Instruments without optics: an integrated photonic spectrograph Joss Bland-Hawthorn & Anthony Horton Anglo-Australian Observatory Ground-based and Airborne Instrumentation for Astronomy SPIE 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 ~ μm, 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 )

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 ( ) dIFU dIFS

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)

Relative merits From finesse argument in paper, I think PEGs win out because they are folded (more compact), and you can get a much higher finesse from them compared to AWGs.. Can I leave you to capture those comments? AWG does have important uses as order sorter feed to PEGs

How many resolution elements? Grab some text from our paper about this. Point out that we are not coupling back into fibres so that doubles number of output channels immediately.

Integrated photonic spectrograph Bland-Hawthorn & Horton (2006) We are investigating array waveguide gratings and photonic echelle gratings integrated onto a chip. Typical device working at R~1000, say, will be 6mm in size for near IR. Each circuit to be fed by a single-mode or few-mode fibre. The light on exit is dispersed onto an IR array.

Diffraction limited performance F = f/ratio l Diffraction limited psf The spot size is in the regime of photonics at near IR, i.e. P = 5 to 50 microns

We go to great effort to create a diffraction limited spot. Do we really want to treat the wavefront with macrosopic optics? Possible solution: integrated photonic spectrograph F.G. Watson

Stellar interferometry:

AWGs hold the record

Number of modes e.g. FBG grating resolution Midwinter Higher modes

Looking forwards Point 1: Feed OH suppressing fibre directly into it, since we’ve gone to all the trouble of focussing light into fibre. (see next slide which will give you a chance to plug this stuff) Point 2: Use these as endpoint devices on Starbugs (see the following slide) Point 3: MEIFU device?

Story so far… FBG takes out 96% of OH background by suppressing 18 doublets over 75nm JH Bland-Hawthorn et al (2004)

Spectrographs on autonomous robots Add some words in here

Here are some OHSupp slides to squeeze in if need be, i.e. to make the link from IntPhotSpec to OHSupp. You will need to go into display mode to actually see what’s behind what

Photonic OH Suppression Program goals To suppress the OH/O 2 night sky emission at 98% or better over z, J, H bands. To achieve this at high throughput (fibre insertion losses less than 10%). To solve this for single-mode (Strehl=1) and multi-mode (Strehl<1). To produce a device that is reliable and robust for astronomical use. Funding PPARC $1.6M over 3 years, with important stage-gate review in April Bland-Hawthorn (AAO)

What is a Fibre Bragg Grating? n1n1 n2n2 Fresnel reflection from single refractive index change in a SMF: Bragg condition: small but multiple reflections in phase with each other, build up to be highly reflective at one wavelength, B – Bragg wavelength EiEi ErEr EtEt If  = refractive index period, then B = 2 n   n ~ 10 -3, L ~ 10 cm, 10,000 layers!

taper transition Bland-Hawthorn (2005)

Next step Present: To suppress 50 doublets within 75nm window (99% OH Suppression) : To double window to 150nm and suppress 100 doublets (99%) : To extend window to 300nm (full J, H) and suppress 200 doublets (99%) 2009: To deliver OH Suppression fibres