1890 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 10, OCTOBER 1999 Etched Cavity InGaAsP/InP Waveguide Fabry–Perot Filter Tunable by Current Injection H. K. Tsang, Mark W. K. Mak, L. Y. Chan, J. B. D. Soole, C. Youtsey, and I. Adesida Abstract—A short cavity Fabry–Perot (FP) tunable filter was fabricated by chemically assisted ion beam etching (CAIBE) of an InGaAsP/InP waveguide. The FP cavity mirrors were formed by high reflectivity dielectric coatings deposited onto the two etched facets of the cavity. High-speed (less than 3 ns) wavelength-tuning of the filter was achieved by current injection. A super-linear dependence on current injection was observed in the wavelength tuning. The filter was tunable over a continuous range of 4 nm by up to 50 mA current injection. The device had a free- spectral range of 9 nm and gave a contrast ratio of about 10 dB. A theoretical simulation of the filter was developed using a beam propagation method calculation to estimate the reflection efficiency at different mirror tilt angles. The calculated contrast ratio of the filter was in good agreement with the measured result. The calculations showed that 20 dB contrast ratio should be possible by having a facet reflectivity of over 90% and ensuring that the etched facets are off-vertical by less than 1.8 . Index Terms—Chemically assisted ion beam etching (CAIBE), Fabry–Perot, integrated waveguides, tunable filter, wavelength division multiplexing (WDM) filter. I. INTRODUCTION W AVELENGTH division multiplexing (WDM) is an attractive solution for broad-band optical communica- tion systems because it can enable the expansion of existing networks and allow efficient network architectures. A key component of a WDM system is a tunable optical filter for use in optical cross-connects [1] and wavelength add–drop multiplexers [2]. A wide range of approaches have been investigated for tunable filters in WDM systems including the use of fiber Fabry–Perot filters [3], [4], electromechanically tuned micromachined semiconductor Fabry–Perot filters [5], [6], Bragg grating filters [7], acoustooptic filters [8], liquid crystal filters [9] and arrayed waveguide grating devices in- tegrated with phase shifters [10]. The switching times of the electromechanically tuned filters range from milliseconds to tens of microseconds [5], [6]. Faster switching speeds are desirable for use in systems such as a high throughput packet switched wavelength division multiple access (WDMA) network [11]. Channel access times as short as a few tens of Manuscript received October 13, 1998; revised June 23, 1999. The work of H. K. Tsang, M. W. K. Mak, and L. Y. Chan was supported by a Hong Kong Research Grant Council Earmarked Grant CU-288-94E. H. K. Tsang, M. W. K. Mak, and L. Y. Chan are with the Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong, R.O.C. J. B. D. Soole was with Bellcore, Red Bank, NJ 07701 USA. He is now with Lucent Technologies, Bell Laboratories, Murray Hill, NJ 07974 USA. C. Youtsey and I. Adesida are with the Microelectronics Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA. Publisher Item Identifier S 0733-8724(99)08015-9. Fig. 1. Schematic structure of FP filter. nanoseconds may be required and can be accomplished by using an optical demultiplexer with an electronically switched photodetector array [12]. An alternative solution, needing fewer components, is to tune a semiconductor optical filter by electrical current injection. Current injection can offer nanosecond tuning speed, being limited by the carrier lifetime in the material (or the time to sweep out the injected carriers by a reverse bias). Electrical current injection was recently used for tuning an arrayed waveguide grating by controlling the phase of individual waveguides [10]. In this paper, we implement a tunable filter by making an etched cavity InGaAsP waveguide to form a Fabry–Perot filter that can be tuned at high speed by free-carrier injection. Our device is similar to the recently proposed silicon-on-insulator (SOI) waveguide interferometer [13]. We describe some of the theoretical issues in the device design and present experimental results on an InGaAsP/InP waveguide Fabry–Perot tunable filter. II. FILTER DESIGN Fig. 1 depicts the basic concept of the Fabry–Perot wave- guide filter. The FP cavity is formed in a single-mode low- loss waveguide between two etched facets upon which are deposited high-reflectivity dielectric coatings. Antireflection coatings are deposited on the surface facing the FP facet across the mirror trench in order to prevent degradation of the FP transmission spectrum by multiple cavities which can produce undesired additional peaks in the transmission characteristics. The design of the waveguide FP filter involves a tradeoff between the desired free spectral range (FSR), which sets the limit on the number of channels that can be present if crosstalk from channels falling in adjacent FP transmission peaks is to be avoided, and the optical phase change that can be generated to tune the filter. In order to attain a wide FSR it is necessary to have a short cavity length since the FSR of a cavity of length and refractive index is approximately given by (e.g., for a FSR of 8 nm a waveguide 0733–8724/99$10.00  1999 IEEE