531 research outputs found
Metrology of Complex Refractive Index for Solids in the Terahertz Regime Using Frequency Domain Spectroscopy
Frequency domain spectroscopy allows an experimenter to establish optical
properties of solids in a wide frequency band including the technically
challenging 10 THz region, and in other bands enables metrological comparison
between competing techniques. We advance a method for extracting the optical
properties of high-index solids using only transmission-mode frequency domain
spectroscopy of plane-parallel Fabry-Perot optical flats. We show that
different data processing techniques yield different kinds of systematic error,
and that some commonly used techniques have inherent systematic errors which
are underappreciated. We use model datasets to cross-compare algorithms in
isolation from experimental errors, and propose a new algorithm which has
qualitatively different systematic errors to its competitors. We show that our
proposal is more robust to experimental non-idealities such as noise or
apodization, and extract the complex refractive index spectrum of crystalline
silicon as a practical example. Finally, we advance the idea that algorithms
are complementary rather than competitive, and should be used together as part
of a toolbox for better metrology.Comment: 21 pages, 11 figures, 4 appendice
Feasibility of direct digital sampling for diffuse optical frequency domain spectroscopy in tissue
Frequency domain optical spectroscopy in the diffusive regime is currently being investigated for biomedical applications including tumor detection, therapy monitoring, exercise metabolism, and others. Analog homodyne or heterodyne detection of sinusoidally modulated signals have been the predominant method for measuring phase and amplitude of photon density waves that have traversed through tissue. Here we demonstrate the feasibility of utilizing direct digital sampling of modulated signals using a 3.6 Gigasample/second 12 bit Analog to Digital Converter. Digitally synthesized modulated signals between 50MHz and 400MHz were measured on tissue simulating phantoms at six near-infrared wavelengths. An amplitude and phase precision of 1% and 0.6 degrees were achieved during drift tests. Amplitude, phase, scattering and absorption values were compared with a well-characterized network analyzer based diffuse optical device. Measured optical properties measured with both systems were within 3.6% for absorption and 2.8% for scattering over a range of biologically relevant values. Direct digital sampling represents a viable method for frequency domain diffuse optical spectroscopy and has the potential to reduce system complexity, size, and cost
Digital parallel frequency-domain spectroscopy for tissue imaging
Near-infrared (NIR) (650 to 1000 nm) optical properties of turbid media can be quantified accurately and noninvasively using methods based on diffuse reflectance or transmittance, such as frequency-domain photon migration (FDPM). Conventional FDPM techniques based on white-light steady-state (SS) spectral measurements in conjunction with the acquisition of frequency-domain (FD) data at selected wavelengths using laser diodes are used to measure broadband NIR scattering-corrected absorption spectra of turbid media. These techniques are limited by the number of wavelength points used to obtain FD data and by the sweeping technique used to collect FD data over a relatively large range. We have developed a method that introduces several improvements in the acquisition of optical parameters, based on the digital parallel acquisition of a comb of frequencies and on the use of a white laser as a single light source for both FD and SS measurements. The source, due to the high brightness, allows a higher penetration depth with an extremely low power on the sample. The parallel acquisition decreases the time required by standard serial systems that scan through a range of modulation frequencies. Furthermore, all-digital acquisition removes analog noise, avoids the analog mixer, and does not create radiofrequency interference or emission
A smart way to minimize test time for transformer dielectric measurements
Dielectric response measurements for the moisture and oil conductivity assessment of transformers are well understood, internationally accepted and growing in use. Of the two principal dielectric response methods, an AC method called Dielectric Frequency Response (DFR, also Frequency-Domain Spectroscopy, FDS) is preferred due to its robustness against noise. The time requirement of a DFR measurement is lengthy compared to that of other electrical test methods. An earlier approach to accelerate test time combined DC (time domain) and AC (frequency domain) test methods. This article discusses the limitations inherent to that approach and presents today’s multifrequency test solution that minimizes test time for DFR measurements without compromising accuracy
Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields
We propose an experimental technique to generate large amplitude coherent
phonons with irradiation of THz-rate pump pulses and to study the dynamics of
phase transition in GeTe ferroelectrics. When a single pump pulse irradiates
the sample at various pump power densities, the frequency of the soft phonon
decreases sub-linearly and saturates at higher pump powers. By contrast, when
THz-rate pump pulse sequence irradiates the sample at matched time intervals to
forcibly drive the oscillation, a large red-shift of the phonon frequency is
observed without saturation effects. After excitation with a four pump pulse
sequence, the coherent soft phonon becomes strongly damped leading to a near
critical damping condition. This condition indicates that the lattice is driven
to a precursor state of the phase transition.Comment: 4 pages, 3 figure
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