Patent Publication Number: US-2023152452-A1

Title: METHOD FOR SIMULTANEOUSLY MEASURING MULTI DOF GEs BY LASER AND SYSTEM THEREFOR

Description:
FIELD OF THE INVENTION 
     The present invention relates to technical field of optical precision measurement, in particular to a method for simultaneously measuring multiple DOF (degree of freedom) GEs by a laser, and a system therefor. 
     PRIOR ART 
     With development of precisely manufacturing, machining and assembling technology, a measuring accuracy of 6DOF GEs of an object in motion or that of 6DOF GEs of an object at rest must be higher and higher. 
     In prior art, a method for measuring 6DOF GEs is performed to use a laser interferometer, which is for a single parameter measurement. One degree error must be measured each time of installing and adjusting. Different types of accessories for measuring and repeatedly adjusting the interferometer are required for each measuring process. 
     In prior art, a method for measuring the above-mentioned 6DOF GEs has such disadvantages that an optical path structure is complex, and multiple detectors are used to measure different errors, which increases a system cost and complexity, makes a measuring time long, and makes measuring accuracy greatly affected by environmental changes. At the same time, an instability of the optical path structure caused by a circuit heat dissipation is increased, leading to a measuring error. Therefore, it is necessary to use as few optical components and detectors as possible to simultaneously measuring multi DOF GEs (geometric errors). 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a method and a system for simultaneously measuring multiple DOF GEs by a laser, so as to overcome the problems in prior art. 
     In order to achieve the above object, the present invention adopts the following technical solutions. 
     According to one aspect of the present invention, there is provided a system for simultaneously measuring multiple DOF GEs by a laser, the system comprises a measuring unit and a target mirror unit; the measuring unit comprises a laser emitting module, a polarizing beam splitter, a fixed reflector, a first quarter-wave plate, a second quarter-wave plate, a first polarizer, a first photodetector, an interference length measuring module, and a 2D (two-dimensional) angle measuring module; the target mirror unit comprises a beam splitter and a reflector. 
     The laser emitting module is used to generate an emitting light L 1 . 
     The polarizing beam splitter is used for: (1) beam splitting: splitting the emitting light L 1  into a measuring light L 11  and a reference light L 12 . The measuring light L 11  is to hit on the target mirror unit, transmitted by the beam splitter of the target mirror unit, and reflected back by the reflector as a light L 111 , which carries a 3DOF LGEs signal back to the measuring unit as a linear GE measuring light. The reference light L 12  only propagates inside the measurement unit; (2) beam combining: transmitting or reflecting the reference light L 12  that passes by the polarizing beam splitter again and the measuring light L 111  that is reflected back 180° toward its original direction by the target mirror unit according to their polarizing status, so as to make the two beams superposed together in a spatial position, so as to form a combined light L 3 ; (3) beam separating: separating a light L 112  reflected by the beam splitter in the target mirror unit from the measuring light L 11  as a two-dimensional angle measuring light, and the light L 112  is transmitted or reflected by the polarizing beam splitter according to its polarization status. 
     The fixed reflector is used for reflecting backward the reference light L 12  propagating only inside the measuring unit, so as to make the reference light L 12  return to the polarizing beam splitter. 
     The first quarter-wave plate is used to change a polarizing direction of the reference light L 12 , so that when the reference light L 12  passes by the polarizing beam splitter again, a transmitting or reflecting status is switched, that is, the reference light L 12  is reflected at a first passing by, but is transmitted through at the second passing by; or the reference light L 12  is transmitted through the polarizing beam splitter when it passes by the same, while becomes reflected when it passes by the polarizing beam splitter in its coming back way. The second quarter-wave plate is used to change a polarizing direction of a LGE measuring light L 111  and the two-dimensional angle measuring light L 112 , so that when the LGE measuring light L 111  and the two-dimensional angle measuring light L 112  pass by the polarizing beam splitter again, the transmitting or reflecting state is switched. 
     The first polarizer is arranged between the polarizing beam splitter and the first photodetector, and a light transmitting axial direction, i.e., a polarizing axis direction, of the first polarizer can be adjusted to make a combined light L 3  interfere after the combined light L 3  passes through the first polarizer. 
     The first photodetector is used to receive a superimposed light L 3  including the reference light L 12  and the measuring light L 111 , to realize a simultaneous measurement of LGEs along X, Y and Z axes. Specifically: (1) according to a spot offset of the LGE measuring light L 111  on the first photodetector, calculate a relative straightness error between the target mirror unit and the measuring unit along Y axis or Z axis; (2) cooperating with an interference length measuring module to measure a relative position error between the target mirror unit and the measuring unit along X-axis. 
     The two-dimensional (2D) angle measuring module includes a focusing lens/lens group, and a second photodetector; the focusing lens or lens group is used to focus the two-dimensional angle measuring light L 112  on the second photodetector; and a 2D relative angle error between the target mirror unit and the measuring unit rotating about Y-axis or Z-axis is calculated according to the spot offset on the second photodetector. 
     The beam splitter in the target mirror unit is used to split the measuring light L 11 , in which a transmitted light L 111  is reflected back by the reflector in the target mirror unit to return to the measuring unit with straightness and positioning error information; while a reflected light L 112  carries two-dimensional angle information back to the measuring unit. 
     The reflector in the target mirror unit is used to reflect the measuring light L 111  backward, and make the measuring light L 111  return to the polarizing beam splitter to realize: (1) changing a spatial position of the measuring light L 111  along Y-axis and/or Y-axis, its offset in the spatial position is twice a relative displacement between the reflector in the target mirror unit and the measuring unit along Y-axis and/or Y-axis; (2) changing an optical path and frequency of the measuring light L 111 , its drift of the optical path and frequency is proportional to a relative displacement of the reflector in the target mirror unit and the measuring unit along X-axis. 
     Preferably, the laser emitting module emits a single frequency laser, and the interference length measuring module includes a first non-polarizer beam splitter, a phase retarder, and a third photodetector. 
     The first non-polarizer beam splitter is arranged between a first polarizer and the first photodetector, and is used to split the interference light, in which one beam is received by the first photodetector, while the other beam is received by the third photodetector. Light intensities of interference spots on the first photodetector and the third photodetector are I 1 , I 3 , respectively. 
     The phase retarder is arranged in front of the first photodetector or the third photodetector, to make a phase difference 90° between the interference spot signals I 1 , I 3  detected by the two photodetectors, and a phase difference φ(Δx) between the reference light L 12  and the measuring light L 111  is calculated; and a relative displacement Δx between the target mirror unit and the measuring unit along X-axis is calculated according to the phase difference. 
     Preferably, the laser emitting module can generate a dual frequency laser; the interference length measuring module comprises a third non-polarizing beam splitter, a second polarizer, and a fourth photodetector. 
     The dual frequency laser is a polarized light of two beams which are superposed at a spatial position, made with a certain frequency difference and with different polarizing directions. 
     The third non-polarizing beam splitter is disposed between the laser emitting module and the polarizing beam splitter, and the emitting light L 1  of the laser module is split by the third non-polarizing beam splitter, so as to form another laser beam L 2 . 
     The second polarizer is arranged between the third non-polarizer beam splitter and the fourth photodetector, and by adjusting a polarizing axis direction of the second polarizer, the laser light L 2  interferes after passing through the second polarizer, and an interference spot is received by the fourth photodetector as a reference signal for a heterodyne interference length measurement. 
     The combined light L 3  interferes after passing through the first polarizer, and an interference spot is received by the first photodetector as a measuring signal of a heterodyne interference length measurement. According to the reference signal and the measurement signal, a relative displacement between the target mirror unit and the measuring unit along X-axis can be calculated. 
     Preferably, the laser emitting module comprises a multi wavelength laser light source and a heterodyne frequency generating module, the interference length measuring module comprises the 1st to the Nth band-pass filters and the 1st to the Nth phase detectors, where N is a natural number greater than or equal to 3, and the polarizing beam splitter is replaced with a second non-polarizing beam splitter. 
     The multi wavelength laser light source emits multi wavelength laser lights λ 1 , λ 2 , λ 3 , . . . , λ N , their frequencies are v 1 , v 2 , v 3 , . . . , v N . After passing through the heterodyne frequency generating module, their frequencies of the multi wavelength laser become v 1 +f 1 , v 2 +f 2 , v 3 +f 3 , . . . , v N +f N , the multi wavelength laser light is the emitting light L 1 . The second non-polarizing beam splitter is used for: (1) beam splitting: splitting the emitting light L 1  into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is incident on the target mirror unit and is reflected back by the target mirror unit as a light L 111  which carries a 3DOF LGE signal to return to the measuring unit as a measuring light, and the reference light L 12  only propagates within the measurement unit; (2) beam combining: transmitting or reflecting the reference light L 12  that passes by the non-polarizing beam splitter again and the measuring light L 111  reflected by the target mirror unit, so that the two beams are superposed together in a spatial position, so as to form a combined beam L 3 ; (3) beam separating: separating the light L 112  reflected by the beam splitter in the target mirror unit from the measuring light L 11 , as a two-dimensional angle measuring light. 
     The combined light L 3  interferes on the first photodetector, making the heterodyne interference signal spectrum only contains components f 1 , f 2 , f 3 , . . . , f N . 
     After the 1st to the Nth bandpass filters separate the components f 1 , f 2 , f 3 , . . . , f N , a ranging phase information φ 1 , φ 2 , φ 3 , . . . , φ N  corresponding to each wavelength is measured by the 1st to the Nth phase detectors. Taking n pairs (2≤n≤N−1, n is a natural number) thereof to form a series of beat signals, forming n pairs of combined wavelengths and n pairs of phase difference, and a relative displacement Δx between the target mirror unit and the measuring unit along X-axis is calculated. 
     Preferably, the laser emitting module emits a single frequency laser, and a second non-polarizing beam splitter replaces the polarizing beam splitter. The interference length measuring module includes a first non-polarizing beam splitter, a phase retarder and a third photodetector. 
     The second non-polarizing beam splitter is used for: (1) beam splitting: splitting the emitting light L 1  into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is incident on the target mirror unit and is reflected back by the target mirror unit as a light L 111  which carries a 3DOF LGE signal to return to the measuring unit as a measuring light, and the reference light L 12  only propagates within the measurement unit; (2) beam combining: transmitting or reflecting the reference light L 12  that passe by the non-polarizing beam splitter again and the measuring light L 111  reflected by the target mirror unit, making the two beams superposed together in a spatial position, so as to form a combined light L 3 . The combined light L 3  is a superimposed beam of the reference light L 12  transmitted by the non-polarizing beam splitter and the measuring light L 111  reflected by the non-polarizing beam splitter, or a superimposed beam of the reference light L 12  reflected by the non-polarizing beam splitter and the measuring light L 111  transmitted by the non-polarizing beam splitter; (3) beam separating: separating the light L 112  reflected by the beam splitter in the target mirror unit from the measuring light L 11 , as a two-dimensional angle measuring light. 
     The first non-polarizer beam splitter is arranged between the first polarizer and the first photodetector, and is used to split the combined beam L 3  that has been interfered, one beam L 31  is received by the first photodetector, while the other beam L 32  is received by the third photodetector. Light intensities of interference spots on the first photodetector and the third photodetector are I 1 , I 3 , respectively. 
     The phase retarder is arranged in front of the first photodetector or the third photodetector, to make a phase difference 90° between interference spot signals I 1 , I 3  detected by the above two photodetectors, then the phase difference φ(Δx) between the reference light L 12  and the measuring light L 111  is calculated; and a relative displacement Δx between the target mirror unit and the measuring unit along X-axis is calculated according to the phase difference. 
     Preferably, a rolling angle measuring is added, so as to make simultaneously measuring 6DOF GEs, in which 
     adding a third λ/4 wave plate (or called a third quarter-wave plate) at the target mirror unit end, so that the measuring light L 111  passes by the third quarter-wave plate to return to the measuring unit, passing by the second quarter-wave plate, then is divided when passing by the polarizing beam splitter again, one of which is a light L 111 ′ received by the first photodetector, while the other beam is a light L 111 ″ received by the fifth photodetector; 
     when measuring, the third quarter-wave plate is turned about X-axis with a relative angle γ between the target mirror unit and the measuring unit, then a polarizing direction rotation angle of the measuring light L 111  is γ′, light intensities received by the first photodetector and the fifth photodetector varies with the angle γ′, and according to the measured light intensity changes of the first photodetector and the fifth photodetector, a relationship between the light intensity and said angle γ′, and a proportional relationship between said angle γ′ and γ, a relative angle change, i.e., a rolling angle γ between the target mirror unit and the measuring unit about X-axis, can be obtained. 
     Preferably, the fixed reflector is anyone of a pyramid prism, a cat&#39;s eye mirror, a corner cube retroreflector composed of three mutually perpendicular reflecting surfaces, a right angle prism, and a mirror group composed of two plane mirrors; and the reflector of the target mirror unit is anyone of a pyramid prism, a cat&#39;s eye mirror, and a corner cube retroreflector composed of three mutually perpendicular reflecting surfaces. 
     Preferably, the first photodetector, the second photodetector, the third photodetector and the fifth photodetector are anyone of QD (four-quadrant photodetector), PSD (Position Sensitive Detector), CCD (Charge-coupled Component), and CMOS (Complementary Metal Oxide Semiconductor); and a relative straightness error between the target mirror unit and the measuring unit along Y-axis or Z-axis is calculated according to a spot offset on anyone of the first photodetector, the third photodetector and the fifth photodetector; and the fourth photodetector is anyone of QD, PSD, CCD, CMOS, and Pin Detector. 
     According to another aspect of the present invention, there is provided a method for simultaneously measuring 5DOF GEs with a laser, which is applied to the system, the method comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser collimation principle or laser autocollimation 
     Step 1.1: when an emitting light L 1  emitted by a laser emitting module passes by a polarizing beam splitter, it is divided into a measuring light L 11  and a reference light L 12 . 
     Step 1.2: the measuring light L 11  is emitted from the measuring unit and enters the target mirror unit, and is divided by the beam splitter of the target mirror unit; a transmitted light L 111  is reflected back by a reflector of the target mirror unit, a spatial position of the light L 111  drifts with a relative straightness error between the target mirror unit and the measuring unit along Y-axis or Z-axis; the light L 111  carries a two-dimensional straightness error information back to the measurement unit, then the light L 111  passes by the polarizing beam splitter again. 
     Step 1.3: the reference light L 12  is reflected back 180° toward its original direction by the fixed reflector, it passes by the polarizing beam splitter again, and the reference light L 12  is combined with the light L 111  passing by the polarizing beam splitter again in Step 1.2, so as to form a combined light L 3  which is received by the first photodetector. 
     Step 1.4: an initial position of the combined beam spot is measured by the first photodetector. 
     Step 1.5: according to a real-time spot of the combined beam on the first photodetector, comparing with the initial spot of the combined beam, a spot offset of the combined light is obtained; since the spot offset of the combined light is only caused by a position drift of the measuring light L 111 , a relative straightness error between the target mirror unit and the measuring unit along Y-axis or Z-axis is calculated according to the spot offset of the combined light. 
     Step 2: measuring a position error along X-axis based on a laser interference 
     Step 2.1: after the reference light L 12  in Step 1.1 is reflected back 180° toward its original direction by the fixed reflector of the measuring unit, its polarization state, frequency and phase are not changed, so the light L 12  is used as a reference light of the interference length measuring signal. 
     Step 2.2: the frequency and phase of the light L 111  in Step 1.2 change with a relative displacement between the target mirror unit and the measuring unit along X-axis, and the light L 111  carries a straightness error information along X-axis and returns to the measuring unit as a measuring light of the heterodyne interference length measuring signal. 
     Step 2.3: after the reference light L 12  in Step 2.1 and the measuring light L 111  in Step 2.2 pass through the polarizing beam splitter, the two beams are superposed together in a space; after passing through the interference length measuring module, a relative displacement between the target mirror unit and the measuring unit along X-axis is calculated by referring to the signal measured on the first photodetector. 
     Step 3: measuring an angle error for rotating around Y-axis or Z-axis based on laser autocollimation 
     Step 3.1: a reflected light of the beam splitter of the target mirror unit in Step 1.2 is a two-dimensional angle measuring light L 112 . 
     Step 3.2: the light L 112  carries the two-dimensional angle error information back to the measurement unit, and passes by the second quarter-wave plate and the polarizing beam splitter, then are focused on the second photodetector by a focusing lens or lens group. 
     Step 3.3: an initial position of a light spot is measured by the second photodetector. 
     Step 3.4: according to a real-time position of the light spot on the second photodetector, comparing with its initial position of the light spot, a spot offset is obtained, and a two-dimensional relative angle error rotating around Y-axis or Z-axis between the target mirror unit and the measuring unit is calculated according to the spot offset. 
     Preferably, calculating the straightness error along Y-axis or Z-axis according to the spot offset of the combined light comprises: 
     the initial position and real-time position of the light spot on the first photodetector are (y1 0 , z1 0 ), (y1 t , z1 t ), respectively, relative straightness errors between the target mirror unit and the measuring unit along Y-axis or Z-axis are Δy=2(y1 t −y1 0 ), Δz=2(z1 t −z1 0 ), respectively. 
     Preferably, when a single frequency length measurement is applied, the position error along X-axis based on a laser interferometry comprises: 
     Step 1) the reference light L 12  and the measuring light L 111  are superposed together in a spatial position after passing through the polarizing beam splitter or the second non-polarizing beam splitter, so as to form a combined light L 3 ; when a polarizing axis direction of a first polarizer is adjusted, the combined light L 3  interferes after passing through the first polarizer. 
     Step 2) the interfered light L 3  is divided into a light L 31  and a light L 32  after passing through the first non-polarizing beam splitter. 
     Step 3) after one of the light L 31  and light L 32  is delayed 90° by a phase retarder, they are received by the first photodetector and the third photodetector, respectively, and light intensities of interference spots thereon are I 1 , I 3 , respectively. 
     Step 4) processing the I 1 , I 3 , so as to obtain a phase difference φ(Δx) between the reference light L 12  and the measuring light L 111 , a number N(Δx) of light and dark changes of interference fringes caused by φ(Δx), the laser emitting laser wavelength is λ, a relative displacement between the target mirror unit and the measuring unit along X-axis Δx=N(Δx)·λ/2. 
     Preferably, when a double frequency length measurement is performed, a position error along X-axis based on a laser interference measurement comprises: 
     Step 1) the emitting light L 1  emitted from the laser emitting module has two polarized beams with a certain frequency difference, frequencies of the two polarized beams are f 1 , f 2 , respectively, and when the emitting light L 1  is split by the polarization beam splitter, a frequency of the measuring light L 11  is f 1 , while a frequency of the reference light L 12  is f 2 . 
     Step 2) between the target mirror unit and the measuring unit, a relative displacement of the measuring light L 111  along X-axis is Δx, a frequency variation due to Doppler effect is f(Δx), a frequency of the measuring light L 111  is f 1 +f(Δx). 
     Step 3) setting a first polarizer in front of the first photodetector; by adjusting a polarizing axis direction of the first polarizer, the linearly polarized lights L 12  and L 111  interfere after passing through the first polarizer; and an interference spot is received by the first photodetector as a measuring signal of a heterodyne interference length measurement, and a frequency of the measured beat signal is measured as f m =f 1 +f(Δx)−f 2 . 
     Step 4) when the emitting light L 1  passes by the third non-polarizing beam splitter, it is split by the third non-polarizing beam splitter to form another laser beam L 2 ; and a polarizing axis direction of the second polarizer is adjusted, so that a light L 2  interferes after passing through the second polarizer; an interference spot is received by the fourth photodetector as a standard signal for the heterodyne interference length measurement, and a frequency of a standard signal is f s =f 1 −f 2 . 
     Step 5) the frequency of the measured beat signal obtained in Step 3), f m =f 1 +f(Δx)−f 2 , subtracts the standard beat signal frequency obtained in step 4), f s =f 1 −f 2 , to obtain f(Δx)=f m −f s , a number of light and dark changes of interference fringes caused by f(Δx) is N(Δx), a laser emitting laser wavelength is λ, so a relative displacement between the target mirror unit and the measuring unit along X-axis is λx=N(Δx)·λ/2. 
     Preferably, when a multiple wavelength length measurement is applied, determining a position error along X-axis based on a laser interference comprises: 
     Step 1) a multi wavelength laser light source emits multi wavelength laser lights λ 1 , λ 2 , λ 3 , . . . , λ N , their frequencies are v 1 , v 2 , v 3 , . . . , v N ; after passing through the heterodyne frequency generating module, their frequencies of the multi wavelength laser become v 1 +f 1 , v 2 +f 2 , v 3 +f 3 , . . . , v N +f N , in which the multi wavelength laser light is an emitting light L 1 . 
     Step 2) the emitting light L 1  is split into a measuring light L 11  and a reference light L 12  by the second non-polarizing beam splitter, and the measuring light L 11  and the reference light L 12  both contain multi wavelength laser beams v 1 +f 1 , v 2 +f 2 , v 3 +f 3 , . . . , v N +f N . 
     Step 3) the measuring light L 11  is emitted from the measuring unit to enter the target mirror unit, and is reflected back 180° toward its original direction by the reflector of the target mirror unit; a retroreflected light L 111  carries a straightness error information along X-axis to return to the measuring unit as a measuring light of the heterodyne interference length measuring signal. 
     Step 4) after the reference light L 12  is reflected back 180° toward its original direction by the fixed reflector of the measuring unit, it passes by the non-polarizing beam splitter, and then combines with the light L 111 , by adjusting a polarizing axis direction of the first polarizer, the reference light L 12  and the measuring light L 111  interfere with each other on the first photodetector. 
     Step 5) the first photodetector detects components of the heterodyne interference signal spectrum, such as f 1 , f 2 , f 3 , . . . , f N , the 1st to the Nth band-pass filters separate the components f 1 , f 2 , f 3 , . . . , f N , and the 1st to the Nth phase detectors measure those ranging phase information φ 1 , φ 2 , φ 3 , . . . , φ N  corresponding to each wavelength. Taking n pairs of beat signals, and a relative displacement Δx between the target mirror unit and the measuring unit along X-axis is calculated according to n pairs of combined wavelengths and n pairs of phase difference. 
     Preferably, calculation of a two-dimensional angular error for rotating around Y-axis or Z-axis according to the spot offset comprises: 
     an initial position and a real-time position of a light L 112 &#39;s spot on the second photodetector are (y 0 , z 0 ), (y t , z t ), respectively, then relative angle errors between the target mirror unit and the measuring unit for rotating around Y-axis or Z-axis are Δα=(y t −y 0 )/2f, Δβ=(z t −z 0 )/2f, respectively, where f is a focal length of a focusing lens or lens group. 
     Preferably, by adding a third quarter-wave plate in the target mirror unit for measuring a rolling angle in a single frequency length measurement, an angular error resulted from rotating around X-axis can be measured, i.e., a method for measuring 6DOF GEs includes: 
     Step 1) adding a third quarter-wave plate in the target mirror unit, the measuring light L 111  passes by the third quarter-wave plate to return to the measuring unit; after passing through the second quarter-wave plate, the measuring light L 111  is split when passing by the polarizing beam splitter again, in which one light L 111 ′ is received by the first photodetector and the third photodetector, while the other light L 111 ″ is received by the fifth photodetector. 
     Step 2) by blocking the measuring light L 111 , the reference light L 12  is received on the first photodetector and the third photodetector only, light intensities of the first photodetector and the third photodetector I 10 , I 30 . 
     Step 3) recovering the measuring light L 111 ; by the third quarter-wave plate, making a relative angle variation γ between the target mirror unit and the measuring unit around X-axis; when the measuring light passes by the polarizing beam splitter again, its polarizing direction increases γ′ with its original polarizing direction; γ′ and y is in a direct proportion, γ′=k 1 γ; when the measuring light passes by the second quarter-wave plate in the target mirror unit for the first time, k 1 =1; when the measuring light passes by the second quarter-wave plate in the target mirror unit for the second time, k 1 =2. 
     Step 4) the measuring light L 111  passes by the polarizing beam splitter and is split by the polarizing beam splitter; one beam of the light is received by the first photodetector and the third photodetector, light intensities thereon are I 1 (γ), I 3 (γ); the other beam is received by the fifth photodetector, its light intensity is I 5 (γ); there exists such a function: f(γ)=[I 1 (γ)−I 10 +I 3 (γ)−I 30 −I 5 (γ)]/[I 1 (γ)−I 10 +I 3 (γ)−I 30 +I 5 (γ)]. 
     Step 5) rotating the target mirror unit around X-axis by several specific angles γ 1 , γ 2 , . . . , measuring the corresponding f(γ 1 ), f(γ 2 ), . . . , and calibrating a function curve of f(γ) v. γ. 
     Step 6) measuring the light intensities I 1 (γ), I 3 (γ), I 5 (γ) in real time, calculating f(γ) according to Step 4); calculating a relative rotation angle γ between the target mirror unit and the measuring unit about X-axis based on the function f(γ) v. γ according to Step 5). 
     Preferably, for measuring a rolling angle in a dual frequency length measurement, a third quarter-wave plate is added in the target mirror unit, so as to measure an angular error resulted from rotating around X-axis, i.e., a 6DOF GEs can be measured, the method comprises the following steps: 
     Step 1) adding a third quarter-wave plate in the target mirror unit, the measuring light L 111  passes by the third quarter-wave plate to return to the measuring unit, when passing by the second quarter-wave plate again, the measuring light L 111  is split when passing by the polarizing beam splitter, one beam L 111 ′ is received by the first photodetector, while the other L 111 ″ is received by the fifth photodetector. 
     Step 2) blocking the measuring light L 111 , so that only the reference light L 12  is received on the first photodetector, a light intensity of the first photodetector is ho. 
     Step 3) recovering the measuring light L 111 , by turning the third quarter-wave plate, making a relative angle γ between the target mirror unit and the measuring unit around X-axis; when the measuring light passes by the polarizing beam splitter again, its polarizing direction increases by γ′ with respect to its original polarizing direction, γ′ is in a direct proportion of γ, γ′=k 1 γ; when the measuring light passes by the second quarter-wave plate in the target mirror unit for the first time, k 1 =1; when the measuring light passes by the second quarter-wave plate in the target mirror unit for the second time, k 1 =2. 
     Step 4) the measuring light L 111  passes by the polarizing beam splitter and is split by the polarizing beam splitter; one beam is received by the first photodetector, its light intensity is I 1 (γ), while the other beam is received by the fifth photodetector, its light intensity is I 5 (γ); f(γ)=[I 1 (γ)−I 10 −I 5 (γ)]/[I 1 (γ)−I 10 +I 5 (γ)]. 
     Step 5) rotating the target mirror unit around X-axis by several specific angles γ 1 , γ 2 , . . . , measuring the corresponding f(γ 1 ), f(γ 2 ), and calibrating a functional curve of f(γ) v. γ. 
     Step 6) measuring light intensities I 1 (γ), I 5 (γ) in real time, calculating f(γ) according to Step (4), calculating a rotation angle γ of the target mirror unit about X-axis according to the function f(γ) v. γ calibrated in Step (5). 
     It can be seen from the technical solutions provided by the above embodiments of the present invention that the embodiments of the present invention can realize: 1) a simultaneous and rapid measurement of 5/6DOF GEs of a space object moving linearly along a linear axis; 2) longtime monitoring a relative deformation of 5/6DOF position and attitude of two objects in a space. 
     Further aspects and advantages of the present invention will be introduced in the following description, so as to become apparent from the following description, or will be corroborated in practice of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF ACCOMPANYING DRAWING 
       In order to more clearly explain technical solutions of the present invention, the following will briefly introduce embodiments with accompanying drawings. It is obvious that the drawings in the following description are only relevant to some embodiments of the present invention. For those skilled in the art, other drawings can be obtained according to these drawings without creative work. 
         FIG.  1    is a schematic view of a system for simultaneously measuring 5DOF (degrees-of-freedom) GEs (geometric-errors) by a single frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  2    is a schematic view of a system for simultaneously measuring 5DOF GEs by a single frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  3    is a schematic view of a system for simultaneously measuring 5DOF GEs by a single frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  4    is a schematic view of a system for simultaneously measuring 5DOF GEs by a single frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  5    is a schematic view of a system for simultaneously measuring 5DOF GEs by a single frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  6    is a schematic view of a system for simultaneously measuring 5DOF GEs by a single frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  7    is a schematic view of a system for simultaneously measuring 6DOF GEs by a single frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  8    is a schematic view of a system for simultaneously measuring 6DOF GEs by a single frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  9    is a schematic view of a system for simultaneously measuring 6DOF GEs by a single frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  10    is a schematic view of a system for simultaneously measuring 6DOF GEs by a single frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  11    is a schematic view of a system for simultaneously measuring 6DOF GEs by a single frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  12    is a schematic view of a system for simultaneously measuring 6DOF GEs by a single frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  13    is a schematic view of a system for simultaneously measuring 5DOF GEs by a dual frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  14    is a schematic view of a system for simultaneously measuring 5DOF GEs by a dual frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  15    is a schematic view of a system for simultaneously measuring 6DOF GEs by a dual frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  16    is a schematic view of a system for simultaneously measuring 6DOF GEs by a dual frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  17    is a schematic view of a system for simultaneously measuring 6DOF GEs by a dual frequency laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  18    is a schematic view of a system for simultaneously measuring 6DOF GEs by a dual frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  19    is a schematic view of a system for simultaneously measuring 6DOF GEs by a dual frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  20    is a schematic view of a system for simultaneously measuring 6DOF GEs by a dual frequency laser with a reflecting sensitive structure according to an embodiment of the present invention. 
         FIG.  21    is a schematic view of a system for simultaneously measuring 5DOF GEs by a multiple wavelength laser with a transmitting sensitive structure according to an embodiment of the present invention. 
         FIG.  22    is a schematic view of a system for simultaneously measuring 5DOF GEs by a multiple wavelength laser with a reflecting sensitive structure according to an embodiment of the present invention. 
     
    
    
     The drawings show a measuring unit I, a target mirror unit II, a single frequency laser  1 , a polarizing beam splitter  2 , a fixed pyramid prism  3 , a first quarter-wave plate  4 , a second quarter-wave plate  5 , a first polarizer  6 , a first non-polarizing beam splitter  7 , a half-wave plate  8 , a focusing lens  9 , a half transmitting and half reflecting mirror  10 , a moving pyramid prism  11 , a second non-polarizing beam splitter  12 , a third quarter-wave plate  13 , a dual frequency laser  14 , a third non-polarizing beam splitter  15 , a multi wavelength laser light source  16 , a heterodyne frequency generating module  17 , a first bandpass filter  18 , a second bandpass filter  19 , a third bandpass filter  20 , a first phase detector  21 , a second phase detector  22 , a third phase detector  23 , a first photodetector (A), a second photodetector (B), a third photodetector (C), a fourth photodetector (D), and a fifth photodetector (E). 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described in detail together with the accompanying drawings, in which the same or similar reference numerals throughout indicate the same or similar elements having the same or similar functions. Those embodiments referring to the accompanying drawings are exemplary, are only used to explain the present invention, and cannot be interpreted as limiting the present invention. 
     It can be understood by those skilled in the art that a singular form “a”, “one”, “said” and “the” used herein may also include plural components, unless it is specifically stated. It should be further understood that a phrase “including” or “comprising” used in the present description refers to features, integers, steps, operations, elements and/or components shown in the application, but does not exclude presence or addition of one or more other features, integers, steps, operations, elements, components and/or combinations thereof. It should be understood that when an element is “connected” or “coupled” to another element, it may be directly connected or coupled to other elements without or with intermediate elements. In addition, the term “connection” or “coupling” used herein may include wireless connection or coupling. The term “and/or” as used herein includes any unit and all combinations of one or more associated listed items. 
     Those skilled in the art can understand that, unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art. It should also be understood that terms such as those defined in the general dictionary should be understood to have a meaning consistent with that in prior art, and cannot be simply interpreted as its literal meaning. 
     In order to facilitate to understand the present invention, several specific embodiments will be explained together with the accompanying drawings, and each embodiment does not constitute a limitation of the present invention. 
     In a XYZ spatial coordinate system of the present invention, “6DOF GEs” includes three straightness errors in translating along X, Y and Z directions, and three angle errors in rotating around X, Y and Z coordinate axes. In the subsequent embodiments, a direction in which a laser light enters a target mirror unit is set as a direction of X-axis. 
     Embodiment 1 
     According to one aspect of the present invention, as shown in  FIG.  1   , there is provided a system for simultaneously measuring 5DOF (degree of freedom) GEs (geometric errors) by a laser. The system is composed of a measuring unit I and a target mirror unit II. 
     The measuring unit I includes a single frequency laser  1 , a polarizing beam splitter  2 , a fixed pyramid prism  3 , a first quarter-wave plate  4 , a second quarter-wave plate  5 , a first polarizer  6 , a first non-polarizing beam splitter  7 , a half-wave plate  8 , a focusing lens  9 , a first photodetector (A), a second photodetector (B), and a third photodetector (C). The single frequency laser  1  constitutes a laser emitting module. The first non-polarizing beam splitter  7 , the half-wave plate  8 , and the third photodetector (C) constitute an interference length measuring module. The focusing lens  9  and the second photodetector (B) constitute a two-dimensional angle measuring module. 
     The target mirror unit II includes a half transmitting and half reflecting mirror  10  and a moving pyramid prism  11 . 
     In the measuring unit I, 
     the single frequency laser  1  is used to generate an emitting light L 1 , and the emitting light L 1  is a circularly polarized light; 
     1) beam splitting: the emitting light or emergent light L 1  is divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is incident on the target mirror unit, transmitted by a half transmitting and half reflecting mirror  10  of the target mirror unit, and reflected back by the moving pyramid prism  11 , so as to form a light L 111 , the light L 111  carries a 3DOF LGEs (linear geometric errors) signal back to the measuring unit I as a LGE measuring light; the reference light L 12  only propagates within the measuring unit I; 2) beam combining: the reference light L 12  that passes by the polarizing beam splitter  2  again transmits the polarizing beam splitter  2 , and the measuring light L 111  reflected back by the target mirror unit II is reflected by the polarizing beam splitter  2 , so that the two beams are superposed together in a spatial position, so as to form a combined light L 3 ; 3) beam separating: the light L 112  reflected by the half transmitting and half reflecting mirror  10  of the target mirror unit II is reflected as a two-dimensional angle measuring light. 
     The fixed pyramid prism  3  is used to reflect backward the reference light L 12  propagating only inside the measuring unit I, and return the reference light L 12  to the polarizing beam splitter  2 . 
     The first quarter-wave plate  4  is used to change a polarizing direction of the reference light L 12 , so that the reference light L 12  is reflected by the polarizing beam splitter  2 , transmitted through the first quarter-wave plate  4 , reflected backward by the fixed pyramid prism  3 , transmitted back through the first quarter-wave plate  4  again, then transmitted through the polarizing beam splitter  2 , that is, with the first quarter-wave plate  4 , when the reference light L 12  hits on the polarizing beam splitter  2  for the first time, the reference light L 12  is reflected by the polarizing beam splitter  2 , while when the reference light L 12  returns to the polarizing beam splitter  2 , the reference light L 12  becomes transmitted through the polarizing beam splitter  2 . 
     The second quarter-wave plate  5  is used to change polarizing directions of the LGE measuring light L 111  and the two-dimensional angle measuring light L 112 , so that the measuring light L 111  and the two-dimensional angle measuring light L 112  are reflected by the polarization beam splitter  2  when they return to the polarization beam splitter again. 
     The first polarizer  6  is arranged between the polarizing beam splitter  2  and the first photodetector (A), and as a polarizing axis direction of the first polarizer is adjusted, the combined beam L 3  interferes after passing through the first polarizer  6 . 
     The first non-polarizing beam splitter  7  is arranged between the first polarizer  6  and the first photodetector (A) for splitting the interference light L 3 , in which one beam is received by the first photodetector (A), while the other beam is received by the third photodetector (C). Light intensities of interference spots on the first photodetector (A) and the third photodetector (C) are I 1 , I 3 , respectively. 
     The half-wave plate  8  is arranged between the first non-polarizing beam splitter  7  and the third photodetector (C), to make a phase difference 90° between the interference spot signals I 1 , I 3  detected by the two detectors, calculate a phase difference φ(Δx) between the reference light L 12  and the measuring light L 111 , and calculate a displacement Δx of the target mirror unit along X-axis according to the phase difference. 
     The focusing lens  9  is used to focus the two-dimensional angle measuring light L 112  on the second photodetector (B). 
     The first photodetector (A) is used to receive the superimposed light beam L 3  including the reference light L 12  and the measuring light L 111 , so as to 1) calculate straightness errors of the target mirror unit II along Y-axis and Z-axis according to a spot offset of L 111  on the first photodetector (A); and 2) coordinate with the interference length measuring module to measure a position error of the target mirror unit II along X-axis. 
     The second photodetector (B) is configured to receive the two-dimensional angle measuring light L 112  containing the two-dimensional angle error information, and calculate two-dimensional angle errors of the target mirror unit rotating about Y-axis and Z-axis according to a spot offset of the two-dimensional angle measuring light L 112  on the second photodetector (B). 
     The third photodetector (C) is used to receive the superimposed light beam L 3  including the reference light L 12  and the measuring light L 111 , and cooperate with the first photodetector (A) to measure a position error of the target mirror unit II along X-axis. 
     In the target mirror unit II, 
     the half transmitting and half reflecting mirror  10  is used to split the measuring light L 11  directed to the target mirror unit II: a transmitted light L 111 , reflected back by the moving pyramid prism  11  to return to the measuring unit I with two-dimensional straightness and positioning error information; and a reflected light L 112 , an angle offset between the reflected light L 112  and the incident measuring light L 11  around Y-axis or Z-axis is twice an angle offset of the half transmitting and half reflecting mirror  10  itself around Y-axis or Y-axis; the light L 112  carries the two-dimensional angle information back to the measuring unit I. 
     The moving pyramid prism  11  is used to reflect the measuring light L 111  backward and return the measuring light L 111  to the polarizing beam splitter  2  so as to realize: (1) changing a spatial position of the measuring light L 111  along Y-axis or Y-axis, and an offset value in a spatial position is twice a displacement of the moving pyramid prism  11  itself along Y-axis or Y-axis; and (2) changing an optical path and frequency of the measuring light L 111 , and a drift of the optical path and frequency is proportional to a displacement of the moving pyramid prism  11  along X-axis. 
     As shown in  FIG.  1   , a method for simultaneously measuring 5DOF GEs by a laser according to an embodiment of the present invention includes the following steps: 
     Step 1: measure a straightness error along Y-axis or Z-axis based on laser autocollimation principle. 
     Step 1.1: an emitting light L 1  of a single frequency laser  1  passes by a polarizing beam splitter  2 , to be divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  transmits through the polarizing beam splitter  2 , the reference light L 12  is reflected by the polarizing beam splitter  2 ; the measuring light L 11  and the reference light L 12  are both a linearly polarized light; and polarizing directions thereof are perpendicular to each other. 
     Step 1.2: the measuring light L 11  passes by a second quarter-wave plate  5 , the linearly polarized light is changed into a circularly polarized light, which is emitted from the measuring unit I to hit the target mirror unit II, and is split by a half transmitting and half reflecting mirror  10  of the target mirror unit the transmitted light L 111  is reflected back by a moving pyramid prism  11  of the target mirror unit II, a spatial position of the light L 111  drifts with a straightness error of the target mirror unit II along Y-axis or Z-axis and carries the two-dimensional straightness error information back to the measuring unit I, and passes by the second quarter-wave plate  5  again, to shift the circularly polarized light to a linearly polarized light, but a polarizing direction passing through the second quarter-wave plate  5  is rotated by 90°, so that the light L 111  becomes reflected by the polarizing beam splitter  2  when it passes by the polarizing beam splitter  2  again. 
     Step 1.3: the reference light L 12  passes by the first quarter-wave plate  4 , the linearly polarized light is changed into a circularly polarized light; after the light L 12  is reflected back 180° toward its original direction by a fixed pyramid prism  3  to pass through the first quarter-wave plate  4  in a return way, the circularly polarized light is shifted back into a linearly polarized light, but a polarizing direction is rotated by 90°, so that the light L 12  transmits through the polarizing beam splitter  2  when it passes by the polarizing beam splitter  2  again, and is combined with the light L 111  reflected by the polarizing beam splitter in Step 1.2, so as to form a combined light L 3 , then the combined light L 3  passes by a first polarizer  6  and a first non-polarizing beam splitter  7 , and is finally received by the first photodetector (A). 
     Step 1.4: recording an initial position (y1 0 , z1 0 ) of the combined beam L 3 &#39;s spot measured by the first photodetector (A), preferably, the initial position is located at a center of the first photodetector (A). 
     Step 1.5: according to a real-time position (y1 t , z1 t ) of the combined beam L 3 &#39;s spot on the first photodetector (A) and the initial position (y1 0 , z1 0 ) of the combined beam L 3 &#39;s spot, an offset of the combined beam&#39;s spot position is obtained; since the offset of the combined beam&#39;s spot is only caused by a drift of the measuring light L 111 , a straightness error of the target mirror unit along Y-axis or Z-axis can be calculated by Δy=2(y1 t −y1 0 ), Δz=2(z1 t −z1 0 ). 
     Step 2: a position error along X-axis is measured based on a single frequency laser interference. 
     Steps 2.1: after the reference light L 12  in Step 1.1 is reflected back 180° toward its original direction by the fixed pyramid prism  3  of the measuring unit I, its polarization state, frequency and phase are not changed, so the light L 12  can be used as a reference light of the interference length measuring signal; 
     Step 2.2: the frequency and phase of the light L 111  in Steps 1.2 change with a displacement of the target mirror unit II along X-axis, so that the light L 111  carries a straightness error information along X-axis and returns to the measuring unit as a measuring light of the heterodyne interference length measuring signal; 
     Step 2.3: by adjusting a polarizing axis direction of the first polarizer, the combined light L 3  obtained in Step 1.3 interferes after passing through the first polarizer  6 ; 
     Step 2.4: after the interference light L 3  passes by the first non-polarizing beam splitter  7 , it is divided into a transmitted light L 31  and a reflected light L 32 ; 
     Step 2.5: the transmitted light L 31  is received by the first photodetector (A), a light intensity of the interference spot is I 1 , and a phase of the reflected light L 32  is delayed 90° by a half-wave plate  8 , then is received by a third photodetector (C), and a light L 32 &#39;s intensity of the interference spot is I 3 ; 
     Step 2.6: a phase difference between the reference light L 12  and the measuring light L 111  is φ(Δx), based on the I 1  and I 3 , a number of light and dark changes of interference fringes caused by φ(Δx) is N(Δx), a laser emitting laser wavelength is λ, then a displacement of the target mirror unit II along X-axis is Δx=N(Δx)·λ/2. 
     Step 3: measuring an angle error for rotating about Y-axis or Z-axis based on laser autocollimation. 
     Step 3.1: a reflected light of a half transmitting and half reflecting mirror  10  of the target mirror unit II in Step 1.2 is a two-dimensional angle error measuring light L 112 , 
     Step 3.2: the light L 112  carries the two-dimensional angle error information back to the measuring unit I, and passes by the second quarter-wave plate  5 , then the light L 112  is reflected by the polarizing beam splitter  2 , and then is focused on the second photodetector (B) by a focusing lens  9  of a two-dimensional angle measuring module; 
     Step 3.3: recording an initial position (y 0 , z0) of a light spot measured by the second photodetector (B); 
     Step 3.4: according to a real-time position (y t , z t ) of the spot on the second photodetector (B) and the initial position of the spot, an offset of the spot is obtained; according to the offset of the spot, a two-dimensional angle error of the target mirror unit rotating around Y-axis or Z-axis is calculated as follows: Δα=(y t −y 0 )/2f, Δβ=(z t −z 0 )/2f, where f is a focal length of a focusing lens or a lens group. 
     In Embodiment 1, if the half-wave plate  8  is moved to between the first non-polarizing beam splitter  7  and the first photodetector (A), a position error along X-axis can also be measured based on the laser interference. 
     If the light L 1  emitted from the single frequency laser is a linearly polarized light, an additional half-wave plate is required; by rotating the half-wave plate, a polarizing direction of the emitting light L 1  is incident at a certain angle (preferably) 45° with respect to an optical axis of the polarizing beam splitter  2 . 
     Embodiment 2 
       FIG.  2    is a schematic view of a system for simultaneously measuring 5DOF GEs by a laser according to Embodiment 2 of the present invention. As shown in  FIG.  2   , according to one aspect of the present invention, there is provided a system for simultaneously measuring 5DOF GEs by a laser, which is composed of a measuring unit I and a target mirror unit II. 
     The measuring unit I in Embodiment 2 has the same components as the measuring unit I in Embodiment 1, and the target mirror unit II in Embodiment 2 has the same components as the target mirror unit II in Embodiment 1. The difference is that when an emitting light L 1  passes by a polarizing beam splitter  2 , its reflected light is taken as a measuring light L 11 , and the target mirror unit II is arranged in a direction for reflecting the light L 1  by the polarizing beam splitter  2 , while a transmitted light of the polarizing beam splitter  2  is taken as a reference light L 12 , a fixed pyramid prism  3  and a first quarter-wave plate  4  are disposed in a direction in which the polarizing beam splitter  2  transmits through the light L 1 . For convenience, a structure in which a transmitted light of a light L 1  in Embodiment 1 is used as a measuring light L 11  is called a transmitting sensitive structure, while a structure in which a reflected light of a light L 1  in Embodiment 2 is used as a measuring light L 11  is referred to as a reflecting sensitive structure. 
     In the measuring unit I, 
     the polarizing beam splitter  2  is used for: (1) beam splitting: the emitting light L 1  is divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is incident on the target mirror unit, transmitted by the half transmitting and half reflecting mirror  10  of the target mirror unit, and reflected back by the moving pyramid prism  11  as a light L 111 , the light L 111  carries 3DOF LGE signals back to the measuring unit I as a linear GE measuring light; the reference light L 12  only propagates within the measuring unit I; (2) beam combining: the reference light L 12  that passes by the polarizing beam splitter  2  again is reflected, and a measuring light L 111  that is reflected back by the target mirror unit II transmits through the polarizing beam splitter  2 , so that the two beams are superposed together in a spatial position, so as to form a combined light L 3 ; (3) beam separating: the light L 112  reflected by the half transmitting and half reflecting mirror  10  of the target mirror unit II transmits through the polarizing beam splitter  2  as a two-dimensional angle measuring light. 
     The first quarter-wave plate  4  is used to change a polarizing direction of the reference light L 12 . so that the reference light L 12  is reflected by the polarizing beam splitter  2  to pass by the first quarter-wave plate  4 , then is reflected backward by the fixed pyramid prism  3  to pass by the first quarter-wave plate  4  pass again by the polarizing beam splitter  2 , then it becomes reflected by the polarizing beam splitter  2 . 
     The second quarter-wave plate  5  is used to change polarizing directions of the LGE measuring light L 111  and the two-dimensional angle measuring light L 112 , so that the measuring light L 111  and the two-dimensional angle measuring light L 112  become transmit through the polarization beam splitter  2  when they pass by the polarization beam splitter again. 
     The functions of other components are the same as those in Embodiment 1 and will not be described again. 
     The functions of the half transmitting and half reflecting mirror  10  and the moving pyramid prism  11  in the target mirror unit II are the same as those in Embodiment 1 and will not be described again. 
     As shown in  FIG.  2   , the present Embodiment 2 provides a method for simultaneously measuring 5DOF GEs by a laser, which includes the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser autocollimation principle. 
     Step 1.1: when the emitting light L 1  of the single frequency laser  1  passes by the polarizing beam splitter  2 , it is divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is reflected by the polarizing beam splitter  2 , the reference light L 12  transmits through the polarizing beam splitter  2 ; the measuring light L 11  and the reference light L 12  are both a linearly polarized light, and their polarizing directions are perpendicular to each other. 
     Step 1.2: the measuring light L 11  passes by the second quarter-wave plate  5 , the linearly polarized light is changed into a circularly polarized light; the measuring light L 11  is emitted from the measuring unit I to hit the target mirror unit II, and is split by the half transmitting and half reflecting mirror  10  of the target mirror unit the transmitted light L 111  is reflected back by the moving pyramid prism  11  of the target mirror unit II, a spatial position of the light L 111  drifts with a straightness error of the target mirror unit II along Y-axis or Z-axis; the light L 111  carries the two-dimensional straightness error information back to the measuring unit I and passes by the second quarter-wave plate  5  again, then the light L 111  changes from the circularly polarized light to a linearly polarized light, but a polarizing direction after passing by the second quarter-wave plate  5  is further rotated by 90°, so that the light L 111  becomes transmitting through the polarizing beam splitter  2  when it passes by the polarizing beam splitter  2  again. 
     Step 1.3: the reference light L 12  passes by the first quarter-wave plate  4 , the linearly polarized light is changed into a circularly polarized light; the reference light L 12  is reflected back 180° toward its original direction by the fixed pyramid prism  3  and passes by the first quarter-wave plate  4  again, the circularly polarized light is changed into a linearly polarized light, but the polarizing direction is further rotated by 90°, so that the light L 12  becomes reflected by the polarizing beam splitter when it passes by the polarizing beam splitter again, and is combined with the light L 111  transmitted by the polarizing beam splitter in Step 1.2, so as to form a combined light L 3 , then the light L 3  passes by the first polarizer  6  and the first non-polarizing beam splitter  7 , and is finally received by the first photodetector (A). 
     Steps 1.4 and 1.5: they are the same as those in Embodiment 1 and will not be described again. 
     Step 2: a position error along X-axis is measured based on a single frequency laser interference, which is consistent with Embodiment 1 and will not be described again. 
     Step 3: measuring an angle error for rotating around Y-axis or Z-axis based on laser autocollimation, which is consistent with Embodiment 1 and will not be described again. 
     In Embodiment 2, if the half-wave plate  8  is moved between the first non-polarizing beam splitter  7  and the first photodetector (A), a position error along X-axis can also be measured based on a laser interference. 
     If the light L 1  emitted from the single frequency laser is a linearly polarized light, an additional half-wave plate is required; by rotating the half-wave plate, a polarizing direction of the emitting light L 1  is incident at a certain angle (preferably) 45° with respect to an optical axis of the polarizing beam splitter  2 . 
     Embodiment 3 
       FIG.  3    is a schematic view of a system for simultaneously measuring 5DOF GEs by a laser according to Embodiment 3 of the present invention. As shown in  FIG.  3   , according to one aspect of the present invention, a system for simultaneously measuring 5DOF GEs by the laser is provided, which is composed of a measuring unit I and a target mirror unit II. 
     The measuring unit I includes a single frequency laser  1 , a second non-polarizing beam splitter  12 , a fixed pyramid prism  3 , a first polarizer  6 , a first non-polarizing beam splitter  7 , a half-wave plate  8 , a focusing lens  9 , a first photodetector (A), a second photodetector (B), and a third photodetector (C). The single frequency laser  1  constitutes a laser emitting module. The first non-polarizing beam splitter  7 , the half-wave plate  8 , and the third photodetector (C) constitute an interference length measuring module. The focusing lens  9  and the second photodetector (B) constitute a two-dimensional angle measuring module. 
     The target mirror unit II is the same as that of Embodiment 1. 
     In the measuring unit I, 
     the single frequency laser  1  is used to generate an emitting light L 1 , which is a circularly polarized light or a linearly polarized light; 
     the second non-polarizing beam splitter  12  is used for: (1) beam splitting: the emitting light L 1  is divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is emitted to the target mirror unit by transmitting through the half transmitting and half reflecting mirror  10  of the target mirror unit, and reflected back by the moving pyramid prism  11 , so as to form a light L 111 , the light L 111  carries a 3DOF LGE signal back to the measuring unit I as a LGE measuring light; the reference light L 12  only propagates within the measuring unit I; (2) beam combining: a part of the reference light L 12  that is transmitted by the second non-polarizing beam splitter  12  when passing by the same again and a part of the measuring light L 111  that is reflected backward by the target mirror unit II are superposed together in a spatial position, so as to form a combined light L 3 ; (3) beam separating: the light L 112  reflected by the half transmitting and half reflecting mirror  10  of the target mirror unit II is as a two-dimensional angle measuring light. 
     The fixed pyramid prism  3 , the first polarizer  6 , the half-wave plate  8 , the focusing lens  9 , the first photodetector (A), the second photodetector (B), and the third photodetector (C) have the same functions as those of Embodiment 1, and will not be described again. 
     The present embodiment 3 provides a method for simultaneously measuring 5DOF GEs by a laser, which comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser autocollimation principle 
     Step 1.1: an emitting light L 1  of a single frequency laser  1 , passing by a second non-polarizing beam splitter  12 , is divided into a measuring light L 11  and a reference light L 12 ; the measuring light L 11  transmits through the second non-polarizing beam splitter  12  and the reference light L 12  is reflected by the second non-polarizing beam splitter  12 . 
     Step 1.2: the measuring light L 11  is emitted from the measuring unit I to hit the target mirror unit II, and is split by a half transmitting and half reflecting mirror  10  of the target mirror unit a transmitted light L 111  is reflected back 180° toward its original direction by the moving pyramid prism  11  of the target mirror unit a spatial position of L 111  drifts with a straightness error of the target mirror unit II along Y-axis or Z-axis; the light L 111  carries a two-dimensional straightness error information back to the measuring unit I and passes by the second non-polarizing beam splitter  12 , then the light L 111  is divided into a reflection part and a transmission part by the second non-polarizing beam splitter  12 . 
     Step 1.3: the reference light L 12  is reflected back 180° toward its original direction by the fixed pyramid prism  3  to transmit through the second non-polarizing beam splitter  12  when passing by the same again, then the reference light L 12  is combined with the reference light L 111  reflected by the second non-polarizing beam splitter  12  in Step 1.2, so as to form a combined light L 3 ; then the combined light L 3  passes by the first polarizer  6  and the first non-polarizing beam splitter  7 , and finally is received by the first photodetector (A). 
     Steps 1.4 and 1.5: they are the same as those in Embodiment 1 and will not be described again. 
     Step 2: a position error along X-axis is measured based on a single frequency laser interference, which is consistent with Embodiment 1 and will not be described again. 
     Step 3: measuring an angle error for rotating around Y-axis or Z-axis based on laser autocollimation 
     Step 3.1: it is consistent with Embodiment 1 and will not be described again. 
     Step 3.2: the light L 112  returns a two-dimensional angle error information to the measuring unit I; when passing by the second non-polarizing beam splitter  12 , the reflected light is focused on the second photodetector (B) by the focusing lens  9  of a two-dimensional angle measuring module. 
     Steps 3.3 and 3.4: they are the same as those in Embodiment 1 and will not be described again. 
     Embodiment 3 has the other 3 variants. 
     As shown in  FIG.  4   , the reflected light L 12  when passing by the second non-polarizing beam splitter  12  for the second time and the transmitted light L 111  passing by the second non-polarizing beam splitter  12  are combined to form a combined light L 3 ′, and the first polarizer  6 , the interference length measuring module and the first photodetector (A) are arranged in an emitting direction of the combined light L 3 ′. 
     As shown in  FIG.  5   , a transmitting sensitive structure is adopted, that is, when an emitting light L 1  passes by the non-polarizing beam splitter  12 , its reflected light is used as a measuring light L 11 , the target mirror unit II is set in a direction for the polarizing beam splitter  2  to reflect the light L 1 , while its transmitted light is used as a reference light L 12 , the fixed pyramid prism  3  and the first quarter-wave plate  4  is disposed in a direction in which the non-polarizing beam splitter  12  transmits through the light L 1 . 
     As shown in  FIG.  6   , a transmitting sensitive structure is used by setting the first polarizer  6 , the interference length measuring module and the first photodetector (A) in a direction for emitting a combined light L 3 ′. 
     Embodiment 4 
       FIG.  7    is a schematic view of a system for simultaneously measuring 6DOF GEs by a laser according to Embodiment 4 of the present invention, in which a rolling angle measuring unit is added based on Embodiment 1, including adding a fifth photodetector (E) to the measuring unit I; and a target mirror unit II is added a third quarter-wave plate  13 . As shown in  FIG.  7   , according to one aspect of the present invention, there is provided a system for simultaneously measuring 6DOF LGEs by a laser, which is composed of a measuring unit I and a target mirror unit II. 
     In the target mirror unit II, the added third quarter-wave plate  13  is used to: (1) changing a measuring light L 111  reflected backward by a moving pyramid prism  11  from a circularly polarized light to a linearly polarized light, so that the linearly polarized light returns to the measuring unit I and passes by the first quarter-wave plate  4  again, the linearly polarized light is changed back into a circularly polarized light (preferred state) or an elliptical polarized light, and then the measuring light L 111  is split by the polarizing beam splitter  2 ; a ratio of intensity of a reflected light L 111 ′ and that of a transmitted light L 111 ″ is 1:1 (preferred state) or close to 1:1; (2) the third quarter-wave plate rotates γ around X-axis with respect to the target mirror unit II, then a polarizing direction of the measuring light L 111  is also rotated by the same angle γ, light intensities of the reflected light L 111 ′ and the transmitted light L 111 ″ of the measuring light L 111  are both changed, and a rotating angle γ of the target mirror unit II about X-axis is calculated according to the change of direction. 
     In the measuring unit I, the added fifth photodetector (E) is used to receive the transmitted light L 111 ″ of the measuring light L 111  passing by the polarizing beam splitter  2 ; the reflected light L 111 ′ of the measuring light L 111  is combined with the light L 12  so as to form a combined light L 3 ; the light L 3  is divided into a light L 31  and a light L 32 , they are received by the first photodetector (A) and the third photodetector (C), respectively. 
     The present Embodiment 4 provides a method for simultaneously measuring 6DOF LGEs by a laser, which comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser autocollimation principle. 
     Step 1.1: it is consistent with Embodiment 1 and will not be described again. 
     Step 1.2: a measuring light L 11  passes by a second quarter-wave plate  5 , a linearly polarized light is changed into a polarized light; the measuring light L 11  is emitted from a measuring unit I to hit a target mirror unit II, and is split by a half transmitting and half reflecting mirror  10  of the target mirror unit the transmitted light L 111  is reflected back by a moving pyramid prism  11  of the target mirror unit a spatial position of the light L 111  drifts with a straightness error of the target mirror unit II along Y-axis or Z-axis; the light L 111  carries a 2D straightness error information back to the measuring unit I; by adjusting a third quarter-wave plate  13 , the light L 111  passing by the third quarter-wave plate  13  again changes from the circularly polarized light to a linearly polarized light so as to pass by a second quarter-wave plate  5 , so that the light L 111  changes from the linearly polarized light to a circularly polarized light (preferred state) or an elliptical polarized light, then the light L 111  is split by the polarizing beam splitter  2  when it passes by the polarizing beam splitter  2  again; a ratio of a light intensity of a reflected light L 111 ′ and that of a transmitted light L 111 ″ is 1:1 (preferred state) or close to 1:1. 
     Step 1.3: the reference light L 12  passes by the first quarter-wave plate  4 , the linearly polarized light is changed into a circularly polarized light; then the reference light L 12  is reflected back 180° toward its original direction by the fixed pyramid prism  3  to pass by the first quarter-wave plate  4  again, then the circularly polarized light is changed into a linearly polarized light, but a polarizing direction is rotated by 90°, so that the light L 12  can transmit through the polarizing beam splitter when it passes by the polarizing beam splitter again, and is combined with the light L 111 ′ reflected by the polarizing beam splitter in Step 1.2, so as to form a combined light L 3 ; then the combined light L 3  passes by the first polarizer  6  and the first non-polarizing beam splitter  7 , and is finally received by the first photodetector (A). 
     Steps 1.4 and 1.5: they are the same as those in Embodiment 1 and will not be described again. 
     Step 2: a position error along X-axis is measured based on a single frequency laser interference, which is consistent with Embodiment 1 and will not be described again. 
     Step 3: measuring an angle error for rotating around Y-axis or Z-axis based on laser autocollimation, which is consistent with Embodiment 1 and will not be described again. 
     Step 4: measuring an angle error about X-axis based on a polarization difference light intensity. 
     Step 4.1: adding a third quarter-wave plate  13  to the target mirror unit the measuring light L 111  passing by the third quarter-wave plate  13  returns to measuring unit I, then passes by the second quarter-wave plate  5  again, the measuring light L 111  is split when passing by the polarizing beam splitter  2  again; a reflected light L 111 ′ is still received by the first photodetector (A) and the third photodetector (C), while the other transmitted light L 111 ″ is received by the fifth photodetector (E). 
     Step 4.2: blocking the measuring light L 111 , so that only the reference light L 12  is received on the first photodetector (A) and the third photodetector (C), light intensities of the first photodetector (A) and the third photodetector (C) are I 10 , I 30 , respectively. 
     Step 4.3: recovering the measuring light L 111 ; when the target mirror unit II rotates γ around X-axis, the third quarter-wave plate  13  rotates γ accordingly; when the measuring light L 111  passes by the polarizing beam splitter  2  again, its polarizing direction increases by an angle γ′ with respect to its original polarizing direction; the angle is equal to a rotated angle γ of the target mirror unit about X-axis. 
     Step 4.4: the light L 111  of the measuring light passes by the polarizing beam splitter  2  and is split by the polarizing beam splitter  2 ; a reflected light is finally received by the first photodetector (A) and the third photodetector (C), their light intensities are I 1 (γ), I 3 (γ); the other transmitted light is finally received by the fifth photodetector (E), its light intensity is I 5 (γ), f(γ)=[I 1 (γ)−I 10 +I 3 (γ)−I 30 −I 5 (γ)]/[I 1 (γ)−I 10 +I 3 (γ)−I 30 +I 5 (γ)]. 
     Step 4.5: rotating the target mirror unit II around X-axis by several specific angles, γ 1 , γ 2 , . . . , measuring the corresponding f(γ 1 ), f(γ 2 ), . . . , and calibrating a function curve of γ v. f(γ). 
     Step 4.6: measuring light intensities I 1 (γ), I 3 (γ), I 5 (γ) in real time, calculating f(γ) according to Step 4.4, and calibrating the function γ v. f(γ) according to Step 4.5; and calculating a rotation angle γ of the target mirror unit II about X-axis. 
     Embodiment 4 has three other variants. 
     (1) As shown in  FIG.  8   , a quarter-wave plate  13  is moved into the measuring unit I, and placed at a position before the measuring light L 111  returns to the measuring unit I and hits the polarizing beam splitter  2 ; a second quarter-wave plate  5  is moved into the target mirror unit II, and placed in front of the half transmitting and half reflecting mirror  10 ; for convenience, a rolling angle measuring structure as shown in  FIG.  7   , in which the second quarter-wave plate  5  is placed in the measuring unit I while the third quarter-wave plate  13  is in the target mirror unit II, is called a first rolling angle measuring structure; a rolling angle measuring structure as shown in  FIG.  8   , in which a second quarter-wave plate  5  is located in the target mirror unit II while a third quarter-wave plate  13  is arranged in the measuring unit I, is referred to as a second rolling angle measuring structure; the measuring light L 11  passes by the third quarter-wave plate  13  in the target mirror unit II, then is transmitted by the half transmitting and half reflecting mirror  10 , reflected backward by the moving pyramid prism  11 , then passes by the third quarter-wave plate  13  again, and returns to the measuring unit I; when the third quarter-wave plate rotates γ around X-axis with the target mirror unit II, a polarizing direction of the measuring light L 111  is rotated by an angle γ′=2γ, a light intensity change of the reflected light L 111 ′ and the transmitted light L 111 ″ after the measuring light L 111  is split by the vibrating beam splitter  2  is twice that of the first rolling angle measuring structure, and a resolution of an angle γ by which the target mirror unit II rotates about X-axis is doubled. 
     (2) As shown in  FIG.  9   , when a reflecting sensitive structure and a first rolling angle measuring structure are adopted, a rolling angle measuring resolution is the same as that of a transmitting sensitive structure and a first rolling angle measuring structure as shown in  FIG.  7   . 
     (3) As shown in  FIG.  10   , when a reflecting sensitive structure and a second rolling angle measuring structure are adopted, a rolling angle measuring resolution is consistent with that of a transmitting sensitive structure and a second rolling angle measuring structure as shown in  FIG.  8   . 
     Embodiment 5 
       FIG.  11    is a schematic view of a system for simultaneously measuring 6DOF GEs by a laser according to Embodiment 5 of the present invention. As shown in  FIG.  11   , according to one aspect of the present invention, there is provided a system for simultaneously measuring 6DOF LGEs by a laser, which is composed of a measuring unit I and a target mirror unit II. 
     Differing from the system of Embodiment 7, as shown in  FIG.  7   , the measuring unit I is without a second quarter-wave plate  5 , while a third quarter-wave plate  13  of the target mirror unit II moves before the half transmitting and half reflecting mirror  10 , so as to form a third rolling angle measuring structure, in which the third quarter-wave plate  13  of the target mirror unit II replaces the second quarter-wave plate  5  in the measuring unit I of Embodiment 7, to change polarizing directions of a linear GE measuring light L 111  and a two-dimensional angle measuring light L 112 , so that: (1) when the two-dimensional angle measuring light L 112  passes by the polarizing beam splitter  2  again, it is reflected by the polarizing beam splitter  2 ; (2) when the measuring light L 111  passes by the polarizing beam splitter  2  again, it is split by the polarizing beam splitter  2 ; a reflected light L 111 ′ and a transmitted light L 111 ″ have a ratio of about 1:1 in light intensities; (3) when measuring, the third quarter-wave plate rotates γ around X-axis with the target mirror unit II, then a polarizing direction of the measuring light L 111  is rotated by an angle γ′=γ, to make light intensities of the reflected light L 111 ′ and the transmitted light L 111 ″ of the measuring light L 111  changed, so a rotation angle γ of the target mirror unit II about X-axis can be calculated according to the change. 
     Embodiment 5 has another variant. As shown in  FIG.  12   , a reflecting sensitive structure and a third rolling angle measuring structure are adopted, its rolling angle measuring resolution is the same as that of the transmitting sensitive structure and the third rolling angle measuring structure as shown in  FIG.  11   . 
     Embodiment 6 
       FIG.  13    is a schematic view of a system for simultaneously measuring 5DOF GEs by a laser according to Embodiment 13 of the present invention. As shown in  FIG.  13   , according to one aspect of the present invention, a system for simultaneously measuring 5DOF GEs by laser is provided with a measuring unit I and a target mirror unit II. 
     The measuring unit I includes a dual frequency laser  14 , a polarizing beam splitter  2 , a fixed pyramid prism  3 , a first quarter-wave plate  4 , a second quarter-wave plate  5 , a first polarizer  6 , a focusing lens  9 , a third non-polarizing beam splitter  15 , a second polarizer  16 , a first photodetector (A), a second photodetector (B), and a fourth photodetector (D). The dual frequency laser  14  constitutes a laser emitting module. The third non-polarizing beam splitter  15 , the second polarizer  16 , and the fourth photodetector (D) constitute an interference length measuring module. The focusing lens  9  and the second photodetector (B) constitute a two-dimensional angle measuring module. 
     The target mirror unit II includes a half transmitting and half reflecting mirror  10  and a moving pyramid prism  11 , which are consistent with Embodiment 1. 
     In the measuring unit I, 
     the dual frequency laser  14  is used to generate an emitting light L 1 , and the emitting light L 1  is a polarized light with two beams. The two beams are spatially superposed with a certain frequency difference, and the two beams have polarizing directions perpendicular to each other; 
     the third non-polarizing beam splitter  15  is disposed between the dual frequency laser  14  and the polarizing beam splitter  2 ; the emitting light L 1  is split by the third non-polarizing beam splitter  14 , including an emitted light in its original direction, and a reflected light L 2 ; 
     the second polarizer  16  is arranged between the third non-polarizing beam splitter  15  and the fourth photodetector (D); by adjusting a polarizing axis direction of the second polarizer  16 , the reflected light L 2  interferes after passing by the second polarizer  16 , and an interference spot is received by the fourth photodetector (D) as a reference signal for heterodyne interference length measurement; 
     the fourth photodetector (D) is used to receive an interference spot of a light L 2  as a standard signal (or called a reference signal) for the heterodyne interference length measurement. 
     A direction of a polarizing axis of the first polarizer  6  is adjusted, to make the combined beam L 3  interferes after passing by the first polarizer  6 , an interference spot is received by the first photodetector (A) as a measuring signal of the heterodyne interference length measurement; a displacement of the target mirror unit along X-axis is calculated according to the reference signal/the standard signal, and the measuring signal. 
     The polarizing beam splitter  2 , the fixed pyramid prism  3 , the first quarter-wave plate  4 , the second quarter-wave plate  5 , the first polarizer  6 , the focusing lens  9 , the first photodetector (A), and the second photodetector (B) have the same functions as those of Embodiment 1, and will not be described again. 
     The present embodiment 6 provides a method for simultaneously measuring 5DOF GEs by a laser, which comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser autocollimation principle. 
     Step 1.1: when an emitting light L 1  of a dual frequency laser  14  passes by a polarizing beam splitter  2 , it is divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  transmits through the polarizing beam splitter  2 , while the reference light L 12  is reflected by the polarizing beam splitter  2 ; the measuring light L 11  and the reference light L 12  are both a linearly polarized light, and polarizing directions thereof are perpendicular to each other. 
     Step 1.2: it is consistent with Embodiment 1 and will not be described again. 
     Step 1.3: the reference light L 12  passes by the first quarter-wave plate  4 , its linearly polarized light is changed into a circularly polarized light; the reference light L 12  is reflected back 180° toward its original direction by a fixed pyramid prism  3  and passes by the first quarter-wave plate  4  again, the circularly polarized light is changed back into a linearly polarized light, but its polarizing direction is rotated by 90°, so that the light L 12  becomes transmitted through the polarizing beam splitter when it passes by the polarizing beam splitter again, and is combined with the light L 111  reflected by the polarizing beam splitter in Step 1.2, so as to form a combined light L 3 , then the combined light L 3  passes by the first polarizer  6 , and is finally received by the first photodetector (A). 
     Steps 1.4 and 1.5: they are the same as those in Embodiment 1 and will not be described again. 
     Step 2: measuring a position error along X-axis based on a dual frequency laser interference. 
     Step 2.1: the emitting light L 1  has two polarized beams with a certain frequency difference, frequencies of the two beams are f 1 , f 2 , respectively, and when they are separated by the polarizing beam splitter  2 , a frequency of the measuring light L 11  is f 1 , while a frequency of the reference light L 12  is f 2 . 
     Step 2.2: a displacement of the measuring light L 111  along X-axis of the target mirror unit is λx, a frequency variation due to Doppler effect is f(Δx), a frequency of the measuring light L 111  is f 1 +f(Δx). 
     Step 2.3: a first polarizer  6  is arranged in front of the first photodetector (A); by adjusting a polarizing axis direction of a polarizer, a combined beam L 3  (including a light L 12  and a light L 111 ) interferes after passing through the first polarizer  6 ; an interference spot is received by the first photodetector (A) as a measuring signal of a heterodyne interference length measurement, and a frequency of the measuring signal is f m =f 1 +f(Δx)−f 2 . 
     Step 2.4: when the emitting light L 1  passes by the third non-polarizing beam splitter  15 , another laser beam L 2  is formed in a reflecting direction of the third non-polarizing beam splitter  15 ; the light L 2  also contains two polarized lights with a certain frequency difference; the light L 2  interferes after passing by the second polarizer  16 ; an interference spot is received by the fourth photodetector (D) as a standard signal for the heterodyne interference length measurement, and a frequency of the standard signal is f s =f 1 −f 2 . 
     Step 2.5: the measured signal frequency (f m =f 1 +f(Δx)−f 2 ) obtained in Step 2.3 subtracts the standard signal frequency (f s =f 1 −f 2 ) obtained in Step 2.4 to obtain f(Δx)=f m −f s , a number of light and dark changes of interference fringes caused by f(Δx) is N(Δx), a laser emitting laser wavelength is λ, so a displacement of the target mirror unit along X-axis Δx=N(Δx)·λ/2. 
     Step 3: measuring an angle error for rotating around Y-axis or Z-axis based on laser autocollimation, which is consistent with Embodiment 1 and will not be described again. 
     In practical applications, the above Steps 1, 2 and 3 can be changed in their order. 
     In Embodiment 6, a reflecting sensitive structure as shown in  FIG.  14    can be used, so as to realize simultaneously measuring 5DOF GEs by a laser. 
     Embodiment 7 
       FIG.  15    is a schematic view of a system for simultaneously measuring 6DOF GEs by a laser according to Embodiment 7 of the present invention. The system adds a rolling angle measuring unit to the measuring system as shown in  FIG.  13    of Embodiment 6, including adding a fifth photodetector (E) to the measuring unit I; and adding a third quarter-wave plate  13  in the target mirror unit II. That is, the system structure combines a dual frequency length measuring, transmitting sensitive structure and the third rolling angle measuring structure. 
     As shown in  FIG.  15   , according to one aspect of the present invention, there is provided a system for simultaneously measuring 6DOF LGEs by a laser, including a measuring unit I and a target mirror unit II. 
     The present embodiment 7 provides a method for simultaneously measuring 6DOF LGEs by a laser, which comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser autocollimation principle. 
     Step 1.1: it is the same as Embodiment 6 and will not be described again. 
     Step 1.2: it is the same as Embodiment 4 and will not be described again. 
     Step 1.3: the reference light L 12  passes by the first quarter-wave plate  4 , its linearly polarized light is changed into a circularly polarized light, then the reference light L 12  is reflected back 180° toward its original direction by the fixed pyramid prism  3  and passes by the first quarter-wave plate  4  again, the circularly polarized light is changed back into a linearly polarized light, but its polarizing direction is rotated by 90°, so that the light L 12  transmits through the polarizing beam splitter when it passes by the polarizing beam splitter again, and is combined with the light L 111 ′ reflected by the polarizing beam splitter in Step 1.2, so as to form a combined light L 3 , and then the combined light L 3  is finally received by the first optoelectrical detector (A) after passing through the first polarizer  6 . 
     Steps 1.4 and 1.5: they are the same as those in Embodiment 1 and will not be described again. 
     Step 2: measuring a position error along X-axis based on a dual frequency laser interference. 
     Steps 2.1 and 2.2: they are the same as those in Embodiment 6 and will not be described again. 
     Step 2.3: setting a first polarizer  6  in front of the first photodetector (A); by adjusting a polarizing axis direction of the polarizer, a combined beam L 3  (including a light L 12  and light L 111 ′) interferes after passing by the first polarizer  6 ; an interference spot is received by the first photodetector (A) as a measuring signal of a heterodyne interference length measurement, a frequency of the measuring signal is f m =f 1 +f(Δx)−f 2 . 
     Steps 2.4 and 2.5: they are the same as those in Embodiment 6 and will not be described again. 
     Step 3: measuring an angle error for rotating around Y-axis or Z-axis based on laser autocollimation, which is consistent with Embodiment 1 and will not be described again. 
     Step 4: measuring an angle error about X-axis based on a polarization difference light intensity. 
     Step 4.1: adding a third quarter-wave plate  13  in the target mirror unit II; the measuring light L 111  passes by the third quarter-wave plate  13  to return to measuring unit I, then passes by second quarter-wave plate  5  again, the measuring light L 111  is split when passing through the polarizing beam splitter  2 ; a reflected light L 111 ′ is still received by the first photodetector (A), while the other transmitted light L 111 ″ is received by the fifth photodetector (E). 
     Step 4.2: blocking the measuring light L 111 , so that the reference light L 12  is only received on the first photodetector (A), a light intensity of the first photodetector (A) is I 10 . 
     Step 4.3: it is the same as Embodiment 4 and will not be described again. 
     Step 4.4: the light L 111  of the measuring light passes by the polarizing beam splitter  2  and is split by the polarizing beam splitter  2 ; a reflected light is finally received by the first photodetector (A), its light intensity is I 1 (γ), the other transmitted light is finally received by the fifth photodetector (E) and its light intensity is I 5 (γ), f(γ)=[I 1 (γ)−I 10 −I 5 (γ)]/[I 1 (γ)−I 10 +I 5 (γ)]. 
     Step 4.5: it is the same as Embodiment 4 and will not be described again. 
     Step 4.6: measuring light intensities I 1 (γ), I 5 (γ) in real time, calculating f(γ) according to Step 4.4, calibrating f(γ) v. γ according to Step 4.5, and calculating a rotated angle γ of the target mirror unit II about X-axis. 
     Embodiment 7 has the following 5 variants. 
     (1) As shown in  FIG.  16   , with a transmitting sensitive structure and a second rolling angle measuring structure, a rolling angle measuring resolution is twice that of a transmitting sensitive structure and a first rolling angle measuring structure as shown in  FIG.  15   . 
     (2) As shown in  FIG.  17   , a transmitting sensitive structure and a third rolling angle measuring structure are adopted, and a rolling angle measuring resolution is the same as that of a transmitting sensitive structure and a first rolling angle measuring structure as shown in  FIG.  15   . 
     (3) As shown in  FIG.  18   , a reflecting sensitive structure and a first rolling angle measuring structure are adopted, and a rolling angle measuring resolution is the same as that of a transmitting sensitive structure and a first rolling angle measuring structure as shown in  FIG.  15   . 
     (4) As shown in  FIG.  19   , with a reflecting sensitive structure and a second rolling angle measuring structure, a rolling angle measuring resolution is twice that of a transmitting sensitive structure and a first rolling angle measuring structure as shown in  FIG.  15   . 
     (5) As shown in  FIG.  20   , a reflecting sensitive structure and a third rolling angle measuring structure are adopted, and a rolling angle measuring resolution is the same as that of a transmitting sensitive structure and a first rolling angle measuring structure as shown in  FIG.  15   . 
     Embodiment 8 
     As shown in  FIG.  21   , according to one aspect of the present invention, there is provided a system for simultaneously measuring 5DOF GEs by a laser, comprising a measuring unit I and a target mirror unit II. 
     The measuring unit I includes a multi wavelength laser light source  16 , a heterodyne frequency generating module  17 , a second non-polarizing beam splitter  12 , a fixed pyramid prism  3 , a first polarizer  6 , a focusing lens  9 , a first photodetector (A), a second photodetector (B), a first bandpass filter  18 , a second bandpass filter  19 , a third bandpass filter  20 , a first phase detector  21 , a second phase detector  22 , and a third phase detector  23 . The multi wavelength laser light source  16  and the heterodyne frequency generating module  17  constitute a laser emitting module. The first bandpass filter  18 , the second bandpass filter  19 , the third bandpass filter  20 , the first phase detector  21 , the second phase detector  22 , and the third phase detector  23  constitute an interference length measuring module. The focusing lens  9  and the second photodetector (B) constitute a two-dimensional angle measuring module. 
     The target mirror unit II includes a half transmitting and half reflecting mirror  10  and a moving pyramid prism  11 , which is consistent with Embodiment 1. 
     In the measuring unit I, 
     the multi wavelength laser light source  16  is used to generate an emitting light L 1 , and the emitting light L 1  has a multi wavelength laser λ 1 , λ 2 , λ 3 , their frequencies are v 1 , v 2 , v 3 ; 
     the heterodyne frequency generating module  17  is configured to change a frequency of the emitting light L 1  to v 1 +f 1 , v 2 +f 2 , v 3 +f 3 . 
     The non-polarizing beam splitter  12  is used for: (1) beam splitting: splitting the emitting light L 1  into a measuring light L 11  and a reference light L 12 . The measuring light L 11  is incident on the target mirror unit and is reflected back by the target mirror unit as a light L 111  which carries a 3DOF LGE signal back to the measuring unit as a measuring light. The reference light L 12  only propagates inside the measurement unit; (2) beam combining: the reference light L 12  is transmitted through the non-polarizing beam splitter  12  when it passes by the same again, and the measuring light L 111  is reflected by the target mirror unit, so that the two beams are superposed together in a spatial position, so as to form a combined light L 3 ; (3) beam separating: separating the light L 112  reflected by the beam splitter in the target mirror unit from the measuring light L 11 , as a two-dimensional angle measuring light. 
     The first polarizer  6  is arranged between the second non-polarizing beam splitter  12  and the first photodetector (A). By adjusting a direction of the polarizing axis of the first polarizer  6 , the combined beam L 3  interferes after passing through the first polarizer  6 . 
     The first photodetector (A) is used to receive the combined light L 3  so as to realize: (1) calculating a straightness error of the target mirror unit II along Y-axis or Z-axis according to a spot offset of light L 111  on the first photodetector (A); (2) coordinating with an interference length measuring module to measure a position error of the target mirror unit II along X-axis. 
     A response spectrum of the first photodetector (A) cannot reach an optical frequency, and a measured heterodyne interference signal spectrum of the combined light L 3  only contains components f 1 , f 2 , f 3 . The first to third band-pass filters of the interference length measuring module separate the components f 1 , f 2 , f 3  of the first photodetector (A) and measure phase information φ 1 , φ 2 , φ 3  corresponding to each wavelength by the first to third phase detectors. Take two pairs of beat signals, and calculate a displacement Δx of the target mirror unit along X-axis according to two pairs of combined wavelengths and two pairs of phase difference. 
     The present embodiment 8 provides a method for simultaneously measuring 5DOF GEs by a laser, which comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis or Z-axis based on laser autocollimation principle. 
     As the multi wavelength laser light source  16  is used, the emitting light L 1  includes multi wavelength laser beams λ 1 , λ 2 , λ 3 . However, when measuring a straightness error along Y-axis or Z-axis with the laser autocollimation principle, only a spot position on a detector is detected, which is not different from measurement with a single frequency laser. It is consistent with Embodiment 1 and will not be described again. 
     Step 2: measuring a position error along X-axis based on a multi wavelength laser interference. 
     Step 2.1: the emitting light L 1  emitted from the multi wavelength laser light source  16  includes multi wavelength laser lights λ 1 , λ 2 , λ 3 , their frequencies are v 1 , v 2 , v 3 ; after passing through the heterodyne frequency generating module  17 , frequencies of the multi wavelength laser become v 1 +f 1 , v 2 +f 2 , v 3 +f 3 . 
     Step 2.2: the emitting light L 1  is split into a measuring light L 11  and a reference light L 12  by a non-polarizing beam splitter  12 ; both the measuring light L 11  and the reference light L 12  contain multi wavelength laser light v 1 +f 1 , v 2 +f 2 , v 3 +f 3 . 
     Step 2.3: the measuring light L 11  is emitted from the measuring unit I to hit the target mirror unit II, and is reflected back 180° toward its original direction by the half transmitting and half reflecting mirror  10  of the target mirror unit II. The retroreflected light L 111  carries straightness error information along X-axis and returns to the measuring unit I as a measuring light of the heterodyne interference length measuring signal. 
     Step 2.4: the reference light L 12  is reflected back 180° toward its original direction by the fixed pyramid prism  3  of the measuring unit I, passes by the non-polarizing beam splitter  12 , its transmitted part is combined with a reflected part of the light L 111  when passing by the non-polarizing beam splitter  12 , so as to form a combined light L 3 ; by adjusting a polarizing axis direction of the first polarizer  6 , the combined light L 3  passes through the first polarizer  6 , and interference occurs on the first photodetector (A). 
     Step 2.5: the first photodetector (A) detects components f 1 , f 2 , f 3  of the heterodyne interference signal spectrum; the first to third band-pass filters  18 - 20  separate the components f 1 , f 2 , f 3 , and the first to third phase detectors  21 - 23  measure phase information φ 1 , φ 2 , φ 3  corresponding to each wavelength. Take two pairs of beat signals, and calculate a displacement Δx of the target mirror unit II along X-axis according to two pairs of combined wavelengths and two pairs of phase difference. 
     Embodiment 8 has the following three variants. 
     As shown in  FIG.  22   , a reflecting sensitive structure is adopted. 
     With a transmitting sensitive structure, a reflected part when the light L 12  passes by the second non-polarizing beam splitter  12  for the second time and a transmitted part when the light L 111  passes by the second non-polarizing beam splitter  12  are combined to form a light L 3 ′, the first polarizer  6  and the first photodetector (A) are arranged in an emitting direction of the combined light L 3 ′. 
     With a reflecting sensitive structure, a reflected part when the light L 12  passes by the second non-polarizing beam splitter  12  again and a transmitted part when the light L 111  passes by the second non-polarizing beam splitter  12  are combined to form a light L 3 ′, and the first polarizer  6  and the first photodetector (A) are arranged in an emitting direction of the combined light L 3 ′. 
     To sum up, the system according to the embodiments of the present invention can rapidly measure the 5/6DOF GEs of a space object moving linearly along the linear axis; and longtime monitor relative changes of 5/6DOF position and attitude of two objects in space. 
     In each embodiment of this specification, there is provided a system and a method for quickly measuring the 5/6DOF GEs of a space object at the same time when the target mirror unit moves linearly along the linear axis. After completing an optical path debugging, the systems in all embodiments can also (1) keep the target mirror unit II stationary, but linearly move the measuring unit I and the space object along the linear axis, so as also realize the simultaneous and rapid measurement of the 5/6DOF GEs of a space object; (2) by keeping the measuring unit I and the target mirror unit II stationary, monitor data measured by the measuring unit for a long time, thus long-term monitoring relative changes of 5/6DOF position and attitude of the two objects in a space. 
     In the case of a multi wavelength measurement, it is a real single detector measurement. In a single frequency and a double frequency length measurement, at least one detector needs to be equipped for auxiliary measurement. In the present invention, by a first photodetector (A) cooperating with different interference length measuring modules, and by a target mirror composed of a single optical component and a single detector, it is the first time in the world to realize measuring 3 linear errors (i.e., three straightness errors of translation in X, Y and Z directions) at the same time, and further, it is the first time to realize simultaneously measuring 5/6DOF errors based on a single prism and a single target mirror. When a single frequency laser and a dual frequency laser are used for measurement, a quarter wave plate and a rolling angle measuring module can be added to measure three linear errors and rolling angle errors based on a single detector. Based on the above two reasons, the following beneficial effects can be obtained: (1) simplifying an optical path structure, reducing a complexity of measuring system and a volume of a measuring unit and a target mirror unit, so as to facilitate practical applications; (2) reducing a number of detectors so as to reduce circuit power consumption, reduce heat dissipation, improve stability of a measurement system, and reduce cost of a measuring system. 
     The present invention can simultaneously measure the 5DOF or 6DOF errors, and greatly improve measuring efficiency compared with a single DOF measuring systems and methods in prior art. 
     A rolling angle is measured according to a polarizing intensity difference, which has a simple measurement principle, high measuring sensitivity and accuracy. 
     Those skilled in the art can understand that the drawings are only schematic view of an embodiment, so the modules or processes in the drawings maybe not necessary to implement the present invention. 
     As can be seen from the description of the above embodiments, those skilled in the art can clearly understand that the present invention can be realized with software and necessary general hardware platform. Based on this understanding, a technical solution of the present invention can be embodied in the form of a software product, which can be stored in a storage medium, such as ROM/RAM, magnetic disk, optical disk, etc., which includes several instructions for causing a computer device (which may be a personal computer, a server, or a network component, etc.) to perform the method described in each embodiment or some part of the embodiments of the present invention. 
     The above embodiments are described in a progressive manner. The same or similar parts of every embodiment can be referred to each other. Each embodiment focuses on differences from the other embodiments. In particular, for any device or system embodiment, since it is basically similar to its corresponding method embodiment, the description is relatively simple. Please refer to the corresponding description of the method embodiment for relevant parts. The above described device and system embodiments are only schematic, in which a unit described as separate component may or may not be physically individual, and the components displayed as a unit may or may not be a physical unit, that is, they may be located in a single place or may be distributed over multiple network units. Some or all of the modules can be further modified or improved according to actual needs to achieve a purpose of any embodiment. Those skilled in the art can do so with spirit of the present invention it without creative work. 
     The above description is only to the preferred embodiments of the present invention, but the scope of protection of the present invention is not limited to the disclosed embodiments. Any variant or alternative that can be easily done by those skilled in the art within the technical concept disclosed in the present invention should be covered by the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be subject to the appended claims.