Resolving the magnetic hyperfine structure of Hg Isotopes
using a VIPA grating spectrometer
Hubert Jean-Ruel, John Reid, and Greg Dodd
LightMachinery Inc., 80 Colonnade Road North, Ottawa ON CanadaIntroduction
LightMachinery’s new series of high-resolution
spectrometer – the HyperFine spectrometer – isits first to be built around a Virtual Imaged
Phased-Array (VIPA) (1). To test thecapabilities of the HyperFine spectrometer, wechose to measure the hyperfine magnetic
structure of the Hg isotopes in mercury. The
rationale behind our choice was threefold: 1)
Mercury lamps are a common calibration source
in most optical labs, allowing the experiment tobe easily verified. 2) Traditional grating
spectrometers capable of resolving the hyperfine
lines of Hg isotopes are typically large (> 1 m inlength) and expensive. 3) Spectra measurementsof the resolved hyperfine structures of mercury
lamps are readily available in literature.
Mercury has six naturally occurring isotopes (2),two of which show hyperfine splitting in their
atomic emission lines (3; 4). Only by utilizing aspectrometer with a resolution greater than 1picometer can the individual hyperfine peaks ofthese atomic emission lines be resolved.

Experiment
The experiment was performed in two parts. In
the first, the hyperfine magnetic structure of the546.1 nm line of the Hg source was measured
with the HyperFine spectrometer. These
measurements were then compared with two
verified sources from literature to confirm theiraccuracy on the basis of wavelength position andrelative intensity. The experiment also proved agood opportunity to determine how theHyperFine spectrometer’s capabilities matchedup against the complex and expensivelaboratory-based spectroscopy setups (4; 5) usedin the reference experiments.
In part two, measurements of the 577/579.1 nm
doublet lines were taken with the HyperFine
spectrometer and two traditional diffraction
grating spectrometers classified as highresolution (Oriel model 260i, Ocean Optics HR2000). Results from the spectrometers werecompared with one another in terms of theirspectral accuracy and ability to resolve
individual isotope hyperfine peaks.
LightMachinery’s HyperFine spectrometer wasequipped with a 1.68 mm thick VIPA etalon
paired with a diffraction grating. It obtained atypical resolution of 1 pm at 530 nm with asimultaneous measurement over a wavelength
range of 15 nm (the range can be increased to
100 nm with grating rotation). A multimodefiber (MMF) optic port supplied the light input
to the spectrometer’s internal components.
Prior to the measurements, the HyperFine
spectrometer was calibrated using a 532 nm
monochromatic laser of precisely known
wavelength. Measurements made with themonochromatic laser at 532 nm produced a
FWHM of 1.0 pm (Figure 1), and a VIPA finesseof ~55.

resolved and align precisely
measurements from literature.

withtheIt is important to note that the wavelength
calibration was carried out independently, andwas not adjusted to match the hyperfinestructure. We must also emphasize that theobserved central line width of 5 pm is not
instrument-limited; it is the true linewidth of theHg-lamp source (this linewidth is due to Doppler
broadening in the lamp).

Figure 1: Second harmonic of a narrow-linewidthytterbium-doped fiber laser demonstrating theresolution of the Hyperfine spectrometer. The 1 pm
width is instrument-limited.

The Oriel spectrometer uses a traditional
diffraction grating arrangement to achieve anoptimum resolution of ~0.2 nm. The entrance
collimating optics of F/3.9 (focal length of 220mm) guide the light source into the 50 x 50 mm
grating (1200 l/m). An adjustable entrance slit
allows for fine tuning of the resolution down toa minimum of approximately 0.2 nm. The Ocean
Optics spectrometer is considered a state-of-theart fixed grating spectrometer with sub-nmresolution.
The Hg source was provided by an Ocean Optics
spectral calibration lamp (5). The lamp wasimaged directly onto the entrance slit of the Oriel
spectrometer, while the HyperFine spectrometerand Ocean Optics HR2000 were supplied with a
125 µm fiber optic cable.

Results
When overlaid with measurements of thehyperfine structure of mercury – extracted from
Doppler-free saturated absorption and frequency
modulated spectroscopy measurements inultralow pressure mercury cells – the HyperFine
spectrometer’s measurements show excellent
agreement in both relative intensity andwavelength position (Figure 2). In addition, thesatellite peaks of the Hg isotopes are clearly

Figure 2: Hyperfine structure of the 546.1 nm line ofmercury. The spectral width of the observed features
is limited by the pressure of the mercury lamp. The
dark blue lines underneath the spectrum
measurement display the known hyperfine magnetic
structure of mercury (4; 6). It should also be notedthat the known hyperfine structure displayed
underneath the HyperFine’s spectrum is not anexperimental curve; it is the set of extracted lines.

The 577/579.1 nm doublet line of mercury wasmeasured with the Oriel spectrometer at several
different slit sizes. From the spectra results(Figure 3), we can see that the stated resolutionof 0.2 nm was achieved once the input slit wasadjusted to the optimum width. While the 0.2 nm
resolution is sufficient to resolve the 577/579.1
nm doublet line, the spectrometer’s maximum
resolution falls far short of that required todifferentiate the individual hyperfine structuresof the Hg isotopes.

References

Figure 3: Comparison of the 577/579 nm mercury
doublet measured using the HyperFine spectrometerand two standard grating spectrometers categorized
as high-resolution. The slit and grating settings ofhigh-resolution grating spectrometer #1 (Oriel, 260i)
were adjusted to provide the optimal resolution
specified for that model, namely ~0.2 nm (weobtained 0.15 nm here). The settings of highresolution grating spectrometer #2 (Ocean Optics,HR2000) were fixed by the manufacturer and its
resolution was found to be ~0.7 nm.

Despite the natural linewidth of the mercury
lamp, which is ~5 times broader than theHyperFine’s resolution, the difference between
the spectrometers measurements is striking. The
HyperFine spectrometer was able to obtain muchnarrower lines, with the resolution difference
becoming most apparent in the magnified viewof the of the 579 nm line (Figure 3 inset). The
HyperFine spectrometer was the only instrument
capable of rendering any definable hyperfinestructures.

Conclusions
The HyperFine spectrometer clearly defined theindividual hyperfine structures of the Hg lamp at
the 546.1 nm line and 577/579 nm doublet;
confirming its ability to obtain 1 pm resolution
with an accuracy greater than 2 pm.

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