IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 1, JANUARY 2004 257 Efficient Architecture for WDM Photonic Microwave Filters B. Vidal, Student Member, IEEE, V. Polo, Student Member, IEEE, J. L. Corral, Member, IEEE, and J. Martí, Member, IEEE Abstract—A novel technique to reduce the number of compo- nents in photonic microwave filters implemented using multiple optical carriers (wavelength-division multiplexing) and dispersive media is presented and experimentally demonstrated. This tech- nique is based on using jointly dispersive and nondispersive media to reuse the optical carriers and, therefore, to reduce the number of filter components. The technique allows the implementation of filters showing taps using components instead of components, as required in previous proposals. Index Terms—Microwave photonics, optical delay lines (ODLs), photonic microwave filters. I. INTRODUCTION P HOTONIC microwave filters allow the processing of radio frequency (RF) signals in the optical domain avoiding the need for inefficient and costly intermediate optoelectronic con- versions. Moreover, they have low and frequency-independent loss, small size, immunity to electromagnetic interference as well as large time-bandwidth products. Several schemes [1]–[3] exploit wavelength-division-multiplexing (WDM) techniques to simplify the filter architecture by using as many optical carriers as the number of filter taps. These WDM architectures benefit from using low-cost high-performance components available for optical communication systems. Schemes based on dispersive media and as many photodiodes as filter taps [4] have also been proposed. In WDM schemes, dispersive media introduce a time delay between optical carriers (taps) to implement a finite-impulse response (FIR) filter. The quality of any FIR transversal filter in terms of finesse depends directly on the number of signal samples or taps [5]. For instance, for a uniform distribution of taps, the width of the main lobe is given by , where is the number of taps. Therefore, there is a cost limitation on the implementation of high-quality filters since many taps (optical sources) are needed. Although the use of integrated multiwavelength lasers or the spectrum slicing of broad-band sources [1] to obtain multiple optical carriers can reduce the filter cost, a more efficient use of the WDM parallelism should be carried out to obtain cost effective high-performance filters. In this letter, a new technique to reduce the cost of photonic WDM microwave filters is proposed. This technique is based on Manuscript received May 14, 2003; revised July 10, 2003. This work was supported in part by the European Commission through the Project OBANET IST-2000–25390, and in part by the Spanish Research and Technology Com- mission (CICYT) through Projects TIC2000–1674 and TIC 2000–2793-CE. The authors are with the Fiber Radio Group, Universidad Politécnica de Va- lencia, 46022 Valencia, Spain (e-mail: bvidal@ieee.org). Digital Object Identifier 10.1109/LPT.2003.820116 Fig. 1. Conceptual diagram photonic microwave filter architecture. the combination of optical dispersive-based delays and optical absolute propagation delays to reuse the optical carriers of the source. It can be used in different architectures based on WDM signals and dispersive media and even in conjunction with an- other cost reduction technique, as in [1]. II. ARCHITECTURE The concept of the technique is depicted in Fig. 1. A set of equally spaced continuous-wave optical carriers is amplitude modulated and driven to a dispersive medium, such as standard single-mode fiber (SSMF), highly dispersive fiber, or a chirped fiber Bragg grating. The chromatic dispersion of the dispersive medium introduces a relative time delay between optical carriers. In previous proposals [1]–[3], once the set of optical carriers is time delayed in the dispersive medium, it is photode- tected obtaining electrical time-delayed samples (i.e., -tap filter). In the scheme shown in Fig. 1, unlike previous proposals, the optical carriers are equally power-split into signals that are incident on photodiodes by means of a op- tical splitter. This way, there is a set of amplitude-modulated optical carriers with a constant time delay between them at each one of the optical splitter outputs. Then, a progressive ab- solute propagation delay is introduced between each op- tical splitter output and its corresponding photodiode to time-in- terleave these sets of optical carriers. Absolute time delays are generated at each branch of the array of photodiodes by the dif- ference in optical path lengths [which has been represented in Fig. 1 as a nondispersive optical delay line (ODL)]. This abso- lute nondispersive delay is wavelength independent and affects all the carriers within a set equally, allowing one to time inter- leave the sets of taps, as represented in Fig. 2. The ab- solute time delay at branch should be made equal to to obtain the time interleaving of sets of taps. Thus, each photodetector generates a set of electrical samples and due to the time-interleaving, there is 1041-1135/04$20.00 © 2004 IEEE