.
`
`United States Patent [191
`Masuda et al.
`
`|I|l||llllllllIllllllllllllllllllllllllllllllll|||||lllllllllllllllllllllll
`
`5,315,431
`[11] Patent Number:
`[45] Date of Patent: May 24, 1994
`
`S0053l5431A
`
`[54] OPTICAL ISOLATORv
`[75] Inventors: Akihiro Masuda; Ikuo Maeda;
`Hideaki Yuri; Yoiehi Suzuki, all of
`Shizuoka, Japan
`
`4,909,612 3/1990 Scerbak et a1. ................... .. 359/484
`FOREIGN PATENT DOCUMENTS
`
`61-58809 12 1986 J
`/
`‘pan
`
`.
`
`.
`
`‘
`
`..
`
`.
`
`[73] Asslgnec' ig?ikgztrimmw Co" Ltd"
`
`Primary Examiner-—Martin Lerner
`
`Attorney, Agent, or Finn-Keck, Mahin & Cate
`
`ABSTRACT
`[57]
`A non-reciprocal unit and a parallel-surfaced ?at bire
`fringent plate are provided between incident side and
`outgoing side ?ber collimators. The non~reciproca1 unit
`is formed by disposing two tapered birefrigent plates,
`°“e 9" “Ech ‘Side °f ‘1 45' Faraday ‘0mm consisting of
`'‘ °Y1mdnca1 PHmanent "188m and *1 masnetwptical
`element housed in the magnet. The optical axes of the
`parallel-surfaced ?at 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.
`
`[21] Appl. No.: 997,745
`[22] Filed:
`Dec_ 30, 1992
`
`Fords" Applic'?on Prim“? D‘ta
`[30]
`Apr. 20, 1992 [JP]
`Japan ................................ .. 4-126763
`[51] Int Cl 5 ......................... .. 0023 s/w GOZF 1/09
`[52] us‘. ci. .................................. .. 359/281; 359/484;
`359/495; 372/703
`[58] Field of Search ............. .. 359/281, 282, 283, 484,
`359/494’ 495, 496, 497; 372/703
`
`[56]
`
`References Cited
`US. PATENT DOCUMENTS
`
`4,548,478 10/1985 Shirasaki _
`4,852,962 8/1989 Nicia ................................. .. 359/484
`
`5 Claims, 1 Drawing Sheet
`
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`
`Cisco Systems, Inc.
`Exhibit 1043, Page 1
`
`

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`5,315,431
`
`FIG.
`
`I
`
`‘I2 36 32 50 38
`
`22
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`28
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`VIII!‘ '%W ZZJIJ
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`
`Cisco Systems, Inc.
`Exhibit 1043, Page 2
`
`

`
`1
`
`OPTICAL ISOLATOR
`
`5,315,431
`
`2
`rays, by the ?rst tapered birefringent plate. Therefore,
`the separated rays which have passed through the ?rst
`tapered birefringent plate spread and do not become
`parallel rays, so that none of these rays enters the inci~
`dent side ?ber 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 ?ber 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 dif?cult.
`
`SUMMARY OF THE INVENTION
`An object of the present invention is to provide an
`improved polarization-independent optical isolator.
`Another object of the present invention is to provide
`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 ?ber collimators are positioned on both sides
`of the non-reciprocal unit. In the present invention, a par
`allel-surfaced ?at birefringent plate is insserted between
`the non-reciprocal unit and one of the ?ber collimators
`in such an optical isolator and the optical axes of the
`parallel-surfaced ?at 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-surfaced 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
`
`20
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`25
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`30
`
`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 ?ber 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 ?ber 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 re?ected return light
`sent out from an optical part in an optical ?ber commu
`nication system.
`_
`An optical ?ber 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 ampli?ed optical signal by
`35
`utilizing a plurality of optical ?ber ampli?ers 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 ?ber collimators on both sides of a non
`reciprocal unit and does not depend on a plane of polar
`ization as disclosed in Japanese Patent Publication No.
`61-58809/1986 and corresponding US. 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 ?ber collimator are separated into
`ordinary rays and extraordinary rays by a ?rst tapered
`birefringent plate in a non-reciprocal unit, and a plane of
`polarization is turned in a 45-degree are by a Faraday
`rotator, these two kinds of rays being turned into paral
`lel rays by a second tapered birefringent plate, which
`parallel rays therefore enter an outgoing side ?ber colli
`mator. In the opposite direction, the re?ected return
`light is also separated into ordinary rays and extraordi
`nary rays by the second tapered birefringent plate, and
`a plane of polarization 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
`
`40
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`45
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`55
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`60
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`65
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`Cisco Systems, Inc.
`Exhibit 1043, Page 3
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`

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`10
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`5,315,431
`3
`4
`perpendicular plane with respect to the rays of incident
`faced birefringent plate may be provided on either side
`light.
`of the non-reciprocal unit.
`The parallel-surfaced ?at birefringent plate 40 is
`When the light enters the non-reciprocal unit in the
`formed, for example, of monocrystalline rutile, and is
`forward direction, it is separated into ordinary rays and
`cut so that the optical axis thereof is in a perpendicular
`extraordinary rays, which pass through the two tapered
`plane with respect to the rays of incident light. The
`birefringent plates. Since the refractive indexes of the
`thickness of the birefringent plate 40 is substantially
`ordinary rays and extraordinary rays in a tapered bire
`equal to the sum of those of the optical path passing
`fringent plate are different, a difference in velocity oc
`portions of the two tapered birefringent plates 36, 38.
`curs between the respective outgoing rays. When a
`In FIG. 2 in which the relation between the optical
`parallel-surfaced birefringent plated. the optical axis of
`axes of the two tapered birefringent plates 36, 38 and
`which is staggered at 90° from the adjacent tapered
`parallel-surfaced ?at birefringent plate 40 is shown, the
`birefringent plate, is provided on the outgoing side of
`birefringent plates 36, 38 are arranged so that the thick
`the non-reciprocal unit, the ordinary rays are converted
`walled portions and thin-walled portions thereof are
`into extraordinary rays, and the extraordinary rays are
`opposed to each other, and the optical axes of the two
`converted into ordinary rays by this birefringent plate.
`tapered birefringent plates 36, 38 are staggered from
`The resultant rays pass through the parallel-surfaced
`each other at 45°. The optical axes of the parallel-sur
`?at birefringent plate. Therefore, the difference in ve
`faced flat birefringent plate 40 and tapered birefringent
`locity is eliminated, and non-polarization-mode-dis
`plate 38 are staggered from each other at 90°. Referring
`persed ordinary and extraordinary rays are sent out in
`to FIG. 2 in which the incident side and outgoing side
`parallel with each other. When the parallel-surfaced ?at
`tapered birefringent plates 36, 38, respectively, taken in -
`birefringent plate is provided on the incident side of the
`the forward direction (direction of a broken arrow) are
`non-reciprocal unit, the incident light is separated into
`shown, the optical axis of the incident side tapered
`ordinary rays and extraordinary rays, and a difference
`birefringent plate 36 is inclined clockwise at 22.5° with
`in velocity of light occurs, which is eliminated in the
`respect to the Y-axis, while the optical axis of the outgo
`non-reciprocal unit. After all, non-polarization-mode
`ing side tapered birefringent plate 38 is inclined counter
`dispersed ordinary and extraordinary rays are sent out
`clockwise at 22.5° with respect to the Y-axis. Therefore,
`in parallel with each other. If the thickness of the para]
`these two optical axes are staggered from each other at
`lel-surfaced ?at birefringent plate is set substantially
`45°. The optical axis of the parallel-surfaced ?at bire
`equal to the sum of the thickness of the optical path
`fringent plate 40 is inclined counter-clockwise at 22.5"
`passing portions of the two tapered birefringent plates
`with respect to the X-axis, so that it is staggered at 90'
`in the non-reciprocal unit, the polarization mode disper
`from the optical axis of the outgoing side tapered bire
`sion can be eliminated substantially completely.
`fringent plate 38.
`The procedure of assembling the optical isolator is as
`follows. First, the incident side and outgoing side ?ber
`collimators 10, 20 and the non-reciprocal unit 30 are
`prepared. The parallel-surfaced ?at 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 ?ber 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 ?ber
`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 ?at birefringent plate 40. These rays
`are sent out as they are left in a parallel-extending state,
`and condensed on the optical ?ber 24 through the
`spherical lens 22. The re?ected 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
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a sectional view of a non-polarization-rnode
`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 ?at birefringent plate.
`FIG. 3 is a diagram explaining the occurrence and
`extinction of polarization mode dispersion.
`PREFERRED EMBODIMENTS OF THE
`INVENTION
`Referring to FIG. 1, a non-polarization-mode-dis
`persing optical isolator is formed by disposing a non
`reciprocal unit 30 and a parallel-surfaced ?at birefrin
`gent plate 40 between an incident side ?ber collimator
`10 and an outgoing side ?ber collimator 20, and housing
`all of these parts in a cylindrical casing 50 and setting
`them ?rmly therein. In this embodiment, the parallel
`surfaced ?at birefringent plate 40 is provided on the
`outgoing side of the non- reciprocal unit 30. The inci
`dent side and outgoing side ?ber collimators 10, 20 are
`substantially identical, and formed by holding spherical
`lenses 12, 22, and ferrules 16, 26 to which optical ?bers
`14, 24 are connected in metallic sleeves 18, 28.
`The non-reciprocal unit 30 is formed by providing
`two tapered birefringent plates 36, 38, one on each side
`of a Faraday rotator in which a magneto-optical ele
`' ment 34 is housed in a cylindrical permanent magnet 32.
`The magneto-optical element 34 in this embodiment is
`formed, for example, of monocrystalline yttrium iron
`garnet. The tapered birefringent plates 36, 38 are
`formed, for example, of monocrystalline rutile, and are
`cut incliningly so that the optical axes thereof are in a
`
`40
`
`45
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`65
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`Cisco Systems, Inc.
`Exhibit 1043, Page 4
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`

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`5,315,431
`5
`6
`same birefringent plate 38. The plane of polarization is
`?ber collimator with the optical axis of the ?at birefrin
`then turned at 45° by the Faraday rotator in the direc
`gent plate staggered at 90° from that of the adjacent
`tion opposite to the direction, i.e. forward direction in
`tapered birefringent plate as described above. There
`which the plane of polarization is turned in the previ
`fore, the ordinary rays and extraordinary rays are con
`ously-described case. In the tapered birefringent plate
`verted into extraordinary rays and ordinary rays, re
`36, the ordinary rays are refracted into extraordinary
`spectively, in these tapered birefringent plates and par
`rays, and the extraordinary rays into ordinary rays.
`allel-surfaced flat birefringent plate, and a difference in
`Consequently, the outgoing rays do not become parallel
`velocity between the separated rays is eliminated,
`but spread, so that the spherical lens 12 cannot condense
`whereby the polarization mode dispersion can be mini
`these rays on the optical ?ber 14. Namely, the re?ected
`mized. Namely, a conventional polarization-independ
`return light is shut off. Thus, an isolation operation
`ent optical isolator in which polarization mode disper
`similar to a conventional isolation operation is carried
`sion generally occurs can be formed into a non-polariza
`tion-mode-dispersing optical isolator. This can mini
`out.
`mize the collapse and disorder of optical signals passing
`FIG. 3 shows the occurrence and extinction of polar
`ization mode dispersion in the forward direction. As
`through an optical isolator, and attain the accurate
`described above, the incident light is separated by the
`transmission of information in the long distance, and
`tapered birefringent plate 36 into ordinary rays (0) and
`high-speed optical communication (for example, not
`extraordinary rays (e), which then pass through the
`lower than 10 Gbit/s).
`same birefringent plate 36, and the ordinary rays and
`What is claimed is:
`extraordinary rays pass the subsequent tapered birefrin
`1. A non-polarization-mode-dispersing optical isola
`gent plate 38 as they remain to be ordinary rays and
`tor comprising:
`extraordinary rays respectively. Since the refractive
`a cylindrical casing,
`indexes of the ordinary rays and extraordinary rays are
`a non~reciprocal unit, in said casing, having a 45°
`different in the birefringent plates, a difference in veloc
`Faraday rotator and two tapered birefringent
`ity of light occurs. When the birefringent plates are
`plates on opposite sides of said Faraday rotator,
`made of monocrystalline rutile, the refractive index no
`?ber collimators on opposite sides of said non-recip
`of ordinary rays and the refractive index neof extraordi
`rocal unit,
`nary rays are 2.453 and 2.709, respectively, so that the
`said ?ber collimators including a ?rst ?ber collima
`ordinary rays advance more speedily than the extraordi
`tor connected to a ?rst optical ?ber and a second
`nary rays. If the thickness of the optical path passing
`?ber collimator connected to a second optical
`portions of the tapered birefringent plates 36, 38 is about
`?ber, and
`0.5 mm, a difference 8 in optical path between the ordi
`a parallel~surfaced ?at birefringent plate between said
`nary rays and extraordinary rays at the time of leaving
`non-reciprocal unit and one of said ?ber collima~
`the non-reciprocal unit 30 becomes around 0.85 pico
`tors,
`wherein said parallel-surfaced flat birefringent plate
`seconds in terms of time difference. In the parallel-sur
`faced birefringent plate 40, the velocity of the ordinary
`is ?xed within said casing with an optical axis of
`said parallel-surfaced flat birefringent plate is stag
`rays and extraordinary rays reverses, i.e., the delayed
`rays out of the separated rays advance at an increased
`gered at 90° from an axis of an adjacent one of said
`tapered birefringent plate.
`speed, while the preceding rays advance at a decreased
`speed, so that the velocity difference between these rays
`2. A non-polarization-mode-dispersing optical isola
`becomes small. If the thickness of the parallel-surfaced
`tor according to claim 1, wherein said tapered birefrin
`gent plates and said parallel~surfaced birefringent plates
`flat birefringent plate 40 is set to about 1 mm (twice as
`large as the thickness of the tapered birefringent plate),
`are made of monocrystalline rutile, and a thickness of
`said parallel-surfaced birefringent plate is substantially
`the velocity difference is eliminated so that the differ
`ence (8) is nearly equal to zero (8:0), and polarization
`equal to a sum of thickness of optical path passing por
`mode dispersion disappears. Namely, the disorder of the
`tions of said tapered birefringent plates.
`waveforms of optical signals at the time of passage
`3. A non-polarization-mode-dispersing optical isola
`through the optical isolator becomes minimal.
`tor according to claim 2, wherein said tapered birefrin
`In this embodiment, the parallel-surfaced ?at birefrin
`gent plates are inclined so that optical axes of said ta
`gent plate 40 is provided on the outgoing side of the
`pered birefringent plates are positioned in a perpendicu
`non-reciprocal unit 30 in the forward direction, and it
`lar plane with respect to an incident light.
`4. A non-polarization-mode-dispersing optical isola
`can be provided on the incident side as well of the non
`reciprocal unit 30 in the forward direction. In such a
`tor according to claim 1, wherein said ?ber collimators
`case, the parallel-surfaced ?at birefringent plate 40 is
`includes an incident ?ber collimator and an outgoing
`?xed in position, with the optical axis thereof staggered
`?ber collimator, each of said ?ber collimators having a
`at 90° from that of the adjacent (i.e. the incident side)
`metal sleeve, a spherical lens and a ferrule in said metal
`tapered birefringent plate. According to the present
`sleeve.
`invention, the shapes of various optical parts and the
`5. A non-polarization-mode-dispersing optical isola
`construction of the casing in which these parts are held
`tor according to claim 1, wherein said Faraday rotator
`may be suitably modi?ed.
`has a cylindrical permanent magnet and a magneto-opti
`According to the present invention, a parallel-sur
`cal element made of monocrystalline yttrium iron gar
`faced flat birefringent plate is inserted between a non
`116i.
`reciprocal unit using tapered birefringent plates and one
`
`ttttt
`
`50
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`55
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`65
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`20
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`25
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`40
`
`45
`
`Cisco Systems, Inc.
`Exhibit 1043, Page 5