Agustin I. Ifarraguerri

All-Optical Histology Using Ultrashort Laser Pulses

As a means to automate the three-dimensional histological analysis of brain tissue, we demonstrate the use of femtosecond laser pulses to iteratively cut and image fixed as well as fresh tissue. Cuts are accomplished with 1 to 10 microJ pulses to ablate tissue with micron precision. We show that the permeability, immunoreactivity, and optical clarity of the tissue is retained after pulsed laser cutting. Further, samples from transgenic mice that express fluorescent proteins retained their fluorescence to within microns of the cut surface. Imaging of exogenous or endogenous fluorescent labels down to 100 microm or more below the cut surface is accomplished with 0.1 to 1 nJ pulses and conventional two-photon laser scanning microscopy. In one example, labeled projection neurons within the full extent of a neocortical column were visualized with micron resolution. In a second example, the microvasculature within a block of neocortex was measured and reconstructed with micron resolution.

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.

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.

Computation of a spectrum from a single-beam fourier-transform infrared interferogram.

A new high-accuracy method has been developed to transform asymmetric single-sided interferograms into spectra. We used a fraction (short, double-sided) of the recorded interferogram and applied an iterative correction to the complete recorded interferogram for the linear part of the phase induced by the various optical elements. Iterative phase correction enhanced the symmetry in the recorded interferogram. We constructed a symmetric double-sided interferogram and followed the Mertz procedure [Infrared Phys. 7,17 (1967)] but with symmetric apodization windows and with a nonlinear phase correction deduced from this double-sided interferogram. In comparing the solution spectrum with the source spectrum we applied the Rayleigh resolution criterion with a Gaussian instrument line shape. The accuracy of the solution is excellent, ranging from better than 0.1% for a blackbody spectrum to a few percent for a complicated atmospheric radiance spectrum.

Impact of atmospheric boundary layer turbulent temperature fluctuations on remote detection of vapors by passive infrared spectroscopy

A computational model to simulate the effects of boundary layer isotropic atmospheric turbulence on the radiative transfer process is presented. We perform a large number of simulations with stochastic ambient conditions to estimate the statistics necessary to predict the detection limit of a given trace gas. We find that the radiance and transmittance variability are primarily determined by the optical depth of the emitting atmosphere, and that the relative variability of the transmittance is an order of magnitude smaller than that of the radiance. We estimate that the atmospheric detection limit of a DMMP vapor cloud at 30 meters altitude for a ground-based observer ranges from 3.5 to 12 ppb-m, depending on the horizontal range to the cloud. Addition of uncorrelated detector noise has a disproportionate effect on the detection limit over the spectrally correlated turbulence noise. These calculations appear to be the first predictions of vapor detection limits that explicitly incorporate the effects of turbulence.

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.

Ultrahigh-sensitivity passive FTIR sensor (HiSPEC) and initial field results

The HiSPEC instrument was designed to examine the potential for passive detection of sub-lethal concentrations of toxic materials and to test the potential for passive indication of biological agent in air. HiSPEC has been operating since 1999, and after substantial laboratory characterization, taken to the field several times for successful trials against known remote targets. Some subtle differences between laboratory and field performance have been diagnosed for the first time with the aid of HiSPECs precise internal sampling system. Results of these tests may have implications for improving less sensitive passive field systems. Some recent field data is presented to indicate ultimate potential.

Chemical imaging system

The Chemical Imaging System (CIS) is a small, high-speed long-wave infrared (8 - 12 micrometers ) imaging spectrometer which is currently under development by the United States Army. The fielded system will operate at 360 scans per second with a large format focal-plane-array. Currently, the CIS uses the TurboFT FTS in conjunction with a 16-pixel direct-wired HgCdTe detector array. The TurboFT spectrometer provides high-speed operation in a small, lightweight package. In parallel to the hardware development, an algorithm and software development effort is underway to address some unique features of the CIS. The TurboFT-based system requires a non-uniform sampling Fourier transform algorithm in order to preserve signal fidelity. Also, the availability of multiple pixels can be exploited in order to improve the interference suppression capabilities of the system by allowing the detection and identification algorithm to adapt its parameters to the changing background. Due to the enormous amount of data generated, the signal processing must proceed at very high rate. High-speed computers operating with a parallel architecture are required to process the data in real time. This paper describes the current CIS bread box system. It includes some field measurement results followed by a discussion of the issues and challenges associated with meeting the design goals set for the program.

All optical histology: serial ablation and multiphoton imaging of neuronal tissue with femtosecond laser pulses

We use femtosecond laser pulses to serially image and ablate neuronal tissue. This allows histological imaging throughout the entire depth of the brain; a procedure that is unattainable by multiphoton imaging alone.

The effect of turbulent temperature fluctuations on vapor detection by ground-based passive infrared sensors at near horizon line of sight

Passive infrared spectral sensors (7-14 um) measure brightness temperature along a line of sight, and from these measurements the presence of a vapor cloud is deduced. How important are atmospheric temperature fluctuations due to turbulence on the detection of vapors? We developed a stochastic simulation that uses the MODTRAN program to explore this question. We were surprised to find that although temperature brightness fluctuations are not insignificant compared to state-of-the-art sensors noise (modeled as uncorrelated white noise) the effect on detection was very small because turbulence noise is spectrally correlated and thus its effect was largely removed with a regression algorithm. In this work we do not address the detection limit due to atmospheric interferences whose effect on detection limit may is severe.