The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable interest due to their superior physicochemical features, such as large anti-Stokes shifts, low autofluorescence background, low toxicity and high penetration depth, which make them extremely suitable for use as alternatives to conventional downshifting luminescence bioprobes like organic dyes and quantum dots for various biological applications. A fundamental understanding of the photophysics of lanthanide-doped UCNPs is of vital importance for discovering novel optical properties and exploring their new applications. In this review, we focus on the most recent advances in the development of lanthanide-doped UCNPs as potential luminescent nano-bioprobes by means of our customized lanthanide photophysics measurement platforms specially designed for upconversion luminescence, which covers from their fundamental photophysics to bioapplications, including electronic structures (energy levels and local site symmetry of emitters), excited-state dynamics, optical property designing, and their promising applications for in vitro biodetection of tumor markers. Some future prospects and efforts towards this rapidly growing field are also envisioned.

Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection

Nonlinear Optical Crystals: A Complete Survey

Within the field of spectral imaging, the vast majority of instruments used are scanning devices. Recently, several snapshot spectral imaging systems have become commercially available, providing new functionality for users and opening up the field to a wide array of new applications. A comprehensive survey of the available snapshot technologies is provided, and an attempt has been made to show how the new capabilities of snapshot approaches can be fully utilized.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-52/issue-9/090901/Review-of-snapshot-spectral-imaging-technologies/10.1117/1.OE.52.9.090901.pdf

Review of snapshot spectral imaging technologies

Autonomous navigation in cross-country environments presents many new challenges with respect to more traditional, urban environments. The lack of highly structured components in the scene complicates the design of even basic functionalities such as obstacle detection. In addition to the geometric description of the scene, terrain typing is also an important component of the perceptual system. Recognizing the different classes of terrain and obstacles enables the path planner to choose the most efficient route toward the desired goal.

This paper presents new sensor processing algorithms that are suitable for cross-country autonomous navigation. We consider two sensor systems that complement each other in an ideal sensor suite: a color stereo camera, and a single axis ladar. We propose an obstacle detection technique, based on stereo range measurements, that does not rely on typical structural assumption on the scene (such as the presence of a visible ground plane)s a color-based classification system to label the detected obstacles according to a set of terrain classess and an algorithm for the analysis of ladar data that allows one to discriminate between grass and obstacles (such as tree trunks or rocks), even when such obstacles are partially hidden in the grass. These algorithms have been developed and implemented by the Jet Propulsion Laboratory (JPL) as part of its involvement in a number of projects sponsored by the US Department of Defense, and have enabled safe autonomous navigation in high-vegetated, off-road terrain.

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Obstacle Detection and Terrain Classification for Autonomous Off-Road Navigation

For pt.I see ibid., vol.32, no.1, pp.92-103 (1996). Laser performance of Ho:Tm:YLF and Ho:Tm:YAG lasers are compared under a wide variety of experimental situations. Laser diode pumping is simulated using a Cr:BeAl/sub 2/O/sub 4/ laser tuned to the absorption peak of the Tm/sup 3/H/sub 4/ manifold for the respective laser material. Laser performance is characterized by experimentally determining the threshold and slope efficiency, and compared with predicted results of a rate equation model developed in the companion paper. Energy transfer parameters for the rate equation model are calculated using the theory developed in that paper. For these experiments, four different pump pulselengths, six different output mirror reflectivities, different Ho concentrations and different length laser materials were evaluated. Using Ho:Tm:YAG an optical slope efficiency of 0.36 was achieved while with Ho:Tm:YLF an optical slope efficiency of 0.50 was reached.

Ho:Tm lasers. II. Experiments

An optical efficiency of 0.052 under normal mode operation for diode-pumped Ho:Tm:YLiF4 at a pulse repetition frequency of 10 Hz has been achieved. Laser output energy of 30 mJ in single Q-switched pulses with 600-ns pulse length were obtained for an input energy of 3 J. A diffusion-bonded birefringent laser rod consisting of Ho:Tm-doped and undoped pieces of YLF was utilized for 10-Hz operation.

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Diode-pumped long-pulse-length Ho:Tm:YLiF(4) laser at 10 Hz.

Direct diode pumped Ho:YAG generated laser pulses at 2.12 μm with an optical to optical slope efficiency of 0.24. Ho:YAG and Ho:LuAG laser rods were evaluated with both wide and narrow bandwidth pump diodes. The laser wavelength varies with the level of pumping and optical design. This variation was found to be predictable. Second harmonic at 1.06 μm was produced in a 6.0 mm long BBO crystal.

Diode pumped Ho:YAG and Ho:LuAG lasers, Q-switching and second harmonic generation

A ZnGeP2 parametric amplifier was constructed, and the small-signal gain of a 3.39-μm He–Ne laser in excess of 10 was measured. With a 2.06-μm Ho:Tm:Er:YLF laser having a nominal 50-ns pulse length as a pump, small-signal gain was measured as a function of pump power. When compared with the predicted gain, taking into account the beam profile of both the pump and the signal, good agreement was found at low pump powers. However, the observed gain was less than predicted at higher pump powers. Pump-induced loss at 3.39 μm, a phenomenon not observed in other nonlinear crystals such as AgGeSe2, causes the difference. Pump-induced loss was observed when the ZnGeP2 crystal was tilted away from the phase-matching angle. Pump-induced loss at 3.39 μm was characterized and found to be directly proportional to the pump energy.

ZnGeP 2 parametric amplifier

AgGaSe(2) has been grown, annealed, and characterized for the mid-IR. Characterization includes measurement of the average power-limiting factors including absorption and the variation of the refractive indices with temperature. Using specially annealed crystals 20 mm long and a Ho:YAG pump, parametric amplification at 3.39 microm has achieved a gain of 2.9 with a peak power input of only 8 MW/cm(2).

Norman P. Barnes

Papers

Ho:Tm lasers. II. Experiments

Diode-pumped long-pulse-length Ho:Tm:YLiF(4) laser at 10 Hz.

Diode pumped Ho:YAG and Ho:LuAG lasers, Q-switching and second harmonic generation

ZnGeP 2 parametric amplifier

Parametric amplification in AgGaSe 2