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Journal of the Optical Society of America B

High-efficiency blue-light generation by frequency doubling of picosecond pulses in a thick KNbO_3 crystal

Yong-qing Li, Dorel Guzun, Greg J. Salamo, Min Xiao

Published Jun 1, 2003• 8 citations
Publisher

Abstract

We report an experimental demonstration of highly efficient single-pass second-harmonic generation from 859 nm to 429.5 nm with picosecond pulses in a thick KNbO3 crystal. Both the conversion-efficiency and quantum-noise properties of the generated blue pulses are measured at various pump intensities under a strong focusing condition. We find that the variation of the conversion efficiency of the picosecond second-harmonic generation is an oscillatory function of the input pump intensity (with a maximum efficiency of 56.5%) and is sensitive to the position of the input beam focus in the crystal. The quantum noise on the blue beam can be reduced below the shot-noise limit by 20% at low input power.

DOI
10.1364/josab.20.001285

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We report an experimental demonstration of highly efficient single-pass second-harmonic generation from 859 nm to 429.5 nm with picosecond pulses in a thick KNbO3 crystal. Both the conversion-efficiency and quantum-noise properties of the generated blue pulses are measured at various pump intensities under a strong focusing condition. We find that the variation of the conversion efficiency of the picosecond second-harmonic generation is an oscillatory function of the input pump intensity (with a maximum efficiency of 56.5%) and is sensitive to the position of the input beam focus in the crystal. The quantum noise on the blue beam can be reduced below the shot-noise limit by 20% at low input power.
The quantum-noise properties of single-pass second-harmonic blue-light generation with femtosecond pulses have been measured. A conversion efficiency of as much as 63.5% of second-harmonic generation at 428.8 nm was observed in a KNbO(3) crystal with femtosecond (130-fs) pulses with wavelengths centered at 857.6 nm. The quantum noise on the generated blue light was measured to be 1.0 dB (1.4 dB of squeezing inferred) below the shot-noise limit. The noise reduction was found to be sensitive to the average power and the center wavelength of the input fundamental pulses under the condition of strong pump depletion.
Summary form only given. Reports quantum noise measurements in a high-efficiency single-pass sum-frequency generation (SFG) system with a mode-locked femtosecond pulse laser in a noncritical type-I phase-matched KNbO/sub 3/ crystal. The conversion efficiency from fundamental pulses (with a center wavelength at 857.7 nm and a broad spectral linewidth >12 nm) to blue light pulses (with a wavelength at 428.8 nm and a narrow spectral linewidth <9.5 nm) can be greater than 60%. Since the phase-matching width of the KNbO/sub 3/ crystal is only about 0.7 nm, the main mechanism for the blue light generation is the SFG from the pairs of frequency components of the fundamental pulses. The quantum noise on the blue light was observed to be 1.0 dB (1.4 dB inferred) below the shot-noise limit in certain condition. To our knowledge, this is the first quantum noise measurement done in strong pump-depletion region.
We experimentally present a high-efficiency generation of low-noise 447 nm laser by frequency doubling of a Ti:sapphire laser in an external ring cavity with a PPKTP crystal. The maximum blue laser power of 308 mW has been obtained and the corresponding conversion efficiency can reach 70%, when 440 mW infrared laser is injected. Moreover, the noise of the resulting blue laser can reach the quantum noise limit when the analyzing frequency is above 1.5 MHz. Our system provides an ideal pump field for an optical parametric amplifier for the generation of non-classical states of light matching matter absorption line.
We present a new heterodyne technique for detecting weak signals beyond the standard shot-noise limited by employing amplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from a highly efficient (up to 60% conversion efficiency) single-pass second-harmonic generation in a nonlinear crystal (KNbO3) pumped by femtosecond (130 fs) pulses at 857.7 nm. The weak signal field is introduced from the entrance port of the crystal to combine with the generated squeezed LO field. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratio beyond the shot-noise limit is directly observed for this new heterodyne detection scheme. Practical applications of proposed technique in laser radar systems are discussed.
The effect of the quantum properties of light on nonlinear processes has been well studied theoretically. It has been shown that the efficiency of n-photon nonlinear processes in many cases scales as the normalized n-th order correlation function. For light with high intensity correlation function, the efficiency of the n-th harmonic generation will be considerably higher than for coherent light. The experimental observation of this effect remained difficult until recently, because of the absence of bright sources with strong and fast intensity fluctuations. For the experimental demonstration of statistical effects in optical harmonic generation we use as a pump the radiation of high-gain parametric down conversion. Such light shows quantum properties (e.g. quadrature or two-mode squeezing) and has large number of photons in one mode. The normalized n-th order correlation function for this light is (2n - 1)!!, which makes it more attractive for nonlinear processes than both coherent and thermal light. For the generation of optical harmonics we used broadband parametric down conversion around frequency-degeneracy (1600 nm) produced in 1cm BBO crystal from Ti:Sapphire laser (800 nm, 1.6ps, 5kHz, 3W mean intensity). Due to spectral filtering and post-selection technique we could vary the statistics of light from coherent to super-bunched, which allowed us to demonstrate the efficiency enhancement for second-, third-, and fourth-harmonic generation. The obtained experimental results show a good agreement with the theory.
We demonstrate the generation of 20.2 W of continuous-wave, low-intensity noise, single-frequency light at 840 nm, obtained via sum-frequency mixing of two high-power infrared fiber laser systems in an MgO-doped periodically poled lithium niobate (ppLN) crystal. Subsequent cavity-enhanced frequency doubling to 420 nm yields an output power of 15.2 W. Respective conversion efficiencies of 48% and 79% are achieved for the sum-frequency and second-harmonic stages, with good beam quality (M 2 < 1.3), low intensity noise (<0.05% RMS [1 kHz–10 MHz]), and long-term continuous operation. Relative intensity noise and frequency noise of the generated signals are experimentally characterized.