206
4 Light Sources for Fiber Links
If E g = 1.43 eV for GaAs, what is the emission wavelength between the i = j = 1
states if the active layer thickness is d = 5 nm?
[Answer: λ = 739 nm].
4.17 In a multiple quantum-well laser the temperature dependence of the differential
or external quantum efficiency can be described by
η ext (T ) = η i (T )
α end
N w
α w + γ (T − T th )
+ α end
where η i (T ) is the internal quantum efficiency, α end is the mirror loss of the
lasing cavity as given in Eq. (4.28), N w is the number of quantum wells, T th is
the threshold temperature, α w is the internal loss of the wells at T = T th and γ
is a temperature-dependent internal-loss parameter. Consider a six-well, 350μm-long MQW laser having the following characteristics: α w = 1.25 cm
−1 , γ
= 0.025 cm
−1 /K, and T th = 303 K. The lasing cavity has a standard uncoated
facet on the front (R 1 = 0.31) and a high-reflection coating on the near facet
(R 2 = 0.96).
(a) Assuming that the internal quantum efficiency is constant, plot the external
quantum efficiency as a function of temperature over the range 303 K ≤ T ≤ 375 K.
Let η ext (T ) = 0.8 at T = 303 K.
(b) Given that the optical output power at T = 303 K is 30 mW at a drive current
of I d = 50 mA, plot the power output as a function of temperature over the range
303 K ≤ T ≤ 375 K at this fixed bias current.
4.18 A distributed-feedback laser has a Bragg wavelength of 1570 nm, a secondorder grating with = 460 nm, and a 300-μm cavity length. Assuming a
perfectly symmetrical DFB laser, find the zeroth-order, first-order, and secondorder lasing wavelengths to a tenth of a nanometer. Draw a relative amplitudeversus-wavelength plot.
4.19 When a current pulse is applied to a laser diode, the injected carrier pair density
n within the recombination region of width d changes with time according to
the relationship
∂n
∂t
=
J
qd
−
n
τ
Assume τ is the average carrier lifetime in the recombination region when the injected
carrier pair density is n th near the threshold current density J th . That is, in the steady
state ∂n/∂t = 0, so that
n th =
J th τ
qd
4 Light Sources for Fiber Links
If E g = 1.43 eV for GaAs, what is the emission wavelength between the i = j = 1
states if the active layer thickness is d = 5 nm?
[Answer: λ = 739 nm].
4.17 In a multiple quantum-well laser the temperature dependence of the differential
or external quantum efficiency can be described by
η ext (T ) = η i (T )
α end
N w
α w + γ (T − T th )
+ α end
where η i (T ) is the internal quantum efficiency, α end is the mirror loss of the
lasing cavity as given in Eq. (4.28), N w is the number of quantum wells, T th is
the threshold temperature, α w is the internal loss of the wells at T = T th and γ
is a temperature-dependent internal-loss parameter. Consider a six-well, 350μm-long MQW laser having the following characteristics: α w = 1.25 cm
−1 , γ
= 0.025 cm
−1 /K, and T th = 303 K. The lasing cavity has a standard uncoated
facet on the front (R 1 = 0.31) and a high-reflection coating on the near facet
(R 2 = 0.96).
(a) Assuming that the internal quantum efficiency is constant, plot the external
quantum efficiency as a function of temperature over the range 303 K ≤ T ≤ 375 K.
Let η ext (T ) = 0.8 at T = 303 K.
(b) Given that the optical output power at T = 303 K is 30 mW at a drive current
of I d = 50 mA, plot the power output as a function of temperature over the range
303 K ≤ T ≤ 375 K at this fixed bias current.
4.18 A distributed-feedback laser has a Bragg wavelength of 1570 nm, a secondorder grating with = 460 nm, and a 300-μm cavity length. Assuming a
perfectly symmetrical DFB laser, find the zeroth-order, first-order, and secondorder lasing wavelengths to a tenth of a nanometer. Draw a relative amplitudeversus-wavelength plot.
4.19 When a current pulse is applied to a laser diode, the injected carrier pair density
n within the recombination region of width d changes with time according to
the relationship
∂n
∂t
=
J
qd
−
n
τ
Assume τ is the average carrier lifetime in the recombination region when the injected
carrier pair density is n th near the threshold current density J th . That is, in the steady
state ∂n/∂t = 0, so that
n th =
J th τ
qd