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.
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.
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.
We describe planar air-core waveguides with Bragg reflector claddings, fabricated by controlled delamination and buckling of sputtered Si/SiO 2 multilayers. We also report preliminary light guiding experiments in the 1550 nm wavelength range.Index Terms-Hollow Waveguide, Self Assembly. BACKGROUND AND MOTIVATIONAir-core integrated waveguides are attracting interest for lab-on-chip [1] and optical interconnect [2] applications. Conventional approaches for fabrication of these waveguides include sacrificial etching, wafer bonding, and chemical vapour deposition in a pre-defined trench [3]. We recently reported an alternative approach [4] based on the controlled formation of straight-sided delamination buckles [5] within a multilayer thin film stack.Those waveguides were fabricated using a combination of a chalcogenide glass and a commercial polymer. Here, we describe preliminary work on an analogous process using silicon-based materials and MEMS-compatible fabrication steps.Silicon-based processing should expand the scope for practical application of these self-assembled waveguides. FABRICATION PROCESS AND STRUCTURESPRODUCED In our process, controlled thin film buckling is exploited for the self-assembly of a three dimensional air core waveguide using otherwise planar (two-dimensional) processing steps. The process starts with a piranha cleaned silicon wafer, and follows the sequence of steps shown in Fig. 1. Si/SiO 2 Bragg reflectors were deposited using a pulsed magnetron sputtering system and a silicon target. The SiO 2 layers were deposited by reactive sputtering in an oxygen-rich environment. Sputtering allows a large degree of control over the extrinsic stress of the thin films, including the deposition of strongly compressive layers [6]. First, a four period Si-SiO 2 Bragg reflector was deposited at a substrate temperature of 150°C without breaking vacuum. This eventually acts as the bottom cladding of the hollow waveguides. After photoresist patterning, a fluorocarbon low adhesion layer (LAL) was deposited using the passivation process in a DRIE chamber [7]. The LAL was patterned by liftoff, thereby defining regions for subsequent delamination of the upper Bragg mirror. Next, the same sputtering system was used to deposit another four period Bragg reflector, which eventually acts as the upper cladding of the hollow waveguides. Deposition parameters were studied to ensure a desired level of compressive stress in the upper Bragg mirror. Deposition at low background pressure (~ 3.5 mTorr) and relatively high power (~200 W) produced a multilayer with a net compressive stress of ~260 MPa. Fig. 1. Process steps for creating planar waveguides: a) A fourperiod Bragg reflector is sputter deposited b) A LAL layer is applied using CVD and patterned using lithography c) A fourperiod top mirror is sputter deposited under compressive stress d) the sample is heated to reduce the adhesion, allowing the buckles to form along the patterned areas.258 978-1-4244-5369-6/10/$26.00 ©2010 IEEE
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