Osamu Mitomi

Millimeter-wave Ti:LiNbO/sub 3/ optical modulators

The design, fabrication, and characteristics of ridged Ti:LiNbO/sub 3/ optical modulators that work in the millimeter-wave region are presented. A new concept of design under velocity matching is demonstrated for the proposed modulator. It has been shown by calculation that impedance matching is achieved and conductor loss is greatly reduced under velocity matching with wider gaps and a thicker coplanar waveguide electrode in conjunction with a ridge structure. Two types of Mach-Zehnder optical intensity modulators for the wavelength of 1.5 /spl mu/m are developed. A fully packaged module for 40 Gb/s transmission with a half-wave voltage of 3.5 V and a broadband modulator responsible up to 100 GHz with a half-wave voltage of 5.1 V.

A broadband Ti:LiNbO/sub 3/ optical modulator with a ridge structure

We describe the design, fabrication, and characteristics of a Ti:LiNbO/sub 3/ optical modulator with a ridge structure. The structure keeps microwave propagation loss low and enables a large interaction between microwaves and optical waves under the conditions of velocity-matching 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 75 GHz and a driving voltage of 5.0 V at a wavelength of 1.5 /spl mu/m. >

Design of a single-mode tapered waveguide for low-loss chip-to-fiber coupling

The novel waveguide structures described in this paper have nonlinearly tapered shapes that result in low radiation losses despite their relatively short lengths. The core at the waveguide endface connected with the fiber has a very small cross section and an expanded mode field with a non-Gaussian shape. The taper structures are analyzed by using an improved step-transition method. This method is a based on the theory of enclosing a waveguide within electrical walls and that can therefore treat the radiation modes in a tapered waveguide as discrete mode spectra. Analyzing the relationships between the lengths and shapes of the tapers and the radiation loss due to the tapers show that appropriately tapered semiconductor waveguides operating at an optical wavelength of 1.55 /spl mu/m and having a taper length of less than 0.7 mm can have a radiation loss of only 0.1 dB and a coupling loss with a conventional single-mode fiber of less than 0.5 dB. >

A simple laterally tapered waveguide for low-loss coupling to single-mode fibers

Low-loss coupling between semiconductor photonic devices and single-mode fibers is achieved using a simple InP/InGaAsP tapered waveguide. The proposed simple structure has a small and nearly square guiding core at its output facet. In this structure, the output field has a non-Gaussian profile, but low-pass filter coupling can be achieved by optimizing the design of the guiding core sizes. The waveguide is composed of a laterally tapered InGaAsP guiding layer and an InP cladding region on an InP substrate, facilitating integration of the waveguide with active devices using conventional processes. The waveguide is shown to have a total insertion loss of 2.6 dB, including a coupling loss of 0.9 dB and large +or-2.5- mu m misalignment tolerance in lateral and vertical directions with single-mode filters.<<ETX>>

High-speed InGaAlAs/InAlAs multiple quantum well optical modulators

High-speed modulation over 22 GHz for waveguided InGaAlAs/InAlAs multiple quantum well (MQW) optical modulators is described. A large on/off ratio of over 25 dB is demonstrated with a low-drive voltage (6 V) operating in the 1.55- mu m wavelength region. The design and characteristics of MQW p-i-n modulators are discussed. The causes of large-insertion loss and the required drive voltage bandwidth figure of merit for the MQW modulator are discussed. The frequency response measurements show that the response speed is limited by the RC time constant of the device. This suggests that the speed can be further enhanced by decreasing the size and capacitance of the device. >

Design of ultra-broad-band LiNbO/sub 3/ optical modulators with ridge structure

This paper describes novel coplanar waveguide (CPW) electrode and asymmetric coplanar strip line (A-CPS) electrode structures, introducing ridged LiNbO/sub 3/ substrates and thicker electrodes, for ultra-broad-band LiNbO/sub 3/ optical modulators. The structures are designed here with quasistatic analysis using the finite-element method. Ridged-structures with CPW and A-CPS electrodes are shown to be able to reduce the driving-voltage below that of the conventional planar-type electrodes and are suitable for modulation of exceeding 100 GHz and driving voltage of far less than 4 V under an optical wavelength of 1.55 /spl mu/m and 50 /spl Omega/ characteristic-impedance system. The ridge-type CPW can provide a broader bandwidth characteristic with a relatively thinner buffer layer and thinner electrode than the ridge-type A-CPS and is easier to fabricate. The ridge-type LiNbO/sub 3/ modulators are consequently candidate devices for future ultra-high-speed optical fiber transmission systems. >

Highly efficient 40-GHz bandwidth Ti:LiNbO/sub 3/ optical modulator employing ridge structure

The design and fabrication of a Ti:LiNbO/sub 3/ optical modulator employing a ridge structure with a shielding plane are described. The ridge structure offers a relatively low microwave propagation loss and large interaction between the microwave and optical wave under the condition of a 50 Omega characteristic impedance system, resulting in a large modulation bandwidth and low driving voltage. As a result, a 3-dB optical bandwidth of 40 GHz with a driving voltage of 3.6 V is achieved at 1.5 mu m.<<ETX>>

Chirping characteristic and frequency response of MQW optical intensity modulator

The spectral linewidth enhancement factor and frequency responses of electro-absorption-type optical-intensity modulators, especially InGaAs/InAlAs MQW modulators, are described. A method of exactly estimating the value of the alpha factor is presented under the nonlinearity of extinction-ratio characteristics. For measuring the frequency response of modulators, the sideband strength of the modulated output light with an optical spectrum analyzer, is analytically compared with the microwave power of photodiode direct detection with an electrical spectrum analyzer. >

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.