A safety-protected high-power MOPA fiber laser against signal light interruption

Abstract

The application provides a kind of safety protection high-power MOPA fiber laser for signal light interruption, comprising seed source, isolator and at least one power amplification stage connected in sequence, wherein the seed source generates seed light of first wavelength, the seed light enters the multi-stage pre-amplification stage through the isolator, and outputs signal light, the signal light enters the power amplification stage;The power amplification stage includes pump source, beam combiner, gain fiber, cladding light stripper and output head, a first grating is arranged at one end of the gain fiber, and a second grating is arranged at the other end, the center wavelength of the first grating and the second grating is second wavelength, and the first wavelength is different from the second wavelength.No additional active control element is needed, to achieve fast, passive switching to safety state with high response speed when signal light interruption occurs.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to a high-power MOPA fiber laser for safety protection against signal light interruption. Background Technology

[0002] High-power, narrow-linewidth fiber lasers commonly employ a master oscillator power amplifier (MOPA) structure. High-power, narrow-linewidth MOPA fiber lasers have wide applications in industrial processing, scientific research, and national defense. Their typical structure is a master oscillator power amplifier (MOPA), consisting of a low-power, high-beam-quality narrow-linewidth seed source and subsequent multi-stage power amplification stages.

[0003] However, such lasers have a significant failure risk: when the pre-amplifier optical path fails or disconnects—for example, due to seed source failure, main optical path disconnection, or other factors resulting in a sudden lack of signal light output or a significant reduction in signal light power—the pump energy of subsequent power amplifier stages continues to be injected. In this situation, the energy stored in the gain medium cannot be effectively extracted and will be released disorderly through nonlinear mechanisms such as spontaneous emission (ASE). This disordered amplification generates powerful superradiative light, causing a rapid increase in temperature of core components such as the gain fiber, combiner, and cladding stripper, ultimately leading to overheating and permanent damage, resulting in substantial economic losses for the user.

[0004] Currently, common protection measures involve monitoring the seed light power or the signal light power output from the pre-amplifier stage using photodetectors. If an anomaly is detected, the pump drive power to the power amplifier stage is cut off. However, the response speed of this electronic protection method is limited by the circuitry and the pump source itself, and may involve delays. Typically, the braking time when a laser malfunctions is on the order of hundreds of milliseconds (or at least tens of milliseconds), which cannot completely prevent energy accumulation within microseconds, still posing a risk of damage to laser components. Therefore, a fast-responding, passive intrinsic safety protection mechanism based on physical principles is needed. Summary of the Invention

[0005] In view of this, embodiments of this application provide a high-power MOPA fiber laser for security protection against signal light interruption, in order to solve the technical defects existing in the prior art.

[0006] According to a first aspect of the embodiments of this application, a high-power MOPA fiber laser with security protection against signal light interruption is provided, comprising a seed source 1, an isolator 2, a multi-stage pre-amplification stage, and a single-stage power amplifier stage connected in sequence, wherein... The seed source (1) generates seed light of the first wavelength. The seed light passes through the first isolator (2) and enters the multi-stage pre-amplification stage. After pre-amplification, the seed light is output as signal light through the second isolator (2). The signal light enters the power amplification stage. The power amplifier stage includes a pump source 3, a bundler 4, a gain fiber 5, a cladding stripper 6, and an output head 7. A first grating 8 is arranged at one end of the gain fiber 5, and a second grating 9 is arranged at the other end. The center wavelengths of the first grating 8 and the second grating 9 are both the second wavelength, and the first wavelength is different from the second wavelength.

[0007] Optionally, the reflectivity of the first grating 8 and the second grating 9 at the first wavelength is lower than a preset reflectivity threshold.

[0008] Optionally, when the pre-amplifier optical path consisting of the seed source 1, the first isolator 2, and the multi-stage pre-amplifier is working normally, the signal light is amplified by the gain fiber 5 in the power amplifier stage and outputs the first wavelength laser through the output head 7; when the pre-amplifier optical path fails or is disconnected, causing the signal light to be interrupted, the laser resonant cavity consisting of the gain fiber 5, the first grating 8, and the second grating 9 generates the second wavelength clamp laser, which is output through the output head 7.

[0009] Optionally, by configuring the laser resonator, a threshold gain coefficient for the resonator can be obtained. This threshold gain coefficient corresponds to the oscillation condition of the laser resonator. When the front-end optical path is working normally, the laser power output by the seed source 1 is amplified sequentially by the multi-stage pre-amplification stage and the power amplification stage. Based on the gain saturation effect, the first actual gain coefficient of the gain fiber 5 at the second wavelength is suppressed below the threshold gain coefficient of the resonant cavity. When the pre-stage optical path fails or disconnects, causing the signal light to be interrupted, the second actual gain coefficient of the gain fiber 5 at the second wavelength is greater than the laser threshold gain coefficient, the laser resonator starts to oscillate, and generates the clamping laser at the second wavelength, which is output through the output head 7.

[0010] Optionally, the process of setting up the laser resonant cavity includes: Determine the pump power of the pump source 3, and based on the pump power, determine the small signal gain coefficient corresponding to the second wavelength when the front-end optical path fails or is disconnected, causing the signal light to be interrupted; Determine the ratio of the emission cross-section of the gain fiber 5 at the first wavelength to the second wavelength, and the wavelength ratio of the first wavelength to the second wavelength; The intensity of the signal light is determined based on the small signal gain coefficient, the emission cross-section ratio, and the wavelength ratio.

[0011] Optionally, when the preceding optical path fails or disconnects, causing the signal light to be interrupted, the total response time associated with the clamping laser oscillation is much less than the heat accumulation time associated with thermal damage.

[0012] This application provides a high-power MOPA fiber laser for safety protection against signal light interruption, comprising a seed source 1, an isolator 2, a multi-stage pre-amplification stage, and a power amplifier stage connected sequentially. The seed source 1 generates seed light of a first wavelength. The seed light passes through the first isolator 2 and enters the multi-stage pre-amplification stage. After pre-amplification, the seed light passes through the second isolator 2 and outputs the signal light, which then enters the power amplifier stage. The power amplifier stage includes a pump source 3, a combiner 4, a gain fiber 5, a cladding stripper 6, and an output head 7. A first grating 8 is arranged at one end of the gain fiber 5, and a second grating 9 is arranged at the other end. The center wavelengths of both the first grating 8 and the second grating 9 are the second wavelength, but the first wavelength is different from the second wavelength. This protection is used to protect laser components from damage. Specifically, it provides protection by embedding a laser resonator of a specific wavelength in the main power amplifier stage, enabling rapid and passive switching to a safe state when the signal light is interrupted. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of a high-power MOPA fiber laser for security protection against signal light interruption, provided in an embodiment of this application. Detailed Implementation

[0015] Many specific details are set forth in the following description to provide a full understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this application; therefore, this application is not limited to the specific embodiments disclosed below.

[0016] The terminology used in one or more embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the scope of one or more embodiments of this application. The singular forms “a,” “the,” and “the” used in one or more embodiments of this application and in the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” used in one or more embodiments of this application refers to and includes any or all possible combinations of one or more associated listed items.

[0017] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this application, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this application, and similarly, second may also be referred to as first.

[0018] This application provides a high-power MOPA fiber laser for security protection against signal light interruption, which will be described in detail in the following embodiments.

[0019] Figure 1 This diagram illustrates a high-power MOPA fiber laser for security protection against signal light interruption according to an embodiment of this application. Specifically, it includes a seed source 1, an isolator 2, and at least one power amplifier stage connected sequentially. The seed source 1 generates seed light of a first wavelength. The seed light passes through the first isolator 2 and enters the multi-stage pre-amplification stage. After pre-amplification, the seed light is output as signal light through the second isolator 2. The signal light then enters the power amplification stage. The power amplifier stage includes a pump source 3, a bundler 4, a gain fiber 5, a cladding stripper 6, and an output head 7. A first grating 8 is arranged at one end of the gain fiber 5, and a second grating 9 is arranged at the other end. The center wavelengths of the first grating 8 and the second grating 9 are both the second wavelength, and the first wavelength is different from the second wavelength.

[0020] Specifically, seed source 1 is the basic light source component of the laser, used to generate the initial, low-power laser light, i.e., seed light, which is the source for subsequent power amplification. In practical applications, the seed light is a single-frequency laser or a narrow-linewidth laser obtained by phase modulation and spectral broadening of a single-frequency laser. Isolator 2 is an optical passive device whose core function is to allow the optical signal to propagate in one direction and prevent the optical signal from propagating in the opposite direction, thus avoiding damage to the front-end devices by reverse light. The multi-stage pre-amplification stage amplifies the low-power seed light output from the seed source to a signal light at the tens of watts level. The power amplification stage amplifies the signal light into a high-power laser, increasing the signal light from tens of watts to thousands of watts. The first wavelength λ_signal can be abbreviated as λ. s The initial laser wavelength generated by the seed source is the wavelength of the initial laser, which is also the reference wavelength for the final output laser when the laser is working normally. The seed light is a low-power, high-stability laser generated by the seed source, which determines the core parameters such as the center wavelength of the final output laser. The pump source 3 is the light source that provides energy for power amplification. It injects pump light into the gain fiber 5 to excite the gain medium in the gain fiber and realize the power amplification of the signal light. The beam combiner 4 is an optical beam combiner used to couple the pump light output from the pump source with the signal light and input them together into the gain fiber. The gain fiber 5 is the core component of the power amplification. It contains a gain medium, such as rare earth doped ions, which generate stimulated emission under the excitation of the pump light to realize the power amplification of the signal light. The cladding light stripper 6 is used to strip unwanted light transmitted in the cladding of the gain fiber, such as pump light that has not been coupled into the fiber core and stray light, to avoid the accumulation of unwanted light that may cause device heating and damage, and to improve the purity of the laser output. The output head 7 is the laser output port of the laser, which exports the amplified high-power laser for subsequent application scenarios.

[0021] Furthermore, both the first grating 8 and the second grating 9 are fiber gratings, which are periodic refractive index modulation structures formed within the fiber core. They have the function of frequency selection and reflection of specific wavelengths of light. Specifically, a fiber Bragg grating (FBG) can be used. The center wavelength is the core parameter of the grating. The center wavelength of both the first and second gratings is set to the second wavelength λ_clamp, which can be abbreviated as λc. The second wavelength is different from the first wavelength and is the wavelength of the clamping laser output by the laser when the previous stage optical path fails or is disconnected.

[0022] Based on this, a pair of fiber Bragg gratings (FBGs) are disposed at both ends of the gain fiber 5 in the power amplifier stage. These FBGs, together with the gain fiber 5 between them, constitute a built-in laser resonant cavity. The center reflection wavelength λ_clamp of the pair of FBGs differs from the output wavelength λ_signal of the seed source. The center reflection wavelength λ_clamp of the FBGs corresponds to the wavelength of the resonant cavity, while the output wavelength λ_signal of the seed source corresponds to the wavelength during normal laser operation. Therefore, the pair of FBGs has extremely low reflectivity at the output wavelength λ_signal of the seed source, avoiding parasitic oscillations in the main laser.

[0023] So, when the preceding optical path is normal, the laser works normally, the resonant cavity does not work, and it does not affect the laser. When the preceding optical path fails or is disconnected, if there is still no resonant cavity, then high-energy ASE will be generated due to spontaneous emission, burning out the optical path components. If there is a resonant cavity, but the preceding optical path fails or is disconnected, most of the remaining pump energy will exceed the threshold of the resonant cavity, satisfying the resonant cavity oscillation condition, generating laser with a wavelength of λ_clamp, thereby clamping the gain of the power amplifier stage to a safe level, consuming the excess pump energy, and ensuring the safety of the laser.

[0024] In addition, it should be noted that the first isolator is Figure 1 The left isolator shown in the diagram, and the second isolator is... Figure 1 The isolator on the right of the two isolators shown. Figure 1 The laser structure shown is only one example. In actual applications, the main amplifier module is divided into two architectures based on the direction of the pump and signal light: pump and signal light in the same direction, and pump and signal light in opposite directions, corresponding to forward pumping and reverse pumping architectures, respectively. These laser structures can all achieve complete deactivation of the protection mechanism during normal laser operation by arranging a first grating at one end of the gain fiber and a second grating at the other end. The center wavelengths of both the first and second gratings are the second wavelength, and the first and second wavelengths of the seed source are different. This ensures that the protection mechanism is completely deactivated and does not affect the main laser performance when the laser is operating normally. In the event of failure or disconnection of the pre-amplifier optical path, this protection mechanism can automatically and quickly switch the amplification stage to a safe laser oscillation state, thereby fundamentally avoiding thermal damage to optical components from superradiation light by consuming the original pump energy.

[0025] Furthermore, the reflectivity of the first grating 8 and the second grating 9 at the first wavelength is lower than a preset reflectivity threshold.

[0026] The preset reflectivity threshold is a manually set upper limit of reflectivity, used to limit the reflectivity of the grating at the first wavelength, ensuring that the grating will not adversely affect the transmission of the laser at the first wavelength. The specific value can be adjusted according to the overall parameters of the laser. In actual use, the fiber grating is made using an apodization process to suppress the side lobes of its reflection spectrum, ensuring that the reflectivity at the λ_signal wavelength is lower than the preset reflectivity threshold. In actual use, the reflectivity threshold is preferably set to -40dB to avoid parasitic oscillations in the main laser.

[0027] In addition, it should be noted that the laser resonant cavity composed of the first grating, the second grating, and the gain fiber can be set only in the last power amplification stage, or it can be set in the pre-amplification stage.

[0028] For example, with λ_signal=1064nm and λ_clamp=1030nm, or λ_signal=1064nm and λ_clamp=1080nm, when the laser is working normally, the grating with a wavelength of 1030nm has a reflectivity of less than -40dB at 1064nm, so as to prevent the grating from causing unnecessary laser oscillation at the wavelength of 1064nm, which would interfere with the normal operation of the laser.

[0029] Therefore, by reducing the proportion of the first wavelength laser reflected by the grating, the signal light can be smoothly amplified into the gain fiber, and the amplified first wavelength laser can be smoothly transmitted to the output head through the grating, reducing reflection loss and improving power amplification efficiency; avoiding the reflected first wavelength laser from being transmitted back to the seed source, preventing damage to the seed source and extending the device lifespan; eliminating the superposition interference between reflected light and incident light, ensuring the stability of the power and wavelength of the output first wavelength laser, and improving the working reliability of the laser; and retaining the reflection capability of the first and second gratings for the second wavelength without affecting the normal transmission of the first wavelength laser, providing a guarantee for the oscillation of the resonant cavity when the subsequent pre-stage optical path fails or is disconnected.

[0030] Furthermore, when the pre-amplifier optical path consisting of the seed source 1, the first isolator 2, and the multi-stage pre-amplifier stage is working normally, the signal light is amplified by the gain fiber 5 in the power amplifier stage and outputs the first wavelength laser through the output head 7; when the pre-amplifier optical path fails or is disconnected, causing the signal light to be interrupted, the laser resonant cavity consisting of the gain fiber 5, the first grating 8, and the second grating 9 generates the second wavelength clamp laser, which is output through the output head 7.

[0031] Furthermore, by configuring the laser resonant cavity, a threshold gain coefficient for the resonant cavity is obtained. This threshold gain coefficient corresponds to the oscillation condition of the laser resonant cavity, wherein... When the pre-stage optical path is working normally, the laser power output by the seed source 1 is amplified sequentially by the multi-stage pre-amplification stage and the power amplification stage. Based on the gain saturation effect, the first actual gain coefficient of the gain fiber 5 at the second wavelength is suppressed below the threshold gain coefficient of the resonant cavity. When the pre-stage optical path fails or is disconnected, causing the signal light to be interrupted, the second actual gain coefficient of the gain medium at the second wavelength is greater than the laser threshold gain coefficient. The laser resonant cavity oscillates and generates the clamped laser at the second wavelength. The clamped laser is output through the output head 7.

[0032] Furthermore, the process of setting up the laser resonant cavity is specifically implemented as follows in this embodiment: The pump power of the pump source 3 is determined. Based on the pump power, the small-signal gain coefficient corresponding to the second wavelength is determined when the pre-stage optical path fails or is disconnected, causing the signal light to be interrupted. The ratio of the emission cross-section of the gain fiber 5 at the first wavelength to the second wavelength, and the wavelength ratio of the first wavelength to the second wavelength are determined. Based on the small-signal gain coefficient, the emission cross-section ratio, and the wavelength ratio, the light intensity of the signal light is determined.

[0033] The normal operation of the pre-amplifier optical path means that the seed source can stably generate seed light of the first wavelength, the output power, wavelength and other parameters meet the preset standards, the main optical path of the pre-amplifier stage is fault-free, and the signal light is output at normal power. The output laser refers to the high-power laser output from the output head after amplification by the power amplifier stage. Its wavelength is consistent with the first wavelength of the seed light, which is the normal output of the laser. The failure or disconnection of the pre-amplifier optical path means that the pre-amplifier stage cannot output the signal light of the first wavelength normally, which may be caused by device failure, abnormal power supply, optical path disconnection and other reasons. The laser resonator is an optical resonant structure composed of gain fiber 5, first grating 8 and second grating 9. The first grating and second grating are two mirrors of the resonator to reflect the light of the second wavelength. The gain fiber provides stimulated emission gain and generates laser after the laser oscillation condition is met. The clamped laser is the laser with the second wavelength generated by the laser resonator when the seed source fails. Its core function is to release the pump energy accumulated in the gain fiber and achieve safety protection.

[0034] In addition, the laser threshold gain coefficient g_th_clamp, abbreviated as g th_cThe minimum gain coefficient required for a laser resonant cavity to generate laser oscillation is the core criterion for determining the oscillation of the resonant cavity. When the actual gain coefficient provided by the gain fiber is greater than this threshold, the resonant cavity oscillates and generates laser; otherwise, it does not oscillate. The preset gain is a gain standard set manually based on the power, wavelength, and other parameters of the laser to ensure that the resonant cavity can oscillate stably without interfering with normal operation. The oscillation condition is a necessary condition for the resonant cavity to generate laser oscillation. The core condition is that the actual gain coefficient of the gain fiber is greater than the laser threshold gain coefficient, while the reflection and frequency selection conditions of the grating are also met.

[0035] Gain saturation refers to the fact that the gain coefficient of the gain medium is not constant. When the intensity of the incident light reaches a certain level, the population inversion density of the gain medium will decrease, causing the gain coefficient to decrease and tend to stabilize. This phenomenon is called gain saturation. The first actual gain coefficient is the actual gain coefficient provided by the gain medium in the gain fiber to the second wavelength light when the front-end optical path is working normally, which is affected by the gain saturation effect of the signal light. The second actual gain coefficient is the actual gain coefficient provided by the gain medium in the gain fiber to the second wavelength light when the front-end optical path fails or is disconnected. At this time, there is no gain saturation effect of the signal light to suppress it, and the gain coefficient will increase significantly.

[0036] Furthermore, pump power, the optical power output from the pump source, is a core parameter determining the excitation level of the gain medium in the gain fiber. Higher pump power results in a higher population inversion density in the gain medium and a larger gain coefficient. The small-signal gain coefficient, when the intensity of the second wavelength light is sufficiently weak to avoid gain saturation, is the gain coefficient of the gain medium for that wavelength. It serves as the initial reference value for the second actual gain coefficient when the preceding optical path fails or disconnects, and is positively correlated with the pump power. The emission cross-section is the cross-sectional area of ​​the emitted photons when a particle in the gain medium transitions from the excited state to the ground state. It reflects the gain medium's ability to emit photons of a specific wavelength; a larger emission cross-section indicates a stronger gain capability. The ratio of the emission cross-section to the first wavelength is... The ratio of the emission cross section to the emission cross section of the second wavelength reflects the difference in gain capability between the two wavelengths of laser light in the gain medium; the wavelength ratio is the ratio of the first wavelength to the second wavelength, used to correct for the difference in gain characteristics between the two wavelengths of laser light, ensuring the accuracy of parameter calculation; the saturation intensity is the signal light intensity threshold corresponding to the gain saturation effect of the gain medium. When the signal light intensity reaches the saturation intensity, the gain coefficient begins to stabilize, which is the core parameter for calculating the impact of the gain saturation effect; the output laser intensity is the intensity of the first wavelength laser light output by the output head during normal operation, which is the core performance indicator for normal laser operation and needs to be determined by the above parameters to ensure that it meets the design requirements.

[0037] Based on this, the operating modes of the laser are defined in two states: normal operation and failure or disconnection of the front-end optical path. When the front-end optical path, i.e., the seed source and preamplifier module, is working normally, its powerful output signal suppresses the gain of the gain fiber below the threshold of the built-in laser resonator, preventing it from oscillating. When the seed source or preamplifier module fails, i.e., the front-end signal light is interrupted, the gain of the built-in resonator rapidly accumulates and exceeds the oscillation threshold of the built-in laser resonator. The resonator immediately oscillates and outputs a clamping laser. Specifically, the main laser with wavelength λ_signal quickly switches to the clamping laser with wavelength λ_clamp after the front-end optical path fails or disconnects, thereby clamping the gain of the power amplifier stage to a safe level, consuming excess pump energy, and ensuring the safety of the laser.

[0038] In practical scenarios, considering a doped optical fiber of length L as the gain medium, under continuous wave operating conditions, the steady-state rate equation of the upper energy level particle number density N2(z) can be expressed as: , Where N represents the total doped ion density; I p Characterizing the intensity of the pump light; v p The frequency of the pump light; Characterizing the absorption cross section of the pump light; Characterizing the emission cross section of the pump light; I i Characterizes the intensity of the i-th signal light, including the main signal and the potential clamping light; v i Characterizing the frequency of the i-th signal light, including the main signal and the potential clamping light; Characterizes the emission cross-section of the i-th signal light, including the main signal and the potential clamping light; Characterizing the overlap factor between the pump light and the doped region; The overlap factor characterizing the signal light and the doped region; Characterizes the lifetime of the upper energy level.

[0039] For high-power fiber amplifiers, there are typically And the pump light intensity I p To reveal the competition between the main signal and the clamping signal, we can assume the pump is strong enough to excite the gain medium to a high inversion level and focus on the gain saturation effect caused by the signal light. Then, for a single wavelength λ, its small-signal gain coefficient... It is proportional to the number of particles in the upper energy level, N2: , When a strong light signal is present, the gain will saturate, and the actual gain coefficient will decrease. for: , Among them, saturated light intensity .

[0040] When the main signal wavelength λ exists s and resonant cavity wavelength λ c Both share the same upper energy level particle number N2, due to the strong principal signal light I s The presence of N2 will consume N2, thereby suppressing the gain of the corresponding wavelength of the resonant cavity. Considering the main signal light I... s The effect of the saturation gain coefficient g at the clamping wavelength c Corrections should be made accordingly. According to the rate equation, in the strong principal signal I… s and possible clamping signal I c Under the combined effect, the number of particles in the steady-state upper energy level is: , Among them, R p This is the pump rate. During normal operation, before the resonant cavity begins to oscillate, I... c ≈0. Substitute ,in It is the number of particles in the upper energy level when there is no signal. We can obtain: , It is generally believed that ,and Define the wavelength-dependent coupling function. Then the above formula simplifies to: .

[0041] Therefore, in a uniformly broadened gain medium, strong signal light will cause gain saturation, and the actual gain coefficient g_c at the second wavelength will be abbreviated as g. c It follows the following relationship: , Among them, g 0c When there is no master signal light, the small-signal gain coefficient of the second wavelength, i.e., the clamping wavelength, is determined by the pump power; I s The intensity of the main signal light; The saturation intensity of the main signal light; The first wavelength of the output laser, and the corresponding emission cross section. This represents the second wavelength of the clamped laser and the corresponding emission cross section.

[0042] For a clamped resonant cavity composed of a pair of fiber gratings with reflectivities R1 and R2, the threshold condition for oscillation is that the net single-pass gain equals the single-pass loss: , Where L is the length of the gain fiber, g th，cis the threshold gain coefficient for clamping the wavelength. It can be obtained that: , To ensure that the clamping cavity never oscillates during normal operation, it must satisfy . Substituting the expression of g c , we get: , Rearranging the above inequality, the minimum main signal light intensity I s required to suppress the clamping cavity can be solved as: , That is to say, in the normal operation scenario, since g_c < g_th_c, to reliably suppress the clamping resonator, the main signal light intensity needs to satisfy: .

[0043] Therefore, in the normal amplification mode, when a strong main signal light I s is injected, its actual gain coefficient g_clamp of the gain medium at the λ_clamp wavelength is suppressed below the threshold g_th_clamp through the gain saturation effect, making the built-in resonator unable to meet the oscillation condition and being completely suppressed.

[0044] In the safe clamping mode, when the previous-stage optical path fails or is disconnected, the small-signal gain coefficient g0_c of the gain medium at the λ_clamp wavelength quickly recovers. Since g0_c > g_th_c, the built-in resonator immediately meets the laser oscillation condition, generates laser oscillation at the λ_clamp wavelength, and this oscillating laser consumes a large amount of inverted particles, clamping the net gain of the amplification stage above the g_th_c level, thereby safely discharging the pump energy.

[0045] Moreover, the stronger the inherent oscillation tendency of the resonator, that is, the larger the ratio, the higher the required main signal suppression power I s . By designing the resonator to increase , that is, increasing the resonator loss, such as reducing the reflectivity of the rear grating, and selecting wavelengths with a large gain difference, the requirement for I s can be reduced, making the scheme easier to implement.

[0046] Therefore, by defining the laser's operating modes in two states—normal and failed or disconnected in the pre-stage optical path—it is ensured that the laser output in different states is predictable and controllable. This ensures that the resonant cavity does not oscillate when the pre-stage optical path is normal, and oscillates promptly when it fails, avoiding problems of false oscillation or no oscillation. Through multi-parameter integrated calculation, the laser threshold gain coefficient is precisely matched with the laser's own parameters. During normal operation, the resonant cavity does not oscillate, there is no interference from the second wavelength laser, and the power, purity, and stability of the first wavelength laser are guaranteed, improving the normal operating performance of the laser. When the seed source fails, the resonant cavity can quickly oscillate, promptly generating and outputting clamped laser, rapidly releasing the pump energy accumulated in the gain fiber, completely solving the problem of device burnout caused by energy accumulation, and achieving safety protection after signal light interruption. The switching between the two operating states does not require additional active control components, the switching is smooth, and it does not affect the normal use of the laser, improving the overall stability and ease of use of the laser.

[0047] Furthermore, when the preceding optical path fails or disconnects, causing the signal light to be interrupted, the total response time associated with the clamping laser oscillation is much less than the heat accumulation time associated with thermal damage.

[0048] The total response time associated with clamping laser is the total time from the failure or disconnection of the preceding optical path to the start-up of the laser resonator, the generation and output of the clamping laser, including the seed source failure detection time, the resonator start-up time, and the clamping laser transmission time. It is the core indicator for measuring the response speed of safety protection. The heat accumulation time associated with thermal damage refers to the time when the heat in the gain fiber is sufficient to cause the device to burn out under the condition of continuous pump light injection and no effective energy release. It is the maximum time without energy release that the gain fiber can withstand. The total response time of clamping laser is significantly shorter than the heat accumulation time, usually by more than an order of magnitude, ensuring that the clamping laser can release energy in time before the heat accumulates to the point of causing thermal damage.

[0049] Gain recovery τ gain This refers to the time it takes for the gain medium to recover from the suppressed stimulated emission state of the upper energy level to the small-signal gain state after the main signal light disappears. This time constant is related to the upper energy level lifetime τ and the pump rate, and is usually much smaller than the upper energy level lifetime under high-power pumping. The approximate time for this process is: , Among them, R p It is the pump rate, under high pumping conditions. Therefore, this time can be significantly shorter than the lifetime of the upper energy level. .

[0050] Laser setup time τ build This refers to the time required for photons within the resonant cavity to grow from the noise level to a steady state. For high gain... For short cavities, the round-trip time can be as short as several cavity round-trip times, such as a 10-meter optical fiber with a round-trip time of about 100 nanoseconds.

[0051] The time required for a laser to establish itself from a noise level to a steady state is related to the amount by which the net gain exceeds a threshold: , in, It is the round-trip time of light within the resonant cavity. It is the single-pass net gain of the resonant cavity at the moment when the signal light fails or is disconnected.

[0052] Therefore, the total response time τ_response from signal light failure to clamp laser establishment and start to effectively consume energy can be as short as 1-10 microseconds, which is two to three orders of magnitude faster than the time required to cause thermal damage, which is typically >1 millisecond, thus providing essential safety protection.

[0053] For example, the laser includes a seed source 1, an isolator 2, and a main power amplifier stage. The main power amplifier stage consists of a pump source 3, a beam combiner 4, a ytterbium-doped gain fiber 5, a cladding light stripper 6, and an output head 7. A high-reflectivity fiber grating 8 (FBG1) and an output coupling fiber grating 9 (FBG2) are fused to the gain fiber 5. Both have a center wavelength λ_clamp of 1080 nm and a reflectivity of <-40 dB at the seed light wavelength λ_signal = 1064 nm.

[0054] By setting the main signal wavelength , resonant cavity wavelength , Resonant cavity design: R1=0.999, R2=0.1, L=10m, then Estimation under high-power pumping ,in .

[0055] So, substituting into... It can be seen that, For 20 / 400µm double-clad optical fibers, their core area Therefore, the minimum required main signal power is: , For actual lasers, the power range of the output signal light of the pre-amplification stage is usually 10-100W, which is much higher than this value. This provides a sufficient safety margin and ensures that the resonant cavity does not oscillate when the laser is working normally. This proves that the proposed solution is completely feasible under typical engineering parameters.

[0056] Due to the instantaneous loss of signal light, the single-pass net gain of the resonant cavity So, the total response time is: , This response time is more than an order of magnitude faster than the typical time that leads to thermal damage to optical components, thus providing effective physical protection. When the front-end optical path is normal, the 1064nm laser suppresses the 1080nm gain below the threshold; when the front-end optical path fails or is disconnected, the 1080nm gain exceeds the threshold within microseconds, the laser starts to oscillate, and the system outputs a 1080nm clamped laser.

[0057] Specifically, during normal operation, the 1064nm narrow-linewidth laser output from seed source 1, after passing through isolator 2, is pre-amplified and injected into ytterbium-doped fiber 5. This signal light is powerful, strongly saturating the gain of the ytterbium-doped fiber, suppressing its actual gain at 1080nm below the resonant cavity threshold. Therefore, the 1080nm clamped laser cannot oscillate. The system, acting as a standard MOPA amplifier, outputs a high-power 1064nm laser from output head 7. When the seed source fails, the 1064nm seed light disappears, or the pre-amplified signal light disappears, and the gain of ytterbium-doped fiber 5 quickly recovers to a low-signal gain level, immediately exceeding the threshold of the 1080nm resonant cavity. The 1080nm laser quickly oscillates, consuming most of the pump energy, and outputs the 1080nm laser from output head 7. At this time, the system operates as a safe 1080nm laser, avoiding the generation of spontaneous emission (ASE) light and protecting all optical components.

[0058] The foregoing has described specific embodiments of this application. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired results. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0059] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0060] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0061] The preferred embodiments disclosed above are merely illustrative of this application. The optional embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this application. These embodiments are selected and specifically described in this application to better explain the principles and practical applications of this application, thereby enabling those skilled in the art to better understand and utilize this application. This application is limited only by the claims and their full scope and equivalents.

Claims

1. A high-power MOPA fiber laser with safety protection against signal light interruption, characterized in that, It includes a seed source (1), an isolator (2), a multi-stage pre-amplification stage, and a power amplification stage connected in sequence, wherein, The seed source (1) generates seed light of the first wavelength. The seed light passes through the first isolator (2) and enters the multi-stage pre-amplification stage. After pre-amplification, the seed light is output as signal light through the second isolator (2). The signal light enters the power amplification stage. The power amplifier stage includes a pump source (3), a bundler (4), a gain fiber (5), a cladding stripper (6), and an output head (7). A first grating (8) is arranged at one end of the gain fiber (5), and a second grating (9) is arranged at the other end. The center wavelengths of the first grating (8) and the second grating (9) are both the second wavelength, and the first wavelength is different from the second wavelength.

2. The high-power MOPA fiber laser for safety protection against signal light interruption according to claim 1, characterized in that, The reflectivity of the first grating (8) and the second grating (9) at the first wavelength is lower than a preset reflectivity threshold.

3. The high-power MOPA fiber laser for safety protection against signal light interruption according to claim 1, characterized in that, When the seed source (1), the first isolator (2) and the multi-stage pre-amplification stage are working normally, the signal light is amplified by the gain fiber (5) in the power amplification stage and outputs the first wavelength laser through the output head (7); when the pre-stage optical path fails or is disconnected, causing the signal light to be interrupted, the laser resonator composed of the gain fiber (5), the first grating (8) and the second grating (9) generates the second wavelength clamp laser, which is output through the output head (7).

4. The high-power MOPA fiber laser for safety protection against signal light interruption according to claim 3, characterized in that, By configuring the laser resonant cavity, a threshold gain coefficient is obtained. This threshold gain coefficient corresponds to the oscillation condition of the laser resonant cavity. When the pre-stage optical path is working normally, the laser power output by the seed source (1) is amplified sequentially by the multi-stage pre-amplification stage and the power amplification stage. Based on the gain saturation effect, the first actual gain coefficient of the gain fiber (5) at the second wavelength is suppressed below the threshold gain coefficient of the resonant cavity. When the pre-stage optical path fails or is disconnected, causing the signal light to be interrupted, the second actual gain coefficient of the gain fiber (5) at the second wavelength is greater than the laser threshold gain coefficient, the laser resonator starts to oscillate, and generates the clamping laser at the second wavelength. The clamping laser is output through the output head (7).

5. The high-power MOPA fiber laser for safety protection against signal light interruption according to claim 4, characterized in that, The process of setting up the laser resonant cavity includes: Determine the pump power of the pump source (3), and based on the pump power, determine the small signal gain coefficient corresponding to the second wavelength when the front-end optical path fails or is disconnected, causing the signal light to be interrupted; Determine the ratio of the emission cross-section of the gain fiber (5) at the first wavelength to the second wavelength, and the wavelength ratio of the first wavelength to the second wavelength; The intensity of the signal light is determined based on the small signal gain coefficient, the emission cross-section ratio, and the wavelength ratio.

6. The high-power MOPA fiber laser for safety protection against signal light interruption according to claim 1, characterized in that, When the preceding optical path fails or disconnects, causing the signal light to be interrupted, the total response time associated with the clamping laser oscillation is much less than the heat accumulation time associated with thermal damage.

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