3.1.2.1 OEO Construction and Its Operation Principle
The feature of OEO diagrams with the direct modulation (Fig. 3.1a) is the possibility
of operation in the mode of amplitude (or intensity) modulation of the QWLD
emission. Structural diagrams of OEO with external modulation (Fig. 3.1b) operate
in the mode of phase modulation of QWLD emission. At that, the phase modulation
of QWLD emission is performed in one of optical channels of the MZ modulator.
In the OEO DM structure (Fig. 3.1a), the following elements are included
sequentially closed in the loop: the laser, which is the modulated light source on
the base, for example, QWLD; the single-mode low-dispersion optical fiber (OF);
the photodetector (PD); the nonlinear wideband amplifier (A) of microwave or
mm-wave ranges; and the RF high-Q filter (F) of microwave or mm-wave ranges,
for instance, on the base of the dielectric microwave resonator.
The main difference of the diagram with the external laser modulator (Fig. 3.1b)
compared to OEO DM is a presence of the external electro-optical MZ modulator.
OEO with external modulation [2] or OEO MZ is formed by the laser and the
following elements are sequentially closed into a loop: the Mach–Zehnder (MZ)
modulator; the fiber optical system (FOS) containing the optical filter (OF) and the
single-mode optical fiber (FO); the photodetector (PD), for example the quantumdimension photodiode; and the narrowband RF filter (F), the nonlinear amplifier
(A) and directional coupler (C). When sequentially connected with the laser, MZ,
OF, FO, and PD form the block of fiber optical delay line (RF FODL), whose electric
input is the MZ input (input voltage of MZ), and its electric output coincides with the
electric output of PD (we mean the output voltage on the PD load resistance).
The optoelectronic oscillator made according to the diagram with external
modulation can be considered as the oscillator with the retarded feedback, in
which RF FODL for RF oscillation is active. RF FODL has its own amplitude and
phase noises, which are the laser noises reduced on the RF FODL output (or passed
from the laser output through MZ, OF, FO).
The quantum-well laser diode or the fiber laser with activating dopes of erbium,
ytterbium, etc. can be served as the OEO laser. Figure 3.2a shows the laser
represented by the closed into a loop optical amplifier (OA) and the optical filter
(OF). This structure corresponds to the laser with the “traveling-wave” resonator.
The optical pumping power P p passes to the active amplifier. If the excitation
conditions are satisfied, laser produces optical oscillations, which pass to MZ from
the laser output, then pass through two optical channels with different delays, then
integrate together and through OF and FO act in the light-sensitive area of PD. The
effective MZ modulation in the microwave range is possible only for single-mode
single-frequency and linearly polarized emission of the highly coherent laser.
Quantum-well laser diodes and fiber lasers with polarizers on their outputs are
such emission sources and therefore, we fulfill the theoretical analysis for this case
only.
Figure 3.2 presents the structural diagrams of OEO DM (Fig. 3.2b) and OEO MZ
(Fig. 3.2c). In the structure of OEO DM, we see the following blocks, which are
78 3 Modulation Methods of Laser Emission in Optoelectronic oscillator (OEO) and OEO. . .
The feature of OEO diagrams with the direct modulation (Fig. 3.1a) is the possibility
of operation in the mode of amplitude (or intensity) modulation of the QWLD
emission. Structural diagrams of OEO with external modulation (Fig. 3.1b) operate
in the mode of phase modulation of QWLD emission. At that, the phase modulation
of QWLD emission is performed in one of optical channels of the MZ modulator.
In the OEO DM structure (Fig. 3.1a), the following elements are included
sequentially closed in the loop: the laser, which is the modulated light source on
the base, for example, QWLD; the single-mode low-dispersion optical fiber (OF);
the photodetector (PD); the nonlinear wideband amplifier (A) of microwave or
mm-wave ranges; and the RF high-Q filter (F) of microwave or mm-wave ranges,
for instance, on the base of the dielectric microwave resonator.
The main difference of the diagram with the external laser modulator (Fig. 3.1b)
compared to OEO DM is a presence of the external electro-optical MZ modulator.
OEO with external modulation [2] or OEO MZ is formed by the laser and the
following elements are sequentially closed into a loop: the Mach–Zehnder (MZ)
modulator; the fiber optical system (FOS) containing the optical filter (OF) and the
single-mode optical fiber (FO); the photodetector (PD), for example the quantumdimension photodiode; and the narrowband RF filter (F), the nonlinear amplifier
(A) and directional coupler (C). When sequentially connected with the laser, MZ,
OF, FO, and PD form the block of fiber optical delay line (RF FODL), whose electric
input is the MZ input (input voltage of MZ), and its electric output coincides with the
electric output of PD (we mean the output voltage on the PD load resistance).
The optoelectronic oscillator made according to the diagram with external
modulation can be considered as the oscillator with the retarded feedback, in
which RF FODL for RF oscillation is active. RF FODL has its own amplitude and
phase noises, which are the laser noises reduced on the RF FODL output (or passed
from the laser output through MZ, OF, FO).
The quantum-well laser diode or the fiber laser with activating dopes of erbium,
ytterbium, etc. can be served as the OEO laser. Figure 3.2a shows the laser
represented by the closed into a loop optical amplifier (OA) and the optical filter
(OF). This structure corresponds to the laser with the “traveling-wave” resonator.
The optical pumping power P p passes to the active amplifier. If the excitation
conditions are satisfied, laser produces optical oscillations, which pass to MZ from
the laser output, then pass through two optical channels with different delays, then
integrate together and through OF and FO act in the light-sensitive area of PD. The
effective MZ modulation in the microwave range is possible only for single-mode
single-frequency and linearly polarized emission of the highly coherent laser.
Quantum-well laser diodes and fiber lasers with polarizers on their outputs are
such emission sources and therefore, we fulfill the theoretical analysis for this case
only.
Figure 3.2 presents the structural diagrams of OEO DM (Fig. 3.2b) and OEO MZ
(Fig. 3.2c). In the structure of OEO DM, we see the following blocks, which are
78 3 Modulation Methods of Laser Emission in Optoelectronic oscillator (OEO) and OEO. . .