JPH10268153A - Optical fiber type fixed attenuator and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easily manufacturable optical fiber type fixed attenuator having stable quality with the less wavelength dependency of a loss and a method for manufacturing the attenuator. SOLUTION: The optical attenuation zone 21 of double-structured optical attenuation fiber 10 having a first clad part 12 and a second clad part 13 around a core part 11, has a structure of a waveguide optical field expanding to the second clad part 13. Consequently, a part of introduced light is externally diffused through the second clad part 13, thereby functioning as an optical attenuator. The optical attenuation fiber 10 can be manufactured by thermally treating an optical fiber having a refraction factor distribution of a W-type profile, and diffusing a dopant added to the core part 11 for improving a refraction factor, up to the first clad part 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical fiber type fixed attenuator used for adjusting an optical signal level in an optical fiber network.

[0002]

2. Description of the Related Art When constructing an optical communication network, a difference occurs in a light amount level input to a receiving side according to a difference in distance between a transmitting side and a receiving side. If this light amount level difference is large, it will hinder communication. An optical fixed attenuator is used to keep this light level constant.

[0003] As such a fixed optical attenuator, conventionally,
(1) an attenuator with an optical absorption filter inserted between optical fibers, (2) an attenuator with an optical fiber doped with a transition metal that absorbs light into the core, and (3) optical axis misalignment of the optical fiber connection. Attenuators that use coupling loss due to gaps and gaps, (4) attenuators that use splice loss of heterogeneous fibers, and (5) attenuators that use optical axis misalignment loss due to lens optical systems have been used.

FIG. 8 shows a schematic view of the device (1). By inserting a filter 40 that absorbs or reflects a part of the light to attenuate the amount of transmitted light into the optical fiber 30 composed of the core 31 and the clad 32, the amount of light transmitted through the core 31 of the optical fiber 30 is attenuated. ing. Filter 40
The filter 40 is inserted obliquely with respect to the optical axis of the optical fiber 30 to prevent interference due to reflection on the surface. The reflected light is radiated to the cladding part 32 and becomes leaked light, which can attenuate the light.

[0005] (2) is described by Nagase et al. In "Characteristics of SC type optical fixed attenuator" (C-5 Spring Meeting of the Institute of Electronics, Information and Communication Engineers, 1989).
99) and "Reduction of cladding mode of SC-type optical fixed attenuator" (1996 IEICE General Conference C-28)
This is the technique disclosed in 3). FIG. 9 shows a schematic diagram of the structure. The core 31 of the radius a of the optical fiber 30 is doped with a transition metal such as Co, and the cladding 32 is not doped with the transition metal. As a result, the light propagating in the core portion 31 is absorbed by the doped transition metal, so that the amount of light transmitted through the optical fiber 30 is attenuated.

(3) is a method of loss adjustment by fusion splicing by Sasaki et al. (1993 IEICE Autumn Conference B-
790) and Matsuura et al., "Development of a tape-fiber-type optical fixed attenuator" (1995 IEICE General Conference C-23)
This is the technique disclosed in 6). FIG. 10 shows a schematic diagram of each structure. If the connection surfaces of the optical fibers 30 and 35 are shifted as shown in FIG. 7A or if there is a gap between the connection end surfaces of the optical fibers 30 and 35 as shown in FIG. A part of the light transmitted through the core 31 of the optical fiber 30 does not reach the core 36 of the optical fiber 35 on the receiving side and is lost due to scattering or the like. Light can be attenuated using this loss.

[0007] On the other hand, (4) is a technology disclosed in Suzuki et al., "A study on heterogeneous fiber connection" (1993 Autumn Conference of the Institute of Electronics, Information and Communication Engineers, B-789), and is similar to (3). It is. FIG. 11 shows a schematic diagram thereof. By connecting the optical fibers 30 and 35 having different mode field diameters, a part of the light transmitted from the transmission side is radiated to the cladding 37 of the reception side optical fiber 35 and scattered, and does not reach the core 36. Lost to. Light can be attenuated using this loss.

[0008] (5) is "Voltage-controlled variable optical attenuator" by Oguma et al. (1996 IEICE Spring Conference C-
263). FIG. 12 shows a schematic diagram thereof. The light transmitted from the transmission side optical fiber 30 is collected by the lens 41 and reflected by the prism 43,
The light is sent to the optical fiber 35 on the receiving side via the lens 42 on the receiving side. The prism 43 is movable in a direction orthogonal to the optical axis. By adjusting the position of the prism 43,
Only a specific percentage of the light transmitted from the transmitting side can be transmitted to the receiving side. That is, the loss rate can be adjusted.

[0009]

However, these techniques have the following problems, respectively.

According to the technique (1), it is difficult to manufacture a filter 40 having no wavelength dependence, and when the filter 40 is inserted, the optical axes of the input and output optical fibers 30 are aligned accurately. In addition, if the position shift or the angle shift occurs, unnecessary loss may be caused.

On the other hand, in the technique (2), the light propagating in the clad is recombined due to the axial misalignment of the input and output cores and a slight difference in the mode field diameter, and a beat is generated in the wavelength dependence of the attenuation. There is. In addition, in order to cause a loss in a short length, it is necessary to add a high concentration of impurities such as several weight%, but this tends to cause crystallization of the glass, which leads to production of an optical fiber of stable quality. Becomes difficult. Further, as the impurities absorb light, the light energy is converted to heat energy, and the optical fiber generates heat. In particular, in the case of an attenuator that attenuates high-output light at a high attenuation rate, the heat generated may damage the attenuator.

In both of the techniques (3) and (4), since the mode field diameters of the optical fibers 30 and 35 have wavelength dependence, it is difficult to keep the attenuation rate constant over a wide wavelength range. . In general, the mode field diameter increases as the wavelength increases, so that the loss decreases as the wavelength increases. In the case of the heterogeneous fiber connection of (4), the cutoff wavelength (λc
In the vicinity of (wavelength), loss due to mode conversion occurs, and the wavelength dependence of the loss increases.

In the case of (5), there is an advantage that the loss can be varied, but there is a disadvantage that the adjustment of the optical system is difficult, and the number of parts increases, so that the apparatus itself is expensive and complicated.

An object of the present invention is to provide an optical fiber type fixed attenuator of stable quality which has a small wavelength dependence of loss and can be easily manufactured, and a method of manufacturing the same.

[0015]

An optical fiber type fixed attenuator according to the present invention has a double clad portion around a core portion, and a refractive index distribution in an optical axis cross section is inside the double structure. An optical attenuating fiber having a minimum in the first cladding portion and having a guided light field reaching the inside of the second cladding portion outside the double structure in a part of the region divided in the axial direction. And

As a result, the refractive index distribution in the cross section of the optical axis has a so-called W-shaped profile. In the region where the guided light field is enlarged, the mode of guided light disappears, and a part of the guided light becomes leaked light. Function as

Further, the optical attenuation fiber may be sealed in a ferrule fixed in the connector. This places the optical attenuation fiber in the connector.

Alternatively, the optical attenuating fiber and the communication optical fiber connected thereto may be fusion spliced. This implements a fixed attenuator in the communication line.

On the other hand, according to the method of manufacturing an optical fiber type fixed attenuator of the present invention, a dopant for changing a refractive index is added to at least a part of a region divided in a radial direction, and a core having a refractive index N1 is provided. A first cladding portion formed around the core portion and having a lower refractive index N2 than the core portion; and a second cladding portion formed around the first cladding portion and having a higher refractive index N3 than the first cladding portion. By heating a part of the region of the optical fiber divided in the axial direction, the dopant for changing the refractive index is diffused to manufacture the optical attenuation fiber.

Thus, in a part of the region divided in the length direction of the optical fiber, the dopant added to the refractive index added to the fiber diffuses, and the change in the refractive index of the cross section becomes gentle, As a result, the guided light field extends to the second cladding. Therefore, in this region, the mode guided by the light disappears and a part of the light becomes a leaked light, and thus functions as an optical attenuation fiber.

Further, in this optical fiber, the core radius a and the inner radius b of the second cladding portion are such that a ² (N1-N2) ≦
The relationship of b ² (N3-N2) may be satisfied. Thereby, even when the dopant in the core portion is completely diffused, the refractive index of the first cladding portion becomes a minimum value and the refractive index of the second cladding portion exceeds that of the first cladding portion in view of the refractive index distribution of the cross section. .

Alternatively, this optical fiber has a core radius a
Is 2.405 ≧ 2πa (N1 ² −
The relationship of N2 ² ) ^0.5 / λ may be satisfied. Thereby, the refractive index structures of the core portion and the first cladding portion of the optical attenuation fiber satisfy the single mode condition at the used wavelength.

Alternatively, the length of the heated region of the optical fiber may be adjusted to adjust the length of the dopant diffusion region. Thereby, the length of the region where the guided light field is enlarged is adjusted, and the loss value is adjusted.

[0024]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 (a) is a longitudinal sectional view of the optical attenuation fiber, FIG. 1 (b) is a transverse sectional view thereof, and FIG. 1 (c) shows a longitudinal distribution of the outer boundary of the guided light field. Figure, same figure
(d) and (e) are diagrams showing the distribution of the refractive index (solid line) and the guided light field (dashed line) of the cross section at the positions D and E shown in FIGS.

As shown in FIGS. 1 (a) and 1 (b), the silica-based optical attenuation fiber 10 has an outer diameter of 125 μm, and is surrounded by a core portion 11 having an outer radius of 18 μm around a core portion 3.9 having a core radius of 3.9 μm. One clad portion 12 is formed. A second clad portion 13 is further formed around the periphery. The optical attenuating fiber 10 has G, which is a dopant for increasing the refractive index,
eO ₂ and F which is a dopant for lowering the refractive index are added, respectively, and the total amount of the additives in the cross section is:
Although the amount is constant in the axial direction, the cross-sectional distribution continuously changes between the position D and the position E in FIG.

At position D in FIG. 2A, the dopant GeO ₂ for increasing the refractive index exists only in the core portion 11. Then, a dopant F for lowering the refractive index
Exists only in the first cladding portion 12. The second cladding portion 13 does not contain any dopant, and
It consists only of iO ₂ glass. As a result, as shown by the solid line in FIG.
Has the highest refractive index N1, and then the refractive index N3 of the second cladding portion 13 and the refractive index N2 of the first cladding portion 12. Then, the guided light field only spreads over a part of the first cladding part 12 as shown by the broken line in FIG. 4D, and the transmitted light mostly propagates through the core part 11. .

On the other hand, at the position E in FIG. 2A, the dopant for increasing the refractive index exists not only in the core portion 11 but also in the first cladding portion 12. The cross-sectional distribution of the dopant concentration for increasing the refractive index has a maximum value at the center of the core portion, gradually decreases in the radial direction, and is substantially zero outside the boundary between the first cladding portion 12 and the second cladding portion 13. To reach the shape of a chevron. The second cladding 13 has almost no dopant for increasing the refractive index. The total amount of refractive index enhancing dopant in the cross section at position E is
This is the same as the position D, but is characterized in that the concentration of the core portion 11 is lower and the concentration of the first cladding portion 12 is higher than in the case of the position D.

At the position E, the dopant for lowering the refractive index is present not only in the first cladding portion 12 but also in the core portion 11. The cross-sectional distribution of the dopant concentration that lowers the refractive index has the maximum value at the center of the first cladding portion 12 (the middle portion between the interface with the core portion 11 and the interface with the second cladding portion 13). At the center of the first and second cladding portions 13 and 13 is substantially zero outside the boundary between the first and second cladding portions 12 and 13.
Futakoyama shaped shape. The second cladding portion 13 has almost no dopant for lowering the refractive index. Also in this case, similarly to the case of the dopant for increasing the refractive index, the total amount of the dopant for lowering the refractive index in the cross section at the position E is the same as that at the position D, but is different from that at the position D in the core portion 11. Is high, and the concentration of the first cladding portion 12 is low.

As a result, as shown by the solid line in FIG.
, A local minimum (minimum value) N2 ′ near the center of the first cladding portion 12 and the second cladding portion 13,
The maximum value N3 'is taken in the second cladding portion 13.
As a result, the guided light field reaches the second clad portion 13 as shown by the broken line in FIG. As a result, the light guided through the core 11 spreads to the second cladding 13.

FIG. 4C shows the distribution in the length direction of the waveguide light field diameter. In a part of the region divided in the axial direction centered on the position E, as described above, the dopant for increasing the refractive index diffuses to the first clad, while the dopant for decreasing the refractive index diffuses to the core. However, since the respective dopant concentrations in the cross section gradually change, the change in the refractive index in the cross section also becomes gentle, and as a result, the guided light field reaches the second clad portion 13.
This region is hereinafter referred to as a light attenuation region 21.

Next, the operation of the optical attenuation fiber 10 will be described. In the light attenuating region 21, a part of the incident light reaches the second cladding 13 because the guided light field diameter is enlarged. Refractive index N3 'of second cladding 13
Is higher than the refractive index N2 'of the inner first cladding portion 12, the light reaching the second cladding portion 13 leaks to the outside without returning to the inside. Since this leakage becomes a loss, the amount of output light decreases. Therefore, the optical attenuation fiber 10 functions as an optical attenuator.

Subsequently, the optical attenuation fiber 1 of this embodiment
0 will be described. The optical fiber 10 ′, which is a material of the optical attenuation fiber 10, is formed around a core portion 11 having a core radius a of 3.9 μm doped with GeO ₂ as a dopant, as shown in FIGS. , A first clad portion 12 having an outer radius b of 18 μm to which F is added as a dopant and a second clad portion 12 to which no dopant is added are provided around the first clad portion 12. This is a silica-based optical fiber having a diameter of 125 μm. The concentration distribution of each dopant in the optical fiber 10 'is uniform in the axial direction and has a cross-sectional refractive index distribution structure as shown in FIG. The relative refractive index difference between the core portion 11 and the second cladding portion 13 with respect to the first cladding portion is 0.36% for n1-n2 and 0.10% for n3-n2, respectively. Here, n1, n2, and n3 are the core 11, the first clad 12, and the second clad 1, respectively.
3 is n1 = N by the respective refractive indices N1, N2, N3.
1 / N2, n2 = N2 / N2 = 1, and n3 = N3 / N2.

As apparent from FIGS. 2 and 1 (d), the cross-sectional distribution of the refractive index of the optical fiber 10 'is the same as the position D except for the optical attenuation region 21 of the optical attenuation fiber 10 to be produced. It has a cross-sectional distribution. Therefore, the guided light field of the optical fiber 10 'is limited to the vicinity of the interface between the core 11 and the first clad 12, as shown by the broken line in FIG.

After removing the coating of the optical fiber 10 ', a heating treatment is performed using a small burner having a heating length of 5 mm. By this heat treatment, the dopant GeO ₂ added into the core portion 11 diffuses into the first cladding portion 12, while the dopant F added into the first cladding portion 12 mainly diffuses into the core portion 11. As a result of this processing, the refractive index distribution of the optical fiber 10 ′ in the heated region 21 shows that the refractive index of the core 11 decreases while the refractive index of the first cladding 12 decreases, as shown by the solid line in FIG. The refractive index in the vicinity of the core portion 11 in the inside increases and changes continuously. As a result, the intensity of the central portion of the guided light field also decreases as shown by the broken line in FIG. 9E, while the skirt expands in the radial direction to reach the second clad portion 13. Thus, the optical attenuating fiber 10 in which the guided light field of the part 21 is expanded to the second clad portion 13 is obtained.

FIG. 3 shows the change of the loss value with respect to the heating time during the heating process of the optical fiber 10 '. Here, a loss value for input light having a wavelength of 1.55 μm used for communication is shown. The loss value increases almost linearly with respect to the heating time until the heating time reaches about 800 seconds, but thereafter, the loss value gradually increases, and the heating time increases by 100 hours.
Beyond 0 seconds, the loss value is approximately constant at about 3.0 dB. Light attenuating fiber 10 obtained by heating for about 1400 seconds
FIG. 4 shows the wavelength dependence of the loss value of. Loss values for light having wavelengths of 1.3 μm and 1.55 μm used for communication are 2.28 dB and 3.17 dB, respectively, and the wavelength dependence is small.

On the other hand, for comparison with this embodiment, an experiment for heating a single step index type optical fiber was also conducted. FIG. 5 shows the refractive index distribution of the cross section of the optical fiber. The outer radius a of the core portion 11 of the fiber is 3.9 μm, which is the same as that of the fiber shown in FIG. 2, and the outer diameter of the fiber is also the same, 125 μm. The refractive index distribution is such that the relative refractive index difference n1-n2 is 0.36%, which is also the same as the fiber shown in FIG. 2, except that the cladding is of a single-structure single-step index type. When this fiber was subjected to a heat treatment under the same conditions as the above-described optical fiber 10 ', no increase in loss was observed regardless of the heating time. This is probably because even when the dopant in the core part diffuses into the clad part due to the heating, the refractive index distribution changes only from the step index type to the graded index type, and there is no large difference in the loss characteristics.

The optical attenuating fiber 10 of the present invention comprises the core 1
It is desirable that the refractive index structures of the first and first cladding portions 12 satisfy the single mode condition at the used wavelength. If the single mode condition is not satisfied, part of the leaked light is
This is because there is a possibility that the attenuation rate may deteriorate due to recombination at 1. Since the rate at which recombination of leaked light occurs varies depending on the wavelength, there is a problem that the occurrence of recombination increases the wavelength dependence of the attenuation rate. In order for the optical attenuation fiber 10 to satisfy the single mode condition, the optical fiber 1 as a material must be used.
At 0 ′, the relationship of 2.405 ≧ 2πa (N1 ² −N2 ² ) ^0.5 / λ may be satisfied.

The core portion 11 of the optical fiber as a raw material
Of the refractive index N1 of the first cladding portion 12 relative to the refractive index N2 of the first cladding portion 12 is the refractive index N1 of the first cladding portion 12.
2 is larger than the sum of the drops in the refractive index N3 of the second cladding portion 13, the dopant for increasing the refractive index of the core portion 11 is completely diffused into the first cladding portion 12 (or / and, together with the first cladding portion 12). When the high refractive index portion of the core portion 11 fills the low refractive index portion of the first cladding portion 12, the core becomes less dense. Part 1
Since the refractive indices of the first and first cladding portions 12 are higher than the refractive indices of the surrounding second cladding portions 13, loss may not occur. Therefore, in order to reliably manufacture the optical attenuation fiber 10 having a radiation mode, it is necessary to satisfy the relationship of a ² (N1−N2) ≦ b ² (N3−N2). Even if this relationship is not satisfied, dopant diffusion to the first cladding is suppressed, and if a part of the first cladding has a refractive index lower than that of the second cladding, it functions as an attenuator.

First clad portion 12 and second clad portion 13
As the difference in the refractive index is larger, the amount of leakage light increases and the attenuation rate can be increased. Further, the loss can also be adjusted by changing the length of the light attenuation region 21 where the leak light is generated. This adjustment can be easily performed, for example, by performing a heat treatment while moving a burner while transmitting light to an optical fiber serving as a raw material, and ending the heat treatment when a predetermined loss value is reached. It is possible to create an optically attenuating fiber having an attenuation factor.

In the optical fiber type fixed attenuator of the present embodiment, the light that is lost leaks to the second clad portion 13 having a much larger area than the core portion 11, so that the light density becomes extremely low. For this reason, when compared with an attenuator using light absorption, the threshold value for the amount of incident light can be set higher, and it can be used as an attenuator for high-output light.

As an embodiment of mounting this optical fiber type fixed attenuator in a communication line, as shown in FIG. 6, the optical attenuating fiber 10 is directly fusion-spliced with the input / output communication optical fiber 25, and FIG. As shown in FIG. 7, after fixing the optical attenuation fiber 10 in the ferrule 26, the ferrule 26 can be mounted on a connector and connected to an input / output optical fiber.

[0043]

As described above, according to the optical fiber type fixed attenuator of the present invention, a part of the guided light is radiated to the second clad and leaks, so that it functions as an attenuator.
Further, since the leaked light only emits, there is no deterioration of the optical power, and a stable output light can be obtained. Furthermore, since the light density of the leaked light is low, the threshold value for the amount of incident light can be set higher than that of an attenuator using light absorption, even when high-output input light is attenuated at a high attenuation rate. Can be used.

Furthermore, if the optical attenuating fiber satisfies the single mode condition, recombination of the leaked light at the core is prevented, and the wavelength dependence of the attenuation rate is suppressed.

Further, by decreasing the maximum refractive index of the core portion from the maximum refractive index of the second cladding portion, the amount of leaked light can be increased and the attenuation factor can be increased.

If the optical attenuating fiber is fixed in the ferrule, this ferrule can be mounted in the connector and easily mounted on the optical communication line.

Alternatively, it can be mounted on an optical communication line by fusion splicing with a communication optical fiber.

According to the method for manufacturing an optical attenuating fiber of the present invention, an optical attenuating fiber in which a guided light field is extended to the second clad by heat treatment can be easily manufactured.

If an optical fiber satisfying a predetermined condition is used, the optical attenuation fiber can be manufactured so as to satisfy the single mode condition.

When the fiber shape and the refractive index of the optical fiber as a raw material satisfy predetermined conditions, the optical fiber has a portion where the waveguide structure cannot be reliably maintained, and loss due to leakage always occurs.

The attenuation factor of the optical attenuation fiber can be adjusted by adjusting the length of the heating zone. This adjustment can be precisely performed by performing heat treatment while measuring the attenuation factor of transmitted light.

[Brief description of the drawings]

FIG. 1 is a diagram showing a sectional configuration diagram, a refractive index distribution, and a guided light field distribution of an optical attenuation fiber according to the present invention.

FIG. 2 is a diagram showing a refractive index distribution of an optical fiber used as a raw material for producing the optical attenuation fiber of FIG.

FIG. 3 is a diagram illustrating a relationship between a heating time and a loss rate of the optical attenuation fiber according to FIG. 1;

FIG. 4 is a diagram illustrating a wavelength loss characteristic of the optical attenuation fiber according to FIG. 1;

FIG. 5 is a diagram showing a refractive index distribution of a single-step optical fiber used for comparison with the optical fiber according to FIG. 2;

FIG. 6 is a first implementation example of the optical attenuation fiber according to FIG. 1;

FIG. 7 is a second example of mounting the optical attenuation fiber according to FIG. 1;

FIG. 8 is a schematic structural diagram of a conventional fixed attenuator using a light absorption filter.

FIG. 9 is a schematic structural diagram of a conventional fixed attenuator in which a transition metal is added to a core.

FIG. 10 is a schematic structural view of a conventional fixed attenuator using an optical axis shift and a gap loss.

FIG. 11 is a schematic structural view of a conventional fixed attenuator to which optical fibers having different MFDs are connected.

FIG. 12 is a schematic structural diagram of a conventional fixed attenuator using an optical system.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Light attenuation fiber, 11 ... Core part, 12 ... First clad part, 13 ... Second clad part, 21 ... Light attenuation area, 2
5 communication optical fiber, 26 ferrule, 30 optical fiber, 31 core, 32 clad, 35 optical fiber, 36 core, 37 clad, 40 light absorption filter, 41, 42 lens, 43 prism.

Claims (7)

[Claims] 1. A core having a clad portion having a double structure around a core portion, wherein a refractive index distribution of an arbitrary cross section is minimized in a first clad portion inside the double structure. An optical fiber-type fixed attenuator, comprising an optical attenuating fiber in which a guided optical field reaches a second clad portion outside the double structure in a part of the divided region. 2. The optical fiber-type fixed attenuator according to claim 1, wherein said optical attenuation fiber is sealed in a ferrule fixed in a connector. 3. The optical fiber-type fixed attenuator according to claim 1, wherein the optical attenuating fiber and the communication optical fiber connected thereto are fusion-spliced. 4. A dopant for changing a refractive index is added to at least a part of a region divided in a radial direction, and is formed around a core portion having a refractive index N1 and around the core portion, and The optical fiber is divided in an axial direction of an optical fiber including a first cladding portion having a low refractive index N2 and a second cladding portion formed around the first cladding portion and having a higher refractive index N3 than the second cladding portion. A method of manufacturing an optical fiber-type fixed attenuator, comprising heating the partial region to diffuse the dopant for changing the refractive index to manufacture an optical attenuation fiber. 5. The optical fiber according to claim 1, wherein the core radius is a,
When the outer radius of the cladding is b, a ² (N1-N
5. The method according to claim 4, wherein a relationship of 2) ≦ b ² (N3−N2) is satisfied. 6. The optical fiber according to claim 4, wherein a core radius a satisfies a relationship of 2.405 ≧ 2πa (N1 ² −N2 ² ) ^0.5 / λ with respect to a used wavelength λ. A manufacturing method of the optical fiber type fixed attenuator according to the above. 7. The method according to claim 4, wherein the length of the heated region of the optical fiber is adjusted to adjust the length of the dopant diffusion region.