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Abstract
With the growing complexity of astronomical instruments devoted to interferometry, such as MATISSE (a 4 telescope beam combiner) or FIRST (a 9 sub-apertures beam combiner), and the rebirth of space projects such as LIFE (a mid-infrared interferometer), integrated optics devices can be an interesting and complementary approach for beam combination of a large number of apertures. Moreover, one of the approaches for beam combination is pairwise combination of the inputs (either from individual telescopes or from aperture masking on a single telescope), which scales as N(N-1)/2 for an N input system. Astrophotonics devices are attractive to reduce mass and system complexity, while achieving all the beam combination in a single chip, even for a high number of inputs. The aim of this work is to develop a compact photonic device for astronomical applications and demonstrate a proof-of-concept of a spectro-interferometer. In this paper ultrafast laser inscription is used to fabricate three arrayed waveguide gratings (AWGs) stacked vertically. This arrangement enables spectral dispersion and interferometry to be measured simultaneously. Individual AWGs were designed for operation at 633 nm, and demonstrated at 633nm and 830nm. A scan between 790 and 830nm was also achieved to study the wavelength behavior of the AWG. Using a segmented mirror, light at 633nm or 830nm was injected simultaneously into three AWGs layered 40 µm apart, showing analogous behavior for all three layers and no unexpected crosstalk. Finally the three outputs were vertically combined to obtain interference fringes, showing the feasibility of spectro-interferometry and opening the way for compact astrophotonic devices devoted to phase closure studies, used in astronomy to reduce the effect of atmospheric turbulence.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-angular resolution imaging is an important tool in astronomy to study planet forming discs, active galactic nuclei, stellar diameters and exoplanets. To reach this angular resolution, instruments combining spectral dispersion and long baseline interferometry have been used for a long time and newly developed instruments such as MATISSE/VLTI [1] and SPICA/CHARA [2] show the growing interest for astronomy of these techniques combining the light collected from multiple telescopes into a single instrument. Besides, in order to get rid of atmospheric turbulence and also increase the observing time, space interferometers have been studied [3,4] or developed [5,6]. In particular, a revival of the mid infrared space interferometers is today under study with projects such as LIFE [7]. It is therefore interesting to develop integrated optics devices that can reduce size and weight, while combining a large number of inputs. Instruments such as FIRST-LITHIUM [8], GRAVITY [9] or PIONIER [10] have demonstrated the use of integrated optics for on-chip beam combination. However, these instruments still require bulk optics for spectral dispersion. The field of integrated optics has on the other hand proven its versatility in order to develop integrated spectrometers, such as multiple Mach-Zehnder interferometers [11], photonic crystals [12], and surface gratings [13].
Here we propose a first step in the integration of both beam combination and spectral dispersion in a single photonic chip by using a stack of arrayed waveguide grating (AWG) [14–18]. The key difference from the aforementioned instruments is the use of an integrated spectrograph, specifically an AWG, before beam combination to achieve horizontal dispersion. By layering the AWGs vertically we can obtain vertical Young interference fringes between different layers by overlapping the output beams. This allows for simultaneous spectral dispersion and multi-telescope interferometry (in order to extract fringe contrast vs baseline as a function of wavelength). If three AWGs are layered in a non-redundant spacing, phase closure [19–22] can be used to obtain a phase that is mostly independent of atmospheric turbulence, which is an important drawback for ground-based observation. This technique is an important tool for aperture synthesis imaging, and the development of interferometers where the closure phase could be measured with high accuracy is compulsory for exoplanet detection. The goal of the work presented here is to develop an integrated optics device that could help addressing these issues.
Arrayed waveguide gratings are typically made lithographically on a wafer. Since AWGs are stacked vertically the layer spacing would be limited by the wafer thickness. A secondary issue is the requirement to align and secure each wafer/layer. To avoid these restrictions the femtosecond laser direct-write technique was used. This flexible 3D fabrication technique has been used to construct devices including, mode converters [23], photonic lanterns [24], fiber Bragg gratings [25], and AWGs [17]. This fabrication technique is ideal as it enables multiple AWGs to be made at arbitrary depths in a single bulk material. This monolithic configuration has no moving parts making the device robust against temperature and mechanical vibrations. These devices are also compact and lightweight making them optimal for CubeSat projects and drone applications, such as FIRST-LITHIUM [26] (a two-telescope CubeSat interferometer) or ATISE [27] (a micro-optics instrument based on a spectrometer on-chip (SPOC) system [28]).
The manuscript is organized as follows: first, the device design and fabrication are described, next the optical characterization of the layered AWGs in the visible and near infrared is presented, and finally the spectro-interferometer prototype is demonstrated.
2. Device design
The designed AWGs use the classical horse-shoe layout [16,17] as shown in Fig. 1. The input waveguide has also been curved to make the input and output to be parallel, resulting in a chip footprint of 35.5 x 4.3 mm. The input waveguide is single mode at 633 nm and connects to the center of the first FPZ (free propagation zone). The FPZ (7.7 mm x 0.9 mm) confines the light in the vertical direction while allowing it to diffract in the horizontal direction. At the output of the FPZ, 0.7 mm long linear adiabatic tapers guide the light into an array of 19 single mode waveguides. Each waveguide has an incrementally longer optical path length (for this design ΔL = 11.77 μm), with an average length of 12.55 mm. At the end of the waveguide array, linear adiabatic tapers allow the mode size to increase. Light is then recombined in the second FPZ, creating a horizontally dispersed spectrum of the input signal at the output of the chip. The AWG is designed to operate at 632.8 nm, 28th diffraction order, with a FSR of 22.6 nm. If the operational wavelength is changed to 790 nm, the device operates on the 22nd diffraction order with a FSR of 35.9 nm. More details on the principle of these AWGs can be found in [17]. In order to achieve interference between different inputs three identical AWGs were stacked vertically separated by 40 μm at three depths 210, 170 and 130 μm. A vertical spacing of 40 µm (center to center) was chosen to avoid coupling between AWG layers. A full beam propagation model of the device suggests that the vertical spacing could be reduced to 16.10 μm before 1% coupling is observed between layers at 632.8nm.
3. Sample fabrication
Devices were inscribed inside an alkaline earth boro-aluminosilicate sample (Corning Eagle2000) using an ultra-fast Ti:sapphire oscillator (FEMTOSOURCE XL 500, Femtolasers GmbH), 50 fs, 5.1 MHz repetition rate, centered at 800 nm. Laser light was focused into the sample using a 0.65 NA objective inducing a localized refractive index change at the focal spot due to non-linear absorption. The glass sample was placed on a set of Aerotech 3-axis air-bearing translation stages. This enables the position of the focal spot to be moved freely in 3 dimensions with respect to the sample, thus creating waveguides or modified regions.
Arrayed waveguide gratings were inscribed at three depths. Due to spherical aberrations the properties of laser inscribed modifications change as a function of depth. To calibrate for these changes the pulse energy is varied to maintain a constant refractive index of 1.5 x 10−3 at each depth. The refractive index was determined using the inverse Helmholtz technique [29]. Using a constant translation speed of 2000 mm/min, modifications at depths of 130, 170, 210 µm were fabricated with pulse energies of 57, 60.7, 66 nJ respectively. Waveguides at each depth are 4.8 µm wide by 9.0 ± 0.2 µm high, and have a propagation loss of 0.82 dB/cm. Individual waveguides are used to form the input and waveguide array. To reduce bend losses a minimum bend radius of 26 mm was chosen keep bend losses below 0.05 dB/cm. These individual waveguides are multi-scanned to form slab waveguides that act as FPZs and taper regions. These slab regions also have a refractive index contrast of 1.5 x 10−3 with a variation of 1.97% measured using quantitative phase microscopy. To avoid focusing through previous modification layers devices are always fabricated from the deepest to shallowest.
A bright field microscope image of a tri-layered AWG output facet is shown in Fig. 2. The thickness of each modification layer is 9 ± 0.2 µm, which compares well to the designed 9 µm. The spacing between each AWG layer is 35.2 μm (lowest-center) and 38.6 μm (center – highest), which is smaller than the designed spacing of 40 μm. This is due to spherical aberrations affecting the modification depth. All three AWG outputs are 920 µm wide. A previously fabricated AWG with the same design had a measured throughput of 11.5 ± 0.2% at 635 nm across 5 orders [17]. As the wavelength is increases the throughput was found to decrease due to material absorption. All AWG components are fabricated using the laser direct write technique. A more in-depth discussion on the fabrication of the AWG components and laser scanning directions can be found in [15, 17].
Fig. 2 White light image of a tri-layered AWG output facet. Due to spherical aberrations the lower two slabs are closer than the designed 40 μm. The top AWG is 9.1 μm thick while the two lower AWGs are both 8.8 µm thick. All three AWG outputs are 920 µm wide.
4. Results
The optical characterization setup is shown in Fig. 3. It consists on a fiber-coupled source at the focus of an off-axis parabolic mirror. The collimated flux is directed towards a three-segment mirror that simultaneously injects light into the waveguides. An aspheric ZnSe f = 50mm lens was used for injection, and a fused silica x10 microscope objective for re-imaging the outputs on the camera (Lumenera detector). Two of the mirrors (upper and lower) can be independently tip-tilted in order to change the focusing position and therefore inject a different input from that of the central mirror. Besides, these two mirrors can be scanned to vary the relative optical path length, and thus simultaneously obtain interference fringes for all three inputs when a wideband source is used and zero optical path difference must be obtained. In our case, we will be using coherent sources (635nm laser or tunable 800nm laser), therefore we don’t need to scan the mirrors to obtain the interference fringes.
A more detailed description of the set-up can be found in [30].
4.1 Diffracted orders and spectral resolution
Using this setup the three AWGs were tested simultaneously at different wavelengths (a red laser diode at 635 nm and a near IR Sacher tunable laser centered at 790 nm), as shown in Fig. 4. No discernible difference was observed between the near-field outputs of each AWG layer when tested individual or simultaneously, demonstrating that there is no interaction between each layer (i.e. there’s no vertical coupling). The stacked AWGs presented better behavior at longer wavelengths (narrower peaks and better Signal to noise (S/N) ratio), this is consistent with scattering from small fabrication defects that decrease with longer wavelengths.
Fig. 4 TOP: Near-field image on a Lumenera detector of the tri-layered AWG output with each layer simultaneously injected using a 635 nm source. BOTTOM: Near-field image of the same AWG using a 790nm source.
The broadband operation of the devices was tested using a Sacher tunable source between 790 and 830 nm. Cross sections were taken from near field images of the deepest AWG, see Fig. 5. We observe the different diffraction orders at a given wavelength and how this orders are shifted as the wavelength is tuned, allowing to deduce the FSR when the m-order at short wavelength superimposes with the (m-1) order at high wavelength. The FSR of the central order at 790 nm was measured to be 36.09 nm which is comparable to the designed FSR of 35.91 nm. This small deviation can be attributed to a 270 nm incremental path length error in the waveguide array. Besides, the noise between diffraction order peaks is attributed to random path length errors in the waveguide array [17].
Fig. 5 Diffracted orders obtained at different wavelengths from a Sacher tunable source after propagation through the deepest AWG in the tri-layer configuration. Modes are shifted to the right (high pixel number) as the wavelength increases.
4.2 Simultaneous inputs injection: spectro-interferometer
When injecting light simultaneously into two or three input waveguides, the spectrum in the horizontal direction can be obtained for each input channel by directly imaging the AWG output, as shown in Fig. 4. Then, by defocusing the imaging objective each layers’ output can overlap vertically. As the output of the AWG consists on a planar waveguide, the vertical divergence of the output beam will be larger than the horizontal divergence, allowing to obtain vertical Young interference fringes for each diffraction order, while avoiding too strong overlapping between the diffraction orders that could reduce the FSR. As shown in Fig. 6 the diffraction orders are still clearly seen, while vertical Young fringes are obtained. A reduction of roughly a quarter of the FSR is observed, but this effect could be easily reduced by using a cylindrical lens to avoid lateral (horizontal) divergence. A similar concept was studied previously in a mid-IR glass [31], where parallel non-redundant waveguides are sampled using a laser-written grating. The grating extracts the flux from the waveguide and by defocusing, overlapping is obtained on the detector. Here the approach is straightforward (the flux is collected on the propagation direction), resulting in a better signal to noise ratio.
Fig. 6 Different combinations of the three AWG layers when injected with 635 nm light obtained by defocusing the output signal. The output diffraction orders are allowed to spatially overlap and the Young interference pattern is recorded on the detector. Top: Fringes from the upper and lower AWGs. Middle: Fringes from the upper and middle AWGs. Bottom: Fringes from all three AWGs injected simultaneously.
Young fringes were observed at 635 nm, near the designed wavelength, with different baselines. The fringes in Fig. 6 appear in vertical bands corresponding to a given diffraction order (5 orders are visible in Fig. 6). The position of these vertical bands shift horizontally as the wavelength is scanned as shown in Fig. 5. By obtaining the Fourier transform of the bottom image in Fig. 6, we obtain the visibility peaks corresponding to the high frequency (interference between the top and bottom AWG output signals), and low frequency (an experimentally redundant peak, as the vertical distance between the top and middle AWGs is the same as the distance between the bottom and middle AWGs of 40 μm). The results obtained in Fig. 7 show that the interference signal coming from the two non-redundant AWG combinations are clearly identified (one peak at 0.1 pix−1 and another at 0.05 pix−1), without overlapping.
Fig. 7 Fast Fourier transform of the vertical fringes obtained at 635 nm when three vertically stacked AWGs are injected and the outgoing fields are overlapping.
For astronomical applications, this optical concept can therefore be used to extract fringe visibilities from the Fast Fourier Transform (FFT) peak power of each non-redundant pairwise combination of telescopes, without crosstalk, by comparison with the peak at zero frequency that contains the average intensity of the direct image. Contrast could be improved however by correctly balancing the flux from the interacting AWGs, but in our experiment we didn’t have an accurate photometry control stage. The fact that the low frequency peak is roughly twice the amplitude of the high frequency peak is due to redundancy. Indeed, when injecting simultaneously the three stacked AWGs, the upper-middle and middle-lower pairs are identically spaced, contributing to the enhancement of the FFT amplitude at this frequency. For future work the spacing between layers could be made non redundant as the laser direct write technique can fabricate AWG layers at arbitrary depths (within the working distance of the focusing objective).
5. Conclusion and perspectives
In this work a novel concept of spectro-interferometer based of direct laser writing of layered arrayed waveguide grating structures has been presented. Our waveguides show a large spectral transmission range, from visible to near IR (635-820 nm). Besides, the spectrum obtained in the output propagation zone covers 560nm but is however limited by the presence of multiple diffraction orders (with a free spectral range of 36 nm). The flexibility of ultrafast laser inscription enables devices to be tailored for different wavelengths within the transmission window of boro-aluminosilicate (0.4-2 µm). With the growing trend toward mid-IR photonic devices, a new substrate such as chalcogenide with a transmission window from 2 to 8 µm would be required. The compatibility of these materials for ultrafast laser inscription is still a topic of research [32], and first results of mid-IR spectro-interferometers fabricated by direct laser writing in commercial chalcogenides glasses (GLS) have already been shown [31].
Finally, the layered structure allows to easily obtain the interference between spectra, and shows no peak overlapping for the high frequency (80 μm AWG separation) and low frequency (40 μm AWG separation). This opens the way to fringe visibility studies using a compact spectro-interferometer. Future work will be devoted to improve the spectral selectivity of the AWG structure, by increasing the number of waveguides in the waveguide array to have less orders within the diffraction envelope. But also to increase the FSR of the devices, by reducing the diffraction order. This can be done by reducing the path length difference between waveguide arrays. These preliminary results are encouraging for light weight applications, such as drone and CubeSat interferometry projects, where this type of compact spectro-interferometers is expected to be a good tradeoff between performances and weight, robustness and cost.
Funding
Australian Research Council Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems (CE110001018); ARC Discovery Early Career Researcher Award (DEI160100714); MQRES scholarship; Programme Hubert Curien FASIC.
Acknowledgments
We would like to acknowledge the Campus France agency, through its international exchange program PHC FASIC, for funding the research visitor period of G. Martin to Macquarie University. This research was supported by the Australian Research Council Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems (project number CE110001018) and was performed in part at the Optofab node of the Australian National Fabrication Facility utilizing Commonwealth as well as NSW and SA state government funding. G. Douglass acknowledges the support of the MQRES scholarship. S. Gross was supported by an ARC Discovery Early Career Researcher Award (DEI160100714).
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