the basing functions of the frequency versus the OEO parameters. Since the OEO
differs from the traditional RF oscillators by the optoelectronic RF FODL, therefore,
here we pay the main attention to determination of functions of the frequency and the
amplitude versus optical parameters of the optical fiber and optical couplers: geometrical dimensions, the refraction index of the material, the temperature constants.
OEO can have in the RF FODL structure the optical fiber in two or several fibers,
which are connected in inputs and outputs by optical couplers. The one of methods
to increase the control range of retuning in frequency is the transfer from the single
optical fibers in RF FODL to structures on the base of two optical fibers of different
length, which are connected in the input with the help of Х or Y directional couplers.
Such RF FODLs are called differential. In this chapter, together with the OEO
frequency control, we examine the important problem of the long-term frequency
instability in OEO with RF FODL on the base of lengthy quartz optical fibers. We
show that the utilization of differential RF FODL improves the long-term OEO
frequency instability. Investigations of the frequency retuning in OEO with the
differential RF FODL developed on the base of X- and Y-couplers is relevant also
because in OEO with external modulation, the Mach–Zehnder modulator is used,
which has two light guides (optical channels) of the different length, which are
connected by X and Y directional couplers.
7.4.5 The Frequency Control of OEO with the Differential
RF FODL on the Base of the Directional Y-Coupler
In this section, we describe the structures with control of OEO with the differential
RF FODL on the base of single-mode and small-mode optical fibers, and also we
consider the features of the frequency and amplitude calculations taking into account
the complicate directional couplers of the Y-type.
Let us examine the system of the OEO frequency control (Fig. 7.24a) in which the
following units (series-connected in the loop) are included: MLS—the laser diode
LD, the differential RF FODL formed by light guides FOS0, and the light guides of
different length FOS1 and FOS2, the photodiode (PD), the RF nonlinear amplifier
(NA), the RF PBF. At that, light guides FOS0, FOS1 and FOS2 are connected as
shown in Fig. 7.38 with the help of the optical directional coupler of Y-type.
The directional coupler of Y-type can be formed with the help of the single-mode
or the small-mode optical fiber, and it can be created by attachment of FOS0, FOS1,
and FOS2 with the gap between light guides. The gap between light guiders is filled
by an air or the optical material. Earlier, we showed that varying of the excitation
coefficients of FOS1 and FOS2, we can effectively control by the delay time in such
a RF FODL, and, hence, by the generated frequency of OEO.
We can show the location of optical channels and directional optical channels of
Y and X-types and their view in the longitudinal section in MZ. The MZ modulator
made on the base of differential RF FODL represents two strip optical waveguides of
420
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .
differs from the traditional RF oscillators by the optoelectronic RF FODL, therefore,
here we pay the main attention to determination of functions of the frequency and the
amplitude versus optical parameters of the optical fiber and optical couplers: geometrical dimensions, the refraction index of the material, the temperature constants.
OEO can have in the RF FODL structure the optical fiber in two or several fibers,
which are connected in inputs and outputs by optical couplers. The one of methods
to increase the control range of retuning in frequency is the transfer from the single
optical fibers in RF FODL to structures on the base of two optical fibers of different
length, which are connected in the input with the help of Х or Y directional couplers.
Such RF FODLs are called differential. In this chapter, together with the OEO
frequency control, we examine the important problem of the long-term frequency
instability in OEO with RF FODL on the base of lengthy quartz optical fibers. We
show that the utilization of differential RF FODL improves the long-term OEO
frequency instability. Investigations of the frequency retuning in OEO with the
differential RF FODL developed on the base of X- and Y-couplers is relevant also
because in OEO with external modulation, the Mach–Zehnder modulator is used,
which has two light guides (optical channels) of the different length, which are
connected by X and Y directional couplers.
7.4.5 The Frequency Control of OEO with the Differential
RF FODL on the Base of the Directional Y-Coupler
In this section, we describe the structures with control of OEO with the differential
RF FODL on the base of single-mode and small-mode optical fibers, and also we
consider the features of the frequency and amplitude calculations taking into account
the complicate directional couplers of the Y-type.
Let us examine the system of the OEO frequency control (Fig. 7.24a) in which the
following units (series-connected in the loop) are included: MLS—the laser diode
LD, the differential RF FODL formed by light guides FOS0, and the light guides of
different length FOS1 and FOS2, the photodiode (PD), the RF nonlinear amplifier
(NA), the RF PBF. At that, light guides FOS0, FOS1 and FOS2 are connected as
shown in Fig. 7.38 with the help of the optical directional coupler of Y-type.
The directional coupler of Y-type can be formed with the help of the single-mode
or the small-mode optical fiber, and it can be created by attachment of FOS0, FOS1,
and FOS2 with the gap between light guides. The gap between light guiders is filled
by an air or the optical material. Earlier, we showed that varying of the excitation
coefficients of FOS1 and FOS2, we can effectively control by the delay time in such
a RF FODL, and, hence, by the generated frequency of OEO.
We can show the location of optical channels and directional optical channels of
Y and X-types and their view in the longitudinal section in MZ. The MZ modulator
made on the base of differential RF FODL represents two strip optical waveguides of
420
7 Optoelectronic oscillator (OEO) as the Time and Spatial Correlator of Random. . .