KR100332422B1 - Apparatus for measuring residual stress and refractive index of an optical fiber preform - Google Patents

Abstract

Disclosed is a measuring apparatus which can efficiently measure the residual stress and the refractive index of an optical fiber base material simultaneously by one measuring apparatus. The measuring device of the present invention comprises: a laser light source; A polarizer for changing the polarization of the light such that the light from the laser light source is incident on the optical fiber base material with the desired input polarization; Condensing means for injecting light that has passed through the polarizer into the optical fiber base material in a condensed state; A wavelength plate and an analyzer positioned at a next stage of the optical fiber base material to convert the phase difference of the passing light according to the residual stress or the refractive index distribution of the optical fiber base material into the light intensity; An optical sensor for converting the intensity of the transmitted light converted by the wave plate and the analyzer into an electric signal; And an analyzer angle adjusting means for determining a polarization angle in which the passing light intensity represents the maximum or the minimum by using the intensity of the passing light detected by the light detector as a negative feedback signal. According to the present invention, since the residual stress and the refractive index of the optical fiber base material can be measured efficiently, the quality of the optical fiber can be improved.

Description

Apparatus for measuring residual stress and refractive index of an optical fiber preform}

The present invention relates to an apparatus for measuring residual stress and refractive index of an optical fiber base material, and more particularly, to a measuring apparatus capable of simultaneously and efficiently measuring the residual stress and refractive index of an optical fiber base material by one measuring device.

The loss of light propagated through the optical fiber is generated by the residual stress existing inside the optical fiber during optical fiber fabrication. Therefore, if the distribution of the residual stress in the optical fiber and its size are accurately measured and optimally adjusted, it may contribute to the improvement of the optical fiber quality. In addition, the refractive index distribution is a parameter for determining the structure of the optical waveguide, and thus the optical characteristics of the optical fiber can be grasped in the matrix state. Therefore, there is a need for a technique for accurately measuring the residual stress and refractive index distribution of the optical fiber preform.

1 is a schematic configuration diagram of an optical fiber base material residual stress measuring apparatus according to an example of the prior art. Referring to FIG. 1, laser light from a light source 100 such as a helium-neon laser passes through a beam expander 110 and a cylindrical lens 120, and then enters a preform cell 130. do. The light passing through the base material cell 130 passes through the condenser lens 140, the waveplate 150, and the analyzer 160, and is converted into the light intensity by the photo detector 170. Is detected. However, since the light passing through the base material cell 130 has a phase difference according to the stress distribution inside the base material cell 130, the stress distribution in the base material can be known through the intensity of light detected by the detector 170.

2 is a schematic configuration diagram of an optical fiber base material residual stress measuring apparatus according to another example of the prior art. In FIG. 2, the same reference numerals are given to the same components as those of the measuring apparatus of FIG. Referring to FIG. 2, laser light from a light source 100 such as a helium-neon laser may be used to convert a chopper 180 and a chopper 180 to convert the laser light into a pulse instead of the beam diffuser and the cylindrical lens. After passing through the polarizer 190 to change the polarization of the laser pulse passed through the incident to the base material cell 130.

As described with reference to FIGS. 1 and 2, various methods have been studied since 1970's to measure residual stress of the optical fiber base material, but have not been commercialized at the laboratory level, and the structure of the device for implementing the studied method is relatively complicated. There is a problem that takes a long time to measure. In addition, in the case of the prior art, the residual stress and the refractive index were measured individually by a separate measuring instrument.

Therefore, the technical problem of this invention is providing the measuring apparatus which simultaneously measures the residual stress and refractive index of an optical fiber base material as one apparatus.

Another technical problem of the present invention is to provide an optical fiber base material residual stress and refractive index measuring apparatus, by improving and automating the optical mechanical structure of a conventional measuring apparatus using a modified polariscope to improve the efficiency and accuracy of the measurement.

1 is a schematic configuration diagram of an optical fiber base material residual stress measuring apparatus according to an example of the prior art;

2 is a schematic configuration diagram of an optical fiber base material residual stress measuring apparatus according to another example of the prior art;

3 is a schematic diagram showing a polar scope modified for use in an apparatus for measuring residual stress and refractive index according to an embodiment of the present invention;

4 is a schematic configuration diagram of an apparatus for measuring residual stress and refractive index according to an exemplary embodiment of the present invention;

FIG. 5 is a graph showing the results of investigating the refractive index distribution of a base material for making a single mode optical fiber using the apparatus of FIG. 4; FIG. And

6 is a graph of residual stress distribution of the optical fiber base material measured using the apparatus of FIG. 4.

Explanation of symbols on the main parts of the drawings

100: light source 110a, 110b, 110c: condenser lens

130: optical fiber base material 150a: 1/4 wave plate

160a, 160b: analyzer 170: light detector

190a: Polarizer 200: A / D Converter

210: controller

220a, 220b: first and second position control motor

An optical fiber base material residual stress and refractive index measuring apparatus of the present invention for solving the above technical problem comprises: a laser light source; A polarizer for changing the polarization of the light such that the light from the laser light source is incident on the optical fiber base material with the desired input polarization; Condensing means for injecting light that has passed through the polarizer into the optical fiber base material in a condensed state; A wavelength plate and an analyzer positioned at a next stage of the optical fiber base material to convert the phase difference of the passing light according to the residual stress or the refractive index distribution of the optical fiber base material into the light intensity; An optical sensor for converting the intensity of the transmitted light converted by the wave plate and the analyzer into an electric signal; And an analyzer angle adjusting means for determining a polarization angle in which the passing light intensity represents the maximum or the minimum by using the intensity of the passing light detected by the light detector as a negative feedback signal.

At this time, the analyzer angle adjusting means includes: an A / D converter which receives the intensity of the passing light detected by the light detector as a negative feedback signal; A controller for receiving an electrical signal passed through the A / D converter and outputting signals for generating displacements at positions of the optical fiber base material and the analyzer; It is preferable to provide the said optical fiber base material and the analyzer movement motor respectively driven by the output signal of the said controller.

In addition, a knife edge filter is further installed between the analyzer and the light detector, so that the refractive index of the optical fiber base material can be easily measured.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention.

When a mechanical part made of a light-transmitting material is observed under constant polarized light in a stressed state, an interference pattern appears in the passing light because the stress causes birefringence. Polariscopes are devices that are primarily used to measure internal stresses, such as glass, from these interference patterns.

3 shows a polarscope modified for use in an apparatus for measuring residual stress and refractive index according to an embodiment of the present invention. Referring to FIG. 3, the light emitted from the laser light source 100 is made of light polarized by 45 degrees on the vertical axis by the polarizer 190a. Subsequently, the light is incident on the optical fiber base material 130, and the passing light having a phase difference according to the residual stress or refractive index distribution of the optical fiber base material 130 is divided into a quarter waveplate 150a and an analyzer. In step 160, information on the phase difference is expressed as the light intensity. In the quarter wave plate 150a, the X axis represents the fast axis and the Y axis represents the slow axis, respectively.

4 is a schematic diagram of a residual stress and refractive index measuring apparatus according to an embodiment of the present invention. Referring to FIG. 4, the light emitted from the laser light source 100 such as helium-neon is a polarizer 190a for converting it into light polarized at 45 degrees with respect to the longitudinal axis, and a first light for condensing light passing through the polarizer. After passing through the condenser lens 110a in turn, it is incident on the optical fiber base material 130. The passing light passing through the optical fiber base material 130 having different residual stresses or refractive indices according to the position is the quarter wave plate 150a and the first analyzer 160a with phase difference information resulting from the stress or refractive index difference. ), The phase difference is expressed by the intensity of light through the second analyzer 160b. Of course, the second and third condensing lenses 110b and 110c are positioned between the first and second analyzers, and between the second analyzer and the light detector, respectively. The light intensity is then converted by the light detector 170 into an electrical signal. The output signal from the light detector 170 is a negative system consisting of an A / D converter (Analog / Digital Converter) 200, a controller 210, and first and second position control motors 220a and 220b. ) Is provided to the A / D converter 200 as an input signal. Based on the output signal from the light sensor 170, the optical fiber base material 130 and the first analyzer 160a are adjusted to their positions by the first and second positioning motors 220a and 220b. Allows you to find the polarization angle that represents the intensity of the light source. In such a measuring apparatus, since the element for determining the residual stress is expressed by the intensity of light, accurate measurement can be performed from the element of the disturbance. In addition, by inserting a knife edge filter between the analyzer and the light detector, the refractive index distribution present inside the optical fiber can be measured by the same device. In measuring the residual stress, the second analyzer 160b and the third condenser lens 110c of FIG. 4 may be omitted.

After measuring the residual stress of a typical single mode optical fiber base material using the residual stress and refractive index measuring apparatus according to an embodiment of the present invention shown in Figure 4 it is shown in Table 1 compared with the results according to the prior art.

Prior art device Device of the Invention Measuring time More than 240 minutes Less than 120 minutes Measurement error Various Less than 1㎛

The results of examining the refractive index distribution of the base material for making the single mode optical fiber by using the apparatus of FIG. 4 are shown in FIG. 5. Referring to FIG. 5, it can be seen that portions corresponding to the optical fiber cores exhibit high refractive indices and portions corresponding to cladding exhibit low refractive indices, respectively.

Meanwhile, the residual stress distribution of the optical fiber base material measured using the apparatus of FIG. 4 is shown in FIG. 6.

According to the present invention, since the residual stress and the refractive index of the optical fiber base material can be measured efficiently, the quality of the optical fiber can be improved.

Claims (3)

A laser light source; A polarizer for changing the polarization of the light such that the light from the laser light source is incident on the optical fiber base material with the desired input polarization; Condensing means for injecting light that has passed through the polarizer into the optical fiber base material in a condensed state; A wavelength plate and an analyzer positioned at a next stage of the optical fiber base material to convert the phase difference of the passing light according to the residual stress or the refractive index distribution of the optical fiber base material into the light intensity; An optical sensor for converting the intensity of the transmitted light converted by the wave plate and the analyzer into an electric signal; Residual stress and refractive index measuring apparatus of the optical fiber base material comprising an analyzer angle adjusting means for determining the polarization angle of the maximum or minimum of the transmitted light intensity using the intensity of the passing light detected by the light detector as a negative feedback signal. The method of claim 1 wherein said analyzer angle adjusting means comprises: An A / D converter receiving the intensity of the passing light detected by the light detector as a negative feedback signal; A controller for receiving an electrical signal passed through the A / D converter and outputting signals for generating displacements at positions of the optical fiber base material and the analyzer; And an optical fiber base material and an analyzer moving motor respectively driven by an output signal of the controller. 3. The apparatus of claim 2, wherein a knife edge filter is further provided between the analyzer and the light detector.