JP5008148B2 - Optical composite module - Google Patents

Description

  The present invention relates to an optical composite module in which a polarization-independent isolator function unit and an optical filter are combined. More specifically, the filter inclined surface of the optical filter is orthogonal to the polarization separation surface of the isolator function unit. The optical composite module is set as described above. This technique is useful for blocking ASE light (Amplified Spontaneous Emission light; amplified spontaneous emission light) in an optical amplifier.

  As is well known, in an optical communication system, an EDFA (erbium-doped optical fiber amplifier) is used to increase the intensity of light propagating in an optical fiber in order to compensate for optical transmission loss accompanying the extension of the transmission distance through the optical fiber. Has been done. The EDFA is also used in an optical measurement system or the like. By using an EDFA, an optical signal can be amplified as it is, and an optical signal with a very high output is obtained in a wavelength band of 1550 nm, for example.

  An optical communication system and an optical measurement system are configured by combining various optical devices in addition to, for example, an LD (semiconductor laser) and an Er-doped optical fiber. However, when these optical devices are connected, inconvenience occurs such that reflection occurs at the connection point, and malfunction of the LD occurs due to the reflected return light. In view of this, optical isolators are incorporated in various places where necessary to prevent the reflected light from recombining to the input side. In such a case, a polarization-independent optical isolator (see Patent Document 1) is used.

  As shown in FIG. 5, this type of optical isolator typically includes wedge-shaped birefringent crystal plates 12 and 14 on the optical axis before and after the 45-degree Faraday rotator 10 (input side and output side), The input fiber 16 and the lens 18 are arranged on the input side, and the lens 20 and the output fiber 22 are arranged on the output side. Here, the optical axes of the wedge-shaped birefringent crystal plate 12 on the input side and the wedge-shaped birefringent crystal plate 14 on the output side are in a plane perpendicular to the optical axis, and the optical axes of the wedge-shaped birefringent crystal plate 14 on the output side. The orientation is in a state of being rotated 45 degrees with respect to the optical axis orientation of the wedge-shaped birefringent crystal plate 12 on the input side. A forward ray from the input side fiber 16 is focused on the output side fiber 22 when focused by the output side lens 20 as shown in FIG. 5A, but as shown in FIG. When the reverse direction light beam from the side fiber 22 is focused by the input side lens 18, the deflection angle of the light beam is separated from the diameter of the input side fiber 16 by the ordinary light and the abnormal light. Do not combine. That is, an isolator function is obtained. Here, the polarization separation surface, which is a surface through which ordinary light and extraordinary light travel after polarization separation, is a surface parallel to the wedge surface (thus, a surface parallel to the paper surface in FIG. 5).

  By the way, in EDFA, ASE light (Amplified Spontaneous Emission light; amplified spontaneous emission light) is generated and propagates through the system together with signal light. The signal light amplified by the EDFA has a wavelength of 1550 nm, whereas the ASE light has a wavelength of 1540 nm or less. In view of this, a wavelength selection filter (for example, an LPF that transmits signal light having a wavelength of 1550 nm) is incorporated for the purpose of blocking extra light such as ASE light.

In conventional optical communication systems and optical measurement systems, optical isolators are used in places where an isolator function is required, and wavelength selection filters are provided separately in places where unnecessary light blocking function (necessary light passing function) is required. Arrangement has been made. However, when the optical isolator is a polarization independent type, it is necessary to dispose a fiber and a lens on the input side and the output side, respectively, with respect to the isolator function unit in which wedge-shaped birefringent crystal plates are disposed before and after the Faraday rotator. If an optical filter is coupled to the optical isolator using a lens, the structure of the entire system becomes complicated and the size (lengthened) increases. This problem can be improved by combining an optical isolator and an optical filter into an optical composite module having an ASE light blocking function and an isolator function (see Patent Document 2). In that case, the optical filter is tilted in order to suppress the influence of surface reflection. However, by simply tilting, the reflected light blocked by the optical filter is recombined on the input side or output side due to the characteristics of the optical filter. A situation arises, and there arises a problem that the return loss characteristic deteriorates.
Japanese Patent Publication No.61-58809 JP-A-5-224102

  The problem to be solved by the present invention is that the reflected light blocked by the optical filter is prevented from recombining on the input side or output side after passing through the isolator function part, and the deterioration of the return loss characteristic does not occur. Is to do so.

  In the present invention, a polarization-independent isolator function unit in which wedge-shaped birefringent crystal plates are arranged before and after the Faraday rotator (input side and output side) and a flat optical filter are arranged on the same optical axis. One wedge-shaped birefringent crystal plate and an optical filter are combined so as to face each other directly, and the optical filter is an optical composite module having a structure in which a normal line is inclined with respect to the optical axis. An optical composite module characterized in that a filter tilt surface determined by a normal line and an optical axis is set to be orthogonal to a polarization separation surface determined by normal light and abnormal light separated by the isolator function unit. .

  Here, the optical filter may be installed on the output side of the isolator function unit, or may be installed on the input side of the isolator function unit. As an optical filter, there is typically a wavelength selective filter in which a dielectric multilayer film that transmits light in a desired wavelength region is formed on a transparent substrate. For such a combination of an isolator function unit and an optical filter, an input optical fiber and a lens are arranged on the input side, and a lens and an output optical fiber are arranged on the output side, respectively.

  In the optical composite module according to the present invention, the isolator function unit and the optical filter are arranged close to each other without interposing a lens or a fiber, so that the device can be downsized (shortened). In addition, the filter tilt plane determined by the normal line of the optical filter and the optical axis is set to be orthogonal to the polarization separation plane determined by the polarized light separated by the isolator function unit, so that the reflection blocked by the optical filter It is possible to prevent the light from recombining with the input side fiber and the output side fiber, thereby eliminating the deterioration of the return loss characteristic.

  A configuration example of the optical composite module according to the present invention is shown in FIG. A polarization-independent isolator function unit 30 in which wedge-shaped birefringent crystal plates 12 and 14 are respectively arranged before and after the Faraday rotator 10 (input side and output side) and a flat optical filter 32 are connected to the same optical axis. The wedge-shaped birefringent crystal plate 14 on the rear side (output side) and the optical filter 32 are combined so as to face each other directly (without interposing a lens or a fiber). That is, in this embodiment, the optical filter 32 is installed on the output side of the isolator function unit 30. In order to facilitate understanding of the member relationship with the conventional polarization-independent optical isolator, members that may be the same as those in FIG. 5 are given the same reference numerals. In order to clarify the direction and the surface, the optical axis is set in the z direction as shown in the figure, and the x direction and the y direction are set at right angles thereto. The left hand in the drawing is the input side, and the right hand is the output side.

  The Faraday rotator 10 includes a magneto-optical crystal exhibiting a Faraday effect and a permanent magnet that applies a magnetic field in the optical axis direction to the magneto-optical crystal, and rotates the polarization plane of input light by 45 degrees. The wedge-shaped birefringent crystal plate 12 on the input side and the wedge-shaped birefringent crystal plate 14 on the output side have a vertical surface on the inside and an inclined surface on the outside so that one thick portion and the other thin portion face each other. It is arranged on the optical axis in a relation. Here, both wedge-shaped birefringent crystal plates 12 and 14 are installed such that their wedge-shaped surfaces are horizontal (parallel to the xz plane). The optical axes of both wedge-shaped birefringent crystal plates 12 and 14 are in the xy plane, one optical axis is +22.5 degrees with respect to the y axis, and the other optical axis is -22.5. It is set to be a degree. Such an isolator function unit may be the same as a conventional one.

  In the present embodiment, the optical filter 32 is incorporated by being tilted by several degrees (specifically, about 4 degrees) (the filter tilt angle is represented by θ). The tilt direction of the optical filter 32 is a polarization separation plane (xz plane or xz plane) determined by polarized light separated from the filter tilt plane (yz plane) determined by the normal line of the optical filter and the optical axis. -Z plane is set so as to be orthogonal to a plane inclined at an angle θ. The optical filter 32 used here is a long-pass wavelength selective filter in which a dielectric multilayer film that transmits light in a desired wavelength region (1540 nm or more) is formed on a transparent substrate.

  For such a combination of an isolator function part and an optical filter, as shown in FIG. 2, an input optical fiber 16 and a lens 18 are arranged on the input side, and a lens 20 and an output optical fiber 22 are arranged on the output side. To do. In FIG. 2, A is a plan view (the paper surface is the xz plane), and B is a side view (the paper surface is the yz plane).

  Input light from the input side fiber 16 is converted into parallel light by the lens 18, passes through the isolator function unit 30, and reaches the optical filter 32. At this time, due to the characteristics of the optical filter 32, light in the desired wavelength region passes through the optical filter 32, but light outside the desired wavelength region is reflected. The transmitted light is focused on the output side fiber 22 by the lens 20. On the other hand, the reflected light from the optical filter 32 passes through the isolator function unit 30 again, and at that time, it is separated into normal light and abnormal light by the isolator function, and is output. Here, if the incident surface of the optical filter is perpendicular to the optical axis, the reflected light returns to the isolator function unit along the optical axis, so that ordinary light and abnormal light separated by the isolator function are in the xz plane. Return in a parallel plane. This optical path is the same as in the case of the isolator function unit alone (in the case of FIG. 5), but the isolator function is reduced by the amount of reflection by the optical filter. Therefore, the optical filter is tilted with respect to the optical axis. If the optical filter is tilted with respect to the optical axis, the ordinary light and abnormal light separated by the isolator function return according to the direction and the tilt angle of the optical filter, and the separated light May be coupled to the input-side fiber 16. At this time, the return loss on the input side is greatly degraded.

  However, in the present invention, the tilt direction of the optical filter is determined by the polarized light (ordinary light and extraordinary light) separated from the filter tilt surface (yz plane) determined by the normal line of the optical filter and the optical axis. Since it is set so as to be orthogonal to the separation plane (the xz plane or the plane in which the xz plane is inclined by the filter tilt angle θ), the ordinary light and extraordinary light separated by the isolator function pass through the lens 18. However, it is not directly coupled to the input side fiber 16. When the filter tilt angle θ is changed, the separated ordinary light and extraordinary light are only swung from the near side to the far side with respect to the paper surface as viewed in FIG. 2A, for example, and are swung horizontally (within the paper surface). Because there is nothing. Accordingly, at this time, the separated light does not cross or approach the input side fiber, and therefore, the return loss on the input side does not deteriorate.

  This is shown in FIG. A is a comparative example, and the tilt direction of the optical filter is set so that the filter tilt surface determined by the normal line and the optical axis of the optical filter is parallel to the polarization separation surface determined by the polarized light separated by the isolator function unit. This is the case. The tilt angle of the optical filter was 4 degrees. Due to the characteristics of the optical filter, the insertion loss (IL) characteristic is larger on the short wavelength side. In this case, the return loss (RL) characteristic is also deteriorated on the short wavelength side in conjunction with the characteristic. B is the present invention, and the tilt direction of the optical filter is set so that the filter tilt surface determined by the normal line of the optical filter and the optical axis is orthogonal to the polarization separation surface determined by the polarized light separated by the isolator function unit. This is the case. Again, the tilt angle of the optical filter was 4 degrees. Also in this case, it can be seen from the characteristics of the optical filter that the insertion loss (IL) characteristics are larger on the short wavelength side, but the return loss (RL) characteristics are not deteriorated without being linked to the characteristics.

  FIG. 4 is an explanatory view showing another embodiment of the present invention, in which A is a plan view and B is a side view. A polarization-independent isolator function unit 30 in which wedge-shaped birefringent crystal plates 12 and 14 are arranged before and after the Faraday rotator 10 on the optical axis (input side and output side), and a flat optical filter 32, The front (input side) wedge-shaped birefringent crystal plate 12 and the optical filter 32 are combined so as to face each other directly on the same optical axis. That is, in this embodiment, the optical filter 32 is installed on the input side of the isolator function unit 30. In this case as well, in order to clarify the direction and the surface, the optical axis is set to the z direction as shown in the figure, and the x direction and the y direction are set at right angles thereto. The left hand in the drawing is the input side, and the right hand is the output side.

  Since the isolator function unit 30 and the optical filter 32 may be the same as those in the above-described embodiment, description thereof will be omitted. The optical filter 32 is incorporated in an inclined state by several degrees (for example, about 4 degrees). The inclination direction of the optical filter is a polarization separation surface (x) determined by polarized light (ordinary light and extraordinary light) separated from the filter inclined surface (yz plane) determined by the normal line of the optical filter and the optical axis. -Z plane or xz plane is set so as to be orthogonal to a plane inclined by the filter tilt angle θ. For such a combination of the optical filter 32 and the isolator function unit 30, the input optical fiber 16 and the lens 18 are disposed on the input side, and the lens 20 and the output optical fiber 22 are disposed on the output side.

  Input light from the input side fiber 16 is converted into parallel light by the lens 18 and reaches the optical filter 32. At that time, due to the characteristics of the optical filter, light in the desired wavelength region passes through the optical filter 32, but is outside the desired wavelength region. The light is reflected. Since the reflected light has an angle with respect to the optical axis, it is not directly coupled to the input side fiber 16. With this function alone, the optical filter can be tilted in any direction. The light that has passed through the optical filter 32 passes through the isolator function unit 30 and is focused on the output-side fiber 22 by the lens 20. The return light from the output side is separated into ordinary light and extraordinary light by the isolator function unit 30 and reaches the optical filter 32. Light in the desired wavelength region passes through the optical filter 32, but light outside the desired wavelength region is reflected. Is done. The reflected light again passes through the isolator function unit 30 and is output.

  Here, in order to prevent reflection by the optical filter, the optical filter is tilted with respect to the optical axis, but ordinary light and extraordinary light separated by the isolator function return in the plane according to the direction and tilt angle of the optical filter. In some cases, a part of the component reflected by the optical filter of light traveling backward from the output side is coupled to the output side fiber. Even in such a case, the return loss greatly deteriorates.

  However, in the present invention, the tilt direction of the optical filter is such that the filter tilt plane (yz plane) determined by the normal line of the optical filter and the optical axis is the polarization separation plane (x−) determined by the polarized light separated by the isolator function unit. z plane or xz plane is set so as to be orthogonal to the plane inclined by the filter tilt angle θ), so that the normal light and abnormal light separated by the isolator function are reflected by the optical filter and the isolator function section is Even if it passes, the direction in which ordinary light and extraordinary light are swayed by refraction and the direction in which light is swayed by reflection of the optical filter are orthogonal to each other. Absent.

  Since the optical composite module of the present invention has an ASE light blocking function and an isolator function, it is useful, for example, as an optical device in an optical amplifier. The optical filter used in the present invention includes a wavelength selection filter that allows light in a desired wavelength region to pass, and may be a BPF (band pass filter) or an SPF (short pass filter) in addition to the above LPF.

The block diagram of the optical composite module which concerns on this invention. Explanatory drawing which shows one Example of the optical composite module which concerns on this invention. The graph which shows a loss characteristic. Explanatory drawing which shows the other Example of the optical composite module which concerns on this invention. Explanatory drawing of the conventional polarization-independent type optical isolator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Faraday rotator 12, 14 Wedge-shaped birefringent crystal plate 16 Input side fiber 18, 20 Lens 22 Output side fiber 30 Isolator function part 32 Optical filter

Claims (5)

The polarization-independent isolator function part in which wedge-shaped birefringent crystal plates are arranged on the input side and the output side of the Faraday rotator, and the plate-shaped optical filter are arranged on the same optical axis. And the optical filter are combined so that they face each other directly, and the optical filter is an optical composite module having a structure in which the normal is inclined with respect to the optical axis,
An optical composite characterized in that an inclined surface of the filter determined by the normal line of the optical filter and the optical axis is set to be orthogonal to a polarization separation surface determined by the ordinary light and the extraordinary light separated by the isolator function unit. module.   The optical composite module according to claim 1, wherein the optical filter is installed on an output side of the isolator function unit.   The optical composite module according to claim 1, wherein the optical filter is installed on an input side of the isolator function unit.   4. The optical composite module according to claim 2, wherein the optical filter is a wavelength selective filter in which a dielectric multilayer film that transmits light in a desired wavelength region is formed on a transparent substrate.   5. The optical composite module according to claim 4, wherein an input optical fiber and a lens are arranged on the input side and a lens and an output optical fiber are arranged on the output side of the combination of the isolator function unit and the optical filter, respectively.

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