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Optical pulse sources with repetition rate approaching Terahertz is very import for many photonics applications including ultra-high speed optical communication and generation of sub-mm waves. Both active and passive mode locked fiber lasers are the appropriate choice for this purpose because of the availability ofErbium doped fiber amplifier. In general, the mode locking occurs with a repetition rate of nf, where n is an integer and f, is the longitudinal mode frequency spacing. This is called harmonic mode locking. In the case ofrational harmonic mode locking, the repetition rate is (np + 1) f,, where p is also another integer. For the case ofactive mode locking, this is obtained when the modulation frequency to the amplitude or phase modulator used for mode locking is given by (n + l/p) f. For the case of passive mode-locking, the rational harmonic mode-locking occurs when the saturable absorber in a ring laser is offset by a fraction p/L from the center where L is the length of the cavity. We have developed a theory of the rational mode locked fiber laser. The results of the theory are compared with experimental results obtained from a l.5p fiber laser actively mode-locked with a LiNbO3 electro-optic phase modulator.

In an experimental demonstration of four-channel, wavelength-division-multiplexed repeaterless transmission over 235 km, adiabatic soliton propagation allows for precise spectral characterization of cross-phase modulation effects during initial soliton collisions. Theoretical predictions of the spectral shifts that occur are verified for ultrashort solitons. The relationship between the frequency shift and the initial pulse separation is confirmed by measurement of the bit-error ratio as the pulse spacing is varied at the fiber input. Adiabatic expansion of narrow solitons may help to alleviate the channel restrictions that are required for prevention of soliton collisions at the fiber input.