OD1: The Characteristics of LEDs and a Laser Diode
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 the interval.
4. Please ensure that the right cables are used to connect the equipment.
5. Please take note that some circuits need to be grounded in order to work properly.
6. 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 standard characterisation of LEDs and a laser
diode. The objectives of the experiment are to determine the:
1.
2.
3.
4.
I-V characteristics of the device,
Luminance-current (L-I) and Luminance-voltage (L-V) characteristics,
Polarisation (LED and Laser Diode)
Cut-off frequency of the system
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.
50 MHz oscilloscope
Function generator
BNC/adapter/terminator
Semiconductor laser diode (red)
LEDs (Red, Blue, White, Infrared)
5mm square photodiode
Power meter
DC power supply
Theory:
Semiconductors (typically direct band-gap semiconductors) can be used as small, highly efficient photon
sources. Such semiconductor photon sources find application in displays, optical storage,
communications, printing, surveying, and as small efficient optical pumps for other optical processes.
Semiconductor photon sources come in two major categories -- laser diodes and light-emitting diodes.
Although both photon sources are fundamentally the same (in other words, a failed laser diode is an
LED); application-based manufacturing differences mean that laser diode and LED products are
constructed in fundamentally different ways -- and so possess fundamentally different optical properties.
Semiconductor photon sources can be "pumped" by a variety of techniques. These include pumping with
another optical source (photopumping), pumping with an electron beam, or pumping via a pn-junction.
However, the most common technique is via a pn-junction. Consider the energy band diagram for a
simple pn-junction intended for use in a semiconductor photon source. Initially, one side of the junction is
heavily doped n and the other side is heavily doped p. In general, both materials are doped degenerately
(meaning that the quasi-Fermi level is above the bandedge).
When a positive voltage is applied to the p-region and a negative voltage is applied to the n-region, then
the diode is forward biased. The quasi-Fermi levels will misalign by the value of the applied voltage.
Under the influence of the forward bias, the holes will drift to the n-region and the electrons will drift into
the p-region. The holes and electrons are now spatially coincident -- and hole-electron recombination can
occur.
The electrically pumped diodes consist of a pn layer/junction, which gives a characteristic curve as shown
in Fig.1.
A
Vth
V
Fig.1: I-V characteristic of a diode
The diode current is given by
I (V )  I 0 ( e eV / k BT )  1)
(1)
This equation, called the diode equation, gives the current through a p-n junction under forward (V>0) and
reverse bias (V<0). Under reverse bias, the current simply goes towards the value (-I0). Under forward
bias the current increases exponentially with the applied forward bias. Vth is the turn on voltage for the
diode.
Semiconductor lasers are the most basic of the existing laser types. In their simplest form they consist of
a small rectangular slab of semiconductor material with two cleaved facets to act as mirrors. The other
facets are destroyed in some way (etched, ground, sawn or ion implanted) in order to avoid spurious laser
modes.
Semiconductor lasers are quite different from conventional lasers. In particular:
1. The gain of the laser material is very high and is generated by a population inversion between the
conduction and valence bands of the semiconductors. In some sense, a semiconductor laser is a
two-state laser system.
2. Since the electromagnetic mode is on the order of the size of the laser device, then the
transverse mode of the semiconductor laser is quite different from that of a conventional laser. In
particular, the beam is not Gaussian, the beam profile tends to be elliptical, and the beam
divergence tends to be large.
3. The gain spectrum is quite large (many THz or hundreds of angstroms).
4. The short cavity (several hundred microns) means that the longitudinal mode spacing is much
larger than that of a conventional gas or solid state laser (on the order of GHz or angstroms).
5. Laser light is strongly polarised, while LED light is not.
Section One
This section requires the students to perform the following experimental steps in order to carry out
standard characterisation of LEDs and a laser diode.
After performing the experimental steps, students are required to interpret and analyse the results in
order to characterize the following:
1.
2.
3.
4.
I-V characteristics of the device,
Luminance-current (L-I) and Luminance-voltage (L-V) characteristics,
Polarisation (LED and Laser Diode)
Cut-off frequency of the system
Experimental Setup:
The experimental setup and the circuit diagram for the LEDs/laser diode is shown in Fig. 2
Power
supply
Laser/
LED
DC power
supply
Or
Func. gen.
Detector
VR
50 ohm
Va
Power
meter
Sample
Vs
Fig. 2: Schematic diagram of the Luminance-Current characteristic and Current-Voltage characteristic
experiment setup.
Experimental Guidelines:
A) Luminance-Current (L-I) and Current-Voltage (I-V) characteristics
Setup the experiment by referring to Fig. 2. Ensure that the laser diode is powered with a suitable
operating voltage. Align the optical source and the detector properly to ensure maximum light hits the
detector for optimum performance.
Using the oscilloscope and the power meter, measure and record the light output intensity, the apply
voltage Va, the voltage dropped across the 50 ohm resistor Vr and the voltage dropped across the sample
which is Vs=Va-Vr. Plot the I-V and L-I characteristics of the laser diode. Repeat the experiment for the
red/infrared/white/blue LEDs (choose any two LEDs).
B) Polarisation
Power
supply
Polariser
Laser/
LED
Detector
Power
meter
Fig. 3: Schematic diagram of the polarisation measurement.
Setup the polarisation experimental as shown in Fig. 3. Ensure that the laser diode is powered slightly
above its threshold voltage. Align the optical source, polariser and the detector to ensure that maximum
light hits the detector for optimum performance.
Vary the angle of the polariser and record the reading of the power meter. Repeat the experiment for
other LEDs and set the power of the LEDs to slightly above the threshold voltage. Plot the intensity
versus angle plot for the LEDs and laser diode.
C) Cut-off frequency
In this experiment, the laser diode is powered by the function generator. Initially, set the function
generator at 1 kHz, 2 V p-p, offset at 1 V. Connect the detector output to an oscilloscope. Monitor the
wave train using the oscilloscope. Align the optical source and the detector to ensure that maximum light
hits the detector for optimum performance. Next, pulsed the laser diode with a square wave with 1 kHz
and set the amplitude to above the threshold voltage. Record the output voltage from the oscilloscope as
the frequencies are varied (10 Hz – 2 MHz). Plot a log (amplitude)-frequency graph to determine the 3 dB
cut-off point.
Repeat the experiment for 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) Luminance-Current (L-I) and Current-Voltage (I-V) characteristics
Questions:
1. How do you know it is a 50-ohm resistor in the circuit for the LEDs/laser diode?
2. Evaluate the threshold voltage for LED/Laser and the threshold current for the laser diode.
3. Compare and contrast the similarities and differences between the results from the laser diode and
the varying LEDs.
B) Polarisation
Questions:
1. Explain the plots.
2. Compare and contrast on the similarities and differences between the results from the laser diode
and the varying LEDs.
C) Cut-off frequency
Questions:
1. Compare and contrast on the similarities and differences between the results from the laser diode
and the varying LEDs.
2. Evaluate the 3 dB cut-off point for the laser diode and varying 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.