JPH06281448A - Optical displacement sensor - Google Patents

Abstract

PURPOSE:To provide an optical displacement sensor where the structure is simple and the operation of a piezoelectric scanner can be calibrated, and which can be applied to the AFM measurement and calibrate the error signal. CONSTITUTION:A polarized condition changing element 60 which is constituted in a selectively attachable/detachable manner to the optical path between a polarized beam splitter 36 which is a component of an optical displacement sensor and a lambda/4 plate 38. When the polarized condition changing element 60 is not inserted in the optical path, the optical displacement sensor is constituted so as to be capable of measuring the displacement of the piezoelectric scanner 42. When the polarized condition changing element 60 is inserted in the optical path, the polarized condition of the reflected light from the piezoelectric scanner 42 is disturbed, and the signal output of a photo-detector 50 is periodically changed. As a result, the operation of the piezoelectric scanner 42 can be calibrated based on these signal outputs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical displacement sensor for optically measuring the amount of displacement of a cantilever, for example.

[0002]

2. Description of the Related Art A scanning tunneling microscope (STM; Scan) is used as an apparatus for observing a conductive sample with atomic resolution.
The ning tunneling microscope was invented by Binnig and Rohrer et al. In this STM, the samples that can be observed are limited to those that are conductive. Therefore, atomic force microscope (AFM) is used as a device that can observe an insulating sample with atomic-order resolution using STM elemental technology such as servo technology.
pe) was proposed. This AFM is disclosed, for example, in Japanese Patent Laid-Open No. 62-
130302.

The AFM has a cantilever having a sharply pointed protrusion (probe) at its free end. When the probe is brought close to the sample, the free end of the cantilever is displaced by the interaction force (atomic force) acting between the atom at the tip of the probe and the atom at the sample surface. By scanning the probe along the surface of the sample while electrically or optically measuring the displacement of the free end, three-dimensional information of the sample is obtained.
For example, if the probe is scanned while controlling the sample-to-sample distance so that the displacement of the free end of the cantilever is kept constant, the probe tip moves along the unevenness of the sample surface. A three-dimensional image showing the surface shape of the sample can be obtained from.

An optical displacement sensor for measuring the displacement of the cantilever is generally applied to such an AFM device, and is disclosed in, for example, Japanese Patent Laid-Open No. 3-102209. FIG. 3 shows this Japanese Patent Laid-Open No. 3-10220.
The schematic configuration of the AFM device disclosed in Japanese Patent No. 9 is shown. As shown in FIG. 3, such an AFM apparatus is provided with a critical angle type optical displacement sensor 2 disclosed in, for example, Japanese Patent Laid-Open No. 62-36502.

That is, the laser light emitted from the semiconductor laser 4 is regulated by the collimator lens 6 into a parallel light beam, which is then reflected by the polarization beam splitter 8.
It passes through the λ / 4 plate 10.

The laser light transmitted through the λ / 4 plate 10 is condensed on the cantilever 14 via the objective lens 12 in a state in which the polarization direction is converted from linear polarization to circular polarization. The reflected laser light reflected from the cantilever 14 is again applied to the polarization beam splitter 8 via the objective lens 12 and the λ / 4 plate 10.

The reflected laser light emitted to the polarization beam splitter 8 is converted by the λ / 4 plate 10 into linearly polarized light whose polarization direction is rotated by 90 ° from the initial linearly polarized light. As a result, the reflected laser light passes through the polarization beam splitter 8 and is applied to the critical angle prism 16. The reflected laser light applied to the critical angle prism 16 is reflected by the reflecting surface 16 a and is applied to the two-divided photodetector 18.

When the cantilever 14 catches the uneven shape of the surface of the measurement sample 20 and is displaced up and down, with this,
First and second light receiving portions 1 of the two-divided photo detector 18
The balance of the amount of light applied to 8a and 18b changes.

Therefore, the first and second light receiving portions 18a, 18a,
An electrical signal having a magnitude corresponding to the amount of received light is output from 18b, and the difference between these electrical signals is determined by the corresponding terminals 22a,
It is configured to be detectable via 22b. The difference between these electric signals is usually called an error signal, but the AFM device applies the error signal as a displacement signal indicating the displacement of the cantilever. It should be noted that the error signal does not have a constant "rule of thumb" such as the wavelength of light as in an optical interference displacement meter, and so on, so to speak, can be said to be a secondary reference displacement signal.

Further, a cylindrical piezoelectric scanner 24 is applied to such an AFM device, and the piezoelectric scanner 24 allows the probe 14a of the cantilever 14 and the measurement sample 20 to be relatively and highly accurately. Scanning is performed in the XYZ directions.

A photodetector 26 capable of monitoring laser light output is provided adjacent to the semiconductor laser 4, and the signal output from the photodetector 26 is
It can be detected through the terminal 28. A control circuit (not shown) is connected to the semiconductor laser 4 via an input terminal 30, and the driving state of the semiconductor laser 4 is controlled via this control circuit.

[0012]

The above-described piezoelectric scanner 24 is normally in a polarized state. However, for example, when an excessive voltage is applied during scanning, depolarization occurs, and the initial unit applied voltage per unit voltage. The displacement amount of may change. Therefore, it is necessary to periodically calibrate its operation.

In the AFM device, Z of the piezoelectric scanner 24 is used.
It is also conceivable to substitute the optical displacement sensor 2 which measures the displacement of the cantilever 14 for calibration of the directional movement. However, as described above, the optical displacement sensor 2 is used for detecting the error signal. Therefore,
If the performance of the displacement sensor 2 is changed or deteriorated, there is a problem that it is difficult to apply it as a sensor for operation calibration.

On the other hand, when a displacement sensor using optical interference is applied to the AFM device, force curve measurement performed during AFM measurement, that is, large scanning exceeding the distance between the crests of the interference fringes is required. There is a problem that it becomes difficult to read and interpret the data during the measurement. For example, a similar problem exists in the displacement sensor disclosed in US Pat. No. 5,025,658, which uses the external resonance mirror of the semiconductor laser as a cantilever.

4A and 4B respectively show a force curve measured by an error sensing type displacement sensor and a force curve measured by an optical interference type displacement sensor in the atmosphere. Has been done. Figure 4 (B)
It can be seen that the force curve shown in (1) has more complicated characteristics. In AFM measurement, a cantilever with a small spring constant is usually used in order to reduce damage to the measurement sample.

However, when a cantilever with a small spring constant is used, a force called meniscus force causes a large hysteresis formed between points a and c of the force curve (see FIG. 4A). Become.

Therefore, when setting the force, the cantilever needs to be moved at least by the distance indicated by d in the figure, and specifically, the moving amount (d) reaches about 500 nm.

In such a case, the output of the displacement sensor is
The amount of movement of the cantilever while changing periodically (d)
Is longer than the period or wavelength, and as a result, a one-to-one correspondence between the output of the displacement sensor and the movement amount (d) of the cantilever cannot be established. This problem similarly occurs even when the movement amount (d) of the cantilever does not exceed the wavelength or more and the interference waveform and the periphery of the peak are set to be between the points d and c.
Further, the displacement sensor using optical interference is difficult to apply to the AFM measurement performed by vibrating the cantilever in the Z direction for the same reason as described above. Therefore, it is desired to develop an optical displacement sensor capable of calibrating the operation of the piezoelectric scanner and also applicable to AFM measurement.

The present invention has been made to satisfy such a demand, and an object thereof is to provide an optical displacement sensor capable of calibrating the operation of a piezoelectric scanner with a simple structure and also applicable to AFM measurement. To provide.

[0020]

In order to achieve such an object, the present invention is based on a change in the amount of laser light reflected from a movable body on which laser light having a predetermined light output is condensed. In an optical displacement sensor configured to detect the amount of displacement of the movable body,

It is constructed so that it can be selectively inserted into and removed from the optical path of the laser beam, and by being inserted and arranged in the optical path, the polarization state of the laser beam reflected from the movable body can be selectively selected. And a polarization state changing element configured to be changeable.

[0022]

The polarization state of the laser beam reflected from the movable body is appropriately and selectively changed by appropriately and selectively inserting the polarization state changing element into the optical path of the laser beam.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical displacement sensor according to an embodiment of the present invention will be described below with reference to FIGS. FIG. 1 schematically shows the configuration of the optical displacement sensor of this embodiment.

That is, the laser light emitted from the semiconductor laser 32 is restricted by the collimator lens 34 to be parallel restraint, then reflected by the polarization beam splitter 36 and transmitted through the λ / 4 plate 38.

The laser light transmitted through the λ / 4 plate 38 is converted from linearly polarized light into circularly polarized light, and the cylindrical piezoelectric scanner 4 passes through the objective lens 40.
It is focused on 2. The reflected laser light reflected from the piezoelectric scanner 42 is again reflected by the objective lens 40 and the λ / 4 plate 38.
The polarized beam splitter 36 is irradiated with the light through.

The reflected laser light applied to the polarization beam splitter 36 is converted by the λ / 4 plate 38 into linearly polarized light whose polarization direction is rotated by 90 ° from the initial linearly polarized light. As a result, the reflected laser light passes through the polarization beam splitter 36 and is applied to the critical angle prism 44. The reflected laser light applied to the critical angle prism 44 is reflected by the reflecting surface 44 a of the critical angle prism 44 and applied to the two-divided photodetector 46.

The reflected laser light applied to the two-divided photodetector is converted by the first and second light receiving portions 46a and 46b into an electric signal having a magnitude corresponding to the amount of received light. This electric signal is configured to be detectable via the terminals 48a and 48b.

A photodetector 50 capable of monitoring laser light output is provided adjacent to the semiconductor laser 32, and a signal output from the photodetector 50 is converted into a current voltage by a preamplifier 52. Here, when the changeover switch 54 is closed, the voltage signal output from the preamplifier 52 is input to the laser drive circuit 56. The laser drive circuit 56 keeps the optical output of the semiconductor laser 32 constant, for example, based on the input voltage signal.

On the other hand, when the changeover switch 54 is opened, the voltage signal output from the preamplifier 52 is used as the monitor input terminal 5
It is input to a monitor (not shown) via 8. As a result, it becomes possible to monitor the change in the laser light output via the monitor.

The optical displacement sensor of the present embodiment is provided with a polarization state changing element 60 constructed so that it can be selectively inserted into and removed from the optical path between the polarization beam splitter 36 and the λ / 4 plate 38. Has been. When the polarization state changing element 60 is not inserted in the optical path, the optical displacement sensor of this embodiment is configured to measure the displacement of the piezoelectric scanner 42. That is, the difference between the electric signals via the terminals 48a and 48b,
That is, the displacement of the piezoelectric scanner 42 can be measured by detecting the error signal. At this time, the changeover switch 54 is closed, and the semiconductor laser 32 is
The light output of is automatically controlled through the laser drive circuit 56.

When the polarization state changing element 60 is inserted in the optical path and the changeover switch 54 is opened, the piezoelectric scanner 42 is used.
The reflected light reflected from is disturbed in its polarization state through the polarization state changing element 60. Therefore, a part of the light is reflected by the modified beam splitter 36 and returned in the direction of the semiconductor laser 32.

At this time, the semiconductor laser 32 is configured as an external resonator by using the reflection surface 42a of the piezoelectric scanner 42 as an external resonance mirror, so that the photodetector 50 corresponds to the displacement of the piezoelectric scanner 42. It will output a periodic signal. That is, an optical interference-like output is presented.

The external resonator is defined when the semiconductor laser 32 is provided with an external reflection surface constituting the resonator in addition to the both end surfaces 32a and 32b. In the case of the present embodiment, the reflecting surface is the upper surface 42 a of the piezoelectric scanner 42. At this time, a part of the light is guided to the two-divided photodetector 46, but its electric signal is transmitted to the terminals 48a, 48.
Not detected via b.

FIG. 2A shows a two-divided photodetector 46 when the polarization state changing element 60 is not inserted in the optical path.
The characteristic of the difference between the signal outputs (see FIG. 1), that is, the error signal output is shown. Further, in FIG. 2B, when the polarization state changing element 60 is inserted in the optical path, the photodetector 5
A zero signal output characteristic is shown.

The horizontal axis shown in FIGS. 2A and 2B is the distance from the position where the laser beam is focused through the objective lens 40 (see FIG. 1) and focused, that is, the in-focus position. Is shown.

As shown in FIG. 2A, during normal AFM measurement, that is, when the polarization state changing element 60 is not inserted in the optical path, a light spot is formed on the back surface of the cantilever (see FIG. 3). Then, the displacement of the cantilever is measured by AFM by utilizing the fact that the signal characteristic between the position a and the position b in the figure has a one-to-one correspondence with the distance. At this time,
The signal output of the photo detector 50 is automatically output controlled via the laser drive circuit 56.

On the other hand, as shown in FIG. 2B, when the applied voltage-displacement amount characteristic of the piezoelectric scanner 42 is measured, that is, when the polarization state changing element 60 is inserted in the optical path, the signal output of the photodetector 50 is It changes periodically. For this reason,
The operation calibration of the piezoelectric scanner 42 is performed on the basis of the wavelength (T) of the external resonator calculated by counting the peak positions a, b, and c. Since the wavelength (T) is “a ruler of light”, its width is always constant even if the signal output of the photodetector does not pass through the in-focus position (0).

As described above, according to the present embodiment, the operation of the piezoelectric scanner can be easily calibrated simply by inserting and removing the polarization state changing element 60 in the optical path, and the optical displacement sensor applicable to the AFM measurement is also provided. Can be provided. In addition,
The present invention is not limited to the configuration of the above-mentioned embodiment. For example, it is also preferable to apply a λ / 2 plate or another crystal plate as the polarization state changing element. It is also possible to incorporate the displacement sensor of the present invention into a scanning probe microscope to calibrate the applied voltage-displacement amount characteristic of a piezoelectric scanner having no linearity.

In this case, first, the cantilever 14 (see FIG.
(See reference) is removed from the spot position formed by the optical displacement sensor, the polarization state changing element 60 is inserted into the optical path, and the changeover switch 54 is opened.

Next, while monitoring the signal output of the photodetector 50, the peak-to-peak distance of the displacement signal that periodically changes the piezoelectric scanner 42 is plotted against the voltage applied to the piezoelectric scanner 42, and the applied voltage is plotted. -A displacement amount characteristic curve (not shown) is constructed.

Next, after removing the polarization state changing element 60 from the optical path, the changeover switch 54 is closed and the semiconductor laser 3
2 while performing automatic output control on the terminals 48a, 48
The error signal output from the two-division photo detector 46 via b is detected.

After performing a predetermined AFM measurement based on this detection result, the Z direction data of the AFM measurement data is calibrated based on the previously obtained applied voltage-displacement amount characteristic curve, and ideal linearity is obtained. AFM image is reconstructed.

[0043]

According to the present invention, since the polarization state changing element capable of being selectively inserted into and removed from the optical path is provided, the polarization state of the laser light reflected from the movable body can be selected selectively and easily. Can be changed to. That is, it is possible to easily detect the displacement amount of the movable body corresponding to the detection purpose only by selectively inserting and removing the polarization state changing element from the optical path according to the detection purpose.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a configuration of an optical displacement sensor according to an embodiment of the present invention.

FIG. 2A is a diagram showing a characteristic of a difference between signals output from the two-divided photodetector, that is, an error signal output characteristic when the polarization state changing element is not inserted in the optical path, and FIG. 2B is a polarization state changing element. The figure which shows the characteristic of the signal output output from a photodetector, when is inserted in an optical path.

FIG. 3 is a diagram schematically showing a configuration of a conventional optical displacement sensor.

FIG. 4A is a diagram showing a force curve measured by an error sensing type displacement sensor, and FIG. 4B is a diagram showing a force curve measured by an optical interference type displacement sensor.

[Explanation of symbols]

36 ... Polarizing beam splitter, 38 ... λ / 4 plate, 42 ...
Piezoelectric scanner, 50 ... Photo detector, 60 ... Polarization state changing element.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] May 18, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

In the AFM device, Z of the piezoelectric scanner 24 is used.
One of the challenges is calibrating directional motion .
Therefore, it is also possible to substitute the optical displacement sensor 2 for measuring the displacement of the cantilever 14. However, as described above, the optical displacement sensor 2 has a second-order reference variation.
It is used to detect error signals that are position signals . Therefore, when the performance of the displacement sensor 2 is changed or deteriorated, it is difficult to apply it as a sensor for operation calibration.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

On the other hand, when a displacement sensor using optical interference is applied to the AFM device, a reference scale called wavelength is also used.
It is suitable for the above operation calibration,
In M measurement, when it becomes difficult to read and interpret data in force curve measurement that is performed in AFM measurement
I have a problem. This is due to the need for a large scan over the crest-to-crest distance of the fringes . For example, a similar problem exists in the displacement sensor disclosed in US Pat. No. 5,025,658, which uses the external resonance mirror of the semiconductor laser as a cantilever.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

However, when a cantilever having a small spring constant is used, a point called a meniscus force or a force called an adhesive force causes points a and c of the force curve (see FIG. 4).
(See (A)) and the hysteresis formed between the two become large.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

Therefore, when setting the force, the cantilever must be moved at least by the distance indicated by d in the figure. Specifically, the adhesive force is about 1 × 10 ^-7 N.
It is said that a cantilever with a spring constant of 0.2 N / m should be used.
When used, the amount of movement (d) reaches about 500 nm.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0018

[Correction method] Change

[Correction content]

In such a case, the output of the displacement sensor is
The amount of movement of the cantilever while changing periodically (d)
Is longer than the period or wavelength, and as a result, a one-to-one correspondence between the output of the displacement sensor and the movement amount (d) of the cantilever cannot be established. This problem also occurs when using a rigid cantilever
Exists. That is, even if the movement amount (d) of the cantilever does not exceed the wavelength, the interference waveform and the vicinity of the peak are at points d and c.
If it is set to be between the points, it will occur similarly.
Therefore, the displacement sensor using optical interference is for AFM measurement.
It is not always easy to use as a sensor. Further, the displacement sensor using optical interference is difficult to apply to the AFM measurement performed by vibrating the cantilever in the Z direction for the same reason as described above. Therefore, it is desired to develop an optical displacement sensor capable of calibrating the operation of a piezoelectric scanner or the like and applicable to AFM measurement. Further, it is desired to develop an optical displacement sensor that can calibrate an error signal.

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

The present invention has been made to satisfy such a demand, and an object thereof is to provide an optical displacement sensor capable of calibrating the operation of a piezoelectric scanner with a simple structure and also applicable to AFM measurement. To provide. Another object is to provide an optical displacement sensor that can calibrate an error signal.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Name of item to be corrected] 0030

[Correction method] Change

[Correction content]

The optical displacement sensor of the present embodiment is provided with a polarization state changing element 60 constructed so that it can be selectively inserted into and removed from the optical path between the polarization beam splitter 36 and the λ / 4 plate 38. Has been. Either in the case of the polarization state changing element 60 is inserted may not be inserted into the optical path,
The optical displacement sensor of this embodiment is the piezoelectric scanner 42.
It is configured to be able to measure the displacement of. Polarization state changing element 60
Is not inserted in the optical path, the optical displacement sensor of this embodiment is
The sir can measure the displacement of the piezoelectric scanner 42 by detecting the difference between the electric signals, that is, the error signal via the terminals 48a and 48b. At this time, the changeover switch 54 is closed, the light output of the semiconductor laser 32, the automatic light output control via the laser drive circuit 56 is performed.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0033

[Correction method] Change

[Correction content]

[0033] The external resonator, both end surfaces 32a of the semiconductor laser 32, in addition to 32b, is defined when an external reflecting surfaces constituting the resonator. In the case of this embodiment,
The reflecting surface is the upper surface 42a of the piezoelectric scanner 42. At this time, a part of the light is also guided to the two-divided photodetector 46, but the electric signal is transmitted to the terminals 48a, 4a.
Not detected via 8b.

[Procedure Amendment 9]

[Document name to be amended] Statement

[Correction target item name] 0036

[Correction method] Change

[Correction content]

During normal AFM measurement, that is, when the polarization state changing element 60 is not inserted in the optical path, a light spot is formed on the back surface of the cantilever (see FIG. 3). Error signal at this time
The output-distance characteristic of the cantilever is as shown in FIG. 2 (A), and the fact that the signal characteristic between the position a and the position b in the figure has a one-to-one correspondence with the distance is utilized. AFM measurement of the displacement of At this time, the signal output of the photodetector 50 is subjected to automatic light output control via the laser drive circuit 56.

[Procedure Amendment 10]

[Document name to be amended] Statement

[Name of item to be corrected] 0037

[Correction method] Change

[Correction content]

On the other hand, when measuring the applied voltage-displacement amount characteristic of the piezoelectric scanner 42, that is, when the polarization state changing element 60 is inserted in the optical path, the output-distance characteristic is shown in FIG. 2 (B).
As described above , the signal output of the photo detector 50 changes periodically. Therefore, the operation calibration of the piezoelectric scanner 42 can be performed based on the wavelength (T) of the external resonator calculated by counting the peak positions a, b, and c . Since the wavelength (T) is “a ruler of light”, its width is always constant even if the signal output of the photodetector does not pass through the in-focus position (0). It is possible to calibrate the displacement-output characteristics of an optical displacement sensor that outputs an error signal based on wavelength by incorporating the displacement sensor of the present invention into a scanning probe microscope. The method of calibrating the applied voltage-displacement amount characteristic of the optical displacement sensor through the piezoelectric scanner 42 will be described below. First, after the scanning probe microscope measurement state, that is, the state in which the light spot from the optical displacement sensor is connected to the cantilever 14 (see FIG. 3), the cantilever 14 is removed from the spot position, and then the polarization state changing element. 60 is inserted in the optical path and the changeover switch 54 is opened. Next, while monitoring the signal output of the photodetector 50, the peak-to-peak distance of the displacement signal that periodically changes the piezoelectric scanner 42 is determined by the piezoelectric scanner 4.
The applied voltage-displacement amount characteristic curve (not shown) of the piezoelectric scanner 42 is constructed by plotting with respect to the applied voltage. Next, after the polarization state changing element 60 is removed from the optical path, the changeover switch 54 is closed to perform automatic output control on the semiconductor laser 32 and output from the two-divided photodetector 46 via the terminals 48a and 48b. It enables the detection of error signals that occur. At this time, the piezoelectric scanner 4
2 is applied with voltage to form an applied voltage-error signal characteristic curve (not shown), and by further adding the applied voltage-displacement amount characteristic curve of the piezoelectric scanner 42 configured above, finally. , But indirectly, the error signal can be calibrated based on wavelength. By performing AFM measurement based on this result based on the error signal, a highly accurate AFM image is reconstructed using the Z-direction data calibrated based on the wavelength.

[Procedure Amendment 11]

[Document name to be amended] Statement

[Correction target item name] 0038

[Correction method] Change

[Correction content]

As described above, this embodiment is based on the displacement sensor of the present invention.
Incorporating the probe into the scanning probe microscope,
The displacement of the piezoelectric element is measured, and the piezoelectric
Switch to use for calibration of applied voltage-displacement amount characteristic of
It is possible to That is , by inserting / removing the polarization state changing element 60 into / from the optical path, the piezoelectric element based on the wavelength is scanned.
Measurements for motion calibration of cannabis, etc. and cantilever changes during AFM
An optical displacement sensor applicable to both position measurement can be provided. As the polarization changing element, for example, a λ / 2 plate or another crystal plate is preferably used.
The present invention is not limited thereby.

Claims (1)

[Claims] 1. An optical displacement sensor configured to detect a displacement amount of a movable body based on a change in the amount of the laser beam reflected from the movable body on which a laser beam having a predetermined light output is condensed. , The laser beam reflected from the movable body is selectively selectively changed by inserting and arranging the laser beam in the optical path of the laser beam. An optical displacement sensor comprising a polarization state changing element configured as possible.