Because the wavelengths required for most spectroscopic, remote sensing and biophotonics measurements are rarely emitted by conventional lasers, nonlinear wavelength conversion is frequently employed in these applications. Stimulated Raman scattering (see “Raman Lasers Offer Power and Wavelength Versatility,” Photonics Spectra, July, page 52) and optical parametric oscillation are two techniques that have been developed to shift the wavelength of conventional lasers.Figure 1. From left to right, the components of the optical parametric oscillator (OPO) are the 1-in.-diameter dichroic mirror, the periodically poled KTP crystal and the Bragg grating.Recently, researchers at the Royal Institute of Technology in Stockholm, Sweden, demonstrated the use of a bulk Bragg grating as the output coupler of an optical parametric oscillator (OPO), thereby reducing its bandwidth by a factor of 20 from that of a conventional OPO with a mirror output coupler.The 2 × 2 × 1-mm Bragg grating was fabricated holographically in photorefractive glass (Figure 1). Besides the grating, the OPO resonator included an end mirror and a periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal (Figure 2). The end mirror was highly transmissive to the 532-nm pump light and highly reflective of the 975-nm OPO signal. The researchers focused 5-ns pulses from a frequency-doubled, Q-switched Minilase I Nd:YAG laser from New Wave Research of Fremont, Calif., to a 170-µm waist inside the PPKTP crystal. Figure 2. In the linear-resonator configuration, the OPO consisted of a dichroic mirror on one side of the nonlinear crystal and the Bragg grating that served as the output coupler on the other. To tune the OPO’s wavelength, the researchers added a mirror to fold the resonator and tilted the Bragg grating. The folded-resonator configuration is shown with broken lines. ©OSA. The 975-nm output from the OPO had a FWHM bandwidth of 0.16 nm (50 GHz), which was lower by a factor of 20 than the bandwidth of the same OPO with a mirror output coupler (Figure 3). In making this comparison, the researchers were careful to ensure that the OPO was running with 30 percent pump depletion in both cases so that its gain was the same.The optical quality of the OPO output beam (M2 ~3.3) was significantly better than that of the YAG pump laser (M2 ~9). The researchers observed this to be the case whether the OPO output coupler was the Bragg grating or the mirror.Figure 3. The reflectivity of the Bragg grating is indicated in green. The spectral output of the OPO was 20 times wider when a mirror was the output coupler (blue) than when the Bragg grating was the output coupler (red). ©OSA.Under optimal conditions, the OPO produced 0.34-mJ pulses at 975 nm when pumped with 1.7 mJ at 532 nm, corresponding to a 36 percent conversion efficiency of pump energy to signal plus idler; that is, 20 percent of the pump energy was converted to signal at 975 nm, and 16 percent was converted to the idler wavelength at 1170 nm. The researchers observed no degradation of the grating during 80 hours of exposure at 20 Hz to 532-nm pulses whose peak intensity was approximately 200 MW/cm2. The output wavelength of the free-running OPO remained constant for extended periods, without the need for manual adjustments or feedback loops.To wavelength-tune the OPO, they folded the resonator so that the angle of incidence on the grating could be adjusted (as indicated with broken lines in Figure 2). Different angles of incidence led to different effective spacings of the grating’s planes and, therefore, to a different wavelength of reflectivity. A 20° rotation of the grating tuned the output from 975 to 913 nm. The bandwidth remained at 0.16 ±0.01 nm across the wavelength range.
