All-pass filters change the phase, but not (ideally) the amplitude of a signal.2 "Allpass filters are a building block just like Mach-Zehnder interferometers," said Madsen.
All-pass filters can be made using etalons, and several companies are pursuing etalon implementations. But these filters can also be made using ring resonators, and that is the approach that the Bell Labs group is taking now. Ring resonators can be implemented very compactly using planar waveguides and scaling up to a number of stages is easy. In addition, planar waveguide integration provides increased functionality in a compact size and at a reduced cost.
For 40-Gbit/s compensation, however, the rings must be very small because the ring's bandwidth is inversely proportional to the ring size. To make such small rings, one needs a waveguide with a contrast between the core and cladding refractive indices much larger than the standard index-contrast used today in AWG manufacture in silica waveguides.
The group described a fully tunable dispersion compensator that uses all-pass filters made of integrated ring resonators (see figure) that incorporates a Mach-Zehnder interferometer within the ring. (The Mach-Zehnder interferometer is used for tuning, but doesn't contribute to the desired frequency response.) The device compensates for chromatic dispersion in 10- and 40-Gbit/s signals. The two phase-shifters allow the device to be completely tunable. By choosing a multistage filter, with optical parameters chosen for each stage, a constant dispersion can be approximated over much of the free spectral range (FSR).
The compensators were made using germanium-doped silica-on-silicon planar waveguides defined by photolithography and reactive ion etching. The group has achieved core-to-cladding index contrasts as high as 4%. "For comparison," says Madsen, "current high-index-contrast AWG research by NTT uses 1.5% index contrast." The Bell Labs researchers made devices with ring radii as small as 350 µm, and used thin-film heaters to create thermo-optic phase-shifters. The wavelength-dependent filter loss depends on the round-trip loss, measured as low as 0.4 dB. For rings with a FSR of 23 GHz, a passband of 16 GHz was maintained with low group delay ripple.
The researchers also built a four-stage filter with a dispersion-tuning range of 4000 ps/nm and passband width of 15 GHz to compensate 10-Gbit/s NRZ signals launched into a fiber with 0-dBm power from a single laser. Over a range of 17 nm, the filter compensated the dispersion with a penalty of less than 1 dB. More recently, they demonstrated filters with bandwidth utilizations up to 80% and passbands as large as 60 GHz, which compensated for 40-Gbit/s non-return-to-zero and carrier-suppressed return-to-zero signals.
For more information contact Christi K. Madsen at [email protected].
Yvonne Carts-Powell
- C. K. Madsen et al., LEOS Summer Topical Meet. 2002 invited paper TuJ2 (July 15–17, 2002).
- C. K. Madsen and J.H. Zhao, Optical Filter Design and Analysis: A Signal Processing Approach, John Wiley: NY (1999).