CS231247B1 - Optical system of detection unit in laser interferometer - Google Patents

Abstract

The system consists of an approximately non-polarizing splitter and polarizing splitters located in each of the divided light beams. The essence of the system is that the polarizing splitters are oriented in space relative to each other* at an angle 03 = are tg where Rz and R„ are the reflectivities of the approximately non-polarizing splitter in a plane perpendicular and parallel to the plane of incidence of the incident light beam. The optical system is intended primarily for the detection of interference signals in a laser interferometer.

Description

(54) Optical system of the detection unit in a laser interferometer

The system consists of an approximately non-polarizing splitter and polarizing splitters located in each of the split light beams.

The essence of the system is that the polarizing dividers are oriented in space relative to each other* at an angle

0 ₃ = are tg where Rz and R„ are the reflectivities of an approximately non-polarizing splitter in a plane perpendicular and parallel to the plane of incidence of the incident light beam.

The optical system is intended primarily for the detection of interference signals in a laser interferometer.

23,247

The invention relates to an optical system of a detection unit in a laser interferometer intended for detecting interference signals.

A laser interferometer consists in principle of a radiation source, which is a single-frequency laser, the interferometer itself and a detection unit, in which interference signals are obtained from the output beam of light from the interferometer itself, containing information about the direction of movement of the wavefronts of the two interfering beams of light with respect to each other. To obtain two signals in quadrature, the interference of two circularly polarized beams of light of opposite orientation is usually used, from which suitable frequency directions of the beams of light that fall on the detectors are selected.

The current arrangement of detection units usually uses dividing plates with so-called approximately non-polarizing layers, which divide the beam bundles emerging from the interferometer and the selection of suitable frequency directions in each divided beam is carried out using a polarization filter, behind which the detectors are located. The disadvantage of this arrangement is that polarization filters have significant losses and weaken the beam bundles incident on the detectors. The use of polarization filters, i.e. polarizers, has a further disadvantage in that they only suppress and do not allow the use of orthogonal components of circularly polarized beams emerging from the interferometer.

For both of these reasons, the efficiency of the separation in the existing arrangement of the detection units is low. The existing detection units are also not universal and do not allow the use of interferometers operating with orthogonal linearly polarized light beams.

A significant improvement in the arrangement of the detection unit ee is achieved by using an ideally non-polarizing splitter (R„ - R ₂ , T, = T ₄ , = 0) and polarizing splitters, which are rotated relative to each other by an angle of 49° in space. An ideally non-polarizing splitter can be realized only with great difficulty. It is much easier to produce an approximately non-polarizing splitter. The polarizing splitters must then be rotated relative to each other by an angle other than 45°, and both polarizing splitters must be appropriately oriented relative to the approximately non-polarizing splitter. The plane of incidence of the light beam of the approximately non-polarizing splitter and one of the polarizing splitters must be identical.

The existing disadvantages are therefore eliminated by the optical system of the detection unit in the laser interferometer, consisting of an approximately non-polarizing splitter and polarizing splitters, located in each of the divided light beams, the essence of the invention of which is that the polarizing splitters are oriented in space relative to each other at an angle θ= are tg l/w ⁴ · , y ^Λ n where Rj and S, are the reflectances of the approximately non-polarizing splitter in the plane perpendicular and parallel to the plane of incidence of the incident light beam.

The advantage of the optical system of the detection unit according to the invention is that it does not place high demands on the parameters of the optical elements, the overall arrangement is simple and provides full contrast of interference signals.

The invention is further explained in the attached drawing, which explains the principle of the proposed function of the detection unit.

The combined fi fi beams, reference and measurement, emerging from the interferometer itself are either orthogonally linearly polarized or circularly polarized in opposite orientation.

In the first case of linearly polarized beams, a quarter-wave delay plate £ is required at the input of the detection unit, which creates circularly polarized light from the reference or measuring beam with the opposite orientation to the reference or measuring beam.

We express the reference and measurement beams, respectively, which are orthogonally linearly polarized after exiting the interferometer itself, by the Jones vector:

ε

where Ε) is the electric vector of the reference beam a,

E ₂ * K,e ^i< ' lE-j is the electric vector of the measuring beam £.

For the layers of an approximately non-polarizing divider 2, we assume that:

R„ = Tj ; Rj = T „ ; - s„ = ( h - ϊ„ - 180°) = 0, where R„ , Rj is the reflectivity of the approximately non-polarizing divider 2 for the radiation component parallel or perpendicular to the plane of incidence, respectively,

T„ , Tj is the transmittance of the approximately non-polarizing divider 3 for the radiation component parallel or perpendicular to the plane of incidence, and _B , J- _R , is the phase shift for reflection or transmission.

This condition ensures that the light transmitted and reflected by the approximately non-polarizing splitter will have the same intensity, with the azimuth of this elliptically polarized light being 0° or 90°.

For the first beam a) ti of light passing through an approximately non-polarizing splitter 2, which is further reflected by a polarizing splitter, the following applies:

e ¹ !“ ,5T ύ e ¹ 2 • AT

Λ ² . "I" X 2 _i 1 _ -Λ-

E, £ 1 - K ₁ (sin v, + cos é, )J 0 where the Jones matrices for: polarizing divider J:

H OH Lo oJ approximately non-polarizing divider per pass:

quarter-wave delay plate 2:

^ei 2 _r 0 0 1|T, e ^_i T

G P 2 Li iJ

Then the light intensity at the detector £ is:

I ₂ = A ² [i + K ² - 2 K ² sin «>,] where A is the amplitude of the electric component of the field for K, = 1:

i ₂ = A ² [i - sin <p,]

For the second beam g ₂ £ ₂ of light reflected by an approximately non-polarizing splitter which passes through a polarizing splitter, the following holds:

~ ^C 2 ^C 2 ^S Í -fRx e ^i_ 7 0 - β '1 Γ -E,- -c ₂ s ₂ s ² _ r 0 <R, e ^_1 2 2 .and 1. - ^E 2-

C ² βχ e ¹ “ (Ε, + iE2) + C ₂ S ₂ β„ e ^_i T <E2 + i E, .C?S2 Οχ e ¹ ^ (E. + iE2) + S ² O,, e ^_i ^ ^(E 2 ^{+ iE} 1 > _ where: C ₂ = cos &y S ₂ = sin Sj are the cos and sin of the azimuthal angle of the plane of transmission of the polarizing divider £ and the Jones matrix for: polarizing divider 1 ^r c ^{2 C} 2 ^c 2s ₂ at an angle ©y .C ₂ s ₂ s ₂ and approximately non-polarizing divider £ on reflection:

Assuming _8r -> 0, K, = 0 and if the rotation of the polarizing divider holds

0-, = are tan

And further we get for the intensity on the detector £,:

(Rn + r ₂ ) . R _x Cg ₂ .

+ cos <p,) where A is the amplitude of the electric component of the field.

Both signals I, and I ₂ are interfering light beams, i.e. in quadrature and, assuming the same intensities of both (Κ, = 1), have full contrast.

The figure shows a practical example of the implementation. We assume a rectangular coordinate system xvz. where the axis £ is the axis of propagation of the light beam and the planes of oscillation of the reference and measuring beams fall into the axes 2 and y, respectively. In the path of the unified beam g £ of light z' of the interferometer itself, a quarter-wave delay plate J is inserted and behind it an approximately non-polarizing splitter £. The transmitted beam as the first divided beam g, i, of light falls on the first polarizing splitter 2 and its reflected part, i.e. the third beam gj £3, arrives at the first detector £. The second divided beam g ₂ of light reflected from the approximately non-polarizing splitter 2, falls on the second polarizing splitter £, which is rotated around the beam axis by an angle with respect to the axis 4 and its transmitted part, i.e. the fourth beam g ₄ arrives at the second detector 6.

The function of the detection unit according to the figure is as follows: The unified beam g & z of the interferometer itself contains the reference beam g and the measurement beam £, which are orthogonally linearly polarized and have a common propagation axis. Assuming a rectangular coordinate system xvz. the propagation axis of the unified beam g £ of light is the g axis. The electric field vectors of the reference and measurement beams g £ of light fall into the 2> and y axes, respectively. The quarter-wave retardation plate 1 creates at an azimuth θ= 45° from the reference, respectively. measurement beam with an azimuth of 0® and 90°, respectively, circularly polarized light with the opposite orientation for the reference, respectively. measurement beam g. By passing through and reflecting from the approximately non-polarizing splitter £, elliptically polarized light of the opposite orientation is created, both for the first beam g, , and for the second beam.

23.247 len beam fl ₂ ÍÍ2 ^light ' compliance with the conditions for an approximately non-polarizing splitter j*:

R„=T ₁₍ R _x = T „ , « _x - «„ = ( - J·, - ,80°) = 0 the light transmitted and reflected by the non-polarizing splitter 2 will have approximately the same intensity, while the azimuth of this elliptically polarized light created will be 0° or 90° according to the splitter parameters. The first divided beam a, fcj of light is divided longer by the first polarizing splitter J and its reflected part, i.e. the third beam a ₃ fc _3, will create a basic electrical signal with zero phase by impacting the first detector £. The electrical signal in quadrature is obtained from the second detector í, on which the part forming the fourth beam £4 Í4 of the second divided beam to £ ₂ &2 ^{of light} transmitted by the second polarizing splitter 1, rotated around its axis and set to an angle

to the first polarizing divider J. In this case, full contrast is achieved for both signals.

The optical system is intended primarily for the detection of interference sigpials in a laser interferometer.

Claims (1)

SUBJECT OF THE INVENTION Optical array of a detection unit in a laser interferometer, consisting of approximately non-polarizing beams and polarizing beams, located in each of the divided beams, characterized in that the polarizing beams (3, 4) are oriented in space relative to each other at an angle. and R 'are the reflectance of an approximately non-polarizing divider in a plane perpendicular and parallel to the plane of incidence of the incident light beam.