Low bend loss waveguides enable compact, efficient 3D photonic chips

Optics Express (Impact Factor: 3.49). 02/2013; 21(3):2978-2986. DOI: 10.1364/OE.21.002978
Source: PubMed


We present a novel method to fabricate low bend loss femtosecond-laser written waveguides that exploits the differential thermal stabilities of laser induced refractive index modifications. The technique consists of a two-step process; the first involves fabricating large multimode waveguides, while the second step consists of a thermal post-annealing process, which erases the outer ring of the refractive index profile, enabling single mode operation in the C-band. By using this procedure we report waveguides with sharp bends (down to 16.6 mm radius) and high (80%) normalized throughputs. This procedure was used to fabricate an efficient 3D, photonic device known as a "pupil-remapper" with negligible bend losses for the first time. The process will also allow for complex chips, based on 10's - 100's of waveguides to be realized in a compact foot print with short fabrication times.

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Available from: Alexander Arriola, Mar 22, 2014
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    • "Internal modification of transparent material using ultrashort laser pulse (USLP) has attracted much attention due to interesting applications such as microwelding [1] [2] [3], waveguide formation [4] [5], selective etching [6] and so on. Among these applications, microwelding of glass using USLP is considered to be especially promising, since crackfree welding with monolithic structure having high mechanical strength is available even with glass having high thermal expansion coefficient. "
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    ABSTRACT: Dynamic behavior of plasma in internal modification ofglass using fs-laser pulses with high pulse repetition rates up to 1 MHz is observed using high-speed video camera at 50,000frames per second.High-speed pictures reveal striking fact that the localizedplasma isproduced with size much smaller than reported in literatures before, and changes its location, size, shape and brightness periodically at repetition rates of the order of kHz. The mechanism responsible for producing localized plasma and its long-term periodic motion is presented based on the analysis of free-electron density based on rate equation.
    Physics Procedia 12/2014; 56:973-982. DOI:10.1016/j.phpro.2014.08.008
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    • "We have recently utilised a new waveguide fabrication process that has enabled us to significantly increase the throughput of the existing remapper architectures to 69-73% [5]. This technique is based on optimising the refractive index profile of the waveguides via thermal annealing processes, which in turn reduces the bend losses plaguing the device. "
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    ABSTRACT: Over the past few years pupil-remapping interferometry has received growing attention [1,2]. Pupil remapping is a technique that involves collecting the stellar photons from the pupil plane of a telescope with a 2-dimensional array of single mode waveguides and reformatting them at the output so that the emerging beams can be interfered in the appropriate fashion. This interferometric technique is used in astronomy in order to achieve sub-diffraction limited imaging despite the turbulent atmosphere and builds on the highly successful bulk optics analogue aperture masking interferometry in a number of ways. Dragonfly uses a femtoseond laser inscribed integrated photonic pupil remapper, which enables a compact instrument to be realized that operates in the astronomical H-band (1.5-1.8 μm). Dragonfly's prototype photonic chips consist of 8 waveguides that reformat segments of the pupil to an equally spaced linear array at the output. This sparse pupil sampling is not ideal for a final instrument but makes instrument characterisation simpler. The initial prototype chips had reasonably high throughputs (58-62%), cross-coupling of the order of 1% and showed large phase instabilities, which are detrimental to the fidelity of the data that can be recovered [3]. Second generation devices were designed to minimize phase instabilities but in turn reduced the throughputs of the devices significantly (3-47%) [2,4]. In parallel to the phase stability and throughputs of the remappers, the optical path length differences between the waveguides were determined to be <46 μm. This means that light with a bandwidth of up to 55 nm could be used to generate high visibility fringes, which is close to the requirements for the astronomical observing strategy typically implemented. Indeed the second generation of prototype devices were tested and validated on-sky at the Anglo-Australian Telescope (AAT) in mid-2010. We have recently utilised a new waveguide fabrication process that has enabled us to significantly increase the throughput of the existing remapper architectures to 69-73% [5]. This technique is based on optimising the refractive index profile of the waveguides via thermal annealing processes, which in turn reduces the bend losses plaguing the device. In addition, we have fabricated a next generation device utilizing the same waveguide recipe with an extra level of stray light suppression in order to increase the phase stability further. This device has demonstrated high throughputs as well (~60%). By using the optimised index profile guides, the cross-coupling was reduced in the latter device to <1×10-5, a significant improvement on the previous devices. The characterisation of these two prototype devices along with the ultimate phase stability measured within the Dragonfly instrument will be presented. In addition we will elucidate the interesting methods employed for measuring the optical path length differences of the waveguides.
    CLEO Europe, Munich (Germany); 05/2013
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    ABSTRACT: We present a novel way to fabricate low bend loss femtosecond-laser written optical waveguides [1] via a two-step process: We first fabricate large-diameter waveguides which are multimode at 1550 nm and exhibit a relatively complex refractive index distribution. In a second step, thermal post-processing [2] results in the selective removal of part of the previously induced refractive index change, leaving behind a very high index contrast waveguide that enables single mode operation in the C-band. We demonstrate that this procedure enables the fabrication of waveguides with unprecedented small bends (down to 16.6 mm radius) [3] with high (80%) normalized throughputs. Our innovative method utilizes the fact that different physical mechanisms underpin the femtosecond laser induced refractive index change in different and distinctive regions of the glass substrate. We have thus developed a thermal post-processing procedure that allowed us to erase the outermost ring of refractive index modification without affecting the central part of the waveguiding structure. Figure 1 (left) shows the refractive index profile of a typical waveguide before thermal treatment: A central core with a high and positive refractive index change Δn is surrounded by a depressed-cladding region (negative Δn) that, in turn, is surrounded by another ring of small and positive Δn. Such a waveguide is multimode at 1550 nm and, moreover, exhibits relatively large bend losses. Figure 1 (right) shows the successful and selective erasure of the outermost positive refractive index modification, which allowed us to realise single- mode waveguides with high light confinement and ultra-low bend losses. We believe that this novel process will allow for the fabrication of complex 3-D photonic devices, based on 10's - 100's of waveguides in a compact footprint with short fabrication times.
    CLEO Europe, Munich; 05/2013
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