VITRUV premilinary system studies

Transcription

1 1 VITRUV premilinary system studies J-P Berger, I. Carvalho, A. Delboulbé, P. Garcia, E. Herwats, L. Jocou, P. Kern, J-B Lebouquin, P. Labeye, E. Lecoarer, F. Malbet, K. Perraut, E. Tatulli Contents 1 Scope of the document 2 2 Pre-requisites and goals 2 3 VITRUV function analysis 2 4 VITRUV subsystems description Injection Module Beam combination (Poster by Berger) Beam combination strategy Status of the research on the beam combiners (Poster by M. Benisty) Wavelength coverage Combiner positioner Spectrometry and Polarisation control VITRUVSim: an end to end simulator (Poster by Lebouquin) Introduction Model contents The IONIC/IOTA precursor (Poster by Berger) 12 7 VITRUV/VLTI laboratory simulator (Poster by Jocou) 13

2 2 1 Scope of the document The VITRUV system group has carried out a functional analysis of the instrument and then proceeded with a detailed analysis of key sub-systems and functionalities. This works allows to provide a first description of the instrument. In parallel to this analysis effort an important work has been done to develop simulation tools capable of testing numerically and experimentally the instrument performances. This document is a summary and more detailed memos have been written and are available upon request. 2 Pre-requisites and goals The VITRUV system study was based on participating teams acquired experience, scientific drivers and with three main motivations in mind. 1. Providing the astronomical community with a spectro-imager instrument capable of ful-filling a first rank scientific program i.e spectro imaging at with high dynamic range, high angular and spectral resolution; 2. Providing VLTI team with a compact, stable, easy to maintain and operate instrument; 3. demonstrate that VITRUV could be PRIMA beam combiner. The scientific capabilities of the instrument are (see e.g Science Cases): temporal resolution: 1 day Wavelength coverage: initially from 1 to 2.5 microns with a contemplated extension to R/I and L bands ; Dynamic range: 100 to 1000; Spectral resolution, 100 to 30000; Polarisation capability: if required by science group. This overall context has lead us to define some pre-requisites: VITRUV is a non-direct imaging instrument. It will measure the complex coherence of the electro-magnetic field at several baselines baselines. The complex coherence data allow us to reconstruct an image. VITRUV is a single-mode instrument aimed at precision interferometry on compact or slightly resolved sources; VITRUV should be capable of combining from 4 to 8 beams; VITRUV should operate in the wavelength range between 1 to 2.5 µm and possibly from 0.5 to 5 µm; VITRUV combination, if no show stopper, will be based on Integrated Optics (IO) technologies. 3 VITRUV function analysis Figure 1 shows a breakdown of an interferometer + instrument functions. These functions are the responsibility either of the VLTI side or the VITRUV instrument.

3 Figure 1: Interferometer function description. 3

4 4 Detector Polariation unit Spectrometer Software α,θ,ϕ Beam combiner Injection subsystem VLTI Figure 2: VITRUV subsystem description. Function Light collecting Beam transportation OPD compensation Logitudinal chromatic dispersion Wavefront correction Fringe tracking Chromatic refraction Polarisation control Spatial filtering Photometric calibration Beam combination Spectral dispersion Detection Comment VLTI telescopes VLTI VLTI Relaxed considerably by the use of spectral resolution Adaptive Optics (MACAO-UT) Straps (AT) Mandatory/ External instrument ADC module (VITRUV, not studied here) VITRUV VITRUV VITRUV VITRUV VITRUV VITRUV We consider the fringe tracker as a specific instrument. We have therefore focused our efforts on the imaging part. However we are ready to study the concept should it become our responsibility. 4 VITRUV subsystems description Figure 2 displays a schematic representation of the different VITRUV subsystems. NB The VITRUV extensions to the R and L band respectively will suppose separated spectrometers and detectors but it can be evisioned a single injection module.

5 5 4.1 Injection Module The light coming from the eight VLTI beams is injected into fibers thanks to up to eight injection modules. Each module is made of a fiber positioner and an off-axis parabola. The mechanical structure of these modules has been studied to be compact and stable to avoid misalignment. This work has inherited from the design succesfully used at IOTA for the IONIC-3 instrument where parabola and fiber positioner are mechanically linked. This injection module has three main functions: 1. injecting light into single mode fibers; 2. selecting which combiner is in use; 3. allow to modify precisely the sky pointing for wide field imaging through mosaicing techniques. We can see in figure 3 a schematic description of the link between the injection modules and the beam combiner stage. The fiber positioner will have two stages: a first coarse three axis translation module that will allow to center the choosen fiber at the micron level at the focus of the parabole. The second fine positioner will allow positioning at a scale of 10 nm. We have developed under the responsibility of O. Preis (see Poster by O. Preis) a specific compact fine fiber positionner able to ensure accurate positioning and servoing of the fibers at the focus of the parabola. This module has a special mechanical design that avoids high frequency oscillations. This module is able to hold a v-groove with four fibers in each of the fiber positioners. 4.2 Beam combination (Poster by Berger) Integrated optics (IO) technologies allow to integrate in or on a substrate singlemode waveguide circuits thanks to photomasking techniques borrowed to microelectronics industry. An integrated optics beam combiner is made of fibers that are connected to a coin-sied chip. All the work we have carried out was made in the context of the IONIC collaboration linking LAOG to two IO laboratory partners (IMEP and LETI). It has been since long demonstrated that the association of modal filtering with a proper calibration allows to reach visibility accuracies otherwise impossible (FLUOR, VINCI, IOTA/IONIC). The use of IO beam combiners adds several crucial advantages to the quality of the instrument: the combiner is very compact (a few cm 2 ) and therefore very stable and therefore; it is self aligned and therefore requires no internal alignments. All the complexity of the instrument relies in the initial design; the flexibility of the designing technique allows to imagine and test very different combination techniques Beam combination strategy We have carried out a specific work aimed at finding the best combination function for different VLTI configurations. VLTI with 4 AT (4 beams) and 4AT + 4UT (8 beams). This work has taken into consideration a wide range of beam combination strategies and evaluated for each case the signal to noise performance and the complexity from the integrated optics design point of view (Lebouquin et al. SPIE Glasgow, 2004). The conclusions of this work is a selection of one beam combiner for a 4 telescope mode and one for an 5-8 telescope mode. a pairwise matricial beam combination scheme for 4 telescopes (see left side of figure 5); an all-in one multiaxial beam combination scheme for 5 to 8 telescopes (see right side of figure 5); The 4-T pairwise beam combiner is made of a few cm long chip with four entrances splitted so that beams can interfere with each other. The interference function is a newly developed matricial function that will allow to sample four π/2 phase shifted position in the fringes without the current need for optical path modulation. The second strong interest is that this type of combination does not require demodulation between the different baselines. And last the pairwise scheme allows to avoid photometric calibration outputs. This combiner will have 24 outputs that can be directly imaged precisely on 24 pixels of a detector.

6 6 Figure 3: Up: the 4 to 8 injection modules select the fiber in which the incoming light is injected. Each fiber coming from such a module is linked to an integrated optics combiner. Down: In the current state of study we envision to offer the possibility to use 4 beam combiners. Therefore each injection module fiber positioner will have a 3 axis translation stage able to adjust fiber focal position at the micron level. The four fibers will be inserted inside the fiber positioner described below (figure 4) Figure 4: Left: Mechanical study of the fiber fine positioner. Right: First generation prototype succesfully tested at LAOG.

7 7 Figure 5: Left: Schematic drawing of the 4-T pairwise matricial beam combiner. Right: Schematic drawing of the 5-8T all-in one beam combiner. The 5-T/8T combination function will be done using a multiaxial scheme already described in (Berger et al, (SPIE Munich 2000). All beams will be propagated inside the chip towars a circularly shaped planar waveguides where diffraction will occur. Electromagnetic fields will overlab and interfere. A proper choice of non-redundant set of axis will provide fringe encoding at different spatial frequencies Status of the research on the beam combiners (Poster by M. Benisty). We have carried out extensive tests since 1996 on a number of different beam combination strategies. This allowed us to validate most of the building-blocks functions needed to build VITRUV s beam combiners. The performances achieved are: throughputs of our best 3T and 4T combiners are around 70%. We expect that the multiaxial beam combiner, intrinsically more simple should have at least comparable throughputs; instrumental broadband constrast after combiner optimiation and with polarisation splitting are above 90%. IOTA experiments have confirmed that this contrast is above 75% under real conditions. optical path equaliation inside an IO circuit is better than the micron level. IO combiners performances (from the astronomical interferometry standpoint) are almost insensitive to temperature variations. visibilities accuracies as measured at IOTA are 1%. Closure phase accuracy is 1 We have recently tested a first prototype beam combiner able to combine pairwisely four beams. The IO circuit and first fringes can bee seen on figure 6. This combiner design is at the heart of our 4T beam combiner proposition. Instead of couplers it will use a the matricial encoding function. As for the multiaxial beam combiner. We have not yet started the manufacture of the beam combiner although CNES has funded a first R&D effort that started this year. The theoretical electromagnetic study is on its way.

8 8 Figure 6: Up: An example of prototype 4T pairwise combiner using couplers for beam combination. The four beams come from the right and are splitted to that they can interfered with each other thanks to optical path temporal modulation. Bottom: Resulting interferograms for both polarisations using our three telescopes laboratory interferometer. Figure 7: BPM numerical simulation of the electric field propagation inside a 8-way all in one beam combiner.

9 9 Figure 7. We have already tested 3-way pairwise multiaxial beam combiners wich showed remarquable instrumental constrasts (greater than 90%) and closure phases stabilities Wavelength coverage. A single-mode beam combiner posseses a cutoff wavelength under which the circuit looses its singlemode properties. On the other extreme part of the spectrum a specific single-mode beam combiner becomes less transparent to light as the wavelength increases. Therefore, even with transmissive materials an IO circuit has a limited wavelength coverage (as any modally filtered instrument). This has led us to propose to use two combiners to cover the J/H/K bandpass. One combiner will cover the J/H band and one the K band. Extensions Most of the IO beam combiners that have been made until now were optimied for the H and K bands using well known technologies (silica fibers and glass or silica on silicon IO technology). The extrapolation to the I band is not seen as a major difficulty and would only require a redimensioning of the waveguides. Extension towards more visible wavelengths (R/I bands) requires research and developments efforts that have not been undertaken yet. We have selected Lithium Niobate IO technology as the best candidate to provide the L band extension. The bulk material has a good transmission throughout the band and waveguide implementation is mature to an industrial level. We have already achieved 2 telescope beam combination with an H band combiner and plan to carry out L band experiments in the coming months (Poster by L.Labadie) Combiner positioner Due to the extreme compacity of the IO circuits we envision that the four beam combiners (recall: one J/H 4T combiner, one K 4T combiner, one J/H 8T combiner, one K 8T combiner) can be implemented on the same chip or in a very compact stack measuring less than 1 cm 2. The assembly will be translatable and orientable in order to be aligned with the spectrometer and the detector. 4.3 Spectrometry and Polarisation control As of today both spectrometry and polarisation analysis sub-systems have not yet be fully studied. This should be linked to the conclusions and future work of the science group. However since the IO circuits have intrinsic high-birefringence we have particularly studied (and are still doing it) both experimentally and numerically the consequences of mis-managed polarisation. We have ruled out previous instruments choices (e.g VINCI, SMART...) that have decided to use mechanical constrains to force polarisation state. We have decided instead to carry the electromagnetic signal properly all the way to the detector. Any mechanical constrain is susceptible of affecting the phase measurement which is an essential element if one is to reconstruct images. VITRUV will have the following properties: fiber and IO combiner fast and slow axis oriented with a 1 2 accuracy at lest; limited polarisation crosstalk (at least 20-30dB); polarisations states will be splitted at the output. Two interferometric signals will be detected for each polarisation. Our numerical simulator: VITRUVSim will allow us to test intensively different cases. 5 VITRUVSim: an end to end simulator (Poster by Lebouquin) 5.1 Introduction An end to end numerical simulator has been developed 1 collaboration with UdoPorto with three goals in mind: under the responsibility of Jean Baptiste Lebouquin in provide a realistic physical model of the instrument to assist laboratory experiments interpretation. A particular emphasis has been put into modelling precisely waveguides (fiber+io) behaviour. 1 Written in yorick language- GPL license

10 Figure 8: Simulated fringes in a 2-beam multi-axial combiner in the S (top), P (middle) and total intensity (bottom) with aligned input fiber axis (Left) ; and in the case of a rotation of 30 of one fiber input (Right). The fiber length is 1 meter, the length difference is 2mm, birefringence is set to and the dispersion is taken into account. The input light is perfectly coherent (unresolved star) and partially circularly polaried (50%). 10

11 p Figure 9: Example of set of data simulated by VITRUVsim. Top-Left, the U-V plane is explored during a complete night with 4-ATs on a star closed to the south (dec=50 ) by 10 differents pointing (no array reconfiguration). Top- Right, example of a noisy frame obtained on the last pointing. The accumulation of 100 frames clearly shows the fringe patterns. The star is at a small altitude and the phase are strongly curved by the dispersion in the DL s. Bottom, reduced flux, visibilities and over this 100 frames are displayed. The visibilities show the decrease due to the different spatial resolution of the source at the different wavelength. The operation were performed in the H-K band, with a bright reference star for AO (strehlsim0.5). 11

12 12 allow to test different instrumental setup and assist the system team in its choices of final instrument subsystems; produce synthetic observables of VITRUV of astrophysical targets in order to test data reduction and image reconstruction softwares. 5.2 Model contents The following boxes are currently included in VITRUVsim : atmosphere : transmission, refraction, turbulence (tip/tilt + strehl) telescopes : collecting area VLTI train : transmission, dispersion due to air in the DL s, polariation axis rotation due to transmitted Field-Of-View. VLTI wavefront control : simple model of MACAO/STRAP to compute resulting strehl + tip/tilt. No model of PRIMA/FINITO, the group delay is supposed perfectly tracked at a given wavelength. This can be easily changed thanks to realistic model of FT s. single mode fibers coupling, transmission, dispersion, polariation cross-talk (mismatch of input/outputs angles), birefringence.y integrated optics combiner combination scheme : all-in-one multi-axial scheme have been tested from 2 to any number of telescopes ; pairwise ABCD have to be checked. The polariation is currently not simulated (birefringence not well known). polariation device : the 0 90 polariation states can be separated or D-G separation can be easily implemented. spectrometer : detector : quantum efficiencies, noises. Figure 8 shows an example of detailed simulation of a two telescope configuration where realistic fiber properties (dispersion, birefrigence, difference in length) have been implemented. We can see that the contrast is not constant over the wavelength because of the differential birefringence (left). This effect is obviously not dependent on the input polariation and can is calibrated as the well-known static instrumental contrast. On the other side, more complicated effects occur when the input fiber axis are not perfectly aligned (right) revealing the need for a careful treatment of polariation. Figure 9 shows a simulation of sequence of observations with the 4 VLTI ATs and the extraction of interferometric observables with a draft data reduction software. 6 The IONIC/IOTA precursor (Poster by Berger) We have carried since 2000 a collaborative effort with SAO-Harvard Center for Astrophysics interferometry team aimed at demonstrating on real-sky experiments the power of IO instruments, this is the IOTA/IONIC project. This effort culminated with the first closure phase measurements made at IOTA and first image reconstruction. Figure 10(up) shows the IO 3-way beam combiner at the focus of the interferometer. Figure 10(bottom) shows the first IOTA/IONIC reconstructed image. The experience gained from the numerous observational campaign is unvaluable. We have learned a lot from the stability of the instrument which is extremely high. The accuracy of the instrument compares with best performances in any interferometer. The sensitivity of the instrument has allowed to measure publication quality visibilities and closure phase on H = 7 sources which is a remarkable achievement in the current international context (even when compared with VLTI and Keck giant telescopes). In addition the instrument is being used by numerous observers who do not belong to the collaboration providing us with extremely useful feedback information.

13 13 Conventional Hybrid Mapping - λ Vir using IONIC* at IOTA( Harvard Smithsonian Center for Astrophysics) Stefan Kraus (University of Massachusetts, Amherst) Figure 10: Up: At the heart of the IONIC/IOTA instrument * The IONIC is 3T acombiner 3-way used at IOTA has beam been developed combiner. by a joint collaboration between Note LAOG that and LETIthe IO chip contains 4 circuits which shows that multiwavelength beam combiners could be potentially integrated on the chip. Bottom: Reconstructed image of the binary star Lambda Virginis (Credit: S. Kraus, from Monnier et al. APJ 2004.) as observed with IONIC/IOTA 7 VITRUV/VLTI laboratory simulator (Poster by Jocou) Since 1996 we have developed several laboratory testbeds aimed at testing photometric and interferometric properties of our beam combiners. a 3-way Mach-Zehnder interferometer: characteriation of the interferometric and dispersion properties; a 3-way imaging interferometer: simulation of binary object and exploration of our beam combiners spatial coherence measurement capability; a polarisation bench: measurements of IO and fiber waveguides birefringence axis; a connectoriation bench: allows home-made accurate fiber to IO combiner connexion. In the process of validating the VITRUV concept we are building a full VLTI simulator operating in the near infrared (effort funded by CNES) that will integrate: a star simulator; an eight telescope interferometer including delay lines an up to eight IO beam combiner; a spectro-polarimeter; a laboratory PICNIC detector. Figure 11 shows some elements of the optical bench. Most of them have alredy been manufactured and the bench should be operational by the end of summer This bench will allow to test, among others, our ability to control chromatic dispersion, our ability to reconstruct images from non-point objects with an IO combiner, our ability to reach given dynamic range on image reconstruction and our ability to achieve mosaicing imaging in order to increase field of view.

14 Figure 11: Up: Mechanical study of the Star Simulator and Interferometer optomechanical structure. Middle: Closer view on telescope + associated delay line. Bottom: Telescope footprints and corresponding uv plane (in mm) 14