The development of single mode chalcogenide glass fibers as wavefront filter for the DARWIN mission is reported. Melting procedures and different preform techniques for manufacturing core-cladding chalcogenide fibers are described. Bulk glass samples on the basis of Te-As-Se- and high Te-compositions have been characterized optically, by measurement of the absorption spectrum and the refractive index in the region 4 – 20 μm. Several chalcogenide core-cladding glass fiber configurations have been drawn and examined by microscopy. The mode propagation behaviour and numerical aperture of fiber samples have been determined by far field intensity distribution measurements, using a CO<sub>2</sub> laser at 10.6 μm. Single mode waveguide performance has been demonstrated for several fiber samples, coated with an absorbing Gallium layer for stripping of cladding modes.
Chalcogenide glasses are known for their large transparency in the mid infrared and their high refractive index (>2).
They present also a high non linear refractive index (n<sub>2</sub>), 100 to 1000 times larger than for silica. An original way to
obtain single-mode fibers is to design photonic crystal fibers (PCFs). Until now, chalcogenide PCFs are realized using
the stack and draw process. However this technique induces defects, like bubbles, at the capillaries interfaces, causing
significant scattering losses. Until now, the best transmission obtained was 3dB/m at 1.55μm. The poor PCF
transmission reduces significantly their application potential. So, we present a new efficient method to realize low-loss
chalcogenide PCFs. This original method by molding permits to reduce the optical losses down to 1dB/m at 1.55μm and
less than 0.5dB/m between 3 and 5μm for an As-Se PCF. Furthermore, this molding method can be used for different
compositions. Single mode fibers were realized. Moreover, very small core fibers were realized with this method,
obtaining a non linear coefficient of 15 000W<sup>-1</sup>km<sup>-1</sup> with an As-Se PCF. We also observed self phase modulation at
1.55μm on a fiber with a 2.3μm<sup>2</sup> mode area.
Various space telescope array systems are being considered to investigate other terrestrial planets orbiting around
nearby stars in order to find extra-terrestrial life. One of them is the DARWIN mission of the European Space Agency
(ESA). The required technology is the nulling interferometer. The challenge of nulling is making the null in the
interferometric signal sufficiently deep to cancel the light from the bright star during the collection of light from its
surrounding planets. The performance of the nulling is limited by the wavefront quality of the beams. The wavefront
error can be reduced by filtering using a single mode fiber. For the DARWIN mission, the operational wavelength range
is 6.5-20μm. Within the current ESA project, this is covered by a dual-band fiber system. A chalcogenide glass fiber
based on the Te-As-Se (TAS) composition is selected to be used for the short wavelength band. For the long wavelength
band up to 20 μm, Tellurium based glass is proposed. Different samples of various composition based on Te glass are
manufactured and tested. The fibers are designed by TNO and different prototypes have been manufactured by the
University of Rennes. Test setups are developed to demonstrate/investigate the single mode operation. Cladding modes
are found to disturb the single mode operation. The effect of cladding modes is modeled. Solutions to eliminate the
cladding modes are investigated and tested.
In this work, we review recent progress on the realization of chalcogenides Photonic Crystal Fibers (PCFs). We present
the fabrication of chalcogenide PCFs with a solid core for three different glass compositions containing a variety of
chalcogens. We show that the Stack and Draw technique currently used for silica PCFs can be problematic in the case of
chalcogenides glasses. We present correct PCF design enables a significant improvement of final fiber losses. We
obtained a lowest attenuation of 3 dB/m at 1.55 μm, of 4.5 dB/m at 3.39 μm and 6 dB/m at 9.3 μm. We also present
experimental demonstration of self phase modulation spectral broadening around 1,55 μm. Moreover, we investigate the
Brillouin and Raman scattering properties of a GeSbS PCF.
Chalcogenide glasses present several original properties when being compared to the reference silica glass. They are
very non linear, hundred to thousand times more non linear than the standard silica, they are very transparent in the
infrared, until 10 μm to 20 μm depending on their composition, and they can be drawn into optical fibers. Thus, the case
of chalcogenide photonic crystal fibers (PCF) is of particular interest. Indeed, the effective modal area is adjustable in
PCF thanks to geometrical parameters. Then chalcogenide microstructured fibers with small mode area could lead to
huge non linear photonic devices in the infrared by the combination of the intrinsic non linearity of these glasses with
the non linearity induced by the PCF. Chalcogenide photonic crystal fibers offer therefore a great potential for
applications in the fields of Raman amplification or Raman lasers and supercontinuum generation in the mid infrared
until at least 5 μm. The possibility to design PCF exhibiting a working range in the mid infrared and more specifically
in the 1-6 μm wavelength range opens also perspectives in the optical detection of chemical or biochemical species.
This contribution presents the advances in the elaboration of such chalcogenide photonic crystal fibers.
Mid-infrared (IR) lasers are of interest for a variety of applications including environmental sensing, LIDAR and
military counter measures. However, this wavelength range lacks powerful, coherent, robust and compact sources. A
solution can lie in chalcogenide glasses as host materials for rare earth ions. With an extended infrared transparency, low
phonon energy limiting the non radiative multiphonon relaxation rates and suitable rare earth solubility, sulfide glasses
based on Ge-Ga-Sb-S system make available radiative transitions in the mid-IR range. The glasses with nominal
composition of Ge<sub>20</sub>Ga<sub>5</sub>Sb<sub>10</sub>S<sub>65</sub> doped with Er<sup>3+</sup> (500 to 10000 ppm) were prepared by means of conventional melting
and quenching method. The Er<sup>3+</sup>, widely studied in glass fibers for near-IR amplification, was initially selected for the
transition <sup>4</sup>I<sub>9/2</sub> to <sup>4</sup>I<sub>11/2</sub> emitting at around 4.5 &mgr;m in order to demonstrate the ability of this sulfide composition for midinfrared
fiber lasers application. In these objectives, absorption and emission spectra have been recorded and the
radiative decay lifetime of excited levels (<sup>4</sup>I<sub>9/2</sub>, <sup>4</sup>I<sub>11/2</sub> and <sup>4</sup>I<sub>13/2</sub>) has been determined. These last experimental results were
compared with those obtained by Judd-Ofelt model from absorption cross-sections of all observable transitions.
Therefore, the <sup>4</sup>I<sub>9/2</sub> radiative quantum efficiency was estimated at 67 %. The emission cross-section was 2.6x10<sup>-21</sup> cm<sup>2</sup> at
4.6 &mgr;m obtained by Fütchbauer-Ladenburg theory. The product of measured lifetime and emission cross-section for <sup>4</sup>I<sub>9/2</sub>
-> <sup>4</sup>I<sub>11/2</sub> transition is about 1.87x10<sup>-24</sup> cm<sup>2</sup>.s is comparable with that for GaLaS glasses. The fiber drawing of the Er<sup>3+</sup>
doped Ge<sub>20</sub>Ga<sub>5</sub>Sb<sub>10</sub>S<sub>65</sub> glasses and measurements of optical losses in mid-IR are currently in progress and first results
A vital function of the space interferometer foreseen in the DARWIN mission is the so-called "nulling" operation. The challenge of nulling is making the null in the interferometric signal sufficiently deep to cancel the light from the bright star during the collection of light from its surrounding planets. The performance of the nulling is limited by the wavefront quality of the beams. The wavefront error can be reduced by filtering. One promising concept for nulling wavefront filtering is using a single mode fiber. For the wavefront filtering in the DARWIN mission, the fiber has to cover the operational wavelength range of 4-20 μm. Furthermore, a minimal insertion loss is required to ensure a minimum exposure time. This results in the separation of the complete wavelength range into several separate wavelength bands in the nulling system. Within an ESA project, a chalcogenide glass fiber based on the Te-As-Se (TAS) composition is selected to be used for the short wavelength band. TNO has designed and tested several TAS fibers that have been manufactured by the University of Rennes. Single mode operation is demonstrated. Furthermore, the effect of bending the fiber and light coupling are investigated. For the long wavelength band up to 20 μm, Tellurium based glass is proposed. Different samples of various composition based on Te glass are manufactured. Accurate temperature control to avoid crystallization is found to be essential for the manufacturing process. For the bulk material, a transmission window up to 20 μm is measured.
Nulling interferometry is the baseline technique for the DARWIN planet finding mission of the European Space Agency. Using this technique it will be possible to cancel, by destructive interference, the light from the bright star and look directly at its surrounding planets and eventually discover life on them. To achieve this goal wavefront errors need to be reduced to a very high degree in order to achieve the required nulling quality. Such a high wavefront quality can only be achieved with adequate wavefront filtering measures. Single mode fibers in general have excellent mode filtering capabilities, but they were not recently available for the broad infrared wavelength region of Darwin (4-20 um). Within an ESA technology development project, TNO has designed and tested an infrared single mode fiber based on chalcogenide glasses that has been manufactured by the University of Rennes. Several tests are carried out to characterize the materials used and the IR single mode fiber. Far field intensity distribution measurement at 10.6 um reveals the single mode operation of the manufactured fiber. Influence of coating, length, light coupling and bending of the fiber are also investigated.
Infrared chalcogenide glasses are studied with respect to their non linear optical properties. These glasses are sulfur or selenide glasses synthesized in the binary or ternary systems of the Ge-As-S-Se family and are transparent from the end of the visible region to wavelengths above 10 μm depending on the composition. The non linear optical characteristics are firstly determined through a spatially resolved Mach Zender interferometer with the help of a Nd-YAG laser at 1064 nm. Non linearities three order of magnitude above the non linearity of silica glass are achieved. Then, the non linear imaging technique has been used to characterize the glasses at the telecommunication wavelength of 1.55μm. This one shot technique has allow us to obtain values for the non linear refractive index n2 as high as 14 10-18 m2/W. The non linear absorption at 1.55 μm has also been evaluated and is below 1 cm/GW for all the glasses. These third order non linear optical properties make these glasses suitable candidates for integrated ultra fast all optical devices. On the basis of the GeSe4 vitreous composition, an optical fiber, single mode at 1.55 μm, is achieved.
A new glass in the fluoroarsenate family was fabricated and doped with Er<SUP>3+</SUP> ions. The compositions made are (Na<SUB>4</SUB>As<SUB>2</SUB>O<SUB>7</SUB>)<SUB>40</SUB> (BaF<SUB>2</SUB>)<SUB>30</SUB> (YF<SUB>3</SUB>)<SUB>x</SUB> with xequals 0, 0.1, 0.5, 1, 3, and 5. The optical properties of Er<SUP>3+</SUP> ions were established in terms of absorption and emission spectra, Judd-Ofelt calculations, and lifetime measurements in the visible and infrared domains. Because of the presence of arsenic, these glasses show lower phonon energies than their fluorophosphate counterparts. As a result, the green emission from the <SUP>4</SUP>S<SUB>3/2</SUB> level of Er<SUP>3+</SUP> can be observed. At concentration of 0.1 mol $ ErF<SUB>3</SUB>, the experimental lifetime for <SUP>4</SUP>S<SUB>3/2</SUB> is well accounted for by thermalization processes and the occurrence of multiphonon relaxations. For the <SUP>4</SUP>I<SUB>11/2</SUB> and <SUP>4</SUP>I<SUB>13/2</SUB> levels, the lifetime is maximum for 1% erbium concentration, due to the occurrence of signal self-absorption. Excellent agreement is obtained between the radiative and experimental lifetimes of the <SUP>4</SUP>I<SUB>13/2</SUB> level, which validates the application of the Judd-Ofelt theory.