Instruments without optics: an integrated photonic spectrograph

Joss Bland-Hawthorn & Anthony HortonAnglo-Australian Observatory

Ground-based and Airborne Instrumentation for AstronomySPIE Orlando May 2006

Overview

• What is an Integrated Photonic Spectrograph?• Why are they of interest?• Photonic Echelle Grating• Array Waveguide Grating• Relative merits• Issues to be resolved• Synergy with other technologies• Summary

What is an IPS?

• An integrated optical device which combines all the functions of a spectrograph, i.e. a ‘spectrograph on a chip’

• Based on photonic technology developed for the telecommunications industry (DWDM)

• IPS input is a single or few-mode waveguide, likely fed by a matching fibre

• Output spectra edge-coupled to a detector array• (Later development could also integrate detector into IPS)

What is an IPS?

Interest in IPS

• A possible alternative to increasingly large instruments and the associated problems

• Small, integrated devices• True mass production• Scaleable• Low scattering

Photonic Echelle Grating

Facets ~15-500μm, m>10,~100

Concave Echelle grating, operating in Littrow configuration

Photonic Echelle Grating

This assumes a Littrow configuration and is for a common grating angle of 60º. The physical seperation of spectral resolution elements at the output (dy) is taken to be 15μm.

For R~1000 a PEG need only be ~1cm in size.

Array Waveguide Grating

AWG output spectrum

Relative Merits

• PEGs offer higher finesse than AWGs (~number of facets vs ~number of waveguides)

• PEGs are more compact than AWGs• AWGs currently capable of producing more spectral

resolution elements (~2k vs ~0.5k) however expect improvements from PEGs

• Overall PEGs are preferred, however AWGs may have some uses, e.g. order sorting ahead of high resolution PEGs

Issues to resolve• Number of modifications and developments needed to

turn current devices into astronomical spectrographs• Throughput improvements to get losses below 3dB• Remove output waveguides and flatten field for use with

edge-coupled detector arrays• Simplify grating design to decrease size of device• Use of higher order modes will help fibre coupling

efficiency (see 6269-58)

Synergy with other tech 1 Natural to include OH

suppresion fibres in the IPS feeds.

Suppression of 18 doublets at R=10000 gives 96% reduction in OH background over a 75nm range, with 4% fibre losses(Bland Hawthorn et al 2004)

Synergy with other tech 2

• Small size makes IPS a potential payload for robotic positioner systems, e.g. Starbugs (6273-70)

dIFUdIFS

Summary

• Possible to produce integrated photonic spectrographs for astronomy only ~1cm in size

• These devices avoid many of the problems associated with building larger and larger conventional instruments

• Two types have been investigated, photonic echelle gratings and array waveguide gratings.

• Photonic echelle gratings appear more useful for astronomy• Developments and refinements are needed before the use

of IPS is practical, however the outlook is promising• Much more detail, especially theory, in the paper (see me to

get a sneak preview)