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4. Operation
4.1 Fringe identification and optimisation
The host software includes a prominent display of the interference
fringes used to compute the laser wavelength. Understanding the
fringe structure is important in ensuring that the wavelength mea-
surement is accurate. The two primary causes of reduced measure-
ment reliability are laser multi-moding, and poor spatial profile of
the light emitted by the fibre.
The presence of multiple frequency components during a measure-
ment can change the structure of the interference pattern and cause
the measurement to fail. Typically this is evident by the presence of
secondary peaks in the fringes, a significant widening of the peak
widths, and/or a significant reduction in the amplitude of the fringes
compared to the background level (Figure 4.1). Multimode behaviour
may be evident in only one of the etalons (Figure 4.2) so it is im-
portant to periodically verify the fringe shape.
Figure 4.1:
Examples of fringes measured with a single-mode laser (left)
and multimode laser (right). The presence of secondary peaks and reduc-
tion in contrast indicate the laser is not single-mode.
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