JPH03132726A - Rare earth element-added optical fiber - Google Patents

Abstract

PURPOSE:To provide the rare earth element-added optical fiber which has high efficiency and can make stable amplification without degrading the amplification degree by the intensity distribution of stimulating light by providing a 1st core added with the rare earth element in he central part of the core and providing a 2nd core on the outside thereof, then forming a clad having the refractive index lower than the refractive index of the 2nd core on the outside of the 2nd core. CONSTITUTION:The core is divided to two regions; the 1st core 5 and the 2nd core 6. The rare earth element is added only to the inside of the 1st core 5. The 2nd core 6 is formed to flatten the power density distribution of the stimu lating light in the 1st core 5 by confining the light transmitted in the optical fiber 1 into the 1st core 5 and is designed to have the refractive index higher than the refractive index of he clad 7. Further, Er which is a laser active mate rial is added only into the 1st core 5 and, therefore, the Er in any part of the 1st core 5 is uniformly and sufficiently stimulated by the stimulating light, by which the stable light amplification is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a rare earth element-doped optical fiber with high efficiency and high amplification rate, which is used as an amplification medium for lasers, optical power amplifiers, and the like.

``Conventional technology 1'' In recent years, single-mode optical fibers with rare earth elements such as Nd (neodymium) and Er (erbium) added to the core have been developed. It has been reported that it has potential for use in the field of communications.

FIG. 17 shows an example of an optical amplifier using this rare-earth element-doped optical fiber, and reference numeral 10 indicates the rare-earth element-doped optical fiber. This rare earth element-doped optical fiber 10 has a core uniformly doped with a rare earth element as a laser active substance, and an optical fiber coupler 2 is connected to one end of the fiber, and a wavelength filter 3 is connected to the other end. There is. The optical fiber coupler 2 receives a signal light (incident light) having a wavelength λa and a wavelength λ for exciting this signal light.
After combining the pump light with the p excitation light, it is made to enter the rare earth element-doped optical fiber IO. A light source (not shown) is connected to the input ends 2 a and 2 b of the optical fiber coupler 2, respectively, so that a signal light having a wavelength λa and a pumping light having a wavelength λp can be input thereto.
A rare earth element-doped optical fiber 10 is provided at one output end 2c.
are connected, and the other output end 2d serves as a non-reflection termination 4. Wavelength filter 3 is rare earth element doped optical fiber IO
Inside, there is a trap for separating the signal light amplified by the excitation light, and it is designed to transmit only the light having the wavelength λa.

In order to operate this optical amplifier, a signal light having a wavelength λa and a pumping light having a wavelength λp are made to enter the optical fiber coupler 2 from the input ends 2a and 2b, respectively, and enter the rare earth element-doped optical fiber 10 connected to the output end 2c. to guide the wave. Then, the rare earth element doped into the rare earth element-doped optical fiber 10 is excited by this excitation light. When the signal light is propagated here, the signal light is amplified by several dB due to the rare earth element in an excited state. And rare earth element doped optical fiber I
Since both the amplified signal light and the excitation light are emitted from O, only the signal light is extracted by the wavelength filter 3, and thus the signal light is amplified.

In order to operate the rare earth element-doped optical fiber 10 as a laser medium with high efficiency, it is important to inject excitation light with sufficient intensity to excite the rare earth element doped into the core.

[Problems to be Solved by the Invention] By the way, the power density distributions of light at wavelengths λ2 and λ3 (where λ1>λ2>λ3) transmitted through a single mode optical fiber are shown in Fig. 18, respectively. As shown by the one-dot chain line, two-dot chain line, and the broken line, all of them show distributions close to Gaussian distribution. As the wavelength of the transmitted light becomes shorter, the confinement of light within the core becomes higher, and the steepness of the rise of the distribution curve at the boundary 7 between the core and the cladding becomes larger. Therefore, although the power density of the transmitted light at the center of the core becomes very high, the power density at the boundary between the core and the cladding decreases rapidly compared to the center of the core.

Taking an erbium-doped optical fiber that becomes a three-level laser as an example, signal light (wavelength λa) and pumping light (wavelength λp)
Since the relationship λa>λp always holds between
The wavelength of the excitation light is λp21.48 μm, which is 0.9
8 μm or the like is selected. However, since the wavelength of the excitation light is short, sufficient power density for excitation of erbium cannot be obtained in the core at the boundary with the club.

On the other hand, the signal light with wavelength λa = 1.535 μm has wavelength λp =
The relationship between the power density of the pumping light and the amplification degree of the signal light when amplified with the pumping light of 1.48 μm is as shown in FIG. 19. When the power density of the pumping light is below a certain value, the signal light is not amplified and becomes an attenuation region. This attenuation is a factor that reduces the gain of the amplification of the rare earth element-doped optical fiber 10, so in the rare earth element-doped optical fiber IQ, the amplification at the boundary with the cladding is equal to the amplification at the center of the core. It is very different. Furthermore, since the degree of amplification varies greatly depending on the wavelength of the pumping light, it has been difficult to amplify signal light with high efficiency and stability.

The present invention has been made to solve the above problems, and it is an object of the present invention to provide a rare earth element-doped optical fiber that is highly efficient and can perform stable amplification without decreasing the amplification degree due to the intensity distribution of excitation light. The purpose is

[Means for Solving the Problems] A rare earth element-doped optical fiber according to claim I of the present invention comprises a core and a cladding, and a first core doped with a rare earth element in the center of the core. The solution is that a second core is provided outside the second core, and a cladding having a refractive index lower than that of the second core is formed outside the second core.

The rare earth element-doped optical fiber according to claim 2 of the present invention is a single mode fiber in which the refractive index of the first core is lower than the refractive index of the second core.

[Action Since the second core is interposed between the first core and the cladding, which are made by adding a rare earth element as a colaser active material,
The power density of the transmitted light changes sharply within the second core. Therefore, the power density distribution of the pumping light transmitted within the first core becomes uniform, and the factors that reduce the amplification degree are eliminated, so that the signal light can be amplified with high efficiency and stability.

The present invention will be described in detail below.

FIG. 1 shows an example of a rare earth element-doped optical fiber according to claim 1 of the present invention.

This rare earth element-doped optical fiber 1 differs from the conventional one shown in FIG. 17 by dividing the core into two regions, a first core 5 and a second core 6, and Rare earth elements have been added only to.

This second core 6 confines the light transmitted within the optical fiber 1 within the first core 5 so that the power density distribution of the excitation light within the first core 5 becomes flat. It is designed to have a higher refractive index than the cladding 7.

Er (erbium) as a rare earth element is added to the first core 5 at a concentration of t o ppm, and the refractive index of the first core 5 is n5, the refractive index of the second core 6 is n6, and the refractive index of the cladding 7 is n7. and, n, -1i, = I, 458, n7=
1.452, the refractive index distribution and Er doping concentration distribution of the rare earth element-doped optical fiber l and the power density distribution of the signal light with wavelength λa and the pumping light with wavelength λp are also shown in FIG.

In the rare earth element-doped optical fiber 1 having such a configuration, the transmitted light is confined within the first core 5 by the second core 6, and the power density distribution of the light within the first core 5 is flat. In addition, since the laser active material Er is added only in the first core 5, the Er in any part of the first core 5 can be uniformly and sufficiently excited by the excitation light. . Therefore, stable optical amplification can be performed.

In addition to Er, rare earth elements added to the first core 5 include Nd (neodymium), yb (ytterbium), and S.
m (samarium) can be used. The doping concentration of these rare earth elements is appropriately selected depending on the core diameter and refractive index of the rare earth element-doped optical fiber I, but it ranges from 0.1 to
Approximately 2000 ppm is suitable. However, depending on the required performance, it is also possible to add about 110,000 pp.

The rare earth element-doped optical fiber according to claim 2 of the present invention is a single mode fiber in which the refractive index n of the first core 5 is lower than the refractive index n8 of the second core 6.

Er (erbium) as a rare earth element is added to the first core 5 at a concentration of 200 ppm, and the refractive index n5 of the first core 5, the refractive index n8 of the second core 6, and the refractive index n7 of the cladding 7 are set. ,n,=1.473,n5=r+t=I,
FIG. 3 also shows the refractive index distribution, Er doping concentration distribution, and power density distribution of the signal light of wavelength λa and the pumping light of wavelength λp of the rare earth element-doped optical fiber 1 in the case of 458. In the rare earth element-doped optical fiber shown in Figure 3,
Although the refractive index n5 of the first core 5 is set as low as the refractive index n7 of the cladding 7, the optical power density distribution is sufficiently large. The energy of the transmitted light is retained in the core 6 of the fiber 2, and the power density distribution of the light transmitted in a single mode does not greatly depend on the refractive index of the fiber. By making the refractive index of the first core 5 equal to the refractive index of the cladding 7 in this way, the first core 5
It is possible to make the power density distribution of the excitation light even more uniform within the interior. Furthermore, by doing this, the refractive index of the first core 5 may be low, so there is no need to add a dopant to the first cladding 5 to adjust the refractive index.
Another advantage is that rare earth elements can be easily added at a desired concentration during production.

Further, the rare earth element-doped optical fiber of the present invention has a 1.45
The wavelength dispersion can be made zero in the wavelength range of ~1.60 μm. 1.45, the lowest loss band of silica-based optical fiber
To set the wavelength dispersion to O at ~1°60 μm, the first
This can be realized by appropriately adjusting the refractive index distribution shape and core diameter of the core 5 and the second core 6. Thereby, it is also possible to use the rare earth element-doped optical fiber of the present invention as a dispersion-shifted optical fiber.

For example, in the rare earth element-doped optical fiber shown in FIG. 2 above, the maximum value of the relative refractive index difference between the second core 6 and the cladding 7 is approximately 1%, and the core diameter is approximately 4.2 μm. As a result, a dispersion-shifted fiber with almost zero chromatic dispersion in the 1.55 μm wavelength band can be obtained.

In order to manufacture the above-described rare earth element-doped optical fiber, a mandrel serving as the first core 5 is prepared, and glasses serving as the second core 6 and the cladding 7 are attached externally to the outer peripheral surface of this mandrel. In addition to the method of forming an optical fiber by melt-spinning the preform into a preform, or by inserting the mandrel into a separately prepared glass tube to form a preform, A method such as melt spinning into an optical fiber can be used.

In addition, although all of the above examples are formed by forming a second core and a clad around the outer periphery of the first core, the rare earth element-doped optical fiber of the present invention is not limited to these examples. Third and fourth cores may be provided between the second core and the cladding.

Each manufacturing method will be explained below along with the manufacturing process.

(Manufacturing method 1) (Step 1) First, a mandrel to which a rare earth element is added at a predetermined concentration is prepared. This mandrel becomes the first core 5, and is made of quartz glass with a predetermined refractive index and E3Nd, which is a laser active material.
, Yb, and Sm are added at predetermined concentrations. Such a mandrel can be manufactured using a method using a plasma flame, a method using a VAD method, or the like.

Method using plasma flame (Patent application 15336/1986)
It is reported in No. 5, etc. ), as shown in FIG. 4, silicon tetrachloride and the chloride of the rare earth element to be added are placed in an inert gas such as A+' into a high-temperature plasma flame 9 formed by a high-frequency oscillator 8. A mandrel in which rare earth elements are added to quartz can be obtained by blowing raw material gas diluted into quartz and oxidizing it.

In addition, in a method using the VAD method, hydrogen, oxygen, and silicon tetrachloride are first heated in an oxyhydrogen burner 12 as shown in FIG.
The soot 14 is produced by supplying soot to the glass and burning it in an oxyhydrogen flame 13 to deposit glass particles. Then, as shown in FIG. 6, this soot 14 is immersed in an immersion liquid 15 made by dissolving or dispersing rare earth elements in the liquid,
The rare earth element is sufficiently impregnated into the porous soot 14.

Next, the soot 14 impregnated with the rare earth element is heated and dried as shown in FIG. 7, and then heated in the heating furnace 16 to turn the porous soot 14 into transparent vitrification, so that the rare earth element is contained in the quartz. A mandrel 11 is obtained by adding the above. FIG. 8 shows this process.

In addition to rare earth elements, the mandrel II obtained in this manner contains Geot
Dopants such as P to s, F 2 and B to 3 may be added.

(Step 2) A glass member that will become the second core 6 is formed on the outer peripheral surface of the mandrel It obtained in Step 1 above. As a method of forming this glass member, as shown in FIG.
Soot 17 is deposited on the outer peripheral surface of. This soot I7 can be deposited by burning hydrogen, oxygen, and silicon tetrachloride supplied to the oxyhydrogen burner 12 in the oxyhydrogen flame I3. Then, by repeatedly moving the oxyhydrogen burner 12 back and forth along the longitudinal direction of the mandrel 11 while rotating the mandrel 11, the soot I7 can be uniformly deposited on the outer peripheral surface of the mandrel 11. It goes without saying that a dopant may be added for the purpose of adjusting the refractive index of the second core when depositing soot.

Next, the mandrel 11 on which a predetermined amount of soot 17 has been deposited is placed in the first
0Heating furnace as shown in figure! The soot 17 is heated in a chamber 6 to make the soot 17 transparent and vitrified to form a core preform 18.

(Step 3) Next, a glass member that will become the cladding 7 is formed on the outer peripheral surface of the core preform 18. To manufacture the glass member that will become the cladding 7, first, as in step 2 above, soot is placed on the outer peripheral surface of the core preform 18. Deposit 7.

Next, core preform 1 on which this soot 17 was deposited
8 is heated to make it transparent and vitrified, and its refractive index is adjusted.

For this, inside the heating furnace where soot 17 is made into transparent vitrification,
A method of adding a dopant such as fluorine that lowers the refractive index of glass can be used.

In this way, the soot 17 deposited on the core preform I8 can be made transparent and vitrified, and only the soot 17 can be impregnated with the dopant. In this way, a preform having a refractive index distribution as shown in FIG. 11 can be obtained.

Note that by repeating this process a plurality of times as necessary, a preform having a predetermined core diameter and cladding diameter can be manufactured.

(Step 4) The preform obtained in Step 3 above can be melt-spun into a rare earth element-doped optical fiber having a predetermined diameter.

(Manufacturing Method 2) In addition to the above manufacturing method I, the rare earth element-doped optical fiber of the present invention can also be manufactured by a rod-in-tube method.

The manufacturing method using the rod-in-tube method will be explained below.

(Step 1) Mandrel 11 made of quartz glass doped with a rare earth element is manufactured in exactly the same manner as Step 1 of manufacturing method 1 above.

(Step 2) Separately from Step 1 above, a base material 19 consisting of a core made of a high refractive index material and a cladding made of a low refractive index material is produced. This base material 19 can be manufactured by a conventional method such as a conventional VAD method or an external attachment method.

A tube 21 is formed by forming a hole 20 passing through the core of the base material 19 thus obtained.

The diameter of this hole 20 is determined by the diameter of the mandrel 11 produced in step 1 above.
is larger than the diameter of the base material 19 and smaller than the core diameter of the base material 19.

Then, as shown in FIG. 13, the mandrel It is inserted into the hole 20 formed in the tube 21.

(Step 3) Next, as shown in FIG. 14, while rotating the tube 21 into which the mandrel 11 is inserted, the heating burner 22 is
By heating the tube 21 by reciprocating it along the longitudinal direction of the tube 1, the diameter of the tube 21 can be reduced, and the mandrel 11 and the tube 21 can be integrated into a preform.

(Step 4) The rare earth element-doped optical fiber of the present invention can be obtained by melt-spinning the preform in exactly the same manner as Step 4 of the manufacturing method 1 above.

In this manufacturing method 2 as well, a dopant for adjusting the refractive index may be appropriately added to the core and cladding portions of the base material 19 as well as to the mandrel 11 in exactly the same manner as in the manufacturing method 1 described above.

[Example] (Example 1) Using a high frequency plasma device similar to that shown in Fig. 4,
Er (erbium) is about 1 in quartz with a refractive index of 1.458.
Mandrels doped at 0.00 ppm were manufactured with a diameter of 4 mm.

Next, on the outer surface of this mandrel, a glass member having a refractive index of -1,458, which will become a second core, is deposited by the process shown in FIGS. 9 and 10 to form a diameter of about 6+n.
A core preform of m was obtained. Furthermore, a glass member having a refractive index of 1.452, which will serve as a cladding, was deposited on the outer surface of this core preform in the same manner as the glass member which would serve as a second core, thereby forming a preform with a diameter of 40 mm.

This preform was then melt-spun to a diameter of 125 μm.
A rare earth element-doped optical fiber of this invention was obtained. Refractive index distribution of this rare earth element-doped optical fiber, wavelength 1.55μ
When the power distribution of the signal light of m wavelength and the pumping light of wavelength of 1.48 μm was investigated, the distribution as shown in FIG. 2 was obtained.

An optical amplifier similar to that shown in FIG. 17 was manufactured using 15 m of this rare-earth element-doped optical fiber, and the wavelength dependence of the gain of this optical amplifier was investigated using a pumping light power of 45 mW, as shown in FIG. 15. The results shown above were obtained, and it was confirmed that optical amplification could be performed with a high gain of 25'dB at a wavelength of 1.535 μm.

(Example 2) A mandrel with a diameter of 4 mm was manufactured in the same manner as in Example 1 above, except that erbium was added at a concentration of 200 ppm.

It is illegal to attach a glass member with a refractive index of 1.473, which will become the second core, to this mandrel and add germanium tetrachloride to the raw material gas consisting of 5iCI24, H2,0, as shown in Figure 9. germanium-doped soot was deposited. Then, on top of this, a soot which is not doped with germanium is deposited by an external method.
Heated at 1600℃ to make it transparent and vitrified, making it 30m in diameter.
A preform of m was produced.

This preform was then melt-spun in the same manner as in Example 1 to obtain a rare earth element-doped optical fiber having a diameter of 125 μm.

Refractive index distribution of the rare earth element-doped optical fiber obtained in this way, signal light with a wavelength of 1.55 μm and signal light with a wavelength of 1.48 μm.
When the power density distribution of the excitation light of m was investigated, it showed a distribution as shown in FIG.

In addition, in the rare earth element-doped optical fiber of Example 2, the first
Even though the refractive index of the core is equal to that of the cladding, the transmitted light is sufficiently confined within the first core because the power density distribution of the mode of the single mode fiber changes to the refractive index of the fiber. This is largely due to the fact that it has a property that does not depend on it.

An optical amplifier similar to that shown in Fig. 12 was manufactured using 12 m of this rare-earth element-doped optical fiber, and the wavelength dependence of the gain of this optical amplifier was investigated using a pumping light power of 40 mW, as shown in Fig. 16. The results shown were obtained, and even though the intensity of the excitation light was low, the wavelength was 1535.
It was confirmed that a high amplification of 25 dB in μm could be achieved.

(Example 3) In exactly the same manner as in Example 1 above, 100 p of erbium was added.
Mandrels were prepared doped at a concentration of pm.

On the other hand, a base material with a refractive index of 1.458 at the center and 1.452 at the periphery was manufactured, and a hole was formed through the core of this base material to form a tube. . The mandrel was then inserted into the hole of this tube and collapsed in the same manner as shown in FIG. 14 to obtain a preform. When this preform was melt-spun in exactly the same manner as in Example 1, a rare earth element-doped optical fiber having the same structure as in Example 1 was obtained.

The rare earth element-doped optical fiber obtained in this way is
It was confirmed that it exhibited exactly the same characteristics as in Example 1, and exhibited stable optical amplification characteristics.

Further, as an application of the present invention, a polarization maintaining fiber can be manufactured by providing stress applying portions on both sides of the core.

[Effects of the Invention] As explained above, the rare earth element-doped optical fiber of the present invention has a rare earth element-doped optical fiber made of quartz glass doped with a rare earth element.
A second core and a cladding having a lower refractive index than the second core are sequentially formed on the outer periphery of the core, so light with uniform power density is transmitted within the first core. You will be able to do this. Therefore, it is possible to eliminate attenuation due to insufficient power density, which was a factor in degrading optical amplification characteristics, and it is possible to uniformly excite the rare earth element added in the first core to perform stable optical amplification. .

Further, the rare earth element-doped optical fiber according to claim 2 of the present invention is a single mode fiber in which the refractive index of the first core is lower than the refractive index of the second core. The transmitted light exhibits a more even power distribution, and stable optical amplification can be performed.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an example of the rare earth element-doped optical fiber of the present invention, and Fig. 2 shows the refractive index distribution, rare earth element doping concentration distribution, and optical power distribution of the rare earth element-doped optical fiber of the present invention. FIG. 3 is a graph showing the refractive index distribution, rare earth doping concentration distribution, and optical power distribution of the rare earth element-doped optical fiber according to claim 2 of the present invention. . FIG. 4 is an explanatory diagram showing one step of manufacturing the mandrel that becomes the first core portion of the rare earth wood-doped optical fiber of the present invention;
Figures 8 to 8 are explanatory diagrams showing each process in order of manufacturing a mandrel, which is the first core of the rare earth element-doped optical fiber of the present invention, and Figure 9 shows a second core formed on the outer peripheral surface of the mandrel. Figure 1O is an explanatory diagram showing the process of depositing soot on the mandrel, and Figure 11 is an explanatory diagram showing the process of transparently vitrifying the soot deposited on the mandrel. It is a graph showing the refractive index distribution of the preform obtained through each process. 12 to 14 show the steps of manufacturing the rare earth element-doped optical fiber preform of the present invention by the rod-in-tube method, and FIG. FIG. 13 is an explanatory diagram showing the process of inserting the mandrel into the tube, and FIG. 14 is an explanatory diagram showing the process of integrating the tube and the mandrel. FIG. 15 is a graph showing the wavelength dependence characteristics of the optical amplification gain of the rare earth element-doped optical fiber of the present invention obtained in Example 1, and FIG. 3 is a graph showing wavelength-dependent characteristics of optical amplification gain of an optical fiber. Fig. 17 is a schematic configuration diagram showing an example of an optical amplifier using a rare earth element-doped optical fiber, and Fig. 18 shows the refractive index distribution and transmission optical power density distribution of a conventional rare earth element-doped optical fiber. FIG. 19 is a graph showing the dependence of amplification and attenuation on pumping light power density in a rare earth element-doped optical fiber. l... Rare earth element doped optical fiber, 5... First core, 6... Second core, 7... Clad.

Claims (2)

[Claims] (1) Comprising a core and a cladding, a first core doped with a rare earth element is provided in the center of the core, a second core is provided on the outside of the first core, and a second core is provided on the outside of the first core. The second one
A rare earth element-doped optical fiber characterized in that a cladding having a lower refractive index than the core is formed. (2) The rare earth element-doped optical fiber according to claim 1, which is a single mode fiber in which the refractive index of the first core is lower than the refractive index of the second core.