JP2008128899A - Output stabilization method of tracking laser interferometer and tracking laser interferometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of a tracking laser interferometer employing a homodyne laser interferometer in measuring displacement wherein the measurement of displacement and the SN ratio degrades, when tracking errors increase. <P>SOLUTION: The distance between the optical axis of a laser beam, incident on a retroreflector 108 and the center position of the retroreflector 108 is detected as a tracking error quantity ΔTr. The quantity, that adjusts the light intensity of the laser beam for keeping the amplitude of a litharge signal to be used in measuring the displacement by the interferometer 107 constant, is calculated from the detected tracking error quantity ΔTr, and the light intensity of the laser beam is adjusted, depending on the tracking error quantity ΔTr. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a tracking laser interferometer for measuring the displacement and position of a moving body with high accuracy while tracking the moving body. In particular, using a Lissajous signal obtained by using the interference of the laser beam irradiated toward the retroreflector that is the object to be measured and reflected in the return direction by the retroreflector, the displacement of the retroreflector is detected, The present invention relates to an improvement of a tracking type laser interferometer that performs tracking using a change in the position of the optical axis of the laser beam.

  Patent Documents 1 and 2 disclose tracking laser interferometers for measuring the displacement and position of a moving body with high accuracy while tracking the moving body. In these tracking laser interferometers, when using a so-called homodyne type laser interferometer that uses a Lissajous signal that is a two-phase sine wave signal with a phase difference of 90 degrees for measuring displacement, when the tracking error amount increases, There is a problem that the SN ratio of the displacement measurement value is lowered.

  The above contents will be described in detail below.

  FIG. 1 shows, as an example, an apparatus using a homodyne Michelson laser interferometer as a laser interferometer proposed by the applicant in Japanese Patent Application No. 2005-216110 (unpublished at the time of filing) as a laser interferometer for measuring displacement. .

  This apparatus measures the displacement ΔL of the retroreflector 108 using the interferometer 107 and the displacement meter 106 when the center of the fixed reference sphere 105 is a datum. Specifically, a change ΔL1 in the distance between the interferometer 107 and the retroreflector 108 is measured using the interferometer 107, and a change ΔL2 in the distance between the reference sphere 105 and the displacement meter 106 is measured using the displacement meter 106. . ΔL can be obtained by adding ΔL1 and ΔL2.

  The laser incident on the interferometer 107 is incident from, for example, a frequency-stabilized He-Ne laser light source 101 and is incident on the interferometer 107 via the lens 102 and the optical fiber 103. The laser incident on the interferometer 107 is divided into two, one of which is used as reference light for interferometric measurement, and the other is emitted toward the retroreflector 108. The laser beam emitted toward the retroreflector 108 is reflected by the retroreflector 108 and then enters the interferometer 107 again. The laser re-entering the interferometer 107 is divided into two, one of which interferes with the reference light as measurement light. The intensity change of the interference light is detected by a phase meter, processed by the signal processing circuit 111, and the displacement ΔL1 is measured using a personal computer (PC) 116.

  The displacement meter 106 is, for example, a capacitance displacement meter or an overcurrent displacement meter, and measures ΔL2 by measuring a signal after signal processing by the signal processing circuit 112 by the PC 116.

  ΔL can be obtained by adding ΔL1 and ΔL2 on the PC 116.

  The other of the lasers re-entering the interferometer 107 enters the optical position detector 238 shown in FIG. The optical position detector 238 is a two-dimensional PSD or quadrant photodiode (QPD), specifically, detecting the distance between the optical axis of the laser incident on the retroreflector 108 and the center position of the retroreflector 108. Can do. This distance is hereinafter referred to as tracking error amount ΔTr. In FIG. 2, 222 is a collimator lens, 226 and 234 are λ / 4 plates, 230 is a polarizing beam splitter (PBS), 236 is a non-polarizing beam splitter (NPBS), 240 is a polarizing plate, 270 is a plane mirror, and 272 is light detection. It is a vessel.

  The optical position detector 238 can measure ΔTr by dividing it into two orthogonal components. For example, as shown in FIG. 1, when the optical axis of the laser emitted from the interferometer 107 is Z, the horizontal axis perpendicular to the Z axis is the X axis, and the Y axis is perpendicular to Z and X, The optical position detector 238 can detect the X direction component ΔTrX and the Y direction component ΔTrY of ΔTr. Signals corresponding to ΔTrX and ΔTrY are taken into the PC 116 via the signal processing circuit 113, and the elevation angle motor 109 and the azimuth angle motor 110 are controlled via the motor drive circuits 114 and 115 according to the values of ΔTrX and ΔTrY. Thus, the carriage 104 is rotated in the elevation direction and the azimuth direction, and control is performed so that the center position of the retroreflector 108 coincides with the optical axis of the laser emitted from the interferometer 107.

  FIG. 3 shows a functional block diagram of the signal processing circuit 111. A signal corresponding to the intensity change of the interference light input from the interferometer 107 is processed by the preamplifier circuit 201 to be a Lissajous signal (two-phase sine wave signal having a phase difference of 90 degrees). The Lissajous signal output from the preamplifier circuit 201 is subjected to AD conversion, Lissajous correction and interpolation in the Lissajous correction / interpolation circuit 202, and is output as a two-phase square wave having a phase difference of 90 degrees. This two-phase square wave is input to the counter 203 and counted. Since this count value changes in accordance with ΔL1, ΔL1 can be measured by reading this count value with the PC 116.

Japanese Patent No. 2603429 US Pat. No. 6,147,748

  However, as described above, in the tracking type laser interferometer using a homodyne type laser interferometer for displacement measurement, there is a problem in that the SN ratio of the displacement measurement value decreases as the tracking error amount increases.

  FIG. 4 shows measured values of the relationship between the tracking error amount ΔTr and the amplitude of the Lissajous signal. The horizontal axis is ΔTr, and the vertical axis is the normalized Lissajous signal amplitude. As ΔTr increases, the deviation between the position of the optical axis of the reference light and the position of the optical axis of the measurement light increases, and the proportion of lasers that cause interference decreases. Therefore, as ΔTr increases, the amplitude of the Lissajous signal decreases. When the amplitude of the Lissajous signal becomes small, the SN ratio of the displacement signal of the interferometer decreases.

  The tracking laser interferometer performs control so that ΔTr becomes 0. However, since the position of the retroreflector 108 changes, the amplitude of the Lissajous signal may actually decrease.

  In the above description, an example of the device is shown. However, even in the case of the devices of Patent Documents 1 and 2, if a homodyne type laser interferometer is used for measuring the displacement, the device is similar to the case of the device of FIG. It is considered that there is a problem that the SN of the measurement value of the displacement decreases with an increase in the tracking error amount.

  The present invention has been made to solve the above-described conventional problems. When a homodyne laser interferometer is used for displacement measurement, the SN ratio of the displacement measurement value decreases as the tracking error amount increases. The purpose is to solve the problem.

  The present invention detects a displacement of a retroreflector by using a Lissajous signal obtained by irradiating a retroreflector as an object to be measured and using interference of a laser beam reflected in the return direction by the retroreflector. And a tracking laser interferometer that performs tracking using a change in the position of the optical axis of the laser beam, between the optical axis of the laser beam incident on the retroreflector and the center position of the retroreflector. The distance is detected as a tracking error amount. From the detected tracking error amount, an adjustment amount of the laser beam light amount for keeping the amplitude of the Lissajous signal constant is obtained, and the laser beam light amount is adjusted according to the tracking error amount. By doing so, the above-mentioned problems are solved.

  The tracking error amount can be calculated from the X direction component and the Y direction component.

  The present invention also irradiates the retroreflector that is the object to be measured, and uses the Lissajous signal obtained by utilizing the interference of the laser beam reflected in the return direction by the retroreflector, to detect the displacement of the retroreflector. In a tracking laser interferometer that detects and tracks using a change in the position of the optical axis of the laser beam, the optical axis of the laser beam incident on the retroreflector and the center position of the retroreflector A means for detecting a distance between them as a tracking error amount, a means for obtaining an adjustment amount of a laser beam light quantity for keeping the amplitude of the Lissajous signal constant from the detected tracking error amount, and depending on the tracking error amount And a tracking laser interferometer, characterized in that it comprises means for adjusting the amount of laser beam light. It is intended to.

  In a tracking laser interferometer that uses a homodyne laser interferometer for displacement measurement, the tracking error amount ΔTr has a relationship as shown in FIG. 4 between the tracking error amount ΔTr and the amplitude of the Lissajous signal. By controlling the amount of laser light incident on the interferometer according to the value of ΔTr, even if ΔTr changes, the amplitude of the Lissajous signal is kept constant, and the SN ratio of the measured displacement value is Decline can be prevented.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the present embodiment, as shown in FIG. 5, an attenuation variable optical attenuator 401 is inserted in the middle of the optical fiber 103 in the same configuration as the apparatus of FIG. Driven by the calculated attenuation amount variable optical attenuator drive signal, the amplitude of the Lissajous signal is kept constant even if the tracking error amount ΔTr changes.

  As the attenuation variable optical attenuator 401, a monitor PD built-in attenuation amount having a structure in which the input light is attenuated to an arbitrary value by moving the ND filter 402 using a stepping motor 403, as illustrated in FIG. A variable optical attenuator can be used.

  In FIG. 6, the laser incident on the variable optical attenuator 401 via the optical fiber 103A is collimated by the lens 404 to become parallel light. The amount of light of the collimated laser is attenuated by passing through the attenuation variable ND filter 402. The laser beam that has passed through the ND filter 402 enters the optical fiber 103B via the lens 405.

  The attenuation variable ND filter 402 is an ND filter whose attenuation changes according to the position of the ND filter in the longitudinal direction (vertical direction in FIG. 6). The ND filter 402 is indicated by an arrow A using the motor 403. Thus, the amount of attenuation of the transmitted laser can be controlled by moving in the longitudinal direction.

  Two PDs (photodetectors) 410 and 411 monitor the attenuation amount of the ND filter 402 using beam splitters 412 and 413. The amount of attenuation can be calculated by taking the ratio of the amounts of laser light detected by the two PDs 410 and 411.

  The signal processing circuit 420 controls the motor 403 so that the amount of attenuation commanded by the command signal matches the amount of attenuation of the ND filter 402 detected by the two PDs 410 and 411. Therefore, the variable optical attenuator 401 can change the attenuation amount so as to become the attenuation amount commanded by the command signal.

  The operation will be described below.

  First, ΔTr is calculated using the PC 116 from the signal corresponding to ΔTrX and the signal corresponding to ΔTrY input from the signal processing circuit 113 to the PC 116. Since ΔTrX and ΔTrY are components obtained by decomposing ΔTr in two orthogonal directions, ΔTr can be calculated using the following equation.

ΔTr = √ {(ΔTrX) ² + (ΔTrY) ² } (1)

  Next, an optical attenuator control signal for controlling the attenuation amount of the variable optical attenuator 401 is output from the PC 116 according to the calculated value of ΔTr.

Here, the relationship between ΔTr and the amplitude of the Lissajous signal as shown in FIG. 4 is obtained in advance, and the attenuation amount R of the variable optical attenuator that makes the value of ΔTr and the amplitude of the Lissajous signal constant from this relationship. A relational expression with _(ΔTr) is obtained in _advance . For example, when the amplitude of the Lissajous signal when ΔTr = 1 mm is attenuated by 80% compared to the amplitude of the Lissajous signal when ΔTr = 0 mm, it is variable when ΔTr = 1 mm compared to when ΔTr = 0 mm. The attenuation amount R _{(ΔTr = 1 mm)} of the variable optical attenuator may be determined so that the amount of laser light transmitted through the optical attenuator 401 is increased by 80%. R _(ΔTr) is determined while changing _ΔTr, and a relational expression between ΔTr and R _(ΔTr) is obtained.

  This relational expression is expressed by, for example, the following expression (2).

R _(ΔTr) = 100− (100−R _{(ΔTr = 0)} ) / A _(ΔTr) (2)

Here, R _{(ΔTr = 0)} is R when ΔTr = 0, and can be arbitrarily set. A _(ΔT) is the amplitude of the Lissajous signal normalized by the amplitude of the Lissajous signal when ΔTr = 0, and the relationship between ΔTr and A is shown in FIG. From the graph, it can be approximately obtained in the form of a polynomial.

The unit of attenuation R _(ΔTr) is%, and the attenuation R can be defined by the following equation (3).

R = (I ₀ −I ₁ ) / I ₀ × 100 (3)

Here, I ₀ is the amount of laser light incident on the variable optical attenuator, and I ₁ is the amount of laser light emitted from the variable optical attenuator.

By using this relational expression, R _(ΔTr) can be calculated from the value of ΔTr calculated by the PC 116 using the PC 116, so that the attenuation amount of the variable optical attenuator 401 becomes R _(ΔTr). The optical attenuator control signal is output from the PC 116 to the variable optical attenuator 401.

  Finally, the variable optical attenuator 401 adjusts the amount of laser light incident on the interferometer 107 in accordance with the optical attenuator control signal from the PC 116.

  As described above, the amplitude of the Lissajous signal can be maintained at a constant value by monitoring the tracking error amount ΔTr and appropriately adjusting the attenuation amount of the variable optical attenuator 401 in accordance with the change in ΔTr. Therefore, even if ΔTr changes, it is possible to suppress a decrease in the SN ratio of the measurement value.

  In the above embodiment, the present invention is applied to the apparatus shown in FIG. 1. However, the application target of the present invention is not limited to this. For example, even in the case of the apparatuses of Patent Documents 1 and 2, measurement of displacement is performed. If a homodyne type laser interferometer is used, the same applies.

  The means for adjusting the amount of laser beam light is not limited to the monitor PD built-in attenuation variable optical attenuator illustrated in FIG.

Perspective view including a partial block diagram showing an example of an apparatus using a homodyne laser interferometer as a laser interferometer for measuring displacement 1 is an optical path diagram showing a configuration example of the interferometer of the apparatus of FIG. Similarly, a block diagram showing a configuration example of a signal processing circuit The figure which shows the example of the relationship between tracking error amount and the amplitude of a Lissajous signal for demonstrating the conventional problem The perspective view including the partial block diagram which shows the structure of embodiment of the tracking type laser interferometer which concerns on this invention An optical path diagram including a partial block diagram showing a configuration example of an attenuation variable optical attenuator used in the embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Laser light source 103 ... Optical fiber 104 ... Carriage 105 ... Reference sphere 106 ... Displacement meter 107 ... Interferometer 108 ... Retroreflector 109 ... Motor for elevation angle 110 ... Motor for azimuth angle 111-3 ... Signal processing circuit 114, 115 ... Motor Drive circuit 116 ... Personal computer (PC)
401: Attenuation variable optical attenuator

Claims (3)

The Lissajous signal is obtained by using the Lissajous signal obtained by using the interference of the laser beam irradiated to the retroreflector that is the object to be measured and reflected in the return direction by the retroreflector. In a tracking laser interferometer that performs tracking using a change in the position of the optical axis of the beam,
Detecting the distance between the optical axis of the laser beam incident on the retroreflector and the center position of the retroreflector as a tracking error amount;
From the detected tracking error amount, obtain an adjustment amount of the laser beam light amount to keep the amplitude of the Lissajous signal constant,
A method for stabilizing the output of a tracking laser interferometer, wherein the amount of laser beam light is adjusted in accordance with the amount of tracking error.   2. The tracking laser interferometer output stabilization method according to claim 1, wherein the tracking error amount is calculated from an X direction component and a Y direction component. The Lissajous signal is obtained by using the Lissajous signal obtained by using the interference of the laser beam irradiated to the retroreflector that is the object to be measured and reflected in the return direction by the retroreflector. In a tracking laser interferometer that performs tracking using a change in the position of the optical axis of the beam,
Means for detecting the distance between the optical axis of the laser beam incident on the retroreflector and the center position of the retroreflector as a tracking error amount;
Means for obtaining an adjustment amount of the laser beam light amount for keeping the amplitude of the Lissajous signal constant from the detected tracking error amount;
Means for adjusting the amount of laser beam light according to the tracking error amount;
A tracking laser interferometer characterized by comprising:

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