Quasi-phase Matched Second Harmonic Generation Research Articles

Wavelength Conversion Devices Using Periodic Domain-Inverted Structures and Their Applications to Photonic Networks

Recent progress in waveguide-type quasi-phase matched (QPM) nonlinear-optic wavelength conversion devices is reviewed with emphasis on research work of the author's group. A new technique for forming periodic domain-inverted structures in optical-damage-resistant MgO:LiNbO3 crystal is presented. Improvements of conversion efficiencies of QPM second-harmonic generation (SHG) and difference-frequency generation (DFG) devices are demonstrated. Experimental works toward applications to photonic network systems are reviewed, including those with the cascaded SHG-DFG wavelength conversion as well as the fundamental DFG wavelength conversion.

Second-harmonic green generation from two-dimensional χ(2) nonlinear photonic crystal with orthorhombic lattice structure

We report the synthesis of nonlinear photonic crystals (NPCs) with a periodical distribution of inverted χ(2) nonlinearity having an orthorhombic lattice structure on Z-cut congruent-grown lithium niobate (LiNbO3) substrate. The quasiphase-matching (QPM) mechanism of nonlinear wave interaction is examined by monitoring the far-field emission pattern of second-harmonic generation (SHG) as the NPC is pumped by a Nd:yttritium–aluminum–garnet laser beam. We observe (i) a series distribution of green SHG in a direction transverse to the fundamental beam, and (ii) an increase of phase-matching temperature in the SHG peak signal with the azimuth rotation angle in the x–y plane. These observations are ascribed to the high-order reciprocal lattice vectors assisted QPM–SHG process in a NPC that has a distribution of χ(2) nonlinearity with an orthorhombic crystal symmetry.

Quasiphase matched second-harmonic generation from periodic optical randomization of poled polymer channel waveguides

We present experimental evidence of quasiphase-matched second harmonic generation in a polymer channel waveguide consisting of disperse red one (DR1) grafted to a poly(methylmethacrylate) (PMMA) main chain polymer at 30% molecular weight concentration. The ability of DR1 to photoisomerize is used to define a periodic segment of linear and nonlinear domains along the waveguide by photodepoling through a photolithographic mask of 10 μm periodicity, following quasiphase-matching requirements. A broad spectral scan between 1450 and 1800 nm evidences a peaked enhancement of second harmonic generation at 1536 nm, with experimental efficiency of 0.023% W−1, further compared to theoretical values. Wavelength dispersion of the quasiphase matched second harmonic peak as well as the penetration depth of the photodepoling beam in DR1 PMMA are also discussed.

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 in two-dimensional periodically poled lithium niobate using second-order quasiphase matching

Two-dimensional nonlinear photonic crystals of lithium niobate with square lattices were fabricated. In these crystals, efficient quasiphase-matched second-harmonic generation was demonstrated by using second-order quasiphase matching. As a mode-locked Nd:yttritium–aluminum–garnet laser at 1064 nm with 35 ps pulse was used, we obtained an average power of 0.86 mW at 532 nm with 2.73 mW input, which corresponds to 42% internal conversion efficiency. The variations of second-harmonic output with crystal temperature and incident angle were measured. In addition, the relation between the second-harmonic output and reversed duty cycle was studied. All the experimental results are well consistent with simulations.

Efficient KTiOPO4 blue-light converter for monochromatic 1.3188-μm emission line of pulsed Nd:YAG laser

A frequency converter is designed for producing efficient blue light at 0.4396 μm from a monochromatic, pulsed, 1.3188-μm Nd-doped emission line by quasi-phase-matched (QPM) second-harmonic generation (SHG), followed by sum-frequency generation, in an integrated, periodically poled KTiOPO4 (KTP) and monodomain KTP crystal. An average 55-mW (peak power of 43.0 W) blue light is achieved by us for an average 2.5-W (peak power of 2.0 kW) pump power; the overall conversion efficiency is thus 2.2%. Through polarization optimization, the same device produces an average 435 mW (peak power of 339.8 W) for the red light through QPM SHG, with a corresponding conversion efficiency of 16%. As a practical blue-light converter, the angle- and temperature-tuning characteristics and period tolerance have been fully investigated.

High Power SHG Blue-Violet Laser Using Distributed Bragg Reflector Laser Diode and Waveguide-Type Wavelength Conversion Device.

We demonstrate highly efficient frequency doubling of a 160 mW-AlGaAs three-section tunable distributed Bragg reflector (DBR) laser diode with real refractive-index-guided structure in a quasi-phase-matched second harmonic generation (QPM-SHG) ridge-type waveguide device. We realized high coupling efficiency of the DBR laser diode and the QPM-SHG waveguide device using a planar-type butt-coupling configuration. High-power SHG blue violet laser was fabricated, whose volume is 0.3 cc. This SHG blue-violet laser satisfied the several specifications for optical disk systems, and the pulsed blue-violet light (410 nm) power of 62 mW was generated.

Cavity-enhanced and quasiphase-matched multi-order reflection-second-harmonic generation from GaAs/AlAs and GaAs/AlGaAs multilayers

We have observed quasiphase-matched second-harmonic generation in the reflection geometry from GaAs/AlAs multilayers. By using GaAs/AlGaAs multilayers and Fresnel reflection as a cavity, we have also achieved cavity-enhanced nonphase-matched second-harmonic generation from GaAs/AlAs multilayers. The linewidth for the first order reflection-second-harmonic generation is limited only by wave-vector mismatch. In addition, we have demonstrated two-order-of-magnitude enhancement on the conversion efficiency by using the cavity.

Quasiphase-matched second-harmonic generation using periodically photoisomerized azo dye doped polymer film waveguide

A process to form a periodically poled polymer film by photoisomerization is described. A poled film of polymethylmetaclyrate containing 5 wt % of 4-(4-nitrophenylazo)aniline (Disperse Orange 3) was exposed to an Ar+ laser beam using a photomask of a 100 μm period grating. Disorientation of the chromophore molecule resulted in decrease of d33 in the photoisomerized domain. Quasiphase-matched second-harmonic generation is demonstrated experimentally using a processed film. Phase-matching was confirmed by tuning the fundamental wavelength around 1200 nm. The effective propagation length due to a tightly focused fundamental wave and the influence of the absorption loss are discussed. The depoling process by photoisomerization is compared with photobleaching, optical poling, and electron beam treatment.

3.5 W frequency-doubled fiber-based laser source at 772 nm

A 3.5 W fiber-based laser source at 772 nm is demonstrated by using quasiphase-matched second-harmonic generation of a 40 dBm seeded ytterbium–erbium fiber amplifier in PPKTP. A 40% conversion efficiency is achieved across the entire output power range of the EDFA. No optical damage has been observed in the PPKTP over long-term high-power exposure.

Quasi-phase-matched self-frequency-doubling waveguide laser in Nd:LiNbO/sub 3/

A Nd:LiNbO/sub 3/ quasi-phase-matched self-frequency-doubling waveguide laser is demonstrated for the first time. Fabrication of domain-inverted gratings in Nd:LiNbO/sub 3/ for quasi-phase-matched second-harmonic generation (QPM-SHG) was a key for implementation of the device. A simple fabrication method was found and employed in a prototype device fabrication. The device was optically-pumped at 814-nm wavelength. A laser oscillation took place at 1084 nm and green light emission was obtained at 542 nm by intracavity QPM-SHG.

Optical Waveguide SHG Devices Using LiNbO3 Epitaxial Grown and Ultraprecision Machining Technique.

Quasi-phase-matched second-harmonic generation (QPM-SHG) devices using LiNbO3 optical waveguides show great potential as low noise and high beam quality blue-violet light sources applicable for high-density optical disks. Optical waveguides with a step index profile are desired for achieving strong confinement and sufficient overlap between the propagation modes in order to realize a high conversion efficiency. Liquid phase epitaxy of LiNbO3 is a novel technique which yields a step index thin-film waveguide structure by doping ZnO in the film. Ultraprecision machining is a powerful technique for processing three-dimensional optical waveguides. The maximum SHG power of 50 mW at 425 nm wavelength was obtained using a ridgetype waveguide device which was fabricated from a periodically domain-inverted MgO: LiNbO3 crystal by ultraprecision machining.

Integrated optical device with second-harmonic generator, electrooptic lens, and electrooptic scanner in LiTaO/sub 3/

This paper reports the first demonstration of an integrated optical device in z-cut LiTaO/sub 3/ that contains the following three functional parts: a quasiphase-matched second-harmonic generation (SHG) grating, an electrooptic (EO) lens, and an electrooptic scanner. The SHG device consists of channel waveguides passing through periodic domain-inverted gratings. The frequency of the input infrared (IR) light at 864 nm was doubled into blue light at 432 nm. A stack of EO lenses was used to collimate the light from the channel waveguide. The measured beam size at the output facet for various applied voltages to the lenses agreed with simulation. After collimation, light passes through an EO scanner that controls the angle of the output beam. A scanning sensitivity of 17 mrad/kV was measured for the scanner, compared to the calculated value of 15 mrad/kV.

Waveguide-Type SHG Blue Laser for High-Density Optical Disk System.

A high power SHG blue laser with direct modulation capability at high frequency has been demonstrated usinga tunable distributed Bragg reflector (DBR) laser diode and a waveguide-type quasi-phase-matched secondharmonic generation (QPM-SHG) device in x-cut Mg doped LiNbO3. Continuous blue light power of 12 mWand pulsed peak power of 26 mW were generated. The SHG blue laser was installed to an optical head, andreadout of ROM disk and recording on phase change optical disk were performed. A butt-coupling-type 0.8cc SHG blue laser was also fabricated. It was indicated that SHG blue laser is applicable to high-density opticaldisk system.

Quasi-Phase-Matched Third-Harmonic Generation in a Quasi-Periodic Optical Superlattice

Quasi-periodic structure can be introduced into nonlinear optical materials such as LiTaO 3 crystals. Such structures were used for quasi–phase-matching second-harmonic generation. These materials are now shown to be able to couple second-harmonic generation and sum-frequency generation through quasi–phase-matching. The approach led to a direct third-harmonic generation with high efficiency through a coupled parametric process. The result verifies that high-order harmonics may be generated in a quadric nonlinear medium by a number of quasi–phase-matching processes, and therefore, exhibits a possible important application of quasi-periodic structure materials in nonlinear optics.

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).

Integrated intracavity quasi-phase-matched second harmonic generation based on periodically poled Nd:LiTaO3

Integrated intracavity quasi-phase-matched second harmonic generation (QPM-SHG) has been demonstrated in a periodically domain inverted Nd:LiTaO3 crystal which functions simultaneously as a gain and nonlinear medium. Lasing at 1.082 μm and simultaneous QPM-SHG at 541 nm has been established by pumping near 806 nm. The effect of doping on the domain reversal voltage of LiTaO3 and the photorefractive effect have also been investigated. It is found that optical damage can be avoided by heating above 80 °C.

Optimizing KTP and LiTaO/sub 3/ channel waveguides for quasi-phase-matched second harmonic generation with high conversion efficiency

The conversion efficiency of quasi-phase-matched second harmonic generation (QPM-SHG) in waveguides can be enhanced significantly by optimizing the linear properties of the guiding structure. We describe here a method for fast and accurate computation of the modal properties of continuous, periodically poled, ferroelectric channel waveguides and a multiparameter optimization algorithm which we have used to maximize normalized internal SHG conversion efficiency. We present the phase-matching characteristics of these designs and discuss the possibility of high SHG conversion efficiency between lowest order transverse modes at the fundamental and second harmonic wavelengths near noncritically phase-matched regimes of operation.

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.