JP2004120002A - Optical fiber booster and optical fiber laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a connection loss of both signal light and exciting light in a joint of a rare-earth-elements-doped optical fiber of an optical fiber booster, which is prepared with the rare-earth elements optical fiber, doped with the rare-earth-elements into the core, as the medium. <P>SOLUTION: Normal frequencies of both rare-earth-elements-doped optical fiber 12 and optical fiber, made up of a optical fiber type coupler 11 connected with the optical fiber 12 are set to be equal. A dopant, which tends to be diffused by heating, is doped in the cores of both fibers, and the diameters of cores are enlarged in those joints, by diffusing the dopant to make the mode field diameters of both fibers match each other. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to an optical fiber amplifier and a laser using a rare-earth-doped optical fiber, and relates to a technique capable of reducing a connection loss at a connection portion between the rare-earth-doped optical fiber and another optical fiber.

FIG. 1 illustrates the basic configuration of an optical fiber amplifier. In the figure, reference numeral 11 denotes a wavelength division multiplexing (WDM type) optical fiber coupler, 12 denotes an amplification fiber, 13 denotes an optical isolator, and 14 denotes a wavelength selection filter. The amplification fiber 12 is a rare earth-doped optical fiber, for example, an erbium-doped optical fiber (EDF). In this optical fiber amplifier, the excitation light having a wavelength of lambda _P to EDF12 via the optical fiber coupler 11 is incident, erbium (Er) ions are added to the core of the EDF12 is excited. The energy stored in the excited Er ions is converted into light of the same wavelength (λ _S ) by stimulated emission when the signal light of the wavelength λ _S enters the EDF 12 via the optical fiber coupler 11. This causes an amplification effect. The optical signal amplified by the stimulated emission is emitted through an optical isolator 13 and a wavelength selection filter 14 provided as necessary.
Such excitation light used in the optical fiber amplifier using the EDF is generally used to select one of the two wavelengths of lambda _{P =} 0.98 .mu.m and lambda _{P =} 1.48 .mu.m, the wavelength lambda An amplified light of _S = 1.55 μm is obtained.
Generally, in order to obtain stable operation characteristics, a single-mode optical fiber is substantially used for an optical amplifier using a rare-earth-doped optical fiber.

In such an optical fiber amplifier using the EDF 12, Er (rare earth) is usually added almost uniformly in the core, and the pumping light power density in the core must be high in order to increase the pumping efficiency of the EDF 12. Is preferred. Therefore, in the EDF 12, the relative refractive index difference between the core and the clad is generally as large as 1% or more, and the core diameter is set to be small.
For example, in an EDF having a step-type refractive index distribution as shown in FIG. 2 (solid line in FIG. 2 indicates refractive index distribution, broken line indicates mode field distribution at excitation light wavelength), excitation light intensity (dashed line) is axially symmetric. It has a distribution, the strength of which decreases with distance 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, it is possible to completely excite Er added to the core and obtain an optical amplification effect efficiently.

On the other hand, in a structure generally used for an optical fiber constituting the optical fiber coupler 11 or an optical fiber other than the EDF constituting the optical fiber amplifier, as shown in FIG. In addition, the core diameter is slightly smaller than the mode field diameter. If the EDF 12 also has a structure as shown in FIG. 2B, Er near the center of the core is completely excited, but Er near the edge of the core is insufficiently excited due to a weak excitation light intensity. Will be. When signal light is incident in such a state that a spatially nonuniform population inversion is formed, an optical signal is amplified by stimulated emission at the center of the core, but is not excited near the edge of the core. Induced absorption by Er occurs, and a sufficient light amplification effect cannot be obtained.

By the way, there is a relationship represented by the following formula (I) between the core diameter 2a and the mode field diameter (hereinafter abbreviated as MFD).

ＶHere, V is called a normalized frequency, and is a parameter indicating the 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 equation (II).

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

However, there is a core diameter difference between the EDF whose core diameter is set to be small as described above and another optical fiber constituting the optical fiber amplifier, which 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 the two fibers.

As one of the techniques for matching the MFD, there is a technique for thermally diffusing a dopant contained in a 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 ends of both optical fibers are heated to diffuse germanium contained in the core. These MFDs are controlled by substantially increasing the core diameter.

At this time, even if the core diameter is enlarged, for example, by a factor of two, the amount of the dopant that has contributed to maintaining the relative refractive index difference between the core and the clad has not changed since it has only been diffused. Becomes 1/2 ² = １ ／, so that the relative refractive index difference between the core and the clad becomes 1/4. In the above formula (II), when the core diameter 2a is doubled and the relative refractive index difference 間 の between the core and the clad becomes １ ／, it is understood that V is constant and does not change.
Therefore, since V is constant in the above equation (I), it can be seen that if the core diameter 2a is increased, the MFD is increased in proportion to this.
In this way, as shown in FIG. 3B, fusion splicing of an optical fiber whose mode field diameter (MFD) is matched is performed with the end faces abutting each other, so that the MFD is matched. Connections can be formed.

As described above, the method of thermally diffusing the dopant in the core to expand the substantial core diameter and thereby achieve the matching of the MFD at the connection portion is effective when only one wavelength is used. When two lights having different wavelengths of signal light and pump light are propagated as in an optical fiber amplifier, the MFD consistency at both wavelengths cannot be obtained at the same time, and the connection loss of one light is large. There was a problem that sometimes it became.

That is, the normalized frequency V _S of the signal light wavelength lambda _S as shown in equation (II), a value different from the normalized frequency V _P at the excitation wavelength lambda _P, thus from equations (I), different The MFD also has a different value for the wavelength. Moreover, lambda difference larger the difference between the _S and lambda _P V _S and V _P is increased, thus resulting in larger difference in MFD at both wavelengths.
FIG. 4 is a graph showing the relationship between the ratio (MFD / 2a) of the MFD to the core diameter 2a (MFD / 2a) represented by the formula (II) and the normalized frequency V. there has been set to the EDF, a difference of MFD relative difference between V _S and V _P is found to be significant.

For example, the optical fiber amplifier formed by using the EDF as shown in FIG. 1, effective to obtain an optical amplifying action, in the connection point S ₁ of the optical fiber coupler 11 and EDF12, the signal light wavelength and excitation It is required that the connection loss is small for both optical wavelengths.
The pump light used in the optical fiber amplifier using the EDF 12 is selected from two wavelengths, λ _P = 0.98 μm and λ _P = 1.48 μm, and the signal light is λ _S = 1.55 μm. It is. Therefore, λ _P = 1.48μm, in the case of lambda _S = 1.55 .mu.m, less the difference between _{V S} and _{V P} the difference between lambda _S and lambda _P is relatively small, MFD / 2a is substantially close values . Therefore, at this time, if the MFD matching is achieved at one of the wavelengths of the signal light and the pumping light, the MFD matching can be substantially obtained at the other wavelength.
However, lambda _P = 0.98 .mu.m, in the case of λ _S = 1.55μm, 0.98 / 1.55 the difference between ≒ 63% and both wavelengths is large, the difference between _{V S} and _{V P} is large, MFD / 2a also increases. In this case, even if MFD matching at one wavelength is achieved, MFD matching at the other wavelength cannot be obtained. This is shown in FIG. For example, in an EDF in which the pump light wavelength λ _P = 0.98 μm and V _P = 2.2 are set small, the signal light wavelength λ _S = 1.55 μm, V _S = 1.39, and the MFD at both wavelengths The difference of / 2a becomes as large as 0.83 (indicated by ● in the figure).
In contrast, for example, in a typical optical fiber which is greatly set with _V P = 3.0 in excitation wavelength _{λ P = 0.98μm, V S =} 1 in the signal light wavelength lambda _S = 1.55 .mu.m .89, and the difference between MFD / 2a at both wavelengths is relatively small at 0.4 (shown by ○ in the figure).

The problem of connection loss at two different wavelengths is the same for an optical fiber laser.
By using a rare-earth-doped optical fiber as a gain medium, spontaneous emission light obtained by injecting pump light into the gain medium and then re-entering the rare-earth-doped optical fiber and feeding back the gain medium, laser light oscillation can be obtained. Are known.
In an optical fiber laser using such a rare earth-doped optical fiber, light having two different wavelengths, that is, excitation light and oscillation light, is propagated at a connection between the rare earth-doped optical fiber and another optical fiber constituting the laser. In such a case, it is required to simultaneously achieve low connection loss for these lights.

The present invention has been made in view of the above circumstances, and in a connection portion between a rare-earth-doped optical fiber and another optical fiber, it is possible to reduce connection loss for both signal light (or oscillation light) and pump light. It is an object of the present invention to provide an optical fiber amplifier and a laser that can be used.

In order to solve the above problem, an optical fiber amplifier according to the present invention is an optical fiber amplifier using a rare earth-doped optical fiber obtained by adding a rare earth element to a core as a gain medium, and includes a rare earth-doped optical fiber and other connected thereto. At the connection with the optical fiber, the MFD at the pump light wavelength and the MFD at the signal light wavelength of both fibers are matched.

The optical fiber laser of the present invention is an optical fiber laser using a rare earth-doped optical fiber in which a rare earth is added to a core as a gain medium, and a connection portion between the rare earth-doped optical fiber and another optical fiber connected thereto. The MFD of both fibers at the pump light wavelength and the MFD of the oscillation light wavelength are matched.

The optical fiber amplifier using the rare earth-doped optical fiber of the present invention has a MFD at a pump light wavelength and an MFD at a signal light wavelength of both fibers at a connection portion between the rare earth-doped optical fiber and another optical fiber connected thereto. All are consistent.
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 pump light can be reduced.
Therefore, even when the ratio of the pumping light wavelength to the signal light wavelength (λ _P / λ _S × 100) is 80% or less and these wavelengths are apart from each other, a highly efficient optical amplification operation can be obtained.

Further, the optical fiber laser using the rare earth-doped optical fiber of the present invention comprises an MFD at an excitation light wavelength and an MFD at an oscillation light wavelength of both fibers at a connection portion between the rare earth-doped optical fiber and another optical fiber connected thereto. Are all 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 oscillating light can be reduced.
Therefore, even if the ratio of the pumping light wavelength to the oscillation light wavelength (λ _P / λ _S × 100) is 80% or less and these wavelengths are apart from each other, a highly efficient laser light oscillation action can be obtained.

Specifically, in both the optical fiber amplifier and the optical fiber laser, the normalized frequency of the rare-earth-doped optical fiber and the other optical fibers connected thereto are set to be equal, and the rare-earth-doped optical fiber and By adding a dopant that is easily diffused by heating into the core of another optical fiber connected to the optical fiber and by heating and diffusing the dopant to expand the core and form a connection portion, the MFD of light having different wavelengths is reduced. Consistency can be obtained at the same time.

Hereinafter, the present invention will be described in detail.
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 significantly different from the conventional one in that the MFD and the pumping light wavelength at the signal light wavelength λ _S of both fibers at the connection point S ₁ between the rare earth-doped optical fiber 12 and the optical fiber type coupler 11. The point is that the MFDs at λ _P are all matched.
As the rare earth-doped optical fiber 12, a core obtained by adding a rare earth element, preferably erbium (Er), is used.

In the optical fiber amplifier of the present embodiment, the normalized frequency V _S and the pump light at the signal light wavelength λ _S of at least the rare earth-doped optical fiber 12 and the optical fiber constituting the optical fiber coupler 11 connected thereto. normalized frequency V _P is set equal to none at the wavelength lambda _P.
Normalized frequency V is intended to be calculated by the formula (II), it is set by adjusting the core diameter 2a, core refractive index n _1, and a cladding refractive index n ₂ as appropriate. Note that the optical fiber and the rare earth doped optical fibers constituting the optical fiber coupler may equal the V _S and V _P, core diameter, relative refractive index difference may have different.
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 by setting, V _P also V _S is also consistent with a value between the two fibers.
Here, the normalized frequency V _S and V _P is preferably equal between the two fibers, may or may not be necessarily exactly equal, because these differences leads to increased connection loss, connection loss by the application or the like that What is necessary is just to make it fall within an allowable range.

The rare earth-doped optical fiber 12, the optical fiber constituting the optical fiber type coupler 11 connected thereto, and the optical fiber connected to the output side of the rare earth-doped optical fiber 12 are preferably diffused into the core by heating. Dopant that is easily added is added. As the dopant used here, germanium is preferable. Then, the core diameter of the connection portion is enlarged by the diffusion of the dopant, and the MFD is matched as necessary.
Here, the connection portion of the rare earth doped optical fiber 32 and the optical fiber coupler 31, and these normalized frequency V _P and V _S is set equal to either the normalized frequency as described above the thermal diffusion of the dopant Does not change. Therefore, according to the above formula (I), if the MFDs are matched for either one of the pump light wavelength (λ _P ) and the signal light wavelength (λ _S ), the MFD consistency is obtained for the other wavelength at the same time. Can be In practice, the MFDs can be matched by making the core diameters (2a) of both fibers substantially equal.

The connection between the rare-earth-doped optical fiber 12 and another optical fiber is achieved by first heating the end of the optical fiber to be connected, thereby diffusing the dopant contained in the core, as shown in FIG. This substantially increases the core diameter. The heating temperature at this time can vary depending on the composition of the glass constituting the optical fiber and the like, but is preferably set to about 1700 to 2000 ° C. If necessary, the heating time is controlled while observing the MFD at the wavelength of the light propagating through the connection point (the signal light wavelength λ _S or the pump light wavelength λ _P ) to obtain a desired MFD.
In this manner, the end faces of the connection ends whose MFDs are matched with each other are joined together, and they are fusion-spliced by a known method.

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

Thus, in this embodiment the connecting point S ₁ of the rare earth doped optical fiber 12 and the optical fiber coupler 11, MFD at a wavelength of both the signal light and pumping light are aligned. Therefore, the pump light and the signal light can be efficiently incident on the rare-earth-doped optical fiber.
In the connection point S ₂ of the optical fiber connected to the exit side of the rare earth-doped optical fiber 12, by thermal diffusion of dopants in the core, and integrity of the MFD only the signal light wavelengths is satisfied, the rare-earth-doped It is possible to efficiently output the signal light pumped by the optical fiber while suppressing the connection loss. Thus, this connecting point S ₂ does not need to set particular equal to the normalized frequency V.

In such an optical fiber amplifier, the ratio of the pumping light wavelength to the signal light wavelength (λ _P / λ _S × 100) because the connection loss is suppressed to be small for both the signal light wavelength and the pumping light wavelength. Is 80% or less, a highly efficient optical amplification action can be obtained even when these wavelengths are apart from each other.

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 via the optical fiber coupler 21. The amplified signal light emitted from the rare-earth-doped optical fiber 22 is output via an optical fiber coupler 21 and a preferably provided isolator 23.
In this optical fiber amplifier, the connecting point S ₄ of the rare earth-doped optical fiber 22 and the optical fiber coupler 21, the normalized frequency is set equal to the signal light wavelength and the pumping light wavelength between the two fibers. Also, at the time of these connections, the MFDs at the connection end faces of both fibers are formed so as to coincide with each other due to thermal diffusion of the dopant in the core.
At the connection point S ₃ between the rare earth-doped optical fiber 22 and the signal light source and at the connection point S ₆ between the optical fiber type coupler 21 and the optical isolator 23, the MFD at the signal light wavelength of both fibers due to thermal diffusion of the dopant in the core. There is formed so as to coincide, the connection point S ₅ of the optical fiber coupler 21 and pumping light source are formed so as MFD match at the excitation light wavelength of the two fibers in a similar manner.
Need not be set equal to the normalized frequency V of both fibers are particularly connected in other than the connection point S _4.

(5) In such an optical fiber amplifier, the connection loss at each connection point is suppressed to be small for both the signal light wavelength and the pump light wavelength, so that the optical amplification function can be obtained efficiently.

(Example 1)
Using an erbium-doped optical fiber (EDF) having the parameters shown in Table 1 below as the rare-earth-doped optical fiber 22, an optical fiber amplifier having the configuration shown in FIG. As shown in Table 1 below, the optical fiber A of the WDM optical fiber coupler 21 connected to the rare-earth-doped optical fiber 22 has a core diameter different from that of the EDF, a relative refractive index difference, and V at the excitation light wavelength. V _S at _P and the signal light wavelength is formed as either equal to that of EDF.
At the connection point _{S 4} of the EDF and optical fiber A, was measured splice loss at the pumping light wavelength lambda _P = 0.98 .mu.m and the signal light wavelength lambda _S = 1.55 .mu.m, as shown in Table 1, any It was also a small value at the wavelength.
In addition, the connection loss at the connection points S ₃ , S ₅ , and S ₆ other than S ₄ was suppressed to 0.1 dB or less.

Further, laser light having a wavelength of 0.98 μm was made incident on the optical fiber amplifier as excitation light, and light having a wavelength of 1.55 μm was made incident as signal light to obtain an amplified signal light (wavelength 1.55 μm). . At this time, the ratio (conversion efficiency) of the saturation output light to the pump light power was examined. When the signal light input was -5 dBm and the pump light power was 60 mW, the conversion efficiency was 53%.
Incidentally, the conversion efficiency, there is obtained by measuring the signal light output power at the point S ₇ with respect to the excitation light incident power in FIG midpoint S _5, do not include losses in the isolator 23 and the like.

(Comparative Example 1)
An optical fiber amplifier was constructed in the same manner except that the optical fiber B having the parameters shown in Table 1 above was used as the optical fiber of the WDM optical fiber coupler 21 connected to the rare earth-doped optical fiber 22.
The connection points S ₄ of the EDF and optical fiber B, was measured splice loss in the same manner as in Example 1, was a large value as shown in Table 1 above.
Further, the ratio (conversion efficiency) of the saturation output light to the pump light power was examined in the same manner as in Example 1 above, and it was 38%.
From the results of Example 1 and Comparative Example 1, 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. 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 drawing, reference numeral 31 denotes a WDM type optical fiber type coupler, 32 denotes a rare earth doped optical fiber, and 33 denotes an optical isolator. The pumping light (wavelength λ _P ) is incident on the ring-type resonator including the rare-earth-doped optical fiber 32 via the optical fiber-type coupler 31 and excites the rare-earth-doped optical fiber 32. Excited rare-earth doped optical fiber 32 emits spontaneous emission light of the wavelength lambda _S, the light of the wavelength lambda _S via the optical fiber coupler 31 is again incident on the ring resonator 32, the rare-earth doped optical fiber By applying optical feedback to 32, oscillated light of wavelength λ _S is obtained. A part of the oscillated light (wavelength λ _S ) oscillated here is taken out via the optical fiber coupler 31.

In the optical fiber laser, at least at the connection point S ₈ of the optical fiber coupler 31 and a rare earth doped optical fiber 32, different for the light of the two wavelengths of the excitation light and the oscillated light, so that the MFD of consistency can be obtained at the same time Is configured.
In other words, at least a rare-earth doped optical fiber 32, the optical fibers constituting the optical fiber coupler 31 to be connected thereto, the normalized frequency V _S at the normalized frequency V _P and the oscillation wavelength lambda _S at the excitation wavelength lambda _P And are both set equal.
The rare earth-doped optical fiber 32 and the optical fiber constituting the optical fiber coupler 31 connected thereto are doped with a dopant which is easily diffused by heating in the core. Then, the dopant is diffused by heating the connection end so that the MFD at the connection end of the rare-earth-doped optical fiber 32 and the optical fiber connected to the rare-earth-doped optical fiber 32 match.
Various kinds of rare earth elements can be used as the rare earth element added to the core of the rare earth-doped optical fiber 32, and oscillation light having different wavelengths can be obtained depending on the type of the rare earth element.
The connection between the rare earth-doped optical fiber 32 and the optical fiber coupler 31 can be made in the same manner as in the case of the above-described optical fiber amplifier.

Thus formed, the connection point S ₈ of the rare earth-doped optical fiber 32 and the optical fiber coupler 31, since these normalized frequency V _S and V _P is set equal to the excitation wavelength (lambda _P ) Or the oscillating light wavelength (λ _S ), the MFD can be matched for the other wavelength at the same time.
Therefore, the excitation light and the oscillation light are efficiently incident on the rare-earth-doped optical fiber.
In such an optical fiber laser, the connection loss can be suppressed both for the excitation light wavelength and the oscillation light wavelength. An efficient laser light oscillation action can be obtained.

Note that 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-described configurations, and various configurations are possible. Then, at the connection point between the rare-earth-doped optical fiber and another optical fiber, where a plurality of lights having different wavelengths are simultaneously propagated, the normalized frequency V is set to be equal, and the MFD is increased by the thermal diffusion of the dopant. It is effective to match and match the MFDs for both wavelengths at the same time.

FIG. 2 is a schematic configuration diagram illustrating an example of an optical fiber amplifier. It is explanatory drawing which shows the core diameter and mode field diameter in a rare earth addition optical fiber and other optical fibers. FIG. 4 is a process chart showing an example of fusion splicing of optical fibers having different core diameters. 4 is a graph showing a relationship between a ratio of a mode field diameter to a core diameter and a normalized frequency. FIG. 9 is a schematic configuration diagram illustrating another example of the optical fiber amplifier. FIG. 2 is a schematic configuration diagram illustrating an example of an optical fiber laser.

Explanation of reference numerals

# 11, 21, 31 ... optical fiber type coupler, 12, 22, 32 ... rare earth doped optical fiber.

Claims (2)

An optical fiber amplifier using a rare earth-doped optical fiber in which a rare earth is added to a core as a gain medium,
At the connection between the rare-earth-doped optical fiber and another optical fiber connected to it, both the mode field diameter at the pump light wavelength and the mode field diameter at the signal light wavelength of both fibers are matched. Optical fiber amplifier. An optical fiber laser using a rare earth-doped optical fiber in which a rare earth is added to a core as a gain medium,
At the connection between the rare-earth-doped optical fiber and the other optical fiber connected thereto, both the mode field diameter at the excitation light wavelength and the mode field diameter at the oscillation light wavelength of both fibers are matched. Optical fiber laser.

Images