ECEN 4606, UNDERGRADUATE OPTICS LAB

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1 ECEN 4606, UNDERGRADUATE OPTICS LAB Lab 10: Photodetectors Original: Professor McLeod SUMMARY: In this lab, you will characterize the fundamental low-frequency characteristics of photodiodes and the circuits used around them. PRELAB: HOMEWORK PROBLEM 1: Pedrotti through The answers to all of these are in the back of the book and they are each a single equation use them to cement your understanding of the relationships between photons, electrons, optical power and electrical current. HOMEWORK PROBLEM 2: Impact of bias on photodiodes. The idealized IV relationship for a photodiode is given by i i d i ev K BT id irs e 1 P ip e, h where i d = Dark current Amps i P = Photocurrent Amps i rs = reverse saturation current Amps e = electron charge Coulombs K B = Boltzmann s constant J/K quantum efficiency of detector electrons per photon, 0 to 1 P = optical power on detector W A d = detector area m 2 h = Planck s constant Js = Photon frequency Hz. Note that h = Energy per photon J K B T/e = Thermal voltage (26 mv at room T) V. P 1. Calculate the detector responsivity in amps per watt at wavelengths of 400, 500 and 600 nm assuming perfect quantum efficiency. 2. What is the minimum photon energy that the photodiode will be sensitive to? Is this consistent with the wave nature of light? Version 1.3, 8/19/12 McLeod and Gopinath 1

2 3. Assume this detector is reverse biased in series with a resistor as shown in Figure 1. Plot the diode IV relationship for a 1 mm 2 detector with incident intensities of 0, 100, 200 and 300 nw/mm 2 assuming a reverse saturation current of 5 na m, a wavelength of 600 nm, and perfect quantum effiiency for -0.5V < V B < 20V. On this same graph, plot the (two) load lines for R = 1 k and 1 M using bias voltages of 20 V. 4. (a). From the previous plot, sketch the voltage on the load resistor (1 k and 1 M versus the incident intensity for the biased (V B =20V) case. (b). Discuss the impact of the load resistance on a) sensitivity (dv Load / d P) and dynamic range (maximum P that can be measured). 5. Given the capacitance of the diode, discus the impact of load resistance on bandwidth. P vb 0 v R i Figure 1. Photodiode bias circuit. Note that bias also linearly decreases the capacitance of the photodiode by increasing the thickness of the depletion region. This improves bandwidth via reduction of the RC time constant. DESIGN PROBLEM 1: Design photo-diode receiver that uses a transimpedance amplifier. Discus in your design the impact of the transimpedance (Ohms) on sensitivity and dynamic range. Your circuit should include the bias voltage on the photo-diode and the supply voltages for the op-amp. TECHNICAL RESOURCES: TEXTBOOK: Chapter 17 LECTURE NOTES: Lecture 9, Photdetectors. EQUIPMENT AVAILABLE: A spatially-filtered, JDS Uniphase 1103P-3020 Helium Neon laser. The laser wavelength is 633 nm. We will use this as a light source. A range of neutral density filters for attenuating the laser. An optical power meter. Benchtop electronics: DC power supply, multimeter, oscilliscope A Thorlabs FDS100 Si Photodiode: S01.pdf Version 1.3, 8/19/12 McLeod and Gopinath 2

3 LAB PROCEDURE: STEP 1: RESPONSIVITY AND DARK CURRENT OF THE PHOTODIODE Align the HeNe, a neutral density filter and an iris in front of the photodiode. The iris can be placed directly in front of the diode to insure the beam diameter is less than the diode sensitive area and to serve as a block for stray light. You may need additional shielding (e.g. a cardboard box) at the low light levels. Measure the laser power with the power meter and no ND filters, then use the quoted attenuation of the filters to calculate optical power as you attenuate. Insure the beam is normally incident on the detector. Measure and make a table of the short circuit current and open-circuit voltage over three orders of magnitude of optical power. At the lower optical powers, the multi-meter may reach the limit of its sensitivity. To fix this, you can rig up a quick transimpedance amplifier with high gain (say 10K) to convert the small current into a measurable voltage. Verify and calibrate your transimpedance gain by measuring the same current both ways, then continue attenuating. In your lab book: 1. Plot the short-circuit current versus the optical power on a log-log plot and fit the data to a line to estimate the responsivity in amps per watt. Compare to the data sheet. 2. Take the natural logarithm of the idealized IV relationship for a photodiode operated in open-circuit and for voltages >> than the thermal voltage. Plot the open-circuit voltage versus the natural logarithm of the photocurrent. Use the equation you derived to estimate the thermal voltage and compare to the expected value. Project the line to zero open-circuit voltage: in your approximate equation, this will give you the dark-current (which would really be zero at zero volts, but is not in your approximation). Compare this to the spec sheet for the diodes. STEP 2: NOISE FLOOR AND SATURATION OF TWO CIRCUIT CONFIGURATIONS In this section you will examine the two primary specifications of photodiode receivers at low temporal frequencies. 1. Simple reverse bias circuit a. First construct a simple reverse bias circuit as per Figure 1 with ~100Kto 500Kgain. *If you use a resistor > 100 k, please make sure you take the input of the oscilloscope into account. Observe the output voltage across the resistor on an oscilloscope with and without room lights on. Determine the appropriate levels of darkness and/or shielding to reduce the 60 Hz noise to immeasurable levels. b. Measure and plot the output voltage across the resistor as a function of increasing optical power. Continue until the output voltage does not significantly with optical power increase - this is the saturation power. Compare this to your expectation from the circuit load-line. c. Estimate the noise voltage by blocking all light to the detector and observing the output on the oscilloscope. Now decrease the optical power, observing the output voltage with and without the detector blocked. When Version 1.3, 8/19/12 McLeod and Gopinath 3

4 the increase in the voltage (the signal) is equal to the noise, the measurement is at a signal to noise (SNR) of 1, which is the minimum detectable power. Calculate the dynamic range, which is the ratio of the saturation power to the minimum detectable power. 2. Transimpedance amplifier: Repeat steps a, b and c for your transimpedance amplifier circuit with the same gain (~ K). Insure that the amplifier is not oscillating at high frequencies by adding a small capacitance in parallel with the feedback resistor. A good trick to add a very small, variable capacitance is to twist together two fine (e.g. 30 gauge) wires about an inch long, then snip them back to reduce the capacitance 1. In your lab book: Plot and analyze the data. Compare the two circuits including the expected bandwidth, which we will utilize in the optical communication lab. Design an experiment to determine the expected bandwidth of the system. 1 M. Johnson, Photodetection and Measurement, pg. 30, McGraw-Hill 2003 Version 1.3, 8/19/12 McLeod and Gopinath 4

5 Grading Expectations Lab Report 10: Photodetectors (100 total points) Name Name and group members. Abstract (10 points). Introduction (10 points) Methods (35 points) 1. RESPONSIVITY AND DARK CURRENT OF THE PHOTODIODE, 11 PTS a. Figure of setup, 5 pts b. Description of setup and measurement to be taken, 6 pts 2. NOISE FLOOR AND SATURATION OF TWO CIRCUIT CONFIGURATIONS, 24 PTS Part 1: simple reverse bias circuit a. Figure of setup, including circuit diagram, 6 pts b. Description of set up, 6 pts Part 2: Transimpedance amplifier a. Figure of setup, including circuit diagram, 6 pts b. Description of set up, 6 pts Results and Analysis (35 points) 1. RESPONSIVITY AND DARK CURRENT OF THE PHOTODIODE, 15 PTS a. Table of short circuit current, open circuit voltage, 3 pts b. Plot short-circuit current versus the optical power on a log-log plot and fit data to line to estimate responsivity (A/W), 3 pts c. Compare results from (b) with data sheet, 2 pts d. Plot of open-circuit voltage versus the natural logarithm of the photocurrent, 3 pts e. Equation and estimation/comparison of thermal voltage to measurements, 2 pts f. Calculation of dark current and comparison with spec sheet, 2 pts NOISE FLOOR AND SATURATION OF TWO CIRCUIT CONFIGURATIONS, 20 PTS Part 1: simple reverse bias circuit a. Comment on light tolerance of system, 1 pt b. Plot of output voltage across the resistor vs. optical power, 2 pts c. Comparison of plot in (b) to theory, 2 pts d. Calculate the dynamic range, which is the ratio of the saturation power to the minimum detectable power, 3 pts Part 2: Transimpedance amplifier a. Comment on light tolerance of system, 1 pt b. Plot of output voltage across the resistor vs. optical power, 2 pts c. Comparison of plot in (b) to theory, 2 pts Version 1.3, 8/19/12 McLeod and Gopinath 5

6 d. Calculate the dynamic range, which is the ratio of the saturation power to the minimum detectable power, 3 pts Parts 1 and 2 a. Comparison of two circuits in terms of expected bandwidth, 2 pts b. Figure and description of experiment to measure bandwidth of system, 2 pts e. Conclusion (10 points) Summary of lab report f. References Include any references that you used. Version 1.3, 8/19/12 McLeod and Gopinath 6