CN100534011C - Ultra-narrow Dual-Channel Filter Based on Symmetrical Sampling Grating Structure - Google Patents

Abstract

The invention provides an ultra-narrow double-channel filter based on a symmetrical sampling grating structure, which is formed by combining two sampling Bragg gratings (SFBG) and a 3-port optical circulator. One of the bragg gratings is sampled with a period of 0 to pi and the other bragg grating is symmetrically spatially sampled. The former produces two rejection bands on its spectral line and the latter two pass bands. The 3dB bandwidth of the filter is about 1pm, and the value is two orders of magnitude less than the bandwidth of the traditional SFBG filter. The filter has the unique advantages that: the channel space is not changed when the filter is changed.

Description

Ultra-narrow Dual-Channel Filter Based on Symmetrical Sampling Grating Structure

technical field

The invention relates to an optical filter, in particular to an ultra-narrow dual-channel optical filter.

Background technique

Fiber Bragg gratings (FBG) have been used in many fiber optic devices, such as optical filters, optical sensors and so on. At the same time, FBG has the ability to generate multiple channels. But the traditional sampling fiber Bragg grating (SFBG) is performed with simple binary sampling, which realizes multiple channels with different lengths and bandwidths. Although the complex sinC sampling function technology can overcome these problems and realize the same multi-channel Bragg grating filter, the bandwidth of each channel is very large, usually 100pm-300pm. At the same time, a spatial π phase shift is introduced at the center of the uniform fiber grating, and the suppression frequency band of the grating can obtain an ultra-narrow transmission window, but only a single-channel response can be obtained.

Contents of the invention

The purpose of the present invention is to provide a kind of ultra-narrow dual-channel filter based on symmetrical sampling grating structure, which solves the problem that the traditional SFBG in the background technology realizes that the multi-channel strength and bandwidth are different, the bandwidth of each channel of the multi-channel Bragg grating filter is very large and the Can only get one response to a technical question.

The technical solution of the present invention is: an ultra-narrow dual-channel filter based on a symmetrical sampling grating structure, which is special in that it includes a 3-port optical circulator A and 0 to π sampling Bragg grating SFBG ₁ , symmetric structure Bragg grating SFBG ₂ , the specific structure of the symmetric structure Bragg grating SFBG ₂ is: a uniform fiber grating with a sampling length of L ₁ and grating-free parts are alternately distributed, and the grating-free part has two widths , are L ₂ , L ₃ respectively, and the lengths are used to represent each section of the grating and the part without the grating, so that the part with the grating and the part without the grating are ...L ₁ L ₂ L ₁ L ₃ ...L ₃ L ₁ L ₃ ...L ₃ L ₁ L ₂ L ₁ ... arrangement, this arrangement is symmetrical about the center; the specific structure of the 0 to π sampling Bragg grating SFBG ₁ is: uniform discrete fiber gratings with a sampling length of L ₁ and grating-free parts are alternately distributed, and no The grating part has two widths, which are L ₂ and L ₃ respectively. The lengths are used to represent each segment of the grating and the part without the grating, so that the part with the grating and the part without the grating are ... L ₁ L ₂ L ₁ L ₃ L ₁ L ₂ ... arrangement; where L ₂ =mΛ, L ₃ =(n+0.5)Λ, Λ is the Bragg period, and m and n represent integers respectively.

The above-mentioned 3-port optical circulator A, 0 to π sampling Bragg grating SFBG ₁ , and the connection relationship among the symmetrical structure Bragg grating SFBG ₂ are: 0 to π sampling Bragg grating SFBG ₁ is connected to the middle port A2 of the 3-port optical circulator A , the symmetrical Bragg grating SFBG ₂ is connected to the output port Λ3 of the 3-port optical circulator A.

The above-mentioned 3-port optical circulator A, 0 to π sampling Bragg grating SFBG ₁ , and the connection relationship between the symmetric structure Bragg grating SFBG ₂ can also be: 0 to π sampling Bragg grating SFBG ₁ and the middle port of the 3-port optical circulator A A2 connection, the symmetrical Bragg grating SFBG ₂ is connected to the input port A1 of the 3-port optical circulator A.

The optical fiber used in the above grating is an erbium-ytterbium co-doped optical fiber.

The aforementioned Bragg period Λ=534.2 nm, m=600, n=584.

The advantage that the present invention has is:

1. The 3dB bandwidth of each channel is about 1pm (10 ^-12 m), which is two orders of magnitude less than the traditional FBG filter (10 ^-10 m).

2. The transmission characteristics of the two channels are almost the same. Theoretically, the two channels should be the same. The main difference between theory and experiment is that the resolution limit of the spectrum analyzer is 0.01nm in the experiment.

3. When changing the two channels of the filter, its spatial channel remains unchanged. The experimental results are shown in Figure 5, the filter wavelength is 0.3nm as the interval, and the channel spacing of the filter remains at 440pm when the wavelength is changed.

Description of drawings

Fig. 1 (a) is the FBG structural representation of 0 to π sampling in the ultra-narrow dual-channel filter of the present invention, and Fig. 1 (b) is the structural representation of the fiber Bragg grating of symmetrical structure in the ultra-narrow dual-channel filter of the present invention, wherein The horizontal axis (Z axis) is the length direction of the grating, and the vertical axis is the refractive index modulation;

Fig. 2 (a) is the transmission spectrum diagram of 0 to π sampling fiber Bragg grating, Fig. 2 (b) is the transmission spectrum diagram of symmetrical structure fiber Bragg grating, the horizontal axis is the wavelength, and the vertical axis is the power;

Fig. 3 (a) and Fig. 3 (b) are two kinds of structural forms of ultra-narrow dual-channel filter of the present invention, wherein SFBG ₁ is 0 to π sampling fiber Bragg grating, and SFBG ₂ is symmetrical structure fiber Bragg grating; A is 3 Port optical circulator, the input light wave is input from the input port A1 of the optical circulator, reaches the middle port A2, and the reflected wave is output from the output port A3 after being reflected by SFBG ₁ ;

Figure 4 is the transmission spectrum, the dotted line is the theoretical value, and the solid line is the experimental result; the horizontal axis is the wavelength, and the vertical axis is the power;

Fig. 5 shows 5 sets of transmission spectra obtained by tuning the SFBG at intervals of about 0.3nm.

Detailed ways

The structure of the ultra-narrow dual-channel filter of the present invention is shown in Fig. 3 in detail, which is mainly composed of a 3-port optical circulator, a 0 to π sampling Bragg grating SFBG ₁ and a symmetrical structure Bragg grating SFBG ₂ . There are two connection relationships: the first connection relationship is shown in Figure 3(a), the 0 to π sampling Bragg grating SFBG ₁ is connected to the middle port A2 of the 3-port optical circulator A, and the symmetrical structure Bragg grating SFBG ₂ is connected to the 3-port optical ring output port A3 of shaper A. The input light wave is input from the input port A1 of the 3-port optical circulator and reaches the middle port A2. After being reflected by SFBG ₂ , the reflected wave passes through the symmetrical structure Bragg grating SFBG ₂ and then output from the output port A3. The second connection relationship is shown in Figure 3(b). The 0 to π sampling Bragg grating SFBG ₁ is connected to the middle port A2 of the 3-port optical circulator A, and the symmetrical structure Bragg grating SFBG ₂ is connected to the input port A1 of the 3-port optical circulator A. connected, output from output port A3. The input light wave is input from the input port A1 of the 3-port optical circulator, and reaches the middle port A2 after passing through the symmetrical Bragg grating SFBG ₂ , and the reflected wave is output from the output port A3 after being reflected by the SFBG ₂ .

The specific structure of SFBG is shown in Figure 1. A uniform fiber grating (length L ₁ ) is separated by periodic non-grating parts L ₂ and L ₃ , where L ₁ is the sampling length, and L ₂ and L ₃ are the gap length, forming the SFBG. In Fig. 1(a), the entire grating is sampled by 0 to π period, forming a 0 to π SFBG. In Fig. 1(b), when it reaches the middle of the grating, the periodic sampling sequence from 0 to π becomes the periodic sampling sequence from π to 0, forming a symmetrical structure SFBG. In the process of making SFBG, the Bragg period Λ is 534.2nm. By adjusting the lengths of L ₂ and L ₃ , they respectively satisfy L ₂ =mΛ, L ₃ =(n+0.5)Λ, where m and n are both integers. L ₂ and L ₃ respectively correspond to 2mπ and (2n+1)π phases, which are equivalent to "0" and "π" phases. For simplicity, _L2 and _L3 are named 0-sampling and π-sampling, respectively.

Fiber Bragg grating is produced by ultraviolet lithography produced by frequency-doubled argon ion laser, its power is about 130mW, wavelength 244nm, engraving precision is 10 nanometers, Λ=534.2nm, L ₁ =0.64104nm, L ₂ =0.32052mm ( That is, L ₂ =600Λ), L ₃ =0.31224mm (that is, L ₃ =584.5Λ). During the manufacturing process, the optical fiber moves on the platform where the template is fixed, that is, the optical fiber moves laterally, the template is fixed, and the total length of the grating is about 57mm. Sections L ₂ and L ₃ are not UV-irradiated, so our gratings are made in one step without post-processing.

The transmission spectrum line of SFBG in the present invention is shown in Fig. 2, and Fig. 2(a) and Fig. 2(b) respectively correspond to the experimental results of Fig. 1(a) and Fig. 1(b). The result is measured by ADVANTEST Q8384 Spectrum Analyzer (OSA), the resolution limit of OSA is 0.01nm, the ultra-narrow dual-channel peak value is -45dBm, and its theoretical value is -33dBm, as shown in Figure 4.

From Figure 2(a) and Figure 2(b), it can be seen that:

(1) The dual-channel loss factor in Figure 2(a) is 32dB, and the dual-channel loss factor in Figure 2(b) is 30dB.

(2) The distance Δλ between two channels is 440pm.

(3) The channel spectrum characteristics of 1548.8nm are close to those of 1549.2nm.

(4) The narrow transmission line of 3dB bandwidth (Fig. 2(b)) is about 1pm. Compared with the line sampled from 0 to π in Fig. 2(a), the symmetric SFBG line in Fig. 2(b) opens two ultra-narrow passbands in two attenuation bands.

From a physical point of view, the proposed SFBG can be considered as two cascaded SFBGs. Therefore Δλ is close to twice that of a traditional sampled FBG:

where n _eff is the effective refractive index.

According to the relevant parameters of the experiment, Δλ=440pm can be obtained from the above formula. The theoretical values are in good agreement with the experimental results. Therefore, the two narrow channels in Fig. 2(b) approximate the cascade of two conventional π-phase-shifted FBGs.

A new type of ultra-narrow dual-channel filter can be obtained by connecting the 0 to π sampling SFBG in Figure 1(a) and the symmetrical structure SFBG in Figure 1(b). In Fig. 3, SFBG1 and SFBG2 adopt the structures of Fig. 1(a) and Fig. 1(b) respectively. The spectral lines of this new filter are shown in Fig. 4. The dashed line is the theoretical value, and the solid line is the experimental result. It can be seen from Figure 4 that this new type of filter has the following advantages:

(1) The 3dB bandwidth of each channel is about 1pm, which is two orders of magnitude (10 ^-10 m) less than that of traditional FBG filters.

(2) The transmission characteristics of the two channels are almost the same.

(3) In theory, the two channels should be the same. The main difference between theory and experiment is that in the experiment, the OSA resolution can only reach 0.01nm.

In addition, the filter of the present invention is unique in that the channel spacing does not change when the wavelength is changed. The experimental results are shown in Figure 5. The filter wavelength is 0.3 nm apart. When the wavelength is changed, the filter channel spacing remains unchanged, which is still 440 pm.

Working principle of the present invention:

The invention designs an attenuation band and an ultra-narrow dual-channel SFBG based on 0-π sampling and a symmetrical structure, and forms a novel ultra-narrow dual-channel filter by the symmetrical FBG and the 0-π sampling FBG. One of the SFBGs is sampled with a period of 0 to π, and the other is sampled in symmetric space. The former produces two suppression bands on its spectral line, and the latter two passbands. This kind of filter can obtain a symmetrical spectrum, its channel is also doubled, and its grating can be regarded as a cascade of two SFBGs. When changing the two channels of the filter, its spatial channel does not change. The 3dB bandwidth of this filter is about 1pm, which is two orders of magnitude less than the traditional SFBG filter. The remarkable advantage of this filter is that Δλ remains unchanged when changing the filter, and in the experiment, by changing the sampling length L ₁ and the spacing L ₂ , L ₃ , Δλ can be tuned within the range of 0.01nm to 2nm.

This ultra-narrow dual-channel filter will have important applications in the fields of dual-wavelength single longitudinal mode (SLM) fiber lasers, microwave signal generation, optical unilateral modulation, and microwave image signal processing.

Claims (5)

1. An ultra-narrow dual-channel filter based on a symmetrical sampling grating structure, characterized in that it includes a 3-port optical circulator (A) and a 0 to π sampling Bragg grating connected to the 3-port optical circulator (A) (SFBG ₁ ), symmetrical structure Bragg grating (SFBG ₂ ), the specific structure of the symmetrical structure Bragg grating (SFBG ₂ ) is: a uniform fiber grating with a sampling length of L ₁ and non-grating parts are alternately distributed, and the non-grating part has two width, respectively L ₂ , L ₃ , each section of grating and grating-free part is represented by length, so that the part with grating and the part without grating are according to …L ₁ L ₂ L ₁ L ₃ …L ₃ L ₁ L ₃ …L ₃ L ₁ L ₂ L ₁ ...arranged in a manner, this arrangement is symmetrical about the center; the specific structure of the 0 to π sampled Bragg grating (SFBG ₁ ) is: a uniform discrete fiber grating with a sampling length of L ₁ and a grating-free part Alternately distributed, the non-grating part has two widths, which are L ₂ and L ₃ respectively, and the length is used to represent each segment of the grating and the non-grating part, so that the grating part and the non-grating part are according to... L ₁ L ₂ L ₁ L ₃ L _{1 L 2 .} 2. The ultra-narrow dual-channel filter based on a symmetrical sampling grating structure according to claim 1, characterized in that, said 3-port optical circulator (A), 0 to π sampling Bragg grating (SFBG ₁ ), symmetrical structure The connection relationship between the Bragg gratings (SFBG ₂ ) is: 0 to π sampling Bragg grating (SFBG ₁ ) is connected to the middle port (A2) of the 3-port optical circulator (A), and the symmetrical structure Bragg grating (SFBG ₂ ) is connected to the 3 Port The output port (A3) of the optical circulator (A) is connected. 3. The ultra-narrow dual-channel filter based on a symmetrical sampling grating structure according to claim 1, characterized in that, said 3-port optical circulator (A), 0 to π sampling Bragg grating (SFBG ₁ ), symmetrical structure The connection relationship between the Bragg gratings (SFBG ₂ ) can also be: 0 to π sampling Bragg gratings (SFBG ₁ ) are connected to the middle port (A2) of the 3-port optical circulator (A), and the symmetrical structure Bragg gratings (SFBG ₂ ) Connect to the input port (A1) of the 3-port optical circulator (A). 4. The ultra-narrow dual-channel filter based on a symmetrically sampled grating structure according to claim 1, wherein the optical fiber used for the grating is an erbium-ytterbium co-doped optical fiber. 5. The ultra-narrow dual-channel filter based on a symmetrically sampled grating structure according to any one of claims 1 to 4, wherein the Bragg period Λ=534.2nm, m=600, n=584.

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