During the last decade there has been a surge of interest in developing mid-infrared (mid-IR) fiber-based supercontinuum (SC) sources. Such broadband light sources take advantage of extreme spectral broadening of high-intensity laser pulses in infrared optical fibers usually made of soft glasses, such as chalcogenides that offer the widest transmission window and the highest nonlinearity. Beyond their spatial coherence and high brightness, mid-IR fiber SC sources are nowadays operating over some wavelength ranges of thermal sources with superior performances for spectroscopic applications. However, intrinsic limitations of current fibers or even integrated waveguides now appear to impose the long-wavelength edge of mid-IR SC sources around 13-15 μm. The current research has to focus now on extending the wavelength coverage over the entire mid-IR molecular fingerprint region (often defined as from 2 to 20 μm).
We here overcome this limitation by the engineered nonlinear transformation of femtosecond pulses over the full transmission window of a step-index chalcogenide fiber. In contrast to previous works, we reach the long-wavelength transparency edge of Se-rich glass family near 18 μm, and without including arsenic and antimony compounds considered as toxic elements and pollutants. Our end-to-end control of both materials chemistry and nonlinear fiber optics, including glass synthesis and purification, fiber design and drawing, as well as engineering of SC generation, has allowed us to optimize each of these crucial steps in order to demonstrate coherent mid-IR SC generation spanning from 2 to 18 μm.
Development of broadband supercontinuum sources has been studied since decades for its high application potential in various fields like spectroscopy, medical science and others. First experiments were made with silica but the results shown the need to find new materials for supercontinuum generation in the IR wavelength range. Two types of materials have been found interesting for supercontinuum generation: chalcogenide and tellurite glasses. These materials have a high non-linear refractive index and a good transmission in infrared which provides a high potential for applications. Bulks tellurite glasses transmit until 5µm while bulks chalcogenide glasses transmit until 12-20µm depending on their composition. We report here the synthesis of low-OH step-index tellurite fibers and their linear and non-linear characterization. The synthesis is firstly realized by build-in-casting in a glovebox which allows to get a large-core preform(∅clad/∅core 2) and a large core corresponding fiber(∅core 60µm). Then, the rod-in-tube technique allows, from the jacketing of the stretched initial preform, to get a small-core preform and subsequently a small core fiber (∅core 3.5µm). The minimum of losses of the large-core fiber is below 1dB/m, the IR transmission wavelength exceeds 4 µm on several meters of fibers and reaches more than 5 µm on small samples (several centimeters long). We have developed core-clad composition with a large refractive index difference (∆n=0.132) which provides a high confinement in our step-index fibers. We discuss the supercontinuum generation in these fibers exploiting numerical simulations based on the generalized nonlinear Schrödinger equation and then we present the supercontinuum experimental results obtained between 1 and 5µm. Most of pollutant and greenhouse gases emitted by human activity, including methane, carbon dioxide and nitrous oxide, absorbs in the mid-IR. The spectroscopic experiments realized on the gases through supercontinuum generation between 1 and 5 µm are presented.
We present an overview of the fabrication process and characterization of germanate, germanate-tellurite and tellurite microstructured fibers or step-index fibers with different index contrasts. For microstructured fibers, we exploit the generated SC for methane spectroscopy measurements in the mid-infrared by means of the supercontinuum absorption spectroscopy technique. We demonstrate also an ageing process of microstructured fibers in ambient atmosphere. For stepindex fibers, several compatible core/cladding glasses were explored for fiber manufacturing with a dehydration process, allowing the drawing of low-OH tellurite fibers. We report both numerical and experimental demonstrations supercontinuum (SC) generation in these fibers. Finally we report our latest results in the generation of SC until 5.3 μm in a step-index tellurite fiber.
We report the manufacturing and characterization of Tellurite micro structured fibres (MOFs) with low OH content. The different purification processes used during the fabrication of the TeO2 - ZnO - Na2O glass allowed us to reduce the hydroxyl compounds concentration down to 1ppm mass. A suspended core MOF was drawn from this material and then pumped by nanojoule-level femtosecond pulses at 1.7μm, its zero dispersion wavelength (ZDW), and well above it at 2.5μm. We show the related supercontinuum (SC) generated in the two distinct dispersion regimes of the waveguide. Moreover, the SC spanning was extended in both visible and mid-IR regions (between 600nm up to 3300nm) by the taperisation of the previously tested MOFs.
In this work we report our achievements in the elaboration and optical characterizations of low-losses suspended
core optical fibers elaborated from As2S3 glass. For preforms elaboration, alternatively to other processes like the stack and draw or extrusion, we use a process based on mechanical drilling. The drawing of these drilled
performs into fibers allows reaching a suspended core geometry, in which a 2 μm diameter core is linked to the
fiber clad region by three supporting struts. The different fibers that have been drawn show losses close to
0.9 dB/m at 1.55 μm. The suspended core waveguide geometry has also an efficient influence on the chromatic
dispersion and allows its management. Indeed, the zero dispersion wavelength, which is around 5 μm in the bulk
glass, is calculated to be shifted towards around 2μm in our suspended core fibers. In order to qualify their
nonlinearity we have pumped them at 1.995 μm with the help of a fibered ns source. We have observed a strong
non linear response with evidence of spontaneous Raman scattering and strong spectral broadening.
We present the fabrication of tellurite TeO2-ZnO-Na2O (TZN) microstructured optical fibers (MOFs) with a suspended
core and the characterization of their optical properties. The fibers are designed to develop an infrared supercontinuum
generation using a sub-nJ femtosecond pulsed laser at 1.56μm. By pumping a 20 cm long fiber we generate a
supercontinuum (SC) spanning over 800 nm in the 1-2 μm wavelength range. For a MOF with a core size of 2.2 μm the
zero dispersion wavelength (ZDW) is at 1.45 μm. The effective area of TZN MOF is 3.5 μm2 and the nonlinear coefficient is calculated to be 437 W-1km-1.
The NULLTIMATE project developed and realized three concepts of achromatic phase shifters for nulling interferometry.
One of the concepts is based on dispersive plates made of three materials which where fully
characterized regarding their refractive index and thermo-optic behavior between 100K and 330 K. The other
two concepts are based on mirror optics, one of which uses the phase shift of π when crossing a focus, the
other the reversal of electric fields at reflection. An optical bench has been set up to test and characterize these
phase shifters at wavelengths 2 − 2.4 μm with the option of changing to the 10 μm domain. We summarize the
development of the achromatic phase shifters and report on the current status of the test bench.
The development of chalcogenide glasses fibers for application in the infrared wavelength region between 1 and 10
μm is a big opportunity. More particularly, the possibility to generate efficient non linear effects above 2 μm is a real
challenge. We present in this work the elaboration and optical characterizations of suspended core microstructured
optical fibers elaborated from the As2S3 chalcogenide glass. As an alternative to the stack and draw process a
mechanical machining has been used to the elaboration of the preforms. The drawing of these preforms into fibers
allows reaching a suspended core geometry, in which a 2.5 μm diameter core is linked to the fiber clad region by
three supporting struts. The zero dispersion wavelength is thus shifted towards 2 μm. At 1.55 μm our fibers exhibit a
dispersion around -250 ps/nm/km. Their background level of losses is below 0,5 dB/m. By pumping them at 1.55 μm
with a ps source, we observe self phase modulation as well as Raman generation. Finally a strong spectral
enlargement is obtained with an average output power of - 5 dbm.
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.
The temperature-dependent dispersion of the refractive index of optical materials is analyzed by introducing an
appropriate formulation of what we call their "normalized
thermo-optic coefficients" (NTOC). These parameters are
obtained experimentally by performing accurate interferometric measurements of both thermal expansion and changes in
optical thickness of parallelepipedic shaped samples of millimetric size which are submitted to a linear ramp of
temperature. Changes in optical thickness are recorded as function of temperature at a few discrete laser wavelengths;
they are expressed as power series of temperature and the use of a simple vectorial formalism allows then to determine
the thermal behavior of the dispersion law over the whole transparency range of the material. The validity and reliability
of the proposed method has been confirmed by modeling the temperature dependence of the type II second harmonic generation 1.064 μm - 0.532 μm in a KTiOPO4 single crystal, recorded up to 120°C.
The Darwin/TPF mission aims at detecting directly extra solar
planets. It is based on the nulling interferometry, concept proposed
by Bracewell in 1978, and developed since 1995 in several European and
American laboratories. One of the key optical devices for this
technique is the achromatic phase shifter (APS). This optical
component is designed to produce a π phase shift over the whole
Darwin spectral range (i.e. 6-18 μm), and will be experimentally
tested on the NULLTIMATE consortium nulling test bench (Labèque et
al). Three different concepts of APS are being simulated: dispersive plates focus crossing and field reversal. In this paper, we show how thermal, mechanical and optical models are merged into a single robust model, allowing a global numerical simulation of the optical component performances. We show how these simulations help us to optimizing the design and present results of the numerical model.
A proper implementation of electro-optic materials in laser systems requires an accurate knowledge of their electro-optic coefficients, along with their temperature dependence which could be of importance at high power level. A new technique has been developed for this purpose, which takes advantage of the thermodynamic equivalence of two
intensive parameters, namely the temperature and applied electric field. A suitably oriented parallelepipedic shaped sample is exposed to a laser beam and acts as a Fabry-Perot interferometer which is submitted to a linear ramp of temperature. The interference pattern is observed by reflection and the shift of interference fringes generated by the thermo-optic effect is detected through amplitude modulation of the light beam and recorded as a function of
temperature. We then switch from amplitude- to phase- modulation by applying a suitable electric field to the crystal: the signal features now the derivative of the thickness fringes generated by the electro-optic effect. The thermo- and electro-optic coefficients are obtained from the fringe shift recorded respectively through amplitude- and phase- modulated operating modes. The study of both KTiOPO4 and LiInS2 single crystals is given as an example to illustrate the so-called Fabry-Perot Thermal Scanning Interferometric (FPTSI) method.
All-optical poling technique permits purely optical orientation of dye molecules in a polymer film. The experiment includes two phases: the writing (seeding) period and the readout one. In seeding phase two beams, the fundamental (omega) and its second harmonic (SH, 2(omega) ) irradiate the sample and as a result of the coherent interference between them the second order (chi) (2)-susceptibility grating is encoded, with a period satisfying the phase matching condition for SH generation. During the readout step only the fundamental beam is incident onto the material and the second harmonic beam generated by the medium is observed at the back side of the sample. The coherent superposition of two beams at (omega) and 2(omega) frequencies results in a presence of a polar field E(t) inside the material, which can break the centrosymmetry of the medium. The physical origin of the effect lies in the orientational hole-burning in the initially isotropic distribution of dye molecules. It has been demonstrated that efficient all-optical poling requires optimization of relative intensities and relative phase of the seeding beams. An original technique of non-perturbative monitoring of the all-optical poling process without any necessity of taking care of the phase difference between seeding beams is presented. This new technique was applied to several new dye-polymer systems.
All-optical poling technique permits purely optical orientation of molecules. The experiment consists of two steps: the writing (so-called seeding) period and the readout one. In the seeding phase two beams, the fundamental one ((omega) ) and its second harmonic (SH, 2(omega) ) illuminate the sample and print in the medium the second order (chi) (2) susceptibility grating with a periodicity satisfying the condition of phase matching for SH generation. In the readout period only the fundamental beam irradiates the sample, and the second harmonic generation is observed at the sample output. Efficient all-optical poling requires optimisation of the seeding beam intensities and their relative phase difference. We propose a novel technique for non-perturbative monitoring of the all-optical poling process and the easy method of orienting molecules without any necessity of taking into account the phase difference between seeding beams.
An experimental method enabling a simultaneous determination of the linear and quadratic electro-optic coefficients in thin film is described. The technique is based on the interference between electric field induced modulation of transverse electric (TE) and transfer magnetic (TM) fields propagating through a stratified medium consisting on a substrate, semitransparent electrode, and the measured thin film. It allows us to determine both the real and the imaginary parts of corresponding electro-optic coefficients as well as their dispersion. The use of thin film enables both electro-optic coefficients measurements in the thin film absorption band allowing nonlinear spectroscopy studies. The technique is applied to a poled functionalized polymer thin film and the results are interpreted in terms of a two level model. This allows a determination of microscopic parameters such as: excited and fundamental state permanent dipole moment difference and transition moment between fundamental and first excited state. A presence of an excited state in the bottom of the absorption band is revealed.
Nonlinear optical response of copolymers bearing pendant active mesogenic units have been studied. The A and C polymers are nematic and smectic A, respectively, while the B and D polymers resemble typical amorphous polymers. Spin-coated films have been activated using the corona poling technique. From the optical absorption spectra, order parameters in the range 0.4 - 0.6 have been determined. The rather large values obtained for the B and D polymers suggest that an isotropic/nematic transition occurs during poling. Second harmonic generation measurements relative to quartz have been performed to determine the values of second order coefficients d33 and d31.
Different types of poled polymer thin films (guest-host, grafted and crosslinked) are studied. The optical birefringence is related to electrooptical coefficients. Those coefficients are measured in the absorption range. Their value off resonance is equal to 10 pm/V. Loss and time relaxation experiments are also presented.
In this paper, a review of some different experimental methods employed for measuring the electrooptic coefficients (Pockels effect and Kerr effect) are presented, together with the most recent technique proposed by Teng and Man. For this last method, the limit of validity of a simplified method is established. A rigorous derivation is also given for studying such polymer films when the wavelength of the light beam is close to the absorption band.
Second order nonlinear optical properties of doped polymer thin films are studied by optical second harmonic generation and electro-optic coefficient measurements. The studied thin films are guest-host systems and cross linked polymers with charge transfer noncentrosymmetric amines as curing agents. The in situ second harmonic generation (SHG) measurements during poling optimize the poling process and give corresponding components of (chi) (2)(- 2(omega) ;(omega) ,(omega) ) tensor depending on the fundamental harmonic electric field polarization configurations. At the same time they follow the stability of induced dipolar orientation by poling. Obviously, crosslinked polymers are more stable, whereas orientation in guest-host systems decay rapidly with time. The electro-optic measurements in thin films by reflection using the technique proposed by Teng and Man and by Schildkraut measures r33 coefficient even in the doped polymer absorption band and give its real and imaginary parts.
Poled polymer thin films are studied by SHG, THG, and ATR experiments. The ATR method is used for determining the linear and quadratic electro-optic (chi) (2) (-(omega) ; 0, (omega) ) and (chi) (3) (-(omega) ; 0, 0, (omega) ) coefficients. The part due to the piezoelectricity and electrostriction is characterized and it is shown that in some cases the mechanical strains are more important than the direct electro-optic effect. From SHG experiments, the degree of orientation linked to the (chi) (2) (-2 (omega) ; (omega) , (omega) ) susceptibility can be evaluated. The various results are presented in this paper.
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