IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 4, APRIL 2005 771 Cascadability of Large-Scale 3-D MEMS-Based Low-Loss Photonic Cross-Connects Volkan Kaman, Member, IEEE, Xuezhe Zheng, Senior Member, IEEE, Shifu Yuan, Senior Member, IEEE, Jim Klingshirn, Chandrasekhar Pusarla, Roger J. Helkey, Senior Member, IEEE, Olivier Jerphagnon, Member, IEEE, and John E. Bowers, Fellow, IEEE Abstract—The performance of cascaded low-loss ( 3.5 dB) 256 256 three-dimensional microelectromechanical system (3-D MEMS) photonic cross-connects (PXCs) is experimentally inves- tigated in a recirculating loop. After 60 transitions through the PXC, a power penalty of 1.7 dB is observed, which is attributed to the accumulation of the low polarization-dependent loss in the optical switch. The use of 3-D MEMS PXCs as a wavelength-selec- tive switch (WSPXC) for transparent all-optical networks is also demonstrated. Measured -factors for all 16 100-GHz-spaced wavelengths at 10 Gb/s over eight spans of 75-km single-mode fiber and eight transitions through the WSPXC nodes are better than 17 dB. Index Terms—Microelectromechanical devices, optical fiber communication, photonic switching systems, wavelength-division multiplexing. I. INTRODUCTION F OR FUTURE high-capacity dense wavelength-divi- sion-multiplexed (DWDM) metro and long-haul networks, all-optical photonic cross-connects (PXC) have emerged as an attractive low cost and low power consuming alternative to optoelectronic switches [1]–[3]. Other advantages of PXCs also include bit rate and wavelength transparency for future network upgrades as well as realizing future dynamically reconfigurable and transparent mesh optical networks [4], [5]. An optical cross-connect node schematic for future trans- parent all-optical networks is shown in Fig. 1. The core of the node is a nonblocking large-scale PXC with the ability to sup- port several services and functionalities. The inputs and out- puts to the node are several fiber trunks carrying DWDM chan- nels in the - and -bands. The optical demultiplexer (DMUX) and multiplexer (MUX) at the input and output of the PXC can support fiber, bands of wavelengths, or wavelength granular fil- tering [6]. The PXC then acts as a blocking switch based on the configuration of the MUX/DMUX as well as the number of fiber trunks. The nonblocking capability of the PXC becomes signifi- cant especially in wavelength-selective PXC (WSPXC) applica- tions for network level cost and node level power consumption reduction by sharing the resources around the PXC among all the wavelength channels based upon network demand. These shared wavelength resources include add–drop tunable trans- mitters and receivers as well as all-optical signal processors, such as 2R/3R regenerators and wavelength converters [7], [8]. Manuscript received September 24, 2004; revised December 21, 2004. The authors are with Calient Networks, Goleta, CA 93117 USA (e-mail: vkaman@calient.net). Digital Object Identifier 10.1109/LPT.2005.843654 Fig. 1. Schematic of an optical cross-connect node in an optically transparent network. In order to support multiple fiber trunks as well as shared re- sources, larger than 200 port nonblocking PXCs become im- perative. For example, a four-degree node carrying 40 DWDM channels per fiber and a typical 25% add–drop ratio requires 200 ports and additional optical signal processing ports [8], [9]. With their low loss, optical transparency, and large-scale capability, three-dimensional microelectromechanical system (3-D MEMS)-based PXCs have become the leading candidate for the switching node of next-generation transparent networks [10], [11]. A low loss PXC is imperative in maintaining high optical signal-to-noise ratio (OSNR) as well as low polar- ization-dependent loss (PDL), polarization-mode dispersion (PMD), wavelength-dependent loss (WDL), and crosstalk when several PXCs are cascaded in a network. We have recently reported on the optical design and charac- teristics of a 256 256 nonblocking 3-D MEMS PXC [10]. In this letter, we first investigate the optical performance of the PXC with the goal of determining the power penalty introduced by cascading PXCs. We then demonstrate this PXCs feasibility as a wavelength-selective node in optically transparent DWDM networks. II. CASCADABILIY OF 3-D MEMS PXC Fig. 2 shows the experimental setup used for investigating the cascadability of the 256-port PXC. A 10-Gb/s optical signal at a wavelength of 1550 nm with a pseudorandom bit sequence (PRBS) of was launched into the recirculating loop using an acoustooptic modulator (AOM) and a 3-dB coupler. The loop 1041-1135/$20.00 © 2005 IEEE