JP2002359419A - Optical fiber coupler and optical amplifier using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means, capable of improving/changing amplification characteristics, such as gain/wavelength characteristics, output/wavelength characteristics, and the like, related to an optical fiber amplifier using an optical fiber coupler formed of a rare-earth-doped fiber itself. SOLUTION: The optical fiber coupler is provided, where a plurality of fibers are fused and drawn, at least one of which comprises a first rare-earth-doped fiber. A single mode fiber is fused to one end near the fusing/drawing part of the first rare-earth-doped fiber, while a second rare-earth-doped fiber is fused to the other end.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber coupler used in the field of optical communication and an optical fiber amplifier using the same.

[0002]

2. Description of the Related Art Conventionally, in an optical communication system, an optical fiber amplifier for amplifying incident light as it is has been used. As shown in FIG. 8, the structure is such that a multiplexer (WDM) 7 composed of an optical fiber coupler and a pumping light source 6 are combined to form a rare-earth-doped fiber 4 in which a rare-earth element represented by Er, Nd, or Pr is doped in a core. It is fusion spliced, and is fusion spliced to a passive component such as the in-line optical isolator 8 at the output portion. Then, the input signal light is multiplexed with the pump light by the multiplexer 7, amplified in the rare-earth-doped fiber 4, and output.

However, in this configuration, since each component is fusion-spliced, it takes time and effort for splicing and the like by fusion, resulting in loss due to splicing, and is not preferable in terms of amplification characteristics such as gain and noise characteristics. In view of this, the present applicant has provided a means for solving the above-mentioned problem in Japanese Patent Application No. 11-243244 by forming an optical fiber coupler with the rare earth-doped fiber 4 itself. This is because the pump light from the pump light propagation fiber 2 and the single mode fiber 1
Are combined by the multiplexer 7 and amplified by the rare-earth-doped fiber 4. Also, as shown in FIG. 9, the multiplexer 7 is formed at both ends of the rare earth-doped fiber,
Thereby, an optical amplifier having a configuration capable of bidirectional pumping can be configured.

[0004]

However, the above configuration is not enough to improve the amplification characteristics of the optical amplifier, and it is required to shift the amplification band to a longer wavelength side to widen the band. .

The present invention has been made to cope with such an improvement in the amplification characteristics of an optical amplifier.

[0006]

According to the present invention, there is provided an optical fiber coupler in which at least one of a plurality of fibers made of a first rare-earth-doped fiber is fused and drawn. A single mode fiber is fused to one end in the vicinity, and a second rare earth-doped fiber is fused to the other end.In other words, the present invention relates to the above-described rare earth-doped fiber itself. When a characteristic different from the conventional amplification characteristics is required by fusion-splicing a second rare earth-doped fiber having different characteristics to the rare earth-doped fiber used for the base based on the optical fiber coupler configured Means can be provided.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. The optical fiber coupler according to the present invention,
As shown in FIG. 1, a single mode fiber 1, a first rare earth-doped fiber 4a fused to the single mode fiber 1, and a pumping light propagation fiber 2 are provided. A fusion-stretched portion 3 is formed between the propagation fibers 2. Further, a second rare-earth-doped fiber 4b is connected to the other end of the first rare-earth-doped fiber via a fused portion 4c. Excitation light propagation fiber 2
Has a structure in which the pump light can maintain a single mode basically, has a mode field diameter close to that of the rare-earth-doped fiber 4a, and can be connected to the rare-earth-doped fiber 4a with low loss. Specifically, a single-mode fiber and a rare-earth-doped fiber in which the end faces are fusion-spliced, or a fiber having the same core diameter, refractive index, and the like as the rare-earth-doped fiber 4 and having no added rare-earth element are used. Is also good.

As shown in FIG. 1 (b), a single-mode fiber 1 having a protective coating on the end face is first removed.
Similarly, at least one fiber in which the end faces of the first rare-earth-doped fiber 4a from which the protective coating has been peeled off is connected by the fusion splicing part 1a is prepared. Similarly, at least one excitation light propagation fiber 2 is prepared. Next, the single-mode fiber 1 and the first rare-earth-doped fiber 4a are fusion-spliced, and the pumping light propagation fiber 2 is installed in parallel, and the rare-earth-doped fiber 4 is heated and fused to draw. Forming / drawing section 3 is formed. At this time, the fibers to be fusion-stretched have almost the same structure, and have the same propagation constant. Therefore, the fibers can be completely coupled at a specific wavelength by fusion-stretching. In this case, the specific wavelength is an excitation light wavelength.

The fusion-spreading section 3 is formed on the rare-earth-doped fiber 4 side near the fusion splicing section 1a. And the fusion stretching section 3
And the distance between the fusion splicing portion 1a and 30mm or less. This is because the absorptance of signal light when the rare earth element is Er in such an optical fiber is usually 3 to 8 dB / m and the length is 3 to 8 dB / m.
In the case of 0 mm or more, the signal light component incident on the rare-earth-doped fiber 4 from the single mode fiber 1 side is absorbed and attenuated by 0.1 to 0.25 dB or more by the rare-earth element before being combined with the excitation light, which is preferable in terms of amplification characteristics. Because there is no. After the above heat fusion stretching, a portion including the fusion stretching portion 3 is set on the substrate 5 by a predetermined process as shown in FIG.
a) to complete the optical fiber coupler. still,
Before or after the fusion stretching step, a second rare earth-doped fiber 4b is fused to the other end of the first rare earth-doped fiber 4a.

Next, an optical fiber amplifier using this fiber coupler will be described. As shown in FIG. 3, the signal light is input to the single mode fiber 1 of the multiplexer 7 comprising the optical fiber coupler of the present invention, the pump light source 6 is connected to the pump light propagation fiber 2, and the second rare earth doped fiber An optical isolator 8 is connected to 4b to output amplified light. When an optical amplifier is configured using the optical fiber coupler of the present invention, the following features are obtained.

(1) The signal light from the single mode fiber 1 directly enters the end face of the first rare earth doped fiber 4a.

(2) The pumping light from the pumping light propagation fiber 2 is evanescently coupled directly with the signal light in the rare-earth-doped fiber 4 via the fusion extending portion 3 between the rare-earth-doped fibers 4.

That is, the signal light and the pump light are directly coupled in the rare-earth-doped fiber 4. As a result, the signal light component propagates directly through the single mode fiber without passing through the multiplexer and the like, enters the erbium-doped fiber 4, and the pump light component passes directly through the pump light transmission fiber 2 to the erbium-doped fiber 4. And a highly efficient coupling structure. That is, since the loss due to the connection is reduced, the amplification efficiency is improved and the noise characteristics are also improved.

Further, by connecting the second rare earth-doped fiber 4b to the other end of the first rare earth-doped fiber 4a, the amplification characteristics can be improved. For example, the band can be shifted by connecting the rare-earth-doped fiber 4a to another rare-earth-doped fiber 4b and increasing the length. For example, when it is desired to shift the amplification band from the C band (1530 to 1565 nm) amplification band to the L band (1565 to 1625 nm), a second rare earth doped fiber having substantially the same material and structure as the first rare earth doped fiber 4a.
It can be realized by connecting 4b. In other words, by increasing the length of the rare-earth-doped fiber, the C-band component of the signal light component in the 1.55 μm band is amplified by the pump light, but when the fiber is elongated, the amplified light is conversely increased by the rare-earth element in the fiber core. And is gradually attenuated. Conversely, since the L band portion having a small amplification degree is gradually amplified, the gain characteristic shifts to a long wavelength band having a long wavelength. As shown by the line B in FIG. 6, the wavelength characteristic of the gain shifts to the L band and the band can be widened.

On the other hand, in the case of realizing gain flatness and broadband / flattening, it is better to connect rare earth doped fibers 4b having different characteristics. For example, if the core of the second rare-earth-doped fiber 4b is made of a fiber in which the amount of aluminum (Al) added is increased, the gain characteristics become flatter. Also, when a material having a high refractive index is used for the fiber core and a fiber having a small core diameter is used, the gain of the stimulated emission cross-sectional area is improved, so that the overall gain is increased, and the gain of the conventionally weak portion is also improved. Therefore, amplification in a wide band becomes possible.

For example, when the second rare earth-doped fiber 4b using a glass material such as fluoride (refractive index 1.53) or tellurite (Te: refractive index 2 or more) for the fiber core is used, a longer wavelength band side is obtained. Amplification can be realized. An example of the gain characteristic is shown in line C of FIG. On the other hand, when the second rare earth doped fiber 4b corresponds to the S band (1470 to 1520 nm) having a short wavelength, the additive to the core of the second rare earth doped fiber 4b is not erbium (Er) but thulium (T).
This can be realized by connecting a fiber added with m). The gain characteristic is shown in line D of FIG.

By connecting the second rare-earth-doped fiber 4b having different characteristics as described above, a necessary amplification band can be obtained. At this time, it can be coped with by adjusting the length of the first rare earth-doped fiber 4a in order to obtain necessary characteristics. For example, by lengthening the second rare-earth-doped fiber 4b, it is possible to make full use of the characteristics of the second rare-earth-doped fiber 4b. In that case, the first rare earth-doped fiber 4a may be shortened.

Next, another embodiment of the optical amplifier will be described. FIG. 4 shows an optical fiber amplifier of bidirectional pumping using fibers having different characteristics. An optical fiber coupler of the present invention is formed by the second rare-earth-doped fiber 4b, and is connected to the second pumping light source 6 as a second multiplexer 7a. Can be increased. In this case, if the second rare-earth-doped fiber 4b is made of the same material as the first rare-earth-doped fiber 4a, the length of the rare-earth-doped fiber is increased, so that the gain-wavelength characteristics can be broadened and the gain can be increased simultaneously.

FIG. 3 shows an embodiment of the forward pumping system. However, the present invention may be applied to a backward pumping system as shown in FIG.
Also, when a plurality of forward-pumped or backward-pumped optical amplifiers shown in FIGS. 3 and 5 are combined in series, it is effective to increase the necessary gain and realize a wider band. Examples are also included.

Next, the operation of the amplifier using the multiplexer according to the present invention will be described in detail with reference to FIG. The signal light enters the multiplexer 7 comprising the optical fiber coupler of the present invention via the single mode fiber 1. The light incident into the multiplexer 7 is incident on a fusion spliced portion 1a between the single mode fiber 1 and the first rare earth doped fiber 4a. Loss occurs because the mode field diameter of the single mode fiber 1 and the first rare earth-doped fiber 4a are different in the fusion spliced portion 1a. Usually, the loss is about 0.1 to 0.2 dB. The loss of the fusion spliced portion 1a is increased by repeating discharge heating a plurality of times during fusion splicing, thereby diffusing the refractive index increasing additive Ge of each fiber core, thereby enlarging the mode field diameter of each fiber propagating light. In addition, connection loss can be reduced.

Thereafter, the signal light propagates in the first rare earth doped fiber 4a. The rare earth element such as erbium added in the core of the first rare earth doped fiber 4a is:
In order to absorb the signal light component (3 to 8 dB / m), the pump light is desirably propagated by about 5 to 15 mm to the coupling section composed of the fusion-stretching section 3 for multiplexing the excitation light. Therefore, by coupling the pumping light, the coupling is completed by evanescent coupling by fusion-spreading of the rare-earth-doped fibers or fibers having substantially the same structure (isolation: 15 to 20).
dB).

Thereafter, electrons of the rare earth element in the rare earth doped fiber core are excited, and change from the ground level to a high energy excited level. When the excited rare earth element electrons transition to a more stable low energy level ground level, they are induced by signal light, emit stimulated emission light, and amplify the signal light component.

As described above, the first rare earth doped fiber 4a
The signal light amplified in the inside enters the second rare earth doped fiber 4b at the fusion splicing part 4c in the splice part, is further absorbed by a different rare earth element added in the fiber core, and is similarly amplified. You.

However, when the length of the rare-earth-doped fiber becomes longer, the pump light is absorbed and attenuated by the rare-earth element, and the amplified signal light is also absorbed, so that the gain is reduced. In order to suppress this, it is necessary to provide a configuration for inputting another excitation light as shown in FIG. As shown in FIG. 4, by pumping light by another pumping light source 6 by backward pumping, it is possible to suppress the attenuation of the pumping light and suppress the decrease in gain. In order to widen the band, an optical fiber coupler using the second rare earth doped fiber 4b having different additives in the core may be used.

[0025]

As described above, the present invention has the following excellent effects.

(1) By connecting a second rare earth-doped fiber of substantially the same structure and material to the fiber end on the output side of an optical fiber coupler using a first rare earth-doped fiber represented by erbium,・ The wavelength characteristic can be shifted to a longer wavelength band and a wider band can be achieved.

(2) By connecting a second rare earth-doped fiber having a different structure and characteristics to the fiber end on the output side of an optical fiber coupler using a first rare earth-doped fiber represented by erbium, By changing the wavelength characteristics, gain characteristics in a required wavelength range can be obtained.

(3) A fiber end of another fiber coupler using a rare earth-doped fiber having different characteristics is connected to a fiber end on the output side of an optical fiber coupler using a first rare earth-doped fiber represented by erbium. As a result, amplification characteristics such as a wide band and gain in a necessary wavelength range can be obtained.

[Brief description of the drawings]

FIG. 1A is a diagram showing an optical fiber coupler of the present invention, and FIG. 1B is an explanatory diagram of a manufacturing process thereof.

FIG. 2 is a board mounting diagram of the optical fiber coupler of the present invention.

FIG. 3 is a diagram showing an embodiment in the case of forward pumping of the optical amplifier of the present invention.

FIG. 4 is a diagram showing an embodiment in the case of bidirectional pumping of the optical amplifier of the present invention.

FIG. 5 is a diagram showing an embodiment in the case of backward pumping of the amplifier of the present invention.

FIG. 6 is a graph showing amplification characteristics of the amplifier according to the present invention.

FIG. 7 is a graph showing another example of amplification characteristics of the optical amplifier of the present invention.

FIG. 8 is a diagram showing a configuration of forward pumping of a conventional optical amplifier.

FIG. 9 is a diagram illustrating a configuration of bidirectional pumping of a conventional optical amplifier.

[Explanation of symbols]

 1: single mode fiber 1a: fusion spliced part 2: pumping light transmission fiber 3: fusion spliced part 4a: first rare earth doped fiber 4b: second rare earth doped fiber 4c: fusion spliced part 5: substrate 6 : Excitation light source 7: Combiner 8: Isolator

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yukiko Furugan 2-1-1 Kagahara, Tsuzuki-ku, Yokohama-shi, Kanagawa Prefecture F-term in the Kyocera Corporation Yokohama Office (reference) 2H050 AC09 AC81 5F072 AB09 AK06 JJ04 JJ20 KK30 YY17

Claims (5)

[Claims] 1. An optical fiber coupler wherein a plurality of fibers, at least one of which is made of a first rare earth-doped fiber, is fused and drawn, a single end is provided at one end in the vicinity of the fused drawing part of the first rare earth-doped fiber. An optical fiber coupler, wherein a mode fiber is fused, and a second rare earth-doped fiber is fused to the other end. 2. The optical fiber coupler according to claim 1, wherein the second rare-earth-doped fiber has a different structure from the first rare-earth-doped fiber or contains a different additive. 3. The optical fiber coupler according to claim 1, wherein said second rare-earth-doped fiber has substantially the same structure and additive as the first rare-earth-doped fiber. 4. An optical amplifier characterized in that signal light and pump light are input and multiplexed to the optical fiber coupler according to claim 1, and an amplified signal is output. 5. A signal light and a pump light are inputted from one end side of a first rare earth doped fiber, and amplified light is outputted from a second rare earth doped fiber connected to the other end side of the first rare earth doped fiber. An optical amplifier designed to work.