JPH08204257A - Optical fiber amplifier and optical fiber laser - Google Patents

Abstract

PURPOSE: To suppress the connection loss for both signal light and excitation light in the connecting section of a rare-earth-element-doped optical fiber of an optical fiber amplifier which uses the rare-earth-element-doped optical fiber as a gain medium with another optical fiber. CONSTITUTION: The normalizing frequencies of a rare-earth-element-doped optical fiber 12 and another optical fiber constituting an optical fiber type coupler 11 connected to the fiber 12 are set to the same value. An dopant which is easily diffused when it is heated is added to both fibers 11 and 12 and the mode field diameters of the fibers 11 and 12 are matched to each other by enlarging the diameters of the fibers at the connecting section of the fibers by diffusing the dopant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier and a laser using a rare earth-doped optical fiber, and it is possible to reduce the connection loss at the connection between the rare earth-doped optical fiber and another optical fiber. Regarding the technology done.

[0002]

2. Description of the Related Art FIG. 1 illustrates a basic configuration of an optical fiber amplifier. Reference numeral 11 in the figure is a wavelength division multiplexing type (WDM
Type: optical fiber type coupler of wavelength division multiplexing, 12 is an amplifying fiber, 13 is an optical isolator, and 14 is a wavelength selection filter. The amplification fiber 12 is a rare earth-doped optical fiber, and for example, an erbium-doped optical fiber (EDF) is used. In this optical fiber amplifier, when pumping light of wavelength λ _P enters the EDF 12 via the optical fiber type coupler 11, the ED
Erbium (Er) ions added to the core of F12 are excited. Then, the energy stored in the Er ions in the excited state, when the signal light of the wavelength λ _S enters the EDF 12 through the optical fiber type coupler 11,
It is converted into light of the same wavelength (λ _S ) by stimulated emission, and an amplification effect occurs. The optical signal amplified by this stimulated emission is emitted through the optical isolator 13 and the wavelength selection filter 14 provided as necessary. The pumping light used in the optical fiber amplifier using such an EDF is generally λ _P = 0.98 μm and λ _P = 1.48 μm.
One of the two wavelengths of
Amplified light of _S = 1.55 μm is obtained. Further, generally, in order to obtain stable operation characteristics, a single mode optical fiber is substantially used in an optical amplifier using a rare earth-doped optical fiber.

In an optical fiber amplifier using such an EDF 12, Er (rare earth) is usually added almost uniformly in the core, and in order to increase the pumping efficiency of the EDF 12, the pumping light power density in the core is increased. Is preferably high. Therefore, the EDF 12 is generally set to have a large relative refractive index difference of 1% or more between the core and the clad, and a small core diameter. For example, an EDF having a step type refractive index distribution as shown in FIG. 2 (in FIG. 2, the solid line shows the refractive index distribution, and the broken line shows the mode field distribution at the excitation light wavelength).
, The excitation light intensity (dashed line) has an axisymmetric distribution,
Its strength weakens as it moves away from the center of the core. Then, in the EDF 12, as shown in FIG. 2A, the core diameter is set small so that the mode field diameter at the excitation light wavelength λ _p is larger than the core diameter. By doing so, Er added to the core can be completely excited and an optical amplification effect can be efficiently obtained.

On the other hand, in the structure generally used for the optical fiber forming the optical fiber type coupler 11 and the optical fibers other than the EDF forming the optical fiber amplifier, the structure shown in FIG. As shown in, the core diameter is slightly smaller than the mode field diameter. If EDF
When 12 also has the structure shown in FIG. 2B, Er near the center of the core is completely excited, but Er near the edge of the core is not sufficiently excited because the excitation light intensity is weak. turn into. When the signal light is incident with the spatially non-uniform population inversion formed, the optical signal is amplified by stimulated emission in the center of the core, but is not excited near the edge of the core. Induced absorption due to Er occurs, and a sufficient light amplification effect cannot be obtained.

By the way, the core diameter 2a and the mode field diameter (hereinafter abbreviated as MFD) have a relationship represented by the following mathematical expression (I).

[Equation 1] Here, V is called a normalized frequency and is a parameter indicating a mode state in the optical fiber. This is determined by the core diameter of the optical fiber, the core refractive index, the relative refractive index difference between the core and the clad, and the wavelength of light, and is calculated by the following formula (II).

[Equation 2] That is, as shown in FIG. 2A, the EDF having a large MFD ratio (MFD / 2a) to the core diameter 2a is
Is set to be small, and V is set to be large in the optical fiber as shown in FIG.

However, there is a difference in core diameter between the EDF whose core diameter is set small as described above and the other optical fibers constituting the optical fiber amplifier.
It causes connection loss. Therefore, when connecting these optical fibers having different core diameters, the connection loss is reduced by matching the MFDs at the end faces of both fibers.

As one of the techniques for matching the MFD, there is a technique for thermally diffusing the dopant contained in the core.
As such a dopant, one that can increase the refractive index of the core and is easily diffused by heating is used, and germanium is preferably used. Then, as shown in FIG. 3A, when connecting two optical fibers having different core diameters, first, the end portions of both optical fibers are heated to diffuse the germanium contained in the core. These MFDs are controlled by causing the core diameter to substantially increase.

At this time, even if the core diameter is doubled, for example, the dopant that has contributed to maintaining the relative refractive index difference between the core and the clad is diffused, and its amount does not change. Therefore, the concentration becomes 1/2 ² = 1/4, and thus the relative refractive index difference between the core and the cladding becomes 1/4. In the above formula (II), the core diameter 2a is doubled,
It can be seen that when the relative refractive index difference Δ between the clads is ¼, V is constant and does not change. Therefore, since V is constant in the above formula (I), it can be seen that when the core diameter 2a is increased, the MFD is increased in proportion to this. In this way, as shown in FIG. 3B, the optical fibers whose mode field diameters (MFDs) are matched are fusion-spliced with their end faces abutted to each other, whereby the MFDs are matched. A connection can be formed.

[0009]

As described above, the dopant in the core is thermally diffused to increase the substantial core diameter,
This method of achieving MFD matching in the connection section is effective when the number of wavelengths used is one, but two lights having different wavelengths of signal light and pumping light are propagated like an optical fiber amplifier. In this case, there is a problem in that the MFDs of both wavelengths cannot be matched at the same time, and one of the lights may have a large connection loss.

[0010] That is, the normalized frequency V _S of the signal light wavelength lambda _S as shown in equation (II), the pumping light wavelength lambda
_The value is different from the normalized frequency V _{P in P,} and therefore, according to the formula (I), the MFD is also different for different wavelengths. Further, the larger the difference between λ _S and λ _P, the larger the difference between V _S and V _P , and therefore the MF at both wavelengths.
The difference in D also becomes large. Furthermore, FIG. 4 shows the ratio (MF) of the MFD to the core diameter 2a represented by the formula (II).
The relationship between the D / 2a) and the normalized frequency V is shown in the graph. From this, in the EDF in which V is set to be particularly small, the MFD with respect to the difference between V _S and V _P. It can be seen that the difference between is remarkable.

For example, in the optical fiber amplifier using the EDF as shown in FIG. 1, in order to effectively obtain the optical amplification effect, the signal light is connected at the connection point S ₁ between the optical fiber type coupler 11 and the EDF 12. A small connection loss is required for both the wavelength and the pumping light wavelength. EDF12
Excitation light used in the optical fiber amplifier using the, lambda _P
= 0.98 μm and λ _P = 1.48 μm, one of them is selected and used, and the signal light is λ _S = 1.55.
μm. Therefore, λ _P = 1.48 μm, λ _S = 1.5
In the case of 5 μm, the difference between λ _S and λ _P is relatively small, so V _S
The difference between V _P and V _P is also small, and MFD / 2a is a value close to each other. Therefore, at this time, if the matching of the MFD at one wavelength of the signal light or the pumping light is achieved, the matching of the MFD at the other wavelength can be almost obtained. However, when λ _P = 0.98 μm and λ _S = 1.55 μm, the difference between both wavelengths is large at 0.98 / 1.55≈63%, so the difference between V _S and V _P is large, and MFD / The difference of 2a also becomes large. In this case, even if the MFD at one wavelength is matched, the MFD at the other wavelength cannot be matched. This is shown in FIG. For example, in the EDF in which V _P = 2.2 is set small at the pumping light wavelength λ _P = 0.98 μm, the signal light wavelength λ _S = 1.
At 55 μm, V _S = 1.39, and the difference in MFD / 2a at both wavelengths becomes as large as 0.83 (indicated by ● in the figure). On the other hand, for example, the pumping light wavelength λ
_In an ordinary optical fiber in which V _P = 3.0 is set large at _P = 0.98 μm, the signal light wavelength λ _S =
In 1.55μm V _S = 1.89, and the difference between MFD / 2a at both wavelengths is relatively small as 0.4 (in the figure indicated by ○).

The problem of connection loss at two different wavelengths is the same for optical fiber lasers. Laser light oscillation can be obtained by using a rare earth-doped optical fiber as a gain medium, inputting spontaneous emission light obtained by injecting pumping light into the gain medium, and then returning the light to the gain medium. Are known. Even in an optical fiber laser using such a rare earth-doped optical fiber, light having two different wavelengths, pumping light and oscillating light, is propagated in a connection portion between the rare earth-doped optical fiber and another optical fiber constituting the laser. In this case, it is required to simultaneously achieve low splice loss for these lights.

The present invention has been made in view of the above circumstances, and has a small connection loss for both signal light (or oscillated light) and pumping light at a connecting portion between a rare earth-doped optical fiber and another optical fiber. An object is to provide an optical fiber amplifier and a laser that can be suppressed.

[0014]

In order to solve the above problems, an optical fiber amplifier according to claim 1 of the present invention is an optical fiber amplifier using a rare earth-doped optical fiber having a core doped with rare earth as a gain medium. The MFD at the pumping light wavelength and the MFD at the signal light wavelength of both fibers are matched at the connection between the rare earth-doped optical fiber and another optical fiber connected thereto. is there. In the optical fiber amplifier according to claim 2, the rare earth-doped optical fiber and another optical fiber connected thereto have the same normalized frequency, and the rare earth-doped optical fiber and another optical fiber connected thereto. Contains a dopant that is easily diffused by heating in the core, and the core is expanded in diameter by the diffusion of the dopant at these connecting portions to obtain the MFD conformity as described above.

An optical fiber laser according to a third aspect of the present invention is an optical fiber laser using a rare-earth-doped optical fiber having a core doped with rare-earth as a gain medium, and the rare-earth-doped optical fiber and another optical fiber connected thereto. The MFD at the pumping light wavelength and the MFD at the oscillating light wavelength of both fibers are matched at the connection with the optical fiber. Further, in the optical fiber laser according to claim 4, the rare earth-doped optical fiber and another optical fiber connected thereto have the same normalized frequency, and the rare earth-doped optical fiber and another optical fiber connected thereto. Contains a dopant that is easily diffused by heating in the core, and the core is expanded in diameter by the diffusion of the dopant at these connecting portions to obtain the MFD conformity as described above.

[0016]

According to the optical fiber amplifier using the rare earth-doped optical fiber of the present invention, the MFD and the signal at the pumping light wavelengths of both the rare earth-doped optical fiber and the other optical fiber connected thereto are connected. MF at light wavelength
Since all D are matched, it is possible to suppress the connection loss of the signal light and the pumping light at the connection portion,
A highly efficient optical amplification effect can be obtained. Similarly, according to the optical fiber laser of the present invention, at the connecting portion between the rare earth-doped optical fiber and another optical fiber connected thereto, the MFD at the pumping light wavelength and the M at the oscillating light wavelength of both fibers.
Since the FDs are matched, the connection loss between the oscillating light and the pumping light can be suppressed to be small, and a highly efficient laser light oscillating action can be obtained. Specifically, in both the optical fiber amplifier and the optical fiber laser, the normalized frequencies of the rare earth-doped optical fiber and other optical fibers connected thereto are set to be equal to each other, and the rare earth-doped optical fiber and the rare earth-doped optical fiber are also set. A dopant that is easily diffused by heating is added into the core of another optical fiber connected to the optical fiber, and the dopant is heated and diffused to expand the diameter of the core to form a connection portion. Consistency can be obtained at the same time.

[0017]

The present invention will be described in detail below. The optical fiber amplifier of the present invention can use the basic configuration shown in FIG. The optical fiber amplifier of the present invention is greatly different from the conventional one at the connection point S ₁ between the rare earth-doped optical fiber 12 and the optical fiber type coupler 11 at the signal light wavelength λ _S of both fibers and the pumping light wavelength. The point is that all MFDs at λ _P are matched. As the rare earth-doped optical fiber 12, a core to which a rare earth, preferably erbium (Er) is added, is used.

In the optical fiber amplifier of this embodiment,
At least a rare-earth doped optical fiber 12, the optical fibers constituting the optical fiber coupler 11 connected thereto, the normalized frequency V _P in the normalized frequency V _S and the pumping light wavelength lambda _P in the signal light wavelength lambda _S Both are set equally. The normalized frequency V is calculated by the above formula (II) and is set by appropriately adjusting the core diameter 2a, the core refractive index n ₁ and the clad refractive index n ₂ . It should be noted that the optical fiber forming the optical fiber type coupler and the rare earth-doped optical fiber may have the same V _S and V _P, and may have different core diameters and relative refractive index differences. That is, since it is from Equation _{(II) V = (2aπn 1} √2 △) / λ, so that the value of (2aπn ₁ √2 △) of optical fibers connected doped optical fiber 12 and to earth is equal If set, both V _P and V _S have the same value between both fibers. Here, it is desirable that the normalized frequencies V _S and V _P be equal between both fibers, but they need not necessarily be exactly equal, and the difference between them will lead to an increase in splice loss. It may be set within the allowable range.

Further, the rare earth-doped optical fiber 12, the optical fiber forming the optical fiber type coupler 11 connected to the rare earth-doped optical fiber 12, and preferably the optical fiber connected to the emitting side of the rare earth-doped optical fiber 12 are provided in the core thereof. A dopant which is easily diffused by heating is added. The dopant used here is preferably germanium. Then, the diffusion of the dopant expands the core diameter of the connection portion, and the MFD is matched as necessary.
Here, in the connecting portion of the rare earth-doped optical fiber 32 and the optical fiber type coupler 31, these normalized frequencies V _P and V _S are both set to be equal, and the normalized frequency is the thermal diffusion of the dopant as described above. Does not change depending on
Therefore, from the above formula (I), the pumping light wavelength (λ _P )
Alternatively, if the MFDs of one wavelength of the signal light wavelength (λ ^S ) are matched, the MFDs of the other wavelength can be matched at the same time. In practice, by making the core diameters (2a) of both fibers substantially the same, the MFD
Can be matched.

To connect the rare earth-doped optical fiber 12 to another optical fiber, as shown in FIG. 3 above, first, the end portion of the optical fiber to be connected is heated to remove the dopant contained in the core. Diffuse, which substantially expands the core diameter. The heating temperature at this time may change depending on the composition of glass constituting the optical fiber, etc.
It is preferably set to about 700 to 2000 ° C. Then, if necessary, the heating time is controlled while observing the MFD at the wavelength of the light propagating through the connection point (signal light wavelength λ _S or pumping light wavelength λ _P ) to obtain the desired MFD. In this way, the end faces of the connection ends whose MFDs are matched are butted against each other, and these are fusion-spliced by a known method.

Alternatively, as a method of connecting the rare earth-doped optical fiber 12 to another optical fiber, first, after fusion splicing, the connecting portion is heated to diffuse the dopant in the core to obtain MFD matching. You can also In this case, it is necessary to change the thermal diffusion coefficient of the dopant in the rare earth-doped optical fiber 12 and the other optical fiber. For example, germanium is added to the core of the rare earth-doped optical fiber and fluorine is added to the clad. On the other hand, germanium is added to the core of another optical fiber, and the cladding is made of silica containing no dopant.
In this way, by making the composition of the cladding different, the thermal diffusion coefficient of the dopant in the core becomes different. Here, since the rare earth-doped optical fiber 12 has a larger thermal diffusion coefficient, by appropriately setting the heating time, the fusion spliced rare earth-doped optical fiber and the dopant of another optical fiber are simultaneously heated and diffused. MFDs at the connection can be matched.

Thus, in this embodiment, the connection point S ₁ between the rare earth-doped optical fiber 12 and the optical fiber type coupler 11 is
Then, MFD at both wavelengths of signal light and pump light
Are aligned. Therefore, the pumping light and the signal light can be efficiently incident on the rare earth-doped optical fiber. Further, at the connection point S ₂ between the rare earth-doped optical fiber 12 and the optical fiber connected to the output side thereof, due to thermal diffusion of the dopant in the core, only M for the signal light wavelength.
The FD conformity is satisfied, and it is possible to suppress the connection loss of the signal light pumped by the rare earth-doped optical fiber and efficiently emit the signal light. Therefore, it is not necessary to set the normalization frequency V to be equal at this connection point S ² .

In such an optical fiber amplifier,
Since the connection loss is suppressed to be small for both the signal light wavelength and the pumping light wavelength, when the ratio of the pumping light wavelength to the signal light wavelength (λ _P / λ _S × 100) is 80% or less, these wavelengths are separated. In this case, a highly efficient optical amplification effect can be obtained.

FIG. 5 shows a second configuration example of the optical fiber amplifier of the present invention. In this example, the signal light is incident from one of the rare earth-doped optical fibers 22 and the excitation light is incident from the other through the optical fiber type coupler 21. In addition, the amplified signal light emitted from the rare earth-doped optical fiber 22 is the optical fiber coupler 2
1 and an isolator 23 which is preferably provided. In this optical fiber amplifier, at the connection point S ₄ between the rare earth-doped optical fiber 22 and the optical fiber type coupler 21, the normalized frequencies at the signal light wavelength and the pumping light wavelength are set to be equal between both fibers. When connecting these, the MF of the connection end face of both fibers is caused by thermal diffusion of the dopant in the core.
It is formed so that D matches. At the connection point S ₃ between the rare earth-doped optical fiber 22 and the signal light source and the connection point S ₆ between the optical fiber type coupler 21 and the optical isolator 23, the MFD at the signal light wavelengths of both fibers is caused by the thermal diffusion of the dopant in the core. Are formed so as to coincide with each other, and the connection point S between the optical fiber type coupler 21 and the excitation light source is
_{In the case of 5} , similarly, M at the pumping light wavelength of both fibers
The FDs are formed so as to match each other. It is not necessary to set the normalized frequencies V of both fibers to be equal to each other except the connection point S ₄ .

In such an optical fiber amplifier,
With respect to both the signal light wavelength and the pumping light wavelength, the connection loss at each connection point is suppressed to be small, and the optical amplification effect can be efficiently obtained.

Example 1 An erbium-doped optical fiber (EDF) having the parameters shown in Table 1 below was used as the rare earth-doped optical fiber 22 to fabricate an optical fiber amplifier having the configuration shown in FIG. This rare earth doped optical fiber 2
As shown in Table 1 below, the optical fiber A of the WDM optical fiber coupler 21 connected to No. 2 has a core diameter and a relative refractive index difference different from those of the EDF, and has a V _P and a signal light wavelength at the pumping light wavelength. Were formed so that V _S in each of them was equal to that of EDF. At the connection point S ₄ between this EDF and the optical fiber A, the connection loss was measured at the pumping light wavelength λ _P = 0.98 μm and the signal light wavelength λ _S = 1.55 μm. Was also a small value at the wavelength of. Further, the connection losses at the connection points S ₃ , S ₅ , and S ₆ other than S ₄ were all suppressed to 0.1 dB or less.

[0027]

[Table 1]

Further, a laser beam having a wavelength of 0.98 μm and a signal light of 1.55 μm are made incident on the optical fiber amplifier as excitation light, and amplified signal light (wavelength 1.55 μm). Got The ratio of the saturated output light to the pumping light power at this time (conversion efficiency) was examined. Signal light input is -5 dBm, pumping light power is 60 m
When W was set, the conversion efficiency was 53%. Note that this conversion efficiency is obtained by measuring the signal light output power at the point S _{7 with} respect to the pumping light incident power at the point S ₅ in FIG. 5, and does not include the loss in the isolator 23 or the like.

(Comparative Example 1) An optical fiber amplifier was used in the same manner except that the optical fiber B having the parameters shown in Table 1 was used as the optical fiber of the WDM optical fiber coupler 21 connected to the rare earth-doped optical fiber 22. Configured. The connection loss at the connection point S ₄ between the EDF and the optical fiber B was measured in the same manner as in Example 1 above, and it was a large value as shown in Table 1 above. Moreover, when the ratio of the saturated output light to the pumping light power (conversion efficiency) was examined in the same manner as in Example 1, it was 38%.
From the results of Example 1 and Comparative Example 1 above, it was confirmed that according to the present invention, a high-efficiency optical fiber amplifier can be obtained by suppressing the connection loss at the connection between the rare earth-doped optical fiber and another optical fiber. It was

FIG. 6 shows a configuration example of a ring resonance type optical fiber laser as an example of the optical fiber laser of the present invention. In the figure, reference numeral 31 is a WDM optical fiber coupler, 32 is a rare earth-doped optical fiber, and 33 is an optical isolator. The pumping light (wavelength λ _P ) is incident on the ring resonator formed of the rare earth-doped optical fiber 32 via the optical fiber type coupler 31 and pumps the rare earth-doped optical fiber 32. Excited rare earth-doped optical fiber 32
Is the wavelength λ emit spontaneous emission light of _S, the wavelength λ ring resonator 3 again through the optical fiber coupler 31 the light of _S
By oscillating light having a wavelength λ _s , the light is incident on the optical fiber 2 and the optical feedback is applied to the rare earth-doped optical fiber 32. The oscillated light (wavelength λ _S ) oscillated here is partly extracted through the optical fiber type coupler 31.

In this optical fiber laser, at least the optical fiber type coupler 31 and the rare earth-doped optical fiber 3 are used.
At the connection point S ₈ with 2, the MFD matching is simultaneously obtained for the two lights having different wavelengths, the excitation light and the oscillation light. That is, at least the rare-earth-doped optical fiber 32 and the optical fiber forming the optical fiber coupler 31 connected thereto have a normalized frequency V _{P at the} excitation light wavelength λ _P and a normalized frequency V _{S at the} oscillation light wavelength λ _S. Both and are set equally. Further, the rare-earth-doped optical fiber 32 and the optical fiber forming the optical fiber type coupler 31 connected to the rare-earth-doped optical fiber 32 are doped with a dopant which is easily diffused by heating in the core thereof. Then, the rare earth element-doped optical fiber 32 is diffused by heating the connection end to diffuse the dopant.
And M at the connection end with the optical fiber connected to this
The FDs are formed so as to match each other. Various kinds of rare earth may be added to the core of the rare earth-doped optical fiber 32, and oscillated light having different wavelengths can be obtained depending on the kind of rare earth. The rare earth-doped optical fiber 32 and the optical fiber type coupler 31 can be connected in the same manner as in the case of the optical fiber amplifier described above.

At the connection point S ₈ between the rare earth-doped optical fiber 32 and the optical fiber type coupler 31 formed in this way,
Since these normalized frequencies V _S and V _P are set to be equal, if the MFDs of one of the pumping light wavelength (λ _P ) and the oscillating light wavelength (λ ^S ) are matched, the other simultaneously. The MFD matching is also obtained for the wavelengths of. Therefore, the excitation light and the oscillation light are efficiently incident on the rare earth-doped optical fiber. In such an optical fiber laser, it is possible to suppress the connection loss with respect to both the pumping light wavelength and the oscillation light wavelength. Therefore, even when the pumping light wavelength and the oscillation light wavelength are separated from each other, the high It is possible to obtain an efficient laser light oscillation action.

The optical fiber amplifier and the optical fiber laser using the rare earth-doped optical fiber of the present invention are not limited to the above-mentioned configurations, but various configurations are possible. Then, at a connection point between the rare earth-doped optical fiber and another optical fiber,
Regarding the point where multiple lights with different wavelengths are simultaneously propagated,
It is effective to set the normalization frequency V equal to each other, match the MFDs by heat diffusion of the dopant, and simultaneously match the MFDs for both wavelengths.

[0034]

As described above, the optical fiber amplifier using the rare earth-doped optical fiber of the present invention pumps both the rare earth-doped optical fiber and another optical fiber connected thereto. Both the MFD at the light wavelength and the MFD at the signal light wavelength are matched. Therefore, at the connection between the rare earth-doped optical fiber and another optical fiber, the connection loss of both the signal light and the pumping light can be reduced. Therefore, the ratio of the pumping light wavelength to the signal light wavelength (λ _P / λ _S ×
100) is 80% or less, a highly efficient optical amplification effect can be obtained even when these wavelengths are distant from each other.

Further, the optical fiber laser using the rare earth-doped optical fiber of the present invention is a connection portion between the rare earth-doped optical fiber and another optical fiber connected thereto, and the MFD and the oscillation light at the pumping light wavelength of both fibers. M at wavelength
The FDs are all matched. Therefore, at the connection between the rare earth-doped optical fiber and another optical fiber, it is possible to reduce the connection loss for both the signal light and the oscillation light. Therefore, the ratio of the excitation light wavelength to the oscillation light wavelength (λ _P / λ _S × 100) is 80%.
Even if these wavelengths are separated from the following wavelengths, a highly efficient laser light oscillation action can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of an optical fiber amplifier.

FIG. 2 is an explanatory diagram showing a core diameter and a mode field diameter in a rare earth-doped optical fiber and another optical fiber.

FIG. 3 is a process drawing showing an example of fusion splicing of optical fibers having different core diameters.

FIG. 4 is a graph showing the relationship between the ratio of the mode field diameter to the core diameter and the normalized frequency.

FIG. 5 is a schematic configuration diagram showing another example of the optical fiber amplifier.

FIG. 6 is a schematic configuration diagram showing an example of an optical fiber laser.

[Explanation of symbols]

 11, 21, 31 ... Optical fiber type coupler 12, 22, 32 ... Rare earth doped optical fiber

Claims (4)

[Claims] 1. An optical fiber amplifier using, as a gain medium, a rare earth-doped optical fiber in which a rare earth is added to a core, wherein a rare earth-doped optical fiber and another optical fiber connected to the optical fiber An optical fiber amplifier characterized in that both the mode field diameter at the pumping light wavelength and the mode field diameter at the signal light wavelength are matched. 2. The rare earth-doped optical fiber and other optical fibers connected thereto have the same normalized frequency, and the rare earth-doped optical fiber and the other optical fiber connected thereto are heated in the core. 2. The optical fiber amplifier according to claim 1, further comprising a dopant which is easily diffused, wherein the core is expanded in diameter by diffusion of the dopant at these connecting portions. 3. An optical fiber laser using a rare earth-doped optical fiber having a core doped with rare earth as a gain medium, wherein the rare earth-doped optical fiber and another optical fiber connected to the optical fiber are connected to each other. An optical fiber laser characterized in that both the mode field diameter at the pumping light wavelength and the mode field diameter at the oscillating light wavelength are matched. 4. The rare earth-doped optical fiber and another optical fiber connected thereto have the same normalized frequency, and the rare earth-doped optical fiber and the other optical fiber connected thereto are heated in the core. 4. The optical fiber laser according to claim 3, wherein the core contains a dopant which is easily diffused, and the core is expanded in diameter by diffusion of the dopant at these connecting portions.