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Pulsed squeezed light with an incoherent pump: theoretical analysis

Jianming Huang, Prem Kumar, Orhan Aytür

Published May 21, 1990• 0 citations
OpenAlex

Abstract

We present a theoretical study of the generation and detection of incoherent squeezed light via optical parametric downconversion of a frequency doubled Q-switched multimode Nd:YAG laser.1 Our analysis begins by writing the plane-polarized output field of a Q-switched multimode laser as.

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We report generating squeezed light via parametric down conversion of a Q-switched and frequency-doubled multi-longitudinal-mode Nd-doped yttrium-aluminum-garnet laser. A local oscillator with appropriate temporal incoherence is needed to homodyne detect the generated squeezed light. As a first approximation, we derive the local-oscillator beam from the same multimode laser whose harmonic is used to pump the parametric down-conversion process. 0.08\ifmmode\pm\else\textpm\fi{}0.2 dB of squeezing is routinely observed.
Squeezed light with partial coherence is generated via optical parametric amplification (OPA) in a KTiOPO4 crystal that is pumped by the second harmonic of a Q-switched multimode Nd:YAG laser.1 More than 1 dB of squeezing is observed when a proper mode of the downconverted light is homodyned with the 380-ns multilongitudinal-mode local oscillator (LO) pulse. In such a system, large OPA gain is easily obtained in a single-crystal signal-pass setup. The detection of squeezed light is, however, not straightforward. In the usual implementation of a balanced homodyne detection scheme,2 the vacuum-noise level, as measured on a radio frequency spectrum analyzer, is determined by the average LO power. If the LO is derived from a Q-switched laser, as is the case in our experiment, the associated peak power can easily saturate the detectors and the subsequent electronics. To overcome this limitation, we have implemented a hybrid frequency/time domain measurement scheme in which the vacuum noise level is determined by the peak (instead of the average) LO power. The LO average power requirement is thus relaxed and the detector saturation problem avoided.
We report pulsed squeezed-light generation by means of an optical parametric downconverter that is pumped by the second harmonic of a mode-locked Q-switched laser. Using the fundamental beam of the laser as a local oscillator, we observe 2-dB squeezing for a parametric gain of 2.0. This local oscillator, however, is nonoptimal because of its spatiotemporal mode mismatch with the squeezed mode generated by the downconverter. We describe an experiment in which a matched local oscillator is generated with the use of an optical parametric amplifier that is pumped by the same laser as is the downconverter. In this case, 2-dB squeezing is observed for a parametric gain of 1.5. The present experimental setup is limited by the uncontrollable phase fluctuations that prohibit us from making squeezing measurements at higher parametric gains.
Nowadays, the realization of quantum computations and communications based on continuous variables has attracted a significant attention due to a substantial expansion of the system dimensionality. The main progress in this area is attributed to the implementation of multimode systems based on squeezed states of light. One of the simplest ways to generate such states relies upon their producing in a single-pass optical parametric amplifier (OPA) using ultrafast pumping. However, for homodyne detection of such multimode states, the profile of the local oscillator (LO) must perfectly match the profile of the measured mode. Usually, this is not the case; therefore a proper treatment of multimode squeezing is required. In this work, we study both theoretically and experimentally the multimode squeezed light generated in type-0 and type-II OPA. We characterize such sources and investigate the degree of squeezing in dependence on the LO spectral profile, employing a pulse shaping technique. The theoretical analysis is performed using the Schmidt-mode theory. This work might have a significant impact on the realization of multimode quantum protocols
Pulsed squeezed light and intensity-correlated twin beams are generated by means of a traveling-wave optical parametric amplifier (OPA) consisting of a type-II phase-matched KTiOPO4 crystal that is pumped by a Q-switched and frequency-doubled Nd:YAG laser. Q switching of the laser results in a high peak pump intensity and a large peak parametric gain. Squeezed light is generated by amplifying the vacuum input field to the OPA, and it is detected by a balanced homodyne configuration that uses the fundamental beam of the laser as a local oscillator.1 Twin beams are generated by supplying a small input signal to the OPA. The amplified signal and idler beams are direct-detected individually and yield quantum-noise reduction when the two photocurrents are subtracted.
We analyze the spectral properties of squeezed light produced by means of pulsed, single-pass degenerate parametric down-conversion. The multimode output of this process can be decomposed into characteristic modes undergoing independent squeezing evolution akin to the Schmidt decomposition of the biphoton spectrum. The main features of this decomposition can be understood using a simple analytical model developed in the perturbative regime. In the strong pumping regime, for which the perturbative approach is not valid, we present a numerical analysis, specializing to the case of one-dimensional propagation in a beta-barium borate waveguide. Characterization of the squeezing modes provides us with an insight necessary for optimizing homodyne detection of squeezing. For a weak parametric process, efficient squeezing is found in a broad range of local oscillator modes, whereas the intense generation regime places much more stringent conditions on the local oscillator. We point out that without meeting these conditions, the detected squeezing can actually diminish with the increasing pumping strength, and we expose physical reasons behind this inefficiency.