ELEC425/6261
Assignment 4
1.
Photon cavity lifetime and total attenuation Consider a semiconductor Febry-Perot
(FP) optical cavity that has a length L, end mirrors with reflectances R1 and R2, and attenuation
inside the cavity given by s. The total attenuation coefficient for the optical cavity is denoted by
t. Calculate the photon cavity time for an In0.60Ga0.40As0.85P0.15 FP optical cavity that has a
refractive index of 3.7, length of 300 m and s = 20 cm-2.
2. SQW laser Consider a SQW (single quantum well) laser which has an ultrathin active
InGaAs of bandgap 0.70 eV and thickness 10 nm between two layers of InAlAs which has a
bandgap of 1.45 eV. Effective mass of conduction electrons in InGaAs is about 0.04me and that
of the holes in the valence band is 0.44me where me is the mass of the electron in vacuum.
Calculate the first and second electron energy levels above Ec and the first hole energy level
below Ev in the QW. What is the lasing emission wavelength for this SQW laser? What is this
wavelength if the transition were to occur in bulk InGaAs with the same bandgap?
3. Consider a GaAs-AlGaAs DH laser operating at 1310 nm. L = 60 m, W = 10 m and d = 0.25
m, the refractive index is 3.5 and the loss coefficient,  = 10 cm-1.
(i) Find the total loss coefficient in the cavity, t and ph.
(ii) If Jth = 500 A/cm2, and sp = 10 ps, what is the threshold electron concentration?
(iii) Calculate the lasing optical power when the current is 5 mA.
4. InGaAsP-InP Laser Consider a InGaAsP-InP laser diode which has an optical cavity of length
250 microns. The peak radiation is at 1550 nm and the refractive index of InGaAsP is 4. The
optical gain bandwidth (as measured between half intensity points) will normally depend on the
pumping current (diode current) but for this problem assume that it is 2 nm.
a
What is the mode integer m of the peak radiation?
b
What is the separation between the modes of the cavity?
c
How many modes are there in the cavity?
d
What is the reflection coefficient and reflectance at the ends of the optical cavity (faces of
the InGaAsP crystal)?
e
What determines the angular divergence of the laser beam emerging from the optical
cavity?
5.
Laser diode efficiencies The following specifications are provided for a particular
commercial AlGaInP power laser diode (Hitachi HL6323MG) with an emission wavelength of
630 nm (red). The threshold current at 25 °C is 45 mA. Typical operating voltage for this device
is 2.3 V. At I = 95 mA, the output optical power is 30 mW.
(a)
Calculate the external QE, external differential QE, power conversion efficiency and the
slope efficiency of the laser diode. What is the current required for an output of 20 mW?
(b)
The threshold current at 50 °C is measured to be 70 mA. Assuming that Ith = Aexp(T/To)
where To is a characteristic temperature, and given Ith at two temperatures, find the characteristic
To temperature parameter What is the threshold current at 0 °C? (The measured value is 31 mA)
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