Michael Fitch - Academia.edu (original) (raw)

Papers by Michael Fitch

To be published in the proceedings of, Oct 25, 1999

PACS2001. Proceedings of the 2001 Particle Accelerator Conference (Cat. No.01CH37268), Nov 14, 2002

The plasma wake-field mechanism can be used to couple energy at a high rate from a bunched electr... more The plasma wake-field mechanism can be used to couple energy at a high rate from a bunched electron beam into a plasma wave. We will present results from the Fermilab A0 facility where a beam with an initial energy of 14 MeV passes through the plasma to emerge with a much broader energy spread, spanning from a low of 3 MeV to a high of over 20 MeV. Over the 8 cm lenth of the 10 14 cm 3 plasma, this implies a 140 MeV/m deceleration and 72 MeV/m acceleration gradient.

Frequency-domain functional near-infrared spectroscopy (FD-fNIRS) has the potential to improve ne... more Frequency-domain functional near-infrared spectroscopy (FD-fNIRS) has the potential to improve neural imaging of brain hemodynamic responses over conventional magnitude measurements from continuous wave (CW) fNIRS systems by providing additional measurement of phase changes. We evaluated whether phase measurements improved accuracy in decoding motor activity and laterality of movement while recording from motor cortex during a finger tapping task conducted with N=12 subjects. Classification was performed using logistic regression with a single feature derived from hemodynamic response function (HRF) regression. Inspecting the regression results on held-out test data, the majority of subjects showed significant differences between baseline and activity conditions over a typical HRF time course in both magnitude and phase signal components. Combining magnitude and phase information using FD-fNIRS significantly improved classification accuracy of motor conditions at the population level relative to the CW-fNIRS analogue represented by the magnitude signal alone. Our results demonstrate that FD-fNIRS systems can provide benefit over CW-fNIRS for neural decoding applications and are a promising technology for future investigation of non-invasive neural imaging.

Frequency-domain (FD) fNIRS is attractive for non-invasive brain imaging because phase-sensitive ... more Frequency-domain (FD) fNIRS is attractive for non-invasive brain imaging because phase-sensitive detection leads to increased resolution and may exhibit improved robustness to motion artifacts. We present an FD-fNIRS system with silicon photomultiplier (SiPM) receivers, where the sensitivity and dynamic range approach those of a first-class continuous-wave (CW-) fNIRS system. This represents a significant step toward fully exploiting the phase degree of freedom provided by FD-fNIRS. The transmitter subsystem includes 32 channels and each supplies 12.5 mW of coherent light at both 690 and 852 nm. A dedicated radio circuit intensity-modulates each laser, and they are independently configured to operate at frequencies up to 400 MHz. The transmitters are on-off-keyed according to a user-specified pattern to mitigate shot noise and maximize dynamic range. The receiver subsystem also includes 32 channels. Each consists of a large-area (2.16-mm diameter), high-NA (0.66) fiber bundle, which carries light to a custom photo-receiver. A three-lens assembly enhances coupling between the fiber-bundle and the SiPM, and the SiPM (ON Semiconductor MICRORB-10020) converts the signal to the electrical domain. The electrical signal is amplified and down-converted to the audio spectrum, and a transformer balances the signal and provides galvanic isolation. Each of the 32 audio waveforms is digitized at 192 kS/s in a bank of commercial audio digitizers. Using a modulation frequency of 211 MHz, swept-power measurements demonstrate that the average noise-equivalent power of the SiPM photo-receivers is 20.5 fW per square root Hz, with about 6 decades of optical dynamic range. This work was funded by a research contract under Facebook’s Sponsored Academic Research Agreement.

2021 10th International IEEE/EMBS Conference on Neural Engineering (NER)

Optical Techniques in Neurosurgery, Neurophotonics, and Optogenetics, 2021

We present a 32-transmitter, 32-receiver dual-wavelength frequency-domain (FD) fNIRS system compr... more We present a 32-transmitter, 32-receiver dual-wavelength frequency-domain (FD) fNIRS system comprised of commercially available avalanche photodiodes, laser drivers and laser mounts. The custom frequency domain (FD) fNIRS system is used to interrogate cerebral tissue with optodes positioned at the posterior occipital region of the head. Data are collected from human subjects watching movie scenes with no sound. We applied cross-validated PCA to identify the number of dimensions retained in the neural signal recorded using FD-fNIRS for the magnitude, phase, and FD (magnitude and phase combined) components of the recorded signal. Importantly, a comparison of the cross-validation error for each signal allows us quantify the dimensionality of the linear subspace spanned by each data type. The number of principal components producing the minimum cross-validation error for the held-out test runs represents the number of orthogonal signal dimensions preserved across training and held-out test data runs. We find that the FD signal captures a higher dimensional space compared to the magnitude or phase signals in isolation. Previous theoretical and empirical work suggest that signals extracted using FD-fNIRS contain higher fidelity neural information than CW-fNIRS in isolation. The findings reported here further support this hypothesis and extend beyond the findings reported in the literature, demonstrating that a higher dimension linear subspace is covered by FD-fNIRS above and beyond the baseline signal captured using traditional CW-fNIRS, assuming other optical performance metrics such as optical dynamic range, noiseequivalent power and cross-talk are comparable. This work was funded by a research contract under Facebook’s Sponsored Academic Research Agreement.

Micro- and Nanotechnology Sensors, Systems, and Applications IX, 2017

The development of portable non-invasive brain computer interface technologies with higher spatio... more The development of portable non-invasive brain computer interface technologies with higher spatio-temporal resolution has been motivated by the tremendous success seen with implanted devices. This talk will discuss efforts to overcome several major obstacles to viability including approaches that promise to improve spatial and temporal resolution. Optical approaches in particular will be highlighted and the potential benefits of both Blood-Oxygen Level Dependent (BOLD) and Fast Optical Signal (FOS) will be discussed. Early-stage research into the correlations between neural activity and FOS will be explored.

2015 IEEE Aerospace Conference, 2015

Microwave and millimeter-wave ranging systems, waveforms, and experimental results are described ... more Microwave and millimeter-wave ranging systems, waveforms, and experimental results are described for coherent distributed RF systems applications. Measured results show that coherent distributed systems operating at carrier frequencies with coherence at λ/10 are possible well into the millimeter-wave regime by using widely separated two-tone ranging waveforms. The two-tone waveform in the context of continuous-wave ranging is introduced. A method for overcoming the range-ambiguous output of the matched filter processing is described. Microwave and millimeter-wave ranging measurements are shown and compared to the Cramer-Rao lower bound for range accuracy.

The US Army Night Vision and Electronic Sensors Directorate (NVESD) and the US Army Research Labo... more The US Army Night Vision and Electronic Sensors Directorate (NVESD) and the US Army Research Laboratory (ARL) have developed a terahertz-band imaging system performance model for detection and identification of concealed weaponry. The details of this MATLAB-based model which accounts for the effects of all critical sensor and display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active illumination, were reported on at the 2005 SPIE Europe Security and Defence Symposium ...

Proceedings of the 1999 Particle Accelerator Conference (Cat. No.99CH36366)

The AØ Photoinjector at Fermilab can produce high charge (10-14 nC) electron bunches of low emitt... more The AØ Photoinjector at Fermilab can produce high charge (10-14 nC) electron bunches of low emittance (20 mmmrad for 12 nC). We have undertaken a study of the optimal compression conditions. Off-crest acceleration in the 9-cell capture cavity induces an energy-time correlation, which is rotated by the compressor chicane (4 dipoles). The bunch length is measured using streak camera images of optical transition radiation. We present measurements under various conditions, including the effect of the laser pulse length (2 ps sigma Gaussian vs. 10 ps FWHM flat top). The best compression to date is for a 13.2 nC bunch with z = 0 : 63 mm (1.89 ps), which corresponds to a peak current of 2.8 kA.

Physical Review Special Topics - Accelerators and Beams, 2005

The Fermilab photoinjector produces electron bunches of 1-12 nC charge with an energy of 16-18 Me... more The Fermilab photoinjector produces electron bunches of 1-12 nC charge with an energy of 16-18 MeV. Detailed measurements and optimization of the transverse emittance have been carried out for a number of beam line optics conditions, and at a number of beam line locations. The length of the bunches has also been measured, first for an uncompressed beam (as a function of the charge) and then for a compressed beam of 8 nC charge (as a function of the 9-cell cavity phase). These measurements are presented and compared with the simulation codes HOMDYN and ASTRA.

PACS2001. Proceedings of the 2001 Particle Accelerator Conference (Cat. No.01CH37268)

Frontiers in Optics, 2003

Linear optics with photon counting is a prominent candidate for practical quantum computing. The ... more Linear optics with photon counting is a prominent candidate for practical quantum computing. The protocol by Knill, Laflamme, and Milburn [Nature 409, 46 (2001)] explicitly demonstrates that efficient scalable quantum computing ...

The Defense Advanced Research Projects Agency’s Revolutionizing Prosthetics program demonstrated ... more The Defense Advanced Research Projects Agency’s Revolutionizing Prosthetics program demonstrated the potential for neural interface technologies, enabling patients to control and feel a prosthetic arm and hand, and even pilot an aircraft in simulation. These landmark achievements required invasive, chronically implanted penetrating electrode arrays, which are fundamentally incompatible with applications for the able-bodied warfighter or for long-term clinical applications. Noninvasive neural recording approaches have not been as effective, suffering from severe limitations in temporal and spatial resolution, signal-to-noise ratio, depth penetration, portability, and cost. To help close these gaps, researchers at the Johns Hopkins University Applied Physics Laboratory (APL) are exploring optical techniques that record correlates of neural activity through either hemodynamic signatures or neural tissue motion as represented by the fast optical signal. Although these two signatures dif...

Arrays of spiral resonators are used in the design of frequency-selective surfaces operating in t... more Arrays of spiral resonators are used in the design of frequency-selective surfaces operating in the frequency range 2-18 GHz. The transmission spectrum of linearly polarized radiation incident on each array exhibits a multiplicity of resonance features, the number and frequency of which is a function of the design of the spiral geometry. The transmission spectra are also functions of the

Chemical Physics Letters, 2004

We have investigated the terahertz (THz) spectrum of 2,4-DNT by using Fourier transform infrared ... more We have investigated the terahertz (THz) spectrum of 2,4-DNT by using Fourier transform infrared spectroscopy in the 0.2-19.5 THz region. We also examined low-frequency intermolecular or phonon modes between 0.2 and 1.8 THz via THz time-domain spectroscopy. The extracted absorption coefficient and refractive index of an intermolecular band at 1.08 THz are $110 cm À1 and 1.67, respectively. Density functional theory (DFT) was applied to obtain structure and vibrational frequencies of 2,4-DNT. The calculated results are in agreement with the experimental data. Observed vibrational frequencies have been interpreted using DFT. Two intermolecular or phonon modes were identified at 1.08 and 2.52 THz.

Chemical Physics …, 2004

AIP Conference Proceedings

In this work we investigate the propagation of terahertz radiation through polyurethane foam and ... more In this work we investigate the propagation of terahertz radiation through polyurethane foam and porous alumina ceramics to understand the effects of structure on the optical properties of these materials at terahertz frequencies. A terahertz time domain system with a GaAs photoconductive emitter and a ZnTe electro-optic crystal was used to generate and detect the transmitted terahertz signal. Using the amplitude and phase characteristics of these signals, the thickness, index of refraction, and other physical and optical properties of the materials were determined.