JP3058896B2 - Polarization crosstalk measurement system for polarization-maintaining optical fiber - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a polarization crosstalk measuring device for measuring a polarization crosstalk of a polarization-maintaining optical fiber (hereinafter simply referred to as an optical fiber). Things.

[Prior Art] Next, the configuration of a conventional polarization crosstalk measuring device will be described with reference to FIG. 4 is a laser diode (LD), 2 is a spherical lens, 3 is a polarizer, 4 is a λ / 4 plate,
5 is a polarizer, 6 and 8 are fiber adapters with collimators (hereinafter simply referred to as adapters), 7 is an optical fiber to be measured, 9 is an analyzer, 10 is a light receiving element, 11 is an optical power meter, and 12 and 13 is a servo motor.

The light emitted from the LD 1 is made into a parallel beam by the spherical lens 2 and becomes a linearly polarized light by the polarizer 3. The axis of the λ / 4 plate 4 is inclined by 45 ° with respect to the axis of the polarizer 3, and the light passing through the λ / 4 plate 4 becomes circularly polarized light. The polarizer 5 is rotated by a servomotor 12.
Light that has passed through the polarizer 5 becomes linearly polarized light. Since the incident light on the polarizer 5 is circularly polarized light, the level of linearly polarized light passing through the polarizer 5 is constant regardless of the rotation angle of the polarizer 5. The linearly polarized light that has passed through the polarizer 5 passes through the optical fiber 7 from the adapter 6, forms a parallel beam again at the adapter 8, and enters the analyzer 9. The analyzer 9 is rotated by a servomotor 13, a linearly polarized light component at an arbitrary angle is extracted from the analyzer 9, and the level is measured by a light receiving element 10 and an optical power meter 11.

 Next, the operation of FIG. 4 will be described.

(A) The polarizer 5 is set at an arbitrary angle, and is incident on the optical fiber 7.

As shown by the line a in FIG. 5, the polarization state at the incident surface of the optical fiber 7 is linearly polarized light having an arbitrary angle.

(A) Rotate the analyzer 9 to minimize the value of the optical power meter 11. When it is difficult to find the minimum point, set the polarizer 5 to 20-
Turn the analyzer 30 ° to adjust the analyzer 9.

As shown in FIG. 6B, the polarization state of the exit surface of the optical fiber 7 is generally elliptically polarized light because an arbitrary phase difference is given in the optical fiber 7. The value of the optical power meter 11 becomes minimum when the analyzer 9 coincides with the minor axis c of the elliptically polarized light.

(C) Rotating the polarizer 5 to minimize the value of the optical power meter 11.

By rotating the polarizer 5 as shown in FIG. 7, the elliptically polarized light b becomes elliptically polarized light d having a higher ellipticity, and the value of the optical power meter 11 becomes smaller. When the value of the optical power meter 11 increases, the polarizer 5 may be turned in the opposite direction.

(D) Rotate the analyzer 9 to minimize the value of the optical power meter 11.

By adjusting the analyzer 9 to the minor axis e of the elliptically polarized light d as shown in FIG. 8, the value of the optical power meter 11 is minimized.

(E) Adjust both the polarizer 5 and the analyzer 9 to search for the minimum value of the power.

At this time, the polarization state at the emission surface is linearly polarized light f as shown in FIG. 9, and the position of the analyzer 9 is at a position g perpendicular to the linearly polarized light f. In addition, the axis of the optical fiber 7 and the linearly polarized light on the entrance surface and the exit surface coincide with each other.

(F) a power -P ₁ was measured in the state of (e), then the analyzer 9
After a rotated 90 °, to measure the power -P ₂ again. At this time
10log (P ₁ / P ₂₎ is polarized Kurosuto - a click.

[Problems to be Solved by the Invention] To measure the polarization crosstalk, an optical fiber 7 is required.
Must be aligned with the main axis of the optical fiber 7. However, in the prior art shown in FIG. 4, the polarizer 5 on the input side and the analyzer 9 on the output side are alternately adjusted so that the power is minimized. And it takes time to measure.

The present invention relates to an optical fiber polarization crosstalk measuring device, wherein the angle between the polarizer 5 and the axis of the optical fiber 7 is α.
It is an object of the present invention to find the main axis of the optical fiber 7 by two measurements of α + 45 ° so that the polarization crosstalk can be measured in a short time.

[Means for Solving the Problems] In order to achieve this object, a polarizer that causes linearly polarized light at a predetermined angle to enter a polarization-maintaining optical fiber, and a light emitted from the polarization-maintaining optical fiber in a predetermined direction An analyzer that allows only polarized light to pass therethrough, a conversion unit that converts the light that has passed through the analyzer into an electric signal, a first changing unit that changes the polarization direction of the polarizer, and a polarization direction of the analyzer. And the converting means at three points where the angle of the analyzer forms an angle of 45 degrees with each other by controlling the second changing means when the polarizer is at the predetermined angle. Measure the electrical signal output from the
The first changing means is controlled to control the second changing means in a state where the polarizer forms an angle of 45 degrees with respect to the predetermined angle. Control arithmetic means for measuring a signal, calculating the measurement result, and calculating the direction of the main axis of the polarization-maintaining optical fiber by calculation.

Next, the configuration of the polarization crosstalk measuring device according to the present invention will be described with reference to FIG.

In FIG. 1, 12 is a servo motor (first changing means), 13 is a servo motor (second changing means), 14 and 15 are motor interfaces, 16 is a CPU (control calculating means), and It is the same as the figure.

[Operation] The light emitted from the LD 1 in FIG.
The light enters the optical fiber 7 from the four plates 4 and the polarizer 5 through the adapter 6. The light exiting the optical fiber 7 is parallel-beamed by the adapter 8.
To be The polarization state at this time is generally elliptically polarized light due to the phase difference of the optical fiber 7. Up to this point, it is the same as FIG.

The output light of the adapter 8 is taken out by the analyzer 9 as a linearly polarized light component at an arbitrary angle, and the light level is measured by the light receiving element 10 and the optical power meter 11 (the light receiving element 10 and the power meter 11 form conversion means). You.

In FIG. 1, the angle of the analyzer 9 is measured at three points of 0 °, + 45 °, and + 90 ° based on an arbitrary angle α (predetermined angle) of the polarizer 5 and stored in a memory in the CPU 16. Next, the polarizer 5
Is rotated 45 degrees from the angle α in an arbitrary direction, and the angle of the analyzer 9 is measured at three points of 0 °, 45 °, and 90 °. From these measured values, the input axis and the output axis are calculated.

 Next, the calculation method will be described.

The polarizer 5 is set at an arbitrary angle (predetermined angle), and linearly polarized light enters the optical fiber 7. As shown in FIG. 2, xy coordinates are set on the axis of the optical fiber 7, the components of the linearly polarized light γ on the xy axis are Ax and Ay, respectively, and the angle between the linearly polarized light γ and the x axis is α.
And

Assuming that the relative phase difference in the optical fiber 7 is δ, the polarization state emitted from the optical fiber 7 is generally elliptically polarized light.
As shown in FIG. 3, the major axis of the elliptically polarized light is a and the minor axis is b,
Let x be the angle between the major axis a and the x axis. Also, the initial optical axis is X
To the corner of x-axis and the X and x _0, the angle between the x axis and the major axis a to theta. Therefore, the θ = x + x _0.

The power P (θ) when passing through the analyzer 9 rotated by θ with respect to the initial optical axis X is represented by the following equation.

P (θ) = A + Bcos2θ + Csin2θ (1) If θ is measured at 0 °, 45 °, and 90 °, A, B, and C can be obtained.

A = {P (0 °) + P (90 °)} / 2 (2a) B = {P (0 °) −P (90 °)} / 2 (2b) C = {2P (45 °) ) becomes -P (0 °) -P (90 °)} / 2 ...... (2c) formula (1) θ = x + x _0, so the following manner.

P (θ) = P (x + x 0) = A + B 0 cos2 x + C 0 sin2 x ...... (3) B 0 = Bcos2 x 0 + Csin2 x 0 ...... (4a) C 0 = -Bsin2 x 0 + Ccos2 x 0 ...... ( 4b) From equation (4), the following equation holds.

B ² + C ² = B ₀ ² + C ₀ ² (5) On the other hand, the following relationship is established between Ax, Ay, a, b, and δ.

a ² + b ² + Ax ² + Ay ² a ² −b ² = (Ax ² −Ay ² ) cos2 x + 2AxAy sin2 x cosδ ± ab ± ab = AxAy sinδ (6) Further, the power P (x ) Becomes as follows.

P (x) = A + B 0 cos2 x + C 0 sin2 x cosδ ...... (7) A = (Ax 2 + Ay 2) / 2 B 0 = (Ax 2 -Ay 2) / 2 C 0 = AxAy cosδ ...... (8) from Figure 2, Ax = γcosα ...... (9a) Ay = γsinα ...... (9b) equation 8, from the equation _{(9), B 0 = (} γ 2/2) cos2α ...... (10a) C 0 = ^{(γ 2/2) sin2αcosδ ......} (10b) equation (5), from equation (10), the following equation holds.

^{A 2 (cos 2 2α + sin} 2 2αcos 2 δ) = B 2 + C 2 ...... (11) A = γ 2/2 , where the B _1, the C B when the angle of the polarizer 5 is alpha C ₁
Assuming that B at α + 45 ° is ₂ and C is C ₂ , when equation (11) is α, cos ² 2α + sin ² 2αcos ² δ = (B ₁ ² + C ₁ ² ) / A ² (12) when alpha + 45 ° ^{sin 2 2α + cos 2 2αcos 2} δ = (B 2 2 + C 2 2) / a 2 ...... (13) (B 1 2 + C 1 2) / a 2 = M 1, (B 2 2 + C 2 2 ) / A ² = M ₂ , relative phase difference δ of optical fiber 7, angle α between polarizer 5 and axis of optical fiber 7, angle x between major axis a and axis of optical fiber 7
Are represented by the following equations.

^{δ = (1/2) cos -1 (} 2M 1 + 2M 2 -3) α = (1/4) cos -1 {(M 2 -M 1) / (M 1 + M 2 -2)} x = (1 / 2) tan ^-1 (tan2αcosδ) As described above, the polarizer 5 is measured at an arbitrary angle.
If the measurement is performed by rotating by 45 °, δ, α, and x can be obtained by calculation. Thereby, the main axis of the optical fiber 7 can be found, and the polarization crosstalk can be measured.

According to the present invention, in the polarization crosstalk measuring device for an optical fiber, the angle between the polarizer and the axis of the optical fiber is α.
And the main axis of the optical fiber can be found by two measurements of α + 45 °, so that the polarization crosstalk can be measured in a short time.

[Brief description of the drawings]

FIG. 1 is a diagram showing the configuration of an optical fiber polarization crosstalk measuring apparatus according to the present invention, FIG. 2 is a diagram showing a component on the XY axis of linearly polarized light 〓, and FIG. FIG. 4 is a block diagram of a conventional polarization crosstalk measuring apparatus, FIG. 5 is a polarization state diagram at an incident surface of an optical fiber 7, and FIG. ,
FIG. 7 is a polarization state diagram of light emitted from the optical fiber 7 when the polarizer 5 is rotated, FIG. 8 is a state diagram when the analyzer 9 is aligned with the short axis e of the elliptically polarized light d, and FIG. When both the polarizer 5 and the analyzer 9 are adjusted to find the minimum value of the power, the exit surface of the polarizer 5 becomes linearly polarized light f, and the position of the analyzer 9 is a position g perpendicular to the linearly polarized light f. FIG. 1 ... LD (laser diode), 2 ... sphere lens, 3 ...
... Polarizer, 4 ... λ / 4 plate, 5 ... Polarizer, 6 ... Adapter (fiber adapter with collimator), 7 ... Optical fiber (Polarization maintaining optical fiber), 8 ... Adapter, 9 …
... Analyzer, 10 ... Light receiving element, 11 ... Optical power meter, 12
・ 13 …… Servo motor, 14 ・ 15 …… Motor interface, 16 …… CPU.

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Claims (1)

(57) [Claims] 1. A polarizer for inputting linearly polarized light having a predetermined angle to a polarization-maintaining optical fiber, and an analyzer for passing only light polarized in a predetermined direction among light emitted from the polarization-maintaining optical fiber. A conversion unit that converts light passing through the analyzer into an electric signal; a first changing unit that changes a polarization direction of the polarizer; a second changing unit that changes a polarization direction of the analyzer; When the polarizer is at the predetermined angle, the second changing means is controlled to measure electric signals output from the conversion means at three points at which the angles of the analyzer form an angle of 45 degrees with each other. Controlling the first changing means to control the second changing means in a state where the polarizer forms an angle of 45 degrees with respect to the predetermined angle, and outputting the three points at the three points from the converting means. Measure the electrical signal and make the measurement Polarization crosstalk measuring apparatus of the polarization maintaining optical fiber, characterized in that by calculating the result and a control arithmetic unit for calculating the operation direction of the main axis of the polarization maintaining optical fiber.