182 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 3, FEBRUARY 1, 2010 On Frequency-Doubled Optical Millimeter-Wave Generation Technique Without Carrier Suppression for In-Building Wireless Over Fiber Applications Hung-Chang Chien, Member, IEEE, Yu-Ting Hsueh, Student Member, IEEE, Arshad Chowdhury, Member, IEEE, Jianjun Yu, Senior Member, IEEE, and Gee-Kung Chang, Fellow, IEEE Abstract—A novel optical millimeter-wave (mm-wave) gen- eration scheme based on double-sideband phase modulation (DSB-PM) to achieve frequency doubling without suppressing the carrier is proposed. Theoretical analysis shows that the generated 60-GHz optical mm-wave can tolerant 0.016-nm wavelength drifting with filter bandwidth ranging from 70 to 100 GHz to sustain first to second harmonic suppression ratio of 18 dB. In addition, error-free transmission of 60-GHz mm-wave at 2.5 Gb/s is experimentally demonstrated over a combined distance of 3-m wireless with 21-dBm equivalent isotropically radiated power, and 250-m fiber to best emulate an in-building network environment. Dispersion effect on the frequency-doubled PM optical mm-wave without carrier suppression is also analyzed and experimentally studied by comparing the link performance over single-mode fiber (SMF-28) and dispersion-shifted fiber (DSF), respectively. Index Terms—Carrier suppression, frequency doubling, mil- limeter wave (mm-wave), phase modulation (PM). I. INTRODUCTION T HE millimeter-wave (mm-wave) radio-over-fiber (RoF) access system has gained much attention recently due to its potential ability to offer ubiquitous multigigabit wireless services through centralized, simplified remote access units (RAUs) [1]–[3]. One of the major challenges for such a system is to acquire a stable and cost-effective optical mm-wave at the head end, and several practical approaches have been pre- sented [4]–[6]. The concept of suppressing the central carrier to multiply the beating frequency of an optical lightwave is generally accepted and can be commonly realized by either tuning the bias of an optical intensity modulator [4], or using a narrowband optical notch filter or an optical interleaver [5], [6]. However, such a rule might not fully generalize to a phase-modulator-based optical mm-wave generation, where two first-order sidebands of a phase-modulated optical carrier are out-of-phase in nature [7]. That means whether or not the optical carrier is suppressed, the first harmonic can always be eliminated after the photodetection as long as the out-of-phase condition can be maintained from end to end. Therefore, in Manuscript received September 03, 2009; revised October 22, 2009; accepted November 15, 2009. Current version published January 15, 2010. This work was supported in part by Georgia Tech Broadband Institute (GTBI). H.-C. Chien, Y.-T. Hsueh, A. Chowdhury, and G.-K. Chang are with the School of Electrical and Computer Engineering, Georgia Institute of Tech- nology, Atlanta, GA 30332 USA (e-mail: hchien3@ece.gatech.edu). J. Yu is with the NEC Laboratories America, Princeton, NJ 08540 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2037333 Fig. 1. Block diagram of the proposed optical mm-wave generation method using DSB-PM without suppressing the central carrier. this letter, we proposed a novel frequency-doubled optical mm-wave generation scheme based on double-sideband phase modulation (DSB-PM) without carrier suppression, which requires no bias control and narrowband carrier-suppressing filtering. Characteristics like wavelength detuning range, filter bandwidth requirement, and harmonics fluctuation due to fiber dispersion induced unequal phase rotation of first-order side- bands are theoretically analyzed. In addition, link performance of the generated optical mm-wave at 60 GHz is experimentally investigated through a combined wired and wireless testbed with different fiber dispersion coefficients. II. PRINCIPLE Fig. 1 illustrates the block diagram of the proposed optical mm-wave generation scheme using DSB-PM without carrier suppression. A phase-modulated optical carrier is generated by modulating the phase of a continuous-wave (CW) light source with a sinusoid microwave signal, and its complex electrical field can be approximated by [7] (1) where , , and are the amplitude, angular frequency, and phase noise of the CW, is the phase modulation index, is the Bessel function of the first kind of order , and is the angular frequency of the modulating microwave signal. The phase-modulated optical carrier features multiple upper and lower optical sidebands centered at with adjacent frequency spacing of . After rejecting higher order sidebands by using an optical filter with a wide bandwidth larger than but smaller than , the resulting phase-modulated optical carrier mainly consists of a central carrier and first- and second-order 1041-1135/$26.00 © 2010 IEEE