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Optical Comb Techniques for Hyperfine Spectroscopy

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Home / Mission / Site-Directed Research and Development / FY 2023 SDRD Annual Report Index / Optical Comb Techniques for Hyperfine Spectroscopy

Project # 23-014 | Year 1 of 3

Rusty Trainhama, Manuel Manarda, Hans Schuesslerb, Alexandre Kolomenskiib, James Boundsb

aSpecial Technologies Lab (STL), bTexas A&M University
This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy, the NNSA Office of Defense Programs, and supported by the Site-Directed Research and Development Program. DOE/NV/03624–1928.

Abstract

This project has two goals for implementing optical comb techniques in a spectral measurement of xenon isotopes. The first is to measure hyperfine structure and isotope shifts of Xe using saturation absorption spectroscopy with a traditional laser scanning technique and with the aid of a broadband frequency comb to provide a frequency ruler in spectral domain. The second goal is to utilize a dual optical comb technique to replace frequency scanning of the laser and to down-mix the optical response signal into the radio frequency by a heterodyne technique. This would permit downsizing and simplification of the experimental apparatus for hyperfine measurements.

Background

We are using atomic hyperfine laser spectroscopy to detect and quantify the isotopes of Xe. The method is potentially several orders of magnitude more sensitive than traditional nuclear radiation measurements because we can measure “dark” atoms before they decay. No radiation shielding is required for this measurement, so the technique can be engineered into a mobile device. However, the energy shifts of the hyperfine “fingerprints” are small, so a Doppler compensation technique is required.

Technical Approach

Doppler-free saturated absorption spectroscopy with a broadly tunable narrow-line Ti:sapphire laser has been utilized to resolve hyperfine spectra of Xe for all near-infrared transitions of the 6s-6p series in the 820-841 nm spectral interval. An inductively coupled discharge creates a xenon plasma to populate the excited metastable 6s state, from which the hyperfine resonances are observed by scanning a Ti:sapphire tunable laser. Energy calibration of the laser is achieved by a wavemeter, a Fabry-Perot interferometer, and a stabilized single comb mode locked reference laser. The saturation absorption signal is sampled by splitting the scanning laser into a pump beam and a probe beam and counter-propagating them through the xenon plasma. The pump beam is modulated (currently by a chopper wheel), and the probe beam is sampled by a photodiode connected to a lock-in amplifier referenced at the modulation frequency. The lock-in signal shows Doppler broadened “shoulders” with sharp features of the natural linewidth at the resonance centers. The pattern of resonance centers is the hyperfine “fingerprint” of an isotope, and the concentration of a particular isotope is inferred by the spectral shape and intensity of the resonances. This requires careful modeling of the nonlinear spectral response of the saturated absorption technique.

The dual comb technique, which is just getting underway, replaces the tunable Ti:sapphire scanning laser with an external cavity diode laser modulated by a dual comb technique. The first comb is generated by mode-locking the laser, and the second comb by use of an electro-optic modulator. The two combs are close in frequency, but not identical. The “beating” of the two combs then scans an optical spectral region at a radio frequency rate. The optical resonances of the hyperfine absorption act as a heterodyne mixer, and the hyperfine structure appears as sideband structure. The photodiode signal from the hyperfine resonances is then sampled by a spectrum analyzer.

Results and Technical Accomplishments

The initial hyperfine experiment has produced a journal article, and the single comb “frequency ruler” work is advancing well. The dual comb work is just getting started.

Conclusions and Path Forward

We have demonstrated that optical hyperfine resonances can be used to measure isotope ratios of xenon, but that careful modeling of the nonlinear saturation absorption signal is required. The combs work is not yet sufficiently advanced to evaluate its effectiveness.

The saturation absorption experiment of xenon currently occupies half of a laser table. Once the dual combs technique has been implemented, the experiment should fit onto an optical breadboard on top of a rolling cart.

Publications

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