Chalcogenide glasses appear as good candidates to build all optical gas sensors due to their wide infrared transparency and the possibility of incorporate rare earth active in MWIR spectral range. To detect and quantify gases, one way is to develop chalcogenide glasses presenting transparency compatible with the molecules absorption band frequency. Two domains of interest can be distinguished: MWIR and LWIR corresponding respectively to the 3–5 μm and 8–12 μm spectral ranges. Selenide and sulfide based chalcogenide glasses are known for their excellent infrared transmission properties in the 1-15 µm region with good thermo-mechanical properties. Doped with Dy3+or Pr3+, sulfide glass fibers have been used as MWIR source for gas sensor for CO2 detection. To probe the far infrared beyond 12 µm, telluride chalcogenide glasses appear as a very interesting material due to it low phonon energy and a broad transparency (up to 25 µm). While these attractive optical properties of telluride glasses, particularly for LWIR, there is few study about rare earth incorporation for luminescence explained a challenging synthesis process avoiding crystallization. To get more stability in the glass state it is essential to add selenium. Thus for each system, it is required to determine the best compromise between the transparency domain and the glass state stability by playing on the ratio between selenium and tellurium atoms. Regarding the energy level of Tb3+, we can expect to have a radiative emission from 3.1 µm up to 8 µm. For gas sensor application, it is a range of interest regarding the LWIR absorption band of some hazardous gases. Thus, Tb3+ doped chalcogenide glasses with nominal composition of Ga5Ge20Sb10Se(65-x)Tex (x = 0, 10, 20, 25, 30, 32.5, 35, 37.5) were synthetized. Their physico-chemical properties (chemical composition, density, thermal characteristics) and optical properties (transmission and ellipsometry spectroscopies) are clearly modified by tellurium substitution to selenium. Based on a detailed study of the Ga5Ge20Sb10Se(65-x)Tex bulk glass and fiber properties, the optimal composition of seleno-telluride glass fiber was found to be Ga5Ge20Sb10Se45Te20. The luminescence properties of Tb3+ (500, 1000 and 1500 ppm) doped Ga5Ge20Sb10Se65 and Ga5Ge20Sb10Se45Te20 were studied in glass bulk and fiber samples. Radiative transitions calculated from Judd-Ofelt (J-O) theory were compared to the experimental values. Although an expected lower phonon energy for telluride glasses, selenide glasses stay more suitable for MWIR emission with a strong emission at 4.8 and 3.1 µm. The emission at 8 µm was successfully observed with careful luminescence investigations.
The 2-15 μm spectral range hosts many optical sensing applications from biology to environmental monitoring, and infrared spectroscopy is a simple and reliable way to provide fast and in-situ analysis method. Rare-earth ion emissions within chalcogenide glasses with low phonon energies proved to be efficient to address mid-IR luminescence based sensing applications. In particular, they give promising results for the development of all-optical gas sensors in the 3 to 5 μm spectral range based on IR conversion into visible light using rare earth excited state absorption mechanisms. In this article, we report the wavelength conversion of 3.4 μm radiation into 660 nm in Er3+:KPb2Cl5 bulk crystal, Er3+:Ga5Ge20Sb10S65 and Er3+:Ga5Ge20Sb5S70 glasses using an excited state absorption process. This wavelength conversion is the result of the excitation of Er3+ ions following the excited state absorption of IR photons and the Er3+ ions subsequent spontaneous emission in the visible domain. Using an 808 nm pumping, a 3.4 μm photon excited state absorption gives rise to a 660 nm emission. This wavelength conversion device can be further implemented for methane all-optical sensing at 3.4 μm, for the development of remote “all-optical” methane mid-IR sensors with only visible and near-IR input and output signals. This “all-optical concept” enables the use of silica fibers over large distances, thus considerably increasing the scope of possible applications.
A review of our work on all-optical gas sensors is presented with an emphasis on the development of both new infrared (IR) sources and IR to visible converters. Many radicals spectroscopic signatures associated to gases of interest are in the 2.5 -15 μm spectral range (4000-350 cm-1). This spectral domain matches rare-earth ions emissions when embedded into chalcogenide glasses which are well- known for having low phonon energies. We present here results concerning the development of IR sources and IR to visible converters based on rare earth doped chalcogenide fibers. The development of all-optical gas sensors in the 3 to 5 µm spectral range is described showing IR signal conversion into visible light using specific excited state absorption mechanisms in rare earth doped materials. This wavelength conversion enables the use of silica fibers to transport the “gas” signal over large distances considerably increasing the scope of possible applications. An example of all-optical sensor using this photon conversion is presented in the case of CO2 detection. The implementation of this type of sensor for different gases such as methane is finally discussed. This all-optical sensor can be typically used over a kilometer range, with sensitivity around hundreds of ppm with cost effective detection heads, making this tool suitable for field operations. Finally, the photon conversion at the heart of this all-optical sensor is discussed as a general mean to detect infrared radiations avoiding the use of infrared detectors for a large span of applications.
Luminescence properties of Pr3+ and Dy3+ doped GaGeSbSe(S) vitreous systems have been studied. The synthesis process to obtain homogeneous glasses has been determined and fibers have been successfully drawn from the produced preforms and characterized. Fibers show a mid-IR luminescence matching with the CO2 absorption band at 4.3 μm and can be used in an environmental monitoring sensor for the CO2 underground storage. The luminescence and glasses properties have been investigated on bulk samples and fibers in order to improve the efficiency of an optical CO2 sensor prototype operating from high to low concentration, down to the ppm level.
The design of two pumping schemes for mid-IR lasers based on photonic crystal fibers (PCFs) is illustrated. The PCFs considered in both pumping schemes are made of dysprosium-doped chalcogenide glass Dy3+:Ga5Ge20Sb10S65. The two optical sources are accurately simulated by taking into account the spectroscopic parameters measured on a rare earth-doped glass sample. A home-made numerical model based on power propagation equations and solving the ion population rate equations of the rare earth is developed and employed to perform a feasibility investigation. The first pumping scheme is based on optical power pumping at 1700 nm wavelength and allows beam emission close to 4400 nm wavelength, the efficiency is increased till about η = 22% by integrating a suitable optical amplifier after the laser cavity. The second pumping scheme exploits two pump beams at the wavelengths close to 2800 nm and 4100 nm and enables a laser emission close to 4400 nm wavelength with an efficiency higher than η = 30%. Both these sources could promote a number of promising applications in different areas such as satellite remote sensing, laser surgery, chemical/biological spectroscopy and mid-IR optical communication.
High optical quality rare-earth-doped LiYF4 (YLF) epitaxial layers were grown on pure YLF substrates by liquid phase epitaxy (LPE). Thulium, praseodymium and ytterbium YLF crystalline waveguides co-doped with gadolinium and/or lutetium were obtained. Spectroscopic and optical characterization of these rare-earth doped waveguides are reported. Internal propagation losses as low as 0.11 dB/cm were measured on the Tm:YLF waveguide and the overall spectroscopic characteristics of the epitaxial layers were found to be comparable to bulk crystals. Laser operation was achieved at 1.87 μm in the Tm3+ doped YLF planar waveguide with a very good efficiency of 76% with respect to the pump power. Lasing was also demonstrated in a Pr3+ doped YLF waveguide in the red and orange regions and in a Yb3+:YLF planar waveguide at 1020 nm and 994 nm.