GB2138585A

GB2138585A - Faraday rotator

Info

Publication number: GB2138585A
Application number: GB08333405A
Authority: GB
Inventors: Irl W Smith
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1980-10-17
Filing date: 1983-12-15
Publication date: 1984-10-24
Also published as: FR2492522A1; JPH02870B2; GB2138585B; GB2087638A; IT1142901B; NL8104726A; JPS5796581A; GB8333405D0; GB2087638B; CA1189600A; IT8149504A0; FR2492522B1; DE3141175A1; DE3141175C2

Abstract

A Faraday rotator 156 for a four-frequency laser gyroscope consists of a cylindrical shell 154 with a stepped bore at an angle to the shell axis. A rare earth doped glass slab 165, Verdet constant exceeding 0.25 min./cm./Oe, is laterally secured by a ring 169 and pressed against an annular internal face by a pair 188 of annular permanent magnets arranged with like poles facing, backed by a spring 175, a spacer 177 a ring 191 of light absorbing material, and a circular clip 193. A further ring 190 of light absorbing material is similar located at the other side of the slab 165. The rings 190 and 191 absorb light reflected off the faces of the slab 165. <IMAGE>

Description

1 GB 2 138 585 A 1

SPECIFICATION Faraday rotor

This invention relates to Faraday rotators.

According to the present invention there is provided a Faraday rotator comprising a supporting shell, means for providing a Faraday rotation disposed within-the shell and means for absorbing electromagnetic waves disposed on at least one side of the Faraday rotation means for allowing a substantial portion of the waves to pass through the Faraday rotation means which are further positioned to direct any reflected portion of the waves toward the absorbing means.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional side view showing the details of construction of a Faraday rotator embodying the invention, and 20 Fig. 2 is a top view of a portion of a laser gyro in 85 the region of the Faraday rotator of Fig. 1, and showing a top view of the Faraday rotator. A Faraday rotator 156 is shown in Fig. 1 which is intended to be used in a four-frequency laser gyroscope.

In Fig. 2, the Faraday rotator 156 is shown mounted in a bore 113 in a gyroblock of a fourfrequency laser gyroscope. The gyro block forms the frame upon which the gyroscope is constructed, and is constructed with a material having a low thermal coefficient of expansion such as a glass-ceramic material to minimize the effects of temperature change upon the laser gyroscope. A preferred commercially available material is sold under the name of Cer-Vit (Trade Mark) material C-1 01 by Owens-Illinois Company; alternatively Zerodur (Trade Mark) by Schott may be used.

A gaseous gain medium fills the passages of the gyro block. A mixture 3 He '21Ne and 22 Ne in the ratio of 8:0.53:0.47 is preferred.

The Faraday rotator mount 154, preferably formed of the same material as laser gyro block, forms the base upon which the structure is constructed. Rotator mount 154 is cylindrical in shape and has several cylindrical apertures of varying diameter at an angle to the longitudinal axis of mount 154 for providing support at predetermined locations to all the elements of Faraday rotator 156 and for providing a clear path along the longitudinal center axis of mount 154. Faraday rotator slab 165 is positioned on shelf 166 formed by the central portion of mount 154. Ring 169 prevents lateral movement of slab 166. Faraday rotator slab 165 may be preferably formed of a rare earth-doped glass or a material of 120 similarly high Verdet constant. A Verdet constant of magnitude in excess of 0.25 min./cm. /Oe. at the operating wavelength is preferred to reduce the thickness of the slab required to produce the desired amount of frequency splitting. Traditional 125 Faraday rotators have employed a thick slab of material, often fused quartz. Any solid material in the path of the counter-rotating beams of a laser gyroscope will introduce scatter points which exhibit a sensitivity to thermal fluxes. This sensitivity may be due to the thermal expansion of the material or to a change in the optical path length due to the temperature dependence of the refractive index of the material. The effective temperature dependence of the optical path length, and therefore the thermally induced drift, has been found to be a strong positive function of the thickness of the solid material in the path of the beams. Thus, it is desirable to use as thin a slab as possible and a thickness of 0.5mm or less is preferred to reduce drift to an acceptable level resulting in a variation of the thickness due to temperature or other causes substantially less than one wavelength of the laser waves over the operating region. A commercially available material is Hoya Optics, Inc. material No FR-5 which is a glass doped with paramagnetic material to provide for the Faraday rotation and results in a rotator having an isotropic refractive index. This was found to be important since a problem of a traditional Faraday rotator is that a crystal material such as quartz has an anisotropic refractive index which introduces elliptical birefringence. This depolarizes the nominally circularly polarized waves and leads to increased coupling between the counter-rotating waves. Thus, it is important to use an isotropic material for the Faraday rotator to eliminate depolarization of the resonant modes. Operating as close to circular polarization as possible reduces cross-coupling and therefore reduces thermally induced drifts due to any remaining scatter centers. This allows a gyro system to achieve stability levels corresponding to a variation in time of the output frequency of a few Hz or better.

Faraday rotator slab 165 is held against shelf 166 by magnet assembly 188. Two hollow cylindrical permanent magnets 186 and 187 are positioned endto-end with like poles adjacent one another at the juncture between the two magnets. The two magnets can be fastened together by any known means, such as solder bonding or welding. The Faraday rotator slab 165 is then adjacent one end of the two magnet pair. A longitudinal magnetic field is produced in the slab, but this field attenuates rapidly upon moving a short distance away from the slab or magnets. This embodiment has the advantage that essentially no stray magnetic field is produced which could extend into the gaseous discharge region and, by the Zeeman effect, produce unwanted modes or frequency offset. Alternatively, a single magnet may be used to provide the required magnetic field to the slab. The permanent magnet structure might also be replaced by a few coils of wire to allow an electrical current to establish a magnetic field in Faraday rotator slab 165. Pushing on magnet structure 188 is spring 175. The other side of spring 17 5 rests along the periphery of hollow cylindrical spacer 177, which in turn rests partially on one side of hollow cylindrical absorber 19 1. Absorber 191 is made of a material such as black glass and being antireflection-coated is used to absorb any electromagnetic wave, such as the

2 GB 2 138 585 A 2 specular reflections from the Faraday rotator slab, incident on its surfaces. Absorber 191 is held in place by circular clip 193, which rests by friction along the periphery of aperture 18 1. This, it can be 55 seen that the elements just described form an assembly positioned by circular clip 193 against the right side of shelf 166, with spring 17 5 providing a sufficient longitudinal force to keep all the elements tightly in place. On the opposite side of shelf 166, there is a similar arrangement of elements with the exception of Faraday slab 165 and magnet 188. Circular clip 192 forms the stationary base against which a second absorber 190 rests. Spring 174, with one end resting on the left side of shelf 166, pushes spacer 196 against absorber 190 and thus keeps all the elements on the left side of shelf 166 in their predetermined position.

Rotator mount 154 is held in place against the shelf formed by the change of diameter of passages 112 and 113, by helical spring 199. A portion of the first smaller diameter turn of spring 199 rests on the body of rotator mount 154, while the other larger diameter end expands circumferentially and frictionally engages the wall of bore 113. The arrangement of the elements of rotator 156 provides for thermal stability, since the optical elements are elastically held against a stable material used for the gyro block. A substrate 140 closes one end of passage 113.

As described above, the axes of apertures provided in rotator mount 154 are at an angle with respect to the longitudinal axis of mount 154, and thus the plane of Faraday rotator slab 165 also describes an angle with respect to the longitudinal axis of mount 154. This contributes to the elimination of coupling between countertravelling waves since any reflections off the two surfaces of Faraday slab 165 are now intercepted and absorbed by the two black glass absorbers. Waves circulating from left to right in the rotator of Fig. 1 will have a reflection from slab 165 intercepted and absorbed by the lower portion of absorber 190, while waves circulating in the opposite direction will have reflections from slab 165 absorbed by the top portion of absorber 19 1. The two absorbers, 190 and 19 1, and rotator slab 165 are also coated with an anti-reflection coating to further reduce the amount of reflections.

Besides providing the frequency splitting between the clockwise and counter-clockwise circulating beams, Faraday rotator 156 performs a second function. Because of the close fit provided in the region of shelf 166, Faraday rotator 156 blocks the longitudinal flow of gas through passage 112. Because there can be no net circulation of gas through the closed path, the possibility of circulation of scatter particles carried by the gas is substantially reduced, as are drifts due to the Fresnel-Fizeau effect.

It is found that the Faraday rotator of the present embodiment produces a Faraday bias having a characteristic that is inversely proportional to temperature.

A thin Faraday paramagnetic glass slab is used instead of the traditional thick Faraday rotator to provide for the non-reciprocal split between the cw and cww waves. Use of a glass Fraday rotator avoids elliptical birefringence and thus maintains the circular polarization. The use of the Faraday rotator 156 in the laser gyroscope described in detail in our co-pending application no. 8131273, from which the present application is divided, eliminates coupling among the different waves since a perfectly I-CP wave on reflection will become an RCP wave and thus it will not couple into the counter-rotating wave. Use of the minimum rotator slab's thickness that achieves a predetermined amount of rotation ensures a minimal temperature dependence of any scattering centres introduced by the slab.

Claims

1. A Faraday rotator comprising a supporting shell, means for providing Faraday rotation disposed within said shell and means for absorbing electromagnetic waves disposed on at least one side of said Faraday rotation means and for allowing a substantial portion of said waves to pass through said rotation means, said rotation means being further positioned to direct any reflected portion of said waves toward said absorbing means.

2. A rotator according to claim 1, in which said supporting shell comprises a low thermal expansion material having a plurality of stops and said rotation means and said absorbing means are held in place against said stops by elastic means.

3. A Faraday rotator substantially as herein described with reference to the accompanying 100 drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Demand No. 8818935, 1011984. Contractor's Code No. 6378. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.