JPH0971431A - Production of silica glass-based multicore optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily obtain a high-quality optical fiber in high accuracy by drawing a preform which is produced by powder molding of silica powder around plural core rods arranged in parallel with one another in the longer side direction of a clad with nearly rectangular section followed by purifying and transparently vitrifying the molded form. SOLUTION: In Figure (a), the upper and lower metallic lids 12, 13 of a rubber mold 11 are provided, in advance, with holes 14 to be threaded with core rods 16 therethrough, and the metallic rod 12 is also provide with a hole 15 through which silica powder is to be filled. In Figure (b), the four core rods 16 are threaded through the holes 14 and arranged in parallel with one another in the rubber mold 11. Granulating powder 17 prepared by blending silica powder with water and PVA is filled via the hole 15. In Figure (c), the metallic lids 12, 13 are removed and exchanged with rubber lids 12a, 13a followed by conducting a pressure molding, and the resultant porous molded form 18 is taken out, and then subjected to cutting work followed by specified heat treatment to obtain a preform, which is then drawn into the objective optical fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a silica glass type multi-core optical fiber whose outer cross section is substantially rectangular.

[0002]

2. Description of the Related Art In recent years, optical fiber cables have been required to have smaller diameters and higher densities as information communication speeds and densities have increased. In response to this request, an optical fiber element wire (usually having an outer diameter of 250 μm) composed of a core 1, a clad 2 and a primary coating layer 3 as shown in FIG. 3 of the related art is further secondary-coated with nylon or the like. Instead of the optical fiber cable accommodating the single-core optical fiber core wire, a plurality of the optical fiber element wires are arranged in parallel as shown in FIG. 4 and then a common core layer 4 is applied to form a tape core wire. An optical fiber cable containing a core wire has been put into practical use.

However, although the tape core wire can be made higher in density in an optical fiber cable than a single-core optical fiber core wire, its width (= long side length) is defined as “optical fiber strand outer diameter × It cannot be less than or equal to the number of optical fiber strands. Therefore, in order to further increase the density, it is considered to reduce the outer diameter of the optical fiber element wire forming the tape core wire. From this point of view, by setting the outer diameter of the optical fiber strand to 180 μm, it is
16 in 3mm width where two optical fiber wires were arranged
Techniques for arranging thin optical fiber strands of a book have been studied. However, if the coating layer of the optical fiber strand is thinned to reduce the diameter of the optical fiber strand, the transmission loss of the optical fiber increases, so it is difficult to achieve an extremely small outer diameter of the optical fiber strand. Is.

Therefore, at present, a multi-core optical fiber in which a plurality of cores are arranged in a clad has been proposed in order to realize high density. Regarding the multi-core optical fiber, for example, in Japanese Unexamined Patent Publication No. 60-225104, a plurality of core rods are arranged in a flat quartz glass tube and arranged in a row adjacent to the flat quartz glass tube in the major axis direction. After heating the quartz glass tube to solidify it,
A method for producing a multi-core optical fiber which is drawn and has a flat cross section is described.

[0005]

However, in the above-mentioned method for manufacturing a silica glass multi-core optical fiber, it is difficult to solidify the preform at the time of manufacturing the preform, and bubbles are likely to be generated at the interface between the silica glass tube and the core rod. There was a problem that the formed preform was difficult to be rectangular.

On the other hand, it is desirable that the cross-sectional shape of the multi-core optical fiber in which the preform is drawn is a rectangle as thin as possible for high-density storage in the optical fiber cable, but the cross-sectional shape of the conventional multi-core optical fiber is circular or A multi-core optical fiber having an oblong shape and a rectangular cross section has not been realized. It is considered that this is because the preform having a rectangular cross section could not be produced for the above reason.

Further, this is the first time that the present inventors have found out that they have succeeded in producing a preform having a rectangular cross section. However, in order to obtain a multi-core optical fiber having a rectangular cross section, the conventional drawing process is used. I found that there is a problem. That is, it was found that the drawing temperature needs to be optimized so that the cross section does not become round in the drawing process.

In addition, the dimensional ratio of the long side to the short side in the cross section of the preform before drawing (hereinafter referred to as the cross-section long-short side ratio)
It was also found that there is a difference between the cross-sectional long and short side ratios of the multi-core optical fiber after drawing and this difference depends on the drawing temperature. Therefore, in order to manufacture a multi-core optical fiber having a desired size, it was necessary to select a suitable preform size and drawing temperature.

Therefore, it is an object of the present invention to provide a method for manufacturing a multi-core optical fiber having a rectangular cross section.

[0010]

SUMMARY OF THE INVENTION The present invention provides a method for manufacturing a silica glass multi-core optical fiber which solves the above problems.

The first invention of the present application is a method for manufacturing a silica glass multi-core optical fiber in which a plurality of cores are arranged in parallel in a long side direction of a clad having a substantially rectangular cross section. A silica glass-based multi-core optical fiber, characterized in that a porous molded body is molded by powder molding silica-based powder, and then the porous molded body is refined and drawn using a transparent vitrified preform. It is a manufacturing method.

Examples of the powder molding method include a pressure molding method, a casting molding method and an extrusion molding method. The reason why these powder molding methods are adopted is that a porous preform of a preform in which a plurality of core-corresponding parts are arranged in the long side direction in a clad-corresponding part having a substantially rectangular cross section can be easily manufactured. Further, the porous molded body manufactured by these methods has an advantage that the side surface thereof can be easily ground and can be processed into a molded body having a clad equivalent portion having a substantially rectangular cross section.

Among the powder molding methods described above, the pressure molding method has advantages that molding is easy, molding accuracy is good, and lead time required for molding is short. In addition, the shape of the molded body can be easily changed by changing the molding rubber die, which is preferable.

The porous base material of the preform in the pressure molding method is manufactured as follows. That is, first, a plurality of core rods are arranged in parallel inside a molding die made of rubber (hereinafter referred to as a molding rubber die) using an appropriate jig. Next, the space in the molding rubber die around the core rod is filled with silica-based powder. Next, using a so-called hydrostatic pressure method in which the filled rubber mold is externally pressure-molded with the liquid pressure of a pressure container, a core rod (a core-corresponding portion is formed)
And a silica-based powder (forming a portion corresponding to the clad) to form a porous compact. Here, the silica-based powder forming the clad equivalent portion is not only silica glass,
Silica glass to which a small amount of dopant is added may be used.
These are preferably high-purity synthetic glass powders, and the average particle diameter is preferably in the range of 1 to 20 μm. The reason why these are preferable is to enhance the purification effect in the subsequent step. Further, the silica powder can be filled as it is, but more preferably, the binder (molding aid) is added to the silica-based glass powder and the granulated powder granulated by a granulator such as a spray dryer is filled. Is good. The average particle size of the granulated powder is 75 to 150 in terms of fluidity and packing density.
It is preferably in the range of μm.

The core rod may be the core-corresponding portion itself, or may be the core-corresponding portion having a part of the clad-corresponding portion. These are manufactured by existing vapor phase processes such as the VAD process.

The porous molded product having a rectangular cross section can be obtained by molding using a mold having a rectangular cross section, but as another method, a porous molded product having an appropriate shape, for example, a circular cross section, is prepared in the molding step. Then, the side surface of the porous molded body can be ground to obtain a porous molded body having a rectangular cross section.

Next, purification of the porous molded body and transparent vitrification will be described. The purification step is performed for the purpose of removing organic substances and water contained in the porous molded body by heat treatment. The former is preferably carried out in an oxygen-containing atmosphere at a temperature up to 500 ° C., and the latter is preferably carried out in a helium atmosphere containing about 1 to 40% chlorine in the range of 1000 to 1200 ° C. Further, it is desirable that the transparent vitrification step is performed in a range of 1500 to 1700 ° C. under a helium atmosphere.

The preform thus obtained has a rectangular cross section, and a plurality of core-corresponding portions are arranged in the long side direction of the clad-corresponding portions. Next, the preform is drawn to manufacture a silica glass based multi-core optical fiber.
It is also possible to purify and vitrify a porous molded body having a circular cross section as it is to form a preform having a circular cross section, and grind the side surface of the preform having a circular cross section to obtain a preform having a rectangular cross section. . However, this method is not economically advantageous as compared with grinding a porous molded body.

In the second invention of the present application, a plurality of cores have long sides L _Fl.
And a short side L _{Fs has} a length ratio of L _Fl : L _Fs = N: 1. The ratio L _Pl : L _Ps of the length of the long side L _Pl and the short side L _Ps of the preform having a substantially rectangular cross section before _pulling is L _Pl : L _Ps = N × M: 1 (where M ≧ 1 .01).

The second invention was made by the present inventors experimentally finding the following facts. That is, even if a preform having the same rectangular cross section is drawn, the cross-sectional long-to-short side ratio of the obtained multicore optical fiber is
We found that it changes depending on the drawing temperature.

FIG. 5 shows the relationship between the drawing temperature and the obtained cross-sectional long / short side ratio of the multi-core optical fiber when a preform having two cores (preform cross-sectional long / short side ratio = 2: 1) is drawn. The relationship obtained is shown. Further, in FIG. 6, the core is 4
The relationship obtained between the drawing temperature and the obtained cross-sectional long-to-short side ratio of the multicore optical fiber when the preform of the core (the cross-sectional long-to-short side ratio of the preform = 4: 1) is drawn is shown.
As can be seen from these figures, the higher the drawing temperature, the smaller the cross-sectional long / short side ratio of the obtained multi-core optical fiber. That is, the higher the drawing temperature, the smaller the cross-sectional length ratio of the obtained multi-core optical fiber becomes than the cross-sectional length short side ratio of the preform.

Therefore, in order to make the cross-sectional long / short side ratio of the obtained multi-core optical fiber equal to the cross-sectional long / short side ratio of the preform, the drawing temperature must be considerably low, specifically about 1830 ° C. However, drawing at such a low temperature causes problems such as a slow drawing speed, which is economically disadvantageous, a high loss of the multicore optical fiber, and deterioration of mechanical strength. In addition, drawing stress may remain in the multi-core optical fiber. Therefore, it is desirable to make the drawing temperature as high as possible.

Therefore, in order to avoid the above-mentioned problems, it is necessary to make the cross-section long / short side ratio of the preform larger to some extent than that of the multicore optical fiber after drawing. As a result of examining this size, the present inventors have reached the second invention of the present application. According to the manufacturing method of the second invention, since the wire drawing can be performed at a higher temperature than the conventional method, the wire drawing speed can be increased, which is economically advantageous. Further, the obtained multi-core optical fiber has low loss and high strength, and the problem of residual drawing stress can be avoided.

In the second aspect of the invention, an appropriate condition for the drawing temperature must be selected. That is, when the preform is drawn, the higher the drawing temperature is, the lower the cross-sectional long / short side ratio of the multi-core optical fiber obtained becomes smaller than the cross-sectional long / short side ratio of the preform. Therefore,
In order to manufacture a multi-core optical fiber having a desired cross-sectional length / short ratio, an appropriate drawing temperature must be selected according to the cross-sectional long / short side ratio of the preform.

If the drawing temperature is not adjusted to an appropriate value, the cross-sectional long / short side ratio of the obtained multi-core optical fiber may be larger or smaller than a desired value. In addition, it is not preferable to unnecessarily increase the drawing temperature. This is because if the drawing temperature is made too high, the cross section of the multi-core optical fiber collapses from the rectangle, and the four sides of the cross section become rounded. In addition, the long-to-short side ratio of the cross-sectional shape of the multi-core optical fiber becomes small, the deformation of the clad becomes remarkable, and the core also deforms, which causes a problem that the transmitted light has polarization dependence.

As a solution to this problem, assuming that the cross-sectional long / short side ratio of the desired multi-core optical fiber of the present application is N: 1, the pre-form cross-sectional long / short side ratio is N × M: 1 ( However, it is preferable that M ≧ 1.01). A more preferable range of M is 1.01 to 1.10. If this condition is observed, M
Since a drawing temperature of 2000 ° C. or less can be selected according to the above, the cross section of the fiber as described above does not suddenly collapse from a rectangular shape and the four sides of the cross section are not rounded. However, the drawing temperature here means the temperature of the central portion in the drawing furnace.

In the third invention of the present application, a plurality of cores have long sides L _Fl.
And a short side L _{Fs has} a length ratio of L _Fl : L _Fs = N: 1. To form a porous molded body by powder molding silica-based powder around a plurality of core rods arranged in
Then, the ratio L _Pl : L _Ps of the lengths of the long side L _Pl and the short side L _Ps of the clad portion of the preform obtained by purifying and vitrifying the porous molded body is L _Pl : L _Ps = N × M: 1 (however, M ≧ 1.01).

The third invention of the present application is a combination of the invention described in claim 1 and the invention described in claim 2. By this, a multi-core optical fiber having a desired size can be efficiently manufactured with high performance. .

Here, it is assumed that the preform and the multicore optical fiber have a substantially rectangular cross section. The reason is that the vertices 22a shown in FIG. 2 (a) are slightly rounded, or each of the vertices 22a shown in FIG. Sides 22b, 22
This is because it is preferable in terms of strength that c has a shape that bulges slightly toward the outside.

A method of manufacturing a silica glass type multi-core optical fiber according to the present invention will be described in detail based on examples.

[0031]

Example 1 In Example 1, a pressure molding method in which silica powder and a core rod were integrally molded was used as a method for manufacturing a preform. Compared to other powder molding methods such as casting molding method and extrusion molding method, the pressure molding method has the advantage that the lead time required for molding is short, and the shape of the molded body can be easily changed by changing the molding rubber mold. There is an advantage that you can.

The manufacturing process of Example 1 is shown in FIGS.
This will be described with reference to FIG. The steps are as follows.

1) Core diameter: clad diameter = 1: 2, relative refractive index difference Δ = 0.3% between core and clad, outer diameter 2 mmφ, length 250 mm, core rod 16 was produced by VAD method.

2) As shown in FIG. 1A, the molded rubber mold 11 on which the core rod 16 is installed has upper and lower metal lids 1 as shown in FIG.
Holes 14 into which the core rod 16 is previously inserted in 2 and 13
The upper metal lid 12 is provided with a filling hole 15 for filling with silica powder.

3) As shown in FIG. 1B, the hole 14
The four core rods 16 were arranged in parallel in the molding rubber mold 11 by inserting the core rods 16 into the rubber mold 11. Also,
The molding rubber mold 11 was filled with the granulated powder 17 of silica powder through the filling hole 15.

The granulated powder 17 was formed as follows. That is, the silica powder (average particle size 8 μm) produced by the gas phase synthesis method, pure water, PVA (polyvinyl alcohol), and glycerin in a weight ratio of 100.0: 66.
After mixing and stirring at a ratio of 0: 1.6: 1.0, a granulated powder 17 was obtained using a spray dryer device. The granulated powder 17 thus obtained has an average particle diameter of 100 μm.
Met.

The average particle size of the silica powder is preferably in the range of 1 to 20 μm in order to enhance the purification effect in the subsequent steps. The average particle size of the granulated powder 17 is
From the viewpoint of fluidity and packing density, it is preferably in the range of 75 to 150 μm.

4) After filling the molding rubber mold 11 with the above-mentioned granulated powder 17, as shown in FIG. 1 (c), the upper and lower metal lids 12 and 13 are removed one by one to remove the rubber lids 12a and 13 respectively.
exchanged for a. Next, after sealing the molding rubber mold 11, the molding rubber mold 11 is made to 98 M using a hydrostatic pressure device.
Pressurized with a pressure of Pa. After the pressure molding, as shown in FIG. 1D, the porous molded body 18 in which the core rod 16 and the granulated powder 17 were integrally molded was taken out from the molding rubber mold 11.

5) The porous molded body 18 was cut by a milling machine to make a cross section into a rectangle having a cross sectional long side ratio of 4.2: 1. In other words, the desired value of the cross-sectional long / short side ratio of the finally obtained multi-core optical fiber was set to 4: 1.

6) The above-mentioned porous molded body having a rectangular cross section was heat-treated in a dry air stream at 500 ° C. for 5 hours, and then, in a helium gas atmosphere containing chlorine (chlorine concentration 1%), 12
Heat treatment was performed at 00 ° C. The heat treatment is a refining process for removing the organic matter in the molded body in the former case and a water content in the silica glass powder for the latter case. After purification, transparent vitrification treatment was performed in the range of 1500 to 1700 ° C. in a helium gas atmosphere. After this treatment, the obtained preform had a rectangular cross section with a long side of 63 mm and a short side of 15 mm, and the cross section long / short side ratio was 4.2: 1.

7) The preform obtained by the above refining and vitrification was drawn at a drawing temperature of 1895 ° C. and a drawing speed of 20 m / min to prepare a multi-core optical fiber 20 as shown in FIG. 1 (e). The cross-sectional shape of the obtained multi-core optical fiber was a rectangle having a long side of 500 μm and a short side of 125 μm, and a cross-section long / short side ratio of 4: 1. Further, four cores 21 are arranged at equal intervals in the long side direction of the clad 22. The multi-core optical fiber is covered with a coating layer 23 having a long side of 625 μm and a short side of 250 μm. As shown in FIG. 2B, the four sides of the cross section were slightly bulged outward, but had almost flat surfaces. Also,

The multicore optical fiber manufactured in Example 1 was a single mode fiber, and the transmission loss at a wavelength of 1.3 μm was 1 dB / km or less.

In Example 1, the preform having the cross-section long-to-short side ratio of 4.2: 1 was used to manufacture the multi-core optical fiber having the cross-section long-to-short side ratio of 4: 1.
2 = 4 × 1.05, that is, a preform having a cross section long / short side ratio which is 1.05 times larger than a cross section long / short side ratio of a desired multi-core optical fiber was used.

[0044]

Second Embodiment The manufacturing process of the second embodiment will be described with reference to the drawings. The steps are as follows. 1) Core diameter: clad diameter = 1: 2.5, relative refractive index difference Δ = 1.1% between core and clad, outer diameter 7.3 mmφ, length 2
VAD of core rod with GI type refractive index distribution of 50 mm
It was produced by the method.

2) The molding rubber mold for installing the core rod has an inner diameter of 50 mm, and similarly to the first embodiment, holes for inserting the core rod are previously inserted in the upper and lower metal lids, and silica powder is provided in the upper metal lid 12. A filling hole for filling is provided. However, the core rod has two insertion holes.

3) By inserting a core rod into the insertion hole in the same manner as in Example 1, two core rods were arranged in parallel in the molding rubber mold with a space of 21.9 mm. Further, the molding rubber mold was filled with granulated powder of silica powder through a filling hole. Here, the same granulated powder as in Example 1 was used.

4) After filling the molding rubber mold with the above-mentioned granulated powder, as in Example 1, the upper and lower metal lids were removed one by one and replaced with rubber lids. Next, after sealing the molded rubber mold, the molded rubber mold was pressed at a pressure of 98 MPa for 1 minute using a hydrostatic pressure device. After the pressure molding, the porous molded body in which the core rod and the granulated powder were integrally molded was taken out from the molding rubber mold. The molded body has an outer diameter of about 40 mm, and two core rods are inserted at an interval of 17.68 mm.

5) The above porous molded body was cut with a milling machine to make a cross-section into a rectangle having a cross-section long / short side ratio of 1.6: 1.

6) The porous molded body having a rectangular cross section was heat-treated in a dry air stream at 500 ° C. for 5 hours, and then in a helium gas atmosphere containing chlorine (chlorine concentration 1%), 120
Heat treatment was performed at 0 ° C. The heat treatment described above is a refining step for removing organic substances in the molded body, and a refining step for removing water in the silica glass powder. After purification
Transparent vitrification treatment was performed in the range of 1500 to 1700 ° C. in a helium gas atmosphere. After this treatment, the cross section of the obtained preform has a long side of 23.36 mm and a short side of 14.6 mm.
mm, and the core rod spacing was 14.6 mm.

7) The preform obtained by the above refining and vitrification was drawn at a drawing temperature of 1850 ° C. and a drawing speed of 20 m / min. The cross-sectional shape of the obtained multi-core optical fiber has a long side of 400 μm and a short side of 253 μm, and a diameter of about 5 μm.
It was a rectangle with 0 μm cores spaced 260 μm apart. Therefore, the cross-sectional long / short side ratio was 1.58: 1. As shown in FIG. 2A, the vertices on the four sides of the cross section were slightly rounded, but the four sides had flat faces.

The multi-core optical fiber manufactured in Example 2 is a GI type multi-mode fiber and has a wavelength of 1.3 μm.
The transmission loss of m was 1 dB / km or less.

In the second embodiment, the cross-sectional long / short side ratio is 1.58:
Since a preform having a cross-section long-to-short side ratio of 1.6: 1 was used to manufacture the multi-core optical fiber of No. 1, 1.6 = 1.58 × 1.01, that is, the cross section of the desired optical fiber. A preform having a cross-sectional long / short side ratio of 1.01 times larger than the long / short side ratio was used.

[0053]

Example 3 In Example 3, core diameter: clad diameter = 1:
2, core and clad relative refractive index difference Δ = 0.3%, outer diameter 2
Example 1 using a core rod of mmφ and a length of 250 mm
In the same procedure as above, the long side with four cores is 32 mm and the short side is 7.
A rectangular preform having a cross section of 3 mm and a length of 250 mm was produced. The cross-sectional long / short side ratio of this preform is 4.
It was 4: 1.

The above preform was drawn at a drawing temperature of 1950.
Drawing was carried out at a temperature of 0 ° C and a linear velocity of 20 m / min. The cross-sectional shape of the obtained multi-core optical fiber has a long side of 500 μm and a short side of 1
It was a rectangle of 25 μm, and its cross-sectional long / short side ratio was 4: 1. The cores were arranged at intervals of 125 μm. As shown in FIG. 2 (b), the four sides of the rectangular cross section were rounded at their vertices, and the four sides were also slightly bulged outward, but had almost flat faces.

This multi-core optical fiber is a single mode fiber, and the transmission loss at a wavelength of 1.3 μm is 1d.
It was B / km or less.

In this embodiment, the preform having the cross-section long-to-short side ratio of 4.4: 1 was used in order to manufacture the multi-core optical fiber having the cross-section long-to-short side ratio of 4: 1.
A preform having a section length / short side ratio of 4.4 = 4 × 1.1, that is, 1.1 times larger than a section length / short side ratio of a desired optical fiber was used.

[0057]

As described above, according to the present invention, there is an excellent effect that a desired silica glass multi-core optical fiber having a substantially rectangular cross section can be easily manufactured with high quality and accuracy.

The silica glass type multi-core optical fiber having a substantially rectangular cross section manufactured according to the present invention can be advantageously used for increasing the density of an optical fiber cable. It is advantageously used for various purposes. This is a flat surface,
It is easy to enter light and sound from this surface,
This is because it is easy to adhere to other objects.

[Brief description of drawings]

FIG. 1A to FIG. 1E are explanatory views of a manufacturing process of a multi-core preform used in an embodiment of a method of manufacturing a multicore optical fiber according to the present invention.

2 (a) and 2 (b) are cross-sectional views of the multi-core optical fiber manufactured in the above embodiment.

FIG. 3 is a cross-sectional view of a conventional optical fiber strand.

FIG. 4 is a cross-sectional view of a conventional tape core wire.

FIG. 5 is a diagram showing two cores (a cross-sectional long / short side ratio of the preform =
It is a figure which shows the relationship between the drawing temperature and the cross-sectional long-short side ratio of the obtained multi-core optical fiber about the case where the 2: 1) multi core preform is drawn.

FIG. 6 is a diagram showing four cores (a cross-sectional long / short side ratio of the preform =
It is a figure which shows the relationship between the drawing temperature and the cross-sectional long-to-short side ratio of the obtained multi-core optical fiber, when the multi-core preform of 4: 1) is drawn.

[Explanation of symbols]

 11 Molded Rubber Mold 12, 13 Metal Lid 12a, 13a Rubber Lid 14 Hole 15 Filling Hole 16 Core Rod 17 Granulated Powder 17a Vitrified Part 18 Porous Molded Body 20 Multicore Optical Fiber 21 Core 22 Cladding 23 Coating Layer

Claims (3)

[Claims] 1. A method for producing a silica glass-based multi-core optical fiber in which a plurality of cores are arranged parallel to each other in a long side direction of a clad having a substantially rectangular cross section, and silica-based powder is provided around a plurality of core rods arranged in parallel. A method for producing a silica glass multi-core optical fiber, which comprises forming a porous molded body by powder molding and then drawing the porous molded body using a preform that has been purified and made into a transparent glass. 2. A plurality of cores are arranged in parallel with a long side direction of a clad having a substantially rectangular cross section in which a ratio of lengths of long sides L _Fl and short sides L _Fs is L _Fl : L _Fs = N: 1. the method of manufacturing a silica glass-based multi-core optical fiber have a line before the substantially rectangular cross section of the long sides of the clad corresponding portion of the preform L _Pl and the ratio of the length of the short side L _Ps L _Pl: L _Ps is L _Pl: L _A method for manufacturing a silica glass multi-core optical fiber, wherein _Ps = N × M: 1 (where M ≧ 1.01). 3. A plurality of cores are arranged parallel to the long side direction of a clad having a substantially rectangular cross-section, wherein the ratio of the lengths of the long side L _Fl and the short side L _Fs is L _Fl : L _Fs = N: 1. In a method for producing a silica glass-based multi-core optical fiber, a porous molded body is molded by powder molding silica-based powder around a plurality of core rods arranged in parallel, and then the porous molded body is purified and a transparent glass is formed. When the ratio L _Pl : L _Ps of the length of the long side L _Pl and the length of the short side L _Ps of the clad equivalent part of the converted preform is L _Pl : L _Ps = N × M: 1 (M ≧ 1.01) A method of manufacturing a silica glass based multi-core optical fiber, characterized in that