J.N. McMullin

High index contrast waveguides in chalcogenide glass and polymer

We review various properties of chalcogenide glasses that make them promising materials for passive and active microphotonics. We then describe two processes for channel waveguide fabrication, using the chalcogenide glass As/sub 2/Se/sub 3/ as a core material and compatible polymers as claddings. In the first approach, waveguides are patterned directly in the chalcogenide film by photoexposure through a mask followed by selective wet etching. This technique has produced shallow rib waveguides with losses as low as 0.26 dB/cm and small modal area photonic wire waveguides with losses on the order of 10 dB/cm. In the second approach, waveguide patterning is achieved by using an organic photoresist as a mask for selective photodoping of silver into the chalcogenide glass. Selective wet etching produced strip waveguides with smooth and highly vertical sidewalls. We report preliminary light guiding results for these latter structures.

Photoinduced refractive index change in As2Se3 by 633nm illumination.

Photodarkening of amorphous As2Se3 thin films was generated by a 633-nm HeNe laser. The refractive index and absorption coefficient of the chalcogenide glass was determined, both before and after exposure, by analyzing the materials transmission spectrum. In order to accurately determine the optical constants, the thin films non-uniform thickness was accounted for. The increase in the refractive index and the coefficient of absorption was investigated and was found to demonstrate saturation with increased exposure time. Index changes as high as 0.05, or 2%, were obtained in As2Se3, a promising glass for all-optical switching.

Chip-scale spectrometry based on tapered hollow Bragg waveguides

We describe a micro-spectrometer that exploits out-of-plane radiation at mode cutoff in a tapered leaky waveguide clad by omnidirectional Bragg reflectors. The device can be viewed as a side-coupled, tapered Fabry-Perot cavity. An effective-index transfer-matrix model reveals that optimal resolution is dependent on the reduction or mitigation of back-reflection and standing waves leading up to the cutoff point. We address this by insertion of low numerical aperture optics between the taper and the detector, and demonstrate an experimental resolution as small as approximately 1 nm and operating bandwidth >100 nm in the 1550 nm range, from a tapered waveguide with footprint approximately 50 microm x 500 microm. The device combines the small size of a Fabry-Perot instrument with the detector array compatibility and fixed optics of a grating-based instrument.

Small core rib waveguides with embedded gratings in As2Se3 glass.

Low-loss shallow-rib waveguides were fabricated using As2Se3 chalcogenide glass and polyamide-imide polymer. Waveguides were patterned directly in the As2Se3 layer by photodarkening followed by selective wet etching. Theory predicted a modal effective area of 3.5-4 microm2, and this was supported by near-field modal measurements. The Fabry-Perot technique was used to estimate propagation losses as low as ~0.25 dB/cm. First-order Bragg gratings near 1550 nm were holographically patterned in some waveguides. The Bragg gratings exhibited an index modulation on the order of 0.004. They were used as a means to assess the modal effective indices of the waveguides. Small core As2Se3 waveguides with embedded Bragg gratings have potential for realization of all-optical Kerr effect devices.

Hollow Bragg waveguides fabricated by controlled buckling of Si/SiO 2 multilayers

We describe integrated air-core waveguides with Bragg reflector claddings, fabricated by controlled delamination and buckling of sputtered Si/SiO2 multilayers. Thin film deposition parameters were tailored to produce a desired amount of compressive stress, and a patterned, embedded fluorocarbon layer was used to define regions of reduced adhesion. Self-assembled air channels formed either spontaneously or upon heating-induced decomposition of the patterned film. Preliminary optical experiments confirmed that light is confined to the air channels by a photonic band-gap guidance mechanism, with loss ~5 dB/cm in the 1550 nm wavelength region. The waveguides employ standard silicon processes and have potential applications in MEMS and lab-on-chip systems.

Direct UV patterning of waveguide devices in As2Se3 thin films

Photodarkening of thermally evaporated amorphous As2Se3 chalcogenide thin films was generated by a UV mercury light source in a standard mask aligner. The refractive index modification of the chalcogenide glass was determined by applying Swanepoel’s method. Index changes of +0.04 were obtained for 500 s exposure. Using these photoinduced index changes, waveguides with losses of approximately 1 dB/cm at 980 nm were fabricated. Another set of waveguides was fabricated by UV exposure and subsequent selective etching to form rib structures. Those waveguides exhibited loss of approximately 2 dB/cm at 980 nm. Silver photodoping of As2Se3 was also performed on a mask aligner with index increases on the order of 0.3 obtained. Due to the rapid and large photomodification obtainable with standard photolithographic equipment, these processes are promising for integrated optic device fabrication.

Photoluminescence and Thermal Properties of Er-DOPED As-Se-Ga-Ge Based Glasses

We studied the photoluminescence characteristics and thermal properties of Er-doped As-Se-Ga-Ge based chalcogenide glasses alloyed with different amounts of Ge (1 to 15 at. %) for use as optical amplifiers at 1550 v nm operation. We used a 980 v nm laser diode to optically excite the Er-doped glass samples. The 980 v nm pumping induced a 1550 v nm photoluminescence output signal. We measured the lifetime of the exited Er state in these chalcogenide glasses as a function of Er doping up to 3 at. %, and Ge composition from 1 to 15 at. %. We also carried out Temperature-Modulated Differential Scanning Calorimetry measurements to study the thermal stability of these glasses.

Photoluminescence measurements of Er-doped chalcogenide glasses

We have investigated the photoluminescence (PL) emission spectra, photoluminescence lifetimes, and relative photoluminescence intensities of various Er-doped chalcogenide glass alloys, designed for possible use in optical waveguide amplifier applications at the telecommunications wavelength of 1550 nm. Bulk samples were prepared using melt-quenching techniques, and the samples were doped with varying amounts of Er2S3 or ErCl3, in the range of 0.3−3u2009at.u200a%. Using a 980 nm pump beam, we measured the lifetime of the 1550 nm (4I13/2 to 4I15/2) luminescence and PL spectra. For two of the alloys, the luminescence efficiency as a function of erbium concentration was observed by scaling the luminescence intensity by the weight of each sample. For GaGeS, the luminescence lifetime exhibited quenching at concentrations of erbium greater than 1 at.u200a%, but for GaGeSe, this behavior was not observed for concentrations as high as 3 at.u200a%. However, the photoluminescence efficiency measurements for both glasses showed quen...

Theory and simulation of a concave diffraction grating demultiplexer for coarse WDM systems

Reflective concave diffraction gratings in multimode planar waveguide geometries are considered as wavelength separation devices for coarse wavelength division multiplexing (WDM) systems. General expressions are derived relating the number of channels, the wavelength and channel wavelength separation, and the numerical aperture of the planar slabwaveguide. Scalar electromagnetic simulations performed for a particular grating design show that acceptable performance is theoretically achievable.

Thermal tuning of hollow waveguides fabricated by controlled thin-film buckling

We describe the thermal tuning of air-core Bragg waveguides, fabricated by controlled formation of delamination buckles within a multilayer stack of chalcogenide glass and polymer. The upper cladding mirror is a flexible membrane comprising high thermal expansion materials, enabling large tuning of the air-core dimensions for small changes in temperature. Measurements on the temperature dependence of feature heights showed good agreement with theoretical predictions. We applied this mechanism to the thermal tuning of modal cutoff conditions in waveguides with a tapered core profile. Due to the omnidirectional nature of the cladding mirrors, these tapers can be viewed as waveguide-coupled, tunable Fabry-Perot filters.