WA-1150 and WA-1650 Wavemeter Operating Manual
4 Theory of Operation & Calibration
EXFO Burleigh Products Group Inc. (09042-M-00 Rev C)
4-1
4. Theory of Operation & Calibration
This section reviews the theory behind the operation of the WA-1150 and WA-1650. It
also discusses the factors that influence the accuracy of the measurement.
Operating Principles
The WA-1150/1650 instruments use a Michelson interferometer to generate interference
fringes from the input radiation. The input laser beam enters the instrument through a
fiber-optic connector on the front panel. After collimation, the beam is directed into the
Michelson interferometer. At the beamsplitter, the input beam is divided into two beams
that follow separate optical paths. One beam reflects off the moving retroreflector
assembly and returns to the beamsplitter. The other beam reflects off of a fixed
retroreflector and returns to the beamsplitter where it interferes with the first beam. The
relative lengths of the two paths govern whether constructive or destructive interference
occurs at the beamsplitter. As the retroreflector moves back and forth, the detected
intensity for each wavelength of the input beam varies sinusoidally.
In order to extract all the spectral components in the input beam from the Michelson
interferometer fringe pattern, it is necessary to sample the detected intensity at precisely
calibrated intervals in the scan of the interferometer. This is accomplished by
introducing into the interferometer a second “reference” laser with an accurately known
wavelength. This laser measures the displacement of the moving arm of the
interferometer and provides a signal for accurately sampling the input laser fringe
pattern.
After a large number of intensity values have been recorded from one scan of the
interferometer, a Fast Fourier Transform is performed on the data to produce the
spectrum of the input laser source. The spectral peak with the highest intensity is used to
calculate the input laser wavelength.
A small portion of the input beam entering the instrument is split off into a power meter.
This measurement of the total input power is used to calculate the absolute power of the
input laser source.
Wavelength Accuracy
The wavelength accuracy of the peaks in the instrument spectrum is dependent on the
knowledge of the reference laser wavelength and appropriate corrections for the
dispersion in the refractive index of air. The WA-1150/1650 uses a HeNe reference laser
with refractive index correction based on temperature and pressure measurements inside
the interferometer. The absolute accuracy of the reference laser wavelength used in the
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