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Annual Meeting Optical Society of America

Squeezed amplification in a gain-saturated parametric amplifier

N. C. Wong, Ke-Xun Sun, Jeffrey H. Shapiro

Published Jan 1, 1991• 0 citations
Publisher

Abstract

Recently optical parametric amplifiers (OPAs) with vacuum inputs have been extensively studied and shown to generate a significant amount of squeezing, but little is known about the quantum noise behavior of an OPA with an injected signal field. We have performed a linearized quantum analysis of a nondegenerate OPA operated in the saturated gain regime showing that the amplified output intensity of a coherent-state input signal becomes squeezed when the classical small-signal gain falls below unity. In uses such as master-oscillator power amplifiers, for which the signal of interest is the mean output intensity, the output signal-to-noise ratio (SNR) can be higher than the input SNR over a broad operating range. For an OPA pumped at 90% of threshold, the SNR improvement is about a factor of 3, and with a 10-Db amplitude squeezed pump, the improvement increases to about a factor of 7. Such a tunable gain-saturated OPA with large-signal gain and improved SNR can be potentially useful in digital communication.

DOI
10.1364/oam.1991.mbb5

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Optical amplifiers, phase-shift keying, and signal-to-noise ratio

Jeffrey H. Shapiro, N. C. Wong

Recently, there has been some controversy concerning the role of signal-to-noise ratio (SNR) calculations in conjunction with optical amplifiers and phase-shift keying. Analysis has shown that the large-signal SNR at the output of an optical parametric amplifier (OPA) or optical parametric oscillator (OPO) can exceed the shot-noise limit when an injected signal beam is used to elicit small-signal gain saturation. Although no small-signal SNR gain is obtained in this way, the large-signal SNR improvement does translate into a communication performance improvement if binary phase-shift keying is applied to the resulting output beam. Analysis has also shown that the large-signal, shot-noise limited SNR can be exceeded when a binary phase-shift keyed coherent-state beam is applied to a preamplify-feedforward-homodyne receiver. In this case, however, there is no communication performance improvement. Indeed, the preamplify-feed-forward-homodyne receiver requires twice as many signal photons, on average, to equal the error probability of ordinary homodyne detection. In this talk we will show that there is no contradiction between these two examples.

Annual Meeting Optical Society of America•Jan 1, 1992•0 citations
Recently optical parametric amplifiers (OPAs) with vacuum inputs have been extensively studied and shown to generate a significant amount of squeezing, but little is known about the quantum noise behavior of an OPA with an injected signal field. We have performed a linearized quantum analysis of a nondegenerate OPA operated in the saturated gain regime showing that the amplified output intensity of a coherent-state input signal becomes squeezed when the classical small-signal gain falls below unity. In uses such as master-oscillator power amplifiers, for which the signal of interest is the mean output intensity, the output signal-to-noise ratio (SNR) can be higher than the input SNR over a broad operating range. For an OPA pumped at 90% of threshold, the SNR improvement is about a factor of 3, and with a 10-Db amplitude squeezed pump, the improvement increases to about a factor of 7. Such a tunable gain-saturated OPA with large-signal gain and improved SNR can be potentially useful in digital communication.
Optical parametric amplifiers (OPA's) and optical parametric oscillators (OPO's) can exhibit squeezed amplification when an injected signal beam is used to elicit small-signal gain saturation. This paper extends previous work on OPA/OPO noise behavior under signal injection. Doubly resonant and triply resonant, single-ended cavities are considered in the good-cavity limit. Owing to the nonlinear coupling between the signal and idler beams within the cavity, both are sensitive to the detuning of each beam from its respective resonance. Signal and idler output noise spectra are found by linearizing the quantum coupled-mode equations about the classical mean-field solutions. Cavity detunings lead to frequency dependent phase shifts between the low-noise and high-noise quadratures of both the signal and idler beams. Moreover, the phase of the low-noise quadrature need not coincide with that of the corresponding mean field. The resulting large-signal, signal-to-noise ratio improvement afforded by squeezed amplification is therefore sensitive to the cavity detunings. The effects of classical excess noise on the pump beam and the injected signal beam are also considered.
Quantum fluctuations in optical amplification are investigated with a nondegenerate optical parametric amplifier whose internal idler mode is coupled to a squeezed vacuum. Reductions of the inherent quantum noise of the amplifier are observed with a minimum noise level 0.7 dB below the usual noise level of the amplifier with its internal idler mode in a vacuum state. With a small coherent field as the signal input, the amplified output exhibits an improvement in signal-to-noise ratio of 0.5 dB for the case of a squeezed-vacuum as compared to a vacuum state for the amplifier's internal mode.
Quantum fluctuations in an optical amplifier are of fundamental as well as technological significance and lead to basic limitations on noise performance. It is well-known that the signal-to-noise ratio (SNR) of an injected signal is degraded in the amplification process due to added noise from thermal or (at best) vacuum fluctuations of the internal modes of the amplifier.[1] However, if the internal modes are coupled to a squeezed vacuum, the added noise in amplification can be reduced.[2] With this motivation, we have constructed a nondegenerate parametric amplifier whose idler mode is itself coupled to a squeezed vacuum, which is generated by a second parametric amplifier. Phase-sensitive noise amplification of a vacuum input to the signal mode is observed with the minimum amplified noise level 0.5dB below the usual phase-insensitive amplified noise level for the amplified signal of the same amplifier but with the idler mode coupled to a vacuum state. Prospects for improvement by increasing the squeezing that is coupled into the idler mode of the amplifier beyond the current level of 2dB as well as surprising results from our theoretical analysis which includes losses will be discussed.
Optical parametric amplifiers (OPA's) can exhibit squeezed amplification when an injected signal beam is used to elicit small-signal gain saturation. This paper extends previous work on OPA mean-field behavior under signal injection. Doubly resonant and triply resonant, single-ended cavities are considered in the good-cavity limit. Owing to the nonlinear coupling between the signal and idler beams within the cavity, both are sensitive to the detuning of each beam from its respective resonance. Signal and idler output mean fields are found by solving the nonlinear coupled-mode equations and are numerically evaluated for a variety of detunings and injection levels. Gain saturation is shown to occur for both beams and to be dependent on the cavity detunings. Comparisons are made with experimental results obtained from a doubly resonant MgO-doped, LiNbO3 OPA/OPO.
Recently, there has been some controversy concerning the role of signal-to-noise ratio (SNR) calculations in conjunction with optical amplifiers and phase-shift keying. Analysis has shown that the large-signal SNR at the output of an optical parametric amplifier (OPA) or optical parametric oscillator (OPO) can exceed the shot-noise limit when an injected signal beam is used to elicit small-signal gain saturation. Although no small-signal SNR gain is obtained in this way, the large-signal SNR improvement does translate into a communication performance improvement if binary phase-shift keying is applied to the resulting output beam. Analysis has also shown that the large-signal, shot-noise limited SNR can be exceeded when a binary phase-shift keyed coherent-state beam is applied to a preamplify-feedforward-homodyne receiver. In this case, however, there is no communication performance improvement. Indeed, the preamplify-feed-forward-homodyne receiver requires twice as many signal photons, on average, to equal the error probability of ordinary homodyne detection. In this talk we will show that there is no contradiction between these two examples.