KR100334801B1 - Long period optical fiber grating filter device - Google Patents

Abstract

The present invention relates to a long period optical fiber grating filter device, the disclosed long period optical fiber grating filter device includes a core formed with a long distance of the long period optical fiber grating; A cladding surrounding the core; A coating part surrounding a cladding of a portion where the long period optical fiber gratings are not formed; And a coating part surrounding a cladding of a portion where the long period optical fiber gratings are formed, the long period optical fiber grating filter device comprising: a core having a negative wavelength shift of a coupling wavelength due to temperature increase according to an amount of a dopant added to the core; / Cladding refractive index change portion; And a cladding / recoating portion refractive index change portion in which the refractive index decreases with increasing temperature so that the coupled wavelength has a positive wavelength shift.

Therefore, according to the present invention, the core exhibits a negative wavelength shift as much as the coupling wavelength shift due to the recording material exhibiting a positive wavelength shift due to the decrease in refractive index with increasing temperature. .

Description

Long Cycle Fiber Optic Grating Filter System {LONG PERIOD OPTICAL FIBER GRATING FILTER DEVICE}

The present invention relates to a long period optical fiber grating filter device, and more particularly, to a temperature compensation long period optical fiber grating filter device having no coupling movement characteristic according to temperature change.

In general, a filter for selecting a specific wavelength that proceeds to the core of an optical fiber, using a laser in the ultraviolet region to induce a periodic change in the refractive index in the optical fiber to Fiber optic gratings are used that can remove or reflect light. Examples of such optical fiber gratings include short period fiber gratings and long period fiber gratings.

A short period fiber grating performs a filtering function by reflecting only a specific wavelength, while a long period fiber grating is a device that couples a core mode that proceeds to the core of an optical fiber to a cladding mode. . Among them, long-period fiber gratings having a distance period of several tens to hundreds of micrometers can be used to remove the light of a specific wavelength desired by coupling the propagating core mode light to the cladding mode in the advancing direction. It is used as a filter for gain flattening of Doped Fiber Amplifier.

These long-period fiber gratings are fabricated by varying the refractive index at periodic distances to the core of the UV-sensitive fiber. At this time, since the refractive index is increased in the portion exposed to ultraviolet rays, and the other portion is not changed in the refractive index, periodic refractive index changes occur in the longitudinal direction of the optical fiber.

On the other hand, long-period fiber gratings have a temperature-sensitive characteristic change, and optical properties are also affected by the refractive index outside the optical fiber cladding. In addition, the center wavelength and extinction ratio of long-period optical fiber gratings determined by mode coupling of core and cladding modes are greatly influenced by micro bending of the optical fiber and the like.

In order to use long period optical fiber gratings, it is necessary to record a stable optical property even under the influence of the external environment. External environmental factors include temperature, moisture, dust penetration, micro cracks, and micro bending of optical fibers.

In such a long period optical fiber grating filter device, the coupling occurs when the phase matching condition as shown in Equation 1 is satisfied.

In Equation 1, Is the propagation constant of core mode, Is the propagation constant of the m-th cladding mode, Is the grating period.

In Equation 1 (here, Silver refractive index, (Wavelength) is expressed as in Equation 2.

To change the light of a certain wavelength into cladding mode, the lattice period (Λ) and the refractive index difference This is done.

The refractive index difference can be obtained by appropriately exposing an ultraviolet laser to an ultraviolet-sensitive optical fiber. In other words, when masking an optical fiber sensitive to ultraviolet rays with a mask having a specific lattice period Λ, and the ultraviolet laser is scanned on the mask, the photosensitive optical fiber reacts to increase the refractive index of the core. As the refractive index increases, the coupling wavelength increases toward the longer wavelength. In order to obtain the spectrum of the desired long-period fiber grating filter, that is, the desired coupling wavelength and extinction ratio, it is necessary to precisely control the mask period and irradiate the ultraviolet laser for a proper time.

The coupling wavelengths of the long-period fiber gratings thus produced are also affected by temperature. The shift of the coupling wavelength with temperature changes is determined by the change in refractive index with the temperature change and the thermal expansion of the length with the temperature change. This is represented by Equation 3 as follows.

In Equation 3, T represents a temperature.

When a long-period grating filter device is fabricated in a general communication optical fiber or a distribution shifted fiber, the value of the first term on the right side of Equation 3 is about ten times larger than the value of the second term on the right side. I never do that. For example, in the case of Corning's Flexcor 1060, the coupling wavelength shifts by about 5 nm per 100 ° C. In the case of a typical dispersion transition optical fiber, the coupling wavelength shift due to length expansion is about 0.3 nm per 100 ° C., whereas the coupling wavelength shift is about 5 nm due to the change in refractive index. However, the gain flattening filter, which is one of the applications of the long-period fiber grating filter in a practical application system, requires a temperature stability of about 0.3 nm per 100 ° C.

In order to compensate for such temperature, conventionally, the refractive index distribution in the optical fiber is designed or The period of the fiber grating was chosen so that the part had a negative value. Alternatively, the equation (3) There is a way to add B ₂ O ₃ to make.

ConventionalIn the method of adjusting the refractive index in the filter so that the part has a negative value, in the case of Λ <100 μm in general long period optical fiber grating filtersIs known as yin. In the case of Corning's Flexcor 1060 fiber, the temperature dependence of the wavelength for Λ = 40㎛ is 0.15 ~ 0.45nm / 100 ℃, Mode It is out of the communication area by 1.1 mu m.

On the other hand, the temperature compensation long period optical fiber grating filter device is described in detail in Patent Application No. 99-8332 (temperature compensation long period optical fiber grating filter) filed by the applicant.

However, the coating part of the long period optical fiber grating filter disclosed in the patent application No. 99-8332 has been described as applying a material in which the refractive index increases as the temperature is increased, but in general, the coating part, in particular the polymer material As it increases, thermal expansion occurs and the refractive index decreases. Therefore, when the long-period grating filter made of ordinary fiber is coated, the long-wave moving effect of the long-period fiber grating filter is added to the long-wave moving effect by the recording, which results in a larger long-wave moving property. There is a difficulty in using a special reducing coating material. Moreover, such a coating material has not been developed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a temperature compensation long period optical fiber grating filter device having no coupling movement characteristic according to temperature change.

It is another object of the present invention to provide a temperature compensation long period optical fiber grating filter device which can use a polymer material having an initial refractive index value smaller than cladding as a recording material.

It is another object of the present invention to provide a temperature compensation long period optical fiber grating filter device that does not penetrate moisture well and is very soft so that no micro bending occurs during curing.

1A is a perspective view illustrating a packaged long period optical fiber grating filter device;

Figure 1b is a perspective view showing a long period optical fiber grating filter device peeled off the coating.

Fig. 1C is a cross-sectional view showing the structure of a long period optical fiber grating filter device with a peeling coating portion.

2A to 2D are graphs showing coupling wavelength shift according to cladding external refractive index.

3 is a graph showing coupling wavelength shift according to cladding external refractive index change.

4 is a graph illustrating coupling wavelength shift according to a change in the cladding external refractive index when the cladding external refractive index is smaller than the refractive index of the cladding.

Figure 5a is a graph showing the change in refractive index of the recording material according to the temperature change, when the coating is a general polymer material.

5B is a graph showing a change in refractive index of a recording material according to temperature change when the coating part is a silicone resin.

Figure 6 is a graph showing the coupling wavelength shift of the recording material with temperature changes.

7 is a graph showing a change in refractive index with temperature change according to the dopant concentration added to the optical fiber core.

8 is a graph showing the temperature dependence of the wavelength according to the dopant concentration added to the optical fiber core.

9 is a graph showing the temperature compensation effect of the long period optical fiber grating filter device according to the present invention.

Figure 10a is a graph showing the temperature dependence of the long period optical fiber grating filter device, when the coating portion of the general long period optical fiber grating is peeled off.

10b is a graph showing the temperature dependence of a long period optical fiber grating filter device when a general long period optical fiber grating is coated.

11 is a graph showing the temperature dependence of the long period optical fiber grating filter device according to the present invention.

12 is a cross-sectional view showing the configuration of a long period optical fiber grating filter device according to the present invention.

In order to achieve the above object, the present invention is a core having a long period optical fiber grating formed with a periodic distance;

A cladding surrounding the core;

A coating part surrounding a cladding of a portion where the long period optical fiber gratings are not formed; And

In the long-period optical fiber grating filter device comprising a coating portion surrounding the cladding of the portion where the long-period optical fiber gratings are formed,

A core / cladding refractive index change portion in which a coupling wavelength due to temperature increase has a negative wavelength shift depending on the amount of dopant added to the core; And

The refractive index decreases with increasing temperature so that the coupled wavelength consists of the cladding / recoating portion refractive index change portion having a positive wavelength shift.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention; In describing the present invention, detailed descriptions of related well-known functions or configurations are omitted in order not to obscure the subject matter of the present invention.

In general, in order to form long period optical fiber gratings on an optical fiber, the optical fiber coating is removed to a predetermined length. Thereafter, long-period fiber gratings are fabricated using a UV laser and an amplitude mask on the stripped portion of the coating. The peeled state of the long-period fiber gratings thus produced is affected by the external environment (temperature, moisture, dust penetration, micro cracks, micro bends, etc.), so a separate protection device is required to keep the optical properties from changing. Lose.

Furthermore, since the long-period optical fiber gratings, which are formed in the longitudinal direction of the optical fiber in a predetermined period, perform a filtering function that couples from the core mode to the cladding mode, the refractive index of the material to be coated is increased. Should be considered.

As shown in FIGS. 1A to 1C, the packaged long period optical fiber grating filter device 100 includes a core 10 formed at a predetermined distance period in which the long period optical fiber gratings 16 are formed, and the long period grating filters 16 are formed. A cladding 12 surrounding the formed core 10, and a coating part 14 covering the cladding 12 and not peeled off, and a coating part 18 formed on the long period optical fiber gratings 16. In other words, the peeled coating part is coated to protect the packaged long period optical fiber gratings.

At this time, as shown in Figure 1c, the arrow indicating the direction of the wavelength shows the operation coupled from the core mode to the cladding mode in the long-period grating filter device. The thickness of an arrow means the light intensity of an advancing wavelength.

The center wavelength traveling in the basic waveguide mode of the core 10 is scattered while encountering the refractive index change portion, that is, the long period optical fiber grating filters 16. As the scattered light is coupled to the cladding 12, light of a wavelength satisfying the phase matching condition is coherently reinforced. As light having such a wavelength exits the cladding, the long period optical fiber grating filter device operates as a wavelength dependent attenuator.

The light traveling in the fundamental guided mode (fundamental guide mode) decreases in intensity (thinner as the thickness of the arrow proceeds in the longitudinal direction) passing through the change part of the refractive index, that is, the grating part (long period optical fiber grating filter), It can be seen that the intensity of the light having a wavelength coupled to the cladding 12 increases gradually (the thickness of the arrow becomes thicker).

As shown in FIG. 1C, the external conditions of the cladding 12 in which these long-period fiber grating filters 16 are formed, namely air, have a value of refractive index 1. After the formation of the long-period fiber gratings, when the cladding is recoated with a material having a refractive index of n, the coupling conditions are changed and thus the coupling wavelength is shifted to a long wavelength or a short wavelength.

2A to 2D are graphs showing coupling wavelength shifts according to refractive indices outside the cladding.

FIG. 2A is a graph illustrating light transmission characteristics when the refractive index outside the cladding surrounding the long period optical fiber gratings is 1 (air).

FIG. 2B is a graph illustrating the transmission characteristics when the cladding external refractive index surrounding the long period optical fiber gratings is 1.400. FIG. 2B shows that the light transmission is increased compared to FIG. 2A, and the coupling wavelength is shifted by 4.8 nm toward the short wavelength. It can be seen that.

2C is a graph showing transmission characteristics when the cladding external refractive index is 1.448. Referring to FIG. 2A, it can be seen that the coupling wavelength has moved toward the short wavelength of about 16.5 nm.

2D is a graph illustrating the transmission characteristics when the cladding external refractive index is 1.484, and it can be seen that the coupling wavelength is shifted toward the longer wavelength than when the cladding external refractive index is 1.

When the cladding external refractive index is 1, the cladding external refractive index increases as shown in FIG. 2B or 2C, but when the cladding external refractive index is smaller than the refractive index of the cladding, the coupling wavelength moves to a short wavelength. On the other hand, when the external refractive index of the cladding increases and becomes larger than the refractive index of the cladding, it can be seen that the coupling wavelength moves to a long wavelength as shown in FIG. 2D. In addition, when the cladding external refractive index is equal to the cladding refractive index, the total reflection condition is broken, and the coupling peak disappears.

3 shows the shift of the coupling wavelength with respect to the cladding external refractive index change. As the external refractive index of the cladding increases from 1.0, the coupling wavelength shifts to a shorter wavelength, and when the same as the refractive index of the cladding, the coupling peak disappears.

4 is a graph showing coupling wavelength shift with respect to the cladding external refractive index change when the cladding external refractive index is smaller than the refractive index of the cladding. Referring to FIG. 4, the coupling wavelength shifts toward the longer wavelength as the cladding external refractive index decreases. However, the cladding external refractive index is shown only when the refractive index of the cladding is small.

The results shown in FIGS. 2A to 4 show the results of the present inventor's paper (1997 Optics Letters: December 1, 1997 / Vol. 22, No. 23, 'Displacement of the resonant peaks of a long-period fiber grating induced by a change of ambient' refractive index ').

Figure 5a is a graph showing the refractive index change characteristics of a typical recording material with temperature changes. 5B is a graph showing a change in refractive index of a silicone resin with respect to a temperature change, taking a silicone resin as an example of a general coating material.

Referring to FIG. 5A, a general recording material, that is, a polymer, undergoes thermal expansion as the temperature increases, thereby decreasing the refractive index. Referring to FIG. 5B, it can be seen that the silicone resin undergoes thermal expansion as the temperature increases, thereby decreasing the refractive index. The silicone resin has a change in refractive index with respect to temperature of -2.4 x 10 ^-2 / 100 deg.

6 is a graph illustrating coupling wavelength shift characteristics of a recording material according to temperature change. It can be seen that as the temperature increases, the coupling wavelength due to the decrease in the refractive index of the recording material moves toward the longer wavelength. Here, the shifting of the coupling wavelength toward the long wavelength means that the coupling wavelength has a positive wavelength shift range.

7 is a graph showing the wavelength shift with respect to temperature change according to the dopant concentration added to the core of the optical fiber. At this time, a method for obtaining a temperature compensation effect by adding B ₂ O ₃ and GeO ₂ to the core is disclosed in detail in EP 0 800 098 A2 (OPTICAL WAVEGUIDE GRATING AND PRODUCTION METHOD THEREOF). As shown in FIG. 7, when B ₂ O ₃ is added to the core relatively more than GeO ₂ , the long period optical fiber gratings have a negative wavelength shift range as the temperature increases. In other words, the change of the refractive index according to the temperature change has a negative value. In the present invention, the long period optical fiber gratings have a negative wavelength shift range, and the effect of the coating material has a positive wavelength shift range, thereby achieving temperature compensation.

For example, assuming that 20 mol% of GeO ₂ is added to the core and 15 mol% of B ₂ O ₃ is added to the core, the long-cycle optical fiber grating filter device formed in the core has a negative refractive index change with temperature change. It will have a value and have a negative wavelength range. A graph showing this result is shown in FIG. 8.

8 is a graph showing that the coupling wavelength shifts toward the shorter wavelength with respect to the temperature increase by adding more B ₂ O ₃ to the core than GeO ₂ . Here, the long-period fiber gratings are unrecorded. As shown in FIG. 8, it can be seen that as the temperature increases, the coupling wavelength moves toward the shorter wavelength. Here, the shift of the coupling wavelength toward the short wavelength means that it has a negative wavelength shift range.

9 shows a long wavelength shift effect by a coating material such as a silicone resin as the temperature increases in a long period optical fiber grating filter device and a short wavelength shift effect by adding B ₂ O ₃ to the core relatively more than GeO _2. This graph shows the temperature compensation effect. 9 is a line indicating the long wavelength shift of the coupling wavelength due to the refractive index change portion of the cladding / recoating portion according to the temperature change, and ③ is a coupling by the core / cladding refractive index change portion according to the temperature change. This line represents the short-wavelength shift of the wavelength.

Accordingly, it can be seen that the long-wavelength shift and the short-wavelength shift of the coupling wavelength occur simultaneously in the long period optical fiber grating filter apparatus according to the present invention to perform temperature compensation.

10A and 10B are graphs showing wavelength shift characteristics according to temperature change when a core is not coated in a general long period optical fiber grating filter device in which a short wavelength shift effect does not occur and when a core is coated with a silicone resin. .

8 and 11 according to the present invention is a case in which the long period optical fiber grating filter device is not recorded in the state of showing a negative wavelength shift range by adding more B ₂ O ₃ to the optical fiber core than GeO ₂ (Fig. 8) and a graph showing the wavelength shift characteristic according to the temperature change generated in the case of coating with silicone resin (FIG. 11).

Hereinafter, the temperature compensation effect according to the present invention will be described by comparing the general FIGS. 10A and 10B and FIGS. 8 and 11 according to the present invention.

As shown in FIG. 10A, when the conventional long period optical fiber grating filter device is not coated, it can be seen that the wavelength moves toward the longer wavelength as the temperature increases, and the temperature dependency at this time is about 5.08 nm / 100 ° C. .

On the other hand, as shown in Figure 10b, in the case of the coating on the conventional long-period optical fiber grating filter device with a silicone resin it can be seen that the wavelength is shifted toward the longer wavelength as the temperature increases, the temperature dependence at this time is about 10nm / It is 100 degreeC or more.

When the long-term optical fiber gratings are coated with a silicone resin, the long-wave shift effect by the core of the optical fiber and the long-wave shift effect by the silicone resin are added, resulting in a larger long-wave shift effect. That is, it can be seen that the temperature dependency is further increased.

In FIG. 8, when the recording is not performed in a state in which B ₂ O ₃ is added to the optical fiber core in a state where Ge ₂ is relatively higher than that of GeO ₂ , as the temperature increases, the wavelength shifts toward the shorter wavelength. The dependency is about -4.7 nm / 100 ° C.

On the other hand, in Figure 11 is coated with a silicone resin in the state in which B ₂ O ₃ is added to the optical fiber core with a dopant as a relatively larger than GeO ₂ according to the present invention, the short wavelength shift effect by the core and the long wavelength shift by the coating material It can be seen that the effect occurs at the same time, so that the temperature compensation is performed and there is almost no wavelength change as the temperature increases. The temperature dependence at this time is 0.7 nm / 100 degreeC.

The long period optical fiber grating filter device manufactured as described above is illustrated in FIG. 12. Reference numeral 120 denotes a core in which appropriate B ₂ O ₃ is added relatively more than GeO ₂ , reference numeral 122 denotes a cladding surrounding the core, and 126 denotes a long period optical fiber grating formed in the core in the longitudinal direction. do. In addition, reference numeral 128 denotes a recording part in which the long period optical fiber gratings are packaged, and the recording part is a silicone resin.

As a result, when B ₂ O ₃ is added to the optical fiber core relatively more than GeO ₂ so that the coupling wavelength shift due to the temperature increase has a negative wavelength shift range, the refractive index decreases with the temperature increase and the coupling wavelength shift is increased. By combining the positive wavelength shift range with the recording unit, a temperature compensation effect with little coupling wavelength shift can be obtained.

As described above, in the present invention, the coupling wavelength shift due to the increase of the dopant is added to the core having a negative wavelength shift range and the refractive index decreases according to the temperature increase, so that the coupling wavelength shift is positive. It is composed of a recording unit having a wavelength shift range can compensate for the coupling wavelength shift of the long period optical fiber gratings with increasing temperature. Therefore, temperature compensation of the long period optical fiber grating filter device can be performed more easily.

Claims (5)

A core having long period optical fiber gratings formed at periodic distances; A cladding surrounding the core; A coating part surrounding a cladding of a portion where the long period optical fiber gratings are not formed; And In the long-period optical fiber grating filter device comprising a coating portion surrounding the cladding of the portion where the long-period optical fiber gratings are formed, A core / cladding refractive index change portion in which a coupling wavelength due to temperature increase has a negative wavelength shift depending on the amount of dopant added to the core; And A long-period optical fiber grating filter device having a temperature compensation by changing a refractive index of a cladding / recoating part having a positive wavelength shift because a refractive index decreases with increasing temperature. The method of claim 1, The dopant comprises GeO ₂ and B ₂ O ₃ , coupling due to the shift of the coupling wavelength due to the refractive index increased with the amount of GeO ₂ added, and the coupling due to the refractive index decreased with the amount of B ₂ O ₃ added A long period optical fiber grating filter device having a sum of wavelength shifts having a negative wavelength shift. The method of claim 1, The recording unit is a long period optical fiber grating filter device of a polymer material that the refractive index decreases with increasing temperature. The method of claim 3, The polymer material is a silicon resin long cycle optical fiber grating filter device. The method of claim 1, And a long period optical fiber grating filter device having a refractive index smaller than that of the cladding.