JPH10281717A - Light wave interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light wave interferometer wherein the displacement of a moving mirror is measured with higher precision by, relating to the light wave interferometer for measuring object's length, displacement, density, etc., reducing measurement error caused by fluctuation in drive frequency of frequency shifter to measure only a true fluctuation in refraction factor. SOLUTION: In oder to provide a specified drive frequency to acoustic optical elements(AOM) 200, 201, 202, and 203, respectively, one AOM driver 605 is provided. The AOM driver 605 divides a 160 MHz reference drive signal to obtain drive frequency signal of four frequencies 0.1, 2, 4, and 80 MHz. Then the AOM driver 605 combines five frequency signals 0.1, 2, 4, and 80 as well as 160 MHz to provide them as drive frequency to AOMs 200, 201, 202, and 203.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical interference measuring apparatus for measuring the length, displacement, density, etc. of an object.

[0002]

2. Description of the Related Art There is a heterodyne type interferometer as a typical light wave interference measuring device for measuring the length, displacement, density and the like of an object with high accuracy. This conventional heterodyne interferometer measures light of two frequencies, which have slightly different frequencies and whose polarization directions are orthogonal to each other, with a polarization beam splitter, and converts the light of one frequency into an object moving on the measurement optical path. The light is reflected by a fixed moving mirror, and the light of the other frequency is reflected by a fixed mirror on the reference optical path. The reflected light is made to interfere with the light and received by a light receiver, and the beat frequency is detected to measure the displacement of the object with high accuracy.

In order to further improve the measurement accuracy, this light wave interference measuring apparatus is provided with an interferometer for measuring a change in the refractive index of a gas such as air on a measuring optical path. For example,
The light of the different frequency from the light for length measurement and the light of the second harmonic thereof are guided to the above-described measurement optical path and the reference optical path, and then interfere with each other. The light is received by the light receiver and the beat frequency is detected. Thereby, the measurement error of the measured displacement amount based on the refractive index fluctuation of the air on the optical path is corrected.

As described above, in the lightwave interference measuring apparatus using the heterodyne method, since a beat frequency is detected by using a plurality of lights of different frequencies, as shown in FIG. A frequency shift unit 611 that generates light having a frequency of? In FIG. 3, light having a frequency f <b> 1 emitted from a light source (not shown) passes through a wave plate 210 and enters a PBS 121. This wave plate 2
The light (p) transmitted through the PBS 121 by rotating
The intensity ratio between polarized light, horizontal to the paper surface, and reflected light (s-polarized light, perpendicular to the paper surface) can be changed appropriately. PBS1
The p-polarized light transmitted through 21 is used as a frequency shifter
For example, s-polarized light that is incident on an acousto-optic device (hereinafter, referred to as AOM) 200 and reflected by the PBS 121 is AOM2
01, and are frequency-shifted to frequency f1
0 and f11 light. The lights having the frequencies f10 and f11 are made substantially coaxial by the PBS 122, and then pass through the frequency separation element (hereinafter, referred to as DM) 101.

Similarly, a frequency f emitted from a light source (not shown)
The light of No. 2 passes through the wave plate 211 and enters the PBS 123. By rotating the wave plate 211, the PBS 123 is rotated.
The intensity ratio between the light transmitting (p-polarized light, horizontal to the paper surface) and the reflected light (s-polarized light, vertical to the paper surface) can be appropriately changed. The p-polarized light transmitted through the PBS 123 is AO
The s-polarized light incident on M202 and reflected by PBS 123 is incident on AOM 203, and is frequency-shifted to light of frequencies f20 and f21, respectively. The lights of the frequencies f20 and f21 are made substantially coaxial by the PBS 124,
The light is reflected at 101 and is made substantially coaxial with the light of frequencies f10 and f11.

[0006] AOMs 200, 201, 202, and 203 are provided with a predetermined drive frequency for modulating incident light into light of a predetermined frequency. Each AOM200
There are provided four AOM drivers 601, 602, 603 and 604 in order to supply a predetermined drive frequency to. For example, AOM driver 601 is AOM20
0 supplies a driving frequency signal of 82 MHz, the AOM driver 602 supplies a driving frequency signal of 80 MHz to the AOM 201, and the AOM driver 603 supplies 1 signal to the AOM 202.
A driving frequency signal of 64.1 MHz is supplied, and the AOM driver 604 supplies a driving frequency of 160.1 MHz to the AOM 203. In this way, a predetermined drive frequency is supplied to the AOMs 200 to 203 from the separate drive frequency drivers 601 to 604.

The frequency of the frequency-shifted light is f10
= F1 + 82 MHz, f11 = f1 + 80 MHz, f2
0 = f2 + 164.1 MHz, f21 = f2 + 160.
1 MHz. Note that the frequency f transmitted through the DM 101
The lights 10 and f11 are separated by the BS 111 and pass through the frequency filter 181.
01 is incident. The interference light having the frequencies f10 and f11 is photoelectrically converted by the detector 401, and the frequency difference f10-f11
(= 2 MHz) is input to an arithmetic unit (not shown) as a reference signal.

The light beam emitted from the frequency shift unit 611 substantially coaxially in FIG. 3 travels along a predetermined optical path (not shown), and is used for measuring the displacement of an object and measuring the fluctuation of the air refractive index on the optical path.

[0009]

In the above-mentioned conventional light wave interference measuring apparatus, the influence of the frequency fluctuation of the frequency shifters 200 to 203 is not considered at all. However,
When the fluctuation of the driving frequency in each of the frequency shifters 200 to 203 independently occurs, the fluctuation component finally appears as a reference of the fluctuation of the refractive index and superimposed on the fluctuation of the phase difference between the length measurement signals. However, there is a problem that it cannot be distinguished whether the variation of the phase difference is caused by the variation of the refractive index of the gas in the length measuring optical path or the variation of the driving frequency of the frequency shifters 200 to 203.

For example, in order to measure the change in the refractive index of air, the frequency of the light having the frequency f10 that has returned in the measurement optical path is converted into a second harmonic f10 ',
In the case where the beat frequency is detected by causing interference with 20 lights, it is assumed that a drive frequency fluctuation Δf occurs in the AOM driver 601 that supplies the AOM 200 with a drive frequency of 82 MHz. Here, f10 ′ = 2 × (f1 + △ f +
82) MHz, f20 = f2 + 164.1 MHz, and 2 · f1 = f2, the beat frequency detected by the detector is f20−f10 ′ = 0.1 MHz−2Δ.
f. On the other hand, when the frequency of the light of the frequency f11 that has returned in the reference optical path is frequency-converted and converted into the second harmonic f11 ′ and interfered with the light of the frequency f21 to detect the beat frequency, f11 ′ = 2 × ( f1 + 80) MHz, f
Since 21 = f2 + 160.1 MHz, the beat signal detected by the detector is f21−f11 ′ = 0.1 MH
z. Here, the arithmetic unit measures the phase difference between the two beat signals, and measures the phase difference caused by the shift of the beat signals of the frequencies f20 and f10 'by 2 · Δf. That is, the arithmetic unit cannot discriminate whether this 2 · Δf is due to variation in the frequency of the AOM or variation in the refractive index of air.

As described above, in the conventional light wave interference measuring apparatus, when the driving frequency of each AOM fluctuates, the fluctuation component of the driving frequency of the AOM is superimposed on the original refractive index fluctuation measuring component. Since the fluctuation amount of the fluctuation of the refractive index is calculated including the fluctuation component of the above, the measurement accuracy of the stage position is reduced. This AO
The change in the M drive frequency is caused by electrical noise or AO
There is a problem that it is unavoidable because it is generated by the influence of the temperature of the oscillation element and the like of the M driver, and that the fluctuation component of the driving frequency cannot be removed by calculation or the like.

SUMMARY OF THE INVENTION It is an object of the present invention to reduce a measurement error based on a variation in a driving frequency of a frequency shifter, measure only a true refractive index variation, and measure a displacement of a movable mirror with higher accuracy. Is to provide.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to measure a light beam for measuring a displacement of an object and a fluctuation of a refractive index of a gas on an optical path of the light beam for measuring a refractive index. A light source unit that emits a light beam, a beam splitter that is provided on a common optical path of a length measuring light beam and a refractive index fluctuation measuring light beam, and that is provided on the reference optical path, and a beam splitter that separates the optical path into a measuring optical path and a reference optical path. Light wave interference comprising a reference light reflecting mirror, a measuring light reflecting mirror movably provided on the measuring light path, and a plurality of light receiving means for receiving respective interference lights of the reflected light beams from the reference light path and the measuring light path. In the measurement device, the light source unit generates a plurality of frequency modulation elements that respectively generate light of a plurality of different frequencies as a length measurement light beam and a refractive index fluctuation measurement light beam, and generates a plurality of different frequency drive signals from a reference frequency. Multiple frequencies It is accomplished by optical interference measuring apparatus according to claim that a frequency modulation element driving system for supplying the respective modulation elements.

As described above, according to the present invention, a driving frequency is generated by one frequency shifter driver and supplied to each frequency shifter to correct the influence of the frequency fluctuation on the measurement of the refractive index fluctuation of the gas. This makes it possible to accurately measure the change in the refractive index of the gas, and to accurately measure the true displacement of the movable mirror in which the influence of the change in the refractive index of the gas on the measurement optical path is corrected.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical interference measuring apparatus according to an embodiment of the present invention will be described with reference to FIGS. FIG.
1 shows a schematic configuration of the lightwave interference measuring apparatus according to the present embodiment. FIG. 2 shows a configuration of the frequency shift unit 610 of the optical interference measurement apparatus according to the present embodiment. The light wave interference measuring apparatus according to the present embodiment is characterized in that a single driver for providing a driving frequency to a frequency modulation element (hereinafter, an AOM is used as an example) is provided. Further, the light wave interference measurement apparatus according to the present embodiment is configured to use the measurement light for refractive index fluctuation also for length measurement and to measure the refractive index fluctuation of gas in the reference optical path.

The light source 300 has a frequency f1 (wavelength 1.064).
μm) and the frequency f2 (= 2
(F1, wavelength 532 nm). Light source 300
The light emitted from is incident on the frequency shift unit 610.

In FIG. 2, the light of frequency f 1 emitted from the light source 300 passes through the wave plate 210 and enters the PBS 121. By rotating the wave plate 210, the PBS 12 is rotated.
The intensity ratio of the light passing through 1 (p-polarized light, horizontal to the paper) and the light reflected (s-polarized light, perpendicular to the paper) can be changed appropriately. The p-polarized light transmitted through the PBS 121 is A
The s-polarized light that is incident on the OM 200 and reflected by the PBS 121 is incident on the AOM 201 and is frequency-shifted to light of frequencies f10 and f11. The lights of the frequencies f10 and f11 are made substantially coaxial by the PBS 122,
101.

Similarly, the light of frequency f2 emitted from the light source 300 passes through the wave plate 211 and enters the PBS 123. By rotating the wave plate 211, the intensity ratio of the light (p-polarized light, horizontal to the paper surface) transmitted through the PBS 123 and the reflected light (s-polarized light, perpendicular to the paper surface) can be appropriately changed. The p-polarized light transmitted through the PBS 123 is AOM20
The s-polarized light that has entered A.2 and reflected from the PBS 123 is A
The light enters the OM 203 and is frequency-shifted to become light of frequencies f20 and f21. The lights of the frequencies f20 and f21 are made substantially coaxial by the PBS 124, then reflected by the DM 101, and made substantially coaxial with the lights of the frequencies f10 and f11.

In order to give a predetermined drive frequency to each of the AOMs 200, 201, 202, and 203, one AOM
A driver 605 is provided. AOM driver 60
5 is a frequency dividing frequency of the reference driving signal of 160 MHz, four frequencies, 0.1 MHz, 2 MHz, 4 MHz, and 80 MHz.
To generate a driving frequency signal. The AOM driver 605 generates the generated 0.1 MHz, 2 MHz, and 4 MHz.
Each of the AOMs 200, 201, 202, 20 is constructed by combining five frequency signals of z, 80 MHz, and 160 MHz.
3 as the driving frequency. Specifically, AOM20
82 MHz for 0, 80 MHz for AOM201, AO
164.1 MHz for M202 and 16 for AOM203
Provides a drive frequency of 0.1 MHz. As described above, since one AOM driver supplies a predetermined drive frequency to each AOM, the reference frequency 16
When a frequency fluctuation occurs in the driving frequency signal of 0 MHz, the fluctuation component is supplied to each AOM.

The frequency of the frequency-shifted light is f10
= F1 + 82 MHz, f11 = f1 + 80 MHz, f2
0 = f2 + 164.1 MHz, f21 = f2 + 160.
1 MHz. The lights of frequencies f10 and f11 coaxial with the PBS 122 are separated by the BS 111, pass through the frequency filter 181, and then enter the detector 401 with a polarizer. The interference light having the frequencies f10 and f11 is photoelectrically converted by the detector 401, and the frequency difference f10−
A beat signal equal to f11 (= 2 MHz) is input to the arithmetic unit 500 as a reference signal.

In FIG. 1, a light beam emitted from the frequency shift unit 610 substantially coaxially enters a polarizing beam splitter (hereinafter referred to as PBS) 120. The light beams incident on the PBS 120 are separated according to the respective polarization directions. The s-polarized light of frequencies f11 and f21 is PBS1
The light is reflected at 20, is circularly polarized by the wave plate 212, and is then coaxially reflected by the fixed mirror 150. And again the wave plate 21
2, the plane of polarization is rotated 90 degrees to be p-polarized light.
After passing through the PBS 120, the corner cube 220
After the optical path is displaced, the light passes through the PBS 120 and again enters the reference optical path. The light having the frequencies f11 and f21 incident on the reference optical path is again rotated by 90 degrees by the wave plate 212 and the fixed mirror 150, becomes s-polarized light, and is reflected by the PBS 120 toward the DM 110.

On the other hand, the p-polarized light having the frequencies f10 and f20 is transmitted through the PBS 120, and the wave plate 213 and the movable mirror 14 are moved.
At 1, the polarization plane is rotated by 90 degrees and becomes s-polarized light and returns coaxially to the PBS 120. And PBS120
The light path is shifted by the corner cube 220,
The light is reflected by the PBS 120 and reenters the length measuring optical path. Thereafter, the polarization plane is rotated by 90 degrees by the wave plate 213 and the moving mirror 140, passes through the PBS 120, and enters the DM 110 almost coaxially with the light of the frequencies f11 and f21.

The light beam emitted coaxially from the PBS 120 is DM
Part of the light of frequencies f10 and f11 is transmitted by 110, and the rest of the light of frequencies f10 and f11 is
The light of f21 is reflected. Frequency f transmitted through DM110
The lights of 10 and f11 pass through the frequency filter 180 and enter the detector 400. At this time, the light having the frequencies f20 and f21 transmitted through the DM 110 is cut by the frequency filter 180. Although not particularly shown, the detector 400 is provided with a polarizer, and its polarization plane has frequencies f10 and f10.
It is inclined 45 degrees with the polarization direction of No. 11. Therefore, the frequency f
The interference light of 10, f11 is photoelectrically converted by the detector 400,
The frequency difference f10-f11 (the reference frequency is 2M
(E.g., Hz) is input to the arithmetic unit 500 as a length measurement signal.

The luminous flux reflected by the DM 110 is a PBS 125
, The p-polarized light at frequencies f10 and f20 is transmitted, and the s-polarized light at frequencies f11 and f21 is reflected. s-polarized light is SH
A part of the light having the frequency f11 which is incident on the G conversion element 170 is converted into the second harmonic, and becomes the light having the frequency f11 ′, and the light having the frequency f21 is transmitted as it is. After each light passes through the frequency filter 182, the detector 40 with a polarizer
2 is incident. At this time, the frequency filter 182 transmits only light of frequencies f11 ′ and f21. Frequency f1
The lights 1 ′ and f21 interfere after transmitting through the polarizer, and have a frequency difference f21−f11 ′ (the reference frequency is 0.1 MHz).
Is input to the arithmetic unit 500 as a measurement signal of the change in the refractive index of the gas in the reference optical path.

Similarly, the p-polarized light is supplied to the SHG conversion element 17.
1 and a part of the light having the frequency f10 is converted into the second harmonic to become light having the frequency f10 ', and the light having the frequency f20 is transmitted as it is. Each light is a frequency filter 1
After passing through 83, the light enters a detector 403 with a polarizer. At this time, the frequency filter 183 outputs the frequency f10 ′,
Only the light of f20 is transmitted. Frequency f10 ', f20
Light interferes after passing through the polarizer, and the frequency difference f20−f
A beat signal equal to 10 ′ (the reference frequency is 0.1 MHz) is input to the arithmetic unit 500 as a measurement signal of the change in the refractive index of the gas in the length measurement optical path.

The arithmetic unit 500 compares the reference signal for measuring the displacement of the moving mirror from the detector 401 of the frequency shift unit 610 with the length measurement signal from the detector 400, and calculates the displacement ΔD of the moving mirror from the phase change. I do. Also, detectors 402 and 4
The measurement signals from the sensors 03 are compared, and the amount of change in the refractive index of air is obtained from the amount of phase change.

Here, a method of simply correcting the refractive index fluctuation of air will be described. The optical path lengths measured with the light having the frequencies fa and fb are defined as D (fa) and D (fb). At this time, each optical path length is

D (fa) = {1 + NF (fa)} D (1) D (fb) = {1 + NF (fb)} D (2)

## EQU2 ## Where D is the geometric distance,
N is the density of the air, and F (f) is a function determined by the frequency f of the light regardless of the density of the air unless the composition ratio of the air changes. From the above equation, the geometric distance D is

D = D (fa) −A {D (fb) −D (fa)} (3) where A = F (fa) / {F (fb) −F (fa)}

## EQU1 ## From this, the displacement of the geometric distance desired by this interferometer, that is, the true displacement ΔD is

ΔD = △ D (fa) −A ｛△ D (fb) − △ D (fa)｝ (4) where A = F (fa) / ｛F (fb) -F (fa) )｝

## EQU1 ## The above equation means that, based on the displacement of the movable mirror measured with the light of the frequency fa, the influence of the change in the refractive index determined with the light of the frequencies fa and fb is corrected, and the true displacement is determined. In this case, the light having the frequencies f10 and f11 corresponds to the light having the frequency fa, and the lights having the frequencies f20 and f21.
Corresponds to the light of the frequency fb. Frequency f1
The displacement ΔD of the movable mirror (f (f)
a) are measured, and the frequencies f11 and f2 passing through the reference optical path are measured.
1 from the phase change of the beat signal produced by the beat signal produced by the frequency signal f10 and f20 produced by the frequencies f10 and f20 passing through the length measuring optical path.
ΔD (fa)} is measured. This is calculated by equation (4) to find the true displacement ΔD.

Now, consider the case where the AOM driver 605 has a frequency variation Δf at a reference frequency of 160 MHz. Under the influence of this frequency fluctuation, each AO
The frequency of each light generated at M is f10 = f1 + △ f /
2 + 82 MHz, f11 = f1 + △ f / 2 + 80 MH
z, f20 = f2 + Δf + 164.1 MHz, f21 =
f2 + Δf + 160.1 MHz. However, 0.1
The effect of Δf at MHz to 4 MHz is ignored.

After each light passes through a predetermined optical path,
Separated according to each polarization direction by S125,
The light of f11 and f21 enters the SHG conversion element 170,
Lights of f10 and f20 enter the SHG conversion element 171. In the SHG conversion element, a part of the light of f10 and f11
HG conversion, f10 ′ (= 2 · f10), f11 ′
(= 2 · f11). The respective lights pass through the frequency filters 182 and 183 and then enter the receivers 402 and 403 with polarizers. The light transmitted through the polarizer interferes, and the receivers 402 and 403 input a signal equal to the frequency difference between the respective lights to the arithmetic unit 500.

Here, the receiver 402 has f11 'and f21
The receiver 403 outputs a beat signal based on a frequency difference of 0.1 MHz (= f21−f11 ′).
A beat signal based on a frequency difference of 0.1 MHz (= f20-f10 ') is generated. The frequency fluctuation Δf generated by the AOM driver 605 in the above process is included in the lights of the frequencies f21 and f11 ′, respectively, and the beat signals f21−f1 detected by the detector 402 due to the interference of the two lights.
At 1 ′, the frequency fluctuation Δf is canceled and removed.
Similarly, in the beat signals f20-f10 ′, the frequency fluctuation Δf is canceled and removed. Therefore, the receiver 40
Reference numerals 2 and 403 can output to the arithmetic device 500 a signal in which the influence of the drive frequency fluctuation is removed and reduced. The arithmetic unit 500 can measure the phase variation of each signal, remove the influence of the drive frequency variation, calculate the reduced refractive index variation, and finally calculate the geometric displacement of the stage 160 with high accuracy. can do.

As described above, the AOMs 200, 201, 20
2 and 203 are driven at the drive frequency generated based on the reference frequency in the AOM driver 605,
Even if the reference driving frequency of the driver 605 fluctuates, A
The OMs 200, 201, 202, and 203 reflect the frequency fluctuations equally. For this reason, according to the optical interference measuring apparatus of the present embodiment, the influence of the phase fluctuation between the measurement signals can be remarkably reduced by referring to the refractive index fluctuation measurement. This makes it possible to reduce the variation in the phase of the measurement signal due to the variation in the frequency of the AOM driver, and to more accurately measure the variation in the refractive index of the gas in the reference and length measuring optical paths. Therefore, the displacement of the movable mirror can be measured more precisely.

In the present embodiment, the displacement of the movable mirror 140 is measured using the light of the frequency f1 from the light source 300, but may be measured at another frequency f2 in principle. Here, the reason why the length measurement is performed using the light having the frequency f1 is as follows. One of the causes of a decrease in the measurement accuracy of this type of interferometer is a problem in the extinction ratio of the PBS. The light that should be transmitted is reflected, or the light that should be reflected is transmitted, and the measurement accuracy is reduced due to the error light. This error light finally enters the SHG conversion elements 170 and 171. The intensity of the emitted light from the light source is 200 mW for the light of frequency f1,
Assuming that the light of the frequency f2 is 200 mW, the conversion efficiency of the light of the frequency f1 to be originally detected by the SHG conversion element is at most about 1% (the intensity of the converted SHG light is proportional to the square of the incident light intensity. To do). Therefore, the error light reaching the detector is weaker and does not significantly affect the measurement accuracy of the refractive index measurement. On the other hand, if light of frequency f2 is used for displacement measurement, light of frequency f2, which is error light,
The light is directly incident on the detectors 402 and 403 without being G-converted. For this reason, there is a possibility that the measurement accuracy may be affected by the error light. This is the reason why light having the frequency f1 is used for displacement measurement.

[0039]

As described above, according to the present invention, it is possible to realize an optical interference measuring apparatus in which a measurement error due to a variation in a driving frequency of a frequency shifter is reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a light wave interference measurement device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of a frequency shift unit of the optical interference measurement apparatus according to one embodiment of the present invention.

FIG. 3 is a diagram illustrating a configuration of a frequency shift unit of a conventional light wave interference measurement device.

[Explanation of symbols]

100, 101, 110 Frequency separation element (dichroic mirror: DM) 111 Beam splitter (BS) 120, 121, 123, 124 Polarization beam splitter (PBS) 140 Moving mirror 150 Fixed mirror 160 Stage 170, 171 SHG conversion element 180, 181 , 182, 183 Frequency filters 200, 201, 202, 203 Frequency shifters (AO
M) 210, 211, 212, 213 Wave plate 220 Corner cube prism (CCP) 300 Light source 400, 401, 402, 403 Detector 500 Arithmetic unit 601, 602, 603, 604, 605 Frequency shifter (AOM) driver 610, 611 Frequency shift section

Claims (1)

[Claims] 1. A light source section for emitting a length measuring light beam for measuring a displacement of an object and a refractive index fluctuation measuring light beam for measuring a refractive index fluctuation of a gas on an optical path of the length measuring light beam. A beam splitter provided on a common optical path of the length measuring light beam and the refractive index fluctuation measuring light beam, and separating a light path into a measuring light path and a reference light path; and a reference light reflection provided on the reference light path. A light wave interference measurement comprising: a mirror; a measurement light reflecting mirror movably provided on the measurement light path; and a plurality of light receiving means for receiving respective interference lights of reflected light beams from the reference light path and the measurement light path. In the apparatus, the light source unit may include a plurality of frequency modulation elements that respectively generate light of a plurality of different frequencies as the length measurement light beam and the refractive index fluctuation measurement light beam, and a drive signal of the plurality of different frequencies from a reference frequency. Generating and optical interference measuring apparatus characterized by comprising a frequency modulation element driving system for supplying to each of the plurality of frequency modulation element.