1. An engineer has two Ga(1-x)Al(x)As LEDs: one has band gap energy of 1.540 eV and the other has x = 0.015.
a. Find the aluminium mole fraction x and the emission wavelength for the first LED
answer :

Eg = 1,424 + 1.266X + 0.266X^2
1.540 = 1.424 + 1.266X+0.266X^2
X = 0.09
b. Find the band gap energy and the emission wavelength of the other LED
answer :
Eg = 1.424 + 1.266x + 0.266×2
Eg = 1.424 + 1.266(0.015) + 0.266(0.015)2
Eg = 1.443 eV

2. A double heterojunction InGaAsP LED emitting at a peak wavelength of 1310 nm has radiative and nonradiative recombination time sof 25 and 90 ns, respectively. The drive current is 35 mA.
answer :
a. Find the internal quantum efficiency and the internal power level.
Internal quantum efficiency :

Internal power level :

b. If the refractive index of the light source material is n = 3,5, find the power emitted from the device.
answer :
Power emitted from device :
Po = n2 = 3.52 = 12.25 Watt

3. a GaAl As laser diode has a 500cavity length which has an effective absorption coeffient of 10 cm-1. For the uncoated facets the reflectives are 0.32
a. What is the optional gain at the lasing threshold?
answer :

b. If one end of the laser is coated with a dielectric reflector so that its reflectivityis now 90%, what is the optical gain at the lasing threshold?
answer :
R = 0.9 x 0.32 = 0.288

c. If the internal quantum efficiency is 0,65, what is the external quantum effiency is cases (a) and (b)?
answer :

4. A GaAs laser emitting at 800 nm has a 400 cavity length with a refractive index n = 3.6. If the gain exceeds the total loss throughout the range 750 nm < < 850 nm, how many modes will exist in the laser?
answer :

5. A distributed feedback laser has aBragg wavelength of 1570 nm, a second-order grating with = 460 nm, and a 300 cavity length. Assuming perfecttly symmetrical DFB laser, find the zeroth-, first-, and the second-order lasing wavelength to a tenth of a nanometer. Draw a relative amplitude-versus wavelength plot.
answer :