Jas S. Sanghera

Chalcogenide Glass-Fiber-Based Mid-IR Sources and Applications

The Naval Research Laboratory (NRL) is developing chalcogenide glass fibers for applications in the mid-and long-wave IR wavelength regions from 2 to 12 mum. The chalcogen glasses (i.e., glasses based on the elements S, Se, and Te) are transparent in the IR, possess low phonon energies, are chemically durable, and can be drawn into fiber. Both conventional solid core/clad and microstructured fibers have been developed. Chalcogenide glass compositions have been developed that allow rare earth doping to enable rare-earth-doped fiber lasers in the IR. Also, highly nonlinear compositions have been developed with nonlinearities ~1000times silica that enables nonlinear wavelength conversion from the near IR to the mid-and long-wave IR. In this paper, we review rare-earth-doped chalcogenide fiber for mid-and long-wave IR lasers, and highly nonlinear chalcogenide fiber and photonic crystal fiber for wavelength conversion in the mid-and long-wave IR.

Applications of chalcogenide glass optical fibers

Abstract Chalcogenide-glass fibers based on sulfide, selenide, telluride and their rare-earth-doped compositions are being actively pursued worldwide. Great strides have been made in reducing optical losses using improved chemical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite this, chalcogenide-glass fibers are enabling numerous applications that include laser power delivery, chemical sensing, and imaging, scanning near field microscopy/spectroscopy, IR sources/lasers, amplifiers and optical switches.

Chemically Resolved Imaging of Biological Cells and Thin Films by Infrared Scanning Near-Field Optical Microscopy

The infrared (IR) absorption of a biological system can potentially report on fundamentally important microchemical properties. For example, molecular IR profiles are known to change during increases in metabolic flux, protein phosphorylation, or proteolytic cleavage. However, practical implementation of intracellular IR imaging has been problematic because the diffraction limit of conventional infrared microscopy results in low spatial resolution. We have overcome this limitation by using an IR spectroscopic version of scanning near-field optical microscopy (SNOM), in conjunction with a tunable free-electron laser source. The results presented here clearly reveal different chemical constituents in thin films and biological cells. The space distribution of specific chemical species was obtained by taking SNOM images at IR wavelengths (lambda) corresponding to stretch absorption bands of common biochemical bonds, such as the amide bond. In our SNOM implementation, this chemical sensitivity is combined with a lateral resolution of 0.1 micro m ( approximately lambda/70), well below the diffraction limit of standard infrared microscopy. The potential applications of this approach touch virtually every aspect of the life sciences and medical research, as well as problems in materials science, chemistry, physics, and environmental research.

10% Yb3+-Lu2O3 ceramic laser with 74% efficiency.

We demonstrate laser oscillation at 1080 nm with more than 16 W of output power and with an optical-to-optical slope efficiency of up to 74% using a 10% Yb3+ doped Lu2O3 ceramic made by hot pressing. This represents the highest output power and efficiency obtained for a Yb3+ doped Lu2O3 ceramic and demonstrates the feasibility for power scaling.

All-fiber mid-IR supercontinuum source from 1.5 to 5 µm

An all-fiber supercontinuum source extending from 1.5 to 5 μm has been demonstrated in single-mode step-index As2S3 fiber using a Raman shifted erbium doped mode-locked silica fiber laser pump source. 140 mW broadband power was demonstrated with a spectral intensity variation of 10 dB from 1.9 to 4.4 μm and 20 dB from 1.65 to 4.78 μm.

Development of chalcogenide glass fiber optics at NRL

Abstract We have fabricated unclad sulphide and telluride chalcogenide fibers with losses of 0.047 dB/m at 2.4 μm and 0.11 dB/m at 6.6 μm, respectively. Core/clad fibers with losses approaching 0.5 dB/m at 4.8 μm have been fabricated in long fiber lengths (~50 m). While theoretical minimum losses are estimated to be about 4 dB/km at around 5 μm for sulphide fibers, we have identified the absorption and scattering mechanisms which limit the practical losses. The bending strengths of these fibers have been measured and are satisfactory for short length applications. For longer lengths, both the loss and strength need to be improved.

BaOGa2O3GeO2 glasses with enhanced properties

Abstract Varying substitutions of In2O3 for Ga2O3 in the BaOGa2O3GeO2 glasses were made and their effects on the glass transition temperature, density, hardness, chemical durability and the infrared transmission were studied. In2O3 substitution for Ga2O3, in general, resulted in improvement in the physical properties of these glasses. The glass transition temperature, density and hardness increased with the substitution. The chemical durability of these glasses improved with the substitutions and the infrared (IR) cut-off edge shifted to longer wavelengths. Phase separation was observed in compositions with higher In2O3 substitutions. The observed changes in physical properties and phase separation with In2O3 substitutions are explained on the basis of a structural model suggesting that octahedrally coordinated In3+ replaces tetrahedrally coordinated Ga3+ in the glass structure. The observed results are in good agreement with the model which also explains the phase separation observed at higher In2O3 substitutions.

Anti-reflective surface structures for spinel ceramics and fused silica windows, lenses and optical fibers

Anti-reflective surfaces structures (ARSS) have been successfully fabricated on fused silica windows, lenses and fibers, and spinel ceramics. The reflection loss for spinel was reduced from 7% per surface to 0.9%. For fused silica with ARSS, the reflection loss was reduced to 0.02% near 1 µm. Pulsed laser damage thresholds at 1.06 µm were measured and thresholds as high as 100 J/cm2 were obtained for fused silica windows of up to 10 cm in diameter with ARSS and 850 J/cm2 for silica fibers with ARSS on the end faces. Spinel samples with ARSS showed damage thresholds more than two times higher than that of spinel with traditional AR coatings.

Fabrication of microlenses in bulk chalcogenide glass

Abstract A new method for fabricating microlenses in bulk, glassy Ge 20 Se 80 is demonstrated. Based upon a laser-induced thermal runaway effect, the process has been used to fabricate lenses with diameters between 70 and 180 μm and numerical apertures of about 0.4.

Chalcogenide glass fiber-based MID-IR sources and applications

The Naval Research Laboratory (NRL) is developing chalcogenide glass fibers for applications in the mid-and long-wave IR wavelength regions from 2 to 12 mum. The chalcogen glasses (i.e., glasses based on the elements S, Se, and Te) are transparent in the IR, possess low phonon energies, are chemically durable, and can be drawn into fiber. Both conventional solid core/clad and microstructured fibers have been developed. Chalcogenide glass compositions have been developed that allow rare earth doping to enable rare-earth-doped fiber lasers in the IR. Also, highly nonlinear compositions have been developed with nonlinearities ~1000times silica that enables nonlinear wavelength conversion from the near IR to the mid-and long-wave IR. In this paper, we review rare-earth-doped chalcogenide fiber for mid-and long-wave IR lasers, and highly nonlinear chalcogenide fiber and photonic crystal fiber for wavelength conversion in the mid-and long-wave IR.