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

Sunday, April 27, 2003

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 volt-ampere 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 r-c 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 4-2 multimode light pack

4) Function generator #02-00004

5) Oscilloscope #01-00045

6) Gain – phase meter #09-00002

7) Decade box #05-00009

8) Adjustable voltage source #1022

9) Various components from the OP-301 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 R₁ 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 R₁ 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 R₁ 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 R₁, 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	I₁	I₂	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	2V_L*	3V_L*	4V_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 turn-on 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.

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.

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.

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
Rise-Time	.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?

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 fiber-optic 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 fiber-optic 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 rise-and-hold 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 common-collector or Darlington configuration as opposed to a common emitter.

Attachments

Original lab handout
Original lab data
Calculations