Integrated photonic building blocks for next-generation astronomical instrumentation I: the multimode waveguide

Abstract

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.

Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications

“…Thomson et al was the first to demonstrate a laser-inscribed photonic lantern with 16 ports [6], as shown in Figure 7. Laser inscription requires careful optimisation of the multi-mode section of the lantern in order to achieve low losses [119]. With optimised design of the length as well as the shape of the tapered transition, a multi-mode to slit reformatting integrated photonic lantern can achieve 95% efficiency at a wavelength of 1550 nm excluding the glass absorption losses [120].…”

Design of optically path-length-matched, three-dimensional photonic circuits comprising uniquely routed waveguides

“…The bend losses were isolated by removing the affects of coupling and bulk material absorption losses. Loss due to the coupling of light into and out of the chip was determined to be 9 ± 2%, with absorption loss in the Eagle2000 glass at 1550 nm also accounted for ( = 0.0075 ± 0.0003 mm -1 ) [23]. Although the series of guides had varying lengths, the bend losses were rescaled to a waveguide length of 30 mm for comparative purposes.…”

Photonic lanterns

“…So far in published results, photonic lantern core geometries have consisted of hexagonal lattices, rounded array or square lattices with the number of cores approximately equal to the number of final waveguide modes as required [12,13,[17][18][19]. However, it has been theoretically demonstrated that the best starting point for a low loss lantern design is an uncoupled core geometry that best approximates or samples the geometry of the modes of the final MM fiber [14,20,21].…”

“…In this case, the isolated singlemode waveguide cores are created and gradually brought together such as they couple strongly. This creates the adiabatic optical transition required for the single-mode to multimode conversion, and the strongly coupled cores form the final multimode composite waveguide [18,19,23]. ULI photonic lanterns, also called integrated photonic lanterns, are a very versatile approach.…”