Multi-longitudinal-mode tunable fiber laser

Abstract

The invention discloses a multi-longitudinal-mode tunable fiber laser comprising a pumping injection unit which comprises a semiconductor laser pumping source, a pumping protector and a wavelength division multiplexer; the light generation unit comprises a high-reflection fiber bragg grating, a rare earth doped fiber and a collimating lens; the grating mode selection and output unit comprises a blazed grating and a reflecting mirror which can rotate around a vertical axis of the grating mode selection and output unit; wherein an input port of the rare earth doped optical fiber, the collimating lens and the blazed grating form an external resonant cavity, an optical fiber laser beam in the collimated resonant cavity is guided to the blazed grating, first-order diffracted light is reflected back into the laser resonant cavity, and zero-order diffracted light is reflected and output through the reflector. According to the invention, it can be ensured that the first-order diffracted light at the oscillation starting wavelength can return to the rare earth-doped optical fiber, so that a corresponding optical fiber laser can effectively provide corresponding tuning performance, stability and output power, and problems in the prior art are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of laser and optical fiber communication, and in particular to a multi-longitudinal mode tunable optical fiber laser. Background Art

[0002] With the rapid development of fiber laser technology, fiber lasers have been widely used in fields such as communications, sensing, and precision measurement. Traditional fiber lasers are usually single-longitudinal-mode fiber lasers, which operate in a single-longitudinal-mode mode and have high frequency stability and narrow spectral bandwidth, making them suitable for applications that require high output wavelength accuracy. However, the tuning range of single-longitudinal-mode fiber lasers is usually narrow, and the tuning process is relatively complex. Therefore, it is difficult for them to meet the application requirements of fiber lasers that require wide-spectrum tuning.

[0003] To address the limited tuning range of single-longitudinal-mode fiber lasers, multi-longitudinal-mode tunable fiber lasers have emerged. Multi-longitudinal-mode fiber lasers can provide multiple different laser frequency outputs by exciting multiple longitudinal modes (i.e., different laser modes) in the optical fiber, thereby extending their tuning range. This allows the corresponding multi-longitudinal-mode fiber laser to have higher output power and greater tuning flexibility.

[0004] The core technology of multi-longitudinal mode fiber lasers is to enable fiber lasers to support the coupling and excitation of multiple longitudinal modes by precisely designing the waveguide structure and gain medium of the optical fiber (such as rare earth ion-doped fiber). In addition, external tuning mechanisms (such as acousto-optic modulators, gratings, temperature control systems, etc.) are used to precisely adjust the output wavelength, thereby achieving a wide spectrum tuning function. However, multi-longitudinal mode tunable fiber lasers still face certain challenges in the implementation process, such as competition between longitudinal modes, control of tuning accuracy, mode selectivity and stability. How to effectively solve the corresponding problems to improve its tuning performance, stability and output power is still a technical problem that needs to be solved.

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

[0006] The purpose of the present 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 purpose of the present invention is achieved through the following technical solutions:

[0008] A multi-longitudinal mode tunable fiber laser comprises 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 generating unit includes a high-reflection Bragg fiber grating, a rare-earth-doped fiber and a collimating lens, wherein the rare-earth-doped fiber meets the predetermined high-doping quantitative 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 constitute an external resonant cavity; within the external resonant cavity, the collimated fiber laser beam in the resonant cavity is guided to the blazed grating, wherein the first-order diffraction light is reflected back into the laser resonant cavity, and the zero-order diffraction light is reflected and output by the reflector; and the blazed grating is rotated around its vertical axis to tune the central wavelength of the laser output through the zero-order diffraction, and the length of the rare-earth-doped fiber is determined according to the reflectivity of the high-reflection Bragg fiber grating, the diffraction efficiency of the blazed grating and the coupling efficiency of the first-order diffraction light returning to the fiber.

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

[0013] The reflectivity of the high-reflection Bragg fiber grating is greater than 90%, and its grating bandwidth is 1018nm to 1042nm or 1056nm to 1070nm; the length of the doped rare-earth fiber is 5-6cm and its output end is beveled, and the predetermined high-doping quantitative index of the doped rare-earth fiber is that the fiber core absorption coefficient used is 800-1500dB / m@976nm, and the doped rare-earth 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 8 mm or 11 mm, and is used to collimate the light emitted from the optical fiber and couple the first-order diffraction light returned by the blazed grating back into the optical fiber; the line number of the blazed grating is 800 lines / mm to 2400 lines / mm, and the diffraction efficiency is 60% to 80%.

[0016] The different intervals Δυ of the multiple longitudinal modes q The calculation formula is:

[0017]

[0018] Where L is the effective optical path of light propagating in the resonant cavity, l′ is the distance between the output end of the rare-earth-doped fiber and the blazed grating, n is the refractive index of the medium inside the rare-earth-doped fiber, and c is the propagation speed of light in a vacuum.

[0019] The calculation formula for the shortest length of the highly doped rare earth optical fiber is:

[0020]

[0021] Where R represents the effective reflectivity of the external resonant cavity, R = R1R2, R1 is the reflectivity of the fiber input end, and R2 is the effective reflectivity of the blazed grating; ∈ represents the effective transmittance of the external resonant cavity, ∈ = ε1ε2, ε1 represents the transmittance of the highly reflective Bragg fiber grating, and ε2 represents the efficiency of the first-order diffraction light of the blazed grating being recoupled back into the fiber; α p and α s Represent the pump light and signal light fiber attenuation coefficients respectively; δ is the saturation power ratio used to represent the energy transfer efficiency and saturation characteristics of pump light and signal light, and the saturation power ratio δ is in represents the saturation power of the pump light during single beam transmission, The cross saturation power of the signal is a parameter that characterizes the energy transferred from the absorbed pump power to the signal. Alternatively, the saturation power ratio δ is the ratio of the pump light absorption cross section to the signal light emission cross section.

[0022] The process of rotating the blazed grating around its vertical axis to tune the central wavelength of the laser output through zero-order diffraction comprises:

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

[0024]

[0025] Wherein, α is the incident angle of the reflection grating of the blazed grating; d is the grating period, which is the inverse of the groove density; λ is the central wavelength of the laser output by the zero-order diffraction;

[0026] The central wavelength of the laser outputted through the zero-order diffraction is tuned based on the determined grating equation.

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

[0028] The multi-longitudinal mode tunable fiber laser may further include a thermistor, which is arranged at the center of the fiber disk composed of the high-reflection Bragg fiber grating, and a thermoelectric cooler is also arranged 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 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 to existing technologies, the multi-longitudinal-mode tunable fiber laser provided by the present invention achieves optical amplification and diffraction grating mode-selective output by optically pumping and exciting energy-level transitions of rare-earth ions in a relatively 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. This enables the corresponding fiber laser to effectively provide corresponding tuning performance, stability, and output power, resolving the problems of existing technologies. Compared with semiconductor lasers, this device has greater potential and can promote the development of laser technology and optical fiber communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 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 an equivalent linear external resonant cavity model of a light generating unit provided in an embodiment of the present invention;

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

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

[0035] Figure 5 Spectrum at 1056 nm for the test provided in an embodiment of the present invention;

[0036] Figure 6 This is a graph showing the relationship between the wavelength tunable range and the laser output power in the test provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the specific content of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments, and do not constitute a limitation of the present invention. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0038] First, the following terms may be used in this article:

[0039] The term “and / or” means that either or both of them can be realized at the same time. For example, X and / or Y includes both “X” or “Y” and “X and Y”.

[0040] The terms "include," "comprises," "contains," "has," or other similar expressions should be interpreted as non-exclusive. For example, "including certain technical features (such as raw materials, components, ingredients, carriers, dosage forms, materials, dimensions, parts, components, mechanisms, devices, steps, procedures, methods, reaction conditions, processing conditions, parameters, algorithms, signals, data, products, or manufactured articles, etc.) should be interpreted as including not only the technical features explicitly listed, but also other technical features known in the art that are not explicitly listed.

[0041] The term "consisting of" excludes any technical features not explicitly listed. If used in a claim, this term renders the claim closed, excluding any technical features other than those explicitly listed, except for conventional impurities associated with them. If this term appears only in a clause of a claim, it limits only the elements explicitly listed in that clause; elements listed in other clauses are not excluded from the claim as a whole.

[0042] Unless otherwise specified or limited, the terms "mounted," "connected," "connect," and "fixed" should be interpreted broadly. For example, they can refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediary; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in this document based on specific circumstances.

[0043] When concentration, temperature, pressure, size or other parameters are expressed in the form of a numerical range, the numerical range should be understood to specifically disclose all ranges formed by the pairing of any upper limit, lower limit, or preferred value within the numerical range, regardless of whether the range is explicitly stated. For example, if a numerical range of "2 to 8" is stated, the numerical range should be interpreted as including ranges of "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 specified, the numerical ranges stated herein include both their endpoints and all integers and fractions within the numerical range.

[0044] The terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings and are only for the convenience and simplification of description, and do not explicitly or implicitly indicate that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be understood as a limitation to this document.

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

[0046] Specifically, during the implementation of the present invention, pump light is forward-pumped into a fiber Bragg grating (FBG) with high reflectivity at the emission wavelength. It then passes through a short section of highly doped rare-earth optical fiber, where the rare-earth elements can be ytterbium, erbium, or other materials. The light is then collimated by a lens, and a high-diffraction-efficiency reflective diffraction grating is used as the external cavity to form a laser resonant cavity. The emission wavelength is selected by rotating the diffraction grating, and the emitted laser light is reflected and output from the grating, realizing a tunable multi-longitudinal-mode laser. This invention leverages the wide tuning range of a blazed grating while avoiding the significant coupling losses associated with spatial optical components. By combining a highly reflective fiber Bragg grating with an ultrashort, highly doped rare-earth single-mode optical fiber, the invention develops a multi-longitudinal-mode tunable fiber laser that achieves high precision, stability, multiple longitudinal modes, narrow linewidth, and tunability over a wide wavelength range. This approach holds significant significance in fields such as optical communications, spectral analysis, sensing, medical treatment, and scientific research.

[0047] The following will be combined Figure 1 The optical path structure diagram shown in the figure introduces the implementation process and principle of multi-longitudinal mode tunable fiber laser.

[0048] like Figure 1 As shown in FIG, it is a schematic diagram of the optical path structure of a multi-longitudinal mode tunable fiber laser provided by 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 central wavelength of the semiconductor laser pump source 1 can be 976 nm (usually a parameter value greater than or equal to 915 nm and less than or equal to 976 nm can be selected). The output power of the pump source as a stimulated laser generally needs to reach the starting power, for example, the output power can be set to be greater than 50 mW;

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

[0052] The wavelength division multiplexer 3 is used to couple the pump light (e.g., 976 nm or 915 nm) 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 in the laser cavity, and improve output stability;

[0053] (2) Light generating unit: including a high-reflection Bragg fiber grating 4, a rare earth-doped fiber 5, and a collimating lens 6. The highly doped rare earth fiber (i.e., the doped rare earth fiber) can be made of rare earth elements such as ytterbium or erbium. The fiber length can be controlled within the range of 5-6 cm, and the corresponding output end can be an angled end.

[0054] The high-reflection fiber Bragg grating 4 is used to transmit the 976nm pump light and highly reflect the laser excited in the gain fiber. The corresponding reflectivity can reach more than 90%. The optional grating bandwidth can be 1018nm to 1042nm, 1056nm to 1070nm, etc.

[0055] The doped rare earth optical fiber 5 may be a highly doped ytterbium-doped optical fiber or other doped rare earth optical fiber. The doped rare earth optical fiber meets the predetermined high-doping quantitative index requirement. When the doped rare earth optical fiber is an ytterbium-doped optical fiber, the predetermined high-doping quantitative index may be an optical fiber core absorption coefficient of 800-1500dB / m@976nm, for example, 1200dB / m@976nm, which is much higher than that of ordinary doped optical fibers. Furthermore, if applied to other output laser wavelengths, erbium-doped optical fibers or neodymium-doped optical fibers may be used.

[0056] The collimating lens 6 may have a focal length of 8 mm or 11 mm, etc., to collimate the light emitted from the optical fiber and couple the first-order diffracted light returned by the blazed grating 7 back into the optical fiber to improve the spatial coupling efficiency;

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

[0058] Among them, 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, etc.) serve as gain media in the optical fiber, and the energy level transitions of the rare earth ions are excited by optical pumping, thereby achieving light amplification and laser output. The collimated fiber laser beam in the external resonant cavity is guided to the blazed grating 7, and the first-order diffraction light is reflected back into the external resonant cavity of the laser; the laser output is output through the zero-order diffraction of the blazed grating 7; specifically, the blazed grating 7 can be rotated around its vertical axis so that the central wavelength of the laser output through the zero-order diffraction can be tuned, thereby achieving the corresponding tunable function;

[0059] The blazed grating 7 may have a line count of 800 lines / mm to 2400 lines / mm, for example, 1200 lines / mm. Its diffraction efficiency, affected by polarization, may be 60% to 80%, which 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 to output laser light.

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

[0062] In the implementation process of the embodiment of the present invention, the interval between different modes of the multi-longitudinal mode laser can be expressed as Δυ q , in theory, it can be roughly calculated by the following formula:

[0063]

[0064] Where L is the effective optical path of light propagating in the resonant cavity, l′ is the distance between the output end face of the rare-earth-doped fiber and the blazed grating, n is the refractive index of the medium inside the rare-earth-doped fiber, and c is the propagation speed of light in a vacuum.

[0065] like Figure 2 As shown in the figure, the external resonant cavity tunable fiber laser can be equivalent to a simple linear cavity model. Based on this model, the minimum length design of the corresponding highly doped rare earth fiber can be roughly calculated by the following formula:

[0066]

[0067] Where R represents the effective reflectivity of the resonant cavity, R = R1R2, R1 is the reflectivity of the fiber input end, and R2 is the effective reflectivity of the blazed grating; ∈ represents the effective transmittance of the cavity, ∈ = ε1ε2, ε1 represents the transmittance of the highly reflective Bragg fiber grating, and ε2 represents the efficiency of the first-order diffraction light of the blazed grating being recoupled back into the fiber; α p (α s ) represents the pump light (signal light) fiber attenuation coefficient, and δ is the saturation power ratio used to represent the energy transfer efficiency and saturation characteristics of pump light and signal light, which can be expressed as in represents the saturation power of the pump light during single beam transmission, It represents the cross-saturation power of the signal, a parameter that characterizes the energy transferred from the absorbed pump power to the signal. Alternatively, δ can be approximated as the ratio of the pump light absorption cross section to the signal light emission cross section.

[0068] In the embodiment of the present 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 diffraction 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 forms a resonant cavity with the blazed grating 7, and the laser is output through the zero-order reflected light.

[0069] The blazed grating 7 plays an important role in the tunable fiber laser process. When the reflection grating (i.e., the blazed grating) diffracts at the mth order, the relationship between the incident angle α and the diffraction angle βm can be expressed as:

[0070]

[0071] Wherein, d is the grating period of the blazed grating, which can be specifically the inverse of the groove density; λ is the central wavelength of the laser output by 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 working orders, namely the zeroth and first diffraction orders, and when it is in the Littrow configuration, β1 = α. 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 the grating equation, so that the central wavelength of the laser output through zero-order diffraction can be tuned based on the determined grating equation.

[0075] It should be noted that, in the specific implementation process of the embodiment of the present invention, its specific implementation structure can refer to Figure 4 As shown, Figure 4 This is a top view of the Ytterbium-doped fiber tunable multi-longitudinal mode laser. Figure 4 The light output from the corresponding pump injection unit passes through an ordinary optical fiber Hi1060, then through a high-reflection Bragg fiber grating 4 with a reflectivity of 90% near 1030nm of about 5cm and a 5.5cm long ytterbium-doped fiber 5 (Yb1200 4 / 125), and then is connected to a collimating lens through the FC / APC connector of the ytterbium-doped fiber to collimate the output of the 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 sleeve containing the ytterbium-doped fiber connector and the collimating lens and adjust its pitch position.

[0076] Furthermore, a temperature control module can be added to the optical fiber spiral disk assembled by the high-reflection fiber Bragg grating. Specifically, a TEC chip (i.e., a thermoelectric cooler) can be placed under the optical fiber disk, and a thermistor 10 can be placed in the central depression of the optical fiber disk. The thermistor is used to accurately measure the temperature and provide a feedback signal to adjust the driving current of the thermoelectric cooler, thereby achieving stable temperature control and better temperature control effect. In this way, when the temperature of the optical 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 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 the present invention mainly includes the following steps:

[0079] (1) It is necessary to design an appropriate length of rare earth-doped optical fiber;

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

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

[0082] (2) What needs to be designed is the rotation angle of the blazed grating;

[0083] The corresponding rotation angle needs to ensure that the first-order diffraction light at the starting wavelength can return to the doped optical fiber; specifically, the grating equation of the Littrow structure can calculate that the blaze angle of 1064nm laser oscillation can be 39.67 degrees; further, affected by the bandwidth of the high-reflection Bragg fiber grating, the corresponding tunable range can be 1056nm to 1070nm, and the corresponding incident angle can be 39.32 degrees to 39.94 degrees.

[0084] During the implementation of the embodiment of the present invention, because the highly reflective fiber Bragg grating is sensitive to temperature changes, the fiber optic disc and the external resonant cavity need to be temperature controlled after being assembled into a fiber optic disc. It is necessary to wait until the temperature of the fiber optic disc and the external resonant cavity reach stability, such as when the temperature error is less than 0.5mK, before rotating the rotation angle of the blazed grating and adjusting the corresponding pitch adjustment frame to adjust the output laser.

[0085] In summary, the multi-longitudinal-mode tunable fiber laser provided in the embodiments of the present invention can provide a more flexible and efficient light source for application fields such as spectral analysis, fiber-optic communication, lidar, and medical treatment, and has broad application prospects.

[0086] Specifically, they may include:

[0087] In the field of optical fiber communications, it can significantly improve the performance of wavelength division multiplexing (WDM) systems, achieve multi-wavelength signal transmission by precisely adjusting the wavelength, and greatly increase communication bandwidth and flexibility;

[0088] In the field of fiber optic communication sensing, tunable lasers can achieve high-sensitivity and high-precision multi-parameter measurements, and are suitable for fields such as structural health monitoring and environmental monitoring;

[0089] In the medical field, corresponding tunable fiber lasers can provide high-resolution imaging in optical coherence tomography (OCT). In laser therapy, they can act on diseased tissues through selective wavelengths, reducing damage to healthy tissues and improving treatment accuracy.

[0090] In the manufacturing industry, tunable fiber lasers offer high flexibility and precision in laser processing, making them suitable for high-precision processing applications such as micromachining and 3D printing, 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.

[0091] In summary, the multi-longitudinal-mode tunable fiber laser provided by the embodiment of the present invention can demonstrate its unique advantages and broad application prospects in multiple technical fields due to its unique advantages.

[0092] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims. The information disclosed in the background technology section of this article is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or any form of implication that the information constitutes prior art already 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-reflection Bragg fiber grating, a rare-earth-doped fiber and a collimating lens, wherein the rare-earth-doped fiber meets the predetermined high-doping quantitative index requirements; 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 constitute an external resonant cavity; within the external resonant cavity, the collimated fiber laser beam in the resonant cavity is guided to the blazed grating, wherein the first-order diffraction light is reflected back into the laser resonant cavity, and the zero-order diffraction light is reflected and output by the reflector; and the blazed grating is rotated around its vertical axis to tune the central wavelength of the laser output through the zero-order diffraction, and the length of the rare-earth-doped fiber is determined according to the reflectivity of the high-reflection Bragg fiber grating, the diffraction efficiency of the blazed grating and the coupling efficiency of the first-order diffraction light returning to the fiber.

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

3. The multi-longitudinal mode tunable fiber laser according to claim 1, characterized in that: The reflectivity of the high-reflection Bragg fiber grating is greater than 90%, and its grating bandwidth is 1018nm to 1042nm or 1056nm to 1070nm; the length of the doped rare-earth fiber is 5-6cm and its output end is beveled, and the predetermined high-doping quantitative index of the doped rare-earth fiber is that the fiber core absorption coefficient used is 800-1500dB / m@976nm, and the doped rare-earth fiber is ytterbium-doped fiber; if applied to other output laser wavelengths, erbium-doped fiber or neodymium-doped fiber is used.

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

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

6. The multi-longitudinal mode tunable fiber laser according to any one of claims 1 to 5, characterized in that: The different intervals Δυ of the multiple longitudinal modes q The calculation formula is: Where L is the effective optical path of light propagating in the resonant cavity, l′ is the distance between the output end of the rare-earth-doped fiber and the blazed grating, n is the refractive index of the medium inside the rare-earth-doped fiber, and c is the propagation speed of light in a vacuum.

7. The multi-longitudinal mode tunable fiber laser according to claim 5, characterized in that: The calculation formula for the shortest length of the highly doped rare earth optical fiber is: Where R represents the effective reflectivity of the external resonant cavity, R = R1R2, R1 is the reflectivity of the fiber input end, and R2 is the effective reflectivity of the blazed grating; ∈ represents the effective transmittance of the external resonant cavity, ∈ = ε1ε2, ε1 represents the transmittance of the highly reflective Bragg fiber grating, and ε2 represents the efficiency of the first-order diffraction light of the blazed grating being recoupled back into the fiber; α p and α s Represent the pump light and signal light fiber attenuation coefficients respectively; δ is the saturation power ratio used to represent the energy transfer efficiency and saturation characteristics of pump light and signal light, and the saturation power ratio δ is in represents the saturation power of the pump light during single beam transmission, The cross saturation power of the signal is a parameter that characterizes the energy transferred from the absorbed pump power to the signal. Alternatively, the saturation power ratio δ is the ratio of the pump light absorption cross section to the signal light emission cross section.

8. The multi-longitudinal mode tunable fiber laser according to claim 5, characterized in that: The process of rotating the blazed grating around its vertical axis to tune the central wavelength of the laser output through zero-order diffraction comprises: The grating equation that determines the relationship between the laser center wavelength of the tuned zero-order diffraction output and the rotation angle of the blazed grating is: Wherein, α is the incident angle of the reflection grating of the blazed grating; d is the grating period, which is the inverse of the groove density; λ is the central wavelength of the laser output by the zero-order diffraction; The central wavelength of the laser outputted through the zero-order diffraction is tuned based on the determined grating equation.

9. The multi-longitudinal mode tunable fiber laser according to claim 5, characterized in that: It also includes a pitch adjustment frame for fixing and adjusting the pitch of a sleeve of a rare-earth-doped optical fiber connector and a collimating lens including a ferrule contactor FC / angle physical contact APC; wherein, the collimating lens is connected to the output external resonant cavity through the FC / APC connector, and then the blazed grating is rotated to a set angle and the output laser is adjusted by adjusting the pitch adjustment frame.

10. The multi-longitudinal mode tunable fiber laser according to claim 9, characterized in that: The system further comprises a thermistor disposed at the center of the optical fiber disk formed by the high-reflection fiber Bragg grating, and a thermoelectric cooler is also disposed below the optical 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 optical fiber disk and the temperature of the external resonant cavity meet 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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