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IEEE Journal of Quantum Electronics

Amplitude squeezing from singly resonant frequency-doubling laser

Joji Maeda, T. Numata, S. Kogoshi

Published Jul 1, 1997• 5 citations
Publisher

Abstract

We analyze amplitude squeezing from a singly resonant frequency-doubling laser oscillating at a single frequency. In this laser system, the cavity loss depends on the intensity of the oscillating fundamental field, so that conventional analyses based on a mean-field approximation become invalid in a highly pumped regime. To avoid this inconvenience, we consider spatial evolution of fields both in a laser medium and in a nonlinear crystal. It is predicted for the first time that a combination of excess nonlinearity and modest laser saturation can increase the output noise. We propose novel indices to evaluate the possible noise enhancement and suggest a design rule for squeezed light generation.

DOI
10.1109/3.594866

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We analyze amplitude squeezing from a singly resonant frequency-doubling laser oscillating at a single frequency. In this laser system, the cavity loss depends on the intensity of the oscillating fundamental field, so that conventional analyses based on a mean-field approximation become invalid in a highly pumped regime. To avoid this inconvenience, we consider spatial evolution of fields both in a laser medium and in a nonlinear crystal. It is predicted for the first time that a combination of excess nonlinearity and modest laser saturation can increase the output noise. We propose novel indices to evaluate the possible noise enhancement and suggest a design rule for squeezed light generation.
We analyze the squeezing characteristics of a singly resonant second-harmonic-generation system, in which only the fundamental light is confined in a plain cavity resonator, with consideration for fundamental depletion in a nonlinear crystal. The system was analyzed under the mean-field approximation, which neglects the variation of the field in the crystal, and as much as 9.5 dB of squeezing was predicted in the second-harmonic output. On the basis of the self-consistent method, we analyze the system, which is driven by such an intense fundamental input that the mean-field approximation is not valid, and predict that an arbitrary amount of squeezing will be available in this regime.
We discuss different approaches to the generation of bright amplitude-squeezed light using second-order nonlinear effects in optical cavities. A 0.2 mW beam at 1064 nm exhibiting 4 dB squeezing has been generated using a phase-sensitive type-I parametric amplifier pumped by a frequency-doubled Nd:YAG laser. In applying frequency-doubling processes to the generation of amplitude-squeezed light, the appearance of parasitic parametric oscillation must be considered. This effect has been observed in a singly-resonant frequency doubler. We also describe recent developments in optical cavity design for doubly-resonant frequency-doubling. Finally, we show theoretically that phase-mismatched second-harmonic generation in an optical cavity can be employed to generate a strong effective Kerr nonlinearity. This system appears promising for achieving optical bistability and for generating squeezed light of high power. Keywords: Non-classical light, squeezed light, amplitude squeezing, nonlinear op...
Quantum noise in a model of singly resonant frequency doubling including phase mismatch and driving in the harmonic mode is analyzed. The general formulae about the fixed points and their stability as well as the squeezing spectra calculated linearizing around such points are given. The use of a nonlinear normalization allows to disentangle in the spectra the dynamic response of the system from the contributions of the various noisy inputs. A general ``reference'' model for one-mode systems is developed in which the dynamic aspects of the problem are not contaminated by static contributions from the noisy inputs. The physical insight gained permits the elaboration of general criteria to optimize the noise suppression performance. With respect to the squeezing in the fundamental mode the optimum working point is located near the first turning point of the dispersive bistability induced by cascading of the second order nonlinear response. The nonlinearities induced by conventional crystals appear enough to reach it being the squeezing ultimately limited by the escape efficiency of the cavity. In the case of the harmonic mode both, finite phase mismatch and/or harmonic mode driving allow for an optimum dynamic response of the system something not possible in the standard phase matched Second Harmonic Generation. The squeezing is then limited by the losses in the harmonic mode, allowing for very high degrees of squeezing because of the non-resonant nature of the mode. This opens the possibility of very high performances using artificial materials with resonantly enhanced nonlinearities. It is also shown how it is possible to substantially increase the noise reduction and at the same time to more than double the output power for parameters corresponding to reported experiments.
We discuss quantum-fluctuation characteristics of a second-harmonic generation system that uses an optical resonator to confine only the fundamental field. It was predicted that the available amount of squeezing is limited to 1.76 dB for the fundamental output and 9.54 dB for the second-harmonic output, respectively, irrespective of system parameters. However, these predictions are based on low-order perturbation theory and are unavailable if the nonlinear coupling between the fundamental and the second harmonic becomes large. We consider the spatial evolution of the fields and reveal that the maximum amount of squeezing depends on the cavity loss: Even an arbitrary amount of squeezing is possible for both the fundamental output and the second-harmonic output. We also consider the possibility of parametric oscillation of empty cavity modes pumped by the generated second harmonic and derive the following prediction: In a system that uses a type I phase-matched crystal, the oscillation of the side mode hardly occurs because of sum-frequency generation with the fundamental and scarcely imposes any limitations on squeezing. On the other hand, a system with a type II phase-matched crystal suffers from the oscillation of the polarization mode orthogonal to that of the fundamental input, and squeezing of the second-harmonic output is limited to 3 dB.
A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.
We have generated squeezed states of light by the process of second harmonic conversion within an optical cavity resonant at both fundamental and harmonic frequencies. The experiment consists of a standing-wave cavity containing a crystal of lithium niobate doped with magnesium oxide and driven by the field from a frequency-stabilized Nd:YAG laser (incident power, ~10 mW). The total field reflected from the cavity illuminates a pair of high quantum efficiency photodiodes, whose individual photocurrents are summed. Observations of squeezing are made by analyzing the spectral density of fluctuations for this sum photo-current while the cavity detuning is scanned through resonance. An intensity regulator holds the reflected light intensity constant during the scan by control of the incident light level. Reductions in photocurrent fluctuations of 12% relative to the vacuum-state or shot-noise level are achieved for frequency offsets near 4 MHz. Since significantly higher degrees of squeezing are predicted by our theoretical analysis, we discuss those processes that currently limit the observed noise reduction, including phase noise produced by the intracavity crystal in the absence of nonlinear conversion.