JP2002006169A - Optical amplification fiber module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplification fiber module consisting of an optically amplifying fiber and an optical input/output fiber, having a high efficiency of optical coupling of the fibers and simple to assemble. SOLUTION: The optically amplifying fiber 1 and the optical input/output fiber 2 of mutually different field modes are optically coupled with a rod lens whose refractive index varies in the radial direction with respect to the optical axis (GRIN rod lens 3). These have especially the same outside diameters and are aligned for their respective optical axes on the basis of the diameters. The optically amplifying fiber and the GRIN rod lens are coupled with an adhesive and the reflectance of the optically amplifying fiber is controlled by the refractive indexes and the thicknesses of adhesive layers 4a and 4b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber having the function of amplifying an optical signal (hereinafter referred to as an optical amplifier fiber) and an optical fiber for inputting and outputting an optical signal and pump light to and from the optical amplifier fiber. (Hereinafter, referred to as an optical input / output fiber). The present invention relates to an optical amplifier fiber module having high optical coupling efficiency and easy assembly.

[0002]

2. Description of the Related Art An optical amplification fiber used in an optical fiber amplifier is provided with an active ion such as a rare earth ion such as erbium to impart an optical amplification effect.
In particular, erbium-doped optical fibers (EDF: Erbium Doped
Fiber) has been widely used in long-distance, large-capacity trunk optical fiber communication systems, since an optical amplification effect appears in the 1.5 μm band, which is the lowest loss wavelength band of silica glass optical fibers.

[0003] Erbium-doped optical fibers are usually made of silica-based glass. On the other hand, multicomponent glass such as phosphate glass and fluoride glass can add erbium at a higher concentration than silica-based glass. When an optical fiber or a planar optical waveguide doped with erbium is manufactured using these glasses, light amplification characteristics equivalent to those of silica glass can be obtained with a very short length. That is,
For silica glass, a length of at least several meters is required, while for phosphate glass and fluoride glass, a length of 10 cm or less is sufficient. Thereby, a small and highly efficient optical amplification module can be manufactured. Such a small optical amplification module is expected to be applied to a subscriber network. Therefore, the development of low-priced modules with good mass productivity has been rapidly advanced (for example, Y. Jaouen et al., IEEE Photonics Technology).
Letters, vol. 11, no. 9, pp. 1105-1107, 1999.).

[0004] In order to increase the amplification efficiency, the optical amplifying fiber usually has a structure in which the pump light and the signal light for exciting the core region to which active ions are added are concentrated. That is, by increasing the relative refractive index difference between the core and the clad and reducing the core diameter, the mode field is reduced to confine light in a narrow region.
On the other hand, a normal optical fiber is used as the optical input / output fiber. Therefore, if the optical fiber is directly connected to the optical amplification fiber, the mode field is greatly different, and the connection loss is increased. The quality of the coupling efficiency between the optical amplifying fiber and the optical input / output fiber greatly affects the efficiency of the optical amplifying fiber module, and the method of connecting the two is an extremely important technical issue.

In order to efficiently couple optical fibers having different mode fields, the size of the mode field may be adjusted using a lens system. However,
The use of a lens system generally complicates the optical coupling part,
It takes time to adjust and assemble the optical axis of the module, and it is difficult to reduce the size, which may not lead to a reduction in price.

Conventionally, with respect to the technical problem relating to the optical coupling between an optical amplification fiber having different mode fields and an optical input / output fiber, a high numerical aperture optical fiber having a large relative refractive index difference is used between the two. Connection method was often used. That is, a high numerical aperture optical fiber is first connected to an optical amplification fiber. Since the high numerical aperture optical fiber has a small mode field and is close to that of the optical amplification fiber, the two can be optically coupled with low loss. Next, the other end of the high numerical aperture fiber is expanded by a core expansion process (so-called TEC) so that the mode field becomes almost equal to that of a normal optical fiber. Then, it is connected to the input / output fiber (for example, Y. Nishida et al., IEEE Photo
nics Technology Letters, vol.11, no.12, pp.1596-15
98, 1999.).

According to this method, high-efficiency optical coupling between the optical amplification fiber and the optical input / output fiber can be achieved with a small loss. However, since a high numerical aperture optical fiber is used between the two, the extra length processing is required. Therefore, although an optical amplification fiber module having a high amplification efficiency can be obtained, there is a problem that it is difficult to reduce the size.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical amplification fiber module which solves high efficiency and miniaturization in optical coupling between an optical amplification fiber having different mode fields and an optical input / output fiber. Is to do.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a rod-shaped lens (hereinafter referred to as a rod-shaped lens) whose optical coupling between an optical amplifying fiber and an optical input / output fiber having different mode fields has a refractive index that changes radially with respect to the optical axis. , A GRIN rod lens), which is the main feature of the prior art. Unlike the prior art, in which the optical coupling between the two is performed directly or through a high numerical aperture optical fiber, the efficiency of the optical coupling is increased. In addition, the assembly is simplified.

Hereinafter, the present invention will be described in detail.

FIG. 1 shows a schematic view of a module according to the present invention. 1 is an optical amplification fiber, 2 is an optical input / output fiber, 3 is a GRIN rod lens, 4a and 4b are
Each shows an adhesive layer. The principle of implementing the present invention in such a configuration will be described below.

Optical amplification fibers often have a relative refractive index difference of 1% or more, and a mode field diameter of 5 microns or less. In contrast, the optical input / output fiber has a relative refractive index difference of about 0.3% and a mode field diameter of about 9 microns. When these are directly connected, the coupling efficiency is reduced because the size of the mode field is different. On the other hand, the GRIN rod lens has the function of a convex lens or a concave lens depending on the length, so that the mode field can be converted by making the convex lens function by appropriately selecting the length. for that reason,
If a GRIN rod lens is used, the optical coupling between the two can be performed with high efficiency.

First, a case in which a change in coupling efficiency due to mode field conversion when a GRIN rod lens is used is theoretically evaluated will be described. In this case, an optical amplification fiber,
It is assumed that the GRIN rod lens and the optical input / output fiber are directly connected. Such mode field conversion can be performed using the following ray determinant for the GRIN rod lens.

[0014]

(Equation 1)

Here, n is a refractive index, and suffixes 0, 1, 2
Represents a GRIN rod lens, an optical amplification fiber, and an optical input / output fiber, respectively, P represents a convergence parameter of the GRIN rod lens, and L represents a GRIN rod lens length L0.
And the length including the beam waist positions t1 and t2 in each fiber. As can be seen from this equation, when a GRIN rod lens having a certain P is used, the mode field diameter can be changed according to the length. Therefore, by increasing the mode field of the optical amplification fiber to be equal to that of the optical input / output fiber, the coupling efficiency between the two can be maximized.

FIG. 2 shows the result of theoretically evaluating the change in coupling efficiency due to the change in the length of the GRIN rod lens. The wavelength of light is assumed to be 1.55 microns, the optical amplification fiber is assumed to be a ZrF4-based fluoride, and the optical input / output fiber is assumed to be a silica-based single mode fiber.
The core refractive index, relative index difference, and cutoff wavelength are:
1.501, 1.455, 1%, 0.35% respectively
And 0.9 microns and 1.2 microns. G
The RIN rod lens is a silica graded index multimode fiber (50 / 125GI Fiber) having a core diameter of 50 microns and a relative refractive index difference of 1%.
P is 32.

As can be seen from FIG. 2, the coupling efficiency is GR
It changes depending on the length of the IN rod lens. Length 1
mm, the coupling efficiency becomes almost 100%. Also,
It can be seen that the range of the length of the lens giving high coupling efficiency is wide.

Since this indicates that the tolerance in assembling the module is large, it means that the assembling of such a module is easy. By using the GRIN rod lens in this manner, highly efficient connection that is easy to assemble becomes possible.

On the other hand, when the optical amplification fiber is made of the same material as the optical input / output fiber, for example, when it is made of quartz glass, the above results are directly reflected since both can be fused. However, when the materials are different, connection using an adhesive is common. In this case, reflection at the end face may be a problem. To prevent this, the end face is polished obliquely or an antireflection film is provided. In the present invention, antireflection is achieved by controlling the refractive index and thickness of the adhesive layer.

FIG. 3 is a schematic diagram showing a case where the connection between the GRIN rod lens 3 and the optical amplification fiber 1 is made with an adhesive layer 4 (adhesive) in the present invention. The state of light transmitted through the adhesive layer 4 changes depending on the refractive index and thickness of the adhesive, and can be expressed by the following equation.

[0021]

(Equation 2)

Here, e and h denote an electric field and a magnetic field, respectively, and the suffixes 1 and 2 denote the optical amplification fiber side and the GRIN rod lens side (the GRIN rod lens and the optical input / output fiber are of the same type (silica glass)). Therefore, the refractive index is the same). Also, ka is the wave number in the adhesive layer,
d is the thickness and na is the refractive index. As can be seen from this equation, the state of light in the adhesive layer changes depending on the optical path length (= wave number × thickness). Therefore, when the optical path length is close to the wavelength, the transmission characteristics of the light including the adhesive layer include its refractive index and thickness. It depends on the size.

FIG. 4 shows that the refractive index of the optical amplification fiber and the GRIN rod lens (the same applies to the optical input / output fiber) are 1.501 and 1.455, respectively, and the thickness of the adhesive layer is 3 μm. When the refractive index was changed (when the refractive index was changed to 1.47 to 1.50), the reflection characteristics in the optical amplification fiber were theoretically evaluated in a wavelength range of 1.5 to 1.6 microns. Things. As can be seen from FIG. 4, near the wavelength of 1.55 μm, the reflectance becomes smallest when the refractive index of the adhesive layer is 1.48.

FIG. 5 shows that the refractive index of the adhesive layer is 1.48,
It shows the result of theoretically evaluating the change in reflectance when the thickness is 1 to 5 microns. As can be seen from FIG. 5, when the thickness of the adhesive layer is 5 microns, 1.55
The reflectance around the micron becomes the smallest. In this case, the reflectance is 0.00001 or less.

When the optical amplification fiber having a different refractive index and the GRIN rod lens are connected via an adhesive, the reflection at the interface of the optical amplification fiber is changed by changing the refractive index and the thickness of the adhesive layer. The rate can be controlled.

[0026]

EXAMPLE 1 FIG. 6 is a schematic diagram showing a case where a ZrF4-based Er-doped fluoride is used for the optical amplification fiber 1 and a silica-based single mode fiber is used for the optical input / output fiber 2. FIG. The refractive index, relative refractive index difference, and cutoff wavelength of the core were 1.50, respectively.
1, 1.455, 1%, 0.35%, and 0.9 microns, 1.2 microns. GRIN rod lens 3
Is a silica graded index multimode fiber with a core diameter of 50 microns and a relative refractive index difference of 1%.
P is 32 and the length is 0.85 mm. GRIN rod lenses are manufactured by embedding a large number of commercially available polishing resins.
After the disk-shaped product is polished on both sides, the resin is melted and taken out.

These were arranged in a V-groove of a V-groove plate 5 formed on the surface of a glass plate having a low coefficient of thermal expansion, and a small amount of an ultraviolet-curing adhesive was dropped and covered with a glass plate of the same type (not shown).
Light having a wavelength of 1.55 μm was incident from the optical amplification fiber 1 and the output from the optical input / output fiber 2 was received by a power meter. The optical input / output fiber 2 was moved in the axial direction using a micrometer, and the connection portion was irradiated with ultraviolet light at the position where the amount of received light was maximized to cure and fix the adhesive. The adhesive is a commercially available epoxy-based adhesive having a refractive index of 1.48.
It is. After curing, light with a wavelength of 1.55 μm is incident from the optical amplification fiber 1 using a high precision reflectometer,
The reflectance at the interface of the adhesive layer and the transmission loss at the output end of the optical input / output fiber were measured. The reflectance was about 43 dB, and a value close to the theoretical value was obtained. This value is equivalent to the reflectance when the end face is obliquely polished or when an antireflection film is provided. Further, the transmission loss in the adhesive layer was 0.1 dB or less, and extremely high coupling efficiency was obtained.

This is equivalent to the loss in the case where the same type of silica-based optical fiber having the lowest connection loss is selected by fusion splicing. When the bonded portion was observed with a microscope, the thickness of the bonded layer was 4 to 5 microns.

Embodiment 2 The same optical amplification fiber and GRIN as described in Embodiment 1
The rod lens and the optical input / output fiber are made of a commercially available glass capillary with an inner diameter of 128 microns (outer diameter x length = 2 x 15 m
m), and the adhesive was dropped. These were connected in the same procedure as in Example 1, and the reflectance and transmission loss at the adhesive layer were measured. As a result, a reflectivity of about 41 dB and a transmission loss of about 0.12 dB were obtained. Although a little inferior to the case of the V groove, a high coupling efficiency is obtained. This is due to the fact that the inner diameter of the capillary was slightly large, causing axial displacement in the radial direction.

[0030]

As described above, when an optical amplifying fiber having the same outer diameter but the same mode field and a different refractive index is connected to an optical input / output fiber, the mode field is adjusted via the GRIN rod lens to adjust the light. Since the coupling is performed, extremely high coupling efficiency is obtained, and since the outer diameter is the same, connection based on the outer diameter can be performed, and the GRIN rod lens has a large tolerance in the optical axis direction, so that assembly is extremely easy. In addition, the reflectivity of the adhesive layer can be significantly reduced only by appropriately selecting the refractive index and thickness of the adhesive layer. There is an advantage that a low-cost and high-performance optical amplification fiber module can be manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a module according to the present invention.

FIG. 2 is a diagram illustrating a result of theoretically evaluating a change in coupling efficiency due to a change in the length of a GRIN rod lens.

FIG. 3 is a schematic diagram showing a case where a connection between a GRIN rod lens and an optical amplification fiber is made with an adhesive layer (adhesive).

FIG. 4 is a diagram illustrating a relationship between a reflection characteristic in an optical amplification fiber and a wavelength when a refractive index of an adhesive layer is changed.

FIG. 5 is a diagram showing the relationship between the reflection characteristics and the wavelength when the refractive index of the adhesive layer is 1.48 and the thickness is changed.

FIG. 6 shows an optical amplification fiber, an optical input / output fiber,
It is the schematic in the case of connecting with GRIN rod lens using a V groove.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical amplification fiber 2 ... Optical input / output fiber 3 ... GRIN rod lens 4, 4a, 4b ... Adhesive layer 5 ... V groove plate

Claims (3)

[Claims] 1. An optical amplification fiber module comprising an optical fiber having an action of amplifying an optical signal and an optical fiber for inputting and outputting an optical signal and pump light to and from the optical amplification fiber, wherein the optical signal is amplified. And an optical fiber for inputting / outputting an optical signal and pumping light to / from the optical amplification fiber have different mode fields, and the optical coupling between the two is in a radial direction with respect to the optical axis. An optical amplifying fiber module characterized by using a rod-shaped lens whose refractive index is changed. 2. An optical fiber having an action of amplifying an optical signal, an optical fiber for inputting and outputting an optical signal and pump light to and from the optical amplification fiber, and a refractive index changing in a radial direction with respect to an optical axis. 2. The optical amplifying fiber module according to claim 1, wherein each of the rod-shaped lenses has an equal outer diameter, and the optical axis is adjusted based on each outer diameter. 3. An optical fiber having an action of amplifying an optical signal, and a rod-shaped lens whose refractive index is changed in a radial direction with respect to an optical axis are joined by an adhesive, and a refractive index of an adhesive layer; 2. The optical amplifying fiber module according to claim 1, wherein the reflectance of the optical fiber having the function of amplifying the optical signal is controlled by the thickness.