All-Optical Tunable Wavelength Shifting of a 128-Gbit/s 64-QAM Signal

Wavelength shifting of a 21.4-Gbaud 64-QAM signal is achieved using Bragg scattering with dither-free pumping in a longitudinally stressed highly nonlinear silica fiber. Three wavelengths on a 10-nm tuning range centered on a 24-nm shift are demonstrated with a penalty of less than 2 dB at a BER of 10-3. Introduction The evolution to ever more complex modulation formats for optical signals has increased the cost and complexity of transmitters and receivers and consequently the cost of changing the wavelength of a signal using ubiquitous OEO (optical-to-electrical-tooptical) techniques. 

In contrast, the cost of alloptical techniques for wavelength shifting is relatively insensitive to the complexity of the modulation format. It becomes attractive therefore to explore the application of these techniques to complex modulation formats. Here we use parametric mixing in a highly nonlinear silica fiber to shift the wavelength of a 64-QAM (quadrature amplitude modulation) signal1-3. Parametric mixing in highly nonlinear silica fiber has been used in numerous demonstrations of manipulations of on/off keyed signals, but as we evolve to higher-order modulation formats, there exists a concern about the wavelength-shifting performance of silica-fiber-based mixers. 

The optical powers of the pump signals driving the parametric process in these fibers generally exceed the threshold for stimulated Brillouin scattering (SBS), requiring a dithering of the frequency of the pumps to broaden the pumps to bandwidths exceeding many-fold the ~25-MHz bandwidth of the Brillouin process4.