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Enabling Precision Astronomical Spectroscopy with Laser Frequency Combs
The laser spectroscopy enabled by optical frequency combs has been key to many developments in atomic and molecular physics and the advancement of related technologies such as atomic clocks and sensors. However, for several important spectroscopic scenarios, such as astronomy and remote sensing, phase-sensitive measurements against frequency combs are inaccessible as the light is broadband and thermal in origin. This work demonstrates how the precision and accuracy of optical frequency combs can be made to benefit spectroscopy in this regime. This thesis details the 30~GHz laser frequency comb designed and deployed for the Habitable Zone Planet Finder~(HPF) spectrograph. An achievement which overcame the challenges involved with high repetition rates, broad spectral bandwidths, and robust autonomous operation. Also included is an exploration of the theory and a presentation of related work associated with two of the most critical aspects of the astrocomb and of optical frequency combs in general: frequency stabilization and supercontinuum generation. A particular emphasis is given to the nonlinear optical theory necessary for numeric simulation of the second and third order nonlinear effects accessible in newly developed waveguide platforms. And in a different approach to the measurement of thermal light, frequency combs are brought to laser-based heterodyne radiometry for the purpose of near-infrared spectroscopy of the Sun. This simple and compact apparatus enables the prospect of high-resolution, quantum-limited spectroscopy of incoherent light wherever a comb can be generated
Precision Doppler Shift Measurements with a Frequency Comb Calibrated Laser Heterodyne Radiometer
We report precision atmospheric spectroscopy of using a laser
heterodyne radiometer (LHR) calibrated with an optical frequency comb. Using
the comb-calibrated LHR, we record spectra of atmospheric near 1572.33
nm with a spectral resolution of 200 MHz using sunlight as a light source. The
measured spectra exhibit frequency shifts by approximately 11 MHz over
the course of the five-hour measurement, and we show that these shifts are
caused by Doppler effects due to wind along the spectrometer line of sight. The
measured frequency shifts are in excellent agreement with an atmospheric model,
and we show that our measurements track the wind-induced Doppler shifts with a
relative frequency precision of 100 kHz (15 cm/s), equivalent to a fractional
precision of a few parts in . These results demonstrate that
frequency-comb-calibrated LHR enables precision velocimetry that can be of use
in applications ranging from climate science to astronomy.Comment: 4 pages, 3 figure
Direct Kerr-frequency-comb atomic spectroscopy
Microresonator-based soliton frequency combs - microcombs - have recently
emerged to offer low-noise, photonic-chip sources for optical measurements.
Owing to nonlinear-optical physics, microcombs can be built with various
materials and tuned or stabilized with a consistent framework. Some
applications require phase stabilization, including optical-frequency synthesis
and measurements, optical-frequency division, and optical clocks. Partially
stabilized microcombs can also benefit applications, such as oscillators,
ranging, dual-comb spectroscopy, wavelength calibration, and optical
communications. Broad optical bandwidth, brightness, coherence, and frequency
stability have made frequency-comb sources important for studying comb-matter
interactions with atoms and molecules. Here, we explore direct microcomb atomic
spectroscopy, utilizing a cascaded, two-photon 1529-nm atomic transition of
rubidium. Both the microcomb and the atomic vapor are implemented with planar
fabrication techniques to support integration. By fine and simultaneous control
of the repetition rate and carrier-envelope-offset frequency of the soliton
microcomb, we obtain direct sub-Doppler and hyperfine spectroscopy of the
manifold. Moreover, the entire set of microcomb modes are
stabilized to this atomic transition, yielding absolute optical-frequency
fluctuations of the microcomb at the kilohertz-level over a few seconds and < 1
MHz day-to-day accuracy. Our work demonstrates atomic spectroscopy with
microcombs and provides a rubidium-stabilized microcomb laser source, operating
across the 1550 nm band for sensing, dimensional metrology, and communication.Comment: 5 pages, 3 figure
Coherent ultra-violet to near-infrared generation in silica ridge waveguides
Short duration, intense pulses of light can experience dramatic spectral broadening when propagating through lengths of optical fibre. This continuum generation process is caused by a combination of nonlinear optical effects including the formation of dispersive waves. Optical analogues of Cherenkov radiation, these waves allow a pulse to radiate power into a distant spectral region. In this work, efficient and coherent dispersive wave generation of visible to ultraviolet light is demonstrated in silica waveguides on a silicon chip. Unlike fibre broadeners, the arrays provide a wide range of emission wavelength choices on a single, compact chip. This new capability is used to simplify offset frequency measurements of a mode-locked frequency comb. The arrays can also enable mode-locked lasers to attain unprecedented tunable spectral reach for spectroscopy, bioimaging, tomography and metrology
Versatile silicon-waveguide supercontinuum for coherent mid-infrared spectroscopy
Infrared spectroscopy is a powerful tool for basic and applied science. The
molecular spectral fingerprints in the 3 um to 20 um region provide a means to
uniquely identify molecular structure for fundamental spectroscopy, atmospheric
chemistry, trace and hazardous gas detection, and biological microscopy. Driven
by such applications, the development of low-noise, coherent laser sources with
broad, tunable coverage is a topic of great interest. Laser frequency combs
possess a unique combination of precisely defined spectral lines and broad
bandwidth that can enable the above-mentioned applications. Here, we leverage
robust fabrication and geometrical dispersion engineering of silicon
nanophotonic waveguides for coherent frequency comb generation spanning 70 THz
in the mid-infrared (2.5 um to 6.2 um). Precise waveguide fabrication provides
significant spectral broadening and engineered spectra targeted at specific
mid-infrared bands. We use this coherent light source for dual-comb
spectroscopy at 5 um.Comment: 26 pages, 5 figure
Thermal-light heterodyne spectroscopy with frequency comb calibration
Precision laser spectroscopy is key to many developments in atomic and molecular physics and the advancement of related technologies such as atomic clocks and sensors. However, in important spectroscopic scenarios, such as astronomy and remote sensing, the light is of thermal origin, and interferometric or diffractive spectrometers typically replace laser spectroscopy. In this work, we employ laser-based heterodyne radiometry to measure incoherent light sources in the near-infrared and introduce techniques for absolute frequency calibration with a laser frequency comb. Measuring the solar continuum, we obtain a signal-to-noise ratio that matches the fundamental quantum-limited prediction given by the thermal photon distribution and our system’s efficiency, bandwidth, and averaging time. With resolving power ∼106, we determine the center frequency of an iron line in the solar spectrum to sub-MHz absolute frequency uncertainty in under 10 min, a fractional precision 1/4000 the linewidth. Additionally, we propose concepts that take advantage of refractive beam shaping to decrease the effects of pointing instabilities by 100×, and of frequency comb multiplexing to increase data acquisition rates and spectral bandwidths by comparable factors. Taken together, our work brings the power of telecommunications photonics and the precision of frequency comb metrology to laser heterodyne radiometry, with implications for solar and astronomical spectroscopy, remote sensing, and precise Doppler velocimetry.</p
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