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Theoretical and experimental investigations of squeeze-state generation in χ(3) semiconductor waveguides

M. K. Udo, Xiaolong Zhang, Seng‐Tiong Ho

Published May 8, 1994• 0 citations
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Abstract

The realization of a compact squeezed-state-generation device at 1.5 µm may have potential applications in optical communication. The squeezed state has been generated in both χ(2) and χ(3) media. In χ(2) media the frequency of the squeezed vacuum is half that of the pump. Therefore frequency doubling is necessary for generating a local oscillator (LO). Such frequency doubling is not needed in χ(3) media. However, squeezing in y(3) media has not been as successful as in X(2) media, for example, in atomic vapor,1 self-focusing seriously limits the amount of squeezing, whereas in optical fibers,2 guided-acoustic-wave Brillouin-scattering (GAWBS) noise causes difficulties that can be circumvented through low- temperature cooling of the fiber or the use of soli ton pulses.

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Acta Physica Sinica•Jan 1, 2018•2 citations
The realization of a compact squeezed-state-generation device at 1.5 µm may have potential applications in optical communication. The squeezed state has been generated in both χ(2) and χ(3) media. In χ(2) media the frequency of the squeezed vacuum is half that of the pump. Therefore frequency doubling is necessary for generating a local oscillator (LO). Such frequency doubling is not needed in χ(3) media. However, squeezing in y(3) media has not been as successful as in X(2) media, for example, in atomic vapor,1 self-focusing seriously limits the amount of squeezing, whereas in optical fibers,2 guided-acoustic-wave Brillouin-scattering (GAWBS) noise causes difficulties that can be circumvented through low- temperature cooling of the fiber or the use of soli ton pulses.
Squeezed vacuum (SV) generation through third-order optical nonlinearity chi(3)is useful for generating a quantum entanglement pair based on continuous variable. However, third-order nonlinearity is usually small. Therefore, the typical interaction length should be large and ultrashort laser pulses should be used. Then, it becomes important to avoid excess phase noise, especially guided-acoustic-wave Brillouin scattering (GAWBS) in optical fibres. In fact, so far, GAWBS has been preventing from generating SV pulses at sufficiently high squeezing levels. In this paper, we examine insusceptible chi(3)squeezing to GAWBS.
We analyze a new scheme for generating squeezed states in a short semiconductor AlxGa1−xAs waveguide with χ(3) nonlinearity at below half the band-gap energy. We find that for a Gaussian pulse the amount of squeezing achievable is limited by the squeezed-state detection phase mismatch caused by the pump self-phase modulation and is also degraded slightly by the pump–probe phase mismatch that is due to the different nonlinear refractive indices experienced by the pump and the probe beams. We show theoretically that the amount of squeezing observed can be increased by use of either a short pulse or a pulse with matched phase variation as the local oscillator. In a centimeter-long AlxGa1−xAs waveguide more than 85% (8.2 dB) of squeezing potentially can be obtained, limited mainly by two-photon absorption.
We give a quantum theoretical analysis of squeezed light generation via nondegenerate four-wave mixing in an optical fiber. The medium is modeled by an ensemble of anharmonic oscillators and loss is included. We include the coupling to acoustic phonons which gives rise to guided acoustic wave Brillouin scattering (GAWBS). The GAWBS introduces random noise which destroys the squeezing at room temperatures. By cooling the fiber to temperatures ~2 K the GAWBS is substantially reduced and the observation of squeezing becomes possible. A comparison is made with the recent experimental results of Levenson et al.1, 2 A scheme to suppress the GAWBS at room temperatures using a two-frequency pump and difference detection is analyzed and the conditions in which good squeezing results are determined.
Squeezing of 100 ps pulses from a Nd:YAG laser operating at 1.3 μm in a fiber ring has led to a reduction of noise 5 dB below the shot noise level. This reduction occurs in a spectral window between 40 and 90 kHz, a frequency range particularly convenient for instrumentation. Guided acoustic wave Brillouin scattering (GAWBS) was found to be the process limiting our squeezing to 5 dB. The G AWBS measurements will be presented. In the meantime promising ways of reducing the GA WBS have been found and will be described. Fiber ring squeezing permits the reuse of the pump as local oscillator (in principle in full if Faraday rotators are used). A simple system of a fiber ring squeezer with an interferometer in cascade is being constructed to perform a phase measurement below the standard quantum limit. Experimental results will be presented.
The signal to noise ratio of optical systems is ultimately limited by the shot noise due to the Poissonian statistics of the light field. In recent years there has been a considerable effort to find ways to overcome this limitation by generating non-classical light with sub-shot noise statistics. Several groups have demonstrated non-classical light generation in diode lasers or light emitting diodes, and the amplitude fluctuations have been reduced substantially below the shot noise level [1,2]. However, for phase-sensitive measurements it is necessary to generate the more general quadrature squeezed states. Such quadrature squeezed light can be generated either by second-order or third-order nonlinear processes provided the nonlinear material has very low losses at the optical wavelength. To date, the successful generation schemes for quadrature squeezed light have been restricted to atomic vapours, optical fibres, or second-order nonlinear crystals [3]. The possibility of using the large χ(3) nonlinearities of semiconductors at frequencies below the band gap where the optical absorption is small has been discussed in the literature, but so far there have been no successful experiments [4-6]. In this paper we report measurements on GaAs/Al0.3Ga0.7As multiple quantum well waveguides and bulk ZnS in the wavelength range 750 - 890nm. Although we measure much larger nonlinearities in the quantum wells, the strong two-photon absorption is detrimental for squeezed light generation. However, in the ZnS where the laser frequency is below the two- photon absorption edge, we have succeeded in generating quadrature squeezed light in a semiconductor for the first time. On the basis of our results, we would expect that squeezed light could be generated in many other semiconductor systems provided the photon energy is less than half the band-gap energy.
Continuous variable (CV) audio-band frequency squeezed states at the fiber telecommunication wavelength is an important quantum resource for the practical applications based on optical fiber. As is well known, the optical power attenuation and phase diffusion effect of light at 1.3 μm in standard telecommunication fibres are low and small, respectively. The audio-band frequency squeezed light at 1.34 μm can be utilized to realize quantum precision measurement, such as quantum-enhanced sensing in the low-frequency range, laser interferometer for gravitational wave detection. In this paper, CV audio-band frequency vacuum squeezed states at 1.3 μm are experimentally generated by using a type-I degenerate optical parametric oscillator (DOPO) below the threshold. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1.34 μm) Nd:YVO4/LBO laser is used as a pump source for DOPO based on a type-I quasi-phase-matched periodically poled KTiOPO4 (PPKTP) crystal. Mode cleaners with a finesse of 400 and linewidth of 0.75 MHz are used to filter the noise of lasers at 671 nm and 1.34 μm, respectively. The intensity noises of the two lasers reach a shot noise level for analysis frequencies higher than 1.0 MHz and their phase noises reach shot noise level for analysis frequencies higher than 1.3 MHz, respectively. The low noise single-frequency 671 nm laser is utilized as a pump of the DOPO. The threshold power of the DOPO is 450 mW. In order to detect the audio-band frequency vacuum squeezed states, the power of local oscillator of a homodyne detector system is optimized to 60 μupW. Furthermore, the effect of common mode rejection ratio (CMRR) of detectors is discussed in detecting the audio-band frequency vacuum squeezed states. Improvement of CMRR of detectors is a good way to detect the audio-band frequency vacuum squeezed states effectively. When the phase matching temperature of PPKTP crystal is controlled at 53℃ by using a home-made temperature controller and the pump power is 95 mW, the vacuum squeezed states are generated at analysis frequency ranging from 8-100 kHz. A maximum measured squeeze of 5.0 dB is obtained at analysis frequency of 36 kHz. A 3.0 dB squeezed light is obtained at an audio-band frequency of 8 kHz.