Patent Publication Number: US-2023152451-A1

Title: SYSTEM FOR SIMULTANEOUSLY MEASURING 3DOF LGEs BY LASER AND METHOD THEREFOR

Description:
FIELD OF THE INVENTION 
     The present invention relates to a technical field of optical precision measurement, in particular, the present invention relates to a system for simultaneously measuring 3DOF (degrees-of-freedom) LGEs by a laser; and a method for simultaneously measuring 3DOF LGEs. 
     BACKGROUND 
     With development of precision manufacturing, machining and assembling technologies, an accuracy for measuring 3DOF LGEs of an object in motion or drifts of 3DOF LGEs of an object at rest needs to be higher and higher. 
     In prior art, a laser interferometer is the most often used tool to measure the 3DOF LGEs. However, the laser interferometer works only for a single parameter. Only one error component can be measured each time of installation and adjustment. In each measurement process, different types of measuring accessories and new adjustment of the interferometer are required, making a long time for each measurement, and the measurement accuracy is greatly sacrificed by environmental fluctuation. Therefore, it is necessary to develop a strategy for simultaneously measuring 3DOF LGEs. 
     In the prior art, a system for simultaneously measuring 3DOF LGEs has such drawbacks that its optical path is too complex, and multiple detectors are used to measure different errors, which increases a cost and complexity of the system, and increases the instability of the optical path caused by circuit heat dissipation, thus introducing extra possible measurement errors. 
     SUMMARY OF THE INVENTION 
     The present invention provides some embodiments of a system and a method for simultaneously measuring 3DOF LGEs by a laser, so as to realize simultaneous and rapid measurement of the 3DOF LGEs of a 3D object moving linearly along any linear axis. 
     In order to achieve the above object, the present invention has the following technical solutions. 
     According to one aspect of the present invention, there is provided a system for simultaneously measuring 3DOF LGEs by a laser, comprising a measuring unit and a target mirror unit, the measuring unit includes a laser emitting module, a polarizing beam splitter, a fixed reflector, a first photodetector, and an interference length measuring module; the target mirror unit includes a reflector; 
     the laser emitting module is used to generate an emergent light or emitting light L 1 ; 
     the polarizing beam splitter is used for: 1) beam splitting: splitting the emergent light L 1  into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is hitting on or passing through the target mirror unit and is reflected back by the target mirror unit, and then returns to the measuring unit with a 3DOF LGEs signal, while the reference light L 12  only propagates inside the measuring unit; 2) beam combining: according to one of two polarizing directions, transmitting or reflecting the reference light L 12  that hits on or passes through the polarizing beam splitter again and the measuring light L 11  that is reflected back 180° toward its original direction by the target mirror unit, so that the two beams of the measuring light L 11  and the reference light L 12  are superposed with each other in a spatial position, so as to form a combined light L 3 ; 
     the fixed reflector is used for backward reflecting the reference light L 12  propagating only inside the measuring unit, so as to return the reference light L 12  to the polarizing beam splitter; 
     the first photodetector is used to receive the combined light L 3  including the reference light L 12  and the measuring light L 11 , so as to realize a simultaneous measurement of linear geometric errors along the X, Y and Z axes. Specifically, 1) according to a spot offset of the measuring light L 11  on the first photodetector, a relative straightness errors between the target mirror unit and the measuring unit along Y or Z axis is calculated; 2) coordinating with the interference length measuring module, a relative position error between the target mirror unit and the measuring unit along X-axis is obtained; 
     the reflector in the target mirror unit is used to reflect the measuring light L 11  backward, and return the measuring light L 11  to the polarizing beam splitter, so as to realize: 1) changing spatial positions of the measuring light L 11  in Y and Z directions, so that a spatial position offset becomes twice relative displacements between the reflector of the target mirror unit and the measuring unit along the Y and Z axes; 2) changing the optical path and frequency of the measuring light L 11 , to make a drift of the optical path and frequency be proportional to relative displacement between the reflector of the target mirror unit and the measuring unit along X-axis. 
     Preferably, when a single frequency is applied, the laser emitting module emits a single frequency laser, and the interference length measuring module includes a first polarizer, a first non-polarizing beam splitter, a phase retarder, and a second photodetector; 
     the first polarizer is arranged in the emitting direction of the combined light L 3 , and the polarizing axis direction or light transmitting axial direction of the first polarizer is adjusted, so that the reference light L 12  and the measuring light L 11  interfere with each other after the combined light L 3  hits on or passes through the first polarizer; 
     the first non-polarizing 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, in which one beam L 31  is received by the first photodetector, while the other beam L 32  is received by the second photodetector, and light intensities of interference spots on the first photodetector and the second photodetector are named as I 1 , and I 2 , respectively; 
     the phase retarder is arranged in front of the first photodetector or the second photodetector, so as to make a phase difference 90° of the interference spot signals I 1  and I 2  detected by the two photodetectors, then calculate the phase difference φ(Δx) between the reference light L 12  and the measuring light L 11 , and calculate the relative displacement Δx between the target mirror unit and the measuring unit along X-axis according to the phase difference. 
     Preferably, when a single frequency laser measurement is applied, the laser emitting module emits a single frequency laser. The polarizing beam splitter is replaced with a second non-polarizing beam splitter. The interference length measuring module includes a first polarizer, a first non-polarizing beam splitter, a phase delay, and a second photodetector; 
     the second non-polarizing beam splitter is used for: 1) beam splitting: splitting the emergent light L 1  into a measuring light L 11  and a reference light L 12 , in which the measuring light L 11  is hitting on or passing through the target mirror unit and is reflected back by the target mirror unit, and returns to the measuring unit with a 3DOF LGEs signal, while the reference light L 12  only propagates inside the measuring unit; 2) beam combining: transmitting and reflecting the reference light L 12  that hits on or passes through the non-polarizing beam splitter again and the measuring light L 11  reflected by the target mirror unit, so that the two beams are superposed with each other in a spatial position, so as to form the combined beam L 3 , and the combined beam L 3  is a superimposed beam of the two beams of the reference light L 12  transmitted through the non-polarizing beam splitter and the measuring light L 11  reflected by the non-polarizing beam splitter, or a superimposed beams of the two beams of the reference light L 12  reflected by the non-polarizing beam splitter and the measuring light L 11  transmitted through the non-polarizing beam splitter; 
     the first polarizer is arranged in an emitting direction of the combined light L 3 , and the polarizing axis direction of the first polarizer is adjusted, so that the reference light L 12  and the measuring light L 11  interfere with each other after the combined light L 3  hits on or passes through the first polarizer; 
     the first non-polarizing beam splitter is arranged between the second non-polarizing beam splitter and the first photodetector, so as to split the combined beam L 3  that has been interfered, in which one beam L 31  is received by the first photodetector, while the other beam L 32  is received by the second photodetector, and light intensities of interference spots on the first photodetector and the second photodetector are indicated as I 1  and I 2 , respectively; 
     the phase retarder is arranged in front of the first photodetector or the second photodetector, and is used to make a phase difference 90° of the interference spot signals I 1  and I 2  detected by the two photodetectors, then a phase difference φ(Δx) between the reference light L 12  and the measuring light L 11  can be calculated, and a relative displacement Δx between the target mirror unit and the measuring unit along X-axis can be calculated according to the phase difference. 
     Preferably, when dual frequency laser measurement is applied, the laser emitting module emits a dual frequency laser light with a certain frequency difference and different polarization directions; 
     the interference length measuring module comprises a third non-polarizing beam splitter, a first polarization detector, a second polarization detector, and a third photodetector; 
     the third non-polarizing beam splitter is disposed between the laser emitting module and the polarizing beam splitter, so that the light L 1  emitted by the laser emitting module is split by the third non-polarizing beam splitter, so as to form another laser beam L 2 ; 
     the first polarizer is arranged in an emitting direction of a combined light with the reference light L 12  and the measuring light L 11  reflected by the target mirror and hitting on or passing through the polarizing beam splitter, and a polarizing axis direction of the first polarizer is adjusted, so that the combined light L 3  with the light L 12  and the light L 11  hits on or passes through the first polarizer, then the reference light L 12  and the measuring light L 11  interfere with each other, and an interference spot is received by the first photodetector, as a length measuring signal for a heterodyne interferometry; 
     the second polarizer is arranged between the non-polarizing beam splitter and the third photodetector; by adjusting a polarizing axis direction of the second polarizer, the laser light L 2  interferes after hitting on or passing through the third polarizer, and an interference spot is received by the third photodetector as a length measuring reference signal for the heterodyne interference; 
     the polarizer is used to determine a relative displacement of the target mirror unit and the measuring unit along X axis according to the reference signal and the measurement signal. 
     Preferably, when a multi wavelength measurement is applied, the laser emitting module includes a multi wavelength laser light source and a heterodyne frequency generating module, the interference length measuring module includes 1st to Nth bandpass filters and 1st to Nth phase detectors, N is a natural number greater than or equal to 3, and the polarizing beam splitter is replaced by a second non-polarizing beam splitter; 
     the multi wavelength laser light source emits multi wavelength laser lights λ 1 , λ 2 , λ 3 , . . . , A N  with frequencies v 1 , v 2 , v 3  . . . v N ; after passing through the heterodyne frequency generating module, the 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 emergent light L 1 , the second non-polarizing beam splitter is used for: 1) beam splitting: splitting the emergent light L 1  into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is hitting on or passing through the target mirror unit, and then is reflected back by the target mirror unit; the light L 11  carries a 3DOF LGEs signal and returns to the measuring unit as a measuring light; the reference light L 12  only propagates within the measuring unit; 2) beam combining: the reference light L 12  that hits on or passes through the second non-polarizing beam splitter again and the measuring light L 11  reflected by the target mirror unit are transmitted and reflected, so that the two beams are superposed with each other in a spatial position, so as to form a combined beam L 3 ; 
     the light L 3  interferes on the first photodetector, and the obtained heterodyne interference signal spectrum only contains components f 1 , f 2 , f 3 , . . . , f N ; 
     after the 1st to Nth bandpass filters separate the components f 1 , f 2 , f 3 , . . . , f N , the length measuring phase information φ 1 , φ 2 , φ 3 , . . . , φ N  corresponding to each wavelength is measured by the 1st to Nth phase detectors; taking n pairs of the length measuring phase data (2≤n≤N−1, n is a natural number) to form a beat signal, and calculating the relative displacement Δx between the target mirror unit and the measuring unit along X-axis according to n pairs of wavelength and n pairs of phase difference. 
     Preferably, the fixed reflector is any one of a pyramid prism, a cat eye mirror, an angular cube retroreflector composed of three mutually perpendicular reflecting surfaces, a right angle prism, and a mirror group composed of two plane mirrors; and the target mirror unit reflector is any one of a pyramid prism, a cat eye mirror, and an angular cube retroreflector composed of three mutually perpendicular reflecting surfaces. 
     Preferably, the first photodetector, the second photodetector, the fourth photodetector, and the fifth photodetector are any one of QD (Four-quadrant photodetector), PSD (Position Sensitive Detector), CCD (Charge-coupled Component) and CMOS (Complementary Metal Axide Semiconductor); and a relative straightness error between the target mirror unit and the measuring unit along Y-axis and/or Z-axis is calculated according to the spot offset on any one of the four photodetectors; while the third photodetector is any one of QD, PSD, CCD, CMOS and pin. 
     According to another aspect of the present invention, there is provided a method for simultaneously measuring 3DOF LGEs with a laser, comprising: 
     Step 1) measuring a straightness error along Y-axis and/or Z-axis based on laser collimation principle 
     Step 1.1) when a light L 1  emitted by a laser emitting module hits on or passes through a polarizing beam splitter, it is divided into a measuring light L 11  and a reference light L 12 ; 
     Step 1.2) after the measuring light L 11  is emitted from the measuring unit, it is hitting on or passing through the target mirror unit; after being reflected back 180° toward its original direction by a reflector of the target mirror unit, a spatial position of the light L 11  drifts with a relative straightness error between the target mirror unit and the measuring unit along Y-axis and/or Z-axis; the light L 11  carries the two-dimensional straightness error information back to the measuring unit, and the light L 11  hits on or passes through the polarizing beam splitter again; 
     Step 1.3) after the reference light L 12  is reflected back 180° toward its original direction by a fixed reflector, it hits on or passes through the polarizing beam splitter again, then is combined with the light L 11  hitting on or passing through the polarizing beam splitter again in step 1.2), so as to form a combined light L 3 , and is received by the first photodetector; 
     Step 1.4) an initial spot position of the combined beam is measured by the first photodetector; 
     Step 1.5) according to a real-time spot position of the combined beam on the first photodetector, comparing the real-time spot position with the initial spot position of the combined beam, it is obtained a spot offset amount of the combined beam; since the spot offset of the combined beam is only caused by a position drift of the measuring light L 11 , a relative straightness error between the target mirror unit and the measuring unit along Y-axis and/or Z-axis is calculated according to the spot offset of the combined beam; 
     Step 2) measuring a position error along X-axis based on laser interference 
     Step 2.1) after the reference light L 12  in Step 1.1) is reflected back 180° toward its original direction by a 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 11  in Step 1.2) drift with a relative displacement between the target mirror unit and the measuring unit along X-axis, and the light L 11  carries the relative 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 in Step 2.1) and the measuring light in step 2.2) hit on (pass through) the polarizing beam splitter, the two beams are superposed with each other in a spatial position; after passing through the interference length measuring module, a relative straightness error between the target mirror unit and the measuring unit along X-axis is calculated by the signals measured on the first photodetector. 
     Preferably, calculating the relative straightness error along Y-axis and/or Z-axis according to the spot offset of the combined beam comprises: 
     the initial position and the real-time position of the light L 11 &#39;s spot on the first photodetector are (yI o , zI o ), (yI t , zI t ), respectively, then the relative straightness errors between the target mirror unit and the measuring unit along Y-axis and/or Z-axis are Δy=2(y1 t −y1 o ), Δz=2(z1 t −z1 o ), respectively. 
     Preferably, when a single frequency length measurement is applied, measuring the position error along X-axis based on the laser interferometry comprises: 
     Step 1) the reference light L 12  and the measuring light L 11  are superposed with each other in a spatial position after hitting on or passing through the second non-polarizing beam splitter, so as to form a combined light L 3 ; a polarizing axis direction of the first polarizer is adjusted, so that the combined light L 3  interferes after hitting on or passing through the first polarizer; 
     Step 2) the interference light L 3  is divided into lights L 31  and L 32  after hitting on or passing through the first non-polarizing beam splitter; 
     Step 3) after one of the beams L 31  and L 32  is delayed 90° by a phase retarder, they are received by the first photodetector and the second photodetector, respectively, and light intensities of the interference light spots on the first photodetector and the second photodetector are I 1  and I 2 , respectively; 
     Step 4) by processing the I 1  and I 2 , the phase difference between the reference light L 12  and the measuring light L 11  is φ(Δx), the number of light and dark changes of interference fringes caused by φ(Δx) is N(Δx), a laser wavelength emitting from a laser 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 applied, measuring the position error along X-axis based on the laser interference measurement comprises: 
     Step 1) the light L 1  emitted from the laser emitting module includes two polarized lights with a certain frequency difference, the two frequencies are f 1  and f 2 , respectively; and when the light L 1  is split by the polarizing beam splitter, the frequency of the measuring light L 11  is f 1 , and the frequency of the reference light L 12  is f 2 ; 
     Step 2) a relative displacement of the measuring light L 11  between the target mirror unit and the measuring unit along X-axis is Δx; the frequency variation due to Doppler effect is f(Δx), the frequency of the measuring light L 11  is f 1 +f(Δx); 
     Step 3) setting a first polarizer in front of the first photodetector, adjusting a direction of the light transmitting axis of the first polarizer, so that the lights L 12  and the light L 11  interfere after hitting on or passing through the first polarizer, and the interference spot is received by the first photodetector as a measuring signal of the heterodyne interference length measurement, and a frequency of a measuring beat signal is f m =f 1 +f(Δx)−f 2 ; 
     Step 4) when the emergent light L 1  hits on or passes through the third non-polarizing beam splitter, it is split by the third non-polarizing beam splitter, so as to form another laser beam L 2 ; the light L 2  also contains two polarized lights with a certain frequency difference, and a polarizing axis direction of the second polarizer is adjusted, so that the light L 2  interferes after hitting on or passing through the second polarizer; the interference spot is received by the second photodetector as a standard signal for the heterodyne interference length measurement; then the standard signal frequency is f s =f 1 f 2 ; 
     Step 5) f m =f 1 +f(Δx)−f 2 , the frequency of the measuring beat signal obtained in Step 3), is subtracted by f s =f 1 −f 2 , the standard beat signal frequency obtained in Step 4), so as to obtain f(Δx)=f m −f s , the number of light and dark changes of interference fringes caused by f(Δx) is N(Δx), a laser wavelength emitting from a laser is λ, so a relative displacement between the target mirror unit and the measuring unit along X-axis is Δx=N(Δx)·λ/2. 
     Preferably, when measuring a multiple wavelength measurement is applied, measuring the position error along X-axis based on the 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 a heterodyne frequency generating module, the 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 emergent light L 1 ; 
     Step 2) the light L 1  emitted from the laser emitting module is divided 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 lights 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 a reflector of the target mirror unit as the reflected back 180° toward its original direction light L 11 , the light L 11  carries the straightness error information along X-axis and returns to the measuring unit as a measuring light of a heterodyne interference length measuring signal; 
     Step 4) after the reference light L 12  is reflected back 180° toward its original direction by a fixed reflector of the measuring unit, it hits on or passes through the non-polarizing beam splitter, and then combines with the light L 11 , and a polarizing axis direction of the first polarizer is adjusted, so that the reference light L 12  and the measuring light L 11  interfere with each other on the first photodetector; 
     Step 5) the first photodetector detects components f 1 , f 2 , f 3 , . . . , f N  of the heterodyne interference signal spectrum, and the 1st to Nth band-pass filters separate the components f 1 , f 2 , f 3 , . . . , f N , and the 1st to Nth phase detectors measure the length measuring phase information φ 1 , φ 2 , φ 3 , . . . , φ N  corresponding to each wavelength; taking n pairs of a beat signal composed of a wavelength and a phase difference, 2≤n≤N−1, n is a natural number, a relative displacement Δx between the target mirror unit and the measuring unit along X-axis is calculated according to the n pairs of wavelength and n pairs of phase difference. 
     It can be seen from the above technical solutions provided by the embodiments of the present invention that the system and method for simultaneously measuring 3DOF LGEs by a laser can realize simultaneous and rapid measurement of 3DOF LGEs of a 3D object moving linearly along any linear axis, so that it can long time monitor 3DOF linear relative position drifting between two objects in a space. 
     Additional aspects and advantages of the present invention will be given in the following description, which will become apparent from the following description, or will be learned from the embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In order to more clearly explain the technical solutions of the embodiments of the present invention, the following will briefly introduce drawings that need to be used in the description of the embodiments. It is obvious that the drawings in the following description show only some embodiments of the present invention. For those skilled in the art, other embodiments could also be obtained according to these drawings without inventive efforts. 
         FIG.  1    is a structural view of a system for simultaneously measuring 3DOF LGEs (linear-geometric-errors) by a single frequency laser according to one embodiment of the present invention. 
         FIG.  2    is a structural view of an interference length measuring module of a single frequency laser with dual channels according to one embodiment of the present invention. 
         FIG.  3    is a structural view of an interference length measuring module of a single frequency laser with four channels according to one embodiment of the present invention. 
         FIG.  4    is a structural view of a system for simultaneously measuring 3DOF LGEs by a single frequency laser according to one embodiment of the present invention. 
         FIG.  5    is a structural view of a system for simultaneously measuring 3DOF LGEs by a single frequency laser according to the other embodiment of the present invention. 
         FIG.  6    is a structural view of a system for simultaneously measuring 3DOF LGEs by a single frequency laser according to another embodiment of the present invention. 
         FIG.  7    is a structural view of a system for simultaneously measuring 3DOF LGEs by a dual frequency laser according to another embodiment of the present invention. 
         FIG.  8    is a structural view of a system for simultaneously measuring 3DOF LGEs by a dual frequency laser according to the other embodiment of the present invention. 
         FIG.  9    is a structural view of a system for simultaneously measuring 3DOF LGEs by a dual frequency laser according to another embodiment of the present invention. 
         FIG.  10    is a structural view of a system for simultaneously measuring 3DOF LGEs by a multi wavelength laser according to one embodiment of the present invention. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described in detail below. In the embodiments shown in the drawings, the same or similar reference numerals indicate the same or similar elements having the same or similar functions. The embodiments described by referring to the accompanying drawings are exemplary and are only used to explain the present invention, but 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 parts, unless it is specifically stated. It should be further understood that the word “including” or “comprising” used in the description of the present invention refers to have those mentioned features, integers, steps, operations, elements, and/or components, but does not exclude having 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, or there may be intermediate elements. In addition, the words “connecting” or “coupling” used herein may include wireless connecting or coupling. The term “and/or” used herein includes any unit and all combinations of one or more listed items. 
     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 defined in a general dictionary should be understood to have a meaning consistent with the prior art, and will not be interpreted literally unless defined as herein. 
     In order to facilitate understanding of those embodiments of the present invention, several specific embodiments will be further explained together with the accompanying drawings, and each embodiment does not constitute a limitation of the present invention. 
     The embodiments will corroborate again and again that the present invention can realize simultaneously measuring 3DOF LGEs with optical components and detectors as less as possible. 
     Embodiment 1 
       FIG.  1    is a structural view of a system for simultaneously measuring 3DOF LGEs by a single frequency laser according to an embodiment of the present invention.  FIG.  2    is a structural view of an interference length measuring module with a single frequency and dual channels according to an embodiment of the present invention. As shown in  FIG.  1   , according to one aspect of the present invention, there is provided a system for simultaneously measuring 3DOF LGEs by a laser, comprising 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 photodetector (A) and an interference length measuring module with a single frequency and dual channels. The single frequency laser  1  constitutes a laser emitting module. As shown in  FIG.  2   , the interference length measuring module includes a first polarizer  6 , a first non-polarizing beam splitter  7 , a half-wave plate  8 , and a second photodetector (B). 
     The target mirror unit II includes a moving pyramid prism  9 . 
     In the measuring unit I, 
     the single frequency laser  1  is used to generate an emergent light L 1 ; 
     the polarizing beam splitter  2  is used for: 1) beam splitting: the emergent light L 1  is divided into a transmitted light recorded as a measuring light L 11 , and a reflected light recorded as a reference light L 12 , the measuring light L 11  is hitting on or passing through the target mirror unit II, and is reflected back by the moving pyramid prism  9  of the target mirror unit II, and then is returned to the measuring unit I with a 3DOF LGEs signal; the reference light L 12  only propagates within the measuring unit I; 2) beam combining: the reference light L 12  that will hit on (pass through) the polarizing beam splitter  2  again is transmitted, and the measuring light L 11  reflected back by the target mirror unit II is further reflected, so that the two beams of the measuring light L 11  and the reference light L 12  are superposed with each other in a spatial position, which is denoted as L 3 ; 
     the fixed pyramid prism  3  is used for backward reflecting the reference light L 12  propagating only inside the measuring unit I, so as to return the reference light L 12  to the polarizing beam splitter  2 ; 
     the first quarter-wave plate  4  is used to change a polarization direction of the reference light L 12 , the reference light L 12  is reflected by the polarizing beam splitter  2 , i.e., transmitting and hitting on or passing through the first quarter-wave plate  4 , being reflected backward by the fixed pyramid prism  3 , transmitting the first quarter-wave plate  4  again, then hitting on or passing through the polarizing beam splitter  2  again. While the light L 12  used to be reflected by the polarizing beam splitter  2 , the light L 12  transmits the polarizing beam splitter  2  when it returns and hits on or passes through the polarizing beam splitter  2  again; 
     the second quarter-wave plate  5  is used to change a polarization direction of the linear error measuring light L 11 , so that the measuring light L 11  is reflected by the polarizing beam splitter  2  when it hits on or passes through the polarizing beam splitter  2  again; 
     the first polarizer  6  is arranged between the polarizing beam splitter  2  and the first photodetector (A), and is used to make the combined light L 3  interfere on the polarizer; 
     a 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 ; one of the split interference light L 3  is received by the first photodetector (A), while the other is received by the second photodetector (B); light intensities of interference spots on the first photodetector (A) and the second photodetector (B) are recorded as I 1  and I 2 , respectively; 
     a half-wave plate  8  is arranged between the first non-polarizing beam splitter  7  and the second photodetector (B), so as to make a phase difference 90° of the interference spot signals I, and  12  detected by two detectors, calculate a phase difference φ(Δx) between the reference light L 12  and the measuring light L 11 , and calculate a displacement Δx of the target mirror unit along X-axis according to the phase difference. 
     The first photodetector (A) is used to receive L 31  to realize: 1) according to a spot offset of the light L 11  in L 31  on the first photodetector (A), calculating a straightness error of the target mirror unit II along Y-axis and/or Z-axis; 2) obtaining the interference spot signal I 1 , and cooperating with the interference length measuring module to realize a position error measurement of the target mirror unit II along X-axis. 
     The second photodetector (B) is used to receive L 32 , obtain the interference spot signal  12 , and realize a position error measurement of the target mirror unit II along X-axis, except for the I 1  measured by the first photodetector (A). 
     In the target mirror unit II, 
     the moving pyramid prism  9  is used to reflect the measuring light L 11  backward and return the measuring light L 11  to the polarizing beam splitter  2 , so as to realize: 1) changing a spatial position of the measuring light L 11  in Y direction and Z direction, and the amount of the spatial position offset is twice the amount of displacement of the pyramid prism  9  itself in the Y direction and the Z direction; 2) changing an optical path and frequency of the measuring light L 11 , and the amount of change in the optical path and frequency is proportional to a displacement amount of the pyramid prism  9  itself in X direction. 
     The embodiment 1 provides a method for simultaneously measuring 3DOF LGEs by a laser, comprising the following steps: 
     Step 1: measure the straightness error along Y-axis and/or Z-axis based on laser collimation principle 
     Step 1.1) when an emergent light L 1  of a single frequency laser  1  hits on or passes through a polarizing beam splitter  2 , it is divided into a measuring light L 11  and a reference light L 12 , in which the measuring light L 11  is transmitted 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 linearly polarized light, and polarization directions of the two lights are perpendicular to each other; 
     Step 1.2) the measuring light L 11  hits on or passes through the second quarter-wave plate  5 , the linearly polarized light is changed into a circularly polarized light, which is emitted from the measuring unit I and incident on the target mirror unit II; after being reflected back 180° toward its original direction by the pyramid prism  9  of the target mirror unit II, a spatial position of the light L 11  changes with a straightness error of the target mirror unit II along Y-axis and/or Z-axis; the light L 11  carries a two-dimensional straightness error information back to the measuring unit I and hits on or passes through the second quarter-wave plate  5  again, the light L 11  changes from the circularly polarized light to a linearly polarized light, but a polarization direction at hitting on or passing through the second quarter-wave plate  5  again is rotated by 90° with respect to that when hitting on or passing through the second quarter-wave plate  5  the first time, so that the light L 11  becomes reflected by the polarizing beam splitter  2  when it hits on or passes through the polarizing beam splitter  2  again; 
     Step 1.3) the reference light L 12  hits on or passes through the first quarter-wave plate  4 , the linearly polarized light is changed into a circularly polarized light, and is reflected back 180° toward its original direction by a fixed pyramid prism  3  and hits on or passes through the first quarter-wave plate  4  again, it changes from the circularly polarized light to a linearly polarized light, but a polarization direction at hitting on or passing through the first quarter-wave plate  4  again is rotated by 90° with respect to that for hitting on or passing through the first quarter-wave plate  4  for the first time, so that when reaching the polarizing beam splitter  2 , the light L 12  becomes transmitted through the polarizing beam splitter  2 , and is combined with the light L 11  reflected by the polarizing beam splitter in Step 1.2, so as to form the combined light L 3 ; the light L 3  is divided into two lights L 31  and L 32  after going over a first polarizer  6  and hitting on or passing through a first non-polarizing beam splitter  7 , and the lights L 31  and  32  are received by a first photodetector (A) and a second photodetector (B), respectively; 
     Step 1.4) recording an initial position (yI o , zI o ) of the light L 31  measured by the first photodetector (A), and the initial position is preferred located at the center of the first photodetector (A); 
     Step 1.5) according to a real-time position (y1 t , z1 t ) of the light L 31  on the first photodetector (A), comparing it with the initial position (yI o , zI o ) of the light L 31 , so as to obtain an offset amount of a L 31  spot position; the L 31  spot position offset is only caused by a position drift of the measuring light L 11 , a straightness error of the target mirror unit along Y-axis and/or Z-axis is calculated according to the L 31  spot position offset Δy=2(y1 t −y1 o ), Δz=2(z1 t −z1 o ); in Step 1.4 and Step 1.5, an offset amount of the light L 32  spot position can also be measured by the second photodetector (B) to calculate a straightness error of the target mirror unit along Y-axis and/or Z-axis. 
     Step 2: measuring the position error along X-axis based on laser interference Step 2.1) after the reference light L 12  in Step 1.1 is reflected back 180° toward its original direction by a fixed pyramid prism  3  of the measuring unit I, its polarization direction, frequency and phase are not changed, and the light L 12  is used as a reference light of an interference length measurement signal; 
     Step 2.2) the frequency and phase of the light L 11  in Step 1.2 change with a displacement of the target mirror unit II along X-axis, and the light L 11  returns a straightness error information along X-axis to the measuring unit I as a measuring light signal for the heterodyne interference length measurement; 
     Step 2.3) adjusting a polarizing axis direction of a first polarizer  6 , so that the combined light L 3  in Step 1.3 interferes after hitting on or passing through the first polarizer  6 ; 
     Step 2.4) after the interference light L 3  hits on or passes through 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), its intensity of an interference spot light is I 1 , and the reflected light L 32  hitting on or passing through a half-wave plate  8  is received by the second photodetector (B), in which the phase is delayed by 90°, its intensity of the interference spot is 13; 
     Step 2.6) a phase difference between the reference light L 12  and the measuring light L 11  is φ(Δx), by processing I 1  and I 3 , the number N(Δx) of light and dark changes of interference fringes caused by φ(Δx) is obtained; the emitting laser wavelength of the single frequency laser  1  is λ, the displacement of the target mirror unit II along X-axis is Δx=N(Δx)·λ/2. 
     In Step 2.5, the half-wave plate  8  can also be arranged before the first photodetector (A), so that the phrase of the transmitted light L 31  can be delayed by 90° through the half-wave plate  8 . 
     As shown in  FIG.  3   , this embodiment can also adopt a single frequency laser with four channels in an interference length measuring module, including a first non-polarizing beam splitter  7 , a half-wave plate  8 , a second polarizing beam splitter  11 , a third quarter-wave plate  12 , a third polarizing beam splitter  13 , a second photodetector (B), a third photodetector (C), and a fourth photodetector (D). 
     The combined light L 3  hits on or passes through the non-polarizing beam splitter  7 , dividing into a transmitted light L 31  and a reflected light L 32 . 
     The transmitted light L 31  hits on or passes through the third quarter-wave plate  12 , and is divided into lights L 311  and L 312 . The transmitted light L 311  is received by the first photodetector (A), its light intensity of interference spot is I 1 . The reflected light L 312  is received by the fourth photodetector (D), its light intensity of interference spot is 14; and a phase difference between I 1  and I 4  is 180°. 
     The reflected light L 32  hits on or passes through the half-wave plate  8 , with a phase delay of 90°, hits on or passes through the second polarizing beam splitter  11  and is split; the transmitted light L 321  is received by the second photodetector (B), its light intensity of the interference spot is I 2 . The reflected light L 322  is received by the fifth photodetector (E), the light intensity of the interference spot is 15; and a phase difference between 12 and 15 is 180°; 
     The phases of I 1 , I 2 , I 4  and I 5  are different by 90° from one to next, and the phase difference between the reference light L 12  and the measuring light L 11  is φ(Δx), by processing I 1 , I 2 , I 4  and I 5 , the number N(Δx) of light and dark changes of interference fringes caused by φ(Δx) is obtained; an emitting laser wavelength of the single frequency laser  1  is λ, a displacement of the target mirror unit II along X-axis is Δx=N(Δx)·λ/2; compared with the dual channel interference length measuring module, the four channel interference length measuring module can also judge a moving direction of the target mirror unit. 
     Embodiment 2 
       FIG.  4    is a view of a system for simultaneously measuring 3DOF LGEs by a single frequency laser according to an embodiment of the present invention. As shown in  FIG.  4   , according to another one aspect of the present invention, there is provided a system for simultaneously measuring 3DOF LGEs by a single frequency laser, including a measuring unit I and a target mirror unit II. 
     The measuring unit I includes the same components as the measuring unit I in embodiment 1, and the target mirror unit II has the same components as the target mirror unit II in embodiment 1. The difference therebetween is in that when an emergent light L 1  hits on or passes through the 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 that the polarizing beam splitter  2  reflects the light L 1 , and its transmitted light is taken as a reference light L 12 , a fixed pyramid prism  3 , and the first quarter-wave plate  4  are disposed in a direction in which the polarizing beam splitter  2  transmits the light L 1 . For convenience of description, a structure in which the transmitted light L 1  used as the measuring light L 11  in Embodiment 1 is referred to as a transmission sensitive structure, while a structure in which the reflected light L 1  used as the measuring light L 11  in this Embodiment 2 is referred to as a reflection sensitive structure. 
     In the measuring unit I, 
     the polarizing beam splitter  2  is used for: 1) beam splitting: the emergent light L 1  is divided into a reflected light recorded as the measuring light L 11 , and a transmitted light recorded as the reference light L 12 , the measuring light L 11  is hitting on or passing through the target mirror unit II, and is reflected back by a moving pyramid prism  9  of the target mirror unit II, and then returns to the measuring unit I with a 3DOF LGEs signal; the reference light L 12  only propagates within the measuring unit I; 2) beam combining: the reference light L 12  that hits on or passes through the polarizing beam splitter  2  again is transmitted, while the measuring light L 11  is reflected back by the target mirror unit II, so that the two beams of the measuring light L 11  and the reference light L 12  are superposed with each other in a spatial position, which is denoted as a light L 3 ; 
     a first quarter-wave plate  4  is used to change a polarization direction of the reference light L 12 , so that the reference light L 12  is transmitted through the polarizing beam splitter  2 , then hitting on or passing through the first quarter-wave plate  4 , backward reflecting by a fixed pyramid prism  3 , and transmitting the first quarter-wave plate  4  again; whenever the light L 12  hints on the polarizing beam splitter  2  again, it becomes reflected by the polarizing beam splitter  2 , that is, an original transmitted state is switched to the reflected state; 
     a second λ 4  wave plate  5  is used to change a polarization direction of a linear error measuring light L 11 , so that the measuring light L 11  is transmitted through the polarizing beam splitter  2  when it meets the polarizing beam splitter  2  again. 
     The functions of other components are the same as those of Embodiment 1, and will not be described in detail again. 
     The target mirror unit II includes a moving pyramid prism  9 , whose function is the same as that in Embodiment 1, and will not be described in detail again. 
     The present Embodiment 2 provides a method for simultaneously measuring 3DOF LGEs by a laser, which comprises the following steps: 
     Step 1: measure a straightness error along Y-axis and/or Z-axis based on laser collimation principle 
     Step 1.1) when the emergent light L 1  of a single frequency laser  1  hits on or passes through the polarizing beam splitter  2 , it is divided into a measuring light L 11  and a reference light L 12 , in which the measuring light L 11  is reflected by the polarizing beam splitter  2 , while the reference light L 12  is transmitted through the polarizing beam splitter  2 , and the measuring light L 11  and the reference light L 12  are both a linearly polarized light, and polarization directions of the two lights are perpendicular to each other; 
     Step 1.2) the measuring light L 11  hits on or passes through the second quarter-wave plate  5 , the linearly polarized light is changed into a circularly polarized light, which is emitted from the measuring unit I and incident on the target mirror unit II; after being reflected back 180° toward its original direction by the pyramid prism  9  of the target mirror unit II, a spatial position of the light L 11  changes with a straightness error of the target mirror unit II along Y-axis and/or Z-axis; the light L 11  carries the two-dimensional straightness error information back to the measuring unit I, and after hitting on or passing through the second quarter-wave plate  5  again, the light L 11  changes from a circularly polarized light to a linearly polarized light, but a polarization direction after hitting on or passing through the second quarter-wave plate  5  again is rotated by 90°, so that the light L 11  is transmitted through the polarizing beam splitter  2  when meeting it again; 
     Step 1.3) the reference light L 12  hits on or passes through the first quarter-wave plate  4 , then the linearly polarized light is changed into a circularly polarized light, which is reflected back 180° toward its original direction by a fixed pyramid prism  3 ; and after hitting on or passing through the first quarter-wave plate  4  again, it changes from the circularly polarized light to a linearly polarized light, but a polarization direction after hitting on or passing through the first quarter-wave plate  4  again is rotated by 90° with respect to that hitting on or passing through the polarizing beam splitter  2  before, so that the light L 12  becomes reflected by the polarizing beam splitter  2 , and is combined with the light L 11  transmitted through the polarizing beam splitter in Step 1.2, to form the light L 3 ; the light L 3  is divided into lights L 31  and L 32  after transmitting through a first polarizer  6  and hitting on or passing through a first non-polarizing beam splitter  7 , so as to be received by the first photodetector (A) and the second photodetector (B), respectively. 
     Steps 1.4 and 1.5 are the same as those in Embodiment 1, and will not be described in detail again. 
     Step 2: a position error along X-axis is measured based on the laser interference, which is consistent with Embodiment 1, and will not be described again in detail. 
     Similar to Embodiment 1, this Embodiment 2 can also use the single frequency laser with four channels in the interference length measuring module as shown in  FIG.  3   , so as to measure the position error along X-axis. 
     Embodiment 3 
       FIG.  5    is a view of a system for simultaneously measuring 3DOF LGEs by a single frequency laser. As shown in  FIG.  5   , in a further aspect of the present invention, there is provided a system for simultaneously measuring 3DOF LGEs by a single frequency laser. The system comprises a measuring unit I and a target mirror unit II. 
     The measuring unit I of this Embodiment 3 includes a single frequency laser  1 , a polarizing beam splitter  2 , a fixed pyramid prism  3 , a first photodetector (A), and an interference length measuring module of the single frequency laser. The interference length measuring module adopts any one of the single frequency laser with dual channels in the interference length measuring module as shown in  FIG.  2    and the single frequency laser with four channels in the interference length measuring module as shown in  FIG.  3   . Compared with the Embodiment 1, the first embodiment quarter-wave plate  4  and the second quarter-wave plate  5  are omitted; the light L 11  is still transmitted when returning to the polarizing beam splitter  2 , and the light L 2  is still reflected when returning to the polarizing beam splitter  2 . Compared with the Embodiment 1, an emitting direction of a combined beam L 3  is rotated by 90°, and locations of the interference length measuring module of the single frequency laser and the first photodetector (A) are changed accordingly. 
     The target mirror unit II includes a pyramid prism  9 . The structure and function are the same as those of the Embodiment 1, and will not be described again in detail. 
     In the measuring unit I, 
     the polarizing beam splitter  2  is used for: 1) beam splitting: an emergent light L 1  is divided into a transmitted light recorded as a measuring light L 11  and a reflected light recorded as a reference light L 12 , the measuring light L 11  is hitting on or passing through the target mirror unit II, is reflected back by the pyramid prism  9  of the target mirror unit II, and returns to the measuring unit I with a 3DOF LGEs signal, while the reference light L 12  only propagates within the measuring unit I; and 2) beam combining: the reference light L 12  that hits on or passes through the polarizing beam splitter  2  again is reflected, while the measuring light L 11  that is reflected back by the target mirror unit II is transmitted, so that the two beams of lights are superposed with each other in a spatial position, to form a combined light L 3 . 
     The functions of other components are the same as those in Embodiment 1, and will not be described again in detail. 
     Based on a single frequency laser with dual channels in the interference length measuring module, Embodiment 3 provides a method for simultaneously measuring 3DOF LGEs by a laser, which includes the following steps: 
     Step 1: measure a straightness error along Y-axis and/or Z-axis based on laser collimation principle 
     Step 1.1) being consistent with Embodiment 1, this step will not be described again in detail; 
     Step 1.2) a measuring light L 11  exits from the measuring unit I and hits on or passes through the target mirror unit after being reflected back 180° toward its original direction by a pyramid prism  9  of the target mirror unit II, a spatial position of the light L 11  changes with a straightness error of the target mirror unit II along Y-axis and/or Z-axis; the light L 11  carries two-dimensional straightness error information back to the measuring unit I, and the light L 11  is then transmitted through the polarizing beam splitter  2 ; 
     Step 1.3) the reference light L 12  is reflected back 180° toward its original direction by a fixed pyramid prism  3 , then is reflected by the polarizing beam splitter  2  when hitting on or passing through the same again; the light L 12  is combined with the light L 11  transmitted through the polarizing beam splitter  2  in Step 1.2, so as to form a combined light L 3 ; after passing through a first polarizer  6  and hitting on or passing through a first non-polarizing beam splitter  7 , the light L 3  is divided into the lights L 31  and L 32 , and is received by the first photodetector (A). 
     Steps 1.4 and 1.5 are the same as those in Embodiment 1, and will not be described again in detail. 
     Step 2: a position error along X-axis is measured based on the laser interference, which is consistent with Embodiment 1 and will not be described again in detail. 
     Based on the single frequency laser with four channels in the interference length measuring module, the method for simultaneously measuring 3DOF LGEs by a laser in Embodiment 3 is similar to the above methods, with the following differences: 1) a straightness error of the target mirror unit II along Y-axis and/or Z-axis can be calculated by a spot offset on any one of the first photodetector (A), the second photodetector (B), the third photodetector (C), and the fourth photodetector (D); 2) the phases of the light intensities I 1 , I 2 , I 4  and I 5  of the four detectors differ by 90° between any two in turn; by processing I 1 , I 2 , I 4  and I 5 , there is determined N(Δx), a number of light and dark changes of interference fringes caused by φ(Δx), an emitting laser wavelength of the single frequency laser  1  is λ, a displacement of the target mirror unit II along X-axis is Δx=N(Δx)·λ/2, at the same time, a moving direction of the target mirror unit can be judged. 
     Embodiment 4 
       FIG.  6    is a view of a system for simultaneously measuring 3DOF LGEs by a dual frequency laser according to Embodiment 4 of the present invention. Its overall structure is similar to  FIG.  5   , except that the second non-polarizing beam splitter  10  replaces the polarizing beam splitter  2 , and a combined beam L 3  comprises a reflected part of the reference beam L 12  hitting on or passing through the second non-polarizing beam splitter  10  again and a transmitted part of the measurement beam L 11  hitting on or passing through the second non-polarizing beam splitter  10  again. 
     Further, in the combined beam L 3  of Embodiment 4, there is a transmitted part of the reference light L 12  when passings through the second non-polarizing beam splitter  10  again, and a reflected part of the measuring light L 11  hitting on or passing through the second non-polarizing beam splitter  10  again. The interference length measuring module of the single frequency laser is arranged in the emitting direction of the combined beam L 3 . 
     Further, Embodiment 4 adopts a reflection sensitive structure. 
     Further, the interference length measuring module of the single frequency laser of Embodiment 4 can also adopt the interference length measuring module of the single frequency laser with four channels as shown in  FIG.  3   . 
     Embodiment 5 
       FIG.  7    is a view of a system for simultaneously measuring 3DOF LGEs by a dual frequency laser according to Embodiment 5 of the present invention. As shown in  FIG.  7   , according to the other aspect of the present invention, there is provided a system for simultaneously measuring 3DOF LGEs by a dual frequency laser, comprising 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 , and a first quarter-wave plate  4 , a second quarter-wave plate  5 , a first photodetector (A), a third photodetector (C) of a dual frequency interference length measuring module; the dual frequency laser  14  constitutes a laser emitting module; the interference length measuring module includes a first polarizer  6 , a third non-polarizing beam splitter  15 , a second polarizer  16 , and a third photodetector (C). 
     The target mirror unit II includes a pyramid prism  9 , which is consistent with Embodiment 1. 
     In the measuring unit I, 
     the dual frequency laser  14  is used to generate an emergent light L 1 , and the emergent light L 1  is a polarized light superposed by two beams at a spatial position, the two beams has a certain frequency difference, and their polarization directions are perpendicular to each other; 
     the third non-polarizing beam splitter  15  is arranged between the dual frequency laser  14  and the polarizing beam splitter  2 ; the emergent light L 1  is split by the third non-polarizing beam splitter  15 , not only remaining in the original direction, but also having a reflected light denoted as L 2 ; 
     the second polarizer  16  is arranged between the third non-polarizing beam splitter  15  and the third photodetector (C); the polarizing axis direction of the second polarizer  16  can be adjusted; the reflected light L 2  interferes after hitting on or passing through the second polarizer  16 , and an interference spot is received by the third photodetector (C) as a reference signal for heterodyne interference length measurement; 
     the combined beam L 3  interferes after hitting on or passing through the first polarizer  6 , and an interference spot is received by the first photodetector (A) as a measuring signal of heterodyne interference length measurement; according to the reference signal and the measurement signal, a displacement of the target mirror unit along X-axis can be calculated; 
     the third photodetector (C) is used to receive an interference spot of the light L 2  as a standard signal for heterodyne interference length measurement; 
     the functions of 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 , and the first photodetector (A) are the same as those of Embodiment 1, and will not be described again in detail. 
     The present Embodiment 5 provides a method for simultaneously measuring 3DOF LGEs by a dual frequency laser, which comprises the following steps: 
     Step 1: measuring a straightness error along Y-axis and/or Z-axis based on laser collimation principle, which is consistent with Embodiment 1, and will not be described again in detail; 
     Step 2: measuring a position error along X-axis based on laser interference 
     Step 2.1) in the light L 1 , the two polarized lights with a certain frequency difference have the frequencies f 1  and f 2 , respectively, and when the light L 1  is separated by the polarizing beam splitter  2 , the frequency of the measuring light L 11  is f 1 , while the frequency of the reference light L 12  is f 2 ; 
     Step 2.2) a displacement of the target mirror unit of the measuring light L 11  along X-axis is Δx, a frequency variation due to Doppler effect is f(Δx), so that the frequency of the measuring light L 11  is f 1 +f(Δx); 
     Step 2.3) setting a first polarizer  6  in front of the first photodetector (A) and adjusting a direction of a light transmitting axis of the first polarizer  6 , so that a combined light L 3  (including the light L 12  and the light L 11 ) interferes after hitting on or passing through the first polarizer  6 ; the interference spot is received by the first photodetector (A) as a measuring signal of the heterodyne interference length measurement; a beat signal is measured to have a frequency f m =f 1 +f(Δx)−f 2 ; 
     Step 2.4) when the emergent light L 1  hits on or passes through the third non-polarizing beam splitter  15 , another laser beam L 2  is formed in a reflected direction of the third non-polarizing beam splitter  15 ; the light L 2  also contains two polarized lights with a certain frequency difference; the polarizing axis direction of the second polarizer  16  is adjusted, so that the two polarized lights with a certain frequency difference in the light L 2  interfere with each other; the interference spot is received by the third photodetector (C) as a standard signal for the heterodyne interference length measurement, then the standard signal frequency is f s =f 1 −f 2 ; 
     Step 2.5): the frequency of the beat signal measured in Step 2.3, f m =f 1 +f(Δx)−f 2 , subtracts the standard beat signal frequency obtained in Step 2.4, f s =f 1 −f 2 , so as 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 wavelength emitting from a laser is λ, so a displacement of the target mirror unit along X-axis is Δx=N(Δx)·λ/2. 
     Further, this Embodiment 5 can adopt a reflection sensitive structure as shown in  FIG.  8   . 
     Further, in this Embodiment 5, on the basis of adopting the transmission sensitive structure, the first quarter-wave plate  4  and the second quarter-wave plate  5  are omitted; the light L 11  after returning to the polarizing beam splitter  2  is still transmitted, while the light L 2  after returning to the polarizing beam splitter  2  is still reflected. Compared with  FIG.  7   , an emitting direction of the combined beam L 3  is rotated by 90°, and locations of the first polarizer  6  and the first photodetector (A) are changed accordingly, as shown in  FIG.  9   . 
     Further, in this Embodiment 5, on the basis of adopting the reflection sensitive structure, the first quarter-wave plate  4  and the second quarter-wave plate  5  are omitted, the light L 11  after returning to the polarizing beam splitter  2  is still transmitted, while the light L 2  after returning to the polarizing beam splitter  2  is still reflected. Compared with  FIG.  8   , an emitting direction of the combined beam L 3  is rotated by 90°, and locations of the first polarizer  6  and the first photodetector (A) are changed accordingly. 
     Embodiment 6 
       FIG.  10    shows a system for simultaneously measuring 3DOF LGEs by a multi wavelength laser according to Embodiment 6 of the present invention. As shown in  FIG.  10   , according to another aspect of the present invention, a system for simultaneously measuring 3DOF LGEs by a multi wavelength laser is provided to comprise a measuring unit I and a target mirror unit II. 
     The measuring unit I includes a multi wavelength laser light source  17 , a heterodyne frequency generating unit  18 , a second non-polarizing beam splitter  10 , a fixed pyramid prism  3 , a first polarizer  6 , a first photodetector (A), a first bandpass filter  19 , a second bandpass filter  20 , a third bandpass filter  21 , a first phase detector  22 , a second phase detector  23 , and a third phase detector  24 . 
     The multi wavelength laser light source  17  and the heterodyne frequency generating unit  18  constitute a laser emitting module. The first polarizer  6 , the first bandpass filter  19 , the second bandpass filter  20 , the third bandpass filter  21 , the first phase detector  22 , the second phase detector  23 , and the third phase detector  24  constitute an interference length measuring module. 
     The target mirror unit II includes a pyramid prism  9 , which is consistent with Embodiment 1. 
     In the measuring unit I, 
     the multi wavelength laser light source  17  is used to generate the emergent light L 1 , and the emergent light L 1  includes multi wavelength lasers λ 1 , λ 2 , λ 3 , and their frequencies are v 1 , v 2 , v 3 ; 
     the heterodyne frequency generating unit  18  is used to change the frequencies of the emergent light L 1  to v 1 +f 1 , v 2 +f 2 , v 3 +f 3 ; 
     the second non-polarizing beam splitter  10  is used for: 
     1) beam splitting: the emergent light L 1  is divided into a measuring light L 11  and a reference light L 12 , the measuring light L 11  is hitting on or passing through the target mirror unit II and is reflected back by the target mirror unit II, and then returns to the measuring unit I with a 3DOF LGEs signal; while the reference light L 12  only propagates inside the measuring unit; 
     2) beam combining: a transmitted part of the reference light L 12  hitting on or passing through the second non-polarizing beam splitter  10  again, and a reflected part of the measuring light L 11  hitting on or passing through the second non-polarizing beam splitter  10  again, are superposed with each other in a spatial position, to form a combined light L 3 . 
     The first photodetector (A) is used to receive the combined light L 3  to realize: 1) calculating a straightness error of the target mirror unit II along Y-axis and/or Z-axis according to a spot offset of the light L 11  on the first photodetector (A); 2) coordinating with the interference length measuring module to measure a position error of the target mirror unit II along X-axis. 
     A low frequency response spectrum of the first photodetector (A) cannot be used to test a much higher optical frequency (a much higher frequency signal is divided into a few low frequency signals which become to be detected by photodetectors and to mathematically form the much higher frequency signal); and the measured heterodyne interference signal spectrum of the combined light L 3  contains only f 1 , f 2 , f 3 , etc.; the first to third band-pass filters of the interference length measuring module separate the f 1 , f 2 , f 3  of the first photodetector (A), to measure the length measuring phase information φ 1 , φ 2 , φ 3  corresponding to each wavelength by the first to third phase detectors; taking two pairs of beat signals, and calculating a displacement Δx of the target mirror unit along X-axis according to two pairs of wavelength and two pairs of phase difference. 
     The present Embodiment 6 provides a method for simultaneously measuring 3DOF LGEs by a laser, which comprises the following steps: 
     Step 1: measure a straightness error along Y-axis and/or Z-axis based on laser collimation principle 
     A multi wavelength laser light source  17  is used, an emergent light L 1  includes multi wavelength lasers λ 1 , λ 2 , λ 3 , however, when measuring a straightness error along Y-axis and/or Z-axis with the laser collimation principle, only the spot offset on the detector is detected, it is not different from the measurement with a single frequency laser, and it is consistent with Embodiment 1, so will not be described again in detail; 
     Step 2: measure a position error along X-axis based on a multi wavelength laser interference 
     Step 2.1) the light L 1  emitted from the multi wavelength laser light source  17  includes multi wavelength lasers λ 1 , λ 2 , λ 3 , their frequencies are v 1 , v 2 , v 3 ; after hitting on or passing through the heterodyne frequency generating unit  18 , the frequencies of the multi wavelength laser becomes v 1 +f 1 , v 2 +f 2 , v 3 +f 3 ; 
     Step 2.2) the emergent light L 1  is split into a measuring light L 11  and a reference light L 12  by the second non-polarizing beam splitter  10 ; both the measuring light L 11  and the reference light L 12  contain multi wavelength lasers v 2 +f 2 , v 3 +f 3 ; 
     Step 2.3) the measuring light L 11  is emitted from the measuring unit I, and once reaching the target mirror unit II, the measuring light L 11  is reflected back by the target mirror unit II, returning to the half transmitting and half reflecting mirror  10  of the measuring unit I with a straightness error information along X-axis as a signal of a measuring light of the heterodyne interference length measurement; 
     Step 2.4) the reference light L 12  is reflected back 180° toward its original direction by a fixed pyramid prism  3  of the measuring unit I, then a transmitted part after hitting on or passing through the non-polarizing beam splitter  12 , and a reflection part of the light L 11  turning on the non-polarizing beam splitter  12  is combined with each other, to form a combined light L 3 , and a polarizing axis direction of the first polarizer  6  is adjusted, so that the light L 3  interferes on the first photodetector (A) after hitting on or passing through the first polarizer  6 ; 
     Step 2.5) the first photodetector (A) detects components such as f 1 , f 2  and f 3  of the heterodyne interference signal spectrum, and the first to third band-pass filters  18 - 20  separate the components f 1 , f 2  and f 3 , and the first to third phase detectors  21 - 23  measure the length measuring phase information φ 1 , φ 2 , φ 3  corresponding to each wavelength; taking two pairs of beat signals, and calculating a displacement Δx of the target mirror unit II along X-axis according to the wavelengths and the phase difference; 
     There are three other structures in this Embodiment 6: 
     1) adopting a reflection sensitive structure; 
     2) with a transmission sensitive structure, a combined light L 3 ′ is composed of a reflected part of the light L 12  after hitting on or passing through the second non-polarizing beam splitter  10  for the second time and a transmitted part of the light L 11  after hitting on or passing through the second non-polarizing beam splitter  12  again; and the first polarizer  6  and the first photodetector (A) are arranged in an emitting direction of the combined light L 3 ′; 
     3) with a reflection sensitive structure, a combined light L 3 ′ is composed of a reflected part of the light L 12  after hitting on or passing through the second non-polarizing beam splitter  10  for the second time and a transmitted part of the light L 11  after hitting on or passing through the second non-polarizing beam splitter  12  again; 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 and method for simultaneously measuring 3DOF LGEs by a laser according to the embodiments of the present invention can be realized to perform simultaneous and rapid measurement of 3DOF LGEs of space objects moving linearly along any linear axis; and a longtime monitoring 3DOF linear position change of two objects in space. 
     Each embodiment in this specification describes a system and method for rapidly measuring 3DOF LGEs of a space object while the measuring unit I remains stationary and the target mirror unit II and the space object move linearly along a linear axis. After an optical path adjusting is completed, the systems in all embodiments will perform the following actions. 1) keeping the target mirror unit II stationary, and making the measuring unit I and the space object move linearly along a linear axis, it can also realize the simultaneous and rapid measurement of 3DOF LGEs of space objects; 2) by keeping the measuring unit I and the target mirror unit II stationary, and monitoring data measured by the measuring unit for a long time, a long-term monitoring 3DOF linear position drift of two objects in space can be realized. 
     In the case of a multi wavelength measurement, it is each single detector measurement. For single frequency and double frequency length measurement, at least one detector shall be equipped for auxiliary measurement. According to the present invention, the first photodetector (A) cooperates with different interference length measuring modules, and the target mirror and every single detector each has a single optical component, it is the first time in the world to realize simultaneously measuring three linearity errors (i.e., three linearity errors in X, Y and Z axis translation). Compared with prior art multi degree of freedom measurement systems and methods, it has the following beneficial effects: 1) simplifying an optical path structure, reducing complexity of the measurement system and volume of the measuring unit and the target mirror unit, and facilitating practical application; 2) reducing a number of detectors, so as to reduce circuit power consumption, reduce heat dissipation, improve stability of the measurement system, and reduce cost of the measurement system. 
     The invention can simultaneously measure the linear error of 3DOF, and greatly improves a measurement efficiency compared with the prior art single degree of freedom measurement system and method. 
     Those skilled in the art can understand that the drawings are only schematic views of embodiments, and some modules or processes in the drawings might be possibly not necessary at all to implement the present invention. 
     It can be seen from the description of the above embodiments that those skilled in the art can clearly understand that the present invention can be realized by means of software and necessary general hardware platform. Based on this understanding, the technical solution of the present invention can be embodied in software, which can be stored in a storage medium, such as ROM/RAM, magnetic disk, optical disk, etc., including several instructions for causing a computer equipment (which may be a personal computer, a server, or a network device, etc.) to perform the method described in each embodiment or some part of the embodiments of the present invention. 
     The embodiments in this specification are described in a progressive manner. The same and similar parts of each embodiment can be referred to each other. Each embodiment focuses on those differences from other embodiments. In particular, as for the apparatus or system embodiment, it is basically similar to the method embodiment, so their description is relatively simple. For relevant parts, please refer to the corresponding description of the method embodiment. The above described apparatus and system embodiments are only schematic, those units described as separate components may or may not be physically separated, and the components displayed as a unit may or may not be a physical unit, that is, they may be located in the same place or may be distributed on multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the embodiment. Those skilled in the art can understand and implement 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 this. Any change or alternative solution that can be easily derived by those skilled in the art within the spirit of the present invention shall be covered by the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.