JP2010151903A - Optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical isolator using a reflector type polarizer in which deterioration of the isolation is suppressed. <P>SOLUTION: The optical isolator includes the reflector type polarizer 2 disposed on the laser oscillation part side, a Faraday rotator 4, and an absorption type polarizer 1. The reflector type polarizer 2 is formed on one of the light transmission surfaces of a glass substrate 3 with a generally wedge-shaped cross section. The polarizer surface of the reflector type polarizer 2 is set at a tilt angle θ1 (0.12-0.29 degrees) to the light transmission surface on the absorption type polarizer 1 side in the Faraday rotator 4, and the polarizer surface of the reflector type polarizer 2 is set at a tilt angle θ2 (0.12-0.29 degrees) to the light transmission surface on the Faraday rotator 4 side in the absorption type polarizer 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical isolator used as a countermeasure for return light of a semiconductor laser used for optical communication, and more particularly to an improvement of an optical isolator that suppresses deterioration of isolation in an optical isolator to which a reflective polarizer is applied. It is.

  As an optical module used for optical communication, a laser diode (LD) module for transmitting an optical signal and a photodiode (PD) module for receiving an optical signal are known. An electrical signal is converted into an optical signal. It is converted and emitted from the LD module, received by the PD module through the optical fiber, and converted into an electrical signal.

  By the way, when a large-capacity optical signal is transmitted over a long distance, a distributed feedback semiconductor (DFB) laser is used as the LD. This DFB laser has a feature that it has a narrow oscillation spectrum width and excellent dispersion characteristics, but it is sensitive to the return light by reflected light, and the reflected light from the coupling end face and other discontinuous interfaces to the optical fiber oscillates. Returning to the part, there is a drawback that the characteristics become unstable. Therefore, the optical isolator is used to prevent the reflected light from returning to the laser oscillation unit.

  There are several items required for this optical isolator, and among them, the demand for low cost has recently become stronger. An optical isolator is composed of two polarizers and a Faraday rotator as optical elements. In a polarization-dependent optical isolator applied to a DFB laser, an electroabsorption polarizer (hereinafter simply referred to as absorption) is used. Type polarizer).

However, in recent years, reflective polarizers that can be produced on large glass substrates and that can be expected to be of low quality and uniform quality are being put into practical use. Such reflective polarizers include photonic crystal polarizers and wire grid polarizers, and optical isolators using these reflective polarizers have also been proposed (Patent Document 1, Patent Document 1). 2).
JP 2000-241762 A JP 2008-003189 A

  By the way, when the reflective polarizer is used in an optical isolator, it is generally used in combination with an absorptive polarizer as described in Patent Documents 1 and 2.

  Hereinafter, the principle of preventing the optical isolator using the reflective polarizer from blocking the return light and returning to the laser oscillation unit will be described.

  First, the blocking principle of the optical isolator shown in FIG. 2 in which the reflective polarizer 2 is disposed on the laser oscillation unit side will be described. As shown in FIG. 2, the return light 7 passes through the absorption polarizer 1 and becomes polarized light having a polarization plane parallel to the transmission axis of the absorption polarizer 1. This light is polarized by the Faraday rotator 4. The polarization plane of the light rotates 45 degrees and enters the reflective polarizer 2. Since the light incident on the reflective polarizer 2 is polarized light having a polarization plane perpendicular to the transmission axis of the reflective polarizer 2, the light is reflected by the reflective polarizer 2 and enters the Faraday rotator 4 again. The light incident on the Faraday rotator 4 is incident on the absorption polarizer 1 after its polarization plane is further rotated by 45 degrees, but is polarized with a polarization plane perpendicular to the transmission axis of the absorption polarizer 1. It is absorbed by the absorption polarizer 1 and functions as an optical isolator. In FIG. 2, the arrows shown in the circles indicate the directions of the polarization planes.

  Similarly, the principle of blocking the optical isolator shown in FIG. 3 in which the absorption polarizer 1 is disposed on the laser oscillation unit side will be described. First, as shown in FIG. 3, the return light 7 passes through the reflective polarizer 2 and becomes polarized light having a polarization plane parallel to the transmission axis of the reflective polarizer 2, and this light is converted into the Faraday rotator 4. Thus, the polarization plane of polarized light rotates 45 degrees and enters the absorption polarizer 1. The light incident on the absorption polarizer 1 is polarized light having a polarization plane perpendicular to the transmission axis of the absorption polarizer 1 and thus is absorbed by the absorption polarizer 1. As in the optical isolator of FIG. Functions as an isolator. In FIG. 3, the arrows shown in circles also indicate the directions of the polarization planes.

  In this way, it is possible in principle to prevent the return light from returning to the laser oscillation section by the action of the optical isolator shown in FIGS. In practice, however, return light that slightly passes through the optical isolator is generated due to various factors.

  First, the reflection polarization of the optical isolator shown in FIG. 2 is caused by a deviation from the design value of the Faraday rotation angle when the Faraday rotator 4 is manufactured, a deviation due to the temperature dependence or wavelength dependence of the Faraday rotation angle, and the like. As shown in FIG. 4, the return light 7 incident on the optical element 2 includes component light having a polarization plane parallel to the transmission axis of the reflective polarizer 2 (indicated by a broken line in FIG. 4). Light passes through the optical isolator. This light will be referred to as first transmitted light 8. Similarly, the return light 7 incident on the absorptive polarizer 1 of the optical isolator shown in FIG. 5 also has a component light having a polarization plane parallel to the transmission axis of the absorptive polarizer 1 (indicated by a broken line in FIG. 5). This light also passes through the optical isolator. This light is also referred to as first transmitted light 8.

  In addition, although optical elements such as the Faraday rotator 4 and the absorption polarizer 1 are usually provided with an antireflection film, the reflection of light cannot be prevented 100% by the antireflection film. A slight amount of reflected light is generated from the antireflection film of the child 4 and the absorbing polarizer 1. In the optical isolator shown in FIG. 2, the reflected light reflected by the reflective polarizer 2 and generated by the antireflection film on the absorption polarizer 1 side in the Faraday rotator 4 is again as shown in FIG. 4. Since it passes through the Faraday rotator 4, the polarization plane of the polarized light is rotated by 45 degrees, whereby the light of the component (shown by a broken line in FIG. 4) having a polarization plane parallel to the transmission axis of the reflective polarizer 2 And pass through the optical isolator. This light will be referred to as second transmitted light 9. Similarly, in the optical isolator shown in FIG. 3, a slight amount of reflected light generated by the antireflection film on the Faraday rotator 4 side in the absorbing polarizer 1 is reflected by the reflective polarizer 2 as shown in FIG. Then, since it passes through the Faraday rotator 4 again, the polarization plane of the polarization is rotated by 45 degrees, and thereby a component having a polarization plane parallel to the transmission axis of the absorption polarizer 1 (shown by a broken line in FIG. 5). ) And pass through the optical isolator. This light is also referred to as second transmitted light 9.

  The amount of the second transmitted light 9 is greater than −30 dB with respect to the return light 7 in some cases. In the optical isolator shown in FIG. 4, although not shown, the reflected light reflected by the reflective polarizer 2 and generated by the antireflection film on the side of the Faraday rotator 4 in the absorption polarizer 1 is again applied. Since it passes through the Faraday rotator 4, it passes through the optical isolator. Similarly, in the optical isolator shown in FIG. 5, even a slight amount of reflected light generated in the antireflection film on the absorption polarizer 1 side in the Faraday rotator 4 passes through the Faraday rotator 4 again. Come through the isolator.

  When the optical isolator of FIGS. 2 and 3 to which the reflective polarizer is applied is arranged between the collimating lenses, the return light is incident on the optical isolator as collimated light, so the second transmitted light is the laser oscillation part of the LD. It was a factor that deteriorated isolation.

  The present invention has been made paying attention to such problems, and the object of the present invention is to provide an optical isolator using a reflective polarizer that suppresses degradation of isolation. .

  Therefore, as a result of intensive studies on the influence of the present inventor on the isolation between the first transmitted light and the second transmitted light, the first transmitted light is a region deviated from the isolation peak, that is, the temperature dependence of the Faraday rotation angle, Although the Faraday rotation angle increases in a region deviating from 45 degrees due to wavelength dependence, the isolation is reduced. However, the second transmitted light is approximately -30 dB with respect to the return light regardless of the deviation of the Faraday rotation angle. In other words, it has been found that the isolation is lowered over the entire temperature range and wavelength range in which the optical isolator is used.

  The present invention has been completed based on such technical knowledge.

That is, the invention according to claim 1
In the optical isolator incorporated in the laser diode module, the Faraday rotator disposed on the optical path includes a reflective polarizer on one light transmission surface side, an absorption polarizer on the other light transmission surface side, and
The polarizer surface of the reflective polarizer is 0.12 to 0.29 degrees with respect to the light transmission surface on the absorption polarizer side of the Faraday rotator and the light transmission surface of the absorption polarizer on the Faraday rotator side. It is characterized by being inclined.

The invention according to claim 2
In the optical isolator according to claim 1,
The reflective polarizer is composed of a wire grid polarizer,
The invention according to claim 3
In the optical isolator according to claim 1,
The reflective polarizer is composed of a polarizer made of a photonic crystal.

Next, the invention according to claim 4 is:
In the optical isolator according to any one of claims 1 to 3,
A reflection polarizer is bonded to one light transmission surface of the Faraday rotator with an adhesive, and an absorption polarizer is bonded to the other light transmission surface with an adhesive.
The invention according to claim 5
In the optical isolator according to claim 4,
The thickness of the adhesive layer between the reflective polarizer and the Faraday rotator and between the absorption polarizer and the Faraday rotator is 3 μm or less,
The invention according to claim 6
In the optical isolator according to any one of claims 1 to 5,
A reflective polarizer formed on one light transmission surface of a glass substrate having a substantially wedge-shaped cross section in which one light transmission surface is inclined by 0.14 to 0.29 degrees with respect to the other light transmission surface The reflective polarizer is configured.

  The optical isolator of the present invention comprising a Faraday rotator, a reflective polarizer, and an absorptive polarizer is such that the polarizer surface of the reflective polarizer has a light transmission surface and an absorptive polarization on the side of the absorptive polarizer in the Faraday rotator. It is characterized in that it is inclined 0.12 to 0.29 degrees with respect to the light transmission surface on the Faraday rotator side of the child.

  Then, the second transmission that returns to the laser oscillation part side due to the reflected light generated on the light transmission surface such as the Faraday rotator or the absorption type polarizer due to the slight inclination of the polarizer surface of the reflection type polarizer. Since the traveling direction of the light and the traveling direction of the first transmitted light returning to the laser oscillation unit due to the temperature dependence and wavelength dependence of the Faraday rotation angle are not parallel, the optical isolator of the present invention is collimated. Even if it is disposed between the lenses, the second transmitted light does not return to the laser oscillation part of the LD, and therefore it is possible to prevent deterioration of isolation in the optical isolator to which the reflective polarizer is applied.

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

  As described above, the optical isolator of the present invention including the Faraday rotator, the reflective polarizer, and the absorptive polarizer is such that the polarizer surface of the reflective polarizer is the light transmitting surface and the absorption side of the Faraday rotator on the absorption polarizer side. Inclined by 0.12 to 0.29 degrees with respect to the light transmitting surface on the Faraday rotator side of the type polarizer.

  FIG. 1 shows a schematic configuration of an optical isolator of the present invention in which a reflective polarizer is arranged on the laser oscillation unit side.

  That is, as shown in FIG. 1, this optical isolator includes a reflective polarizer 2, a Faraday rotator 4, and an absorptive polarizer 1 arranged on the laser oscillation unit side, and the reflective polarizer. 2 is formed on one light transmission surface of the glass substrate 3 having a substantially wedge-shaped cross section, and the polarizer surface of the reflective polarizer 2 is the light transmission surface of the Faraday rotator 4 on the absorption polarizer 1 side. On the other hand, the tilt angle θ1 (0.12 to 0.29 degrees) is set, and the polarizer surface of the reflective polarizer 2 is relative to the light transmission surface of the absorption polarizer 1 on the Faraday rotator 4 side. The inclination angle θ2 (0.12 to 0.29 degrees) is set.

  In the optical isolator of the present invention shown in FIG. 1, the reflected light generated on the light transmitting surface of the Faraday rotator 4, the absorption polarizer 1, etc. is obtained by slightly tilting the polarizer surface of the reflective polarizer 2. Returned to the laser oscillation part due to the traveling direction of the second transmitted light 9 returning to the laser oscillation part side due to (a part of the return light 7) and the temperature dependence or wavelength dependence of the Faraday rotation angle. Since the traveling direction of the transmitted first transmitted light 8 is not parallel, the second transmitted light 9 does not return to the laser oscillation part of the LD even if this optical isolator is arranged between the collimating lenses. It is possible to prevent the deterioration of isolation in the optical isolator to which 2 is applied.

  FIG. 6 shows a schematic configuration of the optical isolator of the present invention in which the absorption polarizer is arranged on the laser oscillation unit side.

  That is, as shown in FIG. 6, this optical isolator includes an absorptive polarizer 1, a Faraday rotator 4, and a reflective polarizer 2 arranged on the laser oscillation unit side, and the reflective polarizer. 2 is formed on one light transmission surface of the glass substrate 3 having a substantially wedge-shaped cross section, and the polarizer surface of the reflective polarizer 2 is the light transmission surface of the Faraday rotator 4 on the absorption polarizer 1 side. With respect to the light transmitting surface on the side of the Faraday rotator 4 in the absorbing polarizer 1, the reflecting polarizer 2 has a polarizer surface of 0.12 to 0. It is set to 29 degrees.

  Also in the optical isolator of the present invention shown in FIG. 6, the reflected light generated on the light transmitting surface of the Faraday rotator 4, the absorbing polarizer 1, etc., when the polarizer surface of the reflective polarizer 2 is slightly inclined. Returned to the laser oscillation part due to the traveling direction of the second transmitted light 9 returning to the laser oscillation part side due to (a part of the return light 7) and the temperature dependence or wavelength dependence of the Faraday rotation angle. Since the traveling direction of the transmitted first transmitted light 8 is not parallel, the second transmitted light 9 does not return to the laser oscillation part of the LD even if this optical isolator is arranged between the collimating lenses. It is possible to prevent the deterioration of isolation in the optical isolator to which 2 is applied.

  Hereinafter, the reflective polarizer 2 is disposed on the laser oscillation unit side, and the reflective polarizer 2 and the absorbing polarizer 1 are bonded to each light transmitting surface of the Faraday rotator 4 using an adhesive. The present invention will be specifically described with reference to an example in which the optical isolator of FIG. 7 is disposed between the collimating lenses 12 and 13 shown in FIG.

  First, in the collimated beam optical system in which the light emitted from the optical fiber 10 shown in FIG. 8 is once converted into parallel light by the lens 12, and the parallel light is collected by the lens 13 and incident on the optical fiber 11. The result of having an optical isolator arranged in the parallel light portion and evaluating the isolation will be described. The optical fiber 11 is a substitute for LD, and the greater the amount of light incident on the optical fiber 11, the worse the isolation.

  The reflective polarizer 2 is formed on one light transmission surface of the glass substrate 3 having a substantially wedge-shaped cross section in which one light transmission surface is inclined at an inclination angle θ with respect to the other light transmission surface. Yes.

  In the optical isolator shown in FIG. 7, since the Faraday rotator 4 and the absorption polarizer 1 are bonded together with an adhesive as described above, the distance between the Faraday rotator 4 and the absorption polarizer 1 and the Faraday rotation. If the distance between the glass element 3 and the glass substrate 3 of the reflective polarizer 2 can be made parallel, the inclination angle θ of the glass substrate 3 itself is the same as the inclination angles θ1 and θ2.

  FIG. 9 shows an example of the wavelength dependence of isolation in the optical isolator having the tilt angle θ of 0.10 degrees. As shown in FIGS. 7 and 8, since the first transmitted light 8 and the second transmitted light 9 pass through substantially the same optical path, the first transmitted light 8 and the second transmitted light 9 cause interference, and the wavelength dependence of isolation. As shown in FIG. 9, the vibration phenomenon of the isolation due to the interference is caused as shown in FIG. As the amount of the second transmitted light incident on the optical fiber 11 increases, the vibration width of this vibration phenomenon increases.

  FIG. 10 shows the result of preparing a plurality of optical isolators having different inclination angles θ and evaluating the vibration width of the isolation. From the graph of FIG. 10, when the inclination angle θ is 0.12 degrees or more, the vibration width becomes small, and in order to prevent the second transmitted light 9 from being coupled to the optical fiber 11, the inclination angle θ is 0.12. It was found that it was necessary to be at least.

  In general, a polarizer or a Faraday rotator constituting an optical isolator is cut into a 1 mm square or less after a polarizer or a Faraday rotator having a size of about 11 mm square is bonded individually or with an adhesive. use. The second transmitted light 9 is not coupled to the optical fiber 11 as the tilt angle θ increases, but the individual difference in the thickness of the reflective polarizer 2 increases as the tilt angle θ increases. If the thicknesses of the reflective polarizers 2 are individually different, not only does the problem arise due to the difference in dimensions when assembling the optical isolator, but the effective optical path length differs between the optical isolators. Problems also occur when assembling an optical module that incorporates. For this reason, the inclination angle θ needs to be 0.29 degrees or less.

  When a reflective polarizer is used by adhering to a Faraday rotator, a glass substrate having a wedge-shaped substantially cross-sectional shape in which one surface of a flat substrate is inclined with respect to the other surface (that is, A glass substrate in which one light transmission surface is inclined with respect to the other light transmission surface and having a reflection type polarizer on the surface opposite to the Faraday rotator is used. Although it is necessary, the inclination angle of the glass substrate at this time is preferably 0.14 to 0.29 degrees. The lower limit of the inclination angle of the glass substrate is larger than the inclination angle of the polarizer surface (0.12 to 0.29 degrees) because the effective inclination angle is 0.12 degrees due to variations in the thickness of the adhesive layer. This is in order not to fall below.

  When the thickness of the adhesive is large, the effective inclination angle is likely to fluctuate due to variations in the thickness of the adhesive layer. Therefore, between the reflective polarizer and the Faraday rotator and between the absorption polarizer and the Faraday rotator. The thickness of the adhesive layer is desirably 3 μm or less.

  Examples of the present invention will be specifically described below.

  First, a reflective polarizer made by Nano Nouveau, in which a wire grid polarizer is formed on a glass substrate made of optical glass (BK7) as a reflective polarizer, is prepared, and is opposite to the surface on which the polarizer is formed. The inclination angle θ was set to 0.17 degrees by polishing the side surface.

  Further, a Faraday rotator made of a general bismuth-substituted rare earth iron garnet crystal was applied to the Faraday rotator, and a polar core (registered trademark), which is a glass polarizer manufactured by Corning, was prepared as the absorbing polarizer. In addition, the reflection type polarizer, the Faraday rotator, and the absorption type polarizer were prepared as 11 mm square.

  Then, after reflecting and absorbing polarizers are bonded to both surfaces of the Faraday rotator using an epoxy-based optical adhesive, the light transmitting surface is cut to 1 mm square by a dicing saw and used for an optical isolator. An optical element was obtained. When the thickness of the adhesive layer between the reflective polarizer and the Faraday rotator and between the absorption polarizer and the Faraday rotator was observed by microscopic observation of the cut surface, both of the adhesive layers were 2.5 μm and 3 μm It was the following.

  Next, an SmCo magnet (see reference numerals 5 and 6 in FIG. 7) that magnetically saturates a bismuth-substituted rare earth iron garnet crystal (Faraday rotator) is incorporated on both sides of the optical element for an optical isolator to obtain an optical isolator, And the isolation was evaluated by the optical system shown in FIG. The evaluation results are shown in FIG.

  From the graph of FIG. 11, the isolation peak was 38 dB, and good characteristics without the vibration phenomenon of isolation were obtained.

  In the same manner as in this example, optical isolators having inclination angles θ of 0.14 degrees, 0.22 degrees, and 0.29 degrees were manufactured and the isolation was evaluated. Similar results were obtained.

  The optical isolator according to the present invention has industrial applicability to be incorporated in a laser diode module because it prevents deterioration of isolation despite the use of an inexpensive reflective polarizer. is doing.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic structure explanatory drawing of the optical isolator which concerns on this invention by which a reflection type polarizer is arrange | positioned at the laser oscillation part side. Explanatory drawing which shows the principle of interruption | blocking of the return light in the conventional optical isolator with which the reflection type polarizer was arrange | positioned at the laser oscillation part side. Explanatory drawing which shows the principle of interruption | blocking of the return light in the conventional optical isolator by which the absorption type polarizer is arrange | positioned at the laser oscillation part side. Explanatory drawing which shows the leakage of the return light in the conventional optical isolator with which the reflection type polarizer was arrange | positioned at the laser oscillation part side. Explanatory drawing which shows the leakage of the return light in the conventional optical isolator with which the absorption type polarizer was arrange | positioned at the laser oscillation part side. BRIEF DESCRIPTION OF THE DRAWINGS Schematic structure explanatory drawing of the optical isolator which concerns on this invention by which an absorption type polarizer is arrange | positioned at the laser oscillation part side. The optical isolator according to the present invention has a structure in which a reflective polarizer is arranged on the laser oscillation unit side and a reflection polarizer and an absorption polarizer are bonded to each light transmitting surface of the Faraday rotator using an adhesive. FIG. FIG. 8 is a configuration explanatory diagram of an isolation measurement optical system in which the optical isolator according to the present invention shown in FIG. 7 is disposed between collimating lenses. The graph which shows the wavelength dependence of the isolation in the optical isolator which concerns on this invention using a reflection type polarizer. The graph which shows the relationship between the inclination-angle of the polarizer surface in the reflection type polarizer of the optical isolator which concerns on this invention, and the vibration width of isolation. The graph which shows the relationship between the wavelength of the optical isolator which concerns on the Example of this invention, and isolation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Absorption-type polarizer 2 Reflection-type polarizer 3 Glass substrate 4 Faraday rotator 5 Magnet 6 Magnet 7 Return light 8 First transmitted light 9 Second transmitted light 10 Optical fiber 11 Optical fiber 12 Lens 13 Lens

Claims (6)

In the optical isolator incorporated in the laser diode module, the Faraday rotator disposed on the optical path includes a reflective polarizer on one light transmission surface side, an absorption polarizer on the other light transmission surface side, and
The polarizer surface of the reflective polarizer is 0.12 to 0.29 degrees with respect to the light transmission surface on the absorption polarizer side of the Faraday rotator and the light transmission surface of the absorption polarizer on the Faraday rotator side. An optical isolator characterized by being inclined.   The optical isolator according to claim 1, wherein the reflective polarizer is a wire grid polarizer.   2. The optical isolator according to claim 1, wherein the reflective polarizer is composed of a polarizer made of a photonic crystal.   4. The reflective polarizer is bonded to one light transmitting surface of the Faraday rotator by an adhesive, and the absorbing polarizer is bonded to the other light transmitting surface by an adhesive. The optical isolator according to any one of the above.   The optical isolator according to claim 4, wherein the thickness of the adhesive layer between the reflective polarizer and the Faraday rotator and between the absorption polarizer and the Faraday rotator is 3 µm or less.   By a reflective polarizer formed on one light transmission surface of a glass substrate having a substantially wedge-shaped cross section in which one light transmission surface is inclined by 0.14 to 0.29 degrees with respect to the other light transmission surface The optical isolator according to claim 1, wherein the reflective polarizer is configured.

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