358 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 3, MARCH 2002 High-Resolution Microwave Phonon Spectroscopy of Dispersion-Shifted Fiber Ilwhan Oh, Member, IEEE, Siva Yegnanarayanan, Member, IEEE, and Bahram Jalali, Senior Member, IEEE Abstract—First measurements of acoustic modes in disper- sion-shifted fiber are presented. High-resolution microwave phonon spectroscopy, performed using an improved method for resolving resonant spectral features, reveals the presence of the mode. The use of external cavity tunable lasers for pump and probe enables kilohertz range spectral resolution and high sensitivity. In addition, this method enables down conversion of the phonon spectrum to lower frequencies through the use of beat-brillouin effect. Index Terms—Acoustic mode, Brillouin amplification, disper- sion-shifted fiber, phonon spectroscopy. I. INTRODUCTION T HE LOW THRESHOLD and narrow bandwidth of Brillouin scattering may be used in applications such as Brillouin amplification for coherent systems and tunable optical filters for wavelength-division-multiplexing (WDM) systems [1], [2]. Experimental methods to resolve the Brillouin spectra in bulk material have been reported both in time and frequency domains [3]–[6]. In optical fibers the spectra can be analyzed in terms of normal acoustic modes [7], [8]. It has been observed that Brillouin spectrum in optical fibers has fine structure resulting from the discrete nature of longitudinal acoustic modes [8], [9]. In the optical heterodyne method, acoustic modes in optical fiber were resolved by tuning the probe frequency of a distributed feedback (DFB) laser [9] with the resolution being limited to the linewidth of the DFB laser. Nikles et al., have proposed a technique to resolve Brillouin spectrum by using a swept microwave side band generated by an electrooptic modulator acting on a single Nd : YAG laser [10]. In this letter, we propose and demonstrate an improvement to this technique that provides not only fine resolution via the use of microwave side band as a probe signal, but also a high sensitivity through the amplification of weak acoustic modes. In addition, the use of external cavity tunable lasers for pump and probe enables down conversion of the phonon spectrum to lower frequencies through the use of the beat-brillouin effect. We also report, for the first time, the acoustic mode spectrum of the dispersion-shifted fiber (DSF) and the first experimental observation of the third higher order acoustic mode of dispersion-shifted optical fiber. Manuscript received June 11, 2001; revised October 25, 2001. I. Oh is on leave from the Electronics Engineering Department, Mokpo Na- tional University, Republic of Korea (e-mail: ilwhanoh@yahoo.com). S. Yegnanarayanan and B. Jalali are with the Department of Electrical Engi- neering, University of California, Los Angeles, CA 90095-1594 USA (e-mail: jalali@ucla.edu). Publisher Item Identifier S 1041-1135(02)00811-X. Fig. 1. Experimental setup for the microwave sideband phonon spectroscopy (MSPS). EDFA: Erbium-doped fiber amplifer. PM: Phase modulator. Circ.: Circulator. Iso.: Isolator. PD: Photodetector. II. PRINCIPLE OF OPERATION Stimulated Brillouin amplification is a well-known nonlinear effect in optical fibers [1]. A pump wave induces a moving grating by means of the electrostrictive effect in optical fiber. The grating backscatters the pump wave into the Stokes wave, down shifted in frequency from the pump by approximately 11 GHz. When a counterpropagating modulated probe wave is introduced, modulation sidebands that coincide with the band- width of the Stokes wave will be amplified [11]. To describe the principle, we refer to the experimental setup in Fig. 1. A tem- perature-stabilized 4.46-km DSF (Corning SMF/DS) was used as the gain medium. Two External Cavity Lasers (ECL) were used as the pump (ECL-pu) and the probe beam (ECL-pr). The ECLs exhibit linewidth below 150 kHz, as confirmed by the standard heterodyne linewidth measurement technique. The use of an ECL offers us two orders of magnitude better frequency resolution than a DFB laser. In the optical domain, the backscattered signal contains the following three components: 1) the Fresnel and Rayleigh backscattered pump signal at frequency ( ); 2) the Brillouin signal at , where is the Stokes shift; and 3) the probe signal at . After photodetection, the microwave spec- trum contains the Brillouin signal centered at , a beat tone at ( ), and a beat-Brillouin tone at . When the phase modulation frequency equals , phase to amplitude conversion occurs resulting in a peak in the measured microwave spectrum. Similarly, an amplitude modulation sideband falling under the Brillouin gain curve results in amplification of the modulation signal. By choosing phase or amplitude modulation, real or imaginary part of the Brillouin spectrum can be mapped. Fig. 2 shows the microwave spectrum for the beat-Brillouin signal centered at , with and without phase 1041–1135/02$17.00 © 2002 IEEE