maximal value depends on the ratio T FOS /T c . At long length of the optical fiber (1 km
and longer), the ratio T FOS /T c is not small and is about T FOS /T c % 1 and more. The
noise suppression effectiveness in OEO is not only defined by the fiber length, but it
also depends on the ratio of the laser coherence time to the delay time in the optical
fiber. When the Sw switch closes, in the case of steady-state mode in OEO, the
effective statistical suppression occurs in the case, if the laser coherence time T c is
comparable to the delay time in the optical fiber T FOS , i.e., T c % N Á T FOS , where for
instance, N ¼ 1–5. In this case, statistically related phase fluctuations of laser
oscillations will be suppressed. At small T FOS delays, when T c ) T FOS , the noise
suppression does not happen; i.e., the correlator, as a device, does not fulfill its
functions of the phase noise suppression. When T c ( T FOS , the noise suppression is
not effective as well. In this case, the correlation coefficient is close to zero and the
correlator effectiveness is low.
In experimental investigations of OEO, the minimal level of the noise spectral
density can be achieved at utilization of high-coherent lasers (with the spectral line
width of 10–100 kHz) and at geometrical lengths of optical fibers of 1–2 km, which
provides the delay in the optical fiber of 10
À5 s and more.
3.1.3.4 Comparison of OEO and the Multi-Tapped Delay Line
The OEO structure can be compared in the noise suppression mechanism with the
structure of the multi-tapped delay line, which has N taps through time intervals
(Fig. 3.6). Such multi-tapped networks for delays are used for radio-frequency signal
processing in radar technologies. Let these time intervals be equal to the time delay
T FOS . If at the electrical input of such the delay line, the stationary random process
ξ(t) with zero mean value affects (passes), which is defined by the correlation
function R ξ (τ) with the spectral density S ξ ( f ), then the spectral density S η ( f ) of
the η(t) process in the adder output is defined as:
Fig. 3.6 The block diagram of RF signal processing on the base of the laser, the MZ modulator, the
photodetector, the combined delay line from N delay elements and the adder (+)
3.1 Direct and External Laser Modulation in OEO
87
and longer), the ratio T FOS /T c is not small and is about T FOS /T c % 1 and more. The
noise suppression effectiveness in OEO is not only defined by the fiber length, but it
also depends on the ratio of the laser coherence time to the delay time in the optical
fiber. When the Sw switch closes, in the case of steady-state mode in OEO, the
effective statistical suppression occurs in the case, if the laser coherence time T c is
comparable to the delay time in the optical fiber T FOS , i.e., T c % N Á T FOS , where for
instance, N ¼ 1–5. In this case, statistically related phase fluctuations of laser
oscillations will be suppressed. At small T FOS delays, when T c ) T FOS , the noise
suppression does not happen; i.e., the correlator, as a device, does not fulfill its
functions of the phase noise suppression. When T c ( T FOS , the noise suppression is
not effective as well. In this case, the correlation coefficient is close to zero and the
correlator effectiveness is low.
In experimental investigations of OEO, the minimal level of the noise spectral
density can be achieved at utilization of high-coherent lasers (with the spectral line
width of 10–100 kHz) and at geometrical lengths of optical fibers of 1–2 km, which
provides the delay in the optical fiber of 10
À5 s and more.
3.1.3.4 Comparison of OEO and the Multi-Tapped Delay Line
The OEO structure can be compared in the noise suppression mechanism with the
structure of the multi-tapped delay line, which has N taps through time intervals
(Fig. 3.6). Such multi-tapped networks for delays are used for radio-frequency signal
processing in radar technologies. Let these time intervals be equal to the time delay
T FOS . If at the electrical input of such the delay line, the stationary random process
ξ(t) with zero mean value affects (passes), which is defined by the correlation
function R ξ (τ) with the spectral density S ξ ( f ), then the spectral density S η ( f ) of
the η(t) process in the adder output is defined as:
Fig. 3.6 The block diagram of RF signal processing on the base of the laser, the MZ modulator, the
photodetector, the combined delay line from N delay elements and the adder (+)
3.1 Direct and External Laser Modulation in OEO
87