Displacement measurement system and method of use

1. A measurement displacement system, comprising:

a sensor head configured to transmit input optical beams and to receive measurement beams;

a transmission grating configured to diffract the input optical beams into sub-beams comprising more than one diffraction order, wherein the transmission grating is adapted move in a direction; and

a reflective element comprising two reflective surfaces and a diffraction grating on another surface, the reflective element configured to diffract the sub-beams from the transmission grating and to return the sub-beams to the transmission grating, wherein the reflective element is substantially stationary relative to the sensor head and the transmission grating selectively recombines the sub-beams to form the measurement beams and returns the measurement beams to the sensor head.

2. A measurement displacement system as claimed in claim 1 , wherein the reflective element is a roof reflector comprising a cross-section of a right isosceles triangle.

3. A measurement displacement system as claimed in claim 1 , further comprising a prism comprising the transmission grating, wherein the transmission grating comprises substantially uniform line spacing.

4. A measurement displacement system as claimed in claim 3 , wherein the prism comprises a cross-section of a right triangle.

5. A measurement displacement system as claimed in claim 1 , wherein the measurement beams comprise a first measurement beam and a second measurement beam.

6. A measurement displacement system as claimed in claim 5 , wherein the first measurement beam comprises only first order diffraction components.

7. A measurement displacement system as claimed in claim 5 , wherein the second measurement beam comprises both zeroth order and first order diffraction components.

8. A measurement displacement system as claimed in claim 5 , further comprising detector arrays configured to measure a phase difference for the first and second measurement beams, wherein an x-displacement is calculated as:

x

=

N

⁢

⁢

1

⁢

p

4

;

and a z-displacement is calculated as:

z

=

1

4

⁢

(

2

⁢

N

⁢

⁢

2

-

N

⁢

⁢

1

)

⁢

p

⁢

⁢

cos

⁡

(

α

)

⁢

cot

⁡

(

α

2

)

wherein p is a pitch of the transmission grating, α is an angle of incidence, N 1 is the number of fringes of the first measurement sub-beam and N 2 is the number of fringes of the second measurement sub-beam.

9. A measurement displacement system as claimed in claim 1 , wherein the measurement beams comprise a first measurement beam and a second measurement beam and the first measurement beam comprises a phase difference only due to a grating phase difference of components of the first measurement beam.

10. A measurement displacement system as claimed in claim 7 , wherein the second measurement beam comprises a phase difference only due to a difference in optical path length of components of the second measurement beam.

11. A method of measuring a displacement, the method comprising:

directing an optical input beam toward a movable object;

at the movable object: diffracting the optical input beam into sub-beams comprising more than one diffraction order, and reflecting the sub-beams toward a stationary reflective element, the reflective element comprising two reflective surfaces and a diffraction grating on another surface;

at the reflective element: diffracting the sub-beams from the transmission grating, and returning the sub-beams to the movable object;

at the movable object selectively recombining the sub-beams to form measurement channels; and

directing the measurement beams toward a sensor head.

12. A method as claimed in claim 11 , wherein the measurement beams comprise a first measurement beam comprising diffraction components and a second measurement beam comprising diffraction components.

13. A method as claimed in claim 12 , wherein the diffraction components of the first measurement beam comprise only first order diffraction components.

14. A method as claimed in claim 12 , wherein the diffraction components of the second measurement beam comprise zeroth and first order diffraction components.

15. A measurement displacement system, comprising:

sensor head configured to transmit input optical beams and to receive a first measurement beam and a second measurement beam, wherein the second measurement beam comprises both zeroth order diffraction components and first order diffraction components;

a transmission grating configured to diffract the input optical beams into sub-beams comprising more than one diffraction order, wherein the transmission grating is adapted move in a direction; and

a reflective element configured to diffract the sub-beams from the transmission grating and to return the sub-beams to the transmission grating, wherein the sensor head is substantially stationary and the transmission grating selectively recombines the sub-beams to form the measurement beams and returns the measurement beams to the sensor head.

16. A measurement displacement system as claimed in claim 15 , wherein the first measurement beam comprises only first order diffraction components.

17. A measurement displacement system as claimed in claim 15 , wherein the reflective element is a roof reflector having a cross-section of a right isosceles triangle.

18. A measurement displacement system as claimed in claim 15 , wherein the reflective element comprises two reflective surfaces and a diffraction grating on another surface, the reflective element.

19. A measurement displacement system as claimed in claim 15 , further comprising a prism comprising the transmission grating, wherein the transmission grating comprises substantially uniform line spacing.