JP2001281091A - Method of inspecting multi-core optical fiber, apparatus for executing the same method and optical attenuator element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for inspecting a multi-core optical fiber, which can simply and verify accurately in a time that the input arranging order matches with the output arranging order of the multi-core optical fiber. SOLUTION: Outputs of a multi-output light source are switches one after another, the optical output of a multi-core optical fiber is measured by a signal detector, a continuous light attenuator element is inserted between the optical fiber and an optical sensor and is aligned in the arranging direction of the emitting ends of the optical fiber with the attenuation varying continuously, the outputs of the light source are re-switched one after another and the optical output of the optical fiber is measured by the signal detector. An arithmetic unit computes the loss for each line number from the optical output values of when the attenuator element is not inserted and when it is inserted, and analyzes the result to execute the line number check and obtain the loss of the optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-core optical fiber inspection method, an apparatus for implementing the method, and an optical attenuator, and more particularly, to an input alignment order and an output alignment of a multi-core optical fiber used for optical communication. Multi-fiber optical fiber inspection method that matches the order and measures the transmission loss of each core in the same process,
An apparatus for performing the method and a continuous light attenuating element suitable for use in the method and apparatus.

[0002]

2. Description of the Related Art A multi-core optical fiber is obtained by aligning a plurality of optical fiber cores on a plane to form a tape. This multi-core optical fiber is required to have the input order and the output order matched. If the order does not match, the optical signal cannot be transmitted to the target transmission line. However, in the manufacturing process of the multi-core optical fiber, defective products whose input order and output order do not match rarely are manufactured, so it is required to confirm that the input line number and the output line number match. . Further, in order to improve the production efficiency of the multi-core optical fiber, there is a need to measure the transmission loss, which is a basic characteristic of the optical fiber, in a short time without complicated work.

A conventional multi-fiber optical fiber line number confirmation method will be described with reference to FIG. 4 is a light source, 104 in FIG.
The optical power meter 102 is a multi-core optical fiber. Conventionally, when confirming the wire number of a multi-core optical fiber and measuring the transmission loss, the output of the light source 100 is input to the respective core wires of the multi-core optical fiber 102, and the respective outputs from the multi-core optical fiber 107 are output. Light was measured by the optical power meter 104. For example, the light source 100 is connected to the first input cord, and 1
The output of the output core No. was checked with the optical power meter 104.

[0004]

According to the prior art, the output side of the multi-core optical fiber 102 is separated into single fibers, and each is separately connected to the optical power meter 104 for measurement. Was very complicated and required a very long measurement time. SUMMARY OF THE INVENTION It is an object of the present invention to provide a multi-core optical fiber inspection method and apparatus for easily measuring the coincidence between the input alignment order and the output alignment order and / or transmission loss of the multi-core optical fiber. It is another object of the present invention to provide a continuous light attenuating element suitable for use in the above method and apparatus.

[0005]

In order to achieve this object, a multi-output light source unit having a plurality of output terminals, a continuous light attenuating element, an optical sensor, and a signal detection unit are provided. The optical sensors are arranged such that the respective outputs are input to the respective optical fibers of the multi-core optical fiber, and the outputs of the multi-core optical fiber are input to the optical sensor. The output of the multi-output light source unit is sequentially switched, the optical output of the multi-core optical fiber is measured by a signal detector, and a continuous light attenuating element is output between the multi-core optical fiber and the optical sensor. The insertion is performed in such a manner that the amount of attenuation changes continuously with respect to the alignment direction of the ends, the output of the multi-output light source is sequentially switched again, and the optical output of the multi-core optical fiber is measured by the signal detection unit. The arithmetic processing unit calculates the loss amount for each wire number from the optical output value when the continuous light attenuating element is not inserted and when it is inserted, and analyzes the result to confirm the wire number of the multi-core optical fiber.

According to the present invention, the above object can be attained by configuring a multi-core optical fiber inspection method and apparatus as follows. 1: light is input one by one in the input alignment forward direction of the multi-core optical fiber, and the output power of each line number of the multi-core optical fiber is measured by an optical sensor to obtain a first measurement value.
The continuous light attenuating element is inserted with the output alignment forward direction of the multi-core optical fiber and the direction in which the amount of attenuation of the continuous optical attenuator changes, and light is input one by one in the input alignment forward direction of the multi-core optical fiber again. Then, the output power of each wire number is measured to obtain a second measurement value, and the input and output alignment orders of the multi-core optical fiber of the multi-core optical fiber are obtained from the first and second measurement values. Confirmation of coincidence and / or calculation of connection loss of the multi-core optical fiber. 2: Light is input one by one in the input alignment forward direction of the multi-core optical fiber according to claim 1, and the output power of each wire number of the multi-core optical fiber is measured by an optical sensor to obtain a first measurement value. From the obtained operation, from the first and second measured values, it is confirmed whether the input alignment order and the output alignment order of the multi-core optical fiber match and / or the connection loss of the multi-core optical fiber is calculated. Perform the above operations twice. 3: Before the inspection of the multi-core optical fiber, a reference multi-core optical fiber composed of the same number of multi-core optical fibers is connected between the light incident means and the optical sensor, The connection loss of the multi-core optical fiber is measured with a reference value for confirming the coincidence of the input alignment order and the output alignment order of the multi-core optical fiber and / or calculating the connection loss of the multi-core optical fiber. I do. 4: The means for injecting light one at a time in the input alignment direction of the multi-core optical fiber turns on the multi-output light source one by one. 5: Means for inputting light one at a time in the input alignment direction of the multi-core optical fiber is to switch light from a single light source by an optical switch and to input light one at a time. 6: The optical sensor is a single optical sensor. 7: The optical sensors are a plurality of optical sensors provided corresponding to each fiber of the multi-core optical fiber. 8: The means for confirming the match between the input alignment order and the output alignment order of the multi-core optical fiber of the multi-core optical fiber and / or calculating the transmission loss from the respective measurement results is signal detection using the output signal of the optical sensor as an input. And a system control unit for sequentially lighting the multi-output light source one by one or turning on the optical switch sequentially, and an arithmetic processing unit. 9: Confirmation of matching between the input alignment order and the output alignment order of the multi-core optical fiber of the multi-core optical fiber is based on whether the transmission loss of each wire number increases or decreases monotonically when the input loss is aligned in the input alignment order. Check more. 10: The continuous light attenuating element is made of a light attenuating material, and is formed in a shape in which the light transmission distance decreases steplessly. 11: The shape that decreases steplessly is a shape formed in a triangle or a shape in which a light incident side and a light emission side are formed in a curved surface. 12: The continuous light attenuating element is made of a light attenuating material, and is formed in a shape in which the amount of light attenuation changes continuously as the irradiation position changes. 13: The continuous light attenuating element is formed by laminating a wedge glass body and a light attenuator made of the same light attenuating material as the glass body.

According to the present invention, the above object can be achieved by configuring a multi-core optical fiber inspection apparatus as follows. 14: a light source, and a light incident unit for inputting light from the light source one by one in the input alignment direction of the multi-core optical fiber,
First measuring means for measuring the output power of each wire number of the multi-core optical fiber by an optical sensor, a continuous light attenuating element,
A continuous light attenuating element insertion unit for inserting the continuous light attenuating element after the measurement by the optical sensor so that the output alignment forward direction of the multi-core optical fiber and the direction in which the attenuation of the continuous light attenuating element changes are aligned; After the continuous light attenuating element is inserted by the attenuating element inserting means, the re-light incident means for inputting the light one by one in the input alignment forward direction of the multi-core optical fiber again, and the output power for each line number of the multi-core optical fiber Is measured by the optical sensor, and the measured values obtained by the first and second measuring means are analyzed to confirm that the input alignment order and the output alignment order of the multi-core optical fiber match. And / or an analysis unit for calculating the transmission loss of the multi-core optical fiber,
Consists of 15: The light source is a multi-output light source, and the light incident means turns on the multi-output light source one by one. 16: The light source is a single light source, and the light incident means switches light from the single light source by an optical switch and causes light to be incident one by one. 17: The optical sensor is a single optical sensor. 18: The optical sensor is a plurality of optical sensors provided corresponding to each fiber of the multi-core optical fiber. 19: The analysis unit is a signal detection unit to which an output signal of an optical sensor is input, a system control unit that sequentially turns on the multi-output light source one by one or sequentially turns on the optical switch, and an output signal of the signal detection unit. And an arithmetic processing unit for analyzing 20: In addition to the multi-core optical fiber, an optical fiber is further provided between the light incident means and the optical sensor, and third measuring means for measuring the optical power incident on the fiber by the optical sensor is provided. From the measured values of the first to third measuring means, it is confirmed whether the input alignment order and the output alignment order of the multi-core optical fiber coincide and / or the transmission loss of the multi-core optical fiber is calculated. 21: The splice loss of the multi-core optical fiber is determined by the output power L0 (m) of each fiber core of the multi-core optical fiber having a predetermined length having the same number of fiber cores as the measured fiber core.
From w) and the output power L1 (mw) of the multi-core optical fiber to be measured, the connection loss is determined by the following equation: connection loss = 10 log (L1 / L0). 22: Confirmation of matching between the input alignment order and the output alignment order of the multi-core optical fiber of the multi-core optical fiber is based on whether the transmission loss of each wire number increases or decreases monotonically when the transmission loss of each wire number is aligned in the input alignment order. Confirm with. 23: The continuous light attenuating element is made of a light attenuating material and is formed in a shape in which the light transmission distance is reduced steplessly. 24: The shape that decreases steplessly is a shape formed in a wedge or a shape in which a light incident side and a light emission side are formed in a curved surface. 25: The continuous light attenuating element is made of a light attenuating material, and is formed in a shape in which the light transmission distance decreases stepwise. 26: The continuous light attenuating element is formed by laminating a triangular glass body and a light attenuator made of a light attenuating material having the same shape as the glass body.

According to the present invention, a continuous light attenuating element suitable for use in a method for inspecting a multi-core optical fiber and an apparatus thereof can be provided by using the following structure of the optical attenuating element. 27: It is made of a light attenuating material, and is formed in a shape in which the light transmission distance decreases in the light transmission direction. 28: The shape to be reduced is a wedge shape or a curved surface on the light incident side and the light emitting side. 29: The decreasing shape is a step shape. 30: A glass body having a triangular planar shape and a light attenuator made of a light attenuating material having the same shape as the glass body are bonded to each other.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a multi-core optical fiber inspection apparatus according to the present invention will be described with reference to FIGS. FIG.
10 is a multiple output light source unit, 40 is an optical sensor, 41 is a signal detection unit, 42 is an arithmetic processing unit, 30 is a continuous light attenuating element, 20
Is a multi-core optical fiber, 21 is a connector, and 50 is a system control unit. In the present embodiment, each input terminal of the multi-core optical fiber 20 is connected to each output terminal of the multi-output light source unit 10 by the connector 21. Further, the optical sensor 40 is arranged so that the light emitted from the multi-core optical fiber 20 enters the optical sensor 40 via the continuous light attenuating element 30. Further, the light emitted from the optical fiber F and emitted from the optical fiber is detected by the optical sensor 40. The output of the optical sensor 40 is connected to the signal detection unit 41, and the signal from the signal detection unit 41 is analyzed by the arithmetic processing unit. Also,
In order to synchronize output switching of the multi-output light source unit 10 and signal detection by the signal detection unit 41, the system control unit 50
Perform control.

The measurement procedure of the embodiment will be described. (1) First, a predetermined length (about 2 m in length) consisting of the same number of optical fibers as the multi-core optical fiber to be measured is connected to the connector 21.
The reference multi-core optical fiber is connected. Thereafter, light is sequentially incident on the fiber core of the reference optical fiber from the multi-output light source unit, the light is received by the sensor 40, and the output power (PW0) of each core is measured. (2) Next, a continuous light attenuating element is inserted between the reference multi-core optical fiber and the sensor 40. Thereafter, light is sequentially incident on the fiber core of the reference multi-core optical fiber from the multi-output light source section,
The light is received by the sensor 40, and the output power (PW1) of each cord is measured. Since the length of the reference multi-core optical fiber is about 2 m and the length of the multi-core optical fiber is much longer, the optical output of the optical fiber has less transmission loss than the optical output of the multi-core optical fiber. Can be considered. (3) Next, the reference multi-core optical fiber is removed, a multi-core optical fiber to be measured is connected instead, and light is sequentially incident on the fiber core of the optical fiber to be measured from a multi-output light source section.
The light is received by the sensor 40, and the output power (PW3) of each cord is measured. (4) Next, a continuous light attenuating element is inserted between the multi-core optical fiber to be measured and the sensor 40, and thereafter, light is sequentially input from the multi-output light source unit to the fiber core of the multi-core optical fiber to be measured. Then, the light is received by the sensor 40, and the output power (PW4) of each cord is measured. The connection loss (Ly) of each core of the multi-core optical fiber to be measured without the continuous light attenuating element is represented by L
y = 10 log (PW1 / PW0), and the connection loss (Lz) of each core of the measured multi-core optical fiber when the continuous optical attenuating element is inserted is obtained by Lz = 10 log (PW4 / PW2). In the present embodiment, when confirming the coincidence of the input alignment order and the output alignment order of the multi-core optical fiber 20, as shown in the embodiment of FIG.
The multi-output light source is sequentially turned on one core at a time by the system control unit 30 and the output power of each line number is measured.

Next, the continuous optical attenuating element is connected to the optical multi-core fiber 2.
0 and the direction in which the amount of attenuation of the continuous light attenuator continuously changes is inserted into the system controller 30.
Thus, the multi-output light sources are sequentially turned on one by one, and the output power of each wire number is measured. The arithmetic processing unit 42 calculates a loss difference for each wire number from the two measurement results of each wire number and performs analysis.

FIG. 3 relates to a specific wire number confirmation method of a multi-core optical fiber, and shows an example of eight cores. FIG. 3A shows the output level of each wire number when the continuous light attenuation element is removed, and FIG. 3B shows the output level of each wire number when the continuous light attenuation element is inserted. c indicates the optical loss level of each line number calculated by ab. When the input alignment order and the output alignment order of the multi-core optical fiber 20 match, the loss of each wire number corresponds to the change in the attenuation of the continuous optical attenuating element, and monotonically increases or decreases in the wire number alignment direction. , Monotonically decreasing. Here, when the four and five cores cross each other, the light loss levels of the four and five cores are reversed as shown in FIG. 3C, and a monotone increase or a monotone decrease cannot be obtained. By combining these, it is possible to confirm whether the alignment order of all the core wires is normal.

FIGS. 5A and 5B show various continuous light attenuating elements used in the present invention, in which a is an isosceles triangular cross section, b is a right triangle, and c is a glass plate bonded to b. 5d and 5c are formed in a curved surface, and f is formed in a step-like shape. This f has difficulty in aligning with the multi-core optical fiber. These continuous light attenuating elements are formed of glass containing chromium metal. FIG. 6 shows a continuous light attenuating element in which glass having the same shape as that of the continuous light attenuating element of FIG.

[0014]

According to the present invention, a multi-output light source unit having a plurality of output terminals, a continuous light attenuating element, an optical sensor, and a signal detection unit are provided, and each output of the multi-output light source unit is multiplied. The optical sensor is arranged so that the optical fiber is input to each of the optical fibers, and the output of the multi-core optical fiber is input to the optical sensor. By measuring and analyzing the level, it is possible to provide a multi-core optical fiber inspection method capable of easily and accurately confirming a wire number in a short time and an apparatus for implementing the method. By using this multi-core optical fiber inspection apparatus, the test time of the multi-core optical fiber wire number confirmation test can be greatly reduced, and the reliability of the test results can be greatly improved. In addition, the transmission loss of the multi-core optical fiber can be measured. Further, according to the present invention, it is possible to provide an optical attenuating element suitable for use in a multi-core optical fiber inspection method and an apparatus for performing the method.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of an embodiment of a multicore optical fiber inspection apparatus according to the present invention.

FIG. 2 is a diagram for explaining an embodiment of a multi-core optical fiber inspection apparatus according to the present invention.

FIG. 3 is a view for explaining an embodiment of a multi-core optical fiber inspection apparatus according to the present invention.

FIG. 4 is a block diagram showing a configuration example of an embodiment of a conventional multi-core optical fiber inspection apparatus.

FIG. 5 is a plan view of various continuous light attenuation elements.

FIG. 6 is a plan view of a second example of the continuous light attenuation element.

[Explanation of symbols]

 Reference Signs List 10 multi-output light source section 20 multi-core optical fiber 21 connector 30 continuous light attenuating element 40 optical sensor 41 signal detection section 42 arithmetic processing section 50 system control section 51 multi-core optical fiber output level 52 multi-core optical fiber output level 53 4-core 5 Multi-core optical fiber output level at cross-core 60 Optical loss level 61 Multi-core optical loss level at 4-core 5-fiber cross 100 Light source 102 Multi-core optical fiber 104 Optical power meter F Optical fiber

Claims (30)

[Claims] 1. A multi-core optical fiber is input with light one at a time in the input alignment forward direction, and the output power of each line number of the multi-core optical fiber is measured by an optical sensor to obtain a first measurement value. After that, the continuous light attenuating element is inserted with the output alignment direction of the multi-core optical fiber and the direction in which the amount of attenuation of the continuous optical attenuating element changes, and one core is again input in the input alignment forward direction of the multi-core optical fiber. Light is incident, the output power of each wire number is measured, and a second measurement value is obtained. From the first and second measurement values, the input alignment order and output of the multi-core optical fiber of the multi-core optical fiber A multi-core optical fiber inspection method, comprising: checking the alignment order and / or calculating the connection loss of the multi-core optical fiber. 2. The multi-core optical fiber according to claim 1, wherein light is input one at a time in the input alignment forward direction, and the output power of each line number of the multi-core optical fiber is measured by an optical sensor. From the operation of obtaining the measured values, the first and second measured values are used to confirm that the input alignment order and the output alignment order of the multi-core optical fiber match and / or the connection loss of the multi-core optical fiber. 2. The method for inspecting a multi-core optical fiber according to claim 1, wherein the operation up to calculating is performed twice. 3. The multi-core optical fiber according to claim 1, wherein a reference multi-core optical fiber comprising the same number of multi-core optical fibers as the multi-core optical fiber is interposed between the light incident means and the optical sensor. After the connection, the connection loss of the optical fiber is measured, and the connection loss is confirmed by checking the input alignment order and the output alignment order of the multi-core optical fiber and / or the connection loss of the multi-core optical fiber. 3. The method for inspecting a multi-core optical fiber according to claim 1, wherein the reference value is used for calculating. 4. A multi-core optical fiber according to claim 1, wherein the means for inputting light one at a time in the input alignment forward direction of the multi-core optical fiber turns on the multi-output light source one at a time. Optical fiber inspection method. 5. The means for inputting light one at a time in the input alignment forward direction of the multi-core optical fiber is a means for switching light from a single light source by an optical switch so that light is input one at a time. 2. The method for inspecting a multi-core optical fiber according to claim 1, wherein: 6. The method according to claim 1, wherein the optical sensor is a single optical sensor. 7. The multi-core optical fiber inspection method according to claim 1, wherein said optical sensor is a plurality of optical sensors provided corresponding to each fiber of said multi-core optical fiber. 8. A means for confirming a match between the input alignment order and the output alignment order of the multi-core optical fiber and / or calculating the transmission loss from the measurement results based on the output signal of the optical sensor as an input. 2. The multi-core optical fiber according to claim 1, further comprising: a signal detection unit for performing the operation, a system control unit for sequentially lighting the multi-output light sources one by one or sequentially turning on the optical switch, and an arithmetic processing unit. Inspection methods. 9. The multi-core optical fiber of the multi-core optical fiber confirms that the input alignment order and the output alignment order coincide with each other by monotonically increasing or monotonically decreasing when the transmission loss of each wire number is aligned in the input alignment order. 2. The method for inspecting a multi-core optical fiber according to claim 1, wherein the method is performed to determine whether or not the multi-core optical fiber is present. 10. The multi-core optical fiber according to claim 1, wherein said continuous light attenuating element is made of a light attenuating material and is formed in a shape in which a light transmission distance decreases steplessly. Inspection methods. 11. The method according to claim 1, wherein the steplessly decreasing shape is a shape formed in a triangle or a shape in which a light incident side and a light emission side are formed in a curved surface.
2. The method for inspecting a multi-core optical fiber according to item 0. 12. The continuous light attenuating element is made of a light attenuating material, and is formed in a shape in which the amount of light attenuation changes continuously as the irradiation position changes. The multi-core optical fiber inspection method described in the above. 13. The multi-layered light attenuating element according to claim 1, wherein the continuous light attenuating element comprises a wedge glass body and a light attenuator made of a light attenuating material having the same shape as the glass body. Optical fiber inspection method. 14. A light source, light input means for inputting the light from the light source one by one in the input alignment direction of the multi-core optical fiber, and the output power of each line number of the multi-core optical fiber First measuring means for measuring with a sensor, a continuous light attenuating element, and after the measurement by the optical sensor, aligning the output alignment forward direction of the multi-core optical fiber with the direction in which the amount of attenuation of the continuous light attenuating element changes. A continuous light attenuating element insertion means for inserting the continuous light attenuating element, and after the continuous light attenuating element is inserted by the continuous light attenuating element inserting means, light is again input one by one in the input alignment direction of the multi-core optical fiber. Light incident means, second measuring means for measuring the output power of each wire number of the multi-core optical fiber by the optical sensor, and analyzing the measurement values obtained by the first and second measuring means, The multi-core optical fiber Bas confirmation of input sort order and output alignment order of matching and / or the multiple-core and analysis unit that calculates a transmission loss of the optical fiber, the multi-fiber optical fiber inspection apparatus characterized by being composed. 15. The multi-core optical fiber inspection apparatus according to claim 14, wherein said light source is a multi-output light source, and said light incidence means turns on said multi-output light source one by one. . 16. The light source is a single light source, and the light incident means switches light from the single light source by an optical switch and causes light to enter one by one. The multi-core optical fiber inspection device according to claim 14, wherein 17. The method according to claim 14, wherein the optical sensor is a single optical sensor. 18. The multi-core optical fiber inspection device according to claim 14, wherein said optical sensor is a plurality of optical sensors provided corresponding to each fiber of said multi-core optical fiber. 19. A signal detection unit to which an output signal of an optical sensor is input, a system control unit that sequentially turns on the multi-output light sources one by one or sequentially turns on the optical switches, and the signal detection unit. The multi-core optical fiber inspection device according to claim 14, further comprising: an arithmetic processing unit that analyzes the output signal of (1). 20. Third measuring means for providing an optical fiber between the light incident means and the optical sensor in addition to the multi-core optical fiber, and measuring the optical power incident on the fiber with the optical sensor. And confirming that the input alignment order and the output alignment order of the multi-core optical fiber coincide with each other and / or calculating the transmission loss of the multi-core optical fiber from the measured values of the first to third measurement means. The multi-core optical fiber inspection device according to claim 14, wherein: 21. The splice loss of the multi-core optical fiber is defined as the output power PWa (mw) of each fiber core of a multi-core optical fiber having a predetermined length having the same number of fiber cores as the measured fiber core. 15. The multi-core optical fiber according to claim 14, wherein a connection loss is determined from an output power PWb (mw) of each fiber core of the multi-core optical fiber to be measured by a connection loss = 10 log (PWb / PWa). Inspection equipment. 22. Confirmation of matching between the input alignment order and the output alignment order of the multi-core optical fiber of the multi-core optical fiber increases or decreases monotonically when the transmission loss of each wire number is aligned in the input alignment order. 15. The multi-core optical fiber inspection device according to claim 14, wherein the determination is made based on whether or not the inspection is performed. 23. The continuous light attenuating element is made of a light attenuating material, and is formed in a shape in which a light transmission distance is reduced steplessly.
The multi-core optical fiber inspection apparatus according to the above. 24. The shape which is reduced steplessly is a shape formed in a wedge or a shape in which a light incidence side and a light emission side are formed in a curved surface.
The multi-core optical fiber inspection method described in the above. 25. The multi-core light according to claim 14, wherein the continuous light attenuating element is made of a light attenuating material and is formed in a shape in which a light transmission distance decreases stepwise. Fiber inspection equipment. 26. The continuous light attenuating element according to claim 14, wherein a triangular glass body and a light attenuator made of a light attenuating material having the same shape as the glass body are bonded to each other. Multi-core optical fiber inspection equipment. 27. A light-attenuating element comprising a light-attenuating material, wherein the light-attenuating element is formed in a shape in which a light transmission distance decreases in a light transmission direction. 28. The light attenuating element according to claim 27, wherein the decreasing shape is formed in a wedge shape, or the light incident side and the light emitting side are formed in curved surfaces. 29. The decreasing shape is stepped,
28. The continuous light attenuating element according to claim 27, wherein: 30. A light attenuating element comprising a glass body having a triangular planar shape and a light attenuator made of a light attenuating material having the same shape as the glass body.