OD2: The Characteristics of a Photodetector
Safety procedures:
1. Do not look directly into the laser light.
2. The surfaces of the power meter, the polarisers, and the photodiode reflect some of the laser light.
The reflected light is a hazard and must be safely contained.
3. Most operating voltages of the LEDs/laser diode fall between 0.6-4.5 V. The provided power supply
can go up to 20 V easily. In order to protect the diodes, please always use the fine-tuning knob with
0.1 V as interval.
4. The power supply for the high speed silicon detector (DET36A) is supplied by a battery. Ensure that
the detector is turned OFF after using it.
5. Please make sure that at the start of the experiment, the battery for the high speed silicon
photodetector (DET36A) is working properly as it can only supply power for 40 hours (according to
the operating manual).
6. Please ensure that the right cables are used to connect the equipment.
7. Please take note that some circuits need to be grounded in order to work properly.
8. Please refer to the instructor if there are any safety concerns while doing the lab experiment.
Objectives:
In this experiment, students are expected to carry out a standard characterisation of a photodetector. The
objectives of the experiment are to measure the following characteristics of a photodetector:
1.
2.
3.
4.
Time constant
Cut-off frequency
Linearity
Sensitivity
Brief guidelines on the principle of the experimental procedure are given. However, students are expected
to perform the experiment on their own. Prior knowledge on how to use the oscilloscope and function
generator is essential.
Equipment:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
50 MHz oscilloscope
Function generator
BNC/adapter/terminator
Semiconductor laser diode (red)
LEDs (Red, Blue, White, Infrared)
High speed silicon detector (DET36A)
Variable terminating resistor
Variable linear ND filter
Power meter
DC power supply
Theory:
Photodetector are devices that convert photons into electric current by means of “electron-hole” pair
generation subsequent to the absorption of light by a semiconductor material. Presently, the most
commonly used material is silicon.
In a semiconductor material, the absorption of light becomes important whenever photon energy is equal
or larger than the forbidden bandgap energy. In the case of visible and near infrared light, incident photon
energy is sufficiently large to create e-h pairs. By this process, carrier density increases in the conduction
band.
Figure 1. illustrates the conversion process. Under reverse bias, the semiconductor junction will pass a
current that is proportional to the incident photon flux, or optical power. The intense electric field E
existing in the depleted region drains off all the carriers created by photon absorption. The current due to
carrier collection is sampled by resistor R and thus can be measured as a voltage. That is how optical
signal is transformed into electric signal.
Figure 1 Basic Structure of a Photodiode and Amplifier Circuit
The principle interesting characteristics of photodetector are quantum efficiency, sensitivity, linearity, time
constant, and leakage current.
Quantum efficiency
Pair generation by photon absorption is a random phenomenon characterised by the mean number of
pairs created by each incoming photon. This probability, named quantum efficiency and denoted as ,
depends upon photon wavelength and semiconductor type. For a typical silicon photodiode, for example,
quantum efficiencies are
 = 85% at 0.9 μm
 = 20% at 1.06 μm
Sensitivity
By definition, sensitivity (S) is the current generated by the photodetector per watt of incident optical
power (Pop). For example, silicon photodiodes exhibit values of 0.6 A/W at 900 nm.
S
Current Intensity
Pop
For a given detector construction, sensitivity varies as a function of wavelength. At short wavelengths,
quantum efficiency is low since absorption occurs very close to the surface. The light-generated
photocarriers recombine very quickly in the N+ region and consequently are not collected. At long
wavelengths, the layer thickness is too small for complete absorption.
Once the quantum efficiency is known, the sensitivity can be deduced from the relationship
S
e
hv
where  is the quantum efficiency, e the electric charge, h planck’s constant, and v the radiation
frequency. Moreover, we already know that a proportional current I0 corresponds to an incident power P0
so that
I o  SP0
Linearity
Linearity is another very important photodetector characteristic. In fact, it is most important that the
photocurrent varies linearly as a function of the incident energy flux, especially for analog signal
reception. Typically, a photodiode exhibits a 100 dB dynamic range while keeping linearity within 1%.
Under zero illumination conditions, the measured current corresponds to the total noise current including
leakage current. This current therefore is named “dark current”. The experiment consists in measuring the
signal output of the photodetector as a function of attenuation of the light source. Since the light source
used approximates a point source, its illumination varies in an inverse square law. Photodiode linearity
then can be evaluated by plotting the amplitude curve on log-log paper. We should then have a straight
sloping line.
Time constant
The photodetector’s time constant corresponds to the carrier travel time within the collection or depletion
region. It depends upon the depletion zone thickness and carrier velocity. With the application of sufficient
bias to the diode, we make sure that carriers reach their limiting velocity within this zone. In principle, the
time constant of a typical photodiode is 0.5 ns for 50 μm layer thickness. The actual values are much
closer to 1 ns because the carriers crated outside the depletion zone are collected at a much slower rate,
due to reduced field intensity in that area.
The determination of the photodetector time constants is performed by measuring the output signal in
response to a step input of light on the device. In practice, we apply a square wave signal to the input of
the transmission module and measure the time interval required for the output signal to rise from 0-63%
of its terminal value as shown in figure 2. This measured value gives the time constant τ of the detector.
100%
63%

Figure 2. Photodetector Time Constant
Section One
This section requires the students to perform the following experimental steps in order to carry out
standard characterisation of a photodetector.
After performing the experimental steps, students are required to interpret and analyse the results in
order to characterize the following:
1.
2.
3.
4.
Time constant
Cut-off frequency
Linearity
Sensitivity
Experimental Setup:
The experimental setup is shown in Fig. 3
Function
generator
Laser/
LED
Detector +
variable
resistor
Oscilloscope
Fig. 3: Schematic diagram of the photodetector characteristic experiment setup.
Experimental Guidelines:
A) Time constant
Setup the experiment as shown in Fig. 3. Powered the red LED with a function generator and monitor the
detector output from the oscilloscope. The detector is connected to the variable load resistor. Ensure that
the red LED is powered with a suitable operating voltage (refer to its threshold voltage). Align the optical
source and the detector properly to ensure that maximum light hits the detector for optimum performance.
Pulsed the red LED with a square wave of 1 kHz (DC source and offset to ensure positive voltage). Set
the variable load resistor to 50 ohm. Measure the time interval corresponding to the detector’s time
constant. Next, vary the value of the load resistor and the frequency of the square wave and measure the
corresponding time interval. Analyze the results.
B) Cut-off frequency
Pulsed the red LED with a square wave of 1 kHz (DC source and offset to ensure positive voltage). Set
the variable load resistor to 50 ohm. Vary the frequency and record the output voltage from the
oscilloscope. Next, vary the value of the load resistor and record the corresponding output voltage.
Plot a log (amplitude)-frequency graph to determine the 3 dB cut-off point. Analyze the results.
C) Linearity
Function
generator
ND Filter
Laser/
LED
Detector+
variable
resistor
Oscilloscope
Fig. 4: Schematic diagram of linearity measurement.
Setup the experimental as shown in Fig. 4. Align the laser diode, ND filter and the detector properly to
ensure that maximum light hits the detector for optimum performance. The variable load resistor is initially
set at 50 ohm.
Vary the angle of the ND filter and record the corresponding output voltage at the oscilloscope. Next, vary
the value of the variable load resistor. Plot the readings on a log-log graph and to check the linearity of
the detector. Analyze the results.
The experiment is repeated for the other LEDs.
D) Sensitivity
Setup the experimental setup as shown in Fig. 4.
In this experiment, vary the angle of the ND filter and record the corresponding output voltage at the
oscilloscope. Next, change the photodetector with a power meter. Record the corresponding value of the
power meter while varying the angle of the ND filter. Next, vary the value of the variable load resistor.
Calculate the sensitivity of the detector. Analyze the results.
The experiment is repeated for the other LEDs.
Section Two
This section contains questions that require the students to examine and evaluate their results following
the steps performed in Section One.
A) Time constant
Questions:
1. Evaluate the time interval corresponding to the detector’s time constant.
2. Compare and contrast the similarities and differences between the results when the value of the
load resistor and frequency of the square wave is varied.
B) Cut-off frequency
Questions:
1. Evaluate the 3 dB cut-off point.
2. Compare and contrast the similarities and differences between the results when the value of the
load resistor and frequency of the square wave is varied.
C) Linearity
Questions:
1. Compare and contrast the similarities and differences between the results when the angle of the ND
filter and the value of the variable load resistor is varied.
2. Compare and contrast on the similarities and differences between the results from the varying
LEDs.
D) Sensitivity
Questions:
1. Compare and contrast the similarities and differences between the results when the angle of the ND
filter and the value of the variable load resistor is varied.
2. Compare and contrast on the similarities and differences between the results from the varying
LEDs.
3. Evaluate the sensitivity of the detector for different LEDs.
Report writing guidelines:
1.
2.
3.
4.
5.
The report should contain general information/comparison about the theory involved.
All the results of the measurements should be noted.
All results must be carefully analysed.
Comments on personal impression/experience during the lab session are to be added.
Report must be done individually and submitted to the technician in Optical Lab 2 at the latest, 2
weeks after the session.
6. Marks will be deducted for any late submissions.
Lab report marking scheme:
The total mark is 10 marks and this will be converted to 5 marks.
1. Title, Objective, Equipment and Conclusion = 1 mark
2. Brief explanation using own words about experimental procedure = 2 marks
3. Results (include plot) and discussion (include explanation for results and compare the results with
theory) for any TWO characteristics = 7 marks (3.5 marks for each characteristic)
4. Marks will be deducted for any late submissions.