JP4911519B2 - Reflective variable optical attenuator - Google Patents

Description

  The present invention relates to a reflection type variable optical attenuator. More specifically, a ferrule having an obliquely polished end face, a separating / combining birefringent element, and a lens are arranged coaxially, and a variable Faraday rotator and a mirror are arranged on the same axis. The present invention relates to a variable optical attenuator having a structure that is combined in an off-axis state.

  In an optical communication system or an optical measurement system, a variable optical attenuator for controlling the amount of transmitted light is required. For example, in wavelength division multiplexing communication, an optical attenuator is incorporated for each wavelength in order to equalize insertion loss. A typical example of this type of variable optical attenuator is a counter type in which polarizers and analyzers are installed on the front and rear (input side and output side) of the optical axis of the variable Faraday rotator. Therefore, the separation / combination birefringence element, the lens, and the mirror are arranged in order, and the input fiber and the output fiber are set on the separation / combination birefringence element side. A reflection type (see Patent Document 1) that can adjust the attenuation of incident light by controlling the rotation angle in the polarization direction with a variable Faraday rotator installed between them has been developed.

  In the conventional reflective variable attenuator, in order to simplify the assembly, each optical component is arranged on the same axis and accommodated in a single holder. The required optical attenuator is adjusted by adjusting the mirror position and angle. The characteristic is expressed.

  By the way, the ferrule holding the input fiber and the output fiber is obliquely polished so as to be a plane inclined about 8 degrees from the vertical plane with respect to the central axis in order to reduce the return loss. By this oblique polishing, the light emitted from the fiber is emitted inclined from the central axis of the ferrule. Due to this inclination, the incident position on the lens is deviated from the lens center. As a result, the focal point formed by the light transmitted through the birefringent element between the ferrule and the lens is deviated from the lens center (ferrule central axis). Become.

For this reason, in the conventional structure in which the optical components are arranged coaxially and housed in a single holder, the light is transmitted through a portion deviated from the center of the variable Faraday rotator, resulting in variations in attenuation characteristics as an optical attenuator. There is a problem that cannot be optimized.
JP 2003-107420 A

  The problem to be solved by the present invention is to stabilize and optimize the attenuation characteristic by suppressing variations in the Faraday rotation angle in each optical path in the variable Faraday rotator.

  The present invention relates to a two-core ferrule in which an input fiber is positioned at an input port and an output fiber is positioned at an output port, and a parallel plane type that separates light of the same optical path whose polarization directions are orthogonal to each other and synthesizes light of different optical paths A birefringent element for separating and synthesizing, a light converging lens, and a mirror disposed at the focal position of the lens, and arranging a variable Faraday rotator between the lens and the mirror, In a variable optical attenuator that controls the amount of reflected output light by controlling the rotation angle in the polarization direction with a variable Faraday rotator, the two-core ferrule has a structure in which an end surface is obliquely polished, and the center of the lens is the center Located on the central axis of the two-core ferrule, thereby forming a fiber collimator. The variable Faraday rotator has its central axis reciprocating between the lens and the mirror. A reflective variable optical attenuator, characterized in that offset from the center axis of said fiber collimators is installed so as to match the center of the road.

  Strictly speaking, the Faraday rotation angle of the variable Faraday rotator is not uniform over the entire surface of the Faraday element, and is symmetrical on the left and right (or up and down) with respect to the center, but has a certain distribution. This is because a small magnetic field applying means incorporated in the variable optical attenuator cannot apply a uniform magnetic field over the entire surface of the Faraday element. Due to the magnetic field distribution, the Faraday rotation angle is also distributed, and the attenuation characteristic is affected as an optical attenuator depending on which position the light is transmitted through. Therefore, in the present invention, by setting the central axis of the variable Faraday rotator to coincide with the center of the optical path that reciprocates between the lens and the mirror, the variation of the Faraday rotation angle in each optical path in the variable Faraday rotator is suppressed, It is possible to stabilize and optimize the attenuation characteristic.

Here, a fiber collimator incorporated in the cylindrical case so that the two-core ferrule, the separating / combining birefringent element, and the lens are positioned coaxially, and a first holder that can be accommodated in a state in which the fiber collimator is tilt-adjusted A variable Faraday rotator and a second holder incorporated so that the mirror is positioned coaxially, the butting surfaces of the first holder and the second holder are movable planes perpendicular to the central axis, in their movable butt planes abutting the first and second holders, a structure for fixing in a state of being adjusted position.

  For example, the variable Faraday rotator includes a rotation angle variable unit and a rotation angle fixing unit, and is configured such that the maximum Faraday rotation angle of the rotation angle variable unit is equal to the Faraday rotation angle of the rotation angle fixing unit. . Alternatively, the variable Faraday rotator is composed of a rotation angle variable part and a rotation angle fixing part, and Faraday elements of the rotation angle variable part and Faraday elements of the rotation angle fixing part having different Faraday rotation directions with respect to the same magnetic field direction. It can also be set as the structure to be used.

  The reflection type variable optical attenuator of the present invention is installed so that the center axis of the variable Faraday rotator is shifted from the center axis of the fiber collimator so as to coincide with the center of the optical path that reciprocates between the lens and the mirror. The variation in the Faraday rotation angle in each optical path in the optical element is suppressed, and the attenuation characteristic can be stabilized and optimized.

  FIG. 1 is a schematic configuration diagram showing an embodiment of a reflective variable optical attenuator according to the present invention. To make the explanation easy to understand, the following coordinate axes are set. The arrangement direction of optical components (the direction in which incident light travels) is defined as the z direction (right direction in the drawing), and the two directions orthogonal thereto are defined as the x direction (horizontal direction) and the y direction (vertical direction). Therefore, A in FIG. 1 is a front view and B is a plan view.

  This reflection-type variable optical attenuator uses a two-core ferrule 14 in which an input fiber 10 is located at an input port and an output fiber 12 is located at an output port, and light on the same optical path in which the polarization direction in the z direction is orthogonal. A parallel plane type separation / combination birefringence element 16 that synthesizes light in different optical paths toward the −z direction in the −y direction, a convex lens 18, and a mirror 20 disposed at the focal position of the convex lens 18. A variable Faraday rotator (only the Faraday element 22 is shown in FIG. 1) is installed between the convex lens 18 and the mirror 20 and the rotation angle in the polarization direction is controlled by the variable Faraday rotator. In this way, the reflected output light quantity is controlled.

  The two-core ferrule 14 has a structure in which the end face is obliquely polished at an angle of 8 degrees from the vertical. In this embodiment, the input fiber 10 is positioned at the input port of the right optical path when viewed in the z direction, and the output port of the left optical path. Assume that the output fiber 12 is positioned. The center of the convex lens 18 is located on the central axis of the two-core ferrule 14, and the fiber collimator 24 is configured by them. The variable Faraday rotator (Faraday element 22) is disposed so as to deviate from the central axis Oc of the fiber collimator 24 so that the central axis Of thereof coincides with the center of the optical path reciprocating between the convex lens 18 and the mirror 20. This is a feature of the present invention.

  Here, the separating / combining birefringent element 16 is made of a parallel plate type rutile crystal. The Faraday element 22 has a Faraday rotation angle that changes in accordance with a combined magnetic field applied from two directions. Although not shown, a fixed magnetic field is applied by a permanent magnet in the direction in which light travels. Magnetic field applying means is installed so that a variable magnetic field is applied by an electromagnet in a vertical direction.

  A specific structural example of this reflective variable optical attenuator is shown in FIG. The fiber collimator 24 is configured by incorporating the two-core ferrule 14, the separating / combining birefringent element 16, and the convex lens 18 into the cylindrical case 30 so as to be positioned coaxially. The cylindrical case 30 has a structure in which a mounting flange 30b is integrally formed on the outer peripheral side of the cylindrical body 30a. The fiber collimator 24 is accommodated in the first holder 32, and is fixed by welding or the like at the flange portion 30b in a state where the center axis of the fiber collimator 24 is tilt-adjusted with respect to the center axis of the first holder 32 (the welding portion is fixed). Indicated by the symbol w). For example, the tilt adjustment corresponds to a structure in which the cylindrical case 30 of the fiber collimator 24 is loosely fitted to the first holder 32. The variable Faraday rotator (Faraday element 22) and the mirror 20 are incorporated in the second holder 34 so as to be positioned on the same axis. The variable Faraday rotator includes a Faraday element 22, a permanent magnet (not shown) for applying a saturation magnetic field in the optical axis direction to the Faraday element 22, and a variable magnetic field applied to the Faraday element 22 in a direction perpendicular to the optical axis. Although not shown, a permanent magnet and an electromagnet are also incorporated in the second holder 34.

  The first holder 32 and the second holder 34 are combined so as to be abutted against each other, and the abutting surfaces thereof are movable planes perpendicular to the central axis, and are relatively moved by the movable abutting planes. Then, the position is adjusted and fixed by welding or the like (the welding location is indicated by the symbol w).

  Next, the operation of this variable optical attenuator will be described. FIG. 3 shows the polarization state between the optical components. The polarization states indicated by a to d in FIG. 3 are views seen in the direction in which light travels at the positions indicated by a to d in FIG.

(Faraday rotation angle: 45 degrees)
First, FIG. 3A shows a case where the Faraday rotation angle by the Faraday element 22 is 45 degrees. The light incident in the z direction from the input fiber 10 travels straight in the birefringent element 16 for separation and synthesis, and the extraordinary light is refracted and separated in the y direction. Then, the light is condensed by the convex lens 18 and passes through the Faraday element 22. Since the Faraday rotation angle is set to 45 degrees, the polarization direction rotates by 45 degrees, and reaches the mirror 20 at the lens focal position to be reflected. The reflected light returning in the −z direction passes through the Faraday element 22 again, and the polarization direction is further rotated by 45 degrees (accordingly, 90 degrees in total), and becomes parallel light by the convex lens 18. In the separating / combining birefringent element 16, the ordinary light in the lower optical path travels straight, and the extraordinary light in the upper optical path is refracted in the −y direction. In this way, when the Faraday rotation angle is 45 degrees, the incident light quantity from the input fiber 10 is almost attenuated and almost all the light quantity is emitted to the output fiber 12.

(Faraday rotation angle: 0 degree)
Next, FIG. 3C shows a case where the Faraday rotation angle by the Faraday element 22 is 0 degree. The light incident in the z direction from the input fiber 10 travels straight in the birefringent element 16 for separation and synthesis, and the extraordinary light is refracted and separated in the y direction. Then, the light is condensed by the convex lens 18 and passes through the Faraday element 22. Since the Faraday rotation angle is set to 0 degree, the polarization direction does not rotate and reaches the mirror 20 at the lens focal position and is reflected. The reflected light returning in the −z direction passes through the Faraday element 22 again, but the polarization direction does not rotate at that time, and becomes parallel light by the convex lens 18. In the separating / combining birefringent element 16, the ordinary light in the upper optical path travels straight, and the extraordinary light in the lower optical path is refracted in the -y direction. Therefore, almost no incident light from the input fiber 10 is coupled to the output fiber 12. That is, almost all of the amount of incident light from the input fiber is attenuated.

(Faraday rotation angle: 22.5 degrees)
When the Faraday rotation angle by the Faraday element 22 is an arbitrary angle between 0 and 45 degrees, it is as follows. For example, 22.5 degrees is shown in FIG. The light incident in the z direction from the input fiber 10 travels straight in the birefringent element 16 for separation and synthesis, and the extraordinary light is refracted and separated in the y direction. Then, the light is condensed by the convex lens 18 and passes through the Faraday element 22. The polarization direction is rotated by an arbitrary angle (22.5 degrees in B of FIG. 3) by the Faraday element 22, and reaches the mirror 20 at the lens focal position to be reflected. The reflected light returning in the −z direction passes through the Faraday element 22 again, and the polarization direction is further rotated by the same angle (here, 22.5 degrees: accordingly, 45 degrees in total), and is reflected by the convex lens 18 as parallel light. become. The separation / combination birefringent element 16 causes the ordinary light component in the lower optical path to travel straight, and the extraordinary light component in the upper optical path is refracted in the −y direction, so that these components are combined in the polarization and coupled to the output fiber 12. However, the ordinary light component in the upper optical path travels straight, and the extraordinary light component in the lower optical path is refracted in the -y direction, so these components are not coupled to the output fiber. Therefore, the incident light from the input fiber 10 is attenuated (when the Faraday rotation angle is set to 22.5 degrees, the incident light amount is almost halved) and emitted to the output fiber 12.

  In this way, by controlling the rotation angle in the polarization direction with the Faraday element 22, the attenuation amount of the incident light (in other words, the reflected output light amount) can be freely adjusted.

  As can be seen from FIG. 3, the light transmitted through the Faraday element 22 is separated into four optical paths. However, as shown in FIG. 4, the Faraday rotation angle of the variable Faraday rotator is not strictly uniform over the entire surface of the Faraday element, but is symmetric with respect to the center on the left and right (or up and down), but has a certain distribution. On the other hand, as described above, the end surface of the ferrule holding the input fiber and the output fiber is obliquely polished so as to be a surface inclined about 8 degrees from the vertical plane with respect to the central axis in order to reduce the return loss. ing. By this oblique polishing, the light emitted from the fiber is emitted inclined from the central axis of the ferrule. Because of this oblique end face, the light emitted from the fiber is emitted inclined from the ferrule central axis. Due to this inclination, the incident position on the lens is deviated from the lens center. As a result, the focal point formed by the light transmitted through the birefringent element between the ferrule and the lens is deviated from the lens center (ferrule central axis). Become.

  When each optical component is coaxially arranged and incorporated in a single holder as in the conventional structure, the light of the four optical paths is transmitted through the Faraday element as shown by the broken-line circle in FIG. The centers of the four optical paths that reciprocate between the mirror and the mirror pass through a portion deviated from the central portion of the variable Faraday rotator (center of the Faraday element). For this reason, the attenuation characteristic is adversely affected by the distribution of the Faraday rotation angle. On the other hand, when the first holder 32 and the second holder 34 are moved relative to each other on the abutting plane and adjusted in position as in the present invention, the lens and the mirror are reciprocated as indicated by a broken-line circle in FIG. The centers of the four optical paths are transmitted through the central portion of the variable Faraday rotator (the center of the Faraday element). That is, the Faraday rotation angles of the polarization planes of the light transmitted through the four optical paths are substantially equal to each other. Therefore, even if there is a distribution of the Faraday rotation angles, the attenuation characteristic is not adversely affected.

  In the above embodiment, the variable Faraday rotator is configured only by the rotation angle variable unit, but the variable Faraday rotator may be configured by a rotation angle variable unit and a rotation angle fixing unit. For example, the rotation angle variable unit typically includes variable magnetic field applying means that can change a combined magnetic field by applying a magnetic field from two directions, and a Faraday element in which the Faraday rotation angle changes according to the combined magnetic field. Consists of. On the other hand, the rotation angle fixing unit is typically composed of a Faraday element having a constant Faraday rotation angle regardless of the applied synthetic magnetic field. The magnetic field applying means of the variable polarization rotating means and the fixed polarization rotating means preferably share at least a part, but may be configured separately. Here, the maximum Faraday rotation angle of the rotation angle variable unit and the Faraday rotation angle of the rotation angle fixing unit may be set to be equal. When the Faraday element of the rotation angle variable part and the Faraday element of the rotation angle fixing part having different Faraday rotation directions with respect to the same magnetic field direction are used, a high attenuation can be realized.

The schematic block diagram which shows one Example of the reflection type variable optical attenuator which concerns on this invention. Explanatory drawing which shows the specific structural example of this reflection type variable optical attenuator. Explanatory drawing which shows the polarization condition between each optical component. Explanatory drawing which shows distribution of Faraday rotation angle, and the position of an optical path.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Input fiber 12 Output fiber 14 Two-core ferrule 16 Separation-combination birefringence element 18 Convex lens 20 Mirror 22 Faraday element 24 Fiber collimator 30 Cylindrical case 32 First holder 34 Second holder

Claims (3)

For two-core ferrules where the input fiber is located at the input port and the output fiber is located at the output port, and parallel plane type separation and synthesis that separates the light of the same optical path whose polarization direction is orthogonal and combines the light of different optical paths A birefringent element, a light converging lens, and a mirror arranged at the focal position of the lens are arranged in that order, and a variable Faraday rotator is installed between the lens and the mirror, and the variable Faraday rotator In a variable optical attenuator that controls the amount of reflected output light by controlling the rotation angle in the polarization direction with
A fiber collimator incorporated in a cylindrical case so that the two-core ferrule, the separating / combining birefringent element and the lens are positioned coaxially, a first holder that can be accommodated with the fiber collimator being tilt-adjusted, and a variable Faraday The second core ferrule has a structure in which an end face is obliquely polished, and the center of the lens is the center of the two-core ferrule. The abutting surfaces of the first holder and the second holder are movable planes perpendicular to the central axis, and the first and second holders are butted against the movable abutting planes. fixed in a state of positional adjustment, the variable Faraday rotator, said Faibako to match the center of the optical path of the central axis reciprocates lenses and mirrors Reflective variable optical attenuator, characterized in that it is placed offset from the center axis of the meter. Variable Faraday rotator is made from the rotation angle fixing unit and the rotation angle varying portion, the mounting according to claim 1 Symbol Faraday rotation angle of the rotation angle fixing unit and the maximum Faraday rotation angle of the rotation angle varying portion is set to be equal Reflective variable optical attenuator. The variable Faraday rotator is composed of a rotation angle variable part and a rotation angle fixing part, and the Faraday element of the rotation angle variable part and the Faraday element of the rotation angle fixing part are different in the Faraday rotation direction with respect to the same magnetic field direction. The reflective variable optical attenuator according to claim 1 or 2 .

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