The spectrum of RF oscillations of OEO is formed not only by noises, which have
the electronic nature, but the phase fluctuations of laser optical emission, which have
the quantum nature and are determined by the laser spontaneous emission. The
spectral line width of RF oscillations is determined by parameters of two oscillating
processes in the laser and on the whole in RF oscillator of OEO.
Figure 5.6b shows of the analog model of OEO МZ as QWLD enclosed by the
positive feedback loop. The positive feedback on AC voltage component is formed
by the circuit “the optical fiber, the photodetector PD, the RF filter F, RF amplifier
A.” The positive FB encloses the modulating light source (MLS). MLS is formed by
the series-connected laser and the Mach–Zehnder modulator. The laser analog model
is composed on the base of the fluctuation differential equations’ system (Eq. 5.66).
In the structure of Fig. 5.6b, the adder and Langevinian noise sources are
depicted. The dotted lines designate the possible additional circuit of FB on DC
current (or voltage). The additional FB loop is intended for adjustment of the laser
phase noise when using the frequency automatic control of the laser optical frequency. The optical laser oscillations pass to the PD and then act on the modulator
after the optical fiber. The light interference on the PD2 area of two oscillations with
the delay difference of some microseconds permits to extract the laser phase noise in
the DC current of PD2, and to perform its adjustment in the block DC PUMP by
means of DC laser pumping changing.
The common feature of analog models of OEO MZ (Fig. 5.6b) and OEO DM is
that in them, the laser is the main energetic element and the main source of the phase
noise caused by the spontaneous emission, which has the quantum character.
Analog models of OEO DM and OEO MZ are constructed on the unified
principle: the enclosure by the positive FB of the modulating source of the coherent
light (the laser). If in OEO DM, we use the internal (inside the optical resonator) or
the direct laser modulation and the positive FB loop in OEO DM is constructed on
AC component of the pumping current, then in OEO MZ, the modulation light
source is the common “inseparable” unified modern construction of the “laser and
the Mach–Zehnder modulator”. The positive FB loop in OEO MZ is arranged on the
AC voltage, and begins from the optical output of the optical modulator and finishes
at the electric input of the MZ modulator.
The noticeable difference of the presented OEO MZ structure (Fig. 5.6b) from the
analog model of OEO DM (Fig. 5.6b) is the fact that the laser itself in OEO MZ can
be enclosed by the additional positive FB loop in DC component of the pumping
current using the additional photodetector PD2, the systems of frequency automatic
control or the phase-locked loop (PLL) system. We should note that in OEO DM
structure (Fig. 5.6b), the implementation of the control system of the laser phase
noise is possible with application of additional photodetector.
The main principal difference of the analog model of OEO MZ (Fig. 5.6b) from
the analog model of OEO DM (Fig. 5.6a) is the potential possibility to use the extralow-noise QWLDs in the OEO MZ structure. This follows from the fact that in the
OEO MZ laser, as we see from the analog model in Fig. 5.6b, the carrier noise
suppression can be realized directly in the internal closed loop (shown in the model)
of the optical feedback or in the optical resonator. For this, we must not only reduce
250
5 Optoelectronic oscillator (OEO) Differential Equations as the Laser System with. . .
the electronic nature, but the phase fluctuations of laser optical emission, which have
the quantum nature and are determined by the laser spontaneous emission. The
spectral line width of RF oscillations is determined by parameters of two oscillating
processes in the laser and on the whole in RF oscillator of OEO.
Figure 5.6b shows of the analog model of OEO МZ as QWLD enclosed by the
positive feedback loop. The positive feedback on AC voltage component is formed
by the circuit “the optical fiber, the photodetector PD, the RF filter F, RF amplifier
A.” The positive FB encloses the modulating light source (MLS). MLS is formed by
the series-connected laser and the Mach–Zehnder modulator. The laser analog model
is composed on the base of the fluctuation differential equations’ system (Eq. 5.66).
In the structure of Fig. 5.6b, the adder and Langevinian noise sources are
depicted. The dotted lines designate the possible additional circuit of FB on DC
current (or voltage). The additional FB loop is intended for adjustment of the laser
phase noise when using the frequency automatic control of the laser optical frequency. The optical laser oscillations pass to the PD and then act on the modulator
after the optical fiber. The light interference on the PD2 area of two oscillations with
the delay difference of some microseconds permits to extract the laser phase noise in
the DC current of PD2, and to perform its adjustment in the block DC PUMP by
means of DC laser pumping changing.
The common feature of analog models of OEO MZ (Fig. 5.6b) and OEO DM is
that in them, the laser is the main energetic element and the main source of the phase
noise caused by the spontaneous emission, which has the quantum character.
Analog models of OEO DM and OEO MZ are constructed on the unified
principle: the enclosure by the positive FB of the modulating source of the coherent
light (the laser). If in OEO DM, we use the internal (inside the optical resonator) or
the direct laser modulation and the positive FB loop in OEO DM is constructed on
AC component of the pumping current, then in OEO MZ, the modulation light
source is the common “inseparable” unified modern construction of the “laser and
the Mach–Zehnder modulator”. The positive FB loop in OEO MZ is arranged on the
AC voltage, and begins from the optical output of the optical modulator and finishes
at the electric input of the MZ modulator.
The noticeable difference of the presented OEO MZ structure (Fig. 5.6b) from the
analog model of OEO DM (Fig. 5.6b) is the fact that the laser itself in OEO MZ can
be enclosed by the additional positive FB loop in DC component of the pumping
current using the additional photodetector PD2, the systems of frequency automatic
control or the phase-locked loop (PLL) system. We should note that in OEO DM
structure (Fig. 5.6b), the implementation of the control system of the laser phase
noise is possible with application of additional photodetector.
The main principal difference of the analog model of OEO MZ (Fig. 5.6b) from
the analog model of OEO DM (Fig. 5.6a) is the potential possibility to use the extralow-noise QWLDs in the OEO MZ structure. This follows from the fact that in the
OEO MZ laser, as we see from the analog model in Fig. 5.6b, the carrier noise
suppression can be realized directly in the internal closed loop (shown in the model)
of the optical feedback or in the optical resonator. For this, we must not only reduce
250
5 Optoelectronic oscillator (OEO) Differential Equations as the Laser System with. . .