The Photonics Research Institute (PRI) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has succeeded in down-sizing a “light-controlled photo-switch” for dividing light signals into two paths without photo-electrical conversion to palmtop size, in collaboration with Dainichiseika Color & Chemicals Mfg. Co., Ltd. (to be designated as Dainichiseika hereinafter). The device makes it possible not only to deliver digital signal light from a single server to two clients through a wavelength-divided multiplex interactive photo-communication path based on 1.31µm and 1.55µm bands by using control light of 660nm wavelength, but also to provide two-way communication from a client to a server. The light-controlled photo-switch is expected to enable constructing a new construction system for a small-sized network covering home, office, hospital, etc.

Switching of signal light by use of 660nm control light is implemented through a micro thermal lens effect (forming a small conical lens) within a thin film device and a holed mirror. The principle is based on selective absorption of control light in a laminated organic film optical device. If N units of light-controlled photo-switch are combined, it is possible to build up an interactive optical communication system of (N+1:1) configuration, and simultaneous illumination with signal light and control light used as a light tag enables to realize an interactive optical packet communications for target destinations through light control only, without resorting to photo-electrical conversion nor to mechanical switching, opening the way to provide a low cost key device for the development of next generation optical communications system.

In optical-switches currently on the way to commercialization, such as Micro Electro- Mechanical System (MEMS) and Planar Light Circuit (PLC), either driving a micro mirror mechanically, or energizing a heater electrically, it is necessary for switching light path by reading a light tag to have gone through a number of steps before processing signal light such as light signal_ conversion to electrical signal_tag readout_generation of electrical signal for switching path_switching of optical path. Moreover, as switching takes a delay of millisecond order, provision of a complicated control is required such as sending “path-switching signal” before sending the main optical signal to have the light path switched in advance, and then, delivering stored optical signal. On the contrary, in the present light-controlled photo-switch, the optical path for signal light is automatically switched when the laminated organic film optical device accepts control light of specific wavelength.

The results of the present study will be presented at the Interopto’04 to be held at the Nippon Convention Center, Makuhari Messe, Chiba on July 13~16, 2004.The newly developed real time communications is implemented on a new protocol installed in the data-link layer, without using upper layer protocol such as TCP/IP. The upper layer protocol is realized by providing a packet relay and a communication route switching. While the real time communication is not used, the ordinary TCP/IP protocol can be used to facilitate software development.

As the utilization of the Internet spreads, demands for data transmission capacity is increasing at a pace of 10 to 100times every 5years. This has increased the expectation for the practical use of “all-optical network” which does not need handling of electrical signals at all. For realizing the all-optical network, it is essential to make a “light-controlled photo-switch” which changes over the destination of light signals delivered through optical fibers without converting into electrical signals. The light-controlled photo-switch has an enormous merit of not only extensively saving the installation cost, but also solving some vital problems such as suppressing power consumption and securing installation space. Moreover, the photo-switch will contribute to removing upper limit of signal processing rate, which is seriously restricted by the semiconductor technology in case of electrical signals. The light-controlled photo-switch will play the most significant role in every sector of optical communications: main trunk lines, local channels (Metropolitan), and fiber-to-the-home (FTTH), and its further development will be counted on. The newly developed palmtop light-controlled photo-switch will implement interactive communications with a single device, owing to lack of dependence on wavelength in contrast to semiconductor-based devices, and ensures enhanced spread owing to simple configuration and lower cost.

The PRI-AIST and Dainichiseika succeeded in the development of light-controlled photo- switch based on the selective absorption of control light by a laminated organic film optical device through the combination of micro thermal lens effect within the film device (forming a micro conical lens) with a holed mirror, press-released February 13, 2003. In the present study, the device is down-sized to palmtop size (to 1/9 in overall volume) through the improved design and manufacturing process, Besides, the wavelength-divided multiplex interactive optical communication path using 1.31µm and 1.55µm bands for digital signal light makes it possible not only to deliver digital signal light from a single server to two clients by using control light of 660nm wavelength emitted from a high power, low cost laser diode, but also to provide two-way communications from a client to a server. The operation at 1.31µm and 1.55µm bands is expected to open the way to the construction of new communications system in small-sized network for home, office, hospital, etc. The dimensions of the device are compared below between the previous and the improved versions.

The operating principle of the light-controlled photo-switch is as following: when an organic film device of thickness 100µm or so (solid or liquid) is illuminated with focused control light, light energy absorbed by the dye makes up a micro thermal lens of which refractive effect creates a transient micro conical lens to turn the cross-section of signal light beam from circular to doughnut shape. The signal light of doughnut-shaped cross-section is reflected by a holed mirror, to change the direction of light progression, while allowing straight beam pass through the hole. In this way, two beam paths are separated.

1. “Light-controlled photo-switch” down-sized to palmtop dimensions.
2. Wavelength-divided multiplex interactive optical communications path simultaneously switched with 660nm control light by using wavelength bands for optical communications: 1.31µm and 1.55µm.
3. Organic film energized by the absorption of focused control light, temperature raised to 200 °C or higher.
4. The temperature rise along a conical face of focused beam reduces refractive index around the focus point.
5. The distribution of reduced refractive index forms an inverted conical lens.
6. The conical lens turns the cross-section of signal beam into doughnut-shape.
7. Optical path of signal light with doughnut-shaped cross-section is switched by a holed mirror.

Photo.1 An outer view of prototype light-controlled photo-switch Size: 120mm x 80mm x 27mm, with a connector included: 212mm x 80mm x 27mm.

Photo.2 An inside view of prototype light-controlled photo-switch