Self-adaptive narrow-bandwidth phase-shift fiber grating filter based on saturable absorber technology

Abstract

The invention discloses a self-adaptive narrow-bandwidth phase-shift fiber grating filter based on a saturable absorber technology. The self-adaptive narrow-bandwidth phase-shift fiber grating filter comprises a pump laser light source, a wavelength division multiplexer, an optical fiber coupler, an erbium-doped optical fiber, an isolator, an optical fiber coupler, an echo wall cavity filter and an optical fiber coupler which are connected with one another. An input port of the wavelength division multiplexer is connected to the laser pumping source, and an output port of the wavelength division multiplexer is connected to an input port of the optical fiber coupler. A resonant cavity of the laser is composed of a main annular cavity and an auxiliary annular cavity, the auxiliary annular cavity forms a resonant cavity for early working of the laser, strong initial signal light is generated, the signal light enters the main annular cavity, line width compression is achieved through an echo wall microcavity, cyclic amplification is achieved through an erbium-doped optical fiber, and finally stable laser oscillation is formed. According to the double-ring-cavity structure, the forming process and the line width compression process of the laser are separated, and the output power of the laser system is remarkably improved on the premise that the line width compression of the echo wall microcavity is guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber communication, and in particular relates to an adaptive narrow bandwidth phase-shifted optical fiber grating filter based on saturated absorber technology. Background Art

[0002] Narrow bandwidth optical filters are widely used in the fields of optical communication and fiber optic sensing, and the most common ones are fiber Bragg gratings, fiber Fabry-Perot cavity filters, whispering gallery mode resonant cavities, etc. However, the central wavelengths of these filters are fixed or limited to a certain range. Therefore, they are not suitable for scenarios that require a large range of filter wavelengths, such as tunable lasers; saturated absorber technology has the characteristic of adaptive filtering center wavelength, but the filter bandwidth is usually tens of MHz and cannot be further reduced; using Brillouin scattering and Rayleigh scattering of optical fibers, filters with bandwidths of tens of MHz or even tens of kHz can be constructed, and the central wavelength is adaptive, but the optical power threshold conditions need to be met, and the optical fiber length often needs to be hundreds of meters or even thousands of meters, which is difficult to integrate and cannot be achieved in some scenarios. Summary of the invention

[0003] The purpose of the present invention is to solve the above technical problems and to provide a whispering gallery microcavity filter fiber laser with a double ring cavity structure.

[0004] The invention discloses an adaptive narrow bandwidth phase-shifted fiber grating filter based on saturated absorber technology, comprising a polarization-maintaining erbium-doped fiber and a common single-mode fiber; a common single-mode fiber is fused between the two polarization-maintaining erbium-doped fibers to form a fiber filter.

[0005] Furthermore, the two polarization-maintaining erbium-doped optical fibers have the same parameters, and one end of the polarization-maintaining erbium-doped optical fiber is coated with a reflector film.

[0006] Furthermore, a weak reflection grating is engraved in the middle of the polarization-maintaining erbium-doped optical fiber, and a reflector film is coated on the right side.

[0007] Furthermore, the reflectivity of the weak reflection grating is at the level of 0.01%.

[0008] Furthermore, one side of the weak reflection grating is coated with a reflective mirror film.

[0009] Beneficial Effects

[0010] (1) Compared with the traditional fiber grating filter, the filtering center wavelength of the present invention is not fixed. Within the C-band range, it can effectively filter light waves of any wavelength and has wavelength adaptability;

[0011] (2) Compared with traditional fiber Bragg grating filters and conventional dynamic fiber Bragg gratings based on saturated absorber technology, it has an extremely narrow filtering bandwidth of MHz;

[0012] (3) Compared with conventional dynamic fiber Bragg gratings based on saturated absorber technology, the dynamic fiber Bragg gratings of this structure require a shorter length of erbium-doped fiber, so the optical loss is smaller;

[0013] (4) Compared with the adaptive filtering based on the fiber Rayleigh / Brillouin scattering principle, this method is simple to manufacture, easy to package, and convenient to integrate;

[0014] (5) The present invention is particularly applicable to the field of tunable technology, such as ring cavity tunable fiber lasers, which can ensure filtering of light waves of all wavelengths in the tuning range while effectively suppressing multi-mode oscillation and mode hopping phenomena of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram of the structure of the adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturated absorber technology of the present invention. The upper part is the fiber filter when no light is passed through. The whole is formed by fusion splicing three sections of optical fiber. The right end face is coated with a reflector film (indicated by black in the figure). The two ends are polarization-maintaining erbium-doped optical fibers with the same parameters, and the middle section is an ordinary single-mode optical fiber. The lower part is the fiber filter after light is passed through. After the signal light is passed through, the erbium-doped optical fiber forms a dynamic grating, which is indicated by a gray pattern in the figure.

[0016] Figure 2 This is a schematic diagram of the structure of the adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturated absorber technology of the present invention. The whole is a section of polarization-maintaining erbium-doped fiber, with a reflector film coated on the right end face (indicated by black in the figure), and a grating of a certain length engraved in the middle section (indicated by a black pattern in the figure). After the signal light is introduced into the erbium-doped fiber, a dynamic grating is formed, which is indicated by a gray pattern in the figure.

[0017] Figure 3 This is the transmission spectrum of the adaptive narrow-bandwidth phase-shifted fiber Bragg grating filter based on the saturated absorber technology of the present invention, and the bandwidth is extremely narrow, about 2.2 MHz.

[0018] Figure 4 It is the fiber Bragg grating reflection spectrum of the adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on the saturated absorber technology of the present invention. DETAILED DESCRIPTION

[0019] The following combination Figures 1 to 4 This embodiment will be described in detail.

[0020] Example 1

[0021] like Figure 1As shown, a structural form of the present invention includes a polarization-maintaining erbium-doped fiber 1, an ordinary single-mode optical fiber, and a polarization-maintaining erbium-doped optical fiber 2, which are fused to form an optical fiber filter. Among them, the various parameters of the polarization-maintaining erbium-doped optical fiber 1 and the polarization-maintaining erbium-doped optical fiber 2 are exactly the same, and the right end of the polarization-maintaining erbium-doped optical fiber is coated with a reflector film. The working process of the invention is that when the filtered light signal is input along the positive direction of the optical fiber filter axis, the light wave is reflected back by the reflector of the optical fiber filter, and interferes with the incident light wave to form a standing wave. The standing wave effect causes a periodic distribution of light intensity in the erbium-doped optical fiber, which causes a periodic distribution corresponding to the absorption coefficient of erbium ions in the erbium-doped optical fiber, that is, the saturated absorber effect, which causes the refractive index of the erbium-doped optical fiber to form a periodic modulation accordingly, which is equivalent to a dynamic fiber grating. Figure 1 Pattern shown in medium grey.

[0022] The basic spectral properties of the dynamic fiber grating are the same as those of the uniform Bragg fiber grating, but the period length is adaptive and is always half the wavelength of the incident light. The middle part of the structure is an ordinary single-mode fiber, which cannot form a saturated absorber effect, so no dynamic grating appears, but the light wave introduces a phase shift during the propagation of this part, and the standing wave fringes mutate here. Therefore, the filter can be equivalent to a phase-shifted fiber grating. According to the principle of phase-shifted fiber grating, its transmission bandwidth is much smaller than the reflection bandwidth of an ordinary uniform Bragg grating, such as Figure 3 and Figure 4 By choosing the appropriate length and concentration of erbium-doped fiber and single-mode fiber length, the final filter transmission spectrum contains only one filter resonance peak and the bandwidth is 2.2MHz.

[0023] Example 2

[0024] like Figure 2 As shown, another structural form of the present invention is used, which introduces the same phase shift effect as structural form 1 during the light wave propagation process in the optical fiber filter by means of writing a weak reflection grating.

[0025] Its specific structure is that a weak reflection grating (reflectivity is at the level of 0.01%) of a certain length is engraved in the middle of the polarization-maintaining erbium-doped optical fiber, and a reflector film is coated on the right side.

[0026] The working process of this structure is that the filtered light signal is input along the positive direction of the fiber filter axis, and the light wave is reflected back by the fiber filter reflector, interfering with the incident light wave to form a standing wave. The standing wave effect causes the periodic distribution of light intensity in the erbium-doped fiber, which causes the periodic distribution of the erbium ion absorption coefficient in the erbium-doped fiber, that is, the saturated absorber effect, resulting in the corresponding periodic modulation of the refractive index of the erbium-doped fiber, which is equivalent to a dynamic fiber grating, such as Figure 2As shown in the middle gray pattern. The basic spectral properties of this dynamic fiber grating are the same as those of the uniform Bragg fiber grating, but the period length is adaptive and is always half the wavelength of the incident light. Since a weak reflection grating is engraved in the middle of the fiber filter, the phase of the light wave changes during propagation, and the standing wave fringes mutate here. Therefore, the filter structure can also be equivalent to a phase-shifted fiber grating, and the filtering effect is the same as that of structure 1.

[0027] When different light waves are incident into the filter, the light wave with the highest intensity will eventually form a unique stable standing wave effect and saturated absorber effect. Therefore, the center wavelength of this filter is adaptive to the wavelength with the highest intensity of the incident light.

[0028] The present invention introduces a structure into the erbium-doped optical fiber that enables an additional phase change during light wave propagation. The optical fiber filter is equivalent to a phase-shifted optical fiber grating, and obtains a filtering effect with a narrower bandwidth while maintaining the wavelength adaptability of the optical fiber grating of the saturated absorber technology.

[0029] The above contents of the present invention are only preferred embodiments of the present invention and are not intended to limit the implementation scheme of the present invention. A person skilled in the art can easily make corresponding changes or modifications based on the main concept and spirit of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope required by the claims.

Claims

1. An adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturated absorber technology, characterized in that: It comprises a polarization-maintaining erbium-doped optical fiber and an ordinary single-mode optical fiber; an ordinary single-mode optical fiber is fused between the two polarization-maintaining erbium-doped optical fibers to form an optical fiber filter.

2. The adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturable absorber technology according to claim 1, characterized in that: The two polarization-maintaining erbium-doped optical fibers have the same parameters, and one end of the polarization-maintaining erbium-doped optical fiber is coated with a reflector film.

3. The adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturable absorber technology according to claim 1, characterized in that: A weak reflection grating is engraved in the middle of the polarization-maintaining erbium-doped optical fiber, and a reflector film is plated on the right side.

4. The adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturable absorber technology according to claim 3, characterized in that: The reflectivity of the weak reflection grating is at the level of 0.01%.

5. The adaptive narrow bandwidth phase-shifted fiber Bragg grating filter based on saturable absorber technology according to claim 3, characterized in that: One side of the weak reflection grating is coated with a reflective mirror film.

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