The PLL system may be successfully applied in OEO [28] for decrease of the
phase noise of OEO. At that, we show that the PLL system in OEO can reduce phase
noises by 7–15 dB/Hz in the offset range from the carrier frequency of 1–10 kHz.
The presence of PLL in OEO, at other advantages, gives a possibility to decrease by
2–3 times the geometrical length of the optical fiber for the narrowband laser source.
In Fig. 7.38 [29, patent (2006)], the closed feedback loop of OEO is formed by
blocks: 20 (a laser), 19 (a modulator), 14 (a phase-shifter), 13 (a photodetector),
15 (an amplifier and the RF filter), 16 (a coupler). To the block 3 (PLL system) the
optical oscillation are applied from blocks 8, 9, 10, which are included into the OEO
structure. Optical oscillations pass to photodetectors 23 and 30. Then signals are
amplified and pass to the phase detector 25. In the output of 25, the electrical voltage
acts (error signal), which is proportional to the level of the phase noise. This error
signal is amplified and passes to the second input of the phase-shifter 14.
The error signal extracted by 25 is proportional to the OEO phase noise. When it
passes on the phase-shifter 14, the regulation of the OEO phase noise level occurs.
The distinctive feature of the PLL system of OEO, which is presented in Fig. 7.38, is
the suppression of RF carrier of OEO oscillations by blocks 21, 22, 23, 27, 28.
Owing to suppression of the RF carrier, the effectiveness of the PLL system
operation increases.
We note that utilization of FLL and PLL systems in the radio electronic equipment including OEO gives advantages at application of RF FODL. At that, requirement to the laser phase noise are reduced, we can decrease of the geometrical length
of the delay line.
7.7.2 PLL Systems in OEO
Figure 7.39a, b shows the structures of PLL systems in OEO:
• At utilization of the electrical output of OEO, from which voltages (or RF
oscillation) passes to the RF FODL input and to the input of the phase detector
(Fig. 7.39a).
• At utilization on the optical output of OEO, from which the optical oscillation
passes to the input of the optical fiber FOS1 and to the input of the phase detector
(Fig. 7.39b).
For utilization of the PLL system in OEO we can use the electronic phase detector
(for example, using diodes as the nonlinear elements) with RF FODL (Fig. 7.39a). In
this case, electrical oscillations are applied to both inputs. We can use another circuit
of the phase detector shown in Fig. 7.39b on the base of the photodetector in the
input of which the optical emission is applied via two optical channels of the
different geometrical length.
454
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .
phase noise of OEO. At that, we show that the PLL system in OEO can reduce phase
noises by 7–15 dB/Hz in the offset range from the carrier frequency of 1–10 kHz.
The presence of PLL in OEO, at other advantages, gives a possibility to decrease by
2–3 times the geometrical length of the optical fiber for the narrowband laser source.
In Fig. 7.38 [29, patent (2006)], the closed feedback loop of OEO is formed by
blocks: 20 (a laser), 19 (a modulator), 14 (a phase-shifter), 13 (a photodetector),
15 (an amplifier and the RF filter), 16 (a coupler). To the block 3 (PLL system) the
optical oscillation are applied from blocks 8, 9, 10, which are included into the OEO
structure. Optical oscillations pass to photodetectors 23 and 30. Then signals are
amplified and pass to the phase detector 25. In the output of 25, the electrical voltage
acts (error signal), which is proportional to the level of the phase noise. This error
signal is amplified and passes to the second input of the phase-shifter 14.
The error signal extracted by 25 is proportional to the OEO phase noise. When it
passes on the phase-shifter 14, the regulation of the OEO phase noise level occurs.
The distinctive feature of the PLL system of OEO, which is presented in Fig. 7.38, is
the suppression of RF carrier of OEO oscillations by blocks 21, 22, 23, 27, 28.
Owing to suppression of the RF carrier, the effectiveness of the PLL system
operation increases.
We note that utilization of FLL and PLL systems in the radio electronic equipment including OEO gives advantages at application of RF FODL. At that, requirement to the laser phase noise are reduced, we can decrease of the geometrical length
of the delay line.
7.7.2 PLL Systems in OEO
Figure 7.39a, b shows the structures of PLL systems in OEO:
• At utilization of the electrical output of OEO, from which voltages (or RF
oscillation) passes to the RF FODL input and to the input of the phase detector
(Fig. 7.39a).
• At utilization on the optical output of OEO, from which the optical oscillation
passes to the input of the optical fiber FOS1 and to the input of the phase detector
(Fig. 7.39b).
For utilization of the PLL system in OEO we can use the electronic phase detector
(for example, using diodes as the nonlinear elements) with RF FODL (Fig. 7.39a). In
this case, electrical oscillations are applied to both inputs. We can use another circuit
of the phase detector shown in Fig. 7.39b on the base of the photodetector in the
input of which the optical emission is applied via two optical channels of the
different geometrical length.
454
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .