Achromatic photonic tricouplers for application in nulling interferometry

Abstract

Integrated-optic components are being increasingly used in astrophysics, mainly where accuracy and precision are paramount. One such emerging technology is nulling interferometry that targets high contrast and high angular resolution. Two of the most critical limitations encountered by nullers are rapid phase fluctuations in the incoming light causing instability in the interference and chromaticity of the directional couplers that prevent a deep broadband interferometric null. We explore the use of a tricoupler designed by ultrafast laser inscription that solves both issues. Simulations of a tricoupler, incorporated into a nuller, result in an order of a magnitude improvement in null depth.

Achromatic design of a photonic tricoupler and phase shifter for broadband nulling interferometry

“…This configuration is ideal for simultaneous broadband null depth measurement and fringe tracking capability. 10 Although careful design of three-way planar waveguides has been shown capable of approaching this ideal, 14 doing so requires considerable control over many parameters governing the detailed waveguide properties. Here, instead, we adopt the strategy of enforcing equal coupling by way of geometrical symmetry.…”

“…Replacing the directional coupler with a symmetric tricoupler, which splits input beams into three outputs, rather than two, has been shown in simulation to be capable of overcoming these limitations. 10 Assuming that the waveguides in the interaction region of the tricoupler are arrayed in an equilateral triangle and that the injected beams are in perfect anti-phase and with equal flux, the central channel will produce a completely nulled output, regardless of the wavelength of incoming light. Underlying this powerful configuration are arguments based on simple symmetry: anti-symmetric input beams are unable to couple to an even-symmetry mode field required to overlap with the central null channel.…”

“…Simulations of a tricoupler able to deliver this achromatic null have been shown to reach a null depth 45 times better than with a single directional coupler. 10 The use of a tricoupler has been previously mooted as a combiner for nulling and stellar interferometry. 11,12 To exploit the equal coupling ratios and phase delays between outputs, early devices made use of three waveguides in a planar, rather than triangular, configuration.…”

“…14 However, devices with an equilateral triangular configuration in the interaction region are able to exploit further symmetries: namely, an equal splitting ratio between bright channels. 10 Early tricouplers made use of fused fiber twisted together to produce this triangular configuration in the central region, which introduced asymmetries, altering the desired coupling ratio. 15 Today, the ultrafast laser inscription (ULI) [16][17][18][19] process enables efficient and precise inscription of tricouplers and other photonic devices requiring three-dimensional design freedom.…”

A report on the status of astrophotonics for optical/IR interferometry and beyond

“…In Martinod et al 46 , the authors demonstrate in the lab an instrumental contrast of ∼10 −3 in dispersed light, while the on-sky observation of α Boo in nulling mode has resulted in the detection of few 10 −2 stellar leakage, in agreement with the expected stellar diameter of this source. In a similar context, the potential of the ULI platform was further analyzed through simulations only by studying the potential of an integrated optics tri-coupler in an equilateral configuration within the interaction region to serve as achromatic nuller and in situ fringetracker simultaneously 47 . These results open new perspectives for nulling interferometry on other observational platforms.…”