Tomoya Sugita

Electric-field poling in Mg-doped LiNbO3

We report on electric-field poling of MgO:LiNbO3 (MgLN) using patterned electrodes. Investigation of electric properties of MgLN reveals that polarization switching causes reversible resistance change and that this phenomenon can explain the physical mechanism of random domain growth. Based on these results, we propose a domain-control method in MgLN single crystals by suppressing resistance reduction during the poling process. Using this method, short periodic structures consisting of submicron domain geometries have been achieved in X-, Y-, and Z-cut MgLN crystals. High performances of these periodic domain-inverted structures are also demonstrated by evaluating their potentials as second-harmonic generation devices.

31%-EFFICIENT BLUE SECOND-HARMONIC GENERATION IN A PERIODICALLY POLED MGO :LINBO3 WAVEGUIDE BY FREQUENCY DOUBLING OF AN ALGAAS LASER DIODE

We present a method of controlling the shape of the domain-inverted structure in an off-cut MgO:LiNbO(3) crystal by utilizing a two-dimensional high-voltage application. With this technique a periodically domain-inverted structure with a period of 3.2 microm and a thickness of 2.0 microm was fabricated over a 10-mm interaction length. This structure has made possible sufficient overlaps between propagation modes and domain inversion in the waveguide. Using this structure, we demonstrated cw blue second-harmonic generation of 17.3 mW of power at a wavelength of 426 nm with single-pass 55-mW cw AlGaAs laser diode input, which corresponded to 31% power-conversion efficiency.

Efficient 340-nm light generation by a ridge-type waveguide in a first-order periodically poled MgO :LiNbO3

Quasi-phase-matched (QPM) UV second-harmonic generation (SHG) in a periodically poled MgO:LiNbO3 waveguide is presented. A ridge-type waveguide with high nonlinearity and strong resistance to photorefractive damage was achieved by use of an ultraprecision machining technique. By use of this waveguide in 1.4-microm periodically poled MgO:LiNbO3, a first-order QPM SHG device for 340-nm UV radiation was demonstrated. In a single-pass configuration, continuous-wave 22.4-mW UV light was generated for a fundamental power of 81 mW, corresponding to a normalized conversion efficiency of 340%/W.

Room-temperature, continuous-wave 1-W green power by single-pass frequency doubling in a bulk periodically poled MgO:LiNbO 3 crystal

Continuous-wave high-power green light generation at room temperature is reported in a single-pass frequency-doubling configuration with bulk periodically poled MgO:LiNbO3 crystal placed outside a diode end-pumped Nd:GdVO4 laser. The MgO:LiNbO3 samples of 6.95-microm domain period, uniform periodicity, and 50% duty cycle along the entire crystal length are fabricated by use of a high-voltage multipulse poling method. A maximum power of 1.18 W at 531 nm with 16.8% conversion efficiency is obtained from a 2-mm-thick, 25-mm-long MgO:LiNbO3 crystal; the corresponding internal green power and conversion efficiency are 1.38 W and 19.6%, respectively, whereas the normalized conversion efficiency is 3.3%/W.

Ultraviolet Light Generation in a Periodically Poled MgO:LiNbO3 Waveguide

We demonstrated the highly efficient UV light generation of 386 nm wavelength in a periodically poled MgO:LiNbO3 waveguide. Utilizing multiple short pulses for electric poling to off-cut MgO:LiNbO3, we succeeded in fabricating a uniform periodically poled structure with a period of 2.2 µm and a thickness of 1.5 µm over a 10 mm interaction length. By using this structure with a waveguide, the harmonic ultraviolet light generation with a high conversion efficiency of 1000 %/W was achieved in a single-pass configuration.

Efficient Second-Harmonic Generation of 340-nm Light in a 1.4-µm Periodically Poled Bulk MgO:LiNbO3

Ultra-violet second-harmonic generation in a 1.4 µm periodically poled MgO:LiNbO3 is presented. Using a high-voltage multi-pulse application method, a first-order nonlinear grating with a period of 1.4 µm was fabricated over 10-mm interaction length in a 2-mm thick MgO:LiNbO3 substrate. In a single pass configuration, cw 1.2 mW of ultraviolet light at 341.5 nm was successfully obtained for a fundamental power of 141 mW, corresponding to a normalized conversion efficiency of 5.4%/W.

High-Power Continuous Wave Green Generation by Single-Pass Frequency Doubling of a Nd:GdVO4 Laser in a Periodically Poled MgO:LiNbO3 Operating at Room Temperature

Continuous-wave power of 0.89 W at 531 nm with 14.3% conversion efficiency is reported at room temperature from a 2-mm-thick periodically polled uncoated MgO:LiNbO3 by single-pass frequency doubling of a diode end-pumped Nd:GdVO4 laser; the corresponding internal green power and conversion efficiency were 1.03 W and 16.5%, respectively. The MgO:LiNbO3 device of 6.95-µm domain period was fabricated by using a high-voltage multi-pulse poling method; uniform periodicity and 50% duty cycle were obtained on the entire 10-mm-long crystal. The sample exhibited an effective nonlinear coefficient of 14 pm/V (~88% of the ideal nonlinear coefficient), resulting in a measured normalized conversion efficiency of 2.7%/W. To the authors best knowledge this is the highest power ever obtained by quasi-phase matching process at room temperature.

Compact Intracavity Green Light Source with Wide Operation Temperature Range Using Periodically Poled Mg:LiNbO3

A compact and high power green-light source has been developed in diode-pumped solid-state laser based on periodically poled Mg:LiNbO3 (PPMgLN). Optimization of the PPMgLN length in Nd:YVO4 intracvity can achieve wide operation temperature range over 30 °C with maintaining high conversion efficiency. Based on this result, we demonstrated 3 cm3 laser module which generated continuous-wave 0.7-W green light with wall-plug efficiency as high as 11.8%.

Wavelength stabilization of a distributed Bragg reflector laser diode by use of complementary current injection

We have demonstrated wavelength stabilization in an 821-nm AlGaAs three-section tunable distributed Bragg reflector (DBR) semiconductor laser diode (LD) that consists of active, phase-controlled, and DBR regions. We injected two separate, complementary currents into the active and the phase-controlled regions in the DBR-LD to suppress wavelength shift. This modulation method was applied to the LD fundamental wave in a second-harmonic-generation (SHG) laser, and the oscillating wavelength was maintained within the phase-matching acceptance range of the SHG device during modulation. A peak blue-violet light power of 62 mW was obtained for the ideal modulation waveform.

Generation of 360-nm ultraviolet light in first-order periodically poled bulk MgO:LiNbO3.

Ultraviolet second-harmonic generation in first-order periodically poled MgO:LiNbO3 is presented. Using a high-voltage multipulse application method, we fabricated a first-order nonlinear grating with a period of 1.8 microm and depth of approximately 150 microm over 10-mm interaction length in a 1-mm-thick MgO:LiNbO3 substrate. In a single-pass configuration, continuous-wave 30-mW UV light at 362.5 nm was generated for a fundamental power of 522 mW, corresponding to a normalized conversion efficiency of 11%/W.