IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 45, NO. 8, AUGUST 1997 1473 All-Optical RF Filter Using Amplitude Inversion in a Semiconductor Optical Amplifier F. Coppinger, S. Yegnanarayanan, P. D. Trinh, and B. Jalali, Senior Member, IEEE Abstract—We present an all-optical delay-line radio-frequency (RF) notch filter. The filter exploits cross-gain modulation in a homogeneously broadened laser medium to obtain a negative tap in an optically incoherent system. Applications including moving-target indication (MTI) in optically controlled radars are discussed. Index Terms— Feed forward filter, microwave photonics, op- tical signal processing, MTI, semiconductor optical amplifier (SOA). I. INTRODUCTION S IGNAL processing using optic-fiber delay lines is a very powerful technique for processing high bandwidth signals [1] . Optic fiber is an excellent propagating medium for time delays. It has low loss, low dispersion, light weight, and is inexpensive. Moreover, the signal carried by the intensity- modulated optical beam is insensitive to electromagnetic ra- diation. Various optical delay-line configurations have been pro- posed to achieve different radio-frequency (RF) filter func- tions. These configurations can be either optically coherent [2], [3] or incoherent [4]–[6]. Coherent optical processing allows more flexibility in shaping of the filter-transfer function because negative tap weights are possible. Nevertheless, the phase of the optical beam has to be precisely controlled since the filter is highly sensitive to any phase variation. Moreover, the longest time delay in the filter has to be shorter than the coherence time of the optical source. Incoherent processing does not require any control of the optical phase. The filter is, therefore, less sensitive to external perturbation (e.g., variation of temperature). Long time delays are made possible, but the shortest delay line has to be longer than the coherence length of the optical source. Most important, only positive taps weights are possible since the intensity is a positive quantity. To overcome this limitation, an optoelectronic approach using differential detection has been proposed [1], [7] and experimentally demonstrated [8]. In this approach, optical signals are detected using a pair of matched photodetectors and combined electrically. In this paper, we propose and demonstrate a new technique to realize negative weights in optical-fiber filters. The filter maintains the advantages of incoherent operation but does not Manuscript received December 3, 1996; revised April 28, 1997. This work was supported in part by the Joint Services Electronics Program (JSEP). The authors are with the Optoelectronic Circuits and Systems Laboratory, Department of Electrical Engineering, University of California at Los Angeles (UCLA), CA 90095-1594 USA. Publisher Item Identifier S 0018-9480(97)06017-1. Fig. 1. Schematic representation of the filter—a directly modulated laser beam is split into two parts. One part is combined with a CW beam at , both beams are then amplified in an EDFA and sent to an SOA. The negative tap at is then combined with the delayed positive tap at and detected inside the same photodetector. require optoelectronic conversion in order to attain negative weights. Negative weighting is obtained using signal inversion in a semiconductor optical amplifier (SOA). Due to gain saturation in the homogeneously broadened gain medium of the SOA [9], an inverted copy of an RF-modulated optical beam at wavelength appears on a probe signal at wave- length [10]. Upon combining using an optical coupler, the ac components are subtracted (in the optical domain). Thus, the SOA performs the function of a negative tap. Due to the wavelength conversion, any time delay can be used regardless of the coherence length of the optical source. The main advantage of this technique compared to differential detection is that the signal is kept in the optical domain, which allows cascading to attain higher order filters. In this paper, we demonstrate a single delay-line canceller where an RF signal is subtracted from its delayed version in the optical domain. Two different configurations of the filter are proposed and their performance is discussed in terms of linearity. Application for moving-target indication (MTI) in optically controlled radar technology is also discussed. II. ALL-OPTICAL FILTER APPROACH Fig. 1 describes the schematic of the filter. An external- cavity tunable laser operating at nm is directly RF intensity modulated. The coherence length of the laser is on the order of 1 km. This beam is then split into two parts, one part goes to the long time delay consisting of 500 m of optic fiber. The other part is combined with a continuous wave (CW) beam from a distributed feedback (DFB) laser operating at nm. These beams are then amplified in an erbium-doped fiber amplifier (EDFA) and sent into the SOA. Due to cross-gain modulation in the SOA, the emerging signal at represents the inverted version of the RF signal 0018–9480/97$10.00  1997 IEEE