Interferometer, Grating, Laser Scale
Saturday, September 09, 2006
A Laser-Scale Grating Interferometer has been designed and developed using two light beams and a holographic grating as a beamsplitter.
The Interferometer consists of two collimated light beams that enter the photodetector side of the read head and a positive lens reconverges the light beams into the photodetector. A second small holographic grating divides the light beam into three spots, one for each of three photodetectors. The center photodetector receives the two cross-polarized light beams. The signal from the center photodetector represents the average light intensity without fringes. The outer photodetectors are covered with small plastic chips, one photodetector acting as a polarizer and the other photodetector acting as a quarter-wave plate plus polarizer. The quarter-wave plate shifts the phase of one the light beams, so that the fringe pattern at that photodetector is in phase quadrature with the fringe pattern at the other photodetector. This configuration provides up-down directional information similar the technique used in optical encoders.
The glass scale consists of a thin center plate containing the scale or grating, laminated between two cover glasses that are specially coated for the angles, wavelength, and polarization of the Laser Diode. The actual scale or grating is enclosed in glass to prevent damage. The scale is optically aligned with the edge of the aluminum frame and cemented to the frame at one central point. The frame has a clamp bar and a U-shaped bar to enclose and protect the glass. The design of the scale moves the measuring datum to the exact center of the glass sandwich to assure no stretching or compressing of the scale occurs with bending (the scale is mounted in it’s neutral bending plane).
Source: “Grating Interferometer”, published 1980; Wright Huntley, Holograf, Santa Clara, CA; Ph 1-408-748-9221
Reference: None Available
Industrial Products
posted by JD52 @ 1:44 PM,
