The conventional control system for controlling polarization of a light will be explained briefly by making reference to FIG. 2.
In FIG. 2, 1 and 2 are phase compensators, 4 is a beam splitter, 22 is a Wollaston prism, 23 and 24 are light detectors, 25 is a controller, 26 is a 1/4 wavelength plate, 27 is a Wollaston prism, 28 and 29 are light detectors and 30 is a controller.
An incident light 11 is passed through the phase compensators 1 and 2 and then divided into an exit light 12 and a monitor light 13 by means of a beam splitter 4.
The phase compensators 1 and 2 may utilize a birefringence which is changed by an electric field applied to an electro-optical crystal or by a stress applied to a glass.
The monitor light 13 is split into monitor lights 14 and 15.
The monitor light 14 is further divided into mutually perpendicular polarized light components by the Wollaston prism 22 and the difference in power of these components is detected by the light detectors 23 and 24 and the controller 25. The detected power difference controls the phase compensator 1.
The monitor light 15 is passed through the 1/4 wavelength plate 26 and divided into mutually perpendicular polarized components and the difference in power of these components is detected by the light detectors 28 and 29 and the controller 30. The detected power difference controls the phase compensator 2.
The directions of the principal axes of the phase compensators 1 and 2 are mutually inclined by an angle of 45 degrees.
The directions of the principal axes of the Wollaston prisms 22 and 27 are mutually inclined by an angle of 45 degrees and the principal axis of the Wollaston prism 22 is in coincidence with that of the 1/4 wavelength plate 26.
The outputs of the light detectors 23 and 24 are led to the controller 25 which, in turn, controls the phase compensator 1 so as to make the output difference, that is, the difference between the perpendicular components of the monitor light 14, zero.
By so doing, the direction of the polarization principal axis is caused to coincide with the principal axis of the Wollaston prism 27.
The outputs of the light detectors 28 and 29 enter the controller 30 which, in turn, controls the phase compensator 2 in such manner that the difference between the outputs becomes zero.
Thus, the incident light 11 is adjusted to obtain a linearly polarized light.
The polarization of the output light in the conventional technique illustrated in FIG. 2 is restricted to linearly polarized lights because the respective output differences of the perpendicularly polarized light components are so controlled that they become zeros.
In addition, even if the detection system is replaced with other systems, it is not possible to continuously change the polarized condition since the two phase compensators can merely control the phase difference of perpendicularly polarized components of an incident light.
Accordingly, an object of the present invention is to provide a polarization control system capable of continuously controlling the polarized condition of a light.
Measurement of polarization according to the conventional system is shown in FIG. 12 which includes a light source 41, a rotary analyzer 42 and a light detector 43.
A light consisting of any polarized light exiting from the light source 41 goes through the rotary analyzer 42 to become a specific linearly polarized light, which in turn is detected by the light detector 43.
We assume that the output signal of the light detector 43 is I.sub.MAX when the output becomes the maximum with the rotation by the rotary analyzer 42. Also, when the rotary analyzer 42 is rotated by an angle of 90 degrees from the direction where the maximum output I.sub.MAX was obtained the minimum output I.sub.MIN is obtained. Then, the degree of polarization P is expressed by the following. EQU P=(I.sub.MAX -I.sub.min)/(I.sub.MAX +I.sub.MIN) (1)
When the light output from the light detector 43 becomes the maximum, the rotational angle of the rotary analyzer 42 is the directional angle of the principal polarization axis of the incident light.
In FIG. 12, the measurement is effected on the spacial distribution of light power and, accordingly, it is not possible to know the phase difference between the mutually perpendicular electrical field components. However, it is required to measure the phase difference in any event to know the rotational direction of the polarized light.
Further, the system shown in FIG. 12 necessitates a mechanical drive to rotate the rotary analyzer 42 and thus the accuracy of the measurement is greatly affected by the allowance of the mechanism, which should be eliminated for making the system smaller.
Although it is possible for the system shown in FIG. 12 to measure the degree of polarization and the directional angle of the principal polarization axis among the three parameters but is not possible to measure the angular direction of the rotation of the polarization.
Additional object of the invention is to provide a system to know all three parameters, that is, the degree of polarization and the directional angle of the principal polarization axis, as well as the angular direction of the rotation of the polarized light.