Coherent light generators – Particular beam control device – Optical output stabilization
Patent
1991-01-22
1992-06-23
Epps, Georgia Y.
Coherent light generators
Particular beam control device
Optical output stabilization
372 19, 372 20, 372 29, 372 97, H01S 313
Patent
active
051249944
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a device for generating light, in particular light for use in length measurement using an interferometer. The frequency of the laser light emitted is controlled as a function of the wavelength in the surrounding gaseous medium (air) in such a way that the wave length in air remains constant.
With interferomic determination of displacement of a mobile component, it is of decided meaning to precisely know the wavelength of the employed laser light in the surrounding medium (henceforth called wavelength in air), since the displacement path exists in units of the wavelength in air. If one knows the exact frequency of the light wave beforehand, the possibility arises to determine the wavelength in air through an ascertainment of the refractive index.
The refractive index can be determined using the "parameter method"-the temperature, air pressure, and air moisture are measured and the index is formulmericly calculated. However, this parameter procedure has the disadvantage that important influences such as the composition of gas are not incorporated and thus eliminates possible operation of the interferometer in the workshop or open areas.
A device is already known where the wavelength in air is measured by an open etalon and the frequency of the light source is adjusted as a function of the refractive index of air, in such a way that the wave length in air for interferomic measurement remains constant. In this device, a Helium-Neon-Laser is employed as the light source. The variation of the emitted laser light's frequency is possible through adjustment of the laser resonator's length by means of warming and cooling or through utilization of the "Piezo-Effect."
Apart from a relatively high required power supply and size of a Helium-Neon-Laser a particular disadvantage appears that the range of frequency adjustability merely suffices for determining fluctuations in the refractive index in the magnitude of .DELTA.n.apprxeq.2.10.sub.-6, while in practice the resulting fluctuations are far higher, namely in the range .DELTA.n.apprxeq.10.sub.-4.
The object of the invention is to provide a device for generating light, in particular light for use in length measurement using an interferometer, that is able to hold the wavelength constant in the surrounding medium through steady changes in frequency with the existing fluctuations in the refractive index (magnitude of .DELTA.n.apprxeq.10.sup.-4). In addition, the device should present a compact model, in particular a small light source and if need be, the possibility of battery operation.
According to the invention, this is accomplished by the fact that the light source is a laser diode and a device is provided external to the laser resonator, for feeding back to the resonator, at selected frequencies, light emitted by the laser diode.
With the feeding back of the laser light at selected frequencies according to the invention's device, it is possible, within certain limits, to "lock" the emissions-frequency of the laser diode's light to the preferred feedback frequency, when alterations of the wave length in air or the refractive index in the surrounding medium occur, in such a way that the wavelength in air actually remains constant, whereby by this procedure the otherwise typically unsteady (discontinuous) behavior of the emission-frequency of the laser diode as a function of the operative parameters (injection current, temperature) can be avoided within a certain "locking range."
With this "locking" of the laserdiode's emission-frequency within a range of control, the device utilizes the known effect that feedback light at selected frequencies amplifies the corresponding emission-frequency-mode of the laserdiode or represses the unwanted build-up of other modes, in such a way that it is even possible to "lock" the emitted light of the laser diode to each frequency that corresponds to the preferred feedback frequency from the frequency selective feedback device.
The frequency selectivity of a feed back device can then
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Article in "Wissensspeicher Lasertechnik", VEB, Fachbuchverlag Leipzip, 1987 pp. 417-420.
Article in the book "Laser" by Knenbuhl and Sirgrist; Verlag Teubner, Stuttgart, 1988, pp. 190-193.
Lazecki Rene
Leuchs Gerhard
Epps Georgia Y.
Tabarelli Werner
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