An adaptive narrow bandwidth phase-shifted fiber grating filter based on saturable absorber technology

Abstract

The application relates to a self-adaptive narrow-bandwidth phase-shift fiber grating filter based on a saturated absorber technology, which comprises polarization-maintaining erbium-doped optical fibers and common single-mode optical fibers; common single-mode optical fibers are fused between the two polarization-maintaining erbium-doped optical fibers; incident light and reflected light form standing waves in the above structure; and the self-adaptive phase-shift fiber grating filter is formed by using the saturated absorber principle and the phase-shift characteristic; the parameters of the two polarization-maintaining erbium-doped optical fibers are the same; and the polarization-maintaining erbium-doped optical fibers are coated with mirror films at one end. Compared with the traditional fiber grating filter, the filter center wavelength is not fixed, the filter can effectively filter any wavelength of light in the C-band range, and the filter has wavelength self-adaptability; compared with the traditional fiber grating filter and the conventional dynamic fiber grating based on the saturated absorber technology, the filter has an extremely narrow filter bandwidth of MHz.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber communication, and particularly relates to a self-adaptive narrow-bandwidth phase-shifted fiber grating filter based on a saturable absorber technology. BACKGROUND

[0002] Narrow-bandwidth optical filters are widely used in the fields of optical communication and optical fiber sensing, and common ones include fiber gratings, fiber Fabry-Perot cavity filters, whispering gallery mode resonant cavities and the like. However, the center wavelengths of these filters are fixed or limited within a certain range, and therefore, they are not applicable to scenarios requiring a large range of filtered wavelengths, such as tunable lasers. The saturable absorber technology has the feature of self-adaptive filtering center wavelength, but the filtering bandwidth is usually tens of MHz, which cannot be further reduced. The Brillouin scattering and Rayleigh scattering of optical fibers can be used to construct filters with a bandwidth of tens of MHz or even tens of kHz, and the center wavelength is self-adaptive, but the optical power threshold condition needs to be met, and the length of the optical fiber used is usually hundreds of meters or even thousands of meters, which is difficult to integrate, and some scenarios cannot be achieved. SUMMARY

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

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

[0005] Further, the two polarization maintaining erbium-doped optical fibers have the same parameters, and a reflective mirror film is coated on one end of the polarization maintaining erbium-doped optical fiber.

[0006] Further, a weak reflection grating is inscribed in the middle of the polarization maintaining erbium-doped optical fiber, and a reflective mirror film is coated on the right side.

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

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

[0009] Advantages

[0010] (1) Compared with the conventional fiber grating filter, the filtering center wavelength of the present application is not fixed, and any wavelength of light can be effectively filtered in the C-band range, and the wavelength is self-adaptive;

[0011] (2) Compared with the conventional fiber grating filter and the conventional dynamic fiber grating based on the saturable absorber technology, the present application has an extremely narrow filtering bandwidth of MHz;

[0012] (3) Compared with the conventional dynamic fiber grating based on the saturated absorber technology, the dynamic fiber grating of the structural form needs shorter erbium-doped fiber length, and thus has smaller light loss;

[0013] (4) Compared with the self-adaptive filter based on the fiber Rayleigh / Briouin scattering principle, the method is simple and easy to make, easy to package and convenient to integrate;

[0014] (5) The application is particularly suitable for the tunable technical field, for example, the ring cavity tunable fiber laser, which can filter the light waves of all wavelengths in the tuning range, and effectively suppress the multi-mode oscillation and mode hopping of the laser. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a structural schematic diagram of the self-adaptive narrow-bandwidth phase-shift fiber grating filter based on the saturated absorber technology of the application, the upper part is the optical fiber filter when no light is passed, the whole is three-section optical fiber fusion, the right end surface is coated with a mirror film (represented by black in the figure), and the two ends are polarization maintaining erbium-doped optical fibers with the same parameters, and the middle section is a common single-mode optical fiber. The lower part is the optical fiber filter after passing the light, the erbium-doped optical fiber forms a dynamic grating after passing the signal light, which is represented by a gray pattern in the figure.

[0016] Figure 2 It is a structural schematic diagram of the self-adaptive narrow-bandwidth phase-shift fiber grating filter based on the saturated absorber technology of the application, the whole is a polarization maintaining erbium-doped optical fiber, the right end surface is coated with a mirror film (represented by black in the figure), and the middle section is engraved with a certain length of grating (represented by a black pattern in the figure). The erbium-doped optical fiber forms a dynamic grating after passing the signal light, which is represented by a gray pattern in the figure.

[0017] Figure 3 It is a transmission spectrum diagram of the self-adaptive narrow-bandwidth phase-shift fiber grating filter based on the saturated absorber technology of the application, the bandwidth is extremely narrow, about 2.2 MHz.

[0018] Figure 4 It is a fiber grating reflection spectrum of the self-adaptive narrow-bandwidth phase-shift fiber grating filter based on the saturated absorber technology of the application. DETAILED DESCRIPTION

[0019] The following will be combined Figures 1 to 4 The present embodiment will be specifically described.

[0020] Example 1

[0021] As Figure 1The diagram illustrates one structural form of the present invention, comprising a polarization-maintaining erbium-doped fiber 1, a standard single-mode fiber, and a polarization-maintaining erbium-doped fiber 2, which are fused together to form a fiber filter. The polarization-maintaining erbium-doped fiber 1 and the polarization-maintaining erbium-doped fiber 2 have identical parameters, and the right end of the polarization-maintaining erbium-doped fiber is coated with a reflective film. The working process of this invention is as follows: when a filtered optical signal is input along the positive direction of the fiber filter axis, the light wave is reflected back by the fiber filter's reflective mirror, interfering 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 fiber, resulting in a corresponding periodic distribution of the erbium ion absorption coefficient, i.e., a saturable absorber effect. This leads to a corresponding periodic modulation of the refractive index of the erbium-doped fiber, equivalent to a dynamic fiber grating. Figure 1 As shown in the medium gray pattern.

[0022] The basic spectral properties of this dynamic fiber grating are the same as those of a uniform Bragg fiber grating, but its period length is adaptive, always being half the wavelength of the incident light. The middle section of this structure is made of ordinary single-mode fiber, which cannot form a saturable absorber effect, therefore no dynamic grating appears. However, a phase shift is introduced during the propagation of the light wave in this section, causing abrupt changes in the standing wave fringes. Therefore, this filter can be considered equivalent to a phase-shifting fiber grating. As can be seen from the principle of phase-shifting fiber gratings, its transmission bandwidth is much smaller than the reflection bandwidth of an ordinary uniform Bragg grating, such as... Figure 3 and Figure 4 As shown, by selecting appropriate erbium-doped fiber length and concentration, as well as single-mode fiber length, the final filter transmission spectrum contains only one filter resonance peak, with a bandwidth of 2.2 MHz.

[0023] Example 2

[0024] like Figure 2 As shown, another structural form of the present invention is introduced in the optical fiber filter by writing a weak reflection grating to introduce the same phase shift effect as structural form 1 during the propagation of light waves.

[0025] Its specific structure is as follows: a weakly reflective grating (with a reflectivity of 0.01%) of a certain length is etched in the middle of the polarization-maintaining erbium-doped fiber, and a reflective film is coated on the right side.

[0026] The working process of this structure is as follows: a filtered optical signal is input along the positive direction of the fiber filter axis. The light wave is reflected back by the fiber filter's mirrors, interfering with the incident light wave to form a standing wave. This standing wave effect causes a periodic distribution of light intensity in the erbium-doped fiber, which in turn causes a corresponding periodic distribution of the erbium ion absorption coefficient, i.e., a saturable absorber effect. This results in a corresponding periodic modulation of the erbium-doped fiber's refractive index, equivalent to a dynamic fiber grating, such as... Figure 2The basic spectrum property of the dynamic fiber grating is same as that of the uniform Bragg fiber grating, but the period length has self-adapting characteristics, and is always half of 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 in the propagation process, and the standing wave fringe changes at this point. Therefore, the filter structure can also be equivalent to a phase-shift fiber grating, and the filtering effect is same as that of the structure 1.

[0027] Different light waves are incident into the filter, and finally the light wave with the maximum light intensity forms a unique stable standing wave effect and saturated absorber effect. Therefore, the central wavelength of the filter is the wavelength with the maximum light intensity, and therefore the filter has self-adapting characteristics.

[0028] The application introduces a structure into the erbium-doped fiber, which enables additional phase change when the light wave propagates. The fiber filter is equivalent to a phase-shift fiber grating. On the basis of keeping the wavelength self-adapting characteristics of the saturated absorber technology fiber grating, the filtering effect with narrower bandwidth is obtained.

[0029] The above description of the application is only the preferred embodiment of the application, and is not used to limit the implementation of the application. Those skilled in the art can easily make corresponding changes or modifications according to the main idea and spirit of the application. Therefore, the protection scope of the application should be subject to the protection scope required by the claims.

Claims

1. An adaptive narrow bandwidth phase-shifted fiber grating filter based on saturable absorber technology, characterized in that, The application relates to a kind of optical fiber filter, comprising a first polarization maintaining erbium-doped optical fiber, a common single-mode optical fiber, and a second polarization maintaining erbium-doped optical fiber, which are fused together to form the optical fiber filter, wherein the first polarization maintaining erbium-doped optical fiber and the second polarization maintaining erbium-doped optical fiber have the same parameters, the second polarization maintaining erbium-doped optical fiber is coated with a mirror film at the right end, when a filtered optical signal is input along the positive direction of the axis of the optical fiber filter, the light wave is reflected back under the action of the mirror of the optical fiber filter, and the reflected light wave interferes 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 optical fiber, causes the corresponding periodic distribution of the absorption coefficient of erbium ions in the erbium-doped optical fiber, i.e., the saturable absorber effect, and causes the corresponding periodic modulation of the refractive index of the erbium-doped optical fiber, which is equivalent to a dynamic fiber grating; the middle part of the structure is a common single-mode optical fiber, which cannot form the saturable absorber effect, and therefore no dynamic grating appears, but the phase shift is introduced in the propagation process of the light wave in the middle part, and the standing wave fringes are changed at the middle part, so that the filter is equivalent to a phase-shift fiber grating.

2. An adaptive narrow bandwidth phase-shifted fiber grating filter based on saturable absorber technology, characterized in that, The application relates to a kind of optical fiber filter, comprising a first polarization maintaining erbium-doped optical fiber, a common single-mode optical fiber, and a second polarization maintaining erbium-doped optical fiber, which are fused together to form the optical fiber filter, wherein the first polarization maintaining erbium-doped optical fiber and the second polarization maintaining erbium-doped optical fiber have the same parameters, the second polarization maintaining erbium-doped optical fiber is coated with a mirror film at the right end, when a filtered optical signal is input along the positive direction of the axis of the optical fiber filter, the light wave is reflected back under the action of the mirror of the optical fiber filter, and the reflected light wave interferes 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 optical fiber, causes the corresponding periodic distribution of the absorption coefficient of erbium ions in the erbium-doped optical fiber, i.e., the saturable absorber effect, and causes the corresponding periodic modulation of the refractive index of the erbium-doped optical fiber, which is equivalent to a dynamic fiber grating; the middle part of the structure is a common single-mode optical fiber, which cannot form the saturable absorber effect, and therefore no dynamic grating appears, but the phase shift is introduced in the propagation process of the light wave in the middle part, and the standing wave fringes are changed at the middle part, so that the filter is equivalent to a phase-shift fiber grating.

3. A self-adapting narrow bandwidth phase-shifted fiber grating filter based on saturable absorber technology as claimed in claim 2, wherein, The reflectivity of the weak reflection grating is 0.01%.

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