摘要
本文讨论用ps光脉冲简并四波混频测量非线性介质弛豫率和在适度或强的相位调制情况下测量激光脉冲宽度的方法，通过用计算机模拟及与实验结果比较，得出结论：对tp》τr和tp《τr两种极限情况，该方法是足够精确的．对τr／tp的中间情况，该方法只能给出一个估计．
In this paper, a method for estimating the relaxation time of nonlinear media, or the ratio of the relaxation time and the laser pulse duration in the presence of moderate or strong phase modulation, is proposed. its principle is based upon degenerate four-wave mixing (DFWM) with ps light pulses from mode-locked Nd: YAG laser. At first,the theoretical background of this method is discussed. DFWM produced by picosecond pulses which are characterized by duration tp,coherence time re and magnitude of phase modulation a is considered. The signal beam appearing as a phase - conjugateoreflection of the probe beam isproportional to the spatial complex-conjugate of the probe. By introducing delays between the pump E1and the probe E, laser pulses,the generated signal may be related to a pulse autocorrelation function. The energy Q4 of the phase - conjugate wave E4 as a function of the delay time r of the probe beam is determined by the temporal profile of the incident wave E1, E2 and E3. The analysis of this energy distrbution starts from the extreme and simplest case: for the long - pulse limit (tp>>τ r., where τr is the relaxation time of the medium), the system's response immediately follws the driving field. The FWHM of Q4 (τ) =tFWHM is therefore determined by the duration tp (tFWHM≈tp);for the short-pulse limit (tp<<τr),the medium's response can' t follow the electro-magnetic field. The FWHM of Q4 (r) =tFWHM is approximately equal to-the coherence time To (tFWHM≈ τc ). It is therefore possible to estimate therelaxation time of the medium by considering the form of the DFWM signal as a function of the delay time r. Alternatively,if the duration of pulse tp is unknown,the ratio of the relaxation time to the pulseduration can be determined. Another important problem connected with the magnitude of the phase modulation a is discussed too. In a case of moderate modulation(a= 2~5),the coherence time τc and pulse duration tp are related approximately by τc≈2tp/a. Then,the validity of this method is verified using computer simulation of the model and comparing them with the experimental data for CS2,nitrobenzene and 8CB liquid crystal. The experimental configuration for relaxation-rate measurements using a 90° phase-conjugate geometry is given,and the relationship between t and a is explored by computer simulation.At last,we have a conclusion:this method can estimate the relaxation time of nonlinear media,or the ratio of this time to the pulse duration in the presence of moderate or strong phase modulation. The most precise results are obtained in two extreme cases:for tp>>τp, and tp<<τr, For intermediate cases the value of τr,/tp can only be estimated.
出处
《光子学报》
EI
CAS
CSCD
1998年第4期312-316,共5页
Acta Photonica Sinica
