Patent ID: 12241739

DETAILED DESCRIPTION OF EMBODIMENTS

In order to make the purpose, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present disclosure and do not constitute limitations to the present disclosure.

FIG.1shows a structure of a bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings according to an embodiment of the present disclosure.

As shown inFIG.1, the bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings provided by the present disclosure includes a light source1, a transmission two-dimensional grating2, a reflection two-dimensional grating3, an X-direction quarter-wave plate4, a Y-direction quarter-wave plate5, a first optical system, a second optical system, a third optical system, and a fourth optical system.

The light source1is a dual-frequency laser, which is used to generate X-direction measurement laser beams with frequencies f1 and f2 respectively that have mutually perpendicular polarization states and Y-direction measurement laser beams with frequencies f1 and f2 respectively that have mutually perpendicular polarization states. The transmission two-dimensional grating2and the reflection two-dimensional grating3are placed in parallel on a path of the X-direction measurement laser beam and the Y-direction measurement laser beam emitted by the light source1, and the transmission two-dimensional grating2and the reflection two-dimensional grating3are perpendicular to the X-direction measurement laser beam and the Y-direction measurement laser beam. The X-direction quarter-wave plate4and the Y-direction quarter-wave plate5are located between the transmission two-dimensional grating2and the reflection two-dimensional grating3, and on the paths of the diffracted light generated by the transmission two-dimensional grating2and the reflected light generated by the reflection two-dimensional grating3.

The first optical system and the third optical system have the same structure, each including a polaroid sheet6used to remove a stable interference signal of a horizontal component and a photodetector7used to receive a stable interference signal of a vertical component. The second optical system and the fourth optical system have the same structure, each including a polaroid sheet6used to remove a stable interference signal of a vertical component and a photodetector7used to receive a stable interference signal of a horizontal component.

In the bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings proposed in the present disclosure, the polaroid sheet6may be replaced by a polarizer8according to actual needs, and corresponding changes may be made to the structure of the measurement device proposed in the present disclosure according to the polarizer8.

First Specific Embodiment

FIG.2aandFIG.2brespectively show principles of the bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings in the x-z direction and the y-z direction when a polaroid sheet is used.

As shown inFIG.2a, +1storder diffracted light obtained by vertically injecting the X-direction measurement laser beam into the transmission two-dimensional grating2passes through the X-direction quarter-wave plate5and is incident on the reflection two-dimensional grating3at a Littrow angle to obtain new +1storder diffracted light, and the new +1storder diffracted light passes through the X-direction quarter-wave plate5and returns to the transmission two-dimensional grating2in a direction of the +1storder diffracted light to obtain X-direction diffracted light, so that horizontal polarization light having the frequency f1in 0thorder diffracted light and −2ndorder diffracted light generated from the new +1storder diffracted light is converted into vertical polarization light, and vertical polarization light having the frequency f2in the 0thorder diffracted light and the −2ndorder diffracted light generated from the new +1storder diffracted light is converted into horizontal polarization light.

−1storder diffracted light obtained by vertically injecting the X-direction measurement laser beam into the transmission two-dimensional grating2is incident on the reflection two-dimensional grating3at a Littrow angle to obtain new −1storder diffracted light, and the new −1storder diffracted light returns to the transmission two-dimensional grating2in a direction of the −1storder diffracted light to obtain 0thorder diffracted light and −2ndorder diffracted light after diffraction by the transmission two-dimensional grating2.

The polaroid sheet6in the first optical system may filter out a stable interference signal of a horizontal component in the −2ndorder diffracted light generated from the new +1storder diffracted light and the 0thorder diffracted light generated from the new −1storder diffracted light, so that diffracted light of a vertical component having the frequency f2and diffracted light of the vertical component having the frequency f1form a stable interference in the photodetector7of the first optical system.

The polaroid sheet6in the second optical system may filter out a stable interference signal of a vertical component in the −2ndorder diffracted light generated from the new −1storder diffracted light and the 0thorder diffracted light generated from the new +1storder diffracted light, so that diffracted light of the horizontal component having the frequency f2and diffracted light of the horizontal component having the frequency f1form a stable interference in the photodetector7of the second optical system, and then a displacement change of the reflection two-dimensional grating3in the X direction may be obtained.

A phase change ϕxcaused by a displacement of the reflection two-dimensional grating3in the X direction may be obtained according to:

ϕx=2⁢π⁢m·Sxd
where Sxrepresents a theoretical displacement of the reflection two-dimensional grating3in the X direction, m represents an order of the diffracted light obtained after the X-direction measurement laser beam has passed through the transmission two-dimensional grating2, and d represents a grating pitch of the transmission two-dimensional grating2and the reflection two-dimensional grating3.

When the order m is equal to +1 and −1 respectively, a Doppler frequency shift phase value ϕ1of the +1storder diffracted light and a Doppler frequency shift phase value ϕ2of the −1storder diffracted light may be obtained according to:

{ϕ1=2⁢π·Sxdϕ2=-2⁢π·Sxd.

A displacement change Sxof the reflection two-dimensional grating3in the X direction is then obtained according to:
2×(ϕ1−ϕ2)=ϕ;

Sx=d8⁢π·ϕ;
where ϕ represents the phase change of the +1storder diffracted light and the −1storder diffracted light. When the phase change ϕ is 2π, Sx=d/4, and the interference measurement device provided by the present disclosure has a 4-fold optical subdivision.

The optical path in the y-z direction in the bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings provided in this specific embodiment is shown inFIG.2b. The optical path of the Y-direction measurement laser beam passing through the transmission two-dimensional grating2, the reflection two-dimensional grating3and the Y-direction quarter-wave plate5, the method of generating the corresponding diffracted light from the Y-direction measurement laser beam, and the method performed by the third optical system and the fourth optical system for the corresponding diffracted light are consistent with the method performed by the first optical system and the second optical system. A displacement change of the reflection two-dimensional grating3in the Y direction may be obtained according to:

Sy=d8⁢π·ϕ.

In a process of performing a displacement detection, in order to prevent other diffracted light and reflected light from interfering with a detection result, all light other than the light mentioned above is removed by blocking.

Second Specific Embodiment

FIG.3shows a principle of a bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings in which a polarizer is used.

As shown inFIG.3, the polaroid sheet6is replaced by a polarizer8, and the polaroid sheet6is placed between the transmission two-dimensional grating2and the reflection two-dimensional grating3. In this case, two optical systems each consisting of a polarizer8and a photodetector7are required to receive stable interference light in the X direction and stable interference light in the Y direction respectively.

The +1storder diffracted light obtained by vertically injecting the X-direction measurement laser beam into the transmission two-dimensional grating2is incident on the reflection two-dimensional grating3at a Littrow angle to obtain new +1storder diffracted light. In this process, the +1storder diffracted light passes through the polaroid sheet6to filter out +1storder diffracted light having the frequency f1, and only +1storder diffracted light having the frequency f2is retained. The new +1storder diffracted light returns to the transmission two-dimensional grating3in a direction of the +1storder diffracted light to obtain X-direction diffracted light.

The −1storder diffracted light obtained by vertically injecting the X-direction measurement laser beam into the transmission two-dimensional grating2is incident on the reflection two-dimensional grating3at a Littrow angle to obtain new −1storder diffracted light. In this process, the −1storder diffracted light passes through another polaroid sheet6to filter out new −1storder diffracted light having the frequency f2, and only new-1st order diffracted light having the frequency f1is retained. The new −1storder diffracted light returns to the transmission two-dimensional grating2in a direction of the −1storder diffracted light, and +2ndorder diffracted light is obtained after diffraction by the transmission two-dimensional grating. The 0thorder diffracted light and the +2ndorder diffracted light in the X-direction diffracted light pass through the polarizer8to form stable interference light carrying a displacement information, and the stable interference light is received by the photodetector7. The interference signal may be processed by the photodetector to obtain the displacement information of the grating in the X direction. Similarly, the displacement information of the grating in the Y direction may be obtained.

The optical path in the y-z direction in the bidirectional Littrow two-degree-of-freedom grating interference measurement device based on double gratings provided in this specific embodiment is consistent with the optical path in the x-z direction in this specific embodiment.

The optical path of the Y-direction measurement laser beam passing through the transmission two-dimensional grating2, the reflection two-dimensional grating3and the polaroid sheet6, the method of generating the corresponding diffracted light from the Y-direction measurement laser beam, and the method performed by the third optical system and the fourth optical system for the corresponding diffracted light are consistent with the method performed in the first optical system and the second optical system, and the displacement information of the reflection two-dimensional grating in the Y direction may be obtained.

In a process of performing a displacement detection, in order to prevent other diffracted light and reflected light from interfering with a detection result, all light other than the light mentioned above is removed by blocking.

The above specific embodiments do not constitute limitations on the protection scope of the present disclosure. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions may be made according to design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.