.5W 
500mW 
Yes 
.85V 
142mA 
220W 
121mW 
Yes 
6) Table 4: Gain  Phase Observations using RL = 932W
Frequency 
Gain 
Phase 

1KHz 
0dB 
0.1^{o} 
2KHz 
0dB 
4.3^{o} 
5KHz 
0.2dB 
13.7^{o} 
10KHz 
0.7dB 
27.6^{o} 
20KHz 
2.3dB 
49.8^{o} 
23.06KHz 
3.0dB 
55.1^{o} 
50KHz 
8.1dB 
85.7^{o} 
100KHz 
14.8dB 
111.6^{o} 
200KHz 
22.3dB 
137.2^{o} 
500KHz 
37.3dB 
173.6^{o} 
7) Table 5: Dark Current:
Dark Voltage: 
Dark Current: 

.9mV 
1.55mA 
8) Table 6: Gain  Phase Observations using RL = 1864W
Frequency 
Gain 
Phase 

1KHz 
+3.1dB 
4.4^{o} 
2KHz 
+3dB 
12.8^{o} 
5KHz 
+2.1dB 
31.5^{o} 
10KHz 
0dB 
53.8^{o} 
18.284KHz 
3.0dB 
74.5^{o} 
20KHz 
3.7dB 
77.3^{o} 
50KHz 
11.6dB 
101.5^{o} 
100KHz 
19.0dB 
120.4^{o} 
200KHz 
27.4dB 
142.8^{o} 
500KHz 
41.7dB 
+181.5^{o} 
9) Table 7: Gain  Phase Observations using RL = 2796W
Frequency 
Gain 
Phase 

1KHz 
6.4dB 
8.2^{o} 
2KHz 
8.1dB 
16.9^{o} 
2.165KHz 
9.4dB 
17.1^{o} 
5KHz 
16.9dB 
13.8^{o} 
10KHz 
22.7dB 
57.3^{o} 
20KHz 
27.6dB 
77.6^{o} 
50KHz 
35.6dB 
102.2^{o} 
100KHz 
42.8dB 
120.1^{o} 
200KHz 
49.7dB 
180.1^{o} 
500KHz 
52.7dB 
72.6^{o} 
10) Table 8: Gain  Phase Observations using RL = 3728W
Frequency 
Gain 
Phase 

1KHz 
9.5dB 
4.5^{o} 
2KHz 
27.2dB 
14.0^{o} 
5KHz 
28.9dB 
35.1^{o} 
10KHz 
32.3dB 
56.6^{o} 
20KHz 
37.3dB 
75.1^{o} 
50KHz 
45.1dB 
96.7^{o} 
100KHz 
50.3dB 
107.2^{o} 
200KHz 
52.7dB 
120.5^{o} 
500KHz 
53.4dB 
126.5^{o} 
11) Diagram 3: Schematic 3 enlarged view:
12) Table 9: Transmitter Input Voltage Table (Voltage):
V_{in} 
V_{R1} 
V_{B} 
V_{RE} 
V_{LED} 
V_{L} 

4.0V 
1.87V 
2.13V 
1.40V 
1.34V 
5.80V 
3.5V 
1.60V 
1.90V 
1.17V 
1.25V 
5.79V 
3.0V 
1.24V 
1.76V 
.90V 
1.24V 
5.77V 
2.5V 
1.15V 
1.35V 
.63V 
1.23V 
5.27V 
2.0V 
1.2V 
.80V 
.38V 
1.21V 
2.97V 
1.5V 
1.16V 
.34V 
.14V 
1.16V 
.88V 
1.24V 
1.1V 
.14V 
.03V 
1.1V 
.11V 
13) Table 10: Transmitter Input Voltage Table (Current):
V_{in} 
I_{1} 
I_{2} 
I_{B} 
I_{C} 
I_{L} 

4.0V 
2.92mA 
2.88mA 
40.9mA 
6.32mA 
550mA 
3.5V 
2.50mA 
2.47mA 
34.4mA 
5.32mA 
549mA 
3.0V 
1.94mA 
1.91mA 
26.5mA 
4.09mA 
547mA 
2.5V 
1.80mA 
1.78mA 
18.5mA 
2.86mA 
500mA 
2.0V 
1.88mA 
1.86mA 
11.2mA 
1.73mA 
282mA 
1.5V 
1.81mA 
1.81mA 
4.12mA 
.636mA 
83.5mA 
1.24V 
1.72mA 
1.72mA 
.883mA 
.136mA 
10.4mA 
14) Table 11: Average Voltages for Digital Transmission:
Frequency: 
Data Rate: 
V_{B} 
V_{RC} 
V_{LED} 
V_{L} 
2*V_{L} 
3*V_{L} 
4*V_{L} 

1KHz 
2Kbps 
.61V 
.26V 
1.03V 
.11V 
.22V 
.32V 
.43V 
2KHz 
4Kbps 
.60V 
.26V 
1.04V 
.11V 
.21V 
.32V 
.43V 
4KHz 
8Kbps 
.58V 
.26V 
1.04V 
.11V 
.21V 
.32V 
.43V 
8KHz 
16Kbps 
.55V 
.25V 
1.05V 
.11V 
.21V 
.32V 
.42V 
16KHz 
32Kbps 
.50V 
.25V 
1.07V 
.10V 
.21V 
.31V 
.42V 
32KHz 
64Kbps 
.42V 
.26V 
1.09V 
.10V 
.21V 
.31V 
.42V 
64KHz 
128Kbps 
.33V 
.26V 
1.14V 
.10V 
.20V 
.31V 
.41V 
128KHz 
256Kbps 
.26V 
.26V 
1.18V 
.10V 
.20V 
.30V 
.40V 
15) Table 12: Rise Time for signals
Load Resistor: 
1.0KW 
1.5KW 
2KW 
500W 

Rise Time: 
negligible 
negligible 
negligible 
negligible 
16) Diagram 4: Rise time signal for R_{L} = 500W
17) Diagram 5: Rise time signal for R_{L} = 1KW
18) Diagram 6: Rise time signal for R_{L} = 1.5KW
19) Diagram 7: Rise time signal for R_{L} = 2KW
20) Diagram 8: Maximum Data Rate signal for R_{L} = 500W
21) Diagram 9: Maximum Data Rate signal for R_{L} = 1KW
22) Diagram 10: Maximum Data Rate signal for R_{L} = 1.5KW
23) Diagram 11: Maximum Data Rate signal for R_{L} = 2KW
Answers to Lab Question
1) Q: What wavelength does the LED emit?
A: 1310nM
2) Q: Plot I_{D} vs. V_{D} for the LED, what is the turnon voltage?.
A: About 1.1V
3) Q: Using the answer to Q1, what material is the LED made of?
A: InGaAs
4) Q: Plot the LED's optical power vs. forward current.
A:
5) Q: Using an assumed efficiency of 1%, calculate and plot the expected optical power output of the LED, and overlay it with the existing plot.
A:
6) Q: Calculate the difference, in dB, for the data in Q5.
A: Power Output
Observed: 
Expected: 
Difference: 

14.41dBm 
6.23dBm 
8.18dB 
14.86dBm 
6.61dBm 
8.25dB 
15.20dBm 
6.99dBm 
8.21dB 
15.81dBm 
7.40dBm 
8.41dB 
16.32dBm 
7.89dBm 
8.43dB 
16.60dBm 
8.83dBm 
8.21dB 
17.80dBm 
9.00dBm 
8.80dB 
17.82dBm 
9.67dBm 
8.15dB 
18.63dBm 
10.5dBm 
8.18dB 
19.56dBm 
11.4dBm 
8.17dB 
20.96dBm 
12.6dBm 
8.40dB 
22.68dBm 
14.2dBm 
8.50dB 
25.72dBm 
16.7dBm 
9.00dB 
33.82dBm 
22.9dBm 
10.9dB 
Average: 

8.56dB 
7) Q: Assuming a 3dB connection loss, what, then, is the coupling loss of the system?
A: 5.56dB
8) Q: Assuming the LED is a Lambertain source, what is the NA of the fiber?
A: NA Z .49
9) Q: Plot the optical power vs. the load current, what is the responsivity of the detector?
A: br Z 112
10) Q: Assuming the phototransistor has a b of 200, calculate r for the base of the transistor, and show the base current for Table 2.
A: r Z .56
Power is in dB Current is in mA
P_{i} 
14.4 
14.9 
15.2 
15.8 
16.3 
16.6 
17.8 
17.8 
18.6 
19.6 
21.0 
22.7 
25.7 
33.8 

I_{B} 
19.8 
18.3 
17.0 
15.3 
13.8 
12.2 
10.7 
9.10 
7.40 
5.90 
4.35 
2.90 
1.50 
.242 
11) Q: Calculate the quantum efficiency of the photodiode.
A: h Z .541
12) Q: Plot the transfer function for all four values of R_{L}.
A:
13) Q: Using the 3dB point, calculate the capacitance of the photodetector, and the rise time, also show the phase.
A: Average Capacitance: 89.1mF
Resistance: 
932W 
1864W 
2796W 
3782W 

3dB Point: 
23.06KHz 
5.01KHz 
2.165KHz 
7KHz 
Capacitance: 
46.5mF 
107mF 
165mF 
37.8mF 
Phase Angle: 
55.1^{o} 
74.5^{o} 
17.1^{o} 
45.9^{o} 
RiseTime 
.0952s 
.438s 
1.01s 
.314s 
14) Q: If the saturation voltage of the phototransistor is .4V and the maximum power dissipation is 150mW, what is the maximum load current, and minimum R_{L}? What, then would be the maximum bandwidth?
A: I_{L} = 2.67A; R_{L} = 2.1W; Infinite bandwidth
15) Q: Plot R_{L} vs. the 3dB bandwidth. What would the bandwidth be for R_{L} = 1KHz? Also, plot the peak output voltage.
A: BW Z 9.5KHz
16) Q: What would be the optimal load resistance?
A: Optimal Resistance: 2.2KW
17) Q: What would be the expected output voltage , relative to input voltage, and bandwidth for the resistance selected in Q16?
A: Gain : 4dB Bandwidth: 4KHz
18) Q: What is the minimum transmitter voltage that will produce an output signal?
A: 1.343V
19) Q: What voltage would be required at the input of schematic 3 to saturate the LED?
A: V_{sat} = 5.56V
20) Q: When the LED is saturated by this voltage, how much current flows through it?
A: I_{LED} = 21.1mA
21) Q: What is the b of the transistor?
A: b = 154
22) Q: What are the rise and fall times for each RL?
A: Rise and fall times were negligible, and scope was not able to illustrate them.
23) Q: Based on the rise time estimates, what is the capacitance of the photodiode?
A: Insufficient data to perform calculation. Capacitance is < 100nF.
24) Q: How does this capacitance compare to the one discovered in Q13?
A: The capacitances are totally different.
25) Q: What correlation, if any, exist between the bandwidth and the rise time?
A: There is no correlation because the bandwidth is more determined on the ability for the system to hold the voltage for sampling more than transition time.
26) Q: What is the maximum NRZ for each load resistor?
A:
Resistance: 
500W 
1KW 
1.5KW 
2KW 

Maximum Data Rate: 
64Kbps 
64Kbps 
64Kbps 
64Kbps 
27) Q: If 1V were required at the load, what would be the maximum NRZ data rate for the system?
A: 64Kbps
Conclusions
This lab has demonstrated the basic principles surrounding a basic fiberoptic communications system. However, this system does not accurately model the actual world. The equipment used was horrible out of calibration and there were many difficulties associated with the lab. Primarily a 220W resistor was substituted for the 110W resistor in the schematics. Although the direct impact on the lab was mostly easily compensated for, it could explain some of the radical errors observed in the data sets. The analog measurements have little to no foundation and were not reproducible. The reason for this is as of yet, undiscovered. The system, overall exhibited a high level of loss, uncharacteristic of a fiberoptic system. Much of the loss is most likely as a direct result of equipment being out of calibration. This may explain the radical readings observed in the analog section. The analog section suggested two possible data sets. One involving the 3.7KW and 930W load resistance, and one involving the 1.7KW and 2.7KW load resistance. There is insufficient data to support or disprove either of these two systems. As a result, calculations were made in sets to show the most accurate representation of the data possible. The digital system represented a much more accurate set of readings. However, the maximum data rate calculation is uncertain. The observation was made based on the requirement of a PECL quantizer being placed at the receive end which requires only a riseandhold time to determine a 1 or 0 state. For this, 1% of the wave time was used. Additionally, since the low output resistance was used, the rise and fall times were unable to be read. On the oscilloscope they appeared as a perfect straight line. However, it would have been possible to make a more accurate measurement had the time base been properly adjusted. Unfortunately, this was overlooked. Also, the lab handout schematic calls out a 220W resistor to be placed in series with the LED. This was omitted to have a higher power output and to be compatible with other readings. It was also omitted to help compensate for the increase in emitter resistance. As a result, R_{E} voltage was read in lieu of R_{C}. It would be interesting to see if the capacitive effect of the photodiode could be reduced if the phototransistor was arranged in a commoncollector or Darlington configuration as opposed to a common emitter.
Attachments
Original lab handout
Original lab data
Calculations