John J. Johnson

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.

Low drive-power integrated acoustooptic filter on X-cut Y-propagating LiNbO/sub 3/

The authors fabricated very low power integrated acoustically tunable optical filters using walkoff-compensated miniature interdigital transducers, coupled to narrow-width acoustic waveguides. Nearly 100% transverse electric-transverse magnetic (TE-TM) polarization conversion efficiency was achieved using only 8 mW of applied RF power in a 1550-nm filter of 1.8-nm optical bandwidth. A power handling capacity of almost 100 channels was demonstrated by driving these transducers with 760 mW of RF power. These devices suffer little performance degradation in terms of filter width and sidelobe spectrum.<<ETX>>

Bistable optical switching using electrochemically generated bubbles.

We demonstrate a novel bistable optical waveguide switch, which may be suitable for incorporation in large-dimension optical cross connects. The switching is based on the electrolytic creation and catalytic destruction of bubbles and is both wavelength and polarization independent. Calculations indicate that low losses and cross talk, which are crucial for large switching arrays, can be achieved with the combination of materials used in the prototype. The intrinsic bistability of the switch eliminates the need for a holding voltage and permits matrix addressing.

Surface-acoustic-wave directional coupler for apodization of integrated acoustooptic filters

A SAW waveguide directional coupler that was used to provide a raised-cosine apodization of the SAW beam intensity, in order to achieve sidelobe suppression of an integrated acoustooptic filter, is demonstrated. The SAW guide coupler consisted of two closely spaced, evanescently coupled parallel acoustic waveguides. A single guided SAW mode was excited by a miniature transducer imbedded in a 100- mu m-wide waveguide, and in excess of 99% of the SAW energy was transferred from the original waveguide to the coupled guide, and back, across a gap of 20 mu m. The coupling length (for complete crossover) was 9.8 mm and depended exponentially on gap, as expected. This SAW coupler resulted in 10-dB sidelobe suppression compared to an unapodized acoustooptic filter.<<ETX>>

Guided blue and green upconversion fluorescence in an erbium–ytterbium-containing silicate glass

We observe guided blue and green upconversion fluorescence in an ion-exchanged waveguide that is fabricated in an erbium-ytterbium-codoped silicate laser glass. The guides are optically pumped at room temperature by using a Ti:sapphire laser with lambda = 880-980 nm; the observed fluorescence is in two bands, 480-490 and 520-560 nm, and results from multiphoton excitation.

Nonsymmetric Mach-Zehnder interferometers used as low-drive-voltage modulators

Optical communications systems require fast optical modulators which are compatible with existing microwave-frequency drive circuits and which have low voltage requirements. The authors have designed and demonstrated a novel type of integrated optical modulator: a nonsymmetric Mach-Zehnder interferometer, which has both low switching voltage and efficient coupling to microwave-frequency drive circuits. In 2-cm-long devices, made in z-cut LiNbO/sub 3/, switching voltages of 3.2 V and better than 20-dB extinction ratio were measured at lambda =1.3 mu m. Device performance in the nonsymmetric devices was independent of the asymmetry, which indicates that devices of this kind are tolerant of fabrication variations. >

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>>

1 Gb/s system performance of an integrated, polarization-independent, acoustically-tunable optical filter

The first system demonstration of a polarization-independent acoustooptic filter centered at 1500 nm with a narrow bandpass (1.3 nm) and broad continuous tuning range (145 nm) is reported. It is shown that filtered transmission of 990-Mb/s pseudorandom data incurs less than 1 dB of additional noise penalty, less than 0.5 dB of which is dependent on the signal state of polarization. In addition, this versatile polarization-diversity configuration allows for multiple-channel selection and operation as a 2*2 wavelength-channel switch.<<ETX>>

Integrated-optic acoustically tunable reflection filter

We demonstrate a two-stage, integrated-optic, acoustically tunable filter formed by reflecting the output of a single stage acousto-optic filter back through the filter by means of an integral mirror. Relative to a single-stage device, this double-pass configuration provides greatly improved sidelobe suppression (to approximately -20 dB) and a 72% reduction in filter bandwidth (to less than 0.9 nm at a wavelength of 1.52 microm). The performance of this device is shown to be equal to the far more difficult to fabricate cascaded pair of strictly matched acousto-optic filters.