Konopsky V.N., Masalov A.V., Putilin A.A., and Vasilyev M.V.
Quantum noise of light during amplification and oscillation in opto-electronic loop. // Proc.SPIE, 1996, V.2799, pp.172-181;
also in:
Coherence and Quantum Optics.// Eberly, Mandel and Wolf Eds., (Plenum Press, New York 1996), VII, p.167.

Light generation, amplification and detection are always followed by noise. Technical noise can be reduced, while quantum noise is unavoidable. In lasers the quantum noise appears as Poissonian fluctuations of output intensity and as a natural width of emitted spectrum. In optical amplifiers the quantum noise appears as spontaneous emission and may cause degradation of the signal-to-noise ratio. In this paper, we present a new type of optical amplifier and oscillator based on light modulation (without stimulated emission). Amplification process in these devices consists of two steps: at first, the input light spectrum is shifted into radio frequency range by heterodyne photoreceiver, then the amplified RF signal is converted back into initial optical frequencies by means of modulation. The quantum noise in such an amplifier originates from the shot noise of heterodyne photoreceiver. The limitations due to shot noise are studied theoretically and experimentally. Although the physical mechanisms of quantum noise in stimulated emission and heterodyne conversion are different, there is deep analogy in resulting equations. When the output of modulation-based amplifier is partially transmitted to the input, self-oscillation of the device is observed. The oscillator is considered as laser light transformer. It may operate as an external frequency and/or intensity stabilizer of laser light, which requires no feedback to the laser source.

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