590 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 5, MAY 1999 Polymeric Tunable Optical Attenuator with an Optical Monitoring Tap for WDM Transmission Network Sang-Shin Lee, Yong-Sung Jin, Yung-Sung Son, and Tae-Kyung Yoo Abstract— A polymeric tunable optical attenuator has been fabricated using asymmetric Y-branch waveguides integrated with an optical monitoring tap. One arm of the branch serves as the main output port, while the other arm helps remove the residual optical power resulting from attenuation. An optical power tap used for the monitoring port is introduced to take a fraction of the main port power. This monitoring port is useful for observing the main output continuously. Furthermore, it can be fed back to the electrical driver to maintain the attenuated output regardless of variations in input light polarization and power. By utilizing the modal evolution effect in asymmetric branches, the attenuator has enhanced fabrication tolerance and reduced wavelength sensitivity. Also, no electrical bias is needed to achieve maximum optical transmission. The attenuator exhibited a dy- namic range of more than 20 dB at 1550 nm and a response time of about 1 ms. The polarization dependent loss was about 0.9 dB, and the wavelength uniformity was less than 1.2 dB over the range of 1530–1560 nm. Index Terms—Integrated optics, optical communication, optical polymers, optical waveguide components, thermooptic effects. I. INTRODUCTION A TUNABLE optical attenuator (TOA) is one of the essen- tial components for the implementation of wavelength- division-multiplexed (WDM) systems. It is useful for gain con- trol of optical amplifiers in WDM networks and for dynamic channel power equalization in cross-connected nodes. Vari- ous types of attenuators have been researched using sliding- block mechanical systems, microelectromechanical systems (MEMS), side-polished fiber devices, and Faraday rotators [1], [2]. Also a TOA based on thermooptic (TO) Mach–Zehnder intensity modulators in silica was demonstrated [3]. Recently, various types of polymeric waveguide devices like switches (using Mach–Zehnder modulators) [4] and modulators have been widely investigated due to their low cost, simple fabrica- tion, and easy integration with other optical and electronic circuits. Furthermore, the TO coefficient of the polymers ( 0.0001 K ) is an order of magnitude larger than that of silica. Therefore, integrated optic attenuators using polymers have advantages like compact size desirable for arrays, low electrical power consumption, and enhanced environmental stability due to no moving parts. In this paper, we have presented a TOA using polymeric TO devices with Manuscript received December 29, 1998; revised January 29, 1999. The authors are with the Devices and Materials Laboratory, LG Corporate Institute of Technology, Seoul 137-724, Korea. Publisher Item Identifier S 1041-1135(99)03597-1. (a) (b) Fig. 1. Structure of the proposed polymeric tunable optical attenuator. (a) Top view. (b) Cross-sectional view. asymmetric -branch waveguides. Unlike other previous at- tenuators, it has a monolithically integrated monitoring port to tap a fraction of the main optical output. One arm of the branch is used for the main output port, while the other arm helps bypass the residual optical power resulting from attenuation without generating any undesirable radiation modes, which are common in Mach–Zehnder intensity modulators. The moni- toring port is realized by attaching a curved waveguide to the main port waveguide. By performing a continuous electronic feedback control with the port, a stable output can be achieved when the polarization state and power of input light are randomly changed. II. DEVICE CONFIGURATION The proposed polymeric TOA is composed of an asym- metric linear Y-branch waveguide [5] and an optical power tap. Its structure is shown in Fig. 1(a) and (b). For the asymmetric branch, the upper arm has a larger width than the lower arm. The straight waveguide combined with the wide arm is used for the main output port. The optical power tap, which is formed by attaching a curved waveguide to the main port to have a fraction of its power, serves as a mointoring output port. The narrow arm is connected to a tapered S-bend waveguide to enlarge the distance between the two waveguides of the branch. The device is operated based on the modal evolution effect, and thus features improved wavelength sensitivity and enhanced fabrication tolerance. 1041–1135/99$10.00  1999 IEEE