Optoelectronics 1: Devices for optical Communications
PIN and APD Photodiode characteristics
Objective:
To measure a number of the important characteristics of a PIN diode and an Avalanche Photodiode
(APD):
1. Responsivity (Ro) for a PIN diode in both Photoconductive and Photovoltaic modes.
2. Responsivity of an APD
3. Gain versus reverse bias for an APD.
Equipment:
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Cased PIN Photodiode (Black plastic case)
APD Photodiode (Silver metal case)
50/125 µm core SMA-to-SMA optical fibre patchcord.
LED 850 nm reference light source, set to -20 dBm (10 µW) at fibre output.
Optical power meter, Noyes OPM 4 or other
Variable low voltage lab power supply (0-12 volts)
High voltage power supply (Farnell E350 high voltage 350 volt power supply)
Digital voltmeter.
Warning: High Voltages are uses in this exercise and special care should be taken at all times.
Background:
PIN diodes and APDs are the most common devices used to convert light at the output of a fibre into an
electrical current, which can be subsequently processed in an electrical receiver. Further background
material is contained in the Appendix to this exercise.
Method:
To carry out this exercise a stable reference source of light at about 850 nm is needed. A cased LED
source is provided for this purpose. The LED source is mains powered. The LED DC current can be set
by a potentiometer on the front panel of the unit. A pair of terminals is also provided in the front panel of
the LED source to measure the LED current. For this exercise these terminals can be shorted together
using a short lead with 4mm plugs at each end.
Plug in the LED source and switch on using the rear panel switch. Connect the LED to the optical power
meter using the fibre patchcord provided and adjust the LED output power level until the measured output
power from the fibre is a close as possible to 10 µW (-20 dBm). Record the exact value of fibre output
power for reference. Ensure that the fibre optic power meter is set for operation at 850 nm, by pressing the
 key. The optical power level set is now the reference source power for the rest of the exercise and
Optical Communications Systems Laboratory, Dr. Gerald Farrell
2002
When the exercise is complete please return all test equipment and leads used to storage and ensure that all protective
dust covers are replaced on fibres, optical sources etc..
should not be altered again. Make sure also that the fibre is not under stress or tightly wound when
taking the reference as this could make the reference setting invalid.
1. Responsivity for the PIN diode (Photoconductive and Photovoltaic modes).
In the photoconductive mode the PIN photodiode is connected to the 12 V supply. The reference optical
signal is connected to the PIN diode and the optically generated current in the PIN is measured by
measuring the voltage across the 100 k resistor in series with the PIN diode (the voltage can be measured
at the BNC connector).
Calculate the output current from the PIN diode and hence calculate the Responsivity in A/W, based on
the optical input power in W from the optical source.
In the photovoltaic mode the PIN diode operates without an external DC supply. To measure the
responsivity in this mode disconnect the 12 V supply and short circuit the red and black terminals on the
PIN diode case (this effectively places the PIN diode in parallel with the 100 K resistor). Now measured
the responsivity as above. The photoconductive responsivity should be slightly higher than the photovoltaic
responsivity. Why?
2. Responsivity for the APD
Using the same 12 volts supply used for the PIN diode measure the responsivity of the APD, in the same
manner as the PIN diode (photoconductive mode). For an APD this measurment is carried out at a low
voltage to ensure that no gain is present, since APD responsivity is normally defined as the value with the
gain set to 1. The APD is contained in a metal case and is in series with a 1 k resistor. The current in the
APD can be measured by measuring the voltage across this 1 k resistor, available at the BNC connector
on the APD case.
3. APD gain voltage characteristic
Connect the APD to the optical power source (set above) and to the high voltage bias supply, taking care
not to touch leads etc. when the HV supply is on. For an applied bias voltage of about 30 volts
measure the output voltage from the APD developed across an internal 1 k resistor and calculate the APD
current, recording this value as the "NO-GAIN" current. Now increase the bias voltage in 20-30 volt steps
and tabulate the output current from the APD in each case. Note that at some stage the output current will
increase rapidly for small changes in the applied bias voltage. This means that the bias voltage is close to
the breakdown voltage for the device. Do not exceed this operating point as damage to the APD
could result.
When you have completed the measurements find the gain at each bias voltage by dividing the APD output
current at that bias voltage by the NO-GAIN value recorded above. Hence plot the gain versus voltage
characteristic for the APD on graph paper.
Results:
Optical Communications Systems Laboratory, Dr. Gerald Farrell
2002
When the exercise is complete please return all test equipment and leads used to storage and ensure that all protective
dust covers are replaced on fibres, optical sources etc..
Record all of the results listed in the paragraphs above. Compare your responsivity values with the typical
values found in the literature for such devices.
Optical Communications Systems Laboratory, Dr. Gerald Farrell
2002
When the exercise is complete please return all test equipment and leads used to storage and ensure that all protective
dust covers are replaced on fibres, optical sources etc..
Background information on PIN and APD Photodiode characteristics
1. Optical detectors
By definition photodetectors convert light signals to electrical signals which can then be processed further. For fiber
optic applications photodetectors work at standard wavelengths around 850, 1330 and 1550 nanometers. Suitable
photodiodes may be either Pin diodes or avalanche photo diodes (APD's). In either case the operating wavelength
determines the material used, for example Si being employed at 800-900 nm and GE or alloys of In, GA, As and P at 1330
nm.
Pin diodes and APD's are variations on a basic depletion layer photodiode in which reverse current is altered by
absorption of light at the correct wavelength. APD's differ from Pin diodes in that APD's have gain so that with the
correct circuitry better sensitivity can be achieved with APD's.
2. Definitions for Photodiodes:
Quantum efficiency is defined for photodiodes as the fraction of incident photons having sufficient energy to liberate
electrons. The symbol used is R and by definition it is dependent both on wave-length and photodiode material.
Responsivity of a photodiode is a practical measure of output current for a given optical power input. It is defined as
average output current divided by average incident optical power so its units are A/W.
Photoconductive mode and Photovoltaic mode. A PIN diode can be operated with an external bias voltage (typically up
to 15 V DC). Ths is called the photoconductive mode were optically generated carriers are swept out of the device as a
current under the influence of the externally applied field. In the photovoltaic mode no external bias is applied and the
carriers are swept out of the device to form an external current by the internal depletion field in the device.
Gain is the multiplication of the primary photon current. Gain exists in APD's only and is defined as the ratio of the
output current at an operating voltage to the current at a low voltage where the gain is unity, that is no multiplication
occurs. In effect, the primary photon current is multiplied by collision ionisation in a high field area. A typical gain
characteristic is shown below:
Breakdown
Voltage
1000
100
10
1 1
10
100
Gain versus reverse bias voltage for
an Avalanche Photodiode
Dark Current
In the absence of light a small dark current flows in a photodiode which is caused by leakage in the reverse biased
photodiode. It has a very small effect in the performance of a receiver in terms of sensitivity and is ignored in any further
analysis. It has been shown that dark currents less than 0.l nA have very little effect. Finally photodiodes normally
exhibit rise and fall times less than one nano-second. In APD's the depletion capacitance increases rapidly below a
certain reverse bias voltage; so to maintain fast rise and fall times a minimum bias voltage is maintained.
Optical Communications Systems Laboratory, Dr. Gerald Farrell
2002
When the exercise is complete please return all test equipment and leads used to storage and ensure that all protective
dust covers are replaced on fibres, optical sources etc..
Optical Communications Systems Laboratory, Dr. Gerald Farrell
2002
When the exercise is complete please return all test equipment and leads used to storage and ensure that all protective
dust covers are replaced on fibres, optical sources etc..