Diffraction encoded position measuring apparatus

DETAILED DESCRIPTION In FIG.
1 a laser source supplies a light beam signal on a path or beam labeled 10.
This beam is passed through a block 12 labeled AOM for acouso-optic modulator which superimposes a modulated signal on the laser beam at a frequency that can be easily detected.
In one embodiment of the invention, the laser beam had a wavelength of 633 nanometers which is approximately equivalent to 4.
7.
times.
10.
sup.
12 megahertz.
On the other hand, the modulating frequency from the AOM 12 may be a much lower value such as 40 megahertz.
A second laser beam 14 is shown passing through a second AOM 16 and producing an output beam 18 modulated at 41 megahertz.
The source 14 may be the same as source 10 and redirected using mirrors or may be a separate source if it is in phase with 10.
The light beam 18 after passing through a grating 20 continues as a zero order beam 22 and a portion of the original beam 18 is diffracted as an N order beam such as first order beam 24.
The output of AOM 12 is supplied as a zero order beam 26 to the grating 20 and after passage through the grating 20, continues as a zero order beam 28 and a diffracted beam 30.
The phase of the diffracted beams 24 and 30 change phase in opposite directions for a given direction of movement of grating 20.
Again, beam 30 may be any of N orders but for the purposes of explanation will be a first order beam.
The beam 30 is deflected from a mirror 32 to a beam splitter 34.
The beam 24 is reflected off a further mirror 36 and is also supplied to beam splitter 34.
Within the beam splitter 34, the signals are combined to produce a fringe pattern which is detected with a detecting device of the square law type such as a photo detector.
Detector 38 is labeled as the detector of the fringe pattern.
A portion of the beam reflected from beam 32 is labeled as beam 40 and is the remaining portion of beam 30 which is still in existence after the combining or mixing process.
Beam 40 would be supplied to some type of beam dump 42 and would be unused