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Annual Meeting Optical Society of America

Direct detection of photon bunching

Yujiang Qu, Surendra Singh

Published Jan 1, 1991• 0 citations
Publisher

Abstract

Second-harmonic generation (SHG) has been predicted to be very sensitive to photon bunching. Because of its two-photon nature SHG would be more efficient with bunched than with random or antibunched photon sequences of the same mean intensity. We use this phenomenon to demonstrate the bunching of photons in time. The fundamental beam, derived from a laser operating near threshold, was focused into a 90° temperature phase matched nonlinear crystal. The operating point of the laser was varied in the neighborhood of threshold, and a feedback loop stabilized its intensity at each operating point. It is well known that the character of laser light changes in passing the threshold of laser action. Underlying photon sequences are bunched below threshold and random above threshold.

DOI
10.1364/oam.1991.wl16

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Second-harmonic generation (SHG) has been predicted to be very sensitive to photon bunching. Because of its two-photon nature SHG would be more efficient with bunched than with random or antibunched photon sequences of the same mean intensity. We use this phenomenon to demonstrate the bunching of photons in time. The fundamental beam, derived from a laser operating near threshold, was focused into a 90° temperature phase matched nonlinear crystal. The operating point of the laser was varied in the neighborhood of threshold, and a feedback loop stabilized its intensity at each operating point. It is well known that the character of laser light changes in passing the threshold of laser action. Underlying photon sequences are bunched below threshold and random above threshold.
Second-harmonic generation in multimode laser beams is studied experimentally. A multimode laser beam derived from a laser oscillating simultaneously in many longitudinal modes that have the same transverse profile (TEM00{\mathrm{TEM}}_{00}TEM00​) is focused into a nonlinear crystal. Noncritical phase matching is utilized to generate the second harmonic. The number of longitudinal modes is systematically varied from 1 to 7 and the dependence of the efficiency of second-harmonic generation on the number of modes in the fundamental beam is studied. Experimental results clearly demonstrate the increasing degree of photon bunching in the fundamental beam as the number of modes increases. Experimental results are discussed and compared with the predictions of theoretical models.
Statistical properties of second-harmonic photons generated by a single-mode laser operating near threshold are investigated by means of photoelectric counting measurements. It is found that second-harmonic photons display superthermal fluctuations below threshold where pump statistics are thermal. Above threshold, as the pump field becomes coherent, second-harmonic photons approach Poisson statistics. These measurements, interpreted as two-photon statistics of the fundamental beam, provide experimental evidence for the dependence of the fluctuation properties of a light beam on the process of detection.
Temporal photon bunching is the phenomena in which photons tend to cluster together on a short time scale. This effect can be quantified using the second order coherenceg(2)(τ)g^{(2)}(\tau)g(2)(τ)of the light. Photon bunching has been utilized in LIDAR, ghost imaging and simulating star light. In these applications, bunched light is typically generated inefficiently by sending coherent laser light through a phase randomization media, such as a rotating ground glass [1] or a liquid suspension of microspheres [2].
We report the generation of sub-Poissonian pulses of light by means of traveling-wave second-harmonic generation (SHG) in a type II phase-matched nonlinear crystal. On direct detection, the quantum noise on the output light, which is polarized orthogonally to the input fundamental field, is measured to be below the shotnoise limit by as much as 0.3 +/- 0.2 dB (6 +/- 4%). We show that the input fundamental-power dependence of the measured Fano factor, direct-detection noise as a fraction of the shot-noise limit, is in qualitative agreement with the quantum theory of SHG.
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.