Abstract

Typically, placing a quasi-phase matched nonlinear optics crystal in a resonance cavity, such as the cases of optical parametric oscillator (OPO) and second-harmonic oscillator (SHO), can lead to efficient wavelength conversion. With the end faces well polished and HR-coated, compact devices can be achieved. Because we have successfully developed a simple laser ablation method for grating formation on LiNbO₃, here we propose an alternative of making compact OPO’s or SHO’s by using distributed Bragg reflectors (DBR’s). Such a device has the advantage of smaller output spectral widths due to the use of gratings. In this paper, we present the results of our numerical simulations for DBR SHO’s and OPO’s based on periodically poled LiNbO₃ (PPLN). The basic device design is to first fabricate a channel waveguide on a PPLN crystal and then make two gratings at the ends of the waveguide. The longer (shorter) grating serves as the end mirror (output coupler). The fundamental (pump) for a SHO (OPO) is incident from the side of the end mirror. The grating period is designed for the Bragg reflection and hence oscillation of the second harmonic (signal) of a SHO (OPO). We have found that the numerical results of the output of such a device depended on the cavity dimensions, pump power level and phase mismatch parameter of the grating. Therefore, it is expected that for fixed cavity dimensions and pump power, the device should provide the output with the optimum phase mismatch parameter. Figure 1 (filled circles) shows the second harmonic output vs. the normalized phase mismatch parameter (δ/κ) of a DBR SHO when the fundamental input is 10 KW/cm². There are two peak values when the normalized mismatch parameters are 0.225 and 0.668. For comparison, the dashed line shows the result when the gratings are removed (single pass second-harmonic generation). Note that with the DBR’s, the output can be enhanced by at least 13 dB.

© 1998 IEEE