7.3 OEO DM Analysis on the Base of Abbreviated
Differential Equations
7.3.1 Structures of OEO DM and Features of OEO Operation
with the Direct LD Modulation with Coherent Optical
Self-Heterodyning
Let us examine OEO with the direct amplitude modulation of the laser diode. Let the
laser be the high-coherent source of stimulated optical emission, i.e., the spectral line
width of laser emission Δν L is much less than the nominal (reference) operation
frequency f 0 of the microwave subcarrier: Δν L ( f 0 . We assume that the pumping
current of the laser diode J 1L (t), besides the DC bias I 0L of the operating point on the
static watt–ampere modulation characteristic of QWLD, has the additive harmonic
RF component J 1L (t) ¼ J 10L cos (2πft + φ 0 ) with the frequency f, the initial phase
φ 0 and the relatively small amplitude J 10L < J 0L À J thr , where J thr is the threshold LD
pumping current, the exceed of which leads to the QWLD soft excitation and settling
of the steady-state generation mode of quasi-harmonic stimulated laser emission in
the near infra-red (IR) range. In the laser output, in such a small-signal mode of
optical modulation, the stimulated weakly modulated emission is formed. This
optical emission represents (in the amplitude spectrum) the sum of three harmonic
optical carriers, which frequencies are relatively, ν 1 ¼ ν 0 À f 0 , ν 2 ¼ ν 0 , ν 3 ¼ ν 0 À f 0 .
Two of these optical frequencies ν 1 and ν 2 are separated from the central laser
optical frequency ν 0 by the subcarrier frequency f 0 . Now we consider the different
functional structures of OEO with direct modulation of LD emission (Fig. 7.16).
In the structure in Fig. 7.16, the following elements (closed in the loop) are
included: the laser diode; FOS, for example, on the base of the single-mode
low-dispersive optical fiber; the photodetector, for instance, p-i-n–photodiode; the
nonlinear wideband low-noise amplifier (NA); the narrowband RF filter (F), for
example, on the base of the solid-state dielectric resonator and the directional
coupler C.
7.3.2 Quantum-Well Laser Diode in ОEO DM
Appearance in the market of the ultra-wideband mesa-strip quantum-well laser
diodes, which have the modulation frequency band of RF signal up to 12 GHz,
opens the great perspectives for their application not only in systems for information
transmission, but for development of the new class of devices: stabilized in the radiofrequency oscillators with the fiber-optical delay line operating in the microwave
range, as well as for development of phased fiber-optical lines for the microwave
subcarrier transmission.
For creation of such devices, in which the information about the phase of the
transmitting sine radio signal in the microwave range plays the important and
392
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .
Differential Equations
7.3.1 Structures of OEO DM and Features of OEO Operation
with the Direct LD Modulation with Coherent Optical
Self-Heterodyning
Let us examine OEO with the direct amplitude modulation of the laser diode. Let the
laser be the high-coherent source of stimulated optical emission, i.e., the spectral line
width of laser emission Δν L is much less than the nominal (reference) operation
frequency f 0 of the microwave subcarrier: Δν L ( f 0 . We assume that the pumping
current of the laser diode J 1L (t), besides the DC bias I 0L of the operating point on the
static watt–ampere modulation characteristic of QWLD, has the additive harmonic
RF component J 1L (t) ¼ J 10L cos (2πft + φ 0 ) with the frequency f, the initial phase
φ 0 and the relatively small amplitude J 10L < J 0L À J thr , where J thr is the threshold LD
pumping current, the exceed of which leads to the QWLD soft excitation and settling
of the steady-state generation mode of quasi-harmonic stimulated laser emission in
the near infra-red (IR) range. In the laser output, in such a small-signal mode of
optical modulation, the stimulated weakly modulated emission is formed. This
optical emission represents (in the amplitude spectrum) the sum of three harmonic
optical carriers, which frequencies are relatively, ν 1 ¼ ν 0 À f 0 , ν 2 ¼ ν 0 , ν 3 ¼ ν 0 À f 0 .
Two of these optical frequencies ν 1 and ν 2 are separated from the central laser
optical frequency ν 0 by the subcarrier frequency f 0 . Now we consider the different
functional structures of OEO with direct modulation of LD emission (Fig. 7.16).
In the structure in Fig. 7.16, the following elements (closed in the loop) are
included: the laser diode; FOS, for example, on the base of the single-mode
low-dispersive optical fiber; the photodetector, for instance, p-i-n–photodiode; the
nonlinear wideband low-noise amplifier (NA); the narrowband RF filter (F), for
example, on the base of the solid-state dielectric resonator and the directional
coupler C.
7.3.2 Quantum-Well Laser Diode in ОEO DM
Appearance in the market of the ultra-wideband mesa-strip quantum-well laser
diodes, which have the modulation frequency band of RF signal up to 12 GHz,
opens the great perspectives for their application not only in systems for information
transmission, but for development of the new class of devices: stabilized in the radiofrequency oscillators with the fiber-optical delay line operating in the microwave
range, as well as for development of phased fiber-optical lines for the microwave
subcarrier transmission.
For creation of such devices, in which the information about the phase of the
transmitting sine radio signal in the microwave range plays the important and
392
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .