JP2007108344A - Polarization-independent type optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarization-independent type optical isolator capable of suppressing the temperature rise of a Faraday rotator, even if a light of high output is made incident on the Faraday rotator, and capable of preventing deterioration in isolation. <P>SOLUTION: Regarding the polarization-independent type isolator, wherein a sapphire single-crystal plate is brought in contact with the light transmissive surface of a magnetic garnet single-crystal arranged between two wedge-type birefringence crystal plates, the direction of the axis C of the incident-side wedge-type birefringence crystal plate is made to coincide with the direction of the axis C of the sapphire single-crystal plate brought in contact with the incident side of the magnetic garnet single crystal, and also, the direction of the axis C of the exit-side wedge-type birefringence crystal plate is made to coincide with the direction of the axis C of the sapphire single-crystal plate, brought in contact with the exit side of the magnetic garnet single crystal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a polarization-independent optical isolator using a wedge-type wedge-shaped birefringent crystal plate. More specifically, the present invention relates to a magnetic garnet single crystal as a Faraday rotator, resulting from light absorption by the Faraday rotator. The present invention relates to a polarization-independent optical isolator that takes into account the prevention of characteristic deterioration and breakage due to temperature rise.

  An optical isolator is an irreversible optical device that has a function of passing an optical signal in the forward direction and preventing the optical signal from passing in the reverse direction. For example, in an optical communication system using a semiconductor laser as a light source, the optical signal is reflected. Returning to the light source side, it is used to prevent the oscillation of the semiconductor laser from becoming unstable.

The optical isolator includes a polarization-dependent optical isolator (hereinafter abbreviated as “PDOI”) used for a semiconductor laser module and a polarization-independent optical isolator (hereinafter referred to as “PDOI”) used before and after an optical fiber amplifier. And abbreviated as “PIOI”). In a polarization-independent optical isolator, a wedge-shaped birefringent crystal plate such as rutile, YVO ₄ , or LiNbO ₃ is usually used as a polarizer, and a magnetic garnet single unit is used as a Faraday rotator between two wedge-shaped birefringent crystal plates. A flat plate made of crystals is arranged. Here, the Faraday rotator is adjusted to rotate the polarized light by 45 degrees, and the two wedge-shaped birefringent crystal plates are arranged so that their optic axis directions are shifted from each other by 45 degrees. Note that an optical element portion composed of a polarizer and a Faraday rotator is called a nonreciprocal portion.

  Parallel input light from the incident side in the forward direction is separated into ordinary light and extraordinary light by the first wedge-shaped birefringent crystal plate of the nonreciprocal part, but the polarization is rotated 45 degrees by the Faraday rotator, and the second Since the optical axis of the wedge-shaped birefringent crystal plate is deviated by 45 degrees from the first wedge-shaped birefringent crystal plate, ordinary light is incident as ordinary light, and extraordinary light is incident as extraordinary light. The light is emitted from the plate and optically coupled.

  The light from the opposite direction is separated into ordinary light and extraordinary light by the second wedge-shaped birefringent crystal plate, and after being rotated 45 degrees by the Faraday rotator, the first wedge-shaped birefringent crystal plate is ordinary light. Is incident as extraordinary light, and extraordinary light is incident as ordinary light, so the light exiting the first wedge-shaped birefringent crystal plate does not become parallel light and is not optically coupled. Thus, it functions as an optical isolator.

  The magnetic garnet single crystal exhibits excellent optical transparency in the near-infrared wavelength region, particularly in the wavelength region (1.3 μm to 1.6 μm) used in optical communication, and absorbs light if it is about several tens of mW. There is little problem of temperature rise due to, and there is almost no problem.

  However, in the shorter wavelength range, especially in the YAG laser wavelength (1.06 μm) and the pumping light wavelength (around 1 μm) of the optical fiber amplifier, light absorption increases and is ignored even at laser power of several mW. The temperature rise is impossible.

  As the Faraday rotator used for the optical isolator in the above wavelength range (around 1 μm), there are a paramagnetic single crystal and a paramagnetic glass. However, when these are used, not only the size of the Faraday rotator itself increases but also the Faraday rotator. In order to magnetically saturate the rotor, a large magnet is required, and the optical isolator becomes huge. In addition, optical isolators in the 1 μm band are beginning to be required to withstand high output.

As a high-power-resistant optical isolator, the nonmagnetic garnet substrate used when pulling up the garnet single crystal in Patent Document 1 is left, and the nonmagnetic garnet substrate is contacted or adhered to the other to improve thermal conductivity. It is disclosed to deal with this. According to Patent Document 2, the C-axis of a light-transmitting sapphire single crystal plate is brought into contact with and joined to at least one surface of a Faraday rotator so as to be perpendicular to the optical surface of the Faraday rotator. A method for suppressing the temperature rise of the rotor is disclosed.
JP 7-281129 A JP 2005-43853 A

  However, in the method of contacting or adhering a non-magnetic garnet substrate to both side surfaces of a Faraday rotator made of a magnetic garnet single crystal described in Patent Document 1, there is a limit to the power of incident light, and a high-power laser When the light is incident, the temperature rise due to absorption cannot be suppressed.

  The method of contacting and joining the C axis of the sapphire single crystal plate described in Patent Document 2 perpendicularly to the optical surface of the Faraday rotator is because the sapphire single crystal is a birefringent crystal. When used for the PDOI described in No. 2, the problem is not so great, but when used for the PIOI, there arises a problem of deterioration of the extinction ratio in the sapphire single crystal plate.

  When light is incident along the C-axis, the refractive index in the sapphire single crystal plate surface is equivalent and the extinction ratio should be -55 dB or more, but if the incident angle is 8 degrees or more, Depending on the state of incident polarized light, it may fall to about -20 dB. In PIOI using a wedge-shaped birefringent crystal plate as a polarizer, the light between two wedge-shaped birefringent crystal plates varies depending on the wedge angle, but the wedge-shaped birefringent crystal plate has a rutile with a wedge angle of 8 degrees. Since crystals are tilted by about 10 degrees, the extinction ratio is degraded to -20 dB or less, and the performance as an optical isolator, particularly the isolation, is degraded.

  In addition, if the direction of the sapphire single crystal plate is rotated and the inclined light is vertically incident on the sapphire single crystal plate, the light is incident along the C axis, so that the extinction ratio is not deteriorated. The light reflected by the vertical incident surface follows the incident path and returns to the light source side. Although the antireflection film is applied to the surface of the optical crystal, the transmittance cannot be made 100%. Therefore, preventing the deterioration of the extinction ratio by the method of rotating the sapphire single crystal plate as described above, In the first place, it cannot be adopted because it impairs the function of the optical isolator that blocks the reflected return light.

  The present invention has been made in view of the above problems, and the object thereof is to prevent an increase in temperature of the Faraday rotator even when high-power light is incident, and to prevent a decrease in isolation. It is to provide a polarization-independent optical isolator having a reciprocal part.

  In order to solve the above problems, a polarization-independent optical isolator according to the present invention is a polarization-free optical isolator in which a sapphire single crystal plate is brought into contact with a light transmission surface of a magnetic garnet single crystal disposed between two wedge-shaped birefringent crystal plates. In the dependency type optical isolator, the direction of the C axis of the wedge-type birefringent crystal plate on the incident side and the direction of the C axis of the sapphire single crystal plate brought into contact with the incident side of the magnetic garnet single crystal are matched, and the wedge-shaped wedge on the output side The direction of the C axis of the sapphire single crystal plate brought into contact with the exit side of the magnetic garnet single crystal is made to coincide with that of the type birefringent crystal plate.

  In the present invention, the direction of the C-axis of the wedge-type birefringent crystal plate on the incident side and the direction of the C-axis of the sapphire single crystal plate brought into contact with the incident side of the magnetic garnet single crystal, and the wedge-type wedge-shaped compound on the exit side The sapphire single crystal plate brought into contact with the exit side of the refracting crystal plate and the magnetic garnet single crystal is not more than 1 degree away from the orientation of the sapphire single crystal plate. A sapphire single crystal plate is bonded with an adhesive.

  According to the present invention, it is possible to provide a polarization-independent optical isolator having a nonreciprocal portion that can suppress a temperature rise of a Faraday rotator even when high-power light is incident and does not cause extinction ratio deterioration. Is possible.

  FIG. 1 shows a schematic configuration of a nonreciprocal portion in the polarization-independent optical isolator according to the embodiment of the present invention. In order of light transmission direction, the first wedge-shaped birefringent crystal plate 1, the first sapphire single crystal plate 2, the magnetic garnet single crystal 3, the second sapphire single crystal plate 4, and the second wedge-shaped birefringent crystal plate 5 Arranged in the order of. FIG. 2 shows the progress of light in the embodiment of the present invention. The forward light emitted from the optical fiber 6 enters the nonreciprocal part via the lens 7. Incident light to the nonreciprocal part is separated into ordinary light and extraordinary light by the first wedge-shaped birefringent crystal plate 1 and enters the first sapphire single crystal plate 2. The light passing through the first sapphire single crystal plate 2 passes through the second sapphire single crystal plate 4 after the polarization plane is rotated by 45 degrees by the magnetic garnet single crystal 3, and the second wedge-shaped birefringence. Incident on the crystal plate 5.

  Here, in the present invention, the C-axis direction of the first wedge-type birefringent crystal plate 1 and the C-axis direction of the first sapphire single crystal plate 2 and the C-axis of the second wedge-type birefringent crystal plate 5 Since the directions of the C-axis of the second sapphire single crystal plate 4 coincide with each other, the ordinary light separated by the first wedge-shaped birefringent crystal plate 1 is also ordinary light in the second wedge-shaped birefringent crystal plate 5. As described above, the extraordinary light separated by the first wedge-type birefringent crystal plate 1 also enters the second wedge-type birefringent crystal plate 5 as extraordinary light. Therefore, ordinary light and extraordinary light emitted from the second wedge-shaped birefringent crystal plate 5 become parallel light, and can be coupled to the optical fiber 6 'on the emission side by the lens 7'.

  The light in the reverse direction returned from the optical fiber 6 ′ on the emission side passes through the lens and is separated into ordinary light and extraordinary light in the second wedge-shaped birefringent crystal plate 5, and 45 due to the irreversibility of the magnetic garnet single crystal. Rotated degrees. As in the forward direction, the orientation of the C-axis of the first wedge-shaped wedge-type birefringent crystal plate 1 and the C-axis of the first sapphire single-crystal plate 2 and the second wedge-shaped birefringent crystal plate 5 and the C-axis direction of the second sapphire single crystal plate 4 coincide with each other, so that the ordinary light in the second wedge-type birefringent crystal plate 5 is normal in the first wedge-type birefringent crystal plate 1. As extraordinary light, extraordinary light in the second wedge-shaped birefringent crystal plate 5 enters the first wedge-type birefringent crystal plate 1 as ordinary light. Therefore, the light emitted from the first wedge-shaped birefringent crystal plate 1 does not become parallel light, and is not optically coupled to the optical fiber 6 on the incident side, so that high isolation is maintained.

  Further, the deviation between the C-axis of the wedge-shaped birefringent crystal plate and the C-axis of the sapphire single crystal plate is preferably 1 degree or less. Since the sapphire single crystal plate itself is also a wedge-type birefringent crystal plate, if the light emitted from the wedge-type birefringent crystal plate is not incident along the crystal axis of the sapphire single crystal plate, the extinction ratio will deteriorate and Lead to deterioration of the FIG. 3 shows the change in the extinction ratio due to the deviation of the C-axis of the wedge-shaped birefringent crystal plate and the C-axis of the sapphire single crystal plate. If it falls to 2 degrees, it will break 30 dB. Furthermore, the sapphire single crystal plate and the Faraday rotator are preferably bonded with a low-viscosity adhesive, and the thickness of the bonded portion is suppressed to less than 10 μm to bond the heat generated by the Faraday rotator to the sapphire single crystal. Since it becomes easier to convey by the plate, it is possible to withstand higher output light than the close contact arrangement, which is preferable.

The magnetic garnet single crystal is grown by a liquid phase epitaxial method, and the thickness is adjusted by polishing so that the Faraday rotation angle is 45 degrees. As the wedge-type birefringent crystal plate, a rutile single crystal having a wedge angle of 8 degrees was used. Rutile single crystal with respect to light of a wavelength of a YAG laser (1.06 .mu.m), having a refractive index _n e = 2.742 for abnormal light refractive index _n o = 2.480 the ordinary. Therefore, the light path between the first and second wedge-shaped birefringent crystal plates is inclined by about 13 degrees. The sapphire single crystal plate is a parallel plate having a thickness of 0.5 mm, and has a C axis in the light transmission surface.

  As shown in FIG. 1, each of the single crystal plates is a first wedge-type birefringent crystal plate, a first sapphire single crystal plate, a magnetic garnet single crystal, a second sapphire single crystal plate, and a second wedge type. The birefringent crystal plates were arranged in this order to constitute the PIOI. The inclined surfaces of the first and second wedge-shaped birefringent crystal plates face the opposite side of the magnetic garnet single crystal, and are arranged so that the inclined surfaces are parallel to each other. Although not shown in FIG. 1, a holder for holding each crystal and a magnet for magnetically saturating the magnetic garnet single crystal are arranged outside the magnetic garnet single crystal. Yes. In addition, an antireflection film for the wavelength used is applied to the light transmission surface of each single crystal plate.

  The position of the C-axis of the wedge-shaped birefringent crystal plate is 22.5 degrees counterclockwise from the Y direction when the first wedge-shaped birefringent crystal plate is viewed from the incident direction. Is inclined 22.5 degrees clockwise from the Y direction. For this reason, the positions of the C axes are shifted by 45 degrees. The C axis of the two sapphire single crystal plates is aligned with the C axis of the first wedge-type birefringent crystal plate so that the C axis of the first sapphire single crystal plate matches the C axis of the second sapphire single crystal plate. Is aligned with the C-axis of the second wedge-shaped birefringent crystal plate, and is bonded so as to sandwich the magnetic garnet single crystal with an adhesive.

  When a 300 mW YAG laser beam was incident on the polarization-independent optical isolator thus manufactured and the characteristics were evaluated, the wedge-shaped birefringent crystal plate hardly absorbs light and the temperature rises. There was almost no. In addition, for the magnetic garnet single crystal, since the thermal conductivity of the closely adhered sapphire single crystal plate is high, the heat is radiated and the temperature rise can be suppressed to a range that does not affect the characteristics. Since the C-axis of the opposing wedge-shaped birefringent crystal plate and the sapphire single crystal plate coincide, 40 dB isolation can be obtained.

  According to the present invention, it is possible to provide a polarization-independent optical isolator that can suppress the temperature rise of the Faraday rotator even when high-output light is incident, and that is less likely to degrade the optical isolation function. It can be used for optical communication systems.

It is a schematic perspective view of the structure of the non-reciprocal part in this invention. It is a figure showing the advancing condition of the light in this invention. It is a figure which shows the extinction ratio change by C-axis of a wedge type birefringent crystal plate and C-axis of a sapphire single crystal plate having shifted | deviated.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st wedge type birefringent crystal plate 2 1st sapphire single crystal plate 3 Magnetic garnet single crystal 4 2nd sapphire single crystal plate 5 2nd wedge type birefringent crystal plate 6, 6 'Optical fiber 7, 7 'Lens

Claims (3)

  In a polarization-independent optical isolator in which a sapphire single crystal plate is brought into contact with a light transmission surface of a magnetic garnet single crystal disposed between two wedge-shaped birefringent crystal plates, the C axis of the wedge-type birefringent crystal plate on the incident side The direction of the C axis of the sapphire single crystal plate brought into contact with the incident side of the magnetic garnet single crystal is matched, and the C axis of the wedge-shaped birefringent crystal plate on the outgoing side is brought into contact with the outgoing side of the magnetic garnet single crystal. A polarization-independent optical isolator characterized in that the directions of the C-axis of a single crystal plate are matched.   The direction of the C axis of the wedge-type birefringent crystal plate on the incident side and the direction of the C axis of the sapphire single crystal plate in contact with the incident side of the magnetic garnet single crystal, and the C axis of the wedge-type birefringent crystal on the output side and the magnetic garnet single unit The polarization-independent optical isolator according to claim 1, wherein a deviation of the C-axis direction of the sapphire single crystal plate brought into contact with the crystal emission side is within 1 degree. The polarization-independent optical isolator according to claim 1 or 2, wherein the magnetic garnet single crystal and the sapphire single crystal plate are bonded with an adhesive.

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