7.7.3 The Phased Detector on the Base of FOS1
In the structure composition, FOS1 and the phase detector are included. The
photodetector, on the light-sensitive area of which the emissions pass from the
different optical fibers (as shown in Fig. 7.39b), is used as the phase detector.
Let us assume that the field strength E FOS1 (t) in the first optical channel in the
FOS1 input is close to harmonic: E FOS1 (t) ¼ E 0FOS1 cos [2πtν(t) + φ 0L + φ FOS1opt (t)],
or E FOS1 (t) ¼ E 0FOS1 cos [φ FOS1opt (t)], or the voltage in the output of this photodetector (or the phase detector) (Fig. 7.39b), in linear approximation (in small index of
the optical modulation in the output of the optical discriminator), the following
expression is true: U PD ¼ R PD K PD |E 0FOS1 |
2 sin [2πν 0 T FOS1 ] sin (2πf 0 t + φ 0etal + Δφ PD ),
where K PD is the slope of the photodetector equaled to the ratio of the current to the
applied to the PD area of normalized emission power P opt ¼ |E 0FOS1 |
2 , R PD is the
load resistance of the photodetector. We designate the voltage U 0PD as:
U 0PD ¼ R PD K PD |E 0FOS1 |
2 . For the low-frequency component of the voltage
u PDLF (t) ¼ U 0PD sin [2πν 0 (T 1M À T 2M )] sin (Δφ PD ) in the photodetector output
(or the phase detector) the following expression is true: u PDLF (t) ¼ U 0PDLF sin (Δφ PD ).
We note that the low-frequency voltage u PDLF (t) in the photodetector (or the phase
detector) depends on the power of the laser optical emission on the PD area.
The maximal low-frequency voltage U 0FDLF ¼ U 0PD Á sin (2πν 0 T FOS1 ) in the PD
output, at sin(Δφ PD ) ¼ 1. The phase difference in the output of the photodetector
(or the phase detector) under investigation is: Δφ PD (t) ¼ Δφ opt (t) À Δφ gen (t). We see
that the characteristic of the phase detector of the structure under examination (see
Fig. 7.39b) has the sine form, and its slope depends on the delay time T FOS1 .
7.7.4 PLL System of the Laser
One of the features of the automatic control in OEO is a possibility to use the control
of not only OEO, but the laser as well. By means of the laser phase of the frequency
control, we can significantly decrease the laser phase noise by several ten times.
As we show in Chap. 2 of this book (Figs. 2.7–2.10), to decrease the laser phase
noise, we can apply for extraction of the laser phase noise the high-Q discriminators
on the base of diffraction lattices with the period less than 100 nm (in Fig. 2.9), the
disk optical resonators, etc.
Methods and mathematical computations for the frequency and phase control
systems for the laser optical frequency are the same as earlier-considered for the RF
oscillator. The double-channel fiber-optical system with two optical fibers of different lengths, which is presented simplistically in [20], can be concerned to the one of
types of the optical discriminator.
Now we obtain the function Δν con (e con ) of the correcting offset Δν con (t) of the
laser optical emission frequency versus the control voltage e con . In modern compact
laser systems of QWLD type, the electronic control of the optical frequency is used.
7.7 Systems of Frequency and Phase Automatic Control in OEO
457
In the structure composition, FOS1 and the phase detector are included. The
photodetector, on the light-sensitive area of which the emissions pass from the
different optical fibers (as shown in Fig. 7.39b), is used as the phase detector.
Let us assume that the field strength E FOS1 (t) in the first optical channel in the
FOS1 input is close to harmonic: E FOS1 (t) ¼ E 0FOS1 cos [2πtν(t) + φ 0L + φ FOS1opt (t)],
or E FOS1 (t) ¼ E 0FOS1 cos [φ FOS1opt (t)], or the voltage in the output of this photodetector (or the phase detector) (Fig. 7.39b), in linear approximation (in small index of
the optical modulation in the output of the optical discriminator), the following
expression is true: U PD ¼ R PD K PD |E 0FOS1 |
2 sin [2πν 0 T FOS1 ] sin (2πf 0 t + φ 0etal + Δφ PD ),
where K PD is the slope of the photodetector equaled to the ratio of the current to the
applied to the PD area of normalized emission power P opt ¼ |E 0FOS1 |
2 , R PD is the
load resistance of the photodetector. We designate the voltage U 0PD as:
U 0PD ¼ R PD K PD |E 0FOS1 |
2 . For the low-frequency component of the voltage
u PDLF (t) ¼ U 0PD sin [2πν 0 (T 1M À T 2M )] sin (Δφ PD ) in the photodetector output
(or the phase detector) the following expression is true: u PDLF (t) ¼ U 0PDLF sin (Δφ PD ).
We note that the low-frequency voltage u PDLF (t) in the photodetector (or the phase
detector) depends on the power of the laser optical emission on the PD area.
The maximal low-frequency voltage U 0FDLF ¼ U 0PD Á sin (2πν 0 T FOS1 ) in the PD
output, at sin(Δφ PD ) ¼ 1. The phase difference in the output of the photodetector
(or the phase detector) under investigation is: Δφ PD (t) ¼ Δφ opt (t) À Δφ gen (t). We see
that the characteristic of the phase detector of the structure under examination (see
Fig. 7.39b) has the sine form, and its slope depends on the delay time T FOS1 .
7.7.4 PLL System of the Laser
One of the features of the automatic control in OEO is a possibility to use the control
of not only OEO, but the laser as well. By means of the laser phase of the frequency
control, we can significantly decrease the laser phase noise by several ten times.
As we show in Chap. 2 of this book (Figs. 2.7–2.10), to decrease the laser phase
noise, we can apply for extraction of the laser phase noise the high-Q discriminators
on the base of diffraction lattices with the period less than 100 nm (in Fig. 2.9), the
disk optical resonators, etc.
Methods and mathematical computations for the frequency and phase control
systems for the laser optical frequency are the same as earlier-considered for the RF
oscillator. The double-channel fiber-optical system with two optical fibers of different lengths, which is presented simplistically in [20], can be concerned to the one of
types of the optical discriminator.
Now we obtain the function Δν con (e con ) of the correcting offset Δν con (t) of the
laser optical emission frequency versus the control voltage e con . In modern compact
laser systems of QWLD type, the electronic control of the optical frequency is used.
7.7 Systems of Frequency and Phase Automatic Control in OEO
457