Lab VII
Semiconductor Laser Properties
ECE 476
I. Purpose
The electrical and optical properties of a semiconductor laser will be investigated.
II. Background
Laser diode basics
A semiconductor laser is a p-n junction diode constructed from a direct band-gap
semiconductor material. The typical characteristic of a semiconductor laser is shown in
Fig. 1. Below the threshold current the device operates as an LED with an output of light
Figure 1: LED laser Current characteristics.
across a wide spectral region (20-30 nm). In the laser region the spectral output narrows
to a single emission wavelength. The transition of the output from LED characteristics to
laser characteristics is shown in Figure 2.
Figure 2: Radiant output as a function of frequency for a p-n junction laser.
The light output for spontaneous emission in the LED operating region is not polarized
and the light output for stimulated emission in the laser operating region is often
polarized.
Figure 3: Laser LED vector board.
LED laser protection
Semiconductor lasers operate at very high current densities to get stimulated emission. To
help dissipate the heat a small heat sink is attached to each laser diode (the metal ‘star’ in
figure 3). The laser diodes are more expensive (25-30 dollars) than LEDs (few cents), so
use caution to not damage the lasers. (Too much current is the most common damage
mechanism, so limit your current to the rated operating current.) The specification for the
semiconductor laser you will use is given below.
The maximum threshold current is specified as 40 mA. Often the threshold current is
lower in the range of 27-35 mA. It is recommended you not exceed a current rating of 45
mA.
The laser is already connected in series with a resistor of value 470 ohms. This resistor is
used to help protect the laser diode from overcurrent damage.
LED laser datasheet
LN9705
Optical output: 5 mW
Threshold current: 40 mA
Operation current: 50 mA
Wavelength: 788 nm
Radiation Angle (parallel): 10 degrees
Radiation Angle (vertical): 25 degrees
Dark Current: 0.1 uA
Operation Voltage: 1.75 V
III. Procedure
Part A: Power Measurement
Set-up
1. Re-read the background section on LED laser protection. These particular LEDs are no
longer manufactured and would be very difficult to replace if damaged.
Experiment
1. Measure the output light power versus input current. Plot this data and determine the
threshold current value. Note: Be sure to calibrate your power meter. Note: You will
need to measure optical power output for LED current between 0mA to no more than
45mA. You may measure the optical power for every milliamp, however it is not
necessary. Since the power does not change much in the lower range of currents (015mA), you could make readings every 5mA. From 15-25mA, you might take readings
every 2mA. From 25-40mA, I recommend taking readings at every milliamp, since this
is where you will find your critical threshold current.
2. Above the threshold current condition, where the device is operating as a laser,
calculate the external differential quantum efficiency ηex, which is the ratio of the
increase in photon output to the increase in the current input (How many photons are
emitted for each electron input.) The expression for the efficiency is
η ex =
qΔP
hfΔI
where q is the charge of an electron, hf is the photon energy, ΔP is the change in light
output, and ΔI is the change in current input. Note: The best way to judge the value of
threshold current is to examine the rate of change among your measurements in the upper
range (25-40mA). The greatest change among the rates of change (i.e. maximum of the
second derivative) is approximately where the threshold is. For instance, if 25 to 26mA
changes by 20uW, 26 to 27mA changes by 100uW, and 27 to 28mA 120mW, you can
approximate your threshold current to be 26mA.
3. How does the efficiency of the laser LED compare to the efficiency of the LEDs from
the previous lab?
4. Use your infrared detector card to look at the shape of the output laser beam. Describe
this shape in your lab notebook.
Part B: Polarization
Experiment
1. You will measure the polarization percentage of the laser diode. Measure the power
output from the laser for different polarizer angles with a polarizer placed between the
laser and the optical power meter. (Definition: Polarization percentage is the ratio of
(minimum power)/(maximum power) where maximum power and minimum are for
different rotations of a linear polarizing filter.) Perform this measurement for two cases:
(1) a current level 4 mA above the threshold current and (2) a current level of 5 or 10 mA
below the threshold current value. Discuss your results. Note: You need to report 6
measurements: minimum power and maximum power for below threshold, minimum and
maximum power for above threshold, polarization percentages for both cases.
C. Spectroscopy
Set-up
1. You will measure the spectral output of your laser using the monochromator you used
for the LED experiment. Follow the operating instructions for the monochromator found
in the beginning of the previous lab.
Experiment
1. Take a spectral measurement at three current levels including 5 mA below threshold
current, at the threshold current, and 4 mA above the threshold current. For each of these
plots determine the FWHM (full width at half maximum) in nm for the light output.
Discuss your results. Note: Make sure the monochromator is calibrated to the correct
wavelength. Note: You need three plots. Compare them to figure 2 to make sure you’ve
captured them correctly.
IV. Conclusion
Draw conclusions on this experiment. Comment on the performance of the laser LED at
different current levels.