Abstract
We report on the fabrication and characterization of composite multimode waveguide structures that consist of a stack of single-mode waveguides fabricated by ultrafast laser inscription. We explore 2 types of composite structures; those that consist of overlapping single-mode waveguides which offer the maximum effective index contrast and non overlapped structures which support multiple modes via strong evanescent coupling. We demonstrate that both types of waveguides have negligible propagation losses (to within experimental uncertainty) for light injected with focal ratios >8, which corresponds to the cutoff of the waveguides. We also show that right below cutoff, there is a narrow region where the injected focal ratio is preserved (to within experimental uncertainty) at the output. Finally, we outline the major application of these highly efficient waveguides; in a device that is used to reformat the light in the focal plane of a telescope to a slit, in order to feed a diffraction-limited spectrograph.
Highlights
Over the past three decades there has been an increase in the incorporation of photonic technologies into world-class astronomical observatories which prompted the branding of a new field: "astrophotonics" in 2009 [1]
We report on the fabrication and characterization of composite multimode waveguide structures that consist of a stack of single-mode waveguides fabricated by ultrafast laser inscription
We demonstrate that both types of waveguides have negligible propagation losses for light injected with focal ratios >8, which corresponds to the cutoff of the waveguides
Summary
Over the past three decades there has been an increase in the incorporation of photonic technologies into world-class astronomical observatories which prompted the branding of a new field: "astrophotonics" in 2009 [1]. The throughputs of the waveguides were normalized to the amount of light that was collected by the 100 μm collection fiber when it was placed directly at the focus of the injection lens, with the photonic chip removed By using this method a consistent number of air/glass interfaces, which cause Fresnel reflections, was maintained between the waveguide and calibrator measurements with the only difference being the presence of the waveguide itself. The FWHM size of the launch field was 46 ± 2 μm, it did not have the ideal flat-topped cross-sectional profile as can be seen in Fig. 3 below (red curve), and there was significant power outside the physical extent of the waveguides This power did not couple into the waveguides and should not be included in our measurements of throughput.
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