Norman C. Anheier

Single-mode low-loss chalcogenide glass waveguides for the mid-infrared

We demonstrate the design, fabrication, and characterization of single-mode low-loss waveguides for mid-infrared (MIR) wavelengths. Planar waveguide structures were fabricated from multilayer thin films of arsenic-based chalcogenide glasses followed by the creation of channel waveguides by using the photodarkening effect. Propagation losses as low as 0.5 dB/cm were measured for a quantum cascade laser end-fire coupled into the waveguides. This is a first step toward the design and fabrication of integrated optical components for MIR applications.

First fringes with an integrated-optics beam combiner at 10 μm - A new step towards instrument miniaturization for mid-infrared interferometry

Context. Observations of milliarcsecond-resolution scales and high dynamic range hold a central place in the exploration of distant planetary systems in order to achieve, for instance, the spectroscopic characterization of exo-Earths or the detailed mapping of their protoplanetary disc birthplace. Multi-aperture infrared interferometry, either from the ground or from space, is a very powerful technique to tackle these goals. However, significant technical efforts still need to be undertaken to achieve a simplification of these instruments if we wish to recombine the light from a large number of telescopes. Integrated-optics concepts appear to be a suitable alternative to the current conventional designs, especially if their use can be extended to a higher number of astronomical bands. Aims. This article reports, for the first time to our knowledge, the experimental demonstration of the feasibility of an integrated-optics approach to mid-infrared beam combination for single-mode stellar interferometry. Methods. We fabricated a two-telescope beam combiner prototype integrated on a substrate of chalcogenide glass, a material transparent from ∼1 μ mt o∼14 μm. We developed laboratory tools to characterize in the mid-infrared the modal properties and the interferometric capabilities of our device. Results. We obtain interferometric fringes at 10 μm and measure a mean contrast V = 0.981 ± 0.001 with high repeatability over one week and high stability over a time-period of ∼5 h. We show experimentally – as well as on the basis of modeling considerations – that the component has a single-mode behavior at this wavelength, which is essential to achieve high-accuracy interferometry. From previous studies, the propagation losses are estimated to be 0.5 dB/cm for this type of component. We also discuss possible issues that may impact the interferometric contrast. Conclusions. The IO beam combiner performs well at the tested wavelength. We also anticipate the requirement of a closer matching between the numerical apertures of the component and the (de)coupling optics to optimize the total throughput. The next step foreseen is the achievement of wide-band interferograms.

Measurement of the refractive index dispersion of As2Se3 bulk glass and thin films prior to and after laser irradiation and annealing using prism coupling in the near- and mid-infrared spectral range

The prism coupling technique has been utilized to measure the refractive index in the near- and mid-IR spectral region of chalcogenide glasses in bulk and thin film form. A commercial system (Metricon model 2010) has been modified with additional laser sources, detectors, and a new GaP prism to allow the measurement of refractive index dispersion over the 1.5-10.6 μm range. The instrumental error was found to be ±0.001 refractive index units across the entire wavelength region examined. Measurements on thermally evaporated AMTIR2 thin films confirmed that (i) the film deposition process provides thin films with reduced index compared to that of the bulk glass used as a target, (ii) annealing of the films increases the refractive index of the film to the level of the bulk glass used as a target to create it, and (iii) it is possible to locally increase the refractive index of the chalcogenide glass using laser exposure at 632.8 nm.

Chalcogenide glasses and structures for quantum sensing

Chalcogenide glasses are formed by combining chalcogen elements with IV-V elements. Among the family of glasses, As2S3, and As2Se3 are important infrared (IR) transparent materials for a variety of applications such as IR sensors, waveguides, and photonic crystals. With the promise of accessibility to any wavelengths between 3.5 and 16 μm using tunable quantum cascade lasers (QCL) and chalcogenides with IR properties that can be compositionally adjusted, ultra-sensitive, solid-state, photonic-based chemical sensing in mid-wave IR region is now possible. Pacific Northwest National Laboratory (PNNL) has been developing quantum cascade lasers (QCLs), chalcogenides, and all other components for an integrated approach to chemical sensing. Significant progress has been made in glass formation and fabrication of different structures at PNNL. Three different glass-forming systems, As-S, As-S-Se, and As-S-Ag have been examined for this application. Purification of constituents from contaminants and thermal history are two major issues in obtaining defect-free glasses. We have shown how the optical properties can be systematically modified by changing the chemistry in As-S-Se system. Different fabrication techniques need to be employed for different geometries and structures. We have successfully fabricated periodic arrays and straight waveguides using laser-writing and characterized the structures. Wet-chemical lithography has been extended to chalcogenides and challenges identified. We have also demonstrated holographic recording or diffraction gratings in chalcogenides.

Simultaneous microscopic measurements of photodarkening and photoexpansion in chalcogenide films

A near-field scanning optical microscopic analysis is performed on As2S3 film gratings in order to simultaneously collect index and topography images with sub-micrometre resolution. This technique makes it possible to unambiguously study the correlation between photodarkening and photoexpansion at the local scale. The development of a positive index change concomitantly with expansion or contraction in films of different thermal histories suggests that homopolar bonds are not a major contributor to the darkening effect. Photodarkening is instead associated with structurally stable defects that appear to be largely decoupled from the volume change mechanism. While photoexpansion and photodarkening follow the same growth kinetic during irradiation of annealed films with band-gap light, it is clearly shown that their structural origin is different. These results have relevance for grating fabrication since both the relief and the index patterns contribute to the grating efficiency, yet they appear to have distinct behaviour during processing or long-term stability.

Emission and Propagation Properties of Midinfrared Quantum Cascade Lasers

We report divergence, astigmatism, and beam propagation factor (M2) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 mum. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62deg and 32degplusmn2deg for the fast and slow axes, respectively. Diffraction-limited Ge aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure M2 yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 mum, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M2 reported for midinfrared QCLs.

Advances in the Development of Mid-Infrared Integrated Devices for Interferometric Arrays

This article reports the advances on the development of mid-infrared integrated optics for stellar interferometry. The devices are fabricated by laser writing techniques on chalcogenide glasses. Laboratory characterizaton is reported and analyzed.

Compact quantum cascade laser transmitter

In this paper we present design considerations, thermal and optical modeling results, and device performance for a ruggedized, compact laser transmitter that utilizes a room temperature quantum cascade (QC) laser source. The QC laser transmitter is intended for portable mid-infrared spectroscopy applications, where the 3 to 5 μm and 8 to 12 μm atmospheric transmission window is relatively free of water vapor interference and where the molecular rotational vibration absorption features can be used to detect and uniquely identify chemical compounds of interest. Initial QC laser-based sensor development efforts were constrained by the complications of cryogenic operation. However, improvements in both QC laser designs and fabrication processes have provided room-temperature devices that now enable significant miniaturization and integration potential for national security, environmental monitoring, atmospheric science, and industrial safety applications.

Rapid assessment of mid-infrared refractive index anisotropy using a prism coupler: chemical vapor deposited ZnS

A state-of-the-art mid-infrared prism coupler was used to study suspected anisotropy in the refractive index of forward-looking-infrared grade chemical vapor deposited (CVD) zinc sulfide. Samples were prepared with columnar grain structure in and perpendicular to the sample plane, as well as from different depths in the CVD growth body. This study was motivated by the growing industry concern among optical design engineers, as well as developers of mid-infrared systems, over the reliability of historically accepted index data. Prior photoluminescence and x-ray diffraction measurements have suggested that refractive index may vary according to sample orientation with respect to the grain structure. Measurements were conducted to provide optical dispersion and thermal index (dn/dT) data at discrete laser wavelengths between 0.633 and 10.591 μm at two temperature set points (30 °C and 90 °C). Refractive index measurements between samples exhibited an average standard deviation comparable to the uncertainty of the prism coupler measurement (0.0004 refractive index units), suggesting that the variation in refractive index as a function of sample orientation and CVD deposition time is negligible and should have no impact on subsequent optical designs. Measured dispersion data at mid-infrared wavelengths were also found to agree well with prior published measurements.

Quantum cascade transmitters for ultrasensitive chemical agent and explosives detection

The small size, high power, promise of access to any wavelength between 3.5 and 16 microns, substantial tuning range about a chosen center wavelength, and general robustness of quantum cascade (QC) lasers provide opportunities for new approaches to ultra-sensitive chemical detection and other applications in the mid-wave infrared. PNNL is developing novel remote and sampling chemical sensing systems based on QC lasers, using QC lasers loaned by Lucent Technologies. In recent months laboratory cavity-enhanced sensing experiments have achieved absorption sensitivities of 8.5 x 10-11 cm-1 Hz-1/2, and the PNNL team has begun monostatic and bi-static frequency modulated, differential absorption lidar (FM DIAL) experiments at ranges of up to 2.5 kilometers. In related work, PNNL and UCLA are developing miniature QC laser transmitters with the multiplexed tunable wavelengths, frequency and amplitude stability, modulation characteristics, and power levels needed for chemical sensing and other applications. Current miniaturization concepts envision coupling QC oscillators, QC amplifiers, frequency references, and detectors with miniature waveguides and waveguide-based modulators, isolators, and other devices formed from chalcogenide or other types of glass. Significant progress has been made on QC laser stabilization and amplification, and on development and characterization of high-purity chalcogenide glasses, waveguide writing techniques, and waveguide metrology.