Architectures of OEO can be distinguished according to modulation approach
and to photodetection. In OEO with external modulation and with the direct amplitude modulation, the phase and amplitude (relatively) modulation of the optical
emission and further photodetection at least two optical oscillation are used. The one
of the optical oscillation is modulated in phase by the RF subcarrier signal. These
architectures can be concerned to the circuits with heterodyne photodetection
according to the approach to photodetection. In cases of the traditional heterodyne
photodetection, which is used in the laser radar technology, at reception of the
external optical emission, the oscillation modulated in phase or frequency is
received with using the external optical generator or the heterodyne, which oscillations pass to the light-sensitive area of the photodetector together with the external
receiving optical oscillation. In OEO with external or direct modulation, the selfheterodyne-mixing is used, i.e., on the photodetector area, two (or three) optical
oscillations (or harmonics) with frequency (1) ν 1 ¼ ν 0 À f 0 , ν 2 ¼ ν 0 , ν 3 ¼ ν 0 + f 0 , or
(2) ν 2 ¼ ν 0 , ν 3 ¼ ν 0 + f 0 , which pass from the one QWLD, are combined.
The most important advantage of the heterodyne conversion is its ability to save
information in phase of the optical oscillations and its transfer into the electric
signal of the photodetector current. But at that, at photoreception of the laser
(or QWLD) emission, there is a transfer of phase fluctuations of the laser oscillations,
which are determined by its spontaneous emission, into phase fluctuations of the RF
microwave oscillations, and this is unpleasant and often inadmissible.
As a result of self-heterodyne mixing, in the photodetector, the RF subcarrier, and
phase noises of QWLD (which plays the role of the heterodyne) are extracted. In this
case, the oscillation spectrum of the photocurrent (under condition of smallness of
the own noises of photodetector and the nonlinear amplifier) repeats the spectrum
form of optical oscillations of the field strength of the signal wave, but with the
frequency shift downward exactly by the frequency of the laser-heterodyne. In
spectral representation this can be expressed as: the optical signal spectrum
(or PSD of AN and PN noises) shifts almost without variations into the region of
the RF subcarrier f 0 spectrum, and at finite spectrum width of the laser-heterodyne,
the RF subcarrier f 0 spectrum (the electric signal) is additionally spread.
2.3 Spontaneous Laser and QWLD Emission and Its Role
in OEO Noises’ Formation
The main aim of this section is determination of the ratio of the spontaneous
emission level and the level of stimulated laser emission at its output for different
quantum generators (laser on ruby and neodymium, semiconductor quantum generators, and QWLDs). In essence, we below substantiate and prove the following
statement: the contribution of spontaneous emission of the laser (or QWLD) is the
main contribution into phase noise formation in OEO, and the power spectral density
(PSD) of the phase noise is determined as a ratio of spontaneous emission level and
28
2 Nanostructural Optoelectronic Oscillators with the Fiber-Optical Delay Line
and to photodetection. In OEO with external modulation and with the direct amplitude modulation, the phase and amplitude (relatively) modulation of the optical
emission and further photodetection at least two optical oscillation are used. The one
of the optical oscillation is modulated in phase by the RF subcarrier signal. These
architectures can be concerned to the circuits with heterodyne photodetection
according to the approach to photodetection. In cases of the traditional heterodyne
photodetection, which is used in the laser radar technology, at reception of the
external optical emission, the oscillation modulated in phase or frequency is
received with using the external optical generator or the heterodyne, which oscillations pass to the light-sensitive area of the photodetector together with the external
receiving optical oscillation. In OEO with external or direct modulation, the selfheterodyne-mixing is used, i.e., on the photodetector area, two (or three) optical
oscillations (or harmonics) with frequency (1) ν 1 ¼ ν 0 À f 0 , ν 2 ¼ ν 0 , ν 3 ¼ ν 0 + f 0 , or
(2) ν 2 ¼ ν 0 , ν 3 ¼ ν 0 + f 0 , which pass from the one QWLD, are combined.
The most important advantage of the heterodyne conversion is its ability to save
information in phase of the optical oscillations and its transfer into the electric
signal of the photodetector current. But at that, at photoreception of the laser
(or QWLD) emission, there is a transfer of phase fluctuations of the laser oscillations,
which are determined by its spontaneous emission, into phase fluctuations of the RF
microwave oscillations, and this is unpleasant and often inadmissible.
As a result of self-heterodyne mixing, in the photodetector, the RF subcarrier, and
phase noises of QWLD (which plays the role of the heterodyne) are extracted. In this
case, the oscillation spectrum of the photocurrent (under condition of smallness of
the own noises of photodetector and the nonlinear amplifier) repeats the spectrum
form of optical oscillations of the field strength of the signal wave, but with the
frequency shift downward exactly by the frequency of the laser-heterodyne. In
spectral representation this can be expressed as: the optical signal spectrum
(or PSD of AN and PN noises) shifts almost without variations into the region of
the RF subcarrier f 0 spectrum, and at finite spectrum width of the laser-heterodyne,
the RF subcarrier f 0 spectrum (the electric signal) is additionally spread.
2.3 Spontaneous Laser and QWLD Emission and Its Role
in OEO Noises’ Formation
The main aim of this section is determination of the ratio of the spontaneous
emission level and the level of stimulated laser emission at its output for different
quantum generators (laser on ruby and neodymium, semiconductor quantum generators, and QWLDs). In essence, we below substantiate and prove the following
statement: the contribution of spontaneous emission of the laser (or QWLD) is the
main contribution into phase noise formation in OEO, and the power spectral density
(PSD) of the phase noise is determined as a ratio of spontaneous emission level and
28
2 Nanostructural Optoelectronic Oscillators with the Fiber-Optical Delay Line