Raman amplification extends CWDM's reach

CWDM technology employs wider wavelength spacing and less expensive components than does long-haul DWDM. DWDM’s narrower channel spacing makes the use of thermo-electric coolers to stabilize the laser emissions essential. On the other hand, the wide wavelength spacing of CWDM can accommodate the wavelength fluctuation of uncooled directly modulated laser diodes (DMLs), thus allowing the use of these more cost-effective signal sources. However, owing to the output power limitations of the uncooled DMLs and additional loss of multiplexer/demultiplexer and optical add/drop modules, the loss budget of CWDM systems can be limited to <30 dB. Thus, the typical transmission distance is between 40 and 80 km. Also, in some cases the insertion loss of the installed dark fiber is higher than the expected (calculated) loss, resulting in reduced transmission distance or loss budget. Therefore, CWDM systems can require optical amplifiers.

Figure 1. EDFAs, semiconductor optical amplifiers (SOAs), and fiber Raman amplifiers (RAs) can be used to amplify CWDM signals. However, EDFAs and SOAs suffer from limitations that fiber RAs do not.

Figure 1 illustrates the gain bandwidth of various optical amplifiers that might be used for CWDM systems. EDFAs are common elements in optical networks; however, their gain bandwidth does not match well with the channel spacing typical of CWDM equipment. Semiconductor optical amplifiers (SOAs) can cover the wide gain band but have inherent technical limitations. On the other hand, fiber Raman amplifiers (RAs) can extend the usable optical bandwidth by optimizing the pumping lightwave spectrum to meet CWDM requirements.

The effectiveness of a lumped (discrete) Raman amplifier (LRA) within a CWDM system depends on the gain flatness, which is due to the optimization of the wavelength allocation and the pump conversion efficiency. By employing pumping lasers at two wavelengths, gain flatness of <1.5 dB at the center wavelength of CWDM signals can be achieved. But to achieve effective Raman amplification with conventional pumping lasers, highly nonlinear fibers (HNLFs) with a high Raman gain coefficient are used in the LRAs. The Raman gain coefficient of HNLF is twice that of conventional dispersion compensating fiber.

Table 1 displays the characteristics of the amplifiers that can be used as a booster amplifier for CWDM systems. The multiplexing/demultiplexing filter for a CWDM system typically has a full width at half-maximum (FWDM) of 13 nm. Because the CWDM signals are essentially affected by the beat noise between signals and amplified spontaneous emission noise of the amplifier, booster amplifiers generally are necessary for CWDM systems.

EDFAs have high saturation output power (>20 dBm) with a low noise figure (<6 dB). But as discussed previously, it is evident that EDFAs don’t perform well in the S-band (1460-1520 nm) and upper L-band (1600-1620 nm). SOAs have a small form factor and can be integrated with other functions on an indium phosphide substrate. However, the SOA's nonlinearities cause crosstalk between channels and crosstalk between bits (TDM crosstalk). Furthermore, because SOAs have a relatively low saturated output power, they suffer from a high noise figure and polarization sensitivity.

As Table 1 shows, an LRA is a good choice for a booster amplifier in CWDM systems. The LRA provides both the seamless wideband amplification that an EDFA cannot as well as the good transmission characteristics the SOA lacks.

Figure 2 illustrates an experimental setup of CWDM transmission with a booster LRA. The wavelengths of four-channel CWDM signals from multi-quantum well (MQW) DMLs were arranged from 1511 to 1571 nm with 20-nm spacing. The bit rate of each CWDM signal was 2.488 Gbits/sec (OC-48). The input and output power of the LRA was +0 dBm/channel and +10 dBm/channel (a total of 16 dBm), respectively. The net gain of each channel was more than 10 dB, with a maximum noise figure of 6.5 dB. The polarization-dependent gain of the LRA was <0.3 dB.

Figure 2. In this CWDM transmission experiment, the input and output power of the lumped (discrete) Raman amplifier (LRA) was +0 dBm/channel and +10 dBm/channel, respectively. The LRA was used as a booster amplifier at the transmitter side.

Table 2 lists the experimental results of the receiver sensitivity, which is for a bit-error rate (BER) of 10-9 using a pseudo-random bit sequence (PRBS) with a 231-1 word length. As Table 2 shows, the power penalty of a 100-km transmission over singlemode fiber (SMF) with an LRA was at least 0.3 dB better than that of a similar link without an LRA. The improved transmission characteristics are due to the HNLF’s dispersion and the pulse compression effect created by the self-phase modulation in the HNLF. Moreover, 150-km transmission over SMF with a power penalty of <2 dB and without any repeater stations also can be achieved by employing an LRA. Figure 3 shows potential applications of CWDM systems using LRAs. In the conventional point-to-point CWDM system, the transmission distance is mainly limited by the loss budget (see Figure 3a). To extend the reach of CWDM transmission systems, additional optical-electrical-optical (OEO) repeaters consisting of optical transmitters and receivers (see Figure 3b) normally are required. Along with the equipment itself, additional repeater sites need to be prepared to install and operate the equipment. Figure 3c shows a simple configuration of a long-distance CWDM transmission employing LRAs, which can be placed in the existing terminals as booster amplifiers.

Figure 3. Lumped (discrete) Raman amplifiers (LRAs) can be beneficial to both point-to-point and ring CWDM topologies.

In addition to point-to-point transmission systems, the LRA can be applied to ring networks with optical add/drop modules (OADMs) as shown in Figure 3d. The total number of optical nodes and the size of the ring are limited by the loss budget of the CWDM system and the insertion loss of each OADM. To increase the number of the optical nodes and/or extend the ring size, the addition of OEO repeaters is essential. However, the use of LRAs to increase the loss budget of the CWDM systems enables flexible expansion of the ring without the use of additional repeaters.

The cost of the LRA is comparable to that of a four-channel CWDM repeater. Taking both capital and operational expenses for constructing and maintaining additional repeater sites into consideration, CWDM systems employing LRAs are clearly more economical. The use of LRAs in CWDM systems should expand the application of CWDM into fields where DWDM systems employing EDFAs are commonly used today. The advent of the LRA should help increase the market for CWDM systems in the future.

Reach extension of CWDM systems using LRAs allows a flexible operational arrangement of the CWDM approach with low operating expenses without building repeater stations. LRAs are optimized for CWDM applications with the characteristics of seamless amplification and saturation output power that could not be achieved by employing EDFAs or SOAs. By using a booster LRA, 150-km transmission with conventional SMF has successfully been demonstrated. Moreover, not only is transmission length enhanced, but also the loss compensation of OADMs, and flexible telecommunications systems can be achieved by employing CWDM systems with LRAs.

This work is supported in part by National Institute of Information and Communications Technology (NICT).

Toshiyuki Miyamoto is a research engineer in optical communications for the Optical Communications R&D division of Sumitomo Electric Industries (Yokohama, Japan).Ron Lindsayis a director of engineering at ExceLight Communications (Durham, NC). They can be reached at [email protected] and [email protected].