The feasibility of two types of all-telluride integrated optics devices being able to single-mode guiding of light in the spectral ranges [6-11 µm] and [10-20 µm], respectively, has been demonstrated. The so-called “rib” waveguides show a several micron thick Te82Ge18 film deposited onto a Te75Ge15Ga10 bulk glass substrate by thermal co-evaporation and further etched by reactive ion etching in CHF3/O2/Ar atmosphere. The obtained structures were proved to behave as channel waveguides with a satisfactory confinement of light in the whole spectral ranges. These results allowed validation of our technological solution for the fabrication of micro-components for spatial interferometry.
© 2011 OSA
The search of Earth-like exoplanets, orbiting in the habitable zone of stars other than Sun and showing biological activity, is a very challenging quest. Nulling interferometry from space in the [6-20 µm] spectral range, where the contrast between the exoplanets and their parent star is minimal, seems to be a promising technique for a direct observation of the extra-solar planets. It has been considered for about 10 years by NASA and ESA in the framework of the TPF-I and Darwin missions, respectively . One of the technological challenges remains to develop a “modal filter” for the filtering of the wavefronts in adequacy with the objective of rejecting the central star flux to an efficiency of about 105. Modal filtering is based on the capability of single-mode waveguides to transmit only one complex amplitude function. It should allow eliminating virtually any perturbation of the interfering wavefronts, thus making very high rejection ratios possible . In the present paper, we present one of the two candidate technologies for the fabrication of such modal filters, together with Fiber Optics: Integrated Optics. The potential of Integrated Optics to produce deep nulling, with 105-class performance, was demonstrated by Thales Alenia Space in 2004 using the Multi-Aperture Imaging Interferometer, but in the short-wave infrared region . Specific technological developments were thus started by ESA to transfer the technology to the thermal infrared range [6-20 µm] compatible with the exoplanet observation. Several solutions were considered, including Hollow Metallic Waveguides. Nulling was even demonstrated at λ = 10.6 µm  but the losses created by the skin effect on the gold-coated walls of the waveguides coupled to a poor coupling efficiency were responsible for a low transmission efficiency. In order to obtain an increased transmission and a reduced numerical aperture, dielectric waveguides were thus considered. Different approaches using chalcogenide or silver halide materials had been investigated in literature and tested successfully to realize integrated optics waveguides performing in the mid-infrared [5–9]. The solution based on etched chalcogenide films was eventually considered as the most promising candidate. In this solution, the vertical confinement of the light is achieved in a core layer of higher-index which is deposited onto a lower-index substrate and optionally covered with a lower-index superstrate, whereas the horizontal confinement is obtained by etching the core layer.
The first step in the fabrication of single-mode waveguides by the film deposition/etching solution was to select materials. Indeed, the operation of the waveguides referenced in literature was limited to wavelengths lower than 12 µm due to the loss of the transparency of their constituting materials at longer wavelengths. At present, the selected family of materials is based on a mixture of Tellurium, Germanium and Gallium. The main motivations for its selection were its wide spectral transmission range, which covers the whole [6-20 µm] range , its potential for film manufacturing (by thermal evaporation or co-evaporation) [11–13], its potential for tuning the refractive index by playing on the proportion of the elements , its stability, and its composition exempt of highly toxic or very volatile elements. The selected composition for the single-mode waveguides substrate is Te75Ge15Ga10. Indeed, this remarkable composition was highlighted to be the most thermally stable one , which allowed fabricating large size (50 mm in diameter) homogeneous bulk glasses, in a reproductive manner. Its refractive index is 3.3960 ± 0.0015 at λ = 10.6 µm .
The second step was to define the guiding structures to be fabricated. Indeed, several types of guiding structures can be considered to achieve the goal of transmitting and filtering infrared stellar light. For simplicity reasons, it has been decided to first focus on rib waveguides. More sophisticated designs involving a covering by a superstrate will be envisaged in the future.
The next steps were to choose a film deposition and etching methods. Thermal co-evaporation and reactive ion etching under a gas mixture of CHF3, O2 and Ar were selected. Thermal co-evaporation allowed obtaining amorphous films with very different compositions, both in the Te-Ge-Ga and Te-Ge systems [16,17]. For reasons of composition reproducibility and easiness in manipulation, Te-Ge system was eventually preferred to Te-Ge-Ga one. Moreover, the refractive index of Te-Ge films was shown to increase linearly with the Te content, ensuring the possibility of finding a composition which refractive index will be consistent with that of the Te75Ge15Ga10 substrate . Concerning reactive ion etching under a gas mixture of CHF3, O2 and Ar, it was recently shown to allow the deep etching of Te-Ge films .
Once the materials and technologies were selected, the fabrication of single-mode rib waveguides could be envisaged. The present paper describes its different steps and presents the first results in term of light guiding in the [6-20 µm] range and modal filtering at λ = 10.6 µm obtained in all-telluride rib waveguides.
2. All-telluride rib waveguides fabrication
In order to perform efficiently as modal filters in a nulling interferometer for extra-solar planet direct observation, the rib waveguides to be fabricated are required to provide single-mode propagation solution. As it is impossible on the whole [6-20 µm] spectral band, two sub-bands were considered: [6-11 µm] and [10-20 µm]. Two types of rib single-mode waveguides were thus designed, for both [6-11 µm] and [10-20 µm] spectral bands. The refractive index difference between the high-index core layer and the low-index substrate is a critical parameter, which is linked to the aperture of the guide on the one hand, as well as that of the coupling and collecting optics on the other hand. By contrast with Fiber Optics, Integrated Optics technology implies relatively strong index differences, in order to keep the dimensions of the guiding structures small enough to be technologically feasible and because stronger guiding enables modal filtering efficiency to be achieved along shorter propagation lengths. A compromise needs to be found with the aperture of the coupling optics, which must remain small enough for the optics to remain feasible, small and stable enough for a spaceborne application. We thus settled the numerical aperture to 0.5. As results the core-index gap was fixed at Δn = ncore – nsubs = 4.10−2, leading to two rib waveguides designs (Fig. 1 ). Their respective dimensions and tolerances have been obtained using a commercial vectorial mode solver (OPTIBPM suite from OPTIWAVE). It has been checked that although the refractive index difference was quite strong, the fundamental modes remained mainly linearly polarized and that a semi-vectorial approximation could be used, i.e. guided modes are either quasi TE or quasi TM modes. Hence, these mode fields have been used to compute the coupling efficiency of these waveguides with a diffraction limited spot coming from an optic with a Numerical Aperture (NA) of 0.5. The losses due the differences between the mode shapes of the coupling optic Airy spot, on one hand, and the guided mode, on the other one, have been estimated by computing the power overlap integral of these two fields.
As it can be seen in Fig. 1, the constraints on the RIB structure dimensions and angles are very strong. An important technological effort was thus necessary to achieve such a realization. Preliminary works highlighted that thermal co-evaporation was a very promising deposition method to obtain thick films of optical quality [15,16]. In particular it was shown that this technique allowed varying the film composition very easily and that the refractive index could be adjusted by changing the tellurium content in the Te-Ge film . Taking into account the linear variation of the refractive index versus the tellurium content, we could thus estimate the film composition to be deposited in order to achieve a refractive index nc = 3.436 at λ = 10.6 µm, with nsub = 3.396. This composition was determined to be Te82Ge18.
Nevertheless, it was important to prove the reproducibility of the deposition procedure and the consequent uncertainty on the refractive index. Twelve layers were thus deposited in the same conditions on commercial As2Se3 substrates whose refractive index is known at λ = 10.6 µm. Their composition was checked by microprobe analysis and their refractive index by m-lines measurements at λ = 10.6 µm. The core layer composition was finally estimated to be Te82.0 ± 1.3Ge18.0 ± 1.3 and its refractive index was obtained to be nc = 3.44 ± 0.02. The effect of such an uncertainty in the core layer refractive index on the single mode behavior and in the coupling efficiency of the RIB structures was analyzed. It can be seen in Fig. 2a that the coupling efficiency of the RIB structures for the [10-20 µm] is not affected by a change in the core layer refractive index in the range [3.44 – 0.02 ; 3.44 + 0.02]. On the contrary, the model showed that for nc = 3.44 - 0.02, the coupling efficiency decreased very rapidly with the wavelength in the spectral range [6 – 11 µm] (Fig. 2b). In order to take into consideration the uncertainty of the core layer refractive index into the RIB waveguide fabrication, a specific photolithographic mask including bands with various widths for both the [6 – 11 µm] and [10 - 20 µm] spectral ranges was fabricated. Indeed, as illustrated in Fig. 2c, the coupling efficiency can be corrected by changing the RIB width.
After having determined the reproducibility in the film composition and refractive index, a thick film of nominal composition Te82Ge18 was deposited on a 5 x 5 cm2 microscope slide in order to check its homogeneity in term of thickness and composition, by profilometry and microprobe. The Fig. 3a and Fig. 3b present 3D profiles of thickness and composition, respectively. They highlight that the film homogeneity is very satisfying on a region of about 4 cm in diameter.
As the analysis by microprobe measurements only allows determining the film surface composition, complementary experiments were performed by secondary ion mass spectrometry. In Fig. 4 , one can see that the tellurium and germanium contents are quasi constant in the whole film depth.
Core layers with nominal composition Te82Ge18 were than deposited on Te75Ge15Ga10 substrates, synthetized at the University of Rennes 1 (France) and polished at the University of Villetaneuse (France). The films of 12 µm in thickness for the realization of RIB waveguides for the [6 – 11 µm] spectral band were deposited in one deposition procedure. On the contrary, at least two stacked layers were necessary to obtain the films of 24 µm in thickness for the [10 – 20 µm] spectral band. Nevertheless, a SEM observation of the film stacking cross-section allowed showing that the layers interface was not visible.
Once the core layers were deposited, they were etched by reactive ion etching, under a mixture of CHF3, O2 and Ar. The etching conditions were chosen according to preliminary results . The etching durations were adapted in order to achieve the required etching depths (according to Fig. 1): 4.5 µm and 9 µm for the [6 – 11 µm] and the [10 – 20 µm], respectively. Note that the specific masks as described before were used for the photolithographic procedure. After etching, several samples were then stacked by sticking one to each other using Arcanson wax. It allowed polishing the input and output facets of several samples in the same time, ensuring a good planarity of the facets and a protection of the etched film surface. After wax removal, a SEM analysis allowed verifying the quality of the facets and validating the RIB structure dimensions. The as-obtained pictures are given in Fig. 5 .
One can see on Fig. 5a and Fig. 5b that the RIB dimensions are in good agreement with the design requirements, taking into account the tolerances. It is also the case concerning the RIB angles which are comprised between 82 and 85 °.
3. Light guiding characterization
Thanks to the polishing of the input and output facets, a good coupling of the light inside the waveguiding structures could be achieved, that allowed optical characterization. After having demonstrated the light guiding in an infrared slab waveguide (i.e. guiding layer without etched structure), we recorded the transmission spectrum of rib waveguides in the [6-20μm] spectral range. As an illustration, the transmission spectrum of a rib waveguide of the [10-20 µm] type is presented in Fig. 6a . For these experiments, a dedicated bench built around a Fourier Transform Infrared spectrometer was used.
Moreover, a dedicated laser interferometry bench, powered by a CO2 laser whose 10.6μm wavelength is compatible with both our spectral bands, was used for the characterization of the rib waveguides. In a first configuration, the chopped, spatially filtered and collimated laser beam is coupled to the component guiding channel by means of an off-axis parabola (OAP). The output beam is collected by a second OAP and sent to a liquid-nitrogen cooled Cadmium-Mercury-Telluride photo-detector followed by a lock-in amplifier synchronized to the laser chopping signal. The strong power of the laser added to the low-noise detection unit thus makes large dynamic range measurements possible. Calibration of the bench transmission is simply achieved by removing the component and superimposing the foci of the OAPS. The setup first allowed proving the bi-dimensional confinement of the light into the RIB structures. Indeed, when light was injected (at λ = 10.6 µm) into a rib structure, a bright spot appeared at the exit of the component (Fig. 6b, as shown using an infrared camera) which disappeared when light injection was de-centered. Then we performed a preliminary measurement of the transmission efficiency of a rib waveguide prototype of the type [6 – 11 µm]. A 1.7% transmission was obtained, to be compared with a prediction of max. 4.5% (mainly driven by 30% Fresnel losses at each end, theoretical coupling efficiency and bench/prototype component mismatch). This result shows that the sample losses due to surfaces finish and material losses do not exceed 4dB, only 1dB short of the 3dB objective, a very encouraging result for a preliminary measurement. The maximum 4.5% transmission will be improved in the future, by playing on the surface finish of the sample, the homogeneity between the bench and component geometries, and by implementing anti-reflective coating on the sample. The configuration of the optical bench itself will also be improved in order to discriminate coupling and propagation losses, by means of a more complex method of the “cutback” family.
In a second configuration of the bench, a Mach-Zehnder interferometer is inserted in the collimated laser beam, upstream the coupling OAP. A delay line may be adjusted so that a constructive or destructive interference is produced in the beam transmitted to the component. The interferometric capability of the bench operating at λ = 10.6 µm was adjusted and tested on a very first sample of the [6 – 11 µm] sub-band. A light rejection efficiency of 6.10−5, quite close to the exo-planet observation mission needs, was obtained from the ratio of the signals measured respectively in the destructive and constructive conditions.
Prototype all-telluride rib waveguides for the [6-11 µm] and [10-20 µm] sub-bands were designed, fabricated and tested by means of dedicated benches. Preliminary results were quite encouraging regarding the spectral transmission ranges and the transmission efficiencies. Moreover the first experimental assessments ever of the modal filtering capability of the all-telluride rib waveguides were performed recently using a nulling interferometer operating at 10.6 µm. A light rejection efficiency of 6.10−5 was obtained on a very first sample, confirming the potential of telluride-based Integrated Optics for the fabrication of modal filters for infra-red nulling interferometry. In a next step, we plan to improve the control of the component and test bench parameters in order to improve the performance and the characterization accuracy. We also plan to duplicate the experimental work for the 10-20µm spectral band.
The authors thank Thierry Billeton for his help to overcome the challenges of the sample polishing. They also thank the European Space Agency for its scientific and financial support (contract Integrated Optics 20742/07/NL/IA).
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