|I|||||l||||l|Ill||I||||||ll|||||l||||||||l||l||||l|ll|||lllllllllllllillll
`USC05315431A
`5,315,431
`[11] Patent Number:
`[-15] Date of Patent: May 24, 1994
`
` .
`
`United States Patent
`Masada et a].
`
`[19]
`
`[73] Assignee:
`
`[54] OPTICAL ISOLATORI
`[15]
`Inventors: Akihiro Masada; Ikuo Meeda;
`Hiduki Yuri; Yaielti Suzuki. all of
`Shizuoka, Japan
`Fuji Electrochemical 00., Ltd,
`Tokyo. Japan
`[21] Appl. No: 997,745
`[221 Filed:
`Dec. 30. 1992
`[30]
`Foreign Application Priority Data
`Apr. 20, 1992 [JP]
`Japan
`
`4.125163
`
`Int. CU ...................
`[51]
`
`[52] US. Cl.
`
`[53] Field of Search
`
`. 6023 5/30; GOZF 1/09
`.......... 359/281; 359/484;
`359/495; 372/103
`359/281. 282. 233. 484.
`359/494. 495, 496, 49?; 372/703
`
`4,909,512 mean 5mm etal.
`FOREIGN PATENr DOCUMENTS
`tit-58809 1mm 1.1m.
`
`same:
`
`Primacy Examiner—Manet Lerner
`Attorney. Agent, or Firm—Keck, Mahin 5!. Cate
`
`[5T]
`
`ABSTRACI
`
`A non-reciprocal unit and a parallel-surfaced flat bire-
`fringent plate are provided between incident side and
`outgoing side fiber collianators. The non-reciprocal unit
`is formed by disposing two tapered birefrigent piates.
`one on each side of a 45' Faraday.r rotator consisting of
`a cylindrical permanent magnet and a magneto-optical
`element housed in the magnet. The optical axes of the
`parallel-surfaced fiat birefringent plate and adjacent
`tapered birefringent plate are staggered from each other
`by 90°. The thickness of the parallel-surfaces birefrin-
`gent plate is set equal to the sum of the thicknesses of
`the two tapered birefringent plates.
`
`{56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`4.548.473 102’1985 Shirasaki
`.
`4,852,961
`3/1989 Nicia
`
`359K484
`
`5 Claims, 1 Drawing Sheet
`
`I2 36 32 5O 33
`
`22
`
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`
`20
`
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`
`30 34
`
`4O
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1043, Page 1
`Exhibit 1043, Page 1
`
`

`

`US. Patent
`
`May 24, 1994
`
`5,315,431
`
`FIG.
`
`I
`
`12 35 32 50 38
`
`22
`
`
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`
`‘III'
`
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`
`
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`
`
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1043, Page 2
`Exhibit 1043, Page 2
`
`

`

`1
`
`OPTICAL ISOLATOR
`
`5,315,431
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to an optical device
`such as an optical isolator or an optical modulator, and
`more particularly to improvement in a polarization-
`independent optical isolator provided with fiber colli-
`mators on both sides of a non-reciprocal unit using
`tapered (wedge-type) birefringent plates.
`The present invention provides a non-polarization-
`mode-dispersing optical isolator adapted to eliminate
`the difference in velocity between the ordinary ray and
`extraordinary ray occurring on tapered birefringent
`plates. by inserting a parallel-surfaced birefringent plate
`between a non-reciprocal unit and one fiber collimator
`in a polarization-independent isolator. This optical iso-
`lator is useful for, especially, the long distance, high
`speed communication.
`As generally known, an Optical isolator is a non-
`reciprocal optical device having a function of allowing
`the passage of light in one direction and preventing the
`passage thereof in the opposite direction, and used in
`large quantities for shutting off the reflected return light
`sent out from an optical part in an optical fiber commu-
`nication system.
`_
`An optical fiber communication system used practi-
`cally at present employs a system for sending out infor-
`mation from an output side with the intensity of light
`modulated. and detecting at a reception side the inten-
`sity of an optical signal by a direct
`light detecting
`method and then demodulating the information. Re-
`garding the long distance communication using.
`for
`example, a submarine cable. an attempt has been made
`for transmitting a directly amplified optical signal by
`utilizing a plurality of optical fiber amplifiers incorpo-
`rated in the intermediate portions of an optical cable.
`When such a method is employed, a number of optical
`isolators are required.
`Various types of optical isolators have been devel-
`oped. The optical isolators suitably incorporated in the
`above-mentioned optical cable include an optical isola-
`tor of the type which is provided with incident side and
`outgoing side fiber collimators on both sides of a non-
`reciprocal unit and does not depend (in a plane of polar-
`ization as disclosed in Japanese Patent Publication No.
`61-53809/1986 and corresponding U.S. Pat. No.
`14,548,478. The non-reciprocal unit referred to above
`has a unitary combination of a 45' Faraday rotator. and
`two tapered birefringent plates sandwiching the rotator
`therebetween. These tapered birefringent plates are
`arranged with the optical axes thereof staggered from
`each other at 45'.
`In the forward direction. the parallel input rays from
`the incident side fiber collimator are separated into
`ordinary rays and extraordinary rays by a first tapered
`birefringent plate in a non-reciprocal unit, and a plane of
`polarization is turned in a 4S-degree arc by a Faraday
`rotator. these two Itinds of rays being turned into paral-
`lel rays by a second tapered birefringent plate. which
`parallel rays therefore enter an outgoing side fiber colli-
`mator. In the opposite direction, the reflected return
`light is also separated into ordinary rays and extraordi-
`nary rays by the second tapered birefringent plate. and
`a plane ofpolarization is turned in a —45‘ are due to the
`non-reciprocity of the Faraday rotator. Out of the sepa-
`rated rays. the ordinary rays are converted into extraor-
`dinary rays. and the extraordinary rays into ordinary
`
`5
`
`ID
`
`I5
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`20
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`25
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`3D
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`35
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`‘0
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`45
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`50
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`60
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`65
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`2
`rays, by the first tapered birefringent plate. Therefore,
`the separated rays which have passed through the first
`tapered birefringent plate spread and do not become
`parallel rays. so that none of these rays enters the inci-
`dent side fiber collimator. Thus, the passage of light in
`one (forward) direction is allowed but that of light in
`the Opposite direction is prevented.
`In the transmission of an optical signal in the forward
`direction by the optical isolator described above. the
`incident light is separated into ordinary rays and ex-
`traordinary rays in the non-reciprocal unit. Since the
`velocity of the respective rays is different, a transfer lag
`is about 0.85 picoseconds in terms of time difference
`when tapered birefringent plates consisting. for exam-
`ple, of monocrystalline rutile and having a thickness of
`their optical path passing portions of about 0.5 mm are
`used. In the outgoing side fiber collimator. the ordinary
`rays and extraordinary rays are synthesized. and. there-
`fore, disorder (increase in the width of optical pulse}
`corresponding to the transfer lag occurs in the wave-
`form of an optical signal.
`The transfer lag of an optical signal caused by the
`difference in velocity (polarization mode dispersion)
`between the ordinary rays and extraordinary rays is
`relatively small in a single optical isolator. In fact, this
`transfer lag is negligible and it poses little problem at an
`optical communication speed of around 2.5 Gbit/s cur-
`rently employed. However, since a great number of
`(about 50-150} optical isolators are incorporated in a
`long distance optical communication (optical communi-
`cation using, for example. a submarine cable) system as
`described above, the turbulence of the waveforms of
`optical signals becomes large, so that, when such an
`optical system is operated at a high Speed of for exam-
`ple, 10 Gbit/s, information transmission normally be-
`comes difficult.
`
`SUMMARY OF THE INVENTION
`
`An object of the present invention is to previde an
`improved polarization-independent optical isolator.
`Another object of the present invention is to pr0vide
`a polarization-independent optical. isolator free from
`the drawbacks encountered in a conventional device
`and capable of minimizing polarization mode dispersion
`and the collapse and disorder of optical signals, and
`thereby attaining the accurate transmission of informa-
`tion in long distance, high-speed communication.
`The present invention is directed to improvement in a
`polarization-independent optical isolator having a non-
`reciprocal unit in which wedge type or tapered birefrin-
`gent plates are provided on both sides of a 45' Faraday
`rotator. and fiber collimators are positioned on both sides
`of the non-reciprocal unit. In the present invention, a pa r-
`allel-surfaced flat birefringent plate is insserted between
`the non-reciprocal unit and one of the fiber collimators
`in such an optical isolator and the optical axes of the
`parallel-surfaced flat birefringent plate and adjacent
`tapered birefringent plate are staggered from each other
`at 90' to form a non-polarization-mode-dispersed opti-
`cal isolator.
`The materials for the tapered birefringent plates and
`the parallel-surfaced birefringent plate include. for ex-
`ample, monocrystalline rutile. The thickness of the par-
`allel~surfaoed birefringent plate is preferably set in such
`a manner that it becomes substantially equal to the sum
`of the thickness of the optical path passing portions of
`the two tapered birefringent plates. The parallel-sur-
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1043, Page 3
`Exhibit 1043, Page 3
`
`

`

`5.315.431
`
`3
`faced birefringent plate may be provided on either side
`of the non-reciprocal unit.
`When the light enters the non-reciprocal unit in the
`forward direction. it is separated into ordinary rays and
`extraordinary rays, which pass through the two tapered
`birefringent plates. Since the refractive indexes of the
`ordinary rays and extraordinary rays in a tapered bire-
`fringent plate are different. a difference in velocity oc-
`curs between the respective outgoing rays. When a
`parallel-surfaced birefringent plated the optical axis of
`which is staggered at 90' from the adjacent tapered
`birefringent plate, is provided on the outgoing side of
`the non-reciprocal unit. the ordinary rays are converted
`into extraordinary rays. and the extraordinary rays are
`converted into ordinary rays by this birefringent plate.
`The resultant rays pass through the parallel-surfaced
`flat birefringent plate. Therefore, the difference in ve-
`locity is eliminated, and non-polarization-mode-dis-
`pcrsed ordinary and extraordinary rays are sent out in
`parallel with each other. When the parallel-surfaced flat
`birefringent plate is provided on the incident side of the
`non-reciprocal unit. the incident light is separated into
`ordinary rays and extraordinary rays. and a difference
`in velocity of light occurs. which is eliminated in the
`non-reciprocal unit. After all. non-polarization-mode-
`dispersed ordinary and extraordinary rays are sent out
`in parallel with each other. If the thickness of the para]-
`lel-surfaced flat birefringent plate is set substantially
`equal to the sum of the thickness of the optical path
`passing portions of the two tapered birefringent plates
`in the non-reciprocal unit, the polarization mode disper-
`sion can be eliminated substantially completely.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a sectional view of a non-polarization-mode-
`dispersing optical isolator embodying the present inven-
`tion.
`FIG. 2 is a diagram showing a relation between opti-
`cal axes of two tapered birefringent plates and that of a
`parallel-surfaced flat birefringent plate.
`FIG. 3 is a diagram explaining the occurrence and
`extinction of polarization mode diapersion.
`PREFERRED EMBODIMENTS OF THE
`INVENTION
`
`Referring to FIG. 1. a non-polarization-mode-dis-
`parsing optical isolator is formed by disposing a non-
`reciprocal unit 30 and a parallel-surfaced flat birefrin-
`gent plate 40 between an incident side fiber collimator
`10 and an outgoing side fiber collimator 20. and housing
`all of these parts in a cylindrical casing 50 and setting
`them fmnly therein. In this embodiment, the parallel-
`surfaced flat birefringent plate ‘0 is provided on the
`outgoing side of the non- reciprocal unit 30. The inci-
`dent side and outgoing side fiber collimators 10, 20 are
`substantially identical. and formed by holding spherical
`lenses 12, 22, and ferrules 16, 26 to which optical fibers
`14. 24 are connected in metallic sleeves 18. 28.
`The non-reciprocal unit 3|] is formed by providing
`two tapered birefringent plates 36, 38, one on each side
`of a Faraday rotator in which a magneto-optical ele-
`men: 34 is housed in a cylindrical permanent magnet 32.
`The magnetouoptical element 34 in this embodiment is
`formed. for example, of monocrystalline yttrium iron
`garnet. The tapered birefringent plates 3-6, 38 are
`formed. for example, of monocrystalline rutile, and are
`cut incliningly so that the optical axes thereof are in a
`
`4
`perpendicular plane with respect to the rays of incident
`light.
`The parallel-surfaced flat birefringent plate 40 is
`formed, for example. of monocrystalline rutile. and is
`cut so that the optical axis thereof is in a perpendicular
`plane with respect to the rays of incident light. The
`thickness of the birefringent plate 40 is substantially
`equal to the sum of those of the optical path passing
`portions of the two tapered birefringent plates 36. 33.
`In FIG. 2 in which the relation between the optical
`axes of the two tapered birefringent plates 36. 38 and
`parallel-surfaced flat birefringent plate 40 is shown. the
`birefringent plates 36, 38 are arranged so that the thick-
`walled portions and thin-walled portions thereof are
`opposed to each other. and the optical axes of the two
`tapered birefringent plates 36. 38 are staggered from
`each other at 45°. The Optical axes of the parallel-sur-
`faced flat birefringent plate 40 and tapered birefringent
`plate 38 are staggered from each other at 90’. Referring
`to FIG. 2 in which the incident side and outgoing side
`tapered birefringent plates 3-6, 38. respectively, taken in .
`the forward direction (direction of a broken arrow) are
`shown.
`the optical axis of the incident side tapered
`birefringent plate 36 is inclined clockwise at 22.5' with
`respect to the Y-axis. while the optical axis of the outgo-
`ing side tapered birefringent plate 38 is inclined counter-
`clockwise at 22.5“ with respect to the Y-axis. Therefore,
`these two optical axes are staggered from each other at
`45". The optical axis of the parallel-surfaced flat bire-
`fringent plate 40 is inclined counter-clockwise at 22.5'
`with respect to the X-axis. so that it is staggered at 90'
`from the Optical axis of the outgoing side tapered bire-
`fringent plate 38.
`The procedure of assembling the optical isolator is as
`follows. First. the incident side and outgoing side fiber
`collimators 10. 20 and the non-reciprocal unit 30 are
`prepared. The parallel-surfaced flat birefringent plate
`40 is then regulated so that the Optical axis thereof is
`staggered at 90'. This regulating operation can be car-
`ried out easily by passing polarized light through the
`non-reciprocal unit. The two fiber collimators are then
`installed, and the optical paths are regulated.
`The operation of this optical
`isolator will be ex-
`plained. Referring to FIGS. 1, 2 and 3 of the drawing.
`the forward direction is the direction extending from
`the left-hand portion of the drawing to the right-hand
`portion thereof. The incident light from the optical fiber
`14 passes through the spherical lens 12 and enters the
`non-reciprocal unit 30. The incident light is separated
`into ordinary rays and extraordinary rays by the tapered
`birefringent plate 36. and the plane of polarization is
`turned at 45' by the Faraday rotator. the ordinary and
`extraordinary rays being turned into parallel rays by the
`tapered birefringent plate 38. Since the optical axis of
`the parallel-surfaced birefringent plate 40 is staggered at
`90' from that of the tapered birefringent plate 38. the
`ordinary rays are converted into extraordinary rays.
`and the extraordinary rays into ordinary rays. in the
`parallel-surfaced flat birefringent plate 4-0. These rays
`are sent out as they are left in a parallel-extending state,
`and condensed on the optical fiber 24 through the
`spherical lens 22. The reflected return light advancing
`in the opposite direction is separated into ordinary rays
`and extraordinary rays by the parallel-surfaced flat
`birefringent plate 40. When these rays enter the tapered
`birefringent plate 38. the ordinary rays are converted
`into extraordinary rays, and the extraordinary rays into
`ordinary rays, and the resultant rays pass through the
`
`lo
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`15
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`25
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`30
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`35
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`45
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`55
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`60
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`65
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1043, Page 4
`Exhibit 1043, Page 4
`
`

`

`5,315,431
`
`5
`same birefringent plate 38. The plane of polarization is
`then turned at 45' by the Faraday rotator in the direc-
`tion opposite to the direction, i.e. forward direction in
`which the plane of polarization is turned in the previ-
`ously-described case. In the tapered birefringent plate
`36, the ordinary rays are refracted into extraordinary
`rays, and the extraordinary rays into ordinary rays.
`Consequently, the outgoing rays do not become parallel
`but spread, so that the spherical lens 12 cannot condense
`these rays on the optical fiber 14. Namely, the reflected
`return light is shut off. Thus, an isolation operation
`similar to a conventional isolation operation is carried
`out.
`FIG. 3 shows the occurrence and extinction of polar-
`ization mode dispersion in the forward direction. As
`described above. the incident light is separated by the
`tapered birefringent plate 36 into ordinary rays (0} and
`extraordinary rays (e), which then pass through the
`same birefringent plate 36, and the ordinary rays and
`extraordinary rays pass the subsequent tapered birefrin-
`gent plate 38 as theyI remain to be ordinary rays and
`extraordinary rays respectively. Since the refractive
`indexes of the ordinary rays and extraordinary rays are
`different in the birefringent plates. a difference in veloc‘
`ity of light occurs. When the birefringent plates are
`made of monocrystalline rutile, the refractive index no
`of ordinary rays and the refractive index n.0f extraordi-
`nary rays are 2.453 and 2.709, respectively, so that the
`ordinary rays advance more speedily than the extraordi-
`nary rays. If the thickness of the optical path passing
`portions of the tapered birefringent plates 36, 3-8 is about
`0.5 mm, a difference 5 in optical path between the ordi-
`nary rays and extraordinary rays at the time of leaving
`the non-reciprocal unit 30 becomes around 0.85 pico-
`seconds in terms of time difference. In the parallel-sur-
`faced birefringent plate 40. the velocity of the ordinary
`rays and extraordinary rays reverses, i.e., the delayed
`rays out of the separated rays advance at an increased
`speed, while the preceding rays advance at a decreased
`speed. so that the velocity difference between these rays
`becomes small. If the thickness of the parallel-surfaced
`flat birefringent plate 4-0 is set to about 1 mm (twice as
`large as the thickness of the tapered birefringent plate},
`the velocity difference is eliminated so that the differ-
`ence (6) is nearly equal to zero (5:0), and polarization
`mode dispersion disappears. Namely. the disorder ofthe
`waveforms of optical signals at
`the time of passage
`through the optical isolator becomes minimal.
`In this embodiment, the parallel-surfaced flat birefrin-
`gent plate ‘0 is provided on the outgoing side of the
`non-reciprocal unit 30 in the forward direction, and it
`can be provided on the incident side as well of the non-
`reciprocal unit 30 in the forward direction. In such a
`case, the parallel-surfaced flat birefringent plate 40 is
`fixed in position, with the optical axis thereof staggered
`at 90" from that of the adjacent (Le. the incident side)
`tapered birefringent plate. According to the present
`invention, the shapes of various optical parts and the
`construction of the casing in which these parts are held
`may be suitably modified.
`According to the present invention, a parallel—sur—
`faced flat birefringent plate is inserted between a non-
`reciprocal unit using tapered birefringent plates and one
`
`10
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`15
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`20
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`25
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`fiber collimator with the optical axis of the flat birefrin-
`gent plate staggered at 90° from that of the adjacent
`tapered birefringent plate as described above. There»
`fore, the ordinary rays and extraordinary rays are con-
`verted into extraOrdinary rays and ordinary rays, re-
`spectively, in these tapered birefringent plates and par—
`allel-surfaced flat birefringent plate, and a difference in
`velocity between the separated rays is eliminated.
`whereby the polarization mode dispersion can be mini-
`mized. Namely, a conventional polarization-independ-
`ent optical isolator in which polarization mode diSper-
`sion generally occurs can be formed into a non-polariza-
`tion-mode-dispersing optical
`isolator. This can mini-
`mize the collapse and disorder of Optical signals passing
`through an optical
`isolator. and attain the accurate
`transmission of information in the long distance. and
`high-speed optical communication (for example. not
`lower than 10 Gbit/s}.
`What is claimed is:
`1. A non-polanzation-mode-diSpersing optical isola-
`tor comprising:
`a cylindrical casing.
`in said casing. having a 45°
`a non-reciprocal unit,
`Faraday rotator and two tapered birefringent
`plates on opposite sides of said Faraday rotator.
`fiber collimators on Opposite sides of said non-recip-
`rocal unit.
`said fiber collimators including a first fiber collima-
`tor connected to a first optical fiber and a second
`fiber collimator connected to a second optical
`fiber, and
`a parallel-surfaced flat birefringent plate between said
`non-reciprocal unit and one of said fiber collima-
`tors.
`Wherein said parallel-surfaced flat birefringent plate
`is fixed within said casing with an optical axis of
`said parallel-surfaced flat birefringent plate is stag-
`gered at 90' from an axis of an adjacent one of said
`tapered birefringent plate.
`2. A non-polarization-mode-dispersing optical isola-
`tor aecording to claim 1, wherein said tapered birefrin-
`gent plates and said parallel-surfaced birefringent plates
`are made of monocrystalline rutile. and a thickness of
`said parallel-surfaced birefringent plate is substantially
`equal to a sum of thickness of optical path passing por-
`tions of said tapered birefringent plates.
`3. A nompolarization-mode-dispersing optical isola-
`tor according to claim 2, wherein said tapered birefrin-
`gent plates are inclined so that optical axes of said ta-
`pered birefringent plates are positioned in a perpendicu-
`lar plane with respect to an incident light.
`4. A non-polafization-mode-dispersing optical isola-
`tor according to claim 1, wherein said fiber collimators
`includes an incident fiber collimator and an outgoing
`fiber collimator. each of said fiber collimators having a
`metal sleeve, a spherical lens and a ferrule in said metal
`sleeve.
`
`5. A non-polarization-mode-dispersing optical isola-
`tor acoording to claim I, wherein said Faraday rotator
`has a cylindrical permanent magnet and a magneto-opti»
`cal element made of monocrystalline yttrium iron gar-
`net.
`.
`U
`3
`‘
`‘
`
`JDS UNIPHASE CORPORATION
`JDS UNIPHASE CORPORATION
`Exhibit 1043, Page 5
`Exhibit 1043, Page 5
`
`