JP2989979B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator for intercepting reflected light returning to a semiconductor laser in optical communication and optical measurement, and more particularly to an optical isolator using a Faraday rotator and a polarizer.

[0002]

2. Description of the Related Art In the field of optical communication and optical measurement using a semiconductor laser (laser diode: LD) as a light source, when the light reflected in the middle of a transmission path returns to the active layer of the LD as a light source, the oscillation wavelength and The output fluctuates, and accurate signal transmission and measurement cannot be performed. There are various causes of this reflection, and not only reflection occurs at the optical fiber, optical element, or each input / output surface of the connected device, but also damage to the optical fiber, stress on the optical fiber, bending of the optical fiber. Even unavoidable objects such as a nonuniform refractive index and the like can cause reflection. Therefore, means for preventing reflected light is indispensable in a transmission path using an LD as a light source.

An optical isolator is a device for preventing such reflected light returning to an LD, and generally includes a Faraday rotator and a pair of polarizers, and is often used immediately after the LD.

FIG. 3 is a sectional view showing the structure of a conventional general optical isolator. In FIG. 3, the first polarizer 3
1 is held by a holder 32. A Faraday rotator 33 that rotates the plane of polarization of the incident linearly polarized light by 45 ° around the optical axis is a magnet 34 that applies a magnetic field parallel to the optical axis.
Is held within. Further, the second polarizer 35 is held by the holder 36 with the polarizer 31 and its transmitted polarization direction rotated about the optical axis by 45 ° in the same direction as the Faraday rotation by the Faraday rotator 33.

The operation of the optical isolator will be described. First, when the forward light coming from the left side is incident on the polarizer 31, the light becomes only a certain polarization component, and the Faraday rotator 33 rotates the polarization plane about the optical axis by 45 °. Further, when the light enters the polarizer 35, the light is transmitted as it is.
That is, since the polarization direction of the polarizer 31 and the polarization direction of the transmitted light are shifted by 45 ° around the optical axis and coincide with the polarization plane of the light, the light incident on the polarizer 35 is transmitted as it is.

On the other hand, the light in the opposite direction coming from the right side is the polarizer 3
When the light enters the optical isolator from 5, it passes through the Faraday rotator 33 and reaches the polarizer 31, but is blocked here because it cannot pass through the polarizer 31. This is because the linearly polarized light limited to a certain polarization component by the polarizer 35 has its polarization plane rotated by the Faraday rotator 33. However, the non-reciprocity of the Faraday rotator 33 causes the direction of the magnetic field to be a reference. The light traveling in the opposite direction and the light traveling in the opposite direction have their polarization planes rotated in the same rotation direction, so that the polarizer 31 and the Faraday rotator 33 are rotated.
This is because the light in the forward direction and the light in the opposite direction have different 90 ° polarization planes at an intermediate position between the two. Therefore, the light in the opposite direction is blocked by the polarizer 31.

However, in the conventional optical isolator shown in FIG. 3, since the input / output boundary surfaces of the constituent optical elements are perpendicular to the light rays, even if an antireflection coating is applied, all the reflections cannot be prevented. In addition, there is a problem that the light in the forward direction is slightly reflected on the surface of each component of the optical isolator itself to generate reflected return light. In addition, since there are a number of parallel input / output boundary surfaces, a complicated resonance state may occur inside the optical isolator or between the optical isolator and the LD, and a problem may occur.

In recent years, the long distance, high density, and high precision of optical communication have been advanced, and such a slight reflection cannot be ignored, and the following improvement has been proposed.

FIG. 4 is a cross-sectional view of a conventional optical isolator having a structure for preventing reflection from the surface of an optical element. The optical isolator includes a first polarizer 41, a second polarizer 45, and a Faraday rotator 43. The elements are installed in the same direction with respect to the incident light and inclined by the same angle θ. For this reason, the reflected light slightly generated on the surface when the light enters the optical element does not return to the LD because it travels in a direction different from that of the incident light.

However, in this example, the optical axis shift 49 occurs between the incident light 47 and the output light 48 to the optical isolator,
When the optical isolator is incorporated into a desired device, its alignment becomes difficult.

FIG. 5 is a cross-sectional view of a conventional optical isolator which can eliminate such an optical axis shift of the optical isolator.
Of the first polarizer 51 and the second polarizer 55 are arranged at the same angle θ in opposite directions. The light is a polarizer 51,
The reflected light does not return to the LD because it is obliquely incident on 55, and the two inclinations are at the same angle in the opposite directions, so that the optical axis shift is canceled out and the axial shift between the incident light and the emitted light does not occur. Further, since the Faraday rotator 53 is fixed perpendicular to the optical axis, the first polarizer 51, the second polarizer 55, and the Faraday rotator 53 do not become parallel to each other, but resonate inside. No cause. However, in this example, since the light is perpendicularly incident on the Faraday rotator 53, reflection that returns to the LD occurs here. In particular, the Faraday rotator is made of YIG (yttrium-iron-garnet) or the like, and has a refractive index of 2.0 to 2.3, which is the component having the highest refractive index among the isolator components and is likely to generate reflected light. I am worried about reflected light.

[0012]

As described above, some conventional optical isolators generate reflected return light due to the reflection of the optical isolator itself. In order to prevent the reflection, each optical element is inclined. Further, some optical elements are tilted so that the optical axis shift due to the tilt is canceled. However, those designed to incline each optical element so that this optical axis shift is offset,
There are many optical axis deviations when actually manufactured based on the design.

This is because, when the optical isolator is actually assembled, it is essential to rotate the polarizer around the optical axis and adjust the angle in the direction of the polarization transmission axis. More specifically, while rotating the polarizer around the optical axis, light is incident in the forward direction and the amount of transmitted light is maximized, or the light is incident in the reverse direction and the amount of transmitted light is minimized. The rotation adjustment work for fixing the parts becomes indispensable. In general, it is a well-known fact that the so-called "quenching method" has the highest accuracy in determining the direction of the polarization transmission axis of a polarizer. Therefore, usually, light is incident from the opposite direction while rotating the polarizer. Each component is fixed at a position where the amount of transmitted light is minimized. In other words, a general optical isolator must be assembled so that the amount of transmitted light from the opposite direction is minimized by rotating the polarizer around the optical axis.

This rotation adjustment is performed by, for example, the holder 4 shown in FIG.
2 is performed by rotating the first polarizer 41 held and fixed around the optical axis.
The normal to the incident boundary surface of the polarizer 41, which is tilted from the optical axis by an angle θ and drawn by a dotted line in FIG. 4, rotates while being tilted by this angle θ. For this reason, this normal line does not exist on the paper on which FIG. 4 is drawn due to the rotation. When the holder 42 is fixed in this state and the assembly of the optical isolator is completed, the normal line enters the polarizer 41 from the forward direction. The shifted light also causes an axis shift in a direction perpendicular to the plane of the drawing. Therefore, even if the inclination of each optical element is designed to an angle that cancels the axis shift as shown in FIG. 5, when the optical isolator is actually assembled, the axis shift occurs due to the rotation adjustment work of the polarizer. Will be lost.

An object of the present invention is to solve the above-mentioned problems by providing an optical isolator that requires an angle adjustment of a polarizer during assembly.
An object of the present invention is to provide an optical isolator that eliminates reflected return light returning in the direction D and that does not have an axis shift between incident light and output light.

[0016]

According to the present invention, there is provided an optical isolator in which a first polarizer, a Faraday rotator, and a second polarizer are arranged in this order. And a Faraday rotator inclined in a direction opposite to the optical axis by an angle at which an optical axis deviation of light transmitted through the first polarizer is completely canceled. And a second polarizer perpendicular to the optical axis, wherein the first polarizer and the Faraday rotator are integrally held and the second polarizer is arranged around the optical axis. This is an optical isolator that is arranged so that the rotation of the optical isolator can be adjusted.

That is, the first polarizer and the Faraday rotator are installed at an angle with respect to the optical axis so as to prevent the reflection at the incident boundary surface from returning to the LD, and the second polarizer is used as a light source. The Faraday rotator is an optical isolator whose entrance interface is perpendicular to the axis, and the Faraday rotator is at an angle to the optical axis by an angle that completely cancels the deviation of the optical axis due to the tilt of the first polarizer. It is inclined in the opposite direction. And
In a state where the first polarizer and the Faraday rotator were previously held integrally with each other in an inclination angle relationship in which the optical axis deviation was completely canceled, the incident boundary surface thereof was made perpendicular to the optical axis around the optical axis. The second polarizer is an optical isolator that is assembled by rotation adjustment.

When the light is obliquely (θ) incident on a parallel flat plate having a refractive index N and a thickness T, the magnitude D of the optical axis shift between the incident light and the outgoing light can be expressed by the following equation 1.

[0019]

(Equation 1)

Therefore, the refractive index of the first polarizer is N ₁ , the thickness is T ₁ , the refractive index and the thickness of the Faraday rotator are N ₂ , T ₂
In order to completely cancel the amount of the optical axis shift, the relationship of the following expression 2 may be satisfied. In addition, the second polarizer is installed perpendicular to the incident light.

[0021]

(Equation 2)

[0022]

The first and second embodiments of the present invention will be described below. FIG. 1 is a sectional view of an optical isolator according to a first embodiment of the present invention.
And a holder 2 having an inclined inner surface structure a for holding the polarizer 1 at an angle θ1 from the optical axis, and a parallel surface obliquely cut so that an end surface is inclined at an angle θ2 with respect to the optical axis. A flat Faraday rotator 3, a magnet 4 for holding the side surface of the Faraday rotator on an inner diameter surface, an absorption-type parallel flat plate-shaped second polarizer 5, and a holder 6 for holding the second polarizer 5 perpendicular to the optical axis.
And a case 9 for accommodating them. θ1 and θ2 satisfy the relationship of the above Expression 2, and as an example,
If the refractive index of the first polarizer is 1.5, the thickness is 200 μm, the refractive index of the Faraday rotator is 2.2, and the thickness is 350 μm, and θ1 is 6 °, θ2 is about 2.30 °. Become.

The assembling is performed by adjusting the rotation angle of the holder 1 for holding the first polarizer 1 and the Faraday rotator 3 at the angles θ1 and θ2 as described above, and the magnet 4 in the case 9 while allowing light to enter the case. When the optical axis shift is completely canceled, fix the optical axis. In this case, since the Faraday rotator 3 has no polarization dependency, it is not necessary to consider the direction of the polarization plane. Further, the holder 6 for vertically holding the second polarizer 5 is inserted into the case 9 and rotated, so that the loss of transmitted light that has entered the optical isolator from the second polarizer 5 side, that is, from the opposite direction. When is maximized, it is fixed and assembly is completed. Since the polarizer 5 is installed perpendicular to the optical axis, the optical axis does not shift again due to the rotation adjustment.

In an optical isolator having such a structure and actually assembled, light that has entered in the forward direction first enters the first polarizer 1 and is converted into linearly polarized light having only a specific polarization component. And the inclination angle θ1 of the first polarizer
Is emitted to a position slightly shifted from the position of the incident light. Next, when this light enters the Faraday rotator 3, the polarization plane is rotated by 45 ° around the optical axis. When this light is emitted from the Faraday rotator 3, the Faraday rotator 3
Is set so that the optical axis shift generated by the first polarizer is completely canceled, and the optical axis shift is assembled, so that the optical axis is the same as when the optical axis is incident on the first polarizer 1. It has returned to its original position. When the light enters the second polarizer 5 in this manner, the second polarizer 5 is rotated and adjusted so that the transmission polarization direction of the first polarizer 1 is rotated by 45 ° in the same direction as the Faraday rotation direction. Light is transmitted as it is because it is fixed.

At this time, since the second polarizer 5 is installed perpendicular to the optical axis, a slight reflected light is generated. However, since the rotation of the polarization plane does not occur in the surface reflection of the polarizer 5, the reflected light returning in the reverse direction is further rotated by 45 ° by the Faraday rotator 3, and when the reflected light is emitted from the Faraday rotator 3, The polarization plane is rotated by 90 ° from the polarization transmission direction of one polarizer 1. Therefore,
This reflected light is completely blocked by the first polarizer 1. As described above, setting the second polarizer 5 perpendicular to the optical axis does not cause any problem regarding the reflected light.

Further, since all the elements of the first polarizer 1, the Faraday rotator 3, and the second polarizer 5 are not installed in parallel with each other, no resonance occurs inside.

FIG. 2 is a sectional view of an optical isolator according to a second embodiment of the present invention. This embodiment has the same optical characteristics as the previous embodiment, but has a configuration in which productivity is further improved. That is, the first polarizer 11, the magnet 14, and the magnet 1
4 and the Faraday rotator 13 and the holder 12
And is assembled as a sub-assembly A. Of course, the first polarizer 11 and the Faraday rotator 13 have a predetermined inclination angle in directions opposite to each other so that the reflected light does not return to the LD and the axis shift between the incident light and the output light is completely canceled. Angles θ1 ′, θ that satisfy the relationship (Equation 2)
It is tilted from the optical axis by 2 '. Since the Faraday rotator 13 has no polarization dependency, it is not necessary to consider the polarization direction when assembling the sub-assembly A.

The holder 12 has a large inner diameter portion b for arranging a magnet, and a first polarizer 11 having the same diameter as the inner diameter of the magnet 14.
, And an inclined inner surface structure a ′ for arranging the first polarizer 11 in an inclined manner is provided in the small inner diameter portion c. Further, a spacer 19 is interposed between the first polarizer 11 and the Faraday rotator 13.
One end face of the spacer 19 has an angle θ of the first polarizer 11.
The other end face is tilted so that the Faraday rotator 13 is tilted by the angle θ2 ′, and the other end face is tilted by 1 ′. This end face makes angle adjustment of both optical elements unnecessary. The assembly work can be simplified.

On the other hand, the second polarizer is fixed in the holder 16 perpendicularly to the optical axis and assembled into a sub-assembly B. Then, the pre-assembled sub-assembly A, sub-assembly and B are aligned as shown in FIG. 2 and light is incident from the second polarizer 15, that is, the sub-assembly is set so that the amount of transmitted light in the reverse direction is minimized. When the assembly A is rotated and adjusted and fixed, the optical isolator is completed.

As described above, according to the present invention, the optical axis shift of the tilted first polarizer is completely canceled by the tilt of the Faraday rotator, and the first polarizer and the first polarizer, which are integrally held, are perpendicular to the optical axis. Since the rotation of the two polarizers is adjusted, unlike the conventional optical isolator, the optical axis is not shifted by the rotation adjustment of the polarizer in the final assembly work, and the optical isolator can be manufactured as designed. it can. Moreover, the manufactured optical isolator not only has no optical axis deviation, but also the first polarizer and the Faraday rotator are inclined with respect to the optical axis, and the light reflected on these incident surfaces is different from the optical axis. Although reflected in a different direction and the second polarizer is perpendicular to the optical axis, the light reflected at this plane of incidence is blocked by the first polarizer. Therefore, light reflected on each element surface inside the optical isolator does not become reflected return light. Further, the light reflected inside the optical isolator does not cause resonance inside because the first polarizer, the Faraday rotator, and the second polarizer are not installed in parallel with each other.

Further, according to the present invention, in the manufacture thereof, the first polarizer and the Faddy rotator are preliminarily assembled integrally so as to have an inclination angle relationship for canceling the optical axis shift, and the first polarizer and the Fade rotator are perpendicular to the optical axis. Since the second polarizer and the second polarizer are assembled by rotation adjustment, compared to a conventional optical isolator in which the optical element is inclined and the optical axis shift is eliminated, there is no trouble in the adjustment work, and the yield is high. Is excellent, and as a result, it has a very excellent effect in terms of mass production.

In the conventional optical isolator shown in FIG. 5, for example, in which the reflected return light generated on the surface of each element inside the optical isolator is eliminated and the optical axis is not shifted, the first optical isolator shown in FIG.
The inclination angles of the polarizer 51 and the second polarizer 55 must be adjusted so as to cancel the optical axis shift, and the direction of the polarization plane must be adjusted and held and fixed. However, it is difficult to perform both adjustments at the same time. As a result, priority is given to the rotation adjustment of the polarization plane in order to achieve the function of removing light from the opposite direction, which is the original function of the optical isolator. For this reason, even if it is designed to offset the optical axis shift, many optical isolators actually manufactured have the optical axis shift. Further, in the example of FIG. 5, since the Faraday rotator 53 is perpendicular to the optical axis, there is a high possibility that reflection occurs on this surface. The light reflected by the Faraday rotator 53 is likely to pass through the first polarizer 51 and return to the LD. Therefore, all the elements of the first polarizer 51, the Faraday rotator 53, and the second polarizer 55 are designed to be inclined with respect to the optical axis, and the inclination of all the elements cancels the optical axis shift. Is also possible. However, in this case, the number of optical elements to be tilted increases, so that the production becomes more difficult. In addition, as in the example of FIG. 5, the adjustment of the tilt angle of each element and the adjustment of the rotation of the polarization plane must be performed simultaneously, which makes the manufacture more difficult.

On the other hand, in the optical isolator according to the present invention, the two optical elements of the second polarizer and the Faraday rotator are integrally held in advance in a tilt angle relationship that cancels the optical axis shift. In this case, since the Faraday rotator has no polarization dependence and does not require any adjustment in the polarization direction, the adjustment of the tilt angle of the optical element for canceling the optical axis deviation and the operation of holding the same can be performed very easily and reliably. . Furthermore, while the second polarizer is held perpendicular to the optical axis, the first polarizer and the Faraday rotator that are integrally held in a tilt angle relationship in which the optical axis shift is canceled out. Since the rotation is adjusted, the optical axis does not shift due to the rotation adjustment.

As described above, the optical isolator of the present invention
Since the first and second optical elements to be tilted and held are constituted only by the first polarizer and the Faraday rotator so as to be integrally held in a tilt angle relationship in which the optical axis shift is canceled out, a plurality of elements which are originally difficult to assemble are provided. The tilt holding operation need only be performed for two elements, and this operation does not require any consideration of the angle adjustment of the polarization plane, which is difficult to perform simultaneously. Further, since the second polarizer may be held perpendicular to the optical axis, it can be easily assembled. In the rotation adjusting operation of the second polarizer, since the optical axis shift has already been canceled out by the first polarizer and the Faraday rotator, there is no fear that the optical axis shift occurs.

Therefore, the optical isolator of the present invention
The adjustment work is much less complicated than before, and the optical axis is not deviated by the rotation adjustment of the polarizer, which is the final assembling step, so that the yield is much higher than before. As described above, the optical isolator of the present invention is extremely easy to manufacture and has excellent mass productivity.

[0036]

As described above, the optical isolator of the present invention has the first
The polarizer and the Faraday rotator are tilted in the opposite direction to the optical axis at an inclination angle at which the optical axis shift is completely canceled, and the second and the third optical axes are perpendicular to the optical axis around these optical axes. Since the polarizer and the optical isolator are rotationally adjusted, the optical axis does not shift during the rotation adjustment work, which is the final step of assembling the optical isolator. Can be removed,
Further, no resonance occurs inside the optical isolator. Further, the optical isolator of the present invention requires less labor for adjustment, has a good yield, and is excellent in mass productivity in its manufacture.

[Brief description of the drawings]

FIG. 1 is a sectional view of an optical isolator showing a first embodiment of the present invention.

FIG. 2 is a sectional view of an optical isolator according to a second embodiment of the present invention.

FIG. 3 is a cross-sectional view of a conventional optical isolator.

FIG. 4 is a sectional view of an optical isolator showing another conventional example.

FIG. 5 is a sectional view of an optical isolator showing another conventional example.

[Explanation of symbols]

 1, 11 First polarizer 3, 13 Faraday rotator 4, 14 Magnet 5, 15 Second polarizer

Claims (1)

(57) [Claims] 1. An optical isolator in which a first polarizer, a Faraday rotator, and a second polarizer are arranged in this order, wherein the first polarizer tilted with respect to an optical axis;
A Faraday rotator tilted in a direction opposite to the optical axis by an angle at which the optical axis deviation of light transmitted through the polarizer is completely canceled, and a second polarized light perpendicular to the optical axis And the first polarizer and the Faraday rotator are disposed so as to be rotated and adjusted with respect to the second polarizer around the optical axis in a state where the first polarizer and the Faraday rotator are integrally held. Optical isolator.