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Physical Review A

Phase-sensitive above-threshold laser amplifiers

Yi Mu, Craig M. Savage

Published May 1, 1994• 3 citations
Publisher

Abstract

We present a quantum-mechanical analysis of an above-threshold laser amplifier made phase sensitive by an injected field. The gain and noise are calculated in a high-Q cavity with four-level atoms as the lasing medium. We find that for coherent-state inputs this amplifier can preserve the signal-to-noise ratio thus achieving the theoretical best noise performance.

DOI
10.1103/physreva.49.4093

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We present a quantum-mechanical analysis of an above-threshold laser amplifier made phase sensitive by an injected field. The gain and noise are calculated in a high-Q cavity with four-level atoms as the lasing medium. We find that for coherent-state inputs this amplifier can preserve the signal-to-noise ratio thus achieving the theoretical best noise performance.
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The purpose of a phase-preserving linear amplifier is to make a small signal larger, regardless of its phase, so that it can be perceived by instruments incapable of resolving the original signal, while sacrificing as little as possible in signal-to-noise ratio. Quantum mechanics limits how well this can be done: A high-gain linear amplifier must degrade the signal-to-noise ratio; the noise added by the amplifier, when referred to the input, must be at least half a quantum at the operating frequency. This well-known quantum limit only constrains the second moments of the added noise. Here we derive the quantum constraints on the entire distribution of added noise: We show that any phase-preserving linear amplifier is equivalent to a parametric amplifier with a physical state for the ancillary mode; the noise added to the amplified field mode is distributed according to the Wigner function of the ancilla state.
Quantum mechanical arguments show that any optical amplifier must add noise to the amplified beam of light.For the case of a high gain amplifier with a coherent (Poissonian) input, the signal-to-noise ratio of the amplified beam must be at least two times smaller than that of the incident beam.We review the theoretical basis of this prediction.We also describe the results of our experimental investigations of the noise properties of optical amplifiers that utilize the nonlinear optical response of strongly driven atomic transitions.
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