New optical star-bus network for subscriber

A new optical subscriber loop network for star-bus architecture is proposed. An Optical Frequency Division Multiplex (OFDM) technique is adopted for downstream transmission using a tunable laser and a wavelength selective star coupler. In this architecture, one wavelength from among several wavelengths emitted by a tunable laser located at a central office is uniquely assigned to one of the subscribers during fixed time slots. For upstream transmission, pre-assigned Time Division Multiple Access (TDMA) is adopted using ordinary laser diodes and a star coupler. The proposed network, called “PASS-NET”, has several advantages as well as several problems to overcome, which are discussed in this paper. To investigate the proposed network, fundamental experiments for downstream transmission were executed. Two wavelengths with a 5 GHz (0.04 nm) space between them emitted by a tunable laser (DBR-LD) were directly intensity-modulated at 32 Mbit/s and transmitted over a 10 km single-mode fiber. These two wavelengths were demultiplexed by a 1 x 2 wavelength selective star coupler. A bit error rate of less than 10 ^-9 was measured for a relatively long burst time (144 μs) which proved that the wavelengths were stable. To investigate possible fiber amplifier applications to PASS-NET, a 100 m Er-doped fiber amplifier was examined.