Optoelectronics 1: Devices for Optical Communications
VCSEL Laser characteristics
Objective:
To measure the light current characteristic of a Vertical Cavity Surface Emitting Laser Diode (VCSEL) and
hence to determine the lasing threshold and the differential quantum efficiency, above and below threshold.
WARNING: Class IIIB laser hazard
WARNING: Laser light is always dangerous. The light from this laser is
invisible to human eyes and thus is even more dangerous. DO NOT stare at
any point where the beam may emerge from, such as the fibre output or the
connectorised laser diode package. The laser can be powered up using the key
on the front panel of the Thorlabs laser diode driver. Only power up the laser
while there is a fibre patchcord securely attached to the laser, with the other
end of the patchcord firmly connected to an optical power meter. At all other
times, YOU MUST TURN OFF THE LASER DIODE CURRENT. Do not
allow any person(s) not involved with this specific exercise to interfere with the
setup.
Please reread the above instructions and be certain that you understand the
warning before proceeding.
Equipment:
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Honeywell HFE4080-32X VCSEL Laser mounted in an ST package on a laser heatsink
ILX 3412 precision Laser diode Driver and associated mains supply and laser interconnect lead
Interconnecting ST connectorised 62.5/125 µm fibre patchcord
Fiber Optic power level meter, Megger OTP 620.
WARNING: The maximum laser diode current must never exceed 12 mA otherwise
permanent damage to the laser will result.
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..
Method:
Connect the laser to the ILX laser driver (the front cover is shown below) and the laser optical output to
the optical power meter. The laser driver output can be controlled to within 0.1 mA. Note that to protect
the laser the current output is initially off when the mains power switch is turned on.
Turn on the laser driver using the power button. Turn the laser driver output current control to zero (fully
anti-clockwise) prior to turning on the laser current. To turn on the laser current press the small output
button once (below the current set knob). Rotating the laser output knob clockwise will increase the
current, the value of which is shown on the display. If the display does not show an increase in current
recheck your connections.
To switch off the laser current depress output button once more. To protect the laser from transient
damage do not disconnect the laser from the laser driver at any time when the laser current is enabled. The
correct sequence to disassemble the experiment is to turn off the laser output, then disconnect the laser
from the ILX laser driver, then turn off the ILX driver using the power button.
ILX 3412 Laser driver
Measure the laser diode light-current characteristic, by varying the laser diode current from about 1 mA up
to a maximum of 12 mA, in small increments (typically 0.2 mA, but close to threshold smaller increments of
0.1 mA will be needed for accuracy. Monitor the output of the laser via the supplied ST connectorised
62.5/125 µm optical fibre patchcord, connected to the optical power meter. Set the optical power meter
to measure µW at 850 nm. WARNING: The maximum laser diode current must never exceed 12 mA
otherwise permanent damage to the laser will result.
Precautions: Only power up the laser while there is a fibre patchcord securely
attached to the laser, with the other end of the patchcord firmly connected to an
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 power meter. At all other times and when you are not taking measurements,
YOU MUST TURN OFF THE LASER DIODE CURRENT.
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..
Results:
Plot the laser diode light-current curve as shown in the sample below and compare with that supplied in the
manufacturers data sheet (supplied). Hence determine the laser threshold and the differential quantum
efficiency, above and below threshold.
The laser threshold is the point where the laser changes its operating mode from a spontaneous emission
(like a light emitting diode (LED)) to stimulated emission (Lasing). By convention the threshold can be
found by drawing a line parallel to the characteristic above threshold. The point where the line intercepts
the X or Current axis is the threshold.
The differential slope efficiency (dL/dI) is the slope of the characteristic at a particular current. The SE has
units of µW per mA (or mW per mA depending on the laser optical power). The SE is low below
threshold and high above. In your results estimate the SE below threshold and at least at three
points above threshold.
In principle the SE should be constant above and below threshold, but in practice the SE above threshold
varies as shown in the diagram below. Serious variations in the SE above threshold along with so-called
"kinks" in the laser characteristic may point to internal defects in the laser. It is for this reason that in laser
production each light-current characteristic is analysed and serious slope changes and kinks are identified in
order in order to eliminate faulty lasers prior to packaging. Check your laser characteristic for kinks. Are
there any present in your opinion and at what drive current?
Light output
Light output
dL/dI
slope
efficiency
variation
Stimulated
emission
Kink
Spontaneous
emission
Drive Current
Laser threshold
current
Laser diode characteristics
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..