High Speed Tunable Optical Filter With Variable Channel Spacing

Abstract

_ _ _ _ _ . _ _ _ _ _ _ Optical wavelength division multiplexing (WDM) is a topic of great interest. A wide variety of components for WDM have been proposed and are under development The unbalanced Mach-Zehnder ( M Z ) interferometer on LiNbO3 is a promising candidate having the following advantages: (1) LiNbO3 waveguide fabrication is a mature commercial technology, (2) MZ based components are simple, (3) the number of channels can be increased by cascading multiple MZ devices, and (4) IaiNb03 is electro-optic, thus allowing wavelength selection on a nanosecond time scale suitable for high speed packet switching. We have developed a hybrid fiberbiNb03 interferometer. The hybrid approach is an intermediate step which allows us to test the control electronics during development of the fully integrated L i W 3 MZ chip. In addition, the hybrid MZ has the useful characteristic of adjustable channel spacing, ranging from 2 GHz to >loo0 GHz. The experimental setup is shown in Fig. 1. The hybrid interferometer consists of two 50/50 polarization maintaining (PM) couplers, a high speed LiNbO3 phase modulator, a free space optical attenuator, and 40 meters of single mode PM fiber wrapped around both mechanical and piezoelectric stretchers. The optical attenuator is used to balance the loss in the phase modulator. The mechanical stretchers allow the path length imbalance between the arms of the interferometer to be varied from 0-10 mm, corresponding to filter channel spacings of > 1 GHz). The stretcher assembly is made from Invar and miihtains a stable path length imbalance over long periods of time. A precision reflectometer was used to measilre the optical path length difference (nM) in the interferometer. For this experiment n M was set at 6 mm, Corresponding to a 25 GHz channel spacing.