JP2010140046A - Optical amplifier system and optical variable attenuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical amplifier system for compensating the wavelength dependence of a gain of an optical amplifier, and an optical variable attenuator. <P>SOLUTION: The optical amplifier system amplifying an optical signal having a predetermined wavelength band includes: an optical amplifier having gain properties with the wavelength dependence; and an optical variable attenuator AAT variably attenuating the optical signal by using the rotation in the polarization direction of the optical signal in a magneto-optical crystal and having attenuation properties with the wavelength dependence substantially reverse to that of the gain in the optical amplifier. The optical signal is attenuated in the optical variable attenuator AAT, and the wavelength dependence of the gain in the optical amplifier is reduced. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical amplifying device and an optical variable attenuator, and more particularly to an optical amplifying device and an optical variable attenuator using a magneto-optic crystal.

  In an optical communication system, it is necessary to adjust the light intensity (power) as necessary, and therefore an optical variable attenuator is used. In the conventional variable attenuator, a substance is attached on a glass substrate so that the transmitted light intensity continuously changes, and the amount of attenuation is changed by mechanically moving the transmission position on the glass substrate. Since the conventional variable attenuator has a mechanical structure, it has problems such as low reliability, low response speed, and large shape. For this reason, it is difficult to incorporate such an optical variable attenuator into a transmission apparatus, and it has been mainly used as a measuring instrument.

  In recent years, the technology of optical fiber amplifiers has advanced, and it has become possible to amplify the light intensity relatively easily. For this reason, studies have been made on a wavelength division multiplexing communication system in which a large number of lights having different wavelengths are transmitted in one optical fiber. FIG. 28 shows a system configuration diagram of a typical wavelength division multiplexing communication system. If an optical fiber amplifier is used, it is possible to amplify a plurality of wavelength-multiplexed optical signals at once, and it is possible to construct an economical optical communication system.

  In a wavelength division multiplexing communication system using an optical fiber amplifier, the quality of a transmission signal is determined by the ratio of optical signal intensity to noise intensity (optical SNR). In order to keep the optical SNR of each optical signal above a predetermined value, it is necessary to align the optical intensities of the optical signals. The level of each optical signal varies due to variations in output power of a light source (generally using a laser diode (LD)), variations in insertion loss of various optical components provided for each light source, and the like. Further, the optical fiber amplifier also has a wavelength dependency of gain, and when passing through the optical fiber amplifier, the power of the optical signal changes for each wavelength. For this reason, it is necessary to incorporate an optical variable attenuator for adjusting and suppressing variations in the power of the optical signal in the optical transmission apparatus.

  As an optical variable attenuator for the above purpose, Japanese Patent Laid-Open No. 6-51255 “Optical Attenuator” has been proposed. FIG. 29 shows a first configuration example of a conventional optical variable attenuator. The optical variable attenuator includes a magneto-optical crystal 1, a polarizer 2, and a first magnetic field applying means 3 that applies a magnetic field parallel to the optical axis to the magneto-optical crystal 1 (in this case, permanent). A magnet is used), and is composed of a second magnetic field applying means 4 for applying a magnetic field perpendicular to the optical axis to the magneto-optical crystal 1. The magneto-optical crystal 1, the first magnetic field application means 3, and the second magnetic field application means 4 constitute a Faraday rotator 9. Here, a permanent magnet is used for the first magnetic field applying means 3, and an electromagnet capable of adjusting the strength of the generated magnetic field by the applied current is used for the second magnetic field applying means 4. The linearly polarized light beam 5 that has passed through another polarizer (not shown) passes through the magneto-optical crystal 1 and the polarizer 2 in that order.

  In this optical variable attenuator, a combined vector (synthetic magnetic field) of the magnetic field vector generated by the first magnetic field applying means 3 and the magnetic field vector generated by the second magnetic field applying means 4 is added to the magneto-optical crystal 1. It is done. At this time, when the magnetic field vector generated by the first magnetic field applying unit 3 is larger than the saturation magnetic field, the combined vector is also larger than the saturation magnetic field. In this case, the magneto-optical crystal 1 is substantially in a state where the internal magnetic domains are integrated into one, and the loss of the light beam 5 that occurs when the magneto-optical crystal 1 has many magnetic domains is reduced.

When the strength of the magnetic field of the second magnetic field applying means 4 is adjusted by the applied current, the direction of the combined magnetic field also changes according to the applied current. Depending on the intensity of the component (magnetization vector) in the same direction as the light beam 5 of the combined magnetic field, the polarization direction of the light beam 5 is rotated by the Faraday effect. This Faraday rotation angle θ is generally expressed by the following equation.
θ = V ・ L ・ H (1)
V is a Verde constant and is determined by the material of the magneto-optical crystal 1. L indicates the optical path length in the magneto-optical crystal 1, and H indicates the strength of the magnetic field. The light beam 5 whose polarization direction has been rotated by the magneto-optical crystal 1 proceeds to the polarizer 2. At this time, when the polarization direction of the polarizer 2 matches the polarization direction of the light beam 5, all the light beams 5 pass through the polarizer 2. If the polarization directions of both do not match, only the component of the polarization direction of the polarizer 2 of the light beam 5 passes. When the two polarization directions have an angle difference of 90 degrees, the light beam 5 does not pass through the polarizer 2 and the attenuation is maximized.

  JP-A-6-512255 “Optical Attenuator” shows another variable optical attenuator. FIG. 30 shows a second configuration example of a conventional optical variable attenuator. The optical variable attenuator includes an optical fiber 6a, a lens 7a, a wedge-shaped birefringent crystal 8a, a Faraday rotator 9 shown in FIG. 29, a wedge-shaped birefringent crystal 8b, a lens 7b, a light It is comprised with the fiber 6b. In this optical variable attenuator, a part of the light beam is guided to the optical fiber 6b by utilizing the birefringence of the light beam supplied from the optical fiber 6a by the birefringence crystals 8a and 8b. Since the amount guided to the optical fiber 6b can be adjusted by the rotation angle of the polarization direction of the light beam in the Faraday rotator 9, the power of the light beam can be variably attenuated.

  Unlike the optical variable attenuator shown in FIG. 29, the present optical variable attenuator can operate regardless of the polarization direction of the light beam supplied from the optical fiber 6a. The variable optical attenuator described above does not have a mechanical movable part and can be miniaturized.

JP-A-6-512255

  In the first configuration example of the conventional optical variable attenuator shown in FIG. 29, a garnet thick film having a YIG or Faraday effect is generally used as a magneto-optical crystal (Faraday element) used for the optical variable attenuator. . Such a Faraday element generally has wavelength dependency and temperature dependency with respect to a rotation angle. Table 1 shows the wavelength dependency and temperature dependency of the Faraday rotation angle of the Faraday element.

  Table 1 shows changes in the Faraday rotation angle with respect to changes in wavelength and temperature when the Faraday rotation angle in the 1550 nm band is 45 degrees. The characteristics of the garnet thick film vary depending on its composition. The above example shows a relatively large case. Table 1 above shows that the Faraday rotation angle decreases as the wavelength or temperature increases.

  FIG. 31 shows the relationship between the magnetic field H and the Faraday rotation angle. In FIG. 31, when the magnetic field H is increased, the Faraday rotation angle increases with a gradient V × L, and the Faraday rotation angle is saturated with a magnetic field of a predetermined magnitude or more. This indicates that the magnetic domain inside the magneto-optic crystal has become a single magnetic domain. In FIG. 31, the slope V × L changes as the temperature or wavelength changes. This indicates that the Verde constant has wavelength dependency and temperature dependency.

  Furthermore, in order to incorporate the optical variable attenuator into the optical transmission apparatus, it is necessary to reduce the size of the optical variable attenuator and reduce the drive current.

  Further, in the second configuration example of the conventional optical variable attenuator shown in FIG. 30, a slight loss due to polarization dependency (Polarization Dependent Loss: PDL) still occurs.

  A first object of the present invention is to provide an optical amplifying apparatus and an optical variable attenuator that can compensate for the wavelength dependence of the gain of an optical amplifier in view of the above problems.

  A second object of the present invention is to provide an optical variable attenuator capable of correcting an attenuation amount.

  A third object of the present invention is to provide an optical variable attenuator that can be miniaturized.

  A fourth object of the present invention is to provide an optical variable attenuator capable of reducing power consumption.

  A fifth object of the present invention is to provide an optical variable attenuator that can reduce loss due to polarization dependence.

  In order to solve the above problems, the present invention is characterized by the following measures.

  According to the first aspect of the present invention, there is provided an optical amplifying apparatus for amplifying an optical signal having a predetermined wavelength band, an optical amplifier having a wavelength-dependent gain characteristic, and a polarization direction of the optical signal in the magneto-optical crystal. The optical signal is variably attenuated by using rotation, and an optical variable attenuator having an attenuation characteristic having a wavelength dependency substantially opposite to the wavelength dependency of the gain in the optical amplifier. In the attenuator, the optical signal is attenuated, and the wavelength dependence of the gain in the optical amplifier is reduced.

  According to a second aspect of the present invention, in the optical amplifying device according to the first aspect, the optical variable attenuator includes a magneto-optical crystal that variably rotates a polarization direction of the optical signal, and light that has passed through the magneto-optical crystal. An analyzer that allows a signal to pass in accordance with the polarization direction of the analyzer, the polarization direction of the analyzer, the polarization direction of the optical signal, and the magneto-optic crystal have the reverse wavelength dependence at a predetermined attenuation. Is set so as to be substantially obtained.

  According to a third aspect of the present invention, there is provided an optical variable attenuator connected to an optical amplifier for reducing wavelength dependence of gain in the optical amplifier and attenuating an optical signal, wherein the polarization direction of the optical signal is variable. A magneto-optical crystal that rotates the optical signal and an analyzer that passes the optical signal that has passed through the magneto-optical crystal according to the polarization direction of the magneto-optical crystal, the polarization direction of the analyzer, the polarization direction of the optical signal, and the magnetic The optical crystal is set so as to obtain a wavelength dependence substantially opposite to the wavelength dependence of the gain of the optical amplifier at a predetermined attenuation.

  According to a fourth aspect of the present invention, there is provided an optical variable attenuator for attenuating the power of the light beam, the magneto-optical crystal variably rotating the polarization direction of the light beam, and the light beam that has passed through the magneto-optical crystal. An analyzer for guiding at least a part to the output of the optical variable attenuator; and an output-side light receiver for branching a part of the output light of the optical variable attenuator to monitor the output power. The polarization direction of the light beam in the magneto-optic crystal is controlled so that the output power of the monitored variable optical attenuator becomes a predetermined value.

  According to a fifth aspect of the present invention, there is provided a variable optical attenuator for attenuating the power of the light beam, the magneto-optical crystal that variably rotates the polarization direction of the light beam, and the light beam that has passed through the magneto-optical crystal. An analyzer that leads at least a part to the output of the optical variable attenuator, an input-side light receiver that monitors the input power of the light beam input to the magneto-optical crystal, and the output power of the optical variable attenuator An output-side photoreceiver, and a ratio between an input power of the light beam monitored by the input-side photoreceiver and an output power of the optical variable attenuator monitored by the output-side photoreceiver becomes a predetermined value. Further, the polarization direction of the light beam in the magneto-optic crystal is controlled.

  According to a sixth aspect of the present invention, in the optical variable attenuator according to the fourth or fifth aspect, the analyzer includes a birefringent crystal, and a part of the light beam whose polarization is separated by the analyzer is output as the output. It further has an aperture leading to the side light receiver.

  According to a seventh aspect of the present invention, there is provided an optical variable attenuator for attenuating the power of a light beam, a magneto-optical crystal that variably rotates the polarization direction of the light beam, and a magnetic field applied to the magneto-optical crystal. And a housing for accommodating the magneto-optic crystal and the magnetic circuit and mounting the magnetic circuit on a substrate, wherein the direction of the gap is substantially the housing. It is mounted on the housing so as to be in the height direction of the body.

  According to an eighth aspect of the present invention, there is provided a variable optical attenuator for attenuating the power of a light beam, comprising a magneto-optical crystal that variably rotates the polarization direction of the light beam, and at least partly a semi-hard magnetic material. And a magnetic circuit that electrically generates a magnetic field to be applied to the magneto-optic crystal by a driving current, and the magnetic field is maintained even when the supply of the driving current is stopped. And

  According to a ninth aspect of the present invention, in the optical variable attenuator according to the eighth aspect, the yoke partially includes a plurality of semi-hard magnetic bodies having different magnetizations in a saturated state, and the semi-hard magnetic bodies are provided for each semi-hard magnetic body. The magnetic field generated in the magnetic circuit can be changed stepwise by controlling the magnetization.

  The apparatus according to claim 10 is an optical variable attenuator for attenuating the power of the light beam, the first wedge-shaped birefringent crystal birefringing the light beam, and the first wedge-shaped birefringent crystal. A magneto-optical crystal that variably rotates the polarization direction of the light beam separated by polarization, a magnetic circuit that generates a magnetic field to be applied to the magneto-optical crystal substantially perpendicularly to the light beam, and the magnetism A second wedge-shaped birefringent crystal that birefringes the light beam output from the optical crystal, and is substantially in a plane constituted by the light beam separated in polarization by the first wedge-shaped birefringent crystal. In particular, the magnetic field of the magnetic circuit is applied to the magneto-optical crystal vertically.

  3. The optical amplifying device according to claim 1 or 2, and the optical variable attenuator according to claim 3, wherein the wavelength of the attenuation is adjusted by adjusting the polarization direction of the analyzer, the polarization direction of the optical signal, and the magneto-optical crystal. Dependencies can be set arbitrarily. Therefore, the wavelength dependence of the gain of the optical amplifier can be compensated without using an optical filter for gain equalization. Further, in this optical variable attenuator, the wavelength dependency can be increased as the attenuation amount is increased. Accordingly, it is possible to satisfactorily cancel the wavelength dependence of the gain of the optical amplifier when the upper limit value of the optical pumping power is small. Therefore, the power of the pumping light of the optical fiber amplifier can be set small, and the optical fiber amplifier can be reduced in size and power consumption.

  7. The optical variable attenuator according to claim 4, wherein the output power of the optical variable attenuator monitored by the output side photoreceiver is controlled so as to become a predetermined value, or the input side photoreceiver. The ratio of the power of the light beam monitored in step 1 to the output power of the variable optical attenuator monitored by the output side light receiver is controlled to be a predetermined value. Accordingly, it is possible to correct the temperature characteristics of the attenuation amount of the optical variable attenuator, deterioration with time, polarization loss fluctuation, and the like.

  In the optical variable attenuator according to the seventh aspect, when the magnetic circuit is constituted by the ring-shaped yoke, it is only necessary to secure a space corresponding to the radius of the ring-shaped yoke on the upper side and the lower side of the light beam. Therefore, the height of the optical variable attenuator can be reduced.

  In the optical variable attenuator according to claim 8, the yoke includes a semi-hard magnetic material at least partially. Therefore, the yoke is magnetized by applying the pulse current, and the magnetization is maintained even when the supply of current is stopped. Therefore, the power consumption of the optical variable attenuator can be reduced.

  In the optical variable attenuator according to the ninth aspect, the magnetic field generated by the electromagnet can be set in a stable and stepwise manner by controlling a plurality of semi-hard magnetic bodies having different magnetizations.

  In the variable optical attenuator according to the tenth aspect, a bias magnetic field for unifying the magnetic domains of the magneto-optic crystal is applied substantially perpendicular to the refractive plane. Thereby, polarization dependent loss can be reduced.

  As described above, the present invention has the following effects.

  3. The optical amplifying device according to claim 1 or 2, and the optical variable attenuator according to claim 3, wherein the wavelength of the attenuation is adjusted by adjusting the polarization direction of the analyzer, the polarization direction of the optical signal, and the magneto-optical crystal. Dependencies can be set arbitrarily. Therefore, the wavelength dependence of the gain of the optical amplifier can be compensated without using an optical filter for gain equalization. Further, in this optical variable attenuator, the wavelength dependency can be increased as the attenuation amount is increased. Accordingly, it is possible to satisfactorily cancel the wavelength dependence of the gain of the optical amplifier when the upper limit value of the optical pump power is small. Therefore, the power of the pumping light of the optical fiber amplifier can be set small, and the optical fiber amplifier can be reduced in size and power consumption.

  7. The optical variable attenuator according to claim 4, wherein the output power of the optical variable attenuator monitored by the output side photoreceiver is controlled so as to become a predetermined value, or the input side photoreceiver. The ratio of the power of the light beam monitored in step 1 to the output power of the variable optical attenuator monitored by the output side light receiver is controlled to be a predetermined value. Accordingly, it is possible to correct the temperature characteristics of the attenuation amount of the optical variable attenuator, deterioration with time, polarization loss fluctuation, and the like.

  In the optical variable attenuator according to the seventh aspect, when the magnetic circuit is constituted by the ring-shaped yoke, it is only necessary to secure a space corresponding to the radius of the ring-shaped yoke on the upper and lower sides of the light beam. Therefore, the height of the optical variable attenuator can be reduced.

  In the optical variable attenuator according to claim 8, the yoke includes a semi-hard magnetic material at least partially. Therefore, the yoke is magnetized by applying the pulse current, and the magnetization is maintained even when the supply of current is stopped. Therefore, the power consumption of the optical variable attenuator can be reduced.

  In the optical variable attenuator according to the ninth aspect, the magnetic field generated by the electromagnet can be set in a stable and stepwise manner by controlling a plurality of semi-hard magnetic bodies having different magnetizations.

  In the variable optical attenuator according to the tenth aspect, a bias magnetic field for unifying the magnetic domains of the magneto-optic crystal is applied substantially perpendicular to the refractive plane. Thereby, polarization dependent loss can be reduced.

1 is a configuration example of an optical variable attenuator according to the present invention. An arrangement example of a polarizer (P), a Faraday element (FR), and an analyzer (A). (A) is called 0 degree arrangement, and when the polarization direction of the polarizer and the polarization direction of the analyzer are parallel, (B) is called 45 degree arrangement, the polarization direction of the polarizer and the polarization direction of the analyzer. (C) is called 90 degree arrangement, and the polarization direction of the polarizer and the polarization direction of the analyzer are orthogonal to each other. Calculation result of attenuation with respect to Faraday rotation angle in the case of 0 degree arrangement. Calculation result of attenuation with respect to Faraday rotation angle in the case of 45 degree arrangement. Calculation result of attenuation with respect to Faraday rotation angle in the case of 90 degree arrangement. The typical relationship figure of the amount of change of the Faraday rotation angle with respect to the change of a Faraday rotation angle and a wavelength or temperature. Attenuation characteristics with respect to wavelength when the angle difference between the polarization directions of the polarizer and the analyzer is 0 degree. (A) is a change in an arbitrary attenuation with respect to the wavelength, and (B) is a deviation of an arbitrary attenuation with respect to the wavelength. Attenuation characteristics with respect to wavelength when the angle difference between the polarization directions of the polarizer and the analyzer is 45 degrees. (A) is a change in arbitrary attenuation with respect to the wavelength, and (B) is a deviation of arbitrary attenuation with respect to the wavelength. The attenuation characteristic with respect to the wavelength when the angle difference between the polarization directions of the polarizer and the analyzer is 70 degrees. (A) is a change in arbitrary attenuation with respect to the wavelength, and (B) is a deviation of arbitrary attenuation with respect to the wavelength. Attenuation characteristics with respect to wavelength when the angle difference between the polarization direction of the polarizer and the analyzer is 80 degrees. (A) is a change in arbitrary attenuation with respect to the wavelength, and (B) is a deviation of arbitrary attenuation with respect to the wavelength. Attenuation characteristics with respect to wavelength when the angle difference between the polarization directions of the polarizer and the analyzer is 90 degrees. (A) is a change in arbitrary attenuation with respect to the wavelength, and (B) is a deviation of arbitrary attenuation with respect to the wavelength. 6 shows another configuration example of an optical variable attenuator according to the present invention. The figure for demonstrating the 2nd principle of the optical variable attenuator concerning this invention. 14 shows a modification example of the optical variable attenuator shown in FIG. The example of a change of the optical variable attenuator shown in FIG. 1 is a configuration example of a magnetic circuit of an optical variable attenuator according to the present invention. 17 shows a modification example of the magnetic circuit of the optical variable attenuator shown in FIG. (A) is sectional drawing seen from the top, (B) is sectional drawing seen from the side. The other structural example of the optical variable attenuator of this invention. Typical EDFA amplification characteristics. 2 is a configuration example of an optical transmission device in which an optical variable attenuator is incorporated. Attenuation characteristics of an optical variable attenuator adjusted to cancel the wavelength dependence of an optical fiber amplifier. The structural example for demonstrating the 5th principle of the optical variable attenuator concerning this invention. The example of a change of the optical variable attenuator shown in FIG. The structural example for demonstrating the 6th principle of the optical variable attenuator concerning this invention. (A) is an external view, (B) is a top view, and (C) is a front view. The figure which shows the structure of the electromagnet used for the optical variable attenuator concerning this invention. The structural example for demonstrating the 7th principle of the optical variable attenuator concerning this invention. (A) is a top view, (B) is a side view. The figure which shows the direction pattern of the bias magnetic field for demonstrating the 7th principle of the optical variable attenuator concerning this invention. 1 is a system configuration diagram of a typical wavelength multiplexing communication system. 1 is a first configuration example of a conventional optical variable attenuator. The 2nd structural example of the conventional optical variable attenuator. Relationship between magnetic field H and Faraday rotation angle.

  First, the first principle of the present invention will be described. FIG. 1 is a configuration example of an optical variable attenuator according to the present invention. This optical variable attenuator includes a polarizer (P) 10, a Faraday element (FR) 20 that is a magneto-optical crystal, and an analyzer (A) 30. The light beam 5 is supplied in the order of the polarizer 10, the Faraday element 20, and the analyzer 30.

  The optical variable attenuator further includes a permanent magnet 40 for applying a magnetic field to the Faraday element 20, and an electromagnet 50 including a yoke 52 and a coil 54. A magnetic field generated by the permanent magnet 40 is applied to the Faraday element 20 in a direction perpendicular to the direction of the light beam 5, and a magnetic field generated by the electromagnet 50 is applied to the Faraday element 20 in the same direction as the direction of the light beam 5.

  When the light beam 5 is supplied to the polarizer 10, linearly polarized light having the same polarization direction as that of the polarizer 10 is output. The linearly polarized light passes through the Faraday element 20, and at this time, the polarization direction of the transmitted light is rotated by the Faraday effect according to the magnitude of the magnetization vector generated in the direction of the light beam 5. The light beam 5 whose polarization direction has been rotated is supplied to the analyzer 30.

  The magnetic field generated by the permanent magnet 40 is sufficiently large so that the magnetic domain in the Faraday element 20 is single. Therefore, the combined magnetic field of the permanent magnet 40 and the electromagnet 50 is also sufficiently large, so that the loss of the light beam 5 in the Faraday element 20 can be very small. The magnitude of the magnetic field of the electromagnet 50 can be changed by the current applied to the coil 54, and thereby the direction of the combined magnetic field can also be changed. At this time, the polarization direction of the light beam 5 is rotated by the Faraday effect by the component (magnetization vector) in the same direction as the light beam 5 in the combined magnetic field. When the polarization direction of the light beam 5 rotated by the Faraday effect does not coincide with the polarization direction of the analyzer 30, a part or all of the light beam 5 is blocked by the analyzer 30, and the light beam 5 is attenuated.

  The above operation is substantially the same as that of a conventional optical variable attenuator using a magneto-optical crystal. Further, in the present invention, the polarizer 10 and the analyzer 30 are configured such that the polarization direction of the light beam 5 in a state where there is no Faraday rotation in the Faraday element 20 (the magnetic field in the optical axis direction is zero) is substantially the same as the polarization direction of the analyzer 30. It is comprised so that it may be in an orthogonal state. Thereby, the temperature dependence and wavelength dependence of the attenuation amount of the optical variable attenuator can be reduced. The orthogonal state can also be set by inserting a wave plate (which can rotate polarized light) in the optical path and adjusting the arrangement of the polarizer and the analyzer. For example, even if the polarizer and the analyzer are set to 0 degree arrangement, the 90 degree arrangement can be substantially set by inserting a wave plate and rotating the polarized light by 90 degrees.

  The principle and operation will be described below. In the conventional optical variable attenuator, the angle difference between the polarization direction of the polarizer 10 and the polarization direction of the analyzer 30 can be set to an arbitrary value, but here, in order to simplify the explanation, three kinds of angle differences ( Consider placement. FIG. 2 shows an arrangement example of the polarizer (P), the Faraday element (FR), and the analyzer (A). 2A is called 0 degree arrangement, and when the polarization direction of the polarizer and the polarization direction of the analyzer are parallel, FIG. 2B is called 45 degree arrangement and the polarization direction of the polarizer. 2C is referred to as a 90-degree arrangement, and the polarization direction of the polarizer and the polarization direction of the analyzer are orthogonal to each other. The arrangement shown in FIG. 2C is applied to the optical variable attenuator according to the present invention.

  The attenuation amount A of the optical variable attenuator is expressed by the following equation, where θ is the relative angle between the polarization direction of the light rotated by the Faraday element and the polarization direction of the detector.

A = 10 log (cos ² (90−θ + E)) + Lo (2)
E: extinction ratio (true number), Lo: loss (dB)
Here, E is the extinction ratio of the optical component that constitutes the optical variable attenuator, and Lo is the internal loss of the optical component. From this equation, the attenuation amount A of the optical variable attenuator increases in accordance with cos ² θ. FIG. 3 shows the calculation result of the attenuation with respect to the Faraday rotation angle in the case of the 0 degree arrangement. FIG. 4 shows the calculation result of the attenuation with respect to the Faraday rotation angle in the case of the 45 degree arrangement. FIG. 5 shows the calculation result of the attenuation with respect to the Faraday rotation angle in the case of the 90 degree arrangement. In the above diagram, the Faraday rotation angle is referred to as Control Angle (deg).

  In the 0 degree arrangement of (A), when the Faraday rotation angle is 0 degree (applied magnetic field is zero), the attenuation amount is the smallest, and as the Faraday rotation angle increases, the attenuation amount increases, and the Faraday rotation angle of 90 degrees. The amount of attenuation becomes maximum at. In this case, the change in attenuation is moderate for Faraday rotation angles up to about 20 degrees, and the change in attenuation with respect to the rotation angle is large at Faraday rotation angles near 90 degrees. In order to achieve the above control, the length L of the Faraday element needs to be a length that can be rotated by 90 degrees or more.

  In the 45 degree arrangement of (B), when the Faraday rotation angle is 0 degree (applied magnetic field is zero), the attenuation is 3 dB. When the rotation angle is set to -45 degrees, the attenuation is minimized, and when it is set to +45 degrees, the attenuation is maximized. In this case, at the Faraday rotation angle near 45 degrees, the change in the amount of attenuation with respect to the rotation angle is large. In the above control, a reverse Faraday rotation can be obtained by applying a reverse current. Therefore, the length L of the Faraday element may be a length that allows rotation by 45 degrees or more. Therefore, half the length of the Faraday element in the case of (A) is sufficient.

  In the 90 degree arrangement of (C), when the Faraday rotation angle is 0 degree (applied magnetic field is zero), the attenuation amount is the largest, and as the Faraday rotation angle is increased, the attenuation amount decreases and the Faraday rotation angle is 90 degrees. The amount of attenuation is minimum at. In this case, the change in attenuation with respect to the rotation angle is large for the Faraday rotation angle near 0 degrees, and the change in attenuation with respect to the rotation angle is small at the Faraday rotation angle near 90 degrees. In order to achieve the above control, the length L of the Faraday element needs to be a length that can be rotated by 90 degrees or more.

  As described above, in the vicinity of the maximum attenuation, a slight change in the Faraday rotation angle causes a sudden change in the attenuation. However, as a result of examination, it was found that the amount of change in the Faraday rotation angle with respect to changes in wavelength or temperature depends on the Faraday rotation angle. FIG. 6 is a schematic relationship diagram between the Faraday rotation angle and the amount of change in the Faraday rotation angle with respect to a change in wavelength or temperature. The change amount of the Faraday rotation angle is proportional to the Faraday rotation angle. That is, when the Faraday rotation angle is 0 degree (the applied magnetic field is zero), the amount of change in the Faraday rotation angle due to a change in wavelength or temperature is zero, and the amount of change in the Faraday rotation angle increases as the Faraday rotation angle increases. .

  Therefore, when no Faraday rotation occurs (when the component parallel to the light beam of the combined magnetic field of the two magnets is substantially zero), the amount of change in the Faraday rotation angle due to a change in wavelength or temperature is minimal ( In this state, if the polarizer and the analyzer are arranged so as to obtain the maximum attenuation, the change in the attenuation with respect to the change in the Faraday rotation angle becomes small. Therefore, the temperature dependency and wavelength dependency of the attenuation amount can be reduced. Such an arrangement corresponds to the 90 degree arrangement of (C) above.

  That is, when the Faraday rotation angle at which the change amount of the Faraday rotation angle due to a change in wavelength or temperature is minimum is zero degrees, a maximum attenuation amount with a large change amount of the attenuation amount with respect to the Faraday rotation angle is obtained. At a large Faraday rotation angle at which the amount of change increases, a small amount of attenuation indicating a gradual change in attenuation is obtained.

  Table 2 shows the characteristics of the polarizer and analyzer in the case of 0 degree arrangement, 45 degree arrangement, and 90 degree arrangement.

  In the case of 0 degree arrangement and 90 degree arrangement, there is no distinction between input and output ports. The same attenuation characteristic can be obtained from either input. On the other hand, in the case of the 45 degree arrangement, non-reciprocal operation is performed when the input / output ports are replaced. When the current is 0, attenuation is 3 dB regardless of which is input, but when the light beam is transmitted without attenuation from one side, the attenuation is maximum from the opposite direction. That is, it operates as an isolator.

  7 to 11 show the attenuation characteristics with respect to the wavelength when the angle differences between the polarization directions of the polarizer and the analyzer are 0 degree, 45 degrees, 70 degrees, 80 degrees, and 90 degrees, respectively. (A) of each figure shows the change of the arbitrary attenuation with respect to the wavelength, and (B) of each figure shows the deviation of the arbitrary attenuation with respect to the wavelength. In (B), the deviation is normalized at a wavelength of 1545 nm.

  In the 0 degree arrangement of FIG. 7, when obtaining an attenuation of 20 dB or more, the deviation of the attenuation with respect to the wavelength is large. On the other hand, in the 90 degree arrangement of FIG. 11, the deviation of the attenuation amount with respect to the wavelength is very small even for the attenuation amount of 35 dB or more. Further, when the attenuation is 1 dB, the Faraday rotation angle is large. For example, the change in the Faraday rotation angle is about ± 2.5 degrees with respect to a wavelength variation of ± 15 nm. However, as shown in FIG. 11B, the deviation of the attenuation at that time is ± 0.01 dB or less and does not affect the operation of optical transmission.

  When the optical variable attenuator is applied to an optical transmission device, generally, an attenuation of 0 to 20 dB is often used. Therefore, when the deviation with respect to the attenuation of 0 to 20 dB is calculated, it was found that the deviation in the case of the 80 degree arrangement shown in FIG. 10 is the smallest. Furthermore, in consideration of general use conditions, if the angle difference between the polarization directions of the analyzer and the polarizer is arranged within a range of about 80 ° ± 30 °, the wavelength dependence can be sufficiently reduced in practice. it can.

  Therefore, in the optical variable attenuator of the present invention in FIG. 1, the polarization direction of the light beam 5 in the state where there is no Faraday rotation in the Faraday element 20 is approximately orthogonal to the polarization direction of the analyzer. Configured as follows. Furthermore, it is desirable that the angle difference between the polarization directions of the polarizer and the analyzer is 80 degrees ± 30 degrees.

  The present invention can be applied to other configuration examples of the variable optical attenuator in addition to the configuration example shown in FIG. FIG. 12 shows another configuration example of the optical variable attenuator according to the present invention. This optical variable attenuator includes a polarizer 10, a Faraday element 20, and an analyzer 30 as in the optical variable attenuator of FIG. Further, the polarizer 10 and the analyzer 30 are arranged so that their polarization directions are substantially orthogonal. Here, in order to simplify the description, the difference in polarization direction is 90 degrees.

  The optical variable attenuator shown in FIG. 12 further includes a permanent magnet 42 for applying a magnetic field to the Faraday element 20, and an electromagnet 55 including a yoke 57 and a coil 59. The permanent magnet 42 is composed of two magnets having a donut-shaped hole, and the light beam 5 passes through the hole. A magnetic field generated by the permanent magnet 42 is applied to the Faraday element 20 in a direction parallel to the direction of the light beam 5, and a magnetic field generated by the electromagnet 55 is applied to the Faraday element 20 in a direction perpendicular to the direction of the light beam 5.

  The light beam 5 having linearly polarized light output from the polarizer 10 passes through the Faraday element 20 through the hole of the permanent magnet 42. The light beam 5 whose polarization direction has been Faraday rotated is supplied to the analyzer 30 through the hole of another permanent magnet 42. The magnetic field generated by the permanent magnet 42 is sufficiently large so that the magnetic domains in the Faraday element 20 are single. Therefore, the combined magnetic field of the permanent magnet 42 and the electromagnet 55 is also sufficiently large, so that the loss of the light beam 5 in the Faraday element 20 can be very small.

  In this optical variable attenuator, when the current applied to the coil 59 is zero, the magnetic field of the permanent magnet 42 is applied only in the direction of the light beam 5. At this time, the polarization direction of the light beam 5 is largely Faraday rotated, and when the Faraday rotation angle is 90 degrees, the attenuation is minimized. On the other hand, when the current applied to the coil 59 is increased, the magnetization vector in the direction of the light beam 5 is decreased and the Faraday rotation angle is also decreased. When the Faraday rotation angle is substantially 0 degrees (when the direction of the combined magnetic field of the permanent magnet 42 and the electromagnet 55 is substantially perpendicular to the direction of the light beam 5), the amount of attenuation is maximized. The relationship between the Faraday rotation angle and the attenuation is the same as the relationship shown in FIG.

  When the Faraday rotation angle is large, the amount of change in the Faraday rotation angle with respect to the change in wavelength is also large. However, in this case, as shown in FIG. 5, the amount of change in attenuation with respect to the change in Faraday rotation angle is small. Therefore, the amount of change in attenuation with respect to the change in wavelength can be reduced.

  Further, when the Faraday rotation angle is small, the amount of change in the Faraday rotation angle with respect to the change in wavelength is also small. Therefore, in this case, although the amount of change in attenuation with respect to the change in Faraday rotation angle is large, the amount of change in attenuation with respect to change in wavelength can be reduced. In the present optical variable attenuator, the amount of change in attenuation can be similarly reduced with respect to temperature change. When this optical variable attenuator is applied to an optical transmission device, the angle difference between the polarization directions of the polarizer and the analyzer is preferably 80 ° ± 30 °, as in the optical variable attenuator shown in FIG.

  The arrangement of the polarization direction of the polarizer and the analyzer in the variable optical attenuator according to the present invention can be applied to various magnetic circuit configurations regardless of the arrangement of the magnetic circuit. Next, the second principle of the optical variable attenuator according to the present invention will be described. In the optical variable attenuator according to the present invention, the optical variable attenuator is always in the transmission state when the current applied to the electromagnet is zero.

  Although the 90 degree arrangement of the polarizer and the analyzer described above can greatly reduce the wavelength dependency and temperature dependency, in the case of the configuration of the magnetic circuit (permanent magnet 40 and electromagnet 50) of the optical variable attenuator shown in FIG. When the current (drive current) applied to the coil 54 of the electromagnet 50 is zero, the amount of attenuation becomes maximum. When the drive current is cut off due to a failure of the control circuit or the like, the attenuation amount is automatically maximized, and this becomes a fail-safe function. However, since light does not transmit, there is a possibility of affecting the device assembly. In practice, the latter drawback is more common.

  FIG. 13 is a diagram for explaining the second principle of the variable optical attenuator according to the present invention. The optical variable attenuator shown in FIG. 13 is provided with an electromagnet 60 instead of the electromagnet 50 as compared with the optical variable attenuator shown in FIG. The electromagnet 60 includes a yoke 62 that houses a permanent magnet 66 and a coil 64. For convenience of explanation, the permanent magnet 40 is omitted. Other configurations are the same as those of the optical variable attenuator of FIG. Accordingly, the polarizer 10 and the analyzer 30 are installed so that the angle difference between their polarization directions is 90 degrees. Elements having the same functions as those of the optical variable attenuator in FIG. 1 are denoted by the same reference numerals.

  In the variable optical attenuator of FIG. 13, a bias magnetic field is applied in the direction of the light beam 5 by the permanent magnet 66 in the electromagnet 60. The intensity of the bias magnetic field is set so that the Faraday rotation angle in the Faraday element 20 is 90 degrees. Further, the magnetic field of the electromagnet 60 generated when a current is applied to the coil 64 operates to cancel the bias magnetic field of the permanent magnet 66.

  In this optical variable attenuator, when the current applied to the coil 64 is zero, only the bias magnetic field is applied in the direction of the light beam 5 by the permanent magnet 66, and the polarization direction of the light beam 5 is 90 degrees in the Faraday element 20. Rotated. Therefore, the polarization direction of the rotated light beam 5 coincides with the polarization direction of the analyzer 30, and the transmittance of the variable optical attenuator is maximized. On the other hand, when the current applied to the coil 64 is increased, the bias magnetic field is canceled and the Faraday rotation is decreased. As a result, the attenuation is increased.

  Therefore, in this optical variable attenuator, even if the current applied to the electromagnet 60 does not flow due to a failure or the like, the optical beam can be transmitted and the wavelength / temperature dependency can be reduced. FIG. 14 shows a modification of the optical variable attenuator shown in FIG. The optical variable attenuator shown in FIG. 14 is provided with a Faraday element 22 having a half length instead of the Faraday element 20 as compared with the optical variable attenuator shown in FIG. The length of the Faraday element 22 is selected so that the Faraday rotation of −45 degrees is performed by the bias magnetic field of the permanent magnet 66 in the electromagnet 60. Other configurations are the same as those of the optical variable attenuator of FIG. Accordingly, the polarizer 10 and the analyzer 30 are installed so that the angle difference between their polarization directions is 90 degrees. Elements having the same functions as those of the optical variable attenuator in FIG. 13 are denoted by the same reference numerals.

  In this optical variable attenuator, when the current applied to the electromagnet 60 is zero, only the bias magnetic field by the permanent magnet 66 is applied to the Faraday element 22 and -45 degree Faraday rotation is performed. At this time, the transmittance is about 50%. When a current is applied to the electromagnet 60 in the positive direction, the bias magnetic field is reduced by the magnetic field generated by the electromagnet 60, and the Faraday rotation angle is also reduced. When the Faraday rotation angle is 0 degree, the amount of attenuation is maximum.

  On the other hand, when a current is applied to the electromagnet 60 in the negative direction, a magnetic field generated by the electromagnet 60 is added to the bias magnetic field, and the Faraday rotation angle increases in the negative direction. When the Faraday rotation angle becomes −90 degrees, the transmittance is maximized. Accordingly, even in the present optical variable attenuator, even if the current applied to the electromagnet 60 does not flow due to a failure or the like, the optical beam can be transmitted about 50%, and the wavelength / temperature dependency can be reduced. . Furthermore, since the Faraday rotation angle can be reduced to 45 degrees, the power supplied to the electromagnet can be reduced, and the power consumption of the optical variable attenuator can be reduced.

  In the optical variable attenuator shown in FIGS. 13 and 14, the structure in which the permanent magnet is embedded in the yoke is illustrated. However, since the magnetic permeability of the material of the yoke is very high, the same effect can be obtained only by bringing the permanent magnet close to the yoke. FIG. 15 shows a modification of the optical variable attenuator shown in FIG. In the optical variable attenuator shown in FIG. 15, as compared with the optical variable attenuator shown in FIG. 14, an electromagnet 50 that does not include a permanent magnet is provided instead of the electromagnet 60, and the bias field is applied to the Faraday element 22 from an oblique direction. The permanent magnet 70 for adding is provided. Other configurations are the same as those of the optical variable attenuator of FIG. Elements having the same functions as those of the optical variable attenuator in FIG. 14 are denoted by the same reference numerals.

  In the optical variable attenuator of FIG. 14 described above, a bias magnetic field is applied in parallel with the light beam 5 by a permanent magnet 66 provided in the yoke 62. In this case, in order to make a single magnetic domain in the Faraday element 22, a bias magnetic field can be further applied in a direction perpendicular to the light beam 5 by another permanent magnet, as in the optical variable attenuator of FIG. it can. At this time, the Faraday element 22 is applied with a combined magnetic field obtained by vector combining these bias magnetic fields. In the optical variable attenuator shown in FIG. 15, this combined magnetic field can be formed by one permanent magnet 70.

  Therefore, this optical variable attenuator can have the same effect as the optical variable attenuator shown in FIG. 14 with a simpler configuration. Further, the present invention is applicable not only to the optical variable attenuator shown in FIG. 14 but also to other configuration examples including the optical variable attenuator of FIG. Next, the third principle of the optical variable attenuator according to the present invention will be described. When an optical variable attenuator is incorporated in the device, it is necessary to reduce the driving power applied to the coil of the magnetic circuit that controls the attenuation of the optical variable attenuator in order to reduce the power consumption of the device. For this purpose, it is necessary to efficiently apply the magnetic field generated by the magnetic circuit to the Faraday element.

  FIG. 16 is a configuration example of a magnetic circuit of an optical variable attenuator according to the present invention. In FIG. 16, an electromagnet 80 composed of a yoke 82 and a coil 84 and the Faraday element 20 are shown. The Faraday element 20 is inserted into the gap of the yoke 82 without a gap. Therefore, the magnetic field generated in the yoke 82 can be efficiently supplied to the Faraday element 20 without leaking outside, and as a result, a strong magnetic field can be uniformly applied to the Faraday element. Therefore, compared to a configuration in which there is a gap between the Faraday element and the yoke, the current supplied to the coil can be reduced and the driving power of the electromagnet can be reduced.

  FIG. 17 shows a modification of the magnetic circuit of the optical variable attenuator shown in FIG. (A) is sectional drawing seen from the top, (B) shows sectional drawing seen from the side. In the magnetic circuit of the optical variable attenuator shown in FIG. 17, two divided coils 86-1 and 86-2 are provided in the vicinity of the Faraday element 20 instead of the coil 84, as compared with the electromagnet 80 shown in FIG. It has been.

  Since the coil is provided in the vicinity of the Faraday element 20, the influence of the magnetic resistance in the yoke is reduced, and the magnetic field generated in the yoke can be efficiently supplied to the Faraday element 20. Also with this configuration, the driving power of the electromagnet can be reduced. Furthermore, since the height of the yoke 82 on the loop side can be reduced, the height of the optical variable attenuator can also be reduced. As a result, the ease of mounting is improved.

  In the optical variable attenuator shown in FIG. 17, a wedge-shaped birefringent crystal is used as a polarizer and an analyzer, whereby the polarization dependence can be removed. This operation is disclosed in Japanese Patent Laid-Open No. 6-51255 “Optical Attenuator”. The following method is also conceivable as a method for efficiently applying the magnetic field generated in the yoke to the Faraday element. In the magnetic circuits of FIGS. 16 and 17, the narrower the gap of the yoke into which the Faraday element is inserted, the more efficiently the magnetic field can be applied to the Faraday element. Since the relative permeability of the Faraday element is not larger than that of the yoke, a leakage magnetic field may be generated in the space via the Faraday element.

  For this reason, it is necessary to keep the yoke gap as narrow as possible. If the gap is narrowed, the area through which the light beam is transmitted decreases, so the collimated light beam system needs to be small. This requirement can be realized by shortening the focal length of the lens. For example, if the focal length of the lens is 0.7 mm, the collimated beam diameter can be reduced to about 140 μm. For this reason, it is relatively easy to set the gap of the yoke to about 300 μm or less, which is about twice that of the yoke tolerance, considering the assembly tolerance.

  FIG. 18 shows another configuration example of the optical variable attenuator of the present invention. In order to simplify the description, the magnetic circuit is omitted. In the variable optical attenuator shown in FIG. 18, the light beam is converged on the Faraday element by the lens on the incident side. Therefore, the yoke gap can be further narrowed in the Faraday element. The light beam can be narrowed down to about 100 μm. When this optical system is applied to an optical variable attenuator, the gap of the yoke gap can be reduced to about 200 μm. Therefore, the magnetic field generated in the yoke can be efficiently applied to the Faraday element, and the driving power can be further reduced.

  Next, the fourth principle of the optical variable attenuator according to the present invention will be described. In the optical variable attenuator according to the present invention, the wavelength dependency of the gain of the optical fiber amplifier can be compensated by using the wavelength dependency of the attenuation amount. First, problems of the optical fiber amplifier will be described. As an optical fiber amplifier, an Er (Erbium) -doped fiber amplifier (EDFA) is often used. This EDFA has a configuration in which input light is amplified by supplying pumping light from the outside.

  FIG. 19 shows typical EDFA amplification characteristics. FIG. 19 shows a case where a multiplexed signal obtained by multiplexing four optical signals is amplified near 1550 nm. As understood from this figure, the EDFA has a gain peak in the vicinity of 1535 nm, and the amplification characteristic is not flat. Therefore, normally, a wavelength band near 1540 to 1560 nm having a relatively flat gain is used as an optical signal.

  However, even in this wavelength band, there is a risk that the wavelength dependency may increase depending on the operation state of the optical fiber amplifier. As shown in FIG. 19, when the input power is increased while the output power is controlled to be constant, or when the output power is increased while keeping the input power constant (the graph of FIG. 19 corresponds to the lower graph). ), The gain on the short wavelength side of 1540 nm is lower than that on the 1560 nm side.

  In an optical communication system, since the length of an optical fiber is different for each place where it is laid, the power input to the optical fiber amplifier is different. Therefore, when the input power is different for each installation location, the wavelength dependence of the gain of the output power occurs. In order to prevent this wavelength dependence, it is necessary to keep the gain of the optical fiber amplifier constant. When the gain is controlled to be constant, the ratio of ions in the inversion distribution state of Er ions in the EDFA becomes constant, and the change in wavelength dependence can be reduced. In this case, the following two problems occur.

  The first problem is that when the gain in the optical fiber amplifier is always controlled to be constant, the output power changes according to the input power. In this case, in an optical fiber, light is confined in a very small portion and light is propagated over a long distance, so that the influence of the nonlinear optical effect increases. Therefore, in order to avoid the influence of the nonlinear optical effect, it is necessary to perform control so as to reduce the input power to the optical fiber. Therefore, in the first conventional method, as shown in FIG. 20, an optical variable attenuator is connected to the optical fiber amplifier in order to keep the optical output constant, and in order to reduce the change in wavelength dependence, The gain of the optical fiber amplifier is controlled to be constant. In this case, in order to reduce the wavelength dependency of the gain, it is necessary to input sufficient pumping light power, which causes problems such as an increase in power consumption and an increase in the size of the apparatus.

  The second problem is that when the gain of the optical fiber amplifier is controlled to be constant and the wavelength dependence of the gain is reduced, it is necessary to increase the pumping power. When the inversion distribution state is set to a predetermined state, the gain in the wavelength range of 1540 nm to 1560 nm can be made substantially flat. However, for that purpose, it is necessary to increase the excitation power. If the pumping power is low, the inversion distribution becomes incomplete as described above, and the gain on the long wavelength side is increased. Therefore, in the second conventional method, an optical filter having a large loss characteristic on the long wavelength side is inserted, and the wavelength dependency of gain can be reduced with a small pumping power. However, this method requires an optical filter, and the configuration of the apparatus is complicated.

  In order to solve the above problems, the above-described variable optical attenuator according to the present invention can be applied. Specifically, the optical variable attenuator according to the present invention can be applied as the optical variable attenuator ATT in the configuration shown in FIG. In this case, the polarizer and analyzer of the optical variable attenuator have a wavelength dependency opposite to the wavelength dependency of the optical fiber amplifier (wavelength dependency when the output power is large: lower graph in FIG. 19). The parameters such as the angle arrangement and the FR element length are adjusted. FIG. 21 shows the attenuation characteristics of such an optical variable attenuator. The amount of attenuation increases on the long wavelength side.

  By applying this optical variable attenuator to a transmission apparatus, an optical filter for canceling the wavelength dependence can be removed. Further, in the present optical variable attenuator, the wavelength dependency increases as the amount of attenuation increases, so that the wavelength dependency of the gain of the optical fiber amplifier can be canceled satisfactorily.

  The latter advantage will be described in more detail. If there is an ideal optical fiber amplifier that can control the gain constant regardless of the input power, the advantage of the latter will be unnecessary. However, the actual pump power of an optical fiber amplifier is finite. When the input power increases, it is necessary to increase the pumping light power in order to control the gain constant. At this time, the attenuation of the optical variable attenuator is increased in order to keep the optical output constant.

  However, when the input power further increases and the pumping light power reaches the upper limit value, the inversion distribution state cannot be kept constant, and the gain on the long wavelength side of the optical fiber amplifier increases. When the upper limit value of the excitation light power is small, the tendency is further increased. Therefore, if the optical variable attenuator has a characteristic that the wavelength dependency increases as the attenuation amount increases, the wavelength dependency of gain can be effectively canceled even if the upper limit of the pumping light power is small. Can do. Therefore, the power of the pumping light of the optical fiber amplifier can be set small, and the optical fiber amplifier can be reduced in size and power consumption.

  Note that in such an optical variable attenuator having a large wavelength dependency, the temperature dependency of the attenuation amount is expected to be large. Therefore, in this case, it is desirable to add a control circuit for keeping the temperature of the Faraday element constant. Next, the fifth principle of the variable optical attenuator according to the present invention will be described. In the optical variable attenuator using the magneto-optic effect, when controlling to the same attenuation, the drive current applied to the electromagnet differs between the control to increase the attenuation and the control to decrease the attenuation. There is a case. This is due to the rotation angle of the Faraday element and the hysteresis characteristics of the magnetic circuit.

FIG. 22 is a configuration example for explaining the fifth principle of the optical variable attenuator according to the present invention. In this configuration, it is possible to control the input power fluctuation to the optical variable attenuator to be suppressed and the output power to be kept constant. In the configuration example shown in FIG. 22, the variable optical attenuator shown in FIG. 30 is used. In this variable optical attenuator, as a polarizer and an analyzer, an optical material having birefringence such as rutile (TiO ₂ ) or calcite is processed into a wedge shape in order to reduce the polarization dependence of attenuation. I use what I did. When a spatial beam is attenuated without providing a fiber at the input / output, or when a polarization maintaining fiber is used as the input / output fiber, linearly polarized light is input to the variable optical attenuator. In this case, a polarization separator using a normal prism or a dielectric multilayer film can be used as a polarizer and an analyzer. In FIG. 22, a permanent magnet for applying a bias magnetic field is omitted for the sake of simplicity.

  In the optical variable attenuator of FIG. 22, an optical coupler 100 that branches a part of two birefringent light beams, a lens 102, and one of the two birefringent light beams are connected to the output side of the optical variable attenuator. An aperture 104 to be passed through and a light receiver 106 for monitoring the optical power that has passed through the aperture 104 are provided, and the amount of attenuation of the optical variable attenuator is controlled so that the optical power becomes a predetermined value. The light beam that has passed through the output-side analyzer 8 b (birefringent crystal) is separated by the optical coupler 100. The separated light beam is input to the light receiver 106 via the lens 102 and the aperture 104.

  The branching ratio of the optical coupler 100 is set so that the attenuation amount of the main signal supplied to the fiber 6 b is small, and the branched light beam can be sufficiently monitored by the light receiver 106. For example, the branching ratio can be set to about 10: 1 to 20: 1. In the optical variable attenuator of FIG. 22, the light beam separated by the optical coupler is input to the light receiver 106 through the lens 102 and the aperture 104. In the variable optical attenuator using the birefringent taper plate as a polarizer and an analyzer, the polarization direction of the light beam is rotated in the Faraday rotator 9, and the coupling position of the light beam in the output optical fiber 6b is shifted. Therefore, a part of the light beam is not supplied to the optical fiber 6b, and the attenuation operation is performed.

  When the attenuation is zero, the light beam is coupled to the center of the core of the output fiber 6b. When Faraday rotation is applied in the polarization direction of the light beam to cause attenuation, the light beam is combined at a position off the core, and the optical power is attenuated. Therefore, even when the separated light beam is received by the light receiver 106, all the light beams are supplied to the light receiver 106 unless the light receiving area is sufficiently reduced as in the case of the optical fiber, and the power of the light beam is reduced. It cannot be measured accurately. That is, even if the coupling position is changed, the attenuation cannot be measured if the light receiving diameter is wider than the positional deviation. If the focal length of the lens on the monitor side is appropriately set, a light receiving surface having a larger area than the core of the optical fiber can be secured. Therefore, an aperture 104 is provided in front of the light receiver 106. If the light receiving surface is sufficiently small, the aperture is not necessary.

Next, the operation of the control circuit provided outside the optical variable attenuator in FIG. 22 will be described. The electrical signal photoelectrically converted by the light receiver 106 is amplified to an appropriate level of electrical signal by the amplifier 108. The amplified electrical signal is input to the error detection circuit 110. The control voltage generation circuit 112 outputs a voltage corresponding to the desired optical power. The linearizer 114 is provided to correct the relationship between the applied power to the coil of the optical variable attenuator and the attenuation. The Faraday rotation angle is proportional to the applied power, but the attenuation is proportional to the Faraday rotation angle cos ² . Accordingly, the set voltage is corrected so that the relationship between the set voltage and the output optical power is linear or logarithmic. This set voltage is input to the error detection circuit 110 together with the above-described electric signal, and the difference signal between them is output as an error signal to be controlled.

  For the error signal output from the error detection circuit 110, the time constant of the electric circuit is adjusted by the phase compensation circuit 116. Since the electromagnet coil that causes Faraday rotation has inductance, the response characteristics may deteriorate and ringing may occur. Therefore, in the phase compensation circuit 116, the frequency characteristic of the control circuit is adjusted to prevent them. The drive circuit 118 is a power amplification circuit for driving the coil.

  By using the above-described control, output power corresponding to the set voltage generated by the control voltage generation circuit 112 can always be obtained. The control voltage generation circuit 112 can be controlled remotely by applying an external control voltage. In this configuration example, it is possible to correct the temperature characteristics of the optical variable attenuator, deterioration with time, polarization loss fluctuation, and the like.

  FIG. 23 is a modification of the optical variable attenuator shown in FIG. In this optical variable attenuator, light beam branching means and light receiving means are further added to the input side of the optical variable attenuator as compared with the optical variable attenuator shown in FIG. In this configuration example, control can be performed so as to obtain a predetermined attenuation regardless of the input light power. Elements having the same functions as those in FIG. 22 are denoted by the same reference numerals.

  In this optical variable attenuator, an optical coupler 100a and a light receiver 106a are provided on the input side as in the output side. A part (for example, 1/10 to 1/20) of the input optical power is branched and monitored by the light receiver 106a. On the input side, a part of the optical power is branched before passing through a polarizer composed of a birefringent taper plate, so that the aperture 104 for limiting the light receiving diameter provided on the output side is unnecessary.

  In the optical variable attenuator of FIG. 23, the signals received by the input and output light receivers 106 a and 106 b are amplified to appropriate levels by the amplifiers 108 a and 108 b and input to the divider circuit 120. In this division circuit 120, the ratio between the output power and the input power is calculated. This calculation result is input to the error detection circuit 110. At the same time, a set voltage corresponding to the attenuation amount is also input to the error detection circuit 110. The error detection circuit 110 generates a control error signal, which is phase compensated and drives the coil via the drive circuit 118. The control circuit controls the optical power ratio between the input unit and the output unit to be constant, so that the attenuation amount of the optical variable attenuator can be controlled to be constant.

  Next, the sixth principle of the optical variable attenuator according to the present invention will be described. When an optical variable attenuator is mounted on an optical transmission device, the optical variable attenuator needs to be downsized. In particular, since there is a case where the transmission device is configured by mounting on a printed board and overlapping the printed boards, it is necessary to reduce the height of the variable optical attenuator. Furthermore, in order to reduce the power consumption of the optical transmission apparatus, it is important to reduce the power consumption of the optical variable attenuator.

  FIG. 24 is a configuration example for explaining the sixth principle of the optical variable attenuator according to the present invention. (A) is an external view, (B) is a view seen in the a direction, and (C) is a view seen in the b direction. However, in FIG. 24, only the Faraday rotator is shown for the sake of simplicity, and the polarizer and the analyzer are omitted.

  The Faraday rotator shown in FIG. 24 includes a Faraday element 130, an electromagnet 132 having a yoke 134 and a coil 136, and a permanent magnet 138. The yoke 134 and the permanent magnet 138 of the electromagnet 132 have a ring shape (for example, a horseshoe shape) having a gap. The Faraday element 130 is provided in the gap of the yoke 134. The electromagnet 132 applies a magnetic field to the Faraday element 130 in a direction perpendicular to the light beam 140, and the permanent magnet 138 applies a magnetic field to the Faraday element 130 in the direction of the light beam 140.

  FIG. 24C particularly shows a state in which the optical variable attenuator is housed in the housing 142. In this figure, the gap direction of the electromagnet 132 is arranged in the height direction of the housing 142. Therefore, the light beam 140 can be positioned approximately in the middle of the height of the housing 142.

  As described above, it is desirable that the height of the optical device is low for mounting reasons. In the case of the variable optical attenuator, the yoke of the electromagnet has a ring shape, and this diameter greatly affects the height of the variable optical attenuator. Specifically, it is desirable in terms of the interface with the outside that the position of the light beam is set to the middle of the height of the optical variable attenuator. In the conventional optical variable attenuator shown in FIG. 29, a space corresponding to the diameter of the ring-shaped yoke is required on the upper side and the lower side of the light beam, respectively, and the height of the optical variable attenuator increases. However, in the configuration example of the optical variable attenuator shown in FIG. 24, it is only necessary to secure a space corresponding to the radius of the ring-shaped yoke on the upper side and the lower side of the light beam 140, respectively. Therefore, the height of the optical variable attenuator can be reduced.

  In FIG. 24B, the permanent magnet 138 has a horseshoe shape, and is placed close to the Faraday element 130 so as not to block the light beam 140. Further, it is shown that the tip of the yoke of the permanent magnet 138 is narrowed. With the above configuration, the magnetic field of the permanent magnet 138 can be efficiently applied to the Faraday element 130 as compared with the conventional optical variable attenuator shown in FIG. Therefore, the permanent magnet 138 can prevent leakage of the magnetic field to the outside, and can also reduce the influence on the electromagnet. Thereby, it can prevent that control of an electromagnet becomes complicated. Furthermore, in this case, the magnetic force of the permanent magnet 138 can be reduced.

  Furthermore, as shown in FIG. 24A, the yoke 134 of the electromagnet 132 includes a semi-hard magnetic body 144 in a portion close to the gap. In the conventional optical variable attenuator shown in FIG. 29, since the yokes are all made of a soft magnetic material, it is necessary to always supply current to the electromagnet in order to supply a magnetic field. As shown in FIG. 24, when a semi-hard magnetic material is used for the yoke of the electromagnet, the yoke is magnetized by applying a pulse current, and the magnetization is maintained even when the supply of current is stopped. Therefore, the power consumption of the optical variable attenuator can be reduced. In this case, the entire yoke need not be made of a semi-hard magnetic material, and the effect can be obtained by partially providing a semi-hard magnetic material in the yoke as shown in FIG.

  However, the semi-hard magnetic material can obtain stable magnetization in the saturated region, but exhibits a large hysteresis characteristic in the unsaturated region, and it is difficult to obtain stable magnetization. Therefore, control at an intermediate stage of the magnetic field is difficult. In order to solve this problem, the configuration shown in FIG. 25 can be considered. FIG. 25 is a diagram showing a configuration of an electromagnet used in the variable optical attenuator according to the present invention.

  In the present electromagnet, a plurality of semi-hard magnetic bodies 144a to 144e having partially different magnetic forces in respective saturation regions are provided in the yoke of the electromagnet. Each semi-hard magnetic body is provided with a coil, and each semi-hard magnetic body can be independently driven in the saturation region. Therefore, by controlling on / off of the current supplied to these coils, it is possible to operate only the desired semi-hard magnetic material and to stably set the magnetic field generated by the electromagnet step by step.

  Next, the seventh principle of the optical variable attenuator according to the present invention will be described. In the optical variable attenuator using the wedge-shaped birefringent crystal, a slight polarization dependent loss (PDL) is generated as described in the conventional optical variable attenuator of FIG. The variable optical attenuator according to the present invention further reduces this polarization dependent loss.

  FIG. 26 is a configuration example for explaining the seventh principle of the variable optical attenuator according to the present invention. (A) is a top view and (B) is a side view. FIG. 27 is a diagram showing the direction pattern of the bias magnetic field for explaining the seventh principle of the optical variable attenuator according to the present invention. (A) is a case where a bias magnetic field is applied perpendicularly to the refraction plane, and (B) is a case where a bias magnetic field is applied parallel to the refraction plane.

  The optical variable attenuator shown in FIG. 26 shows a bias magnetic field 154 for making the magnetic domain of the Faraday element 150 singular compared to the optical variable attenuator shown in FIG. This bias magnetic field 154 is applied to the Faraday element 150 perpendicular to the light beam. The magnet 152 for generating the bias magnetic field 154 is shown only in (B) of FIG. 26 and is omitted in (A). In practice, a magnetic field parallel to the light beam is also applied to the Faraday element 150 in order to generate Faraday rotation. However, in these drawings, the magnetic field parallel to the light beam is not shown for the sake of simplicity. Other configurations are the same as those of the optical variable attenuator shown in FIG. 30, and elements having the same functions are denoted by the same reference numerals.

  In the optical variable attenuator of FIG. 26, the light beam is birefringent in the birefringent crystal 8a and converted into a light beam having components of ordinary light and extraordinary light having different refraction angles. The ordinary light and the extraordinary light are supplied with a bias magnetic field 154 in the Faraday element 150. The bias magnetic field 154 is applied perpendicular to a plane (referred to as a refraction plane) composed of ordinary light 156 and extraordinary light 158. This state is also shown in FIG. Therefore, the same bias magnetic field 154 is applied to both the ordinary light 156 and the extraordinary light 158.

  On the other hand, as shown in FIG. 27B, the bias magnetic field can be applied in parallel to the refractive plane. However, the bias magnetic field is applied substantially perpendicular to the light beam. In this case, since the ordinary light 156 and the extraordinary light 158 have different refraction angles, the bias magnetic field applied to each light is slightly different. It is considered that a polarization dependent loss occurs due to the difference in the magnitude of the magnetic field.

  Therefore, as shown in FIG. 26 and FIG. 27A, the polarization-dependent loss can be reduced by applying a bias magnetic field substantially perpendicular to the refractive plane. In the optical variable attenuator according to the present invention described above in this specification, the amount of attenuation is controlled using two types of magnetic circuits, so that the magnetic field of the magnetic circuit may leak outside the optical variable attenuator. is there. In particular, the magnetic field of the permanent magnet is strong and has a great influence on the outside. In order to reduce this influence, it is effective to provide a permanent magnet with a yoke as in the case of an electromagnet or a method of magnetically shielding a casing.

  As mentioned above, although it demonstrated by the Example of this invention, it cannot be overemphasized that this invention is not limited to these Examples, and can be improved and deform | transformed within the scope of the present invention.

1 Magneto-optical crystal (Faraday element)
2 Polarizer 3 Permanent magnet 4 Electromagnet 5 Optical beams 6a and 6b Optical fibers 7a and 7b Lenses 8a and 8b Birefringent crystal 9 Faraday rotator 10 Polarizers 20 and 22 Magneto-optical crystal (Faraday element)
30 Analyzer 40, 42 Permanent magnet 50, 55, 60, 80 Electromagnet 52, 57, 62, 82 Yoke 54, 59, 64, 84 Coil 66 Permanent magnet 70 Permanent magnet 86-1, 86-2 Coil 100, 100a, 100b optical coupler 102 lens 104 aperture 106, 106a, 106b light receiver 108, 108a, 108b amplifier 110 error detection circuit 112 control voltage generation circuit 114 linearizer 116 phase compensation circuit 118 drive circuit 120 division circuit 130 Faraday element 132 electromagnet 134 yoke 136 Coil 138 Permanent magnet 140 Light beam 142 Case 150 Faraday element 152 Magnet 154 Bias magnetic field 156 Ordinary light 158 Abnormal light

Claims (10)

An optical amplification device that amplifies an optical signal having a predetermined wavelength band,
An optical amplifier having a wavelength-dependent gain characteristic;
The optical signal is variably attenuated by using the rotation of the polarization direction of the optical signal in the magneto-optic crystal, and the attenuation characteristic has a wavelength dependence substantially opposite to the wavelength dependence of the gain in the optical amplifier. An optical amplifying apparatus comprising: an attenuator; wherein the optical signal is attenuated in the optical variable attenuator, and the wavelength dependency of gain in the optical amplifier is reduced. The optical variable attenuator is
A magneto-optic crystal that variably rotates the polarization direction of the optical signal;
An analyzer that passes an optical signal that has passed through the magneto-optic crystal in accordance with its polarization direction;
The polarization direction of the analyzer, the polarization direction of the optical signal, and the magneto-optical crystal are set so as to substantially obtain the opposite wavelength dependency at a predetermined attenuation. The optical amplification device according to claim 1. An optical variable attenuator connected to an optical amplifier for reducing wavelength dependence of gain in the optical amplifier and attenuating an optical signal,
A magneto-optic crystal that variably rotates the polarization direction of the optical signal;
An analyzer that passes an optical signal that has passed through the magneto-optic crystal in accordance with its polarization direction;
The polarization direction of the analyzer, the polarization direction of the optical signal, and the magneto-optic crystal can obtain a wavelength dependence substantially opposite to the wavelength dependence of the gain of the optical amplifier at a predetermined attenuation. An optical variable attenuator characterized by being set. An optical variable attenuator that attenuates the power of the light beam,
A magneto-optic crystal that variably rotates the polarization direction of the light beam;
An analyzer for guiding at least part of the light beam that has passed through the magneto-optic crystal to the output of the variable optical attenuator;
An output side light receiver for monitoring the output power by partially branching the output light of the optical variable attenuator,
The optical variable attenuator, wherein the polarization direction of the light beam in the magneto-optical crystal is controlled so that the output power of the optical variable attenuator monitored by the output-side photoreceiver becomes a predetermined value. An optical variable attenuator that attenuates the power of the light beam,
A magneto-optic crystal that variably rotates the polarization direction of the light beam;
An analyzer for guiding at least part of the light beam that has passed through the magneto-optic crystal to the output of the variable optical attenuator;
An input-side light receiver that monitors the input power of a light beam input to the magneto-optic crystal;
An output side light receiver for monitoring the output power of the optical variable attenuator,
The light in the magneto-optical crystal is set so that a ratio between an input power of the light beam monitored by the input-side light receiver and an output power of the optical variable attenuator monitored by the output-side light receiver becomes a predetermined value. An optical variable attenuator, characterized in that the polarization direction of the beam is controlled.   6. The light according to claim 4, wherein the analyzer further includes a birefringent crystal, and further includes an aperture that guides a part of the light beam whose polarization is separated in the analyzer to the output-side light receiver. Variable attenuator. An optical variable attenuator that attenuates the power of the light beam,
A magneto-optic crystal that variably rotates the polarization direction of the light beam;
A magnetic circuit for generating a magnetic field to be applied to the magneto-optical crystal in an internal gap;
A housing for housing the magneto-optical crystal and the magnetic circuit and mounting the magneto-optical crystal on a substrate;
The variable optical attenuator, wherein the magnetic circuit is mounted on the housing such that the direction of the gap is substantially the height direction of the housing. An optical variable attenuator that attenuates the power of the light beam,
A magneto-optic crystal that variably rotates the polarization direction of the light beam;
A magnetic circuit that electrically generates a magnetic field to be applied to the magneto-optic crystal by a drive current, the yoke including a semi-rigid magnetic body at least in part.
The optical variable attenuator, wherein the magnetic field is maintained even when the supply of the driving current is stopped. The yoke partially includes a plurality of semi-hard magnetic bodies having different magnetizations in a saturated state,
9. The variable optical attenuator according to claim 8, wherein the magnitude of the magnetic field generated in the magnetic circuit can be changed stepwise by controlling the magnetization of each semi-hard magnetic material. An optical variable attenuator that attenuates the power of the light beam,
A first wedge-shaped birefringent crystal that performs polarization separation of the light beam;
A magneto-optic crystal that variably rotates the polarization direction of the light beam separated from the polarized light by the first wedge-shaped birefringent crystal;
A magnetic circuit for generating a magnetic field for application to the magneto-optic crystal substantially perpendicular to the light beam;
A second wedge-shaped birefringent crystal that birefringes a light beam output from the magneto-optic crystal;
The light, wherein a magnetic field of the magnetic circuit is applied to the magneto-optic crystal substantially perpendicular to a plane formed by the light beam whose polarization is separated by the first wedge-shaped birefringent crystal. Variable attenuator.

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