JPH1138238A - Optical fiber having long period grating and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber having a long period grating, that is little dependent on temperature, and to provide its manufacturing method. SOLUTION: The optical fiber has a core 3, whose main component is quartz glass, and a clad 4, whose main component is a quartz glass having a lower refractive index than the core 3. The core 3 has periodic residual tensile stress in its axial direction. Therefore, the core 3 has the refractive index distribution having refractive index stripes 1 which vary in accordance with the residual tensile stress. Thus, the core 3 has the characteristic in which the signal light beams propagating in the core have attenuation in a prescribed wavelength range centered on the loss wavelength where the loss becomes maximum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber having a long-period grating with little temperature dependence and a method of manufacturing the same, and is suitable for a filter for gain equalization of an optical amplifier.

[0002]

2. Description of the Related Art A typical optical fiber communication system includes an optical signal source, an optical fiber line having one end connected to the optical signal source, and an optical receiver connected to the other end of the optical fiber line. Have. An optical amplifier for amplifying a signal being transmitted is installed in the optical fiber line. In such an optical fiber communication system, 1.5 times is often used.
An optical fiber amplifier in which signal light in the μm band is used and a rare earth element such as erbium (Er) is added as an optical amplifier is used. The rare-earth-doped optical fiber amplifier has a function of inducing stimulated emission to amplify incident light when signal light is incident after a predetermined excitation light is incident to form a population inversion. .

[0005] The gain characteristics of such an optical fiber amplifier include 1530 nm on the short wavelength side which is hardly affected by temperature.
There is an area showing a peak in the vicinity. For this reason, in a wavelength division multiplexing (WDM) multiplexing communication system, different gains are given to the respective channels, which causes a problem that bit error rates of some channels are increased. If the gain in the peak region can be suppressed to a value equal to the gain in the long wavelength region, a wide band optical fiber amplifier can be realized.

[0004] A method for solving this problem is described in a paper "Temperature-independent long-period fiber grating" (1997 IEICE General Conference: C-3-150). The temperature dependency of the long-period grating is adjusted by manufacturing an optical fiber using a core to which germanium oxide, whose refractive index tends to increase with respect to temperature, and boron oxide, whose refractive index tends to decrease, at a predetermined ratio.

[0005]

However, the conventional temperature-independent long-period fiber grating uses a core to which germanium oxide and boron oxide are added at a predetermined ratio, so that the temperature can be finely adjusted. However, the production of optical fibers is complicated. In addition, when a long-period grating is formed by excitation of ultraviolet light in an optical fiber doped with germanium oxide, oxygen deficiency-type defects related to germanium oxide cause thermal relaxation over time, and the refractive index changes. is there. In addition, B ₂
O ₃ has a problem in terms of loss because infrared absorption is caused by the vibration of BO up to the 1.5 μm band.

Accordingly, an object of the present invention is to solve the above-described problems, and an object of the present invention is to provide an optical fiber having a long-period grating with little temperature dependence and a method of manufacturing the same.

[0007]

An optical fiber having a long-period grating according to the present invention has a core mainly composed of silica glass and a clad mainly composed of silica glass having a lower refractive index than the core. In an optical fiber, the core has a periodic tensile stress in the axial direction, thereby having a refractive index distribution that fluctuates according to the residual tensile stress, and the signal light propagating through the core has a loss at which the loss is maximized. It is characterized in that attenuation occurs in a predetermined wavelength range around the wavelength.

According to the optical fiber of the present invention, a portion having a residual tensile stress in the axial direction and a portion having no stress are periodically arranged, so that the oxide dopant is irradiated with ultraviolet rays. As in the case of forming a long-period grating, oxygen-deficient defects are alleviated by heat, and the refractive index does not change over time, resulting in a highly reliable configuration.

An optical fiber having a long-period grating according to the present invention is characterized in that the core and the clad are formed by adding at least one of a halogen element and OH to quartz glass. According to the research results of the present inventors, the change in the refractive index of the core and the clad thus formed with respect to the temperature shows almost the same value as that of quartz glass. Therefore, even if the temperature changes, the relative refractive index difference between the core and the clad hardly changes, so that the long-period grating formed on the optical fiber exhibits characteristics that are not affected by the temperature.

An optical fiber provided with a long-period grating according to the present invention has a portion in which residual tensile stress exists in the axial direction and a portion in which no stress exists, as described above, and a core and a clad. Is a temperature-independent long-period grating in which the relative refractive index difference between the core and the clad does not substantially change in the temperature range of -40 ° C to 80 ° C because a halogen element or OH is added to quartz glass. Can be formed.

According to the method of manufacturing a long-period grating according to the present invention, an optical fiber preform having a clad mainly composed of quartz glass having a lower refractive index than the core is formed on the outer periphery of a core glass mainly composed of quartz. In the first step, the optical fiber preform formed in the first step is drawn to produce an optical fiber having a cladding around the core, and the optical fiber is drawn while applying tension to the core, and the residual tension is drawn to the core. A second step of generating a stress; and a third step of periodically heating the optical fiber formed in the second step in the axial direction to periodically reduce the residual tensile stress of the core. .

In the method of manufacturing a long-period grating according to the present invention, the first step is a step of adding at least one of a halogen element and OH to at least one of a core and a clad to form an optical fiber preform. Therefore, the conventional optical fiber preform manufacturing method (VAD method, C
VD method, etc.) can be applied without introducing a special device and technology.

The second step according to the present invention is a method in which the core is drawn while applying tension to the optical fiber to generate a residual tensile stress in the core, so that a conventional drawing method can be applied. .

In a third step according to the present invention, the optical fiber formed in the second step is periodically heated in the axial direction to relieve the residual tensile stress of the heated portion of the core, and the non-heated portion Is a method of leaving a tensile stress. Therefore, the portion where the stress is relaxed has a refractive index corresponding to the added dopant, but the refractive index of the stressed portion decreases in response to the residual tensile stress, so that the refractive index changes periodically. Long-period fiber gratings can be formed. In the long-period fiber grating formed as described above, the refractive index hardly changes over time, and highly reliable characteristics can be obtained.

[0015] In the third step of the present invention, the temperature for heating the optical fiber in the axial direction is preferably in the range of 1100 to 1900 ° C. By heating at a temperature in this range, the residual tensile stress of the core can be easily reduced.

In the third step of the present invention, it is preferable to heat by arc discharge or CO ₂ laser. By employing such a heating means, local fiber heating can be performed, and the residual tensile stress of the core can be periodically reduced.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of an optical fiber having a long-period grating and a method for manufacturing the same will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is a diagram showing a configuration of an optical fiber provided with a long-period grating according to the present embodiment. In FIG. 1, an optical fiber 5 is formed of a core 3 mainly composed of quartz glass and a clad 4 mainly composed of silica glass having a lower refractive index than the core 3 around the core 3.
A long-period grating 2 is formed by applying a refractive index stripe 1 that periodically changes at a pitch 程度 of about several hundred μm in the axial direction to the refractive index.

The long-period grating 2 includes an optical fiber 5
Is a grating that induces coupling between a core mode and a cladding mode that propagates, and the spatial frequency of the grating is set to be the difference between the propagation constant of the core mode and the propagation constant of the cladding mode. And a strong coupling with the cladding mode. As a result, the long-period grating 2 has the function of radiating the core mode to the cladding mode, and reduces the intensity of the core mode to a predetermined wavelength (hereinafter referred to as “loss wavelength”).
Call. ) To attenuate over a narrow band. FIG. 2 is a graph showing an example of a loss wavelength characteristic of a long period grating having a loss wavelength λ.

In the optical fiber 5 of the present embodiment, at least one of a halogen element such as fluorine or chlorine and OH is added to at least one of the core 3 and the clad 4 so that the gap between the core 3 and the clad 4 is increased. A predetermined refractive index difference is formed. Here, when the dopant added to the optical fiber is an oxide such as germanium oxide, when a long-period grating is formed by excitation of ultraviolet light in the optical fiber doped with germanium oxide, it is related to germanium oxide over time. Oxygen deficiency-type defects cause thermal relaxation and change the refractive index, making it impossible to obtain stable characteristics over time.

On the other hand, the periodically changing refractive index distribution of the optical fiber of the present embodiment is obtained by periodically heating the residual stress generated in the core when the optical fiber is drawn, and forming the heated portion at the heated portion. Since it is formed by relaxing the residual stress, the oxygen-deficient defect does not cause thermal relaxation and the refractive index does not change, and stable characteristics over time can be obtained. In addition, since the dopant added to the quartz glass core and cladding is limited to halogen elements and OH, the temperature characteristics of the refractive index of the core and cladding are substantially equal, so the relative relative refraction of the core and cladding is relatively small. It is extremely small that the rate difference changes with temperature. Table 1 shows -4
It shows the rate of change of the refractive index of a typical substance constituting an optical fiber with respect to temperature in the range of 0 ° C. to 80 ° C.

That is, as shown in Table 1, the change in the refractive index of the quartz glass forming the optical fiber and the quartz glass to which a halogen element such as fluorine or the like and OH are added have almost the same value as shown in Table 1. On the other hand, as shown in Table 1, some conventional oxide dopants such as germanium oxide have a change in refractive index with respect to temperature as compared with quartz glass, or have a tendency opposite to temperature. .

[0023]

[Table 1]

Therefore, an optical fiber made of quartz glass to which a halogen element, OH, or the like is added is hardly affected by temperature, and a long-period grating that is stable against temperature changes can be formed.

The long-period grating 2 in this embodiment is formed by residual stress in which the refractive index of the core 3 changes periodically at a pitch に in the axial direction. The residual stress is formed by periodically heating the residual tensile stress generated in the core when the optical fiber 5 is drawn, and relaxing the residual stress in the heated portion.

As described above, the fact that the refractive index is reduced when a tensile stress is applied to the core of an optical fiber and the optical fiber is restored when the tensile stress is relaxed is described in, for example, a paper "Residual Stress Relaxation Mode Field Diameter Converting Fiber". (1993 IEICE Spring Conference: C-242).

FIG. 3 shows the tension F when a preform for an optical fiber having a quartz glass core and a quartz glass doped with fluorine around the core is drawn into an optical fiber having a core diameter of 9 μm and a cladding diameter of 125 μm. 6 is a graph showing a relationship between a relative refractive index difference (between a core and a clad). As shown in the graph of FIG. 3, in the optical fiber 5, it is understood that the relative refractive index difference between the core and the clad becomes smaller as the tension F at the time of drawing becomes larger and the residual tensile stress becomes larger in the core 3.

In general, when a dopant is added to quartz glass, the softening point temperature of the glass tends to decrease. For example, by making the ratio of halogen element or OH added to the core smaller than the ratio of halogen element or OH added to the clad, as in an optical fiber having a silica glass core and a silica glass clad with fluorine. , The core softening temperature T ₁ and the cladding softening temperature T ₂
And T ₁ > T ₂ , and a residual tensile stress can be generated in the core. Even cladding reaches the state of the softening temperature T _2, the core tensile by drawing stress is built for not reaching the softening temperature T _1.
Such an optical fiber can relieve residual stress by heating it to a temperature higher than the temperature at which core strain can be removed.

FIG. 4 is a graph showing the distribution of the relative refractive index difference when the residual tensile stress periodically remains in the axial direction of the core 3. In FIG. 4, n ₀ indicates the relative refractive index difference at the position where the tensile stress applied to the core 3 is relaxed, and n _F indicates the core 3
Shows the relative refractive index difference at the position where the tensile stress still remains, and shows a value smaller than n ₀ by Δn _F (= n ₀ −n _F ).
Thus, by periodically heating the optical fiber 5 in the axial direction, the core 3 is periodically formed with a portion where the tensile stress is relaxed by heating and a portion where the tensile stress remains without being heated. A long-period grating in which the relative refractive index difference changes in accordance with the heating cycle is formed.

Next, a method of manufacturing a long-period grating according to this embodiment will be described. The optical fiber preform used in the present embodiment includes, but is not limited to, a VAD method,
Glass formed by a CVD method or the like and having, as a main component, quartz glass having a lower refractive index than the core is provided on the outer periphery of a core mainly made of quartz glass. At least one of a halogen element (for example, fluorine, chlorine, etc.) and OH is added to at least one of the core and the cladding, and the ratio of the halogen element or OH added to the cladding is higher than the ratio added to the core.

FIG. 5 is a diagram showing a configuration of an optical fiber drawing apparatus used in the present embodiment. In FIG. 5, the glass fiber 12 is drawn so that a constant tension F is applied by the winding device 16 while the optical fiber preform 10 is heated by the heating device 11. Glass fiber 12
Is coated with an ultraviolet curable resin while passing through a die 13 provided immediately below the heating device 11, and subsequently irradiated with ultraviolet light while passing through an ultraviolet irradiation device 14 provided immediately below the die 13, so that the coated fiber 15 is irradiated. Is produced. The drawing tension F of the glass fiber 12 is determined by the heating temperature of the heating device 11 and the drawing speed. That is, the optical fiber preform 10 is heated by the heating device 11 to a temperature at which it can be drawn. Next, when the glass fiber 12 is drawn and the drawing speed is increased, the drawing is performed before the viscosity of the distal end portion of the optical fiber preform 10 is sufficiently reduced. Therefore, a tensile stress occurs in the core of the glass fiber 12.

At this time, the cladding has a softening point lower than that of the core because a halogen element such as fluorine is added to the cladding relatively more than the core. Therefore, the pulling force F of the optical fiber concentrates on the core. When cured in such a state, a glass fiber 12 having a built-in residual tensile stress in the core is formed.

Next, a method of periodically heating the glass fiber 12 having a built-in residual stress in the axial direction to relieve the residual stress will be described. FIG. 6 is a view showing an apparatus for periodically heating the glass fiber 12 having a residual stress in the core portion in the axial direction. The glass fiber 12 from which the coating has been removed is held by the holding mechanism 22 in the axial direction of the optical fiber in the x direction, and the holding mechanism 22 is moved to the stage 23 movable in the x direction and the y and z directions perpendicular to the x direction. The operation of the stage 23 is controlled by the control device 24.
Is controlled by

The pair of discharge electrodes 26 are connected to the holding mechanism 22.
Are arranged in a direction (Y direction) perpendicular to the axis of the glass fiber 12 held at a predetermined interval with the glass fiber 12 interposed therebetween. An arc discharge occurs between the electrodes 26 to heat the glass fiber 12. The spot size of the arc discharge decreases as the distance between the electrodes 26 decreases, and when the power supplied to the electrodes 26 decreases,
Become smaller. The periodic heating by the arc discharge is performed by moving the stage 23 by the control device 24. 6, a CO ₂ laser can be used in place of the discharge electrode 26 to periodically heat the glass fiber 12 in the axial direction. The spot size of the laser light is adjusted by a lens arranged in front of the CO ₂ laser. Other configurations are the same as those in the case of arc discharge.

The heating temperature for relaxing the residual tensile stress of the core is preferably around the softening temperature T _{1 of the} core. Exceeding the softening temperature T ₁ of the core there is a possibility that the optical fiber be blown, too low a heating time becomes longer. FIG.
4 is a graph showing a relationship between a heating temperature and a heating time required to form a grating using the single mode optical fiber of FIG. From this graph, the heating temperature is 11
Although the residual tensile stress of the core can be relaxed in the range of 00 to 1900 ° C, it is 1600 to 19 from the viewpoint of manufacturing efficiency.
00 ° C. is a preferred range.

(Example) An optical fiber preform in which quartz glass doped with fluorine to be clad was deposited on the outer periphery of quartz glass to be a core was formed by a VAD method.
The refractive index difference between the core and the cladding doped with fluorine is 0.36
%. This optical fiber preform was drawn while applying a tension of 125 g by the drawing apparatus shown in FIG. 5 to form a core of silica glass having a diameter of 9 μm and a cladding of silica glass doped with fluorine having an outer diameter of 125 μm. An optical fiber having the same was manufactured. The difference in refractive index between the core and the cladding of the drawn optical fiber is 0.21%.

The optical fiber thus manufactured was periodically heated by the heating device shown in FIG. 6 to periodically reduce the residual tensile stress of the optical fiber. The spot size of the arc discharge is 100 μm, the heating time at one location is 2 seconds, and the heating pitch is 420 μm. The optical fiber thus heat-treated was measured for the refractive index distribution in the axial direction by a phase contrast microscope. FIG. 8 is a graph showing the measurement results of the refractive index distribution. The relative refractive index difference n ₀ of the portion subjected to the heat treatment is approximately 0.
This shows a relative refractive index difference of 36% between the optical fiber preform and the relative refractive index difference n _{F of the} portion where the residual tensile stress exists.
Is approximately 0.2%, and the variation width Δn _F of the relative refractive index difference is 0.
A long-period grating with a variation period of 15% and a fluctuation period of 420 μm was obtained.

The transmission loss of the long period grating thus obtained was measured. FIG. 9 is a graph of the measurement result showing the wavelength characteristic of the transmission loss, and the characteristic of the clear band rejection filter was measured. In FIG. 9, 1512 nm
And the maximum value of the transmission loss appears at 1542 nm,
This is due to two types of cladding modes that couple with the core mode.

With respect to the optical fiber having the long-period grating manufactured as described above, the loss wavelength characteristic with respect to the temperature was measured. The changed temperature range is -40 ° C to 80 ° C
It is. As a result, the change in the loss peak wavelength with respect to the above temperature range was 1512 nm to 1513 nm, there was substantially no change, and a temperature-independent long-period grating could be obtained.

Incidentally, a single-mode optical fiber is formed of silica glass having a cladding of silica glass and a core of silica glass doped with germanium oxide, and the single-mode optical fiber is periodically irradiated with ultraviolet rays in the core axis direction to change the fluctuation period. Is 420
A long period grating of μm was formed. When the loss wavelength of this optical fiber was measured in a temperature range of −40 ° C. to 80 ° C., the loss wavelength was 1512 nm to 1523 nm.
, And it is confirmed that the change is ten times or more as compared with that of the present embodiment.

[0041]

As described above, the optical fiber provided with the long-period grating according to the present invention has a structure in which the residual tensile stress is periodically arranged in the axial direction, so that the refractive index changes with time. It is a highly reliable configuration with less occurrence. In the optical fiber having the long-period grating according to the present invention, a stable long-period grating can be obtained because the relative refractive index difference between the core and the clad hardly changes even when the temperature changes.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an optical fiber including a long-period grating according to an embodiment.

FIG. 2 is a graph showing a loss wavelength characteristic of the long period grating according to the embodiment.

FIG. 3 is a graph showing a relationship between a tension F at the time of drawing an optical fiber according to the present embodiment and a relative refractive index difference.

FIG. 4 is a graph showing a distribution of a relative refractive index difference with respect to a position in a longitudinal direction of the long period grating according to the embodiment.

FIG. 5 is a diagram showing a configuration of an apparatus for drawing an optical fiber used in the present embodiment.

FIG. 6 shows an apparatus for periodically heating the glass fiber along its axis.

FIG. 7 is a graph showing a relationship between a heating temperature and a heating time necessary for relaxing a residual tensile stress of a glass fiber.

FIG. 8 is a graph showing a distribution of a relative refractive index difference with respect to a position in a longitudinal direction of the long period grating according to the first embodiment.

FIG. 9 is a graph showing transmission loss characteristics of the long period grating according to the first embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Refractive index fringe 2 ... Long period grating 3 ... Core 4 ... Cladding 5 ... Optical fiber provided with long period grating 10 Optical fiber base material 11: heating device, 12: glass fiber, 13: die, 14: ultraviolet irradiation device, 15: coated fiber, 16: winding device, 22: holding mechanism, 23: Stage, 24: Controller, 26: Electrode, F: Tension, n: Refractive index, T
.. Temperature, Λ: spacing (pitch), λ: wavelength

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masashi Onishi 1 Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries Co., Ltd. Yokohama Works (72) Inventor Toru Iwashima 1 Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Ki Kogyo Co., Ltd.

Claims (7)

[Claims] 1. An optical fiber having a core mainly composed of silica glass and a clad mainly composed of silica glass having a lower refractive index than the core, wherein the core has a residual tensile stress periodically in an axial direction. By having, having a refractive index distribution that fluctuates according to the residual tensile stress, the signal light propagating through the core is attenuated in a predetermined wavelength range around the loss wavelength where the loss is maximized, An optical fiber provided with a long-period grating, characterized in that: 2. The optical fiber according to claim 1, wherein the core and the clad are formed by adding at least one of a halogen element and OH to quartz glass. 3. The long-period grating according to claim 1, wherein the relative refractive index difference between the core and the clad of the optical fiber does not substantially change in a temperature range of −40 ° C. to 80 ° C. Equipped with optical fiber. 4. A first step of forming an optical fiber preform having a clad mainly composed of quartz glass having a lower refractive index than the core around an outer periphery of a core mainly composed of quartz glass; Drawing an optical fiber preform formed in the above step to produce an optical fiber having a cladding around a core, and drawing while applying tension to the optical fiber to generate a residual tensile stress in the core. And a third step of periodically heating the optical fiber formed in the second step in the axial direction to periodically reduce the residual tensile stress of the core. Manufacturing method. 5. The method according to claim 4, wherein the third step is a step of periodically heating the optical fiber to 1100 to 1900 ° C. in an axial direction of the optical fiber. 6. The method according to claim 4, wherein the third step is a step of heating by arc discharge. 7. The method according to claim 4, wherein the third step is a step of heating with a CO ₂ laser.