All-optical wavelength conversion works at high speed
A wavelength conversion scheme for return-to-zero data uses semiconductor optical amplifiers (SOAs) but is not limited by the SOA's carrier recombination speed. Future WDM networks will probably need all-optical wavelength converters that work at high speed, and converters based on semiconductor optical amplifiers (SOAs) that operate up to 40 Gbit/s are becoming attractive because of their small size, cascadibility, and ease of integration. These SOA converters, however, don't work well for return-to-zero (RZ) data because their speed is limited by interband carrier recombination time in the SOAs.
Min-Yong Jeon and colleagues at the Electronics and Telecommunications Research Institute (Taejon, Korea) and Telpia (Taejon, Korea) recently described an all-optical wavelength conversion system for 20-Gbit/s RZ format data from 1525 nm to 1563 nm, which is not limited by the speed of the SOA.1 The group used an SOA converter incorporated with a stabilized asymmetric Mach-Zehnder interferometer (AMZI).
The AMZI consists of a 1313-nm DFB laser diode, two identical 1550/1313 nm dichroic couplers of WDM1 and WDM2 couplers, two photo-diodes (PDs), and an electronic stabilizing circuit (see Fig. 1). When the SOA received an RZ input signal (at wavelength of ls), it is driven into the saturation regime and the SOA's gain is accordingly modulated with the reverse polarity. By injecting CW light at another wavelength (lc,) the modulation is encoded on this new wavelength. The rise time of the converted signal is determined by the input pulsewidth, while the fall time is determined by the SOA's relatively long carrier lifetime.
The waveform distortion of the SOA output due to this slow gain recovery is then removed by using destructive interference. The slow gain recovery tail of the SOA (shown in the traces for Port 1 and Port 2 output) can be used to cancel each other out. Therefore, the output pulsewidth is determined by the path difference between two arms of the AMZI, not the SOA recovery time (see Fig. 2). The researchers changed the path length of one arm of the interferometer by fine-tuning the wavelength of the 1313-nm laser diode.
A BER penalty of the converted signals is less than 1.2 dB compared to that of the back-to-back signals was achieved for the entire wavelength conversion range from 1530 to 1560 nm with AMZI. The researchers believe the device will be useful for practical WDM communication systems with a high-speed channel bit rate.
- M-Y Jeon et al., IEEE Phot. Tech. Lett. 12(11), 1528 (Nov. 11, 2000).
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.