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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 4, Iss. 4 — Apr. 1, 2009
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Mid-infrared guided optics: a perspective for astronomical instruments

Lucas Labadie and Oswald Wallner  »View Author Affiliations


Optics Express, Vol. 17, Issue 3, pp. 1947-1962 (2009)
http://dx.doi.org/10.1364/OE.17.001947


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Abstract

Research activities during the last decade have shown the strong potential of photonic devices to greatly simplify ground based and space borne astronomical instruments and to improve their performance. We focus specifically on the mid-infrared wavelength regime (about 5–20μm), a spectral range offering access to warm objects (about 300 K) and to spectral features that can be interpreted as signatures for biological activity (e.g. water, ozone, carbon dioxide). We review the relevant research activities aiming at the development of single-mode guided optics and the corresponding manufacturing technologies. We evaluate the experimentally achieved performance and compare it with the performance requirements for applications in various fields of astronomy. Our goal is to show a perspective for future astronomical instruments based on mid-infrared photonic devices.

© 2009 Optical Society of America

1. Introduction

1.1. Scientific drivers for mid-infrared astronomy

Any celestial object with a non-zero temperature emits infrared radiation (i.e. heat). Because the wavelength at which the object most intensively radiates depends on the temperature, observing in the spectral range that goes from 1μm to 1000μm gives access to objects at a wide range of temperatures from a few tens of degrees Kelvin to several thousands. Although spectral boundaries are not firmly set, the region from 1μm to 5μm is defined as the near-infrared, from 5μm to ~ 30μm we refer to the mid-infrared domain (or thermal infrared), while the far-infrared extends up to 1000μm. The mid-infrared becomes a highly interesting observing regime, where warm objects with temperatures of ~100–600 K can be probed. This corresponds to a tempara-ture range where physical conditions for liquid water are encountered. It is also the range of peak emission of telluric planets around solar-type stars and therefore an appropriate domain for detecting the self-emission of nearby exoplanets. Furthermore, bio-markers such as water, CO2 or O3 (ozone) can be revealed through low-resolution spectroscopic observations. Mid-infrared sources have become unquestionably a powerful source of information for relatively new fields like astrobiology and astrochemistry.

1.2. Photonics, a recent technology for astronomy

From these different points, photonics appears very attractive in the context of instrument miniaturization. However, the strong telecom background of photonics has concentrated the technological effort in few narrow bands of the near-infrared spectrum, where fully mature technology is available. When considering longer wavelengths of the astronomical mid-infrared spectrum, photonics suffers from a significant technological gap, which results from a weak mastering of the materials production and manufacturing processes at these wavelengths. Promising efforts, mainly driven by the potential applications of photonics in fields like astronomy, chemistry and biophysics, are being made to reduce the gap. The increasing use of photonic devices for astronomical instrumentation has led to the neologism astrophotonics.

2. Materials and concepts for mid-infrared photonics

2.1. Accessing the 10-μm spectral range: materials selection

The selection criteria for materials are based on the transparency window and on the corresponding intrinsic and extrinsic losses within this window. The window borders are determined by absorption, both on the visible and mid-infrared part of the spectrum. The intrinsic losses depend only on the material structure and properties and represent the minimum expected losses. However, the presence of impurities and defects further degrades the transmission and these losses are generally more important. We then talk about extrinsic losses. In the particular case of silica in the telecommunications industry, the refinement of the manufacturing technology over several years has led to high level of purity and quality, which makes the material losses close to the intrinsic levels. The physical explanation for intrinsic losses are found in material electronic transitions, phonon interactions and free-carrier effects. The interested reader can refer to well-established reference books on optical and infrared materials [14

14. P. Klocek, Handbook of infrared optical materials (Marcel Dekker inc., 1991).

]. The infrared materials that have been tested so far in the context of dielectric photonics devices are chalcogenide glasses, silver-halide glasses and zinc selenide components. Most of these are also transparent in the visible. This has to be considered in case the resulting components have to operate in an environment non-shielded from optical radiation. On the other hand, this offers an advantage for alignment. All these materials are transparent at least up to 11–12μm. Depending on the chemical composition and the stoichiometry, the transmission range can be extended up to 18–20μm for the chalcogenide and zinc selenide glasses, or even beyond for silver halide glasses. In addition to their intrinsic properties, other factors may influence the optical behavior of these materials. These are:

– mechanical stress: most infrared materials show a certain fragility and high sensitivity to shocks, which may complicate the manufacturing and polishing processes.

– chemical stability: some materials can absorb water vapor, which further affects their transparency. In some cases, vacuum operation is required.

– thermal stress: while fibers are sensitive to temperature, integrated optics are generally more stable with regard to thermal constraints [15

15. J.-P. Berger, P. Benech, I. Schanen-Duport, G. Maury, F. Malbet, and F. Reynaud, “Combining up to eight telescope beams in a single chip,” Proc. SPIE 4006, 986–995 (2000) [CrossRef]

].

– spatial homogeneity: when a large surface is required to manufacture complex devices, it is important to ensure a good degree of homogeneity of the optical properties over the whole sample. Underestimating this aspect might bring in differential effects (dispersion etc…) that must then be carefully controlled.

Infrared glasses are typically classified in amorphous and crystalline forms, depending on the regular (crystalline) or irregular (amorphous) arrangement of the atomic structure.

Amorphous materials include oxide glasses (e.g. SiO2) which present strong absorption in the mid-infrared due to OH vibration modes. Other members of this group are chalcogenide glasses (based on S, Se and Te). The transmission of these materials can be extended beyond 12μm by the addition of heavy compounds (Ag, Te).

Crystalline materials show a regular pattern in their structure (e.g. Quartz, the crystalline form of SiO2). Silver halides – bromide or chloride – are also crystalline materials well known in mid-infrared applications. Semiconductors, also classified in this category, are attractive materials due to their extended transmission range in the infrared.

So far, chalcogenide and silver halide glasses are the two main materials that have been investigated to cover the mid-infrared range. Typical chalcogenide glasses are As2Se33 and As2S3, which present good transparency up to ~12–16μm. Compositions including Germanium (Ge) or Tellurium (Te) can extend the transmission range up to 20μm. Silver halide glasses have excellent transparency up to and beyond 20μm. In terms of materials properties, silver halide glasses may encounter corrosive risks if in contact with metals. Furthermore, they present photosensitive properties, which require particular care if this effect is undesirable. On the other hand, the material photosensitivity can also been exploited for controlled laser writing of waveguides (see Sect. 3.1.2).

2.2. Waveguide concepts

2.2.1. Optical fibers and integrated optics

2.2.2. Hollow waveguides and photonic-crystal fibers

Fig. 1. Left: principles of hollow metallic waveguides and hollow glass waveguides. In the HGW design, the tubing can be glass or a plastic polymer. Right: refractive index profiles of index-guiding and photonic bandgap (PBG) guiding PCF.

– Photonic-bandgap (PBG) guiding PCFs are obtained by locally breaking the periodicity of a photonic crystal by introducing a well-defined defect, e.g. in the form of an extra tube. The light then is confined and thus guided, provided that the surrounding photonic crystal cladding exhibits a photonic bandgap at the operation wavelength [26

26. S. E. Barkou, J. Broeng, and A. Bjarklev, “Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect,” Opt. Lett. 24, 46–48 (1999). [CrossRef]

]. A remarkable difference between PBG-guiding PCFs and conventional fibers or index-guiding PCFs is that they allow waveguid-ance with propagation constants (i.e. photonic bandgaps) below the effective cladding index. The photonic bandgap regions, i.e. the transmission windows, are determined by the cladding structure only. They are usually very narrow. The behavior of the guided modes within the photonic bandgaps is solely determined by the characteristic of the defect. By varying the size and shape of the defect, the frequency of the guided mode can be positioned precisely within a photonic bandgap. At near-infrared wavelengths, substantial work has led to the implementation of PCF in astronomical applications [27

27. S. Vergnole, L. Delage, F. Reynaud, L. Labonté, P. Roy, G. Mélin, and L. Gasca, “Test of photonic crystal fiber in broadband interferometry,” Appl. Opt. 44, 2496–2500 (2005) [CrossRef] [PubMed]

, 28

28. S. Vergnole, L. Delage, and F. Reynaud, “Three-beam photonic crystal fiber imaging interferometer,” Appl. Opt. 45, 6712–6717 (2006) [CrossRef] [PubMed]

].

3. Overview of development activities

3.1. Manufacturing techniques

3.1.1. Optical fibers

Chalcogenide glass fibers: The manufacturing techniques for optical fibers are determined by the the different physical properties of fiber materials. In the mid-infrared we distinguish between glassy fibers and crystalline fibers. For both types, a preform is created by mechanical combination of the core and cladding material. Glassy fibers are drawn from the preform, whereas crystalline fibers are extruded from the preform.

Silver-Halide Fibers Crystalline materials have the advantage of a better long wavelength transmission compared to mid-infrared glasses, but suffer from a difficult fabrication process. The first crystalline fibers were made of hot extruded KRS-5 (Tellurium-Bromide-Iodide). Today, poly-crystalline silver-halide (Silver-Chloride-Bromide) is most widely used. Silver-halide fibers show good transmission up to almost 20 μm. Poly-crystalline fibers are made of crystallike solid solutions of Thallium halides, e.g. KRS-5, or silver halides, e.g. AgClBr. These materials offer such properties as ductility, low melting point, and isotropy. Crystalline fibers can be fabricated via plastic deformation by extrusion from a preform. The rod-in-tube preform is realized by a rod of AgClBr as core and a Cl-rich AgClBr crystal as cladding. The preform can also be made by the casting method or by preform growth methods [33

33. V. ArtiouchenkoART Photonics, Optical fibre and fabrication technique for an optical fibre, European Patent EP/01.09.00/EP 00250290 (2000).

]. The preform is placed in a heated chamber and the fiber is extruded to its final form through a polished die. Typical extrusion temperatures are in the range of 50% to 80% of the melting temperature, which is 457°C for AgCl, for example. The pressure within the container ranges form a few to some ten tons per square centimeter. Similar to the chalcogenide glass fibers, poly-crystalline silver-halide fibers have been and are currently being developed for ESA and NASA. Two independent research activities demonstrated the feasibility of single-mode fibers: – A team at Astrium GmbH (Germany) and ART Photonics (Germany) developed for ESA several samples of silver-halide fibers with different core/cladding geometries and different material compositions [34

34. R. Flatscher, O. Wallner, V. Artjuschenko, and J. Pereira do Carmo, “Manufacturing of chalcogenide and silver-halide single-mode fibres for modal wavefront filtering for Darwin,” Proc. 6th Internat. Conf. on Space Optics (2006).

]. To achieve optimum results, extremely homogeneous crystals have been used and different preform manufacturing techniques have been applied, including mechanical combination, preform growth, capillary drop and capillary suction. Single-mode operation has been successfully demonstrated at a wavelength of 10.6μm for a fiber with a core/clad composition of AgCl75Br25/AgCl60Br40, a core diameter of 20μm, a cladding diameter of 500μm, and a single-mode cutoff wavelength of 5.8μm. In a second activity, the team is currently developing single-mode fibers by an improved and reproducible manufacturing process to obtain fibers with improved performance. The test setup is extended and allows interferometric measurements at wavelengths of 5.3μm, 10.6μm and 16.5μm to verify the performance over almost the entire wavelength range. – A team at Tel Aviv University (Israel) developed for NASA silver-halide fibers for TPF-I. An improved crystal growth technique allowed inhomogeneities of less than 2% for preforms of any composition. An improved fiber has been developed with a double-step index profile. The core composition was AgCl30Br70, the composition of the first cladding AgCl32Br68 and that of the second cladding AgCl5Br95. The core diameter was 50μm, the diameter of the first cladding 250μm and that of the second cladding 900μm (see Fig. 3 and [35

35. S. Shalem, A. Tsun, E. Rave, A. Millo, L. Nagli, and A. Katzir, “Silver halide single-mode fibers for the middle infrared,” Appl. Phys. Lett. 87, 091103–1–091103–3 (2005). [CrossRef]

]). The outer cladding was blackened by exposure to UV radiation.

Fig. 2. Schematic view of the three main technologies for manufacturing integrated optics

3.1.2. Integrated Optics

While Integrated Optics is a well mastered technology below 2μm, the situation is noticeably different at longer wavelengths. The manufacturing techniques need to be adapted to infrared materials with different structural, mechanical and thermal properties, like high fragility. Possible technologies for manufacturing integrated optics are based on ion-exchange, waveguide chemical etching and photo-darkening.

Ion-exchange diffusion In the ion-exchange technique, a glass substrate is first layered with a few hundred nanometers of polysilicon coating with diffusion apertures obtained by ion etching (see Fig 2(a)). The structure is then placed in a molten salt bath with precisely determined ion concentrations and treatment duration. The difference in concentration results in local ion exchange from the bath to the substrate, producing the high index core embedded in the substrate. In the mid-infrared range, this technique has been successfully implemented on Germanate glasses for the 3–4μm spectral range, although the experimental characterization has so far been limited to a wavelength of 1.55μm [36

36. T. Luo, S. Jiang, G. Nunzi Conti, S. Honkanen, S. B. Mendes, and N. Peyghambarian, “Ag+/Na+ exchanged channel waveguides in germanate glass,” Electron. Lett. 34, 2239–2240 (1998). [CrossRef]

, 37

37. J. Grelin, A. Bouchard, E. Ghibaudo, and J. E. Broquin, “Study of Ag+/Na+ ion-exchange diffusion on germanate glasses: Realization of single-mode waveguides at the wavelength of 1.55μm,” Mater. Sci. Eng. B 149, 190–194 (2008). [CrossRef]

].

Chemical etching under controlled atmosphere The chemical etching technique is among the most promising solutions to work with amorphous glasses like chalcogenide (see Fig 2(b)). Dry etching has already produced rib waveguides based As2S3, As2Se3 and Germanium components which were characterized at 1.5μm [38

38. Y. Ruan, W. Li, R. Jarvis, N. Madsen, A. Rode, and B. Luther-Davies, “Fabrication and characterization of low loss rib chalcogenide waveguides made by dry etching,” Opt. Express 12, 5140–5145 (2004). [CrossRef] [PubMed]

, 39

39. S. J. Madden, D-Y. Choi, D. A. Bulla, A. V. Rode, B. Luther-Davies, V. G. Taeed, M. D. Pelusi, and B. J. Eggleton, “Long, low loss etched As2S3 chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 22, 14414–14421 (2007). [CrossRef]

]. Later, analog rib waveguides were obtained using Tellurium compounds and characterized at 10.6μm (see Fig. 4 and [40

40. C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J.-E. Broquin, L. Labadie, and P. Kern, “Transmission measurement at 10.6 μm of Te2As3Se5 rib waveguides on As2S3 substrate,” Appl. Phys. Lett. 90, 011110–011112 (2007). [CrossRef]

]). The first step in the manufacturing process is the deposition of a guiding layer by means of thermal evaporation or sputtering, followed by optical characterization [18

18. L. Labadie, C. Vigreux-Bercovici, A. Pradel, P. Kern, B. Arezki, and J.-E. Broquin, “M-lines characterization of selenide and telluride waveguides for mid-infrared interferometry,“ Opt. Express 14, 8459–8469 (2006). [CrossRef] [PubMed]

]. These two processes influence the film parameters such as density, refractive index and surface roughness. A mask with the waveguide pattern is realized before undergoing chemical etching in Argon or CF4/O2 atmosphere. The process produces sharp, well defined waveguide contours, with step resolution better than 0.1μm. Some authors have demonstrated the feasibility of bent waveguides, which is an important prerequisite for more complex integrated optics functions [39

39. S. J. Madden, D-Y. Choi, D. A. Bulla, A. V. Rode, B. Luther-Davies, V. G. Taeed, M. D. Pelusi, and B. J. Eggleton, “Long, low loss etched As2S3 chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 22, 14414–14421 (2007). [CrossRef]

]. Chemical etching has been also attempted on crystalline glass compounds for slab waveguides made of a Zinc Selenide (ZnSe) film deposited on a Zinc Sulfide (ZnS) substrate [41

41. L. Labadie, Nulling interferometry, integrated optics, infrared instrumentation, extrasolar planets; PhD dissertation (Université Joseph Fourier, 2005). [PubMed]

]. However, the chemical and mechanical stability of the structure could not be preserved during the etching phase, and the layer was irremediably damaged.

Fig. 3. Left: cross section of a single-mode silver halide fiber characterized by output profile imaging [35]. Right: photograph of a conductive waveguide cross-section with dimensions 10×5 μm. This structure maintains a single polarization direction perpendicular to the longer side [45].

Photo-darkening effect and laser writing A third way of producing mid-infrared integrated optics is to use the photo-darkening effect for amorphous glasses (see Fig 2(c)). Photo-darkening results from exposure of the substrate to high-power external radiation, in order to modify locally the refraction index of the glass. Although the correlation between photo-darkening effect and structural changes in the substrate is not well understood, it has been observed that photo-darkening occurs only in disordered amorphous materials, and not in crystalline materials. The process of waveguide manufacturing using the photo-darkening effect is named laser writing. This consists in focusing a visible source such as He-Ne laser in a particular location of the substrate surface leading to a change in the refractive index, which amplitude is a function of the exposure time to the radiation. This technique has been successfully applied to the As–S–Se family of chalcogenide glasses. Based on this technique, some authors could obtain a square cross-section single-mode waveguide with 5.4μm width and Δn~0.04 using laser writing at 632nm [42

42. N. Hô, M. C. Phillips, H. Qiao, P. J. Allen, K. Krishnaswami, B. J. Riley, T. L. Myers, and N. C. Anheier, “Single-mode low-loss chalcogenide glass waveguides for the mid-infrared,” Opt. Lett. 31, 1860–1862 (2006). [CrossRef] [PubMed]

]. This work has been done in the continuation of Efimov et al. (2001) effort to obtain analog results with 850 nm laser writing. We wish also to underline that, depending on the exposition conditions, the photo-darkening effect can be reversible or permanent.

Impact factors towards single-mode behavior Beyond the manufacturing aspects of channel waveguides, the possibility to achieve single-mode behavior depends on the capabilities of manufacturing small cores (i.e. <10μm). This implies a higher refinement and mastering of the various technological parameters. Alternatives to planar dielectric integrated optics exist to produce functions: a possibility is to emulate the near-infrared fiber couplers as this was done by Eyal et al. (1994) who mechanically joined uncladded 900-μm core silver-halide fibers to produce a Y-coupler for the mid-infrared [43

43. O. Eyal, S. Shalem, and A. Katzir, “Silver halide midinfrared optical fiber Y coupler,” Opt. Lett. 19, 1843–1845 (1994). [CrossRef] [PubMed]

]. Another option is to downgrade microwave E-plane and H-plane coupler to the 10-μm range as suggested by Wehmeier et al. (2004) [44

44. U. J. Wehmeier, M. R. Swain, C. Y. Drouet D’Aubigny, D. R. Golish, and C. K. Walker, “The potential of conductive waveguides for nulling interferometry,” Proc. SPIE 5491, 1435–1445 (2004). [CrossRef]

]. However, obtaining a single-mode Y-coupler would require the handling of much smaller cores, while the solution of conductive waveguides needs additional developments to reduce the linear losses. Furthermore, the level of integration appears in both cases lower compared to planar integrated optics.

Fig. 4. Left: output profile of a 2.2-m long single-mode silver halide fiber obtained at 10.6μm [35]. Right: sketch of a chalcogenide rib waveguide showing the position of the injection spot, together with output images at 10.6 μm when the spot is centered and de-centered from the rib [40].

3.1.3. Hollow waveguides

While a certain amount of theoretical work has been produced to investigate the properties of single-mode hollow waveguides for the mid-infrared range, little has been done from the experimental approach. Single-mode hollow waveguides have been limited by the actual micrometer capabilities of micro-machining techniques. Recently, different groups have addressed this manufacturability aspect. The group at Steward Observatory (Arizona) involved in radio and sub-mm instrumentation has tested the manufacturing of conductive waveguides by laser micromachining. This consists in etching a silicon wafer with high power (30 W) Argon laser in order to obtain a preform. These wafers are then gold coated, bonded and cut to obtain the waveguides. The accuracy and quality of the laser etching process is of crucial importance to realize the samples and minimize their losses. At the time of their published work [44

44. U. J. Wehmeier, M. R. Swain, C. Y. Drouet D’Aubigny, D. R. Golish, and C. K. Walker, “The potential of conductive waveguides for nulling interferometry,” Proc. SPIE 5491, 1435–1445 (2004). [CrossRef]

], the group could not achieve better than a 6μm resolution, which is compatible with operation in the 2-THz (150μm) regime. A second group based in Grenoble (France) has investigated a similar idea, but based on well mastered chemical etching process of the silicon wafer. The advantage is to obtain smaller structures with sizes down to 4μm and 50–100 nm resolution, together with smoother waveguide walls. The deposited gold layer shows then a more uniform surface, which reduces scattering. The studied waveguide concept was inspired from well-known microwave rectangular waveguides, but scaled down to typical sizes of ~10μm. Hollow metallic waveguides with rectangular geometry and with cross-section dimensions of 10×5μm were produced with this technique and characterized at 10.6μm (see Fig. 3 and [45

45. L. Labadie, P. Labeye, P. Kern, I. Schanen, B. Arezki, and J.-E. Broquin, “Modal filtering for nulling interferom-etry. First single-mode conductive waveguides in the mid-infrared,” Astron. Astrophys. 450, 1265–1275 (2006). [CrossRef]

]). Later, Y-junctions based on hollow metallic waveguides were also manufactured, but with no sufficient transmission in the mid-infrared spectral range.

3.1.4. Photonic crystal fibers

The manufacturing techniques for photonic crystal fibers are similar to that for optical fibers and therefore are determined by the fiber materials. PCFs made of glassy material are drawn from the preform, PCFs made of crystalline material are extruded from the preform. The major difference with optical fibers is given by the preform fabrication technique. It shall be noted that PCFs for the mid-infrared are at a very experimental stage today. For glassy PCFs the preform is usually obtained by drilling holes in the fiber material. For crystalline PCFs the preform cannot be obtained by drilling holes in the preform as these would be destroyed during the extrusion process. A reduced effective cladding index is obtained by adding a material of lower index to the cladding region [46

46. R. Flatscher, O. Wallner, and V. Artiouchenko, Single-mode fibres for DARWIN, Summary Report, ESA/ESTEC Contract No. AO/1-4023/01/NL/CK (2007).

]. A one-dimensional photonic crystal is achieved by alternately coiling thin layers of silver-chloride and silver-bromide on a central silver-bromide rod. Two dimensional photonic crystals are achieved by filling the holes in a silver-bromide rod with small rods of silver-chloride, or by stacking silver-chloride and silver-bromide core-only fibers around a central rod of silver-bromide.

Experimental PCFs made of silver-halide have been manufactured by several groups. The first index-guiding PCF was drawn from a preform realized by stacking core/clad fibers around an uncladded AgBr fiber [47

47. E. Rave, K. Roodenko, and A. Katzir, “Infrared photonic crystal fiber,” Appl. Phys. Lett. 83, 1912–1914 (2003). [CrossRef]

]. The fiber showed an effective core diameter of 200μm. Other fibers have been realized from a silver-halide preform made of AgCl20Br80 with holes drilled in a hexagonal pattern [48

48. L. N. Butvina, O. V. Sereda, E. M. Dianov, N. V. Lichkova, and V. N. Zagorodnev, “Single-mode microstructured optical fiber for the middle infrared,” Opt. Lett. 32, 334–336 (2007). [CrossRef] [PubMed]

]. The holes are filled with rods made of AgCl50Br50. The extruded fiber showed a hole to distance spacing of d/∧ = 0.74, an effective core diameter of 79μm and a numerical aperture of 0.16.

3.1.5. Considerations on design and technology

In the context of astronomy, photonics devices face a certain number of requirements linked to the original design of astronomical instruments, both from the design and technology point-of-view. For instance, classical optics (lens, mirrors…) used to feed an instrument are slow optics, with high numerical aperture that does not match one of fibers or integrated optics. This requires to use tapers, which in return can introduce additional losses if they are not adiabatic. Another point concern the importance of facet cleaning and anti-reflection coatings to limit Fresnel losses at the waveguide input. This is a technological aspect which is not yet very well accounted for, especially for mid-infrared waveguides. A trade-off between these different considerations is essential in the astronomical context, where transmission losses have to be kept as low as possible.

3.2. Testing techniques and characterization results

3.2.1. Transmission range and excess losses

Transmission range and losses are the two fundamental parameters that define and fix the op-erability of the waveguides in the range of interest. The combined effect of transmission range and losses is very strongly dependent on the material, the applied technological process and the waveguides geometry. This measurement is the first step of any characterization phase. – In the case of dielectric waveguides, the transmission range simply refers to the spectral domain where photons are not absorbed by the material. Obviously, this definition is linked to the transparency window of the bulk material. The effective transmission range is provided through spectroscopy measurements in the region of interest. For dielectric bulks, the transparency window is obtained by FTIR spectroscopy, which is a standard technique and will not be further developed here. – Excess losses regroup propagation losses, coupling losses and Fresnel losses. Excess losses are specific to each waveguide and directly depending on its opto-geometrical parameters (length, numerical aperture, index difference at air-waveguide interface). The different contributions to excess losses are difficult to disentangle for a single waveguide. Overall excess losses can be measured in dedicated Fourier Transform spectroscopy experiments in which light is launched into the waveguide placed in the optical path. In some cases, it is possible to extract separately coupling and propagation losses by differential measurements on similar waveguides with different lengths and numerical aperture. This method is also named “cut-back method”. This method gives good results, but requires then a dedicated set of waveguides planned in the manufacturing phase [49

49. L. Abel-Tiberini, L. Labadie, B. Arezki, P. Kern, R. Grille, P. Labeye, and J.-E. Broquin,“Transmission behaviors of single-mode hollow metallic waveguides dedicated to mid-infrared nulling interferometry,” Opt. Express 15, 18005–18013 (2007). [CrossRef] [PubMed]

] Measurement campaigns on the different type of waveguides presented in Sect. 3.1 have shown a relatively large span in the measured losses. Chalcogenide and silver-halides fibers show the best transmission, even in single-mode regime, with losses at 10μm of 8 dB/m [32

32. A. Ksendzov, O. Lay, S. Martin, J. S. Sanghera, L. E. Busse, W. H. Kim, P. C. Pureza, W. V. Q. Nguyen, and I. D. Aggarwal, “Characterization of mid-infrared single-mode fibers as modal filters,” Appl. Opt. 46, 7957–7962 (2007). [CrossRef] [PubMed]

], 10 to 18 dB/m [31

31. L. K. Cheng, A.-J. Faber, W. Gielesen, C. Boussard-Pledel, P. Houizot, J. Lucas, and J. Pereira Do Carmo, “Test results of the infrared single-mode fiber for the DARWIN mission,” Proc. SPIE 5905, 59051F–1–59051F–8 (2005).

], 20 dB/m [30

30. P. Borde, G. Perrin, T. Nguyen, A. Amy-Klein, C. Daussy, P.-I. Raynal, A. Leger, and G. Maze, “10-mm wavefront spatial filtering: first results with chalcogenide fibers,” Proc. SPIE 4838, 273–279 (2003). [CrossRef]

], 23.2 dB/m [34

34. R. Flatscher, O. Wallner, V. Artjuschenko, and J. Pereira do Carmo, “Manufacturing of chalcogenide and silver-halide single-mode fibres for modal wavefront filtering for Darwin,” Proc. 6th Internat. Conf. on Space Optics (2006).

], 25 dB/m [35

35. S. Shalem, A. Tsun, E. Rave, A. Millo, L. Nagli, and A. Katzir, “Silver halide single-mode fibers for the middle infrared,” Appl. Phys. Lett. 87, 091103–1–091103–3 (2005). [CrossRef]

]. Ex-perimental measurements at longer wavelengths for chalcogenide fibers have led to increased values up to 40 dB/m between 12 and 13μm, and 150 to 300 dB/m between 16 to 20μm [50

50. A. J. Faber, L. K. Cheng, W. L. M. Gielesen, C. Boussard-Pledel, S. Maurugeon, B. Bureau, X. H. Zhang, J. Lucas, and J. Pereira Do Carmo, “Optical characterization of infrared telluride glass fibers for space use,” Proc. Internat. Conf. on Space Optics (2008).

]. Dielectric integrated optics have presented so far higher losses in the range of 1 to 10 dB/cm [40

40. C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J.-E. Broquin, L. Labadie, and P. Kern, “Transmission measurement at 10.6 μm of Te2As3Se5 rib waveguides on As2S3 substrate,” Appl. Phys. Lett. 90, 011110–011112 (2007). [CrossRef]

, 42

42. N. Hô, M. C. Phillips, H. Qiao, P. J. Allen, K. Krishnaswami, B. J. Riley, T. L. Myers, and N. C. Anheier, “Single-mode low-loss chalcogenide glass waveguides for the mid-infrared,” Opt. Lett. 31, 1860–1862 (2006). [CrossRef] [PubMed]

]. However, integrated optics is designed to operate over much shorter propagation distances than fibers. At this stage of the development of mid-IR waveguides, the reason of the high dispersion in the propagation losses is mainly found in the non-optimized manufacturing processes (impurities, defects etc…). More exotic waveguides like PCFs and hollow waveguide show a quite different behavior. In two cases, mid-infrared photonic-crystal fibers have shown very different losses of ~2 dB/m [48

48. L. N. Butvina, O. V. Sereda, E. M. Dianov, N. V. Lichkova, and V. N. Zagorodnev, “Single-mode microstructured optical fiber for the middle infrared,” Opt. Lett. 32, 334–336 (2007). [CrossRef] [PubMed]

] and 60 dB/m [47

47. E. Rave, K. Roodenko, and A. Katzir, “Infrared photonic crystal fiber,” Appl. Phys. Lett. 83, 1912–1914 (2003). [CrossRef]

], but the single-mode behavior is not assessed. For hollow waveguides as well, we found a strong dispersion in the performance. For most multimode mid-infrared hollow waveguides, straight losses can be as low as few dB/m, which is attractive for power transportation. However, single-mode hollow waveguides are less advanced technologically, and the first prototypes have shown high losses of several dB/mm [45

45. L. Labadie, P. Labeye, P. Kern, I. Schanen, B. Arezki, and J.-E. Broquin, “Modal filtering for nulling interferom-etry. First single-mode conductive waveguides in the mid-infrared,” Astron. Astrophys. 450, 1265–1275 (2006). [CrossRef]

]. Understanding such a discrepancy requires further advances from the technological side.

Fig. 5. Measurement procedures to verify single-mode behavior and spatial filtering of optical fibers. See text for details.

3.2.2. Single-mode regime and spatial filtering

Supporting only a single spatial mode is a key feature of optical fibers for astronomical applications, and especially for mid-infrared interferometry [51

51. G.L. Clark and C. Roychoudhuri, “Interferometry through single-mode optical fibers”, Proc. SPIE 192, 196–203 (1979).

]. For mid-infrared fibers, one may think on different procedures which rely either on measuring the fibers’ far field intensity distribution or on measuring the fibers waveguiding properties (see Fig. 5). The procedures differ clearly in complexity and accuracy:

4. Conclusions and perspectives

Acknowledgments

The authors thank the anonymous referee for his useful comments to improve the scientific quality of the paper. The authors also thank Dr. T. M. Herbst for the constructive discussions and the accurate reading of the paper.

References and links

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12.

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17.

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18.

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19.

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20.

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21.

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22.

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23.

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32.

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36.

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37.

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C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J.-E. Broquin, L. Labadie, and P. Kern, “Transmission measurement at 10.6 μm of Te2As3Se5 rib waveguides on As2S3 substrate,” Appl. Phys. Lett. 90, 011110–011112 (2007). [CrossRef]

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42.

N. Hô, M. C. Phillips, H. Qiao, P. J. Allen, K. Krishnaswami, B. J. Riley, T. L. Myers, and N. C. Anheier, “Single-mode low-loss chalcogenide glass waveguides for the mid-infrared,” Opt. Lett. 31, 1860–1862 (2006). [CrossRef] [PubMed]

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45.

L. Labadie, P. Labeye, P. Kern, I. Schanen, B. Arezki, and J.-E. Broquin, “Modal filtering for nulling interferom-etry. First single-mode conductive waveguides in the mid-infrared,” Astron. Astrophys. 450, 1265–1275 (2006). [CrossRef]

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48.

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OCIS Codes
(060.2390) Fiber optics and optical communications : Fiber optics, infrared
(060.2430) Fiber optics and optical communications : Fibers, single-mode
(130.3060) Integrated optics : Infrared
(130.3120) Integrated optics : Integrated optics devices
(130.3130) Integrated optics : Integrated optics materials
(350.1270) Other areas of optics : Astronomy and astrophysics

History
Original Manuscript: November 5, 2008
Revised Manuscript: January 22, 2009
Manuscript Accepted: January 28, 2009
Published: January 30, 2009

Virtual Issues
(2009) Advances in Optics and Photonics
Vol. 4, Iss. 4 Virtual Journal for Biomedical Optics
Focus Issue: Astrophotonics (2009) Optics Express

Citation
Lucas Labadie and Oswald Wallner, "Mid-infrared guided optics: a perspective for astronomical instruments," Opt. Express 17, 1947-1962 (2009)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-17-3-1947


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References

  1. V. Coud’e du Foresto, G. Perrin, J.-M. Mariotti, M. Lacasse and W. Traub, in The FLUOR/IOTA fiber stellar interferometer, F. Malbet, P. Kern, eds. (Bastianelli-Guirimand, 1997), pp.115-125.
  2. H. A. McAlister, T. A. ten Brummelaar, L. Sturmann, J. Sturmann, N. H. Turner and S. T. Ridgway, "Recent progress at the CHARA interferometric array," Proc. SPIE 6268,62680G (2006). [CrossRef]
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  19. M. Benisty, J.-P. Berger, L. Jocou, F. Malbet, K. Perraut, P. Labeye, and P. Kern, "The VSI/VITRUV combiner: a phase-shifted four-beam integrated optics combiner," Proc. SPIE 6268,62682D (2006). [CrossRef]
  20. E. Garmire, T. McMahon and M. Bass, "Propagation of infrared light in flexible hollow waveguides," Appl. Opt. 15,145-150 (1976). [CrossRef] [PubMed]
  21. E. Garmire, E., T. McMahon and M. Bass, "Flexible infrared waveguides for high-power transmission," J. Quantum Electron. QE-16,23-32 (1980). [CrossRef]
  22. F. E. Vermeulen, C. R. James and A. M Robinson, "Hollow microstructural waveguides for propagation of infrared radiation," J. Lightwave Technol. 9,1053-1060 (1991). [CrossRef]
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  26. S. E. Barkou, J. Broeng and A. Bjarklev, "Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect," Opt. Lett. 24,46-48 (1999). [CrossRef]
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