Quasi-continuous thulium-doped fiber laser capable of synchronously suppressing relaxation oscillation and self-pulse

Abstract

The invention provides a quasi-continuous thulium-doped fiber laser capable of synchronously suppressing relaxation oscillation and self-pulse, relates to the technical field of fiber laser, and is used for simultaneously realizing suppression of a self-pulse effect and a relaxation oscillation peak. Comprising a continuous driving module, a pulse driving module, a semiconductor laser pumping source, a high-reflectivity fiber bragg grating, a low-reflectivity fiber bragg grating, a beam combiner, a thulium-doped gain fiber, a passive fiber, a cladding pumping stripper and a laser output port, the continuous pump light and the pulse pump light are respectively driven by the continuous driving module and the pulse driving module to output continuous pump light and pulse pump light; the high-reflectivity fiber bragg grating and the low-reflectivity fiber bragg grating form a resonant cavity structure, the thulium-doped gain fiber serves as a gain medium of the thulium-doped fiber laser and absorbs pump light to generate population inversion, laser oscillation is formed by combining the resonant cavity of the laser, relaxation oscillation suppression is achieved, and meanwhile longitudinal mode distribution of the laser is effectively disrupted.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber lasers, and in particular to a quasi-continuous-time thulium-doped optical fiber laser capable of synchronously suppressing relaxation oscillation and self-pulses. Background Art

[0002] Fiber lasers have the advantages of compact structure, high conversion efficiency, high beam quality, and good heat dissipation performance. For thulium-doped fiber lasers, the thulium ion has a broad emission spectrum, and its output wavelength can cover 1700-2100nm. In recent years, its application in laser medicine, polymer processing, mid-infrared pump sources and other fields has become increasingly widespread. At present, self-pulsation is common in continuous fiber laser systems, that is, random pulse sequences appear in the output laser of continuous fiber lasers without modulation devices. Its time domain is extremely unstable and has a large amplitude, which can easily cause irreversible damage to the optical fiber. At the same time, the appearance of self-pulsation will reduce the threshold of nonlinear effects such as self-phase modulation, stimulated Brillouin scattering and stimulated Raman scattering in the optical fiber, restricting the improvement of fiber laser performance. Due to the high doping characteristics of thulium-doped fiber, its self-pulsation phenomenon is more obvious. In addition, at the moment the laser is turned on, the time domain characteristics of the output are extremely unstable, manifesting as a series of relaxation oscillation spikes. For quasi-continuous thulium-doped fiber lasers, each pulse will have a relaxation oscillation spike with a relatively high peak value. To a certain extent, this affects the stability of the laser output and the peak power distribution, limiting its application in biomedicine, material processing and other fields.

[0003] It is generally believed that the generation of self-pulsation is caused by the saturable absorber of the doped ion pair in the doped fiber, which leads to the generation of self-pulsation. For thulium-doped fiber lasers, it is believed that the Tm 3+ Excited-state absorption and ion-pair absorption are the main reasons for the formation of self-pulses. The generation of relaxation oscillation pulses is mainly due to the repeated establishment and destruction of the dynamic equilibrium of the upper energy level population at the moment the laser is turned on, causing the output laser to exhibit a series of spike pulses. If the self-pulsation and relaxation oscillation phenomena in the laser can be eliminated, the threshold of nonlinear effects in the fiber laser can be further increased, and the stability of the laser can be increased, which has important guiding significance for the realization of high-power fiber lasers.

[0004] For the suppression of the self-pulsation effect, the reported self-pulsation suppression schemes mainly include improving pump uniformity, increasing the photon lifetime in the cavity, introducing FP cavity mode selection and adopting special oscillator structures. Researchers from the University of Rochester in the United States successfully eliminated self-pulsations by adding 2349m of passive optical fiber to the laser cavity and increasing the round-trip time of the laser cavity, thereby increasing the photon lifetime in the cavity. Researchers from the Raja Ramanna Advanced Technology Center found that in ytterbium-doped fiber oscillators of different structures, the generation of self-pulsations is closely related to the pumping conditions, fiber length and laser linewidth of the laser, and that self-pulsations can be effectively suppressed by increasing the photon lifetime in the resonant cavity or increasing the pump power to achieve gain uniformity. Researchers from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences inserted an FP cavity into the all-fiber resonant cavity for mode selection and found that by constructing a fiber FP cavity, the lifetime of photons in the cavity can be increased, thereby suppressing the generation of self-pulsations. TANG and others from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences experimentally observed the self-pulsing behavior of thulium-doped double-clad fiber lasers under different output couplings and found that low output coupling ratio can suppress or even eliminate the self-pulsing mode, and high pump power and low output coupling can both increase the signal light intensity in the cavity, thereby suppressing the pulse behavior and achieving continuous wave output.

[0005] The methods used to suppress relaxation oscillation pulses are: (1) Q-switching technology: using electro-optical switches or acousto-optic modulators to quickly change the cavity loss to prevent the particle inversion number from oscillating repeatedly near the threshold, which can compress nanosecond spike pulses into a single high-intensity pulse, significantly increasing the peak power. (2) Current waveform control: the drive current is divided into two stages. The first stage uses a low-amplitude current (below the threshold) to preheat the laser medium and reduce the initial amplitude of the relaxation oscillation; the second stage adjusts the current to the target value to ensure stable output power. (3) Signal delay circuit design: using a multi-stage delay unit (such as a combination of diodes, potentiometers, and capacitors) to accurately control the delay time to ensure that the pulse width and frequency of the drive signal are consistent, reducing the randomness of the spike pulse.

[0006] Reported methods for suppressing fiber laser self-pulsations, especially those in thulium-doped fiber lasers, lack effective solutions. These solutions also suffer from limitations such as non-fiberization and low output power. Furthermore, for quasi-continuous-current thulium-doped fiber lasers, there are no reported methods for simultaneously suppressing self-pulsations and relaxation oscillations, limiting the effective improvement of the power and performance of quasi-continuous-current thulium-doped fiber lasers. Summary of the Invention

[0007] In order to realize a quasi-continuous thulium-doped fiber laser with time domain stability, the purpose of the present invention is to improve the output performance of the quasi-continuous thulium-doped fiber laser and propose a quasi-continuous thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation. Without changing the pump driving circuit, the optical structure is directly used to regulate the upper energy level particle number of the all-fiber resonant cavity near the laser threshold to achieve relaxation oscillation suppression, while effectively disrupting the longitudinal mode distribution of the laser to achieve self-pulse suppression, and finally achieve time-domain stable quasi-continuous thulium-doped fiber laser pulse output, which can effectively improve the output performance of such lasers. The present invention adopts a continuous / pulse hybrid pumping method to regulate the upper energy level particle number to achieve effective suppression of relaxation oscillation pulses, and adopts a long cavity or a composite cavity structure to achieve self-pulse suppression. In this way, relaxation oscillation pulses and self-pulse effects can be effectively suppressed, the stability of such lasers can be greatly improved, and the cost performance of such equipment can be improved.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] Based on the above objectives, the present invention provides a quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation, which is used to simultaneously suppress the self-pulsation effect and the relaxation oscillation spike. The quasi-continuous-wave thulium-doped fiber laser includes a continuous drive module, a pulse drive module, a semiconductor laser pump source, a high-reflection fiber Bragg grating, a low-reflection fiber Bragg grating, a beam combiner, a thulium-doped gain fiber, a passive fiber, a cladding pump stripper, and a laser output port.

[0010] The semiconductor laser pump source is used to provide pump energy for the thulium-doped fiber laser, and is driven by the continuous driving module and the pulse driving module respectively to output continuous pump light and pulse pump light; the high-reflection fiber grating and the low-reflection fiber grating constitute a resonant cavity structure, and the thulium-doped gain fiber serves as the gain medium of the thulium-doped fiber laser, absorbs the pump light to generate a population inversion, and combines with the resonant cavity of the laser to form laser oscillation; the beam combiner couples the continuous pump light and the pulse pump light to the thulium-doped gain fiber, and the beam combiner includes a forward pump combiner and a reverse pump combiner, which are used to couple the pump light into the thulium-doped gain fiber and couple the pump light into the thulium-doped gain fiber from the opposite direction of the signal output; the cladding pump stripper is used to strip off the unabsorbed pump light, the passive optical fiber is used to increase the cavity length of the resonant cavity, and the laser output port is used to output the thulium-doped signal light generated by the resonant cavity.

[0011] As a further solution of the present invention, the pumping wavelength of the semiconductor laser pumping source is 780-800 nm and is used in a cascade manner.

[0012] As a further solution of the present invention, the continuous drive module is used to provide continuous current drive for the semiconductor laser pump source, so that the semiconductor laser pump source operates in a continuous operation state, provides a continuous low-energy supply for the gain fiber, and enables the thulium-doped fiber laser oscillator to output a low-power continuous laser signal, so that the laser operates near the threshold and the number of upper energy level ions is in a balanced state, thereby suppressing the generation of relaxation oscillation spikes.

[0013] As a further solution of the present invention, the pulse driving module is used to provide pulse current drive for the semiconductor laser pump source, so that the semiconductor laser pump source operates in a pulsed operation state and achieves a larger pumping energy, thereby making the laser operate in a quasi-continuous operation state. Through the peak current control, duty cycle adjustment, and pulse width adjustment of the driving module, combined with the thulium-doped fiber laser oscillator, quasi-continuous signal pulse output with different parameters can be achieved.

[0014] As a further solution of the present invention, the resonant cavity structure is composed of a high-reflection fiber Bragg grating and at least one low-reflection fiber Bragg grating, and the low-reflection fiber Bragg grating includes a first low-reflection fiber Bragg grating and a second low-reflection fiber Bragg grating, forming a composite cavity to destroy the longitudinal mode distribution.

[0015] As a further solution of the present invention, the high-reflection fiber Bragg grating serves as the high-reflection part of the thulium-doped fiber laser resonant cavity, is used to provide positive feedback, and is combined with a low-reflection fiber Bragg grating to make the spontaneous radiation light form laser oscillation. The first low-reflection fiber Bragg grating serves as the coupled output part of the thulium-doped fiber laser resonant cavity, is also used to provide partial positive feedback, and is combined with the high-reflection fiber Bragg grating to make the spontaneous radiation light form laser oscillation, and is also used as a laser output port. The high-reflection fiber Bragg grating respectively forms a composite cavity structure with the first low-reflection fiber Bragg grating and the second low-reflection fiber Bragg grating: two cavities of the first low-reflection fiber Bragg grating and the high-reflection fiber Bragg grating, and the second low-reflection fiber Bragg grating and the high-reflection fiber Bragg grating. There are two groups of longitudinal modes in the oscillator cavity, so that the longitudinal mode spacing has a significant difference, and when the longitudinal mode spacing is not a multiple relationship, the two groups of longitudinal modes will crosstalk with each other and coherently cancel each other, thereby causing the strong pulse formed by the superposition of the longitudinal modes in the cavity to be rapidly quenched, thereby suppressing the generation of self-pulses in the output timing.

[0016] As a further solution of the present invention, the continuous driving module enables the laser to operate near the threshold to suppress relaxation oscillation, and the composite cavity structure or passive optical fiber suppresses self-pulsation by longitudinal mode interference or extending the photon lifetime.

[0017] As a further solution of the present invention, the passive optical fiber is arranged in the long cavity structure to increase the length of the resonant cavity and the photon lifetime in the cavity.

[0018] As a further solution of the present invention, the thulium-doped gain fiber can radiate photons with a wavelength of 1800-2100 nm after absorbing pump energy, forming laser oscillation output under the action of the resonant cavity.

[0019] Compared with the existing technology, the quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation proposed in the present invention has significant advantages in laser output performance, system reliability, and application scenario expansion by synchronously suppressing relaxation oscillation and self-pulsation effects, and has the following beneficial effects:

[0020] The present invention discloses a quasi-continuous-time thulium-doped fiber laser with synchronous suppression of self-pulsation effect and relaxation oscillation spike. The continuous / pulse hybrid pumping technology is used to pre-establish a stable population inversion near the threshold of the laser, thereby avoiding the generation of relaxation oscillation spikes. After hybrid pumping, the output pulse waveform is close to a rectangle, the relaxation oscillation peak intensity is reduced by more than one order of magnitude, and the time domain stability is significantly improved. In addition, a composite cavity or a long cavity structure is used to destroy the coherent superposition of longitudinal modes, suppress the formation of self-pulses, and achieve self-pulse elimination.

[0021] Moreover, the present invention does not require an external modulator or complex circuit control, and only achieves dual-effect suppression through optical structure (such as composite cavity, passive optical fiber) and hybrid pumping, realizing the advantages of full optical fiber system, compact structure, strong anti-interference ability and long-term maintenance-free. Compared with traditional electro-optical Q-switching or acousto-optic modulation schemes, it eliminates high-cost devices, reduces the overall cost, and improves cost-effectiveness.

[0022] Secondly, the random high-intensity pulses of self-pulsation can easily cause local thermal damage to the optical fiber. After the present invention suppresses the self-pulsation, the probability of optical fiber damage is reduced, the device life is extended, and the low-power preheating of continuous pumping reduces thermal shock, avoids the problem of uneven doping caused by thulium ion clustering, and further extends the service life of the gain fiber. The quasi-continuous thulium-doped fiber laser of the present invention achieves high stability, long life, low cost and wide applicability while suppressing relaxation oscillations and self-pulsation through innovative optical structure design and pump control strategy, thereby enhancing the stability of the output performance of such lasers. The present invention adopts an all-fiber structure with a simple and compact structure, no maintenance required, and can greatly improve the suppression effect of self-pulsation effects and relaxation oscillation spikes, improving the cost-effectiveness of such lasers.

[0023] These and other aspects of the present application will be more clearly understood in the following description of the embodiments. It should be understood that the above general description and the following detailed description are merely exemplary and explanatory and cannot limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the following briefly introduces the drawings required for the exemplary embodiments or related technical descriptions. The drawings are used to provide a further understanding of the present invention and constitute part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the drawings:

[0025] Figure 1 Schematic diagram of the structure of a quasi-continuous-time thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to an embodiment of the present invention.

[0026] Figure 2 Schematic diagram of the structure of a single-ended forward pumping structure in a quasi-continuous-time thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to an embodiment of the present invention.

[0027] Figure 3 Schematic diagram of the structure of a single-ended backward pumping structure in a quasi-continuous-time thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to an embodiment of the present invention.

[0028] Figure 4 The figure shows the time domain characteristics of the quasi-continuous-time thulium-doped fiber laser output before relaxation oscillation and self-pulse suppression according to an embodiment of the present invention.

[0029] Figure 5 Schematic diagram of the synchronous suppression effect of relaxation oscillation and self-pulsation in a quasi-continuous-time thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to an embodiment of the present invention.

[0030] Reference numerals:

[0031] 101- continuous driving module, 102- pulse driving module, 103- semiconductor laser pump source, 104- high-reflection fiber Bragg grating, 105- first low-reflection fiber Bragg grating, 106- second low-reflection fiber Bragg grating, 107- thulium-doped gain fiber, 108- forward pumping combiner, 109- reverse pumping combiner, 110- cladding pump stripper, 111- passive fiber, 112- laser output port. DETAILED DESCRIPTION

[0032] Below, the present application is further described in conjunction with the accompanying drawings and specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0033] To make the purpose, technical solutions and advantages of the present invention more clearly understood, the following is a further detailed description of the embodiments of the present invention in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application.

[0034] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are intended to distinguish two non-identical entities or non-identical parameters with the same name. Therefore, "first" and "second" are used for convenience of expression only and should not be understood as limitations on the embodiments of the present invention. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, other steps or units inherent to a process, method, system, product, or device that includes a series of steps or units.

[0035] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0036] The flowcharts shown in the accompanying drawings are for illustrative purposes only and do not necessarily include all contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps may be decomposed, combined, or partially merged, so the actual execution order may vary depending on the actual situation.

[0037] The following describes some embodiments of the present application in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features therein may be combined with each other.

[0038] See also Figures 1 to 3 As shown, an embodiment of the present invention provides a quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation, which is used to simultaneously suppress the self-pulsation effect and the relaxation oscillation spike. The quasi-continuous-wave thulium-doped fiber laser includes a continuous driving module 101, a pulse driving module 102, a semiconductor laser pump source 103, a high-reflection fiber Bragg grating 104, a low-reflection fiber Bragg grating, a beam combiner, a thulium-doped gain fiber 107, a passive fiber 111, a cladding pump stripper 110, and a laser output port 112.

[0039] The semiconductor laser pump source 103 is used to provide pump energy for the thulium-doped fiber laser, and is driven by the continuous driving module 101 and the pulse driving module 102 respectively to output continuous pump light and pulse pump light; the high-reflection fiber Bragg grating 104 and the low-reflection fiber Bragg grating constitute a resonant cavity structure, and the thulium-doped gain fiber 107 serves as the gain medium of the thulium-doped fiber laser, absorbs the pump light to generate a population inversion, and combines with the resonant cavity of the laser to form laser oscillation; the beam combiner combines the continuous pump light and the pulsed light The pump light is coupled to the thulium-doped gain fiber 107. The combiner includes a forward pump combiner 108 and a reverse pump combiner 109, which are devices for coupling the pump light into the thulium-doped gain fiber 107 and coupling the pump light into the thulium-doped gain fiber 107 from the direction opposite to the signal output. The cladding pump stripper 110 is used to strip off the unabsorbed pump light. The passive fiber 111 is used to increase the cavity length of the resonant cavity. The laser output port 112 is used to output the thulium-doped signal light generated by the resonant cavity.

[0040] In this embodiment, the pump wavelength of the semiconductor laser pump source 103 is 780-800 nm. The semiconductor laser pump source 103 is used to provide pump energy for the thulium-doped fiber laser and can be used in a cascade according to the output power.

[0041] In this embodiment, the continuous driving module 101 is used to provide continuous current drive for the semiconductor laser pump source 103, so that the semiconductor laser pump source 103 operates in a continuous operation state, provides a continuous low-energy supply to the gain fiber, and enables the thulium-doped fiber laser oscillator to output a low-power continuous laser signal, so that the laser operates near the threshold and the number of upper energy level ions is in a balanced state, thereby suppressing the generation of relaxation oscillation spikes.

[0042] In this embodiment, the pulse driving module 102 is used to provide pulse current drive for the semiconductor laser pump source 103, so that the semiconductor laser pump source 103 operates in a pulsed operation state and achieves a larger pumping energy, thereby making the laser operate in a quasi-continuous operation state. Through the peak current control, duty cycle adjustment, and pulse width adjustment of the driving module, combined with the thulium-doped fiber laser oscillator, quasi-continuous signal pulse output with different parameters can be achieved.

[0043] In this embodiment, the resonant cavity structure is composed of a high-reflection fiber Bragg grating 104 and at least one low-reflection fiber Bragg grating, wherein the low-reflection fiber Bragg grating includes a first low-reflection fiber Bragg grating 105 and a second low-reflection fiber Bragg grating 106, forming a composite cavity to destroy the longitudinal mode distribution.

[0044] In this embodiment, the high-reflection fiber Bragg grating 104 is used as the high-reflection part of the thulium-doped fiber laser resonant cavity to provide positive feedback, and is combined with the low-reflection fiber Bragg grating to make the spontaneous radiation light form laser oscillation. The first low-reflection fiber Bragg grating 105 is used as the coupling output part of the thulium-doped fiber laser resonant cavity, and is also used to provide partial positive feedback, and is combined with the high-reflection fiber Bragg grating 104 to make the spontaneous radiation light form laser oscillation, and at the same time serves as the laser output port 112; the high-reflection fiber Bragg grating 104 is respectively connected to the first low-reflection fiber Bragg grating 105. The reflective fiber Bragg grating 105 and the second low-reflective fiber Bragg grating 106 form a composite cavity structure: two cavities, namely, the first low-reflective fiber Bragg grating 105 and the high-reflective fiber Bragg grating 104, and the second low-reflective fiber Bragg grating 106 and the high-reflective fiber Bragg grating 104. There are two groups of longitudinal modes in the oscillator cavity, which result in a significant difference in the longitudinal mode spacing. When the longitudinal mode spacing is not in a multiple relationship, the two groups of longitudinal modes will crosstalk with each other and coherently cancel each other, thereby causing the strong pulse formed by the superposition of the longitudinal modes in the cavity to be rapidly quenched, thereby suppressing the generation of self-pulses in the output timing.

[0045] In this embodiment, the continuous driving module 101 enables the laser to operate near the threshold to suppress relaxation oscillation, and the composite cavity structure or passive optical fiber 111 suppresses self-pulsation by longitudinal mode interference or extending the photon lifetime.

[0046] In this embodiment, the passive optical fiber 111 is disposed in the long cavity structure to increase the cavity length of the resonant cavity, thereby increasing the photon lifetime in the resonant cavity and suppressing the self-pulsation effect.

[0047] In this embodiment, the thulium-doped gain fiber 107 serves as the gain medium of the thulium-doped fiber laser. After absorbing the pump energy, it can achieve a particle number inversion, thereby generating spontaneous radiation. Combined with the laser's resonant cavity, laser oscillation is formed. Its doped ions are thulium ions, which can radiate photons with a wavelength of 1800-2100nm, forming laser oscillation output under the action of the resonant cavity.

[0048] In this embodiment, the forward pump combiner 108 couples pump light into the thulium-doped gain fiber. The pump fiber matches the pump source pigtail, and the output fiber matches the core / cladding dimensions of the signal fiber and the thulium-doped gain fiber. The reverse pump combiner 109 couples pump light into the thulium-doped gain fiber from the opposite direction of the signal output, compensating for insufficient forward pump power. The pump fiber matches the pump source pigtail, and the output fiber matches the core / cladding dimensions of the signal fiber and the thulium-doped gain fiber. The cladding pump stripper 110 is used to strip unabsorbed pump light to prevent damage to other components.

[0049] The working principle of the present invention is:

[0050] The quasi-continuous thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation of the present invention can be used to simultaneously achieve the suppression of self-pulsation effect and relaxation oscillation spikes, wherein the semiconductor laser pump source 103 provides pump energy for the quasi-continuous thulium-doped fiber laser, the high-reflection fiber grating 104 and the low-reflection fiber grating constitute a resonant cavity, which provides the necessary conditions for achieving laser oscillation and can determine the output wavelength of the laser; the thulium-doped fiber is the gain medium of the quasi-continuous thulium-doped fiber laser, and after absorbing the pump energy, it can radiate photons with a wavelength of 1800-2100nm, forming laser oscillation output under the action of the resonant cavity; the continuous driving module 101 and the pulse driving module 102 provide driving current for the semiconductor laser pump source, which can enable it to output continuous and pulsed pump light, and then enable the resonant cavity to output continuous and pulsed signal light.

[0051] The core of the present invention lies in the synchronous suppression of the self-pulsation effect and the relaxation oscillation peak. For the suppression of the self-pulsation effect, the following methods can be used: (1) a composite cavity structure, a first low-reflection fiber grating 105 and a second low-reflection fiber grating 106 form a composite cavity with a high-reflection fiber grating 104. At this time, there are two groups of longitudinal modes in the oscillator cavity, so that there is a significant difference in the longitudinal mode spacing. When the longitudinal mode spacing is not in a multiple relationship, the two groups of longitudinal modes will crosstalk with each other and cancel each other out, thereby causing the strong pulse formed by the superposition of the longitudinal modes in the cavity to be rapidly quenched, thereby suppressing the generation of self-pulses in the output sequence; (2) a long cavity structure, by adding a certain length of passive optical fiber 111 in the cavity, the length of the resonant cavity is increased, thereby increasing the photon lifetime in the cavity. The photon lifetime in the cavity can be expressed by the following formula: Where, τ c is the intracavity photon lifetime; n is the fiber core refractive index; L is the cavity length; c is the speed of light in a vacuum; R1 and R2 are the reflectivities of the cavity grating pair; and α is the cavity scattering loss. Increasing the intracavity photon lifetime can alter the relaxation dynamics, thereby suppressing the self-pulsation effect.

[0052] The core technology of this invention for suppressing relaxation oscillation spikes is the use of a hybrid continuous and pulsed pumping method to regulate the upper energy level population and maintain a stable state. The dynamic characteristics of the laser at the moment of activation can be described by the following equation, where ξ is the ratio of the number of oscillating mode photons in the spontaneous emission to the total number of photons. The physical meaning of this term is the number of signal light photons in the cavity before the laser is activated.

[0053]

[0054] Numerical calculations using the above equation reveal that if signal light is flowing through the fiber before the laser is turned on, the shape of the output pulse can be significantly altered. By increasing ξ by an order of magnitude and then inserting it into the above equation, the relaxation oscillation peak disappears and the entire single pulse approaches a rectangular pulse. Based on this, the present invention proposes hybrid pumping of the oscillator, which simultaneously introduces continuous and quasi-continuous pump light. The continuous pump source is set to operate at low power, allowing the oscillator to emit light near the threshold power. This ensures that signal light is present in the resonant cavity before the quasi-continuous pumping is turned on, corresponding to the increase in ξ, ultimately achieving suppression of the relaxation oscillation pulse.

[0055] The present invention provides a quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of self-pulsation effect and relaxation oscillation spikes. By enhancing the stability of the output performance of such lasers, the solution adopts an all-fiber structure with a simple and compact structure and no maintenance. It can also greatly improve the suppression effect of self-pulsation effect and relaxation oscillation spikes, thereby improving the cost-effectiveness of such lasers.

[0056] For the suppression of self-pulses, the long cavity structure and the dual cavity structure of the present invention can be selected or used simultaneously, that is, Figure 1 In the structure shown, the passive optical fiber 111 is retained, and the first low-reflection fiber Bragg grating 105 or the second low-reflection fiber Bragg grating 106 can be removed; the passive optical fiber 114 can also be removed, and the first low-reflection fiber Bragg grating 105 or the second low-reflection fiber Bragg grating 106 can be retained. Figure 1 In addition to the structure shown, you can also use Figure 2 The single-ended forward pumping structure shown, or Figure 3 The single-ended backward pumping structure shown in the figure can be used in the process of suppressing the self-pulse, and the long cavity structure and the dual cavity (composite cavity) structure can be used at the same time. In particular, the structure for synchronous suppression of the self-pulse effect and the relaxation oscillation pulse is within the scope of protection of the present invention, not limited to Figure 1 、 2 , the structure shown in 3.

[0057] The suppression effect of the relaxation oscillation peak of the present invention is as follows Figure 4 As shown, Figure 4 The relaxation oscillation peak suppression effect of the present invention is compared, wherein, Figure 4 The time domain diagram of the pulse before and after suppression is shown in Figure 2. Figure 4 (a) is the whole pulse sequence, (b) is the single pulse case, (c) is the relaxation oscillation peak details, and (d) is the self-pulse details. It can be seen that in the case of single pulse pumping, the pulse leading edge has an obvious relaxation oscillation peak, which is large in intensity and unstable. The pulse and continuous synchronous pumping scheme proposed in this invention is adopted. The continuous pumping power makes the laser work near the threshold to form a stable laser oscillation. The pulse pumping works at a higher power to generate the required output power. Figure 4It can be seen that the intensity of the relaxation oscillation peak is greatly reduced, and the entire pulse shows a stable rectangular waveform.

[0058] The suppression effect of the self-pulsation effect of this scheme is as follows: Figure 5 As shown in the figure, conventional cavity structures exhibit extremely unstable self-pulsations in the time domain, and the intensity varies, which can easily cause damage to the optical fiber system. The long cavity and composite cavity structures proposed in this invention can effectively destroy the longitudinal mode superposition and suppress the self-pulsation effect, such as Figure 5 As shown, according to Figure 5 The time-domain characteristics after suppressing the self-pulsation effect and relaxation oscillation spikes show that (a) is the overall pulse sequence, (b) is the single pulse case, (c) is the relaxation oscillation spike detail, and (d) is the self-pulsation detail. The present invention achieves time-domain stable quasi-continuous-wave thulium-doped fiber laser output by synchronously suppressing the self-pulsation effect and relaxation oscillation spikes.

[0059] Compared with the existing technology, the quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation proposed in the present invention has significant advantages in laser output performance, system reliability, and application scenario expansion by synchronously suppressing relaxation oscillation and self-pulsation effects, and has the following beneficial effects:

[0060] The present invention discloses a quasi-continuous-time thulium-doped fiber laser with synchronous suppression of self-pulsation effect and relaxation oscillation spike. The continuous / pulse hybrid pumping technology is used to pre-establish a stable population inversion near the threshold of the laser, thereby avoiding the generation of relaxation oscillation spikes. After hybrid pumping, the output pulse waveform is close to a rectangle, the relaxation oscillation peak intensity is reduced by more than one order of magnitude, and the time domain stability is significantly improved. In addition, a composite cavity or a long cavity structure is used to destroy the coherent superposition of longitudinal modes, suppress the formation of self-pulses, and achieve self-pulse elimination.

[0061] Moreover, the present invention does not require an external modulator or complex circuit control, and only realizes dual-effect suppression through optical structures such as a composite cavity, passive optical fiber 111 and a hybrid pump, thereby achieving the advantages of a fully optical system, a compact structure, strong anti-interference ability and long-term maintenance-free. Compared with traditional electro-optical Q-switching or acousto-optic modulation schemes, it eliminates high-cost devices, reduces the overall cost, and improves cost-effectiveness.

[0062] Secondly, the random high-intensity pulses of self-pulsation can easily cause local thermal damage to the optical fiber. After the present invention suppresses the self-pulsation, the probability of optical fiber damage is reduced, the device life is extended, and the low-power preheating of continuous pumping reduces thermal shock, avoids the problem of uneven doping caused by thulium ion clustering, and further extends the service life of the gain fiber. The quasi-continuous thulium-doped fiber laser of the present invention achieves high stability, long life, low cost and wide applicability while suppressing relaxation oscillations and self-pulsation through innovative optical structure design and pump control strategy, thereby enhancing the stability of the output performance of such lasers. The present invention adopts an all-fiber structure with a simple and compact structure, no maintenance required, and can greatly improve the suppression effect of self-pulsation effects and relaxation oscillation spikes, improving the cost-effectiveness of such lasers.

[0063] The above are exemplary embodiments disclosed in the present invention, but it should be noted that various changes and modifications may be made without departing from the scope of the embodiments disclosed in the claims. The functions, steps and / or actions of the method claims according to the disclosed embodiments described herein do not need to be performed in any particular order. In addition, although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as multiple unless expressly limited to the singular.

[0064] It should be understood that, as used herein, the singular form "a" or "an" is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, "and / or" refers to any and all possible combinations of one or more of the items listed in association. The serial numbers of the embodiments disclosed in the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0065] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples. Within the spirit of the embodiments of the present invention, the technical features of the above embodiments or different embodiments may be combined, and there are many other variations of different aspects of the above embodiments of the present invention, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the scope of protection of the embodiments of the present invention.

Claims

1. A quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation, characterized in that: It comprises a continuous driving module (101), a pulse driving module (102), a semiconductor laser pump source (103), a high-reflection fiber Bragg grating (104), a low-reflection fiber Bragg grating, a beam combiner, a thulium-doped gain fiber (107), a passive fiber (111), a cladding pump stripper (110), and a laser output port (112); The semiconductor laser pump source (103) is used to provide pump energy for the thulium-doped fiber laser, and is driven by the continuous driving module (101) and the pulse driving module (102) respectively to output continuous pump light and pulse pump light; the high-reflection fiber grating (104) and the low-reflection fiber grating constitute a resonant cavity structure, and the thulium-doped gain fiber (107) serves as the gain medium of the thulium-doped fiber laser, absorbs the pump light to generate a population inversion, and combines with the laser resonant cavity to form laser oscillation; the beam combiner combines the continuous pump light and the pulse pump light Coupled to a thulium-doped gain fiber (107), the combiner comprises a forward pump combiner (108) and a reverse pump combiner (109), a device for coupling pump light into the thulium-doped gain fiber (107) and a device for coupling pump light into the thulium-doped gain fiber (107) from a direction opposite to a signal output, the cladding pump stripper (110) is used to strip unabsorbed pump light, the passive fiber (111) is used to increase the cavity length of the resonant cavity, and the laser output port (112) is used to output the thulium-doped signal light generated by the resonant cavity.

2. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 1, characterized in that: The pumping wavelength of the semiconductor laser pump source (103) is 780-800 nm and is used in a cascade manner.

3. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 2, characterized in that: The continuous driving module (101) is used to provide continuous current driving for the semiconductor laser pump source (103), so that the semiconductor laser pump source (103) operates in a continuous operation state, provides a continuous low-energy supply for the gain fiber, and enables the thulium-doped fiber laser oscillator to output a low-power continuous laser signal, so that the laser operates near the threshold and the number of upper energy level ions is in a balanced state, thereby suppressing the generation of relaxation oscillation peaks.

4. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 3, characterized in that: The pulse driving module (102) is used to provide pulse current driving for the semiconductor laser pumping source (103), so that the semiconductor laser pumping source (103) operates in a pulse operation state and the laser operates in a quasi-continuous operation state.

5. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 1, characterized in that: The resonant cavity structure is composed of a high-reflection fiber grating (104) and at least one low-reflection fiber grating, wherein the low-reflection fiber grating includes a first low-reflection fiber grating (105) and a second low-reflection fiber grating (106), forming a composite cavity to destroy the longitudinal mode distribution.

6. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 5, characterized in that: The high-reflection fiber Bragg grating (104) serves as a high-reflection part of the thulium-doped fiber laser resonant cavity, and is used to provide positive feedback. In combination with the low-reflection fiber Bragg grating, the spontaneously emitted light forms laser oscillation. The first low-reflection fiber Bragg grating (105) serves as a coupled output part of the thulium-doped fiber laser resonant cavity, and is also used to provide partial positive feedback. In combination with the high-reflection fiber Bragg grating (104), the spontaneously emitted light forms laser oscillation, and serves as a laser output port (112).

7. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 6, characterized in that: The high-reflection fiber Bragg grating (104) respectively forms a composite cavity structure with the first low-reflection fiber Bragg grating (105) and the second low-reflection fiber Bragg grating (106): two cavities of the first low-reflection fiber Bragg grating (105) and the high-reflection fiber Bragg grating (104), and the second low-reflection fiber Bragg grating (106) and the high-reflection fiber Bragg grating (104).

8. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 7, characterized in that: The continuous driving module (101) enables the laser to operate near the threshold to suppress relaxation oscillation, and the composite cavity structure or passive optical fiber (111) suppresses self-pulsation by longitudinal mode interference or extending photon lifetime.

9. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 8, characterized in that: The passive optical fiber (111) is arranged in the long cavity structure to increase the length of the resonant cavity and the photon lifetime in the cavity.

10. The quasi-continuous-wave thulium-doped fiber laser with synchronous suppression of relaxation oscillation and self-pulsation according to claim 1, characterized in that: After absorbing pump energy, the thulium-doped gain fiber (107) can radiate photons with a wavelength of 1800-2100 nm, forming laser oscillation output under the action of the resonant cavity.

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