JPS63197905A - Detecting method for core of optical fiber - Google Patents

Abstract

PURPOSE:To simply detect a core of an optical fiber without using a special device, by detecting the core from a temperature radiation image of the optical fiber which is seen brightly by each different light intensity, at the time of heating the optical fiber. CONSTITUTION:In optical fibers 2, 2' in which absorption losses of a core 2-2 and a clad 2-1 are different from each other, at the time of heating the optical fibers 2, 2', the core 2-2 is detected from temperature radiation images 6, 6' of the optical fibers 2, 2' seen brightly by each different light intensity. Also, the virtual misalignment quantity of the core 2-2 against an optical fiber center axis 7 is multiplied by 1/refractive index of the core 2-2, and the virtual misalignment quantity of the core 2-2 is corrected. In such a way, the device constitution of an optical fiber connecting device is simplified and miniaturized, and also, the axial alignment can be executed stably and with high accuracy extending over a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for accurately detecting the core position of an optical fiber, particularly a single mode optical fiber with a small core diameter.

<Technical Background of the Invention> When connecting optical fibers, especially single-mode optical fibers with small core diameters, it is necessary to align the optical fibers after aligning the optical fibers based on their cores. Further, even after connection, it is necessary to evaluate the amount of axis deviation of the core of the optical fiber to determine whether the connection is good or bad.

Conventional optical fiber core detection methods include the following.

■ 2007゜9.7-388 of the collection of sJI presentation proceedings distributed at the 1981 Electronic Communication Society General National Conference.
As can be seen in the report "Design of a core-side view fusion splicer for rsMF" by Koichi Rizou and two others published at the same conference, the fluorescence phenomenon caused by ultraviolet excitation of Ge-doped optical fibers was utilized. ■ Also, 2009, p. 7-39 of Lecturer 88m distributed at the 1988 IEICE Comprehensive National Conference.
As shown in the report by Masaharu Haibara et al., “Estimation of Splice Loss by Direct Observation of the Core of Single-mode Optical Fiber,” which was published in A method of detecting the core of an optical fiber and aligning the core in air, also published in ``Transactions of the Institute of Electronics and Communication Engineers J'83/12゜vol J 66-B, No. 12 pp, 1
520-1521, written by Toshiaki Katagiri et al., ``A study on the core detection conditions of single-mode optical fibers,'' a uniform parallel beam of light is applied to an optical fiber, and the This method detects the core by observing transmitted light.

<Problems to be Solved by the Invention> However, in the conventional optical fiber core detection methods described above, for example, method (2) requires a laser light source in the ultraviolet region and its irradiation location, which has the disadvantage of increasing the size of the device. there were.

Furthermore, since the apparent axis alignment was performed using the upper core, there was a drawback that accurate axis alignment was not possible.

In addition, method (2) requires a special device called a differential interference microscope, and has the disadvantage that the design of the optical fiber alignment fine movement mechanism is limited by the size of the microscope body. Furthermore, like the above method @, since the axis is apparently aligned at the upper core position, there is a drawback that accurate axis alignment cannot be achieved.

Furthermore, method (2) requires an illumination source that emits uniform parallel light beams, which has the disadvantage of complicating the device configuration. Furthermore, in order to always generate a constant core image, illumination conditions such as uniform parallel light beams, conditions for aligning the 112Q1 surface of the microscope at a position approximately 1 of the outer diameter of the optical fiber from the center of the optical fiber toward the microscope side, There are various error factors when detecting the core position of an optical fiber, such as conditions where the depth of focus of the microscope is extremely small (for example 1 μ) and conditions where the observation surface of the microscope is set at the correct position using a fine movement mechanism, and all of these conditions must be met. There was a drawback that the accurate core position could not be detected unless the following conditions were satisfied.

The present invention was made in order to eliminate the drawbacks of the conventional optical fiber core detection method as described above, and provides a simple method for detecting the core of an optical fiber without using any special equipment. This is what I am trying to do.

Means for Solving the Problems> The method for detecting the core of an optical fiber according to the present invention for achieving the above-mentioned object is such that when heating the optical fiber, the core and the cladding have different absorption losses.
The core is detected from the temperature radiation image of the optical fiber, which appears to shine with different light intensities, and the apparent core relative to the central axis of the optical fiber is calculated by multiplying the above positional deviation by 1/1 of the refractive index of the core. The appearance of the core is characterized by correcting the amount of positional deviation above.

Embodiments Next, typical embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view showing a specific method of detecting the core of an optical fiber by thermal radiation according to the present invention.
' is a V-groove on an optical fiber fixing table supported by a fine movement mechanism driven by an electric motor, 2 and 2' are optical fibers whose tips have been completely removed and cut, and a pair of 3-bar electrodes 4 and 4 A high frequency power source is used to cause a discharge between the electrodes 4 and 4', and 5 indicates a heating range due to the discharge of the electrodes 4 and 4'.

In order to detect, align, and connect the cores of the optical fibers 2, 2' using the configuration shown in FIG.
With the cut end surfaces of the optical fibers facing each other,
．． 1' (fixed in pV groove.

Next, by operating the fine movement mechanism, the tip of the optical fiber 2゜2' is connected to the electrodes 4, 4' which are supplied with power from the high frequency power source.
into the discharge heating section 5.

Then, the tips of the optical fibers 2 and 2' are heated to a temperature determined by the power supplied from the high frequency power source 3 (for example, 1,500°C).
) to be heated.

At this time, as shown in FIG. 2, which is a cross-sectional view of an optical fiber, an optical fiber in which the cladding 2-1 is silica glass and the core 2-2 is silica glass containing Ge dopant will be explained as a typical example. Core 2-2 is cladding 2
Since the absorption loss is larger than -1, it has a larger emissivity than the cladding. As a result, the core 2-2 in this optical fiber emits light with a higher light intensity than the cladding 2-1.

That is, in FIG. 1, the optical fibers 2 and 2' in the heating section 5 are visualized as a temperature radiation image 6°6'. 7
It can be detected by

Therefore, the amount of axis deviation of the cores of the optical fibers 2, 2' can be detected from the X-axis and the y-axis direction perpendicular thereto. In this case, as shown in FIG. 2, the magnitude of the positional deviation ΔX of the core 2-2 from the central axis 7 of the optical fiber 2 (2') is determined by the refractive index of the cladding 2-1 due to the lens effect of the cladding 2-1. It is the apparent size magnified by twice and vASed,
The exact amount of core position deviation is the apparent amount of core position deviation Δ
It is necessary to multiply X by 1/1 of the refractive index of the cladding (for example, n=1.46) and correct it to Δx/n. Figure 2 shows the case of observation from the X direction, but similarly from the X direction it is 1E! If measured, the apparent positional deviation amount Δy can be obtained,
Accurate positional deviation amount Δy is calculated by multiplying by 1/1 of the refractive index of the cladding.
/n is obtained.

As described above, the core axes can be aligned before the fusion splicing of the optical fibers 2 and 2' based on the accurate amount of core position deviation detected from two orthogonal directions, and after the fusion splicing, the core axes can be aligned. The quality of the connection can be determined from the resulting amount of core axis deviation.

In the above explanation, glow discharge heating was taken as an example, but other heating means such as a nichrome heater, a carbon heater, a propane gas flame, etc. are also applicable.

Next, we will discuss theoretical and experimental considerations regarding the thermal radiation of a silica glass fiber whose core is doped with Ge, and an investigation regarding the observation error of the core position shift l due to the lens effect of the cladding.

[In] Temperature radiation of Ge-doped optical fiber by heating (1) Spectral radiant emittance of optical fiber core In general, the spectral radiant emittance Pλ of a temperature radiator is calculated using the spectral emissivity C(λ) and the blank radiation equation. It is given as the product of the sum and the following formula.

Pλ= 1 (λ) (1) In order to calculate the spectral emissivity ε(λ) of the core and cladding of the optical fiber, first, the absorption losses a, b
, , that is, the procedure for determining the absorption coefficient α is as follows (a) (b)
Shown below.

(Al　Absorption loss a, b of silica glass.

Literature 0 people, Pinnow, T., C., Rich, F.
,W.

0ster+mayor, Jr, and M, DiD
omenico, Jr.

-Fundamental 0ptical＾tten
ationLia+its in the Liquor
id and Ga55y Statewith ap
Application to Fiber Optica
lWavwguide Materials, +Personp
p1. Phys, Lett.

22, 10. From pp527-529 (1973), the absorption loss 'abs (dB/m) of silica glass is calculated using the following formula (%). However, A and ΔE are constants, A=4.29X10 ΔE=0.704 (31, E is the effective energy gap 7, Reference S, Takahashi and S, 5hi
bata,”Ther+mal Variation
of Attenuation for Optic
al Fibers, +J, Non-Crystal
lineSolids, 30. pp, 359-
370 (1979), the effective energy gap between the cladding and the core, namely E, cd and Ellell, is given by:

E, cd=13-38 + dE, c7 d T (
T 20 ) E, c, = 12.91 + dE, C
, / dT (T-201 where T is the temperature of the silica glass, and the temperature gradient of its effective energy gap is given by the following equation.

dE, c/dT = -1,2X 10-3dE, .

/dT = -1.8 did.

Furthermore, in equation (2), E is photon energy (eV
) and the wavelength λ(-) has the following relationship.

E = 1.23985 /λ (6) or more (
From equations 2) to 6), absorption losses a and b with respect to wavelength λ and temperature T of silica glass.

can be obtained by calculation.

(b) Absorption coefficient a of silica glass The absorption coefficient a (1/m) of silica glass has a relationship with its absorption loss αmba (dB/m) as shown in the following equation.

a=(log 10/10)a=0.2303
cr, b.

- Figure 3 shows the relationship between the temperature absorption coefficient a of silica glass and the wavelength from 300n+s to 700nm, using wavelength as a parameter. Figure 3: The magnitude of the absorption coefficient of the core and cladding of the optical fiber can be calculated from the above equation (4), that is, (5)
It is determined by the temperature gradient of the effective energy gap in the equation, and since the gradient of the core is larger than that of the cladding at least at T>0°C, the magnitude of the absorption coefficient of the core is larger than that of the cladding.

(C) Spectral emissivity C(λ) The spectral emissivity ε(λ) of an optical fiber is modeled using a silica glass plate as shown in FIG. 4. Now, when light with unit light intensity l is incident on a silica glass plate, there is reflection and transmission at the air-silica glass interface, and some of the light that has entered the interior of the silica glass is reflected and transmitted according to Lambert's law. Absorbed. At this time, if the light intensity of the light emitted to the outside of the silica glass plate is determined as the sum of an infinite series, then the spectral emissivity C(λ) can be determined using the following formula.

ε(λ) = 1-r-(1-r)2exp (-a
t)/(1-reexp (-at)). Here, r is the reflectance when light is perpendicularly incident on the silica glass, a is the absorption coefficient (1/m), and t is the thickness of the silica glass plate. Here, the reflectance r was set to r = 0.03477 since the refractive index of the cladding was 1.4584.

However, the spectral emissivity of the core is calculated below under the following conditions. In other words, the refractive index difference between the core and the cladding is less than 1%, ignoring reflection at the interface between the core and the cladding, and since the absorption loss of silica glass is extremely small, the absorption loss in the cladding is I ignored it.

(2) Contrast between core and cladding (a)
Calculation When heating an optical fiber, the emissivity ε of the core for each wavelength, at least at T>0 °C, as can be seen in FIG. . is the emissivity ε of the cladding independent of temperature. Greater than 6. Therefore, the core should appear to glow with a higher light intensity than the cladding. Figure 5 shows the calculation results of the emissivity on the surface of the optical fiber when the optical fiber is heated, using the optical fiber temperature as a parameter. ) is for single mode optical fiber. However, in order to simplify the emissivity, the emissivity is calculated by substituting the thickness of the optical fiber in the direction of the tip axis when viewed from the a'a mirror into equation (8) as shown in the figure. It was obtained by using , and ignoring the lens effect of the core and cladding.

In Figure 5, when considering the contrast between the core and the cladding, it is clear that the core of a multimode optical fiber is easier to observe than that of a single mode optical fiber, and that the contrast improves as the fiber temperature increases for both fibers. . It can also be seen that the optical fiber appears to shine more intensely as the temperature rises. Note that since this is temperature radiation, the contrast is constant for both fibers regardless of wavelength.

(bl Experimental results Figure 6 (al) (bl is the measured actual light intensity distribution of the temperature radiation image of the optical fiber, Figure 6 (al is the light intensity distribution of the temperature radiation image of the multimode optical fiber) Figure 6 (fb) is the light intensity distribution of the temperature radiation image of a single-mode optical fiber.Comparing Figures 6 and 5 with respect to the contrast between the core and cladding, it can be seen that the optical fiber has been heated to about 1500°C. It is estimated that there are.

By the way, in the case of a single-mode optical fiber, the amount of Ge dopant is smaller than that in a multi-mode optical fiber, so absorption loss is reduced. In other words, since tb+ in Figure 5 is a calculation result based on the absorption loss of a multimode optical fiber, it is thought that the actual contrast between the core and cladding shown in Figure 6 is smaller by the difference in the amount of Ge dopant. It will be done. In addition, in the case of Figure 5, the core diameter remains at its original size because the lens effect of the cladding is ignored, but in contrast, in the case of the actual measurement result (Figure 6), it is as follows. , the core diameter is enlarged by the refractive index of the cladding to 1i! 1111m.

From the above, the optical fiber should be placed close to the heating section. What you can see when you touch the core is the core. The emissivity of the core is greater than that of the cladding. This is because there is stronger temperature radiation from It became clear that

[&] Consideration of observation errors in core position When measuring an optical fiber core image using a microscope, the lens effect of the cladding becomes a problem.

The amount of core positional deviation from the central axis of the optical fiber is measured at a magnification different from that of the microscope due to the lens effect as well as the core diameter.

Considering this in Figure 7, core RR with center coordinates (xo, yo) and diameter 2n can be seen from the microscope as (mommy) and (king p).
The line segment VV connecting y) is measured by multiplying the refractive index of the cladding. d is the diameter of the optical fiber.

y=(a-χ)-w2 y = (x-a) toa ll.

Here, cho and 1 are the angles shown in Fig. 7, and are also the angles at positions corresponding to ri, dan; yo; and 10,000.

As described above, the thermal radiation phenomenon of Ge-doped optical fibers due to heating and the exact observation conditions for the core have been clarified. In the above explanation, temperature radiation was explained using the case of Ge dopant as an example.
Even in the case of a silica fiber doped with , the core can be detected from its temperature radiation image. Generally, if the absorption loss of the core and cladding of an optical fiber differs depending on the dopant content, the core can be detected from the temperature radiation image. Also,
In the above explanation, we considered the case of observing the core with a microscope W1, but if you introduce a CCD camera, image processing device, etc. in addition to the microscope, you can achieve more efficient core alignment by linking it with the fine movement mechanism. Also, the quality of the connection can be efficiently determined based on the amount of core axis deviation after connection.

□By the way, above we considered the case where the core of the optical fiber contains a dopant, but if the dopant is included in the cladding and the absorption loss of the cladding is increased (in this case, the cladding is relative to the core). However, in the above explanation, the heating temperature of the optical fiber was about 150°C, but from the relationship of the contrast between the core and the cladding, a temperature of 1000°C or higher is desirable. At a splicing temperature (for example, 2200° C.), splicing loss can be estimated and evaluated by detecting the amount of axial misalignment between cores during fusion splicing, but this is the upper limit temperature for practical use.

<Effects of the Invention> As is clear from the above description, ■ The method for detecting the core of an optical fiber using thermal radiation according to the present invention is capable of emitting light with different intensities when heating optical fibers with different absorption losses in the core and cladding. Because it uses an existing house that can be seen as a fusion splicer, and because it can reuse the heating means that is always installed in equipment that fusion splices optical fibers,
There is no need for special equipment such as an illumination device to always generate a constant core image or a laser and its irradiation device to make the core emit light. Therefore, the configuration of the optical fiber connecting device is simplified, and the device can be made smaller and more economical.

■ Furthermore, in the optical fiber core detection method of the present invention,
Since the core image is generated by simply heating the optical fiber, it is completely unaffected by defects or poor adjustment of means such as illumination equipment for generating the core image. Therefore, there is an advantage that there is no error factor at the stage of generating the most important core image, and since axis alignment can be effectively performed stably over a long period with high accuracy, optical fibers can be connected with low loss.

■Furthermore, the method for detecting the core of an optical fiber according to the present invention is such that during the fusion splicing operation of optical fibers, especially just before the completion of the operation, when the positional relationship between the optical fibers to be spliced is fixed, the Since the amount of core axis misalignment can be detected and evaluated, it is possible to simultaneously perform optical fiber connection operations and detect and evaluate core axis misalignment. Therefore, there are various advantages such as shortening the optical fiber connection time.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing the implementation procedure of the method for detecting the core of an optical fiber using thermal radiation according to the present invention, Fig. 2 is a principle explanatory diagram showing a situation in which the core of the optical fiber appears to shine against the cladding, and Fig. 3 The figure is a characteristic diagram showing the relationship between the optical fiber temperature and the absorption coefficient of the core and cladding. Figure 4 shows the reflection at the interface between air and the silica glass plate when light of light intensity I is incident on the silica glass plate. Explanatory diagrams showing the relationship between transmission and absorption, Figures 5 (a) and (b) are diagrams showing the calculation results of emissivity on the surfaces of multimode optical fibers and single mode optical fibers, respectively, and Figure 6 (al (b) are diagrams showing actual measurement results of temperature radiation images of multi-mode optical fiber and single-mode optical fiber, respectively, and Figure 7 is an explanatory diagram showing the lens effect of the cladding of the optical fiber. V full, 2 (overall code)...Optical fiber 7/ipa, 2-1...Clad, 2-2...Core, 3...High frequency power supply, 4...Electrode, 5...Heating 6. Temperature radiation image of the core, 7. Central axis of the optical fiber. Patent applicant: Agent of Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] When heating an optical fiber in which the core and cladding have different absorption losses, the core is detected from the temperature radiation image of the optical fiber, which appears to glow with different light intensities, and the core is detected relative to the central axis of the optical fiber. A method for detecting the core of an optical fiber, comprising: multiplying the apparent positional deviation of the core by 1/1 of the refractive index of the core to correct the apparent positional deviation of the core.