2.1.1 Optoelectronic Oscillator
OEO, which diagram is shown in Fig. 2.1a, from the scientific and engineering point
of view is the self-oscillating system with delayed feedback, in which RF FODL is
formed by the series-connected modulated light source (MLS), fiber-optical system
(FOS), and photodetector (PD). In our case, the quantum-well laser diode (QWLD)
is the MLS. The QWLD, which is described in detail in Chap. 4 of this book, has
following serious features due to quantization of energy zones in the transition area:
• compared to the traditional micro-strip laser diode, it has the output optical
power, which is several times more,
• its threshold pumping current is less by an order,
• less noises determined by the laser spontaneous emission, and
• less by the order bandwidth of the optical generation line.
The OEO block diagram presented in Fig. 2.1a consists of series-connected into a
loop:
• the modulated light source on the base of QWLD,
• the FOS consisting of the one or several optical fibers,
• the PD,
• the nonlinear wideband amplifier for the RF signal,
• the narrowband RF filter (F) (the selective RF system), and
• the directional coupler for RF signal extraction.
At that, in this book, we consider OEO types, which differ on the modulation
type: (1) OEO with direct modulation of QWLD emission [1], and (2) OEO with
external QWLD emission modulation by the MZ electro-optical modulator (MZ in
Fig. 2.1a). Both types of modulators are shown in upper insets in Fig. 2.1a. The OEO
diagram with external modulation is constructed on the base of the electro-optical
MZ modulator [2]. At that, the optical phase modulation (PM) is used in the one
from two channels of MZ modulator. In addition, we can use the acousto-optical
modulator with frequency modulation (FM) of laser emission for optical emission
modulation in OEO. Application of various ways (direct and external) of optical
modulation and its types: amplitude, phase, or frequency (AM, PM, FM) is defined
by the OEO practical destination and its correspondence to technical specifications
of the system.
For instance, in the high-frequency OEO, dedicated to systems for measurement
of optical fiber parameters, in particular, at investigation of temperature dependences
of the optical fibers, it is expedient to use the laser diode or the light-emitting diode
(LED) with low-frequency internal modulation. In microwave OEO, suitable for
application in the communication structures as, for example, low-noise devices for
generation of reference oscillations with operation frequencies 5–73 GHz, it is
expedient to use QWLD with direct and external modulation. In the ultralow-noise
oscillators of microwave and mm-wave ranges (8–30 GHz) we need to use QWLD
with the external MZ modulator.
2.1 Operation Principle and Functional Diagram of OEO with RF FODL
17
OEO, which diagram is shown in Fig. 2.1a, from the scientific and engineering point
of view is the self-oscillating system with delayed feedback, in which RF FODL is
formed by the series-connected modulated light source (MLS), fiber-optical system
(FOS), and photodetector (PD). In our case, the quantum-well laser diode (QWLD)
is the MLS. The QWLD, which is described in detail in Chap. 4 of this book, has
following serious features due to quantization of energy zones in the transition area:
• compared to the traditional micro-strip laser diode, it has the output optical
power, which is several times more,
• its threshold pumping current is less by an order,
• less noises determined by the laser spontaneous emission, and
• less by the order bandwidth of the optical generation line.
The OEO block diagram presented in Fig. 2.1a consists of series-connected into a
loop:
• the modulated light source on the base of QWLD,
• the FOS consisting of the one or several optical fibers,
• the PD,
• the nonlinear wideband amplifier for the RF signal,
• the narrowband RF filter (F) (the selective RF system), and
• the directional coupler for RF signal extraction.
At that, in this book, we consider OEO types, which differ on the modulation
type: (1) OEO with direct modulation of QWLD emission [1], and (2) OEO with
external QWLD emission modulation by the MZ electro-optical modulator (MZ in
Fig. 2.1a). Both types of modulators are shown in upper insets in Fig. 2.1a. The OEO
diagram with external modulation is constructed on the base of the electro-optical
MZ modulator [2]. At that, the optical phase modulation (PM) is used in the one
from two channels of MZ modulator. In addition, we can use the acousto-optical
modulator with frequency modulation (FM) of laser emission for optical emission
modulation in OEO. Application of various ways (direct and external) of optical
modulation and its types: amplitude, phase, or frequency (AM, PM, FM) is defined
by the OEO practical destination and its correspondence to technical specifications
of the system.
For instance, in the high-frequency OEO, dedicated to systems for measurement
of optical fiber parameters, in particular, at investigation of temperature dependences
of the optical fibers, it is expedient to use the laser diode or the light-emitting diode
(LED) with low-frequency internal modulation. In microwave OEO, suitable for
application in the communication structures as, for example, low-noise devices for
generation of reference oscillations with operation frequencies 5–73 GHz, it is
expedient to use QWLD with direct and external modulation. In the ultralow-noise
oscillators of microwave and mm-wave ranges (8–30 GHz) we need to use QWLD
with the external MZ modulator.
2.1 Operation Principle and Functional Diagram of OEO with RF FODL
17