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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Reconstruction of blurred images via controlled spatial-solitons formation

Alberto Bramati, Walter Chinaglia, Stefano Minardi, P. Di Trapani

Published Nov 11, 2002• 0 citations
Publisher

Abstract

Summary form only given. Quadratic non-linear interaction is known to support the process of spatial soliton formation. A very simple set-up consists of a single pass, single crystal, quantum noise optical parametric amplifier (OPA) pumped by a focussed beam. The diffraction-free propagation achieved in the solitary-wave regime implies that the beam-waist plane of the pump beam can arbitrarily move inside the crystal without any relevant change in the size of the mutually trapped pump, signal and idler beams at the crystal output face. We present a very useful application of this property to the reconstruction of digital images. The experimental set-up is based on a 30 mm temperature tuned lithium triborate (LBO) crystal, operated in noncritical type I phase matching and pumped by a 15 ps, 527 nm pulse from an amplified frequency doubled Nd:glass laser. We set the crystal temperature to 107/spl deg/C to obtain generation at 1.8 /spl mu/m (idler) and 0.75 /spl mu/m (signal), ensuring minimum group velocity mismatch among interacting pulses.

DOI
10.1109/cleoe.2000.909702

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Summary form only given. Quadratic non-linear interaction is known to support the process of spatial soliton formation. A very simple set-up consists of a single pass, single crystal, quantum noise optical parametric amplifier (OPA) pumped by a focussed beam. The diffraction-free propagation achieved in the solitary-wave regime implies that the beam-waist plane of the pump beam can arbitrarily move inside the crystal without any relevant change in the size of the mutually trapped pump, signal and idler beams at the crystal output face. We present a very useful application of this property to the reconstruction of digital images. The experimental set-up is based on a 30 mm temperature tuned lithium triborate (LBO) crystal, operated in noncritical type I phase matching and pumped by a 15 ps, 527 nm pulse from an amplified frequency doubled Nd:glass laser. We set the crystal temperature to 107/spl deg/C to obtain generation at 1.8 /spl mu/m (idler) and 0.75 /spl mu/m (signal), ensuring minimum group velocity mismatch among interacting pulses.
In a single 15-mm lithium triborate crystal, operated at high gain and strong diffraction, we obtained the spatial mode locking of the quantum noise, its amplification up to the pump level, and quasisoliton propagation of the mutually trapped beams. Visible pulses of 1 $\ensuremath{\mu}\mathrm{J}$ 1.5 ps were converted, with 5050%50 efficiency, into diffraction-limited superfluorescence, tunable in the range 0.67-- $2.5\ensuremath{\mu}\mathrm{m}$.
The nondegenerate optical parametric amplifier (OPA) has many uses for investigating quantum fluctuations of light. Among them are EPR-type quadrature-phase correlations,1 QND measurement,2 and phase-sensitive quantum amplification.3 To study these problems, a reliable high-gain low-loss optical parametric amplifier (OPA) is crucial. Although OPAs operated at 1064 nm with KTP crystals pumped by a frequency doubled YAG laser exist,1 their performance is compromised because of problems with beam walk-off and polarization mixing for pairs of critically phase-matched KTP crystals. To avoid such problems, we have constructed a different nondegenerate OPA, which is operated at 1080 nm and consists of a single KTP crystal with type II noncritical phase matching. The pump beam at 540 nm for the amplifier is obtained from intracavity frequency doubling of a single-mode Nd: YAP laser with a similar KTP crystal as in the OPA.
Summary form only given.We describe the generation and detection of broadband squeezed states at 1047 nm in a lithium triborate (LiB/sub 3/O/sub 5/) degenerate optical parametric amplifier (DOPA). The light source used is a laser-diode-pumped additive pulse mode-locked Nd:LiYF/sub 4/ laser, which produces pulses 3 ps in duration at repetition rate of 140 MHz, and the average output power is 1 W. The fundamental light is frequency doubled in an external ring enhancement cavity, and the resulting high peak power of the doubled light means that it is possible to produce significant squeezing even in a single-pass or traveling-wave parametric amplifier.
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 have constructed a mode-locked optical parametric oscillator (OPO), synchronously pumped by the second harmonic of a continuously mode-locked Nd:YAG laser and based on type II phase matching in a KTP crystal. The two subharmonics (signal and idler waves) are nearly degenerate in frequency but are distinct orthogonal polarization states, and the oscillator is thus a nondegenerate OPO. Above threshold, the intensities of the signal and idler output beams should be highly correlated, as recently demonstrated for a similar single-frequency OPO.1 Ideally, the quantum noise on the corresponding photocurrent difference signal is completely suppressed for frequencies that are small compared to the OPO cavity bandwidth. Since this configuration produces intense light beams with potentially very good noise suppression, it is an important prototype for possible applications, e.g., to spectroscopy. Initial results with our OPO show an excess uncorrelated polarization noise, possibly due to depolarized light scattering in the KTP crystal. When the OPO cavity is stabilized with the signal and idler degenerate in frequency, they can be mixed to form a superposition by manipulating their polarization. When the oscillator is pumped below threshold, a pulsed squeezed vacuum state is produced in two such mixtures.2 When the polarizations are mixed inside the OPO cavity by a Faraday rotator, the backaction evasion necessary for a quantum nondemolition (QND) measurement of one of the field quadratures of an externally injected beam is obtained.3 The pulsed local oscillator used in observing the squeezing or QND effects is derived from the fundamental output of the pump laser. Progress in implementing these experiments with our mode-locked OPO is described as well as possible applications to time-domain measurements with noise below the shot-noise limit.
In high-precision optical measurements, squeezed vacuum states are a promising resource for reducing the shot noise. To utilize a squeezed vacuum, it is important to lock the phase of the local oscillator (LO) to the squeezed light. The coherent control sideband (CCSB) scheme has been established for the precise phase locking, while the previous CCSB scheme was designed for the squeezed vacuum generated with an optical parametric oscillator (OPO). Thus the previous CCSB scheme is not applicable to squeezing by a single-pass optical parametric amplifier (OPA), which is attractive for generating broadband squeezed vacuum states. In this study, we propose a variant of CCSB scheme, which is applicable to the squeezing by single-pass OPA. In this scheme, we inject pump light and frequency-shifted signal light into an OPA crystal in the same way as the previous CCSB scheme. The parametric process in the OPA crystal generates a squeezed vacuum, amplifies the signal light, generates an idler light, and causes the pump depletion reflecting the interference of the amplified signal light and the idler light. Through the lock-in detection of the pump depletion, we can phase-lock the injected signal light to the pump light. Then, after the heterodyne detection of the signal and the idler light, we get the error signal of LO and realize the precise phase locking of LO to the squeezed quadrature. We show the feasibility of the proposed scheme by deriving the signal-to-noise ratio (SNR) of the modulated pump signal. We experimentally demonstrate the proposed scheme on pulsed squeezing by a single-pass OPA.