Analysis of double-parallel amplified recirculating optical-delay lines

Abstract

A novel method of analysis of double-parallel amplified recirculating optical-delay lines (DPAROD) is presented. The location of the maxima and the minima of the transfer function for this configuration is calculated and experimentally demonstrated. The influence of different parameters, such as the coupling coefficients, gains, lengths of the fiber loops and fractional losses of the directional couplers, on the shape of the transfer function are analyzed. Different measurements have been taken to verify this model. The potential application of these interconnected delay loops as filters is a reason for developing this method. lntroductionSingle, amplified recirculating optical-delay lines have been widely analyzed,ly2 showing their capability for the development of signal-processing functions such as filtering, pulse-train generation, conv~lution,~ and memo^-y.4 A higher degree of design flexibility can be achieved with more complex structures, such as the double-parallel amplified recirculating opticaldelay line (DPAROD),l which is depicted in Fig. 1.New features obtained with DPAROD allow one to reshape their frequency response. For instance, one can obtain a predetermined relative amplitude between the main and secondary maxima and minima of the frequency response of DPAROD. This performance may be fairly useful for filtering, when discrimination is made on the basis not only of the signal frequency, but also on its signal-to-noise ratio. Another application of the DPAROD may be in the field of fiber-optic sensors, owing to the dependence of the DPAROD response on the gain of the fiber loops. If the results to be shown are extrapolated to integrated optics, large free spectral ranges should be achieved with DPAROD, as compared with singlestructure configurations. ~revious reports have been made in the coherent analysis of a three-couplerThe authors are with the Departamento de Tecnología Fotónica, Escuela Técnica Superior de Ingenieros Telecomunicación, Universidad Politécnica de Madrid, Ciudad Universitaria, E-28040 Madrid, Spain.Received 9 February 1993; revision re ical So.configuration,5 while with our DPAROD only two couplers are needed. An incoherent analysis (i.e., we consider intensities rather than fields) of the DPAROD has been carried out. Our main interest lies in predicting the frequency-domain location of the maxima-minima and the shape of the transfer function, we have developed a new model to do so. Some experimental results are also shown. Novel Method for Analysis of a Double-Amplified Recirculating Optical-Delay LineTo describe the signal in fiber-optic recirculating delay lines one usually utilizes the Z t r a n~f o r m .~ This usage is possible as long as a basic time delay T can be defined such that the other relevant system delays are integer multiples of this basic delay, and the system can be considered discrete in time. Such is the case in the single-structure configuration,ly2 or in Nth-order fiber-optic lattice structures, where the delay T is the same for al1 se~tions.~ This is n...

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