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Physical Review A

Parametric amplification and squeezing of a mode-locked pulse train: A comparison of MgO:LiNbO3 with bulk periodically poled LiNbO3

E. M. Daly, A. I. Ferguson

Published Sep 13, 2000• 9 citations
Publisher

Abstract

We report on an initial squeezing of 3.3 dB below the shot noise level, generated in bulk, quasi-phase-matched lithium niobate. This squeezing was generated by degenerate parametric amplification, at 1047 nm, of a mode-locked pulse train. We present a detailed comparison of the process performed in both the quasi-phase-matched material and in ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3},$ which was phase matched using conventional birefringent methods. The single-pass gain measured was 2.5 from ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}$ and 3.9 from periodically poled lithium niobate (PPLN), although the former was affected by photorefractive damage. We directly recorded $11%$ of noise reduction in the vacuum using the PPLN parametric amplifier, purely because the detection efficiency was so low in our experiment. The experimental data was fitted to a model which includes the initial squeezing level and the homodyne detector efficiency as free parameters. The observed squeezing agrees well with the a total detection efficiency of just $16%$ and an initial squeezing level of 3.3 dB (53%) in PPLN.

DOI
10.1103/physreva.62.043807

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Conference on Lasers and Electro-Optics•Jan 1, 2006•0 citations
We report on an initial squeezing of 3.3 dB below the shot noise level, generated in bulk, quasi-phase-matched lithium niobate. This squeezing was generated by degenerate parametric amplification, at 1047 nm, of a mode-locked pulse train. We present a detailed comparison of the process performed in both the quasi-phase-matched material and in MgO:LiNbO3,{\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3},MgO:LiNbO3​, which was phase matched using conventional birefringent methods. The single-pass gain measured was 2.5 from MgO:LiNbO3{\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}MgO:LiNbO3​ and 3.9 from periodically poled lithium niobate (PPLN), although the former was affected by photorefractive damage. We directly recorded 1111%11 of noise reduction in the vacuum using the PPLN parametric amplifier, purely because the detection efficiency was so low in our experiment. The experimental data was fitted to a model which includes the initial squeezing level and the homodyne detector efficiency as free parameters. The observed squeezing agrees well with the a total detection efficiency of just 1616%16 and an initial squeezing level of 3.3 dB (53%) in PPLN.
An optical parametric amplifier (OPA) has been demonstrated in bulk, periodically poled lithium niobate and is shown to operate with a noise figure well below the classical limit. In contrast to conventional OPA's, this device uses quasi-phase matching to provide the coupling between the pump and the signal. Comparison of the measured performance with that of a theoretical model reveals that the main intrinsic contribution to the output noise is due to spatial and temporal mode mixing, which arises as a consequence of tight focusing of the incident beams. Factors that affect the performance of this amplifier are identified theoretically and their relative importance investigated for both amplification and squeezing.
We report traveling-wave quadrature squeezing at 1064 nm in a quasi-phase-matched LiNbO(3) waveguide, avoiding the gain-induced diffraction encountered in bulk squeezing experiments. The 10-mm-long singlemode waveguide parametric amplifier exhibited a gain of 1.9, averaged over the 20-ps mode-locked pulses, with only 0.5 W of peak pump power at 532 nm. Independent waveguides were employed for the degenerate parametric amplifier squeezer and for the second-harmonic generator that produced the pump for the parametric amplifier. We measured phase-sensitive amplification versus pump power and found close agreement with theory. The observed 14% squeezing correlates closely with the predicted value, based on the measured phase-sensitive amplification and 40% overall detection efficiency. Impedance-matched electrically resonant detection was used to boost our squeezing measurement signals substantially above the background level.
Although noiseless optical amplification has been demonstrated in bulk KTP crystals, and is well understood, there have been no observations of noiseless optical amplification in bulk quasi-phase-matched (QPM) systems. That such an amplifier is possible was predicted recently and indeed squeezing has already been observed in QPM waveguides (using KTP and LiNbO3). QPM materials are attractive for use in quantum-optical devices because they give access to certain (large) components of the nonlinear susceptibility tensor that cannot be used in conventional birefringent phase-matching schemes. They also extend the range of materials accessible for use in parametric amplifiers to include some low-birefringence crystals exhibiting large nonlinearities. Even with the technological advantages offered by certain QPM crystals, efforts to find practical applications for optical parametric amplifiers are hampered by the need for enormous pump-field strengths. The obvious solution to the problem of the pump-field requirement is to confine the incident energy to a narrow interaction region thereby increasing the energy density. However, this approach can only be realised at the expense of sacrificing the homogeneity of the parametric interaction between pump and signal waves. This in turn causes vacuum field fluctuations from parasitic modes to be mixed into the signal mode thereby degrading the signal to noise ratio and negating the advantage of using an optical parametric amplifier. In the experiments presented here, optical signals have been amplified with up to 9dB of gain, yet with a noise figure (NF) of only 1.09, as compared to the minimum possible NF of a classical amplifier with the same gain which would equal 1.86. Furthermore, the interaction is achieved with tightly focused incident fields in a sample of bulk QPM lithium niobate. The entire interaction region is only 200µm thick and 2.8mm long. This performance was achieved with the aid of a theoretical model m which the effect of wavefront curvature and mode distortion are treated within a quantised mode framework using a spatial mode basis. Such a simulation permits the fundamental origin of the excess quantum noise to be easily traced to zero-point fluctuations of high-order spatial modes which couple to the signal mode via the parametric interaction. Furthermore the simulation enabled an experimental geometry to be selected in which an optimal compromise is made between wavefront matching bump beam smaller than signal) and pump-homogeneity (pump larger than signal), so that the overall noise figure remains well into the quantum-performance region.
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.