WRD-19020-1
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8.2
Operating Principles
The ClarityPlus primarily consists of a semiconductor laser diode array, an internal reference gas cell
and controlling circuitry. Laser chip temperature, drive current and SOA current are all controlled in
concert while local ambient temperature and the gas cell output are monitored and accounted for. By
incorporating the gas cell in its control loops, the ClarityPlus builds on the inherent physical standard
asserted by the internal reference gas cell.
At the start of all lock modes, the laser first stabilizes to a molecular absorption line. These lines are
relatively insensitive to ambient temperature conditions and long-term aging effects
– both for the gas
cell and the laser diode array. Thermal and current control loops maintain a wavelength output at the
peak of the absorption line, and once the laser is stabilized, characterization data is recorded. If in
Reference Lock, the process is complete. If in Wavelength or Channel Frequency Lock, the laser
instrument applies the characterization data as calibration data to the new target wavelength or
frequency.
To achieve longer term stability in Wavelength or Channel Frequency Lock, ambient temperature is
monitored and compensated for.
8.3
Accuracy
Line location and accuracy are determined by applying NIST pressure shift data (measurement and
uncertainty) to the extrapolated “zero-pressure” line position. Uncertainties are added in quadrature.
Pressure uncertainty in the gas fill also contributes to the line location and uncertainty, though not by a
measureable amount. A 1 Torr change in pressure, well above our pressure uncertainty, shifts the line
by -.02 to +0.01pm.
Locked lasers are measured in-house with an Agilent 86122C wavemeter to ensure locking and
stability. The fundamental reference line lock routine has been verified by measurements at NIST to be
well within 0.1pm accuracy.
8.4
NIST Traceability
The ClarityPlus is considered NIST traceable by virtue of the internal reference gas cell used for
calibration and locking stabilization.
NIST traceability describes a process by which an instrument is referenced to a NIST SRM (Standard
Reference Material). This can be due to a direct measurement with the SRM or through an unbroken
chain of instrumentation that ends with a NIST SRM. This often applies to laser instruments and
metrology instruments such as wavemeters and spectrum analyzers.
NIST traceability of gas cells is obtained instead through verification of the gas sample within the
reference cell. The absorption lines seen in the gas cell (NIST or Wavelength References) are based
on fundamental energy levels of molecules and as a result are not subject to calibration, only
verification. The specified accuracy of the gas cell lines also generally exceeds the accuracy of the
measurement equipment.
The principle variations in gas cell environment are chemical purity and pressure. Each are treated
below:
