A two-waveguide tapered velocity coupler is presented for a variable divider of optical beams. The coupler consists of one tapered slab waveguide in dimension and the other slab waveguide with a constant film thickness. It is assumed that the device is fabricated on a LiNbO3 substrate, with a push/pull external electric field parallel with the optic axis applied only in the film regions of the coupler. Various numerical simulations through the finite difference beam propagation analysis show that a wide range of dividing ratios from - 15 dB to 15 dB or more can be achieved with considerably small values of driving-voltage electrode-length product and that the dividing characteristics are stable over a wide range of frequencies.

External modulators, which have smaller chirping characteristics than laser diode direct modulation, are desired for high-speed and long-distance optical fiber communication systems. This paper reviews semiconductor and Ti:LiNbO3 guided-wave high-speed optical modulators. Since several effects exist for semiconductor materials, various kinds of semiconductor optical modulators have been investigated. Among these, absorption type intensity modulators based on Franz-Keldysh effect in bulk materials and quantum confined stark effect in multiple quantum well materials, are promising because of compactness, low drive voltage nature and integration ease with DFB lasers. Recent progress on semiconductor absorption modulators and DFB-LD integrated semiconductor modulators is discussed with emphasis on a novel fabrication method using selective area growth by MOVPE (Metal Organic Vapor Phase Epitaxy). The Ti:LiNbO3 optical modulators are also important, due to the advantage of superior chirping characteristics and wide bandwidth. Since the Ti:LiNbO3 optical modulator has low propagation loss and low conductor loss natures for optical waves and microwaves, respectively, the traveling-wave electrode configuration is suitable for high-speed operation. Here, broadband Ti:LiNbO3 optical modulators are discussed with emphasis on traveling-wave electrode design.

We describe the design, fabrication, and characteristics of a push-pull type ridged Ti:LiNbO3 optical modulator with two electrodes. The structure keeps microwave propagation loss low and enables a large interaction between microwaves and optical waves under the condition of velocity and impedance matching, resulting in a large modulation bandwidth and low driving voltage. Using this structure, we have developed an optical intensity modulator with an optical 3-dB bandwidth of 45 GHz (an electrical 3-dB bandwidth of 30 GHz) and a half-wave voltage of 3.9 V in single-electdoe operation (a half-wave voltage of 1.95 V in push-pull operation)at a wavelength of 1.55µm.

A new type of variable beam splitter at optical frequencies is proposed. The basic structure of the device utilizes a tapered velocity coupler which is composed of a center slab waveguide of constant-thickness, constant-index type and two identical outer slab waveguides of constant-thickness, variable-index type. The coupler is assumed to be fabricated on a LiNbO3 substrate, whith an external electric field applied in parallel with the optical axis. The numerical results obtained with the finite difference method show that a wide range of splitting ratios can be obtained with moderate drive voltages and that the splitting characteristics are stable over a wide range of frequencies.

A new polarization splitter at optical frequencies is proposed. The basic structure of the device is a tapered velocity coupler which is composed of a straight and a dimensionally tapered slab waveguide on a LiNbO3 substrate. The numerical results obtained with the finite difference method indicate that extinction ratios of polarization less than 2% for both TE and TM modes are possible of realization under moderate control voltages and that the splitting characteristics are stable over a wide range of frequencies.

Microwave characteristics of a LiNbO3 optical modulator using a superconductor (Pb-In-Au) as a resonant electrode has been studied experimentally at low temperatures down to 4.2 K. It is shown that at the resonance frequency of 14.8 GHz the obtained modulation depth takes a maximum value as expected from theory when the electrode becomes superconducting. The present results demonstrate the possible applications of superconducting electrodes to high performance LiNbO3 optical modulators.

Microwave characteristics of a LiNbO3 optical modulator employing superconductor electrodes (Pb-In-Au) as a transmission line of a traveling signal has been studied experimentally in the temperature range from 300 K to 4.2 K. At frequencies between 8 GHz and 12 GHz it is shown that the obtained modulation efficiency increases as expected from theory when the superconductor undergoes the transition from a normal state to a superconducting state. The present results dumonstrate the possible applications of superconducting electrodes to high performance LiNbO3 optical modulators.

This paper reviews Ti:LiNbO3 and semiconductor optical waveguide modulators. The operating principle of the Ti:LiNbO3 optical waveguide modulator is based on the Pockels effect. On the other hand, semiconductor optical modulators can be operated based on the Pockels effect, the Franz-Keldysh effect, or the quantum confined stark effect (QCSE) by employing multiple quantum wells (MQWs). Ti:LiNbO3 optical waveguide modulators are discussed with emphasis on the velocity-matching and optical waveguides. MQW optical waveguide modulators are discussed in view-point of design of a MQW structure, waveguide and modulation bandwidth.

The inverted slot line (ISL) has been propoaed for millimeter-wave LiNbO3 optical modulator. It is simple in structure, and capable of achieving the perfect velocity matching between carrier and modulating waves. The excellent performance of the ISL optical modulator has been demonstrated at 100 GHz, and the extension into the 50 GHz range is being expected. This paper addresses the analysis of the ISL based on the spectral domain approach. The major results obtained here are the demonstration of the perfect velocity matching not only at 10 GHz but also at 50 GHz, and the characterization of the ISL in terms of effective refractive index, characteristic impedance, overlap integral factor and transmission loss. The depth of optical phase modulation is also estimated at 50 GHz to show a promising performance in the millimeter-wave frequency range. The effective refractive index and the characteristic impedance are found to be theoretically predictable, but the field profile, the overlap integral factor and the transmission loss are not necessarily in good agreement with measurements. As a result of analysis, it can be concluded that the Y-cut substrate is superior to the Z-cut substrate in the following respects: 1. Coupling with the surface wave mode hardly occurs near the operating frequency range. 2. The perfect velocity matching can be attained with a larger spacing between the electrode and the ground plane. 3. The transmission loss is smaller. 4. The field intensity and the voerlap integral factor do not seem to be much deteriorated in the actual ISL.

A shielded velocity-matched Ti:LiNbO3 optical modulator with a ridge is investigated. The analytical method is based on the second-order triangular element finite element method. The thickness of the coplanar waveguide traveling-wave electrode is taken into consideration and the relationship between the electrode thickness and optimum overlaid layer thickness is clarified. Incorporating a ridge into these Ti:LiNbO3 optical modulators can improve not only their modulation bandwidth but also their driving voltage.

This paper describes on the transmission properties of the superconducting coplanar waveguide on LiNbO3 (LN) substrates, fabricated by YBa2Cu3Oy (YBCO) superconducting films. The films have been prepared by the reactive co-evaporation method and patterned by a wet etching process. The surface resistance of the obtained film was 0.04 Ω at 18 GHz and 77 K. It was confirmed from X-ray diffraction (XRD) that these films were highly oriented to the direction of c-axis without a secondary phase. The microwave transmission properties of these YBCO coplanar waveguides were investigated at frequencies up to 20 GHz and compared with that of the aluminum coplanar waveguide. The characteristic impedances of both coplanar waveguides were designed to be 50 Ω. It was found that the attenuation constants of these samples at 77 K were less than that of the aluminum coplanar waveguide for frequencies below 18 GHz.

Low temperature experiments have been made to demonstrate the operation of LiNbO3 optical modulator with superconducting electrodes. The operation of the modulator for applied dc signals as well as microwave signals in the frequency range between 8 GHz and 12 GHz has been observed at temparatures as low as 4.2 K. The present results indicate a possibility of realizing high performance LiNbO3 optical modulators employing superconducting electrodes.