MOGlabs FSW4, Manual, Page: 20 / 72

14
Chapter 4. Operation
Figure 4.2:
A multimoding laser might only be evident in one of the inter-
ference patterns. In some circumstances this will be clear from an obvious
change in fringe spacing (left), whereas at other times the secondary peaks
might be smaller amplitude (right).
Note that while the wavemeter may be able to produce a value for the
wavelength of the strongest frequency component of a multimoding
laser, the accuracy of this value should not be relied upon.
In many situations, multi-mode optical fibres are convenient for
achieving good coupling efficiency quickly. However, they produce
a non-Gaussian beam shape that introduces bias and reduces accu-
racy of the measurement. Single-mode fibres are therefore strongly
preferable where accurate measurements are required.
With multimode fibre, the structure of the fringes fluctuates with both
the fibre-coupling alignment and mechanical strain on the fibre, as
can be seen by fluctuations in the measured wavelength when dis-
turbing the fibre. Wherever possible the fibre should be restrained
to the table and the coupling alignment should be optimised to make
the peaks as close to equal height as possible (Figure 4.3).
Fibres with very large core diameters (e.g.
>
100
µ
m) should be
avoided as the increased core size causes distortion in the interfer-
ence fringes to the point where interpretation of the fringes becomes
impossible (Figure 4.4).