JP2000081307A - Fast, high resolution double heterodyne interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a system and an apparatus which achieve a higher resolution while permitting coping with a higher speed of a target by generating a synthetic wavelength a half as much as the wavelength of a laser light of a light source in an optical heterodyne interferometer. SOLUTION: A light source employs lasers with the azimuth of polarization orthogonal to each other and having longitudinal mode frequency intervals different from each other. The light obtained by rotating the light source and the polarization of the light source by 90 deg. is admitted into a heterodyne interferometer to make two sets of interference light. One set is selected from the two sets of interference light depending on the moving direction of a target thereby permitting handling of the measurement of the fast moving target.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-speed heterodyne interferometer which uses a synthetic wavelength shorter than the wavelength actually used as a light source in an optical heterodyne interferometer and provides a Doppler shift higher than a carrier frequency inherent to the light source. It is.

[0002]

The resolution of an interferometer increases as the wavelength of the light source decreases. On the other hand, the measurement speed of the interferometer is limited by a frequency difference inherent to the laser, such as the Zeeman beat frequency. The present invention provides a high-speed heterodyne interferometer that is equivalent to using a short wavelength light source using a synthetic wavelength shorter than the actual laser wavelength, and using a low laser-specific frequency. No one has dealt with such issues in the past.

[0003]

According to the present invention, a light source has spectra having polarization directions which are alternately orthogonal to each other and have different frequency intervals. Here, three spectra will be described. Of the three spectra, two at both ends and one at the center are sent to the two optical arms of the optical heterodyne optical system, and the optical beat of the interference light (having two power spectra) is demodulated as a square. By doing so, it is possible to obtain a composite light having a wavelength half the wavelength of the original laser light.

[0004] In the present invention, two sets of light to be combined are produced, which is provided for speeding up the interferometer. That is, one set of light having the three spectra and a second set of light, which is obtained by rotating the polarization direction of the other light by 900 °, simultaneously pass through the two arms of the interferometer, Of the two sets of light, only the set on the side where the optical heterodyne frequency becomes high due to Doppler shift due to the high-speed change of the interference arm is selectively switched and picked up to realize a high-speed interferometer.

The above selection switching has many pitfalls and pits, but this problem has already been solved. Shuk
o Yokoyama et. al. , SPIE, (1
990) P212

[0006]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is an example of the present invention. In the figure, Π and ⊥ represent the directions of the polarized light. Here, Π is called p, and ⊥ is called s. It is assumed that the spectrum of the three-mode laser is composed of two p and one s. The frequencies of the light are ν1, ν2, ν3, the frequencies between the longitudinal modes are f1, f2, and the Doppler shift due to a change in the arm length of the interferometer is Δν.

In an actual three-mode HeNe laser, f1
And f2 is about 430 MHz f1-f2 is about 500 kHz. Such a laser beam (a) is split by a non-polarized beam splitter BS into two optical arms of the interferometer. The light reflected by the movable mirror M1 receives the Doppler shift, and becomes ν1 ± Δν, ν2 ± Δν, and ν3 ± Δν, respectively. The polarization direction has a spectrum as shown in FIG.

On the other hand, the light (d) reflected by the reference mirror M2 is 4
Since the light passes through the half-wave plate twice, (a) and p and s are reversed. After the light passing through the two optical arms is superimposed by the BS and then split again by a polarizing beam splitter having the same orientation as P and s, the P component becomes The spectrum has the same polarization of ± Δν. f1 and f2 are 4
Δν does not approach this value even with a high-speed interferometer of at least 00 MHz.

It is said that a wave detector is not sensitive to its frequency but to the square of the amplitude in wave optics (accurately, the number of photons). Therefore, the output current of the detector is f1-f2 = 5.
When this current having two power spectra obtained by adding and subtracting Δf to 00 kHz is square-detected, the spectrum (e) is f1−f2 ± 2Δf, that is, the s1 spectrum ( can get. This operation is so-called double heterodyne. This AC signal passes through 0 and becomes a negative value as Δν increases. That is, a part of the incident light (a) is extracted as a reference signal, and a similar process is performed to create a reference signal SR having a frequency of f1-f2, and an AC signal of s1 and s2 on which the frequency is not negative is selected. It is to deal with a high-speed interferometer by measuring a frequency difference and a phase difference between two AC signals.

[0010]

FIG. 2 shows an embodiment of the present invention. Light such as G emitted from the three-mode laser 1 is split by the non-polarizing beam splitter 2 and travels to the reference mirror 3 and the moving mirror 4. The light directed to the reference mirror is rotated 900 ° by the quarter-wave plate 5 and then re-overlaid by the beam splitter 2. The superimposed light is split by the polarization beam splitter 6, the first light beat by the first light detector 7, the second light beat by the second light detector 8, and the reference light detector 9. A reference light beat is required.

Further, the first, second, and reference light beats have two power spectra, which are squared by the integrator 10 to become first and second reference AC signals having a single power spectrum. The frequencies of the first and second AC signals vary depending on the speed of the movable mirror 4, and one of the frequencies may become negative beyond the zero point. Therefore, here, the value of the difference between the first and second AC signals and the reference AC signal is obtained, and the first or second side that does not become lower than the value of the reference AC signal is selected.
Prevents the passage of points.

To prevent this, the circuit composed of the 90 ° phase shifter 20, the frequency difference detection unit 21, and the pulse arrangement unit 22 switches the first and second AC signals without error.

The switched AC signal has correct information as long as the Doppler shift Δν does not exceed 400 MHz, and the position of the movable mirror can be known by the frequency measuring device 23 and the phase difference measuring device 24 with respect to the reference signal.

[0014]

By making the combined wavelength shorter than the wavelength of the laser, the resolution of the optical heterodyne interferometer can be increased, and high-speed interferometric measurement can be performed at a low laser-specific beat-to-beat frequency. Further, in this case, since a plurality of laser beams are formed by a perfect common path, the problem of wavefront mismatch does not occur at all.

[0015]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention.

FIG. 2 is an embodiment of the present invention.

[Explanation of symbols]

 1, 3-mode laser 2, non-polarized beam splitter 3, reference mirror 4, moving mirror 5, quarter-wave plate 6, polarized beam splitter 7, 8, 9, photodetector 10, integrator 20, 90 ° Phase shifter 21, Frequency difference detection unit 22, Pulse arrangement unit 23, U / D pulse counter 24, Phase difference meter

Claims (1)

[Claims] 1. A laser beam comprising three or more spectral lines having alternately orthogonal polarization directions and mutually different longitudinal inter-mode frequencies has a non-polarized beam splitter, and one optical arm has a quarter wavelength. Incident on an optical heterodyne interferometer having a plate, and splits the output light of the optical heterodyne interferometer into two sets of optical heterodyne interference light with a polarizing beam splitter having the same direction as the orthogonal polarization direction. The two sets of optical heterodyne interference light are detected by first and second two photodetectors to generate two sets of optical heterodyne signals, and a part of the incident light is taken out and detected by a third detector. A reference heterodyne signal was created and the 3
Demodulating by a method such as squaring each of the sets of heterodyne signals, and measuring a frequency difference and a phase difference between the demodulated reference AC signal waveform and one of the two sets of AC waveforms. Heterodyne interferometer