JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 13, JULY 1, 2008 1835 Characterization of the Dynamic Absorption of Electroabsorption Modulators With Application to OTDM Demultiplexing Andrew D. Gallant and John C. Cartledge, Senior Member, IEEE, Fellow, OSA Abstract—In this paper, a technique for characterizing the dy- namic absorption of electroabsorption modulators (EAMs) under high-frequency sinusoidal modulation is described. By using an optical sampling oscilloscope (OSO), the nonlinear response of a modulator to 10- and 40-GHz modulation is accurately measured. The results for the dynamic absorption are used in a measurement- based model to calculate optical gating windows for the demulti- plexing of a 160-Gb/s optical time-division multiplexed (OTDM) signal to its 10- and 40-Gb/s tributary signals. Good agreement is achieved between the calculated and measured gating windows. Index Terms—Dynamic absorption, electroabsorption modu- lator (EAM), gating window. I. INTRODUCTION O PTICAL time-division multiplexing (OTDM) is a tech- nique that can be used in conjunction with wavelength division multiplexing (WDM) to decrease the cost of spectrally efficient optical communication systems [1]. By performing the high-speed multiplexing and demultiplexing operations in the optical domain, the bandwidth limitation of electrical time-di- vision multiplexed systems is avoided. OTDM transmission at a bit rate of 2.56 Tb/s for a single channel has been reported [2], as has OTDM/WDM transmission of eight 170-Gb/s channels [3]. Demultiplexing in OTDM systems is performed by creating a gating window in the optical path at the receiver. The shape of the gating window is an important measure of the performance of an OTDM demultiplexer as it must pass one of the tributary signals and suppress all the others. In 160-Gb/s OTDM systems, EAM-based optical gates are an attractive choice for demul- tiplexing [4], [5]. EAMs have also been used to construct an OTDM transmitter that is capable of simultaneously modulating and multiplexing four 40-Gb/s tributary signals to a 160-Gb/s OTDM signal [5]. In order to design an EAM-based OTDM demultiplexer and assess its impact on system performance, it is important to have an accurate characterization of the gating window. Manuscript received September 2, 2007; revised January 22, 2008. Published August 29, 2008 (projected). This work was supported by the Natural Sciences and Engineering Research Council of Canada. A. D. Gallant was with the Department of Electrical and Computer Engi- neering, Queen’s University, Kingston, ON K7L 3N6, Canada. He is now with Gennum Corporation, Burlington, ON L7L 5M4, Canada. J. C. Cartledge is with the Department of Electrical and Computer Engi- neering, Queen’s University, Kingston, ON K7L 3N6, Canada (e-mail: john.car- tledge@queensu.ca). Digital Object Identifier 10.1109/JLT.2008.922190 While the static absorption properties of an EAM [only a direct current (dc) voltage applied] are simple to measure, they do not adequately describe the device when driven by a modulated electrical signal. Rather, the dynamic absorption properties under high-frequency sinusoidal modulation are needed. Re- ported models for the dynamic absorption require an in-depth knowledge of the device structure [6], or are computationally intensive and require the measurement of multiple EAM char- acteristics [7], [8]. In this paper, an accurate experimental characterization of the dynamic absorption properties under high-frequency sinu- soidal modulation is presented. The results are incorporated into a measurement-based device model that allows the calculation of the gating window for OTDM demultiplexing. Such a model is particularly useful for system performance studies because of its accuracy and computational efficiency. Good agreement is demonstrated between calculated and measured gating win- dows for the demultiplexing of a 160-Gb/s OTDM signal to its 10- and 40-Gb/s tributary signals. II. DYNAMIC ABSORPTION The accurate measurement of high-frequency (10 and 40 GHz) dynamic absorption requires a large measurement bandwidth to capture the nonlinear response of the EAM absorption to the applied voltage. The nonlinear response allows optical gating windows to be obtained with a full-width half-maximum (FWHM) of about 5 ps. Fig. 1 compares the optical transmission of an EAM measured using an Agilent electrical sampling oscilloscope (ESO, 86116C) with a band- width of 65 GHz to the optical transmission measured using an Agilent optical sampling oscilloscope (OSO, 86119A) with a bandwidth exceeding 700 GHz. The OSO has a dynamic range of over 30 dB. The combination of bandwidth and dynamic range allows accurate measurement of the optical response at extinction [9]. When the EAM is driven with a 40-GHz sinusoidal signal, the conventional ESO is not able to accu- rately measure the time dependence of the change in optical absorption. The OSO is indispensable for this purpose. The dynamic absorption of an EAM is calculated from mea- surements of the sinusoidal electrical drive waveform using an ESO with a bandwidth of 80 GHz and the resultant output op- tical waveform using the OSO. The stimulus and response mea- surements are temporally aligned using a fast Fourier transform (FFT)-based implementation of time-domain circular convolu- tion between the two waveforms. The time for which the convo- lution is maximized corresponds to the relative delay between 0733-8724/$25.00 © 2008 IEEE