A common optical path interferometric gauge.

9 3 07 Interferometric measuring device with a common optical path Interferometrisk mätanordning med gemensam optisk bana

Object of the invention

The present invention relates to interferometric measuring devices and more particularly to optical measuring devices having interferometers with common optical paths.

Background of the invention

With the advent of optical fibers, numerous interferometric fiber optic measuring devices have been developed. These devices can be widely applied to the measurement of vibration and dynamic distortion of mechanical components. In addition, fiber optic interferometers are particularly suitable for metrology due to their versatility and the size of the components involved.

Prior art fiber optic interferometers also include those characterized as modified Mach-Zehnder or Twyman-Green interferometers. In their simplest form, these interferometers use light from a coherent source that is divided into two optical beams. The first beam is used as a comparator and traverses a fixed-length optical path. The second beam is guided along an optical path whose length is changed by the external workpiece to be measured. The beams are later combined to produce an interference pattern that indicates vibration or dynamic distortion of the mechanical element.

Optical auxiliary vibrator interferometers are also well known in the art. These devices are similar in nature to Mach-Zehnder and

Twyman-Green basic interferometers, but modified to include an optical modulator that changes the optical frequency of the reference and / or measurement beam. As with Mach-Zehnder or TwymanGreen basic interferometers, the length of the optical path of the measuring beam is changed by an oscillating external element. Both rays are combined

39307 later again to obtain a frequency modulated beam having a carrier frequency equal to the frequency of the optical modulator and deviations from the carrier caused by vibration or dynamic distortion of the mechanical element. The component due to the motion of the mechanical element is separated by conventional frequency modulation demodulation techniques.

In prior art fiber optic measurement systems, both reference and measurement beams are guided along optical paths that include discrete optical fibers. This configuration ensures that the actually paid reference wavefront is used for comparison with an unknown Doppler-switched wavefront returning from the workpiece. However, optical fibers act as microphones picking up ambient sounds. The ambient noise signal consists of unwanted Doppler shifts due to ambient vibrations and slight changes in the refractive index of the optical fiber carrying the measurement or reference beam, and is added to the vibration signal measured from the workpiece. typical manufacturing environments.

Description of the invention

The present invention relates to an interferometric measuring device having a common optical path for measuring the movement of a workpiece surface.

According to the present invention, an interferometric measuring device having a common optical path is characterized in that it comprises:

- an optical source unit for generating an optical beam having a coherent length;

- an optical device for dividing the optical beam into the first and second beams and providing them with modulation, the first and second beams being guided along the first and second optical orbits in this order, the length difference of which is selected to be greater than the coherent length;

- a unit having a common optical path corresponding to the first and second beams, forming a combined beam, directing the combined beam and dividing the combined beam into reference and measuring beams each comprising first and second beam components guided along the optical measuring path, including the workpiece surface and optical reference track; the length difference between the optical measurement and reference paths is selected to be approximately the same as the length difference between the first and second optical paths so that coherence between them is restored, and the unit having a common path combines the reference and measurement beams to form an interference beam;

- a detector unit for receiving the interference beam and providing a corresponding electrical signal; and

- a signal processing unit for receiving and demodulating the electrical signal to determine a component indicating the movement of the workpiece surface.

Brief description of the drawings

Figure 1 is a block diagram of a common track interferometric measuring device in accordance with the present invention; and

Figure 2 is an alternative embodiment of a common track interferometric measuring device according to Figure 1.

Best Mode for Carrying Out the Invention

Figure 1 is a simplified block diagram of an interferometric measuring device having a common path according to the present invention. This common track laser interferometer includes an optical source 12 of partially coherent light, such as a conventional laser diode. The optical source generates an optical beam 14 on the lens 16 which focuses the light beam splitter 18 into the input fiber, which splitter represents a fiber optic beam splitter in the best embodiment, but those skilled in the art will recognize that corresponding separate beam splitters may be used in place. In the best embodiment, the laser diode comprises a Sharp LTO23MC that emits light at a wavelength of 780 nanometers.

The fiber optic beam splitter divides the optical beam into first and second beams which are guided along the first and second optical paths, respectively. The first optical path 20 includes a fiber optic section 22, a collimation lens 24, and an acoustic optical modulator 26. The modulator is of the type known in the art and in the best embodiment comprises a Brag cell such as a Hoya A-100 modulator that lowers the first beam optical frequency by about 75 MHz. The alternating frequency first beam is applied to a lens 28 which focuses the beam into the fiber optic section 32. The lens 28 as well as the above-mentioned lenses may be of any type known in the art and in the best embodiment comprise microscope object sets. The optical fiber portion 32 represents typical fiber optic components used in the interferometric measuring devices of the present invention and comprises a Corning glass-to-glass type fiber having an inner diameter of about 5 to 9 microns and a degree of refractive index and a numerical aperture of 0.1, as noted by those skilled in the art. that the corresponding multimode or monofilament fiber may be selected instead.

The second beam 34 exits the beam splitter and is received by an acoustic optic modulator 36, similar to the acoustic optic modulator 26 described above, after traversing the fiber optic section 38 and the lens 40. In the best embodiment, the modulator 36 raises the optical frequency to about 85 MHz. It will be appreciated by those skilled in the art that a common orbit interferometric measuring device according to the present invention may be provided with only one modulator located on either the first or second optical orbit. As will be explained in detail below, an additional modulator has been added to reduce the carrier signal throughput during demodulation of the optical signals.

The alternating frequency second beam emanates from the lens 42 and focuses on the optical fiber portion 44. Those skilled in the art will recognize that although optical fibers have been used to direct the first and second optical beams, corresponding optical units may be used in place on the respective optical paths.

A partially coherent optical source can be characterized by a coherent length, which is defined as the difference in the length of the optical path to which the optical beams emanating from the same source interfer. If the lengths of the optical paths traversed by the rays differ by an amount that exceeds the coherent length, the rays are said to be incoherent. In the common-path interferometric measuring device of the present invention, the difference between the lengths of the optical paths of the first and second beams is selected to be greater than the coherent length of the optical source so that the two beams become incoherent. In the best embodiment, the first beam traverses a longer optical path than the second beam, and this is expressed as follows:

Li - La> l_c where L1 is the length of the optical path of the reference beam and La is the length of the optical path of the measuring beam, and l_c is the coherent length of the optical source.

Both beams are arranged in a bidirectional optical switch 45 of a type known in the art, for example ITT T-7270. The beams connected thereto are directed to a common optical path, which in the best embodiment comprises a common optical fiber portion 46. Since the difference between the optical paths of the measurement beams exceeds the coherent length of the optical source, no interference can occur between the beams propagating in the common optical fiber portion. The identity of the individual rays is not lost when the coherence between the selected parts of the combined beam is sufficiently maintained.

A portion of the combined signal is reflected internally from the end surface 48 of the common optical fiber portion. This reflected portion comprises a reference beam and includes force from the first and second beams. Reflection can be accomplished by techniques known in the art and may involve the use of common optical surface layers or only from the end surface of the Fresnel helical coating. Figure 3 illustrates a portion 50 of a common optical fiber portion and an end surface 48 having an optical surface layer 52 formed thereon.

The remaining force of the combined beam comprises a measuring beam 54 and includes the remaining force of both the first and second beams. The measuring beam originates from a common optical fiber portion and is directed to a conventional collimating lens 56 and later to a focusing lens 58 where it is focused on a moving workpiece surface 60. It should be noted that in some configurations a single lens can be used to collect and focus light. The reflected light from the workpiece surface travels back through lenses 58 and 56 and returns to the common optical fiber portion.

The length of the optical path traversed by the measuring beam is selected so that its portion of the first beam has a total length of the optical path that is approximately equal to the total length of the optical path of the second beam, including the coherence length of the laser diode. Upon returning to the common optical fiber portion, the portion of the second beam of the reference beam reflected at the end face and the first component of the measuring beam are interfered because they are again coherent.

The interference beam formed between them travels back to the optical switch, where part of it is distributed and directed via lines 62 to a detector 64 of a type known in the art, such as a conventional silicon diode. The detector provides an electrical signal equivalent to the interference beam on lines 66 to signal processor 68. It will be appreciated by those skilled in the art that mold-like detectors may be used on the farm, which may include conventional additional preamplifiers or filtration devices.

The signal processor can demodulate either the sum or the difference of the frequencies given by the modulators. In the best embodiment, the signal processor comprises a filter having a passband of 160 MHz and a conventional radio frequency mixer that lowers the detection signal to 100 MHz of the frequency modulated frequency band. Signaali7

The S93C7 processor also comprises a conventional FM receiver. The FM receiver provides signals that indicate movement of the workpiece surface. Those skilled in the art will recognize that demodulation at differential frequency requires appropriate modifications to the signal processor described in the best embodiment. Those skilled in the art will further recognize that other similar signal processor units, both analog and digital, may be used on the farm.

Fig. 2 shows an alternative embodiment 70 of the common path interferometric measuring device of Fig. 1 illustrating the use of certain different optical components and similarly including a light source 72 which provides a partially coherent beam 74 to the first beam splitter 76. This produces first and second beams 78 and 80 which are given for acoustic optical modulators 82 and 84. Lenses 86 and 88 focus both beams on optical fiber portions 90 and 92, respectively. As in the interferometric measuring device having a common path in Fig. 1, the length of the optical path traversed by each beam is selected so that the difference between them exceeds the coherent length of the coherent light source. The beams are combined in a beam switch 94. The components of the first and second beams of the combined beam do not interfere as they travel along a common optical path such as a common optical fiber portion 96 for the reasons set forth above with reference to Figure 1.

In addition to components and structures that are identical to the common path interferometer described above in Figure 1, the alternative common path interferometric measuring device of Figure 2 comprises a second beam splitter 98 or similar fiber optic switch that receives the combined beam after collimation from lens 100 fiber parts. The second beam splitter produces a reference beam 102 comprising a portion of both the first and second beams and reflected from the mirror 104. The remainder of the common beam comprises a measuring beam 106 which the lens focuses on and from the surface of the workpiece 110.

Both the measuring and reference beams contain light from the first and second beams, which have previously been made incoherent by adjusting the length difference of the optical path of the 9 9C 7 larger than the coherent length of the partially coherent source. The optical coherence between the portion of the second beam of the reference beam and the portion of the first beam of the measuring beam can be restored by adjusting the length difference between the optical paths between them to be smaller than the coherent length of the partially coherent source.

The interference beam 112 is produced in the second beam splitter by a portion of the coherent beams and collimated by a lens 114 and directed to an optical control unit such as a fiber optic section 116 and finally a detector 118 which generates an electrical signal corresponding to the interference beam. The signal is routed via lines 120 to a signal processor 122 identical to the signal processor shown in FIG. The FM receiver component produces a signal that indicates movement of the workpiece surface.

Those skilled in the art will recognize that the optical interferometric measuring device of the present invention is symmetrical in the placement of the detector and the optical source. In the embodiments of Figures 1 and 2, the detector and optical source can be interchanged without affecting the performance of either interferometric measuring device.

By using a common optical path for the reference and measurement beams, the small changes in the coefficient caused by the ambient noise sources affect both beams equally, thus eliminating the problem of microscopic picking. Due to the way in which the rays are delayed, a fixed reference wavefront, relative to the returning wavefront, is created in any case, which ensures an accurate measurement of the Doppler shift produced by the vibrating workpiece.

Although the invention has been and will be described with reference to the best mode, those skilled in the art should likewise appreciate that numerous other changes, omissions, and additions may be made therein without departing from the spirit and scope of the invention.