ETM 4106 Optoelectronics and optical communication
Tutorial 6
Solve the following problems:
Q1. Discuss the principles of operation of the injection semiconductor laser. Your discussion should
also include diagrams that might be useful to explain the operation.
Q2. The longitudinal modes of a galium arsenide injection laser emitting at a wavelength of 0.87m are
separated in frequency by 278GHz. Determine the length of the optical cavity and the number of
longitudinal modes supported within the structure. The refractive index of gallium arsenide is 3.6.
Ans. 150m, 1241
Q3. An injection laser has GaAs active region with a bandgap energy of 1.43eV. Estimate the
wavelength of optical emission from the device and determine its linewidth in hertz when the
measured spectral width is 0.1nm.
Ans. 0.87m, 39.6GHz.
Q4. A DH injection laser has an optical cavity of length 50m and width 15m. At normal operating
temperature the loss coefficient is 10cm-1 and the current threshold is 50mA. When the mirror
reflectivity at each end of the optical cavity is 0.3, determine the gain factor for the device.
Assume that the current is confined to the optical cavity.
Ans. 3.6710-2cmA-1
Q5. Give the definition of internal quantum efficiency. A galium arsenide injection laser with a cavity
of length 500m has a loss coefficient of 20cm-1. The measured differential quantum efficiency of
the device is 45%. Calculate the internal quantum efficiency of the laser. The reflectivity at each
end of the optical cavity is 0.32.
Ans. 84.5%
Q6. The threshold current density for a stripe geometry AlGaAs laser is 3000Acm -1 at a temperature of
15oC. Estimate the required threshold at a temperature of 60 oC when the threshold temperature
coefficient To for the device is 180K, and the contact stripe is 20 x 100m.
Ans. 77.0mA
Extra Questions
Q8. If the cavity of a given InGaAsP laser has length L = 500m and the wavelength interval is [1.54,
1.56] m within which the gain profile is greater than the minimum required gains, find the
longitudinal mode separation and the number of possible modes. Also, what is the corresponding
mode separation in GHz? Assume the reflactive index is 3.63.
Ans. 0.6618nm, 30, 82.6GHz.
Q9. A C3 laser has two cavities of lengths 800m and 1000m. Find the longitudinal mode separation
for each cavity and also the laser common mode separation. Assume the central wavelength is
1.5mm and the refractive index of the two cavities is 3.0.
Ans. 0.469nm, 0.375nm, 1.875nm
Q10. If the tunable wavelength range of a DBR laser is from 1.552mm to 1.558mm, find the
corresponding frequency range. If the channel separation is 10Ghz, how many WDM channels
can this laser be tuned to?
Ans. 70