1560 1580 wavelength (nm) CThW2 Fig. 3 Calculated crosstalk isolation (fraction of power not switched) for a device with L = 25 mm, X = 1550 nm, and f = 0.5 MHz,where the waist diameter profile (a) varies linearly from 15.8 pm to 16.5 pm, and (b) is uniform at 16.15 pm. The acoustic powers required are 39.8 p W and 7 pW, respectively. tuned by changing the acoustic frequency. From Eq. 3, the bandwidth of a typical switch' withf= 10 MHz and L = 25 mm is 3.6 nm. For a bandwidth of 0.8 nm (the dense WDM channel spacing), higher frequencies and lengths are required; for example, f= 40 MHz and L = 45 mm, which is achievable in The spectrum can be apodized5 by varying the diameter or acoustic amplitude along the coupler, or by concatenating successive filter stages.Static filters are possible by replacing the acoustic wave with a blazed long-period grat-ing6 The fiber cladding must be photosensitive, because light fills the cladding in the coupler waist, but apodization is simpler.In contrast, a broader bandwidth is preferred for simple routing switches and frequency shifters. Equation 3 indicates that short couplers and low acoustic frequencies are required. However, the 20 dB bandwidth is 1/8 of the FWHM bandwidth, so responses as broad as 30 nm (say) with crosstalk isolation better than -20 dB are impractical. The solution is to chirp the resonance with a nonuniform coupler waist. Figure 3 is the response calculated for a simple linear variation of waist diameter. The 20 dB bandwidth of 36 nm is over 10 times that for an equal-length uniform coupler. The penalty is a six-fold increase in the acoustic power required, but this is small anyway.Fused couplers with controllably nonuniform waists can be made by use of a "flame brush" setup7 by varying the elongation rate of the fibers as the traveling flame moves along them (or by varying the flame speed). The waist will end up narrower where the fibers are stretched more quickly. The profile of Fig. 3 can be formed by varying the elongation rate between 0 and 5 mm min-' on the final pass only, for a constant flame speed of 2 mm s-'. T. A. Birks, D. 0. Culverhouse, S. G. Farwell, P. StJ. Russell, Opt. Lett. 21, 722 (1996). T. A. Birks, P. St.J. Russell, D. 0. Culverhouse, "The acousto-optic effect in singlemode fiber tapers and couplers," to appear in IEEE J. Lightwave Technol. 14, issue 11 (1996). D. 0. Culverhouse, T. A. Birks, S. G. Farwell, J. Ward, P. St.J. Russell, "40 MHz all-fiber acousto-optic frequency shifter,"~ 4.
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7.Optical code division multiple access (CDMA) communication systems are a viable choice for local area optical networks because of their unique attributes of optical processing, asynchronous transmission, high information security, and the capability for multiple access, which results in high overall data transmission rates.' Here we report results on a nonlinear fiber-optic receiver for an optical CDMA scheme based on encoding and decoding of coherent ultrashort pulses. As proposed previously: ultrashort pulse CDMA transmitters would use a sp...
