Valerie C. Coffey
Researchers at Lucent Technologies Bell Laboratories (Holmdel, NJ) have demonstrated the transmission of a 10-Gbit/s modulated WDM signal over 500 km of nonzero dispersion-shifted fiber (NZDSF) using a semiconductor optical amplifier (SOA) as an in-line amplifier. Thanks to a new wavelength modulation technique that maintains constant channel power in the SOA, this may be the longest distance transmission with the most cascading on a saturated SOA.
Semiconductor optical amplifiers have the advantages of compact size, ease of integration with other devices, ultrawide available gain bandwidth covering most of the optical communication band, and potentially low cost. However, very fast gain dynamics strongly limit the use of SOAs as amplifying devices in WDM systems because of a cross-gain-modulation (CGM) effect. Previous attempts to reduce the CGM effect were limited by a low optical signal-to-noise ratio, which results from cascading the SOAs. In the Bell Labs approach, the applied wavelength modulation technique reduces the CGM effect by maintaining a constant channel power.
Generated by a dual-input single-output Mach-Zehnder LiNbO3 modulator (see Fig. 1), the signal has four channels (1550.6 to 1555.5 nm) 1.6 nm apart. The corresponding dummy signals are shifted 0.3 nm from the main signal. A long-distance transmission is simulated using a recirculating loop. In the loop, one SOA compensates for the 15-dB loss induced in 50 km of NZDS fiber and the related dispersion-compensating fiber, and one EDFA is used to compensate the loss (7 dB) in the devices used for the loop operation. The WDM signal and related main and dummy signals are demultiplexed and discriminated in front of the receiver by using an optical filter having a 3-dB linewidth of 0.25 nm.
The resulting bit-error rate (BER) is better than 10-14, and even after 600 km transmission, the estimated BER for the worst channel is approximately 10-11, which is mainly limited by the optical signal-to-noise ratio (~21.7 dB). The resulting eye pattern of the worst channel (channel 2) after 500 km transmission shows the advantages of the wavelength modulation (see Fig. 2).
Lucent will continue to investigate limitations of this modulation method, including the accumulated dispersion before the SOA, which could alter the constant power characteristics of the wavelength-modulated signal. For more information contact Hyang K. Kim at firstname.lastname@example.org.
FIGURE 1: Two signals are fed into a continuous-loop system, successfully modulating a 10-Gbit/s WDM signal over 500 km of fiber.
FIGURE 2: The eye pattern for the worst channel is shown on the left with wavelength modulation after 500 km, compared to the nearly closed eye pattern on the right without wavelength modulation.
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