JP4578733B2 - Long-period optical fiber grating using polarization-maintaining optical fiber - Google Patents

Description

【００２０】
図２の結果から、この０．９８μｍ帯域用低複屈折ＰＡＮＤＡファイバは温度による中心波長の変動が小さいことが分かる。
また、表１の結果から、ｓ偏波の中心波長の温度による変動は−０．００８９ｎｍ／℃と、−０．０１〜＋０．０１ｎｍ／℃以内に抑えられている。
このようなＰＡＮＤＡファイバを用いて作製された長周期光ファイバグレーティングは、温度による残留応力の変化が小さいため、ｓ偏波の温度による中心波長の変動が低減されることになる。
【００２１】
（比較例）
０．９８μｍ、１．３３μｍ、１．５５μｍ帯域用ＰＡＮＤＡファイバの被覆を除去した部分に紫外光を照射し、長周期光ファイバグレーティングを形成し、これらの長周期光ファイバグレーティングの温度による損失スペクトルの変動を測定した。この長周期光ファイバグレーティングを形成した際における、ＰＡＮＤＡファイバの紫外光に対する露光条件を表２に示す。
温度によるｓ偏波の中心波長の変動の結果を図３（ａ）に、ｆ偏波の中心波長の変動の結果を図３（ｂ）示す。また、長周期光ファイバグレーティングの温度による中心波長の変動量を表３に示す。
【００２２】
【表２】

【００２３】
【表３】

【００２４】
図３の結果から、温度の上昇に伴って、これらのＰＡＮＤＡファイバにｓ偏波を入射した際の損失ピークの中心波長は短波長側へ、ｆ偏波の中心波長は長波長側へそれぞれ大きく変動している。
また、表３の結果から、この光ファイバグレーティングの温度による中心波長の変動量は、−０．０１〜＋０．０１ｎｍ／℃の範囲を大きく超えており、温度依存性が大きい。
【００２５】
【発明の効果】
以上説明したように、本発明の長周期光ファイバグレーティングに用いられる偏波保持光ファイバは、コアを囲むクラッド内にコアに対して対称的に配された２個の応力付与部を有する偏波保持光ファイバであって、ｓ偏波のみからなる入射光に対して、偏波保持光ファイバの残留応力の温度による変動によって生じる前記入射光の実効屈折率の変化量と、前記偏波保持光ファイバを形成する材料の屈折率の温度による変動によって生じる前記入射光の実効屈折率の変化量とが、逆符号を有し、かつこれらの実効屈折率の変化量の絶対値が近接しているから、偏波保持光ファイバの残留応力の温度による変動によって生じる前記入射光の実効屈折率の変化量が、偏波保持光ファイバを形成する材料の屈折率の温度による変動によって生じる前記入射光の実効屈折率の変化量を打ち消すから、偏波保持光ファイバ全体として前記入射光の実効屈折率がほとんど変化しない。したがって、このような偏波保持光ファイバは、温度依存性の少ないものとなる。
【００２６】
前記偏波保持光ファイバにｓ偏波のみが入射されれば、偏波保持光ファイバの残留応力の温度による変動によって生じる前記入射光の実効屈折率の変化が打ち消される。前記偏波保持光ファイバの複屈折率が１．３×１０^−４〜２．５×１０^−４であれば、この偏波保持光ファイバを用いて作製された長周期光ファイバグレーティング中を伝搬するｓ偏波の光の損失スペクトルの中心波長の変動の温度依存性を低減することができる。前記偏波保持光ファイバを用いて製造された長周期光ファイバグレーティング中を伝搬するｓ偏波の光の損失スペクトルの中心波長の温度変化による変動が−０．０１〜＋０．０１ｎｍ／℃であれば、このファイバグレーティングを用いた光増幅器用利得等化器などの温度依存性が低減する。さらに、この光増幅器用利得等化器は、光通信システムなどに好ましく用いられる。
【図面の簡単な説明】
【図１】ＰＡＮＤＡファイバの一例を示したものである。
【図２】本発明の偏波保持光ファイバを用いて作製した長周期ファイバグレーティングの温度によるｓ偏波の中心波長の変動を示すグラフである。
【図３】従来の偏波保持光ファイバを用いて作製した長周期ファイバグレーティングの温度による中心波長の変動を示すグラフで、図３（ａ）はｓ偏波の中心波長の変動を示し、図３（ｂ）はｆ偏波の中心波長の変動を示す。
【符号の説明】
１１・・・コア、１２・・・クラッド、１３・・・応力付与部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polarization maintaining optical fiber used in the field of optical information communication and an optical component using the same, and more particularly to a polarization maintaining optical fiber with reduced temperature dependency and an optical component using the same.
[0002]
[Prior art]
A polarization maintaining optical fiber coupler is used as a polarization multiplexer used for a multiplexer for pumping light for optical amplification. In addition, as an external resonator for stabilizing a laser diode or an optical fiber laser, a strain sensor, etc., a polarization maintaining fiber grating (FG) is used.
Various types of polarization-maintaining optical fibers used for such optical components have been proposed. A typical example is a PANDA fiber (polarization maintaining and absorption reducing fiber).
[0003]
FIG. 1 shows an example of a PANDA fiber. The PANDA fiber 10 includes a core 11 provided in the center, and is provided around the core 11 in a concentric manner with the core 11. A clad 12 having a lower refractive index, and two stress-applying portions 13 and 13 having a circular cross section that are arranged symmetrically around the core 11 and have a larger coefficient of thermal expansion than the clad 12. It is configured.
In this example, the core 11 is made of germanium-added quartz glass, the cladding 12 is made of pure quartz glass, and the stress applying portion 13 is made of quartz glass to which a relatively large amount of boron is added. The outer diameter of the core 11, the outer diameter of the stress applying portion 13, the relative refractive index difference between the core 11 and the clad 12, and the relative refractive index difference between the clad 12 and the stress applying portion 13 are appropriately set according to desired characteristics.
[0004]
The stress applying part 13 has a larger thermal expansion coefficient than the clad 12 and has a low melting point. Therefore, since the clad 12 is hardened before the stress applying part 13 in the process of cooling the optical fiber drawn at the time of manufacturing the optical fiber, the shape is not distorted so much, and the clad 12 and the stress applying part in the fiber cross section. The stress resulting from the difference of a thermal expansion coefficient with 13 is produced. This stress causes stress anisotropy with respect to the core 11.
As a result, in this PANDA fiber 10, the X polarization propagates while holding the electric field vector in the X-axis direction of the PANDA fiber 10, and the Y polarization holds the electric field vector orthogonal to the X polarization and PANDA fiber 10 is propagated.
In the PANDA fiber, the effective refractive index of X polarization is larger than that of Y polarization and the propagation speed is slow, so the X polarization axis may be referred to as the slow axis and the Y polarization axis may be referred to as the fast axis. In general, in a polarization maintaining optical fiber, since the effective refractive index of one polarization is larger than that of the other and the propagation speed is slow, a polarization having an electric field in the slow axis direction is changed to the s polarization and fast axis directions. A polarization having an electric field may be referred to as f polarization.
[0005]
[Problems to be solved by the invention]
By the way, with recent increases in the density of optical transmission systems, optical components with low temperature dependence are required.
However, the stress generated in the polarization-maintaining optical fiber uses the difference in thermal expansion coefficient between the cladding and the stress-applying part, and the stress-applying part contracts strongly during the process of cooling the polarization-maintaining optical fiber. appear. Therefore, the stress generated in the polarization maintaining optical fiber shows temperature dependence. Therefore, an optical component manufactured using such a polarization maintaining optical fiber exhibits temperature dependence due to the polarization maintaining optical fiber. That is, in such an optical component, as the temperature rises, the stress in the polarization-maintaining optical fiber is released, and the characteristics of propagating s-polarized light and f-polarized light change. Other factors that change the s-polarized wave and f-polarized wave propagation characteristics of the optical component with changes in temperature include changes in the refractive index of the material forming the polarization maintaining optical fiber and thermal expansion. There is a change in the length of the optical component in the longitudinal direction.
[0006]
Japanese Laid-Open Patent Publication No. 9-64440 proposes a method for suppressing fluctuations in X polarization and Y polarization, which cause fluctuations in oscillation frequency in an optical fiber laser, using a polarization maintaining optical fiber grating. . However, no proposal has been made for a polarization maintaining optical fiber and a method for reducing the temperature dependence of an optical component using the same.
[0007]
This invention is made | formed in view of the said situation, and makes it a subject to provide the polarization-maintaining optical fiber which reduced temperature dependence, and an optical component using the same.
[0008]
[Means for Solving the Problems]
The object is achieved by a long-period fiber grating using polarization-maintaining optical fiber having two stress applying portions that symmetrically arranged with respect to the core in a cladding surrounding the core, from the s polarization only For the incident light, the amount of change in the effective refractive index of the incident light caused by the temperature variation of the residual stress of the polarization maintaining optical fiber and the temperature variation of the refractive index of the material forming the polarization maintaining optical fiber The change amount of the effective refractive index of the incident light caused by the above has an opposite sign, and the absolute values of the change amounts of the effective refractive index are close to each other, and the polarization maintaining optical fiber is a PANDA fiber The polarization maintaining optical fiber has a birefringence at room temperature of 1.3 × 10 ^{−4 to} 2.5 × 10 ⁻⁴ , and the distance between the two stress applying portions is that of the core or the cladding. Concentric , Determined based on the diameter of the largest circle that does not affect the stress applying part and does not include the stress applying part therein, the diameter is 25-30 μm, the core is germanium-added quartz glass, The clad is made of pure silica glass, and the stress applying portion is made of quartz glass to which boron is added, and only s-polarized light is incident on the polarization-maintaining optical fiber and propagates through the long-period optical fiber grating. The fluctuation due to the temperature change of the center wavelength of the loss spectrum of the s-polarized light can be solved by a long-period optical fiber grating characterized in that it is −0.01 to +0.01 nm / ° C.
The object is achieved by a long-period fiber grating using polarization-maintaining optical fiber having two stress applying portions that symmetrically arranged with respect to the core in a cladding surrounding the core, from the s polarization only For the incident light, the amount of change in the effective refractive index of the incident light caused by the temperature variation of the residual stress of the polarization maintaining optical fiber and the temperature variation of the refractive index of the material forming the polarization maintaining optical fiber The change amount of the effective refractive index of the incident light caused by the above has an opposite sign, and the absolute values of the change amounts of the effective refractive index are close to each other, and the polarization maintaining optical fiber is a PANDA fiber The polarization maintaining optical fiber has a birefringence of 1.3 × 10 ^{−4 to} 2.5 × 10 ⁻⁴ at room temperature, the stress applying portion has a diameter of 15 to 35 μm, and the core is doped with germanium. stone British glass, the cladding is made of pure silica glass, and the stress applying portion is made of silica glass to which boron is added, and only s-polarized light is incident on the polarization-maintaining optical fiber, and the long-period optical fiber grating Fluctuation due to temperature change of the center wavelength of the loss spectrum of s-polarized light propagating through the inside can be solved by a long-period optical fiber grating characterized in that it is −0.01 to +0.01 nm / ° C.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below.
Here, a PANDA fiber is shown in FIG. 1 as an example of the polarization maintaining optical fiber of the present invention.
The PANDA fiber 10 in this example includes a core 11 provided at the center, a cladding 12 provided concentrically with the core 11 around the core 11, and having a lower refractive index than the core 11, and the cladding 11. 12 includes two stress applying portions 13 and 13 which are arranged symmetrically around the core 11 and have a circular cross section having a larger coefficient of thermal expansion than the clad 12.
In the PANDA fiber 10 of this example, when s-polarized light is incident, the amount of change in the effective refractive index of incident light (hereinafter referred to as “amount of change A”) caused by fluctuation of the residual stress due to temperature, and PANDA. The amount of change in effective refractive index of incident light (hereinafter referred to as “change amount B”) caused by the temperature variation of the refractive index of the material forming the core 11, the cladding 12 and the stress applying portion 13 of the fiber 10 is reversed. And the absolute values of the change amount A and the change amount B are substantially equal.
[0010]
In the polarization maintaining optical fiber of this example, the effective refractive index change amount A caused by the temperature change cancels the effective refractive index change amount B. Hardly changed.
[0011]
Further, in the PANDA fiber 10, the amount of change A and the amount of change B have opposite signs, and there is a method of reducing the residual stress of the PANDA fiber 10 at room temperature as a method of making the absolute values substantially equal. According to this method, the change amount of the residual stress due to the temperature change of the PANDA fiber 10 can be reduced, and the absolute value of the change amount A can be brought close to the change amount B.
Specific methods for reducing the residual stress of the PANDA fiber 10 at room temperature include a method of increasing the distance between the stress applying portions 13 and 13, a method of reducing the stress applying portions 13 and 13, and the stress applying portions 13 and 13. And a method of reducing the difference in thermal expansion coefficient with the cladding 12. In the polarization maintaining optical fiber of this example, a method of increasing the distance between the stress applying portions 13 and 13 is preferably used.
[0012]
A method for increasing the distance between the stress applying portions 13 and 13 will be described below.
As shown in FIG. 1, the distance between the stress applying portions 13 and 13 is a concentric circle of the core 11 or the clad 12, does not reach the stress applying portions 13 and 13, and the stress applying portions 13 and 13 are provided inside thereof. It is determined based on the diameter A of the largest circle 15 not including. The diameter A is 20 μm or more, preferably 25 to 30 μm.
In a polarization maintaining optical fiber for ordinary communication or the like, the diameter A is about 12 to 17 μm. On the other hand, when the stress applying portions 13 and 13 are separated as described above, the stress applied to the core 11 by the stress applying portions 13 and 13 is reduced, and the birefringence is compared with a normal polarization maintaining optical fiber. The rate is lowered.
[0013]
Further, when the stress applying portions 13 and 13 are reduced, the diameter of the stress applying portion 13 is preferably 15 to 35 μm.
[0014]
In the PANDA fiber 10, when the s-polarized light is incident on the PANDA fiber 10, the amount of change A and the amount of change B caused by the temperature change cancel each other. Therefore, in order to reduce the temperature dependence of the polarization maintaining optical fiber of this example, it is preferable that only s-polarized light is incident on the polarization maintaining optical fiber.
[0015]
In addition, the polarization maintaining optical fiber of the present invention has a reduced birefringence at room temperature, and the birefringence is preferably 1.0 × 10 ^{−4 to} 3.0 × 10 ⁻⁴ . More preferably, it is 1.3 × 10 ^{−4 to} 2.5 × 10 ⁻⁴ . If the birefringence is within the above range, the fluctuation due to temperature change of the center wavelength of an optical fiber grating, which will be described later, manufactured using this polarization maintaining optical fiber can be suppressed to within ± 0.01 nm / ° C. .
If the birefringence is less than 1.0 × 10 ⁻⁴ , the temperature variation of the loss center wavelength of the optical fiber grating is +0.01 nm / ° C. or more. On the other hand, when the birefringence exceeds 3.0 × 10 ⁻⁴ , it becomes −0.01 nm / ° C. or less.
By the way, it is known that the center wavelength of the optical fiber grating formed in the polarization maintaining optical fiber is proportional to the birefringence of the polarization maintaining optical fiber.
[0016]
Further, the polarization maintaining optical fiber of the present invention is not limited to a PANDA fiber, but may be a polarization maintaining optical fiber such as a Bow-Tie fiber or an elliptical jacket fiber.
Furthermore, the polarization maintaining optical fiber of the present invention can be applied to optical components such as an optical fiber grating such as a long-period fiber grating and a short-period fiber grating, and an optical fiber coupler.
[0017]
Here, long-period fiber grating is described as an example of an optical component.
In long-period fiber gratinging, a refractive index modulation with a period of several tens of μm to 1000 μm is formed in the longitudinal direction of the optical fiber, so that light propagating in the core (guided mode light) is transmitted in the clad in the same direction. Coupled to propagating light (clad mode light) and radiation mode light in the same direction. When long-period fiber grating is formed in a polarization maintaining optical fiber, the main factors that the center wavelength fluctuates with temperature are due to fluctuations in residual stress and fluctuations in the refractive index of the material.
When the long-period fiber grating is formed in the polarization maintaining optical fiber of the present invention, the fluctuation of the center wavelength due to the fluctuation of the residual stress accompanying the temperature rise when either the s polarized wave or the f polarized wave is incident. However, it moves in a direction that cancels the fluctuation of the center wavelength due to the refractive index fluctuation of the material forming the polarization maintaining optical fiber. Therefore, since this long period fiber grating has almost no temperature dependence, it is preferably used for a gain equalizer for an optical amplifier. Furthermore, the gain equalizer for an optical amplifier is preferably used for an optical communication system or the like.
[0018]
Hereinafter, specific examples will be shown to clarify the effects of the present invention.
(Example)
A low birefringence PANDA fiber for 0.98 μm band in which residual stress at room temperature was reduced was produced by separating the stress applying portion from the core. The produced birefringence of the low birefringence PANDA fiber for 0.98 μm band is 2.4 × 10 ^{−4 at} 25 ° C., and the birefringence of the normal 0.98 μm band PANDA fiber is 4.9 × 10 ^−4. It was about half of.
The portion where the coating of the low birefringence PANDA fiber for 0.98 μm band was removed was irradiated with ultraviolet light to form a long-period optical fiber grating, and the fluctuation of the loss spectrum due to the temperature of the long-period optical fiber grating was measured. When this long-period optical fiber grating was formed, the exposure conditions of the PANDA fiber for ultraviolet light were a refractive index modulation period of 320.3 μm and a grating stage number of 118.
FIG. 2 shows the result of fluctuations in the center wavelength of the s-polarization with temperature. Table 1 shows the fluctuation amount of the center wavelength depending on the temperature of the long-period optical fiber grating.
[0019]
[Table 1]

[0020]
From the result of FIG. 2, it can be seen that the low birefringence PANDA fiber for 0.98 μm band has a small variation in the center wavelength due to temperature.
Further, from the results in Table 1, the fluctuation of the center wavelength of the s-polarized light due to the temperature is suppressed to −0.0089 nm / ° C. and within −0.01 to +0.01 nm / ° C.
Since a long-period optical fiber grating manufactured using such a PANDA fiber has a small change in residual stress due to temperature, the fluctuation of the center wavelength due to the temperature of s-polarized light is reduced.
[0021]
(Comparative example)
The portion of the PANDA fiber for 0.98 μm, 1.33 μm, and 1.55 μm bands is irradiated with ultraviolet light to form long-period optical fiber gratings, and the loss spectrum due to the temperature of these long-period optical fiber gratings Variation was measured. Table 2 shows the exposure conditions of the PANDA fiber for ultraviolet light when this long-period optical fiber grating is formed.
FIG. 3A shows the result of fluctuation of the center wavelength of the s-polarization due to temperature, and FIG. 3B shows the result of fluctuation of the center wavelength of the f-polarization. Table 3 shows the amount of fluctuation of the center wavelength depending on the temperature of the long-period optical fiber grating.
[0022]
[Table 2]

[0023]
[Table 3]

[0024]
From the results of FIG. 3, as the temperature rises, the center wavelength of the loss peak when s-polarized light is incident on these PANDA fibers increases toward the short wavelength side, and the center wavelength of the f-polarized wave increases toward the long wavelength side. It has fluctuated.
Further, from the results of Table 3, the fluctuation amount of the center wavelength due to the temperature of the optical fiber grating greatly exceeds the range of −0.01 to +0.01 nm / ° C., and the temperature dependency is large.
[0025]
【The invention's effect】
As described above, the polarization maintaining optical fiber used in the long-period optical fiber grating of the present invention has a polarization having two stress applying portions disposed symmetrically with respect to the core in the cladding surrounding the core. a maintaining optical fiber, to the incident light comprising s-polarized only the the variation of the effective refractive index of the incident light caused by variations due to the temperature of the residual stress of the polarization-maintaining optical fiber, the polarization maintaining optical and the amount of change of the effective refractive index of the incident light caused by variations due to the temperature of the refractive index of the material forming the fiber has an opposite sign, and the absolute value of the change amount of the effective refractive index is close from before the variation of the effective refractive index of the incident light caused by variations due to the temperature of the residual stress of the polarization-maintaining optical fiber is caused by variations due to the temperature of the refractive index of the material forming the polarization-maintaining optical fiber Since canceling the variation of the effective refractive index of the serial incident light, the effective refractive index of the incident light as a whole polarization-maintaining optical fiber is hardly changed. Therefore, such a polarization maintaining optical fiber is less temperature dependent.
[0026]
When the polarization maintaining optical fiber to only s-polarized is incident, a change in the effective refractive index of the incident light caused by variations due to the temperature of the residual stress of the polarization-maintaining optical fiber is canceled. If the birefringence of the polarization maintaining optical fiber is 1.3 × 10 ^{−4 to} 2.5 × 10 ⁻⁴ , it propagates through a long-period optical fiber grating manufactured using this polarization maintaining optical fiber. It is possible to reduce the temperature dependence of the fluctuation of the center wavelength of the loss spectrum of the s-polarized light. If the fluctuation of the center wavelength of the loss spectrum of s-polarized light propagating in the long-period optical fiber grating manufactured using the polarization-maintaining optical fiber is −0.01 to +0.01 nm / ° C. For example, the temperature dependency of an optical amplifier gain equalizer using the fiber grating is reduced. Furthermore, the gain equalizer for an optical amplifier is preferably used for an optical communication system or the like.
[Brief description of the drawings]
FIG. 1 shows an example of a PANDA fiber.
FIG. 2 is a graph showing fluctuations in the center wavelength of s-polarized light with temperature of a long-period fiber grating manufactured using the polarization maintaining optical fiber of the present invention.
FIG. 3 is a graph showing a change in center wavelength due to temperature of a long-period fiber grating manufactured using a conventional polarization maintaining optical fiber. FIG. 3 (a) shows a change in center wavelength of s-polarized wave. 3 (b) shows the fluctuation of the center wavelength of f-polarized light.
[Explanation of symbols]
11 ... Core, 12 ... Cladding, 13 ... Stress applying part

Claims (2)

A long-period optical fiber grating using a polarization maintaining optical fiber having two stress applying portions disposed symmetrically with respect to the core in a clad surrounding the core,
For incident light consisting only of s-polarized light, the amount of change in the effective refractive index of the incident light caused by temperature fluctuations in the residual stress of the polarization-maintaining optical fiber, and the refraction of the material forming the polarization-maintaining optical fiber The change amount of the effective refractive index of the incident light caused by the variation of the refractive index due to temperature has an opposite sign, and the absolute value of the change amount of the effective refractive index is close,
The polarization maintaining optical fiber is a PANDA fiber, and the polarization maintaining optical fiber has a birefringence of 1.3 × 10 ^{−4 to} 2.5 × 10 ⁻⁴ at room temperature, and the two stress applying portions The distance between them is a concentric circle of the core or the clad, and is determined on the basis of the diameter of the largest circle not applied to the stress applying portion and not including the stress applying portion therein. ~ 30 μm,
The core is made of germanium-added quartz glass, the clad is pure quartz glass, and the stress applying part is made of quartz glass to which boron is added, respectively.
The fluctuation due to the temperature change of the center wavelength of the loss spectrum of the s-polarized light that is incident only on the polarization-maintaining optical fiber and propagates in the long-period optical fiber grating is -0.01 to +0. A long-period optical fiber grating characterized by being 01 nm / ° C. A long-period optical fiber grating using a polarization maintaining optical fiber having two stress applying portions disposed symmetrically with respect to the core in a clad surrounding the core,
For incident light consisting only of s-polarized light, the amount of change in the effective refractive index of the incident light caused by temperature fluctuations in the residual stress of the polarization-maintaining optical fiber, and the refraction of the material forming the polarization-maintaining optical fiber The change amount of the effective refractive index of the incident light caused by the variation of the refractive index due to temperature has an opposite sign, and the absolute value of the change amount of the effective refractive index is close,
The polarization maintaining optical fiber is a PANDA fiber, a birefringence index of the polarization maintaining optical fiber at room temperature is 1.3 × 10 ^{−4 to} 2.5 × 10 ⁻⁴ , and a diameter of the stress applying portion is 15-35 μm,
The core is made of germanium-added quartz glass, the clad is pure quartz glass, and the stress applying part is made of quartz glass to which boron is added, respectively.
Only the s-polarized light is incident on the polarization-maintaining optical fiber, and the fluctuation due to the temperature change of the center wavelength of the loss spectrum of the s-polarized light propagating through the long-period optical fiber grating is −0.01 to +0. A long-period optical fiber grating characterized by being 01 nm / ° C.

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