We report on our efforts to develop a whispering gallery mode resonator etalon as a tool for precision radial velocity observations to detect exoplanets. The crystalline MgF2 etalon will be referenced to a compact fiber laser frequency comb, and will serve as the wavelength calibration source for a stabilized, high resolution, visible band spectrograph. The extreme stability required for the detection and characterization of exo-Earths orbiting solar-type stars will be achieved by employing a composite resonator structure with a compensating material to balance the resonator’s coefficients of thermal expansion and thermal refractivity. Progress in modeling the etalon to achieve single mode-like performance, and experiments to demonstrate broad-band (octave-spanning) ling to a white light source, are described.
The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here I will touch on the application of this system for open-path measurements of atmospheric trace gases (CH4, CO2, CO, NH3, water, ethane, and N2O) and volatile organic compounds (acetone, isopropanol, propane) with field applications targeting industrial oil and gas monitoring and agriculture.
Dual-comb spectroscopy has recently attracted significant interest due to its fast acquisition times, absolute frequency accuracy, negligible lineshape, and coherent probe light. We have recently expanded our near-infrared dual-comb spectroscopy efforts to the mid-infrared, which offers significantly improved sensitivity for many trace gas species and access to other species which cannot be measured in the Near-Infrared.
Our Mid-Infrared spectrometer is based on two erbium fiber optical frequency combs that generate light spanning from about 3 to 5 microns using a two-branch difference frequency generation (DFG) design with a periodically poled lithium niobate crystal (PPLN). The data product of the spectrometer are interferograms. Once digitized, the interferograms are corrected for residual phase noise of the frequency combs and coadded in real-time on a field-programmable gate array (FPGA). Finally, the optical spectrum is calculated through a Fourier transform of the coadded interferogram.
I will present three measurement modalities we implemented with this spectrometer. In a laboratory gas cell measurement, we characterized low-pressure gas phase propane, demonstrating excellent agreement with literature spectra obtained with high-resolution FTIR. In a separate measurement, we performed in-situ monitoring of a chemical reaction using attenuated total reflection spectroscopy. Finally, open-path measurements of atmospheric trace gases (methane, CO2, water, ethane) and volatile organic compounds (acetone, isopropanol) demonstrate the spectrometer's capability to monitor atmospheric trace gases and quantify emissions from sources like oil and gas, forest fires and industry.
The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here we use a DCS systems to measure the atmospheric absorption over long open-air paths with 0.007cm-1 resolution over 1.57 to 1.66 um, covering absorption bands of CO2, CH4, H2O and isotopologues. Inter-comparison of instruments shows that we can measure CO2 and CH4 with precision of 0.14% and 0.35%, respectively, relative to their natural abundance. In addition, this novel spectroscopy source can be employed for regional (~kilometer scale) monitoring using an array of stationed retros or in conjunction with an unmanned aerial systems (UAS). Both fixed and UAS systems combine the high-precision, multi-species detection capabilities of open-path DCS and have proven to be extremely successful at locating and sizing very small gas leaks. Here we focus on two long-term field deployments where a near-IR spectrometer is used to detect methane leaks to support oil and gas and CO2 traffic emissions from the city of Boulder.
The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here we show that this novel spectroscopy source can be employed for regional (~kilometer squared) monitoring using an array of stationed retros or in conjunction with an unmanned aerial systems (UAS). Both fixed and UAS systems combine the high-precision, multi-species detection capabilities of open-path DCS with the spatial scanning capabilities to enable spatial mapping of atmospheric gas concentrations. The DCS systems measure the atmospheric absorption over long, 100m to 1 km, open air paths with 0.007cm-1 resolution over 1.57 to 1.66 um, covering absorption bands of CO2, CH4, H2O and isotopologues.
Optical time and frequency transfer offers extremely high precision wireless synchronization across multiple platforms for untethered distributed systems. While large apertures provide antenna gain for wireless systems which leads to robust link budgets and operation over increased distance, turbulence disrupts the beam and limits the full realization of the antenna gain. Adaptive optics can correct for phase distortions due to turbulence which potentially increases the total gain of the aperture to that for diffraction-limited operation. Here, we explore the use of adaptive optics terminals for free-space time and frequency transfer. We find that the requirement of reciprocity in a two-way time and frequency transfer link is maintained during the phase compensation of adaptive optics, and that the enhanced link budget due to aperture gain allows for potential system operation over ranges of at least tens of kilometers.
Laura Sinclair, William Swann, Jean-Daniel Deschênes, Hugo Bergeron, Fabrizio Giorgetta, Esther Baumann, Michael Cermak, Ian Coddington, Nathan Newbury
Synchronization of optical clocks via optical two-way time-frequency transfer across free-space links can result in time
offsets between the two clocks below tens of femtoseconds over many hours. The complex optical system necessary to
support such synchronization is described in detail here.
We present dispersive dual-comb spectroscopy of atmospheric CO2 across a 2-km open-air path. By sending a single comb through the open-air path, both molecular phase spectrum and conventional absorbance spectrum are obtained. The measured phase spectra match expected molecular lineshape models.
We discuss precision spectroscopy with a comb-based spectrometer at 3.4 μm. Our goal is to explore comb-based
spectroscopy as an alternative method for fast, highly resolved, accurate measurements of gas line shapes. The
spectrometer uses dual 1.5 μm frequency combs down converted to 3.4 μm via difference frequency generation (DFG)
with a stabilized 1 μm fiber laser. One 3.4 μm comb is transmitted through methane and heterodyned against the second,
offset comb to measure the gas absorption and dispersion. Doppler-broadened methane spectral lines are measured to
below 1 MHz uncertainty. We also discuss the higher sensitivity alternative of a comb-assisted swept-laser DFG
spectrometer.
A coherent dual fiber-comb spectrometer centered at 1.5 μm wavelengths is transferred to 3.4 μm by differencefrequency
generation with a 1064 nm cw laser. It is shown that the residual linewidth between the comb teeth at 3.4 μm
is resolution-limited to 200 mHz; such narrow linewidths can enable coherent dual-comb spectroscopy at high-precision
and signal-to-noise ratio. We then discuss different interferometric configurations for coherent dual-comb spectroscopy.
We find that a two-branch interferometric setup is appropriate to measure both the magnitude and phase spectrum of
purely Doppler-broadened absorption lines. An initial measurement of methane lines in the υ3 band P-branch with a resolution of 114 MHz is demonstrated.
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