JP2530823B2 - Fiber type single mode lightwave circuit element and method of manufacturing the same - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber type single mode lightwave circuit element and a method for manufacturing the same, and more particularly to a fiber having a non-uniform refractive index distribution along the traveling direction of the lightwave. Type single-mode lightwave circuit element used as a single-mode matching device.

[Prior Art] Conventionally, as an optoelectronic circuit element, one having a structure in which the refractive index distribution is uniform in the traveling direction of the light wave has been used, and therefore, a plurality of elements having different materials or geometric structures are used. It has been pointed out that the connection causes a radiation loss due to a mismatch of the electromagnetic field distribution. For example, when an element having a flat electromagnetic field distribution such as a semiconductor laser is connected to an element having a circular electromagnetic field distribution such as an optical fiber, the coupling loss is large.

In addition, when inserting between various optical elements that do not have a waveguiding effect, which are usually used, the spread of the electromagnetic field distribution of the light wave propagating through the fiber is small, so the radiation loss due to diffraction between the fibers is large. . For this reason, a lens has been conventionally used as a coupling element,
It is difficult to design and manufacture a lens system in which the mutual electromagnetic field distributions are matched, and it is not suitable from the viewpoint of miniaturization and stabilization.

With respect to this problem, for an optical integrated circuit type, that is, a planar type optical circuit element, a lens system is used by applying a means of gradually changing the refractive index distribution of the waveguide in the traveling direction of light. Without this, a branch circuit with a small radiation loss, a high-efficiency laser / fiber coupler, a single-mode optical fiber / thin-film optical circuit coupler, etc. has been developed, and its manufacturing technology has also been developed. (JP-A-60-191208). This lightwave circuit element is a glass substrate containing sodium in which potassium ions are diffused at a position where an optical waveguide is to be formed in an optical integrated circuit, a glass film formed on the glass substrate, and diffused in the glass substrate. An optical waveguide is formed by re-diffusing potassium ions into the substrate and the coating.

The optical integrated circuit type optical circuit element has an advantage that the matching of the electromagnetic field distribution with the optical integrated circuit is good and the integrated circuit process technology can be used in the manufacture thereof, but It is difficult to obtain a large waveguide, and since it is an optical integrated circuit, it is necessarily lacking in flexibility.

[Problems to be Solved by the Invention] By the way, the above-mentioned optical circuit of optical integrated circuit type has a problem of radiation loss due to mismatch of electromagnetic field distribution when a plurality of integrated circuit type elements are connected. Although it is a solution for the time being, the mechanical and geometrical compatibility with the single mode optical fiber at the input and output is not necessarily good.

INDUSTRIAL APPLICABILITY The present invention is capable of suppressing not only optical integrated circuit type elements but also radiation loss due to mismatch of electromagnetic field distribution when various optical circuit elements are connected, to a low level. The purpose is to provide a device.

[Means for Solving the Problems] The invention of the present application consists of the following three inventions related to the fiber type single mode lightwave circuit element and four inventions related to the manufacturing method thereof.

A first aspect of the present invention is a fiber-like element including a core region and a cladding region, wherein the total amount of dopant contained in the core region and / or the cladding region in the cross section in order to generate a refractive index distribution is the total length of the device. The present invention relates to a fiber-type single-mode lightwave circuit element, characterized in that the refractive index distribution continuously changes in the axial direction while satisfying the single-mode condition while keeping constant over the entire length.

A second invention is a fiber-like element consisting of a core region and a cladding region, wherein one end or the other end thereof or a refractive index distribution in at least one surface in any cross section between them is non-circular. , While keeping the total amount of the dopant contained in the core region and / or the cladding region in the cross section for generating the refractive index distribution constant over the entire length of the device, while the refractive index distribution satisfies the single mode condition in the axial direction. The present invention relates to a fiber type single mode lightwave circuit element characterized by being continuously changing.

A third aspect of the present invention diffuses a dopant in a core material and / or a cladding material by subjecting a single mode fiber having a uniform refractive index distribution in the axial direction to distributed heating along the axial direction. The present invention relates to a method for manufacturing a fiber type single mode lightwave circuit element, characterized in that the refractive index distribution of the fiber is continuously changed in the axial direction while satisfying the single mode condition.

[Operation] The basic concept of the fiber type single mode lightwave circuit element of the first invention is shown in FIG. In the figure, 1 is a fiber type single mode lightwave circuit element of the present invention, which is configured as a fiber-like element composed of a core region 2 and a cladding region 3. The total amount of the dopant contained in the core region and / or the cladding region in the cross section in order to generate the refractive index distribution is kept constant over the entire device length. Then, as shown in FIGS. 2a to 2c as an example, the refractive index distribution in each cross section continuously changes while satisfying the single mode condition in the axial direction, that is, the dopant distribution also has the total amount in the cross section. The distribution shape is continuously changed in the axial direction while maintaining constant, and the outer shape of the element is formed so as to be maintained substantially constant as a whole.

Here, the single mode condition means that, assuming that the cross section of the core is circular, for example, the radius of the core is a, the refractive indices of the core and the cladding are n ₁ and n ₂ , respectively, and When the wavelength in vacuum is λ, it means that the normalized frequency V defined by the relational expression of V = (2π / λ) a (n ₁ ² −n ₂ ² ) ^1/2 is 2.4 or less.

Therefore, when a light wave having an eigenmode corresponding to the refractive index distribution on the one end 1a side of the element 1 is incident from the one end 1a,
As it propagates through the element, it changes to a dopant distribution, that is, an electromagnetic field distribution corresponding to the refractive index distribution, retains its eigenmode and reaches the other end 1b, and the refractive index distribution in the axial direction changes smoothly. Therefore, the coupling of the light wave propagating through the element to the radiation mode can be suppressed, and the electromagnetic field distribution from one end 1a to the other end 1b can be deformed with low loss.

In addition, "the refractive index distribution is continuously changing" means the first
The present invention is not limited to the taper-like change as shown in the figure, but the point is that a smooth change can be configured while satisfying the single mode condition (hereinafter, the same interpretation will be made).

The basic concept of the fiber type lightwave circuit element of the second invention is shown in FIG. In the figure, 4 is a fiber type single mode lightwave circuit element of the present invention, which is configured as a fiber-shaped element comprising a core layer 5 and a cladding layer 6.

Then, as an example, X _m −X _m and Y _m −Y _{m in} FIG.
Graphing each refractive index distribution of the surface indicated by (m; 1,2,3), the refractive index distribution in the surface of m = 1 has a non-circular shape as shown in Figs. 4a and 5a. On the other hand, the refractive index distribution on the surface of m = 3 has a substantially circular shape as shown in FIGS. 4c and 5c.

The refractive index distribution continuously changes between the surface of m = 1 and the surface of m = 3, and FIGS. 4b and 5b show one cross section in the middle of this continuous change. 3 shows the refractive index distribution of. In the above, one example of the refractive index distribution is a non-circular shape at one end and a substantially circular shape at the other end, but in the present invention, both ends are non-circular shapes,
It also includes one in which both ends are substantially circular, but any cross section is non-circular.

In the device of the present invention as well, the total distribution of the dopant in the cross section is kept constant over the entire length of the device, and the refractive index distribution in the axial direction changes smoothly while satisfying the single mode condition. Can be suppressed from coupling to the radiation mode, and the electromagnetic field distribution from one end to the other can be deformed with low loss. Another advantage of this device is that when various optical circuit devices are connected, it is possible to suppress the radiation loss due to the geometrical mismatch in the input / output parts, that is, the mismatch of the electromagnetic field distribution. For example, when the element to be connected is an optical integrated circuit type element and has a non-circular refractive index distribution, the refractive index distribution at the connection end of the element of the present invention is matched with the refractive index distribution. If so, the radiation loss can be suppressed to a very low level.

The basic concept of the use example or the embodiment of the fiber type single mode lightwave circuit device of the present invention is shown in FIG. In the figure, reference numerals 7 and 8 denote first and second element portions composed of core layers 9a and 9b and cladding layers 10a and 10b, respectively.
The refractive index distribution is continuously changed in the axial direction so that it is sufficiently widened at b. A gap 12 is formed between the cross sections 11a and 11b.

Therefore, the spot size of the propagating light is
12 is sufficiently large, and even when another element having no waveguiding effect is interposed in the gap 12, the loss due to the diffraction of the light wave can be suppressed low.

When an optical element having no waveguiding action is inserted between general optical fibers, a large radiation loss due to diffraction occurs between the fibers because the spread of the electromagnetic field distribution of the light wave propagating through the fibers is small. Since the fiber type single mode lightwave circuit element has the refractive index distribution continuously changed in the axial direction so that the electromagnetic field distribution is sufficiently widened in the cross sections 11a and 11b, the radiation loss in that part is made extremely low. It will be possible.

The basic concept of the method of manufacturing the fiber type single mode lightwave circuit element of the third invention is shown in FIG. In the figure, 13 is a fiber preform having a configuration in which a core preform 14 having a non-circular cross-sectional shape is arranged at the center and a cladding preform 15 is arranged around the core preform 14.

Then, the fiber preform 13 is subjected to drawing heating 16 and drawn to form a single mode fiber 17. This line drawing is a well-known technique. The present invention is characterized in that the drawn fiber is further subjected to distributed heating 18 along the axial direction.

As shown in FIG. 8, the distributed heating 18 diffuses the dopant contained in the core material 14a and / or the cladding material 15a in the vicinity 19 of the boundary surface of the core material 14a and / or the cladding material 15a. It serves to continuously change the refractive index distribution in the cross section in the axial direction. That is, in the distributed heating 18, for the portion heated by a large amount of heat,
Since the diffusion length of the dopant in the radial direction in the cross section becomes longer and the diffusion length in the radial direction in the cross section of the dopan becomes shorter in the portion heated by a small amount of heat, the heating distribution should be controlled appropriately. This makes it possible to change the refractive index distribution continuously along the axial direction of the single mode fiber 17.

The term "distributed heating" used here refers to the single mode fiber 17
Not only when heating with a temperature distribution in the axial direction of
This is a concept including heat treatment for locally heating a predetermined portion of the single mode fiber 17. In addition, the amount of dopant contained in the core material 14a and / or the cladding material 15a is essentially uniform, and therefore the single mode fiber 17
Since the total amount of dopants per unit length in is constant, the single mode fiber 17 after distributed heating 18
In the lightwave circuit element constituted by, the refractive index distribution changes along the axial direction while satisfying the single mode condition.

In the present invention, since it is necessary to draw the fiber preform 13, glass is suitable for each of the preforms 14 and 15, and silver ions, potassium ions or the like can also be used as the dopant for changing the refractive index. But,
In order to obtain a large change in the refractive index, it is desirable to use, for example, thallium ions having a large electronic polarizability.

[Embodiment] Embodiment 1 (corresponding to the first invention) FIG. 9 shows an embodiment of the fiber type single mode lightwave circuit element of the first invention of the present application, and 51 is a core layer. Yes, 52 is a cladding layer. Where both ends 51
The cross-sectional shape of the core layer 51 in a, 51b is substantially circular, and between both ends 51a, 51b, the refractive index distribution has a single mode in the axial direction so that the total amount of dopant in the cross-section is kept constant. It changes continuously while satisfying the conditions. The state of this change is the cross section indicated by Zm-Zm (m; 1,2,3) in FIG. 9, and the electromagnetic field is continuously changed as shown in FIGS. 9a, 9b and 9c, respectively. The distribution is set to widen.

Therefore, the light wave incident from one end 51a retains its eigenmode while suppressing the radiation loss during propagation, and the other end
Reach 51b. Further, the core layer 5 at both ends 51a, 51b is made to correspond to the electromagnetic field distribution of the lightwave circuit element connected to both ends 51a, 51b.
If the cross-sectional shape of 1 is set, the radiation loss due to the mismatch of the electromagnetic field distribution due to the connection can be suppressed, so that it has a function as a matching element.

Embodiment 2 (corresponding to the second invention) FIG. 10 shows an embodiment of the fiber type single mode lightwave circuit element of the second invention of the present application, 61 is a core layer, and 62 is It is a cladding layer.

Here, the one end 61a of the core layer 61 has a substantially rectangular cross section, and the other end 61b has a substantially circular cross section. Therefore, the refractive index distribution is a rectangular distribution about the Z1-Z1 axis at one end 61a as shown in FIG. 11a, and a non-circular distribution about the axis, and the other end 61b at FIG. 11c. The refractive index distribution is circular as shown in. The core layer 6 is provided between the end surfaces 61a and 61b.
The cross-sectional shape of 1 continuously changes from a substantially square shape to a substantially circular shape. The refractive index distribution of the surface indicated by Z2-Z2 in FIG. 10 is shown in FIG. 11b, which shows a transitional step from the rectangular distribution to the circular distribution.

Therefore, when the refractive index distribution of the optical waveguide or the optical element connected to the one end 61a has a substantially rectangular shape and that of the other end 61b has a substantially circular shape, a single mode matching device with low radiation loss is obtained. Can be used.

As shown in FIG. 12 as an embodiment of the second invention, the shape of the core layer 71 is substantially circular at both ends 71a, 71b, and is substantially square at a certain cross section 72 (indicated by Zn-Zn). It is also possible to have a shape. Although this element also has a function as a matching element, a part of the fiber type single mode circuit element in the third invention (corresponding to 7 and 8 in FIG. 6) is obtained by cutting at the cross section 72. Can be used as.

Example of Use FIG. 13 shows an example of use of the fiber type single mode lightwave circuit element of the present invention. In the figure, 81 is a fiber type circuit element, which includes a core layer 82 and a cladding layer.
83 and a gap 84 formed by removing a part of the cross section including the core layer 82 from the element 81.

The end faces 81a and 81b and the end faces 81c and 81d have different refractive index distributions, and the end faces 81a and 81b and the end faces 81c and 81d have continuous refractive index distributions. What is important here is that the refractive index distribution is set so that the electromagnetic field distribution of the eigenmode of the propagating light is sufficiently widened at the end faces 81b and 81c, that is, the spot size of the propagating light is sufficiently large. Is.

By setting in this way, the light wave propagating through the element 81 can propagate with a small diffraction loss from the end face 81b to the end face 81c, even though there is no waveguiding effect in the gap 84 portion. FIG. 14 shows the width of the gap 84 (that is, the length of the interposing material) and the diffraction loss when a material having a refractive index of 2,33 (for example, a magneto-optical crystal for an optical isolator) is interposed in the gap 84. Is obtained for light waves having various spot sizes and a wavelength of 1.3 μm.

Since the spot size is about 5 μm at a wavelength of 1.3 μm in a typical communication single-mode fiber, the gap width must be 40 μm or less to suppress the diffraction loss to 0.1 dB.

However, if the spot size is expanded to, for example, 15 μm using the element 81 of this example, the gap width can be increased to about 400 μm with the same diffraction loss value. Therefore, various elements such as an optical isolator and an optical switch can be inserted in the gap 84.

Example 3 (corresponding to the third invention) FIGS. 15a and 15b show an outline of a manufacturing apparatus for manufacturing the fiber type single mode lightwave circuit element, and 91 is a fiber preform, By being heated by the heater 92, it is drawn and becomes a fiber 93. The produced single mode fiber 93 is further subjected to distributed heating in which the amount of heat changes in the axial direction by the heater 94.

In this example, BK7 glass containing thallium was used for the core and BK7 glass containing no thallium was used for the cladding. This is because the refractive index of glass changes significantly depending on the content of thallium, and the relative refractive index difference is 1.8%.
And the element having a large value of about 2.9% is obtained, and the coefficient of thermal expansion, the transition point, and the yield point are almost the same in the BK7 glass and the BK7 glass containing thallium. This is because it is suitable for line drawing.

FIG. 16 is a cross-sectional view of the fiber preform 91 (15a
It is a sectional view taken along the line AA in the drawing), and is manufactured through the following steps. First, a groove is provided in one of the cladding materials 95 and 96 made of BK7 glass, the cladding materials 95 and 96 are joined, and the mixture is held at a temperature of 640 ° C. for 30 minutes and fused to each other. To produce a cladding preform 98 having a rectangular hole 97. A core preform 99 made of BK7 glass containing thallium and having a rectangular cross section is inserted into the hole 97 via a quartz spacer 100.

The fiber preform 91 thus produced is heated by the heater 92 to be drawn as shown in FIG. 15a, and is softened by heating as shown in FIG. At a portion where the cladding layer 102 is in contact, ion exchange occurs between thallium in the core layer 101 and potassium in the cladding layer 102, and thallium diffuses into the cladding layer 102 as a dopant.

The diffusion at this stage is a drawn single mode fiber 93
Causes a blunted refractive index distribution, but in reality it has the effect of reducing the diffusion length of thallium by drawing, and further suppressing this effect by lowering the drawing temperature and decreasing the diffusion coefficient of thallium. be able to. Next, the drawn single mode fiber 93 is cut into a predetermined length, and the cut single mode fiber 93 is subjected to distributed heating by a heater 94.

That is, at this stage, thallium in the core layer 101 and potassium in the cladding layer 102 are ion-exchanged to obtain a desired element having a different refractive index distribution in each axial cross section. The heater 94 is capable of adjusting the temperature distribution along the length direction of the fiber 93,
It plays a role of controlling the above-mentioned ion exchange.

FIG. 18 shows the relationship between the thallium concentration and the amount of change in the refractive index. The refractive index increases almost in proportion to the concentration of thallium, and the thallium concentration distribution shape corresponds to the refractive index distribution shape. I understand.

Fig. 19 shows a multimode device in which it is easy to measure the dopant distribution due to heating.
The result measured by MA is shown.

Also, FIG. 20 is an EPMA measurement result showing the distribution of thallium after the fiber was held at a temperature of 600 ° C. for 3 hours. As is clear from these figures, the diffusion of thallium is caused by heating, and the distribution of the dopant, that is, the distribution of the refractive index can be controlled by performing the controlled distributed heating on the single mode fiber 93, Single mode fiber
It can be seen that the refractive index distribution in each of the 93 axial cross sections can be varied continuously.

In this way, various fiber-type single-mode lightwave circuit devices described above could be manufactured, and a single-mode device at a wavelength of 1.52 μm, for example, could be manufactured in the same manner.

[Effects of the Invention] As described above, according to the first invention, it is possible to suppress the radiation loss due to the mismatch of the electromagnetic field distribution in the case of connecting the single-mode optical circuit element to a low level. Further, since only the dopant distribution, that is, the refractive index distribution needs to be changed, the outer shape of the element can be made constant.

Further, since it is a fiber type, it is small and lightweight and highly flexible.

Further, according to the second aspect of the invention, it is possible to suppress the radiation loss due to the extreme geometrical mismatch in the input / output section.

According to the third invention, when manufacturing a fiber-type single-mode lightwave circuit element, a single-mode fiber having a uniform refractive index distribution in the axial direction is prepared, and the amount of heat for heating can be controlled. A simple method of controlling the diffusion of the dopant by uniform distributed heating can easily realize the change in the refractive index distribution in the axial direction under the single mode condition.

In general, according to the present invention, there is provided a circuit element most suitable for use as a single mode matching device between a semiconductor laser and an optical fiber or a fiber type single mode functional element having various elements interposed in the fiber. You can

Further, one end of the lightwave circuit element according to the present invention is processed into a lens shape, or a lens is added to provide more functions or characteristics, thereby expanding the present invention. It is possible to do so.

According to the above usage example, even when various elements are interposed in the single-mode lightwave waveguide, the matching can be performed while suppressing the radiation loss, and the lightwave circuit optimal for use as the fiber-type functional element is provided. An element can be provided.

[Brief description of drawings]

FIG. 1 is a view showing the basic concept of the present invention (first invention), and is a perspective view of a fiber type single mode lightwave circuit element. Figures 2a to 2c show the position on the cross section of the fiber type single mode lightwave circuit element on the horizontal axis in the upper figure, the refractive index on the vertical axis, and the fiber type single axis on the vertical axis in the lower figure. 6 is a graph showing the refractive index distribution in each cross section, with the horizontal axis representing the position on the cross section of the one-mode lightwave circuit element, and the refractive index taken along the horizontal axis. FIG. 3 is a diagram showing the basic concept of the present invention (second invention) and is a perspective view of a fiber type single mode lightwave circuit element. 4A to 4C are graphs showing the refractive index distribution in each cross section, with the horizontal axis representing the position of the fiber type single-mode lightwave circuit element on the horizontal cross section and the vertical axis representing the refractive index. . 5a to 5c, the horizontal axis represents the position on the cross section of the fiber type single mode lightwave circuit element, and the horizontal axis represents the refractive index.
It is a graph which showed the refractive index distribution in each cross section. FIG. 6 is a view showing the basic concept of one use example of the present invention, and is a perspective view of a fiber type single mode lightwave circuit element. FIG. 7 is a diagram showing a basic concept of the present invention (third invention), and a diagram showing a manufacturing process of a fiber type single mode lightwave circuit element. FIG. 8 is a perspective view of a single mode fiber in a heating distribution state. FIG. 9 is a perspective view of the fiber type single mode lightwave circuit element of the embodiment. FIGS. 9a to 9c are graphs in which the horizontal axis represents the position on the cross section of the fiber type single-mode lightwave circuit element, and the horizontal axis represents the refractive index, showing the refractive index distribution in each cross section. FIG. 10 is a perspective view of a fiber type single mode lightwave circuit element of an example. FIGS. 11a to 11c are graphs showing the refractive index distribution in each cross section with the horizontal axis representing the position on the horizontal cross section of the fiber type single mode lightwave circuit element and the vertical axis representing the refractive index. FIG. 12 is a perspective view of a fiber type single mode lightwave circuit element of an example. FIG. 13 is a perspective view of a fiber type single mode lightwave circuit element of the example. FIG. 14 is a graph showing the loss due to diffraction on the horizontal axis and the width of the gear axis on the horizontal axis, and showing the diffraction loss due to the difference in the spot size of the light wave. FIG. 15a and FIG. 15b are schematic views of a fiber type single mode lightwave circuit device manufacturing apparatus (a drawing heating unit and a distribution heating unit after drawing, respectively). FIG. 16 is a cross-sectional view of the fiber pre-firm (a view taken along the line AA in FIG. 15a). FIG. 17 is a side view of the fiber pre-firm during drawing. FIG. 18 is a graph showing the relationship between the thallium concentration on the horizontal axis and the refractive index change on the horizontal axis. FIG. 19 is an EPMA measurement graph showing a thallium concentration distribution after drawing a multimode element. FIG. 20 is an EPMA measurement graph when the multimode element after drawing is heat-treated at 600 ° C. for 3 hours to diffuse thallium. (Explanation of symbols) 1 ... Fiber type single mode lightwave circuit element, 1a, 1b ... End face, 2 ... Core layer, 3 ... Cladding layer, 7 ... First element part, 8 ... 2 element part, 9a, 9b ... core layer, 10a, 10b ... cladding layer, 11a, 11b ... cross section, 12 ... gap, 13 ... fiber preform, 14 ... core preform, 14a …… core material, 15 …… cladding preform, 15a …… cladding material, 16 …… heating, 17 …… single mode fiber, 18 …… distributed heating, 19 …… core material and cladding material Near the boundary of.

Continuation of the front page (56) Bibliographic references Sho 48-91948 (JP, U) Japanese Patent Sho 56-7202 (JP, B2)

Claims (9)

(57) [Claims] 1. A fiber-shaped element comprising a core region and a cladding region, wherein the total amount of dopant contained in the core region and / or the cladding region in the cross section in order to generate a refractive index distribution is measured over the entire length of the device. A fiber-type single-mode lightwave circuit element, characterized in that the refractive index distribution continuously changes in the axial direction while satisfying the single-mode condition while keeping constant. 2. The fiber type lightwave circuit element according to claim 1, wherein the refractive index distributions at both ends are substantially circular. 3. A fiber-shaped element comprising a core region and a cladding region, wherein one end or the other end thereof, or at least one surface in an arbitrary cross section between them has a non-circular refractive index distribution, and While keeping the total amount of the dopant contained in the core region and / or the cladding region in the cross section to generate the refractive index distribution constant over the entire length of the device, the refractive index distribution is continuous in the axial direction while satisfying the single mode condition. Fiber-type single-mode lightwave circuit element characterized in that it is changing over time. 4. The refractive index distribution at one end is non-circular and the refractive index distribution at the other end is substantially circular, and the dopant contained in the core region and / or the cladding region in order to generate the refractive index distribution. The fiber according to claim (3), wherein the total amount in the transverse cross section is kept constant over the entire length of the element, and the refractive index distribution between both ends continuously changes while satisfying a single mode condition in the axial direction. Type single mode lightwave circuit element. 5. The refractive index distribution at both ends is non-circular, and the total amount in the cross section of the dopant contained in the core region and / or the cladding region for generating the refractive index distribution is kept constant over the entire length of the device. The fiber type single mode lightwave circuit element according to claim (3), wherein the refractive index distribution between both ends continuously changes while satisfying the single mode condition in the axial direction. 6. The refractive index distribution in an arbitrary cross section is non-circular, and the refractive index distribution at both ends is substantially circular, and is contained in the core region and / or the cladding region to generate the refractive index distribution. Wherein the total amount of the dopant in the cross section is kept constant over the entire length of the device, and the refractive index distribution between both ends and the cross section continuously changes in the axial direction while satisfying the single mode condition. A fiber type single mode lightwave circuit element according to the item (3). 7. A core region and a cladding region, wherein one end of the core layer has a substantially rectangular cross-sectional shape and the other end has a substantially circular cross-sectional shape.
A fiber type single mode lightwave circuit element according to the paragraph. 8. A single-mode fiber having a uniform refractive index distribution in the axial direction is subjected to distributed heating along the axial direction to diffuse the dopant in the core material and / or the cladding material. , A method for manufacturing a fiber type single mode lightwave circuit element, characterized in that the refractive index distribution of the fiber is continuously changed in the axial direction while satisfying the single mode condition. 9. A fiber type single mode lightwave circuit device according to claim 9, wherein the dopant in the core material is thallium ion and the dopant in the cladding material is ion other than thallium ion. Method.