We demonstrate an all-fiber nonreciprocal comb filter by using two acoustooptic frequency shifters and a twomode fiber interferometer. The transmission spectra for two opposite optical propagation directions are both periodic functions of optical wavelength, but different in transmission peak wavelengths. Methods for controlling the spacing and position of the transmission peak wavelengths are demonstrated. Index Terms-Acoustooptic devices, optical fiber communication, optical fiber filters,

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... able filters, wavelength-division multiplexing. M OST OF THE nonreciprocal devices widely used in present optical systems are based on the magnetooptic Faraday rotation. Optical isolators and circulators are two representative nonreciprocal devices where transmission losses are different for two opposite propagation directions [1], [2]. The optical transmission directions in such devices are fixed by the direction of applied magnetic field for a range of operating wavelengths offered by Faraday materials. Recently, we have proposed and demonstrated an active nonreciprocal device based on an all-fiber interferometer employing two acoustooptic frequency shifters [3] . Unlike the conventional magnetooptic devices, the nonreciprocity in the new device is provided by the energy transfer between the acoustic and optical waves in the frequency shifters. The device has a unique capability of tuning or modulating the magnitude and transmission direction of the nonreciprocity using a simple electronic control. In this letter, we describe the spectral characteristics of such an all-fiber nonreciprocal device and demonstrate its novel function as a tunable nonreciprocal comb filter having periodic wavelengthdependent transmission directions. The device may open up new possibilities in wavelength-division-multiplexed (WDM) communication technologies. Fig. 1(a) shows the schematic of the basic device which consists of two frequency shifters, FS1 and FS2, and a twomode fiber (TMF) interferometer. The two FS's are positioned along a strand of the TMF, separated from each other by a length . The FS's produce mode coupling (with an efficiency of 50%) between two propagation modes of the TMF, the LP and the LP modes, via a flexural acoustic wave with frequency [4], [5] . Let us consider an optical wave of Manuscript received June 1, 1998.