JPH01318902A - Measuring apparatus for displacement by two-wavelength optical heterodyne system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical heterodyne displacement measuring device, and in particular,
The present invention relates to a two-wavelength optical heterodyne displacement measuring device suitable for highly accurate displacement measurement.

[Conventional technology]

Research on measuring minute displacements using optical heterodyne interference technology is progressing. Conventional methods of optical heterodyne interference include a method of branching a single wavelength laser and shifting the frequency using a Bragg cell or the like, and a method of creating and using a two-wavelength laser beam using the 2eeman effect. The former method is described, for example, in Abride Optex, Vol. 18, No. 11 (1979) p. 1797-1803 (
Applied 0ptics Vol, 18°No,
11 (1979) pp. 1797-1803), the light emitted by a single wavelength laser source is split into two lasers, and a Brag laser is inserted into each laser optical path.
In this method, a gCell is placed, the frequency is shifted, two wavelengths of laser light are obtained, and the displacement of the object light is measured using a Twyman Green interferometer. Since this method includes an optical system that shifts the frequency midway, there is a problem that the entire optical system becomes large. In other words, if this measurement technology is used for measurements on the order of angstroms (person), even if there is a slight temperature change during measurement, a difference will occur in the optical path, and the wavelength of λ/100 (1: L/- =5o
0 to 700 nm), that is, a resolution of 5 to 7 nm was considered to be the limit. For this reason, active research is being carried out to improve the configuration of the optical system and to reduce the size of the entire system.

For example, I-N-N-N-I, Ultrasonic Symposium Proceedings (1985) pp. 432-435.
Page (IEEE, VltrasonicSymposiu
m Proceedings in San Fran
Cisco, (1985) pp. 432-435) proposes a method of fitting the entire optical system within 80 x 50 x 30 nm by combining a beam splitter, a doped prism, a polarizing beam splitter, and a Bragg cell.

On the other hand, the latter method is described, for example, in Ablide Optics, Vol. 19, No. 3 (1980) 435-441 (
Applied Optics, Vol. 19. No,
3 (1980) pp. 435-441), this method utilizes the Zeeman effect to generate two-wavelength laser light, thereby eliminating the need for a frequency shifter.

As described above, the miniaturization of optical systems has been progressing, but this has not been sufficient for high-precision displacement measurement on the order of angstroms (persons).

[Problem to be solved by the invention]

As mentioned above, research and development of miniaturization of optical systems has been progressing in the past, but it has not been sufficient for high-precision displacement measurement on the angstrom order (human).

An object of the present invention is to provide an optical heterodyne displacement measuring device that integrates a means for separating and combining two wavelength lasers and a detector for interference light.

[Means to solve the problem]

The above purpose is to separate and combine the two wavelength lasers at the same point in space using two 1/4 wavelength plates, a total reflection mirror, and a polarizing beam splitter using a single laser source with two wavelengths, and the resulting interference This is achieved by focusing the light onto a detector with a thin film lens.

[Effect]

Since the 1/4 wavelength plate and the total reflection mirror can be manufactured as thin film elements, they can be attached to a substantially cubic polarizing beam splitter.

Furthermore, although a bulk type lens has conventionally been used as a means for condensing light onto a detector, if this is made into a thin film, a condensing detector with an integrated detector can be produced. Therefore, by combining these two means, it is possible to create an interference optical system integrated with the detector, and it is possible to downsize the entire device.

(Example) Hereinafter, an example of the present invention will be explained with reference to Fig. 1.
The figure shows the basic configuration of the present invention. In the figure, dual wavelength laser 1
Linearly polarized beams (P-polarized light and S-polarized light) 10oO emitted from the polarized light beam 10oO that are tilted at 90 degrees to each other are separated into P-polarized light and S-polarized light by the polarization beam splitter 120. That is, the P-polarized light passes through the polarizing prism 120 and becomes 1
/4 wavelength plate converts it into circularly polarized light 1010, and the measurement target 5
It leads to 0. The reflected beam from this measurement object undergoes a Doppler shift due to the movement of the measurement object 50, and is 1/4
Return to wave plate 140. Here, the circularly polarized light changes to linearly polarized light, but since it passes through the 1/4 wavelength plate twice, the returned light from the measurement object 5o is tilted 90 degrees and is reflected by the polarizing beam splitter 120 and sent to the detector 20 be led to. on the other hand,
Of the two-wavelength laser 1000, the S-polarized light 102o reflected by the polarization beam splitter 12o passes through the quarter-wave plate 160, is reflected by the total reflection mirror 180, and is again
The light passes through the quarter-wave plate 160 and returns to the polarizing beam splitter 120. In this case as well, since the polarized light passes through the 174-wave plate 160 twice, the polarized light is rotated by 90 degrees and transmitted through the polarizing beam splitter 120. This transmitted light interferes with the return light from the measurement target 50 and is guided to the detector 200. In this case, it is necessary to narrow down the interference light 1o30 for photodetection, but since conventional lenses were bulk-type lenses, it was difficult to integrate them with a polarizing beam splitter.

Therefore, in the present invention, by developing a dual-wavelength multiple hologram lens 260 to be used, the lens is made thinner. In the case of a hologram lens, the shape of the hologram differs depending on the wavelength, but since there is almost no difference in the wavelength of the laser used here, it is also possible to create a hologram based on one wavelength. In this case, since the efficiency of the laser beam of the wavelength not used for hologram design decreases, it is desirable to select a laser beam with a smaller amount of light as the design wavelength. Interference light 1030 detected in this way
is converted into an electrical signal 1o40 and detected as a Doppler shift amount by the signal processing 20. The content of this signal processing 2o is the same as that of a commonly used optical heterodyne detection circuit.

FIG. 2 shows another embodiment of the present invention. In this example, the polarizing beam splitter 121 is rotated by 90 degrees from the polarizing beam splitter 120 of FIG. Suitable for use when the amount of light on one side is significantly smaller than the other. That is, in this example, the S-polarized light emitted from the two-wavelength laser is reflected by the polarization beam splitter 121 and immediately guided to the detector, so there is little loss in the optical system. On the other hand, the P-polarized light initially passes through the polarization beam splitter, but the return light from the measurement object 50 is reflected and guided to the mirror 18'O. Furthermore, the return light from the mirror 180 is transmitted to the polarizing beam splitter 1.
21 and is guided to the detector, the optical path is longer than that of the initial S-polarized light, and there are many optical elements through which it passes, resulting in a large loss. Therefore, the optical system is suitable for using the smaller light quantity of the two wavelength lasers as S-polarized light and the larger light quantity as P-polarized light.

FIG. 3 shows another embodiment of the present invention. 3 is characterized in that a half mirror 122 is used in place of the polarizing beam splitter 120 of FIG. 1, and a dichroic mirror 123 is used for separating and combining two wavelength lasers. That is, the two-wavelength laser 1000 emitted from the laser source 10 passes through the half mirror, but is separated by the dichroic mirror 123, and one is reflected and the other is transmitted. The transmitted light 1010 undergoes a Doppler shift at the measurement target, returns, and interferes with the reflected light at the dichroic mirror 123. This interference light 1030 is focused by the hologram lens 260 and guided to the detector 200.

FIG. 4 is a modification of FIG. 1. This example is suitable when the measurement target 50 is a minute part, that is, the laser beam 1010 that irradiates the measurement target is transmitted through the hologram lens 240.
narrowed down by Here, too, it is preferable to use a hologram lens, which allows the interference optical system to be integrated.

FIG. 5 is an example of an application system using the interference optical system 100 of the present invention. This example is an example used to measure the unevenness of the sample 60. When the lever 50 to be measured and the sample 60 approach, the lever 50 bends due to van der Waals force or magnetic flux. At this time, if the lever 50 is slightly vibrated by the vibration source 40, the sample 6
Since the vibration frequency shifts due to the interaction force between the lever 50 and the lever 50, the vibration frequency is detected by the interference optical system 100. Furthermore, ShiA-
If the feedback control 30 is performed so that the frequency of the sample 50 becomes a predetermined value, this feedback signal will be applied to the sample 6o.
corresponds to the unevenness signal.

〔Effect of the invention〕

According to the present invention, the two-wavelength heterodyne optical system including the detector can be integrated, making it easier to adjust the optical axis and making it more compact, so it is possible to suppress measurement errors due to temperature changes during measurement. I can do it.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an embodiment of the present invention, and FIG. 3 and 4 are explanatory diagrams of other embodiments, and FIG. 5 is a block diagram of FIG. 1. DESCRIPTION OF SYMBOLS 10... Dual wavelength laser light source, 20... Signal processing circuit, 30... Feedback circuit, 40... Vibration source,
50...Measurement object, 60...Sample, 100...Interference optical system. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. An optical heterodyne displacement measuring device having a light source that emits laser beams of two wavelengths and means for separating and combining the laser beams, comprising: a hologram that focuses interference light onto a detector; and the detection device. A two-wavelength optical heterodyne displacement measuring device, characterized in that the device is integrally constructed with the laser beam separation and combination means. 2. The two-wavelength optical heterodyne displacement measuring device according to claim 1, characterized in that a dichroic mirror and a half prism are used as means for separating and combining the two-wavelength lasers. 3. A two-wavelength interference optical element with an integrated structure of two quarter-wave plates, a total reflection mirror, a focusing hologram, and a polarizing beam splitter. 4. The dual-wavelength optical heterodyne displacement measuring device according to claim 1, further comprising means for focusing one of the dual-wavelength lasers onto a single point on a measurement target.