JPH10288715A - Two-wavelength stop type optical fiber grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the grating small-sized by forming a grating part in a polarization plane maintaining fiber. SOLUTION: A PANDA fiber 10 consists of a core 11 made of quartz glass to which germanium is added, a clad 12 made of pure quartz glass, and a stress imposing part 13 made of quartz glass to which a relatively large amount of boron is added. At one place of the PANDA fiber 10 which is different in effective refractive index to an Xpolarized wave, the core 11 is given periodical variation in refractive index along the length of the PANDA filter 10 to constitute one kind of grating part. Consequently, different grating characteristics are shown to the X-polarized wave and Y-polarized wave of light transmitted through the grating part, and a transmission spectrum having two loss peaks is obtained, so that this grating can be used as a two-wavelength stop type optical fiber grating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a two-wavelength blocking optical fiber grating, and more particularly to a two-wavelength blocking optical fiber grating that can be downsized.

[0002]

2. Description of the Related Art An optical fiber grating is formed by forming a constant periodic change in the length direction of an optical fiber, for example, a periodic change in the refractive index of the core or a periodic change in the core diameter.

Conventionally, as one method of manufacturing an optical fiber grating, when a quartz glass to which germanium is added is irradiated with strong ultraviolet light, the refractive index increases in accordance with the amount of irradiation, and a periodic wave is applied to the core. There is known a method of forming a significant refractive index change. For example, a germanium-doped core / silica-clad optical fiber or a germanium-doped core / fluorine-doped optical fiber is used, and this optical fiber is placed in a hydrogen pressurized container (100a).
tm), a method of irradiating ultraviolet light at a constant period in the optical fiber length direction using a photomask, or a method of sequentially irradiating ultraviolet light at equal intervals in the optical fiber length direction is used. Have been.

In general, optical fiber gratings include a radiation mode coupling type and a reflection mode coupling type. The radiation mode coupling type grating couples a mode propagating in a core and a mode propagating in a cladding to generate light of a specific wavelength. Is emitted to the outside of the optical waveguide to attenuate the characteristic. The reflection mode coupling type grating reflects light of a specific wavelength by coupling a mode propagating in the core in a positive direction and a mode propagating the core in the opposite direction (negative direction). The characteristic is obtained. A major difference in the configuration between the radiation mode coupling type grating and the reflection type grating is a difference in a period of a periodic change (hereinafter, sometimes referred to as a grating pitch). For example, in the case of an optical fiber grating in which a periodic change in the refractive index is formed in a core, a radiation type grating is obtained by setting the grating pitch to several hundred μm, and a reflection type grating is obtained by setting the grating pitch to about 1 μm. Have been obtained. In particular, when the grating pitch is made constant at equal intervals in a reflection type grating, a transmission spectrum as schematically shown in FIG.
It is known that a relatively sharp narrow band transmission loss peak (hereinafter referred to as a loss peak) is obtained. The width of the wavelength band of the loss peak is called a stop band width, the center wavelength is called a center wavelength (indicated by λc in the figure), and the magnitude of the transmission loss is called a stop ratio.

[0005] Therefore, a reflection type grating having a narrow band loss peak is used, and two loss peaks are obtained in its transmission spectrum.
Wavelength blocking optical fiber gratings have been proposed. FIG. 5 is a partial cross-sectional view schematically showing an example of a two-wavelength blocking optical fiber grating in which a reflection grating is formed on a normal optical fiber. FIG. 6 is a transmission diagram obtained in the two-wavelength blocking optical fiber grating. It is the figure which showed the spectrum typically. In the figure, reference numeral 1 denotes an optical fiber, 2 denotes a core, 3 denotes a clad, and 4 denotes a coating layer. In the optical fiber 1, the core 2 has a higher refractive index than that of the clad 3, and is made of quartz glass doped with at least germanium. The clad 3 is made of quartz glass having a lower refractive index than that of the core. For example, pure quartz glass or fluorine-doped quartz glass is preferably used. This two-wavelength blocking type optical fiber grating has two types of grating portions 5A, in which the refractive index of the core 2 is periodically changed in the length direction of the optical fiber 1, in a part of the optical fiber 1 from which the coating layer 4 is peeled off. 5B is formed. Since the grating portions 5A and 5B have different grating characteristics, a transmission spectrum having two loss peaks can be obtained as shown in FIG. In FIG. 6, λA and λB indicate the center wavelengths of the two loss peaks, respectively. These [lambda] A and [lambda] B depend on the grating pitch, the amount of change in the refractive index, and the like, and the wavelength distance a between [lambda] A and [lambda] B can be adjusted appropriately.
Can be separated by, for example, about 1 nm.

However, in such a conventional two-wavelength blocking type optical fiber grating, two types of grating portions 5A and 5B are formed along the length of the optical fiber 1, so that the accommodation space becomes large, and the element space of the device is increased. This was an obstacle to miniaturization. Further, since one of the grating portions 5A and 5B is generally formed after the other is formed, the manufacturing time is long and the manufacturing efficiency is low. In order to manufacture the above-mentioned two-wavelength blocking type optical fiber grating, first, since the coating layer 4 is peeled off, the tensile strength of the optical fiber 1 increases as the length of the grating portions 5A and 5B becomes longer. In some cases, it was easily reduced.

Since the center wavelengths λA and λB change in response to the strain or temperature change of the optical fiber 1, the center wavelengths λA and λB can be used as strain sensors or temperature sensors by monitoring these λA and λB. That is, when the effective refractive index of the optical fiber 1 changes due to a temperature change, λA,
Since λB changes, a temperature change can be detected. At this time, the inter-wavelength distance a indicating the relative positional relationship between λA and λB changes with the fluctuations of λA and λB. When stress is applied to one or both of the grating portions 5A and 5B, λA and λB depending on the grating pitch change as the grating pitch changes according to the respective strains. Also at this time, since the distortion in each of the grating portions 5A and 5B varies, the inter-wavelength distance a changes.
Therefore, in the case of the two-wavelength blocking optical fiber grating having such a configuration, it was not possible to determine whether the center wavelengths λA and λB changed due to a temperature change or a distortion.

[0008]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a two-wavelength blocking type optical fiber grating that can be downsized. Also,
An object of the present invention is to provide a two-wavelength blocking optical fiber grating that can determine whether the center wavelength of a loss wavelength is affected by temperature or strain when used as a temperature sensor or a strain sensor.

[0009]

In order to solve the above problems, the present invention is characterized in that a grating portion is formed on a polarization maintaining fiber.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The polarization-maintaining fiber has a different refractive index in the horizontal and vertical directions in a cross section perpendicular to the length direction of the optical fiber, that is, the polarization direction of the propagating light ((X polarization (lateral polarization)). The effective refractive index differs depending on the Y-polarization (also referred to as longitudinal polarization), which is referred to as birefringence.Polarization-preserving fibers having various structures have been proposed. However, a PA having two stress applying portions symmetrically with respect to the core in a clad outside the core is described.
NDA fibers are widely used because of their large birefringence and excellent polarization preserving characteristics.

FIG. 1 shows an example of a PANDA fiber. FIG. 1 (a) is a cross-sectional view in a direction perpendicular to the length direction of the PANDA fiber, and FIG. 1 (b) is XX 'in the figure.
The refractive index distribution in the cross section indicated by the line, FIG.
The refractive index distribution at the cross section indicated by the Y 'line is shown. The PANDA fiber 10 of this example includes a core 11 made of quartz glass to which germanium is added, a clad 12 made of pure quartz glass, and a stress applying portion 13 made of quartz glass to which boron is added in a relatively large amount. Germanium is usually germanium oxide (Ge
O ₂ ) can increase the refractive index of quartz glass. Boron is usually added as boron oxide (B ₂ O ₃ ) and can reduce the refractive index of quartz glass.

The refractive index distribution in the section indicated by the line XX ′ in FIG. 1B is composed of a core 11 and a clad 12 having a lower refractive index than the core 11. FIG.
The refractive index distribution in the cross section indicated by the line YY ′ in the figure shown in FIG.
A cladding 12 having a lower refractive index than the cladding 12, a stress applying portion 13 adjacent to the outer periphery of the cladding 12 and having a lower refractive index than the cladding 12, and a cladding 12 located outside the stress applying portion 13. Have been. As described above, since the respective refractive index distributions in the cross section indicated by the line XX ′ or the line YY ′ in the drawing are different, the effective refractive index of the X polarized wave and the effective refractive index of the Y polarized wave are different. The coupling between the X-polarized light and the Y-polarized light of the light propagating as a result is suppressed, and the optical fiber has characteristics as a polarization-maintaining fiber that retains the respective polarization planes.

On the other hand, it is known that the center wavelength of the loss peak of the optical fiber grating depends on the effective refractive index. Therefore, as described above, the refractive index of the core 11 is periodically changed along the length direction of the PANDA fiber 10 at one location of the PANDA fiber 10 where the effective refractive index of the X polarization and the effective refractive index of the Y polarization are different. When a change is formed and one type of grating portion is formed, the X-polarized light and the Y-polarized light of light transmitted through the grating portion show different grating characteristics, respectively, for example, as shown in FIG. Then, a transmission spectrum having two loss peaks is obtained, and can be used as a two-wavelength blocking optical fiber grating. This will be described below with reference to FIG.

The PANDA fiber 10 is manufactured according to a conventional method. In addition, PANDA fiber 10
The method of forming the grating portion on the core 11 is the same as that of the conventional method. When a strong ultraviolet light is applied to quartz glass to which germanium is added, the refractive index increases in accordance with the amount of irradiation. The refractive index change is formed. That is, since the core 11 of the PANDA fiber 10 is made of silica glass to which germanium is added, for example, after hydrogenation treatment is performed in a hydrogen pressurized container (about 100 atm), the PANDA fiber 10 is
Method of irradiating ultraviolet light at a constant cycle in the length direction of
The grating portion can be formed by using a method of sequentially irradiating ultraviolet light at equal intervals in the length direction of the NDA fiber 10 or the like. At this time, the wavelength of the ultraviolet light is 200 to 300.
nm, and a KrF laser (wavelength: 248 nm) or the like is suitably used as a light source.

By the way, in the transmission spectrum obtained in the two-wavelength blocking optical fiber grating of the present invention, the center wavelengths λA and λB of the two loss peaks, respectively.
Varies depending on the amount of change in the refractive index of the grating portion, the grating pitch, or the effective refractive indexes of the X-polarized light and the Y-polarized light. Further, the inter-wavelength distance a depends on the difference between the effective refractive index of X-polarized light and the effective refractive index of Y-polarized light. In designing a two-wavelength blocking optical fiber grating,
These are adjusted so that the desired center wavelengths λA and λB and the inter-wavelength distance a can be obtained, particularly so that the two loss peaks are separated. The amount of change in the refractive index can be adjusted by the energy density of the ultraviolet light and the irradiation time. In addition, 2 of this invention
A radiation grating can be applied to the wavelength-blocking optical fiber grating. However, in order to sufficiently separate the two loss peaks, it is preferable to use a reflection grating capable of obtaining a loss peak in a narrow band.
In this case, the grating pitch is constant at regular intervals, and 0.3
About 0.6 μm. The difference in the effective refractive index between the X-polarized light and the Y-polarized light of the PANDA fiber 10 is generally 0.0001 to 0.0015.
In the wavelength blocking type optical fiber grating, the distance a between wavelengths can be adjusted in the range of 0.05 to 2 nm by arbitrarily selecting from the above range.

The two-wavelength blocking type optical fiber grating of the present invention can be used for the following applications. When used as a two-wavelength blocking optical filter capable of blocking two-wavelength light, an optical system as shown in FIG. 2 is used. Using a linearly polarized light source having a rotating part as the light source, and controlling the intensity for each polarization, X
Polarized light and Y-polarized light can be supplied. PANDA fiber 1 having grating section 15 formed on this light source
0, and for each of the X, Y polarizations of similar intensity,
Light is received by the light receiver so that loss peaks of different center wavelengths are obtained.

Further, the two-wavelength blocking type optical fiber grating of the present invention is connected to an ordinary light source which does not control the intensity for each polarization, and the intensity of X polarization and Y polarization of this light source is determined by the loss peak in the transmission spectrum. Can also be measured by comparing

Further, when the two-wavelength blocking type optical fiber grating of the present invention is used as a temperature sensor and a strain sensor, the influence of temperature or strain can be determined. That is, the two-wavelength blocking type optical fiber grating of the present invention has a characteristic that the effective refractive index differs between X-polarized light and Y-polarized light, and the difference between these effective refractive indices approaches as the temperature rises. . Therefore, when the temperature rises, the positions of the center wavelengths λA and λB change, and the distance a between the wavelengths decreases. Further, since the grating portion is formed at one position of the PANDA fiber, the grating pitch that changes according to the strain is constant regardless of the polarization. Therefore, when distortion is applied, the positions of the center wavelengths λA and λB change, but the distance a between wavelengths does not change. Therefore, the center wavelength λ
By monitoring A and λB and the wavelength distance a, it is possible to determine whether the change is due to a temperature change or a distortion.

Although the PANDA type is described here as an example of the polarization maintaining fiber, the two-wavelength blocking type optical fiber grating of the present invention is not limited to the PANDA type, but is symmetrically arranged with respect to the core. A structure having a stress applying portion, that is, a so-called bow-tie type or an elliptical jacket type is also applicable.

As described above, according to the present invention, a two-wavelength blocking type optical fiber grating can be obtained by forming one kind of grating portion at one position of the polarization maintaining fiber, so that the size can be reduced. Thus, the manufacturing efficiency can be improved. Further, when this two-wavelength blocking type optical fiber grating is used as a temperature sensor or a strain sensor, the relationship between the center wavelengths λA and λB and the change in the distance a between wavelengths is different depending on the temperature change and the strain change. By doing so, it is possible to determine whether the change is due to a temperature change or a distortion.

[0021]

EXAMPLE A two-wavelength blocking optical fiber grating was manufactured using a PANDA fiber having the structure shown in FIG. The difference between the effective refractive indices of the X and Y polarizations of this PANDA fiber was 4 × 10 ⁻⁴ . A KrF laser was used as a light source, and ultraviolet light of 248 nm was irradiated from above the mask. The grating pitch was 0.533 μm, and the grating length was 10 mm. Irradiation was performed for 3 minutes with the energy density of the laser beam set to 1 mJ / mm ² . FIG. 3 shows the measurement results of the transmission spectrum of the two-wavelength blocking optical fiber grating. The center wavelength and rejection of the two loss peaks are 1551.1 nm and 2.
They were 1 dB, 1551.5 nm and 2.0 dB. The center wavelengths of the two loss peaks were separated by 0.4 nm. As described above, by forming the grating portion in the PANDA fiber (polarization preserving fiber), a transmission spectrum having two well-separated loss peaks can be obtained, and the transmission spectrum can be used as a two-wavelength blocking type optical fiber grating. It could be confirmed.

[0022]

As described above, according to the present invention, a two-wavelength blocking type optical fiber grating can be obtained by forming one kind of grating portion at one position of the polarization-maintaining fiber, so that the size can be reduced. Thus, the manufacturing efficiency can be improved. Further, when this two-wavelength blocking type optical fiber grating is used as a temperature sensor or a strain sensor, the relationship between the center wavelength and the change in the distance between the wavelengths at two loss peaks differs depending on whether the temperature changes or the strain. Therefore, by monitoring these, it can be determined whether the change is due to a temperature change or a distortion.

[Brief description of the drawings]

FIG. 1 shows a PAND as an example of a polarization-maintaining fiber used in a two-wavelength blocking optical fiber grating of the present invention.
FIG. 1A shows a structure of an A fiber, and FIG.
Sectional view in the direction perpendicular to the length direction of the NDA fiber,
FIG. 1B is a refractive index distribution diagram at a cross section indicated by line XX ′ in the drawing, and FIG. 1C is a refractive index distribution diagram at a cross section indicated by line YY ′ in the drawing.

FIG. 2 is an explanatory diagram showing an example of an optical system necessary when the two-wavelength blocking optical fiber grating of the present invention is used as a two-wavelength blocking optical filter.

FIG. 3 is a graph showing a transmission spectrum of a two-wavelength blocking optical fiber grating obtained in an example.

FIG. 4 is a graph schematically showing a transmission spectrum having a narrow-band loss peak obtained in a reflection grating.

FIG. 5 is a partial cross-sectional view schematically showing an example of a two-wavelength blocking optical fiber grating in which grating portions are formed at two places of a normal optical fiber.

FIG. 6 is a graph schematically showing a transmission spectrum obtained in a two-wavelength blocking optical fiber grating.

[Explanation of symbols]

10 PANDA fiber (polarization preserving fiber), 1
DESCRIPTION OF SYMBOLS 1 ... Core, 12 ... Clad, 13 ... Stress application part, 15 ...
Grating part

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Ryozo Yamauchi 1440 Mutsuzaki, Sakura-shi, Chiba Pref.

Claims (1)

[Claims] 1. A two-wavelength blocking type optical fiber grating, wherein a grating portion is formed in a polarization maintaining fiber.