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Optical and Quantum Sensing and Precision Metrology II

Numerical analysis of the effects of higher-order modes and loss in waveguide optical parametric amplifiers

Yuki Sano, Yoshitaka Taguchi, Kenichi Oguchi, Yasuyuki Ozeki

Published Mar 3, 2022• 0 citations
Publisher

Abstract

Waveguide-based optical parametric amplification (OPA) is advantageous in generating broadband or pulsed squeezed vacuum. To enhance the squeezing level, it is crucial to clarify its limiting factors. In this study, we develop a numerical method which allows us to derive the squeezing level in waveguide OPA. Based on the developed method, we investigate the effect of the higher-order mode of the pump light, which generates noise due to the OPA between the fundamental mode and the higher-order mode. We also investigate the effect of optical loss in the waveguide. Specifically, we formulated coupled equations that take into account the degenerate OPA between the pump and signal both in the fundamental mode, and the non-degenerate OPA between the pump in the higher-order mode, the signal in the fundamental mode, and the idler in the higher-order mode. By solving the coupled equations, we can express the quantum state as a linear combination of annihilation and creation operators, and the squeezing level can be derived from its coefficients. The analysis allowed us to quantify the effect of OPA on the fundamental and higher-order modes, and the requirement for mode matching of pump light with the fundamental mode. We also analyzed the effect of loss in the waveguide based on the same procedure. The results show the loss in the waveguide to have less effect than the same loss at the output of the waveguide. These results provide important design criteria of waveguide OPA for squeezing.

DOI
10.1117/12.2608430

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Waveguide-based optical parametric amplification (OPA) is advantageous in generating broadband or pulsed squeezed vacuum. To enhance the squeezing level, it is crucial to clarify its limiting factors. In this study, we develop a numerical method which allows us to derive the squeezing level in waveguide OPA. Based on the developed method, we investigate the effect of the higher-order mode of the pump light, which generates noise due to the OPA between the fundamental mode and the higher-order mode. We also investigate the effect of optical loss in the waveguide. Specifically, we formulated coupled equations that take into account the degenerate OPA between the pump and signal both in the fundamental mode, and the non-degenerate OPA between the pump in the higher-order mode, the signal in the fundamental mode, and the idler in the higher-order mode. By solving the coupled equations, we can express the quantum state as a linear combination of annihilation and creation operators, and the squeezing level can be derived from its coefficients. The analysis allowed us to quantify the effect of OPA on the fundamental and higher-order modes, and the requirement for mode matching of pump light with the fundamental mode. We also analyzed the effect of loss in the waveguide based on the same procedure. The results show the loss in the waveguide to have less effect than the same loss at the output of the waveguide. These results provide important design criteria of waveguide OPA for squeezing.
We investigate a limiting factor of waveguide optical parametric amplifiers used for generating broadband squeezed light. We found that spatially non-degenerate optical parametric amplification (OPA) between the fundamental modes and the higher-order modes generates noise, and that this effect can be avoided by detecting the squeezed light with the optimal spatial mode. The analysis is conducted assuming the waveguides with different dimensions, offering valuable insight into the waveguide design for squeezing.
Optical parametric amplifiers (OPAs) configured as phase-sensitive amplifiers (PSAs) can be used for noiseless optical image amplification, generation of non-classical states of light, and, in particular, for multimode squeezed light generation. For all these applications and for effective use of the quantum properties of the multimode PSA, we need to know its independently squeezed (or amplified) eigenmodes. First we present the quantum theory of a spatially multimode traveling-wave PSA pumped by a high-power pump beam with arbitrary spatial profile. To quantitatively identify the Green's function of the PSA, we have developed a semi-analytical coupled-mode-theory of the PSA using 2D Hermite-Gaussian mode expansions of the signal- and pump-beam spatial distributions. By using Green's functions of the classical OPA, we calculate the normallyordered quadrature correlators at its output, which provide complete quantum description of the phase-sensitive OPA and enable determination of its independently squeezed eigenmodes. We find the number of the supported eigenmodes and their shapes for a spatially broadband frequency-degenerate optical parametric amplifier with elliptical Gaussian pump. We conclude by discussing our recent extensions of the coupled-mode-theory to study higher-order pump modes, compact representation of the PSA eigenmodes (especially convenient for experiments), and effect of non-zero phase mismatch on the PSA eigenmodes. We expect that the results from our model can be used for optimum mode matching in phase-sensitive image amplification and multimode squeezing generation.
We report generation of 10.1\pm0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al., Appl. Phys. Lett. 122, 234003 (2023)], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a novel phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with high-signal-to-noise ratio and suppressing degradation of squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12\% to 8\%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards realization of fault-tolerant ultra-fast universal optical quantum computation.
We analyze the influence factors that affect the observed degree of squeezing in generation and detection stages theoretically, including the fluctuations of pump power, the intra-cavity loss of the optical parametric amplifier (OPA) cavity , the phase fluctuation on balanced homodyning stage, the propagation efficiency, the mode matching efficiency, the out-put mirror transmission of the OPA cavity and the quantum efficiency of the detector. The theoretical results show that different loss sources appearing at various stages of the squeezed light generation and detection are far more dominant and in the end limit the squeezing level achievable. The analysis of these parameters will be extremely useful for subsequent experiments.
We report detection of 10.1 ± 0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report, where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al ., Appl. Phys. Lett. 122 , 234003 ( 2023 ) 10.1063/5.0144385 ], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with a high signal-to-noise ratio and suppressing the degradation of the squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12% to 8%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards the realization of fault-tolerant ultra-fast universal optical quantum computation.
Optical parametric amplification is expected as a key technique for high-speed optical fiber communication and optical quantum computing.We developed a high-gain and broadband optical parametric amplifier (OPA) module by using a periodically poled lithium niobate (PPLN) waveguide, the single-mode core of which is directly bonded to a lithium tantalate substrate.The OPA is assembled as a pump-combiner-integrated module with four optical-fiber input/output ports.By improving the fabrication processes of the PPLN waveguide and the assembly techniques of OPA modules, we achieved over-30-dB phase sensitive amplification with a continuous-wave pump.By using the PPLN waveguide, we successfully obtained over-6-dB squeezed light, the spectral bandwidth of which was 2.5 THz.This high-gain broadband OPA module will help build an advanced information society.