A 1030 nm band 9-shaped cavity fiber laser and a design method thereof

Abstract

The application provides a 1030nm waveband 9-shaped cavity fiber laser and a design method thereof. In the design method, the total cavity length is determined through dispersion matching, and then the length ratio of the gain fiber and the passive fiber is adjusted under the condition of the fixed total cavity length, and the splitting ratio and the phase shift amount are optimized in combination with the nonlinear interference principle, so that stable self-starting mode locking is realized, the problems of difficult mode locking starting and poor stability of the traditional 9-shaped cavity laser are solved, the 9-shaped cavity laser has the advantages of high cavity length fault tolerance and fast mode locking speed, and can stably output 1030nm waveband ultrashort pulses, and has good application prospect in the fields of industrial processing, laser radar and the like.

Description

Technical Field

[0001] This invention relates to laser technology, specifically to a 1030nm band figure-9 cavity fiber laser and its design method. Background Technology

[0002] The figure-9 cavity fiber laser, named for its overall optical path structure resembling the number "9," is an ultrafast laser based on a nonlinear amplifying ring mirror (NALM) mode-locking mechanism. It has garnered widespread attention in the optical field due to its compact structure, stable mode-locking, and excellent pulse quality. The 1030nm wavelength, as a typical output center wavelength for this type of laser, offers several advantages in practical applications: this wavelength falls within the effective gain range of ytterbium-doped fiber, improving pump conversion efficiency and laser output power, thus helping to reduce overall system costs; simultaneously, the 1030nm laser is compatible with commonly used 1060–1100nm gain range double-clad large-mode-field ytterbium-doped fiber amplifiers, facilitating subsequent power amplification. In terms of applications, the 1030nm laser covers multiple fields such as biomedicine, precision materials processing, and lidar, and can be used for cell imaging, laser surgery, tissue cutting, micro / nano fabrication, laser cutting and welding, target detection, and distance measurement, demonstrating broad applicability and strong engineering feasibility.

[0003] From a working principle perspective, the figure-9 cavity fiber laser utilizes the nonlinear phase shift difference generated when two beams propagate in a NALM to cause nonlinear interference, thereby forming an equivalent saturable absorber and achieving dynamic modulation and frequency selection of the intracavity pulse. When the accumulated nonlinear phase shift in the cavity reaches a specific threshold, the amplitude and phase of the intracavity pulse are locked, ultimately outputting a stable mode-locked pulse sequence.

[0004] However, the mode-locking self-starting performance of existing figure-9 cavity fiber lasers is sensitive to cavity length parameters: autonomous mode-locking without external intervention can only be achieved when the lengths of key fiber components such as the gain fiber and passive fiber are adjusted to the optimal matching range; if the cavity length is too long or too short, it cannot start naturally and must be triggered by external disturbances such as manually applying fiber stress or adjusting the physical position of the optical path. Therefore, existing technologies usually require repeated testing of the effects of different passive fiber lengths and different fiber placement positions on the output pulsed laser, while verifying the mode-locking start-up speed, stability duration, and other performance characteristics under different cavity length conditions. The entire debugging process is time-consuming, labor-intensive, and inefficient.

[0005] Patent CN119674690A discloses a self-starting 9-cavity laser and device based on motor disturbance. This device introduces controllable noise using motor disturbance, which to some extent shortens the mode-locking start-up time and alleviates the difficulty of self-starting. However, this solution essentially still triggers mode-locking through external disturbance and does not fundamentally solve the core problem of cavity length and dispersion parameter matching optimization. It not only adds extra components such as motors and control modules, leading to structural complexity and increased cost, but its mode-locking stability still depends on external disturbance, resulting in insufficient reliability in high-stability scenarios. Furthermore, it cannot avoid the low efficiency of traditional cavity length adjustment.

[0006] Therefore, how to overcome the shortcomings of the existing technology is the subject of this invention. Summary of the Invention

[0007] The purpose of this invention is to provide a 1030nm band figure-9 cavity fiber laser and its design method to solve the above-mentioned technical problems.

[0008] To achieve the above objectives, a first aspect of the present invention provides a design method for a 1030nm band figure-9 cavity fiber laser. The 1030nm band figure-9 cavity fiber laser is composed of a pump laser, a wavelength division multiplexer, a gain fiber, a phase shifter, a coupling beam splitter, and a chirped fiber Bragg grating, which are passively fused together via optical fibers. The wavelength division multiplexer, the gain fiber, the phase shifter, and the coupling beam splitter are connected in a ring. The chirped fiber Bragg grating is connected to one output end of the coupling beam splitter, and the pump laser is connected to one input end of the wavelength division multiplexer.

[0009] The design method includes the following steps:

[0010] S1. Obtain the center wavelength λ and dispersion coefficient D of the chirped fiber Bragg grating using the formula...

[0011] ;

[0012] The second-order dispersion value β provided by the chirped fiber Bragg grating was calculated. CFBG , where c is the speed of light;

[0013] S2. Obtain the dispersion β per unit length of the gain fiber. YDF and the dispersion β per unit length of passive optical fiber GDF ;

[0014] Determine the target β for intracavitary dispersion control total The target is located in the negative region of zero dispersion;

[0015] S3. According to the total dispersion calculation formula:

[0016] ;

[0017] in, The length of the gain fiber. The length of the passive optical fiber;

[0018] Take β YDF and β GDF All are equal to the common dispersion value β0 in the 1030nm band, and are substituted into the formula;

[0019] Then, according to the formula for calculating the adjustable total cavity length:

[0020] ;

[0021] The adjustable total cavity length was calculated.

[0022] ;

[0023] S4. While keeping the total adjustable cavity length L constant, adjust the lengths of the gain fiber and the passive fiber to design a 1030nm band 9-shaped cavity fiber laser that can self-start mode-locking.

[0024] In a further technical solution, the intracavity dispersion control target β total The value ranges from -0.002ps² to -0.004ps².

[0025] In a further technical solution, in step S4, the length of the gain fiber... The adjustment range is 75cm to 85cm.

[0026] In a further technical solution, in step S4, the length of the gain fiber is determined by interpolation.

[0027] In a further technical solution, after step S4, the following is also included:

[0028] S5. Establish the correlation curve between the nonlinear phase shift difference and the intracavity transmittance, and select the splitting ratio with better transmission efficiency; based on the selected splitting ratio, match and determine the corresponding phase shift amount of the phase shifter.

[0029] According to a second aspect of this application, a 1030nm band figure-9 cavity fiber laser is provided, comprising a pump laser, a wavelength division multiplexer, a gain fiber, a phase shifter, a coupling beam splitter, and a chirped fiber Bragg grating. The wavelength division multiplexer, the gain fiber, the phase shifter, and the coupling beam splitter are fused together with passive optical fibers to form a closed loop optical path. The chirped fiber Bragg grating is connected to one output end of the coupling beam splitter via a passive optical fiber. The pump laser is connected to one input end of the wavelength division multiplexer.

[0030] The pump light emitted by the pump laser is coupled into the ring cavity through a wavelength division multiplexer and then propagates clockwise through the gain fiber, phase shifter, and beam splitter. The gain fiber amplifies the signal light within the cavity. When the optical signal reaches the chirped fiber Bragg grating, part of the light is reflected back into the ring cavity and propagates counterclockwise, while the other part passes through the chirped fiber Bragg grating as the output light of the laser. The clockwise propagating optical signal and the counterclockwise propagating optical signal reflected back into the cavity by the chirped fiber Bragg grating circulate within the ring cavity and interfere in the nonlinear amplifying ring mirror structure formed by the beam splitter, thereby achieving stable mode-locked pulse output.

[0031] The lengths of the gain fiber and the passive fiber are determined according to the design method described in the first aspect.

[0032] In a further technical solution, the gain fiber is a single-clad polarization-maintaining highly doped ytterbium-doped fiber, and the passive fiber is a PM980 polarization-maintaining fiber.

[0033] In a further technical solution, the reflectivity of the chirped fiber Bragg grating is 15%-25%.

[0034] In a further technical solution, the coupling beam splitter is a 2×1 polarization-maintaining coupler with a splitting ratio of 4:6.

[0035] In a further technical solution, the phase shifter is a bias-maintaining phase shifter with a phase shift of -π / 2.

[0036] The 1030nm band 9-cavity fiber laser and its design method provided in this application have the following technical advantages:

[0037] By first determining the total cavity length and then adjusting the length ratio of the gain fiber and the passive fiber, the cavity length adjustment process is simplified, providing a structural basis for achieving mode-locked self-starting. Controlling the total dispersion within the zero-dispersion-biased range provides a suitable dispersive environment for the formation of stable optical soliton mode-locked pulses within the cavity, suppressing pulse broadening and distortion. Combining the dispersion characteristics of the 1030nm band, the gain fiber and passive fiber are calculated using a common dispersion value, significantly simplifying the design derivation logic and enabling rapid and accurate determination of the total cavity length. Relying on precise total cavity length design, the accumulation rate and distribution position of the nonlinear phase shift within the cavity can be optimized, effectively promoting and accelerating the mode-locked self-starting process and shortening the laser's response time from power-on to stable output. Attached Figure Description

[0038] Figure 1 This is a flowchart of the method provided in an embodiment of the present invention;

[0039] Figure 2 This is a schematic diagram of the laser structure provided in an embodiment of the present invention;

[0040] Figure 3 The mode-locked spectrum obtained by the laser provided in the embodiments of the present invention;

[0041] Figure 4 The mode-locked pulse sequence obtained by the laser provided in the embodiments of the present invention;

[0042] Figure 5 This is a graph showing the relationship between intracavity transmittance and nonlinear phase shift difference in an embodiment of the present invention. Detailed Implementation

[0043] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0044] The terms "first," "second," etc., used in this article do not specifically refer to order or sequence, nor are they intended to limit this case; they are merely used to distinguish components or operations described using the same technical terms.

[0045] The terms "connection" or "positioning" as used in this article can refer to two or more components or devices making direct physical contact with each other, or making indirect physical contact with each other, or to two or more components or devices operating or moving with each other.

[0046] The terms “include,” “including,” and “have” used in this article are all open-ended, meaning they include but are not limited to.

[0047] The terms “front,” “back,” “up,” “down,” “left,” and “right” used in this article are directional terms. In this case, they are only used to describe the positional relationship between the structures and are not intended to limit the specific direction of the protection scheme or its actual implementation.

[0048] See Figure 1 This embodiment provides a design method for a 1030nm band figure-9 cavity fiber laser, which is as follows: Figure 2 As shown, it consists of a pump laser, a wavelength division multiplexer, a gain fiber, a phase shifter, a coupling beam splitter, and a chirped fiber Bragg grating, all fused together via passive fiber fusion. The wavelength division multiplexer, gain fiber, phase shifter, and coupling beam splitter are connected in a ring. The chirped fiber Bragg grating is connected to one output of the coupling beam splitter, and the pump laser is connected to one input of the wavelength division multiplexer.

[0049] This design method includes the following steps:

[0050] S1. Obtain the center wavelength λ and dispersion coefficient D of the chirped fiber Bragg grating. These two values ​​are parameters of the chirped fiber Bragg grating itself and can be obtained through experiments or by searching. In this embodiment, λ is 1028.654 nm and D is 0.42 ps / nm.

[0051] Through formula

[0052] ;

[0053] This formula is a commonly used formula for calculating the second-order dispersion value, where c is 299,792,458 m / s. The second-order dispersion value β provided by the chirped fiber Bragg grating is calculated. CFBG -0.2357ps 2 .

[0054] S2. Obtain the dispersion β per unit length of the gain fiber through device datasheets, simulation, or experimental measurements. YDF and the dispersion β per unit length of passive optical fiber GDF The dispersion per unit length β of gain fiber YDF and the dispersion β per unit length of passive optical fiber GDF It is related to the operating wavelength. At the 1030nm wavelength range, the dispersion β per unit length of the gain fiber (taking a single-clad polarization-maintaining highly doped ytterbium-doped fiber as an example) is... YDF Approximately +0.025 to +0.027 ps 2 / m; Dispersion per unit length β of passive optical fiber GDF It is approximately +0.0260 to +0.0265 ps² / m.

[0055] Then, the intracavity dispersion control target β was determined. total , making β total Located in the zero-dispersion negative bias region. The zero-dispersion negative bias region is the optimal operating region for optical soliton mode locking. The weak negative dispersion and the nonlinear effect of self-phase modulation (SPM) can form a dynamic balance, which allows the pulse to be naturally compressed during transmission and maintain a stable soliton waveform.

[0056] Intracavitary dispersion control target β total The preferred value for β is between -0.002 ps² and -0.004 ps². -0.002 to -0.004 ps² are commonly used empirical values ​​in experiments / simulations. Controlling the total intracavity dispersion within this negative-to-zero dispersion range of -0.002 to -0.004 ps² can suppress multi-pulse splitting, improve pulse stability, and reduce dependence on external stress. In this embodiment, β... total Take -0.003ps².

[0057] S3. According to the total dispersion calculation formula:

[0058] ;

[0059] in, The length of the gain fiber. The length of the passive optical fiber.

[0060] The above formula indicates that the total intracavity dispersion is composed of the dispersion of the chirped fiber Bragg grating, the dispersion of the gain fiber, and the dispersion of the passive fiber. The formula ignores the dispersion contribution of the internal fiber of passive devices such as wavelength division multiplexers and couplers, because their length is short and their dispersion ratio is extremely low, and can be ignored in engineering calculations.

[0061] It can be seen that, due to and Both are unknowns, and the total cavity length cannot be directly calculated using this formula alone. We can first fix one length and then calculate the other, for example, assuming... It is 80cm, then calculate. However, this method can lead to the total dispersion and total cavity length not being synchronized optimally, making it difficult to balance dispersion target and mold-locking start-up performance.

[0062] The inventors discovered that at a working wavelength of 1030 nm, the difference in the second-order dispersion coefficient between highly doped ytterbium-doped gain fiber (YDF) and PM980 polarization-maintaining passive fiber is small, and can be approximated as equal in engineering calculations, thus significantly simplifying the cavity length adjustment process. Therefore, β can be taken as... YDF and β GDF All of these are equal to a common dispersion value in the 1030nm band, for example, β0 = +0.0262ps. 2 Substituting / m into the formula, we get:

[0063]

[0064] Then, according to the formula for calculating the adjustable total cavity length:

[0065] ;

[0066] The adjustable total cavity length was calculated.

[0067] ;

[0068] It should be noted that the adjustable total cavity length does not include the length of the internal fiber of the device, but only represents the sum of the lengths of the gain fiber and the passive fiber between the device.

[0069] S4. While keeping the adjustable total cavity length L=8.893m constant, the nonlinearity and net dispersion matching relationship of the figure-9 cavity fiber laser is optimized by adjusting the length ratio of the gain fiber and the passive fiber, so as to achieve stable self-starting mode-locking.

[0070] The specific implementation method is as follows: keep the total cavity length constant, select multiple groups of gain fibers of different lengths, calculate the corresponding passive fiber lengths, test the time required for laser self-starting mode locking under each length ratio, and select the optimal combination of gain fiber and passive fiber lengths by comprehensively considering the mode locking start-up speed, pulse stability and output power.

[0071] In this embodiment, keeping the total cavity length L=8.893m constant, the lengths of the gain fiber are set to 70cm, 80cm, 90cm, and 100cm, respectively, and the corresponding lengths of the passive fiber are 8.193m, 8.093m, 7.993m, and 7.893m, respectively. The self-starting mode-locking time required for each combination is tested, and the optimal combination with the fastest start-up speed and the most stable operation is selected. The corresponding lengths of the gain fiber and the passive fiber are 80cm and 8.093m, respectively.

[0072] Preferably, in step S4, the length of the gain fiber... The adjustment range can be limited to 75cm to 85cm to shorten testing time and improve testing efficiency. In addition, the shorter gain fiber length, combined with a precise total cavity length design, can optimize the rate and distribution of nonlinear phase shift accumulation within the cavity, effectively promoting and accelerating the self-starting process of mode-locking and shortening the mode-locking establishment time.

[0073] In step S4, the length of the gain fiber can be further refined within the optimal range using interpolation. For example, using 75cm, 80cm, and 85cm as sampling points, the self-starting mode-locking time for each group is measured. Linear or quadratic interpolation is used to fit the curve of mode-locking time versus gain fiber length, and the length corresponding to the minimum value of mode-locking time is taken as the optimal L. YDF And constrained by the total cavity length L GDF =LL YDF The length of the matched passive fiber is determined to optimize the self-starting performance while ensuring that the total dispersion remains unchanged.

[0074] To ensure that the calculated cavity length can be used for mold locking within a certain error range, after step S4, the following steps are also included:

[0075] S5. Establish the correlation curve between the nonlinear phase shift difference and the intracavity transmittance, and select the splitting ratio with better transmission efficiency; based on the selected splitting ratio, match and determine the corresponding phase shift amount of the phase shifter.

[0076] Figure 5 The curves showing the relationship between intracavity transmittance and nonlinear phase shift difference are illustrated. The horizontal axis represents the nonlinear phase shift difference, indicating the phase shift difference caused by nonlinear effects during light propagation within the cavity. The vertical axis represents the intracavity transmittance, i.e., the proportion of energy transmitted after the optical signal passes through the resonant cavity. Figure 5As shown, it can be seen that under different splitting ratios (e.g., splitting ratio 4:6 and splitting ratio 3:7), the intracavity transmittance of the splitting ratio 4:6 is higher, with the highest transmittance approaching 1.0, and it maintains a high transmittance level in a wider range of nonlinear phase shift differences. This characteristic can effectively improve the intracavity photon accumulation efficiency and accelerate the establishment of mode-locked pulses. Therefore, a coupler with a splitting ratio of 4:6 is selected in this embodiment.

[0077] Based on this, by theoretically calculating the nonlinear phase shift difference and introducing a -π / 2 phase shifter, the cavity length can be kept at a stable operating point with high transmission of mold locking within a wide range. This greatly improves the system's tolerance to errors in cavity length processing and assembly, ensuring that the total cavity length calculated by the aforementioned method can still reliably achieve self-starting mold locking within the error range.

[0078] like Figure 2 As shown in the embodiment of this application, a 1030nm band figure-9 cavity fiber laser is also provided, including a pump laser (976nm, 400mW), a wavelength division multiplexer (WDM), a gain fiber (YDF), a phase shifter, a coupling beam splitter, and a chirped fiber Bragg grating (CFBG). The WDM, gain fiber, phase shifter, and coupling beam splitter are fused together with passive fiber (GDF) to form a closed loop optical path. The chirped fiber Bragg grating is connected to one output end of the coupling beam splitter through a passive fiber, and the pump laser is connected to one input end of the WDM.

[0079] The pump light emitted by the pump laser is coupled into the ring cavity through a wavelength division multiplexer and then propagates clockwise through the gain fiber, phase shifter, and beam splitter. The gain fiber amplifies the signal light within the cavity. When the optical signal reaches the chirped fiber Bragg grating, part of the light is reflected back into the ring cavity and propagates counterclockwise, while the other part passes through the chirped fiber Bragg grating as the output light of the laser. The clockwise propagating optical signal and the counterclockwise propagating optical signal reflected back into the cavity by the chirped fiber Bragg grating circulate within the ring cavity and interfere in the nonlinear amplifying ring mirror structure formed by the beam splitter, thereby achieving stable mode-locked pulse output.

[0080] The lengths of the gain fiber and the passive fiber are determined according to the design method described in Example 1. In this example, the gain fiber is selected as a single-clad polarization-maintaining highly doped ytterbium-doped fiber with a length of 80 cm. The passive fiber is PM980 polarization-maintaining fiber with a total length of 8.093 m. The shorter gain fiber length (compared to the conventional design) combined with a precise total cavity length design can optimize the rate and position of nonlinear phase shift accumulation, effectively promote and accelerate the mode-locking self-starting process, reduce the start-up time, and improve practicality.

[0081] In this embodiment, the reflectivity of the chirped fiber Bragg grating is 15%-25%. By using a low-reflectivity CFBG, the coupler port function used for output in the traditional structure can be replaced, thereby reducing one port of the coupling beam splitter. The coupling beam splitter adopts a 2×1 polarization-maintaining coupler with a splitting ratio of 4:6.

[0082] The chirped fiber Bragg grating (CFBG) in this laser simultaneously achieves three functions:

[0083] 1. The key reflection point of the mode-locking mechanism: providing the necessary reflected light to form the reverse propagation optical path required by NALM.

[0084] 2. Intracavity spectral filter: The inherent wavelength selectivity of CFBG (chirped characteristics can provide a certain bandwidth) can effectively suppress amplified spontaneous emission (ASE) noise, stabilize mode-locking and optimize the output spectrum without the need for an additional filter.

[0085] 3. Laser Output Terminal: The light transmitted through the CFBG is directly used as the laser output. This design significantly simplifies the optical path structure, reduces at least one fusion splice point and one optical fiber (a traditional 2×2 coupler has one more port than a 2×1 coupler), improves the system's compactness, stability, and reliability, and reduces splice loss and potential failure points.

[0086] In this embodiment, the phase shifter is a polarization-maintaining phase shifter with a phase shift of -π / 2. Introducing a fixed -π / 2 phase shift into the NALM loop (typically referring to the path from the coupler, through the phase shifter, part of the passive fiber, WDM, gain fiber, and back to the coupler) can artificially break the symmetry of the NALM loop, biasing the operating point of the nonlinear interference to the region most favorable for self-starting mode locking, greatly improving the reliability and success rate of self-starting.

[0087] Preferably, this laser employs a fully polarization-maintaining fiber structure: the entire laser cavity (including CFBG, coupler, WDM, gain fiber, passive fiber, and phase shifter) uses polarization-maintaining (PM) fiber and devices. This ensures that the polarization state of the light remains stable during transmission, effectively suppressing mode competition and noise caused by polarization fluctuations, and significantly improving the stability, signal-to-noise ratio, and repetition frequency accuracy of the output pulse, making it more suitable for applications with extremely high stability requirements.

[0088] Furthermore, the aforementioned design (CFBG integration, 2×1 coupler, precise cavity length, and full polarization maintenance) collectively achieves a highly integrated, all-fiber "9"-shaped cavity seed source. All key functional components are connected via fiber optic fusion splicing, eliminating spatial optical paths and free-space adjustment components. This not only makes the structure extremely compact and robust but also greatly simplifies manufacturing, packaging, and maintenance processes, improving environmental adaptability and long-term stability.

[0089] The 1030nm band 9-cavity fiber laser provided in this embodiment operates as follows, with each component working together to achieve stable mode-locked pulse output:

[0090] 1. Pumping and Gain Process: Continuous pump light emitted from the 976nm pump laser is efficiently coupled into the figure-9 cavity ring structure through a 980 / 1030nm wavelength division multiplexer, and propagates clockwise within the cavity. When the pump light is incident on a single-clad polarization-maintaining highly doped ytterbium-doped fiber (PMYDF), its energy is absorbed by ytterbium ions in the fiber core, causing the ytterbium ions to transition from the ground state to the excited state, achieving population inversion. The ytterbium ions in the population-inverted state undergo stimulated emission, amplifying the optical signal propagating within the cavity, ultimately generating a laser signal in the 1030nm target wavelength band, completing the replenishment and amplification of optical energy.

[0091] 2. Looping and Beam Splitting Process: The 1030nm laser signal, amplified by the gain fiber, continues to propagate clockwise within the cavity until it reaches the chirped fiber Bragg grating (CFBG). Based on the CFBG's low reflectivity and unique reflection / transmission characteristics, the incident 1030nm laser is split into two parts: one part is reflected by the CFBG and propagates counter-clockwise within the cavity, re-entering the cavity loop; the other part is directly transmitted through the CFBG, serving as the laser's final output signal.

[0092] 3. Mode-locking process: The clockwise propagating laser signal and the counterclockwise propagating laser signal after reflection by the CFBG continuously circulate within the figure-9 cavity. After the two counter-propagating laser signals meet, they are both incident on a 2×1 coupler and undergo nonlinear interference. The coupler splits the two counter-propagating laser beams into two paths according to a preset ratio, further optimizing the interference conditions. When the two laser beams pass through 80cm long highly doped ytterbium-doped fibers, due to the high nonlinearity of the fiber, especially the self-phase modulation (SPM) effect, the laser signals will accumulate nonlinear phase shifts during transmission. To optimize the nonlinear interference conditions within the cavity and promote self-initiation of mode-locking, a -π / 2 phase shifter installed within the cavity introduces a fixed -π / 2 phase shift into one of the propagating laser signals (clockwise or counterclockwise), ensuring that the phase matching degree of the two counter-propagating laser beams meets the mode-locking requirements.

[0093] When two back-propagating laser beams carrying linear phase shifts, nonlinear phase shifts, and a fixed -π / 2 phase shift meet again at the CFBG, their amplitude and phase information superimposed and interfere. Under the preset optimal cavity parameters in this embodiment (including a total cavity length of 8.893m, fiber length ratio, coupler splitting ratio of 4:6, appropriate pump power, and nonlinear coefficient, etc.), this superimposed interference forms an equivalent saturable absorber effect. Only the high-intensity pulse peak portion can pass through the interference region with low loss, while the continuous background light and low-intensity light signals in the cavity are suppressed by interference (exhibiting high-loss characteristics). Through this intensity-dependent loss modulation, the continuous light or noise pulses in the cavity are continuously compressed and shaped, eventually locking into a stable ultrashort mode-locked pulse sequence, completing the formation of mode lock.

[0094] 4. Output and Maintenance Process: The portion of the laser signal transmitted through the CFBG is the stable 1030nm band mode-locked pulse output, realizing the normal signal output function of the laser. Simultaneously, the mode-locked pulse sequence formed within the cavity continuously circulates within the main cavity, which is composed of PM980 polarization-maintaining passive fiber and single-clad polarization-maintaining highly doped ytterbium-doped fiber. Each time it passes through the gain fiber, the mode-locked pulse receives energy replenishment, offsetting the energy loss during transmission within the cavity. Upon passing through the CFBG, a portion is split off as the output signal at a fixed ratio, while the remaining portion is reflected back into the cavity to continue circulating. Under the beam-splitting effect of the 2×1 coupler and the phase modulation effect of the -π / 2 phase shifter, the amplitude, phase, and pulse width of the mode-locked pulse remain stable, ensuring the laser continuously outputs a high-quality mode-locked pulse sequence.

[0095] Figure 3 The mode-locked spectrum obtained by the laser provided in this embodiment of the invention is shown, where the horizontal axis represents wavelength in nanometers (nm), and the vertical axis represents optical power in decibels (dBm). The REF at the vertical axis is the reference level. Figure 3As shown, the mode-locked spectrum output by this laser exhibits a typical plateau-type broadband spectral shape, a typical characteristic of passively mode-locked ultrashort pulse lasers. The dashed box is a visualization marker for spectral width analysis (threshold mode), representing the spectral width range calculated at a 3.00 dB threshold. The left and right boundaries of this dashed box correspond to two wavelength points where the spectral intensity drops by 3.00 dB from the peak; the difference between these two wavelength points is the 3 dB bandwidth of the spectrum. The center position of the dashed box corresponds to the peak wavelength of the spectrum. In this embodiment, the measured center wavelength of the spectrum is 1029.63 nm, which is highly consistent with the target operating wavelength of 1030 nm; the 3 dB spectral width is approximately 9.45 nm, indicating a relatively wide bandwidth and corresponding to narrow pulse output in the time domain. The overall spectral profile is smooth, without obvious mode jumps, parasitic peaks, or intensity distortions, indicating that the laser is operating in a stable, single-pulse passively mode-locked state. The flat-top structure and steep edges of the spectrum reflect a good balance between intracavity nonlinearity, dispersion management and phase matching, verifying the effectiveness of the cavity structure design, splitting ratio selection and phase shifter configuration scheme adopted in this embodiment.

[0096] Figure 4 The figure shows the mode-locked pulse sequence output by the laser, where the horizontal axis represents time in nanoseconds (ns) and the vertical axis represents voltage in millivolts (mV). As can be seen from the figure, the pulse sequence has a uniform period, regular intervals, and good amplitude consistency, without amplitude jitter, pulse loss, or multi-pulse competition, further proving that the laser is in a stable passive mode-locked operating state. The narrow pulse width and low inter-pulse floor noise indicate that the intracavity pulse shaping, saturable absorption equivalent mechanism, and energy compensation process are stable and reliable. This stable pulse output result verifies that the overall scheme of the total cavity length design, gain / passive fiber length ratio, 4:6 coupler splitting ratio, and -π / 2 phase shifter in this embodiment is reasonable and feasible, and the laser can stably output high-quality ultrashort mode-locked pulses for a long period.

[0097] In summary, the 1030nm band figure-9 cavity fiber laser and its design method provided in this application achieve precise total dispersion control through systematic parameter calculation and length matching. By optimizing the fiber length ratio under the premise of a fixed total cavity length, it avoids mode-locking failure or pulse instability caused by dispersion deviation, significantly improving the stability and success rate of mode-locking self-starting. Reliable self-starting can be achieved without external stress, solving the problems of stringent start-up conditions and insufficient practicality of traditional figure-9 cavity lasers. At the same time, the simplified calculation process reduces design and assembly difficulty, improves engineering feasibility, and is more suitable for mass production and practical engineering applications.

[0098] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A design method for a 1030nm band figure-9 cavity fiber laser, characterized in that, The 1030nm band figure-9 cavity fiber laser is composed of a pump laser, a wavelength division multiplexer, a gain fiber, a phase shifter, a coupling beam splitter, and a chirped fiber Bragg grating, which are fused together by passive fiber. The wavelength division multiplexer, the gain fiber, the phase shifter, and the coupling beam splitter are connected in a ring. The chirped fiber Bragg grating is connected to one output end of the coupling beam splitter, and the pump laser is connected to one input end of the wavelength division multiplexer. The design method includes the following steps: S1. Obtain the center wavelength λ and dispersion coefficient D of the chirped fiber Bragg grating using the formula... ； The second-order dispersion value β provided by the chirped fiber Bragg grating was calculated. CFBG , where c is the speed of light; S2. Obtain the dispersion β per unit length of the gain fiber. YDF and the dispersion β per unit length of passive optical fiber GDF ; Determine the target β for intracavitary dispersion control total The target is located in the negative region of zero dispersion; S3. According to the total dispersion calculation formula: ； in, The length of the gain fiber. The length of the passive optical fiber; Take β YDF and β GDF All are equal to the common dispersion value β0 in the 1030nm band, which is then substituted into the total dispersion calculation formula. Then, according to the formula for calculating the adjustable total cavity length: ； The adjustable total cavity length was calculated. ； S4. While keeping the total adjustable cavity length L constant, adjust the lengths of the gain fiber and the passive fiber to design a 1030nm band 9-shaped cavity fiber laser that can self-start mode-locking.

2. The design method of the 1030nm band figure-9 cavity fiber laser according to claim 1, characterized in that: The intracavity dispersion control target β total The value ranges from -0.002ps² to -0.004ps².

3. The design method for a 1030nm band figure-9 cavity fiber laser according to claim 1, characterized in that: In step S4, the length of the gain fiber... The adjustment range is 75cm to 85cm.

4. The design method of the 1030nm band figure-9 cavity fiber laser according to claim 3, characterized in that: In step S4, the length of the gain fiber is determined by interpolation.

5. The design method of the 1030nm band figure-9 cavity fiber laser according to claim 1, characterized in that, Following step S4, the following is also included: S5. Establish the correlation curve between the nonlinear phase shift difference and the intracavity transmittance, and select the splitting ratio with better transmission efficiency; based on the selected splitting ratio, match and determine the corresponding phase shift amount of the phase shifter.

6. A 1030nm band figure-9 cavity fiber laser, characterized in that: The device includes a pump laser, a wavelength division multiplexer, a gain fiber, a phase shifter, a coupling beam splitter, and a chirped fiber Bragg grating. The wavelength division multiplexer, the gain fiber, the phase shifter, and the coupling beam splitter are fused together with passive optical fibers to form a closed loop optical path. The chirped fiber Bragg grating is connected to one output end of the coupling beam splitter via a passive optical fiber. The pump laser is connected to one input end of the wavelength division multiplexer. The pump light emitted by the pump laser is coupled into the ring cavity through a wavelength division multiplexer, and then propagates in a clockwise direction through the gain fiber, phase shifter and beam splitter, wherein the gain fiber amplifies the signal light in the cavity. When an optical signal is transmitted to a chirped fiber Bragg grating, part of the light is reflected back into the ring cavity and propagates in a counterclockwise direction, while the other part of the light is transmitted through the chirped fiber Bragg grating as the output light of the laser. The clockwise propagating optical signal and the counterclockwise propagating optical signal reflected back into the cavity by the chirped fiber Bragg grating are transmitted in a loop within the ring cavity and interfere with each other in the nonlinear amplifying ring mirror structure formed by the coupling beam splitter, thereby achieving stable mode-locked pulse output; The lengths of the gain fiber and the passive fiber are determined by the design method according to any one of claims 1 to 5.

7. The 1030nm band figure-9 cavity fiber laser according to claim 6, characterized in that, The gain fiber is a single-clad polarization-maintaining highly doped ytterbium-doped fiber, and the passive fiber is a PM980 polarization-maintaining fiber.

8. The 1030nm band figure-9 cavity fiber laser according to claim 6, characterized in that: The reflectivity of the chirped fiber Bragg grating is 15%-25%.

9. The 1030nm band figure-9 cavity fiber laser according to claim 8, characterized in that: The coupled beam splitter is a 2×1 polarization-maintaining coupled beam splitter with a splitting ratio of 4:

6.

10. The 1030nm band 9-cavity fiber laser according to claim 9, characterized in that: The phase shifter is a bias-maintaining phase shifter with a phase shift of -π / 2.

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