JPH08220378A - Fiber with lens - Google Patents

Abstract

PURPOSE: To obtain a fiber with a lens which is small in the number of constituting parts, easy in center alignment, has a large working distance and tolerance and is stable in coupling efficiency by adopting the constitution formed by connecting a coreless fiber and a core enlarged fiber expanded in the core diameter at one end with the other end of this coreless fiber and the one end of this core enlarged fiber. CONSTITUTION: This fiber 1 with the lens is constituted by connecting the coreless fiber 2 which is formed by working one end of a coreless fiber having an isotropic refractive index to a convex curve and the core enlarged fiber 3 which is enlarged in the core diameter at one end with the other end of the coreless fiber 2 and the one end of the core enlarged fiber 3. The coreless fiber 2 converges the exit light from the optical element by the lens effect of the convex curved face and emits this light to the one end of the core enlarged fiber 3. The core enlarged fiber 3 is enlarged in the outside diameter of the core at one end and, therefore, the light emitted from the coreless fiber 2 is made incident thereon. Then, the alignment is easy and the coupling efficiency is stabilized.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a lensed fiber.

[0002]

2. Description of the Related Art With the development of optical fiber networks in homes,
It is desired to develop a technique for coupling an optical semiconductor element and an optical fiber under a high coupling efficiency which is a transfer efficiency of light energy. Conventionally, as a method for coupling an optical semiconductor element and an optical fiber, one or two microlenses are arranged between the optical element and the optical fiber and coupling is performed, or they are gradually reduced toward the tip side. A method is known in which a tip-end of an optical fiber with a core is processed into a spherical shape to connect it to an optical element.

[0003]

The first method using a microlens has the advantage that the coupling efficiency is relatively high, but on the other hand, the optical axis alignment (adjustment) between the optical element, the lens and the optical fiber is performed. (Heart) becomes complicated, and
There has been a problem that the module constituted by these essential components becomes large and the cost becomes high.

On the other hand, in an optical element, for example, a general semiconductor laser, light emitted at an angle of about 30 degrees deviates from the core of the optical fiber and does not enter the core. For this reason, in the second method using a spherical fiber, while concentrating the emitted light from the optical element by utilizing the lens effect of which the tip is processed into a spherical shape, it is possible to improve the coupling efficiency with the optical element. The distance in the optical axis direction (hereinafter, referred to as "working distance") is made as close as possible. For example, in the case of a single mode fiber (hereinafter referred to as “SMF”), the working distance is 5 to 10 μm.
Although it is coupled with the optical element after taking about m, there is a problem that there is a high risk of breaking the optical element due to expansion and contraction of the fiber due to heat.

In addition, in any of the above methods, the tolerance in the direction orthogonal to the optical axis is very small, the components are displaced from each other when they are fixed after alignment, and the coupling efficiency varies. There was also the problem of being easy. The present invention has been made in view of the above points, the number of components is small, the alignment between the components is easy, the size of the assembled module can be suppressed, the working distance can be increased, and the optical axis can be increased. It is an object of the present invention to provide a fiber with a lens, which has a large tolerance in a direction orthogonal to and a stable coupling efficiency.

[0006]

In order to achieve the above object, the lensed fiber of the present invention comprises a coreless fiber in which one end of a fiber having an isotropic refractive index without a core is processed into a convex curved surface, and The core expanding fiber having an expanded core diameter is connected to the other end of the coreless fiber and one end of the core expanding fiber.

Preferably, the convex curved surface of the coreless fiber is processed into a spherical shape by heat treatment.

[0008]

The coreless fiber converges the light emitted from the optical element by the lens effect of the convex curved surface and emits the light to one end of the core expanding fiber. At this time, since the coreless fiber has no core and has an isotropic refractive index, even if the distance from the optical element is large, the axial center determined by the diameter of the fiber, the refractive index, and the shape of the convex curved surface. Light within a range of a predetermined angle is incident on.

Since the outer diameter of the core at one end of the expanded core fiber is expanded, the light emitted from the coreless fiber enters in a wider range than in the case of an optical fiber in which the core is not expanded. Therefore, in the coreless fiber, the shape, the refractive index, and the length of the convex curved surface are determined so that the light incident from the convex curved surface is converged at an angle suitable for the numerical aperture corresponding to the expanded core of the core expanding fiber. Further, the core-expanded fiber is heated at one end so that the dopant of the core is diffused, and processed so that the diameter of the core is expanded.

At this time, as a material of the coreless fiber having an isotropic refractive index, synthetic resin such as acrylic resin or silicone resin, quartz, optical glass or the like can be used. On the other hand, as the core expanding fiber, a normal single mode fiber or the like can be used. Coreless fiber
When a synthetic resin is used as a material, the core expanding fiber is connected by adhesion using an optical adhesive. As for the connection between the coreless fiber and the core-expanding fiber, when both materials are glass, fusion splicing by arc discharge or the like is easy.

On the other hand, the light transmitted through the core-expanded fiber enters the optical element through the route opposite to the above. When the convex curved surface of the coreless fiber is processed into a spherical shape by heat treatment, a micro torch or arc discharge can be used, and the processing is easy.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to FIGS. The lens-equipped fiber 1 is formed by connecting a coreless fiber 2 and a core expanding fiber 3 as shown in FIG. 1, and as shown in the figure, is arranged opposite to an optical element, for example, a laser diode 5 with a working distance D. Then used.

In the coreless fiber 2, one end of a coreless isotropic fiber made of quartz or the like is heat-treated to form a convex curved surface 2a, and the other end is cut along a plane orthogonal to the axis. It is a fiber of length L. Here, in the coreless fiber 2, it is effective to process the convex curved surface 2a into an aspherical surface rather than a spherical surface in order to improve the coupling efficiency. The core-expanding fiber 3 is a so-called TEC (Thermal E) obtained by heating one end of the SMF so that the dopant of the core 3a is diffused and the diameter of the core 3a is expanded outward.
xpanded-Core) fiber.

The lens-equipped fiber 1 constructed as described above is manufactured as follows. First, one end of the SMF is heated to diffuse the dopant in the core to produce a TEC fiber in which the diameter of the core expands outward,
A core-expanded fiber 3 shown in FIG. 3 (a) was obtained by cutting the core at the maximum expansion portion.

Next, one end of a quartz coreless fiber 2 having the same diameter was fusion-spliced to the expanded core fiber 3 and then cut into a predetermined length L to obtain the shape shown in FIG. 3 (b). Next, as shown in FIG. 3 (c), electric discharge machining was applied in the vicinity of the cut surface to machine one end of the coreless fiber 2 into a convex curved surface to manufacture the lens-equipped fiber 1 shown in FIG. 1.

With regard to the lens-equipped fiber 1 of the present invention configured as described above, high coupling efficiency is obtained in the case where the convex curved surface 2a of the coreless fiber 2 is formed into a spherical shape having a radius of curvature R in consideration of easiness of manufacture. The structural parameters that define the bond properties for obtaining are modeled based on the geometrical optics method. That is, regarding the core expansion fiber 3, FIG.
Incident angle φ (degree) of the light beam on the end face of the core 3a shown in
And the incident position h (μm) measured from the optical axis La with the exit angle θ (see FIG. 1) of the ray from the end face of the laser diode 5 as a parameter, the refractive index of the coreless fiber 2 is 1.45, and the ray The wavelength was set to 1.3 μm and the working distance D (μm), the length L (μm) of the coreless fiber 2 and the radius of curvature R (μm) were changed and analyzed. The results are shown in FIGS. In each figure, the output angle θ is plotted by changing it every 1 °.

5 to 7, when the emission angle θ of the light beam emitted from the laser diode 5 is changed,
The finite value of the incident position h on the core expanding fiber 3 is due to aberration. As is clear from the above figures, when the absolute value | θ | of the output angle θ is small, the absolute value | φ | of the incident angle φ with respect to the change of θ remains small while the absolute value | φ | There is a tendency that the absolute value | h | of h increases. Also, as is clear from FIG.
Assuming that the radius of curvature R and the working distance D of the coreless fiber 2 are constant, in the range of the emission angle θ of −10 ≦ θ ≦ 10 °,
The value of the incident angle φ is substantially constant regardless of the length L of the coreless fiber 2. Therefore, the lensed fiber 1 of the present invention
In, it was found that the structural parameters that define the coupling characteristics are mainly the working distance D and the radius of curvature R.

Thus, in order to couple the light beam emitted from the laser diode 5 to the core expanding fiber 3,
The incident position h and the incident angle φ must be values within a certain fixed range. Therefore, as a standard SMF, the incident angle φ
Is | φ | ≦ 4.8 ° and the incident position h is | h | ≦ 4.0 μm
MF and the core expansion fiber, the incident angle φ is | φ | ≦
Based on analysis results, assuming a core expansion (TEC) fiber with an incident position h of 1.2 ° and | h | ≦ 16.8 μm.
The range of the SMF, the incident angle φ of the core expanding fiber, and the incident position h is indicated by a rectangle on the line when the radius of curvature R = 75 μm, and is shown as SMF and TEC in FIG. 8, respectively.

As shown in FIG. 8, the SMF tends to have a large incident angle | φ | and a small incident position | h |, as compared with a core expansion (TEC) fiber. Therefore, FIG.
~ Judging from the analysis results shown in Fig. 7, it can be seen that a lens-equipped fiber using SMF cannot receive a large number of light rays. On the other hand, the lens-equipped fiber using the core expanding fiber can receive the light beam emitted from the laser diode 5 up to a larger emission angle | θ | than the lens-equipped fiber using the SMF.
Coupling efficiency is SMF because it can support a wide range of output angles θ.
It is expected to be higher than the fiber with a lens using

In the core expanding fiber, since the normalized frequency of the propagated light is preserved, the product of the maximum light receiving angle and the core diameter is constant regardless of the expansion ratio of the core. Therefore, in FIG. 8, the rectangular areas determined by the ranges of the incident angle φ and the incident position h of the SMF and the core expanding fiber are equal. From the above analysis results, the working distance D = 150 μm,
It is expected that a lens-attached fiber with high coupling efficiency can be obtained by fusion-splicing based on the design values of radius of curvature R = 75 μm and length L = 1000 μm and emitting a light beam with a large spot size from the laser diode 5. It

Next, based on the above design values, a fiber with a lens was manufactured according to the procedure shown in FIG. First, S
Heating one end of the MF to diffuse the core dopant,
A TEC fiber whose core diameter is expanded outward is produced, and the core is cut at the maximum expanded part to have an outer diameter of 1
A 25 μm core-expanded fiber 3 shown in FIG. 3 (a) was obtained. Here, the core expansion fiber 3 has a light wavelength of 1.49.
The core was enlarged to about 4 times the original value so that the spot radius at 2 μm was 21.5 μm.

Next, one end of the quartz coreless fiber 2 having the same diameter is fusion-spliced to the expanded core fiber 3, and then the length L = 980 μm is cut according to the above design value, as shown in FIG. 3 (b). The shape shown is used. Then, FIG. 3 (c)
As shown in FIG. 3, electric discharge machining was performed in the vicinity of the cut surface, and one end of the coreless fiber 2 was processed into a spherical surface having a radius of curvature R = 75 μm to manufacture the lens-equipped fiber 1 shown in FIG.

With respect to the lens-equipped fiber 1 manufactured in this way, the coupling loss (d
The change in B) was measured based on the coordinate system shown in FIG.
In the coordinate system shown in FIG. 4, in the laser diode 5, the full width half maximum (FWHM) in the far field pattern (FFP) is 23 in the vertical direction (x) of the joint surface.
And the horizontal direction (y) was 18 °, and the wavelength of the emitted light was 1.49 μm. Here, the coupling loss was obtained by measuring the amount of light emitted from the other end of the core expanding fiber 3 with respect to the amount of light directly emitted from the laser diode 5 as a dB value.

The measurement result of the coupling loss (dB) with respect to the working distance D is shown by a circle in FIG. As is clear from FIG.
It was found that even when the working distance D is as large as 160 μm, a relatively low coupling loss of 4.2 dB is obtained, and the working distance can be made 10 times or more as compared with the conventional case using the spherical fiber. In addition, the calculation results of the coupling loss based on the geometrical optics and the coupling loss based on the wave optics are respectively shown by the solid line L.
It is also shown in FIG. 9 as RT and LWT.

Here, the coupling loss based on geometrical optics is multiplied by a Gaussian weight according to the exit angle θ of the light beam from the end face of the laser diode 5, and the light beam is incident on the core region of the core expanding fiber 3 at a critical angle or less. Was obtained by integrating. At this time, the full width half maximum of the laser diode 5 was set equal to 21 ° both in the vertical direction (x) and the horizontal direction (y) of the junction surface, and the meridional ray was assumed in the calculation.

As a result, as shown in FIG. 9, the coupling loss based on geometrical optics was in good agreement with the actual measurement result when the design value was larger than the design value D = 150 μm. In this case, since the laser diode 5 and the spot diameter of the light beam at the core end are not taken into consideration, it is difficult to say highly accurate calculation, but it has been found that the approximate value of the coupling loss can be easily obtained based on the geometrical optics. .

On the other hand, regarding the coupling loss based on the wave optics, it is considered that a Gaussian wave is emitted from the core end faces of the laser diode 5 and the core expanding fiber 3, and the radiation field of the ray emitted from the laser diode 5 is the coreless fiber 2. Was obtained by superposition integration of the field created immediately after passing through the convex curved surface 2a and the virtual radiation field created by the light beam emitted from the core expanding fiber 3 on the convex curved surface 2a. However, unlike the calculation result obtained by the actual measurement value and the geometrical optics, the spherical aberration on the convex curved surface 2a is not taken into consideration in this calculation.

Next, the tolerance characteristics for misalignment, which is a problem in the production of the lens-equipped fiber 1 of the present invention, were investigated. The coupling loss with respect to the axis deviation and the coupling loss with respect to the inclination when the working distance D = 160 μm was measured, and the coupling loss with respect to the axis deviation is shown in FIG. 10 and also in the x direction (vertical direction) shown in FIG. 4, that is, xz. Coupling losses for the tilt in the plane and the tilt in the y-z plane, which is the y direction (horizontal direction), are shown in FIGS.

Here, FIGS. 11 and 12 show the x direction and the y direction.
In the measurement of the coupling loss with respect to the inclination of the direction, the case where the optimization process was performed (marked with ◇) and the case where the optimization process was not performed (marked with ○) are shown. The optimization process is as follows:
When the optical axis of the lens-equipped fiber 1 and the laser diode 5 are aligned, the fiber 1 is tilted at an angle θtx or an angle θty with respect to the optical axis, and the measured coupling loss is not optimized in the x direction. And values in the y direction (marked with a circle in the figure). Next, the angle θ at which the fiber 1 is tilted
With tx or the angle θty held, the fiber 1 was translated so that the coupling loss was minimized, and the minimum coupling loss was taken as the value in the x direction and the y direction when the optimization processing was performed ( (Marked with ◇).

Then, based on the actual measurement values shown in FIGS. 9 to 12, the range of the working distance D, the axis deviation amount, and the inclination amount when the coupling loss increases by 1 dB from the optimum coupling state in which the maximum light output is obtained. Was calculated as the tolerance. As a result, the tolerance TD of the working distance D, the tolerance TL relating to the x-direction (vertical direction) axis deviation amount dx and the y-direction (horizontal direction) axis deviation amount dy, the inclination relating to the x-direction angle θtx and the y-direction angle θty. The tolerance TT of each is TD = ± 17
Since μm, TL = ± 1.3 μm and TT = ± 0.4 °, the tolerance can be made larger than that of the conventional general spherical fiber.

In the above embodiment, the coreless fiber 2 is described as having a convex curved surface formed into a spherical shape at one end. However, a convex curved surface is an aspherical surface such as a convex parabolic surface or a convex elliptical surface. May be.

[0032]

As is apparent from the above description, according to the present invention, the number of components is small, the alignment between the components is easy, the size of the assembled module is suppressed, and the working distance is increased. It is possible to provide a lens-equipped fiber having a large tolerance in the direction orthogonal to the optical axis and stable coupling efficiency.

At this time, the convex curved surface of the coreless fiber is
Since it is processed into a spherical shape by heat treatment, a micro torch or arc discharge can be used, and processing can be easily performed.

[Brief description of drawings]

FIG. 1 is a front view showing a configuration and an example of use of a lens-equipped fiber of the present invention.

FIG. 2 is an enlarged view showing a portion A of FIG. 1 in an enlarged manner.

FIG. 3 is an explanatory diagram showing a method of manufacturing the lens-equipped fiber of FIG. 1.

4 is a perspective view showing a coordinate system used for measuring a coupling loss of the fiber with lens of FIG.

5 is a working distance characteristic diagram showing an analysis result concerning working distance dependency of an incident angle φ and an incident position h of the lens-equipped fiber of FIG. 1 on a core-enlarging fiber.

FIG. 6 is also a length characteristic diagram showing an analysis result regarding the length dependency of the coreless fiber.

FIG. 7 is a curvature radius characteristic diagram showing an analysis result regarding a curvature radius dependency when the convex curved surface of the coreless fiber is spherical.

FIG. 8 is a light collection characteristic diagram showing an effective light collection range with respect to an incident angle φ and an incident position h of a standard SMF and a core expanding fiber.

FIG. 9 is a change characteristic diagram showing a change in coupling loss with respect to a change in working distance.

FIG. 10 is a coupling loss diagram showing a coupling loss regarding an axis shift in a direction orthogonal to an optical axis.

FIG. 11 is a coupling loss diagram showing coupling loss with respect to inclination in the x direction.

FIG. 12 is a coupling loss diagram showing coupling loss with respect to y-direction tilt.

[Explanation of symbols]

 1 Fiber with lens 2 Coreless fiber 2a Convex curved surface 3 Core expanding fiber 3a Core 5 Laser diode D Working distance L Length (of coreless fiber) La Optical axis R Curvature radius (of convex curved surface) h Incident position θ Emission angle φ Incident angle

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kazuhito Matsumura Kazuhito Matsumura 2-12 Wakakusa, Utsunomiya City, Tochigi Prefecture Wakakusa No. 2 Housing 1-8 (72) Inventor Naoto Koyama 2-13-7 Yoto, Utsunomiya City, Tochigi Prefecture No. Residence Kotobuki 102 (72) Inventor Isamu Oishi 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd. (72) Shigeru Suga 2-6-1 1-1 Marunouchi, Chiyoda-ku, Tokyo Kawa Electric Industry Co., Ltd.

Claims (2)

[Claims] 1. A coreless fiber obtained by processing one end of a fiber having an isotropic refractive index without a core into a convex curved surface, and a core expansion fiber having an expanded core diameter at one end and the other end of the coreless fiber. A lens-attached fiber connected to one end of the core expanding fiber. 2. The fiber with lens according to claim 1, wherein the convex curved surface of the coreless fiber is processed into a spherical shape by heat treatment.