85 % efficiency for cw frequency doubling from 1 . 08 to 0 . 54 , um

Because of the nonlinear nature of the process and the generally small susceptibilities involved, harmonic generation with high conversion efficiency is most often realized only for pulsed radiation with high intensity for a short period.1 By contrast, the intensity available for cw operation is usually limited to much smaller values than for pulsed operation, with a concomitant reduction in conversion efficiency. A possible remedy to this circumstance that was recognized long ago is to employ a highfinesse resonator in either a passive2 or active3 configuration to enhance the circulating power and hence the overall conversion efficiency. While the quest for efficient conversion in a cw setting might at first sight seem to be best pursued with crystals with large nonlinear coefficients, such as lithium niobate and potassium niobate,4'5 unfortunately the advantages of large nonlinear coefficient are often mitigated by deleterious effects such as linear and nonlinear absorption and subtle intensity-dependent changes in refractive index, which although small can quickly degrade the performance of a highfinesse cavity. On the other hand, crystals with greater transparency, with fewer thermal problems, and with a higher damage threshold such as the newly developed lithium triborate usually have much smaller nonlinear coefficients and therefore require correspondingly higher powers for the fundamental wave.6 For many years, potassium titanyl phosphate (KTP) has been a well-known crystal for frequency doubling of Nd:YAG lasers (1.064 Am). 7 The large nonlinear coefficient of KTP together with the wide temperature width, large acceptance angle, and extremely low absorption loss make KTP a promising candidate for high conversion efficiency. However, because type II critical angle tuning is the only possible phase-matching scheme at 1.064 pum, the performance of KTP inside a high-finesse resonator is significantly degraded owing to the walk-off of ordinary and extraordinary beams within the crystal. To circumvent this difficulty, recently Garmash et al.8 reported type II 90° noncritical phase matching in an a-cut KTP crystal at 1.08 Am, thus suggesting new possibilities for better performance of KTP in intracavity cw frequency doubling. Following this lead, we report in this Letter experiments for frequency doubling from 1.08 to 0.54 Am with a single KTP crystal inside an external ring cavity of extremely low passive loss. Our measurements are carried out over a range of input fundamental powers up to 700 mW, and a cw conversion efficiency of 85% is achieved. The experimental results are in good absolute agreement with a simple theory, which suggests that efficiencies greater than 90% should be obtainable in our current system once the focusing geometry is optimized. As illustrated in Fig. 1, our frequency-doubling experiment consists of a folded ring cavity of total length 38 cm with two curved mirrors (Ml, M2) of 10-cm radius of curvature and two flat mirrors (M3, M4). A KTP crystal is placed between the two curved mirrors at the position of the collimated waist (wo0 60 Am), with a small folding angle (<60) used in the arrangement to reduce both astigmatism and polarization imbalance. Three of the four mirrors have high reflectivity at 1.08 Am (R > 99.98%) and high transmissivity at 0.54 Am (T 94-96%). Infrared excitation power Pi is coupled into the cavity through the remaining flat mirror M4 with reflectivity 97% at 1.08 Am. The fundamental wave in our experiment comes from a single-frequency, TEMO0 -mode Nd:YO 3 laser operating at 1.08 ,m, with the frequency of the laser locked to a stable external cavity and with 700 mW of infrared power available to pump the doubling cavity. The length of the doubling cavity is servo controlled with a standard locking system9 to obtain coincidence between the fundamental input frequency and the frequency of a TEMoo longitudinal-mode resonance for the ordinary polarization. The harmonic light generated in the KTP crystal is transmitted principally through the curved mirror M2 (with transmission at 0.54 Aum of 94%) as shown in Fig. 1. The nonlinear medium for frequency doubling is a 3 mm X 3 mm X 10 mm flux-grown, a-cut KTP crystal with two faces antireflection coated for low loss at both fundamental and harmonic wavelengths. Type II noncritical 90° phase matching at 1.08 ,um [2n0 (2co) = nfe(C) + n0 (w)] is achieved near 630 C with a temperature width of 30°C for our crystal (note that this phase-matching temperature is significantly different from the temperature of 153°C reported in Ref. 8).1o Because type II phase matching requires both ordinary (o) and extraordinary (e) beams for the generation of harmonic light, the in-