Vinay V. Alexander

Supercontinuum generation from ~1.9 to 4.5 μmin ZBLAN fiber with high average power generation beyond 3.8 μm using a thulium-doped fiber amplifier

A mid-IR supercontinuum (SC) fiber laser based on a thulium-doped fiber amplifier (TDFA) is demonstrated. A continuous spectrum extending from ∼1.9 to 4.5 μm is generated with ∼0.7 W time-average power in wavelengths beyond 3.8 μm. The laser outputs a total average power of up to ∼2.6 W from ∼8.5 m length of ZrF4─BaF2─LaF3─AlF3─NaF (ZBLAN) fiber, with an optical conversion efficiency of ∼9% from the TDFA pump to the mid-IR SC. Optimal efficiency in generating wavelengths beyond 3.8 μm is achieved by reducing the losses in the TDFA stage and optimizing the ZBLAN fiber length. We demonstrate a novel (to our knowledge) approach of generating modulation instability-initiated SC starting from 1.55 μm by splitting the spectral shifting process into two steps. In the first step, amplified approximately nanosecond-long 1.55 μm laser diode pulses with ∼2.5 kW peak power generate a SC extending beyond 2.1 μm in ∼25 m length of standard single-mode fiber (SMF). The ∼2 μm wavelength components at the standard SMF output are amplified in a TDFA and coupled into ZBLAN fiber leading to mid-IR SC generation. Up to ∼270 nm SC long wavelength edge extension and ∼2.5× higher optical conversion efficiency to wavelengths beyond 3.8 μm are achieved by switching an Er:Yb-based power amplifier stage with a TDFA. The laser also demonstrates scalability in the average output power with respect to the pulse repetition rate and the amplifier pump power. Numerical simulations are performed by solving the generalized nonlinear Schrodinger equation, which show the long wavelength edge of the SC to be limited by the loss in ZBLAN.

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.

Power adjustable visible supercontinuum generation using amplified nanosecond gain-switched laser diode

Supercontinuum covering 0.45-1.2 mum is scaled from 250-740 mW by varying the repetition rate of a frequency doubled telecom laser diode. Efficient SC generation requires minimal non-linearity in the amplifier and fiber dispersion matched to the pump.

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

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.

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.

Modeling, development, and testing of a shortwave infrared supercontinuum laser source for use in active hyperspectral imaging

A fundamental limitation of current visible through shortwave infrared hyperspectral imaging systems is the dependence on solar illumination. This reliance limits the operability of such systems to small windows during which the sun provides enough solar radiation to achieve adequate signal levels. Similarly, nighttime collection is infeasible. This work discusses the development and testing of a high-powered super-continuum laser for potential use as an on-board illumination source coupled with a hyperspectral receiver to allow for day/night operability. A 5-watt shortwave infrared supercontinuum laser was developed, characterized in the lab, and tower-tested along a 1.6km slant path to demonstrate propagation capability as a spectral light source.

Surface roughness measurement of flat and curved machined metal parts using a near infrared super-continuum laser

We describe a system for performing high-accuracy, noncontact rms roughness measurements of flat and curved machined parts in the industrially relevant range of ∼0.05 to 0.35 μm. The system uses a near infrared (NIR) super-continuum laser to measure the intensity of specular reflection versus wavelength, at relatively long (∼1 m) stand-off distances and has the potential to be used in high speed, in-line manufacturing applications. The surface roughness value is extracted from the slope of the normalized specular intensity using the Beckmann-Kirchhoff (BK) model. According to the BK model, the normalized specular intensity in the NIR mostly depends on the surface roughness parameter alone and is independent of the absolute reflectance due to the normalization process. We discuss the benefits of performing the reflectance measurements in the NIR versus the commonly used visible spectrum. These include measurements at lower angles of incidence and the lack of need for a reference of the same metal composition. The roughness measurements performed by this system are in very good agreement with comparative data from a stylus profilometer and a white light interferometer. A potential industrial application is also demonstrated where the system is used to detect polishing defects in automotive engine crankshaft journals.

Renal denervation using focused infrared fiber lasers: A potential treatment for hypertension

Renal denervation has recently become of great interest as a potential treatment for resistant hypertension. Denervation techniques using radio frequency (RF) or ultrasound energy sources have already been explored in literature. In this study, we investigate the use of lasers as a potential energy source for renal denervation. In vitro studies are performed in porcine/ovine renal arteries with focused laser beams at 980 nm, 1210 nm, and 1700 nm to study the ability to damage renal nerves without causing injury to non‐target tissue structures like the endothelium. Then, a 980 nm laser catheter prototype is built and used to demonstrate in vivo renal denervation in ovine renal arteries.

Non-contact surface roughness measurement of crankshaft journals using a super-continuum laser

We measure the RMS roughness of crankshaft journals from 0.05-0.13microns at a 45degree angle of incidence and 70cm standoff distance. The system is used to detect and sort journals not polished to specifications.

Power scalable visible supercontinuum generation using amplified nanosecond gain-switched laser diode

Supercontinuum covering 0.45-1.2 mum is scaled from 250-740 mW by varying the repetition rate of a frequency doubled telecom laser diode. Efficient SC generation requires minimal non-linearity in the amplifier and fiber dispersion matched to the pump.