33 research outputs found
Comb-mode-resolving broadband Fourier transform spectroscopy
No full textThis report demonstrates progress made to develop a robust turn-key astrocomb architecture for the calibration package of the HIRES spectrograph planned for installation on the upcoming Extremely-Large Telescope (ELT) which intends to enable novel research across a range of scientific disciplines. Here, we demonstrate progress towards a laser frequency comb covering optical and infra-red wavelength ranges and made possible by use of nonlinear interactions driven by a single stabilised mode-locked laser source. We also detail a proof-of-concept experiment carried out with a compact broadband Fourier-transform spectrometer to identify the subset of filtered frequency-comb modes, an important ancillary technology for any future broadband Fabry-P´erot-based astrocomb
Mid-infrared photonic imaging strategies
No full textImaging at mid-infrared (MIR) wavelengths between from 3–12 µm can provide unique
insights and contrast mechanisms because of the low scattering of MIR light and the chemical
specificity of MIR absorption. Consequently, new light sources and spectroscopic methods
applied in the MIR offer previously unavailable capabilities for MIR hyperspectral (HS) or
depth-resolved imaging. In this thesis, I report the development of three new MIR imaging
techniques, configured in particular for applications in heritage science.
Current MIR HS imaging technologies are expensive, as they employ complex cooled
MIR detectors, or slow, in terms of acquisition rates. These factors limit widespread use
in certain applications such as pigment mapping of paintings for cultural heritage. Here, I
demonstrate two relatively inexpensive and fast HS imaging systems which utilise novel
compressive sensing strategies.
The first system is an imaging Fourier-transform spectromter (IFTS) based on recording
a set of microbolometer camera frames of a sample’s response to illumination by uniquely
spectrally structured blackbody radiation from a Michelson interferometer. The developed
non-uniform sampling strategy was, to my knowledge, the first practical implementation of
compressive sensing in Fourier-transform spectroscopy and enabled a sampling rate as low as
15% Nyquist-limited sampling with a generic prior. The instrument was used in a campaign
at the Hunterian Museum on the artwork “Uplands in Lorne” by David Young Cameron.
A second system utilised a fast digital micromirror device (DMD) to arbitrarily shape
MIR spectra, providing sample illumination by optimised spectral structures for material
identification.
OCT has seen extensive work in the visible and near-infrared (NIR) but not as much
in the MIR. This is, in part, due to the limitation of suitable MIR ultrafast sources. MIR
OCT has the advantage of greater sample penetration depth which could find applications
in the security sector. Results are presented from an MIR time-domain OCT system with
an ultrafast orientation-patterned gallium phosphide (OP-GaP) optical parametric oscillator
(OPO)
A decade of astrocombs: recent advances in frequency combs for astronomy
No full textA new regime of precision radial-velocity measurements in the search for Earth-like exoplanets is being facilitated by high-resolution spectrographs calibrated by laser frequency combs. Here we review recent advances in the development of astrocomb technology, and discuss the state of the field going forward
PISTACHIO (Photonic Imaging Strategies For Technical Art History And Conservation)
No full textIR spectral database of artists’ pigments to be used as spectral library for compound identification purposes. Spectra were acquired using a benchtop FTIR microscope in ATR and external reflection, and the novel hyperspectral Fourier-transform spectrometer built within the PISTACHIO project, a collaboration between the University of Glasgow and Heriot-Watt University in Edinburgh
High-Speed Compressive Spectroscopy at 3.3 µm by Spectral Shaping with a Digital Micromirror Device
No full textWe demonstrate compressive spectroscopy at 3.3 µm by projecting a sparse set of spectra shaped by a digital micromirror device. Acquisition at 280 Hz is achieved. Reconstructed spectroscopy shows good agreement with ground truth measurements.</p
Continuous Wavelength Tuning across 3.9-12.0m from a 1040-nm-Pumped Optical Parametric Oscillator Based on Orientation-Patterned GaP Grown on GaAs
No full textOrientation-patterned gallium phosphide [1] (OPGaP) is a wide-bandgap material enabling two-photonabsorption-free pumping using 1040-nm Yb-lasers, and such OPGaP optical parametric oscillators (OPOs) have been applied in Fourier-transform [2] and dual-comb [3] spectroscopies across the 5-12-m molecular fingerprint region. Hetero-epitaxial growth of GaP on orientation-patterned gallium arsenide (OPGaAs) templates has been developed [4] , to mitigate issues with GaP substrates, but no nonlinear frequency conversion has yet been demonstrated. Here we report the first efficient frequency conversion in OPGaP-on-OPGaAs. OPGaP layers up to 1.2-mm thick were grown by hydride vapor-phase epitaxy (HVPE) on a 3-inch MBE-grown OPGaAs template. Several multi-grating and fan-out crystals of lengths 1.1 mm and 2.9 mm were diced from the wafer and polished into plane-parallel chips, and a broadband anti-reflection coating was applied to both faces.</p
Broadband Fourier-transform spectrometer enabling modal subset identification in Fabry-Pérot-based astrocombs
No full textA multi-GHz frequency comb (astrocomb) is typically realized by filtering modes of a sub-GHz frequency comb (source comb) in a Fabry-Pérot etalon, which can lead to ambiguities in determining which subset of source comb modes has been filtered. Here we demonstrate a broadband Fourier-transform spectrometer (FTS) with a resolving power of R = 430,000 at 550 nm, and apply it to the identification of comb subsets from a filtered 1‑GHz supercontinuum. After apodization the FTS demonstrated an instrument line shape width of 1.26 GHz which enabled individual comb-line positions to be identified with an uncertainty of 17.6 MHz, a relative precision of 5 × 10−8. Correcting for air dispersion allowed the instrument to determine the comb-mode spacing to an accuracy of 300 Hz and filtered subsets of source comb modes to be uniquely distingished across the entire comb bandwidth from 550 to 900 nm. The inherently broadband design of the FTS makes it suitable in future applications for calibrating ultra-broadband astrocombs employed by instruments such as ELT HIRES
Broadband Astrocomb with Feed-Forward Integration of a cw Laser Frequency Marker
No full textCalibrating high-resolution spectrographs traditionally relies on absorption cells or hollow cathode lamps, but their precision is constrained by their irregular spacing and line intensities [1]. Laser frequency combs (LFCs) offer a transformative alternative with atomically referenced, drift-free, and equally spaced lines [2], [3]. While LFCs with comb-mode spacings (frep ) between 250–1000 MHz can deliver thousands of low-noise, high-contrast calibration lines, the multi-GHz resolution of astronomical spectrographs requires Fabry-Pérot (FP) pre-filtering to extract a sub-set of this primary spacing to a multiple, mfrep (typically > 10 GHz), that allows individual comb modes to be resolved. A several-nm length change is sufficient to adjust the FP transmission condition between adjacent comb-mode sub-sets, but there is no a priori way to determine which of the m sub-sets has been filtered to the spectrograph. In this work, we present a Ti:sapphire-based astrocomb in RIZ band (650–1050 nm) integrated with a single-frequency laser using a feed-forward technique. We show how this approach precisely integrates the cw laser mode into the LFC, providing a fiducial marker and enabling Pound-Drever-Hall (PDH) locking of the FP cavity by the cw laser to automatically ensure broadband LFC filtering when the FP length is chosen appropriately
Compressive spectroscopic long-wave infrared imaging
No full textWe report compressive spectroscopic imaging from 7-12 µm with a 4 cm-1 optical resolution, sampled at 25% of the Nyquist rate. Compressed measurements of plastics are presented with 640×512 pixels observed and reconstructed simultaneously.</p
Mid-infrared optical coherence tomography with a stabilized OP-GaP optical parametric oscillator
No full textWe demonstrate mid-infrared time-domain optical coherence tomography (OCT) with an orientation-patterned GaP optical parametric oscillator. Instantaneous broadband mid-infrared spectra provide reduced scattering for OCT applications including cultural heritage, quality assurance, and security. B-scan calibrations performed across the wavelength tuning range show depth resolutions of 67 µm at 5.1 µm and 88 µm at 10.5 µm. Volumetric imaging inside a plastic bank card is demonstrated at 5.1 µm, with a 1 Hz A-scan rate that indicates the potential of stable broadband OPO sources to contribute to mid-infrared OCT.</p
