ETM 4106 Optoelectronics and optical communication
Tutorial 7
Solve the following problems :
1.
Determine the total carrier recombination lifetime and the power internally generated within an LED if the
peak emission wavelength is 0.87m at a drive current of 35mA. It is given that the radiative and
nonradiative recombination lifetimes of the minority carriers are 62ns and 100ns respectively.
Ans. 30.8mW
2.
The power generated internally within a double heterojunction LED is 28.4mW at a drive current of 60mA.
Determine the peak emission wavelength from the device when the radiative and nonradiative recombination
lifetimes of the minority carriers in the active region is equal.
Ans. 1.31m
3.
Determine the optical power emitted into air by a planar LED as a percentage of the internal optical power for
the device when the transmission factor at the semiconductor-air interface is 0.65. If the power generated
internally is 55% of the electrical power supplied, calculate the external power efficiency. It is assumed that
the LED is fabricated from GaAs material with a refractive index of 3.6.
Ans. 1.25%, 0.69%
4.
Light from an LED is coupled into a step index fiber with a numerical aperature of 0.25, a refarctive index of
1.48 and a diameter that is bigger than the diameter of the LED. Determine the coupling efficiency ans loss in
dB when the device is close to the fiber.
Ans. 6.25%, 12.0dB
5.
(a) Determine the optical loss due to Fresnel reflection at the semiconductor-fiber interface when LED is but
joint to the fiber. It is given that the refractive indices of of fiber and GaAs (LED) are 1.5 and 3.6
respectively.
(b) Calculate the optical power coupled into a 200m diameter core step index fibre with an NA of 0.3 from a
GaAs surface-emitting LED with an emission diameter of 90m and a radiance of 40Wsr-1cm-2.
(C) Estimate the optical power emitted into air for the device in 5(b).
Ans. (a) 0.81dB, (b) 600W, (c) 5.44mW
6.
Estimate the electrical modulation bandwidth for an LED with a carrier recombination lifetime of 8ns. The
frequency response of the device may be assumed to be Gaussian, where the optical bandwidth is a factor of
2 greater than the electrical bandwidth.
Ans. 24.4MHz
7.
The minority carrier recombination lifetime for an LED is 6.5ns. When a constant d.c drive current is applied
to the device the optical output power is 300W. Determine the optical output power when the device is
modulated with an rms drive current corresponding to the d.c drive current at frequencies (a) 25MHz and (b)
115MHz.
Ans. (a) 209.9W, (b) 62.5W