23.4 Lasers
677
Fig. 23.52 Lasing spectra of quantum dot laser (L = 1.2 mm, stripe width w = 75 µm) at room temperature. The active
medium is a three-fold stack of InGaAs/GaAs QDs. The current density as labeled is given in units of the threshold
current density ( j thr = 230 A/cm 2 ). Adapted from [2000]
(a)
n
p
active
medium
(b)
n
DBR
grating
p
DBR
grating
pump
region
Fig. 23.53 Schematic drawing of a DFB (distributed feedback) and b DBR (distributed Bragg reflection) lasers. The
active medium is schematically shown as a triple quantum well, the waveguide is shown as a grey area
present [2001]. The lasing spectrum takes a hat-like shape when homogeneous broadening is small
compared to the inhomogeneous broadening[2000, 2002] (Fig. 23.52) as predicted theoretically [2001,
2003].
23.4.11 Longitudinal Single-Mode Lasers
In order to achieve a high SSR or single longitudinal mode lasing, the feedback must offer a higher
wavelength selectivity than a simple mirror. A preferential feedback for certain modes can be obtained
using a periodic dielectric structure that ‘fits’ to a particular wavelength, similar to a Bragg mirror. The
periodic modulation of the refractive index can be made within the cavity (distributed feedback, DFB,
Fig. 23.53a) or at the mirror (distributed reflection, DBR, Fig. 23.53b). In this way, monochromatic
lasers with SSR 30 dB are possible (Fig. 23.51b).
It is possible to couple several hundred mW optical power of a laterally and spectrally monomode
laser into a monomode optical fiber [2004] (Fig. 23.54).
677
Fig. 23.52 Lasing spectra of quantum dot laser (L = 1.2 mm, stripe width w = 75 µm) at room temperature. The active
medium is a three-fold stack of InGaAs/GaAs QDs. The current density as labeled is given in units of the threshold
current density ( j thr = 230 A/cm 2 ). Adapted from [2000]
(a)
n
p
active
medium
(b)
n
DBR
grating
p
DBR
grating
pump
region
Fig. 23.53 Schematic drawing of a DFB (distributed feedback) and b DBR (distributed Bragg reflection) lasers. The
active medium is schematically shown as a triple quantum well, the waveguide is shown as a grey area
present [2001]. The lasing spectrum takes a hat-like shape when homogeneous broadening is small
compared to the inhomogeneous broadening[2000, 2002] (Fig. 23.52) as predicted theoretically [2001,
2003].
23.4.11 Longitudinal Single-Mode Lasers
In order to achieve a high SSR or single longitudinal mode lasing, the feedback must offer a higher
wavelength selectivity than a simple mirror. A preferential feedback for certain modes can be obtained
using a periodic dielectric structure that ‘fits’ to a particular wavelength, similar to a Bragg mirror. The
periodic modulation of the refractive index can be made within the cavity (distributed feedback, DFB,
Fig. 23.53a) or at the mirror (distributed reflection, DBR, Fig. 23.53b). In this way, monochromatic
lasers with SSR 30 dB are possible (Fig. 23.51b).
It is possible to couple several hundred mW optical power of a laterally and spectrally monomode
laser into a monomode optical fiber [2004] (Fig. 23.54).