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The Review of Laser Engineering

New Method for Pulse Squeezed Light Generation.

Jun Zhou

Published Jan 1, 1995• 0 citations
Publisher

Abstract

A new method has been developed for pulse squeezed light generation in parametric process by using low repetition pulse laser. With this new technique, it becomes very easy to realize the condition for obtaining substantial squeezing, i. e. the damping constant for pump field should be much smaller than that for the parametric signal field. This condition, however, was difficult to be realized with conventional experimental setup by using low repetition pulse laser. Semiclassical approach indicates that deep degree squeezing may be obtained over a wide frequency range by increasing the ratio of the dumping constant for signal field to that for the pump field in the new experimental setup. At the same time, the conversion efficiency of the second-harmonic generation can also be increased remarkably by this new method in the squeezed light generation process.

DOI
10.2184/lsj.23.1131

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A new method has been developed for pulse squeezed light generation in parametric process by using low repetition pulse laser. With this new technique, it becomes very easy to realize the condition for obtaining substantial squeezing, i. e. the damping constant for pump field should be much smaller than that for the parametric signal field. This condition, however, was difficult to be realized with conventional experimental setup by using low repetition pulse laser. Semiclassical approach indicates that deep degree squeezing may be obtained over a wide frequency range by increasing the ratio of the dumping constant for signal field to that for the pump field in the new experimental setup. At the same time, the conversion efficiency of the second-harmonic generation can also be increased remarkably by this new method in the squeezed light generation process.
Squeezed light is expected to enable new quantum metrology and quantum computation applications, but the very high degree of experimental control required in practice places severe demands on the physical devices used to generate it. As ultrafast squeezed light sources are reaching an unprecedented degree of brightness and modal control, there is a strong interest in developing methods for their experimental characterization and theoretical understanding. In this thesis, we demonstrate both a new application and a new characterization technique for pulsed squeezed light. In the first part of the thesis, we present a new metrological application of pulsed squeezed light: quantum-enhanced label-free nonlinear microscopy. Stimulated emission microscopy (SEM) is a state-of-the-art pump-probe microscopy technique whose sensitivity is ultimately limited by shot noise in the probe beam. We propose to overcome this fundamental limitation by using a sub-Poissonian probe generated via optical parametric amplification. We demonstrate an intensity-squeezed sub-picosecond pulsed probe, and a simple stimulated emission microscope. Using this squeezed probe, sub-Poissonian statistics are observed in the SEM signal. After this proof-of-principle experiment, we aim to provide an improved design for a source of intensity-squeezed light. First, we develop a theoretical framework to simulate the photon statistics of a general sub-Poissonian source, considering pulsed and focused pump and seed fields in a unified spatio-temporal description. Next, we use this framework to suggest possible improvements to our previous squeezed probe. In the second part of this thesis, we provide new theoretical and experimental tools to analyze the high gain regime of pulsed spontaneous parametric down-conversion (SPDC). We study a waveguided source capable of generating twin-beams with tens of photons in a single spatio-temporal mode. We develop a framework to efficiently simulate SPDC for an arbitrary nonlinear gain, including the effect of parasitic nonlinearities such as self-phase modulation (SPM) of the pump and cross-phase modulation (XPM) between pump and twin-beams. We experimentally characterize our source using a new technique based on stimulated difference-frequency generation (DFG). In a type-II down-conversion process, we measure the frequency generation produced not only in the orthogonal polarization to the seeding beam, but also in its same polarization, which we interpret as cascaded difference-frequency generation. We argue that the additional information obtained in this measurement allows for a self-referenced estimation of the squeezing gain. Using these measurements, we accurately fit our theoretical model, revealing that, in the high gain regime, parasitic nonlinearities strongly affect the spectral structure of the twin-beams. We propose a simple strategy to reduce the impact of these nonlinearities on the modal structure of SPDC.
Squeezed light has evolved into a powerful tool for quantum technology, ranging from quantum-enhanced sensing and quantum-state engineering based on partial postselection techniques.The pulsed generation of squeezed light is of particular interest, as it can provide accurate time stamps and a physically defined temporal mode, which are highly preferred in complex communication networks and large-scale information processing.However, the multimode feature of pulsed squeezing limits the purity of the output state, negatively impacting its application in quantum technology.Previous demonstrations and analysis of pulsed squeezing focus on single-pass configurations and synchronously pumped free-space cavities.In this paper, we propose a different approach to generate pulsed squeezing with high temporal purity, where a parametric down-conversion process in integrated photonic cavities is pumped by a single-pass pulse.We show that the effective mode number of the output pulsed squeezing approaches unity.Such a high-purity squeezed light can be realized with broad parameters and a low pump power, providing a robust approach to generating large-scale quantum resources.
The degree of squeezing generated by parametric downconversion or four-wave mixing can be limited by the ratio of the nonlinear coupling to the linear losses in many practical systems. This ratio can be enhanced by increasing the pump power used for the nonlinear coupling in many solid nonlinear systems where the linear losses are not directly related to the nonlinear squeezing process. Pulsed pump sources are attractive for optimizing the nonlinear coupling-to-loss ratio. We have shown1 that homodyne detection can be used to detect the pulsed squeezing generated by a mode-locked pulse train from the pump laser. A portion of the pump beam is used as the local oscillator which stroboscopically samples portions of the squeezed light pulse train. The amount of squeezing which can be measured as a function of the relative durations of the pump and local oscillator pulses is shown. Progress of recent experiments using both four-wave mixing and parametric downconversion is described.
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.
Frequency-degenerate pulsed type-I parametric downconversion is a widely used source of squeezed light for numerous quantum optical applications. However, this source is typically spectrally multimode, and the generated squeezing is distributed between many spectral modes with a limited degree of squeezing per mode. We show that in a nonlinear crystal, where the condition of complete group velocity matching (GVM) for the pump and the signal is satisfied, the number of generated modes may be as low as two or three modes. We illustrate the general theory with the example of the MgO-doped lithium niobate crystal pumped at 775 nm and generating squeezed light at 1.55 µm. Our model includes the derivation of the degree of squeezing from the properties of the pump and the crystal and shows that 12 dB of squeezing can be obtained in a periodically poled crystal at a length of 80 mm.
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.