CS215697B1 - Phase Shifting Method and Optical Compensator for Phase Shifting - Google Patents

Abstract

Method for creating a phase change and an optical compensator for creating a phase shift. The invention relates to a method for creating a phase change and a compensator for creating a phase shift between components of polarized light that are orthogonal to each other. The method provides in particular for obtaining a phase shift that is the same throughout the entire cross-section of the light beam and for very fine adjustment of the phase shift with high accuracy. The essence of the invention is that the light beam is acted upon by two identical anisotropic optical media placed one after the other, the axes of anisotropy of which are rotated symmetrically with respect to the components of the input light beam. The optical compensator for implementing this method consists of two linear delay plates provided with a mechanism for their mutual counter-rotation. The method and the device are illustrated in the accompanying drawing.

Description

The invention relates to a method of creating a phase change between components of polarized light that are orthogonal to each other and to the use of this method for an optical phase shift compensator.

Existing methods of obtaining phase shift and optical compensators are usually specially designed for this purpose and use the mutual displacement of two combined birefringent elements, or possibly tilting the entire compensator, composed of birefringent elements.

A disadvantage of some existing types of compensators is that the phase shift they cause is not the same across the beam cross-section, but varies somewhat from place to place, depending on the beam diameter. The phase shift caused by the compensator is also usually strongly dependent on the angle of incidence of the measuring beam on the compensator and on the rotation of the compensator relative to the planes of oscillation of the incident light,

This disadvantage is eliminated by the method of creating a phase shift between the orthogonally linearly polarized components of the electric vector E , E of the incoming light beam according to the invention, the essence of which is that the light beam is acted upon by two identical anisotropic optical media placed one after the other, the axes of anisotropy of which are rotated symmetrically with respect to the components of the incoming light beam.

The optical compensator for implementing this method consists of two linear retardation plates, provided with a mechanism for their precise counter-rotation.

The advantage of the method is that the obtained phase shift is the same in the entire beam cross-section and is not critically dependent on the angle of incidence of light on the polarizing element, furthermore, the compensator, assembled according to this method, uses two conventional linear retardation plates and does not require special combined birefringent elements for its implementation. Another advantage is that the compensator is very fine, i.e. the phase shift can be reliably adjusted with high accuracy by rotating the azimuth of the linear retardation plates relative to each other. Mechanical rotation of both linear retardation plates relative to each other represents a simply feasible movement, which provides the prerequisites for miniaturization of the entire device.

The invention will be further explained by the drawing, which shows the spatial arrangement of linear retardation plates and the phase shift between orthogonally linearly polarized light components in front of and behind these plates. The input light beam, which is formed by orthogonally linearly polarized components of the electric vector E , E , propagates in the direction of the z -,-ž -JL axis. " and first impinges on the first linear retardation plate RP 1. whose fast axis is rotated by an angle β with respect to the x-axis, and then passes through the second identical linear retardation plate RP 2 , which has a fast axis rotated by an angle - 6 with respect to the x-axis. In the output beam of light, the orthogonally linearly polarized components of the electric vector Ε , E are phase-shifted by an angle 2t[ with respect to each other. Both linear retardation plates RP 1 and RP 2 are counter-rotating with respect to each other. Their control mechanism can be implemented by any known structural arrangement. For the sake of clarity of the drawing, the control mechanism is not indicated in this drawing.

The compensator works as follows:

Let us assume a rectangular coordinate system x, χ, z, where the z axis is the beam propagation axis and

The Jones vector of the incoming light is given by the form

E^ = Ε _χ x... component of the electric field vector E _? ^{s E} v Υ···° ^ν ύ component of the electric field vector For the output light behind the compensator, the following applies:

£-

Γ ιδ C* . ^{+ with} ? e^ C-jS-^i son 2 * +s| e 2+ Sge C ₂ S ₂ 2i ď' sin^ * ^E 1 .s With C ₁ S ₁ 2i son^ dj.- ¹ '. ?1 C ₂ S ₂ 2i son* ^C 2* ^? 2 +S*e * ^E 2 - • •

A E^ + B Eg

C Ε _χ + D Eg

Where C _? = cos ©j, = sin ©.,, C _? = what © _? , S _? = sin © _? ,

1’ 2

2' 2 where: & = phase delay of linear delay plates, © _lt © ₂ ...... azimuth of the fast axis of the linear delay plates.

So:

-Í7

A = (C*e * + S*e *) (C^e * + S*e 2) * CjS-^i sin » C ₂ Sg2i sin

B = (C^e ^? _{+ s} 2 _e f )«CgSg2i sin ⁵ + (Cfje ⁵ + S^e · 0^21 sin ²

-J J / -ií

C = (C?e 1 + S?e 2) . c_s„2i sin + (C^e + S?e *)· ^J l ^c ' 1 ' 2 2 .«r .<r _y l® ^{T ů} l ^e ' ^2®

S*e )· CjS^i sin <r i

D = (C?e + S?e ·(C^e + S^e % + 0^3^21 sin ^c . C _o S,2i sin ^2 2

If we require that only a phase shift occurs between the x- and y-components of the output electric field vector and not a change in the form of polarization of the light, then the following must hold:

B = C = 0 A = to ¹ ^

D - k ě ¹ ^ , where k ... constant

2^.. phase shift.

By calculation we get:

Cg = C^, Sg ⁼ -S^ , so the azimuth ©^ of the first delay plate and the azimuth ©g of the second delay plate must be symmetrical with respect to the components of the input vector.

The following applies to the phase delay of the compensator:

2.Γ = are sin Γ sinJ(2: coé^ejjř.; l/]

For example, using two delay plates with £ = 50°, it is possible to change the phase delay continuously in the range from zero to 100° while simultaneously rotating the azimuth 0^ from 45° to zero and from -45° to zero. In the vicinity of the maximum phase shifts, the compensator is very gentle, which is its great advantage, e.g. in the vicinity of the phase delay 90°, the sensitivity for this case is 1.5° of phase shift per two degrees in the difference in rotation between the azimuths and - Θ^. According to the desired maximum range of phase shift adjustment, we select the phase delay of the plates.

In practical implementation, polarizing elements are not exactly identical, or they may exhibit, in addition to different phase velocities for linearly polarized light, residual optical rotation. For these reasons, it is advisable to leave the possibility of independent adjustment of both counter-rotating elements, or the first element is made of left-handed quartz crystal and the second element of right-handed quartz crystal.

Claims (2)

SUBJECT OF THE INVENTION A method of generating a phase shift 2 ^ between orthogonally linearly polarized components of an electric light input beam vector Ε _χ , E _y , characterized in that the light is acted on by two superimposed identical anisotropic optical environments whose anisotropy axes are rotated symmetrically to the components input light beam. Optical compensator for carrying out the method according to claim 1, characterized in that it comprises two linear delay plates (RP 1 and RP 2) provided with a mechanism for their counter-rotating relative to each other.