Patent GB-1450692-A (GB)
Title: Optical dilatometer
IPC: G01N25/16
Filed: 1973-09-24
Abstract: 1450692 Measuring expansion photoelectrically HEWLETT-PACKARD CO 24 Sept 1973 [2 Oct 1972] 44629/73 Heading G1A In an optical dilatometer for measuring the expansion of a test object 15, Fig. 1, mounted on a platen 9, light from a source 1 is split into four beams, the first of which is incident on the platen and then interferes with the second while the third beam is incident on the free end of the test object and then interferes with the fourth one; by observing the two interference patterns thus formed motion of the free end of the object and of the platen can be measured, so that the expansion of the object can be found. The paths of the first and third beam are arranged so that the first beam strikes the platen twice, and the third beam strikes the test object twice, so that if the platen tilts the overall path length of the first and third beams is not affected and the interference patterns do not respond to the tilting. In Fig. I the source is a two frequency laser  which produces a frequency f 1  polarized in the plane of the drawing as shown by the double head arrow, and a frequency f 2  polarized normal to the drawing as shown by the encircled dot. The third and fourth beams passes directly through beamsplitter 2 to a polarizing beam-splitter 4. The first and second beams are reflected via beam-splitter 2 and mirror 6 to the polarizing splitter 4. The first beam is of frequency f 2  and is so polarized that the splitter 4 directs it to the left hand side of the platen; it is reflected back but its polarization has changed due to passing twice through a quarter wave plate 10 and instead of following exactly the same path it now enters the corner cube reflector 11 where it is reflected so that is strikes the right hand side of the platen. From there it is reflected back to the splitter 4, the double passage through the quarter wave plate changing the polarization so that the splitter 4 directs the beam towards detector 12. The second beam, of frequency f 1 , has meanwhile passed directly into the corner cube 8 and is reflected to interfere with the first beam when the latter has been reflected from the right hand side of the platen. It is the resulting interference pattern which is viewed by detector 12. A similar set of paths is followed by the third and fourth beams so that a second detector 13 views an interference pattern formed by reflection from the free end of the test object. Detector circuits: as seen in Fig. 1 the output of detector 12 is f 1 -f 2 #f p , where #f p  is a doppler shift due to platen motion, while the output of detector 13 is f,-f 2 #f s  where #f s  is the doppler shift due to motion of the free end of the test specimen. A reversible counter 14 integrates the difference to give an output representing the expansion of the test specimen. The sensitivity of the counter can be extended as in Fig. 2 by multiplying the signal of detector 12 by a factor n and the signal of detector 13 by n-1 to form a signal f 1 -f 2 (n#f p -[n-1] #f s ) which is then fed to counter 14 for comparison with the signal from detector 13, the result being that the counter output is increased by a factor n.