DE3006580A1 - Ring interferometer reflection and polarisation rotation suppression - using short source coherence time and input-output polarisers - Google Patents

Abstract

A ring interferometer using the Sagnac effect for absolute measurement of rotations or rotation speeds has simple provision for the suppression of the unwanted effects of reflections and polarisation rotation. The ring interferometer light source coherence time is selected as being smaller than the transition time of the light around the light path (L). In particular, the coherence time of the light source may be selected to be smaller than the difference in transition times of both natural wavelengths of the light path. The light from the source (Q) passes through a polariser (POL1) before being coupled into the light path. It is then passed through the same or another polariser (POL2) on being coupled out of the light path after its transition.

Description

Ring interferometer

The invention relates to a ring interferometer according to the preamble of claim 1.

Ring interferometer and its application for the absolute determination of spatial Rotations using the Sagnac effect are known - see e.g. B. in the Reference: "Reviews of Modern Physics" 39, no. 2 (1967), pages 475-493, Sagnac effect by E.J. Post.

A light path is used to measure the rotational speed, which encloses an area once or several times. The guidance of light in the light path takes place through mirrors and / or light guides. The implementation is particularly advantageous a light path with the help of optical fibers, as this results in a large number of Circumnavigations of the light are made possible.

Ring interferometers are known from the literature - see e.g. B. V. Vali, R.W. Shorthill, M.F. Berg: "Fresnel-Fizeau effect in a rotating optical fiber ring interferometer, Applied Optics 16, No. 10 (1977), pages 2605-2607 known designs of ring interferometers occur and disturbances caused by them Falsification of the measurement results mainly due to reflections in the light path (e.g. B. due to Rayleigh scattering) and due to polarization rotations in the Fiber on. The disturbing influence of the reflections is described in the publication: "Applied Optics "18, No. 6 (1979), pages 915-931," Sensitivity analysis of the Sagnac effect optical-fiber ring interferometer "by S.-C. Lin and T.G. Giallorenzi. The harmful influence of the polarization rotations is described in the publication: Bptics Letters 4 (1979), pages 152-154, "Fiber-ring interferometer: polarization analysis" described by R. Ulrich and M. Joluison.

In this publication measures to reduce the by Disturbances caused by polarization rotations by switching on polarization filters described. The complete elimination of those caused by polarization rotations However, interference makes the use of a polarization control device necessary. Polarization control devices are both in the document: "Applied Physics Letters "35, No. 11 (1979), pages 840-842" Polarization stabilization on single-mode fiber "described by R. Ulrich as well as in German patent application P 29 34 794.2 ("Procedure for measuring absolute rotations and arrangement for implementation of the procedure ") already proposed.

The object of the invention is to provide a ring interferometer of the type described Specify type in which with simple Means the disturbing influences can be suppressed by reflection and polarization rotations.

The invention is described in claim 1. The subclaims include advantageous developments and embodiments of the invention.

The invention is explained in more detail below.

FIG. 1 shows the arrangement of a ring interferometer, which in known Way consists of a light source (Q), a beam splitting arrangement T, one connecting the light source Q with the arrangement T dividing the beam Light path a, a light path L, which encloses an area F one or more times and which opens into the beam splitting arrangement T at both ends, as well as a photodetector D and a light path b, which the beam splitting arrangement g connects to the photodetector D, the light emitted by Q via a and T is coupled into both ends of L, L traverses in both directions and afterwards it is re-frozen via T and thereby brought to interference, and then is forwarded via b to D and detected there. The beam splitting arrangement can be a simple beam splitter as well as an arrangement consisting from two radiators.

Likewise, U can use an arrangement with single-sided bane modulators, as in the German patent application P 29 34 794.2 proposed to be.

The interference from reflections occurs because the in L and T reflected portions of the light in g also interfere and lead to incorrect measurements lead in D.

To reduce the interference caused by reflection according to the invention proposed a light source Q with low temporal coherence of the emitted To use light. If Rc is the coherence time of the light source Q and R is the transit time of light through the light path L, then the condition z, «rL (a) has to be fulfilled. As a result, there are only reflection and scattered light components, their transit time difference between Q and D is less than #o, capable of interference. Are the scattering centers over L. evenly distributed, so is the portion of the scattered light that comes from one end comes from L, only with a fraction of the size tc / bL of that from the other end of the fiber coming scattered light capable of interference.

The portion of scattered light capable of interference and the one caused by it Measurement error is therefore reduced by # o / tL.

Likewise, what is reflected in T and passed on directly from a to b Light cannot interfere with the light conducted via L and therefore cannot Cause interference signal.

Real single-mode optical fibers show a manufacturing-related deviation from the cylindrical symmetry. This leads to the fact that in the nonomode optical waveguide the two eigenwaves with mutually orthogonal polarization different Have propagation velocities.

The difference in transit time of the two eigenwaves is given below with aZL. Accordingly, the light, which is in the first eigenwave 1 propagates from L, through L a transit time tLl and the light that is in the Eigenwave 2 propagates a transit time through L ZL2 = # L1 + ## L (2), where ## L is small against zL1 tL2 is (typically is ß tL / # L # 10-6 ..... 10).

A single mode optical fiber with this property is said to be birefringent designated. Since in general both natural waves in the light path L in both directions are excited, is an interference of partial beams in T, the L in one Direction in eigenwave 1 traversed, with partial beams, the L in the opposite Direction of rotation in Eigenwelle 2 possible. Such interference results to incorrect measurements. To avoid such incorrect measurements, a Further development of the invention a light source with a coherence time Zc «## L L (3) used. As a result, only between light.

rays that move in both directions in the same eigenwave spread, interference is possible. Incorrect measurements due to interference from Light rays that have spread in different natural waves, are therefore excluded. Because of A TL or tL2, with inequality (3) is also satisfies inequality (2). The activation of polarization filters in the Beam path for suppressing a natural wave can be dispensed with, albeit this measure can also lead to a higher accuracy.

Switching on polarization filters makes sense if #o is not small enough that complete depolarization is achieved at the end of L c will. Incorrect measurements are then caused by polarization filters in the light path between Q and w or inserted between T and D are avoided. Such arrangements are already in the German patent applications P 29 06 870, P 29 34 794.2, P 29 41 618.0 has been proposed.

In FIG. 1, two polarizers POL1 and POL2 are indicated by dashed lines. The depolarization caused by the small Tc is also useful in this case, since Disturbances caused by fluctuations in the polarization rotation in L can be reduced considerably.

The condition formulated by inequality (3) has the consequence that the light is completely depolarized after passing through the light path L. That means, that there is no correlation between the light carried in both eigenwaves of L and no interference between the light beams guided in different eigenwaves is possible. The mechanism of depolarization of light during the passage by L is illustrated in a simple manner with the help of the in FIG. 2 shown Põincare ball. The representation of polarization states with the help of the Poincare sphere is z. B.

in the document: "Optics Letters" 1, no. 3 (1977), pp. 109 - 111, "Representation of codirectional coupled waves" described by R. Ulrich.

In FIG. 2 are the following polarization states as points on the spherical surface highlighted: G1: horizontally polarized wave, G2: vertically polarized wave, H1, H2: +450 polarized waves, F1, F2: oppositely circularly polarized Waves.

Is the single mode optical fiber used as the light path L z. -B. linear birefringent with the two horizontally and vertically polarized natural waves, so during the propagation of the light through L there is a rotation of the on the Surface of the PoincarB sphere of polarization state around the x-axis the Poincare sphere.

A state P of the Poincaré sphere is shown in FIG. 2 rotated by an angle # around the x-axis into a state P 'of the Poincare sphere. If the transit time difference between the two natural waves is A rL, then = 2 # f0 A ZL (4), where f0 is the mean frequency of the light emitted by the light source Q. If the light source Q has a finite coherence time tc, this corresponds to the finite spectral width Af = 1 (5), o Z, c where equation (5) applies except for a definition-dependent factor of the order of magnitude. Since the rotation angle # depends on the light frequency, the polarization rotation varies in an angular range A + given by Inserting equation (5) into equation (6), then with inequality (3) This means that for Zc # OZL the rotation of the polarization state varies within the spectral range #fo by an amount larger than 2 #. If the polarization state of the light coupled into L lies on the great circle lying in the yz plane of the Poincaré sphere, the light is completely depolarized. In general, the following applies: If the polarization state in the Poincaré sphere is rotated around any axis, the variation in the angle of rotation within the spectral range of light is large compared to 2 # and the original polarization state lies on a great circle of the Poincaré sphere, which is in the plane normal to the axis of rotation complete depolarization occurs.

In an advantageous embodiment of the invention, the light path L uses a single-mode birefringent optical fiber and that derived from Q. fully or partially polarized light coupled into L in such a way that the polarization state of the light coupled into L is on a great circle the Poincare sphere is located, which lies in a surface normal to the axis of rotation, which is formed by the connecting line between the two eigenstates of L (i.e. the polarization states of the eigenwaves of L).

In a special embodiment, the light path L is linear birefringent single mode optical fiber with the two horizontally and vertically polarized Eigenwaves are used and the light coupled into L is circularly polarized or below +450 or below -4-50 linearly polarized or with less than + 45 ° inclined Main axis elliptically polarized.

To implement a linear birefringent fiber, it is advantageous a single mode optical fiber with an elliptical core is used. Such optical fibers are known from the literature and z. B. in the publication: "Electronics Letters" 15, No. 13 (1979), pages 380-382 "Preservation of polarization in optical-fiber waveguides with elliptical cores "by R.B. Dyott, J.R.

Cozeus and D.G. Morris described. The one described in this work Single mode optical fiber has a transit time difference of AtL = 5.10 ZLl. Does L have a Length of about 200 m, then tL1 is 10-6s and AZ L = 5x? O 10 It follows that the Spectral width Afo of the light source Q in this example can be large towards 2 GHz got to.

There are still others for the realization of linear birefringent fibers Proposals known, e.g. B. from the publication: "Appl. Phys. Bett." 33, no. 8 (1978), Pages 699-701 "Linear polarization in birefringent singlemode fibers" by R.H. Stolen, V. Ramasway, P. Kaiser and W. Pleibel and from the German patent applications P 29 30 781.1, P 29 30 791.3, P 29 30 704.8 and P 29 34 794.2.

In a further embodiment it is proposed as a light path L to use a circular birefringent single mode optical fiber and in the Light path L to couple completely or partially linearly polarized light.

The realization of a circular birefringent single mode optical fiber takes place z. B. in a known manner by twisting a single mode optical fiber.

In a further development of the invention it is proposed that the light path L to train in such a way that it consists of two or more each birefringent Sections consists, each of these sections having the polarization around a different one The axis of the Poincaré-Eugel rotates. In the individual sections of the light path L becomes the polarization state rotated around different axes of the Poincaré sphere.

For a certain light frequency the polarization state P is after Passing through L to the state P 'rotated, with the angular coordinates 20 and 2 wien 2§ 'and 24)' are transferred, see FIG. 3. At a finite spectral Width are both 'and 0' frequency dependent. The birefringence of each The consequence of fiber sections is that, for a given Afo, it is independent of the polarization state of the light coupled into L, the light at the output of L is completely depolarized is.

A necessary condition for this is As a function of fO with variation of fO on the Poincaré sphere, P 'is intended to describe a curve which covers the Poincaré sphere as symmetrically as possible to the origin of the coordinates. In addition to condition (8a), (8b) it is therefore ordered that P 'does not describe a closed curve on the Poincaré sphere. This is e.g. B. to achieve that the sizes of can be chosen very differently. The condition of the overlap of the Poincaré sphere symmetrically to the origin can be passed through mathematically exercise, where 1 (f0) is the power spectral density of Q and P (fO)) is the vector directed from the origin of the coordinates to the point P '(fO) of the Poincaré sphere. The requirement of the zero-point symmetrical overlap is weaker than that of the uniform overlap.

FIG. 4 shows the schematic representation of an arrangement of a ring interferometer with different sections of L. The light path L consists of two sections L1 and L2, where one of the two sections consists of a linearly birefringent single-mode optical fiber and the other of the two sections consists of a circularly birefringent fiber.

The different values of are achieved by different lengths of L1 and L2. For practical implementation z. B. assumed a linear birefringent single-mode optical fiber and this twisted in one of the two sections, so that the optical fiber is circularly birefringent in this section.

To explain the arrangement according to FIG. 4 we assume that the section L1 is significantly shorter than L2, that L1 is circularly birefringent and is linearly birefringent. In section L1, the polarization state is rotated around the z-axis of the Poincare sphere, where {'depends on o. In section L2 there is a rotation about an axis in the xy plane of the Poincard sphere, where 'depends on fO. Since L2 is assumed to be much longer than L1, the following applies The polarization state P of the light coupled into L1 at end 1 of L is converted into state P at end 2 of L as a function of f0 (FIG. 4).

FIG. 5 shows the curve that P 'as a function of f0 on the Poincaré sphere describes. By rotating around z in L1, P first becomes states on the curve C1 transferred. Each state on C1 is then rotated around the axis of rotation f rotated from L2. Because of inequality (10) correspond to one revolution around z in L1 many revolutions around f in L2 so that the curve C2 results.

FIG. 6 shows another arrangement with different sections from L.

The light path L consists of a number N of sections L1. LN, each of the sections being birefringent and rotations in the individual sections take place around different axes of the Poincaré sphere. It is advantageous to if there is a rotation in two successive sections LK, LK + 1 with K = 1 ... N-1 takes place around mutually orthogonal axes. The light path L is realized according to the invention z. B. by a linear birefringent fiber, which at certain intervals is rotated by 450 each (twisted parallel to the axes). The individual sections rotate around polarization axes in the x-y plane of the Poincare sphere, the axis parallel Rotation of the fiber by 450 at the border of two sections, the polarization axis in the Poincare sphere rotated by 900 in the x-y plane of the PoincarO sphere. As another Embodiment of the arrangement according to FIG. 6 it is proposed to light paths L in successive To form sections alternately linear and circular birefringent. Through this a rotation about a polarization axis takes place in the linear birefringent sections in the x-y plane of the Poincare sphere and in the circular birefringent sections a rotation around the z-axis of the Poincaré-Eugel. The circular birefringent sections are z. B. realized in a known manner by twisting the single-mode optical fiber, so that L consists of a single single mode optical fiber, which in every other Section is twisted.

In an advantageous development of the invention, it is proposed that the Form light path L so that L is birefringent over its entire length, the The polarization axis of rotation of the Poincaré sphere is continuous over the entire length of L. varies, the variation of the Polarization axis of rotation a regular or a random function of fiber length. The production of such a Light path L takes place z. B. by systematic or random twisting and / or Twisting a single mode optical fiber over its entire length.

Because of the required short coherence time tc are used as the light source Q advantageous semiconductor disconnection lasers or temporally incoherent light sources to use. If semiconductor lasers are used as the source Q, it is advantageous to meet the necessary incoherence for each individual laser oscillation, with this can also be achieved with the help of a modulation.

In the case of temporally incoherent light sources, only small ones emit light A satisfactory coupling efficiency in the light path L can be achieved.

The use of so-called super-luminescent is advantageous here Edge emitter light-emitting diodes. Such a light emitting diode is z. B. in the Publication: "NTG Bach reports" Volume 59 (1977), pages 148-150, "Superluminescent Diode as Light Source in Optical Fiber Systems "by M.-C. Amann and W. Harth.

The line width of 10 nm at an emission wavelength of 800 nm correspond to the values f0 = 3.75 x 1614, dz Afo = 4.7 x 10 Hz and etc = 2 x 10 Ds, so that inequalities (1), (3), (7), (8a), (8b), (10) in all cases without Difficulties can be met without using a highly birefringent single mode optical fiber must be used. Because of the small size etc., ring interferometer arrangements are suitable, in which the zero order interference maximum is adjusted especially well in connection with the procedures described here.

Ring interferometer with adjustment to zero order interference maximum are z. B. in German Patent applications P 29 06 870, P 29 34 794.2 and P 29 41 618.0 are suggested.

If Tc is not sufficiently small to fulfill the conditions (Eq. (1), (3), (7), (8a), (8b), (10)), the activation is a polarizer advantageous in a and b or in T (as in P 29 06 870). There is another possibility in the use of an unpolarized light source Q or a depolarizer in light path a, if #o is not sufficiently small.

Claims (28)

Claims 1.Ringinterferometer with a light source (Q), a <beam dividing arrangement (T), a light path (L), which covers a surface (F) encloses one or more times and which at both ends in the beam splitting Arrangement (T) opens, as well as with a photodetector (D), which is from the light source (Q) emitted light via the beam splitting arrangement (U) in both ends of the light path (L) is coupled, traverses this in both directions and afterwards the beam-splitting arrangement (T) reunited and thereby to interference is brought, and then forwarded to the photodetector (D) and detected there is, characterized in that the light source (Q) is a light source with a Coherence time #o lt c, where c is significantly smaller than the transit time #L of the light through the light path (L). 2. Arrangement according to claim 1 or 2, characterized in that the Coherence time tc of the light source (Q) is much smaller than the transit time difference az, the two eigenwaves of the light path (L). 3. Arrangement according to claim 1 or 2, characterized in that the light coming from the light source (Q) before coupling into the light path (L) Polarizer (POL1) passes through and after coupling out of the light path (L) the the same or a different polarizer (POL2). 4. Arrangement according to one or more of claims 1 to 3, characterized characterized in that the light path (L) is a birefringent single mode optical fiber and that that originating from the light source (Q) is completely or partially polarized Light is coupled in such a way that the polarization state of the in the The light path (L) of the coupled light is located on a great circle of the Poincare sphere, which lies in a surface normal to the axis of rotation, which is formed by the Line connecting the two eigenstates of the light path (L) (i.e. the Polarization states of the eigenwaves). 5. Arrangement according to claim 4, characterized in that the light path (L) a linearly birefringent single mode optical fiber with the two horizontal and vertically polarized eigenwaves and that the coupled light is circular polarized or below + 4 / in or below -45 ° linearly polarized or below + 45 ° inclined main axis is elliptically polarized. 6. Arrangement according to claim 4, characterized in that the light path (L) is a circular birefringent single mode optical fiber and is in the light path (L) completely or partially linearly polarized light is coupled. 7. Arrangement according to one of the preceding claims, characterized in that the light path (L) is realized by a birefringent single-mode optical fiber, such that the relationships for two orthogonal angles 0 and w of the Poincaré sphere apply, where Afo denotes the spectral width of the light source (Q). 8. Arrangement according to one or more of the preceding claims, characterized in that the light path (L) consists of two or more each birefringent Sections consists, each of these sections having a different polarization aa The axis of the Poincaré-Eugel rotates. 9. Arrangement according to claim 8, characterized in that the light path (L) consists of two sections (L1 and L2), with one of the two sections from a linear birefringent single mode optical fiber and the other of the two Sections consists of a circular birefringent fiber. 10, arrangement according to claim 8, characterized in that the light path (L) composed of two linearly birefringent sections twisted against each other at 450 Consists of fibers of different lengths. 11. The arrangement according to claim 8, characterized in that the light path (L) consists of a number (N) of sections (L1 ...), each of the sections is birefringent and in the individual sections rotations around different Axes of the Poincaré-Eugel. 12. The arrangement according to claim 11, characterized in that in two successive sections (LK + 1) a rotation of the polarization state takes place around mutually orthogonal axes of the Poincaré sphere. 13. Arrangement according to claim 11 or 12, characterized in that a linear birefringent single mode optical fiber is used as the light path and that successive sections are axially rotated with respect to one another. 14. Arrangement according to claims 12 and 13, characterized in that consecutive linear birefringent nonomode optical fiber sections 450 are rotated axially against each other. 15. Arrangement according to claim 12, characterized in that circular and alternate linear birefringent sections. 16. Arrangement according to one of the preceding claims, characterized in that that the light path (L) consists of a single linearly birefringent single-mode optical fiber is established and that the birefringence properties of the fiber by axial Twisting and / or twisting of the single-mode optical fiber are influenced. 17. Arrangement according to one or more of the preceding claims, characterized in that the Polarization axis of the Poincaré-Eugel changes continuously over the fiber length. 18. The arrangement according to claim 17, characterized in that the Change in the polarization axis of the Poincare sphere changes randomly. 19. Arrangement according to one or more of the preceding claims, characterized in that a semiconductor injection laser is used as the light source (Q) is. 20. Arrangement according to claim 19, characterized in that the semiconductor injection laser is longitudinally multi-waved. 21. Arrangement according to claim 20, characterized in that the coherence time tc or the spectral width AfO of each individual laser oscillation in the claims 1, 2 or 7 conditions are sufficient. 22. Arrangement according to one of claims 19 to 21, characterized in that that the semiconductor laser is modulated to degrade the coherence. 23. Arrangement according to one or more of claims 1 to 18, characterized characterized in that a temporally incoherent light source is used as the light source (Q) is. 24. Arrangement according to one or more of claims 1 to 18 and 24, characterized in that the light source (Q) is a light-emitting semiconductor diode is. 25. Arrangement according to claim 24, characterized in that the light source (Q) is a superluminescent diode. 26. Arrangement according to claim 25, characterized in that the light source (Q) is an edge emitter type superluminescent diode. 27. Arrangement according to one or more of the preceding claims, characterized in that the light source (Q) emits unpolarized light. 28. Arrangement according to one or more of claims 1 to 27, characterized characterized in that the light source (Q) has an arrangement for depolarizing the light is downstream.