JP2002039916A - Method and instrument for measuring polarized crosstalk of optical component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately measure the polarized crosstalks of an optical component, having a plane-of-polarization preserving structure. SOLUTION: Light, comprising only a single polarized components, is made to be incident on a plane-of-polarization preserving optical fiber coupler 1 to be measured, and an extinction ratio of the emitted lights from the plane-of- polarization preserving optical fiber coupler 1 is measured, while the phase difference between two polarization components emitted from the plane-of- polarization preserving optical fiber coupler 1 is changed, at least from a value smaller than (2n+1)×(π/2)} to a value larger than (2n+1)×(π/2)} (n: an integer of than 0 larger). The minimum value of this extinction ratio is set to the absolute value of the polarized crosstalk.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for accurately measuring polarization crosstalk of an optical component having a polarization plane preserving structure.

[0002]

2. Description of the Related Art For example, a single mode optical fiber is designed so that only one mode propagates, but in fact, it is transmitted as two independent polarization components. The polarization-maintaining optical fiber is provided with a polarization direction depending on the direction of the polarization in order to eliminate one of the polarization components in the propagation mode or to prevent interference between the used polarization and the unused polarization. The refractive indices are designed to have different birefringence. An optical component having a polarization-maintaining structure, such as a polarization-maintaining optical fiber, is configured such that when only one of the two polarizations is incident on the optical component, the output power also includes only the same polarization component as the incident. Is ideal, but in practice, some power of one polarization component is coupled to the other polarization component, and two polarization components are simultaneously propagated. As described above, the ratio of the power leaking to the other polarization component while passing through the optical component to the power transmitted as the incident polarization component is defined as polarization crosstalk (crosstalk).

[0003] A conventional method for measuring this polarization crosstalk is to use, for example, a polarization-conserving optical system that transmits only one polarization component of two orthogonal polarizations that can be transmitted through a polarization-maintaining optical fiber. This method measures the extinction ratio of light emitted from the polarization-maintaining optical fiber when the light enters the optical fiber. The extinction ratio is the value of the difference between the maximum power and the minimum power when the polarization plate is rotated while measuring the power after the light emitted from the polarization-maintaining optical fiber passes through the polarization plate. The value of the extinction ratio obtained by this measurement method corresponds to the absolute value of the polarization crosstalk, and the polarization crosstalk is obtained as a value obtained by adding a minus sign (-) to the value of the extinction ratio.

[0004]

By the way, for example, when two polarization components propagate simultaneously in a polarization-maintaining optical fiber, the polarization-maintaining optical fiber has birefringence.
The propagation speeds of the two polarization components are different. For this reason,
As light propagates in the length direction of the optical fiber, the phase difference between the two polarization components changes, and as shown in FIG. 4, the polarization direction changes along the length direction of the optical fiber (the Z direction in the figure). The state changes from elliptically polarized light to linearly polarized light to elliptically polarized light. Therefore, the polarization state at the output end face of the polarization-maintaining optical fiber depends on the phase difference between the two polarization components,
Circularly polarized light or linearly polarized light.

In the above-mentioned conventional measuring method, the measured value of the extinction ratio differs depending on whether the polarization state at the output end face of the polarization-maintaining optical fiber is linearly polarized light or elliptically polarized light. There is a problem that the polarization crosstalk of the storage optical fiber cannot be measured accurately. Specifically, when the polarization state at the output end face of the polarization-maintaining single-mode fiber is linearly polarized light, the difference (= extinction ratio) between the maximum value and the minimum value of the optical power measured while rotating the polarizing plate is equal to the polarization. It becomes larger than the value corresponding to the polarization crosstalk of the wavefront preserving optical fiber. The phase difference between two polarization components propagating through the polarization-maintaining optical fiber is 2
Since the polarization state depends on the optical path length difference between the two polarization components, the polarization state at the output end face is affected by the bending or temperature change of the optical fiber, which makes the measured value of the extinction ratio very unstable and causes polarization crosstalk. There was also a problem that it was not possible to measure with high accuracy.

The present invention has been made in view of the above circumstances, and has as its object to accurately measure polarization crosstalk of an optical component having a polarization maintaining structure such as a polarization maintaining optical fiber. And

[0007]

According to the present invention, there is provided a method for measuring the polarization crosstalk of an optical component, comprising the steps of: , A light consisting of only one polarization component is incident on the optical component, and the phase difference between the two polarization components emitted from the optical component is at least ｛(2n + 1) × (π /
2) From the following values, {(2n + 1) × (π / 2)} (n
Is any integer greater than or equal to 0) or more, and the minimum value of the extinction ratio when the extinction ratio of the light emitted from the optical component is measured is defined as the absolute value of the polarization crosstalk. Features. It is preferable that a phase difference between two polarized waves emitted from the optical component is changed by applying an external force and / or a temperature change to the optical component. In addition, the polarization crosstalk measuring apparatus for an optical component according to the present invention comprises: means for causing light consisting of only one polarization component to enter the optical component having a polarization plane preserving structure; and an extinction ratio of light emitted from the optical component. , And means for changing the phase difference between two polarized waves propagating in the optical component. Preferably, the means for changing the phase difference includes means for applying a tension to the optical component, means for bending the optical component, and / or means for applying a temperature change to the optical component. .

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
Hereinafter, an embodiment of the present invention will be described using an optical fiber coupler including a polarization maintaining optical fiber as an example of an optical component having a polarization maintaining structure. The present inventors have studied in detail two polarization components propagating through the polarization-maintaining optical fiber, and as a result, the phase difference between the two polarization components is (2n + 1) times π / 2 (2n + 1) at the output end. Integer greater than or equal to)
When is, that the polarization state at the output end is elliptically polarized light having the smallest oblateness, that an accurate polarization crosstalk value can be obtained by using the value of the extinction ratio at the output end, and The present inventors have found that the value of the extinction ratio at this time is the minimum value among the values of the extinction ratio that can be taken by the polarization-maintaining single-mode optical fiber.

FIG. 1 is a schematic diagram showing an example of an apparatus suitable for measuring the polarization crosstalk of an optical fiber coupler in the present embodiment. In the figure, reference numeral 1 denotes an optical fiber coupler composed of a polarization maintaining optical fiber, 1a, 1b, 1c,
1d is a lead fiber, 2 is a means for entering light, and 3 is a means for measuring an extinction ratio. In order to measure the polarization crosstalk of the polarization-maintaining optical fiber coupler 1 by the method of the present invention, first, light having only one polarization component is incident on the polarization-maintaining optical fiber coupler 1 to be measured.
As this incident light, light composed of only linearly polarized light whose electric field direction is the same as one of the two polarization axes of the optical component to be measured is used. In this embodiment, the direction of the electric field of the incident light is matched with the polarization axis of one lead fiber 1a on the incident side of the polarization-maintaining optical fiber coupler 1. If the degree of polarization of the incident light is smaller than or equal to the value of the polarization crosstalk of the polarization-maintaining optical fiber coupler 1 to be measured, the polarization crosstalk cannot be accurately measured. It is necessary to use light having a polarization degree sufficiently higher than the polarization crosstalk value of the polarization-maintaining optical fiber coupler 1. More specifically, since the specification required for the polarization crosstalk of the optical component is generally about −20 dB, it is approximately 30 dB.
It is desirable that the extinction ratio be above.

As means 2 for making such incident light incident on the polarization-maintaining optical fiber coupler 1 to be measured, there are provided a light source 2a and a polarizer 2b for converting the light from the light source into light consisting only of linearly polarized components. The device may be used. As the light source 2a, a polarization maintaining optical fiber output type LD (laser diode) or SLD (super luminescent diode) is suitable. As the polarizer 2b, an in-line type such as a coil type fiber polarizer is suitable. The degree of polarization of the incident light is defined by the degree of polarization of the light source and the extinction ratio of the polarizer. Further, the direction of the electric field of the incident light can be controlled by matching the polarization axes at the output end of the polarizer 2b and the polarization maintaining optical fiber connection portion of the incident end of the component 2 to be measured.

When light consisting of only one polarization component is incident on the polarization-maintaining optical fiber coupler 1 to be measured as described above, the incident light is propagated through the polarization-maintaining optical fiber coupler 1. Since some power of the polarized light component of the light is coupled to the other polarized light component and two polarized light components are emitted from the emission end, the extinction ratio of the emitted light is measured. Specifically, the emission end of one lead fiber 1c on the emission side of the polarization-maintaining optical fiber coupler 1 may be connected to the extinction ratio monitor device 3. The extinction ratio monitor device 3 is configured to measure the optical power while rotating the polarizing plate so that the emitted light passes through the polarizing plate, and obtain the extinction ratio.

At the time of measuring the extinction ratio, the phase difference between the two polarization components emitted from the polarization-maintaining single-mode fiber coupler 1 is changed from a value of {(2n + 1) × (π / 2)} or less to {(2
(n + 1) × (π / 2)｝ (n is any integer of 0 or more). For example, the phase difference between the two polarization components may be changed from a value of π / 2 or less to a value of π / 2 or more, or from a value of (3π) / 2 or less to a value of (3π) / 2 or more. You may. Alternatively, it may be changed in a wider range. Specifically, it is possible to change the phase difference between two polarization components emitted from the polarization-maintaining optical fiber coupler 1 by applying an external force and / or a temperature change to a part of the polarization-maintaining optical fiber coupler 1. it can.
In FIG. 1, reference numeral 4 denotes a means for changing the phase difference between two polarized waves, and specifically means for applying an external force and / or a temperature change. The portion to which the external force and / or temperature change is applied is preferably near the emission end of the polarization-maintaining optical fiber coupler 1. In the present embodiment, it is preferable to apply an external force and / or a temperature change to the lead fiber 1c on the emission side. The means for applying a temperature change is a means capable of applying heat fluctuation to the optical component, and includes a heating means such as a heater configured to be able to accommodate a part of the lead fiber 1c on the emission side of the polarization-maintaining optical fiber coupler 1. It can be suitably used. As means for applying an external force, for example, the lead fiber 1c on the output side of the polarization-maintaining optical fiber coupler 1 is used.
A means for applying a tension in the length direction and changing the magnitude of the tension continuously and periodically can be used. Specifically, a piezo element can be suitably used. Alternatively, as means for applying an external force, means for bending the lead fiber 1c on the emission side of the polarization-maintaining optical fiber coupler 1 and changing the bending direction continuously and periodically can be used. For example, FIG.
As shown in (1), a device in which the lead fiber 1c is wound around the reel 10 and the reel 10 is periodically moved like a fan can be suitably used.

When the extinction ratio of the emitted light is measured while changing the phase difference between the two polarized components of the emitted light emitted from the polarization-maintaining optical fiber coupler 1 as shown in FIG. 3, for example, as shown in FIG. In addition, the value of the extinction ratio changes periodically.
In the graph of FIG. 3, the horizontal axis represents the heating time (unit: second) of the polarization-maintaining optical fiber, and the vertical axis represents the extinction ratio (unit: dB). As described above, the extinction ratio takes the minimum value when the phase difference between the two polarization components is {(2n + 1) × (π / 2)} (n is an integer of 0 or more) at the emission end. Therefore, in the example of FIG. 3, the extinction ratio is minimum when the heating time is 15 seconds and when the heating time is 45 seconds.
At 5 seconds, the phase difference between the two polarization components is π / 2, and when the heating time is 45 seconds, the phase difference between the two polarization components is (3).
π) / 2, or vice versa. As described above, when the phase difference between the two polarization components is {(2n + 1) × (π / 2)} (n is an integer of 0 or more) at the output end, the polarization state at the output end is In the example of FIG. 3, the heating time is 15 seconds or 45 seconds, since an accurate polarization crosstalk value can be obtained by using the value of the extinction ratio at the emission end at this time. The extinction ratio at that time, that is, the minimum value of the extinction ratio is adopted as the absolute value of the polarization crosstalk. In the example of FIG. 3, since the minimum value of the extinction ratio is 23 dB, the value of the polarization crosstalk is obtained as −23 dB.

As described above, according to the present embodiment, the polarization crosstalk is obtained by measuring the extinction ratio while forcibly changing the polarization state of the light emitted from the polarization-maintaining optical fiber, thereby obtaining accurate polarization crosstalk. The value of wave crosstalk can be obtained stably. In the above embodiment, the polarization-maintaining optical fiber coupler has been described as an example, but the method of the present invention can be similarly applied to optical components having other polarization-maintaining structures. Specific examples of the optical component to be measured in the present invention include a polarization beam splitter and a polarizer (polarizer).

[0015]

EXAMPLES Hereinafter, the effects of the present invention will be clarified by showing specific examples. (Example 1) A polarization-maintaining optical fiber coupler using two PANDA fibers was prepared as an optical component to be measured. This optical fiber coupler has two lead fibers on each of the input side and the output side. First, linearly polarized light having a degree of polarization of about 40 dB is adjusted into one of the lead fibers on the incident side of the optical fiber coupler so that the polarization axis direction of the lead fiber and the electric field direction of the incident light coincide with each other. did. The emission end of one lead fiber on the emission side of the optical fiber coupler was connected to an extinction ratio monitor. In this state, a part of one of the lead fibers on the emission side of the optical fiber coupler was housed in a heater over a length of about 6 cm, and was heated for one minute. At this time, the temperature in the heater before heating was about 20 ° C., and the temperature in the heater after heating was about 100 ° C. FIG. 3 shows a fluctuation state of the measured value of the extinction ratio from the start of heating to the end of heating. In the graph of FIG. 3, the cycle in which the measured value of the extinction ratio decreases → increases twice as the heating time increases. The extinction ratio is such that the phase difference between the two polarization components is (2
Since it becomes the minimum when n + 1) × π / 2 and becomes the maximum when nπ, in this embodiment, it can be estimated that a phase difference of about 2π has been generated by heating for 1 minute. Therefore, the minimum value of the extinction ratio when the phase difference between the two polarization components is changed from 0 to 2π is 23 dB according to the graph of FIG. 3, which indicates that the polarization-maintaining optical fiber coupler which is the optical component to be measured. Is obtained as −23 dB.

(Embodiment 2) Linear polarization having a degree of polarization of about 40 dB is applied to one lead fiber on the incident side of the same optical fiber coupler as used in Embodiment 1 above, with respect to the polarization axis direction of the lead fiber. The light was incident so as to be adjusted so that the direction of the electric field of the incident light coincided therewith. The emission end of one lead fiber on the emission side of the optical fiber coupler was connected to an extinction ratio monitor. In this state, a part of one of the lead fibers on the emission side of the optical fiber coupler was wound around a piezo element, and the piezo element was vibrated at a cycle of 1 Hz to apply a periodic tension to the lead fiber. At this time, the measured value of the extinction ratio fluctuated periodically at a period of about 4 kHz, and the minimum value was 23 dB. In this embodiment, it can be estimated that a phase difference of about 4π occurs in one cycle of the vibration of the piezo element.
Accordingly, when the phase difference between the two polarization components is changed from 0 to 4π, the minimum value of the extinction ratio is 23 dB, which indicates that the polarization crosstalk of the polarization-maintaining optical fiber coupler, which is the optical component to be measured, is smaller. The value is determined to be -23 dB.

(Embodiment 3) Linear polarization having a degree of polarization of about 40 dB is applied to one lead fiber on the incident side of the same optical fiber coupler as used in Embodiment 1 above, with respect to the polarization axis direction of the lead fiber. The light was incident so as to be adjusted so that the direction of the electric field of the incident light coincided therewith. The emission end of one lead fiber on the emission side of the optical fiber coupler was connected to an extinction ratio monitor. In this state, one of the output sides of the optical fiber coupler
Some of the re Dofaiba, about the diameter as shown in FIG. 2 5c
m was wound around a circular reel, and this reel was periodically driven like a fan around a lead fiber to apply a periodic bending stress to the lead fiber.
At this time, the measured value of the extinction ratio periodically fluctuated with the rotation of the reel, and the minimum value was 23 dB. From this, the value of the polarization crosstalk of the polarization-maintaining optical fiber coupler, which is the optical component to be measured, is determined to be −23 dB.

[0018]

As described above, according to the present invention,
It is possible to accurately measure polarization crosstalk of an optical component having a polarization-maintaining structure.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an example of a polarization crosstalk measuring device of the present invention.

FIG. 2 is an explanatory diagram showing an example of a means for applying an external force to an optical component according to the present invention.

FIG. 3 is a graph showing a measurement result of an extinction ratio in an example according to the present invention.

FIG. 4 is an explanatory diagram schematically showing a polarization state in a polarization-maintaining optical fiber.

[Explanation of symbols]

Reference numeral 1 denotes a polarization-maintaining optical fiber coupler (optical component having a polarization-maintaining structure); 2 means for inputting light; 3 means for measuring an extinction ratio; 4 means for changing the phase difference of polarization.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference)

Claims (4)

[Claims] 1. A method for measuring polarization crosstalk of an optical component having a polarization-maintaining structure, comprising: causing light having only one polarization component to be incident on the optical component and being emitted from the optical component. The phase difference between the two polarization components is changed from a value of at least {(2n + 1) × (π / 2)} to a value of {(2n + 1) × (π / 2)} (n is any integer of 0 or more) or more. The polarization crosstalk measurement of an optical component, wherein the minimum value of the extinction ratio when the extinction ratio of light emitted from the optical component is measured while changing the extinction ratio to the absolute value of the polarization crosstalk is measured. Method. 2. A phase difference between two polarized waves emitted from the optical component by applying an external force and / or a temperature change to the optical component.
A method for measuring polarization crosstalk of an optical component as described in the above. 3. An optical component having a polarization plane preserving structure, means for making light consisting of only one polarization component incident thereon, means for measuring an extinction ratio of light emitted from the optical component, and Means for changing a phase difference between two propagating polarized waves, the polarization crosstalk measuring apparatus for an optical component. 4. The means for changing the phase difference includes means for applying tension to the optical component, means for bending the optical component, and / or means for applying a temperature change to the optical component. 4. The apparatus for measuring polarization crosstalk of an optical component according to claim 3, wherein: