WA-1100/WA-1600 Wavemeter Operating Manual
4 Theory of Operation & Calibration
08115-M-00 Rev E
4-5
Fringe Signal
Laser Spectrum
Broad linewidth
Trigger
ZOP
Window
Figure 4-2.
Fringe signal and measurement window for a broad linewidth input
laser
Figure 4-3 shows an example of a bi-modal laser with two strong spectral lines. This
laser produces a fringe pattern that exhibits beating between the two laser frequencies.
Since the fringe amplitude goes to zero on either side of the ZOP, only the center fringe
envelope can be used. Prior to the valid Window surrounding the ZOP point, other
windows may occur but these are ignored for measurement purposes.
Fringe Signal
Laser Spectrum
Bi-modal
Trigger
ZOP
Window
Valid Window
Figure 4-3. Fringe signal and measurement window for a bi-modal input laser
When the number of fringes is reduced by increased bandwidth or lack of coherence in
the laser source, the accuracy of the wavelength measurement is also reduced. Typically,
an increased bandwidth limits the accuracy of measurement to about 10% of the laser
bandwidth for a single broad line as in Figure 4-2 or to 10% of the mode separation in a
multi-mode laser spectrum as in Figure 4-3. When a laser has a complex mode structure,
the measurement precision will be about 10% of the effective bandwidth and the
wavelength reported will correspond to the intensity weighted average of the spectral
distribution.
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