KR100318902B1 - Long period fiber grating - Google Patents

Abstract

The long period optical fiber grating according to the present invention has a different refractive index modulation value, the distribution of the refractive index modulation value is asymmetric about an arbitrary reference section, and is composed of a plurality of sections each having a length of at least two times the grating period. .

Description

Long Cycle Fiber Optic Grating {LONG PERIOD FIBER GRATING}

FIELD OF THE INVENTION The present invention relates to optical fiber gratings, and more particularly to long period gratings.

A typical method of forming an optical fiber grating is to change the refractive index of the optical fiber by irradiating ultraviolet rays to the photosensitive optical fiber.

The photosensitive optical fiber maintains the refractive index of the optical fiber permanently or for several decades when ultraviolet light is incident. Initially, this phenomenon was thought to be limited to germanium-doped optical fibers, but it is now found in optical fibers of various materials that do not contain germanium.

1 is a view showing a long period optical fiber grating having a conventional uniform refractive index modulation value. The long period optical fiber grating 112 shown has a uniform refractive index modulation over the entire grating length L ₁₁ . The long period optical fiber grating 112 has a shadow pattern formed on the optical fiber 111 by an amplitude mask (not shown). The long period optical fiber grating 112 formed in the optical fiber 111 is used for the purpose of attenuating the optical signal by converting the optical signal traveling in the core mode to the clad mode.

FIG. 2 is a diagram for describing a method of manufacturing the long period optical fiber grating 112 shown in FIG. 1. The manufacturing method of the long period optical fiber grating 112 is similar to the photolithography process commonly seen in the semiconductor manufacturing process. That is, ultraviolet rays are irradiated to an amplitude mask 113 composed of a plurality of slits 114 to pattern the shape of the amplitude mask 113 on the optical fiber 111. In this case, ultraviolet rays passing through the slit 114 of the amplitude mask 113 are irradiated onto the optical fiber 111 to change the refractive index of the optical fiber 111. In this process, a long period optical fiber grating 112 having a predetermined period Λ ₁₁ and a length L ₁₁ is formed in the optical fiber 111. In addition, in FIG. 2, a lens system (not shown) including a plurality of lenses is disposed on the amplitude mask 113 to change the length L ₁₁ and the period Λ _{11 of} the long period optical fiber grating 112. There is a number. That is, the shape of the amplitude mask 15 can be enlarged or reduced to pattern the optical fiber 111.

3 is a perspective view of the amplitude mask 113 shown in FIG. 2. As described above, the amplitude mask 113 has a form similar to the long period optical fiber grating 112 shown in FIG. The amplitude mask 113 has a constant period Λ ₁₂ and is composed of a plurality of slits 114 arranged in a line.

In general, the above-mentioned long period optical fiber grating is used for the purpose of flattening the gain of an erbium doped optical fiber amplifier by using the optical signal attenuation characteristic.

4 is a view showing an erbium-doped optical fiber amplifier having a conventional attenuator. The illustrated erbium-doped fiber amplifier 142 amplifies the optical signal traveling into the erbium-doped fiber 146 using the density inversion of the erbium ions. In Fig. 4, the optical fiber 141, which is an optical signal transmission medium, isolators 143 and 149 for blocking backing light, an erbium-doped fiber 146 for amplifying the optical signal, and the erbium-doped fiber Erbium-doped optical fiber amplifiers comprising pumping light sources 144 and 148 for outputting pumping light for exciting erbium ions in 146, and optical couplers 145 and 147 for coupling the pumping light to the optical fiber 141. 142 is shown. A rear end of the erbium-doped fiber amplifier 142 is usually provided with an attenuator for flattening the gain curve according to the wavelength of the erbium-doped fiber amplifier 142. As shown, after the nine-channel optical signal is input to the erbium-doped fiber amplifier 142 and amplified, the power of each channel of the optical signal output from the erbium-doped fiber amplifier 142 is not constant. It is showing. This is because the gain value for each wavelength of the erbium-doped fiber amplifier 142 does not have a single value. Currently, optical communication adopts a wavelength division multiplexing system to transmit or receive a plurality of channels through a single fiber, and the wavelength spacing of the channels is getting narrower due to the limitation of the available wavelength band. It is a trend. Therefore, as described above, if the power of each channel is not constant, the risk of loss of information transmitted to the channel increases due to noise or interference between the channels. Typically, a separate attenuator 150 is used to planarize the power of each channel of the amplified optical signal. The long period optical fiber gratings 151, 152, or 153 constituting the attenuator 150 have a loss curve in the form of a Gaussian-like function representing a peak value at a center wavelength. In the case where the wavelength band of the optical signal to be transmitted is narrow, gain flattening may be performed using only one long-period fiber grating 151, 152, or 153. 151, 152 and 153 are used in combination.

FIG. 5 is a view for explaining a process of flattening a gain curve of an erbium-doped optical fiber amplifier having a conventional attenuator. As described above, the erbium-doped fiber amplifier should have a gain curve somewhat flattened in the wavelength band used. As shown, the gain curve (EDFA) of the erbium-doped fiber amplifier is obtained by using only one loss curve (LPG1, LPG2 or LPG3) having a specific shape, that is, a Gaussian function when the wavelength band used is wide enough to reach 40 nm. It is almost impossible to flatten. The basic concept of gain flattening is that the loss is higher at higher gain wavelengths and less at lower gain wavelengths. Accordingly, as shown in FIG. 5, the total loss curve (attenuator) formed by overlapping the loss curves LPG1, LPG2 and LPG3 of the three long period optical fiber gratings is the gain curve EDFA of the erbium-doped fiber amplifier. And its form is similar. Thus, the overall gain curve (EDFA + attenuator) formed by the combination of the erbium-doped fiber amplifier and the three long-period fiber gratings is somewhat flattened.

However, in the related art, when the gain flattening band of the erbium-doped optical fiber amplifier is wide, a combination of a plurality of long-period optical fiber gratings has to be used as an attenuator. In addition, since the shape of the loss curve of each long-period fiber grating is not varied, there is a difficulty in implementing a loss curve of a desired shape by combining loss curves having only a specific shape such as a Gaussian function.

The present invention has been made to solve the above-mentioned problems, an object of the present invention to provide a long-period optical fiber grating that can implement a variety of wavelength-specific loss curve. Another object of the present invention is to provide a long period optical fiber grating that can be applied over a wide wavelength band.

In order to achieve the above objects, the long period optical fiber grating according to the present invention,

The refractive index modulation values are different from each other, and the distribution of the refractive index modulation values is asymmetric about any reference section, and is composed of a plurality of sections each having a length greater than two times the lattice period.

1 is a view showing a long period optical fiber grating having a conventional uniform refractive index modulation value,

2 is a view for explaining a manufacturing method of the long period optical fiber grating shown in FIG.

3 is a perspective view showing an amplitude mask shown in FIG.

4 is a view showing an erbium-doped fiber amplifier with a conventional attenuator;

FIG. 5 is a view for explaining a process of flattening a gain curve of an erbium-doped optical fiber amplifier having a conventional attenuator. FIG.

6 is a graph showing the growth behavior of a loss curve with a change in length of a long period optical fiber grating;

7 is a graph illustrating the growth behavior of a loss curve with a change in refractive index modulation of a long period optical fiber grating;

8 is a view showing a long period optical fiber grating according to an embodiment of the present invention;

FIG. 9 is a view showing transmittance for each wavelength of the long period optical fiber grating shown in FIG. 8;

FIG. 10 is a view showing an apparatus for manufacturing the long period optical fiber grating shown in FIG. 8. FIG.

Hereinafter, with reference to the accompanying drawings will be described an 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.

6 is a diagram showing the growth behavior of a loss curve with a change in the length of a long period optical fiber grating. Loss curves L12, L13 and L14 are shown for three long period fiber gratings of different lengths. The period and refractive index modulation values of the three long period optical fiber gratings are all the same, and the lengths, L12, L13 and L14 of the long period optical fiber gratings have a relationship of (L12 <L13 <L14). As shown in the figure, as the length of a long period optical fiber grating increases, the peak loss at the center wavelength increases and the bandwidth decreases. However, the form retains the form of a Gaussian function.

FIG. 7 is a graph illustrating growth behavior of a loss curve according to a change in refractive index modulation of a long period optical fiber grating. The method of changing the refractive index modulation value is possible by adjusting the irradiation time of ultraviolet rays when fabricating a long period optical fiber grating using a fabrication apparatus as shown in FIG. In other words, it is to check the loss curve of the long-period fiber grating over time while continuing to irradiate ultraviolet light. The longer the irradiation time of ultraviolet rays, the larger the refractive index modulation value of the long-period fiber grating becomes within a certain limit.

As shown, as the irradiation time of ultraviolet rays (2 minutes, 5 minutes, 7 minutes, 12 minutes or 14 minutes) increases, the peak loss at the center wavelength increases and the center wavelength gradually shifts toward the longer wavelength. Can be.

8 is a diagram illustrating a long period optical fiber grating according to an exemplary embodiment of the present invention. The long period optical fiber grating 212 according to the present invention is divided into 30 sections S ₁ , S ₂ ,..., S _M ,..., S _29, and S ₃₀ along the longitudinal direction. The period Λ ₂₁ of the long period optical fiber grating 212 is constant over the entire grating length. In addition, the periods of the sections S ₁ , S ₂ ,..., S _M ,..., S ₂₉ and S ₃₀ constituting the long period optical fiber grating 212 are also the same. On the other hand, the refractive index modulation values of the 30 sections S ₁ , S ₂ ,..., _{M M} ..., S ₂₉ and S ₃₀ are not identical. Table 1 according to one preferred embodiment of the invention, the 30 sections _{(S 1, S 2, ...} , S M, ..., S 29 and S ₃₀₎ of the grating length 44.445 mm, period 185.1861 ㎛ consisting of the long For the optical fiber gratings, the refractive index modulation values for each section are shown. The length of each section S ₁ , S ₂ ,... S _M ,..., S ₂₉ or S ₃₀ is 1.481 mm.

At this time, each section number is attached in order along the longitudinal direction of the said long period optical fiber grating 212. It can be seen that the lengths of the sections S ₁ , S ₂ ,... S _M ,..., S ₂₉ or S ₃₀ constituting the long period optical fiber grating 212 are very small compared to the grating length of the conventional long period optical fiber grating. have. As can be seen from Table 1, the long-period optical fiber grating 212 according to the present invention has a different refractive index modulation value than the conventional one, and the distribution of the refractive index modulation values is asymmetric about an arbitrary reference section. Consists of sections S ₁ , S ₂ ,..., S _M ,..., S ₂₉ and S ₃₀ .

Section number Index Modulation Value Section number Index Modulation Value One 1.455829 16 1.455640 2 1.455785 17 1.455781 3 1.454974 18 1.455771 4 1.455629 19 1.455882 5 1.455684 20 1.455995 6 1.456376 21 1.455800 7 1.455447 22 1.455732 8 1.455487 23 1.456294 9 1.455712 24 1.455071 10 1.456024 25 1.456459 11 1.454605 26 1.455318 12 1.456622 27 1.455569 13 1.456337 28 1.456979 14 1.455016 29 1.456004 15 1.455835 30 1.455791

FIG. 9 is a diagram illustrating wavelength-specific transmittance of the long period optical fiber grating 212 of FIG. 8. Conventional long-period fiber gratings were transmittance graphs in the form of Gaussian functions, ie, graphs with one local minimum, while the graph shown has two local minimums near 1535 nm and 1557 nm. One long-period fiber grating according to the present invention can produce a wavelength-specific transmittance curve produced by a combination of two conventional gratings. That is, a per-wavelength permeability curve that can be made by a combination of a plurality of conventional long-period fiber gratings can be implemented with one long-period fiber grating according to the present invention.

FIG. 10 is a diagram illustrating an apparatus for manufacturing the long period optical fiber grating 212 shown in FIG. 8. The illustrated manufacturing apparatus has a structure further including a slit 215 in the manufacturing apparatus illustrated in FIG. 2. That is, a slit 215 is shown located between the amplitude mask 215 and the optical fiber 211. The slit 215 serves to block the ultraviolet light that passes through the amplitude mask 213 and enters a portion other than the corresponding section S _M. In addition, the slit 215 maintains a stop state at a position corresponding to each section S _M for a corresponding time to implement a refractive index modulation value set for each section, and when the predetermined time passes, the next section S _{M +1} ) to the position.

Until now, a method of irradiating ultraviolet light to the photosensitive optical fiber has been used to form the long period optical fiber grating according to the present invention. However, the long period optical fiber grating according to the present invention can be implemented in various ways. For example, the residual stress of the optical fiber can be used. Due to the photoelastic effect, the optical fiber reduces the refractive index of the optical fiber core due to the residual stress due to the drawing tension. When the laser light is irradiated to the optical fiber to periodically relax the residual stress of the optical fiber core, the refractive index of the irradiated optical fiber core is changed to form an optical fiber grating. The residual stress is classified into thermal stress and mechanical stress, the former being due to the difference in coefficient of thermal expansion between core and clad, the latter being due to the difference in viscosity of core and clad. The residual stress may be relaxed by irradiating laser light to the optical fiber portion in which the residual stress exists as described above, or by applying an electric arc, thermal energy of the electric arc may be transmitted to the optical fiber portion to relax.

As described above, the long-period optical fiber grating according to the present invention is composed of a plurality of sections, and has an advantage in that various wavelength-specific loss curves can be realized by making the asymmetric structure of the refractive index modulation value distribution for each section.

Claims (8)

Long cycle fiber optic grating, A long-period fiber grating having different refractive index modulation values, wherein the distribution of the refractive index modulation values is asymmetric about an arbitrary reference section, and is composed of a plurality of sections each having a length of at least two times the grating period. The method of claim 1, And the grating periods of the sections all have the same value. The method according to claim 1 or 2, And the lengths of the sections all have the same value. The method according to claim 1 or 2, The long period optical fiber grating is formed by periodically relaxing the residual stress of the optical fiber. The method of claim 4, wherein The remaining stress of the optical fiber is a long period optical fiber grating, characterized in that the laser light is periodically radiated. The method of claim 4, wherein And the residual stress of the optical fiber is periodically relaxed by applying an electric arc. The method according to claim 1 or 2, The long period optical fiber grating is formed by irradiating ultraviolet light selectively transmitted by an amplitude mask to a photosensitive optical fiber. The method of claim 7, wherein The adjustment of the refractive index modulation value of each section constituting the long period optical fiber grating is characterized in that made by the irradiation time of the ultraviolet rays to each section.