JPH061233B2 - Automatic method for measuring the coupling length of polarization-maintaining optical fibers - Google Patents

Description

Description: BACKGROUND AND OBJECT OF THE INVENTION The present invention relates to a method for automatically measuring a coupling length of a polarization-maintaining optical fiber, and is particularly suitable for automatically measuring from a short coupling length to a long coupling length. The present invention relates to an automatic measuring method.

Various methods such as an optical fiber cutting method, a light scattering method, a method using Faraday rotation, and a pulse method have been proposed for measuring the coupling length of a polarization-maintaining optical fiber. There is a limit to the measurable bond length, and there is no one that can accurately measure a short bond length to a long bond length in a short time with a short sample.

As a conventional method of measuring the coupling length, there is known a wavelength change method in which the coupling length is measured from the wavelength characteristic of the polarization degree of the emitted light by changing the incident wavelength, but the polarization degree of the emitted light is set to a plurality of wavelengths. On the other hand, there is a problem that automation by computer control is very difficult in terms of measurement.

An object of the present invention is to solve the above-mentioned drawbacks of the conventional technology and to automatically measure the coupling length of a polarization-maintaining optical fiber, which can easily and accurately measure short to long coupling lengths with a short sample. To provide a method.

[Summary of the Invention] A feature of the present invention is that white light from a light source is split into monochromatic light of an arbitrary wavelength by a spectroscope, and the light is polarized in a 45 ° direction with respect to an intrinsic polarization axis of a polarization-maintaining optical fiber. Is incident on the polarization-maintaining single-mode fiber as linearly polarized light having a wavelength of 45 ° with respect to the specific polarization axis of the light emitted from the polarization-maintaining single-mode fiber, and the light intensity is measured for a plurality of wavelengths. The point is that the coupling length of the polarization-maintaining optical fiber is automatically measured from the periodic change of the ratio of the light intensity and the ratio of the sum with respect to the wavelength.

[Embodiment] An embodiment of the method of the present invention will be described in detail below with reference to FIG.

FIG. 1 is a schematic configuration diagram showing an example of a measuring system of an automatic coupling length measuring device for explaining an embodiment of an automatic measuring method of the coupling length of a polarization-maintaining optical fiber of the present invention. In FIG. 1, 1 is a white light source, 2 is a spectroscope that disperses light from the white light source 1, 3 is a polarizer that linearly polarizes monochromatic light that has been dispersed by the spectroscope 2, 4 is a lens, and these are polarization planes. It is provided on the incident end side of the storage optical fiber 5. Further, on the exit end side of the polarization-maintaining optical fiber 5, an analyzer 6 that cuts out light having an arbitrary polarization azimuth of the emitted light and a multimode light that guides the light to a Ge-APD (german apalanche photodiode) 8 are provided. A voltmeter 9 (light intensity measuring device) for measuring the voltage of the electric signal output from the fiber 7 and the Ge-APD 8 is provided. Reference numeral 10 is a computer introduced for automating the measurement. The computer 10 stores and calculates the measurement data and sends a signal to the monochromator controller 11 to change the wavelength λ of the spectroscope 2 via the monochromator controller 11. In addition, the results are shown by using the XY plotter 12.

The polarizer 3 is adjusted so that the vibration direction of the linearly polarized light transmitted through the polarizer 3 is incident at an angle of 45 degrees with respect to the intrinsic polarization axis of the polarization-maintaining optical fiber 5. The linearly polarized light incident on is separated into two intrinsic polarization modes of the polarization-maintaining optical fiber 5 and propagates while changing its polarization state. The outgoing light emitted from the polarization-maintaining optical fiber 5 passes through the analyzer 6 set in the azimuth of 45 degrees with respect to the intrinsic polarization axis, and the transmitted light intensity I
₁ (λ) is input to the computer 10 via the multimode optical fiber 7 and the Ge-APD 8. Then, this operation is repeated for each wavelength. Next, the analyzer 6 is rotated by 90 degrees, and the light intensity I ₂ (λ) at each wavelength is obtained as in the above operation.
To measure.

Here, the principle of the present invention will be described.

In general, the propagation constant β of an optical fiber is approximately the same for light with wavelength, It is represented by. Given that the propagation constants β _x and β _{y of the} two intrinsic polarization axes of the polarization-maintaining optical fiber, the difference Δβ is It is represented by (n _x , n _y : refractive index of the core of each intrinsic polarization axis) and is a function of the wavelength λ.

Now, if a linearly polarized wave is incident on the polarizing plate at an angle of 45 degrees with respect to the intrinsic polarization axis, the phase difference θ between the x axis and the y axis becomes θ = Δβ · l (l: optical fiber length) (3) The polarization degree P is expressed as P = | cos (Δβ · l) | (4) Since the polarization of the light emitted from the optical fiber changes with the azimuth of 45 ° as the main axis with respect to the intrinsic polarization axis of the polarization-maintaining optical fiber, I ₁ (λ) and I ₂ (λ) measured previously
The difference and the ratio of the sums are expressed by the following equation in consideration of the polarization in the polarization degree P.

Since equation (5) is a function of synchronization 2π, assuming that the wavelengths that give one cycle are λ ₁ and λ ₂ , from equations (2) and (3), Becomes

On the other hand, the coupling length L (λ ₁ ) at the wavelength λ ₁ is Therefore, from equations (2), (7), and (8), Becomes

Therefore, the coupling length of the polarization-maintaining optical fiber can be automatically measured by searching the computer for the wavelengths λ ₁ and λ ₂ that give one cycle.

As described above, according to the present invention, (1) there is no limit to the measurable coupling length of the polarization-maintaining optical fiber 5, and even a short sample can be easily and easily
The bond length can be measured accurately.

(2) Since the automatic measurement is possible by introducing the computer 10, the bond length can be measured in a short time.

(3) Since the bond length can be measured at any wavelength, it is possible to clarify the wavelength characteristic of the bond length which has been unknown in the past.

Since the periodicity of cos θ with respect to the wavelength appears in the single mode region of the polarization-maintaining single-mode fiber 5, the cut-off wavelength of the polarization-maintaining single-mode fiber 5 can be obtained from the disturbance of the period.

Further, by obtaining the coupling length L (λ) at each wavelength, the wavelength characteristic of the mode birefringence B (λ) can be obtained by the following equation.

In addition, when pressure is applied in the axial direction of the polarization-maintaining optical fiber 5, a change in the coupling length appears. Therefore, by preliminarily obtaining the relationship between the amount of change and the pressure, the pressure using the polarization-maintaining optical fiber is reduced. It can be applied to measuring instruments.

[Effects of the Invention] As described above, according to the present invention, there is an effect that a short bond sample to a long bond length can be easily and automatically measured with a short sample.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of a measuring system of an automatic coupling length measuring device for explaining an embodiment of an automatic measuring method of the coupling length of a polarization-maintaining optical fiber of the present invention. 1 ... White light source, 2 ... Divider, 3 ... Polarizer, 5 ... Polarization preserving optical fiber, 6 ... Analyzer, 7 ... Multimode optical fiber, 8 ... Ge-APD, 9 ... Voltage Total, 10 ... Computer, 11 ... Monochromator controller, 12 ... XY plotter.

Claims (1)

[Claims] 1. A white light source, a spectroscope, a polarizer and an incident side of a polarization maintaining optical fiber to be measured, while an analyzer, a light intensity measuring device and a computer are provided on the exit side of the polarization maintaining optical fiber to be measured. The computer controls the spectroscope to separate the white light from the white light source into monochromatic light of the wavelength used for measurement, and the separated light is transmitted through the polarizer to the unique polarization-maintaining optical fiber. A linearly polarized light having a plane of polarization of 45 ° with respect to the polarization axis is incident, and the light emitted from the polarization-maintaining optical fiber to be measured generated by this incident light is analyzed by an analyzer.
By rotating it by 0 °, the polarization-maintaining optical fiber is transmitted in two directions of 45 ° with respect to the intrinsic polarization axis.
The light intensity measurement device measures the intensity of light in the two directions, and the computer calculates and stores the ratio of the difference and the sum of the light intensity of the transmitted light in the two directions. A polarization-maintaining optical fiber characterized by calculating the coupling length of a polarization-maintaining optical fiber from two wavelengths that are changed to operate for a plurality of wavelengths and give one cycle in the periodic change of the output light intensity ratio to the incident wavelength. Automatic method for measuring bond length.