IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 6, JUNE 1998 893 240-km Repeater Spacing in a 5280-km WDM System Experiment Using 8 2.5 Gb/s NRZ Transmission Matthew X. Ma, Howard D. Kidorf, Karsten Rottwitt, Frank W. Kerfoot, III, and Carl R. Davidson Abstract— A repeater spacing of 240 km was experimen- tally demonstrated over a distance of 5280 km in an Gb/s wavelength-division-multiplexing (WDM) transmission experiment. This long-span length was achieved by a combination of Raman amplification, a locally pumped erbium-doped fiber (EDF), and two remotely pumped EDF’s. The total gain obtained from those four gain elements was about 50 dB. The performance demonstrated in this experiment was comparable to that of a system of similar length and capacity using conventional erbium- doped fiber amplifier (EDFA) technology at a repeater spacing of 80 km. Index Terms— Optical fiber amplifiers, optical fiber commu- nication, power lasers, Raman scattering, wavelength-division multiplexing. I. INTRODUCTION I N THE QUEST to improve the performance of long-haul transmission systems, there is a desire to increase both the repeater spacing and the transmission capacity [1]. In general, extending the repeater spacing results in an increase of the accumulated optical noise, which limits total transmission capacity. In this letter, we have significantly increased the repeater spacing of a long-haul transmission system while maintaining high-transmission capacity by using span engi- neering techniques first proposed for repeaterless systems [2] together with wavelength-division multiplexing (WDM). For the first time, the combination of remotely pumped erbium- doped fiber amplifiers (EDFA’s) and Raman gain [3] is used to extend the repeater spacing of a long haul transmission system. We demonstrate a technology that extends the repeater spacing to 240 km for a 8 2.5 Gb/s WDM transmission system operating over 5280 km. High-power pump laser technology based on dual cladded fiber [4], was used to obtain a pump power level of approximately 1.2 W per pump at 1480 nm. This pump power was used to stimulate gain from both erbium doped fiber and from the Raman process within the transmission fiber. II. EXPERIMENTAL SETUP The experiment was performed using an eight-channel WDM transmitter and a 480-km transmission line in a Manuscript received January 8, 1998; revised February 9, 1998. The authors are with Tyco Submarine Systems Ltd. Laboratories, Holmdel, NJ 07733 USA. Publisher Item Identifier S 1041-1135(98)03819-1. circulating loop. The transmitted channels were uniformly spaced between 1554.5 and 1561.5 nm with a 1.0-nm channel separation. All channels were modulated at a bit rate of 2.488 Gb/s using a 2 1 pseudorandom bit sequence (PRBS). Alternate channels were modulated using an inverted pattern to assure uncorrelated channel-to-channel interactions. Fig. 1 shows the configuration of one of the two 240-km transmission spans constructed. There were four equivalent amplifiers within each span. The first amplifier is formed by remotely pumping EDF1 through a low-loss pure silica core fiber. A separate fiber between the pump laser and EDF1 was required to avoid interaction of the pump and the signal copropagating within the same fiber. This would result in the noise from the pump transferring to the signal through the Raman amplification caused by the copropagating pump. The remaining three amplifiers were all pumped by a second pump laser used to launch pump light counter-propagating to the signal. The high-pump power first passed through an EDF3 (located at the repeater station) and was then passed into the transmission fiber. This formed a Raman amplifier with approximately 14-dB net gain in the 80-km fiber section. The pump power remaining after the Raman amplifier pumped an inline EDFA formed by EDF2. There were three isolators in each span to reduce the multipath interference caused by double Rayleigh reflections [5]. At the system design stage, we used two types of computer simulations to optimize the system performance. First, we used a linear simulator to select the location of each EDF within the span and to optimize the gain and noise figure of each EDF. The linear simulator modeled the EDFA and the Raman processes which are the only two amplified spontaneous emis- sion (ASE) noise sources during the transmission. The second simulation used the split step algorithm to solve the nonlinear Schr¨ odinger equation for the transmission line. This nonlinear simulator enabled us to analyze the nonlinear impairment in the transmission. For example, the maximum peak signal power was determined by the nonlinear simulator so the nonlinear transmission penalty was limited. The criterion for this optimization was to achieve the maximum signal-to-noise ratio (SNR) at the output of the repeater span while keeping the peak channel power below 1.25 mW to limit the nonlinear impairment. Based on this computer simulation, the distance between the three EDFA’s was chosen to be 80 km. The transmission fiber consisted of dispersion shifted fiber with zero-dispersion wavelength at about 1580 nm and a chro- 1041–1135/98$10.00  1998 IEEE