Fundamental Researches on Semiconductor Optical Modulators and Optical Switches

Schematic cross-sectional diagram of the world’s first optical modulator of the directional coupler configuration

Fig.1 Schematic cross-sectional diagram of the world’s first optical modulator of the directional coupler configuration

The parabolic quantum well in the world’s first polarization-independent MQW optical waveguide modulator

Fig.2 The parabolic quantum well in the world’s first polarization-independent MQW optical waveguide modulator

Structure of the five-layer asymmetric quantum well (FACQW) with a giant electrorefractive effect

Fig.3 Structure of the five-layer asymmetric quantum well (FACQW) with a giant electrorefractive effect

    The optical modulator for converting electrical signals to optical ones, and the optical switches for directing optical signals to proper routes are key devices in optical electronics. Kunio Tada is one of the pioneers for fundamental researches in this field, and has contributed greatly for putting these devices, especially those based on semiconductors, into practice.

    In the latter half of 1960s, he began research on electrooptic effect of compound semiconductors and its application to bulk type optical modulators. Since early 1970s when integrated optics started, he has been working mainly on guided-wave-optic and integrated modulators/switches and related materials.

    In 1974, his group succeeded in implementing an optical directional coupler modulator/2×2 switch for the first time in the world. This was a GaAs multilayered device with coupled planar waveguides whose waveguide-index could be modulated by linear electrooptic effect and carrier depletion effect at a pn junction. Soon after this, his group improved the original device into more useful devices with coupled channel waveguides, and has long been leading in developing various new kinds of semiconductor directional coupler modulators/switches. A very fast traveling-wave type device with modulation bandwidth of about 10 GHz, and a polarization-independent device on (111) GaAs or InP substrate, are examples of these innovative developments.

    Today, optical modulators/switches with multiple quantum well (MQW) waveguide are very important. Since 1987, Kunio Tada and his group have been pioneering research and development of optical modulators/switches based on potential-tailored QWs. After a detailed analysis of parabolic QWs, they reported on a polarization-independent optical waveguide intensity modulator/switch based on a unique nature of electroabsorptive effect in parabolic QWs. This was the first polarization-independent MQW waveguide modulator device in the world.

    For ultrafast and ultra-low-voltage modulators beyond 40 Gb/s in the future, and for various new and high-performance semiconductor optical modulators and switches based on optical phase modulation, the five-layer asymmetric coupled QW (FACQW) proposed by his group in 1996 seems very promising. In a conventional quantum well with the rectangular potential distribution, the electroabsorptive and electrorefractive effects are large only near the fundamental absorption edge. In the FACQW, a new kind of potential-tailored quantum well, a giant electrorefractive effect in the order of 10-4 cm/kV can be obtained in the transparent wavelength region well apart from the absorption edge.

    For these contributions, Kunio Tada was awarded in 1999 the Achievement Award of the Institute of Electronics, Information and Communication Engineers.



Publications

[1] K. Tada and K. Hirose、A new light modulator using perturbation of synchronism between two coupled guides、1974、Appl. Phys. Lett., vol.25, no.10, pp.561-562, 1974
[2] K. Tada, S. Nishimura and T. Ishikawa、Polarization-independent optical waveguide intensity switch with parabolic quantum well、1991、Appl. Phys. Lett., vo.59, no.22, pp.2778-2780, 1991
[3] H. Feng, J. P. Pang, M. Sugiyama, K. Tada and Y. Nakano、Field-Induced Optical Effect in a Five-Step Asymmetric Coupled Quantum Well with Modified Potential、1998、IEEE J. Quantum Electron., vol.34, no.7, pp.1197-1208, 1998

Related Researches

Category

Electronics & Devices
(Optical Technology)

Events in World

1999
Single currency Euro was introduced to 11 countries of EU.
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