676
23 Electricity-to-Light Conversion
(a)
1.44 1.46 1.48
1.54
1.50 1.52
th
th
th
th
(b)
Fig. 23.51 a Mode spectra of a Fabry–Perot laser, under, at and above threshold (I thr = 13.5 mA). Adapted from [1185].
b Mode spectra of a cw DFB InGaAs/InP laser with 2 mm cavity length at various currents of 200, 400, . . ., 1400 mA
(I thr = 65 mA), SSR>40 dB at T = 293 K. Adapted from [1999], reproduced with permission from SPIE
and quantum wire lasers. Only for quantum dots with a δ-like density of states is the change of Fermi
distribution irrelevant as long as excited states are energetically well separated from the (lasing) ground
state. In Fig. 23.50c, the threshold of a quantum dot laser is indeed temperature independent (T 0 = ∞)
as long as excited states are not thermally populated (for T < 170 K for the present laser).
23.4.10 Mode Spectrum
In Fig. 23.51a, the mode spectrum of a typical edge-emitting laser is shown. Below threshold, the
amplified spontaneous emission (ASE) spectrum exhibits a comb-like structure due to the Fabry–
Perot modes. Above threshold, some modes grow much faster than others, possibly resulting in single
longitudinal mode operation at high injection. The relative strength of the strongest side mode is
expressed through the side-mode suppression ratio (SSR) in dB
SS R = 10 log
I mm
I sm
,
(23.39)
where I mm (I sm ) is the intensity of the maximum (strongest side) mode in the lasing spectrum.
As a tendency a DHS or QW semiconductor laser above threshold develops a narrow spectrum since
the pump power is channeled into one or few modes with large gain. A quantum dot laser behaves
differently when pumped largely above threshold. Since the gain of individual QDs in an inhomogeneously broadened ensemble (due to different QD sizes) is independent, a broad gain spectrum is
23 Electricity-to-Light Conversion
(a)
1.44 1.46 1.48
1.54
1.50 1.52
th
th
th
th
(b)
Fig. 23.51 a Mode spectra of a Fabry–Perot laser, under, at and above threshold (I thr = 13.5 mA). Adapted from [1185].
b Mode spectra of a cw DFB InGaAs/InP laser with 2 mm cavity length at various currents of 200, 400, . . ., 1400 mA
(I thr = 65 mA), SSR>40 dB at T = 293 K. Adapted from [1999], reproduced with permission from SPIE
and quantum wire lasers. Only for quantum dots with a δ-like density of states is the change of Fermi
distribution irrelevant as long as excited states are energetically well separated from the (lasing) ground
state. In Fig. 23.50c, the threshold of a quantum dot laser is indeed temperature independent (T 0 = ∞)
as long as excited states are not thermally populated (for T < 170 K for the present laser).
23.4.10 Mode Spectrum
In Fig. 23.51a, the mode spectrum of a typical edge-emitting laser is shown. Below threshold, the
amplified spontaneous emission (ASE) spectrum exhibits a comb-like structure due to the Fabry–
Perot modes. Above threshold, some modes grow much faster than others, possibly resulting in single
longitudinal mode operation at high injection. The relative strength of the strongest side mode is
expressed through the side-mode suppression ratio (SSR) in dB
SS R = 10 log
I mm
I sm
,
(23.39)
where I mm (I sm ) is the intensity of the maximum (strongest side) mode in the lasing spectrum.
As a tendency a DHS or QW semiconductor laser above threshold develops a narrow spectrum since
the pump power is channeled into one or few modes with large gain. A quantum dot laser behaves
differently when pumped largely above threshold. Since the gain of individual QDs in an inhomogeneously broadened ensemble (due to different QD sizes) is independent, a broad gain spectrum is