This letter shows theoretically and experimentally that Fresnel end reflection of the waveguide under test degrades the sensitivity of low coherence optical time domain reflectometry (OTDR). Optical mixing of end reflection and reference light in the OTDR produces the phase noise in proportion to end reflectivity. With the balanced detection technique, the excess photon noise is subdued and the phase noise becomes the dominant source of sensitivity degradation. At 3.2% end reflection and 300 μA mean photocurrent, the noise floor due to the phase noise is −138 dB/Hz. By reducing the fiber end reflection with matching oil, a shot-noise limited sensitivity of −140 dB at a 3 Hz bandwidth has been demonstrated at submillimeter resolution.

An optical time-domain reflectometer (OTDR) with −136 dB minimum detectable reflectivity and 14 μm spatial resolution is developed based on low coherence interference. The high sensitivity was accomplished by using a high-power superluminescent diode module with a 1.5 mW fiber output and a new system configuration, both for effectively operating the balanced heterodyne detection. The reflectivity of −136 dB is only 6 dB above the shot noise limit. The first observation of 1.3 μm wavelength Rayleigh backscattering in single-mode fibers was successfully made by the OTDR with a 17 dB dynamic range and 14 μm spatial resolution.

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This paper describes a photonic time division multiplexing (TDM) highway switch, called FRONTIERNET, that uses optical frequency as routing information. This switch architecture can be applied to asynchronous transfer mode (ATM) switching systems. The N 2 N switch consists of N tunable frequency convertors and N frequency-division-multiplexed (FDM) output buffers connected through an N 2 N frequency router. The router can interconnect N input highways with N output highways in a completely noninterfering way. It is possible to address each output highway uniquely by the choice of frequency (frequency routing) and each output highway can receive any given frequency from only one input. This switch architecture therefore has three advantages: its novel output buffering scheme achieves the best possible performance, there is no need for a complicated contention resolution mechanism between input highways, and there is no splitting loss of the transmitted optical power. An experimental switch with a one-cell FDM buffer was constructed. The tunable frequency convertor based on fast tunable frequency lasers can transmit high-speed optical cells to which frequencies are assigned on a cell-by-cell basis. The frequency router is an integrated-optic arrayed-waveguide grating 16 2 16 filter produced by using planar-lightwave- circuit (PLC) technology. The one-cell FDM output buffer based on optical fiber delay lines can store FDM cells and selectonly one cell at each timeslot over the output highway. A 2.5-Gb/s experimental switch was successfully operated. And an experimental FDM loop buffer was also demonstrated. This stores two cells at a data rate of 2.5 Gb/s. The bit error rate (BER) of the cells after up to 10 circulations is <10 09 . The performance of the buffer in terms of the probability of cell loss and the cell waiting time in the buffer is analyzed numerically. It is concluded that fewer optical buffers are needed to satisfy the desired probability of the cell loss compared with the conventional electronic buffers. The scale of the switch can be expanded in a modular fashion in two ways: using a multistage frequency router and a multistage switching network. And the multihop FRONTIERNET architecture is proposed to reduce the required frequency channels rather than the single-hop FRONTIERNET. The switch scale is also very easy to expand by connecting the frequency-router-based switching submodules.

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