Dual-wavelength fiber laser based on pre-stressed birefringent fiber grating

Abstract

The invention belongs to the technical field of photoelectrons, and particularly relates to a dual-wavelength fiber laser based on a pre-stress birefringent fiber grating. The invention aims to solve the problems of high manufacturing difficulty, low yield and low wavelength stability in the prior art. According to the dual-wavelength optical fiber laser, a first wavelength division multiplexer, an erbium-doped optical fiber and a second wavelength division multiplexer are sequentially arranged on a pumping light path generated by a pumping source, and an optical fiber isolator, an optical fiber circulator and an optical fiber coupler are sequentially connected to an output path of the second wavelength division multiplexer. A port 2 of the optical fiber circulator is sequentially connected with a uniform Bragg fiber grating and a pre-stressed birefringence fiber grating which have the same output power; and a port 3 of the optical fiber circulator is connected with an optical fiber coupler. The intensity of the two longitudinal modes for oscillation starting is basically consistent, good coherence exists between the two longitudinal modes, the wavelength stability is high, meanwhile, the structure is simple, the manufacturing difficulty is low, and the high yield is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic technology, and in particular relates to a dual-wavelength fiber laser based on a pre-stressed birefringent fiber grating. Background Art

[0002] In the patent document with application number "202210169419.4", a uniform grating or uniform sampled grating dual-wavelength fiber laser with two adjustable phase shifts is disclosed. In the adopted scheme: the production process of phase-shifted fiber grating or sampled fiber grating is complicated and expensive, and the manufacturing, use and maintenance costs are high; and the original grating structure will be destroyed during the fiber grating stretching process, causing longitudinal mode competition misalignment, resulting in the failure of dual-wavelength laser. The spectral image of the dual-wavelength laser is not published in this document, so there is a possibility that it cannot be implemented.

[0003] The patent document with application number "202410691550.6" discloses a high spectral purity dual-wavelength fiber laser system. In the scheme adopted: the two wavelength lasers are generated by two resonant cavities respectively, and there is no good coherence between the two wavelengths; and the two resonant cavities that realize the dual wavelengths are both linear standing wave cavities, which means that there will be a spatial hole burning effect, resulting in multi-longitudinal mode oscillation, affecting the wavelength stability of the laser.

[0004] In the patent document with application number "202011377817.2", a dual-wavelength fiber laser based on a seed source is disclosed. In the scheme adopted: the dual-wavelength laser is mainly based on a multi-mode seed source and a chirped fiber grating. The chirped fiber grating is mainly used for dispersion compensation and cannot effectively control the longitudinal mode competition, affecting the stability of the generated dual wavelengths; in addition, there is no optical resonant cavity in this scheme, that is, it is not a source laser, but only the longitudinal mode selection and power amplification of the existing multi-mode laser. The characteristics of the dual-wavelength laser depend on the seed source, and its parameters are uncontrollable.

[0005] In summary, the existing dual-wavelength fiber laser solutions have the following problems: 1. Special fiber gratings with complex manufacturing processes are required, which have the problems of high manufacturing difficulty and low yield rate; 2. Dual-wavelength lasers are realized by using two independent resonant cavities. There is no coherence between the two wavelengths of lasers, resulting in multi-longitudinal mode oscillation, which will affect the wavelength stability of the laser. Summary of the invention

[0006] The present invention provides a dual-wavelength fiber laser based on a pre-stressed birefringent fiber grating to solve the problems of high manufacturing difficulty, low yield and low wavelength stability in the prior art.

[0007] In order to achieve the above-mentioned object, the technical scheme of the present invention is: a dual-wavelength fiber laser based on prestressed birefringent fiber grating, comprising a first wavelength division multiplexer, an erbium-doped fiber and a second wavelength division multiplexer arranged in sequence on the pump light path generated by a pump source, a fiber isolator, a fiber circulator and a fiber coupler connected in sequence on the 1550nm output path of the second wavelength division multiplexer, a uniform Bragg fiber grating and a prestressed birefringent fiber grating with the same output power are connected in sequence to the two ports of the fiber circulator; and a fiber coupler is connected to the three ports of the fiber circulator.

[0008] Furthermore, the prestressed birefringent fiber grating is obtained by bending a uniform Bragg fiber grating.

[0009] Compared with the prior art, the advantages of the present invention are:

[0010] (1) The present invention uses two fiber Bragg gratings as components for generating dual-wavelength lasers, one of which is a uniform fiber Bragg grating. This type of fiber Bragg grating has mature technology and is cheap, which can effectively reduce the cost of manufacturing, use and maintenance; the other is also a pre-stressed birefringent fiber Bragg grating made based on a uniform fiber Bragg grating. Its essence is to bend the uniform fiber Bragg grating. Bending will generate stress inside the fiber Bragg grating, thereby inducing stress birefringence. In this technical solution, the fiber Bragg grating is wrapped by a heat shrink tube, and after heat shrinkage, it is bent to introduce pre-applied stress birefringence. This birefringence effect will change the competition state of the longitudinal mode in the ring cavity, thereby realizing the dual-wavelength operation of the fiber laser, which is the core component of the present invention. At the same time, the patent solution of the present invention abandons the polarization controller, and adopts the method of applying a heat shrink tube to the fiber Bragg grating and bending to introduce stress birefringence to change the competition conditions of the longitudinal mode in the resonant cavity. Compared with the solution using a polarization controller, bending the fiber Bragg grating is more likely to induce stress birefringence, which is much more sensitive than adding a polarization controller to the optical fiber loop, and is more likely to produce dual-wavelength lasers, and has high wavelength stability.

[0011] (2) Adopt unpumped erbium-doped fiber to adaptively filter the two wavelengths respectively, without designing filters for each wavelength separately, and the narrowband filter formed by saturation absorption effect automatically retains the strongest longitudinal mode, and other weak longitudinal modes are filtered out. The structure is simple and easy to implement.

[0012] (3) Since the resonant cavity of the ring cavity fiber laser is long, multiple longitudinal modes will be formed. Two uniform Bragg gratings and unpumped erbium-doped fiber are used as saturated absorbers to perform adaptive filtering on the two wavelengths. The adaptive filtering will produce an ultra-narrowband filtering effect on each wavelength, which can filter out most longitudinal modes and ensure that only one longitudinal mode is started in each channel, and finally only two longitudinal modes are retained. Then, stress birefringence is introduced by bending a uniform fiber grating to change the competition conditions of the longitudinal modes in the cavity, so that the intensity of the two longitudinal modes that are started is basically the same, and there is good coherence between the two longitudinal modes. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is the principle diagram of the dual-wavelength fiber laser of the present invention;

[0014] Figure 2 This is the output spectrum diagram of the dual-wavelength fiber laser of the present invention.

[0015] The figures are marked as follows: 1-pump source, 2-first wavelength division multiplexer, 3-erbium-doped fiber, 4-second wavelength division multiplexer, 5-fiber isolator, 6-fiber circulator, 7-unpumped erbium-doped fiber, 8-uniform fiber Bragg grating, 9-prestressed birefringent fiber grating, 10-fiber coupler. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific implementation methods.

[0017] See also Figure 1 The present invention provides a dual-wavelength fiber laser based on prestressed birefringent fiber grating, comprising a first wavelength division multiplexer, an erbium-doped fiber and a second wavelength division multiplexer arranged in sequence on the pump light path generated by a pump source, the second wavelength division multiplexer has two output paths, the 980nm output path is not connected to a device, and the residual pump light is directly discharged from the resonant cavity through the path, and the 1550nm output path is connected in sequence to a fiber isolator, a fiber circulator and a fiber coupler, and the two ports of the fiber circulator are connected in sequence to a uniform Bragg fiber grating and a prestressed birefringent fiber grating with the same output power; the three ports of the fiber circulator are connected to a fiber coupler. .

[0018] Specifically, a first wavelength division multiplexer 2, an erbium-doped fiber 3 and a second wavelength division multiplexer 4 are sequentially arranged on the pump light path generated by a 980nm pump source 1 of the fiber laser. The pump light reaches the erbium-doped fiber 3 through the first wavelength division multiplexer 2. The erbium-doped fiber 3 absorbs the 980nm pump light and generates a population inversion. The stimulated radiation generates 1550nm light. The 1550nm stimulated radiation light passes through the second wavelength division multiplexer 4. The 1550nm output path of the second wavelength division multiplexer 4 is sequentially connected with a fiber isolator 5, a fiber circulator 6 and a fiber coupler 10. The excess 980nm pump light leaves the resonant cavity, and the 1550nm light continues to enter the fiber isolator 5. The fiber isolator 5 ensures that the light beam runs unidirectionally in the resonant cavity to avoid spatial hole burning. After passing through the optical fiber isolator 5, the 1550nm light reaches the optical fiber circulator 6. The two ports of the optical fiber circulator 6 are connected in sequence with a uniform fiber Bragg grating 8 and a prestressed birefringent fiber grating 9. The prestressed birefringent fiber grating 9 is wrapped by a heat shrink tube and bent after heat shrinkage to introduce pre-stressed birefringence, which can achieve the stability of the wavelength of the dual-wavelength optical fiber laser. The output powers of the uniform fiber Bragg grating 8 and the prestressed birefringent fiber grating 9 are basically the same, which are the core components of the present invention. 1550nm light enters from port 1 of the fiber circulator 6 and outputs from port 2, and reaches the first wavelength selection device - uniform fiber Bragg grating 8 through the unpumped erbium-doped fiber 7. At this time, the first wavelength 1550nm light is reflected back to port 2 of the circulator through the uniform fiber grating 8, and the remaining 1550nm light continues to move forward to reach the pre-stressed birefringent fiber grating 9, which is also the second wavelength selection device. It is made of a uniform fiber grating wrapped in a heat shrink tube, heated and bent, which will introduce stress birefringence, thereby changing the polarization mode loss between different longitudinal modes, so that the longitudinal mode competition conditions in the resonant cavity change. Since the fiber Bragg grating 9 is wrapped by a heat shrink tube and tightly attached to the steel column in the heat shrink tube, the steel column is bent by applying stress conventionally, and the bending amount of the fiber Bragg grating 9 is the same as the bending amount of the steel column in the heat shrink tube. The output spectrum of the laser is observed by a spectrometer. As the bending amount changes, two laser wavelengths with basically the same optical power will appear on the spectrometer. At this time, the appropriate bending amount is reached, so that only two longitudinal modes of oscillation appear inside the ring resonator. Here, the second wavelength of 1550nm light is also reflected back to the circulator 2 port. The two wavelengths of 1550nm light will appear a standing wave field inside the unpumped erbium-doped fiber 7, thereby inducing a saturated absorption effect to adaptively filter the two wavelengths, ensuring that only one longitudinal mode of each path is oscillated. The three ports of the optical fiber circulator 6 are connected to the optical fiber coupler 10. Light waves of two wavelengths are output from the three ports of the optical fiber circulator 6 and reach the optical fiber coupler 10. A part of the light is coupled out of the resonant cavity as the output of the optical fiber laser, and the other part of the light is transmitted to the first wavelength division multiplexer 2 as the seed light, and continues to operate unidirectionally in the resonant cavity to form laser light by stimulated radiation.

[0019] The preferred components used in the present invention are as follows:

[0020] The central wavelength of pump source 1 is 974.7nm, and the output optical power is 200mW;

[0021] The first wavelength division multiplexer is in the 980 / 1550nm band, with a 980nm isolation of 22.92dB, a 1550nm isolation of 24.96dB, and a return loss greater than 55dB;

[0022] The length of the erbium-doped fiber 3 is 4.5 m, the absorption rate of the pump light at 980 nm is 15.8 dB / m, the numerical aperture is 0.15, the mode field diameter at 1550 nm is 8.8 um, and the cladding diameter is 125 um;

[0023] The second wavelength division multiplexer 4 is in the 980 / 1550nm band, with a 980nm isolation of 22.68dB, a 1550nm isolation of 24.57dB, and a return loss greater than 55dB;

[0024] The optical fiber isolator 5 has a central wavelength of 1550nm, a bandwidth of ±15nm, an isolation of 28dB, and a return loss greater than 50dB;

[0025] The three-port fiber circulator 6 has a center wavelength of 1550nm, a bandwidth of ±30nm, an insertion loss of 0.79dB between port 1 and port 2, an insertion loss of 0.91dB between port 2 and port 3, an isolation of 54dB from port 2 to port 1, and an isolation of 51dB from port 3 to port 2, a return loss greater than 55dB, and a maximum continuous optical power less than 500mW;

[0026] The unpumped erbium-doped fiber 7 has a length of 0.24 m, an optical absorption rate of 6 dB / m at 1530 nm, a numerical aperture of 0.23, a mode field diameter of 5.8 um at 1550 nm, and a cladding diameter of 125 um;

[0027] Uniform fiber Bragg grating 8, grating length 15mm, central reflection wavelength 1564.2nm, reflectivity greater than 99%, 3dB bandwidth less than 0.2nm, side mode suppression ratio greater than 20dB;

[0028] Prestressed birefringent fiber Bragg grating 9, grating area length 15mm, central reflection wavelength 1564.58nm, reflectivity greater than 99%, 3dB bandwidth less than 0.2nm, side mode suppression ratio greater than 20dB, heat shrink tube length 25mm, bending angle 124°;

[0029] The optical fiber coupler 10 has a central passing wavelength of 1550 nm, a splitting ratio of 20:80, an insertion loss of 1.19 dB, and a return loss greater than 55 dB.

[0030] The dual-wavelength fiber laser based on the above preferred components can be used in the fields of microwave photonics, terahertz generation, optical precision measurement and spectroscopy, see Figure 2 Through the spectrometer, two very obvious starting longitudinal modes can be seen, with central wavelengths of 1564.143nm and 1564.518nm respectively, and the optical power of the two is also the same.

[0031] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A dual-wavelength fiber laser based on prestressed birefringent fiber grating, characterized in that: The invention comprises a pump light path generated by a pump source (1), on which a first wavelength division multiplexer (2), an erbium-doped optical fiber (3) and a second wavelength division multiplexer (4) are arranged in sequence; a 1550 nm output path of the second wavelength division multiplexer (4) is connected in sequence to an optical fiber isolator (5), an optical fiber circulator (6) and an optical fiber coupler (10); two ports of the optical fiber circulator (6) are connected in sequence to a uniform optical fiber Bragg grating (8) with the same output power and a prestressed birefringent optical fiber grating (9); and a third port of the optical fiber circulator (6) is connected to an optical fiber coupler (10).

2. A dual-wavelength fiber laser based on prestressed birefringent fiber grating according to claim 1, characterized in that: The prestressed birefringent optical fiber grating (9) is obtained by bending a uniform optical fiber Bragg grating.

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