Source: http://www.google.com/patents/US5844710?dq=4,923,986
Timestamp: 2014-03-16 21:04:40
Document Index: 309983393

Matched Legal Cases: ['application No. 08', 'application No. 08', 'application No. 08', 'application No. 08', 'application No. 08', 'application No. 08']

Patent US5844710 - Faraday rotator and optical device employing the same - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA Faraday rotator capable of maintaining a Faraday rotation angle always constant regardless of temperature variations. The Faraday rotator includes a magneto-optic crystal provided in a light propagation path, a permanent magnet for generating a magnetic field parallel to the light propagation path,...http://www.google.com/patents/US5844710?utm_source=gb-gplus-sharePatent US5844710 - Faraday rotator and optical device employing the sameAdvanced Patent SearchPublication numberUS5844710 APublication typeGrantApplication numberUS 08/803,378Publication dateDec 1, 1998Filing dateFeb 20, 1997Priority dateSep 18, 1996Fee statusLapsedPublication number08803378, 803378, US 5844710 A, US 5844710A, US-A-5844710, US5844710 A, US5844710AInventorsNobuhiro FukushimaOriginal AssigneeFujitsu LimitedExport CitationBiBTeX, EndNote, RefManPatent Citations (49), Non-Patent Citations (14), Referenced by (19), Classifications (8), Legal Events (6) External Links: USPTO, USPTO Assignment, EspacenetFaraday rotator and optical device employing the sameUS 5844710 AAbstract A Faraday rotator capable of maintaining a Faraday rotation angle always constant regardless of temperature variations. The Faraday rotator includes a magneto-optic crystal provided in a light propagation path, a permanent magnet for generating a magnetic field parallel to the light propagation path, and an electromagnet for generating a magnetic field orthogonal to the light propagation path. The strength of a synthetic magnetic field by the permanent magnet and the electromagnet is set to a magnitude large enough to magnetically saturate the magneto-optic crystal. The electromagnet is driven by a drive circuit. The Faraday rotator further includes a temperature sensor provided adjacent to the Faraday rotator, and a controller incorporating data on temperature dependence of a Faraday rotation angle of the magneto-optic crystal to control the drive circuit so that the Faraday rotation angle becomes constant according to a temperature detected by the temperature sensor.
6. A Faraday rotator comprising:a magneto-optic crystal provided in a light propagation path; a first permanent magnet located so that its magnetic field direction forms a first angle with respect to said light propagation path, said first permanent magnet having a first temperature coefficient of a field strength; and a second permanent magnet located so that its magnetic field direction forms a second angle larger than said first angle with respect to said light propagation path, said second permanent magnet having a second temperature coefficient of a field strength larger than said first temperature coefficient; wherein a Faraday rotation angle exhibited on the light passing through said magneto-optic crystal by a synthetic magnetic field generated by said first and second permanent magnets is controlled substantially constant regardless of temperature changes. 7. A Faraday rotator according to claim 6, wherein said first angle is 0 light propagation path.
A saturation magnetic field (a magnetic field strength saturating magnetization of a magneto-optic crystal or a magnetic field strength required to saturate a Faraday rotation angle) can be reduced by using the magneto-optic crystal 4 which has a relatively small thickness enough to transmit an optical beam. As the magneto-optic crystal 4, sliced YIG (yttrium-iron-garnet) or epitaxially grown (GdBi).sub.3 (FeAlGa).sub.5 O.sub.12, for example, may be used.
The direction of the magnetic field to be applied to the magneto-optic crystal 4 by the permanent magnet 6 is parallel to a direction of transmission of an optical beam 12 in the magneto-optic crystal 4, and the direction of the magnetic field to be applied to the magneto-optic crystal 4 by the electromagnet 8 is perpendicular to the application direction of the magnetic field by the permanent magnet 6 and the transmission direction of the optical beam 12 in the magneto-optic crystal 4. The optical beam 12 incident on the magneto-optic crystal 4 is linearly polarized light, and its polarization direction is Faraday-rotated by the Faraday rotator 2. The Faraday rotation angle varies according to the purposes of use of the Faraday rotator 2. In the case of an optical isolator, the Faraday rotation angle is set to 45
In step 110, a Faraday rotation angle indication signal is input into the controller 16. For example, in the case that the Faraday rotator 2 is used as an optical isolator, an indication signal indicating that a Faraday rotation angle of 45 value from the temperature sensor 14 is taken in, and in step 112, the current value is converted into temperature data. In step 113, a normal temperature (room temperature) is subtracted from a temperature detected by the temperature sensor 14 to obtain a temperature difference ΔT. In step 114, ΔT is multiplied by a temperature coefficient of the Faraday rotator to calculate a correction value (step 115).
The temperature coefficient of the Faraday rotator is determined by a material forming the magneto-optic crystal. For example, YIG has a temperature coefficient of -0.06.degree. / coefficient is preliminarily incorporated in the program. After calculating the correction value in step 115, the program proceeds to step 116, in which a drive current value is calculated. The relation between Faraday rotation angle and drive current is preliminarily stored as a table in the program. In step 117, the drive current value is input into a drive circuit (variable current source 10) to supply the drive current from the drive circuit to the coil 9 of the electromagnet 8.
Thus, the current flowing in the coil 9 of the electromagnet 8 is changed according to a temperature change detected by the temperature sensor 14, so that the Faraday rotator 2 according to this preferred embodiment can obtain a desired Faraday rotation angle that is always constant regardless of temperature variations. In the case of a 90 for example, the temperature coefficient of the Faraday rotator 2 is about 0.1.degree. / 20 the case of a 45 changes about 1
As apparent from the graph shown in FIG. 4, a drive current of about 36 mA is required to change a Faraday rotation angle by 60 correction of a drive current of 0.6 mA is required to change a Faraday rotation angle by about 1 the program. By correcting the Faraday rotation angle prior to the calculation of the drive current in step 116, the number of times of correction operations can be reduced to one.
Referring to FIGS. 7A and 7B, there is shown a preferred embodiment of an optical isolator 46 employing the Faraday rotator 2. The configuration of this preferred embodiment is substantially the same as the configuration of the optical attenuator shown in FIG. 5. However, the Faraday rotation angle of the Faraday rotator 2 is set to 45, so that a range of adjustment of a drive current by a drive circuit 64 may be reduced. As shown in FIG. 7A, the optical isolator 46 includes an optical fiber 48 located upstream of a propagation direction of forward light, a lens 50 for converting the light emerging from the optical fiber 48 into a collimated beam, a polarizer 52 formed from a wedged birefringent crystal, a Faraday rotator 2 having a rotation angle set to 45 wedged birefringent crystal, a lens 56, and an optical fiber 58. These components are arranged in this order along the light traveling direction.
The polarizers 52 and 54 are positioned so that the top and bottom portions of the polarizer 52 are opposed to the bottom and top portions of the polarizer 54, respectively, and that the corresponding surfaces of the polarizers 52 and 54 are parallel to each other. While the positional relation between the polarizers 52 and 54 is opposite to the positional relation between the birefringent crystals 22 and 24 of the optical attenuator shown in FIG. 5, the same positional relation as that shown in FIG. 5 may be adopted. The optic axis of the polarizer 54 is pointed at an angle of 45 the same rotational direction as that of Faraday rotation in the Faraday rotator 2. When the forward light emerging from the optical fiber 48 is passed through the lens 50, the polarizer 52, the Faraday rotator 2, and the polarizer 54 in this order and then focused by the lens 56, the focus is formed inside a core end face of the optical fiber 58. Further, when backward light emerging from the optical fiber 58 is passed through the lens 56, the polarizer 54, the Faraday rotator 2, and the polarizer 52 in this order and then focused by the lens 50, the focus is formed outside a core end face of the optical fiber 48.
When the beam emerging from the optical fiber 48 and collimated by the lens 50 enters the polarizer 52 in the forward direction, the incident light on the polarizer 52 is divided into an ordinary ray and an extraordinary ray to be refracted in different directions to enter the Faraday rotator 2, because of different refractive indices in the polarizer 52 according to polarized light components. Since the optic axis of the polarizer 54 is pointed at an angle of 45 polarizer 52 in the same rotational direction as that of Faraday rotation in the Faraday rotator 2, the polarization planes of the ordinary ray and the extraordinary ray in the polarizer 52 are rotated 45 in the Faraday rotator 2, and respectively become an ordinary ray and an extraordinary ray also in the polarizer 54. Accordingly, the ordinary ray and the extraordinary ray passed through the polarizer 54 exit in parallel to each other. A collimated beam of the ordinary ray and the extraordinary ray outgoing from the polarizer 54 is focused by the lens 56 to enter the optical fiber 58.
As shown in FIG. 7B, feedback light reflected on an end face of an optical connector or the like (not shown) enters the polarizer 54, in which the incident light is divided into an ordinary ray and an extraordinary ray to be refracted in different directions to enter the Faraday rotator 2, in which the polarization plane of the incident light is rotated 45 then exiting from the Faraday rotator 2. The ordinary ray in the polarizer 54 whose polarization plane has been rotated 45 refraction as an extraordinary ray in the polarizer 52. Further, the extraordinary ray in the polarizer 54 whose polarization plane has been rotated 45 polarizer 52. Accordingly, the traveling direction of backward light emerging from the polarizer 52 is different from the traveling direction of forward light. As a result, the backward light passed through the lens 50 is not coupled to the optical fiber 48.
As mentioned above, the Faraday rotation angle of the Faraday rotator 2 varies with temperature. Therefore, although the Faraday rotation angle of the Faraday rotator 2 is set to 45 rotation angle changes with temperature variations, so that an optical isolator having a sufficient extinction ratio cannot be realized. To cope with this problem, the optical isolator 46 according to this preferred embodiment employs a temperature sensor 60 to detect an ambient temperature of the Faraday rotator 2 and input the detected temperature into a controller 62. As similar to the previous preferred embodiment, the controller 62 calculates a correction value of a drive current according to the detected temperature, and controls the drive circuit 64 according to the correction value to adjust the drive current flowing in the coil of the electromagnet so that the Faraday rotation angle always becomes 45
The birefringent plate 82 is inserted between the Faraday rotator 2 and a polarizing prism 90 having a beam splitting film 88. The half-wave plate 78 is inserted between the polarizing prism 90 and the birefringent plate 82 so as to be opposed to an upper portion of the birefringent plate 82. In the case that the direction of application of a magnetic field in the Faraday rotator 2 is the same as the light traveling direction, the Faraday rotator 2 rotates a polarization plane by 45 clockwise direction, whereas in the case that the field application direction is opposite to the light traveling direction, the Faraday rotator 2 rotates a polarization plane by 45 direction. Further, a half-wave plate 92 rotates a polarization plane by 45
When an optical beam 126 emerging from an optical fiber 94 enters the polarizing prism 86 in the condition that a magnetic field is applied to the Faraday rotator 2 in a direction shown by an arrow in FIG. 8, the optical beam 126 is divided into P-polarized light 128 and S-polarized light 130 by the polarizing prism 86. The P-polarized light 128 is passed through the half-wave plate 76. At this time, the polarization plane of the P-polarized light 128 is rotated 90 light, which is next passed through the birefringent plates 80 and 82. At this time, a crosstalk component due to the beam splitting film 84 is refracted to exhibit a translational shift as shown by a broken line.
On the other hand, the S-polarized light 130 is passed through the birefringent plates 80 and 82. At this time, a crosstalk component due to the beam splitting film 84 is refracted to exhibit a translational shift as shown by a broken line. The S-polarized light 130 is passed through the half-wave plate 78. At this time, the polarization plane of the S-polarized light 130 is rotated 90 The P-polarized light is transmitted by the beam splitting film 88, and the S-polarized light is reflected by the beam splitting film 88. Therefore, almost all the light is combined as an optical beam 132, which is next coupled to an optical fiber 96.
FIG. 9 shows the case where the field application direction in the Faraday rotator 2 is opposite to that shown in FIG. 8. When the optical beam 126 from the optical fiber 94 enters the polarizing prism 86 in this case, the P-polarized light 128 and the S-polarized light 130 divided by the polarizing prism 86 are passed through the birefringent plate 80 in a manner similar to that shown in FIG. 8. However, when the P-polarized light 128 and the S-polarized light 130 are passed through the Faraday rotator 2 and the half-wave plate 92, the polarization planes of the P-polarized light 128 and the S-polarized light 130 are rotated 90 for each.
Accordingly, after passing the half-wave plate 92, the P-polarized light 128 remains P-polarized light, but the S-polarized light 130 becomes P-polarized light. The light outgoing from the half-wave plate 92 is refracted in the birefringent plate 82, and its optical paths come to coincidence with the crosstalk components shown by broken lines in FIG. 9. Thereafter, the P-polarized light based on the S-polarized light 130 is passed through the half-wave plate 78. At this time, the polarization plane of the P-polarized light is rotated 90 light, which is next reflected by the beam splitting film 88 of the polarizing prism 90. On the other hand, the P-polarized light based on the P-polarized light 128 is not passed through the half-wave plate 78 and then transmitted by the beam splitting film 88. Thus, the two beams of P-polarized light are combined to become a synthetic beam 134, which is next coupled to the optical fiber 98. The crosstalk component shown by the broken line exiting rightward from the polarizing prism 90 has exhibited a translational shift, so that it is not coupled to the optical fiber 96.
On the other hand, the Faraday rotation angle of the magneto-optic crystal 68 generally becomes small at high temperatures. In YIG, for example, the Faraday rotation angle decreases by about 0.06.degree. per 1 increase in temperature. The Faraday rotation angle of the Faraday rotator 66 according to this preferred embodiment can be maintained substantially constant regardless of temperature variations.
The operation of the Faraday rotator 66 according to this preferred embodiment will now be described with reference to FIG. 11. The field strength of the first permanent magnet 70 is denoted by A, which is substantially constant regardless of temperature variations. The field direction of the first permanent magnet 70 is parallel to the propagation direction of the optical beam 74. On the other hand, the field strength of the second permanent magnet 72 at an ordinary temperature is denoted by B.sub.2, and an angle formed between the direction of the synthetic magnetic field and the field direction of the first permanent magnet 70 is denoted by θ2. Letting θF denote a Faraday rotation angle if the field direction coincides with the light traveling direction, the Faraday rotation angle corresponding to the angle θ2 becomes θFcosθ2.
When the temperature rises, the field strength of the second permanent magnet 72 decreases to B.sub.1. Accordingly, the angle between the direction of the synthetic magnetic field and the field direction of the first permanent magnet 70 becomes θ1 smaller than θ2. In this case, the Faraday rotation angle becomes θFcosθ1, which is larger than θFcosθ2, provided that θF is constant. However, since the Faraday rotation angle of the magneto-optic crystal 68 decreases with an increase in temperature, θF decreases with an increase in temperature. That is, the effect of the direction of the synthetic magnetic field by the two permanent magnets 70 and 72 on the Faraday rotation and the effect of the temperature coefficient of the magneto-optic crystal 68 are counteracted.
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Ltd.System, method, and computer program product for structured waveguide including polarizer regionUS7254287Feb 11, 2005Aug 7, 2007Panorama Labs, Pty Ltd.Apparatus, method, and computer program product for transverse waveguided display systemCN100334484C *Aug 12, 2003Aug 29, 2007Tdk株式会社Magneto-optic optical partsWO2002091069A1 *Apr 30, 2002Nov 14, 2002Fujitsu LtdFaraday rotator* Cited by examinerClassifications U.S. Classification359/283, 359/251International ClassificationG02F1/09, G02F1/31Cooperative ClassificationG02F1/09, G02F1/093European ClassificationG02F1/09F, G02F1/09Legal EventsDateCodeEventDescriptionJan 18, 2011FPExpired due to failure to pay maintenance feeEffective date: 20101201Dec 1, 2010LAPSLapse for failure to pay maintenance feesJul 5, 2010REMIMaintenance fee reminder mailedMay 5, 2006FPAYFee paymentYear of fee payment: 8May 9, 2002FPAYFee paymentYear of fee payment: 4Feb 20, 1997ASAssignmentOwner name: FUJITSU LIMITED, JAPANFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:FUKUSHIMA, NOBUHIRO;REEL/FRAME:008493/0403Effective date: 19970205RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google