Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1995-04-24
1997-01-21
Turner, Samuel A.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
345345, 345346, G01B 902
Patent
active
055964109
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention concerns an interferometric measurement process, in particular for length measurement by means of a laser, whereby when a measuring interferometer is in operational state the laser wavelength is controlled by virtue of the fact that the output signal of a regulating interferometer, which changes periodically with the laser wavelength in the form of a wave, adjusts within a wave period to a value in the area of a wave edge, for example, and changes in the operational state of the measuring interferometer, i.e. the laser frequency and/or the wavelength of the laser beam, are detected in the regulating section of the regulating interferometer and compensated via the operating current and/or the temperature of the laser, with a corresponding effect on the measuring interferometer, together with a laser interferometer configuration suitable for implementation of this process, incorporating a laser which is mode jump-free and continuously tunable in a specified wavelength range, an initial measuring interferometer and an initial regulating interferometer, with a measuring and regulating section respectively, whereby a constant difference in length applies with regard to the arms of the regulating interferometer, further incorporating a primary beam splitter to split the incident laser beam so that the radiation from one and the same laser is supplied to the interferometers, whereby for the purpose of forming at least two mutually interfering sub-beams, the measuring interferometer possesses at least one primary non-polarised beam splitter, two retroreflectors or, alternatively, one or more reflectors or 90.degree. prisms, for example, further incorporating means to generate mutually phase-displaced beam components to enable identification of the direction of changes in the length of the measuring section, such as quarter-wave or eighth-wave plates, and means to separate the phase-displaced beam components, such as polarised beam splitters or Wollaston prisms, further incorporating at least one photo-detector to detect the interference signal, and whereby for the purpose of forming at least two mutually interfering sub-beams the regulating interferometer possesses at least one primary beam splitter, two retroreflectors or, alternatively one or more reflectors or 90.degree. prisms, for example, and at least one photo-detector to detect the interference signal, and finally incorporating a control device for the laser wavelength.
Mode jump-free, variable semiconductor lasers are being employed to an increasing extent in interferometers of the above-stated type. One reason for this is the relatively low unit costs for laser diodes in comparison to conventional laser sources. In addition to this, however, laser diodes are continuously variable over a wavelength range which is around twice as broad as that which applies to helium-neon lasers, for example. Conversely, however, semiconductor lasers have the disadvantage that their resonance wavelength is highly dependent on the operating parameters of the laser diode, as a result of which the laser wavelength is subject to time fluctuations. Additionally, fluctuations of the air wavelength may also occur in the measuring or regulating section, due to certain sections of the laser beam passing through open air, the refractive index of which may also be subject to fluctuations, for example on account of a change in the ambient temperature, humidity or the CO.sub.2 content of the air.
The semiconductor interferometer described in DE-PS 3 404 963 counters this problem via the use of a further interferometer to control the air wavelength, in addition to the actual measuring interferometer. This so-called regulating interferometer is configured in such a manner that a constant difference applies with regard to the respective lengths of the interferometer arms. The radiation from one and the same laser diode is supplied to both the measuring interferometer and the regulating interferometer. Should the air wavelength now alter within the measuring sect
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patent: 4984898 (1991-01-01), Hofler et al.
patent: 5177566 (1993-01-01), Leuchs et al.
Hantel Peter
Thiel Jurgen
Kim Robert
Turner Samuel A.
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