J. E. Baran

Integrated-optic acoustically-tunable filters for WDM networks

Recent advances in fully integrated acoustically tuned optical filters (ATOFs) for use in wavelength-division multiplexing (WDM) are reviewed. Experiments in parallel processing of wavelength channels are described. The ATOF uses the photoelastic effect induced by a surface acoustic wave (SAW) to flip the polarization state of a narrow phase-matched band of wavelengths. A unique feature of the ATOF is that many wavelengths can be simultaneously and independently processed, because the associated SAW beams can be superimposed with negligible interaction at the drive levels required for complete polarization conversion. This parallel processing capability has been used to select as many as five out of eight laser channels in an experiment in which each channel was tagged by a separate microwave subcarrier. Polarization-independent devices are also discussed. A polarization-diversity ATOF that decomposes an input beam into TE (horizontal) and TM (vertical) components, interchanges TE and TM roles for the resonant wavelength, and reroutes the filtered beam into a spatially distinct port is described. >

Polarization‐independent acoustically tunable optical filter

A polarization‐independent acoustically tunable optical filter is described, with 1.3 nm bandwidth and 145 nm tuning range demonstrated about a center wavelength of 1525 nm. Filtered and unfiltered beams exit from different ports of this 2×2 integrated‐optic structure fabricated on a LiNbO3 substrate. The filter conversion efficiency was 97% for unpolarized light, limited by imperfect splitting of the directional‐coupler polarizing beamsplitters. Using an unpolarized, 105‐nm‐wide light‐emitting diode as the optical source, 16 dB isolation of the filtered port was achieved, both with respect to the nearest nulls and the remote background spectrum.

Acousto-optic tunable filters (AOTFs) for multiwavelength optical cross-connects: crosstalk considerations

Acousto-optic tunable filters (AOTF) have been investigated as a potential basis for multiwavelength cross-connects in optical networks. In this paper, we discuss crosstalk issues, some of which are common to other cross-connect technologies, and some of which are unique to the AOTF, which will determine the suitability of the AOTF technology for this application. In particular we show how the interactions between wavelength channels make the AOTF sensitive to switch architectures, we conclude that significant performance improvements will be required to diminish crosstalk if the AOTF is to be useful in any but small-size cross-connects, even when spare and wavelength dilation are used.

Multiwavelength performance of an apodized acousto-optic switch

Use of the acousto-optic switch as a wavelength-routing element in WDM networks is described. The need for deep isolation between wavelength channels has resulted in the search for efficient sidelobe suppression techniques such as tapering of the acousto-optic profile by imbedding an optical waveguide in one arm of an acoustic directional coupler. The theory of sidelobe-suppression in the optical transmission function of a SAW-coupler apodized acousto-optic filter is presented. A low-sidelobe SAW-coupler-apodized AO switch was fabricated and used in a 1.5 micron multi-wavelength WDM switching demonstration employing four 4 nm-spaced optical channels. Crosstalk was below -19 dB for single wavelength selection and below -15 dB for arbitrary routing patterns, a dramatic improvement over unapodized AO switches.

Wavelength-selective circuit and packet switching using acousto-optic tunable filters

Wavelength-selective circuit and packet switching applications of acoustooptic tunable filters (AOTFs) in wavelength-division multiplexing (WDM) networks are discussed. For circuit switching applications, the AOTFs are used as 1*2 or 2*2 wavelength-selective space-division switches. It is shown how large switches can be built from these basic building blocks both schematically and physically. Such switch fabric can be used for routing and cross connect between central offices. For packet switching applications, the AOTFs are used as wavelength-tunable receivers in a broadcast and select cross-connect configuration. It is shown how such a configuration can accomplish packet communication with high throughput. Various system issues are also discussed.<<ETX>>

Integrated acoustically-tuned optical filters for filtering and switching applications

The recent evolution of integrated acoustooptic filters, which has been driven by the successive integration of ever more sophisticated guided-wave structures on a single X-cut lithium niobate substrate, is reviewed. The most highly integrated device to date is a versatile low-power, polarization-independent four-port wavelength-division switch. Acoustooptic filter applications are discussed, including laser tuning, spectroscopy, frequency shifting, and wavelength filters and switches for wavelength multiplexing.<<ETX>>

Observation of Modulated Crosstalk in Multichannel Acoustooptic Switches

Crosstalk in acousto-optic switches is modulated at frequencies related to that of the acoustic waves driving the device. We present measurements of the modulation, a brief discussion of its causes and examples of its effect on system performance.

Time-Dependent Model of an Acousto-Optic Tunable Filter for Multiple-Channel Operations

With its wide tunability and multiple-channel switching capability, the integrated version of acousto-optic tunable filters (AOTF’s) has shown healthy development in recent years for wavelength-division-multiplexing (WDM) optical-communication network applications as 2 × 2 and 4×4 frequency-selective optical switches.

Devices for wavelength switching in optical networks

Wavelength routing crossconnects are considered the core of WDM optical networks. They consist of optical switches independently rearrangeable for each wavelength channel and for any input- output configuration so that any path can be chosen almost arbitrarily by the network users. In general, the implementation of the wavelength routing function requires complex switch arrays. Very simple wavelength-selective crossconnects can be realized by using acousto-optic switches (AOS), because of their unique ability of processing several optical signals simultaneously and their low driving power consumption, less than 10 mW/channel. AOSs can be considered a particular evolution of acousto-optical tunable filters, whose integrated optic version on lithium niobate has been developed in several research institutions around the world in the past decade. This paper reviews the last accomplishments of AOSs, whose specifications are directly tied with optical network requirements, the foremost challenge being a strong suppression of crosstalk. Dilated AOSs can reduce interport crosstalk to below -30 dB and apodization of acousto-optic interaction can reduce interchannel crosstalk to below -15 dB during multiwavelength operation.