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Physical Review A

Squeezing limits at high parametric gains

Arthur La Porta, Richart E. Slusher

Published Aug 1, 1991• 94 citations
Publisher

Abstract

The generation of squeezed light in a single-pass parametric amplifier pumped by Gaussian beams is analyzed. Limits to the degree of squeezing that can be obtained are found that arise from spatial distortion of the signal beam that is significant for parametric gains greater than 3 dB. It is found that squeezing is limited to approximately 6 dB for typical experimental configurations. These limits severely constrain the design of quantum noise experiments unless high finesse cavities or waveguides are used.

DOI
10.1103/physreva.44.2013

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The generation of squeezed light in a single-pass parametric amplifier pumped by Gaussian beams is analyzed. Limits to the degree of squeezing that can be obtained are found that arise from spatial distortion of the signal beam that is significant for parametric gains greater than 3 dB. It is found that squeezing is limited to approximately 6 dB for typical experimental configurations. These limits severely constrain the design of quantum noise experiments unless high finesse cavities or waveguides are used.
The single-pass parametric downconver- sion amplifier is a simple optical arrangement that is useful for squeezed-light noise reduction and quantum nondemolition measurement.1 This device is pumped by a beam with a Gaussian spatial mode profile and acts on an input mode with a corresponding spatial profile. High parametric gain is required to achieve large squeezed noise reduction or accurate quantum measurement of a quadrature component of an input field. At large gains (>3 dB) the spatial variation of the pump field causes the parametrically amplified and deamplified quadrature field components to assume strongly non-Gaussian spatial profiles and to be distorted significantly with respect to each other. We analyze this distortion in the classical limit for an input beam with Gaussian profile and find that the gain measured by homodyne or direct detection differs from the low-gain limit. In the quantum limit, the case of squeezing is considered. It is shown that, in addition to the classical modifications, noise from higherorder spatial modes are coupled into the Gaussian mode, which is measured by a homodyne detection scheme using a Gauss- ian-profile local-oscillator beam. This high- gain mode-coupling process limits the degree of squeezing that can be obtained and the accuracy for quantum nondemolition measurement.
We investigate the generation of amplitude-squeezed states with degenerate optical parametric amplifiers that are pumped by focused Gaussian beams. We present a model that facilitates the calculation of the squeezing level for an experimentally realistic configuration in which there is a Gaussian input signal beam that has the same confocal parameter and waist location as the Gaussian pump beam, with no restriction on the interaction length-to-confocal parameter ratio. We show that the 3-dB squeezing limit that was thought to be imposed by the Gaussian pump profile can be exceeded in the (previously uninvestigated) tight-focusing regime. We find the maximum possible amplitude squeezing in this regime to be 4.65 dB. However, it is possible to increase the squeezing level further by spatially filtering the tails of the output signal beam, resulting in squeezing levels in excess of 10 dB.
Summary form only given. The generation of quantum states of light, such as squeezed states, often relies on degenerate optical parametric amplification. The input signal in a degenerate optical parametric amplifier (DOPA) may be amplified or deamplified depending on the phase of the signal field relative to that of the pump field (/spl Delta//spl phi/=/spl phi//sub pump/-2/spl phi//sub signal/). We present the results of an accurate numerical DOPA model for the generation of amplitude-squeezed light with Gaussian pump and signal beams. We find that the widely accepted 3-dB limit imposed by the Gaussian beam profiles can actually be exceeded in the tight focusing regime.
This paper investigates the generation of quadrature-squeezed states of light using a degenerate optical parametric amplifier (DOPA) that is pumped by a focused Gaussian beam. The formulation that is presented facilitates the calculation of squeezing for an arbitrary local oscillator beam. This formulation also establishes a formal equivalence between the classical parametric gain and the measured level of squeezing. The maximum squeezing that can be achieved using a Gaussian-beam local oscillator is determined to be limited to 13.4 dB, as a consequence of gain-induced diffraction. The phase lag of the maximally squeezed quadrature is shown to be significantly different from the plane-wave theoretic value of $\ensuremath{\pi}/2,$ unless the focusing is very weak. The use of a second DOPA for generating a local oscillator beam that is matched to the squeezed field is also investigated. In this case, squeezing is limited only by the available pump power.
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.