Efficient SHG with the Longitudinal Temperature Gradient along the LBO Crystal

Anna Mazhorova is currently working on the experimental realization of the autoresonance effects in nonuniform second order nonlinear materials. One of the potential applications of this research is the design of highly-efficient frequency doubling devices allowing complete depletion of the pump wave [1]. This kind of devices is expected to have a very large conversion bandwidth with a flat conversion spectral profile, similarly to what have been suggested and demonstrated in the case of four-wave mixing in tapered optical fibers [2].

As a first step towards the realization of the project we demonstrate an efficient technique for the second harmonic generation (SHG) based on the temperature gradient along a nonlinear crystal. The characteristics of Type I non-critical phase-matched SHG of broadband radiation in the LiB3O5(LBO) crystal with the temperature gradient imposed along the crystal were investigated.
 
 
 
 

Fig.1. a) Experimental setup, long crystal is mounted inside the temperature-controlled holder. Efficiency of second harmonic generation; b) in a conventional configuration (at optimal temperature for noncritical phase matching ) conversion efficiency is very poor, not higher than 10% was achievedand c) with a temperature gradient along the crystal, a high second-harmonic conversion efficiency was achieved.

  

The use of LBO crystal for second harmonic generation has the following advantages: high damage threshold (18.9 GW/cm2 for a 1.3ns laser at 1.054mm), relatively high deff (0.85 pm/V), no spatial walk-off and large acceptance angle for the case of type-I noncritical phase matching condition  (theta=90° and varphi=0°) at 1 um wavelength. The main limitation to efficient broadband frequency conversion comes from the chromatic dispersion of the nonlinear crystal, basically from the pulse group velocity mismatch. One of the possibilities to improve the efficiency of the frequency conversion process is a) to use shorter crystals or b) to use a higher power. However, the maximum applied power is limited by the damage threshold of the crystal. Also the conversion efficiency is proportional to the square of the interaction length, but the wavelength acceptance bandwidth is inversely proportional to the same interaction length, resulting in a trade-off between the conversion efficiency and bandwidth. One promising concept for improving the frequency conversion efficiency is through the use of a temperature gradient applied along the crystal [3]. With the temperature gradient along the crystal, the phase-matching conditions are satisfied at different positions along the crystal for different wavelengths and efficient second harmonic generation takes place at a small distance until the wave-number mismatch is close to zero at the corresponding wavelength.

 

 
References
[1] O.Yaakobi, M.Clerici, L.Caspani, F.Vidal and R.Morandotti, "Complete pump depletion by autoresonant second harmonic generation in a nonuniform medium", Journal of The Optical Society of America B (accepted).
[2] O.Yaakobi and L.Friedland, "Autoresonant four-wave mixing in optical fibers", Physical Review A, 82, 023820 (2010).
[3] K.-H. Hong, C.-J.Lai, A.Siddiqui, and F. X.Kärtner "130-W picosecond green laser based on a frequency-doubled hybrid cryogenic Yb:YAG amplifier", Optics Express, 19, 16911 (2009).