Impedance Matching Approach to 60 GHz Band Analog Traveling Wave Electro-Absorption Modulator (TWEAM) Module Kwang-Seong Choi, Yong-Duck Chung, Young-Shik Kang, Dong-Suk Jun, Byoung-Tae Ahn, Jong-Tae Moon, and Jeha Kim Basic Research Laboratory, Electronics and Telecommunications Research Institute 161, Gajeong-Dong, Yuseong-Gu, Daejeon 305-700, Korea Abstract A traveling wave electro-absorption modulator (TWEAM) module for 60 GHz band radio-over fiber link was developed. The fractional bandwidth at the operating frequency can be controlled with the termination resistance and it was shown that to estimate the fractional bandwidth, the model should contain the effect of packaging parasitics. Impedance matching in the module was achieved with the double stub design and the laser trimming process. The measured electrical-to-optical (E/O) response showed the increased in the response near 60 GHz. In addition, a filter as an implement of the impedance matching was proven to be an effective solution. Index Terms TWEAM module, impedance matching, E/O response I. INTRODUCTION Radio-over-Fiber (ROF) link technology employs optical carriers that are intensity modulated by the microwave signals through optical modulators and transmitted or distributed to optical receiver via optical fibers. [1, 2] It takes advantage of excellent characteristics of single mode fiber such as low optical attenuation, wide data bandwidth, and no electromagnetic interference (EMI). The 60 GHz band features spectrum availability, frequency reusability between adjacent picocell coverage ranges. Carrying 60 GHz band on ROF link leads to the reduction of power attenuation along the transmission line so that the deployment of the wireless access network with 60 GHz can be easily realized. The Key components in the ROF link are an analog optical source and an optical receiver. As an electrical-to-optical (E/O) converter, an electro- absorption modulator (EAM) has many advantages such as its small size, low driving voltage, large bandwidth and easy integration with other optical devices such as laser diodes or photo diodes so that it can be a good candidate for the component in ROF link. [3] A modulator with a traveling wave electro-absorption modulator (TWEAM) is very attractive for many applications, especially at high frequency because it overcomes the bandwidth limitation of a lumped EAM. To make use of high-speed properties of the TWEAM in a packaged module, an appropriate approach to the RF termination and the proper impedance matching scheme are necessary. In this paper, we presented the characteristics of the TWEAM modulator module optimized in 60 GHz band. The dependence of the fractional bandwidth on the termination scheme was simulated and measured with the packaged module. To estimate the fractional bandwidth of the module, the effect of packaging parasitics should be considered because the frequency dependence of them affects input impedance behavior of the module. With designing a double stub as an impedance matching circuit and applying the laser trimming process for a fine tuning, A good impedance matching was achieved and this matching led to the enhanced E/O response of the module near 60 GHz. Also, a filter was proposed as an implementation of the impedance matching to replace the impedance matching circuit and the tuning process. II. TWEAM DEVICE AND ITS CHARACTERIZATION A 0.5 ptm thick n+-layer grown on a semi-insulating InP substrate was designed for slow wave mode operation. A 0.3 ptm intrinsic layer contained the strain- compensated multiple quantum well (MQW) structure which consisted of 12 wells (8 nm and 12 nm, -0.38 00 tensile strained) and 13 barriers (7 nm, 0.50 00 compressive strained). The photo-luminescence peak was at 1.49 pm. The frequency response of a TWEAM is related to the structural parameters such as the epi and electrode structure. Since the epi structure directly affects the high-frequency response and extinction ratio [4], the epi-layer was carefully designed through the finite difference time domain (FDTD) simulation. The active region of device was formed into a 100 ptm-long waveguide of and butt-jointed with 250 pm-long passive waveguide on both sides to increase the total device length for easy handling during packaging process. For high frequency operation, the coplanar waveguide (CPW) electrodes were designed and formed by the evaporation process. The device fabrication process was reported in [5]. The return loss and the insertion loss of the device were measured with the vector network analyzer (VNA) and Fig. 1 shows the results. The insertion loss was less than 3 dB and the return loss was larger than 10 dB over the entire measured frequency range. The characteristic