JOURNAL OF LIGHTWAVE TECHNOLOGY 1 Reduction of Fiber Chromatic Dispersion Effects in Fiber-Wireless and Photonic Time-Stretching System Using Polymer Modulators Jeehoon Han, Student Member, IEEE, Byoung-Joon Seo, Student Member, IEEE, Yan Han, Bahram Jalali, and Harold R. Fetterman, Fellow, IEEE, Fellow, OSA Abstract—We have investigated the general characteristics of the power penalty due to the fiber chromatic dispersion effects in both fiber-wireless and photonic time-stretching systems. Two dif- ferent modulation schemes have been demonstrated to reduce this penalty using our novel polymer modulators incorporating a multi- mode interference (MMI) structure. A single-sideband (SSB) mod- ulator configuration has almost completely eliminated this penalty without a bandwidth limit. A double-sideband (DSB) modulator configuration with an appropriate quadrature bias has also shown significant improvement in bandwidth limitations for a given fiber link length. Index Terms—Fiber-wireless systems, photonic time-stretching, polymer modulators, power penalty, single-sideband modulation. I. INTRODUCTION I N FIBER-WIRELESS systems, the radio frequency (RF) signals are generated at the central exchange using optical techniques and transmitted to the remote base stations over op- tical fiber links. The simplest and best technique to modulate optical fields with RF signals is an intensity modulation scheme via Mach–Zehnder modulators (MZMs) with continuous-wave (CW) lasers. Using the conventional DSB modulation scheme, the RF power detected at the base station suffers from a pe- riodic degradation due to the fiber chromatic dispersion. As the RF frequency or fiber-link distance increases, this effect is even more severe and limits the system performance. This detrimental effect can be mitigated using alternative modula- tion schemes [1]–[3]. In this paper, we derive more specific and standard expressions and confirmed them by experiments using standard MZM designs. This examination of the power penalty in CW applications can be also utilized for the appropriate and clear understanding of those in pulsed applications. Photonic time-stretching (PTS) utilizes optical systems to en- able high-speed analog-to-digital conversion (ADC) of RF sig- nals at otherwise inaccessible high frequencies. By exploiting chirped optical pulses and chromatic dispersion in standard op- tical fibers, high-frequency RF signals can be stretched in time, without distortion, to lower frequency regimes where conven- tional electronic ADCs are able to digitize with high resolution. However, as in CW applications, the inherent fiber chromatic Manuscript received October 11, 2002; revised February 27, 2003. This work was supported in part by AFOSR and DARPA. The authors are with the Electrical Engineering Department, University of California, Los Angeles, CA 90095 USA (e-mail: hoon@ ee.ucla.edu). Digital Object Identifier 10.1109/JLT.2003.812155 dispersion effects limit the actual bandwidth of PTS system [4]–[6]. We describe the general theory and present for the first time the experimental demonstration of PTS system associated with various modulation conditions including SSB modulation. II. POWER PENALTY IN FIBER-WIRELESS SYSTEM The basic structure for these MZMs is shown in Fig. 1 repre- senting all possible modulation schemes and biases. If the input optical signal at is with unit magnitude, the generalized expression for output optical field from MZM mod- ulated at is given by (1) where is the modulation depth at arm, is the optical phase shift controlled by dc bias, is the half-wave voltage. When this signal travels through the standard fiber with length of , the resulting optical field can be written in terms of three frequency components, , and , with different phase changes due to the chromatic dispersion (2) where , , , are the Bessel func- tion values and is assumed to be small. Each phase change can be specified by usual Taylor expansion of the propagation constants (3) where group velocity dispersion in standard fibers is defined by ps/km nm. At the photodiode, the RF signal at modulation frequency is produced as a result of interference among these components. Normally the detected RF power is associated with their phase 0733-8724/03$17.00 © 2003 IEEE