1506 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 11, NOVEMBER 1999 Triple-Wavelength-Band WDM Transmission Over Cascaded Dispersion-Shifted Fibers J. Kani, Member, IEEE, K. Hattori, Member, IEEE, M. Jinno, Member, IEEE, T. Kanamori, and K. Oguchi, Member, IEEE Abstract— Triple-wavelength-band WDM transmission over cascaded dispersion-shifted fibers (DSF’s) is demonstrated. A total of 54 10-Gb/s signals in the 1470-, 1550-, and 1590-nm bands are transmitted through 240 km (3 80 km) DSF’s. The effect of interwavelength-band nonlinear interactions expected from the stimulated-Raman-scattering-induced excess loss are shown to be negligible. Index Terms—Dispersion-shifted fiber, optical fiber communi- cations, thulium-doped fiber amplifier, wavelength-division mul- tiplexing. I. INTRODUCTION O PTICAL networks based on wavelength-division- multiplexing (WDM) transmission systems with abun- dant number of wavelengths have been recognized as the best way to supply enormous capacity as well as advanced functions such as those offered by the optical path architecture [1]. However, the available number of wavelengths in WDM transmission systems has been limited by the finite gain bandwidth of erbium-doped fiber amplifiers (EDFA’s), even though the low-loss window of silica optical fiber ranges from 1450 to 1650 nm ( 0.3 dB/km). Using frequencies outside the EDFA-gain band is an attractive way of supplying the abundant wavelengths. Moreover, within the conventional EDFA-gain band, WDM transmission systems on dispersion-shifted fibers (DSF’s) experience strong four-wave mixing (FWM) effects due to fiber zero-dispersion in the band which cause strong signal interference. Available number of wavelengths is limited to 12 because unequal channel spacing is needed to avoid matching the FWM wavelengths to a signal wavelength. We recently described design guidelines for ultrawide-band WDM transmission systems that utilize the whole low-loss window. We set five equal wavelength-bands in the low-loss window: S -band (1450 to 1490 nm), S-band (1490 to 1530 nm), M-band (or C-band, 1530 to 1570 nm, corresponds to the conventional EDFA-gain band), L-band (1570 to 1610 nm), and L -band (1610 to 1650 nm) [2]. The guidelines show how to suppress the effect of nonlinear interaction between signals in different wavelength bands (called interwavelength-band nonlinear interactions), such as stimulated Raman scattering (SRS) crosstalk, cross-phase modulation (XPM), and nonde- Manuscript received April 12, 1999. J. Kani, K. Hattori, M. Jinno, and K. Oguchi are with the NTT Network Innovation Laboratories, Yokosuka, Kanagawa 239-0847, Japan. T. Kanamori is with the NTT Photonics Laboratories, Yokosuka, Kanagawa 239-0847, Japan. Publisher Item Identifier S 1041-1135(99)08679-6. Fig. 1. Experimental setup, and spectra (a) before and (b) after transmission. generate FWM (ND-FWM) [4]. According to the guidelines, we successfully demonstrated triple wavelength-band repeater- less WDM transmission; a total of 38 10 Gb/s WDM signals in the S -, M-, and L-bands were transmitted through a 100-km DSF [3]. In repeaterless transmission, the effect of interwavelength-band nonlinear interactions were shown to be minimized because of the large walk off between any two wavelength bands [2], [3]. In this letter, we report triple wavelength-band transmission over a series of DSF’s linked by ultrawide band optical repeaters. A total of 54 Gb/s WDM signals are successfully transmitted through 240 km of DSF. The interwavelength-band interactions are shown to be negligible even though the 80-km fibers are linked by optical repeaters. II. EXPERIMENTAL SETUP Fig. 1 shows the experimental setup. We used 10 signals in the S -band ranging from 1464 to 1478 nm, 12 signals in the M-band ranging from 1535 to 1558 nm, and 32 signals in the L-band ranging from 1574 to 1600 nm. Signals in the S - and L-bands were given the equal channel spacing of 200 and 100 GHz, respectively. Signals in the M-band were given unequal channel spacing, (a 25-GHz frequency grid was used with minimum channel spacing of 125-GHz), in order to suppress the degradation due to the FWM between the signals within the M-band. The laser diode outputs in each band were multiplexed using an arrayed waveguide grating (AWG) and modulated at 10 Gb/s with a Mach–Zehnder inten- sity modulator using 2 1 NRZ pseudorandom bit streams (PRBS). Signal pulses in every band were initially chirped in the modulator to compensate for the waveform distortion created by the finite dispersion of the transmission fiber and 1041–1135/99$10.00  1999 IEEE