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Journal of Physics B Atomic Molecular and Optical Physics

Investigation of fundamental and second harmonic squeezed lights from a singly resonant PPKTP frequency doubler

Sujing Zhang, Yongmin Li, Jianli Liu, Kuanshou Zhang

Published Oct 10, 2006• 3 citations
Publisher

Abstract

Both fundamental and second harmonic amplitude squeezed lights were generated from an external cavity-enhanced singly resonant PPKTP frequency doubler. The effect of input coupling and fundamental power on the squeezing was experimentally investigated. It was shown that the optimal conditions (input coupling and fundamental power) are different for the squeezing of the fundamental and second harmonic lights. An agreement between experiment and theoretical prediction was found.

DOI
10.1088/0953-4075/39/20/014

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Both fundamental and second harmonic amplitude squeezed lights were generated from an external cavity-enhanced singly resonant PPKTP frequency doubler. The effect of input coupling and fundamental power on the squeezing was experimentally investigated. It was shown that the optimal conditions (input coupling and fundamental power) are different for the squeezing of the fundamental and second harmonic lights. An agreement between experiment and theoretical prediction was found.
We present a new technique for the efficient and simple production of amplitude-squeezed light with high coherent excitation, using a frequency doubler, in which only the fundamental wave is resonantly enhanced. Our theory predicts up to nearly 90% squeezing in the second-harmonic wave under ideal conditions. In our first experiment we measured up to 30% squeezing [having taken into account the limited quantum efficiency (63%) of our detection system] in a stable 50 mW beam. Compared to doubly resonant frequency doubling our new approach has considerable technical advantages.
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.
Second-harmonic generation, in a cavity resonant only at the fundamental, produces amplitude squeezing in both the fundamental and the second harmonic, with optimal squeezing of -1/3 and -8/9, respectively, at low frequency. It also produces nonclassical correlations between the two output beams. We show that these correlations can be used to enhance the squeezing in either beam by constructing a feedback loop. By using the second-harmonic photocurrent to control the amplitude of the pump, the optimal fundamental squeezing is enhanced by 50% to -1/2. The limit of -8/9 for the seocnd harmonic (for infinite nonlinearity strength) cannot be improved by feeding back the fundamental. However, for any fixed finite nonlinearity, such feedback can produce significant enhancement, with a maximum of 50%. Even with the losses characteristic of a practical experiment included, the enchancement offered by the feedback is still significant, being greater than 20% in either mode.