network, in which oscillations with frequency about 10 GHz are generated. The
multiplicity of these frequencies is about 20,000.
The laser (quantum-well laser diode) is the pumping source for closed into a loop
RF network formed by a modulator, the optical fiber, the photodetector, the RF
amplifier, the RF filter, and the directional coupler. In result of oscillating processes,
the spectra with fluctuations are formed, which have the different nature, and the
spectral line width of RF oscillations is defined by parameters of both self-oscillating
interacting systems: the laser and the RF oscillator. The interesting OEO feature is
the fact that the generating spectrum of RF oscillations is formed not only by noises,
which have the electronic nature, but also by phase fluctuations of the laser optical
emission. Laser fluctuations have the quantum nature and are defined by the laser
spontaneous emission. OEO can be represented as the system providing the division
of optical frequency of 200 THz to the RF values of 10 GHz.
FODL
FODL
Laser
Laser
MZ
a)
b)
CH1
CH2
Pump
OF
OF
OF
OF
C
F
A
FO
PD
OA
OA
Pump
C
A
U(t)
U(t)
E(t)
E(t)
F
FO
PD
Fig. 3.1 Structural diagrams of the optoelectronic oscillator (a) with the direct modulation by
current and (b) with external Mach–Zehnder modulator
3.1 Direct and External Laser Modulation in OEO
77