Ryerson University

Department of Electrical and Computer Engineering

Optical Communication Systems

Problem Set - Optical Transmitters

1. An InGaAsP laser operating at 1300 nm has dimensions d = 700 nm, w = 1300 nm, L

= 0.5 mm. The absorption coefficient α = 600 m−1; spontaneous emission lifetime τsp

= 4 ns; refractive index n = 3.5; threshold current density Jth = 2 X 106 A/m2.

(a) Calculate the photon lifetime τph.

(b) Calculate the number of emitted steady state photons φs when the current density

J of 3 X 106 A/m2,

(c) Calculate the internal optical power density Pint in mW/m2

(d) Find the output optical power density that will be emitted from the laser = Pint(1−R) (Note (1-R) is the loss due to Fresnel reflection).

(e) Find the output power Pout = Pint(1−R)wd in mW

2. An engineer has two Ga1−xAlxAs LEDs; one has a band gap energy of 1.54 eV and the

other has x = 0.015.

(a) Find the aluminum mole fraction x and the emission wavelength for the first LED.

(b) Find the band gap energy and the emission wavelength of the other LED.

3. Assume that the injected minority carrier lifetime of an LED is 5 ns and, the optical

output power when a constant dc current is applied is 0.30 mW. Plot the frequency

reponse of this LED (the optical output power versus the driving ac current frequency).

(a) What is the optical 3-dB bandwidth of this LED?

(b) What is the electrical 3-dB bandwidth of this LED?

(c) What is the optical output power at 100 MHz?

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4. A laser diode has maximum average output power of 1 mW. The laser amplitude mod-

ulated with a signal x(t) that has a dc component of 0.2 and periodic component of

±2.56. If the relationship between the optical power to driving current is P (t) = i(t)/10,

find the values of Io and m if the modulating current is i(t) = Io[1 + mx(t)]. What is

the peak value of i(t)? What is the minimum value of i(t)? To avoid clipping distortion,

what should the smallest/largest value of the laser threshold current?

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