JPH0321889B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention generally relates to Faraday rotators;
More specifically, the present invention relates to a Faraday rotator that can be used in a four-frequency laser gyroscope that effectively operates two laser gyroscopes simultaneously in a common propagation path.

[Background technology]

The principle of operation of a four-frequency laser gyroscope is described in US Pat. No. 3,741,657 (June 26, 1973). In the device described in this patent,
Electromagnetic waves of four different frequencies are propagated through a propagation circuit formed by three or more mirrors. Two of these beams rotate clockwise around the propagation loop, whereas the other two rotate counterclockwise. One of the clockwise beams and one of the counterclockwise beams are in a first polarization direction, while the other clockwise and the other counterclockwise are in a second polarization direction. For example, a first clockwise beam and a first
The counterclockwise beam consists of right-handed circularly polarized waves, and the second clockwise beam and the second counterclockwise beam consist of left-handed circularly polarized waves. Further, the two right-handed circularly polarized beams consist of, for example, two higher frequencies, and the two left-handed circularly polarized beams consist of two lower frequencies.

Rotation of the laser gyroscope about its axis causes the two right-handed circularly polarized beams to be more separated in frequency than in the rest state, and the two left-handed circularly polarized beams to be more closely spaced.
For rotation in the opposite direction, an opposite frequency shift occurs. The difference in frequency shift between the right-handed and left-handed circularly polarized beams is directly proportional to the rotation speed of the gyroscope. The time integral of this difference is directly proportional to the total amount of rotation about the axis.

To maintain a beam consisting of four frequencies,
Two means for frequency separation are provided in the propagation path. In the device described in the aforementioned U.S. patent,
A crystal rotator separates the average frequencies of the right-handed and left-handed circularly polarized beams. This separation is achieved by a crystal that provides a phase delay to the circularly polarized wave, and this phase delay differs depending on the direction of the circularly polarized wave and is reversible with respect to the propagation direction. Additionally, the Faraday rotator provides frequency separation between the frequencies of the clockwise and counterclockwise beams of the same circular polarization. Faraday rotators are irreversible and produce different phase delays for electromagnetic waves of the same polarization state propagating in opposite directions.

Although the device described in the aforementioned US patent works well, there is room for improvement. For example, it is desirable to remove as much solid material from the propagation path as possible. The reason is that if there is a solid material in the propagation path,
This is because scattering centers occur there, from which light can undesirably couple from one beam to another, causing output frequency drift.

[Summary of the invention]

SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a Faraday rotator that allows as little scattering material to be placed in the propagation path of the circulating beam as possible.

To achieve this objective, a Faraday rotator according to the invention comprises: a base support member having an opening therein; a permanent magnet having an opening therein, the opening being aligned with the opening of said base support member; a slab made of glass doped with a rare earth element and disposed inside an opening in the permanent magnet, the slab and the permanent magnet being supported by the base support member; a retaining member having an aperture therein that is aligned with an aperture in the base support member and a permanent magnet and retaining the spring by the magnetic force of the permanent magnet; a support member is provided with a substantially cylindrical center portion, a substantially cylindrical first end portion having a diameter greater than the center portion, and the opening in the base support member; a second end having a substantially flat portion attached thereto, the opening in the base support member and the longitudinal axis of the permanent magnet being substantially perpendicular to the longitudinal axis of the central portion of the base support member; Ru.

In a preferred embodiment, a Faraday rotator is used as a means of imparting different delays to electromagnetic waves propagating in opposite directions on a propagation circuit, and comprises a slab having a thickness of 0.5 mm or less and generating a magnetic field within the slab. have the means to do so. The slab material is preferably 0.25 (min/cm/Oe) at the operating wavelength.
It has the above Verdet constant. Glass doped with a suitable rare earth element satisfies this condition.

〔Example〕

1 to 4 show the construction of a laser gyroscope device in which a Faraday rotator according to the invention can be used.

Gyroscope block 102 forms the frame of the device. Gyroscope block 102 is constructed of a material with a low coefficient of thermal expansion, such as a glass-ceramic material, thereby minimizing the effects of temperature changes on the laser gyroscope device. A commercially available material is material C-101, manufactured by the Owens-Illinois Company, sold as "Cer-Vit ^™ ".

Gyroscope block 102 has approximately nine planes, as shown in FIGS. 1-4. In particular, FIGS. 3 and 4 omit other components of the device, showing only the gyroscope block 102, and the passages 108, 110, 112 and 11.
4 are provided between the four surfaces of the gyroscope block 102. The passage forms a non-planar propagation loop within the laser gyroscope block 102.

mirrors on surfaces 122, 124, 126 and 12;
8 at the intersection with the passageway. Substrates 140 and 142 have suitable reflective surfaces to provide mirrors disposed on surfaces 124 and 126, respectively. A mirror surface is also provided directly in front of and adjacent to the path length control transducer 160. One of these mirrors is made concave so that the beam is formed stably and precisely in the center of the path. A partially transmissive mirror is provided on surface 122 to couple a portion of each beam propagating along a closed path within gyroscope 102 to output optics 144. The structure of the output optical system 144 may be that disclosed in US Patent Application No. 758,223 (filed January 10, 1977).

Passages 108, 110, 112, and 114 form non-planar propagation paths for the different beams within the device, so that each beam undergoes polarization rotation as it travels around the closed path. Only approximately circularly polarized beams will be able to exist within the non-planar cavity. With circularly polarized beams, drift due to beam scattering or coupling from one beam to another is reduced to a minimum. This reduction phenomenon occurs because, when scattered, light of one circularly polarized state is not in the proper polarized state to combine with or influence other beams. This is not the case for other forms of optical polarization because there is always a component of the scattered beam that couples into other beams.

In a preferred embodiment, the passageway and reflective mirrors are arranged to provide approximately 90 degrees of polarization rotation for each beam.

Because the right-handed and left-handed circularly polarized beams are rotated in opposite directions by the same amount, independent of the propagation direction, for the beam to resonate within the optical cavity, there is a Frequency separation must occur. This is shown in FIG. 7 as the frequency separation of the left-handed and right-handed circularly polarized beams. In the preferred embodiment, a 90 degree rotation corresponds to a 180 degree relative phase shift, although other phase shifts may be used as well based on the desired frequency separation. Rotation occurs as long as the propagation circuit is non-planar. Accurately configuring the passage will determine the amount of rotation.

In conventional devices, such as those disclosed in the aforementioned US patents, frequency separation between right-handed and left-handed circularly polarized beams is achieved using blocks of solid material of substantial optical thickness in the propagation path. achieved by. As mentioned above, the presence of solid material directly in the beam propagation path creates scattering points,
From there, the light undesirably couples together causing errors in the gyro output. This amount of coupling and error is very sensitive to thermal influences. Heretofore, the output frequency of this type of device has been affected by temperature-induced drift, which cannot be compensated for by a fixed output bias. According to the present invention, the solid material conventionally used as a crystal rotor can be completely removed from the beam propagation path, and the error and drift sources arising therefrom can be eliminated.

To help understand how phase shifts occur, it is useful to consider a path-propagating linearly polarized beam. For example, surfaces 122 and 1
The beam traveling between 24 and 24 is linearly polarized by an upwardly directed electric vector. beam is face 1
Since it is reflected from the mirror prepared on 24,
The electric vector is still pointing generally upward, but is tilted slightly forward as passageway 112 dips between surfaces 124 and 128. When the beam is reflected from the mirror on surface 128, the vector points to the left with a slight downward tilt as shown in FIGS. beam is plane 15
1, the electrical vector of the beam in passageway 108 will point to the left with a slight upward slope as shown in FIGS. Thus, when the beam arrives at surface 122,
The polarization rotates approximately 90 degrees. Of course, such a rotated circularly polarized beam is not reinforced and therefore does not resonate along a closed path. Only circularly polarized beams with a frequency shifted from the frequency at which such a beam would resonate in a planar loop of the same length will resonate.

A dual frequency laser gyroscope can be constructed with a non-planar propagation path to provide only frequency separation. Faraday rotators or other such elements are not needed in this case. To detect rotational speed, an output signal is generated by beating together the tapped portions of the two beams to form an output signal having a frequency that is the difference in frequency between the two beams. If it is stationary, the output signal will have a certain value of ₀ . When rotating in one direction, the frequency of the output signal increases to ₀ + Δ (where Δ
is proportional to the rotational speed), and on the other hand, when rotating in the other direction, it decreases to ₀ - Δ. According to the invention,
By using circular polarization, cross-coupling due to backscatter can be significantly reduced, thereby reducing the lock-in range and
Laser gyroscopes can be used in many applications without completely eliminating lock-in.

A second frequency separation between the clockwise and counterclockwise beams is generated by a Faraday rotator 156 according to the present invention. The Faraday rotator 156 is disposed within an opening in the surface 151, as shown in FIGS. 2 and 4.

6 and 6A show the detailed construction of Faraday rotator 156 according to the present invention. Faraday rotator base 154 is preferably made of the same material as laser radio block 102 and provides the base for assembling the structure. The Faraday rotator base 154 has a central cylindrical portion with a flange at one end and is adapted to limit lateral movement of the device within the opening 120 provided in the laser gyroscope block 102. Faraday rotor stand 15
The other end of 4 is cut off to leave a platform for mounting the device. opening 1
55 is placed alongside passageway 112 and has approximately the same diameter as passageway 112. Hollow cylindrical permanent magnet 1
66 are disposed around the opening 155. An opening 155 is provided within the opening of the permanent magnet 166. A slightly wedge-shaped Faraday rotor slab 165 is disposed within the opening of the permanent magnet 166 . Faraday rotor slab 165
is preferably made of rare earth doped glass or a similarly high Werde constant material. The magnitude of the Werde constant at the operating wavelength is
It is desirable to reduce the slab thickness to 0.25 (min/cm/Oe) or higher to create the required amount of frequency separation. Since the amount of temperature drift that occurs in the output of the device is known to be a direct function of the thickness of the solid material in the path of the electromagnetic waves, it is desirable to use as thin a slab as possible. Commercially available materials include Hoya Optics, Inc. material number FR-5.
can be used. A thickness of 0.5 mm or less is desirable to reduce drift to an acceptable level.

The Faraday rotor slab 165 is held on the Faraday rotor stand 154 by a coil spring 168. Pole piece 170 is made of an unmagnetized ferromagnetic material and is held against permanent magnet 166 by its magnetic field (magnetic force). Pole piece 170 has an aperture 155 in its center and an opening approximately the same diameter as passage 112;
2 has a diameter slightly smaller than the opening in permanent magnet 166. Thus the coil spring 168
is held down by the portion of the pole piece that extends into the aperture of permanent magnet 166.

In another embodiment, two cylindrical permanent magnets are disposed end-to-end so that the joints between them have the same magnetic poles. The Faraday rotor slab is located near one end of the two magnet pairs. A longitudinal magnetic field is generated within the slab, but this field decays rapidly with any distance from the slab or magnet. This embodiment does not in principle generate stray magnetic fields, which can be extended into the gas discharge region and cause undesired modes or frequency offsets due to the Zeeman effect.

Additionally, Faraday rotator 156 performs a second function by providing frequency separation between the clockwise and counterclockwise rotating beams. Due to its tight fit within the opening 120 of the gyroscope 102, the Faraday rotator 156 stops longitudinal gas flow through the passageway 112. Since there is no true orbiting gas that follows a closed path, there is little chance that scattering particles carried by the gas will orbit. Backscattering of the incident radiation can be prevented if both sides of the Faraday rotator slab 165 are coated with a non-reflective material. Also, reflection is allowed as long as it is used as reflected radiation that is useful for the output signal. In this case a partially transmitting mirror is not necessary.

In FIGS. 1, 3, and 4, the surface 122
The angle of incidence is small for the beam striking the partially transmitting mirror disposed above. Passage 108, 11
The beams traveling within 0, 112 and 114 respectively become circularly polarized waves as a whole. The closer one of the beams hitting the reflecting mirror or surface is vertical, the more
The polarization of the beam passing through the mirror surface is close to circular polarization. As the angle of incidence moves away from normal, the partially transmitted beam becomes elliptically polarized.

As discussed in the aforementioned U.S. Patent Application No. 758,223, if the output optics and the beam in the detector are made completely circularly polarized, undesirable cross-coupling may occur within the detector. In principle, it is considered that no interference occurs between the upper two frequency beams and the lower two frequency beams. As the degree of ellipse increases, cross-coupling begins to become apparent and appears as an amplitude modulated component in the output signal of detector diode 143. It has been discovered that the undesirable cross-coupling increases monotonically in a non-linear manner with the degree of ellipse. It has also been discovered that cross-coupling is relatively low at angles of incidence below about 15°.
However, the amount of cross-coupling increases very rapidly above this angle of incidence. This cross-coupling can be removed by using a suitable polarizing filter, but the filtered effective power will decrease as the cross-coupling increases before filtering.

Furthermore, as the angle of incidence of each beam on the output mirror increases, the effective power in the detector diode for each beam decreases. The calculation of the power reduction factor, i.e. the ratio of the effective power in the detector for a given angle of incidence to the effective power of the same beam perpendicular to the mirror plane, is shown in the graph of FIG. 8 in the above-mentioned US patent application no. The case of the output structure described in . As already evident, the power reduction factor drops sharply at angles of incidence greater than approximately 15°. In accordance with the principles of the present invention, passageway 108
and 110, the angle of incidence of the beam onto a partially transmitting mirror disposed on surface 122 can be 15° or less. In other words, the angle between passages 108 and 110 is 30 degrees or less.

In FIGS. 1, 3 and 4, the passage 10
An electrode for activating the gas gain medium disposed within 8 and 110 is located within electrode opening 104 . Preferably, the center cathode electrodes 132 and 1
36 is connected to the negative terminal of an external power source, whereas electrodes 127, 130, 134 and 138 are connected to the positive terminal. The cathode electrode is in the form of a hollow metal cylinder capped at the end furthest from the seal to the laser radio block 102, whereas the positive electrode is in the form of a metal rod extending into the other electrode opening 104. being done. By doing so, current flows outwardly toward opposite electrodes 132 and 136 in a single path. Negative electrode 136 connects positive electrodes 134 and 1
38, so that the negative electrode 132 is located midway between the positive electrodes 130 and 127. In this method, a beam that crosses one of the paths in which the electrodes are present will pass through the same length as the current in the opposite direction, so that the beam overlap caused by unequal currents flowing through the gas gain medium The tensile effect almost disappears. However, the distance between the positive electrode and the two negative electrodes in each passage is not exactly equal due to manufacturing tolerances in the position of each electrode. To correct this inequality, the currents between the positive electrode and the nearby negative electrode may be made equal.

The gas gain medium filling passageways 108, 110, 112, and 114 is supplied from an external gas source through gas supply tube 146 and through gas fill opening 106. The ratio of the mixed gases ³ He, ²⁰ Ne, and ²² Ne is preferably 8:0.53:0.47.

The details of the configuration of the laser gyroscope device in one region of the positive electrode are illustrated in detail in the cross-sectional view of FIG. Metal electrode 130 is positioned by glass electrode seal 131 and is disposed within electrode opening 104 . Electrode 130 extends more than halfway between the surface of gyroscope 102 and passageway 110. Electrode opening 10
4 intersects the passageway 110, preferably with a rightward slope. The terminal chamber 125 is formed between the surfaces of the gyroscope block 102 on which the mirror substrate 140 is disposed. The terminal chamber 125 is formed into a cylindrical shape having a diameter at least twice the diameter of the passageway 110. The terminal chamber 125 and the passage 110 are coaxial with each other. The passageway 110 extends slightly beyond the electrode opening 104 before intersecting the terminal chamber 125 such that a buttful 145 connects the electrode 104 and the terminal chamber 12.
It is formed between 5.

Conventional devices do not have a terminal room or a buffer. The passageway extended directly through the electrode aperture to the surface of the laser diamond block. When the electrode is energized, dust and other undesirable particles, caused, for example, by ion bombardment or sputtering of the laser gyroscope, collect around the intersection of the electrode aperture and the beam path. Stagnant particles act as scattering centers that increase structural optical losses. In contrast, according to the present invention, dust and other undesirable particles do not become lodged in the area of the intersection of electrode opening 104 and passageway 110. In this way, potential drift sources can be eliminated.

The preferred embodiments of the present invention have been described above. However, many other modifications and changes may be made without departing from the principles of the invention.

[Brief explanation of the drawing]

1 is a top perspective view of a laser block device that can use the Faraday rotator according to the present invention, seen from the first corner; FIG. 2 is a bottom perspective view of the device of FIG. 1, seen from the second corner; 4 are perspective views showing the internal structure and passages of the device shown in FIG. 1, FIG. 5 is a sectional view showing the terminal chamber and mirror board as the internal structure of the device shown in FIG. 1, and FIG. 6 is a perspective view showing the internal structure of the device shown in FIG. 6A is a partial sectional view of the rotor in FIG. 6,
FIG. 7 is a diagram showing the frequency-gain characteristics of the gas gain medium used in the apparatus of FIG. 1, and FIG. 8 is a curve diagram showing the relationship between the power reduction coefficient and the incident angle of the beam incident on the mirror structure. . 102...Gyroscope, 108,11
0,112,114...Aisle, 122,124,
126, 128... face, 127, 130, 13
4,138... Electrode, 132,136... Cathode electrode, 140... Substrate, 160... Path length control transducer, 156... Faraday rotator, 165... Faraday rotor slab, 166
...Permanent magnet, 168...Coil spring.

Claims (1)

[Scope of Claims] 1. A base support member having an opening therein; a permanent magnet having an aperture therein aligned with the opening of the base support member; and glass doped with a rare earth element. , a slab disposed within an opening in the permanent magnet; a spring holding the slab against the base support member; and an opening therein aligned with the base support member and the opening in the permanent magnet; A Faraday rotor comprising: a holding member that holds the spring by the magnetic force of a permanent magnet; the permanent magnet is attached to the base support member; and the base support member is substantially cylindrical. a substantially cylindrical first end portion having a diameter greater than the center portion; and a substantially flat portion in which the opening of the base support member is provided and to which the permanent magnet is attached. a Faraday rotator, the Faraday rotator comprising: the base support member; and a second end portion in which the central axis of the aperture of the permanent magnet is substantially perpendicular to the central axis of the base support member. 2. The Faraday rotator according to claim 1, wherein the spring is a coil spring. 3. The Faraday rotator of claim 1, wherein the permanent magnet and retaining member have substantially cylindrical inner and outer surfaces. 4. The Faraday rotator according to claim 3, wherein the holding member is made of a ferromagnetic material. 5. The Faraday rotator according to claim 1, wherein a cross section of the second end portion to which the permanent magnet and the slab are attached is not larger than a cross section of the center portion of the base support member.