Method of and device for realtime measurement of the state of polarization of a quasi-monochromatic light beam

Claim 2

1) Method of real-time measurement of the state of polarization of a quasi-monochromatic light beam, where a first and a second quasi-monochromatic light radiation (2a, 2b; .102a, 102b; 202a, 202b), having slightly different frequencies are generated, such radiations are sent along two different paths (hereinafter referred to as "reference branch" and "measurement branch") and the beatings among such radiations are originated at the end of said paths, characterized in that the first radiation, sent onto the reference branch, is converted into a fixedly oriented linearly-polarized radiation which can be split into two equal components with horizontal and vertical polarization respectively, said components being separated and recombined with horizontally and vertically polarized components of the second radiation which has been made to follow the measurement branch and which presents the polarization state to be determined, imposed by a transparent body under test (4, 204), the separation and recombination giving rise to a first and a second beam (20a, 21a; 34, 35) comprising radiations at both frequencies, respectively polarized in the same plane; a first and a second electrical signals are obtained from said two beams (20a, 21a; 34, 35), which signals represent the beatings between the two radiations composing each beam and have a relative phase equal to the phase between the horizontal and vertical components of the radiation having the polarization state to be determined and intensity proportional to the amplitude of such components, and information on such phase and on amplitude ratio is obtained from said electrical signals.

Claim 3

2). Method according to claim 1, characterized in that said first and second beam (20a, 21a) are obtained by separating the two polarization components of the radiations coming from both the reference and the measurement branch,combining the horizontal polarization component of one radiation with the vertical component of the other into beams forwarded along different paths, and polarizing the radiations of each said beam into a common plane.

Claim 4

3) Method according to claim 1, characterized in that said first and second beam (34, 35) are obtained by combining the radiations coming from both the reference and the measurement branch into a single beam (32), separating the horizontally polarized components of the two radiations from the vertically polarized components, and combining into the first and second beam (34, 35) the horizontally and the vertically polarized components, respectively.

Claim 5

4) Method according to any of claims 1 to 3, characterized in that it comprises a calibration phase in which the radiations sent into both branches are converted into radiations of equal amplitude and phase, which radiations are linearly polarized with the same fixed orientation such as to allow the splitting into equal components with horizontal and vertical polarization.

Claim 6

5) Method according to claim 4, characterized in that said radiation having the linear polarization with fixed orientation imposed to the radiation passing through the measurement branch is transmitted to the body under test.

Claim 7

6) Method according to any of claims 1 - 5, characterized in that said two quasi-monochromatic radiations (2a, 2b) are generated by a common source and present orthogonal linear polarizations.

Claim 8

7) Method according to any of claims 1 - 5, characterized in that said two quasi-monochromatic radiations (102a, 102b; 202a, 202b) are obtained by the use of an acousto-optic device operating on a same quasi-monochromatic radiation (102; 202).

Claim 9

8) Device for measuring the state of polarization of a light beam, comprising means (1, 3; 101, 103; 201, 203) for generating a first and a second quasi-monochromatic light radiation (2a, 2b; 102a, 102b; 202a, 202b) having slightly different frequencies and for sending such radiations along two different branches of the device (hereinafter referred to as "reference branch" and "measurement branch") and means (8, 9) for detecting the beatings between such radiations at the end of such branches, characterized in that the reference branch comprises means (13, 24) for converting the radiation traversing such a branch into a linearly-polarized radiation with fixed orientation such that said radiation can be split into two equal components, with horizontal and vertical polarization respectively, and ends at means (7, 14, 15; 30, 31) separating and recombining the polarization, said means receiving also the radiation (2a, 102a, 202a) emerging from the measurement branch, which radiation presents the state of polarization to be determined imposed by a transparent body under test (4, 204) and can be split into a horizontal polarization and a vertical polarization component; in that said separating-recombining means (7, 14, 15; 30, 31) is apt to emit a first and a second beam (20a, 2la; 34, 35) comprising radiations at both frequencies, the radiations of each beam being linearly polarized in a same plane; and in that the first and the second beam (20a, 21a; 34, 35) are sent to a first and a second photodetector (8, 9) which are the means for detecting the beatings, originate a first and a second electrical signal of which the relative phase is equal to the relative phase between the horizontal and vertical components of the radiation having the polarization state imposed by the body (4; 204) under test and intensity proportional to the amplitude of such components, and are connected to a device (18) for measuring such phase and the ratio between the amplitudes and/or for visualizing the polarization state.

Claim 10

8) Device for measuring the state of polarization of a light beam, comprising means (1, 3; 101, 103; 201, 203) for generating a first and a second quasi-monochromatic light radiation (2a, 2b; 102a, 102b; 202a, 202b) having slightly different frequencies and for sending such radiations along two different branches of the device (hereinafter referred to as "reference branch" and "measurement branch") and means (8, 9) for detecting the beatings between such radiations at the end of such branches, characterized in that the reference branch comprises means (13, 24) for converting the radiation traversing such a branch into a linearly-polarized radiation with fixed orientation such that said radiation can be split into two equal components, with horizontal and vertical polarization respectively, and ends at means (7, 14, 15; 30, 31) separating and recombining the polarization, said means receiving also the radiation (2a, 102a, 202a) emerging from the measurement branch, which radiation presents the state of polarization to be determined imposed by a transparent body under test (4, 204) and can be split into a horizontal polarization and a vertical polarization component; in that said separating-recombining means (7, 14, 15; 30, 31) is apt to emit a first and a second beam (20a, 2la; 34, 35) comprising radiations at both frequencies, the radiations of each beam being linearly polarized in a same plane; and in that the first and the second beam (20a, 21a; 34, 35) are sent to a first and a second photodetector (8, 9) which are the means for detecting the beatings, originate a first and a second electrical signal of which the relative phase is equal to the relative phase between the horizontal and vertical components of the radiation having the polarization state imposed by the body (4; 204) under test and intensity proportional to the amplitude of such components, and are connected to a device (18) for measuring such phase and the ratio between the amplitudes and/or for visualizing the polarization state.

Claim 11

9) Device according to claim 8, characterized in that the reference branch comprises at least a mirror (12) mounted on a support allowing the rotation about two axes and a translatory movement for moving it nearer to or farther from said first separating device (7) and in that said mirror (12) and said detectors (8, 9) are placed at a focal distance from a first optical system (10; 33) allowing an independent control of the position and of the incidence angle on-the detectors of the two components of the radiation (2b, 102b, 202b) which has gone through the reference branch.

Claim 12

10). Device according to claim 8 or 9, characterized in that said separating-reco n bining means comprise abeam splitter (30) receiving on a first face the radiation (2a, 102a, 202a) from the measurement branch and on a second face the radiation (2b, 102b, 202b) from the reference branch and combining the two radiations into a single beam (32), and a first polarizer (31) separating the horizontally and vertically polarized components of the two radiations and creating the first and the second beam (34, 35) by combining the horizontally and the vertically polarized components, respectively, of the two radiations.

Claim 13

11) Device according to claim 8 or 9, characterized in the said separating-recombining means comprise a single polarizing beam splitter (7) which combines the horizontally polarized component of one of the two radiations and the vertically polarized component of the other radiation, and sends the beams (20, 21) resulting from the combination to a second and a third polarizer (14, 15), respectively, which linearly polarize the components of the two resulting beams in two parallel planes.

Claim 14

12) Device according to claims 9 and 11, characterized in that said first optical system (10) is inserted in the reference branch between the mirror (12) and the separating-recombining means (7; 14, 15) and said detectors (8, 9) are placed at focal distance from a second optical system (6), analogous to the first and inserted into the measurement branch.

Claim 15

13) Device according to any one of claims 8 to 12, characterized in that the measurement branch comprises, at least or only during a calibration phase of the device, a fourth polarizer (5) which, during such phase, converts the radiation (2a, 102a, 202a) which goes through the measurement branch into a radiation having a linear polarization identical to that of the radiation (2b, 102b, 202b) having gone through the reference branch.

Claim 16

14) Device according to any one of claims 8 to 13, characterized in that the means (1, 3) which generates and separates the radiations at the two frequencies comprises a source (1) of a light beam (2) comprising two radiations linearly polarized in perpendicular planes, and a polarization splitter (3).

Claim 17

15) Device according to any one of claims from 8 to 13, characterized in that the means (101, 103; 201, 203) generating and separating the two radiations comprises a source (101, 201) of a quasi-monochromatic light beam (102, 202) and an acousto-optic frequency shifter (103, 203) which obtains from such a beam (102, 202) the radiations at the two frequencies (102a, 102b; 202a, 202b) and sends them along said two branches.

Claim 18

16) Device according to any one of claims 8 to 15, characterized in that the body under test (4) is inserted into the measurement branch.

Claim 19

17) Device according to claim 16, characterized in that, in the case in which the body under test presents, in the direction of the light radiation it is traversed by, dimensions to be compared to the source coherence length, the reference branch comprises a standard sample body with controlled characteristics, such that the difference among the optical paths of the radiations going through the two branches is negligible with respect to said coherence length.

Claim 20

18) Device according to any one of claims 8 to 17, characterized in that the means (13) giving a fixed linear polarization to the radiation (2b, 102b) going through the reference branch consists of a Soleil-Babinet compensator.

Claim 21

19) Device according to claim 15, characterized in that in the case in which the body under test (204) presents in the direction of the radiation passing through it, dimensions comparable to the coherence length of the source (201), such a body (204) is inserted between the source (201) and the acousto-optic frequency shifter (203), and the means (24) giving a fixed linear polarization to the radiation (202b) passing in said branch comprises a fifth polarizer (24) and a quarter-wave plate (25).