Efficient second-harmonic generation of broadband radiation in the nonlinear crystal with constant axial temperature gradient

Abstract

Summary form only given. Efficiency of second-harmonic generation (SHG) of broadband light pulses is limited by chromatic dispersion of a nonlinear crystal. The group velocity mismatch between first and second harmonic pulses results diminishing overlap between pulses as they propagate in a nonlinear crystal. Particularly, when fundamental pulses are chirped (phase-modulated), the group velocity mismatch causes loss of phase-matching even before complete walk-off of pulse envelopes occurs. This limits the optimal crystal length and, as a result, overall conversion efficiency. In order to maintain the high conversion efficiency in a shorter crystal, pulse irradiance has to be increased. However, the pulse irradiance is limited by the crystal damage threshold. Thus, achieving the high conversion efficiency of broadband pulses is not a trivial task, especially when the pulses are not bandwidth-limited. Concept of improving the frequency conversion bandwidth by inducing a constant temperature gradient along a nonlinear crystal was proposed by R.A. Hass. The phase-matching wavelength of the non-critical phase-matched SHG in a nonlinear crystal depends on the crystal temperature. When a constant temperature gradient is imposed along the crystal, the phase-matching conditions for different wavelengths are satisfied at different positions along the crystal. This explains the broad conversion bandwidth achievable by this method. However, in order to fully analyze possible improvements in the conversion efficiency achievable by this method, numerical calculations are required. In this contribution, we present results of numerical and experimental investigation of the non-critical phasematched SHG in LiB3O5 (LBO) crystal with a constant axial temperature gradient. Numerical calculations were accomplished by integrating (in the plane-wave limit) coupled-wave equations using a split-step technique in which pulse propagation is handled by the fast Fourier transform methods, whereas nonlinear interaction is handled by Runge-Kutta integration. According to the numerical results, by using the SHG method with a constant axial temperature gradient for broadband (phase-modulated) picosecond pulses, higher conversion efficiency could be achieved than using conventional SHG method, when the crystal temperature is uniform. For the experiment, a special crystal oven with two independent heaters at opposite ends of the crystal was developed. Second harmonic of the 12 nm bandwidth (1064 nm center wavelength) 11 ps pulses was generated in the 30 mm long LBO crystal with axial temperature gradient. At pulse energy of 2.5 μJ, the conversion efficiency was higher than 65 % and the whole fundamental spectrum was converted to second harmonic. The obtained experimental results were in agreement with results of our numerical calculations. According to our results, this SHG method can be attractive for implementation in practical applications, because of the high-efficiency broadband operation, robust design and simple wavelength tuning. Particularly, when using the LBO nonlinear crystal, a single frequency conversion module can be designed suitable for operation in the whole ytterbium-doped fiber laser gain wavelength range.