JP2003238192A - Method for producing gradient index lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a gradient index lens, whereby gradient index lenses having excellent properties can be produced in a improved yield and productivity as to reduce a manufacturing cost. <P>SOLUTION: The method produces a gradient index lens which is based on quartz glass and has a gradient index that the refractive index gradually increases toward the direction from the periphery to the inside by the addition of a dopant, wherein a preform of a gradient index lens is heat-treated to thermally diffuse the dopant into the preform. In this way, the flattening and elimination of disturbances such as center dip 1 and ripple dip 2, in the index gradient prevents deterioration in the characteristics of the gradient index lens. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a method of manufacturing a gradient index lens used in an optical fiber collimator or the like in the field of optical communication.

[0002]

2. Description of the Related Art Conventionally, in the field of optical communication, signal light propagating through an optical fiber is taken out as parallel light outside the optical fiber, or conversely, the parallel light is focused on the end face of the optical fiber and An optical fiber collimator is used to make the light incident on the optical fiber.

This kind of optical fiber collimator is provided with a collimator lens in order to change the traveling direction of light as described above. As the collimator lens, lenses of various shapes are used, but since polishing processing at the time of manufacturing and assembling of an optical system such as a collimator are relatively easy as compared with spherical lenses and aspherical lenses, A cylindrical gradient index lens (also called a rod lens or a GRIN lens) is often used.

The refractive index distribution type lens has a function as a lens by gradually changing the refractive index inside the lens so that it becomes larger as it gets closer to the optical axis of the lens and becomes smaller as it gets away from the optical axis and gets closer to the outer circumference. This is because they are distributed. This refractive index distribution can usually be approximately expressed by the following equation (1).

[0005]

[Equation 1]

However, in the equation (1), n (r) is the refractive index, n ₀ is the refractive index on the optical axis, g is the magnitude of the refractive index change (where g> 0), and r is from the optical axis. Distance, h ₄ ,
h _6, ... is the coefficient of the higher-order terms.

In order to manufacture such a gradient index lens, in general, first, a rod-shaped preform having a refractive index distribution in which the refractive index gradually increases from the outer circumference toward the inside is manufactured. Then, a method is employed in which the preform is heated and melted, drawn to a diameter of a lens, cut into a predetermined length, and the end face is polished.

As a method for producing a preform having a refractive index distribution, for example, the following ion exchange method is known. According to the ion exchange method, first, a glass rod made of a multi-component glass doped with a component that increases the refractive index of thallium or the like is produced, and the glass rod is immersed in a molten salt such as sodium or potassium, A preform having a refractive index distribution can be manufactured by exchanging ions in the outer peripheral portion of the rod to reduce the concentration of thallium and reduce the refractive index in the outer peripheral portion.

Further, as disclosed in Japanese Patent Application No. 2001-104929, a PCVD method, an MCVD method or the like is used to change the composition of a material gas such as SiCl ₄ , GeCl ₄ or the like while the quartz tube Glass soot is deposited in layers inside, and then the quartz tube on which this glass soot is deposited is heated and collapsed (solidified) to form a transparent vitrification, which is mainly composed of quartz glass and germanium (Ge). There is a method of obtaining a preform in which the concentration of a dopant such as is gradually changed. In particular, the gradient index lens made of silica-based glass manufactured by using the latter manufacturing method can be directly fused and connected to the silica-based optical fiber, resulting in splice loss and reflection loss at the end face. It has the advantage that it can be reduced.

[0010]

However, when manufacturing the above-mentioned gradient index lens made of quartz glass, it is difficult to form the refractive index profile of the preform of the gradient index lens to be smooth. There is a problem.
When the preform is manufactured by the PCVD method or the MCVD method, for example, as shown in FIG. 1A, when the quartz tube is collapsed, a dopant such as germanium near the center is volatilized as GeO ₂ etc. A drop (center dip) 1 of the refractive index inevitably occurs near the center of the refractive index distribution of. Further, the refractive index of the glass soot deposited on the inner side of the quartz tube becomes non-uniform for each layer, and the refractive index disorder (ripple dip) 2 may occur around the center dip 1.

As described above, if the refractive index distribution of the preform is not smooth, the optical characteristics of the gradient index lens obtained from the preform may be extremely deteriorated and the product may not be used as a product. For this reason, the gradient index lens is inspected after manufacturing, and the lens whose optical characteristics have passed the standard is selected, but this reduces the yield of the gradient index lens and increases the manufacturing cost. become.

The present invention has been made in view of the above circumstances, and it is possible to easily manufacture a gradient index lens having excellent characteristics, improve yield and productivity,
An object of the present invention is to provide a method of manufacturing a gradient index lens that can significantly reduce the manufacturing cost.

[0013]

[Means for Solving the Problems] The above-mentioned problem is to form a preform having a refractive index distribution whose main component is silica glass and whose refractive index gradually increases from the outer periphery toward the inner portion by adding a dopant. After that, this preform is heat-treated to thermally diffuse the dopant in the preform to produce a gradient index lens. By thermally diffusing the dopant, the refractive index profile of the preform is, for example, as shown in FIG.
Since the center dip and the ripple dip are smoothed as shown in (b), the characteristics of the gradient index lens obtained from the preform can be remarkably improved.

The smaller the diameter of the preform, the shorter the moving distance required for the dopant to be thermally diffused to remove the center dip and the ripple dip, and the heat diffusion process time can be shortened. Therefore, the thermal diffusion treatment of the dopant is preferably performed after drawing the preform. At this time, in order to suppress thermal deformation of the preform, the heating temperature is preferably 1400 ° C. or lower.

Further, the heat diffusion treatment can be performed while drawing the preform by utilizing the heat when drawing the preform. According to this method, the preform is pulled by the drawing force, and thermal deformation is prevented. Therefore, as compared with the case where the thermal diffusion treatment is performed after the drawing, the heating temperature can be increased and the time of the thermal diffusion treatment can be further shortened. In this case, the heating temperature is 1800
˜2500 ° C., and drawing speed is preferably 0.5 to 10 m / min.

In this way, the dopant is thermally diffused,
The gradient index lens having a smoothed refractive index distribution has excellent optical characteristics.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail based on the embodiments. According to the first embodiment of the present invention,
The gradient index lens can be manufactured by the following procedure. First, a transparent rod-shaped preform is manufactured using a technique such as PCVD and MCVD. Then, the preform is melted by heating and drawn to a diameter of a lens. Then, the preform after drawing is heat-treated to thermally diffuse the dopant and smooth the refractive index distribution. Further, by cutting this preform into a predetermined length and finishing such as polishing the end face, a gradient index lens can be obtained.

Here, the diameter of the gradient index lens is not particularly limited, but is 125 at the end surface of the gradient index lens.
In order to be able to connect one optical fiber having a diameter of μm, it is preferable that the diameter is at least about 135 μm or more. As the upper limit of the diameter of the gradient index lens, at least the diameter of the gradient index lens is 1.9 mm,
Even when two optical fiber core wires having a coating diameter of 0.9 mm are connected to one end surface of the gradient index lens, the optical fiber core wires can be connected with a sufficient margin without forcibly bending the optical fiber core wires. That is, since it is not necessary for the diameter of the gradient index lens to exceed 1.9 mm, 1.9 m
It is preferably m or less. Further, it is generally preferable that the length of the gradient index lens is short in order to reduce the size of optical components such as a collimator, but if it is too short, processing and handling become difficult. Therefore, it is preferably 1 mm or more.

In the present embodiment, first, the preform is drawn, and then the preform is heat treated to thermally diffuse the dopant, thereby smoothing the refractive index distribution of the preform. The reason why the thermal diffusion treatment is performed on the preform after drawing as described above is as follows.

Generally, the dopants such as germanium (Ge), fluorine (F) and boron (B) in the quartz glass are sodium (Na), potassium (K), thallium (Tl) and the like in the multi-component glass. It is more stable than monovalent modification ions and is less likely to diffuse heat. However, it is known that heat can be diffused by heating to a sufficiently high temperature. The change in the concentration distribution of the dopant due to thermal diffusion can be expressed by the diffusion equation in cylindrical coordinates shown in the following equation (2).

[0021]

[Equation 2]

In the equation (2), u is the concentration distribution of the dopant, which is a function of the radius r and the time t. Also,
The diffusion constant D is expressed as a function of T (K) in absolute temperature as shown in the following expression (3), and changes exponentially with respect to 1 / T.

[0023]

[Equation 3]

In the equation (3), D ₀ (m ² / s) is a coefficient, Q (J / mol) is an activation energy,
R has a gas constant of 8.31 J / K / mol. D ₀ and Q can be regarded as substantially constant when the variation in the components in the glass forming the preform is small.

Now, the relationship between the diameter of the preform and the diffusion rate will be considered. When the diameter of the preform becomes C times, r in the diffusion equation of the equation (2) is replaced with C · r and further rewritten as shown in the equation (4).

[0026]

[Equation 4]

Comparing equations (2) and (4), it can be seen that the temporal change ∂u / ∂t of the concentration of the dopant is when the diameter of the preform is C times (C is a positive arbitrary constant). , C ^-2 times. Therefore, it can be seen that the time required for the thermal diffusion treatment is proportional to the square of the reciprocal of the diameter of the preform.

Based on the diffusion equation of equation (2), the relationship between the diameter of the preform and the time required for the thermal diffusion treatment was calculated using a numerical solution method. Here, as the initial value of the concentration distribution of the dopant, an average value in the preform obtained by the conventional manufacturing method was used as an example. The result is shown in FIG. According to the results shown in FIG. 2, for example,
When the diameter of the preform is 20 mm, the heat diffusion treatment is required at 1200 ° C. for 8000 hours, whereas when the diameter of the preform is 0.5 mm, the heat diffusion treatment is 1200.
The thermal diffusion treatment will be completed in about 5 hours at ℃. Therefore, when the thermal diffusion treatment is performed on the preform after drawing, the treatment time can be significantly shortened as compared with the case where the thermal diffusion treatment is performed on the preform before drawing.

The diameter of the preform during heat diffusion is
The smaller the size, the shorter the time required for heat diffusion, and therefore more preferable. However, if the diameter is reduced to be smaller than the diameter of the target gradient index lens, it is difficult to increase the diameter of the preform after thermal diffusion. Therefore, it is preferable to set the diameter to about the diameter of the gradient index lens.

The thermal diffusion treatment can be carried out by heating the drawn preform by an appropriate heating means such as an oxyhydrogen flame burner or an electric furnace. Regarding the heating temperature, if the heating temperature is too high, the preform may be thermally deformed and may not be processed into a lens. Therefore, the heating temperature is preferably 1400 ° C. or lower. Further, the lower limit of the heating temperature is not particularly limited, but if the temperature is too low, the time required for the heat diffusion treatment becomes long, so the temperature is set to 800 ° C. or higher so that the heat diffusion treatment can be completed within about 20 hours. It is preferable.

When heating using a heating furnace such as an electric furnace, for example, the preform after drawing is 0.1 to 1.0.
By cutting it to a length of about m and aligning it on a V-groove plate made of stackable heat-resistant ceramics etc., it becomes possible to put the preform after drawing into the furnace at a high density and handleability. Can also be improved. Then, the heat diffusion treatment can be carried out by placing it in a heating furnace and heating it under predetermined heating conditions.

As a burner for heating with a burner,
It is not necessary to use a special burner, and for example, a general oxyhydrogen flame burner such as used in manufacturing a fusion splicing type optical fiber coupler can be used. As shown in FIG. 3, the preform 11 after drawing is
By cutting to a length of about 60 mm, grasping both ends with appropriate grasping means, rotating the preform 11 axially, and heating while adjusting the relative distance between the burner 12 and the preform 11 after drawing, It can be heated uniformly. During this heating, tension is applied so that the preform 11 is not bent by its own weight. The applied tension varies depending on conditions such as the diameter and length of the preform 11, the heating temperature and the heating time, but it is particularly preferable to control the magnitude of the tension so that the preform 11 is not stretched during heating.

The method for producing the preform is not particularly limited, and various chemical vapor deposition methods generally used for producing silica optical fibers can be used. That is, the refractive index distribution can be formed inside the preform by changing the concentration distribution of the dopant using a method such as the VAD method, the OVD method, the MCVD method, and the PCVD method. Among these methods, it is preferable to use the MCVD method or the PCVD method because the mixing of water in the preform can be suppressed. In particular, it is preferable to use the PCVD method because the refractive index controllability is good.

An example of a method of manufacturing a preform by the PCVD method will be described below. A quartz tube is used as a starting tube, and SiCl, which is a raw material for quartz glass, is placed in the tube.
₄ , GeCl ₄ which is a dopant for changing the refractive index of quartz glass, and oxygen (O ₂ ) which is an oxidant are fed, and glass fine particles are synthesized by a gas phase oxidation reaction using plasma. Glass fine particles are deposited on the inner wall of the quartz tube.

At this time, in order to deposit the glass fine particles evenly in the circumferential direction of the quartz tube, the quartz tube is rotated around the axis. Further, by moving the plasma in the longitudinal direction of the quartz tube, glass particles are evenly deposited in the longitudinal direction of the quartz tube. Then, it is possible to form a refractive index distribution in the radial direction by repeating this deposition operation while changing the refractive index little by little for each layer.
Specifically, the refractive index of each layer can be changed by changing the ratio of the SiCl ₄ gas and the dopant gas for each layer.

After the deposition, when the quartz tube on which the glass particles are deposited is heated to about 2000 ° C., the viscosity of the glass particles and the quartz tube is reduced, the gap is crushed and solidified, and a transparent rod-shaped preform is obtained. To be This process is called collapse. Further, it is heated and melted in an electric furnace or the like, and then drawn to obtain a desired diameter. After thermal diffusion treatment of the preform after drawing,
A gradient index lens can be obtained by cutting it into a predetermined length and polishing the end face by an appropriate method.

Next, a second embodiment of the present invention will be described. This embodiment, when drawing the preform,
The thermal diffusion process is performed at the same time. That is, conventionally,
When drawing the preform, the preform is heated and melted. However, by appropriately setting the heating temperature and the drawing speed at this time, the thermal diffusion treatment can be performed. Specifically, for example, the heating temperature is 1800 to 250.
By setting the drawing temperature to 0 ° C. and the drawing speed to 0.5 to 10 m / min, it is possible to sufficiently heat the melted portion and perform the thermal diffusion treatment. The diameter of the target gradient index lens is 135 μm or more, as in the first embodiment.
It is preferably 1.9 mm or less.

The preform manufacturing method used in this embodiment is similar to the first embodiment in that the PCV is used.
It can be manufactured by using the D method, the MCVD method, or the like.
As a device used for drawing the preform, a device similar to a conventionally used device such as an electric furnace can be used. Therefore, the manufacturing method according to the present embodiment can be easily performed without increasing the number of steps for heat diffusion treatment or adding a new device, and is excellent in mass productivity. Also, since the thermal diffusion treatment is performed while drawing the preform, there is no risk of thermal deformation of the preform, and the heating temperature can be raised sufficiently to shorten the thermal diffusion treatment time remarkably. The preform after drawing can be made into a gradient index lens by cutting it to the length of the lens and polishing the end face.

Although the method of manufacturing the gradient index lens according to the present invention has been described above based on the preferred embodiments, the present invention is not limited to the above embodiments and various modifications can be made. Needless to say. For example, when the diameter of the target gradient index lens is extremely small, according to the first embodiment, in the method of performing the thermal diffusion treatment after drawing the preform to the diameter of the gradient index lens, In some cases, the preform is likely to be thermally deformed, which may be inconvenient. In that case, for example,
It is also possible to adopt a method of performing the drawing step twice before and after the thermal diffusion treatment. That is, first, the wire is drawn to about 2 to 5 mm so as to be suitable for the heat diffusion process, the heat diffusion process is performed, and then the wire is drawn to a predetermined lens diameter.

Since the gradient index lens obtained by the present invention is made of silica glass, it can be directly fused and connected to the silica optical fiber to reduce the connection loss and the reflection loss at the end face. be able to. Moreover, since the refractive index distribution is smoothed by thermally diffusing the dopant, the optical characteristics such as insertion loss are significantly improved as compared with the conventional one.

That is, the silica-based glass has an extremely low light absorption rate as compared with the multi-component glass, and the loss of the multi-component glass is 0.02 dB / cm, whereas the loss of the silica-based glass is 0. It is as small as 0.00002 dB / cm.
From this fact, it is expected that an extremely low loss gradient index lens can be obtained by using quartz glass. However, according to the conventional manufacturing method, a refractive index profile lens such as a center dip can be obtained. Disturbance could not be prevented, the insertion loss was about 0.2 dB, and the insertion loss could not be reduced to an expected level.

According to the present invention, by smoothing the refractive index distribution, it is possible to greatly reduce the scattering of light and aberrations in the lens, so that the insertion loss of the gradient index lens is about 0.
It is possible to reduce it to 1 dB or less. Conventionally, as a gradient index lens of this type, there is one manufactured by an ion exchange method. Using the ion exchange method has the advantage that ripple dip does not occur inside the lens,
It is said that a lens having a small loss of 1 mm or less makes it difficult to control the refractive index distribution, and a lens with low loss cannot be obtained.

Therefore, a gradient index lens having an outer diameter of 1 mm or less and an insertion loss of about 0.1 dB or less is
This has not been achieved hitherto, and the present invention enables production for the first time. Moreover, the extremely small loss means that
This means that the light absorptivity is extremely low, and even when high-intensity light is incident, the temperature rise due to absorption of the energy of the incident light is reduced, deterioration of optical characteristics is suppressed, and long-term reliability is also improved. It is considered to be a thing.

The gradient index lens of the present invention can be used as an optical fiber collimator by connecting an optical fiber to the end face. Further, two such optical fiber collimators are opposed to each other, and an optical functional element such as a dielectric multilayer film filter element, an isolator element, a circulator element, an amplifier gain equalizer (gain equalization element) is inserted between them. It is possible to manufacture an optical component having low loss and good characteristics.

Next, the present invention will be described with reference to specific examples. First, a preform was manufactured by the PCVD method, and drawn to a diameter of 0.5 mm with an electric furnace. In the gradient index lens of the example, the preform after drawing was placed in an electric furnace for 1
It was manufactured by performing thermal diffusion treatment at 200 ° C. for 10 hours, cutting it to a length of about 2 mm, and polishing the end faces.
Further, the gradient index lens of the conventional example was manufactured by cutting the lens to a length of about 2 mm and polishing the end face without performing heat diffusion treatment.

In order to evaluate the characteristics of the obtained gradient index lens, the characteristics were evaluated by the following methods. First, two gradient index lenses are arranged to face each other with an interval of 5 mm, and a light source is connected to the first gradient index lens via a first single mode optical fiber,
A power meter was connected to the second gradient index lens via a second single mode optical fiber. The first
As the second single mode optical fiber, a fiber having a fiber diameter of 125 μm and a mode field diameter of 10 μm was used. Then, the optical power P ₁ propagating through the _first single-mode optical fiber and the optical power P ₂ propagating through the second single-mode optical fiber are measured, and the insertion loss of each gradient index lens is determined to be −10 log ( P ₂ / P ₁ )
Sought as.

In the gradient index lens of the conventional example, the insertion loss was about 0.25 dB. On the other hand, in the gradient index lens of the example, it was 0.08 dB. From this result, it is understood that the insertion loss of the gradient index lens can be significantly reduced by performing the thermal diffusion treatment of the preform.

[0048]

As described above, according to the present invention,
By the thermal diffusion treatment, the refractive index distribution of the gradient index lens can be smoothed, and the optical characteristics such as insertion loss can be greatly improved as compared with the conventional case. Further, it is possible to easily obtain a gradient index lens having excellent characteristics, significantly improve the yield and the productivity, and reduce the manufacturing cost.

By performing the heat diffusion treatment after drawing the preform, the time of the heat diffusion treatment can be greatly shortened. Further, by performing the thermal diffusion treatment by using the heating when drawing the preform, the preform is pulled by the force of drawing and thermal deformation is prevented, so compared with the case of heating after drawing. Therefore, the heating temperature can be raised to further shorten the time required for the heat diffusion treatment. Moreover, since it does not increase the number of steps and does not need to add a new device, it can be easily carried out,
Excellent mass productivity. By using the gradient index lens obtained by the present invention, it is possible to easily manufacture an optical fiber collimator or an optical functional component having excellent characteristics.

[Brief description of drawings]

FIG. 1A is a graph showing an example of the refractive index distribution of a gradient index lens before the thermal diffusion process. (B) A graph showing an example of the refractive index distribution of the gradient index lens after the thermal diffusion treatment.

FIG. 2 is a graph showing an example of the relationship between the diameter of the preform and the time required for the thermal diffusion treatment.

FIG. 3 is a diagram showing an example of an embodiment of the present invention.

[Explanation of symbols]

11 ... Preform, 12 ... Burner.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Katsutoshi Komoto             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office (72) Inventor Yoichi Kurumiya             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office (72) Inventor Hideyuki Hosoya             Fuji Co., Ltd. 1440 Rokuzaki, Sakura City, Chiba Prefecture             Kura Sakura Office F-term (reference) 4G021 EA03 HA01

Claims (6)

[Claims] 1. A preform containing silica glass as a main component and having a refractive index distribution in which a refractive index gradually increases from an outer circumference toward an inner portion by adding a dopant, and then the preform is heat treated. do it,
A method of manufacturing a gradient index lens, which comprises thermally diffusing a dopant in the preform. 2. A preform having silica glass as a main component and having a refractive index distribution in which the refractive index gradually increases from the outer circumference toward the inner portion by adding a dopant, and this preform was drawn. After that, a heat treatment is performed to thermally diffuse the dopant in the preform, and a method of manufacturing a gradient index lens. 3. The method of manufacturing a gradient index lens according to claim 2, wherein the heating temperature for thermal diffusion is 1400 ° C. or lower. 4. A preform having silica glass as a main component and having a refractive index distribution in which a refractive index gradually increases from the outer circumference toward the inner portion by adding a dopant, and the preform is drawn. However, a method of manufacturing a gradient index lens, wherein the dopant in the preform is thermally diffused. 5. The heating temperature at the time of drawing is 1800 to 25
The method for manufacturing a gradient index lens according to claim 4, wherein the drawing speed is set to 00 ° C and the drawing speed is set to 0.5 to 10 m / min. 6. A gradient index lens, which comprises quartz glass as a main component and has a refractive index distribution in which a refractive index gradually increases from an outer periphery toward an inner portion by adding a dopant, A gradient index lens, wherein the dopant is thermally diffused to smooth the gradient index distribution.