Ultra-High-Capacity Optical Communications and Networking: High-Performance Electrooptic Tunable Filters for Dense Wavelength Division Multiplexing

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

0123367 Eknoyan This proposal was submitted in response to the solicitation NSF 01-65 on "Ultra-High Capacity Optical Communications and Networking." The goal of this project is to demonstrate unprecedented tuning range and speed in electrooptic tunable filters for dense wavelength division multiplexing (WDM) using recently developed design principles and fabrication methods. The filters, which will be produced on LiNb03 substrates, are expected to provide a combination of faster tuning (< 50 ns) and wider tuning range (> 10 THz) than has previously been achieved in any tunable filter technology. Optical frequency selection in the proposed filters is based upon phase-matched polarization mode conversion by a spatially periodic strain perturbation in a single mode optical waveguide. Tuning is accomplished by an applied electric field which alters the waveguide birefringence, and hence the phase-match frequency at which near-complete polarization conversion occurs. The devices are inherently polarization independent because the TE TM and TM TE coupling are reciprocal processes. Innovations from the research at Texas A&M which will be incorporated into these devices include: (1) The use of spatially periodic strips of SiO2 to induce polarization coupling of light via the strain-optic effect. The SiO2 is deposited at an elevated temperature on the LiNb03 substrates and patterned at room temperature, so that longitudinal strain results from thermal expansion mismatch. (2) A new filter design which eliminates the need for polarizing beam splitters and thus gives an extra degree of freedom in the fabrication process. (3) The use of a sparse, apodized strain grating to achieve polarization coupling. This gives an etalon-like periodicity to the filter response, with nulls which are almost evenly spaced in frequency. This design technique should make it possible to extend the spectral range by an order of magnitude over previous EOTF designs. Preliminary laboratory work directed towards the optimization of LiNb03 circuit elements has prepared the way for the proposed research. Phase-matched polarization coupling with > 99.3% efficiency for both TE TM and TM TE conversion has been achieved in straight waveguides at a wavelength of 1537 nm, and electrooptic tuning of the peak conversion wavelength over a range > 15 nm (2 THz) was also demonstrated. Response speed for polarization conversion was measured to be 52 ns. Both polarizing beam splitters with splitting ratios > 25 dB and non-polarizing beam splitters suitable for EOTF application have been produced and characterized. Under this project, four-port spectral slicing filters, two-port bandpass filters, and four-port add-drop filters with 50 GHz and 100 GHz channel spacing will be fabricated on LiNb03 substrates. Spectral and temporal response of the filters and their tuning range will be characterized.