determining significance, it is necessary to investigate the phase-frequency and
amplitude-frequency characteristics of the laser diode, as well as functions of the
slope deviations of the LD phase characteristic (at fixed radio frequencies) versus the
variations of the DC pumping current I 0 .
In this section, we give calculations and experimental measurements of the watt–
ampere characteristic, of optical spectra at different pumping levels, of AFC and
PFC characteristics of the RF modulation of the mesa-strip LD operating in the
single-frequency and multi-frequency optical modes.
QWLD may be attributed to the class of injection semiconductor laser light
sources. These modern optoelectronic devices for generation of the laser optical
emission can be constructed on the base of traditional structure of the laser including
the active optical element, optical mirrors and the RF pumping. In modern fastacting semiconductor lasers, the semiconductor material (active medium) is the
active optical element, and as mirrors, the output faces of the active element are used.
Besides laser diodes with the wide contact, the strip laser structures are used.
Compared to the laser diodes with the wide contact, the strip diodes have the
following advantages: (1) they have the low value of the threshold current owing
to small area of the active strip; (2) they have improved condition for heat sink;
(3) they have small dimensions of the emitting region on the resonator mirror;
(4) they have the simple structure of modes (oscillation types), i.e., the small number
of transverse modes in the resonator (the parallel spatial generation modes are
excluded).
In the fast-acting laser diodes, the strip width is 2–5 μm. Such LDs are called
mesa-strip laser diodes. At present, to increase the modulation frequency band in
microwaves, the quantum-well mesa-strip laser diode is used. We describe below
its main features and properties. It is known that in laser optical spectra on the base of
semiconductors, which have several ultrathin layers with thickness 5–50 nm, the
effect of quantization of energy states is demonstrated. Due to dependence of this
effect upon the layer thickness, this LD can be attributed to the quantum-dimension
laser diodes. At layer thickness comparable with the de Broglie wavelength, the
electron manifests its quantum properties. In radio-physical language, we may speak
that, at that, the oscillating system is formed for carriers, and as a result, in the
amplification loop of the laser structure, the clearly expressed sharp “peaks” appear,
which correspond to resonances.
Observation and creation of the quantum-dimension effect in injection lasers
becomes possible after assimilation of the hetero-structure technology with the
uniform thickness of the active layer of 5–50 nm. Carriers (electrons) in the active
layer are captured from the wide-zone regions into the quantum well. At small layer
thickness, electrons stop to have time for relaxation into the deepest levels before
their recombination with holes. This leads to the growth of the gain slope of the
active layer, and hence, to the decrease of threshold pumping (by 5–20 times), to the
growth of pumping power density in the active region and to the increase of the
power of output emission (by 10–20 times), to improvement of emission polarization
characteristics, to reduction of the spontaneous emission (by 10–100 times), and to
reduction of the phase noise level. In laser diodes, to create quantum wells, the
394
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .
amplitude-frequency characteristics of the laser diode, as well as functions of the
slope deviations of the LD phase characteristic (at fixed radio frequencies) versus the
variations of the DC pumping current I 0 .
In this section, we give calculations and experimental measurements of the watt–
ampere characteristic, of optical spectra at different pumping levels, of AFC and
PFC characteristics of the RF modulation of the mesa-strip LD operating in the
single-frequency and multi-frequency optical modes.
QWLD may be attributed to the class of injection semiconductor laser light
sources. These modern optoelectronic devices for generation of the laser optical
emission can be constructed on the base of traditional structure of the laser including
the active optical element, optical mirrors and the RF pumping. In modern fastacting semiconductor lasers, the semiconductor material (active medium) is the
active optical element, and as mirrors, the output faces of the active element are used.
Besides laser diodes with the wide contact, the strip laser structures are used.
Compared to the laser diodes with the wide contact, the strip diodes have the
following advantages: (1) they have the low value of the threshold current owing
to small area of the active strip; (2) they have improved condition for heat sink;
(3) they have small dimensions of the emitting region on the resonator mirror;
(4) they have the simple structure of modes (oscillation types), i.e., the small number
of transverse modes in the resonator (the parallel spatial generation modes are
excluded).
In the fast-acting laser diodes, the strip width is 2–5 μm. Such LDs are called
mesa-strip laser diodes. At present, to increase the modulation frequency band in
microwaves, the quantum-well mesa-strip laser diode is used. We describe below
its main features and properties. It is known that in laser optical spectra on the base of
semiconductors, which have several ultrathin layers with thickness 5–50 nm, the
effect of quantization of energy states is demonstrated. Due to dependence of this
effect upon the layer thickness, this LD can be attributed to the quantum-dimension
laser diodes. At layer thickness comparable with the de Broglie wavelength, the
electron manifests its quantum properties. In radio-physical language, we may speak
that, at that, the oscillating system is formed for carriers, and as a result, in the
amplification loop of the laser structure, the clearly expressed sharp “peaks” appear,
which correspond to resonances.
Observation and creation of the quantum-dimension effect in injection lasers
becomes possible after assimilation of the hetero-structure technology with the
uniform thickness of the active layer of 5–50 nm. Carriers (electrons) in the active
layer are captured from the wide-zone regions into the quantum well. At small layer
thickness, electrons stop to have time for relaxation into the deepest levels before
their recombination with holes. This leads to the growth of the gain slope of the
active layer, and hence, to the decrease of threshold pumping (by 5–20 times), to the
growth of pumping power density in the active region and to the increase of the
power of output emission (by 10–20 times), to improvement of emission polarization
characteristics, to reduction of the spontaneous emission (by 10–100 times), and to
reduction of the phase noise level. In laser diodes, to create quantum wells, the
394
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .