Xin Mi

Saturation effect in nondegenerate two-wave mixing

Abstract Saturation effects in nondegenerate two-wave mixing are analyzed theoretically for saturable absorbers. A gain saturation effect in the process of energy transfer has been observed. The fact that the frequency difference of two incident waves at which the gain reaches maximum varies with the pump intensity has been predicted and demonstrated. It has also been demonstrated that parameters such as T 1 , I s and χ (3) can be determined sucessfully by nondegenerate two-wave mixing experiments.

Time-delayed laser-induced double gratings

We have employed chaotic and phase-diffusion models to study the effect of laser coherence on time-delayed laser-induced double gratings (TDLIDG). It is found that, although the temporal behavior of the four-wave mixing signal depends on the stochastic properties of the lasers and the relaxation time of the grating, the modulation of the signal is always damped out when the relative time delay between two pump beams is much longer than the laser coherence time. We performed a TDLIDG experiment mediated by thermal effects to study the temporal behavior of the modulation, and we obtained the frequency difference between two light beams with an accuracy limited by the laser linewidth. We also studied the influence of the phase change in the optical path on TDLIDG.

Observation of the beat between two independent light sources by a method of time-delayed laser-induced double gratings

A novel method of time-delayed laser-induced double gratings has been proposed to study the beat between two independent light sources. Our experimental results indicate that we can obtain good interference patterns even when the coherence time of the laser source is only a few picoseconds. This new technique can be used to measure the frequency difference between two laser fields.

NOISE-REDUCTION IN IMAGE AMPLIFICATION IN PHOTOREFRACTIVE BATIO3

Abstract Based on the feature that a signal grating has faster formation rate than noise gratings, we have demonstrated that in the presence of a beam with a frequency different from that of the pump beam, it is possible to reduce the optical noise while maintaining high gain for the two-wave mixing image amplification.

Distinguishing molecular-reorientation gratings from thermal gratings by a time-delayed method

We demonstrate a novel time-delayed method that employs the partially coherent properties of pump beams to distinguish molecular-reorientation gratings from thermal gratings. An extension of this method to the case of population grating is pointed out.

Four-level ultrafast modulation spectroscopy

Abstract Based on the interference between two two-photon processes, we demonstrated a four-level ultrafast modulation spectroscopy in Na vapor, where 3S, 3P, 5S and 4D formed a four-level system. We obtained signal modulation with period of 25 fs, corresponding to the splitting between 5S and 4D.

Ultrafast modulation spectroscopy in cascading three-level and four-level systems

We study theoretically ultrafast modulation spectroscopy in Doppler-broadened cascading three-level and four-level systems. The pump beams consist of two frequency components that drive two-pathway excitation. The four-wave mixing (FWM) signal exhibits damping oscillation as the relative time delay τ between the two pump beams is varied, from which the resonant frequencies between energy levels can be deduced. Furthermore, Doppler-free precision in the measurement of the resonant frequencies can be achieved when the pump beams have either narrow-band or broadband linewidths. We study two-photon resonant FWM induced by time-delayed correlated fluctuating fields in a Doppler-broadened cascading three-level system. It is related intrinsically to the sum-frequency trilevel echo when the pump beams have broadband linewidths. However, unlike the corresponding incoherent-light stimulated photon echo, the maximum of the FWM signal is shifted from τ≃0. If we consider the fluctuations of the pump beams, the FWM signal exhibits damping oscillation when the laser frequency is off resonant from the two-photon transition.

Phase-sensitive detection of Raman-enhanced nondegenerate four-wave mixing

Based on the polarization interference between two four-wave mixing (FWM) processes, we demonstrate a phase-sensitive method for studying Raman-enhanced nondegenerate four-wave mixing (RENFWM). The reference beam is another FWM signal, which propagates along the same optical path as the RENFWM signal. We have employed this method to study the phase dispersion of the third-order susceptibility χ(3) and for optical heterodyne detection of the RENFWM signal.

OBSERVATION OF BEATING BETWEEN CW LASERS BY TIME-DELAYED LASER-INDUCED DOUBLE GRATINGS MEDIATED BY PHOTOREFRACTIVE EFFECT

We have demonstrated in KNbO3: Fe that the time-delayed laser-induced double gratings mediated by photorefractive effect can be employed to measure the frequency difference between two cw light sources. Due to the highly sensitive photorefractive effect in KNbO3: Fe the power of the incident beams can be less than 1 mW. With this technique, we have observed beating between the blue (488 nm) and the green (514.5 nm) lines of an argon ion laser, which shows a beat note of 31.2 fs period.

Beam-size effect in time-delayed laser-induced double gratings

We have studied theoretically the effect of the probe-beam size on time-delayed laser-induced double gratings by assuming that the probe beam is described as a Gaussian beam. LettingEs1 andEs2 be the Four-Wave Mixing (FWM) signals originating from the diffraction of the probe beam by the gratings, the condition for the occurrence of the FWM signal modulation is that the divergence angle ofEs1 andEs2 is larger than half of the intersection angle between the propagation directions ofEs1 andEs2. We have also proposed methods to increase the modulation contrast.