The fiber-optic (FO) switch is a basic building block for many optical applications such as routing in fiber communications networks, photonic signal processing, distributed optical sensing, and optical controls. The desired features for a FO switch include low optical loss (e.g., &lt;1 dB), low interchannel crosstalk (&lt;-30 dB), polarization independence, robustness to catastrophic failure, and simple to align low cost designs for large scale commercial production and deployment. Depending on the application, FO switching speeds can range from nanoseconds to several milliseconds. Fast sub-microseconds (e.g., 100 ns) switching speeds are required in internet type packet switched FO networks.
Similarly, variable fiber-optic attenuators are the basic building blocks for several key optical systems. Presently, these attenuators are required as equalizers in wavelength division multiplexed (WDM) optical communication systems using non-uniform gain optical amplifiers. Other important applications include polarization dependent loss compensation in fiber optic networks, optical component testing, and optical receiver protection. Hence, a variable fiber-optic attenuator with fast sub-microseconds duration speed with exceptionally high attenuation dynamic range (e.g., 50 dB) control is a present challenge to the optical community.
Over the years, attempts have been made to realize acoustooptic (AO) FO switches as AO technology has speeds in the submicrosecond regime. These include works such as W. E. Stephens, P. C. Huang, T. C. Banwell, L .A. Reith, and S. S. Cheng, "Demonstration of a photonic space switch utilizing acousto-optic elements," Opt. Eng. 29 (3):183-190, 1990, D. O. Harris and A. Vanderlugt, "Acousto-optic photonic switch," Opt. Lett. 14 (21):1177-1179, 1989, D. O. Harris, "Multichannel acousto-optic crossbar switch," Appl. Optics 30, 4245-4256, Oct. 10, 1991, D. O. Harris and A. Vanderlugt, "Multichannel acousto-optic crossbar switch with arbitrary signal fan-out," Appl. Optics 32, pp. 1684-1686, April 1992, E. Tervonen, A. T. Friberg, J. Westerholm, J. Turunen, and M. R. Taghizadeh, "Programmable optical interconnections by multilevel synthetic acousto-optic holograms," Opt. Lett. 16:1274-1276, 1991, M. L. Wilson, D. L. Fleming, and F. R. Dropps, "A fiber optic matrix switchboard using acoustooptic bragg cells," SPIE 988, 56-62, 1988, and K. Wagner, R. T. Weverka, A. Mickelson, K. Wu, C. Garvin, and R. Roth, Chapter 14, Low-loss acousto-optic photonic switch," pp.479-492, in Acousto-optic Signal Processing, Editors N. J. Berg and J. M. Pellegrino, 2.sup.nd Edition, Marcel Dekker, 1996. All these switches have been unable to realize the goal for high &gt;50 dB isolation optical switching. Moreover, some approaches require lossy passive N:1 beam combiners, others require multichannel AO devices with limited crosstalk levels, and some even require multimode output fibers that limit signal modulation bandwidths and are incompatible with single mode telecommunication fibers. In addition to making larger N.times.N switches, these prior art design switches do not scale well as, for instance, there is a limit (e.g., 64) to the number of channels presently possible in a multichannel AO device. This type of design also requires N drive frequencies that are different, making the drive hardware complex, costly, and hard to control as the switch scale grows. If a Fourier optics type design is used, there are limitations to the number of spots the system can resolve in the output fiber plane, and the lens focal length and size can become big in order to reduce interchannel spatial crosstalk.
Specifically, because AO devices work on the principle of diffraction to implement 1 to N beam deflection, even a high 99% diffraction leads to a 1% leakage light in the non-switched port, implying a near 100:1 or 20 dB switch isolation. In this case, a single AO device serves to form a minimum 1.times.2 FO switch where N=2. Thus, so far it has not been possible to form a very high optical isolation (e.g., &gt;50 dB) switch using diffraction-based devices like AO devices even for the simple 1.times.2 switch configuration. The 2.times.2 switch is the highly sought after FO switch as many 2.times.2 switches can be combined to form large N.times.N switch matrices. It is also highly desirable to form high dynamic range (e.g., 50 dB) and high resolution (0.1 dB) FO attenuators working at high sub-microsecond domain speeds.