A multi-longitudinal mode tunable fiber laser

Abstract

The application discloses a kind of multi-longitudinal mode tunable fiber laser, including pump injection unit, it includes semiconductor laser pump source, pump protector and wavelength division multiplexer;Light generating unit, it includes high-reflectance Bragg fiber grating, doped rare earth optical fiber and collimating lens;And grating mode selection and output unit, it includes the blazed grating that can be rotated around its vertical axis and mirror;Wherein, the input port of the doped rare earth optical fiber, collimating lens and blazed grating constitute external resonant cavity, collimated resonant cavity in optical fiber laser beam is guided to the blazed grating, wherein first order diffraction light is reflected back into the resonant cavity of laser, and zero order diffraction light is reflected and output by the mirror.This application can guarantee that the first order diffraction light at the start wavelength can return to the doped rare earth optical fiber, so that the corresponding fiber laser can effectively provide corresponding tuning performance, stability and output power, solve the problems existing in the prior art.

Description

Technical Field

[0001] This invention relates to the field of laser and fiber optic communication technology, and in particular to a multi-longitudinal-mode tunable fiber laser. Background Technology

[0002] With the rapid development of fiber laser technology, fiber lasers have been widely used in communication, sensing, and precision measurement. Traditional fiber lasers are typically single-longitudinal-mode fiber lasers, operating in a single-longitudinal-mode mode. They possess high frequency stability and narrow spectral bandwidth, making them suitable for applications requiring high output wavelength accuracy. However, single-longitudinal-mode fiber lasers generally have a narrow tuning range and a complex tuning process, thus making them unsuitable for applications requiring wide-spectrum tuning.

[0003] To address the limited tuning range of single-mode fiber lasers, multi-mode tunable fiber lasers have emerged. Multi-mode fiber lasers can extend their tuning range by exciting multiple longitudinal modes (i.e., different laser modes) within the fiber to provide multiple different laser frequency outputs, thus enabling them to possess higher output power and greater tuning flexibility.

[0004] The core technology of multi-mode fiber lasers involves precisely designing the waveguide structure and gain medium (such as rare-earth-doped fiber) of the fiber, enabling the laser to support the coupling and excitation of multiple longitudinal modes. Furthermore, external tuning mechanisms (such as acousto-optic modulators, gratings, and temperature control systems) are used to precisely adjust the output wavelength, thereby achieving wide-spectrum tuning capabilities. However, the realization of multi-mode tunable fiber lasers still faces certain challenges, such as competition between longitudinal modes, control of tuning accuracy, mode selectivity, and stability. How to effectively solve these problems to improve its tuning performance, stability, and output power remains a key technical challenge.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-longitudinal-mode tunable fiber laser to solve the technical problems existing in the prior art, thereby effectively improving the tuning performance, stability and output power of the fiber laser.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A multi-longitudinal-mode tunable fiber laser includes a pump injection unit, a light generation unit, and a grating mode selection and output unit, wherein:

[0009] The pump injection unit includes a semiconductor laser pump source, a pump protector, and a wavelength division multiplexer;

[0010] The light generation unit includes a high-reflectivity Bragg fiber grating, a rare-earth-doped fiber, and a collimating lens. The rare-earth-doped fiber meets the predetermined high-doping quantization index requirements.

[0011] The grating mode selection and output unit includes a blazed grating and a reflector; wherein, the input port of the rare-earth-doped fiber, the collimating lens, and the blazed grating form an external resonant cavity; within the external resonant cavity, the collimated fiber laser beam is guided to the blazed grating, where the first-order diffracted light is reflected back into the laser resonant cavity, and the zero-order diffracted light is reflected and output through the reflector; and by rotating the blazed grating about its vertical axis, the center wavelength of the laser output through the zero-order diffraction is tuned, and the length of the rare-earth-doped fiber is determined based on the reflectivity of the high-reflectivity Bragg fiber grating, the diffraction efficiency of the blazed grating, and the coupling efficiency of the first-order diffracted light returning to the fiber.

[0012] The center wavelength of the semiconductor laser pump source is greater than or equal to 915nm and less than or equal to 976nm, and its output power is greater than 50mW.

[0013] The high-reflectivity Bragg fiber grating has a reflectivity greater than 90% and a grating bandwidth of 1018nm to 1042nm or 1056nm to 1070nm; the rare-earth-doped fiber has a length of 5-6cm and its output end is beveled; the predetermined high-doping quantification index of the rare-earth-doped fiber is that the absorption coefficient of the fiber core used is 800-1500dB / m@976nm; the rare-earth-doped fiber is ytterbium-doped fiber; if applied to other output laser wavelengths, erbium-doped fiber or neodymium-doped fiber is used.

[0014] The length of the rare-earth-doped optical fiber is 5.5 cm.

[0015] The collimating lens has a focal length of 8mm or 11mm and is used to collimate the light emitted from the fiber and to couple the first-order diffracted light returning through the blazed grating back into the fiber; the blazed grating has a line count of 800 lines / mm to 2400 lines / mm and a diffraction efficiency of 60% to 80%.

[0016] The different intervals of the multi-longitudinal modules The calculation formula is:

[0017] ;

[0018] in, The effective optical path length for light to propagate within the resonant cavity. This refers to the distance between the output end of the rare-earth-doped optical fiber and the blazed grating. denoted as ρ, where ρ is the refractive index of the internal medium of the rare-earth-doped optical fiber, and c is the speed of light in a vacuum.

[0019] The formula for calculating the shortest length of the rare-earth-doped optical fiber is as follows:

[0020] ;

[0021] Where R represents the effective reflectivity of the external resonant cavity. , The reflectivity at the fiber optic input end. The effective reflectivity of the flash grating; Indicates the effective transmittance of the external resonant cavity. , This indicates the transmittance of a high-reflectivity Bragg fiber grating. This indicates the efficiency of recoupling the first-order diffracted light from the blazed grating back into the optical fiber; and These represent the fiber attenuation coefficients of the pump light and signal light, respectively. The saturation power ratio is used to represent the energy transfer efficiency and saturation characteristics between the pump light and the signal light, and the saturation power ratio is... for ,in This represents the pump light saturation power during single-beam transmission. The cross-saturation power of the signal is a parameter characterizing the transfer of energy from the absorbed pump power to the signal, or the ratio of the saturation power to the signal. It is the ratio of the pump light absorption cross section to the signal light emission cross section.

[0022] The process of tuning the center wavelength of the laser output via zero-order diffraction by rotating the blazed grating about its vertical axis includes:

[0023] The grating equation that determines the relationship between the center wavelength of the laser output from the tuned zero-order diffraction and the rotation angle of the blazed grating is:

[0024] ;

[0025] Where α is the incident angle of the reflection grating of the blazed grating; d is the grating period, which is the reciprocal of the groove density; and λ is the center wavelength of the laser output by zero-order diffraction.

[0026] Based on the determined grating equation, the center wavelength of the laser output through zero-order diffraction is tuned.

[0027] The multi-mode tunable fiber laser may further include a pitch adjustment frame for fixing and adjusting the pitch of a sleeve containing a rare-earth doped fiber connector with a ferrule contactor (FC) / angle physical contactor (APC) and a collimating lens; wherein, the collimating lens is connected to the collimating lens to collimate the external resonant cavity, and the output laser is adjusted by adjusting the pitch adjustment frame after the blazed grating is rotated to a set angle.

[0028] The multi-mode tunable fiber laser may further include a thermistor disposed at the center of the fiber disk formed by the high-reflectivity Bragg fiber grating, and a thermoelectric cooler disposed below the fiber disk. The thermistor measures the temperature and provides a feedback signal to adjust the driving current of the thermoelectric cooler. When the temperature of the fiber disk and the temperature of the external resonant cavity meet the predetermined stability requirements, the angle of the blazed grating is rotated and the pitch adjustment frame is adjusted to adjust the output laser.

[0029] Compared with existing technologies, the multi-mode tunable fiber laser provided by this invention can achieve optical amplification and mode-selective output via diffraction grating by optically pumping rare-earth ions to excite energy level transitions in a short, highly doped rare-earth fiber. Furthermore, by designing the rotation angle of the blazed grating, it ensures that the first-order diffracted light at the starting wavelength can return to the rare-earth-doped fiber. Therefore, the corresponding fiber laser can effectively provide corresponding tuning performance, stability, and output power, solving the problems existing in existing technologies. It has greater potential than semiconductor lasers and can promote the development of laser technology and fiber optic communication. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. 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.

[0031] Figure 1 This is a schematic diagram of the optical path of a multi-longitudinal-mode fiber laser provided in an embodiment of the present invention;

[0032] Figure 2 A schematic diagram of the equivalent linear external resonant cavity model of the light generation unit provided in an embodiment of the present invention;

[0033] Figure 3 This is a schematic diagram of the structure of the grating mode selection and output unit of the external resonant cavity provided in an embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram of the structure of a multi-longitudinal-mode fiber laser provided in an embodiment of the present invention;

[0035] Figure 5 The spectrum at 1056 nm provided for testing in an embodiment of the present invention;

[0036] Figure 6 The graph showing the relationship between the tunable wavelength range and the laser output power is provided for the embodiments of the present invention. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the specific content of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments, which do not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0038] First, the following explanations are provided for the terms that may be used in this article:

[0039] The term "and / or" means that either or both can be achieved simultaneously. For example, X and / or Y means that it includes both "X" or "Y" as well as the three cases of "X and Y".

[0040] The terms "comprising," "including," "containing," "having," or other similar semantic descriptions should be interpreted as non-exclusive inclusion. For example, including a technical feature element (such as raw material, component, ingredient, carrier, dosage form, material, size, part, component, mechanism, device, step, process, method, reaction conditions, processing conditions, parameter, algorithm, signal, data, product or article of manufacture, etc.) should be interpreted as including not only the expressly listed technical feature element, but also other technical feature elements that are not expressly listed and are well-known in the art.

[0041] The term "composed of" excludes any technical features not expressly listed. When used in a claim, it closes the claim to exclude all technical features other than those expressly listed, except for associated conventional impurities. If the term appears only in a clause of a claim, it limits the claim to the elements expressly listed in that clause; elements recited in other clauses are not excluded from the overall claim.

[0042] Unless otherwise explicitly specified or limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this document according to the specific circumstances.

[0043] When concentration, temperature, pressure, size, or other parameters are expressed as numerical ranges, such ranges should be understood to specifically disclose all ranges formed by any pairing of upper limits, lower limits, or preferred values ​​within that range, regardless of whether the range is explicitly stated; for example, if the numerical range "2 to 8" is stated, then that range should be interpreted to include ranges such as "2 to 7", "2 to 6", "5 to 7", "3 to 4 and 6 to 7", "3 to 5 and 7", "2 and 5 to 7", etc. Unless otherwise stated, the numerical ranges described herein include both their endpoints and all integers and fractions within that range.

[0044] The terms “center,” “longitudinal,” “lateral,” “length,” “width,” “thickness,” “upper,” “lower,” “front,” “back,” “left,” “right,” “vertical,” “horizontal,” “top,” “bottom,” “inner,” “outer,” “clockwise,” and “counterclockwise” indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience and simplification of description and do not imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this document.

[0045] This invention provides a multi-longitudinal-mode tunable fiber laser, which can specifically realize a multi-longitudinal-mode fiber laser based on the grating-tunable wavelength range, thereby possessing characteristics such as wide tunability range and narrow linewidth.

[0046] Specifically, in the implementation of this invention, the pump light is forward-pumped into a Bragg fiber grating with high reflectivity at the emission wavelength, then passes through a short section of highly doped rare-earth fiber, where the rare-earth element can be ytterbium, erbium, etc. After collimation by a lens, the output is formed. A high-diffraction-efficiency reflective diffraction grating is selected as the external cavity, thus forming a laser resonant cavity. The emission wavelength is selected by rotating the diffraction grating, and the emitted laser is reflected by the grating for output, realizing a tunable multi-mode laser. In other words, this invention utilizes the wide tuning range advantage of blazed gratings while avoiding the large coupling loss of spatial optical elements. By combining a high-reflectivity fiber grating with an ultrashort, highly doped rare-earth single-mode fiber, it develops a multi-mode tunable fiber laser with high precision, stability, multiple modes, narrow linewidth, and tunability over a wide wavelength range. This has significant implications for optical communication, spectral analysis, sensing, medical applications, and scientific research.

[0047] The following will combine Figure 1 The schematic diagram of the optical path structure shown is used to introduce the implementation process and principle of multi-longitudinal-mode tunable fiber laser.

[0048] like Figure 1 The diagram shown is a schematic representation of the optical path structure of a multi-longitudinal-mode tunable fiber laser provided in an embodiment of the present invention, which mainly includes:

[0049] (1) Pump injection unit: including semiconductor laser pump source 1, pump protector 2 and wavelength division multiplexer 3;

[0050] The center wavelength of the semiconductor laser pump source 1 can be 976nm (usually, a parameter value greater than or equal to 915nm and less than or equal to 976nm can be selected). As a pump source for stimulated laser, the output power generally needs to reach the start-up power, for example, the output power can be set to 50mW or more.

[0051] The pump protector 2 is used to protect the fiber laser or amplifier system from backlight, reflected light or spontaneous emission, so as to avoid damage to the pump laser diode.

[0052] The wavelength division multiplexer 3 is used to couple pump light (e.g., 976nm or 915nm) into a rare-earth-doped fiber (e.g., erbium-doped or ytterbium-doped fiber) to filter out unwanted light wavelengths, reduce spontaneous emission (ASE), optimize the gain distribution within the laser cavity, and improve output stability.

[0053] (2) Light generation unit: including high reflectivity Bragg fiber grating 4, rare earth doped fiber 5 and collimating lens 6, wherein the high doped rare earth fiber (i.e., doped rare earth fiber) can be selected from rare earth elements such as ytterbium or erbium, the fiber length can be controlled in the range of 5-6cm, and the corresponding output end can be a bevel.

[0054] The high-reflectivity Bragg fiber grating 4 is used to allow 976nm pump light to pass through, and the laser excited in the gain fiber is highly reflective, with a corresponding reflectivity of over 90%. The selectable grating bandwidth can be 1018nm to 1042nm, 1056nm to 1070nm, etc.

[0055] The rare-earth-doped optical fiber 5 can be selected from highly doped ytterbium-doped fiber or other rare-earth-doped optical fibers; the rare-earth-doped optical fiber meets the predetermined high-doping quantification index requirements, and when the rare-earth-doped optical fiber is ytterbium-doped fiber, the predetermined high-doping quantification index can be that the absorption coefficient of the fiber core used is 800-1500dB / m@976nm, for example, it can be 1200dB / m@976nm, which is much higher than that of ordinary doped optical fibers; furthermore, if it is applied to other output laser wavelengths, erbium-doped fiber or neodymium-doped fiber can be used.

[0056] The collimating lens 6 can be selected with a focal length of 8mm or 11mm, etc., to collimate the light emitted from the fiber and to couple the first-order diffracted light returned by the blazed grating 7 back into the fiber, thereby improving the spatial coupling efficiency.

[0057] (3) Grating mode selection and output unit; including blazed grating 7 and reflector 8;

[0058] The input port of the highly doped rare-earth fiber, the collimating lens 6, and the blazed grating 7 form an external resonant cavity. Rare-earth elements (such as ytterbium, erbium, or neodymium) serve as gain media in the fiber, exciting energy level transitions of rare-earth ions through optical pumping, thereby achieving optical amplification and laser output. The fiber laser beam within the collimated external resonant cavity is guided onto the blazed grating 7, and the first-order diffracted light is reflected back into the external resonant cavity. The laser output is achieved through the zero-order diffraction output of the blazed grating 7. Specifically, the center wavelength of the laser output through zero-order diffraction can be tuned by rotating the blazed grating 7 around its vertical axis, thus achieving the corresponding tunable function.

[0059] The blazed grating 7 can have a line count of 800 lines / mm to 2400 lines / mm, for example, 1200 lines / mm. Its polarization-dependent diffraction efficiency can be 60% to 80%, and it is used to tune the laser center wavelength, thereby achieving a wide range of tunable wavelengths.

[0060] The reflector 8 is used to reflect the zero-order reflected light passing through the blazed grating 7 for laser output.

[0061] Based on the above structure, a corresponding multi-mode tunable fiber laser can be obtained, which can solve problems such as competition between multiple modes, control of tuning accuracy, mode selectivity and stability, and provide a multi-mode tunable fiber laser with high tuning range and accuracy and stable performance.

[0062] In the implementation of this invention, the interval between different modes of multi-longitudinal-mode laser can be expressed as: Theoretically, it can be roughly calculated using the following formula:

[0063] ;

[0064] in, The effective optical path length for light to propagate within the resonant cavity. This refers to the distance between the output end face of the rare-earth-doped optical fiber and the blazed grating. denoted as ρ, where ρ is the refractive index of the internal medium of the rare-earth-doped optical fiber, and c is the speed of light in a vacuum.

[0065] like Figure 2 As shown, an externally resonant tunable fiber laser can be represented by a simplified linear cavity model. Based on this model, the minimum length of the corresponding highly doped rare-earth fiber can be roughly calculated using the following formula:

[0066] ;

[0067] Where R represents the effective reflectivity of the external cavity of the resonant cavity. , The reflectivity at the fiber optic input end. The effective reflectivity of the flash grating; Indicates the effective permeability of the cavity. , This indicates the transmittance of a high-reflectivity Bragg fiber grating. This indicates the efficiency of recoupling the first-order diffracted light from the blazed grating back into the optical fiber; This represents the fiber attenuation coefficient of the pump light (signal light). The saturation power ratio, used to represent the energy transfer efficiency and saturation characteristics of pump light and signal light, can be expressed as: ,in This represents the pump light saturation power during single-beam transmission. The cross-saturation power of the signal represents a parameter characterizing the transfer of energy from the absorbed pump power to the signal, or... It can be approximated as the ratio of the pump light absorption cross section to the signal light emission cross section.

[0068] In this embodiment of the invention, the grating mode selection and output unit can adopt a littrow structure, and its optical path diagram is as follows: Figure 3 As shown, the signal light excited by the rare-earth-doped fiber passes through the collimating lens 6 and is incident on the surface of the blazed grating 7. When the blazed grating 7 rotates to a specific angle, the first-order diffracted light is reflected back to the gain fiber. At this time, the high-reflection Bragg fiber grating 4 on the left side of the gain fiber and the blazed grating 7 form a resonant cavity, and the laser is output through the zero-order reflected light.

[0069] Blazed gratings play a crucial role in tunable fiber lasers. When the reflection grating (i.e., the blazed grating) undergoes diffraction in the m-th order, the relationship between the incident angle α and the diffraction angle βm can be expressed as:

[0070] ;

[0071] Where d is the grating period of the blazed grating, which can be the reciprocal of the groove density; λ is the center wavelength of the laser output from the zero-order diffraction.

[0072] For diffraction gratings used in tunable laser resonators, the most important case is that the grating period of the blazed grating produces only two operating orders: the zeroth order and the first diffraction order. Furthermore, in the Littrow configuration, β1 = α, and the corresponding grating equation can be expressed as:

[0073] ;

[0074] The relationship between the tuning wavelength and the rotation angle of the blazed grating can be obtained through this grating equation, thus allowing the center wavelength of the laser output through zero-order diffraction to be tuned based on the determined grating equation.

[0075] It should be noted that the specific implementation structure of the embodiments of the present invention can be referred to in the specific implementation process. Figure 4 As shown, Figure 4 This can be a top view of a ytterbium-doped fiber tunable multi-longitudinal-mode laser. (Refer to...) Figure 4 The light output from the corresponding pump injection unit passes through a common optical fiber Hi1060, then through a 5cm high-reflectivity Bragg fiber grating 4 with 90% reflectivity near 1030nm and a 5.5cm long ytterbium-doped fiber 5 (Yb1200 4 / 125), and then through the FC / APC connector of the ytterbium-doped fiber to connect to the collimating lens to collimate the output external resonant cavity. After that, the blazed grating can be rotated to a specific angle and the output laser can be adjusted by adjusting the pitch adjustment frame 9. The pitch adjustment frame 9 is used to fix the ytterbium-doped fiber connector containing the FC / APC and the sleeve of the collimating lens and to adjust their pitch position.

[0076] Furthermore, a temperature control module can be added to the fiber spiral disk assembled from high-reflectivity Bragg fiber gratings. Specifically, a TEC chip (i.e., a thermoelectric cooler) can be placed below the fiber disk, and a thermistor 10 can be placed in the central recess of the fiber disk. The thermistor is used to accurately measure the temperature and provide a feedback signal to adjust the drive current of the thermoelectric cooler, thereby achieving stable temperature control and a better temperature control effect. In this way, when the temperature of the fiber disk and the temperature of the external resonant cavity meet the predetermined stability requirements, the angle of the blazed grating is rotated and the pitch adjustment frame is adjusted to adjust the output laser.

[0077] To facilitate a further understanding of the embodiments of the present invention, the specific implementation process of the multi-longitudinal-mode tunable fiber laser provided by the embodiments of the present invention will be described in detail below.

[0078] The implementation of this invention mainly includes the following processing steps:

[0079] (a) It is necessary to design a suitable length of rare-earth-doped optical fiber;

[0080] The corresponding shortest fiber length can be calculated using the formula mentioned above. It is mainly affected by the reflectivity of the high-reflectivity Bragg fiber grating, the diffraction efficiency of the blazed grating, and the coupling efficiency of the first-order diffracted light return fiber. Specifically, the reflectivity of the high-reflectivity Bragg fiber grating used can be 90%, the corresponding diffraction efficiency of the blazed grating can be 60%, and the coupling efficiency can be 20%. Considering mode field mismatch, the transmittance of the collimating lens, and Fresnel reflection, the calculated shortest length for 1064nm laser oscillation initiation is 5.28cm. Therefore, in the implementation of this embodiment, a 5.5cm long doped rare-earth fiber is more suitable. It should be noted that although the calculated shortest length for 1030nm laser oscillation initiation is 2.25cm, considering the need for a larger tunable bandwidth, it is still recommended to choose a longer doped fiber to provide sufficient gain.

[0081] Reference Figure 5 and Figure 6 As shown, Figure 5 The image shows a spectral measurement of a multi-longitudinal-mode fiber laser at 1056 nm. Figure 6 The middle section shows a schematic diagram of the relationship between the wavelength tunable range and the laser output power during the test; the tunable range demonstrated is from 1056nm to 1070nm, and the maximum power is 10.08mW and the minimum power is 40μW when the pump power is 70mW.

[0082] (ii) What needs to be designed is the rotation angle of the blazing grating;

[0083] The corresponding rotation angle needs to ensure that the first-order diffracted light at the oscillation wavelength can return into the doped fiber. Specifically, the blaze angle for 1064nm laser oscillation can be calculated to be 39.67 degrees using the grating equation of the Littrow structure. Furthermore, due to the bandwidth influence of the high-reflectivity Bragg fiber grating, the corresponding tunable range can be from 1056nm to 1070nm, and the corresponding incident angle can be from 39.32 degrees to 39.94 degrees.

[0084] In the implementation of this invention, because the high-reflectivity Bragg fiber grating is sensitive to temperature changes, the fiber grating and the external resonant cavity need to be temperature controlled after assembly into a fiber optic disk. The output laser can only be adjusted by rotating the blazed grating and adjusting the corresponding pitch adjustment frame when the temperature of the fiber optic disk and the external resonant cavity reach a stable state, such as when the temperature error is less than 0.5 mK.

[0085] In summary, the multi-mode tunable fiber laser provided by the embodiments of the present invention can offer a more flexible and efficient light source for applications such as spectral analysis, fiber optic communication, lidar, and medicine, and has broad application prospects. Specifically, it may include:

[0086] In the field of optical fiber communication, it can significantly improve the performance of wavelength division multiplexing (WDM) systems, and achieve multi-wavelength signal transmission by precisely adjusting the wavelength, greatly increasing communication bandwidth and flexibility;

[0087] In the field of fiber optic communication sensing, tunable lasers can achieve high-sensitivity and high-precision multi-parameter measurement, and are suitable for structural health monitoring and environmental monitoring.

[0088] In the medical field, corresponding tunable fiber lasers can provide high-resolution imaging in optical coherence tomography (OCT), and in laser therapy, they can act on diseased tissues with selective wavelengths, reducing damage to healthy tissues and improving treatment precision.

[0089] In manufacturing, laser lasers offer high flexibility and precision in processing, making them suitable for high-precision machining fields such as microfabrication and 3D printing, thus improving processing efficiency and product quality. In basic scientific research, tunable fiber lasers are used for high-resolution spectral analysis, exploration of nonlinear optical phenomena, and precision measurement, driving the development of materials science and physics.

[0090] In summary, the multi-longitudinal-mode tunable fiber laser provided by the embodiments of the present invention has unique advantages and broad application prospects in multiple technical fields.

[0091] 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 scope of the technology 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. The information disclosed in the background section is intended only to enhance the understanding of the overall background technology of the present invention and should not be construed as an admission or implication in any way that such information constitutes prior art known to those skilled in the art.

Claims

1. A multi-longitudinal-mode tunable fiber laser, characterized in that, It includes a pump injection unit, a light generation unit, and a grating mode selection and output unit, wherein: The pump injection unit includes a semiconductor laser pump source, a pump protector, and a wavelength division multiplexer; The light generating unit includes a high-reflectivity Bragg fiber grating, a rare-earth-doped fiber, and a collimating lens. The high-reflectivity Bragg fiber grating has a reflectivity greater than 90% and a grating bandwidth of 1018 nm to 1042 nm or 1056 nm to 1070 nm. The rare-earth-doped fiber has a length of 5-6 cm and its output end is beveled. The predetermined high-doping quantification index of the rare-earth-doped fiber is that the absorption coefficient of the fiber core is 800-1500 dB / m@976 nm. The rare-earth-doped fiber is ytterbium-doped fiber. The grating mode selection and output unit includes a blazed grating and a reflector; wherein, the input port of the rare-earth-doped fiber, the collimating lens, and the blazed grating form an external resonant cavity; within the external resonant cavity, the collimated fiber laser beam is guided to the blazed grating, where the first-order diffracted light is reflected back into the laser resonant cavity, and the zero-order diffracted light is reflected and output through the reflector; and by rotating the blazed grating about its vertical axis, the center wavelength of the laser output through zero-order diffraction is tuned. The formula for calculating the shortest length of the rare-earth-doped fiber is: ； Where R represents the effective reflectivity of the external resonant cavity. , The reflectivity at the fiber optic input end. The effective reflectivity of the flash grating; Indicates the effective transmittance of the external resonant cavity. , This indicates the transmittance of a high-reflectivity Bragg fiber grating. This indicates the efficiency of recoupling the first-order diffracted light from the blazed grating back into the optical fiber; and These represent the fiber attenuation coefficients of the pump light and signal light, respectively. The saturation power ratio is used to represent the energy transfer efficiency and saturation characteristics between the pump light and the signal light, and the saturation power ratio is... for ,in This represents the pump light saturation power during single-beam transmission. The cross-saturation power of the signal is a parameter characterizing the transfer of energy from the absorbed pump power to the signal, or the ratio of the saturation power to the signal. It is the ratio of the pump light absorption cross section to the signal light emission cross section.

2. The multi-longitudinal-mode tunable fiber laser according to claim 1, characterized in that, The center wavelength of the semiconductor laser pump source is greater than or equal to 915nm and less than or equal to 976nm, and its output power is greater than 50mW.

3. The multi-longitudinal-mode tunable fiber laser according to claim 1, characterized in that, The length of the rare-earth-doped optical fiber is 5.5 cm.

4. The multi-longitudinal-mode tunable fiber laser according to claim 1, characterized in that, The collimating lens has a focal length of 8mm or 11mm and is used to collimate the light emitted from the fiber and to couple the first-order diffracted light returning through the blazed grating back into the fiber; the blazed grating has a line count of 800 lines / mm to 2400 lines / mm and a diffraction efficiency of 60% to 80%.

5. The multi-longitudinal-mode tunable fiber laser according to any one of claims 1 to 4, characterized in that, The different intervals of the multi-longitudinal modules The calculation formula is: ； in, The effective optical path length for light to propagate within the resonant cavity. This refers to the distance between the output end of the rare-earth-doped optical fiber and the blazed grating. denoted as ρ, where ρ is the refractive index of the internal medium of the rare-earth-doped optical fiber, and c is the speed of light in a vacuum.

6. The multi-longitudinal-mode tunable fiber laser according to claim 5, characterized in that, The process of tuning the center wavelength of the laser output via zero-order diffraction by rotating the blazed grating about its vertical axis includes: The grating equation that determines the relationship between the center wavelength of the laser output from the tuned zero-order diffraction and the rotation angle of the blazed grating is: ； Where α is the incident angle of the reflection grating of the blazed grating; d is the grating period, which is the reciprocal of the groove density; and λ is the center wavelength of the laser output by zero-order diffraction. Based on the determined grating equation, the center wavelength of the laser output through zero-order diffraction is tuned.

7. The multi-longitudinal-mode tunable fiber laser according to claim 5, characterized in that, It also includes a pitch adjustment frame, used to fix and adjust the pitch of the sleeve containing the rare-earth doped fiber connector with FC / APC contactor and collimating lens; wherein, the collimating lens is connected to the collimating lens to collimate the external resonant cavity, and the output laser is adjusted by adjusting the pitch adjustment frame after the blazed grating is rotated to the set angle.

8. The multi-longitudinal-mode tunable fiber laser according to claim 7, characterized in that, It also includes a thermistor, which is located at the center of the fiber disk formed by the high-reflectivity Bragg fiber grating, and a thermoelectric cooler is also provided below the fiber disk. The thermistor measures the temperature and provides a feedback signal to adjust the driving current of the thermoelectric cooler. When the temperature of the fiber disk and the temperature of the external resonant cavity meet the predetermined stability requirements, the angle of the blazed grating is rotated and the pitch adjustment frame is adjusted to adjust the output laser.

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