448 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 4, APRIL 1999 Thermooptic Planar Polymer Bragg Grating OADM’s with Broad Tuning Range Louay Eldada, Member, IEEE, Robert Blomquist, Mac Maxfield, Deepti Pant, George Boudoughian, Constantina Poga, and Robert A. Norwood Abstract— Tunable optical add/drop multiplexers (OADM’s) were achieved by combining thermally tunable planar polymer Bragg gratings with optical circulators. The gratings exhibit better than 45-dB reflection with no detectable out-of-band re- flection. Apodization was utilized to achieve strong sidelobe sup- pression, and symmetric gratings with uniform strength across the core and cladding layers were used to minimize coupling to cladding modes. These OADM’s have a high-bandwidth utiliza- tion (BWU) factor of 0.92, with a minimum channel spacing of 75 GHz. They are tunable at a rate of 0.256 nm/ C. Tuning over a 20-nm range was demonstrated with a single device. Index Terms—Bragg gratings, dense wavelength-division mul- tiplexing, optical add/drop multiplexers, optical planar waveguide components, optical polymers, thermal tuning, thermooptic effect, tunable filters. I. INTRODUCTION O PTICAL polymers offer both unique possibilities and value in the production of photonic components. We have developed and extensively tested a polymer technology whereby quality gratings can be produced in single-mode channel waveguides on a planar substrate. By combining these chips with optical circulators, high-performance optical add/drop multiplexers (OADM’s) can be achieved. The large variation of the polymer refractive index with temperature permits thermal tuning of the filter channel over a wide wavelength range, a functionality capable of enabling an architectural revolution in the telecommunication industry. II. DESIGN A simple device of the type shown in Fig. 1 acts as a narrow-band thermally tunable OADM that can cover a 15- nm wavelength window with a reasonable temperature range (e.g., 20 C–80 C). The use of two such components in series allows full coverage of the erbium “C” band between 1535 and 1565 nm. The chip consists of a grating printed in an integrated single-mode polymeric waveguide with a deposited thin film heater. This chip, combined with two three-port optical circulators, forms the OADM. In this design, the only critical requirement for the die is to have a good grating. A good grating is one that reflects a narrow band of light efficiently and uniformly with no out-of-band spectral features and has a low insertion loss. The circulators provide the add Manuscript received October 15, 1998; revised December 23, 1998. The authors are with AlliedSignal Inc., Telecom Photonics Division, Mor- ristown, NJ 07962 USA. Publisher Item Identifier S 1041-1135(99)02529-X. Fig. 1. OADM configuration consisting of a chip (which integrates a grating printed in a single-mode polymeric waveguide with a thin-film heater) and two three-port optical circulators. and drop functions in a reliable fashion, circumventing the yield issues associated with 3-dB couplers in Mach–Zehnder- based add/drop filters. These OADM’s offer low, uniform insertion loss for add/drop paths ( 2 dB), efficient dropping of the reflected channel through the use of circulators with high isolation ( 60 dB), and low out-of-band reflections into the drop port through the use of apodized gratings that are symmetric, have uniform strength across the core and cladding layers, and have a uniform dc refractive index. The general design of placing a grating between optical circulators to form an OADM has been implemented in the past with fiber Bragg gratings [1], however due to the small of glass (about 25 times smaller than that of our polymers), those filters cannot be tuned thermally, and must be tuned by less reliable mechanical means. III. EXPERIMENTAL RESULTS The design shown in Fig. 1 was implemented using Al- liedSignal optical polymers. These materials are formed from highly crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, loss, and environmental stability. These monomers are intermisci- ble, providing for precise adjustment of the refractive index from 1.3 to 1.6. In polymer form, they exhibit state-of-the- art loss values (about 0.5 and 0.1 dB/cm for our standard full-CH-content and 80%-halogenated acrylates, respectively, at 1550 nm), temperature resistance [2], humidity resistance (no humidity-induced loss in unprotected waveguides after 600 hours at 85 C 85% RH), low refractive index humidity dependence [3], low refractive index dispersion ( 2 10 nm at 1550 nm) and low birefringence ( at 1550 nm). These materials have also been tested following Bellcore protocols 1209 and 1221. Waveguides are formed photolithographically, with the liquid monomer mixture polymerizing upon illumination in the UV 1041–1135/99$10.00  1999 IEEE