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Annual Meeting Optical Society of America

What's so ultra about ultrasqueezed light?

Bonny L. Schumaker

Published Jan 1, 1988• 0 citations

Abstract

At a previous OSA conference the author introduced the phenomenon of ultrasqueezed light, produced by three-wave and four-wave mixing in nonlinear media pumped at multiple frequencies spaced so as to produce appropriate phase-matching. Here quantitative results are given of the advantages offered by ultrasqueezing in terms of the degree of squeezing obtainable for a given total amount of input pump power. Phase-matching requirements and detection techniques are described, and resulting implications for the usefulness (enhanced squeezing vs ease of production and detection) are indicated. The results differ appreciably for three-wave and four-wave mixing. Several different experimental schemes for producing ultrasqueezed light are described using three-wave and four-wave mixing. Only one of these, four-wave mixing with two pump frequencies in an optical fiber, has been attempted experimentally; its success has been described at a previous OSA conference and is reviewed only briefly. Comparison is made between ultrasqueezed light and pulsed squeezed light, which can be viewed as a special limit of ultrasqueezing with many pump frequencies.

DOI: 10.1364/oam.1988.tua7

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At a previous OSA conference the author introduced the phenomenon of ultrasqueezed light, produced by three-wave and four-wave mixing in nonlinear media pumped at multiple frequencies spaced so as to produce appropriate phase-matching. Here quantitative results are given of the advantages offered by ultrasqueezing in terms of the degree of squeezing obtainable for a given total amount of input pump power. Phase-matching requirements and detection techniques are described, and resulting implications for the usefulness (enhanced squeezing vs ease of production and detection) are indicated. The results differ appreciably for three-wave and four-wave mixing. Several different experimental schemes for producing ultrasqueezed light are described using three-wave and four-wave mixing. Only one of these, four-wave mixing with two pump frequencies in an optical fiber, has been attempted experimentally; its success has been described at a previous OSA conference and is reviewed only briefly. Comparison is made between ultrasqueezed light and pulsed squeezed light, which can be viewed as a special limit of ultrasqueezing with many pump frequencies.

The fundamental and simplest kind of squeezing (broadband squeezing) is produced by pumping a nonlinear medium at a single frequency. The observables squeezed are the electric-field quadrature phases defined relative to the pump frequency. Their measurement requires homodyning against an optical-frequency (the pump frequency) local oscillator. When the medium is pumped at several different frequencies (equally spaced), different kinds of observables exhibit squeezing. Their measurement requires further homodyning, against a radio-frequency (the pump frequency spacing) local oscillator, and, for four or more pump frequencies, against subsequent lower-frequency local oscillators. For a given total pump power, it is possible to achieve greater squeezing by distributing the pump power over several (at least three) different pump frequencies than by pumping at a single frequency. The drawback is that a more sophisticated detection scheme is required (i.e., further homodyning) to measure the maximum squeezing (less squeezing is produced in more easily measured observables, such as the electric-field quadrature phases). The required further homodyning at rf and lower frequencies is straightforward to accomplish, however, and the advantages of using lower pump powers at each optical frequency can be substantial, especially in mediums such as optical fibers, where large pump powers bring with them substantial added noise sources.

The degree of squeezing generated by parametric downconversion or four-wave mixing can be limited by the ratio of the nonlinear coupling to the linear losses in many practical systems. This ratio can be enhanced by increasing the pump power used for the nonlinear coupling in many solid nonlinear systems where the linear losses are not directly related to the nonlinear squeezing process. Pulsed pump sources are attractive for optimizing the nonlinear coupling-to-loss ratio. We have shown1 that homodyne detection can be used to detect the pulsed squeezing generated by a mode-locked pulse train from the pump laser. A portion of the pump beam is used as the local oscillator which stroboscopically samples portions of the squeezed light pulse train. The amount of squeezing which can be measured as a function of the relative durations of the pump and local oscillator pulses is shown. Progress of recent experiments using both four-wave mixing and parametric downconversion is described.

The four-wave-mixing interactions produced by pumping at two well-separated frequencies can couple four different radiation modes to generate a new kind of squeezed state of light. A dual-frequency homodyne detection scheme is described and is used to observe this type of nonclassical correlation. Initial experimental results in an optical fiber show a noise level 20% below the vacuum noise level for the output of the dual-frequency detector, even though the noise at each individual detector remains above the vacuum level.

A scheme for detecting the phase of an incoming optical signal is systematically optimized and demonstrated experimentally. It makes use of four-wave mixing in a highly nonlinear fiber but differs from a phase-sensitive amplifier by its focus on phase detection, rather than on phase-dependent noise reduction. This experimental setup combines the signal with its phase-conjugated idler at the same frequency, before sending the combination to a photodetector. Experimental results show that the ratio of maximum and minimum currents can exceed 1000 even at a relatively low pump power of 10 mW. The proposed scheme may be useful for detecting the signal phase in coherent communication systems without the need of heterodyne detection.