A multi-wavelength mode-locked fiber laser with adjustable wavelength spacing

Abstract

This invention provides a multi-wavelength mode-locked fiber laser with adjustable wavelength spacing. It comprises a nonlinear amplifying ring consisting of a pump source (01), a wavelength division multiplexer (02), an erbium-doped fiber (07), a 3km single-mode fiber (08), a three-ring polarization controller (041), a coupler (03), and an optical fiber conduit (12) to achieve pulse mode-locking. A mode selection coupler (05) is used to convert from the fundamental mode to higher-order modes. An improved Sagnac ring consisting of a coupler (06), an optical fiber conduit (17), a three-ring polarization controller (042, 043), and polarization-maintaining fibers (09, 10) is used to achieve multi-wavelength and adjustable wavelength spacing. This multi-wavelength mode-locked fiber laser can achieve multi-wavelength pulsed mode-locked laser with adjustable output wavelength spacing and can be widely used in wavelength division multiplexing, optical communication, optical sensing systems, and other fields.

Description

Technical Field

[0001] This invention relates to the fields of optical fiber communication and pulsed laser technology, and in particular to a multi-wavelength mode-locked fiber laser with adjustable wavelength spacing. Background Technology

[0002] Erbium-doped fiber is currently the most mature rare-earth-doped fiber, and erbium-doped fiber lasers and amplifiers based on it have seen great development and widespread application in optical communication systems. Erbium-doped fiber lasers operate in the 1550nm band and have advantages such as low dispersion, low loss, high efficiency, and high beam quality factor, thus being widely used in ultra-long-distance transmission, lidar ranging, intelligent fiber optic sensing, coherent optical communication, and gravitational wave detection.

[0003] Pulse mode-locking and multi-wavelength operation have always been important research directions in fiber lasers. Pulse mode-locking and multi-wavelength operation have significant research value in modes division multiplexing (MDF), wavelength division multiplexing (WDM), optical communication, and optical sensing. Wavelength selection mechanisms and pulse mode-locking are key to designing fiber lasers with multi-wavelength pulse output. For wavelength selection, Mach-Zehnder interferometers, Loyt filters, and a series of improved devices based on Sagnac rings can be used. These wavelength selection devices can achieve adjustable wavenumbers, adjustable wavelength spacing, and tunable wavelengths for multi-wavelength output. Common methods for achieving pulse mode-locking include nonlinear optical loop cavities, nonlinear amplifying loop cavities, semiconductor saturable absorber mirrors, nonlinear multimode interference, nonlinear polarization rotation, and absorbers made of graphene. Nonlinear amplifying loop cavities are all-fiber devices that are easier to adapt to optical fibers and have many advantages such as simple fabrication and low insertion loss, and are often used as key devices for pulse mode-locking.

[0004] Currently, existing multi-wavelength mode-locked fiber lasers suffer from problems such as the inability to achieve high-efficiency performance balance in terms of wavelength tunability, wavenumber tunability, and wavelength spacing tunability, as well as low pulse repetition frequency. Summary of the Invention

[0005] This invention provides a multi-wavelength mode-locked fiber laser with adjustable wavelength spacing to achieve multi-wavelength pulsed mode-locked laser with adjustable output wavelength spacing.

[0006] To achieve the above objectives, the present invention adopts the following technical solution.

[0007] A multi-wavelength mode-locked fiber laser with adjustable wavelength spacing, comprising:

[0008] Pump source (01), wavelength division multiplexer (02), first coupler (03), three-ring polarization controller (041, 042, 043), mode selection coupler (05), second coupler (06), erbium-doped fiber (07), 3km single-mode fiber (08), polarization-maintaining fiber (09), polarization-maintaining fiber (10), first fiber duct (11), second fiber duct (12), third fiber duct (13), fourth fiber duct (14), fifth fiber duct (15), sixth fiber duct (16) and seventh fiber duct (17);

[0009] The pump source (01) is connected to one end of the input of the wavelength division multiplexer (02) through the first optical fiber channel (11). One input of the first coupler (03) is connected to the other input of the wavelength division multiplexer (02). The output of the wavelength division multiplexer (02) is connected to the erbium-doped fiber (07), the 3km single-mode fiber (08), and the three-ring polarization controller (041) through the second optical fiber channel (12), and finally returns to the input of the first coupler (03). The output of the first coupler (03) is connected to the mode selection coupler (041) through the third optical fiber channel (13). The input end of the mode selection coupler (05) is connected to the input end of the mode selection coupler (05). The output end of the mode selection coupler (05) has two output ends. One output end is connected to the fourth optical fiber channel (14) and outputs, and then the mode characteristics are detected by the detection device. The other output end is connected to the input end of the second coupler (06) through the fifth optical fiber channel (15). The two output ends of the second coupler (06) are connected to the three-ring polarization controller (041), polarization-maintaining fiber (09), three-ring polarization controller (043) and polarization-maintaining fiber (10) through the seventh optical fiber channel (17). Finally, the output is output through the sixth optical fiber channel (16) and the multi-wavelength output characteristics are detected by the detection device.

[0010] Preferably, the pump light generated by the pump source (01) enters the optical path through a 980 / 1550nm wavelength division multiplexer (02), and the 2.3m erbium-doped fiber (07) achieves population inversion and generates stimulated emission amplification of light. The polarization controller (041) adjusts the polarization state of the mode in the fiber optic channel, and the 2×1 first coupler (03) with a 5:5 splitting ratio achieves interference and output of two beams with opposite transmission directions. The pump source (01), the wavelength division multiplexer (02), the first coupler (03), the erbium-doped fiber (07), and the polarization controller (041) form a nonlinear amplification cavity for realizing pulse mode-locking operation.

[0011] Preferably, the mode selection coupler (05) is used to realize the conversion from the fundamental mode to the higher-order mode. The converted higher-order mode is transmitted through the fourth optical fiber channel (14) and then output. The detection device performs mode spot detection on the higher-order mode. The other output end of the mode selection coupler continues to be transmitted through the fifth optical fiber channel (15). The improved Sagnac ring is composed of a 2×2 coupler (06) with a splitting ratio of 1:9, the seventh optical fiber channel (17), a three-ring polarization controller (042, 043), a 2.2m polarization-maintaining fiber (09), and a 4.35m polarization-maintaining fiber (10).

[0012] The improved Sagnac ring changes the polarization state of the light transmitted in the three-ring polarization control (042, 043) by adjusting the three-ring polarization control (042, 043), thereby changing the gain and loss under different wavelength competition. When dynamic equilibrium is reached and a certain wavelength obtains sufficient gain, the light is finally output in the sixth fiber optic channel (16). The light output by the improved Sagnac ring is then subjected to multi-wavelength characteristic detection by the detection equipment.

[0013] Preferably, the output spectrum of the improved Sagnac ring includes wavelength intervals of 0.5 nm and 1.5 nm. By adjusting the three-ring polarization control (042, 043), the three-ring polarization control (042, 043) is made to work at a wavelength interval of 1.5 nm, so that the wavelength output of the laser is single, dual, or triple wavelength. When the wavelength interval is 0.5 nm, the laser has multi-wavelength outputs of four, five, six, seven, eight, and nine wavelengths.

[0014] Preferably, the mode selection coupler (05) is formed by fused taper of two optical fibers. The two optical fibers selected are a 1550nm single-mode fiber and a ring core fiber, respectively. When the phase matching condition is met, the energy of the fundamental mode transmitted in the single-mode fiber will be transferred to the higher-order mode transmitted in the ring core fiber, realizing mode conversion. The converted higher-order mode is output from the output end of the mode selector through the fourth optical fiber channel (14), and then the mode spot characteristics are detected by the detection equipment. When the laser outputs single, three, five, seven, and nine wavelengths, the corresponding mode spot output is detected respectively, and the mode purity is 95%.

[0015] Preferably, the nonlinear amplification ring cavity is used as a saturable absorber to achieve pulse operation, thereby reflecting high-power light and absorbing low-power light, resulting in a mode-locked pulse output with a constant repetition frequency of 101.4 MHz, a pulse interval of 9.86 ns, and a single pulse width of 5.7 ns. By increasing the pump power by 80 mW-300 mW, the output power is increased from 52 μW to 257 μW.

[0016] As can be seen from the technical solutions provided by the embodiments of the present invention described above, the multi-wavelength mode-locked fiber laser with adjustable wavelength spacing of the present invention utilizes a nonlinear amplifying cavity as the key device for pulse mode-locking, a mode selection coupler to achieve the conversion from the fundamental mode to higher-order modes, and an improved Sagnac ring as the wavelength selection device. The combination of these three devices ultimately achieves multi-wavelength pulsed mode-locked laser output with adjustable wavelength spacing. This multi-wavelength mode-locked fiber laser can play an important role in mode division multiplexing, wavelength division multiplexing, optical communication, optical sensing, and other fields.

[0017] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of a multi-wavelength mode-locked fiber laser with adjustable wavelength spacing, provided as an embodiment of the present invention.

[0020] Figure 2 An improved output spectrum of the Sagnac ring is provided for an embodiment of the present invention. Figure 2 (a) Output spectrum with a wavelength interval of 0.5 nm, Figure 2 (b) Output spectrum with a wavelength interval of 1.5 nm.

[0021] Figure 3 This invention provides a multi-wavelength output spectrum of a laser with a wavelength spacing of 1.5 nm as an embodiment of the present invention. Figure 3 (a) Single-wavelength output spectrum Figure 3 (b) Dual-wavelength output spectrum Figure 3 (c) Three-wavelength output spectrum.

[0022] Figure 4 This invention provides a multi-wavelength output spectrum of a laser with a wavelength spacing of 0.5 nm as an embodiment of the invention. Figure 4 (a) Four-wavelength output spectrum Figure 4 (b) Five-wavelength output spectrum Figure 4 (c) Six-wavelength output spectrum Figure 4 (d) Seven-wavelength output spectrum.

[0023] Figure 5This invention provides a multi-wavelength output spectrum of a laser with a wavelength spacing of 0.5 nm as an embodiment of the invention. Figure 5 (a) Eight-wavelength output spectrum Figure 5 (b) Nine-wavelength output spectrum.

[0024] Figure 6 This invention provides a multi-wavelength output and a corresponding wavelength output mode. Figure 6 The first column shows the output spectra from top to bottom for single, triple, five, seven, and nine wavelengths, respectively. The second column shows the pattern patterns detected at the corresponding wavelength output.

[0025] Figure 7 (a) A pulse sequence within a 100ns time window provided in an embodiment of the present invention. Figure 7 (b) Single pulse sequence within a 10ns time window.

[0026] Figure 8 This is a graph showing the variation of output power with pump power, provided as an embodiment of the present invention. Detailed Implementation

[0027] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0028] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means 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 say an element is “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, “connected” or “coupled” as used herein can include wireless connections or couplings. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0029] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0030] To facilitate understanding of the embodiments of the present invention, the following will provide further explanation and description with reference to the accompanying drawings and several specific embodiments. These embodiments do not constitute a limitation on the embodiments of the present invention.

[0031] This invention provides a multi-wavelength mode-locked fiber laser with adjustable wavelength spacing, comprising: a pump source, a wavelength division multiplexer, an erbium-doped fiber, a single-mode fiber, a three-ring polarization controller, a mode selection coupler, a polarization-maintaining fiber, a coupler, and a detection device, which includes an oscilloscope, a spectrometer, an optical power meter, and a CCD (charge-coupled device) camera.

[0032] The pump source is connected to one end of the input of the wavelength division multiplexer (WDM) via a first optical fiber duct. One input of the first coupler is connected to the input of the WDM. The output of the WDM is connected to an erbium-doped fiber, a 3km single-mode fiber, and a three-loop polarization controller via a second optical fiber duct, finally returning to the input of the coupler. The output of the first coupler is connected to the input of the mode selection coupler via a third optical fiber duct. The mode selection coupler has two outputs. One output is connected to a fourth optical fiber duct, where mode characteristics are detected by a testing device. The other output is connected to the input of the second coupler via a fifth optical fiber duct. The two outputs of the second coupler are connected to a three-loop polarization controller, a polarization-maintaining fiber, another three-loop polarization controller, and another polarization-maintaining fiber via a seventh optical fiber duct, finally outputting through a sixth optical fiber duct, where multi-wavelength output characteristics are detected by a testing device.

[0033] A 980nm pump source is injected into the erbium-doped fiber through a 980 / 1550nm wavelength division multiplexer, and the erbium-doped fiber is then optically amplified. A nonlinear amplification cavity is formed by the pump source, wavelength division multiplexer, erbium-doped fiber, 3km single-mode fiber, and a coupler with a splitting ratio of 1:9. This nonlinear amplification cavity exhibits high-power light reflection and low-power light absorption, serving as a reflective cavity for the laser and a key component for pulse mode-locking. A self-made mode-selection coupler can be used as a mode-switching device. According to coupled-mode theory, when the phase-matching condition is met, the energy of the fundamental mode transmitted in the single-mode fiber is coupled to the higher-order mode transmitted in the ring-core fiber, thus achieving mode conversion from the fundamental mode to a higher-order mode. On the other hand, an improved Sagnac ring is formed by a coupler with a splitting ratio of 5:5, two three-ring polarization controllers, and two segments of polarization-maintaining fiber. This Sagnac ring is used to achieve wavelength selection. By adjusting the three-ring polarization controller and the pump magnitude of the pump source, high-order mode output with adjustable wavelength spacing and wavenumber multi-wavelength pulse mode-locking is achieved.

[0034] A schematic diagram of a multi-wavelength mode-locked fiber laser with adjustable wavelength spacing provided in this embodiment of the invention is shown below. Figure 1As shown, it includes: a pump source (01), a wavelength division multiplexer (02), a first coupler (03), a three-ring polarization controller (041, 042, 043), a mode selection coupler (05), a second coupler (06), an erbium-doped fiber (07), a 3km single-mode fiber (08), a polarization-maintaining fiber (09), a polarization-maintaining fiber (10), a first fiber optic channel (11), a second fiber optic channel (12), a third fiber optic channel (13), a fourth fiber optic channel (14), a fifth fiber optic channel (15), a sixth fiber optic channel (16), and a seventh fiber optic channel (17). The pump source (01) is connected to one end of the input of the wavelength division multiplexer (02) through the first optical fiber channel (11). One input of the first coupler (03) is connected to the other input of the wavelength division multiplexer (02). The output of the wavelength division multiplexer (02) is connected to the erbium-doped fiber (07), the 3km single-mode fiber (08) and the three-ring polarization controller (041) through the second optical fiber channel (12), and finally returns to the input of the first coupler (03). The output of the first coupler (03) is connected to the input of the mode selection coupler (05) through the third optical fiber channel (13). The mode selection coupler (05) has two outputs. One output is connected to the fourth optical fiber channel (14) and outputs, and then the mode characteristics are detected by the detection equipment. The other output is connected to the input of the second coupler (06) through the fifth optical fiber channel (15). The two output ends of the second coupler (06) are connected to the three-ring polarization controller (041), polarization-maintaining fiber (09), three-ring polarization controller (043) and polarization-maintaining fiber (10) in succession through the seventh fiber optic channel (17), and finally output through the sixth fiber optic channel (16), and the multi-wavelength output characteristics are detected by the detection equipment.

[0035] The laser cavity of the fiber laser includes: a 980 / 1550nm wavelength division multiplexer (02), a second fiber optic channel (12), an erbium-doped fiber (07), a 3km single-mode fiber (08), a three-ring polarization controller (041), a first coupler (03), a third fiber optic channel (13), a mode selection coupler (05), a fifth fiber optic channel (15), a second coupler (06), a seventh fiber optic channel (17), a three-ring polarization controller (042, 043), and polarization-maintaining fibers (09, 10).

[0036] Pump light generated by pump source (01) enters the optical path through 980 / 1550nm wavelength division multiplexer (02). 2.3m erbium-doped fiber (07) achieves population inversion and generates stimulated emission amplification of light. Polarization controller (041) adjusts the polarization state of the mode in the fiber optic tube. 2×1 first coupler (03) with a 5:5 splitting ratio achieves interference and output of two beams with opposite transmission directions. The above devices form a nonlinear amplification cavity for pulse mode-locking operation. Self-made mode selection coupler (05) is used to realize the conversion from the fundamental mode to higher-order modes. The converted higher-order modes are transmitted through the fourth fiber optic tube (14) and output. The detection equipment performs mode spot detection on the higher-order modes. The other output end of the mode selection coupler (05) continues to be transmitted through the fifth fiber optic tube (15). An improved Sagnac ring is composed of a 2×2 second coupler (06) with a splitting ratio of 1:9, a seventh fiber optic tube (17), a three-ring polarization controller (042, 043), a 2.2m polarization-maintaining fiber (09), and a 4.35m polarization-maintaining fiber (10). This device can change the polarization state of the light transmitted in it by adjusting the three-ring polarization controller (042, 043), thereby changing the gain and loss under different wavelength competition. When dynamic equilibrium is reached and a certain wavelength obtains sufficient gain, it is finally output in the sixth fiber optic tube, and then the multi-wavelength characteristics are detected by the detection equipment. Figure 2 The image shows the output spectrum of the improved Sagnac ring, which has two wavelength intervals of 0.5 nm and 1.5 nm. By adjusting the three-ring polarization controller to operate at a wavelength interval of 1.5 nm, the laser output wavelengths are obtained as single, dual, and triple wavelengths, as shown below. Figure 3 As shown. When the wavelength interval is 0.5nm, the laser produces multi-wavelength outputs of four, five, six, seven, eight, and nine wavelengths, as shown. Figure 4 , Figure 5 As shown.

[0037] The self-made mode selection coupler (05) is formed by fused taper of two optical fibers, namely a 1550nm single-mode fiber and a ring-core fiber. According to the coupling mode theory, when the phase matching condition is met, the energy of the fundamental mode transmitted in the single-mode fiber will be transferred to the higher-order mode transmitted in the ring-core fiber, thus realizing mode conversion. The converted higher-order mode is output from the output end of the mode selector through the fourth fiber channel (14), and then the mode spot characteristics are detected by the detection equipment. When the laser outputs single, three, five, seven, and nine wavelengths, the corresponding mode spot output is detected respectively. The mode purity is 95%, and the results are as follows. Figure 6 As shown.

[0038] Nonlinear amplification ring cavities can reflect high-power light and absorb low-power light, thus serving as saturable absorbers for pulsed operation. Ultimately, a mode-locked pulse output with a constant repetition frequency of 101.4 MHz, a pulse interval of 9.86 ns, and a single pulse width of 5.7 ns was obtained. Figure 7 As shown, by increasing the pump power from 80mW to 300mW, the output power increased from 52μW to 257μW. The terms "high power" and "low power" are relative; different lasers require different levels of power. High and low power primarily depend on whether the pump power is sufficient to achieve a gain greater than the loss within the laser cavity to achieve laser output. For the laser involved in this invention, the threshold pump power is 40mW, while a relatively stable pulsed laser output requires approximately 80mW. Therefore, a pump power of 40-80mW can be considered low power, and greater than 80mW can be considered high power.

[0039] In summary, the multi-wavelength mode-locked fiber laser of this invention utilizes a nonlinear amplifying cavity as a pulse device to achieve mode-locking, thereby enabling the entire laser to output mode-locked pulses. A self-made mode selection coupler is used as a mode conversion device to achieve the conversion from the fundamental mode to higher-order modes. An improved Sagnac ring is used as a wavelength selection device to achieve multi-wavelength output of the laser. The coordinated use of these devices realizes multi-wavelength pulsed mode-locked laser output with adjustable wavelength spacing. This invention will play an important role in fields such as mode division multiplexing, wavelength division multiplexing, optical communication, and optical sensing.

[0040] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of one embodiment, and the modules or processes shown in the drawings are not necessarily essential for implementing the present invention.

[0041] As can be seen from the above description of the embodiments, those skilled in the art can clearly understand that the present invention can be implemented by means of software plus necessary general-purpose hardware platforms. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in various embodiments or some parts of the embodiments of the present invention.

[0042] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, for apparatus or system embodiments, since they are basically similar to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. The apparatus and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0043] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A multi-wavelength mode-locked fiber laser with adjustable wavelength spacing, characterized in that, include: Pump source (01), wavelength division multiplexer (02), first coupler (03), three-ring polarization controller (041, 042, 043), mode selection coupler (05), second coupler (06), erbium-doped fiber (07), 3km single-mode fiber (08), polarization-maintaining fiber (09), polarization-maintaining fiber (10), first fiber duct (11), second fiber duct (12), third fiber duct (13), fourth fiber duct (14), fifth fiber duct (15), sixth fiber duct (16) and seventh fiber duct (17); The pump source (01) is connected to one end of the input of the wavelength division multiplexer (02) through the first optical fiber channel (11). One input of the first coupler (03) is connected to the other input of the wavelength division multiplexer (02). The output of the wavelength division multiplexer (02) is connected to the erbium-doped fiber (07), the 3km single-mode fiber (08), and the three-ring polarization controller (041) through the second optical fiber channel (12), and finally returns to the input of the first coupler (03). The output of the first coupler (03) is connected to the mode selection coupler (041) through the third optical fiber channel (13). The input end of the mode selection coupler (05) is connected to the input end of the mode selection coupler (05). The output end of the mode selection coupler (05) has two output ends. One output end is connected to the fourth optical fiber channel (14) and outputs, and then the mode characteristics are detected by the detection device. The other output end is connected to the input end of the second coupler (06) through the fifth optical fiber channel (15). The two output ends of the second coupler (06) are connected to the three-ring polarization controller (041), polarization-maintaining fiber (09), three-ring polarization controller (043) and polarization-maintaining fiber (10) through the seventh optical fiber channel (17). Finally, the output is output through the sixth optical fiber channel (16) and the multi-wavelength output characteristics are detected by the detection device.

2. The multi-wavelength mode-locked fiber laser with adjustable wavelength spacing according to claim 1, characterized in that, The pump light generated by the pump source (01) enters the optical path through the 980 / 1550nm wavelength division multiplexer (02). The 2.3m erbium-doped fiber (07) achieves population inversion and generates stimulated emission amplification of light. The polarization controller (041) adjusts the polarization state of the mode in the fiber optic channel. The 2×1 first coupler (03) with a 5:5 splitting ratio achieves interference and output of two beams with opposite transmission directions. The pump source (01), the wavelength division multiplexer (02), the first coupler (03), the erbium-doped fiber (07), and the polarization controller (041) form a nonlinear amplification cavity for realizing pulse mode-locking operation.

3. The multi-wavelength mode-locked fiber laser with adjustable wavelength spacing according to claim 1, characterized in that, The mode selection coupler (05) is used to realize the conversion from the fundamental mode to the higher-order mode. The converted higher-order mode is transmitted through the fourth optical fiber channel (14) and then output. The detection equipment performs mode spot detection on the higher-order mode. The other output end of the mode selection coupler continues to be transmitted through the fifth optical fiber channel (15). The improved Sagnac ring is composed of a 2×2 coupler (06) with a splitting ratio of 1:9, the seventh optical fiber channel (17), a three-ring polarization controller (042, 043), a 2.2m polarization-maintaining fiber (09), and a 4.35m polarization-maintaining fiber (10). The improved Sagnac ring changes the polarization state of the light transmitted in the three-ring polarization control (042, 043) by adjusting the three-ring polarization control (042, 043), thereby changing the gain and loss under different wavelength competition. When dynamic equilibrium is reached and a certain wavelength obtains sufficient gain, the light is finally output in the sixth fiber optic channel (16). The light output by the improved Sagnac ring is then subjected to multi-wavelength characteristic detection by the detection equipment.

4. The multi-wavelength mode-locked fiber laser with adjustable wavelength spacing according to claim 3, characterized in that, The improved Sagnac ring output spectrum includes wavelength intervals of 0.5 nm and 1.5 nm. By adjusting the three-ring polarization control (042, 043) to operate at a wavelength interval of 1.5 nm, the laser's wavelength output is obtained as single, dual, and triple wavelengths. When the wavelength interval is 0.5 nm, the laser's multi-wavelength output is obtained as four, five, six, seven, eight, and nine wavelengths.

5. The multi-wavelength mode-locked fiber laser with adjustable wavelength spacing according to claim 1, characterized in that, The mode selection coupler (05) is made of two optical fibers fused together and tapered. The two optical fibers selected are a 1550nm single-mode fiber and a ring core fiber. When the phase matching condition is met, the energy of the fundamental mode transmitted in the single-mode fiber will be transferred to the higher-order mode transmitted in the ring core fiber, realizing mode conversion. The converted higher-order mode is output from the output end of the mode selector through the fourth optical fiber channel (14). Then the mode spot characteristics are detected by the detection equipment. When the laser outputs single, three, five, seven and nine wavelengths, the corresponding mode spot output is detected respectively. The mode purity is 95%.

6. The multi-wavelength mode-locked fiber laser with adjustable wavelength spacing according to claim 2, characterized in that, The nonlinear amplification ring cavity, acting as a saturable absorber, is used to achieve pulse operation, reflecting high-power light and absorbing low-power light, resulting in a mode-locked pulse output with a constant repetition frequency of 101.4 MHz, a pulse interval of 9.86 ns, and a single pulse width of 5.7 ns. By increasing the pump power by 80 mW to 300 mW, the output power is increased from 52 μW to 257 μW.

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