:
The aim of this experiment is to carry out standard characterisation of a laser diode/LED. You are expected to measure the a) I-V characteristics of the device, b) Light-current (L-I) and light-voltage (L-V) characteristics, c) Polarisation (LED and Laser) d) Cut-off frequency of the system
1. GoodWill 50MHz Oscilloscope Model GOS-652
2. Signal generator
3. BNC/adapter/terminator
4. Semiconductor Lasers (red)
5. LEDs (Red, Blue, White, Infrared)
6. TO10 Sharp Emitter and Silicon Phototransistor (infrared)
7. 5mm square Photodiode
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.
A
The electrically pumped diodes consist of a pn layer/junction, which gives a characteristic curve as shown in Fig.1.
The diode current is given by
I ( V )

I
0
( e eV / k
B
T ) 
1 ) (1)
V th
This equation, called the diode equation, gives us the current
V
Fig.1: I-V characteristic of a diode through a p-n junction under forward (V>0) and reverse bias
(V<0). Under reverse bias, the current simply goes towards the value (-I
0
). Under forward bias the current increases exponentially with the applied forward bias. V th
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, 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.
:
It consists of a semiconductor laser, oscilloscope, detector, polariser, pulse generator, DC power supply and a function generator. The experimental setup is shown in Fig. 2
Power supply
Laser/
LED
Detector
DC power supply
Or
Func. gen.
50 ohm
V
R
Reader
Sample Vs
Fig. 2: Schematic diagram of Light-Current characteristic, Current-Voltage characteristic experiment setup.
V a
A) Light-Current (L-I) and Current-Voltage (I-V) characteristics
1. Place the detector in front of the laser/LED (Back-to-back). Connect the detector output to the reader.
2. Power the laser diode/LED with DC power supply and monitor the detector output.
3. Warning: Most operating voltages of LEDs/Lasers 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. Measure the apply voltage V a
and the voltage dropped across the 50 ohm resistor V r
and hence the voltage dropped across the sample is Vs=V a
-V r
.
5. Comment: How do you know it is a 50-ohm resistor?
6. Record Vs, V a
, V r
and the value of the light output L.
7. Plot I-V and L-I with computer. Please verify your result with your lab instructor (Signature required)
8. Determine the threshold voltage for LED/Laser and the threshold current for the Laser
9. Repeat step 1-4 for the Red/Infrared/White/Blue LED (choose two LEDs) and Laser.
(You need to disconnect and re-connect the wires)
10. Comment on their similarities and differences.

B) Polarisation
Power supply
Laser/
LED
Polariser
Detector
Reader
Fig. 3: Schematic diagram of polarisation measurement.
1. There is a Laser+polariser+detector on the track. Turn on the laser and detector reader and align the optic to optimise the performance.
2. By varying the angle of the polariser, record the reading.
3. Chose an LED. Set up the LED and a detector assembly with a polariser and lens in between as shown in Fig. 3. (As close as possible). Convex lens is
C) Cut-off frequency needed if the light is too weak to be recorded.
4. Power the LED with the DC power supply and set it slightly above the threshold voltage.
5. Record the light intensity by varying the angle of the polariser.
6. Repeat the steps for red Laser under/above the threshold current. (Refer to your measurements in section A)
7. Comment on the plots.
8. Optimise the reception of the emitter and detector
9. Set the Function Generator at 1 kHz, 2 V p-p, Offset at 1 V. Do not make connect with the diode, monitor the wave train using the oscilloscope.
10. Pulse the emitter with square wave with 1 kHz and set the amplitude above the threshold voltage.
11. Connect the photodiode output to the oscilloscope. You should now see the wave train.
12. By varying the frequencies (10 Hz-2 MHz), record the output voltage from the oscilloscope.
13. Plot a Log (amplitude)-frequency graph to determine the 3dB cut-off point.
1. The report should contain general information/comparison about the theory involved.
2. All the results of the measurements should be noted.
3. All results must be carefully analysed.
4. Comments on personal impression/experience during the lab session are to be added.
5. Report must be submitted in group to the technician in optical lab 2 at the latest 2 weeks after the session.