systems with distributed optical coupling. The optical coupling between light guides
in this coupler occurs on the definite segment, which is much more than the optical
emission wavelength 1.55 μm of QWLD, but at that, the length of the coupling
segment is much less than the geometrical length of FOS. As the rule, the length of
the coupling segment between optical channels is 0.5–3 mm.
We note that thanks to the distributed optical coupling on the optical segment of
small length, the directional optical coupler of Х-type represents not only the
division element of the optical power between different channels. The optical
coupler of Х-type represents the optical filter with the “comb” dependence of the
module of the transfer function on RF and optical frequency. The readjustment of
FOS0
FOS2
T 1 =
T 2 =
L 1 n thr
c
L 2 n thr
c
FOS1
FOS0
FOS1
FOS2
P 1
P 2
P 0
NA
PD
LD
RF F
a)
0.5
1
CZ = 6
4
7
5, 5
Phase j = 2p f (T 2 – T 1 ) [rad]
0.45
0.4
0.35
0.3
0.25
AFC |K
FODL | [a.u.]
0.2
0.15
0.1
0.05
0
0
CZ = 7
5, 5
1
6
4
-5
-10
-15
f eF ,T eF
DC X
V = V(t)
V = V(t)
b)
c)
p
2p
3p
Phase j = 2p f (T 2 – T 1 ) [rad]
PFF j
FODL [rad]
p
2p
3p
Fig. 7.27 (a) The system of the generation frequency control in OEO with the help of the
directional optical coupler of X-type. Parameters RF FODL in OEO: delay times in FOS1
T 1FOS ¼ L 1 n thr /c and in FOS2 T 2FOS ¼ L 2 n thr /c, the natural frequency of the RF filter is f eF , the
time constant of RF filter is T eF . (b) The structure of the directional optical coupler of X-type. (c)
AFC of |K FODL | and PFF φ FODL ¼ 2πf(T 2FOS À T 1FOS ) of the differential combined RF FODL with
the directional coupler of Х-type for the single-mode optical fiber with different coefficients of OF:
СZ ¼ 1; 4; 5; 5.5; 7
424
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .