In an ECOC 2001 postdeadline paper, researchers from the University of Paderborn (Germany) and Pirelli Labs S.p.a. (Milan, Italy) reported the simultaneous conversion of four wavelengths with an efficiency of -10 dB. The difference-frequency generation process exploited here takes place inside periodically poled lithium niobate (LiNbO3) waveguides. These have the advantage of having a large bandwidth, potentially covering the S- or L-bands, as well as standard communications wavelengths. In addition, the devices do not deform the shape of the signal (converted or not)—a crucial attribute for communications.
Difference-frequency generation involves using the second-order nonlinearity in LiNbO3 to produce a copy of some signal at another wavelength. The relationship between the signal and its copy is determined by the pump frequency: the new signal has the same "distance" from the pump, but is positive instead of negative (or vice versa). For example, a signal at 1559 nm pumped at 1556 nm will produce an idler (copy) at 1553 nm. The process works very quickly, and thus can accommodate high bit rates and preserve the temporal shape of pulses converted.
The system used in the German/Italian effort has 7-µm-wide stripes of titanium, diffused into a commercially produced 0.5-mm-thick LiNbO3 crystal, to define a waveguide structure. Then the crystal is poled with a period of 16.6 µm (to phase-match the device for the interaction to take place, thus making best use of the intrinsic nonlinearity), after which the ends are polished and antireflection-coated. Fiber pigtails are used to bring in the pump and signal light, but these are positioned a few micrometers from the waveguide to allow the LiNbO3 to be heated (avoiding photorefractive damage) without affecting the fiber.
In one experiment, three different kinds of lasers at four different wavelengths were used to supply the signal, this time to be upconverted by a higher pump beam (see figure). All four, including a 10-GHz, 4-ps-width pulse train, were accurately converted. In another experiment, the team showed that after 500 km, including dispersion compensation, the bit-error rate for both converted and unconverted signals remained below10-13.
For more information contact Hubertus Suche at email@example.com or Alessandro Schiffini at firstname.lastname@example.org.
- H. Suche et al., ECOC 2001, postdeadline paper (September/October 2001).