JPH05302810A - Heterodyne two wave lengths displacement interference meter - Google Patents

Abstract

PURPOSE:To ensure mechanical stability and improve measuring accuracy by providing a modulation means for changing at least one light frequency of reference light and measuring light. CONSTITUTION:The light wave emitted by a semiconductor laser 22 is condensed through a lens 10a in a KNbO3 crystal 23, split into two light waves with common axis and different wave lengths and arranged in parallel light with a lens 10b. The two light waves are separated into light waves 51, 52 by a polarization beam splitter 5 and go into acoustooptical elements 8a and 8b. The elements 8a, 8b form diffraction light for each light wave of basic wave and second harmonic wave. By properly controlling a scattering compensation prism 7 at this moment, the primary diffraction light of the basic wave and the secondary diffraction light of the second harmonic wave become parallel light having almost common axis. Then, only the first order diffraction light of the basic wave and the second order diffraction light of the second harmonic wave in the light waves 51 and 52 are synthesized with a polarization beam splitter 24 and this synthesized light waves are introduced into a Michelson type interference meter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention divides at least two kinds of light waves having different wavelengths, which are set to be parallel in a coaxial or spatially close state, into a reference light and a measurement light, respectively. The present invention relates to a two-wavelength displacement interferometer that measures a change in optical path difference between reference light and measurement light at each wavelength by light wave interference between the reference light and measurement light.

[0002]

2. Description of the Related Art In the precision industry, a precision length measuring machine is an important major element, and its performance greatly influences the performance of the entire device. In a length measuring instrument using an interferometer, the most appropriate method is adopted according to the environment for measurement and the required accuracy.
For example, if the measurement is performed in an extremely stable environment, make the time coherence of the measurement laser as long as possible,
It is considered to introduce a squeezing method. On the other hand, in a very unstable environment, a method has been proposed in which environmental parameters that fluctuate with time, such as air fluctuations, do not affect the length measurement. Two-wavelength displacement interferometry is
This is a typical method.

Next, the principle of the two-wavelength displacement interferometer will be described. Consider a case where two different wavelength laser beams having the same phase characteristic are coaxially incident on a single interferometer. If the optical path lengths observed at the respective wavelengths are D1 and D2, then D1 = n1 (ρ, λ1) d D2 = n2 (ρ, λ2) d. Where ρ is an appropriate thermodynamic parameter, λ1
, Λ2 is the two wavelengths of the laser, n1 and n2 are the refractive indices of air at the respective laser wavelengths, and d is the distance. Further, from the physical empirical rule, n1 / n2 = k (λ1, λ2) approximately holds. Therefore, the above simultaneous equations are
Then, there are two unknowns, and the definite solution d is obtained. in this way,
The two-wavelength interferometer is a method of eliminating an error in observation data due to air fluctuation by using air dispersion.

An experimental example of a two-wavelength interferometer is "J.Jpn.Appl.P".
hys., Vol. 28, L473 (1989) "and JP-A-1-98902. In these experimental examples, the 90 ° phase difference method was adopted to measure the displacement amount.

[0005]

Many methods other than the 90 ° phase difference method can be used for measuring the amount of interference displacement. In particular, the heterodyne method is a typical one.
However, when trying to perform the heterodyne method in a two-wavelength interferometer, it is necessary to create two frequencies with different lightwave frequencies at the two wavelengths (because the wavelength also changes when the frequency changes), and a total of four lightwaves must be handled. The system becomes complicated. In general, a displacement interferometer is required to have mechanical stability such as an optical component (mirror, beam splitter) forming a light source and an interferometer. Therefore, as the number of constituent parts increases and the spatial size of the apparatus increases, it becomes difficult to secure mechanical stability, which hinders improvement of measurement accuracy. The present invention aims to solve such problems.

[0006]

For the above object, in the present invention, at least two kinds of light waves having different wavelengths which are set to be parallel in a coaxial or spatially close state are orthogonally polarized. A two-wavelength displacement interferometer that splits a reference light and a measurement light by a beam splitter and measures a change in optical path difference between the reference light and the measurement light at each wavelength by interference of the light waves of the reference light and the measurement light. A modulation means for changing the optical frequency of at least one of the light and the measurement light is provided.

[0007]

In order to form two light waves having a constant frequency change for heterodyne measurement, an optical system having a structure shown in FIG. 6 has been conventionally used. The light of frequency ω is
After being separated by the polarization beam splitter 3a, the frequencies are (ω + f1) and (ω +) by passing through the acousto-optic elements 1 and 2 driven at different frequencies f1 and f2.
f2). Then, the polarization beam splitter 3
It is adjusted from b.

With this method, two different frequencies of ω1 and ω2
The light waves are (ω1 + f1), (ω1 + f2), (ω2 + f3
4) and (ω2 + f4) are shown in FIG. In the figure, the dichroic mirror 4
Indicates that the light wave of ω1 is reflected and the light wave of ω2 is transmitted. The acousto-optic element 6 has frequencies f1, f2, f, respectively.
It is driven at 3, f4. In this optical system, the frequency ω1,
The two light waves of ω2 are split into the light waves of ω1 and ω2 by the dichroic mirror 4, and then two light waves of which the constant frequency is changed with respect to the light of ω1 and ω2 are formed in the same manner as the optical system of FIG. Obtain a total of 4 light waves. In order to reduce the number of acousto-optic elements 6 to be used and to simplify the optical system, light waves ω1 and ω2 having different frequencies may be incident on a common acousto-optic element (driven at frequency f). FIG. 5 is a diagram showing a state in which the light waves ω 1 and ω 2 having the frequencies are incident on the common acoustooptic device 6. In general, lights of different wavelengths are diffracted in different directions due to the first-order diffraction of the acoustooptic device. On the other hand, in a two-wavelength interferometer, it is desirable that two light waves having different wavelengths be coaxial or close to each other inside (the optical system of) the interferometer. Therefore, in the present invention, as shown in FIG. 3, two optical waves having different wavelengths that are diffracted by the acousto-optic element 6 in different directions are made into parallel light beams that are nearly coaxial by using the optical prism 7. According to the present invention, by appropriately setting the incident condition on the optical prism 7 and the dispersion of the optical prism 7, one acousto-optical element 6 and one prism 7 allow parallel light (ω 1 + f, ω 2 + f) can be obtained.

The present invention is particularly effective when the fundamental wave (ω) of the laser and the second harmonic (2ω) formed by a nonlinear optical crystal or the like are used as the two light waves of different wavelengths.
When the fundamental wave and the second harmonic arranged coaxially enter the same acousto-optic device, the first-order diffracted light (ω +
f) and the second-order diffracted light of the second harmonic (2ω + 2f) are diffracted in almost the same direction, but the direction is slightly different due to the optical dispersion of the acousto-optic element itself. Therefore, the difference in this direction can be easily corrected by using the present invention.
Note that it is possible to perform this correction by another method, for example, a semi-transparent mirror or a combination of mirrors, but in that case, it is practical that the optical system becomes complicated and much light is wasted due to reflection and the like. This is a problem and is not preferable.

[0010]

[Embodiment 1] FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention. In the heterodyne two-wavelength displacement interferometer of the present embodiment, a pair of acousto-optic elements and a dispersion compensating prism are used as the modulation means for forming two light waves having different frequencies. The light wave emitted from the semiconductor laser 22 is reflected by the lens 1
0a causes the light to be condensed in the KNbO3 (potassium niobate) crystal 23 to form two coaxial light waves having different wavelengths, and then is collimated by the lens 10b. The light wave emitted from the semiconductor laser 22 has a wavelength of 860 nm and an output of 100.
mW. The second harmonic generated in the KNbO3 crystal 23 has a wavelength of 430 nm and an output of 100 μW. here,
If necessary, the oscillation wavelength of the semiconductor laser 22 may be fixed, or the laser spectrum may be narrowed.

After the polarization characteristics of these two light waves are appropriately corrected by the phase plate 12, they are split by the polarization beam splitter 5 with approximately equal intensities, respectively.
It becomes 1,52. Each of the light waves 51 and 52 is transmitted by the acousto-optic device 8
It is incident on a and 8b. The acousto-optic element 8a has a frequency f
1 and the acousto-optic element 8b is driven at the frequency f2. These acousto-optic elements 8a and 8b form diffracted light with respect to the respective light waves of the fundamental wave (ω) and the second harmonic wave (2ω). At this time, by appropriately adjusting the dispersion compensating prism 7, the first-order diffracted light of the fundamental wave (ω) and the second-order diffracted light of the second harmonic wave (2ω) become parallel light beams that are nearly coaxial. In this embodiment, PbMoO is used as the acousto-optic elements 8a and 8b.
A 3 (lead molybdate) crystal driven by a piezoelectric element was used. In addition to this, as an acousto-optic element, TeO2
It is possible to use (tellurium dioxide), glass or the like driven by a piezoelectric element. Further, as the dispersion compensating prism 7, a processed optical glass (BK7) was used.

In this embodiment, only the first-order diffracted light of the fundamental wave and the second-order diffracted light of the second harmonic of the light waves 51 and 52 are combined by the polarization beam splitter 24, and the combined light waves are displaced by the Michelson type. It is incident on the interferometer. The Michelson displacement interferometer is composed of a polarization beam splitter 25, a movable mirror 14, a fixed mirror 15, and a quarter-wave plate 13 capable of simultaneously converting two light waves having different wavelengths into circularly polarized light. Here, by matching the polarization planes of the polarization beam splitters 24 and 25 as much as possible, increasing the extinction ratio of each polarization beam splitter, and improving the performance of the quarter-wave plate 13, crosstalk in heterodyne measurement, that is, , Frequency (f1 -f2) and (2f
It is possible to suppress the occurrence of a fixed beat of 1-2f2). The light wave emitted from the Michelson displacement interferometer is dispersed by the dichroic mirror 16 which has a characteristic of transmitting the fundamental wave and efficiently reflecting the second harmonic wave. On the optical path that has passed through the dichroic mirror 16,
A polarizer 26 and an optical filter 17 that transmits only the fundamental wave and absorbs the second harmonic are installed. And
Only the fundamental wave enters the photodetector 18 by the optical filter 17, and the heterodyne signal appearing in the fundamental wave is measured.
Further, a polarizer 27 is installed on the optical path reflected by the dichroic mirror 16, and the second harmonic passes through the polarizer 27 and then enters the photodetector 18 to appear in the second harmonic. The signal is measured. The photodetector 18 detects the heterodyne signal at each wavelength. The heterodyne two-wavelength arithmetic processing system 31 processes the heterodyne signals measured by both photodetectors 18 to calculate the displacement amount (of the movable mirror 14) from which the length measurement error due to air fluctuation is removed.

[0013]

Second Embodiment FIG. 2 is a schematic configuration diagram showing a second embodiment of the present invention. Light source 9 consisting of a semiconductor laser excitation ring Nd: YAG laser (oscillation wavelength 1.06 μm, output 300 mW)
The light wave emitted from the KTP crystal (K
TiOPO4) 11 focused on two different coaxial wavelengths
After becoming one light wave, the light is collimated by the lens 10b. The second harmonic generated in the KTP crystal 11 has a wavelength of 5
The output is 30 nm and the output is 70 μW. If necessary, the oscillation wavelength of the light source 9 may be set to a light absorption line of a molecule, such as iodine molecule (I2).
You may fix by using the absorption line etc. of.

The polarization characteristics of these two light waves are appropriately corrected by the phase plate 12, and then split by the polarization beam splitter 5 with approximately equal intensities. The interference optical system in this embodiment is composed of two polarization beam splitters 5 and 29, a plane moving mirror 14, a quarter wave plate 30 capable of handling two light waves having different wavelengths, and two fixed corner cube prisms 20 and 28. Has been done. A right angle prism or a corner mirror may be used instead of the corner cube prism.

The light wave reflected by the polarization beam splitter 5 is again reflected by the polarization beam splitter 5 via the corner cube prism 20 and becomes reference light 53 of the interferometer. On the other hand, the light wave transmitted through the polarization beam splitter 5 is
Since the light passes through the quarter-wave plate 30 twice before being reflected by the movable mirror 14 and returning to the polarization beam splitter 5, the polarization plane is rotated by 90 °. Therefore, the reflected light from the movable mirror is reflected by the corner cube prism 28, reflected again by the polarization beam splitter 5, and then reflected by the movable mirror 14 again to become the measurement light 54. In the interference optical system, the reference light 53 and the measurement light 54 are not coaxial with each other (so that there is a space in which an acousto-optical element and a dispersion compensation prism described later can be installed for each light). The arrangement of the optical elements (corner cube prism, etc.) that compose is set.

The reference light 53 and the measurement light 54 are respectively collected by the acousto-optic element 8 before being combined by the polarization beam splitter 29.
The a and 8b and the dispersion compensating prism 7 shift the frequency of each light wave by a predetermined value. The same acousto-optic element and dispersion compensation prism as in Example 1 are used. The first-order diffracted light of the fundamental wave and the second-order diffracted light of the second harmonic are
The dispersion compensating prism 7 collimates the light into near parallel rays.
The reference light 53 and the measurement light 54 are both circularly polarized by the quarter-wave plate 13 and then combined by the polarization beam splitter 29. One of the combined light waves is incident on the photodetector 18 through the polarizer 19 and the optical filter 17 that transmits only the fundamental wave, and the heterodyne signal appearing in the fundamental wave is measured. The other light enters the photodetector 18 via the polariser 19 and the optical filter 21 that transmits only the second harmonic, and the heterodyne signal appearing in the second harmonic is measured. The heterodyne two-wavelength arithmetic processing system 31 processes the heterodyne signals measured by both photodetectors 18 to calculate the displacement amount (of the movable mirror 14) from which the length measurement error due to air fluctuation is removed.

[0017]

EFFECTS OF THE INVENTION Generally, a displacement interferometer is required to have mechanical stability such as an optical component (mirror, beam splitter) constituting a light source and an interferometer. As the mechanical size increases, it becomes difficult to ensure mechanical stability. In the present invention, the number of acousto-optic elements used is reduced to half, and the modulation light of different wavelengths is converted into parallel light close to the coaxial axis by using only one prism, which is extremely stable as compared with the conventional one. A heterodyne two-wavelength interferometer can be provided.

[Brief description of drawings]

FIG. 1 is a schematic diagram of one embodiment of a heterodyne dual wavelength displacement interferometer of the present invention.

FIG. 2 is a schematic diagram of another embodiment of the heterodyne two-wavelength displacement interferometer of the present invention.

FIG. 3 is a diagram showing that the dispersion compensating prism compensates the dispersion of the diffraction angle generated when two light waves having different wavelengths are incident on the acoustooptic device driven at the frequency f.

FIG. 4 shows frequencies (ω1 + f1), (ω1 + f1) obtained by using four acousto-optic elements from two light waves of frequencies ω1 and ω2.
2) is a diagram showing an example of the configuration of an apparatus for generating four light waves of (ω2 + f3) and (ω2 + f4).

FIG. 5 shows dispersion of diffraction angles generated when two light waves having different wavelengths are incident on an acoustooptic device driven at a frequency f.

FIG. 6 is a schematic diagram showing a conventional method for generating light waves of frequencies (ω + f1) and (ω + f2) from a light wave of frequency ω by using two acousto-optic elements.

[Explanation of symbols]

1 Acousto-optical element driven at frequency f1 2 Acousto-optical element driven at frequency f2 6 Acousto-optical element (part of modulation means) 7 Dispersion compensation prism (part of modulation means) 8 Acousto-optic element (of modulation means) 9) Light source (semiconductor laser excitation ring Nd: YAG laser) 11 KTP crystal 12 Phase plate 13 Quarter wave plate 14 Moving mirror 15 Fixed mirror 17 Optical filter that transmits fundamental wave and absorbs second harmonic wave 18 Photodetector 19 Polarizer 20 Corner cube prism 21 Optical filter that transmits the second harmonic and absorbs the fundamental wave 22 Semiconductor laser 23 Potassium niobate (KNbO3) crystal 26, 27 Polarizer 28 Corner cube prism 30 Quarter wave plate 31 Heterodyne dual wavelength arithmetic processing system 50 Reflector

Claims (2)

[Claims] 1. An orthogonal polarization beam splitter divides light waves of at least two types of different wavelengths, which are set to be parallel in a coaxial or spatially close state, into a reference light and a measurement light, respectively. In a two-wavelength displacement interferometer for measuring a change in optical path difference between reference light and measurement light at each wavelength by interference of light waves of the reference light and measurement light, at least one of the reference light and the measurement light is changed in optical frequency. A heterodyne two-wavelength displacement interferometer characterized by having a modulation means. 2. A heterodyne two-wavelength displacement interferometer, wherein the modulating means comprises a photoacoustic optical element and a dispersion compensating prism, and changes the optical frequency of at least one of the reference light and the measurement light by a constant value. ..