Absolute interferometer measuring process and apparatus having a measuring interferometer, control interferometer and tunable laser

A method for making absolute measurements, said method comprising the steps of:

• providing a tunable laser having a variable operating current and an output power, the laser emitting a laser beam having a wavelength;
• providing a measuring interferometer having a measuring line of variable length, said measuring interferometer comprising;
• a beam splitter, a first reflector, a second reflector and at least one detector wherein said beam splitter, said first reflector and said detector are mounted on a first object and said second reflector is mounted on a second object, said second object being displaceable with respect to said first object wherein said measuring line extends from said beam splitter to said reflector;
• providing a control interferometer having a control segment of fixed length, said control interferometer comprising;
• a beam splitter, a first reflector, a second reflector and at least one detector wherein said beam splitter, said first and said second reflectors and said detector are mounted on a first object and said control segment extends from said beam splitter to said second reflector;
• directing said laser beam onto said beam splitter of said measuring interferometer and said control interferometer causing interference in said measuring interferometer and control interferometer;
• detecting said interference of said control interferometer and adjusting, based on said detected interference, the first wavelength .lambda..sub.1 until it is free of mode jumps;
• deactivating said control interferometer after said laser is adjusted to said first wavelength .lambda..sub.1 ;
• modulating the wavelength of said laser from said first wavelength .lambda..sub.1 to a second wavelength .lambda..sub.2 ;
• detecting said interference of said measuring interferometer;
• detecting an integral phase change .DELTA..phi. of said detected interference;
• calculating an absolute length L.sub.abs of said measuring line according to equation ##EQU7## and activating said control interferometer when the wavelength of said laser is at said second wavelength .lambda..sub.2.