JP2002328255A - Fiber optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical system which has a smaller reflection quantity in a stable and small-sized fiber optical system in which an optical element is mounted at nearly alignment-free and a loss is small. SOLUTION: In a fiber optical system which is composed of two fiber optical systems, each of which is composed of a single mode fiber for transmission, a graded index fiber, and a coreless fiber for focusing connected in this order, are located facing each other across an optical element in a manner that the coreless fibers are directed to the optical element side and two fiber optical systems are respectively fixed on a base, the refractive index of the coreless fiber is between the refractive index of the core of the graded index fiber and the refractive index of the optical element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber optical system suitably used for optical communication equipment, sensors, and the like, for coupling an optical fiber and an optical element.

[0002]

2. Description of the Related Art With the development of optical technology, transmission of signals and energy using optical fibers has been actively used in fields such as optical communication and optical measurement. In these systems, it is necessary to couple an optical fiber with a light source, a light receiver, a filter and an optical element for sensing. When a filter or an optical element for sensing is inserted into a fiber transmission line, it is necessary to minimize the coupling loss.

Conventionally, as shown in FIG. 2, an example in which an optical element 4 is aligned between two transmission single mode fibers 1 and a lens 8 to constitute an optical system has been used most often. 9 is a holder for holding the lens, and 10 is a package.

Further, there is an example in which a graded index fiber (hereinafter, referred to as a GI fiber) is used as a lens (see, for example, the Institute of Electronics, Information and Communication Engineers 1995 General Conference C283). The GI fiber is a fiber having an axially symmetric refractive index distribution such that the refractive index gradually decreases from the center axis of the fiber, and is originally used for multimode transmission. Most GI fibers have a squared index profile.

Since the above-described refractive index distribution has a lens effect similarly to a graded index lens (also called a GRIN lens), if a GI fiber having an appropriate refractive index distribution is used at an appropriate length, a coupling optical system can be used. Can be formed.

[0006]

However, in the arrangement shown in FIG. 2, the optical element 4, the lens 8 and the like are arranged as independent parts and are individually fixed to the holder and then aligned. There was a problem that it was complicated and large.

In the case of using a GI fiber, the GI fiber is used.
Since the fiber is a lens, it is necessary to adjust the focal direction, which is troublesome. Also, clearance for adjusting the position in the focal direction and mounting the optical element is required, and optical coupling once emitted from the fiber into space must be achieved. If the distance between the GI fibers becomes necessary, the adjustment becomes more troublesome,
When the light is emitted from the I-fiber into the space, the refractive index is different, so that there is a problem that reflection occurs at the light-emitting end face.

[0008]

In order to achieve the above object, a fiber optical system according to the present invention comprises a transmission single mode fiber, a graded index fiber, and a coreless fiber for focusing, which are joined in this order. Two fiber optical systems are arranged so that the coreless fiber faces the optical element side with the optical element interposed therebetween, and each is fixed to a base, and the refractive index of the coreless fiber is gray. By setting the size between the refractive index of the dead index fiber and the refractive index of the optical element, the amount of reflection of a coupling system formed by combining fibers having different refractive indices is reduced.

[0009]

Normally, since the refractive index of the coreless fiber is substantially equal to the refractive index of the quartz glass, it is smaller than the refractive index of the GI fiber, and the refractive index of the optical element arranged in the coupling system is G
Often larger than I-fibers. Therefore, G
I fiber, coreless fiber, and a combination of materials having different refractive indices of the optical element, and there are a plurality of points where reflection occurs at the boundary surface between different refractive index members. G rate
By setting the size between the refractive indices of the I-fiber, the reflection amount of the optical coupling system can be reduced.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. In the drawings, the same members are denoted by the same reference numerals, and description thereof will be omitted.

As shown in FIG. 1, a transmission single mode fiber 14, a GI fiber 11, and a coreless fiber 12 having a mode field diameter (MFD) of about 10 μm.
Are arranged in this order, and the coreless fiber 12 is opposed to the optical element 13 so as to face the optical element 13 with the optical element 13 interposed therebetween, thereby forming an optical coupling system. The end faces of the single mode fiber 14, the GI fiber 11, and the coreless fiber 12 are fusion-spliced.

The optical coupling system includes a groove 22 for arranging and fixing the optical element 13 and a single mode fiber 1.
4. A substrate 21 on which a V-groove 23 for arranging and fixing portions of the GI fiber 11 and the coreless fiber 12 is formed.
It is fixed to. It is preferable that the material of the substrate 21 has a small difference in linear expansion coefficient in order to reduce the stress generated due to a temperature change of the optical fiber or the optical element 13 and the environment, and silicon or quartz glass is used. When the fiber optical system is used in an environment where a severe temperature change does not occur, such as indoors, the stress generation due to the temperature change can be small. It is not necessary to match the expansion coefficient, and for example, a less expensive plastic molded product may be used.

The optical element 13 is fixed to the substrate groove 22 using an ultraviolet curing adhesive, and the optical system is fixed to the V groove 23 using a thermosetting adhesive. In the case where quartz glass is used as the material of the substrate 21, ultraviolet light is applied to the quartz glass substrate 2 when the ultraviolet curing adhesive is cured.
1 has an advantage that the adhesive can be sufficiently irradiated with ultraviolet rays without being blocked by the fixing member 1, so that the fixing operation can be easily performed.

As a method of manufacturing the optical system of the present invention,
In advance, the GI fiber 11, the GI fiber 11, the single mode fiber 14,
The single mode fibers 14 which are fused in the order are fixed in a V-shaped groove 23 on a substrate 21, and then a groove 22 for dividing the middle of the coreless fiber 12 is formed, and the optical element 13 is set in the groove 22. If the method of fixing is adopted, it is possible to manufacture with less man-hour.

Generally, the refractive index N3 of the optical element 13 is GI
The refractive index N2 of the coreless fiber 12 which is larger than the refractive index N1 of the core of the fiber 11 is almost equal to that of quartz glass,
It was even smaller than N1. For this reason, in the present optical system, Fresnel reflection that occurs due to the difference in the refractive index of the materials to be joined at the joining surface of different kinds of materials is likely to occur. Therefore, it is necessary to minimize the difference in the refractive index and reduce the reflectance.

Therefore, in the present invention, the coreless fiber 1
2 is changed to the refractive index N1 of the core of the GI fiber 11.
By setting the size between the optical element N3 and the optical element N3, the amount of reflection can be reduced. As an optimal combination, when the magnitude of the refractive index N2 of the coreless fiber 12 is set to an intermediate value N2 = (N1 + N3) / 2 between the refractive index N1 of the core of the GI fiber 11 and the refractive index N3 of the element 13. The reflectance can be minimized.

The coreless fiber 12 is made of quartz glass doped with GeO ₂ as a dopant in order to realize the above-mentioned desired refractive index N2. The sol-gel method was used as the manufacturing method. In this case, since the type of the material constituting the glass is the same as that of the GI fiber 11, the fiber melting temperature becomes close when the coreless fiber and the GI fiber 11 are fused, so that the operation is facilitated. ing. In addition, it is also possible to adjust the composition of the multi-component glass and melt-draw a material having a refractive index to produce it.

Although the end face where the coreless fiber 12 and the optical element 13 are connected is formed as a flat face, it is actually that all of the end faces are in complete physical contact with the face of the optical element 13. There is a slight gap. Therefore, it is preferable to fill the space between the coreless fiber 12 and the optical element 13 with the ultraviolet curing adhesive used for fixing the optical element 13 and the optical system to the substrate 21.

Since a boundary surface also exists between the adhesive and the coreless fiber 12 and between the adhesive and the optical element 13, if the refractive index of the adhesive and the coreless fiber 12 and the optical element 13 are different, a reflection point is formed. I will. In order to solve this problem, the amount of reflection can be reduced by using an adhesive having a refractive index between the refractive index N2 of the coreless fiber 12 and the refractive index N3 of the optical element 13.

The GI fiber 11 used in this embodiment is a GI fiber that is normally used for optical communication transmission. G
If the refractive index N1 of the core of the I-fiber 11 is too large, the reflection at the interface with the transmission single-mode fiber 14 increases, so the value of N1 is preferably 1.49 or less.

The GI fiber 11 has an axially symmetric refractive index distribution such that the refractive index gradually decreases from the center axis of the fiber. Most of the GI fiber 11 has a squared refractive index distribution. Since the light beam in the GI fiber 11 exhibits a sine curve behavior, the length is represented by a pitch P in correspondence with the period of the light beam behavior. The point light source becomes parallel light is P
= 0.25, and again converging to the point is P = 0.5
It is. In practice, the length of the GI fiber 11 used is desirably such that the pitch P is in the range from 0.25 to 0.5.

In principle, P = 0.25 +
GI having a pitch length of 0.5 × n (n is an integer of 0 or more)
Even if the fiber 11 is used, the light emitted from the fiber is a parallel light, which has a disadvantage that the length of the entire optical system becomes longer. However, the same function can be provided in terms of characteristics.

On the other hand, the length of the coreless fiber 12 is G
If the length of the I-fiber 11 is a pitch length at which the light becomes substantially parallel light, it can have any length.

An optical isolator is disposed as the optical element 13. The optical isolator used in this embodiment includes three elements, a polarizer, a Faraday rotator, and an analyzer. In the case of an optical isolator,
The refractive index N3 of the optical element according to the present invention corresponds to the refractive indexes of the polarizer and the analyzer. Usually, the polarizer and the analyzer are often formed using the same material.

[0025]

EXAMPLES Specific examples will be described below.

This will be described with reference to FIG. MFD about 10μm
The GI fiber 11 having a refractive index N1 = 1.47 and a core diameter of 50 μm is provided at the tip of the transmission single mode fiber 14 of FIG.
Were connected by fusion processing by electric discharge. A coreless fiber 12 having a refractive index of N2 = 1.49 was connected to the end face of the GI fiber 11 by fusion welding with a discharge to the GI fiber 11 and cut.

Subsequently, the same GI fiber 11 and transmission single mode fiber 14 were fusion-spliced in this order to produce a fiber optical system.

Next, the silicon substrate is subjected to anisotropic etching using KOH to form a substrate 21 having a fiber mounting V-groove 23.
Then, the fiber optics produced previously was installed and fixed with a thermosetting epoxy adhesive.

Next, an optical element mounting groove 22 was formed by cutting with a dicer so as to cut the coreless fiber 12.

Next, the optical element 13 is inserted into the optical element mounting groove 2.
2 and the gap between and around the optical element 13 and the coreless fiber 12 was filled with an ultraviolet curable refractive index matching adhesive adjusted to have a refractive index n = 1.49 and fixed. The refractive index N3 at the interface between the mounted optical element 13 and the coreless fiber 12 is 1.51.

From the transmission single mode fiber 14,
As shown in Table 1, the amount of reflection when light is incident on the optical element 13 is -33 dB when a coreless fiber that is substantially equal to the refractive index of conventionally used silica glass is used.
On the other hand, in the embodiment of the present invention, -40d
B was obtained, and the amount of reflection was improved.

In this embodiment, as the resin for fixing the optical element, the refractive index of the refractive index matching adhesive is adjusted to the refractive index of the coreless fiber, but it is adjusted to the refractive index of 1.51 of the optical element. Can obtain similar characteristics.

[0033]

[Table 1]

In the above embodiment, the refractive index N2 of the coreless fiber 12 is
Is used, the reflection amount is -37 dB even when the refractive index N2 of the coreless fiber 12 is equal to the refractive index of the GI fiber 11 or the optical element 13. And improved compared with the conventional configuration. In this case, the interfaces with different refractive indices are
Since there is only one portion between the coreless fiber 12 and the optical element 13 or between the coreless fiber 12 and the GI fiber 11, there is an advantage that the number of locations where reflection occurs is reduced and the influence of multiple reflection can be reduced. However, when focusing only on the magnitude of the reflection amount, the most preferable is the optical element 13 and the GI fiber 11 as in the first embodiment.
Of the coreless fiber 12
Has the least amount of reflection.

[0035]

As described in detail above, according to the present invention,
Two fiber optical systems, in which a single mode fiber for transmission, a graded index fiber, and a coreless fiber for focus adjustment are joined in this order, face each other with the coreless fiber facing the optical element side with the optical element interposed therebetween. In a fiber optical system in which the coreless fibers are arranged and fixed to a base, the amount of reflection in the entire optical system is reduced by setting the refractive index of the coreless fiber to a value between the graded index fiber refractive indexes. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing a fiber optical system according to the present invention.

FIG. 2 is a sectional view showing a conventional fiber optical system.

[Explanation of symbols]

 1, 14: single mode fiber for transmission 11: GI fiber 12: coreless fiber 13: optical element 21: substrate 23: V groove 22: groove 4: optical element 8: lens 9: holder 10: package

Claims (2)

[Claims] 1. A two-fiber optical system comprising a transmission single-mode fiber, a graded-index fiber, and a coreless fiber for focusing in this order,
A fiber optical system in which the coreless fibers are arranged facing each other so as to face the optical element side with the optical element interposed therebetween, and each is fixed to a base, and the refractive index of the coreless fiber is a graded index fiber. A fiber optical system having a size between the refractive index of the core and the refractive index of the optical element. 2. A gap between the optical element and an end face of the coreless fiber is filled with an optically transparent substance, and the refractive index of the optically transparent substance is determined by the refractive index of the coreless fiber and the refractive index of the optical element. The fiber optics according to claim 1, wherein the size of the fiber optics is between: