Amplitude and phase responses of this two-port varies by the QWLD bias current
variations. At that, the joined microwave filter “RFF-QWLD” has been formed, and
it is controlled by the QWLD bias current. The filter control is performed by
displacement of the QWLD amplitude response peak. Therefore, we may speak
about new type of the tuned microwave filter. The filter role is played by the known
(in the balance theory of laser generation) quasi-resonance peak on the highfrequency edge of the small-signal amplitude-frequency curve of used QWLD.
In the quantum-well laser diodes, as already shown in Chap. 4, this peak is caused
by the specific dynamics of electron–photon interaction of the relaxation-reactive
character. A position of this peak apex (the natural frequency) on the frequency axis,
its height and width depend on many parameters of the active medium and the
QWLD optical resonator. The natural frequency of this peak depends on the value of
QWLD bias current excess over its threshold value, which is 12 mA in QWLD used
in this experiment. The natural frequency of relaxation oscillations corresponding to
the peak apex is tuned by the QWLD bias current in very wide limits: from 1 to
9 GHz.
This allows implementation of the OEO generation frequency tuning in the range
8200–8203 GHz at presence of RFF with the natural frequency 8.2 GHz and the
bandwidth about 2 MHz in the OEO circuit in Fig. 8.13. Measured experimental
function of the generation frequency (Fig. 8.13) versus the bias current, is close to
calculated and well approximated by the parabolic function.
At small bias currents, the slope of generation frequency variations versus bias
current is 0.3 MHz/mA. At large QWLD bias currents (by 5–8 times) the slope of
generation frequency versus is 0.003 MHz/mA. Fulfilled researches allow optimization of the QWLD bias current choice. The optimal bias current is 60 mA.
Investigations of frequency functions (Fig. 8.13) versus the bias current at
different light guide length 1, 3, 60, 70, and 4500 m. It is stated that the singlefrequency mode of OEO generation in the whole range of bias currents at filter Qfactor close to 1000 may be obtained at definite lengths of the optical fiber. So, for
instance, at the optical fiber length of 60 m, the single-frequency mode without
jumps occurs in the whole range of bias currents (Fig. 8.13c, the curve 2). At optical
fiber length of 70 m, the single-frequency mode is realized with the jump to the
adjacent oscillation type (Fig. 8.13c). Owing to narrow bandwidth of RFF and the
effect of frequency pulling, the single-frequency mode realizes at the length of
4500 m. Jumps to another type of oscillations at bias current variations are well
agreed by the general theory of oscillators (including OEO with RF FODL). The
graphical image of amplitude and phase responses of QWLD and RFF, shown in
Fig. 7.18 (Chap. 7), well explains a mechanism of the OEO generation frequency
control at variations of the bias current.
A hysteresis character (small) of the frequency and amplitude functions versus the
bias current (at single-frequency mode without frequency jump) is explained by
thermodynamic processes in the active zone of the QWLD at variations of the bias
current. At that, the structure of functions of the generation signal amplitude versus
the bias current at frequency tuning in this range was different for various lengths of
the optical fiber. These experimental functions of frequency and amplitude coincide
8.4 Implementation of OEO in the Microwave Range and Its Experimental. . .
485
variations. At that, the joined microwave filter “RFF-QWLD” has been formed, and
it is controlled by the QWLD bias current. The filter control is performed by
displacement of the QWLD amplitude response peak. Therefore, we may speak
about new type of the tuned microwave filter. The filter role is played by the known
(in the balance theory of laser generation) quasi-resonance peak on the highfrequency edge of the small-signal amplitude-frequency curve of used QWLD.
In the quantum-well laser diodes, as already shown in Chap. 4, this peak is caused
by the specific dynamics of electron–photon interaction of the relaxation-reactive
character. A position of this peak apex (the natural frequency) on the frequency axis,
its height and width depend on many parameters of the active medium and the
QWLD optical resonator. The natural frequency of this peak depends on the value of
QWLD bias current excess over its threshold value, which is 12 mA in QWLD used
in this experiment. The natural frequency of relaxation oscillations corresponding to
the peak apex is tuned by the QWLD bias current in very wide limits: from 1 to
9 GHz.
This allows implementation of the OEO generation frequency tuning in the range
8200–8203 GHz at presence of RFF with the natural frequency 8.2 GHz and the
bandwidth about 2 MHz in the OEO circuit in Fig. 8.13. Measured experimental
function of the generation frequency (Fig. 8.13) versus the bias current, is close to
calculated and well approximated by the parabolic function.
At small bias currents, the slope of generation frequency variations versus bias
current is 0.3 MHz/mA. At large QWLD bias currents (by 5–8 times) the slope of
generation frequency versus is 0.003 MHz/mA. Fulfilled researches allow optimization of the QWLD bias current choice. The optimal bias current is 60 mA.
Investigations of frequency functions (Fig. 8.13) versus the bias current at
different light guide length 1, 3, 60, 70, and 4500 m. It is stated that the singlefrequency mode of OEO generation in the whole range of bias currents at filter Qfactor close to 1000 may be obtained at definite lengths of the optical fiber. So, for
instance, at the optical fiber length of 60 m, the single-frequency mode without
jumps occurs in the whole range of bias currents (Fig. 8.13c, the curve 2). At optical
fiber length of 70 m, the single-frequency mode is realized with the jump to the
adjacent oscillation type (Fig. 8.13c). Owing to narrow bandwidth of RFF and the
effect of frequency pulling, the single-frequency mode realizes at the length of
4500 m. Jumps to another type of oscillations at bias current variations are well
agreed by the general theory of oscillators (including OEO with RF FODL). The
graphical image of amplitude and phase responses of QWLD and RFF, shown in
Fig. 7.18 (Chap. 7), well explains a mechanism of the OEO generation frequency
control at variations of the bias current.
A hysteresis character (small) of the frequency and amplitude functions versus the
bias current (at single-frequency mode without frequency jump) is explained by
thermodynamic processes in the active zone of the QWLD at variations of the bias
current. At that, the structure of functions of the generation signal amplitude versus
the bias current at frequency tuning in this range was different for various lengths of
the optical fiber. These experimental functions of frequency and amplitude coincide
8.4 Implementation of OEO in the Microwave Range and Its Experimental. . .
485