the level of stimulated laser emission, which depends mainly by a ratio of active
carrier population in the upper “emissive” level and the total carrier population. The
bandwidth of PSD of the phase noise line is determined by the spontaneous emission
intensity, which depends on the carrier lifetime on the emissive level.
We emphasize that the influence on OEO phase noise formation, which is caused
by a presence of the relatively large spontaneous emission level, is the one of the
main distinctive properties of optical generators compared to traditional electronic
oscillators. As it well known, the spontaneous emission level has a cubic dependence
upon the photon emission frequency. At that, it essentially exceeds a noise in the
optical range caused by thermal factors, which is proportional kT (where k is the
Boltzmann constant, T is the temperature in Kelvin), and the flicker-noise and
the shot noise, which are traditionally studied in radio physics. The issue of the
OEO phase noise is considered in detail in Chaps. 4, 6, and 7. Here we substantiate
the main relationships.
The new scantily investigated (in radio physics sense) noise source appears,
which is caused by the QWLD spontaneous emission detected by the photodetector
in OEO compared to traditional electronic oscillators investigated in the nonlinear
oscillation theory. For the coherent system of formation and the heterodyne
photodetection, to which OEOs are concerned, the spontaneous emission noise is
determining because its level essentially exceeds the proper electronic noises of the
photodetector and the nonlinear amplifier. Small spontaneous emission examination
and the fact that it is not marked as the separate specific quantum noise in a majority
pulse optoelectronic systems, which are widely used in RF FODLs, can be explained
by the fact that they use the pulse mode of laser (or QWLD) operation. At that, the
detection is usually performed without utilization of heterodyne-mixing. In systems
for information transmission, for example, in usual pulse RF FODLs (in OEO, the
large signal acts on the light-sensitive area of the photodetector) a small signal is
applied to the photodetector area, and the pulse emission power is detected, while
noises in RF FODL are determined by the photoreceiver noises.
A complexity of noise modeling and investigation, which is caused by QWLD
spontaneous emission, can be explained by the fact that spontaneous emission has
the pure quantum nature. Here under the quantum nature, we mean the “postulates,”
which are proved within the limits of the quantum mechanics theory about atom
energy levels at transfer from upper “emissive” energy level E 2 onto the lower level
E 1 , the particle (an atom or an electron at recombination) emits a light quantum
hν ¼ E 2 À E 1 (ν is the optical frequency, h is the Planck constant) and stimulated
emission happens at interaction of the particle with the electromagnetic field with
this field frequency and the same phase of external EMF. We must at that to take in
the consideration the process of spontaneous emission. The correct mathematical
description of spontaneous emission is possible at using the quantum mechanics
apparatus. We would like to remind that spontaneous and stimulated emission in
impossible to describe within the limits of the ordinary electrodynamics. For example, during accelerated electron movement on an orbit around a nucleus, the light
emission does not occur although, according to electrodynamics laws, in this case
the electromagnetic wave must be emitted into a space. Dealing with quantum
2.3 Spontaneous Laser and QWLD Emission and Its Role in OEO Noises’ Formation
29
carrier population in the upper “emissive” level and the total carrier population. The
bandwidth of PSD of the phase noise line is determined by the spontaneous emission
intensity, which depends on the carrier lifetime on the emissive level.
We emphasize that the influence on OEO phase noise formation, which is caused
by a presence of the relatively large spontaneous emission level, is the one of the
main distinctive properties of optical generators compared to traditional electronic
oscillators. As it well known, the spontaneous emission level has a cubic dependence
upon the photon emission frequency. At that, it essentially exceeds a noise in the
optical range caused by thermal factors, which is proportional kT (where k is the
Boltzmann constant, T is the temperature in Kelvin), and the flicker-noise and
the shot noise, which are traditionally studied in radio physics. The issue of the
OEO phase noise is considered in detail in Chaps. 4, 6, and 7. Here we substantiate
the main relationships.
The new scantily investigated (in radio physics sense) noise source appears,
which is caused by the QWLD spontaneous emission detected by the photodetector
in OEO compared to traditional electronic oscillators investigated in the nonlinear
oscillation theory. For the coherent system of formation and the heterodyne
photodetection, to which OEOs are concerned, the spontaneous emission noise is
determining because its level essentially exceeds the proper electronic noises of the
photodetector and the nonlinear amplifier. Small spontaneous emission examination
and the fact that it is not marked as the separate specific quantum noise in a majority
pulse optoelectronic systems, which are widely used in RF FODLs, can be explained
by the fact that they use the pulse mode of laser (or QWLD) operation. At that, the
detection is usually performed without utilization of heterodyne-mixing. In systems
for information transmission, for example, in usual pulse RF FODLs (in OEO, the
large signal acts on the light-sensitive area of the photodetector) a small signal is
applied to the photodetector area, and the pulse emission power is detected, while
noises in RF FODL are determined by the photoreceiver noises.
A complexity of noise modeling and investigation, which is caused by QWLD
spontaneous emission, can be explained by the fact that spontaneous emission has
the pure quantum nature. Here under the quantum nature, we mean the “postulates,”
which are proved within the limits of the quantum mechanics theory about atom
energy levels at transfer from upper “emissive” energy level E 2 onto the lower level
E 1 , the particle (an atom or an electron at recombination) emits a light quantum
hν ¼ E 2 À E 1 (ν is the optical frequency, h is the Planck constant) and stimulated
emission happens at interaction of the particle with the electromagnetic field with
this field frequency and the same phase of external EMF. We must at that to take in
the consideration the process of spontaneous emission. The correct mathematical
description of spontaneous emission is possible at using the quantum mechanics
apparatus. We would like to remind that spontaneous and stimulated emission in
impossible to describe within the limits of the ordinary electrodynamics. For example, during accelerated electron movement on an orbit around a nucleus, the light
emission does not occur although, according to electrodynamics laws, in this case
the electromagnetic wave must be emitted into a space. Dealing with quantum
2.3 Spontaneous Laser and QWLD Emission and Its Role in OEO Noises’ Formation
29