All-fiber pulsed annular spot output mode-locked Raman laser and output method thereof

Abstract

The present invention discloses an all-fiber pulsed annular spot output mode-locked Raman laser and its output method. The laser includes: a pump source, a first wavelength division multiplexer, a highly nonlinear optical fiber, a quasi-saturable absorber, a mode selective coupler, and a first polarization controller. The pump source is connected to the first wavelength division multiplexer; the first wavelength division multiplexer is connected to the highly nonlinear optical fiber, which is connected to the quasi-saturable absorber; the quasi-saturable absorber is connected to the mode selective coupler, which is connected to the first wavelength division multiplexer and the first polarization controller, respectively, for converting the fundamental mode into a high-order mode and outputting it to the first polarization controller; the first polarization controller is used to control the high-order mode output by the mode selective coupler into an annular spot and output it. The all-fiber annular spot output mode-locked Raman laser provided by the present invention is not only low in cost and loss, but also has low thermal effect, thereby obtaining a pulsed annular spot with high slope efficiency and high energy.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular to an all-fiber annular spot output mode-locked Raman laser and an output method thereof. Background Art

[0002] In recent years, cylindrical vector beams and vortex beams with annular spot shapes have attracted widespread attention due to their unique spatial intensity and polarization distribution. They play an important role in various fields and have broad application prospects in material processing, optical tweezers, fiber-optic communications, and data storage.

[0003] There are many ways to obtain an annular spot, which can be roughly divided into two categories: those based on space devices and those based on fiber devices. The first is to use free-space components such as birefringent crystals, variable spiral plates, spiral phase plates, subwavelength gratings, and spatial light modulators to precisely control the phase and polarization state of light to obtain an annular spot beam. However, free-space components are difficult to integrate and have high production costs. All-fiber systems have better flexibility, stability, and compactness, so methods based on fiber-optic output of annular spots have further emerged, such as long-period gratings based on few-mode fibers and offset splicing technology to output annular spots. However, these two methods have the disadvantages of high loss and poor beam quality.

[0004] Therefore, the existing technology still needs to be improved and developed. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide an all-fiber annular spot output mode-locked Raman laser and an output method thereof, so as to solve the problems of high cost, large loss and poor beam quality in the existing methods of obtaining annular spot.

[0006] The technical solutions of the present invention are as follows:

[0007] An all-fiber pulsed annular spot output mode-locked Raman laser comprises: a pump source, a first wavelength division multiplexer, a highly nonlinear optical fiber, a quasi-saturable absorber, a mode selective coupler, and a first polarization controller; the pump source, the first wavelength division multiplexer, the highly nonlinear optical fiber, the quasi-saturable absorber, and the mode selective coupler are connected to form a Raman ring cavity; wherein,

[0008] The output end of the pump source is connected to the short wavelength end of the first wavelength division multiplexer;

[0009] The common end of the first wavelength division multiplexer is connected to the input end of the highly nonlinear optical fiber, and the output end of the highly nonlinear optical fiber is connected to the input end of the saturable absorber-like body;

[0010] The output end of the quasi-saturable absorber is connected to the single-mode input end of the mode selective coupler, the output end of the mode selective coupler is connected to the long wavelength end of the first wavelength division multiplexer, the few-mode fiber end of the mode selective coupler is connected to the first polarization controller, and the mode selective coupler is used to convert the fundamental mode into a high-order mode and output it to the first polarization controller; the first polarization controller is connected to the few-mode fiber end of the mode coupling selector, and the first polarizer is used to control the high-order mode output by the mode selective coupler into a ring-shaped light spot and output it.

[0011] According to a further configuration of the present invention, the quasi-saturable absorber comprises: a second polarization controller, a polarization-dependent isolator and a third polarization controller; wherein,

[0012] The input end of the second polarization controller is connected to the output end of the highly nonlinear optical fiber, and the input end of the second polarization controller is connected to the input end of the polarization-dependent isolator;

[0013] The output end of the polarization-dependent isolator is connected to the input end of the third polarization controller, and the output end of the third polarization controller is connected to the single-mode input end of the mode selective coupler;

[0014] The first polarization controller, the polarization-dependent isolator, and the third polarization controller form a quasi-saturable absorber for obtaining a mode-locked pulse.

[0015] According to a further configuration of the present invention, the all-fiber annular spot output mode-locked Raman laser further comprises: a fourth polarization controller;

[0016] The input end of the fourth polarization controller is connected to the output end of the mode selective coupler, and the output end of the third polarization controller is connected to the long wavelength end of the first wavelength division multiplexer.

[0017] A further arrangement of the present invention further includes: a second wavelength division multiplexer;

[0018] The common end of the second wavelength division multiplexer is connected to the output end of the highly nonlinear optical fiber, the long wavelength end of the second wavelength division multiplexer is connected to the input end of the second polarization controller, and the short wavelength end of the second wavelength division multiplexer is connected to the outside of the Raman ring cavity.

[0019] According to a further configuration of the present invention, the mode selective coupler is made of a single-mode optical fiber and a few-mode optical fiber by using a fused taper method; the mode selective coupler can convert the input fundamental mode into a high-order mode and output it in the few-mode optical fiber, and the fundamental mode is output from the single-mode optical fiber.

[0020] According to a further configuration of the present invention, the bandwidth of the mode selective coupler is 100-150 nm.

[0021] According to a further configuration of the present invention, the operating wavelength of the highly nonlinear optical fiber is located in a near-zero anomalous dispersion region, and the entire laser operates in the near-zero anomalous dispersion region.

[0022] Based on the same inventive concept, the present invention also provides an all-fiber pulsed annular spot output method for the above-mentioned all-fiber high-pulse annular spot output mode-locked Raman laser, which comprises:

[0023] The pump light output by the pump source is injected into the highly nonlinear optical fiber through the first wavelength division multiplexer to generate Raman light;

[0024] The Raman light generated by the highly nonlinear optical fiber forms a mode-locked pulse after passing through a quasi-saturable absorber and is output to a mode selective coupler;

[0025] The mode selective coupler receives the mode-locked pulse output by the quasi-saturable absorber and converts the fundamental mode-locked pulse into a high-order mode-locked pulse and outputs the high-order mode-locked pulse to the first polarization controller;

[0026] According to a further configuration of the present invention, the step of the first polarization controller controlling the received high-order mode-locked pulse to be an annular light spot and outputting the same comprises:

[0027] The mode field of the high-order mode-locked pulse is adjusted to output a cylindrical vector beam and a vortex beam in the shape of an annular spot.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The present invention adopts an all-fiber coupling method to connect various optical fiber devices. The entire device has a compact structure, low cost, easy integration, and high energy output.

[0030] (2) The present invention uses a mode selective coupler as a mode converter to convert the intracavity fundamental mode LP 01 Convert to higher order mode LP 11 , small insertion loss, high mode purity, high efficiency, and broadband mode conversion characteristics.

[0031] (3) The present invention is based on stimulated Raman scattering and uses a highly nonlinear optical fiber with near-zero dispersion as a gain medium. It is a nonlinear conversion system with a femtosecond response that can output any wavelength (with a suitable pump light source) and has high energy and high slope efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary personnel in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0033] Figure 1 It is a structural schematic diagram of the all-fiber pulsed annular spot output mode-locked Raman laser in the present invention.

[0034] Figure 2 It is a structural diagram of the mode selective coupler in the present invention.

[0035] Figure 3 This is a graph showing the variation of the effective refractive index of the mode at 1583nm for single-mode fiber and few-mode fiber with the core diameter.

[0036] Figure 4 The LP output at different wavelengths from the few-mode output end of the mode selective coupler is measured by CCD. 11 Mode field distribution diagram.

[0037] Figure 5 This is the output spectrum at 1250mW pump power.

[0038] Figure 6 This is an oscilloscope diagram of a multi-pulse sequence at a pump power of 1250 mW.

[0039] Figure 7 This is an oscilloscope single pulse sequence diagram at 1250mW pump power.

[0040] Figure 8 It is a radio frequency spectrum analysis diagram.

[0041] Figure 9 is a graph showing the output power and pulse energy changing with pump power.

[0042] Figure 10 This is the mode field distribution diagram of the annular spot at a pump power of 2635mW.

[0043] Figure 11 It is a flow chart of the all-fiber pulsed annular spot output method.

[0044] 100, pump source; 200, first wavelength division multiplexer; 300, highly nonlinear optical fiber; 400, quasi-saturable absorber; 401, second polarization controller; 402, polarization-dependent isolator; 403, third polarization controller; 500, mode selective coupler; 600, fourth polarization controller; 700, second wavelength division multiplexer; 800, first polarization controller. DETAILED DESCRIPTION

[0045] The present invention provides an all-fiber pulsed annular spot output mode-locked Raman laser and its output method. To clarify the objectives, technical solutions, and effects of the present invention, the present invention is further described below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention.

[0046] In the embodiments and patent claims, unless otherwise specified herein, the words "a," "an," "the," and "the" may include plural forms. If the embodiments of the present invention include descriptions of "first," "second," etc., such descriptions are for descriptive purposes only and should not be understood as indicating or implying their relative importance or implicitly specifying the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include at least one of such features.

[0047] It should be further understood that the term "comprising" as used in the description of the present invention refers to the presence of the stated features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when we refer to an element as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or there can be intermediate elements. In addition, "connected" or "coupled" as used herein can include wireless connections or wireless couplings. The term "and / or" as used herein includes all or any units and all combinations of one or more associated listed items.

[0048] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0049] In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the fact that ordinary technicians in this field can implement them. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0050] The inventors discovered that all-fiber systems offer improved flexibility, stability, and compactness. Consequently, fiber-based methods for producing annular beams have emerged, such as Bragg gratings based on few-mode fibers and offset splicing techniques. However, these methods suffer from high loss and poor beam quality. Subsequently, long-period fibers emerged. Although they reduced loss, their inherent narrow operating bandwidth limited their further application. All-fiber pulsed lasers offer advantages such as compact structure, high beam quality, and low cost. High-energy pulsed annular beams have important applications in fields such as material processing and ion acceleration. Generating high-energy pulsed annular beams using an all-fiber approach holds great promise. However, current all-fiber pulsed lasers that produce annular beams mostly use short rare-earth-doped ion fibers as the gain medium. Short gain fibers can cause thermal effects, resulting in low pulse energy and low laser slope efficiency within the resonant cavity. Furthermore, most current high-energy pulsed annular beams are typically achieved by cascading multiple amplifier stages after a pulse seed source, which significantly increases system complexity.

[0051] In response to the above technical problems, the present invention provides an all-fiber pulsed annular spot output mode-locked Raman laser and an output method thereof. The laser includes: a pump source, a first wavelength division multiplexer, a highly nonlinear optical fiber, a quasi-saturable absorber, a mode selective coupler, and a first polarization controller; wherein the output end of the pump source is connected to the short wavelength end of the first wavelength division multiplexer; the common end of the first wavelength division multiplexer is connected to the input end of the highly nonlinear optical fiber, and the output end of the highly nonlinear optical fiber is connected to the input end of the quasi-saturable absorber; the output end of the quasi-saturable absorber is connected to the single-mode input end of the mode selective coupler, the output end of the mode selective coupler is connected to the long wavelength end of the first wavelength division multiplexer, the few-mode fiber end of the mode selective coupler is connected to the first polarization controller, the mode selective coupler is used to convert the fundamental mode into a high-order mode and output it to the first polarization controller; the first polarization controller is connected to the few-mode fiber end of the mode coupler selector, and the first polarizer is used to control the high-order mode output by the mode selective coupler into an annular spot and output it. The pump source, first wavelength division multiplexer, highly nonlinear optical fiber, quasi-saturable absorber, and mode-selective coupler in the present invention are connected to form a Raman ring cavity. This structure is compact, easy to integrate, has high beam quality, and is low in cost. The use of highly nonlinear optical fiber as the gain medium effectively reduces thermal effects. Furthermore, the use of a mode-selective converter offers the advantages of low insertion loss, wide operating bandwidth, high mode purity, and high efficiency. Therefore, the all-fiber annular spot output mode-locked Raman laser provided by the present invention is not only low in cost and loss, but also has low thermal effects, thereby producing a high-slope-efficiency, high-energy pulsed annular spot.

[0052] Please also see Figures 1 to 10 The present invention provides a preferred embodiment of an all-fiber pulsed annular spot output mode-locked Raman laser.

[0053] like Figure 1 As shown, the present invention provides an all-fiber pulsed annular spot output mode-locked Raman laser, which includes: a pump source 100, a first wavelength division multiplexer 200, a highly nonlinear optical fiber 300, a quasi-saturable absorber 400, a mode selective coupler 500 and a first polarization controller 800; the pump source 100, the first wavelength division multiplexer 200, the highly nonlinear optical fiber 300, the quasi-saturable absorber 400 and the mode selective coupler 500 are connected to form a Raman ring cavity; wherein the output end of the pump source 100 is connected to the short wavelength end of the first wavelength division multiplexer 200; the common end of the first wavelength division multiplexer 200 is connected to the input end of the highly nonlinear optical fiber 300 The output end of the highly nonlinear optical fiber 300 is connected to the input end of the quasi-saturable absorber 400; the output end of the quasi-saturable absorber 400 is connected to the single-mode input end of the mode selective coupler 500, the output end of the mode selective coupler 500 is connected to the long wavelength end of the first wavelength division multiplexer 200, and the mode selective coupler is used to convert the fundamental mode into a high-order mode and output it to the first polarization controller 800; the first polarization controller 800 is connected to the few-mode optical fiber end of the mode coupling selector 500, and the first polarization controller 800 is used to control the high-order mode output by the mode selective coupler 500 into a ring-shaped light spot and output it.

[0054] Specifically, the pump light output by the pump source 100 is injected into the highly nonlinear optical fiber 300 through the first wavelength division multiplexer 200. When the pump light power exceeds the Raman effect threshold, stimulated Raman scattering will occur and be converted by the highly nonlinear optical fiber 300 to generate Raman light. The Raman light generated by the highly nonlinear optical fiber 300 forms a mode-locked pulse after passing through the quasi-saturable absorber 400 and is output to the mode selective coupler 500. The mode selective coupler 500 receives the mode-locked pulse output by the quasi-saturable absorber 400 and converts the fundamental mode mode-locked pulse into a high-order mode mode-locked pulse and outputs it to the first polarization controller 800. The high-order mode-locked pulse is then regulated by the first polarization controller 800 (by adjusting the polarization state) to obtain a high-energy pulsed annular spot-shaped cylindrical vector beam and a vortex beam. In addition, by finely controlling the mode field of the high-order mode, radially polarized light or angularly polarized light in the annular spot-shaped cylindrical vector beam can also be output. The pump source 100 can be a 1480nm single-mode laser. The pump source 100, the first wavelength division multiplexer 200, the highly nonlinear optical fiber 300, the quasi-saturable absorber 400, the mode selective coupler 500, and the first polarization controller 800 are connected using conventional optical fiber coupling to form a Raman ring cavity. The entire device is compact, easy to integrate, has high beam quality, is relatively low in cost, is not easily affected by the external environment, and has high operating stability. Furthermore, using the highly nonlinear optical fiber 300 as the gain medium can effectively reduce thermal effects, suppress pulse splitting to a certain extent, and achieve high pulse energy, thereby obtaining a high slope efficiency and high-energy annular light spot. Furthermore, the mode selective converter used in the present invention has the advantages of low insertion loss and wide operating bandwidth. Therefore, the all-fiber annular light spot output mode-locked Raman laser provided by the present invention is not only low in cost and loss, but also has low thermal effects, thereby achieving a high slope efficiency and high-energy annular light spot.

[0055] See also Figure 1 In a further implementation of an embodiment, the quasi-saturable absorber 400 includes: a second polarization controller 401, a polarization-dependent isolator 402 and a third polarization controller 403; wherein, the input end of the second polarization controller 401 is connected to the output end of the highly nonlinear optical fiber 300, and the input end of the second polarization controller 401 is connected to the input end of the polarization-dependent isolator 402; the output end of the polarization-dependent isolator 402 is connected to the input end of the third polarization controller 403, and the output end of the third polarization controller 403 is connected to the single-mode input end of the mode selective coupler 500; the first polarization controller 401, the polarization-dependent isolator 402 and the third polarization controller 403 constitute a quasi-saturable absorber for obtaining a mode-locked pulse.

[0056] Specifically, the second polarization controller 401, the polarization-dependent isolator 402 and the third polarization controller 403 constitute a quasi-saturable absorber based on the principle of nonlinear polarization rotation. Stable mode-locked pulses can be obtained by adjusting the second polarization controller 401 and the third polarization controller 403.

[0057] See also Figure 1 In a further implementation of an embodiment, the all-fiber annular spot output mode-locked Raman laser also includes: a fourth polarization controller 600; the input end of the fourth polarization controller 600 is connected to the output end of the mode selective coupler 500, and the output end of the fourth polarization controller 600 is connected to the long wavelength end of the first wavelength division multiplexer 200. The fourth polarization controller 600 can improve the tuning accuracy and expand the tuning range of the light polarization state.

[0058] See also Figure 1 In a further implementation of an embodiment, the all-fiber ring spot output mode-locked Raman laser also includes: a second wavelength division multiplexer 700; the common end of the second wavelength division multiplexer 700 is connected to the output end of the highly nonlinear optical fiber 300, the long wavelength end of the second wavelength division multiplexer 700 is connected to the input end of the second polarization controller 401, and the short wavelength end of the second wavelength division multiplexer 700 is connected to the outside of the Raman ring cavity.

[0059] Specifically, the common end of the second wavelength division multiplexer 700 is connected to the output end of the highly nonlinear optical fiber 300, and the short wavelength end of the second wavelength division multiplexer 700 is connected to the outside of the Raman ring cavity, which can output the pump light that is not completely converted to the outside of the cavity to protect the optical fiber components in the cavity.

[0060] See also Figure 1 and Figure 2 In some embodiments, the mode selective coupler 500 is made of a single-mode optical fiber and a few-mode optical fiber using a fused taper method; the mode selective coupler 500 can convert the input fundamental mode into a high-order mode and output it in the few-mode optical fiber, and the fundamental mode is output from the single-mode optical fiber.

[0061] Specifically, the mode selective coupler 500 is made of single-mode fiber and few-mode fiber by fusion taper. The appropriate taper parameters can make the fundamental mode LP in the fiber 01 Convert to high-order LP 11 , and outputs the signal at the few-mode fiber end while outputting the fundamental mode at the single-mode fiber end, thus having both mode conversion and mode separation functions. Furthermore, the mode selective coupler 500 has broadband mode conversion characteristics with a wide bandwidth range of up to 100-150nm.

[0062] According to mode coupling theory, if the fundamental mode LP in a single-mode fiber 01 The propagation constant is equal to the high-order mode LP in the few-mode fiber 11 The propagation constant of , then the phase matching condition will be met, and the two modes will perform periodic power exchange, realizing the LP 01 To LP 11 Mode conversion. The propagation constant β can be expressed by the formula β=k0n eff to calculate, where k0 is the propagation constant in vacuum, n eff is the effective refractive index of the mode, that is, if the effective refractive index is equal, the propagation constant is equal. The effective refractive index of the mode is related to the fiber core radius. According to the finite element method, the simulation software COMSOL is used to simulate and calculate the effective refractive index of single-mode fiber and few-mode fiber at different core radii at 1583nm, as shown in the following figure: Figure 3 According to the simulation results, a suitable mode selective coupler 500 was made and the output LP was measured using CCD. 11 Mode field distribution diagrams at different wavelengths, such as Figure 4 As shown. After testing, the LP output from the few-mode fiber end 11 In the 60nm Raman operating wavelength (1540-1600nm), the mode purity exceeds 90%. In particular, at the Raman center wavelength of 1583nm, the mode purity is about 95% and the insertion loss is about 0.55dB.

[0063] like Figure 5 As shown, Figure 5 This is the output spectrum of this embodiment at a pump power of 1250 mW, with a central wavelength of 1587.93 nm and a 3 dB bandwidth of 14.59 nm. Figure 6 and Figure 7 This is the oscilloscope output waveform of this embodiment under a pump power of 1250 mW. The basic pulse repetition frequency is 319.3 kHz, and the single pulse width is 374.6 ns. Figure 8 : is a radio frequency spectrum analysis diagram, from which it can be seen that at the fundamental frequency of 319.3 kHz, the signal-to-noise ratio is as high as 64.6 dB, which illustrates the high stability of the annular light spot obtained in this embodiment. Figure 9 This graph plots output power and pulse energy versus pump power. The slope efficiency reaches 20.3%. At a maximum pump power of 2635mW, the average output power reaches 342mW, corresponding to a pulse energy of 1.1J, the highest energy currently achieved directly from a laser cavity. This indicates that the all-fiber ring-pulse shaped spot output mode-locked Raman laser provided by this invention can produce a high-slope-efficiency, high-energy ring spot.

[0064] Figure 10The mode field distribution diagram of the annular light spot at a pump power of 2635mW is obtained by CCD detection. By adjusting the first polarization controller 800, different types of annular light spot outputs can be obtained. Figure 10 (a) is azimuthally polarized light. After inserting the polarizer, the outline of the lobe is always perpendicular to the transmission axis of the polarizer, as shown in Figure 10 (a1)-(a4); Figure 10 (b) is radially polarized light, and its profile is always parallel to the transmission axis of the polarizer, such as Figure 10 In (b1)-(b4), the pattern purities were 95.5% and 95.6%, respectively. Figure 10 (d) and Figure 10 (f) is the vortex beam, Figure 10 (c) shows the cylindrical lens phase image used to test the topological charge of the vortex light. The corresponding images are (d1) and (f1), with topological charges of +1 and -1, respectively. This indicates that by adjusting the first polarization controller 800 and finely controlling the mode field of the higher-order modes, it is possible to output high-mode-purity radially polarized light or angularly polarized light, as well as a vortex beam, in an annular spot-shaped cylindrical vector beam.

[0065] like Figure 11 As shown, in some embodiments, the present invention further provides an all-fiber pulsed annular spot output method for the above-mentioned all-fiber pulsed annular spot output mode-locked Raman laser, which comprises the steps of:

[0066] S100, the pump light output by the pump source is injected into the highly nonlinear optical fiber through the first wavelength division multiplexer to generate Raman light; the details are as described in an embodiment of an all-fiber annular spot output mode-locked Raman laser, which will not be repeated here.

[0067] S200, the Raman light generated by the highly nonlinear optical fiber forms a mode-locked pulse after passing through a quasi-saturable absorber and is output to a mode selective coupler; the details are as described in an embodiment of an all-fiber annular spot output mode-locked Raman laser, which will not be repeated here.

[0068] S300, the mode selective coupler receives the mode-locked pulse output by the quasi-saturable absorber and converts the fundamental mode mode-locked pulse into a high-order mode mode-locked pulse and outputs it to the first polarization controller; the specific details are as described in an embodiment of an all-fiber annular spot output mode-locked Raman laser, which will not be repeated here.

[0069] S400: The first polarization controller controls the received high-order mode-locked pulse to be an annular light spot and outputs the annular light spot.

[0070] In some embodiments, step S400 includes:

[0071] S410, adjusting the mode field of the high-order mode-locked pulse to output an annular spot cylindrical vector beam and a vortex beam. The details are as described in the embodiment of an all-fiber annular spot output mode-locked Raman laser, which will not be repeated here.

[0072] In summary, the all-fiber pulsed annular spot output mode-locked Raman laser and its output method provided by the present invention combine the Raman effect with a mode-selective coupler and perform mode locking with a quasi-saturable absorber composed of a polarization controller and a polarization-dependent isolator. A pulsed annular spot (column vector beam and vortex beam) with high energy, high slope efficiency, and high mode purity is directly output in the resonant cavity, solving the problems of low pulse energy and low efficiency in the output of pulsed annular spots in current mode-locked all-fiber lasers. The laser can be widely used in fields such as material processing, particle acceleration, optical fiber communication, and data storage.

[0073] It should be understood that the application of the present invention is not limited to the above examples. For those skilled in the art, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. An all-fiber pulsed annular spot output mode-locked Raman laser, characterized in that: include: A pump source, a first wavelength division multiplexer, a highly nonlinear optical fiber, a quasi-saturable absorber, a mode selective coupler, and a first polarization controller; the pump source, the first wavelength division multiplexer, the highly nonlinear optical fiber, the quasi-saturable absorber, and the mode selective coupler are connected to form a Raman ring cavity; wherein, The output end of the pump source is connected to the short wavelength end of the first wavelength division multiplexer; The common end of the first wavelength division multiplexer is connected to the input end of the highly nonlinear optical fiber, and the output end of the highly nonlinear optical fiber is connected to the input end of the saturable absorber-like body; The output end of the quasi-saturable absorber is connected to the single-mode input end of the mode selective coupler, the output end of the mode selective coupler is connected to the long wavelength end of the first wavelength division multiplexer, the few-mode fiber end of the mode selective coupler is connected to the first polarization controller, and the mode selective coupler is used to convert the fundamental mode into a high-order mode and output it to the first polarization controller; The first polarization controller is connected to the few-mode fiber end of the mode selective coupler, and is used to control the high-order mode output by the mode selective coupler to be an annular light spot and output it; The pump light output by the pump source is injected into the highly nonlinear optical fiber through the first wavelength division multiplexer. When the pump light power exceeds the Raman effect threshold, stimulated Raman scattering will occur and the pump light will be converted by the highly nonlinear optical fiber to generate Raman light. The Raman light generated by the highly nonlinear optical fiber forms a mode-locked pulse after passing through a quasi-saturable absorber and is output to a mode selective coupler. The mode selective coupler is made of a single-mode optical fiber and a few-mode optical fiber using a fused taper method; the mode selective coupler can convert the input fundamental mode into a high-order mode and output it in the few-mode optical fiber, and the fundamental mode is output from the single-mode optical fiber; the bandwidth of the mode selective coupler is 100-150nm.

2. The all-fiber pulsed annular spot output mode-locked Raman laser according to claim 1, characterized in that: The quasi-saturable absorber includes: a second polarization controller, a polarization-dependent isolator and a third polarization controller; wherein, The input end of the second polarization controller is connected to the output end of the highly nonlinear optical fiber, and the output end of the second polarization controller is connected to the input end of the polarization-dependent isolator; The output end of the polarization-dependent isolator is connected to the input end of the third polarization controller, and the output end of the third polarization controller is connected to the single-mode input end of the mode selective coupler; The second polarization controller, the polarization-dependent isolator, and the third polarization controller form a quasi-saturable absorber for obtaining a mode-locked pulse.

3. The all-fiber pulsed annular spot output mode-locked Raman laser according to claim 2, characterized in that: Also includes: a fourth polarization controller; The input end of the fourth polarization controller is connected to the output end of the mode selective coupler, and the output end of the fourth polarization controller is connected to the long wavelength end of the first wavelength division multiplexer.

4. The all-fiber pulsed annular spot output mode-locked Raman laser according to claim 2, characterized in that: Also includes: Second wavelength division multiplexer; The common end of the second wavelength division multiplexer is connected to the output end of the highly nonlinear optical fiber, the long wavelength end of the second wavelength division multiplexer is connected to the input end of the second polarization controller, and the short wavelength end of the second wavelength division multiplexer is connected to the outside of the Raman ring cavity.

5. The all-fiber pulsed annular spot output mode-locked Raman laser according to claim 1, characterized in that: The working wavelength of the highly nonlinear optical fiber is located in the near-zero anomalous dispersion region, and the entire laser operates in the near-zero anomalous dispersion region.

6. A method for outputting an all-fiber pulsed annular spot applied to the all-fiber pulsed annular spot output mode-locked Raman laser according to any one of claims 1 to 5, characterized in that: include: The pump light output by the pump source is injected into the highly nonlinear optical fiber through the first wavelength division multiplexer to generate Raman light; The Raman light generated by the highly nonlinear optical fiber forms a mode-locked pulse after passing through a quasi-saturable absorber and is output to a mode selective coupler; The mode selective coupler receives the mode-locked pulse output by the quasi-saturable absorber and converts the fundamental mode-locked pulse into a high-order mode-locked pulse and outputs the high-order mode-locked pulse to the first polarization controller; The first polarization controller controls the received high-order mode-locked pulse into a ring-shaped light spot and outputs it.

7. The all-fiber pulsed annular spot output method according to claim 6, characterized in that: The step of the first polarization controller controlling the received high-order mode-locked pulse to be an annular light spot and outputting the light spot comprises: The mode field of the high-order mode-locked pulse is adjusted to output a cylindrical vector beam and a vortex beam in the shape of an annular spot.

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