Optics Communications 103 (1993) 245-253 OPTICS North-Holland COMMUNICATIONS Full length article An acousto-optic correlator working by coherence modulation of light J.P. Goedgebuer, R. Ferriere and B.E. Benkelfat Laboratoire d'Optique P.M. Duffieux, UA CNRS 214, Facult~des Sciences, Universit~ de Franche-Comt~, 25030 Besan¢on cedex, France Received 1 April 1993 Demonstration of coherence modulation of light is reported for the first time in the area of acousto-optic signal processing. The method allows electro-optic modulation of broadband sources to be used in acousto-optic time integrating correlators, with a modulation depth and a dynamic range comparable to those of coherent systems. Though a single coherence-modulated channel is used, it demonstrates that a generalization of the method to several channels is possible. 1. Introduction A number of papers which demonstrate the use of coherence modulation of light in optical communi- cations and optical sensors have been published. In particular, the use of multiple transmitter and re- ceiver interferometers for multiplexed communica- tions using a broadband source was proposed as early as 1974 [ I ]. Other versions using electro-optic co- herence modulators in optical fiber transmission links were reported in the 1980's [2]. More recently, co- herence modulation of light has been utilized in the area of interferometric fiber sensors [ 3-7 ] matched to sources with short coherence lengths. We recently reported a technique also based on coherence mod- ulation of light which allows full duplex transmis- sions to be carried out on a single fiber [8]. We re- port in this paper the first demonstration of coherence modulation in the area of optical pro- cessing, presenting initial experimental results ob- tained with a coherence-modulated time-integrating correlator, and demonstrating that coherence mod- ulation of light can be used in optical processing. The method allows electro-optic modulation of broad- band sources to be used with a dynamic range com- parable to that of other systems, without incurring the associated speckle problems. The method is demonstrated using a modified version of a time-in- tegrating correlator such as that described by Spra- gue and Koliopoulos [ 9 ]. The peculiarity is to allow the correlation products to be computed independently by each of the wave- length components that are scaled alike and are in registry at the system output. Though the method is demonstrated with low fre- quency signals and bulk electro-optic modulators, integrated modulators (which offer potential band- widths up to 35 GHz) can be used as well. 2. Principle of operation First, let us briefly recall that coherence modula- tion of light (also termed path-difference modula- tion) consists in encoding a signal on a light beam as an optical delay larger than the coherence time, that results in a modulation of the degree of coher- ence of light. This opens the way to the use of mul- tiwavelength or broadband sources in systems that were thought to be restricted to quasi monochro- matic light. Figure 1 shows how the method can be used in acousto-optic correlator for computing the crosscor- relation product between two electric signals f(t) and s(t). To demonstrate the method, the system was ira- 0030-4018/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved. 245