JP2005106865A - Method for fixing plane-of-polarization conservative optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for fixing a plane-of-polarization conservative optical fiber that improves a characteristic of a quenching ratio by suppressing deterioration in quenching ratio and wavelength dependence, temperature dependence, and optical-fiber-length dependence of the deterioration. <P>SOLUTION: The plane-of-polarization conservative optical fiber is joined and fixed to a support member, and satisfies the equation: L = (Lb/2×N) ± Lb/8, where L is the optical fiber length of the plane-of-polarization conservative optical fiber which is stressed as a result of the join fixation in an optical axis direction, Lb is the beat length of the plane-of-polarization conservative optical fiber, and N is an arbitrary integer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a polarization-maintaining optical fiber used for optical communication or the like, and more particularly to a polarization-maintaining optical fiber fixing method for suppressing a decrease in extinction ratio due to stress at the time of bonding and fixing.

  Recently, the use of polarized waves is increasing in optical communication, optical measurement, optical sensors, and the like. Especially in optical communication, transmission capacities from 2.5Gbps to 10Gbps, and even 40Gbps are being put to practical use. When it becomes 10Gbps or more, it becomes difficult to directly modulate the semiconductor laser, and an external modulation method using an LN (lithium niobate) modulator or the like is adopted. In the external modulation method, light from a semiconductor laser is optically coupled to a polarization-maintaining optical fiber (hereinafter simply referred to as an optical fiber if necessary), and light emitted from the optical fiber is guided to an LN modulator. In the modulator, light is switched by an electric signal and transmitted through the optical fiber as an optical signal.

As means for optically coupling light emitted from a light source such as a semiconductor laser to a polarization-maintaining optical fiber, the optical fiber is bonded to a cylindrical support part such as a ferrule or capillary made of ceramic glass or metal, or an adhesive or solder After bonding and fixing with a fixing agent such as an agent, the optical fiber core is aligned with a focus position of the light emitted from the light source with high accuracy, and the support component is fixed to a package in which the light source is stored. The method is generally done. FIG. 9 is a partial cross-sectional view showing an example of an optical fiber fixing method using such a support component (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 04-234710 (page 4, FIG. 1 (b))

In FIG. 9, the coating 2 is removed for a certain length near the tip of the optical fiber 1, and the strand 3 is exposed, and the inside of the strand 3 has a high refractive index as shown in the sectional view of FIG. The core 1a, a relatively low refractive index clad 1c formed concentrically around the core 1a, and two stress applying portions 1b provided in the clad 1c. The stress applying portions 1b are disposed symmetrically around the core 1a in the cladding 1c, and the cross section thereof is circular. Further, its refractive index is lower than that of the cladding 1c. A material having a larger thermal expansion coefficient than that of the cladding 1c is used for the stress applying portion 1b, and in particular, B ₂ O ₃ —SiO ₂ glass is widely used. Internal stress is applied to the core 1a from both sides by the two stress applying portions 1b (in the case of FIG. 2, it is applied in the Y-axis direction), and the internal stress causes the stress distribution inside the core 1a to be in the X and Y axes. It becomes asymmetric and birefringence characteristics appear. Due to this characteristic, when light enters the core 1a, the incident light propagates through the core 1a while maintaining the polarization plane of the light in a certain direction. This stress direction Y-axis and the orthogonal X-axis are called the optical fiber principal axis (polarization preserving axis). Due to the asymmetry of the stress distribution, coupling between the polarization modes is prevented by making a difference in the propagation constant between the X axis and the Y axis.

  The strand 3 is inserted into a cylindrical capillary 4 made of ceramic glass, and is bonded and fixed with an adhesive (not shown). Further, the capillary 4 and the coating 2 are inserted and fixed to a cylindrical ferrule 5 made of stainless steel or the like. The outer peripheral surface of the coating 2 and the inner peripheral surface of the ferrule are also bonded and fixed with the same adhesive as described above.

  However, bonding stress and thermal stress (hereinafter referred to as generic names) due to mismatch in thermal expansion coefficient due to the difference between the material of the optical fiber 1 and the material of the support component (capillary 4 or ferrule 5), curing shrinkage of the adhesive, and the like. If this occurs, the direction of the internal stress preliminarily applied to the optical fiber 1 is disturbed and the birefringence inside the core 1a changes, and the apparent rotation of the birefringent main axis and Degradation of polarization characteristics such as an increase in polarization crosstalk is caused. When such stress is applied to the optical fiber 1, the optical fiber strand covered with the coating 2 is hardly affected by the stress because the coating 2 functions as a kind of stress buffering material. Even if appears, it is small enough to be ignored.

  However, since the sheath 2 is removed from the strand 3, the stress is transmitted to the strand 3 over the optical fiber length L (the length on the optical axis) without being buffered. Therefore, the polarization characteristic is deteriorated as described above, and the extinction ratio of the optical fiber 1 is greatly deteriorated.

  Further, the extinction ratio change characteristic (wavelength dependence) accompanying the change in light wavelength, the extinction ratio change characteristic (temperature dependence) accompanying the temperature change of the optical fiber 1, and the optical axis direction (arrow Z direction) of the optical fiber 1 The characteristic of changing the extinction ratio (depending on the length of the optical fiber) with the change in length in () also becomes violent, and it is difficult to obtain a stable characteristic of the extinction ratio. FIG. 10 is an example of the result of actual measurement of the extinction ratio-wavelength dependence characteristic of the optical fiber 1 of FIG.

  The graph of FIG. 10 was measured as follows. First, a polarization-maintaining optical fiber with a total length of Lf: 1020 mm is prepared as a sample, the optical fiber is inserted into a ferrule (supporting part), the outer peripheral surface of the optical fiber is bonded with an epoxy resin agent, and light is emitted with a tunable laser light source. The extinction ratio of the optical fiber was measured while changing the wavelength band of the incident light to the fiber. As shown in FIG. 10, it can be seen that the extinction ratio fluctuates so as to decrease significantly periodically with respect to the change in the wavelength.

と導かれる。ここで、λ：波長、B：モード複屈折率、Lf：全長であり、λ：1550nm、B：0.0005、Lf：1020mmを代入すると、λb = 4.7nmと導出される。これは図１０のグラフとよく一致している。 Here, if the wavelength period of fluctuation of the extinction ratio is λb, the wavelength period λb of the extinction ratio drop is

It is guided. Here, λ: wavelength, B: mode birefringence, Lf: full length, and λ: 1550 nm, B: 0.0005, Lf: 1020 mm are substituted, and λb = 4.7 nm is derived. This is in good agreement with the graph of FIG.

  The present invention has been made in view of the above problems, and its purpose is to suppress extinction ratio, and to suppress / improve the wavelength dependence, temperature dependence, and optical fiber length dependence of the degradation, and to reduce extinction ratio characteristics. It is an object of the present invention to provide a method for fixing a polarization-maintaining optical fiber that can be improved.

に設定することを特徴とする偏波面保存光ファイバの固定方法である。 According to the first aspect of the present invention, the polarization-maintaining optical fiber is bonded and fixed to a support component, and the length of the optical fiber in the optical axis direction of the polarization-maintaining optical fiber to which stress is applied by the bonding and fixing is L. When the beat length of the wavefront-preserving optical fiber is represented by Lb and an arbitrary integer is represented by N, the L is

The polarization plane preserving optical fiber fixing method is characterized in that it is set as follows.

  Furthermore, the invention according to claim 2 of the present invention is a method of fixing a polarization-maintaining optical fiber, characterized in that the arbitrary integer N is set to an even number.

  According to the method of fixing a polarization-maintaining optical fiber of the present invention, the length of the optical fiber to which stress is applied in the polarization-maintaining optical fiber is set to an integral multiple of 1/2 of the beat length, which is extremely simple and can be manufactured. It is possible to improve the extinction ratio characteristic of the polarization-maintaining optical fiber by suppressing the above cost increase.

  Furthermore, by setting the length of the optical fiber to which the stress is applied to an even multiple of 1/2 of the beat length, it is possible to further prevent the deterioration of the extinction ratio characteristic of the polarization-maintaining optical fiber.

  Hereinafter, a method for fixing a polarization-maintaining optical fiber according to the present invention will be described with reference to FIGS. FIG. 1 is a partial cross-sectional side view of a support component-integrated optical fiber fixed by a polarization-maintaining optical fiber fixing method according to the present invention, and FIG. 2 is a plane perpendicular to the optical axis of the polarization-maintaining optical fiber. FIG. 3 is an explanatory view showing a polarization state of light propagating through the core of the polarization-maintaining optical fiber. In addition, the same number is attached | subjected to the same location as the conventional support component integrated optical fiber shown in FIG. 8, and the overlapping description is abbreviate | omitted or simplified and described.

  1, 2 and 3 (a), linearly polarized light whose polarization direction is deviated by + θ degrees with respect to the Y-axis (the axis in which the two stress applying portions 1b are linearly arranged) is incident on the polarization-maintaining optical fiber. Then, the incident light corresponds to the increase in the phase difference in each of the X-axis and Y-axis polarization components. FIG. 3 (a): + θ degree linearly polarized light → FIG. 3 (b): right (or left) ) Rotating elliptically polarized light → Fig. 3 (c): -θ degree linearly polarized light → Figure 3 (d): Left (or right) rotating elliptically polarized light → Fig. 3 (a): + θ degree linearly polarized light → Propagating through the core 1a of the optical fiber 1 while periodically changing the polarization state as shown in FIG. Note that the rotation direction of the elliptically polarized light in FIGS. 3B and 3D depends on the magnitude relationship of the propagation constants of the respective polarization components of the X axis and the Y axis.

ここで、λ：波長、B：モード複屈折率、Lb：ビート長をそれぞれ表し、λ：1550nm、B：0.0005を代入することにより、Lb＝3.1mmが得られる。 The optical fiber length in the optical axis direction (arrow Z direction) of the polarization-maintaining optical fiber 1 required for one cycle change from FIG. 3 (a) to the next figure (a) is called beat length Lb or coupling length. The length Lb is expressed by the following equation.

Here, λ: wavelength, B: mode birefringence, Lb: beat length, respectively, and by substituting λ: 1550 nm and B: 0.0005, Lb = 3.1 mm is obtained.

  Each polarization state in (a), (b), (c), and (d) shown in FIG. 3 corresponds to a phase difference for each quarter of the beat length Lb. When linearly polarized light having a polarization state coincident with θ = 0 degrees, that is, the Y axis is incident on the optical fiber 1, the incident light is propagated through the core 1a while maintaining the linearly polarized state.

  Here, as shown in FIG. 1, the optical fiber length L (which is the length on the optical axis) is applied to the joint where the strand 3 of the polarization-maintaining optical fiber 1 is joined and fixed with the adhesive 6. Stress caused by bonding is generated and remains. Then, a change occurs in the direction (Y-axis direction) and the magnitude of the internal stress applied in advance in the core 1a of the polarization-maintaining optical fiber 1 due to the stress.

  In a general method of fixing the polarization-maintaining optical fiber 1 and the support component (capillary 4 or ferrule 5), the influence on the extinction ratio by fixing the joint is a major cause of the change in the direction of the internal stress causing the influence. The change of the magnitude (scalar amount) of the internal stress itself is negligible.

  Here, the stress in the direction shifted by + φ degrees (−45 degrees ≦ φ ≦ + 45 degrees: +3 degrees as an example here) from the Y-axis is caused by the capillary 4 and the strand 3 that are one of the support parts. When joining the joint, the optical fiber length L of the joint by the adhesive 6 is set to an even multiple of 1/2 of the beat length Lb (that is, an integral multiple of the beat length Lb, excluding negative numbers and zero), and further Linearly polarized light having a polarization deviation angle of 0 degree with respect to the Y-axis direction was made incident on the polarization-preserving optical fiber, and the polarization state of the propagation light in the core 1a was measured. FIG. 4 shows the polarization state of the light inside the core 1a at the joint, and FIG. 5 shows a graph of the polarization direction of the light propagating inside the core 1a and the optical fiber length L.

  Then, the incident light passes through the core 1a as shown in FIG. 4 (a) → FIG. 4 (b) → FIG. 4 (c) → FIG. 4 (d) → FIG. 4 (a) →. Propagating while repeating a periodic polarization state change with polarization (however, since linearly polarized light with a polarization deviation angle of 0 degree is incident, the polarization state of (a) is the polarization deviation angle with respect to the Y-axis direction. Is a linearly polarized light of 0 degree, and the polarization state of (c) is a linearly polarized light whose polarization deviation angle is + 2φ degrees with respect to the Y axis), and the incident polarization direction every even multiple of 1/2 of the beat length Lb It was measured to return to (0 degree = (a) state). Therefore, by setting the optical fiber length L to an even multiple of 1/2 of the beat length Lb, linearly polarized light whose polarization direction is the Y-axis direction is measured at the end of the joint. Therefore, in the strand covered with the covering 2, the light propagates through the core 1a while maintaining the linearly polarized state.

  Next, FIG. 6 shows a graph of the polarization direction of light propagating through the core 1a and the optical fiber length L when the optical fiber length L is set to an odd multiple of 1/2 of the beat length Lb. Then, the incident light passes through the core 1a, as shown in FIG. 4 (c) → FIG. 4 (d) → FIG. 4 (a) → FIG. 4 (b) → FIG. 4 (c) →. Propagating while periodically changing the state of polarization with respect to the polarization (however, since linearly polarized light with a polarization deviation angle of + 2φ is incident, the polarization state in (a) is the polarization deviation angle with respect to the Y-axis direction. Is a linearly polarized light of 0 degree, and the polarization state of (c) is a linearly polarized light with a polarization deviation angle of + 2φ degrees with respect to the Y axis), and a polarization deviation angle every odd multiple of 1/2 of the beat length Lb It was measured to return to the polarization state of 0 degree. Accordingly, by setting the optical fiber length L to an odd multiple of 1/2 of the beat length Lb, linearly polarized light whose polarization direction is the Y-axis direction is measured at the end of the joint. Therefore, in the strand covered with the covering 2, the light propagates through the core 1a while maintaining the linearly polarized state.

  In either case, linearly polarized light in the Y-axis direction is obtained at the non-joined portion (the strand portion covered with the coating 2). In order to maintain the linearly polarized state at the non-junction portion, it is necessary to change the polarization angle of the incident light between the even times and the odd times as described above. With respect to, it is sufficient to adjust the extinction ratio of the emitted light to be maximum, and there is no need to consider in particular the change in the stress direction due to bonding and fixing. However, there are two axial directions in which the extinction ratio of the emitted light is maximized, that is, the Y axis that is the stress application direction and the X axis that is the orthogonal direction. In the case of the even multiple, since the polarization direction of the incident light coincides with the Y axis, it is easy to determine which axis coincides. On the other hand, in the case of odd multiples, if the angle value of φ increases and the deviation from the Y-axis increases, the polarization direction of the incident light is adjusted based on which axis when the extinction ratio of the emitted light is adjusted to the maximum. It becomes difficult to determine whether it was done. Therefore, considering the possibility of changing the direction of stress (value of angle φ), setting the optical fiber length L to an even multiple of 1/2 of the beat length Lb is more effective in suppressing deterioration of the extinction ratio. Is preferable. Further, in the case of an odd number, since the incident polarized light must be incident after setting the angle of deviation of incident light to 2φ degrees, it is predicted that the deterioration of the extinction ratio is slightly larger than that at the even number. FIG. 7 shows a graph of the extinction ratio and the optical fiber length L. As is clear from the comparison with FIGS. 5 and 6, it has been found that the extinction ratio becomes maximum at an integral multiple of 1/2 the beat length Lb.

  From FIG. 7, since the fluctuation range of the 1/8 extinction ratio of Lb when the optical fiber length L fluctuates by 1/8 of the beat length Lb is sufficiently smaller than the maximum extinction ratio, ± (Lb / 8) can be set as the tolerance of the optical fiber length L.

で与えられる。数４よりビート長Lbは波長λの関数であり、前記接合箇所で生じる位相差δは波長λに依存するが、前記光ファイバ長Lは光ファイバの全長Laに比べると非常に短く、またビート長Lbの数倍以内なので、接合箇所での位相差δの波長依存性は無視できる。 The phase difference δ between the polarization components of the X axis and the Y axis depends on the optical fiber length L,

Given in. From equation (4), the beat length Lb is a function of the wavelength λ, and the phase difference δ generated at the junction depends on the wavelength λ, but the optical fiber length L is very short compared to the total length La of the optical fiber, and the beat Since it is within several times the length Lb, the wavelength dependence of the phase difference δ at the junction can be ignored.

  Fig. 8 shows a method of inserting a strand of polarization-maintaining optical fiber (B: 0.0005) with a total length La: 1018 mm into a ferrule with a length of 3.1 mm (twice Lb / 2) and joining with a resin adhesive. It is the result of having actually measured the wavelength dependence of the extinction ratio of the polarization-maintaining optical fiber when fixed. Compared with FIG. 10, it can be seen that the fluctuation of the extinction ratio drop due to the wavelength change is greatly reduced.

  Since the wavelength dependence of the extinction ratio is caused by the phase difference between the polarization components of the X axis and the Y axis, it is clear that the temperature dependence and optical fiber length dependence caused by the same phase difference can be greatly improved.

  By applying the polarization-maintaining optical fiber fixing method of the present invention to a polarization-maintaining optical fiber used in optical communication, optical measurement, optical sensors, etc., the extinction ratio characteristic of an optical device using polarization can be improved. Can be improved.

The fragmentary sectional side view of the support component integrated optical fiber fixed by the fixing method of the polarization-maintaining optical fiber according to the present invention. Sectional drawing when a polarization plane preserving optical fiber is cut along a plane perpendicular to the optical axis. The figure which shows the polarization state of the light which propagates the inside of the core of a polarization-maintaining optical fiber. The figure which shows the polarization state of the light which propagates the inside of the core in a junction part. The graph of the polarization direction of the light which propagates the inside of a core, and an optical fiber length when the optical fiber length of the junction part of a support component and an optical fiber is set to the even multiple of 1/2 of beat length Lb. The graph of the polarization direction of the light which propagates the inside of a core, and optical fiber length when the optical fiber length of the junction part of a support component and an optical fiber is set to the odd multiple of 1/2 of beat length Lb. The graph of the extinction ratio and optical fiber length of the polarization-maintaining optical fiber fixed by the fixing method of the polarization-maintaining optical fiber according to the present invention. The graph which shows the wavelength dependence of the extinction ratio of the polarization-maintaining optical fiber fixed by the fixing method of the polarization-maintaining optical fiber according to the present invention. The fragmentary sectional side view of the support component integrated optical fiber fixed by the conventional fixing method. The graph which measured the extinction ratio-wavelength dependence characteristic of the polarization-maintaining optical fiber fixed by the conventional fixing method.

Explanation of symbols

1 Polarization plane preserving optical fiber
1a core
1b Stress application part
1c Cladding 2 Coating 3 Wire 4 Capillary 5 Ferrule 6 Adhesive

Claims (2)

The polarization-maintaining optical fiber is bonded and fixed to a support component, and the optical fiber length in the optical axis direction of the polarization-maintaining optical fiber to which stress is applied by the bonding and fixing is L, and the beat length of the polarization-maintaining optical fiber is Lb When the integer of is represented by N, the L is

A method for fixing a polarization-maintaining optical fiber, characterized in that:   The method of fixing a polarization-maintaining optical fiber according to claim 1, wherein the arbitrary integer N is set to an even number of 2 or more.

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