Page 179 - Photonics Essentials an introduction with experiments

Lasers

Lasers 173

Problems and Exercises
7.1 Build and test the circuit shown in Fig. 7.2. Any general-purpose
transistor with a current gain greater than 50 will be satisfac-
tory. Include a potentiometer in series with one of the feedback
resistors. The oscillation frequency using the components shown
should lie close to the audio range. Attach an oscilloscope lead to
the output to measure the oscillation amplitude.
a) Measure the range of frequencies over which oscillation oc-
curs. Plot the amplitude of the oscillation as a function of fre-
quency.
b) How does the oscillation frequency depend on the bias volt-
age?
c) Use a soldering iron to locally heat a feedback resistor. Do not
touch the iron to the resistor, just hold it nearby. What hap-
pens to the frequency of the oscillation?
Record the circuit diagram, components used, and account of the
measurement in your lab book.
7.2 Laser action can occur when the stimulated emission rate ex-
ceeds the spontaneous emission rate (see Eq. 7.5). What would
happen if you reduced the spontaneous emission rate to zero?
Would you have a threshold-less laser? Explain your answer.
7.3 Estimate the threshold current density in A-cm –2 of a GaAs-
based laser with the following properties:
Emission wavelength = 850 nm
Line width of the gain spectrum = 1.5 × 10 13 Hz
Internal losses = 30 cm –1
Index of refraction = 3.5
Cavity length = 400 m
Thickness of the recombination region = 200 nm

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