Introduction to Interferometer Lasers

Laser interferometry is a well established method for measuring distances with great accuracy.  This hasn't always been the case though!  There was a time when there wasn't any way to calibrate the machines other than through tedious mechanical means.  The accuracy of machine tools was mainly dependant upon the operator's ability to precisely turn crank handles.  With the advent of CNC Machines it became of utmost importance to verify the displacement accuracy with the cnc control and to compensate for the errors.  From that need new calibration tools arose.  One of those tools is the Interferometer Laser.

Today, quality control personal, maintenance personal, and machine manufacturers use Laser Interferometry to compensate for CNC Machine and Coordinate Measuring Machine (CMM) linear displacement errors and other errors associated with the all degrees of freedom.  Even though laser interferometry is a modern tool, is not all that new, it originated with the advent of The Michelson Interferometer.
The Michelson Interferometer
The Michelson Interferometer was invented in the 1880s by Albert Abraham Michelson.  The basic principle is the use of the wavelength of light as the unit of measure.  The first Michelson interferometer used a white light as a source, a fixed mirror, and a movable mirror to measure linear displacement.  Later, the white light was replaced by a Helium-neon (HeNe) laser.  The wavelength of laser light from a HeNe laser is 0.633 micron or 0.000025 of an inch.  By further sub-dividing the wavelength, a resolution as small as 0.001 micron or 0.00000004 of an inch can be achieved.
Modern HeNe Interferometer Laser Systems
Two main types of interferometer laser systems have evolved from the use of the HeNe laser, single-frequency and dual-frequency systems.  The two most well known systems today are the Renishaw ML10 and XL-80 models which are single-frequency systems, and the Keysight 5530 (formerly the Hewlett-Packard HP 5529A, then the Agilent 5529A) which are dual-frequency systems.

InterferometerBasically, for measuring linear displacement, the HeNe laser beam is directed at an interferometer which is a polarized beam splitter.  Half of the beam in the single frequency case or one of the frequencies in the dual-frequency case passes through the interferometer to a movable (relative to the interferometer) measurement retroreflector, and the other half of the beam to a fixed reference retroreflector.  The reflected beams return and combine again at the interferometer.  Relative motion between the reference retroreflector and the measurement retroreflector causes a doppler shift in the return beam which is measured with a photodetector.  Since the laser wavelength is known, the displacement of the measurement retroreflector can be accurately determined.
Electronic Doppler Laser Systems
There is also another type of laser used today to calibrate CNC Machines and CMMs, an Electronic Doppler Laser.  Unlike the interferometer lasers mentioned above which use a laser tube to produce the beam.  They use electronics to produce the beam and to measure the doppler shift without an interferometer.  The only optic used for linear displacement measurements is a retroreflector which reflects the return beam right back through the output beam.  There are a number of manufacturers of Doppler Lasers.  Specific to machine tool calibration, metrology, etc; a well known manufacturer is Optodyne.
Wavelength of Light Compensation
Although the technology has been improved over the years, the wavelength of light as a unit of measure has remained.  However, the standard is the wavelength in a vacuum.  In a vacuum the wavelength of light is a stable value.  Outside of a vacuum the wavelength depends upon the optical density of air or the refractive index of air.  In the shop environment, the refractive index of air varies mainly with air temperature, barometric pressure, and relative humidity.  Because of this, wavelength of light compensation is required for linear displacement measurements.  With all the systems previously mentioned, compensation for linear displacement measurements can be automatically accounted for by means of an air sensor connected to the system.

Material Temperature Sensor Placement for Measuring Linear Displacement