- Set the micrometer to approximately 12mm
on its main scale.
- Equalise the path lengths, BS-M1 and
BS-M2, (measuring from the centre of the beam-splitter) by moving the adjustable minor
unit axially on its slide using knob Z.
- Switch on the laser diode and raise the
search pin, SP, to its full height. You should see two images side-by-side through the
ground-glass screen, GG.
- Carefully adjust the 'X' and r knobs at
the rear of mirror unit, M1 to make the images co-incident. You may see interference
fringes immediately, but more likely, you will have to continue making small adjustments
until fringes are seen.
- Once you have located the centre of the
circular fringe pattern, make small adjustments to the axial position of M1 to obtain as
large a fringe spacing as possible. At this position, the path lengths are the same to
within a few wavelengths.
- For experiments involving fringe counting
and wavelength measurement, take micrometer readings as each fringe (or perhaps every 1Oth
fringe) passes the search pin image. There is a small amount of backlash in the micrometer
lever system, so always rotate the micrometer in the same direction and do not make
measurements until a few fringes have passed the search pin image.
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