The Photonics Research Institute (PRI) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has implemented, first in the world, intersubband transition for 1.55 μm (1μm = 1/1,000,000m) optical communication wavelength band in the form of quantum well using II-VI semiconductor materials, and successfully manufactured intersubband transition ultra-fast semiconductor optical, switch meeting requirements both for ultra-fast operation at 1 Tb/s (1Tb = 1terabit = 10¹²bit), the fastest in the world, and for low switching energy. This achievement will contribute to accelerating R&D works on new generation ultra-fast, massive capacity optical communications, as fast as 400Tb/s.

Recently, the information traffic through the internet and other networks is growing rapidly, and the construction of photonic networks for massive capacity, high speed communications has been urgently requested. For implementing massive capacity optical communications, efforts are being made along two lines; increasing the number of channels and making transmission rate per channel faster. However, at a transmission rate higher than about 160Gb/s (1Gb = 1gigabit = 10⁹bit) per channel, the signal processing in the present communication scheme based on electronic circuit is supposed to fail because of speed limit in electronic devices. For this reason, it is necessary to execute the signal processing at 160 Gb/s or faster by using optical signals directly without converting to electronic signals. For this scheme, the development of ultra-fast optical switch will play a crucial role. In the event of using an ordinary semiconductor optical device working on electrons and holes for an optical device, the operational speed is 1ns (1ns = 1nanosecond = 1/10⁹second), because the recombination of electron with hole takes time. Owing to this, it has been difficult to make operation faster than about 6ps (1ps = 1picosecond = 1/10¹²second) as required for 160Gb/s or more signal processing. For solving this difficulty, it has been proposed to confine electron in a quantum well and to utilize the optical transition between energy levels (subbands) formed in the well for achieving fast operation at 1 ps or so. However, with the semiconductor materials used for optical devices, it has been difficult to make transition wavelength between subbands for quantum well correspond to optical communication wavelength band, calling for the development of new semiconductor materials.

The PRI-AIST has succeeded first in the world to realize the intersubband transition corresponding to the optical communication wavelength band with semiconductor quantum well based on II-VI semiconductor materials prepared by the high quality crystal growing technology using molecular beam epitaxy. Furthermore, a process to construct light guide structure has been developed, and succeeded in manufacturing a module of intersubband transition ultra-fast semiconductor optical switch based on II-VI semiconductor quantum well. The new optical switch has achieved ultra-high speed operation of around 200fs (1fs = 1femtosecond = 1/10¹⁵second) and at the same time, very low switching energy of 5.1dB (decibel) extinction ratio for optical input of 10pJ (1pJ = 1picojoule). This result is expected to be a great stride toward the practical application of ultra-fast optical switch.

The future efforts are going to be focused on upgrading the design of quantum well construction and waveguide structure, and on experiment with an optical communication system to verify the switching function.

The result will be announced at the Final Report Meeting of the Femtosecond Technology Project in the Ultra-Fast Photonics Symposium to be held at Tokyo on November 2, 2004.

With the advance of information technology (IT), the development of massive capacity optical communication system has been increasingly requested. It is expected that the ultra-massive capacity communication traffic for 400 Tb/s will be needed around 2015. [Optical Technology Road Map Report for FY2001---Information & Communication Division, Optoelectronic Industry & Technology Development Association (OITDA), ---in Japanese]. Since the conventional scheme of converting optical signals into electronic ones, and returning them to optical after necessary processing takes finite time to be inadequate for 1Tb/s processing, it is the priority demand to develop the ultra-fast optical switch, where optical signals are directly processed as they are, without converting to electronic signals. However, in the semiconductor optical devices utilizing both electrons and holes, the recombination of the two takes time to make the operation rate as slow as 1 ns, and it becomes difficult to realize the ultra-fast operation to meet this requirement. As a means to solve this trouble, it has been proposed to confine electrons in semiconductor quantum well, and to utilize the optical transition between energy levels (subbands) formed in this situation. This would make 1Tb/s signal processing, in principle, through the fast operation of around 1ps. However, with the semiconductor materials having been used for conventional semiconductor optical device for optical communications, it would be difficult to make the intersubband transition wavelength compatible with the optical communication wavelength band (2 μm or shorter), requiring the development of innovative semiconductor materials.

The PRI-AIST has been dedicated to R&D of a variety of ultra-fast optical devices and their characterization technology under the Project “R&D of Femtosecond Technology (FY1995-04)”, sponsored by the Ministry of Economy, Trade and Industry (METI). In the framework of this Project, the PRI-AIST has been engaged in R&D of II-VI semiconductor intersubband transition optical switch.

The PRI-AIST has succeeded in implementing intersubband transition in semiconductor quantum well in compliance with the optical communication wavelength band with II-VI semiconductor materials, leading the world. Moreover, a process for preparing waveguide device based on the quantum well has been developed, and an intersubband transition ultra-fast semiconductor optical switch module with the best performance in the world has been successfully manufactures meeting two requirements simultaneously: for ultra-fast operation and for minimum switching energy.

(1) Development of semiconductor quantum well structure with intersubband transition wavelength in compliance with optical communication wavelength band

For implementing the intersubband transition at the optical communication wavelength band, the quantum well is to be adequately deep, about 2eV (electron volt) or deeper. The PRI-AIST noted on unusually large band offset for the conduction band at the hetero-interface between two kinds of II-VI semiconductor: cadmium sulfide (CdS) and beryllium telluride (BeTe). When a quantum well structure made of these semiconductors was deposited on gallium arsenide (GaAs) substrate through the molecular beam epitaxyial growth process, light absorption due to intersubband transition corresponding to 1.55μm optical communication wavelength band was observed (Fig. 1). Furthermore, it was found that when an intermediate layer of 1~2 atom thick zinc selenide was inserted at the CdS-BeTe interface to make up CdS/ZnSe/BeTe quantum well structure, an atomically flat CdS/BeTe interface could be formed. For reducing the intersubband transition wavelength to 1.55 μm or so for optical communication wavelength band, it is necessary to set the width of CdS quantum well to 3-atom layer or so, requiring film thickness control at the atomic level. The development of interface flattening technology was responsible to the present success. The operation speed of optical switch based on similar structure was approximately 200 fs, demonstrating ultra-fast operation as anticipated.

Fig. 1. The conduction band energy structure of CdS/ZnSe/BeTe quantum well for 1.57μm intersubband transition wavelength and transmission electron microgram of its cross-section.

(2) Development of waveguide preparing process based on II-VI semiconductor quantum well switch and manufacture of intersubband transition ultra-fast semiconductor optical switch module capable of optical fiber I/O

If a light guide structure could be created to confine optical pulse within a space of a few square μm in a multi-layer quantum well switch made of laminated II-VI semiconductor quantum well and to make the pulse travel over hundreds of μm, the switching energy might be reduced to a minimum. Confining optical pulse in a limited space makes it possible to keep intensive light for a long distance to improve switching efficiency. In view of creating a module applicable to the communication system, the preparation of waveguide structure ensuring easy optical coupling with single mode optical fiber to be used for normal signal I/O is of great significance. In order to confine light within a multiple quantum well, it is necessary to construct a multiple quantum well switch layer sandwiched by cladding layers of which refractive index is a little smaller than that of quantum well switch layer. In the present study, a quaternary alloy material, ZnMgBeSe was used for cladding layer to confine light, with magnesium (Mg) and beryllium (Be) added to ZnSe, for sandwiching a CdS/ZnSe/BeTe multiple quantum well layer. Such a triple layer structure was successfully prepared through epitaxial growth. For obtaining high quality epitaxial growth, lattice alignment, that is, equalizing dimensions of crystalline lattice in the substrate and the film is very important. Owing to the use of ZnMgBeSe for cladding layer, it becomes possible to achieve epitaxial growth of quantum well crystal with designed band gap and refractive index, while keeping the lattice alignment with the GaAs substrate. Following this, a process of preparing waveguide structure has been developed through dry-etching with chlorine-based gas the three-layer structure while leaving stripes of a few μm width, for defining the path of light confined in the multiple quantum well switch layer (Fig. 2). Based on this process, the intersubband transition ultra-fast semiconductor optical switch has been manufactured on trial basis. The module is designed so that I/O of optical signals can be made through optical fibers, allowing for demonstration of switching function with various experimental communication systems (Fig. 3). The newly manufactured intersubband transition ultra-fast semiconductor optical switch has achieved a record ultra-fast operation of approximately 200 fs, and at the same time, low switching energy operation with 10 pJ optical input and 5.1 dB extinction ratio, to make it available in various experimental systems aiming at the new generation optical communication system. For the practical application, it is necessary to reduce the switching energy further. While there is room for improvement with the quantum well structure and the light guide construction, it is expected that the switching energy is reduced to 1/10 of the present level.

Fig. 2. Light guide structure chip and its switching characteristics for optical input of subpico second control

Fig. 3. A concept diagram of optical switch operation and manufacture module