LiNbO/sub 3/ waveguide SHG device with ferroelectric-domain-inverted grating formed by electron-beam scanning

A ferroelectric-domain-inverted grating was fabricated by electron beam scanning in LiNbO/sub 3/. A waveguide second harmonic generation (SHG) device with the grating was fabricated and demonstrated for the first time. The experiments were performed using a CW-Nd:YAG laser, and normalised SHG conversion efficiency of 50%/W was obtained.<>     

Quasi-phase-matched second harmonic generation in a LiTaO3 waveguide

The authors report third-order quasi-matched (QPM) second harmonic generation (SHG) in a LiTaO₃ channel waveguide. A deep domain-inverted region is first fabricated by a proton-exchange and heat treatment technique. Then a uniform and low-pass channel waveguide is fabricated by pyrophosphoric acid proton exchange. Consequently, 12 mW of blue light is obtained at 424 nm wavelength with a conversion efficiency of 6%. The observed FWHM (full-width half maximum) temperature acceptance width for SHG power is 3.2°C and FWHM wavelength acceptance bandwidth for that is 0.2 nm. It is also shown that diffraction limited focusing of the generated blue light may be obtained     

Blazed grating couplers on LiNbO3optical channel waveguides and their applications to integrated optical circuits

Blazed grating couplers on LiNbO3optical channel waveguides have been successfully designed, fabricated, and evaluated. A reactive ion beam etching technique was used to shape the groove profile of grating couplers. A large decay factor of 600 dB/cm along optical channel waveguide was obtained. The air coupling intensity ratio of 8:1 between positive and negative light propagation directions for ion beam angle of 55° indicated a strong blaze effect. Highly efficient optical coupling from channel waveguide to Si phototransistor was also demonstrated.     

Lithium–Niobate Channel Waveguide for the Realization of Long-Period Gratings

We propose a special lithium-niobate (LiNbO₃) single-mode waveguide for the realization of long-period gratings, which consists of a channel core embedded in a thin slab cladding. We fabricated the waveguide on a z-cut LiNbO₃ substrate with a two-step proton-exchange process and demonstrated its suitability for grating application with a number of removable photoresist long-period gratings deposited on the waveguide surface. The waveguide fabrication process and the LiNbO₃ waveguide structure could be further explored for the development of electrooptic gratings for high-speed applications.     

Multiple quasi-phase-matched device using continuous phase modulation of /spl chi//sup (2)/ grating and its application to variable wavelength conversion

We propose a new multiple quasi-phase-matched wavelength converter based on the continuous phase modulation of a /spl chi//sup (2)/ grating for use in variable wavelength conversion. A numerical study shows that the proposed device exhibits a high conversion efficiency, flexible design, and robust fabrication tolerance. A waveguide device fabricated by annealed proton exchange agrees well with the numerical design. Fine-tuning the device enabled us to demonstrate variable wavelength conversion between signals on the standard optical frequency grid. Using the device, we also demonstrated fast (<100 ps) wavelength switching of 4-channel 40-Gb/s signals. The obtained results clearly show that the proposed multiple quasi-phase-matched devices will be useful when constructing future flexible photonic networks.     

Double-layered magnetooptic channel waveguide for waveguideisolator application

TE-TM mode conversion based on the nonreciprocal Faraday effect for waveguides fabricated by liquid-phase epitaxially grown (YBiCa)₃Fe₅O₁₂ films is discussed. The phase mismatch is controlled by constructing a double-layer channel waveguide with an intermediate cladding layer of refractive index slightly lower than the core. Care must be taken to compensate any birefringence of materials, e.g. stress- and growth-induced birefringence. It is shown that such a structure is effective in decreasing the effect of waveguide shape deformation on the phase-matching degree. The analysis of the double-layered channel waveguide and experimental results are described     

Optical surface wave mode converters and modulators utilizing static strain-optic effects

Periodic perturbations of dielectric waveguides can be obtained by an evaporated SiO2thin film grating through the static strain-optic effect. These waveguides are applied to TE-TM mode converters and modulators in Ti-diffused LiNbO3waveguides. Using a coupling length of 3 mm in an optical surface waveguide, we have demonstrated a TE-TM mode conversion efficiency of 80 percent and a TE-TM mode modulator with 100 percent modulation with an applied field of 2 V/μm. A theoretical analysis of a reflector for a semiconductor laser waveguide is also described.     

Type-I Quasi-Phase-Matched Waveguide Device for Polarization-Entangled Twin Photon Generation

Quantum-entangled twin photons are expected to play an important role in future quantum communications and information processing. We propose and demonstrate a Type-I quasi-phase-matched (QPM) nonlinear-optic (NLO) LiNbO₃ waveguide device for high-efficiency polarization-entangled twin photon generation. The device consists of a Ti:LiNbO₃ channel waveguide, a ferroelectric domain-inverted grating for Type-I QPM, and a thin-film half-wave plate for polarization conversion at the center of the NLO interaction region. Generation of polarization-entangled twin-photons is demonstrated through quantum interference experiment.     

Broadening of the phase-matching bandwidth in quasi-phase-matchedsecond-harmonic generation

We report on theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG) in LiTaO₃. QPM waveguides consisting of segments, where each segment has a phase-matching condition different from the others, are proposed to obtain a broad bandwidth and simultaneously, a high SHG efficiency. The waveguides were fabricated by adjusting the phase-matching condition either by changing the grating period of the SHG coefficient or by controlling the width of the waveguide. Consequently, the bandwidth of QPM-SHG was broadened to 0.35 nm with an SHG efficiency of 57 percent/W by modulation of the waveguide width and to 1.12 nm with an SHG efficiency of 29 percent/W by modulation of the grating period. The SHG efficiency and phase-matching characteristics in both types of waveguides showed good agreement with theoretical results     

Blue-light generation by frequency doubling of a laser diode in aperiodically domain-inverted LiTaO3 waveguide

The authors report quasi-phase matched second-harmonic generation by frequency doubling of a laser diode in LiTaO₃ having a first-order periodically domain-inverted region and proton-exchanged channel waveguide. A deep domain-inverted region and a low-loss channel waveguide with strong confinement are formed by using proton-exchange and quick heat treatment techniques. Utilizing this structure, a high normalized conversion efficiency of 157%/W is obtained with a Ti:Al₂O₃ laser. Using a temperature-controlled laser diode and AR coating on the input and output facet of the waveguide, the laser diode maintains single-mode oscillation without any mode hopping. Consequently, 1.1 mW of blue light is obtained at a wavelength of 436.5 nm     

Wavelength-selective coupling between vertically integratedthin-film waveguides via supermode by a pair of grating couplers

Grating coupling between vertically integrated waveguides via supermode was discussed and demonstrated. Two thin-film waveguides with grating couplers were stacked on a substrate. A guided wave in one waveguide is converted by a grating coupler to a supermode propagating contradirectionally, and then converted by another grating coupler to a guided wave in the other waveguide. The coupling shows high wavelength selectivity, and the wavelength-division add/drop multiplexing function can be combined. A demonstrator was designed and fabricated. Theoretically predicted coupling efficiency was almost 100% with wavelength selectivity of 2 nm, while the experimentally obtained value was 40% in the efficiency and 1 nm in the selectivity     

n-p-n silicon lateral phototransistors for hybrid integrated optical circuits

An etched mesa silicon lateral phototransistor (EMS-LPT) suitable for detecting the light signal from optical channel waveguides has been designed and fabricated. In this paper both n⁺-p-n⁺uniform base and n⁺-p-p^--n⁺double-diffused EMS-LPT's are reported. The photoactive region of the EMS-LPT is highly localized and can be easily coupled either via an evanescent field or to a grating coupler on a channel waveguide. Light coupling, gain, speed, and signal-to-noise ratio of the device are thereby greatly improved. The fabrication techniques of the EMS-LPT's are compatible with those of MOSFET's, permitting integration of multiple EMS-LPT's and MOSFET load transistors to form optically addressed inverters on the same silicon chip. By flip-chip bonding LiNbO3and silicon substrates and coupling LiNbO3channel waveguides to EMS-LPT's via grating couplers, we produce electrooptic switches with optical input and output.     

Noncritical quasi-phase-matched second harmonic generation in anannealed proton-exchanged LiNbO3 waveguide

We report the demonstration of dimensional noncritical phase matching, a phase-matched interaction length exceeding 10 mm, and an internal conversion efficiency of 204%/W for second harmonic generation of 976 nm radiation in a periodically poled, annealed proton-exchanged LiNbO₃ waveguide. Using models for the linear and nonlinear optical properties of annealed proton-exchanged LiNbO₃ waveguides and the observed ferroelectric domain grating, the phase-matching wavelength was predicted to within several nm and the conversion efficiency to within ≈20% of the measured values. Optimization of waveguide second harmonic generation devices is discussed     

A high-efficiency grating coupler between single-mode fiber and silicon-on-insulator waveguide

We present the design of a diffractive grating structure and get the optimal parameters which can achieve more than 75% coupling efficiency (CE) between single-mode fiber and silicon-on-insulator (SOI) waveguide through 2D finite-different time-domain (FDTD) simulation. The proposed architecture has a uniform structure with no bottom reflection element or silicon overlay. The structure, including grating couplers, adiabatic tapers and interconnection waveguides can be fabricated on the SOI waveguide with only a single electron-beam lithography (ICP) step, which is CMOS-compatible. A relatively high coupling efficiency of 47.2% was obtained at a wavelength of 1562 nm.     

Wavelength multiplexer based onSiO2-Ta2O5 arrayed-waveguide grating

The characteristics of a wavelength multiplexer based on an arrayed-waveguide grating are carefully investigated by using the grating theory and related experiments. A 28-channel multiplexer is designed and fabricated as SiO₂-Ta₂O₅ waveguides on a 1 cm×2 cm substrate. The designed wavelength channel spacing of 1 nm is obtained. The crosstalk to an adjacent channel is -15 dB. The measured minimum loss is 4.2 dB, which is composed of 3.4 dB excess loss and 0.8 dB grating loss     

Coupled-mode analysis of highly asymmetric directional couplerswith periodic perturbation

This paper presents an in-depth analysis of highly asymmetric grating assisted directional couplers. The directional coupler consists of a polymer waveguide with dimensions and refractive indices closely matching a single-mode fiber fabricated atop a Ga_0.6Al_0.4As/GaAs/Ga_0.4Al_0.6 As waveguide. The structure is investigated analytically by means of a new orthogonal coupled-mode theory formulated in terms of the Lorentz reciprocity theorem. For the first time, the analysis includes three distinct loss mechanisms, namely, the leakage of power toward the semiconductor substrate, the power lost to radiation modes (mode mismatching), and the grating radiation loss     

Widely tunable polymer long-period waveguide grating with polarisation-insensitive resonance wavelength

A long-period waveguide grating in a polymer channel waveguide, which consisted of a benzocyclobutene core and an epoxy cladding, has been fabricated. By changing the stress in the waveguide through etching of the cladding width, a polarisation-insensitive resonance wavelength thermo-tunable from 1520 to 1610 nm with a temperature range of only 8/spl deg/C was achieved.     

Optical multiplication using a bisected intersecting waveguide

We present an analysis of a bisected intersecting waveguide in periodically poled lithium niobate that can produce and isolate the second-order optical product E(ω₁+ω₂)~E₁(ω₁ )E₂(ω₂) free from the near degenerate second harmonics. Model calculations of such a device fabricated in quasi-phase matched (QPM) LiNbO₃ predict a conversion efficiency of ≈1.5%/W, about 15% that of a straight waveguide of the same length, and a crosstalk of ≲-30 dB     

New 1.5 ?m wavelength GaInAsp/InP distributed feedback laser

A new 1.5 ?m-wavelength GalnAsP/InP distributed feedback buried-heterostructure laser was fabricated by a three-step LPE growth process. The second-order corrugation grating was formed on the waveguide layer grown on the active layer. High differential quantum efficiency of 13%/facet was obtained. Single-longitudinal-mode operation in the temperature range from ?20°C to 55°C was obtained.     

LiNbO3 waveguide quasi-phase-matching second harmonicgeneration devices with ferroelectric-domain-inverted gratings formed byelectron-beam scanning

The characteristics of the formation of LiNbO₃ ferroelectric-domain-inverted gratings using electron-beam scanning are examined and discussed for application to waveguide quasi-phase-matching second-harmonic-generation (SHG) devices. It is found that the domain inversion tends to occur in segmented regions, and the inversion width is thinner near the crystal surface than inside the crystal. The dependence of the SHG efficiency on the grating structure is examined theoretically. Prototype devices for green and blue light generation have been fabricated and the device performance examined. Normalized SHG efficiencies as high as 50%/W and 70%/W, respectively, are obtained in green and blue light generation devices of 3.3-mm length. The experimental results are compared with the theoretical results