JP2012220816A - Polarization-independent optical isolator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization-independent optical isolator which includes a pair of wedge birefringent crystal plates and a Faraday rotator disposed between them and has improved assembly accuracy and workability.SOLUTION: The polarization-independent optical isolator includes: a first holder 10 which has an inner body thereof divided into space parts 10a and 10b by a partition 12; and a second holder 9 which has an outer body thereof fitted into the space part 10b and has a space part 9a having one end closed by a bottom part 90 and having the other opened. A wedge birefringent crystal plate 3 is stored in the space part 10a of the first holder 10, and a wedge birefringent crystal plate 5 is stored in the space part 9a of the second holder 9, and a Faraday rotator 4 is fixed between the partition 12 and the bottom part 90. A notched concave portion 95 is provided at an open edge part of the second holder 9. A convex portion of a rotational adjustment external jig is engaged with the concave portion 95, and a rotational operation rod of the jig is caused to act to rotate the second holder 9 relative to the first holder 10, whereby wedge surfaces of the wedge birefringent crystal plate 3 and the wedge birefringent crystal plate 5 are arranged in parallel. Because of work using the rotational operation rod of the jig, fine adjustment at one degree or less is possible.

Description

  The present invention relates to an optical isolator used as a countermeasure against return light of a high-power laser used for optical communication and processing, and in particular, is a polarization-free device that has high assembly accuracy of components and simplifies assembly work. The present invention relates to an improvement of a dependent optical isolator.

  Semiconductor lasers used for optical communications and solid-state lasers used for laser processing, etc., do not oscillate when the light reflected from the optical surface or processing surface outside the laser resonator returns to the laser element. Become stable. If the oscillation becomes unstable, signal noise may occur in the case of optical communication, and the laser element may be destroyed in the case of a processing laser. For this reason, an optical isolator for blocking such reflected return light from returning to the laser element is used.

  By the way, in an optical fiber system, since polarization of light emitted from an optical fiber generally changes randomly, a polarization-independent optical isolator is used.

  This type of polarization-independent optical isolator includes a pair of wedge-shaped birefringent crystal plates, a Faraday rotator arranged between these wedge-shaped birefringent crystal plates and rotating 45-degree polarized light, and a Faraday An optical component called an “optical isolator core” composed of a magnet that applies a magnetic field to the rotor is the main part, and “fiber collimators” composed of collimating lenses and fibers are placed at both ends of the “optical isolator core”. The thing of the structure which is made is known.

  Hereinafter, the operation when light in the forward direction passes through a polarization-independent optical isolator including an “optical isolator core” and a “fiber collimator” will be described with reference to FIG.

  First, the light emitted from the incident side optical fiber 1 and passing through the collimating lens 2 becomes parallel light and passes through the wedge-shaped birefringent crystal plate 3 positioned on the forward direction incident side. At this time, the parallel light is separated into two lights of ordinary light and extraordinary light by the wedge-shaped birefringent crystal plate 3 on the incident side. The two lights are orthogonally polarized by 90 °, and when passing through the Faraday rotator 4, the directions of polarization are rotated by 45 ° and enter the wedge-shaped birefringent crystal plate 5 positioned on the forward emission side. At this time, the direction of the optical axis of the wedge-type birefringent crystal plate 3 on the incident side and the wedge-type birefringent crystal plate 5 on the exit side differ from each other by 45 °. And each light separated as extraordinary light is incident as ordinary light and extraordinary light of the wedge-shaped birefringent crystal plate 5 on the emission side. For this reason, each of ordinary light and extraordinary light behaves as if it is incident on an optical crystal of a parallel plate, and enters the collimating lens 6 on the output side as parallel light at the same time as the incident parallel light. To join.

  On the other hand, as shown in FIG. 2, when light in the reverse direction passes through the polarization-independent optical isolator, the ordinary light and extraordinary light that have passed through the wedge-shaped birefringent crystal plate 5 on the emission side are converted into the Faraday rotator 4. , The polarization direction rotates by 45 ° in the same direction as in the forward direction due to the nonreciprocity of the Faraday rotator 4 and enters the wedge-shaped birefringent crystal plate 3 on the incident side. At this time, since the polarization is rotated by 90 ° in the case of the forward direction, the light transmitted as ordinary light when passing through the wedge-shaped birefringent crystal plate 5 on the emission side is wedge-shaped birefringent crystal plate 3 on the incident side. Then, it behaves as extraordinary light, and extraordinary light behaves as ordinary light.

  Then, the two polarized lights have an angle with the incident parallel light by a finite angle, and after passing through the collimating lens 2, they are not coupled to the optical fiber 1 and function as an isolator.

  By the way, in a polarization-independent optical isolator, in order to obtain a sufficiently high coupling efficiency in the forward direction, the wedge angles of the pair of wedge-shaped birefringent crystal plates are equal, and the wedge surfaces (that is, inclined light transmission surfaces) ) Are arranged in parallel.

  The larger the wedge angle of the wedge-shaped birefringent crystal plate and the greater the focal length of the collimating lens used, the more the coupling efficiency is affected (that is, the wedge angle of the wedge-shaped birefringent crystal plate and the focal length of the collimating lens). When becomes larger, the deterioration of the coupling efficiency becomes larger due to the wedge angle and the deviation from the parallel arrangement of the wedge surfaces).

  Therefore, in order to minimize the influence, it is common to use a wedge-type birefringent crystal plate of the same polishing lot. In addition, a wedge-type birefringent crystal plate whose wedge angle is set as small as possible so that return light does not return to the core of the optical fiber, and a method of shortening the focal length of the collimating lens as much as possible are employed.

For example, when inexpensive LiNbO ₃ is used as the wedge-type birefringent crystal, it is common to use a lens having a wedge angle of 13.5 ° and a focal length of 1.8 mm. YVO ₄ having a wedge angle of 8 ° may be used in order to reduce the height.

JP 2010-237268 A Japanese Patent Laid-Open No. 10-274749

  By the way, when a rare earth iron garnet doped with bismuth is applied as a Faraday rotator to a fiber laser optical isolator having a wavelength of around 1 μ which has been attracting attention in recent years, in order to suppress the generation of heat due to light absorption by the Faraday rotator. Reducing the power density of light incident on the Faraday rotator is an effective means.

  However, when reducing the power density of light incident on the Faraday rotator, the above-described collimating lens with a short focal length cannot be used, and a lens with a long focal length is used.

  Further, in the case of an optical isolator for a fiber laser, operation may be hindered when light in the reverse direction returns to the cladding of the optical fiber positioned on the forward incidence side.

  Therefore, in order not to return the light in the reverse direction not only to the core of the optical fiber but also to the cladding, it is necessary to increase the wedge angle of the wedge-shaped birefringent crystal plate, and the above-described forward direction has a sufficiently high coupling efficiency. There was a problem that it was difficult to assemble and adjust the "optical isolator core".

  Under such a technical background, as a method for easily assembling and adjusting the above-mentioned “optical isolator core”, Patent Document 1 discloses that at least one side surface other than the light transmitting surface of the wedge-shaped birefringent crystal plate is mirror-finished As a result, a wedge-type birefringent crystal plate having a wedge angle paired with high accuracy using the optical surface as a position reference is obtained. If a wedge-type birefringent crystal plate using this method is used, adjustment during assembly becomes easy, but the cost of the wedge-type birefringent crystal plate increases.

  Furthermore, Patent Document 2 adopts a structure in which a wedge-shaped birefringent crystal plate on the incident side and a Faraday rotator are arranged on the front and back with a metal holder interposed therebetween, and a wedge-shaped birefringence on the incident side. Into the metal holder on which the crystal plate and the Faraday rotator are mounted, a case on which the wedge-shaped birefringent crystal plate on the output side is mounted is incorporated, and the metal holder or the case is rotated so that the incident side and the output side are rotated. A method for finely adjusting the positional relationship of each wedge-shaped birefringent crystal plate has been proposed.

  However, the fine adjustment method proposed in Patent Document 2 is adjusted when a short collimating lens with a focal length of 1.8 mm is used and a wedge-type birefringent crystal plate with a small wedge angle is used. Although it is possible to use the collimating lens with a short focal length or the wedge-type birefringent crystal plate with a small wedge angle in the above-mentioned application, the accuracy of adjusting the metal holder by rotating (+ / −0.5 °) is required, and it is practically impossible to adjust the rotation with such accuracy, or it takes a very long time.

  The present invention has been made paying attention to such problems, and the problem is that the coupling efficiency can be easily achieved even when a wedge type birefringent crystal plate having a large wedge angle or a collimating lens having a long focal length is applied. An object of the present invention is to provide a polarization-independent optical isolator having a high level of polarization.

That is, the invention according to claim 1
A pair of wedge-shaped birefringent crystal plates and a Faraday rotator provided on the optical path between the wedge-shaped birefringent crystal plates, and a pair of wedge-shaped birefringent crystal plates In the polarization-independent optical isolator arranged such that the inclined light transmitting surfaces and the non-tilted light transmitting surfaces are parallel to each other,
A first holder in which the inner body is partitioned into a first cylindrical space and a second cylindrical space by a partition plate having an optical path opening in the center, and an outer side in the second cylindrical space of the first holder A main body is slidably fitted, and one side of the inner main body is closed at the bottom surface and the other side is opened to form a cylindrical space portion and a second holder provided with an optical path opening on the bottom surface portion,
One wedge-shaped birefringent crystal plate is accommodated in the first cylindrical space portion of the first holder, with the vicinity of the outer peripheral edge thereof being fixed to the partition plate, and the other wedge-shaped double-crystal plate is accommodated in the cylindrical space portion of the second holder. The refractive crystal plate is housed by fixing the vicinity of the outer peripheral edge thereof to the bottom surface portion, and the Faraday rotator is fixed between the partition plate of the first holder and the bottom surface portion of the second holder, and
Rotation adjustment for rotating and sliding the second holder relative to the first holder on the open edge of the inner body of the second holder so that the inclined light transmitting surfaces of the pair of wedge-shaped birefringent crystal plates are arranged in parallel. A notch recess for engaging the external jig is provided, and the distance from the bottom surface in the cylindrical space portion of the second holder to the notch bottom of the notch recess is accommodated in the cylindrical space portion of the second holder. It is characterized by being set larger than the thickness dimension of the wedge-shaped birefringent crystal plate.

The invention according to claim 2
In the polarization independent optical isolator according to claim 1,
One wedge-shaped birefringent crystal plate is fixed to the partition plate in the first cylindrical space portion of the first holder using an epoxy adhesive, and the other wedge-shaped birefringent crystal plate is fixed to the cylindrical space portion of the second holder. The Faraday rotator is fixed to the bottom surface using an epoxy adhesive, and the Faraday rotator is fixed to at least one of the partition plate of the first holder or the bottom surface of the second holder using an epoxy adhesive. ,
The invention according to claim 3
In a two-stage polarization-independent optical isolator,
The polarization-independent optical isolator according to claim 1 or 2 has a structure in which two stages are stacked.

According to the polarization independent optical isolator according to the present invention,
A notch recess is provided in the open edge of the inner body of the second holder, and the second holder can be rotated and slid by engaging a rotation adjusting external jig with the notch recess. The rotation can be adjusted.

  Accordingly, even when a wedge-type birefringent crystal plate having a large wedge angle or a collimating lens having a long focal length is applied, the inclined light transmitting surfaces of the pair of wedge-type birefringent crystal plates can be simply arranged in parallel. The polarization-independent optical isolator with high coupling efficiency can be easily provided.

  Further, the distance from the bottom surface portion in the cylindrical space portion of the second holder to the notch bottom portion of the notch recess is set larger than the thickness dimension of the wedge-shaped birefringent crystal plate accommodated in the cylindrical space portion of the second holder. Therefore, there is an effect that the wedge-shaped birefringent crystal plate accommodated in the cylindrical space portion of the second holder is not damaged during the rotation adjustment by the rotation adjustment external jig.

Explanatory drawing of operation when forward light passes through a polarization-independent optical isolator. Action | operation explanatory drawing when the light of a reverse direction passes the polarization-independent optical isolator. FIG. 3A is an explanatory view showing an “optical isolator core” constituting the main part of the polarization-independent optical isolator according to the present invention, and FIG. 3B is a schematic diagram of a second holder in the “optical isolator core”. Perspective view. Schematic explanatory perspective view of rotation adjustment work performed by engaging a rotation adjustment external jig with a notch recess of the second holder in the “optical isolator core”.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 3A is an explanatory view showing an “optical isolator core” that constitutes a main part of the polarization-independent optical isolator according to the embodiment of the present invention, and FIG. It is a schematic perspective view of a two holder.

  First, as shown in FIG. 3A, the "optical isolator core" according to this embodiment has an inner body that is separated from the first cylindrical space portion 10a by a partition plate 12 provided with an optical path opening 11 in the center. The metal first holder 10 partitioned into the second cylindrical space portion 10b, the outer body is slidably fitted into the second cylindrical space portion 10b of the first holder 10, and one side of the inner body is the bottom surface A second holder 9 made of metal, which is closed by the portion 90 and opened on the other side to form a cylindrical space portion 9a and an opening 91 for the optical path is provided on the bottom portion 90, and the first holder A cylindrical magnet 8 is fitted to the outer body 10.

  Further, the wedge-shaped birefringent crystal plate 3 on the forward incidence side is accommodated in the first cylindrical space portion 10a of the first holder 10 by fixing the vicinity of the outer peripheral edge to the partition plate 12 with an adhesive. The exit-side wedge-shaped birefringent crystal plate 5 is housed in the cylindrical space portion 9a of the second holder 9 with the vicinity of the outer peripheral edge thereof being fixed to the bottom surface portion 90 via an adhesive, and the first holder The Faraday rotator 4 is fixed to the partition plate 12 of the first holder 10 between the partition plate 12 and the bottom surface portion 90 of the second holder 9 with an adhesive.

  The incident side wedge-type birefringent crystal plate 3 has a non-tilted light transmission surface (a flat surface that is not a wedge surface), the output wedge-type birefringent crystal plate 5 has a non-tilt light transmission surface, and the Faraday rotator 4 has a flat surface. Are arranged in parallel to each other.

  Furthermore, a notch recess 95 is provided at the open edge of the inner body of the second holder 9 as shown in FIGS. 3 (A) and 3 (B). As shown in FIG. The convex portion 100b of the rotation adjustment external jig 100 is engaged, and the rotation holder 100a of the rotation adjustment external jig 100 is operated to rotate the second holder 9 relative to the first holder 10. By this rotation operation, the inclined light transmitting surface (wedge surface) of the incident-side wedge-shaped birefringent crystal plate 3 accommodated in the first cylindrical space portion 10a of the first holder 10 and the second holder 9 The inclined light transmitting surfaces of the exit-side wedge-shaped birefringent crystal plate 5 accommodated in the cylindrical space portion 9a are arranged in parallel. In addition, since the adjustment operation using the rotation operation rod 100a of the rotation adjustment external jig 100 is performed, a fine rotation adjustment within one degree is possible.

  3A and 3B, the distance from the bottom surface portion 90 in the cylindrical space portion 9a of the second holder 9 to the notch bottom portion of the notch recess 95 is such that the second holder 9 Since it is set larger than the thickness dimension of the exit-side wedge-shaped birefringent crystal plate 5 accommodated in the cylindrical space portion 9a, during the adjustment work using the rotation operation rod 100a of the rotation adjustment external jig 100, The exit-side wedge-shaped birefringent crystal plate 5 accommodated in the cylindrical space 9a of the second holder 9 is not damaged.

  Hereinafter, the present invention will be described specifically by way of examples.

The material constituting the metal first holder 10 and the second holder 9 is arbitrary as long as it can be finely processed, but in this embodiment, nickel-plated copper was used. In addition, the incident-side wedge-shaped birefringent crystal plate 3 and the exit-side wedge-shaped birefringent crystal plate 5 have a 2 mm square size, the wedge angle is 8 °, and the thickest part is 0.5 mm. A certain YVO ₄ was applied, and as the Faraday rotator 4, a GLB film with a GGG substrate (both surfaces with air non-reflective coating) manufactured by Granopt Co., Ltd. was used.

  First, an epoxy adhesive is applied in advance to the vicinity of the outer peripheral edge of the incident-side wedge-shaped birefringent crystal plate 3 and the Faraday rotator 4, and the incident-side wedge is applied to the partition plate 12 of the first cylindrical space portion 10 a in the first holder 10. The mold birefringent crystal plate 3 is fixed, and the Faraday rotator 4 is fixed to the partition plate 12 of the second cylindrical space 10b in the first holder 10, and an epoxy adhesive is applied to the outer main body of the first holder 10. In use, the cylindrical magnet 8 is fixed. The light transmitting surfaces of the incident-side wedge-shaped birefringent crystal plate 3 and the Faraday rotator 4 were not covered with an adhesive.

  On the other hand, the emission-side wedge-shaped birefringent crystal plate 5 was fixed to the bottom surface portion 90 of the cylindrical space portion 9a in the second holder 9 using an epoxy adhesive. Note that the adhesive was also applied to the vicinity of the outer peripheral edge of the exit-side wedge-shaped birefringent crystal plate 5 so that the light transmission surface was not covered with the adhesive.

  Then, in the second cylindrical space portion 10b of the first holder 10 in which the incident-side wedge-shaped birefringent crystal plate 3 and the Faraday rotator 4 are accommodated, the output-side wedge-shaped birefringent crystal plate 5 is accommodated. The outer body of the holder 9 is inserted through an uncured epoxy adhesive to assemble the “optical isolator core”, and the structure of the “optical isolator core” before adjustment is changed to the “fiber collimator” (the lens focus of each collimating lens). 4), and the rotation operation rod 100a of the rotation adjustment external jig 100 was acted to adjust the rotation so that the insertion loss was minimized as shown in FIG.

  The “optical isolator core” adjusted in this way was heated with a heater and allowed to stand until the epoxy adhesive solidified, and then this cure was performed in an oven. When the insertion loss was confirmed after fixing, it was confirmed that the insertion loss during adjustment was maintained. The maximum coupling efficiency could be easily obtained by such a method.

  The polarization-independent optical isolator according to the present invention can easily obtain high coupling efficiency even when a wedge-type birefringent crystal plate having a large wedge angle or a collimating lens having a long focal length is applied. It has industrial applicability that can be widely used as an optical isolator in communications and laser processing.

DESCRIPTION OF SYMBOLS 1 Incident side optical fiber 2 Incident side collimating lens 3 Incident side wedge type birefringent crystal plate 4 Faraday rotator 5 Outgoing side wedge type birefringent crystal plate 6 Outgoing side collimating lens 7 Outgoing side optical fiber 8 Magnet 9 Second holder 9a Tube Shaped space part 90 bottom face part 91 optical path opening part 95 notch recessed part 10 first holder 10a first cylindrical space part 10b second cylindrical space part 11 optical path opening part 12 partition plate 100 rotation adjustment external jig 100a rotation operation rod 100b Convex part

Claims (3)

A pair of wedge-shaped birefringent crystal plates and a Faraday rotator provided on the optical path between the wedge-shaped birefringent crystal plates, and a pair of wedge-shaped birefringent crystal plates In the polarization-independent optical isolator arranged such that the inclined light transmitting surfaces and the non-tilted light transmitting surfaces are parallel to each other,
A first holder in which the inner body is partitioned into a first cylindrical space and a second cylindrical space by a partition plate having an optical path opening in the center, and an outer side in the second cylindrical space of the first holder A main body is slidably fitted, and one side of the inner main body is closed at the bottom surface and the other side is opened to form a cylindrical space portion and a second holder provided with an optical path opening on the bottom surface portion,
One wedge-shaped birefringent crystal plate is accommodated in the first cylindrical space portion of the first holder, with the vicinity of the outer peripheral edge thereof being fixed to the partition plate, and the other wedge-shaped double-crystal plate is accommodated in the cylindrical space portion of the second holder. The refractive crystal plate is housed by fixing the vicinity of the outer peripheral edge thereof to the bottom surface portion, and the Faraday rotator is fixed between the partition plate of the first holder and the bottom surface portion of the second holder, and
Rotation adjustment for rotating and sliding the second holder relative to the first holder on the open edge of the inner body of the second holder so that the inclined light transmitting surfaces of the pair of wedge-shaped birefringent crystal plates are arranged in parallel. A notch recess for engaging the external jig is provided, and the distance from the bottom surface in the cylindrical space portion of the second holder to the notch bottom of the notch recess is accommodated in the cylindrical space portion of the second holder. A polarization-independent optical isolator characterized in that it is set larger than the thickness dimension of the wedge-shaped birefringent crystal plate.   One wedge-shaped birefringent crystal plate is fixed to the partition plate in the first cylindrical space portion of the first holder using an epoxy adhesive, and the other wedge-shaped birefringent crystal plate is fixed to the cylindrical space portion of the second holder. It is fixed to the bottom part using an epoxy adhesive, and the Faraday rotator is fixed to at least one of the partition plate of the first holder or the bottom part of the second holder using an epoxy adhesive. The polarization-independent optical isolator according to claim 1.   3. A two-stage polarization-independent optical isolator having a structure in which the polarization-independent optical isolators according to claim 1 or 2 are stacked in two stages.

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