JP3615864B2 - Measuring method of optical axis angle of wedge birefringent plate - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring the optical axis direction of a wedge-shaped birefringent plate used in an optical device such as a polarization-independent optical isolator.
[0002]
[Prior art]
FIG. 5 is a perspective view for explaining a schematic configuration and an operation principle of a polarization-independent optical isolator. As shown in FIGS. 1A and 1B, the polarization-independent optical isolator 2 has a pair of wedge-shaped birefringent plates 6 of the same shape arranged symmetrically with a Faraday rotator 4 interposed therebetween, and further A pair of lenses 8 are arranged on the front and rear sides and coupled to the optical fiber 10.
[0003]
Here, the rotation angle of the polarization plane by the Faraday rotator 4 is set to 45 degrees, and the pair of wedge-shaped birefringent plates 6 before and after the pair of wedge-shaped birefringent plates 6 are arranged in the rear direction with respect to the optical axis direction of the first birefringent plate 6a. The optical axis direction of the refracting plate 6b is shifted by 45 degrees corresponding to the polarization plane rotation angle by the Faraday rotator 4, and the first and second birefringent plates 6a and 6b are parallel with the wedge-shaped inclined surfaces facing each other. Are arranged symmetrically. The wedge-shaped birefringent plate 6 has an end face 62 perpendicular to the light transmission direction and an inclined face 64 that faces the end face 62 at a predetermined angle, and the optical axis is in the plane of the end face 62. Exist.
[0004]
In the polarization-independent optical isolator 2 configured in this way, as shown in FIG. 5A, the forward light from the front optical fiber 10a is converted into parallel rays by the first lens 8a. The light incident on the first birefringent plate 6a and transmitted through the first birefringent plate 6a is separated into ordinary light and extraordinary light. The separated ordinary light and extraordinary light are further rotated by 45 degrees by the Faraday rotator 4 and are incident on the rear second birefringent plate 6b.
[0005]
At this time, in relation to the first birefringent plate, the second birefringent plate 6b is shifted by 45 degrees in accordance with the rotation angle of the polarization plane by the Faraday rotator 4 and the inclined surfaces are offset from each other. Since they are arranged in parallel and symmetrically, the relationship between ordinary light and extraordinary light when passing through the second birefringent plate 6b is the same as when passing through the first birefringent plate 6a. The ordinary light and the extraordinary light transmitted through the refracting plate 6b are returned to parallel rays and enter the rear second lens 8b. Therefore, the normal light and the abnormal light are condensed on the rear optical fiber 10b and propagated in the forward direction.
[0006]
On the other hand, as shown in FIG. 4B, the reflected light in the reverse direction from the rear optical fiber 10b is converted into parallel rays by the second lens 8b and then converted into ordinary light and extraordinary light by the second birefringent plate 6b. After being separated and further rotated by 45 degrees by the Faraday rotator 4, the light is incident on the first birefringent plate 6a.
[0007]
However, in the case of this reverse direction, the optical axis of the first birefringent plate 6a is deviated from the optical axis of the second birefringent plate 6b by 45 degrees in the direction opposite to the rotation direction by the Faraday rotator 4. Therefore, the relationship between the ordinary light and the extraordinary light separated when transmitted through the second birefringent plate 6b is reversed when transmitted through the first birefringent plate 6a.
[0008]
Therefore, even if it passes through the first birefringent plate 6a, the ordinary light and the extraordinary light of the reflected return light do not return to parallel rays, so that the ordinary light and the extraordinary light are collected on the optical fiber 10a in front by the first lens 8a. Therefore, propagation of the reflected return light in the reverse direction is prevented.
[0009]
By the way, the polarization-independent optical isolator 2 has a pair of wedge-shaped birefringent plates 6a and 6b arranged in parallel and in parallel with each other, as is apparent from the operation principle described above, and the optical axis It is necessary to shift the angular directions by 45 degrees from each other, and these precisions must be set very precisely.
[0010]
For this reason, the pair of wedge-shaped birefringent plates 6a and 6b are simultaneously processed in the same process from cutting out of an optical material such as rutile (TiO ₂ ) from the block material to formation of the inclined surface 64 by polishing. After that, the two birefringent plates 6a and 6b are cut and separated, and the two wedge-shaped birefringent plates 6a and 6b combined in a pair are always manufactured in pairs at the same time, so that the shape has an error level of machining accuracy. To be equal.
[0011]
In addition, the pair of wedge-shaped birefringent plates 6a and 6b manufactured in pairs as described above has the inclined surfaces 64 on the front and rear sides of the Faraday rotator 4, and the parallelism is precise as shown in an enlarged view in FIG. In this case, not only the parallelism but also the direction of the optical axis <001> must be accurately shifted by 45 degrees.
[0012]
Here, as described above, the pair of wedge-shaped birefringent plates 6a and 6b are cut and processed in pairs from the same block material, so that they are formed at right angles around the end face 62 where the optical axis <001> exists. If the common peripheral side surface is the reference surface 66, the direction of the optical axis <001>, that is, the angle θ with respect to the reference surface 66 is equal, and the pair of wedge-shaped birefringent plates 6a and 6b are arranged with their inclined surfaces 64 in parallel. If symmetrically arranged, the angle formed by both optical axes <001>, that is, the deviation angle between the optical axes <001> is 2θ. Therefore, in order to set the deviation angle to 45 degrees, the optical axis angle θ from the reference surface 66 may be set to 22.5 degrees.
[0013]
Therefore, from the above, the pair of wedge-shaped birefringent plates 6a and 6b has been manufactured as shown in FIG. That is, first, the optical axis <001> of the block material 12 of the optical material is measured by X-ray diffraction, and two parallel surfaces including the optical axis <001> in the plane are orthogonal to the two surfaces and are optical. A prismatic body 14 is cut out surrounded by two parallel surfaces forming an angle of 22.5 degrees with respect to the axis <001>. Next, one of the two parallel surfaces including the optical axis <001> is polished and inclined by a predetermined angle to form the inclined surface 64. Then, the same wedge-shaped birefringent plates 6a and 6b are obtained by cutting along a plane orthogonal to the four surfaces.
[0014]
[Problems to be solved by the invention]
By the way, in order to satisfy the performance of the polarization-independent optical isolator 2 to a specified value or more, the direction of the optical axis of the wedge-shaped birefringence plates 6a and 6b is about 22.5 ± 10 minutes with respect to the reference plane. Must be within the machining accuracy. However, after processing to the wedge-shaped birefringent plate 6 is too small, the optical axis <001> cannot be measured accurately even by X-ray diffraction. There is no other way to measure the optical axis <001>. Therefore, the dimensional accuracy of the wedge-shaped birefringent plate 6 cannot be managed, and it is substantially impossible to make a pass / fail judgment as a part.
[0015]
Therefore, the wedge-shaped birefringent plate 6 depends on the optical axis measurement by X-ray diffraction at the stage of the block material 12 in the middle of processing, and improves the subsequent processing accuracy as much as possible. As a result, it has been necessary to make a pass / fail judgment in a performance test of a finished product that has been assembled as the polarization-independent optical isolator 2.
[0016]
For this reason, even if the optical axis angle is defective as a component, the assembly is wasted, resulting in a decrease in yield and a factor in inhibiting cost reduction.
[0017]
The present invention has been made in view of such circumstances, and an object of the present invention is to accurately measure the angle formed by the reference surface and the optical axis of the wedge-shaped birefringent plate, and thus pass or fail as a component. An object of the present invention is to provide a method for measuring the optical axis angle of a wedge-shaped birefringent plate that can be easily determined.
[0018]
[Means for Solving the Problems]
In order to achieve the above object, in the method of measuring the optical axis angle of the wedge-shaped birefringent plate according to the present invention, the reference plane (66) parallel to the light transmission direction and the optical axis <001> perpendicular to the transmission direction are provided. The reference surface (66) in the wedge-shaped birefringent plate (6) having the end surface (62) existing in the surface and the inclined surface (64) facing the end surface (62) at a predetermined angle and the optical surface When measuring the angle θ formed with the axis <001>, the birefringent plate (6) is placed on the measurement table (22 ) with its reference surface (66) aligned, and the placed birefringent plate ( The light polarized by the rotary polarizing plate (28) is transmitted to 6) , the transmitted light amount of only ordinary light or abnormal light after the transmission is measured, and the transmitted light amount of the polarizing plate (28) is minimized. after looking for the rotational position seeking optical axis <001> direction, the reference surface of the tapered birefringent plate 6 for mounting (6 ) Is both with the wedge-shaped birefringent plate leave the (6) is rotated 180 degrees by obtaining the optical axis <001> direction, from the end face (62) side and the inclined surface (64) side Then, the optical axis <001> direction is obtained, and the angle θ between the reference plane (66) and the optical axis <001> is measured from the angle difference α between the rotational positions.
[0019]
Here, the rotational position of the rotary polarizing plate (28) at which the transmitted light amount of only ordinary light or abnormal light is the minimum value is obtained by accurately measuring the optical axis <001> direction, and is on the end face (62) side. The optical axis <001> direction when viewed from the side and the optical axis <001> direction when viewed from the inclined surface (64) side are naturally the same angle in the opposite direction with respect to the reference surface (66) . The position is rotated by θ. Accordingly, if the rotational positions in the respective optical axis <001> directions when viewed from these two directions are obtained and the angular difference α between them is obtained, the angular difference α is determined from the reference plane (66) and the optical axis <001. Since it is obtained by subtracting twice the angle θ formed by> from π, the angle θ is (π−α) / 2 and can be easily calculated. In addition, each optical axis direction obtained by measuring from two directions, that is, the rotational position, is a direct measurement of the optical axis <001>. Therefore, the angular difference α can be measured with extremely high accuracy. Therefore, the angle θ can be measured with high accuracy. For this reason, highly accurate component accuracy management becomes possible, and it becomes possible to appropriately perform pass / fail determination of components.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a method for measuring an optical axis angle of a wedge-shaped birefringent plate according to the present invention will be described in detail with reference to the accompanying drawings.
[0021]
FIG. 1 is a diagram showing a schematic configuration of a measurement system used in carrying out the measurement method according to the present invention. As shown in the figure, the measurement system 20 irradiates parallel beams toward a measuring table 22 on which a wedge-shaped birefringent plate 6 as a measurement object is placed and a wedge-shaped birefringent plate 6 mounted on the measuring table 22. Irradiator 24 for transmitting the light and its light source 26, a rotating polarizing plate 28 arranged between the measuring table 22 and the irradiator 24 and capable of polarizing a parallel light beam to a polarization plane of an arbitrary angle, and this rotation It comprises an angle measuring means 30 for measuring the rotation angle of the polarizing plate 28, a detector 32 for measuring the amount of light transmitted through the wedge-shaped birefringent plate 6, and its power supply 34.
[0022]
Further, as described in the above-mentioned prior art, the wedge-shaped birefringent plate 6 faces the end face 62 that is perpendicular to the light transmission direction and includes the optical axis <001>, and is inclined at a predetermined angle with respect to the end face 62. It has a sloped surface 64 and a reference surface 66 that is orthogonal to the end surface 62 and parallel to the light transmission direction, and specifically has a wedge shape in which one surface of a rectangular parallelepiped is formed on the sloped surface.
[0023]
By the way, in order to measure the optical axis <001> direction of the wedge-shaped birefringent plate 6 with respect to the reference surface 66, that is, the angle θ between the reference surface 66 and the optical axis <001>, as shown in FIGS. First, the wedge-shaped birefringent plate 6 is placed on the measuring table 22 with the reference surface 66 aligned, and the end surface 62 or the inclined surface 64 of the wedge-shaped birefringent plate 6 faces the irradiator 24 (FIG. 2 (a). )), And irradiates and transmits parallel rays from the irradiator 24. Then, the position of the detector 32 is aligned on one of the optical axes of ordinary light and extraordinary light separated by the wedge-shaped birefringent plate 6, and the amount of transmitted light after transmission is measured. In the illustrated example, the detector 32 is disposed on the optical axis of ordinary light so that the transmitted light amount of only ordinary light is measured, and when viewed from the end face 62 side including the optical axis <001>. In order to obtain the optical axis <001> direction first, the end face 62 side is directed to the irradiator 24 side.
[0024]
Next, the rotation type polarizing plate 28 is rotated to find the rotation position where the measured value of the transmitted light amount is the minimum value, the optical axis <001> direction is obtained, and if the optical axis <001> direction is obtained, The rotation position of the rotary polarizing plate 28 at this time is set as the angle measurement start point, and the scale of the angle measurement means 30 is reset.
[0025]
Next, the wedge-shaped birefringent plate 6 to be placed is left as it is, and the wedge-shaped birefringent plate 6 is rotated 180 degrees so that the inclined surface 64 faces the irradiator 24 (see FIG. 2B). The optical axis <001> direction when viewed from the inclined surface 64 side is obtained in the same manner as described above. If the rotation angle of the rotary polarizing plate 28 at this time is read by the angle measuring device 30, the scale is reset in advance in the angle direction of the optical axis <001> when viewed from the end face 62 side. The value is the angle difference α in each optical axis <001> direction when viewed from two directions as it is. Since the angle difference α thus obtained is substantially equal to directly measuring the angle between the two optical axes <001>, the measurement accuracy is extremely high.
[0026]
Further, since the angle difference α is obtained by subtracting twice the angle θ formed by the reference surface 66 and the optical axis <001> from π, the angle θ is (π−α) / 2. It can be calculated and measured, and a highly accurate measurement value can be obtained.
[0027]
In determining the angle difference α, the initial scale value of the angle measuring means 30 is set arbitrarily, and the scale display value of the rotational position when the optical axis <001> direction from each direction is found is read as it is. Then, the angle difference α may be obtained from the difference between the read values. Even if the calculation is performed in this way, the initial value set arbitrarily is lost when subtracting, and thus has no effect. That is, when the direction of each optical axis <001> is found by rotating the rotary polarizing plate 28, the scale of the angle measuring means 30 does not need to be initially set. Further, any of the four peripheral surfaces of the inclined surface can be selected as the reference surface 66 of the wedge-shaped birefringent plate 6.
[0028]
When measuring the optical axis direction of a pair of wedge-shaped birefringent plates 6a and 6b incorporated in the polarization-independent optical isolator, one wedge-shaped birefringent plate 6a has a wedge-shaped trapezoidal cross section as shown in FIG. In FIG. 3, the circumferential side surface on the long side which is wide is measured as the reference surface 66, and the other wedge-shaped birefringent plate 6b is formed on the circumferential side surface on the short side which is narrow in the wedge-shaped trapezoidal cross section as shown in FIG. The measurement may be performed as 66. In this way, when the reference plane is measured separately, the optical axis <001> directions of the pair of wedge-shaped birefringent plates 6a and 6b are both within the allowable dimensional accuracy (22.5 degrees ± 10 minutes as a specific example). If the pass judgment is made, the pair of wedge-shaped birefringent plates 6a and 6b are placed on the reference surface 36a of the assembly jig base 36 when the polarization-independent optical isolator is assembled. The optical axis directions can be shifted from each other by 45 degrees simply by placing them, eliminating the need for optical axis adjustment.
[0029]
【The invention's effect】
As described above in detail in the embodiments of the present invention, according to the wedge-shaped birefringent plate optical axis angle measuring method according to the present invention, the rotating polarizing plate in which the transmitted light amount of only ordinary light or extraordinary light becomes the minimum value. The optical axis direction can be accurately found from the rotation position of the lens, and the angle difference α between the optical axis direction when viewed from the end surface side and the optical axis direction when viewed from the inclined surface side can be measured. In this case, the angle difference α in the optical axis direction is obtained by subtracting twice the angle θ formed by the reference surface and the optical axis from π, and the angle θ is (π−α) / 2. The angle θ can be easily calculated from the difference α and measured.
[0030]
Further, the rotational positions in the respective optical axis directions obtained by measuring from the two directions substantially measure the optical axis directly, so that the angle difference α can be measured with extremely high accuracy, and therefore the angle θ Can be measured with high accuracy.
[0031]
For this reason, highly accurate component accuracy management becomes possible, and it becomes possible to appropriately perform pass / fail determination of components.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of a measurement system used in carrying out a measurement method according to the present invention.
FIG. 2 shows a wedge-shaped birefringent plate placed on a measurement table with a reference surface aligned, where FIG. 2A shows a case where measurement is performed from the end surface side, FIG. 2I is a side view thereof, and FIG. ) Is a front view thereof, FIG. 5B is a case where measurement is performed from the inclined surface side, (i) is a side view thereof, and (ii) is a front view thereof.
FIG. 3 shows a wedge-shaped birefringent plate placed on a measuring table with different reference planes, wherein FIG. 3 (a) shows a case where measurement is performed from the end face side, and FIG. 3 (i) is a side view thereof. , (Ii) is a front view thereof, FIG. 4B is a case of measurement from the inclined surface side, (i) is a side view thereof, and (ii) is a front view thereof.
FIG. 4 is a diagram showing a state in which a polarization-independent optical isolator is assembled on an assembly jig table.
FIG. 5 is a perspective view for explaining a schematic configuration and an operation principle of a polarization-independent optical isolator.
FIG. 6 is an enlarged perspective view showing a main part of a polarization-independent optical isolator.
FIG. 7 is a diagram illustrating a procedure for forming a wedge-shaped birefringent plate used in a polarization-independent optical isolator.
[Explanation of symbols]
6 Wedge-shaped birefringent plate 22 Measuring table 28 Rotating polarizing plate 62 End surface 64 Inclined surface 66 Reference surface <001> Optical axis

Claims (1)

A reference plane (66) parallel to the light transmission direction, an end face (62) having an optical axis <001> in-plane perpendicular to the transmission direction, and inclined by a predetermined angle with respect to the end face (62) In measuring the angle θ between the reference surface (66) and the optical axis <001> in the wedge-shaped birefringent plate (6) having the inclined surface (64) facing each other, the compound surface is placed on the measurement table (22). The refracting plate (6) is placed with its reference surface (66) aligned, and the polarized light by the rotary polarizing plate (28) is transmitted through the placed birefringent plate (6) . The transmitted light amount of only ordinary light or extraordinary light is measured, the rotational position of the polarizing plate (28) where the transmitted light amount becomes the minimum value is found, the optical axis <001> direction is obtained, and then the wedge-shaped birefringence to be placed The wedge-shaped birefringent plate (6) is rotated 180 degrees with the reference surface (66) of the plate 6 as it is, and the optical axis <0. 1> by obtaining the direction, both in the optical axis between the end face (62) side from the inclined surface (64) side <001> the determined direction, the reference plane from the angle difference α of the both rotation position The method of measuring an optical axis angle of a wedge-shaped birefringent plate, comprising measuring an angle θ between (66) and the optical axis <001>.

Images