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Optics Express

Optical signal to noise ratio improvement through unbalanced noise beating in phase-sensitive parametric amplifiers

Rohit Malik, Ales Kumpera, Samuel L. I. Olsson, Peter A. Andrekson, Magnus Karlsson

Published Apr 23, 2014• 15 citations
Publisher

Abstract

We investigate the beating of signal and idler waves, which have imbalanced signal to noise ratios, in a phase-sensitive parametric amplifier. Imbalanced signal to noise ratios are achieved in two ways; first by imbalanced noise loading; second by varying idler to signal input power ratio. In the case of imbalanced noise loading the phase-sensitive amplifier improved the signal to noise ratio from 3 to 6 dB, and in the case of varying idler to signal input power ratio, the signal to noise ratio improved from 3 to in excess of 20 dB.

DOI
10.1364/oe.22.010477

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Flat and wide-bandwidth gain spectrum, along with phase-sensitive gain with no need to generate amplifier input idlers, can be achieved with a coupled dual-core fiber optical parametric amplifier. In this paper, we analyze the noise properties of such an amplifier. We achieve a 3 dB noise figure in the phase-insensitive case and a minimum of -6 dB noise figure in the phase-sensitive configuration. An alternative phase-sensitive configuration is also studied, that avoids the use of idlers at the input of the amplifier, leading to a 3 dB flat spectrum combined noise figure for the output signals. Pump transfer noise in phase-insensitive and phase-sensitive configurations is also studied along with the noise figure variation against the length of the amplifier.
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.
A cascaded parametric amplifier consists of a first parametric amplifier, which amplifies an input signal and generates an idler, which is a copy of the signal, a signal processor, which controls the phases of the signal and idler, and a second parametric amplifier, which combines the signal and idler in a phase-sensitive manner. In this paper, cascaded parametric amplification is modeled and the conditions required to maximize the constructive-destructive extinction ratio are determined. The results show that a cascaded parametric amplifier can be operated as a filter: A desired signal-idler pair is amplified, whereas undesired signal-idler pairs are deamplified. For the desired signal and idler, the noise figures of the filtering process (input signal-to-noise ratio divided by the output ratios) are only slightly higher than those of the copying process: Signal-processing functionality can be achieved with only a minor degradation in signal quality.