Generation of near-diffraction-limited, high-power supercontinuum from 1.57 µm to 12 µm with cascaded fluoride and chalcogenide fibers.

We generate a supercontinuum (SC) spectrum ranging from 1.57 µm to 12 µm (20 dB bandwidth) with a soft glass fiber cascade consisting of ZrF4-BaF2-LaF3-AlF3-NaF fiber, As2S3 fiber, and As2Se3 fiber pumped by a nanosecond thulium master oscillator power amplifier system. The highest on-time average power generated is 417 mW at 33% duty cycle. We observe a near-diffraction-limit beam quality across the wavelength range from 3 µm to 12 µm, even though the As2Se3 fiber is multimode below 12 µm. Our study also shows that parameters of the As2Se3 fiber, such as numerical aperture, core size, and core/cladding composition, have significant effects on the long wavelength edge of the generated SC spectrum. Our results suggest that the high numerical aperture of 0.76 and low-loss As2Se3/GeAs2Se5 core/cladding material all contribute to broad SC generation in the long-wave infrared spectral region. Also, among our results, 10 µm core diameter selenide fiber yields the best spectral expansion, while the 12 µm core diameter selenide fiber yields the highest output power.

Mid-infrared supercontinuum generation from 1.6 to >11 µm using concatenated step-index fluoride and chalcogenide fibers.

We demonstrate an all-fiber supercontinuum source that generates a continuous spectrum from 1.6 µm to >11 µm with 417 mW on-time average power at 33% duty cycle. By utilizing a master oscillator power amplifier pump with three amplification stages and concatenating solid core ZBLAN, arsenic sulfide, and arsenic selenide fibers, we shift 1550 nm light to ∼4.5 µm, ∼6.5 µm, and >11 µm, respectively. With 69 mW past 7.5 µm, this source provides both high power and broad spectral expansion, while outputting a single fundamental mode.

Field tests for round-trip imaging at a 1.4 km distance with change detection and ranging using a short-wave infrared super-continuum laser.

Field tests have been conducted of a broadband illuminator for active hyperspectral imaging (HSI) using a short-wave infrared supercontinuum laser (SWIR-SCL). We demonstrated irradiance comparable to the sun for two-way measurements at a 1.4 km distance between laser and target, and performed change detection and ranging. The experimental results suggest that the range resolution of our method is ∼1.5 cm even at the 1.4 km distance. Hence, we demonstrated the possibility to perform HSI with active broadband illumination using the SWIR-SCL. To our knowledge, this experiment is the first-ever to test two-way propagation of the active HSI illumination over a long distance. The 64 W SWIR-SCL provides near sunlight-equivalent illumination over multiple square meters, and the laser could enable HSI 24 h a day, even under a cloud cover, as well as enhanced capabilities such as change detection and ranging.

Field trial of active remote sensing using a high-power short-wave infrared supercontinuum laser.

Field trial results of a 5 W all-fiber broadband supercontinuum (SC) laser covering the short-wave infrared (SWIR) wavelength bands from ~1.55 to 2.35 µm are presented. The SC laser is kept on a 12 story tower at the Wright Patterson Air Force Base and propagated through the atmosphere to a target 1.6 km away. Beam quality of the SC laser after propagating through 1.6 km is studied using a SWIR camera and show a near diffraction limited beam with an M(2) value of <1.3. The SC laser is used as the illumination source to perform spectral reflectance measurements of various samples at 1.6 km, and the results are seen to be in good agreement with in-lab measurements using a conventional lamp source. Spectral stability measurements are performed after atmospheric propagation through 1.6 km and show a relative variability of ~4%-8% across the spectrum depending on the atmospheric turbulence effects. Spectral stability measurements are also performed in-lab and show a relative variability of <0.6% across the spectrum.

Power scalable >25 W supercontinuum laser from 2 to 2.5 µm with near-diffraction-limited beam and low output variability.

A power scalable thulium-doped fiber-amplifier-based supercontinuum (SC) laser covering the shortwave infrared region from 2 to 2.5 µm is demonstrated. The SC laser has an average power up to 25.7 W and a spectral density of >12 dBm/nm. Power scalability of the laser is proven by showing that the SC laser maintains a nearly constant spectral output, beam quality (M(2) measurements), and output spectral stability as the SC average power is scaled from 5 to 25.7 W average output power. We verify that the SC laser beam is nearly diffraction limited with an M(2)<1.2 for all power levels. Output spectral stability measurements with power scaling show a radiometric variability of <0.8% across the entire SC spectrum.

Stand-off detection of solid targets with diffuse reflection spectroscopy using a high-power mid-infrared supercontinuum source.

We measure the diffuse reflection spectrum of solid samples such as explosives (TNT, RDX, PETN), fertilizers (ammonium nitrate, urea), and paints (automotive and military grade) at a stand-off distance of 5 m using a mid-infrared supercontinuum light source with 3.9 W average output power. The output spectrum extends from 750-4300 nm, and it is generated by nonlinear spectral broadening in a 9 m long fluoride fiber pumped by high peak power pulses from a dual-stage erbium-ytterbium fiber amplifier operating at 1543 nm. The samples are distinguished using unique spectral signatures that are attributed to the molecular vibrations of the constituents. Signal-to-noise ratio (SNR) calculations demonstrate the feasibility of increasing the stand-off distance from 5 to ~150 m, with a corresponding drop in SNR from 28 to 10 dB.

Photothermolysis of sebaceous glands in human skin ex vivo with a 1,708 nm Raman fiber laser and contact cooling.

BACKGROUND AND OBJECTIVES: Wavelengths near ∼1,720 nm are of interest for targeting fat/lipid-rich tissues due to the high absorption coefficient of human fat and low water scattering and absorption. In this study, a 1,708 nm laser was built and shown to selectively target fat/lipid adjacent to porcine heart and dermis and then used to damage dermal sebaceous glands in human skin. STUDY DESIGN AND MATERIALS: An all-fiber 1,708 nm laser with ∼4 W maximum power was designed and built. Selectivity for targeting fat/lipid was studied by exposing porcine heart and skin tissue cross-sections to the 1,708 nm laser. Human skin treatments to damage sebaceous glands were performed both with and without cold window cooling. Histochemical evaluation on the frozen sections was performed using methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay. RESULTS: Histochemical analysis of porcine tissue cross-sections showed that 1,708 nm laser can selectively damage pericardial fat(heart) and subcutaneous fat(skin) with little to no damage to the myocardium and the dermis, respectively. In human skin, histochemical evaluation without contact cooling showed damage to both epidermis and dermis. With cooling, epidermis was spared and damage was observed in dermis extending ∼0.4-1.65 mm from the skin surface at an average laser fluence of ∼80 J/cm(2). Selective damage of sebaceous glands was suggested but not definitively demonstrated. CONCLUSIONS: We have developed an all-fiber 1,708 nm laser capable of damaging majority of the sebaceous glands in the dermis and thus may have potential applications in the treatment of conditions such as acne vulgaris whose pathophysiology involves disorders of sebaceous glands.

Mid-infrared absorption spectroscopy and differential damage in vitro between lipids and proteins by an all-fiber-integrated supercontinuum laser.

We identify and differentially damage lipids and proteins using wavelengths between 2.6 and 3.8 mum from a fiber-based supercontinuum (SC) laser. Absorption spectroscopy of the constituents of normal artery and atherosclerotic plaque, including adipose tissue, macrophages and foam cells, are measured by a SC laser in the mid-infrared. By using the laser light within the C-H fatty acid and cholesterol esters absorption band, we also demonstrate differential damage of lipid-rich adipose tissue without damaging the protein-rich blood vessel wall. The experiments use a novel SC laser that is all-fiber-integrated with no moving parts, covers a continuous spectrum ranging from approximately 0.8 to beyond 4.2 microm, and outputs a time-averaged power scalable up to 10.5 W.