One essential element of a technical laser interferometer is the secure detection of the bright / dark transitions (fringes) even for variable contrast conditions which may establish due to modifications of the initial adjustment during the displacement of the measuring reflector. To compensate for these influences a fringe signal C phase shifted by 180° with respect to fringe signal A is optically produced.
By means of an subsequent electronic comparator disturbing DC-offset parts are eliminated from the resulting signal. To detect the direction of the displacement of the measuring reflector a signal B, phase shifted by 90° with respect to signal A, is created optically. Furthermore such a 90° phase shifted signal D with respect to signal C is created. Also signal B and D are treated with a comparator to remove any offset. As a result two offset free fringe signals are created having a phase shift of 90° to each other allowing the directional discrimination by applying the quadrature encoder principle.
Another important difference to the classical Michelson interferometer lies in the fact that instead of mirrors triple reflectors are used in such a way that no beam travels back into the laser source. Such back reflections lead to frequency and thus intensity fluctuation of the laser source falsifying the counted number of fringes.
This extension provides all necessary optical components to turn the existing Michelson interferometer to a technical one. One triple reflector is mounted onto a translation stage. The piston pin of the provided micrometre gauge firmly touches the back of the moveable triple reflector. The idea of the measurement is to calibrate the display of the gauge against the wavelength of the laser source which forms the secondary standard of a metre. The fringes generated by the movement are counted by the provided forward backward counter in fractions of the wavelength λ like λ/4, λ/8 or even λ/16.
LM-0120 Laser interferometer technical extension
The technical interferometer is a refinement of the Michelson interferometer. To avoid any undesired back reflection into the laser source, triple reflectors are used instead of flat mirrors. Furthermore the technical interferometer needs a mechanism for reliable counting of the fringes, even if the movement direction (M) is reversed. For this purpose optical quadrature signals are required. The superimposed waves of the reference arm (R) and (M) are leaving the interferometer at the deflecting prism. Both orthogonal linear polarised waves are converted into opposite circular polarisation by the quarter wave plate (QWP). In a next step the intensity is split into two equal parts, whereby one part travels to the channel (A) and the other to channel (B). In channel B a quartz polarizer turns the polarisation of channel B by 90° to channel A. In this way the required phase shift of 90° for the quadrature encoding is achieved. In addition, a polarizing beam splitter in each channel provides a 180° phase shift which is used to become independent of varying contrast of the moving interferometer. A comparator converts the A, A’ and the B,B’ into the quadrature signal C and D which are counted by a quadrature counter.