2810 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 15, AUGUST 1, 2008 Wideband Adaptive Feedforward Photonic Link Sean R. O’Connor, Member, IEEE, Thomas R. Clark, Jr., Senior Member, IEEE, Member, OSA, and Dalma Novak, Fellow, IEEE Abstract—We present the first demonstration of an adaptive wideband radio-frequency (RF) photonic link architecture suitable for high-dynamic-range microwave signal fiber optic transport. The architecture incorporates feedforward (FF) linearization to correct the electrical-to-optical (E/O) conversion nonlinearity as well as optimized control loops that enable the system perfor- mance to be maintained in real time during changing operating conditions. Third-order distortion suppression of greater than 25 dB has been demonstrated over the input frequency range of 2 GHz to 17 GHz. Under manual control of the wideband FF linearized photonic link, a spur-free dynamic range (SFDR) in excess of 120 dB-Hz is demonstrated, while an SFDR of 116 dB-Hz is achieved with adaptive control. We present experimental results of a fully automated wideband FF photonic link and also discuss the potential for the technique. Index Terms—Feedforward, linearization, microwave photonics, photonic link. I. INTRODUCTION T HE transport of microwave signals over optical fiber is important for many applications, including broadband radio, television, and Internet signal distribution, radar signal transport, and hybrid fiber-wireless communications networks. These applications can benefit substantially from the low-loss and wide-bandwidth transport over flexible, lightweight, and nonconducting point-to-point links or through multisignal fiber distribution networks. A key impediment to realizing these ben- efits are the high-dynamic-range targets of many applications, typically limited by the linearity of the electrical–optical (E/O) conversion process [1]. Most linearization schemes suitable for analog correction of microwave photonic link nonlinearities are derived from common radio-frequency (RF) amplifier design techniques [2] and include predistortion [3]–[7] and feedforward (FF) [8]–[13] architectures. In addition, novel electrooptical modulator designs [14]–[16] have been proposed and demonstrated. The majority of previous linearization implementations for RF photonic links have been characterized by limited opera- tional and/or instantaneous bandwidths, as well as achievable SFDR. Feedforward linearized photonic links have so far been limited to microwave signals at frequencies 6 GHz, and al- though a broadband (multioctave) predistortion circuit was re- ported almost a decade ago [5], the SFDR was limited to 110 Manuscript received January 30, 2008; revised May 8, 2008. Current version published October 10, 2008. This work was supported under Navy Contract N68335-06-C-0321. S. R. O’Connor and T. R. Clark, Jr., are with the Applied Physics Labora- tory, The Johns Hopkins University, Laurel, MD 20723 USA (e-mail: Sean. OConnor@jhuapl.edu; Thomas.Clark@jhuapl.edu). D. Novak is with Pharad LLC, Glen Burnie, MD 21061 USA (e-mail: dnovak@pharad.com). Digital Object Identifier 10.1109/JLT.2008.927189 dB-Hz . Another issue with any of the above architectures is the strong sensitivity to the amplitude and phase variations of the circuit components, particularly in the presence of environ- mental changes. Various approaches to implementing adaptive linearization schemes for RF power amplifiers in microwave systems have been reported [17], [18] to deal with similar is- sues. Adaptive predistortion circuits for photonic transmitters have also been demonstrated [4], [6]; however, the operating frequency was limited to less than 2 GHz with SFDR 100 dB-Hz . In this paper, we describe and demonstrate a high-dy- namic-range adaptive wideband photonic link architecture. Our approach is based on the use of feedforward linearization to correct distortion introduced in the nonlinear E/O encoding. Unlike other linearization schemes, no a priori knowledge of the particular order of distortion to be corrected is required. Our architecture, utilizing control loops to optimize phase and amplitude balancing within the circuit, has the capability to be real-time adaptive, maintaining system performance during changing operating conditions that arise due to variations in input signal, environmental changes, component parameter tolerances, and device aging. This is the first demonstration of a feedforward linearized photonic link with adaptive func- tionality, as well as multioctave operational bandwidth and an SFDR approaching 120 dB-Hz . We present here the experi- mental evaluation of a demonstration system that incorporates a single-hardware, multiple-frequency band, computer-con- trolled feedforward linearized photonic link and discuss the present limitations and directions for future work. II. FEEDFORWARD LINEARIZATION ARCHITECTURE Feedforward linearization is a two-loop architecture that re- quires no previous knowledge of the input signal and can theo- retically suppress all orders of distortion with no response time limit. It is, however, generally regarded as the most complex lin- earization topology to design due to the larger number of circuit elements, strong sensitivity to amplitude and phase balancing, and stringent linearity requirements on feedforward circuit com- ponents. The first loop in an FF linearization scheme isolates any error created by a nonlinear circuit component [e.g., the electrooptic modulator (EOM) of Fig. 1]. This error signal is then subtracted from the output of the nonlinear component in the second loop, improving the output linearity. A generalized feedforward analog photonic link can be seen in Fig. 1. As shown in Fig. 1, the input RF signal to a FF photonic link is split into two paths. For this paper, the upper branch of the signal loop is an intensity modulation direct-detection (IMDD) photonic link [1], where an EOM encodes the RF signal with 0733-8724/$25.00 © 2008 IEEE