Dense WDM techniques that exploit the enormous bandwidth of dispersion-shifted fibers (DSFs) while avoiding the impairments due to nonlinear effects are described. First, the nature of four-wave mixing (FWM), the dominant impairment factor in WDM transmission systems, is investigated using DSF installed in the field and laboratory experiments. This provides useful information for the practical design of WDM networks based on DSF. Second, practical techniques to reduce FWM impairment, unequal channel allocation and off-lambda-zero channel allocation (equal channel allocation in the novel 1580 nm band) along with gain-shifted erbium-doped fiber amplifiers for the 1570 to 1600 nm band, is described. Comparisons between off-lambda-zero and unequal channel allocation are provided in terms of the maximum transmission distance for various numbers of channels. Two schemes to immunize WDM systems against group velocity dispersion, span-by-span dispersion compensation and optical duobinary format, are presented. The combination of unequal channel allocation with off-lambda-zero channel allocation as well as the combination of two bands: the conventional 1550 nm band and the novel 1580 nm band are proven to be very useful in expanding the usable bandwidth of DSFs.

A novel 1470-nm-band (S+ band) wavelength-division multiplexing (WDM) transmission system is described. The first advantage of S+-band transmission is suppression of degradation caused by four-wave mixing (FWM), which has been the dominant impairment factor in WDM transmission systems on dispersion-shifted fibers (DSFs). FWM suppression by using the S+ band instead of the conventional 1550-nm-band (M band) is successfully demonstrated. The second advantage is expansion of the usable bandwidth by using the S+ band together with other wavelength bands. A triple-wavelength-band WDM repeaterless transmission experiment using the S+ band, the M band and the L band (1580-nm-band) is conducted over DSF, and it is shown that degradation due to inter-wavelength-band nonlinear interactions is negligible in the transmission. Moreover, the transmission performance of an S+-band linear repeating system is estimated by computer simulation, and compared with that of other wavelength-band systems. In the experiments, thulium-doped fiber amplifiers (TDFAs) are used for amplification of signals in the S+ band.

A novel 1470-nm-band (S+ band) wavelength-division multiplexing (WDM) transmission system is described. The first advantage of S+-band transmission is suppression of degradation caused by four-wave mixing (FWM), which has been the dominant impairment factor in WDM transmission systems on dispersion-shifted fibers (DSFs). FWM suppression by using the S+ band instead of the conventional 1550-nm-band (M band) is successfully demonstrated. The second advantage is expansion of the usable bandwidth by using the S+ band together with other wavelength bands. A triple-wavelength-band WDM repeaterless transmission experiment using the S+ band, the M band and the L band (1580-nm-band) is conducted over DSF, and it is shown that degradation due to inter-wavelength-band nonlinear interactions is negligible in the transmission. Moreover, the transmission performance of an S+-band linear repeating system is estimated by computer simulation, and compared with that of other wavelength-band systems. In the experiments, thulium-doped fiber amplifiers (TDFAs) are used for amplification of signals in the S+ band.

We have developed a wavelength-swept laser source with ultrahigh phase stability. A potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystal was employed as an electro-optic deflector for a high-speed wavelength sweep in the laser cavity. The device structure and performance of a KTN deflector is described. The device includes the beam-shaping optics, which can enhance the resolution of a KTN deflector. A 200-kHz sweep rate was obtained with an average output power of 20mW and a coherence length of 8mm for a wavelength range exceeding 100nm. We demonstrated a swept source with ultrahigh phase stability in the 1.3µm wavelength range as a result of the low-jitter operation of the deflector. The standard deviation of timing jitters measured between adjacent A-lines was confirmed to be less than 78ps, which corresponds to a phase difference of 0.017 radians at a Michelson interferometer path difference of 1.5mm. In addition to realizing the phase stability of neighboring A-lines, a long-term stable sweep was demonstrated by eliminating the refresh operation that was previously needed to prevent output power decay.