Yasuo Kitaoka

High power blue light generation by frequency doubling of a laser diode in a periodically domain‐inverted LiTaO3 waveguide

We report high power and stable blue‐light generation by frequency doubling a laser diode, stabilized by a grating feedback technique, in a periodically domain‐inverted LiTaO3 waveguide. Due to the deep domain‐inverted regions and strong waveguide confinement formed by proton‐exchange and a quick heat treatment, high efficiency second harmonic generation (SHG) has been obtained. Locking the oscillation wavelength of a laser diode through the waveguide by a grating feedback technique, 8 mW of stable blue SHG has been demonstrated.

Stable and efficient green light generation by intracavity frequency doubling of Nd:YVO4 lasers

A green light source of 11 mW (maximum) has been obtained by frequency doubling in 3 at. % Nd:YVO4 lasers pumped by a single longitudinal mode laser diode (LD), for a 50 mW incident pumping power, with grating feedback optics. When the temperature of the LD was changed within the range of 23±10 °C, the oscillated wavelength of the LD remained unchanged, and the relative intensity noise of the green light was maintained to less than −140 dB/Hz (at 2.5 MHz).

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.

Intracavity second-harmonic generation with a periodically domain-inverted LiTaO3 device

Efficient intracavity second-harmonic generation (SHG) based on a bulk-type quasi-phase-matched (QPM) SHG device in a LiTaO(3) crystal is demonstrated. The periodically domain-inverted regions were fabricated by a high-voltage application technique. The internal loss from the refractive-index change in the domain-inverted regions was reduced to less than 0.1% by an annealing technique. Intracavity frequency doubling of a Nd:YVO(4) laser with the QPM SHG device pumped by a laser diode was effected to generate 13-mW green light for a pump power of 130 mW without suffering transverse mode distortion from a photorefractive effect.

Frequency doubling of wavelength locked laser diOde with a transmission-type filter in a confocal optical configuratiOn

Direct frequency doubling of a wavelength locked laser diode with an optical bandpass filter in a confocal optical configuration is demonstrated. The wavelength of the laser diode was locked in single longitudinal mode oscillation and tuned to phase-matching wavelength of a quasi-phase-matched second harmonic generation device based on a LiTaO3 Waveguide. Stable blue light of 4.2 mW was obtained for incident power of 48 mW.

Intracavity frequency doubling of a Nd:YVO(4) laser pumped by a wavelength-locked laser diode using a transmission-type optical filter.

We demonstrate that intracavity frequency doubling of Nd:YVO(4) pumped by a laser diode can be stabilized by use of a wavelength-locking system of the laser diode with a transmission-type optical filter in a confocal optical configuration. The wavelength locking of the laser diode to the peak wavelength of the absorption spectrum of Nd:YVO(4) yielded stable green light of 10-mW power for an incident pumping power of 55 mW.

Quasi-phase-matched second harmonic generation device in Mg-doped LiNbO3 and its application to high-density optical disk system

The resistance to photorefractive damage is investigated for several nonlinear crystals. A quasi-phase-matched (QPM) second harmonic generation (SHG) waveguide device is fabricated in 5 mol% Mg-doped LiNbO3 which has high resistance to optical damage. The SHG blue laser of the QPM-SHG waveguide device and a tunable distributed Bragg reflector (DBR) laser diode is demonstrated, wherein output stability of continuous blue light is measured. The SHG blue laser, using the QPM-SHG waveguide device with broadened flat matching response, shows good modulation characteristics. The pulsed peak power of 23 mW with rectangular modulated waveform is generated. The SHG blue laser is installed to an optical head, and good recording and readout characteristics are demonstrated. Moreover, butt-coupled SHG blue laser is examined to gain a miniature module with volume of 0·8 cc.

High-powered green light generation by intracavity frequency doubling using grating feedback optics

Grating feedback optics is shown to contribute to narrowing the spectral bandwidth of a multilongitudinal-mode laser diode to less than 0.2 nm and tuning the lasing wavelength to the peak absorption wavelength of Nd:YVO(4). A continuous green light of 31 mW was efficiently generated by intracavity frequency doubling of the Nd:YVO(4) laser with a KTiOPO(4) crystal. A relative intensity noise of less than -140 dB/Hz was obtained in the frequency region greater than 2 MHz. The noise characteristics of generated green light are discussed as compared with the case of using a single-longitudinal-mode laser diode as the pumping source.

Efficient planar-type butt coupling of a proton-exchanged waveguide on a Z-cut LiTaO3 substrate

A half-wave thin film was formed on the facet of a proton-exchanged waveguide by the oblique deposition of Ta2O5. Planar-type butt coupling of the waveguide on the z-cut LiTaO3 substrate to the AlGaAs laser diode in transverse electric mode oscillation was realized with coupling efficiency as high as 53%.

Compact SHG green lasers

The combination of a laser material Nd:YVO4 and a nonlinear crystal KTiOPO4 (KTP) is a promising method for intracavity second harmonic generation (SHG) of laser diode pumped solid state lasers. A wavelength locking system, based on an optical feedback technique, is shown to contribute to locking the lasing wavelength of laser diode to the peak absorption wavelength of Nd:YVO4 and generating stable green light. By applying the wavelength locking system to multi-longitudinal mode laser diode, a continuous green light of 31 mW is efficiently generated for pumping power of 121 mW. The noise characteristics of generated green light are discussed in two cases of using a single or a multi-longitudinal mode laser diode as a pumping source. Moreover, SHG green laser of 3.1 cc is fabricated to gain output power of 2.8 mW using the wavelength locking with an optical bandpass filter as a wavelength selective optical component, which has advantages as more compactness and less accurate alignment.