JPH0566178A - Method for evaluating optical fiber coupler - Google Patents

Abstract

PURPOSE:To enhance evaluation efficiency by forming an optical fiber coupler by means of fusion and drawing of plural long-size polarization retaining optical fibers through heating, and allowing linearly polarized light coinciding with a polarization axis to be incident on the coupler from one end of each fiber to obtain a predetermined branching ratio, and then measuring the crosstalk of light emitted from the coupler. CONSTITUTION:A plurality of long-size polarization retaining optical fibers 5, 5' are sequentially unwound and are fused together through heating and drawn to form an optical fiber coupler 6 and linearly polarized light coinciding with the poralization axes of the fibers is incident on the coupler from one end of each long-size optical fiber 5 via a light source 1 and a polarizer 4 or the like and, after a predetermined branching ratio is obtained, the crosstalk of light emitted from the coupler is measured by a polarizing beam splitter 7 and power meters 8,8' or the like. The necessity of setting fibers on the incident side and adjusting an optical system is thus eliminated and enhancement of evaluation efficiency is achieved. When a poralization retaining optical fiber couper 6 is used, the work of adjusting a stage and making polarization axes coincide with each other is unecessary and the effect of enhancing the work efficiency is remarkable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating an optical fiber coupler formed by heating a part of a plurality of polarization-maintaining optical fibers and fusing and extending the parts.

[0002]

2. Description of the Related Art An optical fiber coupler is a device that splits and couples light between a plurality of optical fibers, and is manufactured by heating a part of the plurality of optical fibers to fuse and stretch them. The characteristics of the optical fiber coupler, such as branching ratio and excess loss, may change depending on the polarization state of the incident light, so that the polarization characteristics must be evaluated. In a coupler using a polarization-maintaining optical fiber, crosstalk indicating how much the polarization state of light incident on the coupler is preserved is an important characteristic parameter. In order to evaluate the polarization characteristics of such a coupler, conventionally, measurement was performed using a measurement system as shown in FIG. 2 for each completed coupler.

[0003]

In order to evaluate polarization characteristics, it is necessary to collect light whose polarization state is adjusted by a polarizer, a wave plate, etc., by a lens and make it enter a fiber at one end of the coupler. .. In order to make the light incident on the coupler efficiently, it is necessary to focus the light on the core of the fiber. Therefore, it is usually necessary to place the fiber on the fine movement stage and monitor the output light power, The stage is adjusted so that Conventionally, it was necessary to reset the fibers at both ends of the coupler in the measurement system and adjust the stage each time the coupler was manufactured or evaluated. In addition, in the case of a polarization-maintaining optical fiber coupler, in addition to adjusting the stage, it is necessary to match the incident light and the direction of the polarization main axis peculiar to the fiber, and there is a problem that these adjustments take a lot of time. It was

[0004]

In order to solve the above-mentioned problems, according to the present invention, as shown in FIG. 1, a plurality of long polarization-maintaining optical fibers 5 and 5'are successively drawn out, heated, fused and drawn. To form the optical fiber coupler 6, and the linearly polarized light, which coincides with the polarization main axis of the fiber, is incident from one end of the long optical fiber 5 through the light source 1, the polarizer 4, etc. After that, the crosstalk of the light emitted from the coupler is continuously measured by the polarization beam splitter 7, the power meters 8 and 8 ', and the like.

A second invention is that the measured value of the crosstalk can be corrected and evaluated by the value of the crosstalk of the long optical fiber measured in advance before the manufacture of the coupler. In addition,
FIG. 1 shows an example of a measurement system applied to the present invention, and the branch side of the coupler can measure the optical power for each polarized wave as well as the main line side to obtain crosstalk.

[0006]

In the conventional polarization characteristic evaluation, it is necessary to adjust the optical system by setting the coupler on every time the coupler is manufactured and evaluated, and the output side fiber is set to the measuring system again, which causes a decrease in working efficiency. .. On the other hand, in the method according to the present invention, one end is already set in the measurement system, and the coupler is manufactured and evaluated while sequentially feeding out the optical system-adjusted fibers. Therefore, it is necessary to adjust the fiber set on the incident side and the optical system. There is no Therefore, the evaluation efficiency is improved as compared with the conventional case.

Further, in the case of the polarization maintaining optical fiber coupler, not only the adjustment of the incident side fine movement stage but also the work of matching the polarization main axes of the polarizer and the fiber becomes unnecessary.
The effect of improving work efficiency is remarkable. Depending on the measurement conditions, the change in the polarization state in the bobbin-wound fiber cannot be ignored, and this may cause a measurement error.However, this problem is caused by the output light from the bobbin-wound fiber before the coupler is manufactured. This can be solved by measuring the polarization state and correcting the measurement result during or after the production of the coupler based on this result.

[0008]

EXAMPLE A polarization-maintaining optical fiber coupler was actually manufactured and evaluated in the measurement system shown in FIG. An LD light source having a wavelength of 0.85 μm that emits linearly polarized light is used as the light source 1, and after being made into parallel light by a lens, λ / 4 for 0.85 μm
The plate 3 converts the light into circularly polarized light, and finally, the linearly polarized light that coincides with the polarization main axis of the long polarization-maintaining optical fiber 5 wound by the polarizer on the main line side is condensed by the lens 2 and is incident on the fiber 5. ing. At the emission end of the main-line-side bobbin-wound optical fiber 5, the light emitted from the fiber 5 is collimated by the lens 2 and separated by the polarization beam splitter 6 into an incident polarization component and a component orthogonal thereto, and the respective optical powers are separated. Crosstalk is obtained by measuring with optical power meters 8 and 8 '. Further, when the branching ratio and the excess loss are evaluated, the optical powers emitted from the main line side and the branch line side are measured by the optical power meters 8, 8 ′ and 8 ″.
At this time, as for the outgoing light power on the main line side, the optical powers of the incident polarization and the orthogonal polarization are added, and the loss due to the optical system such as the lens and the polarization beam splitter is corrected.

Fiber length 100 m, crosstalk-39
A polarization maintaining optical fiber for 0.85 μm of dB is used as the bobbin-wound fibers 5, 5 ', and a polarization maintaining optical fiber coupler with 50% branching is manufactured by measuring the crosstalk by the method according to the present invention. The crosstalk was measured again by the conventional method to confirm whether the two agree with each other, and also to confirm the effect of efficiency improvement by the present invention. The relationship between the main line crosstalk measurement results according to the present invention and the conventional method is summarized in FIG. As shown in FIG. 3, it was confirmed that the two agree extremely well. While the measurement time was 30 minutes in the conventional method, it was 12 minutes in the method of the present invention, and the effect of improving the measurement efficiency was confirmed.

Next, a bobbin-wound fiber is used, and the fiber length is 3
A coupler similar to the above was manufactured and evaluated in place of the polarization maintaining optical fiber for 000 m and crosstalk-23 dB for 0.85 μm. The relationship of the crosstalk measurement results on the main line side according to the present invention and the conventional method is summarized in FIG. 4. However, due to the influence of the crosstalk of the bobbin-wound fiber itself, the measured value by the method of the present invention is about −20 dB from the conventional method. The gap has become large. Therefore, based on the fact that the crosstalk of the bobbin-wound fiber itself is -23 dB, the coupler crosstalk measurement value according to the present invention was corrected, and the comparison result of this correction value and the conventional method is shown in FIG.
The correction results and the values of the conventional method are in good agreement, and even if there is polarization state variation such as crosstalk deterioration in the bobbin wound fiber, the polarization state of the light emitted from the bobbin wound fiber should be measured in advance. Then, it was confirmed that there would be no problem if correction was made based on this result.

A method of correcting the above-mentioned measured value by the crosstalk of the bobbin wound fiber will be described below. When the X-polarized light is incident on the bobbin-wound fiber and the optical powers of the X-polarized light component and the Y-polarized light component of the bobbin-emitted light are respectively P _XO and P _YO , the crosstalk CT _{O of} only the bobbin-wound fiber is given by ( It is represented by 1). CT _O = 10 log (P _YO / P _XO ) …… (1)

Meanwhile, the optical power of the X polarization and Y polarization of the emitted light after the coupler manufacturing P _'X, P' by placing a _{Y P 'X = S X (} P XO -γ'P XO) ...... (2) P ′ _Y = S _Y P _YO + S _X γ′P _XO (3) Here, S _X and S _Y are branching ratios for the X and Y polarized waves, and γ ′ is a ratio of power conversion from the X polarized wave to the Y polarized wave in the coupler unit, that is, crosstalk of the coupler itself. The power conversion from Y polarization to X polarization is negligible because it is very small.

Now, in the case of P _XO >> γ′P _XO , S _X ≈S _Y , crosstalk CT ′ after the production of the coupler is calculated. From equations (2) and (3), CT ′ = 10log (P ′ _Y / P ′ _X ) ≈10 log (P _YO / P _XO + γ ′) (dB) (4) When ^{γ'is calculated} using equations (1) and (4), γ '= 10 ^{CT' / 10} -10 ^{CTo / 10} (5). Therefore, the crosstalk CT of the coupler itself is given by CT = 10logγ '(dB) (6).

For example, the crosstalk of the bobbin wound fiber is -23 dB, and the crosstalk after the coupler is manufactured is -20 d.
In the case of B, the corrected crosstalk is γ ′ = 10 ^−20/10 −10 ^−23/10 = 0.00499 CT = 10 ^log 0.00499 = −23 (dB).

[0015]

As described above, according to the present invention, it is possible to improve the efficiency of the evaluation of the optical fiber coupler because it is not necessary to adjust the incident stage, the polarizer and the like when evaluating the optical fiber coupler.

[Brief description of drawings]

FIG. 1 is a diagram showing a crosstalk measurement system of a polarization maintaining optical fiber coupler that is a specific example of the present invention.

FIG. 2 is a diagram showing a conventional polarization measurement system for a coupler.

FIG. 3 is a comparison diagram of the results of the present invention and the conventional method.

4 is the same as FIG.

FIG. 5 is the same as FIG.

[Explanation of symbols]

 1: Light source 2: Lens 3: 1/4 wavelength plate 4: Polarizer 5, 5 ': Bobbin wound long fiber 6, 6': Coupler 7: Polarization beam splitter 8, 8 ': Optical power meter 9: Analyzer

Claims (2)

[Claims] 1. A plurality of long polarization-maintaining optical fibers are successively drawn out, heated, fused and stretched to form an optical fiber coupler, and a straight line from one end of the long optical fiber coincides with the polarization main axis of the fiber. A method for evaluating an optical fiber coupler, which comprises inputting polarized light, obtaining a predetermined branching ratio, and subsequently measuring the crosstalk of the coupler output light. 2. The evaluation method of an optical fiber coupler according to claim 1, wherein the measured value of the crosstalk is corrected by the value of the crosstalk of the long optical fiber measured in advance before manufacturing the coupler.