Nonlinear converter makes 10-GHz RZ source tunable

June 1, 2001

Researchers in Denmark have developed a technique that allows a new highly nonlinear fiber to tune the wavelength of a fixed-wavelength source. A collaboration between researchers at the Technical University of Denmark, Lyngby, and Lucent Technologies Denmark, BrØndby, the system produces tuned pulses that have transmission characteristics similar to the original source and can be used to produce more than one wavelength channel at a time. Though the tuning bandwidth has so far only been demonstrated at 14 nm, researchers say that the maximum set by the nonlinear fiber used is much higher, up to 76 nm. Their current limitation, they say, is set only by the gain of the erbium-doped fiber amplifiers (EDFAs) used.

The wavelength conversion is performed through four-wave mixing inside a new fiber that has a high nonlinear coefficient.¹ This significantly reduces the length of fiber necessary for the nonlinear interaction to take place: in this case, to about 500 m. A 10-GHz return-to-zero (RZ) source (a gain-switched distributed feedback laser in the Danish experiments) is used to provide the pump beam at the zero-dispersion wavelength (1553.6 nm) of the highly nonlinear fiber (HNLF). Before entering this fiber, the signal is first compressed, amplified, and then bandpass-filtered. The probe is supplied by a commercially available, continuous-wave source, tunable between 1525 and 1610 nm and polarization-controlled to match the RZ pulse-source signal.

Inside the HNLF, energy from the pump is converted into the wavelength of the probe or probes (see figure), producing a series of 7 ps full-width half-maximum pulses (from 6.3 ps at the original wavelength) that widen by up to 0.4 ps depending on the wavelength. The conversion penalty was about 0.7 dB, with transmission penalties of 0.7 and 1 dB for the converted and original-wavelength pulses respectively. In a demultiplexing experiment, researchers say the converted pulses had a conversion penalty that was negligibly higher than that of the original signal.

For additional information, contact A. T. Clausen at [email protected].

Sunny Bains

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