Minoru Imaeda

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.

High-speed and low-driving-Voltage thin-sheet X-cut LiNbO/sub 3/ Modulator with laminated low-dielectric-constant adhesive

We present a novel thin-sheet X-cut LiNbO/sub 3/ optical modulator structure which can be fabricated by precise polishing and lapping to obtain a thinner LiNbO/sub 3/ substrate for a lower driving voltage in addition to velocity matching and impedance matching. We demonstrated that the fabricated modulator had a driving voltage V/spl pi/ of 2 V and zero chirp for 40-Gb/s operation and had a high potential for suppressed dc drift, and long-term reliability.

40-Gb/s X-cut LiNbO/sub 3/ optical modulator with two-step back-slot structure

We propose a newly designed X-cut lithium niobate (LiNbO/sub 3/) optical modulator. It has a two-step back-slot structure to satisfy the velocity-matching condition without the buffer layer of silicon dioxide (SiO/sub 2/). Accordingly, this modulator can achieve low drive voltage and low optical insertion loss. In addition, the dc-drift phenomena due to the buffer layer can be suppressed. This structure is fabricated with micromachining technology using excimer laser ablation. The optical 3-dB bandwidth of the fabricated modulator reaches 30 GHz, and the drive voltage is less than 3 V at 1 kHz. From the measurement of the optical eye diagram at 43.5-Gb/s, clear eye openings were obtained. This modulator is sufficient for 40-Gb/s optical transmission systems.

Homogeneity and SHG properties of K3Li2 − xNb5 + xO15 + 2x single crystals grown by micro-pulling-down technique

Homogeneity of potassium lithium niobate single-crystal fibers grown by micro-pulling-down technique is examined via second-harmonic-generation measurement both in axial and radial directions. It is found that fluctuation of phase-matching wavelength along the growth axis is small and independent of growth rate. On the other hand, relatively large variation of phase-matching wavelength is found along the radial direction. The phase-matching wavelength shows its minimum around the center of the section of the fiber and increase toward the periphery. Compositional inhomogeneity and internal strain in the radial direction are investigated in association with variation of phase-matching wavelength. X-ray rocking-curve analysis, absorption measurement and polarized beam transmittance are also reported.

Growth of high crystalline quality LiNbO3 thin films by a new liquid phase epitaxial technique from a solid-liquid coexisting melt

Abstract LiNbO3 films of higher crystalline quality than underlying LiNbO3 substrates have been successfully grown from a solid-liquid coexisting melt using a new liquid phase epitaxial technique. The dependence of crystallinity and lattice mismatch on film thickness and growth temperature has been investigated in detail through X-ray rocking curve analysis.

Liquid-phase epitaxial growth of Zn-doped LiNbO3 thin films and optical damage resistance for second-harmonic generation

ZnO-doped LiNbO3 (LN) liquid-phase epitaxial thin films grown on LN substrates were investigated as modified LN substrates for optical waveguide devices. The potential for optical use as substrates compatible with conventional ones was examined in quasi-phase-matched second-harmonic generation (SHG) devices from the viewpoint of the domain inversion process and optical damage properties. The formation of a periodically inverted-domain structure and the increased optical damage resistance in SHG were demonstrated.

High-performance optical modulator with a wide center electrode and thin x-cut LiNbO/sub 3/ substrate

We propose an excellent large-bandwidth back-slot lithium niobate (LN) modulator with a wide center electrode of typically 50 /spl mu/m or wider and relatively thin electrodes. From the calculation, a modulator with a 3-dBe bandwidth of 34 GHz and a half-wave voltage of 2.0 V for a 50-/spl Omega/ characteristic impedance system was realized theoretically. This means when their half-wave voltages are identical, by applying a wider electrode, the modulator bandwidth becomes 2.6 times larger than that of a conventional one. We also confirmed experimentally that the wide center electrode structure is effective for back-slot LN modulators.

Growth-temperature dependence of the LiNb ratio in LPE-grown LiNbO3 films estimated by second-harmonic generation

The relationship between the Li/Nb ratio of liquid-phase epitaxial (LPE) grown LiNbO 3 films and the growth temperature is investigated by means of second-harmonic generation measurements. Uniform 3-inch-wafer-size epitaxial films, which are available in conventional device fabrication processes, were successfully grown.

Crystal structure of LPE-grown LiNb1-xTaxO3 epitaxial films

Abstract LiNb 1− x Ta x O 3 epitaxial films, which were grown on LiNbO 3 substrates at various growth temperatures by the liquid-phase epitaxial method, were characterized using X-ray rocking curves, diffraction-space maps and absolute lattice constants. The films had relaxed structures regardless of the growth temperature, and the density of the dislocations due to the lattice mismatch between the films and the substrates increased with increasing growth temperature. Therefore, the crystallinity of the films became poor with increasing growth temperature.

A double-die modification of micro-pulling-down method for in situ clad/core doping of fiber crystal

Abstract A novel technique for single-crystal fibers cladding immediately in the course of the growth process based on the micro-pulling-down (μ-PD) method has been proposed and verified. The principle of the double-die-μPD implies that the capillary nozzle provides capillary feeding to the growing fiber simultaneously from the inner pipe channel with undoped melt and from the outer surface with doped melt. The menisci shape, melt flow outside the nozzle, control of the doped/undoped melt ratio and process stability are discussed in terms of capillarity. Examples of practical crucible design are discussed in connection with radial dopant distribution. Mn-doped lithium niobate fiber, 0.5 and 0.7 mm in diameter, having 40–60 μm doped outer area were grown successfully.