Quasi-phase-matched Second-harmonic Generation Device Research Articles

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.

Study on Formation of Small Polarization Domain Inversion for High-Efficiency Quasi-Phase-Matched Second-Harmonic Generation Device

One of the fabrication methods for a coherent blue light source is a quasi-phase-matched second-harmonic generation (QPM-SHG) device. The key technology for producing an efficient QPM-SHG device is fabrication of the periodic domain inversion in approximately 3 µm range. With the lift-off method used widely, fabrication of fine periodic electrodes is difficult. Recently, to overcome these problems, groups from Stanford University and Osaka University have proposed the following. A LiNbO3 (LN) crystal surface with a periodic resist pattern is filled with a liquid electrode or metal electrode, and periodic domain inversion is obtained when a high voltage is applied throughout the crystal surface through the electrode. We call this technique the “full cover electrode (FCE) method”. It has been proved that this method has a design flexibility which does not exist in the conventional lift-off method, since there are many parameters, such as the resist pattern form with a rectangular section or corrugated section, the sizes of the intervals and periods, the thickness of the resist, and the dielectric constant of the resist. These parameters will have a big influence on periodic domain inversion. The above-mentioned results were reported under specially fixed conditions. Can these conditions be the best state? To what extent will fine periodic domain inversion be obtained? These questions have not been answered. Thus we investigated “an extended concept of the FCE method”. At first, we determined the relationship between many resist parameters and electric-field distribution. In the case of appropriately setting several parameters, when a periodic electrode line width is designed to be as large as possible, it was proved that the ratio of the electric-field intensity directly under an electrode to that directly under a resist is larger than that obtained by the lift-off method. This result can be obtained for a fine periodic domain inversion. Experimentally, it was found that the domain-inversion shape depended on a large value of E/ER. It was found that a domain-inversion form approximated to an electrode form can be obtained with a large value of E/ER, and uniformity over the whole surface and sufficient polling current can be secured, which is difficult in the lift-off method. We proved that the FCE method has useful features which do not exist in the lift-off method. Recently, we have fabricated a periodic domain inversion with a 4.0 µm period in a 500-µm-thick LT crystal.

Second-harmonic generation with a high-index-clad waveguide

Theoretical and experimental analyses of second-harmonic generation (SHG) with a high-index-clad waveguide are reported. It was found that confinement of the propagation modes and the overlap between the fields of fundamental and second-harmonic waves could be increased in this waveguide. This structure was achieved in an x-cut MgO:LiNbO (3) quasi-phase-matched (QPM) waveguide by use of Nb(2)O(5) as a cladding layer. With the QPM SHG device, harmonic blue light of 5.5 mW at the 434-nm wavelength was generated with a normalized conversion efficiency of 1200%/W cm(2).

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 a Laser Diode Using a Domain-Inverted LiTaO3Waveguide with a Monolithic Bragg Reflector

We report wavelength stabilization of a laser diode using a highly efficient distributed Bragg reflector (DBR) grating formed on a LiTaO3 quasi-phase-matched second harmonic generation (QPM-SHG) device. Fabricating the second-order DBR on the LiTaO3 waveguide, the reflectivity of 90% and FWHM wavelength bandwidth of 0.2 nm were obtained. By stabilizing the oscillation wavelength of the laser diode, 3.1 mW of blue light was generated in the QPM-SHG device with the monolithic grating.

Special Issue on Nonlinear-Optic Waveguide Materials and Devices. LiTaO3 Quasi-Phase-Matched Second-Harmonic-Generation Device.

Quasi-phase-matched second-harmonic-generation (QPM-SHG) is an attractive method for obtaining blue light, as phase-matching of an arbitrary wavelength may be achieved by appropriate periodic modulation of the nonlinear polarization. This paper describes recently reported QPM-SHG in waveguides, especially QPM-SHG in LiTaO3, which has large non-linearlity and high resistance to optical damage. Recent experiments in frequency doubling of laser diodes using the QPM-SHG device are also described.

Conversion Efficiency Enhancement Technique for a Quasiphase Matched Second-Harmonic Generation Device

Conversion efficiency enhancement techniques have been demonstrated for quasiphase matched (QPM) second-harmonic generation (SHG). First, we describe a technique for confining the high fundamental optical power density in a waveguide with a domain-inverted grating (SHG waveguide), i.e., a technique for monolithic integration, the SHG waveguide, and a distributed Bragg reflector (DBR). A 40% increase in the conversion compared with a conventional device without a DBR was achieved under QPM conditions. We also describe a method of automatically satisfying QPM conditions, using a laser diode (LD) with antireflection-coated facets. We used the InP/InGaAsP LD and confirmed that the LD oscillated at a wavelength satisfying the QPM conditions. The normalized conversion efficiency was 4.1%·W-1·cm-2.

Highly efficient quasi-phase-matched second-harmonic generation using a first-order periodically domain-inverted LiTaO3 waveguide

We report blue-light generation using a first-order quasi-phase matched second-harmonic generation (QPMSHG) in LiTaO3 waveguide. A first-order periodically domain-inverted structure can be fabricated using a quick heat-treatment technique. We have examined the dependence of the depth of the fabricated domain-inverted region on its width, which critically affects the conversion efficiency of SHG, and designed an optimum domain-inverted structure for the QPMSHG device. As a result, 15 mW of blue light was generated for a conversion efficiency of 10.3%.