Abstract

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.

© 1990 Optical Society of America