192 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 53, NO. 1, FEBRUARY 2004 Rapid Amplitude and Group-Delay Measurement System Based on Intra-Cavity-Modulated Swept-Lasers Sze Y. Set, Member, IEEE, Mark K. Jablonski, Member, IEEE, Kevin Hsu, Member, IEEE, Chee S. Goh, Student Member, IEEE, and Kazuro Kikuchi, Member, IEEE Abstract—In this paper, we present a high-speed wavelength- swept laser for application in a real-time optical device charac- terization system. The system is capable of simultaneous measure- ment of both the spectral amplitude and group-delay responses of the device-under-test, at a scan rate of 22 Hz over a wide wave- length range of 50 nm. This corresponds to a record sweep rate of nm/s. Index Terms—Laser applications, measurement, optical compo- nents, optical fiber lasers, optical fiber measurements, phase mea- surement. I. INTRODUCTION O PTICAL communication systems are evolving toward employing higher channel data-rates at 10 Gb/s, 40 Gb/s and beyond. At such bit-rates, the dispersion or group-delay properties of not only the transmission fibers, but also of each of the optical components used, are important parameters for system designers and component vendors. Various methods have been proposed for group-delay or dispersion measure- ment. The most commonly adopted “industrial” standard for group-delay measurement is based on the modulation phase-shift (MPS) technique [1] as shown in Fig. 1. Typically, a tunable external-cavity laser (ECL) is used as an optical source, and the output is intensity-modulated at a known radio-frequency (RF) signal. The modulated signal is launched through the device-under-test (DUT) and detected through a photo-detector and lock-in RF phase-discriminator. The optical group-delays at each wavelength can then be calculated from the measured RF phase differences. The measurement is usu- ally performed with an RF network analyzer and a tunable ECL which steps each measuring wavelength point-by-point, at a rate which takes seconds per point, due to the ECL wavelength set- tling time. Additionally, a wavelength meter may also be re- quired, due to the poor wavelength accuracy of an ECL, which further increases the measurement time. A fast dispersion measurement technique was proposed re- cently with 10-Hz scan rate [2]. However, the wavelength sweep range was limited to nm, giving a sweep rate of 60 nm/s. Manuscript received March 1, 2003; revised August 15, 2003. S. Y. Set and M. K. Jablonski are with Alnair Laboratories Corporation, Saitama, Japan (e-mail: set@alnair-labs.com). K. Hsu is with Micron Optics Inc., Atlanta, GA 30345 USA. C. S. Goh and K. Kikuchi are with the Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan. Digital Object Identifier 10.1109/TIM.2003.821511 Fig. 1. Experimental setup of conventional MPS dispersion measurement system. Furthermore, 30-times averaging is required for noise reduc- tion, giving an effective sweep rate of only 2 nm/s. One way to achieve a high speed and wideband swept-wavelength source is to employ a fiber ring laser with a fast wavelength tunable filter such as a fiber Fabry–Perot tunable filter (FFP–TF) [3]. However, such a swept laser will passively modelock and gen- erate pulses at a multiple of the cavity fundamental frequency of around several tens of megahertz due to the frequency shifted feedback [3], [4]. Since the output of such laser is pulsing, the MPS technique cannot be applied effectively. In this paper, we propose and demonstrate a high-speed dis- persion measurement system based on an intra-cavity-modu- lated swept-laser (ICM-SL)[5], by placing the modulator within the fiber ring cavity. The intra-cavity modulation suppresses the natural mode-locking of the swept-laser and directly produces synchronous wavelength-swept pulses for dispersion measure- ment. It is capable of a continuous scan over 50-nm wavelength range at 22 Hz, giving a “real-time” display of both the am- plitude and phase response simultaneously. Measurement at a record sweep rate of nm/s has been achieved. II. FAST WAVELENGTH TUNABLE FILTERS One key element to achieve a high sweep-rate in the swept- laser is a fast wavelength-tunable filter. However, typical filter technologies are not suitable for high-speed operation. For ex- ample, diffraction gratings used in standard ECLs are limited in speed due to larger optics load and slow dc motors or stepping motors. Tunable filters such as the angle-tunable thin-film inter- ference filter, and the bulk Fabry–Perot (FP) filter, suffer from their weight load and hence limited tuning speed. MEMS-based FP filters could probably be driven as fast, but may be unstable, and its nonlinearity limits its use in laser cavities. The acousto- 0018-9456/04$20.00 © 2004 IEEE