This paper provides an overview of recent developments affecting in-vehicle optical networks. Visible light sources are now being used in this field, bringing advantages in both visibility and workability. In particular, current trends and subjects affecting the key devices used in these networks, such as visible light sources, optical fibers, optical circuits and transceiver modules, are explained. We consider that the use of a wavelength division multiplexing (WDM) technology is promising for realizing further advances in high-speed communications. We will briefly introduce the optical devices for WDM that have been developed in our research group.

Satoru Kato, Osamu Fujishima,
Takahiro Kozawa, Tetsu Kachi

The performance of gallium nitride (GaN) green LEDs that have been developed for use in plastic optical fiber (POF) data links is described. The LEDs consist of a conventional surface-emitting structure with a single quantum well (SQW) that emits at a wavelength of 495 nm. In order to verify the performance of an optical data link based on SQW-type green LEDs and a polymethyl methacrylate (PMMA) POF, we evaluated the temperature dependence and the time response of the injection current of the SQW-type LED samples compared with commercially available display-type GaN green LEDs with a multi-quantum well (MQW) structure. As a result, 250 Mbit/s transmission over 20 meters of a PMMA-POF was successfully demonstrated with a BER of less than 10^-12. This optical device can be applied for high-speed digital interfaces, such as IEEE1394.

Tatsuya Yamashita, Manabu Kagami

A fabrication technique for producing light-induced self-written (LISW) waveguides for large-core optical fibers is proposed. The proposed technique employs a photopolymerizable resin consisting of two kinds of photopolymerizable monomers that differ from each other in terms of both refractive index and polymerization mechanism. The core portion is formed by virtue of a self-trapping effect, in which visible light is irradiated to the resin through an optical fiber that is inserted into the resin. Only the low refractive index monomer can be radically polymerized to form the LISW waveguide, which is generated from one end of the fiber. After the irradiation is stopped, the concentration gradient induced by the comsumption of the low refractive index monomer initiates a counter-diffusion phenomenon between the residual monomers. The low refractive index radically polymerizable monomer diffuses into the core region, while the high refractive index cationic polymerizable monomer diffuses out of the core region. The residual monomers are subsequently cured by exposure to UV light, and the region with decreased concentration of high refractive index monomer becomes a cladding layer. The resultant refractive index profiles of the waveguides were experimentally confirmed to be "W-shaped". The measured propagation loss of the waveguide was 1.7 dB/cm at 680 nm wavelength.