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Nonlinear Guided Waves and Their Applications

Parametric Amplification and Squeezing in Quasi-Phase-Matched Waveguides

Matthew E. Anderson, M. Beck, Michael G. Raymer, John D. Bierlein

Published Jan 1, 1995• 0 citations
Publisher

Abstract

Nonlinear optical waveguides hold promise as parametric amplifiers and as sources of quadrature-squeezed light. The confinement of the pump field drastically increases the efficiency of the nonlinear conversion. In addition, the control of the spatial mode profiles by the waveguide geometry should diminish the effect of gain-induced diffraction, which has been seen to limit the amount of attainable squeezing in bulk parametric crystals.1 Using subpicosecond laser pulses, we have observed parametric gain as well as quadrature squeezing in a quasi-phase matched KTP waveguide.

DOI
10.1364/nlgw.1995.nthc3

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Physical Review A•Aug 1, 1997•23 citations
Nonlinear optical waveguides hold promise as parametric amplifiers and as sources of quadrature-squeezed light. The confinement of the pump field drastically increases the efficiency of the nonlinear conversion. In addition, the control of the spatial mode profiles by the waveguide geometry should diminish the effect of gain-induced diffraction, which has been seen to limit the amount of attainable squeezing in bulk parametric crystals.1 Using subpicosecond laser pulses, we have observed parametric gain as well as quadrature squeezing in a quasi-phase matched KTP waveguide.
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.
Parametric down-conversion of light (PDC) in a quasi-phase-matched (QPM) nonlinear crystal can be spectrally very broad when the spatial frequency of periodical poling is linearly chirped. The present day technology makes possible the fabrication of chirped QPM crystals phase-matched for PDC in all optical spectrum. In the low-gain regime this allows for generation of biphotons with the correlation time of the order of a single optical cycle. A natural next step is the study of the PDC light produced by such a crystal in the high-gain regime, where PDC is widely used as a source of squeezed states of light. Up to now only a classical treatment of this regime was undertaken. In the present work we demonstrate a possibility to generate single-cycle squeezing in a chirped QPM nonlinear crystal with squeezing spectrum covering almost all the optical octave.
It is shown that coherent quantum simultons (simultaneous solitary waves at two different frequencies) can undergo quadrature-phase squeezing as they propagate through a dispersive ${\ensuremath{\chi}}^{(2)}$ waveguide. This requires a treatment of the coupled quantized fields including a quantized depleted pump field. A technique involving nonlinear stochastic parabolic partial differential equations using a nondiagonal coherent state representation in combination with an exact Wigner representation on a reduced phase space is outlined. We explicitly demonstrate that group-velocity matched ${\ensuremath{\chi}}^{(2)}$ waveguides which exhibit collinear propagation can produce quadrature-phase squeezed simultons. Quasi-phase-matched KTP waveguides, even with their large group-velocity mismatch between fundamental and second harmonic at 425 nm, can produce 3 dB squeezed bright pulses at 850 nm in the large phase-mismatch regime. This can be improved to more than 6 dB by using group-velocity matched waveguides.