JP5227044B2 - Rare earth doped fluorophosphate fiber - Google Patents

Description

  The present invention relates to an optical fiber used for a rare earth-doped optical fiber amplifier for optical communication and a high power processing laser, and more particularly to a rare earth-doped fluorophosphate glass fiber.

  In order to expand the transmission capacity and improve the function of an optical communication system, optical signals of a plurality of wavelengths are combined and transmitted in one optical fiber, or conversely, a plurality of optical signals propagated through one optical fiber. Currently, research and development of wavelength division multiplexing (WDM) technology for demultiplexing an optical signal with a wavelength of λ is performed for each wavelength. In this transmission method, it is necessary to transmit a plurality of optical signals having different wavelengths through a single optical fiber and to perform relay amplification in accordance with the transmission distance. Therefore, in order to increase the transmission capacity by increasing the optical signal wavelength, it is necessary to transmit signals with a larger number of channels.

In the Er ³⁺ doped fiber amplifier (EDFA), the C band having a band of 1530 to 1565 nm has been conventionally used. However, in order to further increase the capacity, the L band having a band of 1565 to 1625 nm is used. Expansion to the belt has been promoted. In order to operate the EDFA in the L band, it is necessary to lower the inversion distribution of Er ions to about 40%. For this purpose, it is necessary to use a fiber that is about 5-20 times longer than the C band.

During operation of the L-band EDFA, signal power increases in EDF (Er ³⁺ doped optical fiber) due to signal amplification and propagates through a long fiber length, so that four-wave mixing (FWM), which is a nonlinear interaction between signals, is performed. : Four-Wave Mixing) generates noise light. This noise light causes signal crosstalk and significantly deteriorates transmission quality. In order to suppress the occurrence of this FWM, attempts have been made to shorten the fiber length by increasing the cutoff wavelength (Non-Patent Document 1) or increasing the concentration of Er (Non-Patent Document 2). The required fiber length has been shortened to about 40 m and is currently used as a standard.

  However, the transmission capacity is becoming larger and larger to compensate for a loss of 10 dB or more due to a composite device such as ROADM (Reconfigurable Optical Add Drop multiplxer). Furthermore, since higher output characteristics are required, there is a problem that signal degradation due to FWM cannot be avoided even with an EDF fiber length of about 40 m.

  Fluorophosphate glass has been applied to laser glass because it can add a rare earth at a high concentration and has a small nonlinear refractive index coefficient (Patent Document 1). Similarly, research has been advanced as a host glass of an optical fiber amplifier in which rare earth can be added at a high concentration (Patent Document 2, Patent Document 3, and Patent Document 4). However, the composition glass used for the laser glass has a temperature dependence of viscosity abruptly changed, so that it is difficult to make it into a fiber as it is, and it is not possible to reduce the loss to withstand practical use. Therefore, in Patent Document 2, Patent Document 3, and Patent Document 4, glass is obtained by adding 5 mol% or more of an alkaline element such as Li, Na, and K to the rare earth-added fluorophosphate glass used in the laser glass. The fiber has been successfully reduced by reducing the viscosity of the fiber. However, these glasses have a problem in water resistance, and it is difficult to clear Telcordia-GR468, which is a standard reliability test for optical communication components.

  As a connection technique between the quartz fiber and the low melting point multi-component glass fiber, there is a fusion connection. This fusion splicing of dissimilar fibers is not a technology in which both fiber glasses are melted and connected like quartz fibers. The tip of the low melting glass fiber side melts or softens, but the silica fiber side is the tip of the fiber. Connection is made in a state where the portion is not melted (Patent Document 5). Research and development using such a fusion technique is being carried out for fluoride fibers, tellurite fibers, and bismuth fibers.

  Since the refractive index of the fluoride fiber can be almost equal to that of the quartz fiber, the return loss can be 50 dB or more even if the fiber is cut vertically and connected. However, the fluoride fiber has a low glass bending temperature of 300 ° C. or lower, and the anion of the glass component is fluorine. Therefore, the connection strength is weak, which is not practical.

On the other hand, in the tellurite fiber and the bismuth-based fiber, the yield temperature of the glass is 300 ° C. or higher, and the anion of the glass component is oxygen, so the connection strength is relatively strong. However, the refractive index of the glass of these tellurite fibers and bismuth-based fibers is n _D (refractive index at the D line) of 2 or more, so that a return loss of about 50 dB and a low connection loss of 0.5 dB or less are realized. In order to achieve this, not only the oblique cut connection but also the angle of the connection surface must be set to follow Snell's law (Patent Document 5). Such a connection not only has a poor product yield, but also has an angle in the fiber after connection, so it is difficult to attach a fusion reinforcing sleeve and requires more space on the assembly. there is a possibility.

S. Ishikawa et al. "High Gain Per Unit Length Silica-Based Erbium Doped Fiber for 1580nm Band Amplification", OAA1998 TuC4 pp.1-4 (111-114) Keiichi Aiso et al. "Erbium Lanthanum co-doped fiber for L-band amplifier with high efficiency, low non-linearity and low NF", OFC2001 TuA6 pp.1-3 Tetsuro Izumiya “Optical Glass and Laser Glass”, published by Nikkan Kogyo Shimbun, March 24, 1998 pp. 203 Japanese Patent Publication No.54-006047 JP 05-238775 A JP 09-211155 A JP-A-10-12952 Japanese Patent No. 3396422

  The present invention has been made in view of the problems to be solved by the prior art as described above, and its purpose is to realize a low reflection / low loss connection even when a fiber is cut and fusion-bonded vertically. An object of the present invention is to provide a low-loss rare earth-doped fluorophosphate fiber using a fluorophosphate glass that is highly reliable and can be doped with Er at a high concentration.

In addition, since the fluorophosphate glass to which the present invention is applied has a nonlinear refractive index coefficient smaller than that of pure quartz glass, an Er high-concentration-added fluorophosphate fiber that satisfies the above-mentioned objectives at the same time has been realized with low nonlinearity and practical use. It is an attendant object of the present invention to provide an L-band Er ³⁺ doped optical fiber amplifier (EDFA).

  In order to achieve the above object, a fluorophosphate fiber according to the present invention and a fluorophosphate glass constituting the fiber have the following configurations.

A double-structured clad in which a first clad and a second clad are sequentially arranged around a central core, and the core, the first clad, and the second clad are made of fluorophosphate glass; A fluorophosphate fiber to which at least a rare earth is added, wherein an alkali element is added to each of the core and the first cladding glass, and the second cladding glass has a composition not containing an alkali element, When the refractive index is n ₁ , the refractive index of the first cladding glass is n ₂ , and the refractive index of the second cladding glass is n ₃ , n ₃ is larger than n ₂ so as to attenuate the cladding mode, And since an alkali element is added to the core and the second clad glass does not contain an alkali element, n ₃ becomes n ₁ or more, and n ₃ ≧ n ₁ > n ₂ and the refractive indexes of the silica glass fiber to be connected and the core and the first clad glass match within 0.2%, respectively, and the core, the first clad, and the second clad glass The yield temperature of the end face is 450 ° C. or higher.

Sectional area of the pre-Symbol second cladding glass, may be so that account for at least 70% of the fiber cross-sectional area of the fluorophosphate fiber.

As the composition of the previous SL core and the first cladding glass, P2O5 + Al2O3 + Y2O3 5~20mol% by mole%, AlF3 + YF3 5~40mol%, MgF2 + CaF2 + SrF2 + BaF2 40~70mol%, in the range of Li + NaF + KF 0~5mol%, the core and the first 1 the cladding glass both, O (oxygen) / F within the atomic ratio of (iron) is 0.1 to 0.45, and a divalent and monovalent as fluoride element can be substituted with the oxide elements It may be .

As the composition of the pre-Symbol second cladding glass, P2O5 + Al2O3 + Y2O3 10~25mol% by mole%, AlF3 + YF3 5~40mol%, in the range of MgF2 + CaF2 + SrF2 + BaF2 40~70mol %, the second cladding glass is the atomic ratio of O / F is within the scope of 0.2 to 0.6, and a divalent and monovalent fluoride elements may be can be substituted with the oxide elements.

  With the configuration as described above, according to the present invention, low-reflection and low-loss connection is possible by vertically cutting and fusion-bonding the fibers, and the reliability is low and Er can be added at a high concentration. An object of the present invention is to provide a fluorophosphate glass that can be made into a lossy fiber. In addition, the fluorophosphate glass has a characteristic that the nonlinear refractive index coefficient is smaller than that of pure quartz glass. An Er high-concentration doped fluorophosphate fiber that simultaneously satisfies these issues can provide an L-band EDFA that is less nonlinear and practical than ever, and promotes higher density and higher performance in wavelength division multiplexing optical transmission systems. be able to. As a result, it can greatly contribute to the sophistication and economy of services using these systems, particularly in the medium-distance system and the trunk line system.

  A preferred embodiment of a fluorophosphate fiber according to the present invention will be described below with reference to the drawings.

The fluorophosphate fiber of the present invention is an optical fiber having a double-structured clad in which a first clad and a second clad are sequentially arranged around a central core, and at least a rare earth added to a core glass. When the refractive indexes of the first clad and the second clad are n ₁ , n ₂ , and n ₃ , the relation of n ₃ ≧ n ₁ > n ₂ is established, and the silica glass fiber to be connected, the core, and the first clad glass The refractive indexes of the glass layers are equal to each other within 0.2%, and the deformation temperature of the glass is 450 ° C. or higher.

  When the fused silica fiber and the fluorophosphate fiber are fusion-spliced, as described in the background art section, the refractive index is not uniformized by melting both glasses. For this reason, when there is a difference in the refractive index at the connection surface, the reflectance R follows the following formula (1).

Here, _ng is the refractive index of the quartz fiber, and n is the refractive index of the fluorophosphate fiber.

  The return loss of a communication device, particularly an amplification fiber that is a gain medium, is required to be 50 dB or more in a WDM amplifier. The value is the minimum value required to prevent signal quality deterioration due to multiple scattering. If the above refractive index difference matches within 0.2%, the return loss can be ensured to be 50 dB or more even when the fibers are cut vertically and the fibers are connected straight.

  On the other hand, commercially available silica fibers range from a single mode fiber (SMF) having a Δn (ratio of refractive index difference between core and clad) of 0.3% to a dispersion compensating fiber of about 3.7%. Actually, when Δn increases to about 3.7%, the core diameter becomes about 2 μm, which not only makes it difficult to connect the silica-based fiber and the fluorophosphate fiber, but also the high Δn quartz fiber and the SMF of the fluorophosphate fiber. In the meantime, it is necessary to sandwich a TEC (Thermally diffuse expanded core) fiber in which the MFD (mode field diameter) of the optical fiber is locally expanded by a thermal diffusion technique, so that the loss increases by about 0.2 dB.

  Therefore, about 0.3 to 1.5% is appropriate as Δn of the quartz fiber facing the fluorophosphate fiber, and the refractive index of the core glass is in the range of 1.448 to 1.47. It is necessary to adjust the refractive index of the core glass of the fluorophosphate fiber to this value, and to adjust the refractive index difference between the quartz fiber, the core, and the first cladding glass within the above 0.2%.

As described above, the fusion splicing of the silica fiber and the fluorophosphate fiber is not performed by remelting the melt that is melted by the fusion of the glass at both ends, but the glass of the end surface of the fluorophosphate fiber is melted and its components Enters the end face of the quartz glass and reacts with quartz in the end face, so that the higher the temperature, the longer the reaction length and the stronger the connection strength. The yield point of glass is the temperature in the temperature range where the state changes from a rigid state to a viscous state. If the yield temperature of the glass is 450 ° C. or higher using this temperature as an index, the anion mainly contains a fluorine component. Also in the fluorophosphate fiber, a tensile strength of 200 MPa (that is, 200 N / mm ² ) or more can be obtained, and the operation of attaching the fusion reinforcing sleeve can be handled without breaking.

  In the above-described multicomponent glass fiber, it is generally performed to facilitate drawing by adding an alkali element to lower the viscosity of the glass and suppressing crystallization. However, as described above in the section of the background art, the glass to which the alkali element is added has a problem from the viewpoint of long-term reliability because the water resistance deteriorates.

  Since quartz glass is a stable glass that does not crystallize, it can be easily processed without adding an alkali element, and is a standard fiber for optical communication. In the case of fluorophosphate fibers, fiberization has been promoted with glass added with alkali elements, but in order to clear Telcordia-GR468, which is a standard reliability test for optical communication components, at least the outer periphery of the fiber is used. It is necessary to fiberize glass with no alkali element added.

  In the production of an optical fiber to which the present invention is applied, a core and a first clad preform are first produced, and then a jacket is drawn with a first clad glass, followed by jacket drawing with a second clad glass. It is assumed that a desired core / cladding diameter ratio is obtained. In that case, since the 2nd clad glass jacket tube which carries out a jacket drawing is the glass which comprises outer periphery, it is necessary to comprise by the composition without an alkali element at least. Also, the thickness of the second clad glass needs to take into consideration the ingress of moisture from the outer periphery and the strength at the connection point, and therefore it is necessary to have a thickness corresponding to 70% or more in terms of the fiber cross-sectional area. .

FIG. 1 shows the composition range of fluorophosphate glass that satisfies Claims 1 and 2 and can produce a low-loss fiber. FIG. 1 is called a triangular diagram, and the components shown at the top are oxides P ₂ O ₅ (+ Al ₂ O ₃ etc.), trivalent AlF ₃ (+ YF ₃ ), and divalent RF ₂ (R is Ba , Ca, Sr, etc.). The solid curve in FIG. 1 is the vitrification range disclosed in Non-Patent Document 3.

  In FIG. 1, X is a core glass composition in which a loss of 1 dB / m or less was not obtained, and Δ was a loss of 1 dB / m or less, but a core glass having a return loss of 40 dB or more by fusion splicing. The composition, O, is a core glass composition having a loss of 1 dB / m or less and a reflection attenuation amount by fusion splicing of 50 dB or more.

From FIG. 1, the glass satisfying claims 1 and 2 has a P ₂ O ₅ (+ Al ₂ O ₃ , BaO, etc.) of 10 to 20 mol%, an AlF ₃ (+ YF ₃ etc.) of 12 to 40 mol%, and an RF _2. (R is Ba, Ca, Sr, Mg, etc.) is 42 to 70 mol%, and the O / F ratio is 0.1 to 0.45.

On the other hand, if P ₂ O ₅ (+ Al ₂ O ₃ , BaO, etc.) is 10 to 20 mol%, or the O / F ratio deviates from 0.1 to 0.45, the refractive index of the glass is 1.448 to 1. Because it deviates by 0.2% or more from the range of .47, it is not possible to obtain a return loss of 50 dB or more. Further, if the sum of AlF ₃ (such as + YF ₃ ) or RF ₂ (where R is Ba, Ca, Sr, Mg, etc.) deviates from the above value, a fiber loss of 1 dB / m or less cannot be obtained.

  Next, with reference to drawings and tables, examples of rare earth-doped fluorophosphate glass fibers according to the present invention will be described in detail together with comparative examples.

  Table 1 below shows examples of the composition of the fluorophosphate glass to which the present invention is applied (in mol%) (No. 1-20) and deviates from the present invention, but is devitrified (crystals are detected during cooling and are transparent) Comparative composition examples (AD) in which glasses that do not lose the glass), O / F ratio, glass transition temperature (Tg: ° C), yield point temperature (Tc: ° C), thermal stability (Tx- Tg: ° C), refractive index (n), measurement test results for surface deterioration state by high temperature and high humidity test (120 ° C, 90RH, 100 hours) (O: no deterioration is observed, x: deterioration is recognized) Show.

  The glass was melted at 1000 ° C. in a nitrogen atmosphere by mixing raw materials in a glove box filled with nitrogen gas and using a platinum crucible. Thereafter, the melt was poured into a mold preheated to 400 ° C.

  Referring to Table 1, the glass of No. 1-20 in Table 1 and the glass of Comparative Example A excluding Comparative Examples B, C and D have Tx-Tg (thermal stability) of 119 ° C. or higher, It can be seen that it has sufficient thermal stability to be processed into a single mode fiber.

  In the triangular diagram of FIG. 1 already shown, the fiber using No. 1-20 glass for the core and the first cladding is all marked with a circle in the measurement test result of the surface degradation state of the high temperature and high humidity test, and the fiber loss is 1 dB. / M or less, the return loss by fusion splicing was 50 dB or more.

  On the other hand, in the optical fiber using the glass of A of the comparative example as the core, the fiber loss was a good value of 1 dB / m or less, but the Δn 1.5% quartz fiber having the refractive index of the core glass connected was used. Since it was higher by 0.2% or more than the core glass, the return loss was 50 dB or less.

  When a fiber was produced using the comparative example B as the core glass, the crystal was deposited on the core glass during reheating of the fiber drawing, resulting in an increase in scattering loss and a loss of 10 dB / m. Even if an EDFA is configured using this fiber, the L band cannot be sufficiently amplified due to a large loss.

  In the fiber using the C glass of the comparative example as the core, the fiber loss was a good value of 1 dB / m or less, but it was 0 as compared with the core glass of the silica fiber having the refractive index of the core glass of 1% connected to the core glass. Since it was lower by 2% or more, the return loss was 50 dB or less.

  Finally, in the case where a fiber was produced using the comparative example D as the core glass, the core was crystallized at the time of fiber drawing as in the case of using the comparative example B, and the fiber loss was 8 dB / m. Even if an EDFA is configured using this fiber, the L band cannot be sufficiently amplified due to a large loss.

  Fibers produced using the glass compositions listed in Table 1 were subjected to a high-temperature and high-humidity test (120 ° C., 90 RH, 100 hours). The measurement results at that time are shown in FIG. As shown in FIG. 2, by the loss measurement before and after the test, only the fiber using Comparative Example C showed an increase in fiber loss due to the OH group. In FIG. 2, the solid line curve shows the measurement result before the high temperature and high humidity test, and the broken line curve shows the measurement result after the high temperature and high humidity test.

  Example composition of fluorophosphate glass to which the present invention is applied (in mol%) (No. 1-10), and comparative composition example (AC) from which glass deviating from the present invention but not devitrified is obtained, O / F ratio, glass transition temperature (Tg: ° C) of these glasses, yield point temperature (Tc: ° C), thermal stability (Tx-Tg: ° C), refractive index (n), high temperature and high humidity test (120 ° C, Table 2 below shows the measurement test results (O: no deterioration is observed, x: deterioration is recognized) for the surface deterioration state by 90RH (100 hours).

  The glass was melted at 1000 ° C. in a nitrogen atmosphere using a platinum crucible by mixing raw materials in a glove box filled with nitrogen gas. Thereafter, the melt was poured into a mold preheated to 400 ° C.

  Referring to Table 2, the glass No. 1-10 in Table 2 has a stable glass composition in which no exothermic peak due to crystallization is observed, and has sufficient thermal stability to be processed into a fiber. I understand. In order to use these glasses for the second cladding of the fiber, a jacket tube was prepared by the rotational casting method (rotational molding method) using the glass No. 1-10 in Table 2, and the suction casting method ( A drawn base material (rod) in which a core and a first clad were made of glass No. 1-20 in Table 1 using a suction molding method) was inserted into the jacket tube and made into a fiber by drawing. As a result, a fluorophosphate fiber (optical fiber) of this example having one core and a double clad around the core was obtained.

  At that time, an X tube having an outer diameter of 15 mm and an inner diameter of 5 mm and a Y tube having an outer diameter of 15 mm and an inner diameter of 10 mm were used as the jacket tube. The cross-sectional area of the fiber using the X-tube is 89% of the glass of No. 1-10 in Table 2, whereas the cross-sectional area of the fiber using the Y-tube is 56% of the No. of Table 2. .1-10 glass. These fibers have a core diameter of about 6 μm, whereas the second cladding inner diameter is about 40 μm, and the clad light is sufficiently attenuated, so that the signal light does not propagate through the second clad glass, The cladding mode is attenuated by the UV coating having a higher refractive index (which is coated on the outside of the second cladding glass). Therefore, if the refractive index of the second cladding glass is higher than that of the first cladding and the refractive index of the second cladding glass is lower than the refractive index of the UV coating, connection loss and cladding mode propagation can be avoided.

  The fusion splicing strength of the fiber using the X tube and the Y tube made of No. 1 glass in Table 2 with the quartz fiber was compared. This evaluation was performed based on the tensile strength.

  Fibers using X tubes had an average tensile strength of 150 MPa, whereas fibers using Y tubes had an average tensile strength of 60 MPa. When the tensile strength is 100 MPa or less, the fiber connection portion is taken out from the fusion splicer and the fiber is damaged during the operation of protecting it with the reinforcing tube, whereas when the tensile strength is 100 MPa or more, the fiber is damaged during the fiber protection operation. It ’s gone. FIG. 3 shows the fiber cross-sectional area dependency of the tensile strength of the fusion spliced portion of the fiber. From the graph of FIG. 3, it can be seen that the cross-sectional area occupied by the glass of Table 2 is required to be 70% or more in order for the tensile strength to exceed 100 MPa on average.

  A jacket tube was prepared with the No. 2 glass of Table 2 and Comparative Example A and glass, respectively, and the stretched base materials prepared with the No. 15 core and No. 16 clad of Table 1 were inserted into the jacket tubes, respectively. Fiber drawing was performed. As a result, when a No. 2 glass jacket tube was used, a low-loss fiber of 1 dB / m or less could be produced. However, when the glass jacket tube of Comparative Example A was used, the drawing temperature increased, Since the crystal was generated in the inner base material, the loss increased to 18 dB / m.

  The jacket tube is made of No. 3 glass of Table 2 and the glass of Comparative Examples B and C, respectively, and the stretched base material made of No. 1-20 glass of Table 1 is inserted into each of the jacket tubes to draw the fiber. went. As a result, the tensile strength of the fiber was measured before and after the fiber 50 m wound up was subjected to a high temperature and high humidity test (120 ° C., 90 RH, 100 hours). Here, “RH” is relative humidity (%). The average tensile strength of the fiber made of No. 3 glass in Table 2 was hardly changed from 700 MPa to 690 MPa, but the average tensile strength of the fiber made of the glass of Comparative Example B was 630 MPa. In the case of Comparative Example C, the average tensile strength was greatly deteriorated from 580 MPa to 120 MPa.

  A fiber made of No. 3 glass in Table 2 and a fiber made of Comparative Example C glass were each fused and connected to a quartz fiber. At that time, the ratio of each glass (second clad glass) to the fiber cross-sectional area exceeded 70%. As a result of evaluating the connection strength by tensile strength, the connection strength of the fiber in which the second clad was made of No. 3 glass of Table 2 was 180 MPa on average, which was sufficient for handling, whereas the fiber of Comparative Example C glass was second. The connection strength of the fiber from which the clad was produced was an average of 70 MPa, which was insufficient.

  A base material was prepared using No. 11 in Table 1 as a core and No. 12 in Table 1 as a first clad, and this base material was inserted into a jacket tube having the same composition as the first clad and stretched. The fiber was further inserted into a drawing jacket tube made of No. 6 glass in Table 2 and drawn to produce a fiber.

  FIG. 4 shows a sectional view and a refractive index distribution of the completed optical fiber. The core diameter, the outer diameter of the first cladding, and the outer diameter of the second cladding were 6.1 μm, 60 μm, and 120 μm, respectively. In addition, Δn and the cutoff wavelength were 0.61% and 1.3 μm. The cross-sectional area of the outermost jacket glass (second clad) accounted for 75%.

This optical fiber was fusion spliced with a silica fiber of Δn: 1%. The fusion splicing was performed using a commercially available arc discharge type connecting device. The average tensile strength of the fusion spliced portion was 172 MPa, and removal from the connecting device and attachment of a connection reinforcing tube (not shown) could be handled smoothly. The connection loss was 0.2 dB and the return loss was 56 dB, and the high refractive index of n ₃ in the refractive index distribution of FIG. 4 did not optically affect the connection loss or return loss. Furthermore, 11 connection fibers fusion-bonded under the same conditions as described above were prepared, and a temperature cycle test (from −40 ° C. to 75 ° C., 500 cycles) was performed. As a result, the loss variation in all the fibers was 0.1 dB or less.

  Next, the 25-dB L-band amplification operation is performed using the fluorophosphate fiber of the present invention having a length of 6 m and the silica-based EDF having a length of MFD and suppressing the generation of four-wave mixing. It was. As a result, the signal of the central 45th channel was extracted during the signal amplification of 90 channels at intervals of 50 GHz, and FWM (4-wave mixing) light was measured. As a result, when the quartz EDF was used, the signal power / FWM power was 25 dB, but in the fluorophosphate EDF of the present invention, it was 45 dB, and the generation of FWM light was suppressed by 20 dB.

  In the above embodiments, specific examples of the composition of the optical fiber are shown, but the present invention is not limited to this.

(Other embodiments and examples)
In the above, preferred embodiments and examples of the present invention have been illustrated and described. However, the embodiments and examples of the present invention are not limited to the above examples, and are within the scope of the claims. If there are, various modifications such as replacement, change, addition, increase / decrease in the number of components, design change of the shape, etc. are all included in the embodiment of the present invention. In addition, the present invention may be applied to a system composed of a plurality of devices, or may be applied to an apparatus composed of one device. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiment. For example, you may delete some components from all the components shown by an Example as needed. Furthermore, constituent elements over different embodiments may be appropriately combined.

It is a characteristic figure which shows the composition dependence of the fluorophosphate glass by this invention with respect to the fiber loss and the reflection amount of a connection. It is a characteristic view which shows an example of the loss spectrum of the fluorophosphate fiber before and behind a high temperature, high humidity test (120 degreeC, 90RH, 100 hours). It is a characteristic view which shows the 2nd clad glass fiber cross-sectional area ratio dependence of the fluorophosphate fiber with respect to the tensile strength of the fusion splicing part of the fluorophosphate fiber and quartz fiber by this invention. It is a conceptual diagram showing the cross section and refractive index profile of the fluorophosphate fiber by this invention.

Claims (4)

A double-structured clad in which a first clad and a second clad are sequentially arranged around a central core, and the core, the first clad, and the second clad are made of fluorophosphate glass; A fluorophosphate fiber to which at least a rare earth is added,
An alkali element is added to each of the core and the first clad glass, the second clad glass has a composition not containing an alkali element, the refractive index of the core glass is n ₁ , and the refractive index of the first clad glass. When the rate is n ₂ and the refractive index of the second clad glass is n ₃ ,
n ₃ is larger than n ₂ so as to attenuate the cladding mode, and since the alkali element is added to the core and the second cladding glass does not contain the alkali element, n ₃ becomes n ₁ or more,
n ₃ ≧ n ₁ > n ₂
Relationship,
The refractive indexes of the silica glass fiber to be connected and the core and the first cladding glass match within 0.2%,
In addition, a fluorophosphate fiber, wherein a bending temperature of end faces of the core, the first cladding, and the second cladding glass is 450 ° C. or higher. 2. The fluorophosphate fiber according to claim 1, wherein a cross-sectional area of the second clad glass occupies at least 70% or more of a fiber cross-sectional area of the fluorophosphate fiber. As the composition of the core and the first cladding _{glass, P 2 O 5 + Al 2} O by _{mole% 3 + Y 2 O 3 5~20mol} %,
AlF ₃ + YF ₃ 5-40 mol%,
MgF ₂ + CaF ₂ + SrF ₂ + BaF ₂ 40 to 70 mol%,
Li + NaF + KF 0-5 mol%
In the range of
Both the core and the first cladding glass have an atomic ratio of O / F in the range of 0.1 to 0.45, and can replace divalent and monovalent fluoride elements with oxide elements. The fluorophosphate fiber according to claim 1 or 2, characterized in that: As a composition of the second cladding glass, P ₂ O ₅ + Al ₂ O ₃ + Y ₂ O ₃ 10 to 25 mol% in mol%,
AlF ₃ + YF ₃ 5-40 mol%,
_{MgF 2 + CaF 2 + SrF 2} + BaF 2 40~70mol%
In the range of
The second clad glass has an O / F atomic ratio in a range of 0.2 to 0.6, and can replace divalent and monovalent fluoride elements with oxide elements. A fluorophosphate fiber according to any one of claims 1 to 3.

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