Fiber Optics

Waveguide losses remain a challenge in the use of traditional methods of photonic crystals based chalcogenide fiber, so that new methods are developed for cast glass fibers that create records with nonlinear coefficients.

chalcogenide glasses are a mixture of elements chalcogens (sulfur, selenium and tellurium) and other elements such as arsenic, germanium, antimony or gallium. In comparison with silica glasses, they have several distinctive optical properties, such as a transmission window as far into the infrared (IR) spectral range (up to 25 h for telluride glasses). chalcogenide glasses show a high nonlinear refractive index coefficient, which can be two or three orders of magnitude greater than that of silica at 1.55 pm. These nonlinear properties can be improved by chalcogenide glasses in photonic crystal fiber (PCF) because of the possibility of designing these fibers with a very small core diameter.1, 2 This new fiber nonlinear active optical functions such as supercontinuum generation perform in the mid-infrared and optical gates.3

photonic crystal fibers also show some other interesting features, including constantly running single-mode, widely tunable chromatic dispersion and a unique orientation mode large effective area in the mid-IR chalcogenide with CPF (CPCFs) may be developed. In the field of passive optical functions CPCFs should be useful for the development of power, nulling interferometry for space applications and detection of pollutants using optical sensors.

Since 2004 the laboratory and PerfOS “Glass and Ceramics” at the University of Rennes (VCA) jointly CPCFs.4 a reliable method for the production worked to develop the first place, was the municipality “stack and draw “technical investigation. This method is usually to more than a decade to silica PCF and includes stacking glass capillaries in a hexagonal lattice of multiple rings that surround a glass jacket, the shape fiber preform. However, this method is not suitable for chalcogenide glasses, we observed the presence of a large number of bubbles and a change in refractive index induced by the interface region in glass diffusion losses (> 15 dB / m) .5 The reasons for reduced optical losses of these fibers remains a challenge and asked PerfOS CVS and examine a new technology for producing chalcogenide glass preforms on the basis of casting process.6

A new method of preform production

In the molding process, made of pure glass rods were first constructed with the method of sealed quartz tube. The various components are expressed in a silica ampoule under vacuum for several hours. The glass is then synthesized at a temperature of 800 ° C for 12 hours in an oven rocking chair. It is then soaked in water and dipped the glass transition temperature Tg (several hundred degrees Celsius). Other specific treatments such as thermal distillation are necessary to remove impurities such as carbon and water.

The form, which is entirely silica containing silica in silica capillaries in a hexagonal threaded leaders silica ampoule. A glass rod is first placed on top form and is then heated, almost liquid. It must be sufficiently flexible to silica in the flow of Mold. Once the glass is in the form, the tube is then dipped into the air and tempered to d. The silica capillaries are removed by applying a hydrofluoric acid (see Figure 1) ..

The focus of the process is the thickness of silica capillaries. If they are too big, they move from broken glass, because the coefficient of thermal expansion of silicon is two orders of magnitude smaller than in chalcogenide glasses. However, if the thickness capillary is less than 20 hours, the capillaries are sufficiently flexible to avoid breakage. The molding process allows CPCF different designs with different types of fibers in suspension base and a common approach with a panel of several circles of holes (see Figure 2). This process improves the optical transmission of CPCFs.

We measure the attenuation of a multi-six-hole arsenic selenide (As) are suspended-core fiber (core diameter, background = 16 mu m) with a Fourier infrared spectroscopy (FTIR) analyzer and the ‘indium antimonide (InSb) detector and was converted to 0.75 dB / m to 1.55 mu m and 0.3 dB / m over 3.5 hours. These values are very close to sinking material losses. This fiber design is particularly suited for applications of sensors for detecting the absorption of various chemical species in the infrared range, such as carbon dioxide (CO2) is based on 4.3 or methane ( CH4) by 3, for example. Recently CPCFs have succeeded in spectroscopy systems evanescent wave fiber used to detect CO2 gas.7

Another fiber design with several circles of holes were of a size and arrangement of holes developed in front, allowing a system of endless single mode propagation. The glass composition was chosen tellurium, arsenic selenide (TeAsSe), which shows good optical transmission in the spectral range from 2-11 hours. The question of the guided mode profiles at 3.39 m to 9.3 mu m mu have been using one of the near field measurements to confirm the single-mode fiber. Such designs could contribute to the signals used mid-IR nulling interferometer for the filter space.

A third project is subject to a basic AsSe three holes preform the production of fiber optics allows a very small core (diameter = 3, loss = 0.9 dB / m to 1.55 mu m) for nonlinear applications. In particular, this interest in the design of supercontinuum generation in the mid-IR region has attracted. The wavelength of zero dispersion (ZDW) of chalcogenide glasses is beyond 4:00 pm, but it can be cut to less than 2 mu m with a suspended-core design.

Record-high nonlinearities

FOTON tested fiber exhibited three-hole-core applications around 1.55 μm.8 nonlinear effective area of the fiber is a far-field method; output intensity with a photodiode was moving in a circular orbit at a distance fixed fiber recorded for different angles of rotation (see Figure 3). To calculate the surface area, the far-field data for the near field using the inverse Hankel converts. With this method, a real space has been calculated at 1.7 ± 0.1 μm2 Arms This is proven one of the smallest values of appointment for a chalcogenide fiber.

For the same fibers, we measured nonlinear coefficient through the characterization of the enlargement of the spectrum by self-phase modulation (SPM) in the fiber. Duration 6 ps pulses at 1550 nm was injected into a fiber 60 cm long, optical input power (1.5W) monitor was recorded using a variable optical attenuator, while the experimental spectrum analyzer with a output optical spectrum was (see Figure 4). It has been compared to simulated spectrum, calculated by solving the nonlinear Schrödinger equation. The data showed good agreement between experimental and simulated spectra with a dispersion value of -1800 ps / km nm () and a record of nonlinear coefficients: 31 300 W-1km-1, the high potential of this fiber for nonlinear applications.

To make developed the best of our knowledge, the only way AsSe CPCF by molding glass PerfOS and Laboratory of Glass and Ceramics, University of Rennes has the highest nonlinear coefficient nontapered reported a value of fiber. Spectral broadening of SPM generated leads to the demonstration of effective nonlinear optical gate at 1.55 mu m Foton. These production advances should open the doors to a wide range of new applications CPCF.