Researchers at Kyushu University in Fukuoka, Japan, have analyzed and demonstrated a quasi-phase-matched lithium-niobate waveguide that incorporates an integrated electro-optic modulator. The compact, monolithic device could find applications in imaging, biological sensing and spectroscopy.Quasi-phase-matched second-harmonic generation with diode lasers operating in the infrared has become a very efficient and popular technique for generating moderate powers in the blue and green spectral regions. Modulating these frequency-doubled beams is problematic, however, for several reasons. One approach would be to modulate the diode laser, which can readily be accomplished, but the varying thermal load this places on the nonlinear material disrupts the phase matching. Another possibility is to place an acousto-optic or electro-optic modulator — perhaps a Mach-Zehnder device — in the second-harmonic beam. Although this will work, the additional components are bulky and awkward, and they tend to decrease system reliability. A single, integrated frequency doubler and modulator provides a more satisfactory solution.Figure 1. The researchers periodically poled the front and rear sections of a lithium-niobate waveguide and applied electrodes to the middle section, resulting in a phase modulator between two frequency-doubling sections (a). To enhance the extinction ratio, they added a second phase modulator and a third frequency-doubling section (b). PP = periodically poled section; PM = phase modulation section. Images ©OSA.The scientists placed a phase modulator between two frequency-doubling sections of a periodically poled lithium-niobate crystal (Figure 1a). Depending on the phase retardation imposed by the modulator, the visible light generated in the second doubling section could be in or out of phase with that generated in the first section. Analyzing the device mathematically, they determined that they could achieve an extinction ratio as great as 23 dB with one phase modulator between two frequency-doubling sections (Figure 2). Their simulation showed that even greater modulation depth was possible with a second phase modulator and a third frequency doubler (Figure 1b).To investigate the concept experimentally, the researchers fabricated lithium-niobate waveguides with three frequency-doubling sections and two phase-modulating sections by annealed proton exchange. They evaporation-coated aluminum electrodes onto the phase-modulating sections and pumped the integrated devices with 940-nm radiation (for the blue second harmonic) and 1060-nm radiation (for the green second harmonic).Figure 2. The second-harmonic power increases through the first periodically poled region (PP 1) and experiences only modest propagation loss (1.2 dB/cm) through the phase-modulator section (PM 1). But the investigators’ simulation shows that the phase retardation imposed by the modulator can cause a 23-dB change in the amplitude of the output from the second periodically poled region (PP 2).The investigators stabilized the diode lasers with fiber Bragg gratings to obtain bandwidths of several picometers. However, when a diode laser was coupled to the lithium-niobate waveguide, reflections from the waveguide’s uncoated surfaces created etalon effects that destabilized the laser. As a result, the measured extinction ratio was only 9 dB. The investigators are working to improve their experimental design to yield better results.Optics Letters, May 15, 2006, pp. 1492-1494.