A method of testing distributed amplifiers is presented; multipath interference (MPI) is detected as a beat spectrum between a multipath signal and a direct signal using a frequency-modulated test signal. A test signal with an approximately 450 MHz frequency deviation at an 80 kHz modulation frequency is emitted from a directly modulated DFB-LD by a pulse stream passing through an equalizer. A receiver consisting of a photodiode and an electrical spectrum analyzer (ESA) detects a baseband power spectrum peak appearing at the frequency of the test signal frequency deviation. MPI is converted from the spectrum peak power using a calibration chart. The test method has decreased the minimum detectable MPI as low as -70 dB, compared with that of -50 dB of conventional test methods. The detailed design and performance of the proposed method are discussed, including the calibration procedure, computer simulations for evaluating systematic errors caused by the repetition rate of the frequency modulated test signal and the fiber length under test, and experiments on single-mode fibers and distributed Raman amplifiers.

We report here a pulsed lightwave frequency synthesizer system that is composed of a pulsed lightwave sweep frequency generator and a tracking generator. The key advance in the sweep generator is the use of a dynamically gain controlled EDFA. The combination of feedback and feed forward dynamic gain control effectively compensates EDFA gain fluctuation and equalizes fiber loop loss so that the initial pulse wave form and amplitude is retained in the loop at large circuit numbers. Over 1000 pulsed lightwave frequencies are synthesized in 250MHz steps by the sweep generator. Almost flat response (0.55dB variation) is realized up to 240GHz. The power spectrum decreases by 67% (1.7dB down) at 250GHz. The peak level of the pulses output from the loop is about -4dBm. Tracking generator and total synthesizer system performance are evaluated by (a) beat frequency between the tracking generator and the master lightwave source, (b) beat frequency between two tracking generators, and (c) a frequency chain between the master lightwave source and another HCN stabilized lightwave source via the synthesizer system. A continuous lightwave frequency locked to a frequency selected from the pulsed sweep frequency signal is demonstrated at over 200GHz to have an instability of 5MHz. Absolute accuracy of the lightwave frequency emitted from the synthesizer system is about 10MHz. Therefore, the relative accuracy of the lightwave frequency is as high as 510-8.

A 161 km non-repeatered transmission and a 216 km transmission with a linear repeater have been achieved at 10 Gbit/s using Er3+-doped fiber amplifiers. It is also verified that GaAs-IC technology is applicable to 10 Gbit/s optical transmission systems.

Basic characteristics of Erbium-doped fiber laser amplifier (EDFA) pumped by laser diodes with oscillating wavelength of 1.48µm are measured. Unsaturated gain of 26 dB, 3 dB saturation output power of 1 mW, and noise figure of 5.5 dB are obtained. As a result of transmission experiment using the EDFA at 1.8 Gbit/s over 210 km, linear repeater gain of 20 dB is achieved by considering loss of optical band pass filter and power penalty due to degradation of extinction ratio. By using the basic parameters obtained experimentally, maximum regenerative repeater spacing are calculated in a direct detection transmission system with EDFA repeaters. It is clarified that theoretical regenerative repeater spacing of 5000 km can be achieved by using 50 amplifiers with gain of 20 dB and noise figure of 6 dB, including coupling loss of 0.5 dB, at a bit rate of 2 Gbit/s.