O-band harmonic mode-locked laser based on dual-pump nonlinear polarization evolution

Abstract

The application discloses a kind of O band harmonic mode-locked lasers based on double-pumping nonlinear polarization evolution, including first pump source, second pump source, first wavelength division multiplexer, second wavelength division multiplexer, gain fiber, first polarization controller, polarization-dependent isolator, second polarization controller and coupler, constitute bidirectional pumping ring laser resonator.Coupled with the first and second polarization controller is placed in polarization-dependent isolator two sides, jointly constitute nonlinear polarization evolution mode-locked structure.By improving the power of double pump source and adjusting the polarization state in the cavity, the switching from fundamental mode-locked to multi-harmonic mode-locked can be realized, with a maximum of 33 orders, breaking through the upper limit of the repetition frequency of O-band all-fiber mode-locked laser, with the advantages of compact structure, high damage threshold, low cost, easy to integrate into all-fiber, suitable for optical communication and other fields.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and in particular to an O-band harmonic mode-locked laser based on dual-pump nonlinear polarization evolution. Background Technology

[0002] Ultrafast fiber lasers, as crucial tools in modern photonics, have demonstrated immense application potential in precision machining, bioimaging, optical communication, and fundamental scientific research. Since the invention of the laser, technological advancements have consistently pursued higher peak power, shorter pulse widths, and more stable operating performance. Traditional solid-state lasers, such as Ti:sapphire lasers, while capable of generating femtosecond-level pulses, suffer from system complexity, large size, and high stability requirements, limiting their industrial applications. In contrast, fiber lasers, with their superior beam quality due to waveguide structures, efficient heat dissipation, and ease of all-fiber integration, have become an ideal platform for achieving high-power, ultrafast laser output. Among these advancements, passive mode-locking within the cavity using nonlinear effects is a key technology for generating ultrashort pulses.

[0003] The O-band (1260 nm–1360 nm) is one of the core windows for optical communication, where single-mode fiber exhibits low loss and near-zero dispersion. However, achieving high-performance ultrafast laser sources in this band faces inherent challenges. Traditional rare-earth ion (such as erbium) doped silica fiber exhibits weak gain in the O-band, making it difficult to use as an effective gain medium. This bottleneck was overcome with the advent of bismuth-doped phosphate silica fiber, which provides effective luminescence gain in the O-band. Nevertheless, the gain coefficient of bismuth-doped fiber is relatively low, and to achieve sufficient laser gain, a longer gain fiber is typically required, leading to an increase in the cavity length. In mode-locked lasers, the cavity length directly determines the pulse cycle time within the cavity, i.e., the fundamental frequency repetition rate. The longer the cavity length, the lower the fundamental frequency repetition rate. Therefore, O-band all-fiber mode-locked lasers based on bismuth-doped fiber generally have low fundamental frequency repetition rates. Currently, the highest repetition rate of O-band all-fiber mode-locked lasers is only around 10 MHz. Harmonic mode-locking is an effective way to increase the repetition frequency, but its stable implementation is highly dependent on the performance of the saturable absorber inside the cavity.

[0004] Among various mode-locking techniques, nonlinear polarization evolution (NPE) is an artificial saturable absorber mechanism based on the fiber Kerr effect. Its working principle involves precisely controlling the polarization state within the resonant cavity, utilizing intensity-dependent nonlinear phase shifts to cause different losses in the pulse peak and flanks as they pass through polarization-dependent components, thereby achieving pulse narrowing and mode-locking. The NPE structure is essentially a fully fiber-based "virtual" absorber, possessing significant advantages such as no wavelength limitations, a high damage threshold, and a relatively simple and compact structure (typically requiring only a few components such as a polarization controller and polarization-dependent isolators). It is well-suited for generating harmonic mode-locked pulses with high repetition rates and high peak power. However, although NPE technology is very mature in the near-infrared band (e.g., 1 μm, 1.5 μm), its application in O-band bismuth-doped fiber lasers, particularly for achieving high repetition rate (megahertz level) harmonic mode-locking, lacks research.

[0005] Currently, the main technical approaches to achieving O-band all-fiber mode-locking include using real material saturable absorbers (such as carbon nanotubes and semiconductor saturable absorber mirrors), nonlinear amplifying ring mirrors, and active mode-locking (such as acousto-optic modulators). These existing solutions all have significant limitations: material-based saturable absorbers have low damage thresholds, making it difficult to operate stably at high pump powers to achieve high-order harmonic mode-locking; nonlinear amplifying ring mirrors have complex structures and are prone to generating disordered multi-pulses at high power; and acousto-optic modulators suffer from high cost, large insertion loss, and difficulty in all-fiber fusion splicing integration. These factors collectively restrict further increases in the repetition rate of O-band all-fiber ultrafast lasers. Therefore, developing a novel all-fiber resonant cavity structure that is compact, has a high damage threshold, is cost-controllable, and can stably generate O-band ultrafast lasers with repetition rates on the order of hundreds of megahertz is of significant technical value and urgently needed in practice. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the difficulty in achieving high repetition rates on the order of hundreds of megahertz in the existing O-band all-fiber ultrafast lasers due to limitations in damage threshold, complexity and cost of existing mode-locking technology or structures.

[0007] To address the aforementioned technical problems, this invention provides an O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution, comprising:

[0008] First pump source, second pump source, first wavelength division multiplexer, second wavelength division multiplexer, gain fiber, first polarization controller, polarization-dependent isolator, second polarization controller, and coupler;

[0009] The first pump source is connected to one end of the gain fiber through the first wavelength division multiplexer, and the second pump source is connected to the other end of the gain fiber through the second wavelength division multiplexer, forming a bidirectional pumped ring laser resonator. The first pump source and the second pump source inject pump light into the ring laser resonator through the first wavelength division multiplexer and the second wavelength division multiplexer, respectively, so that the total pump power injected into the ring laser resonator is the sum of the pump power of the first pump source and the second pump source.

[0010] The output end of the gain fiber is connected to the first polarization controller, the polarization-dependent isolator, the second polarization controller, and the coupler; the output port of the coupler is fed back to the first wavelength division multiplexer, so that the optical path is closed to form the ring laser resonant cavity;

[0011] The first polarization controller and the second polarization controller are respectively disposed on both sides of the polarization-dependent isolator; the first polarization controller, the polarization-dependent isolator and the second polarization controller together constitute a nonlinear polarization evolution mode-locking structure.

[0012] In one embodiment of the present invention, the output power of the first pump source and the second pump source is increased, and the polarization state within the ring laser resonator determined by the first polarization controller and the second polarization controller is adjusted to achieve the switching of the operating state from fundamental frequency mode-locking to multi-harmonic mode-locking.

[0013] In one embodiment of the present invention, the highest order of the multi-harmonic mode-locking is 33.

[0014] In one embodiment of the present invention, both the first pump source and the second pump source are semiconductor lasers with a wavelength of 1240 nanometers.

[0015] In one embodiment of the present invention, the gain fiber is a bismuth-doped phosphate silica fiber.

[0016] In one embodiment of the present invention, the length of the gain fiber is 50 meters.

[0017] In one embodiment of the present invention, both the first wavelength division multiplexer and the second wavelength division multiplexer are 1240 / 1310 nm wavelength division multiplexers.

[0018] In one embodiment of the present invention, the splitting ratio of the coupler is 80:20, wherein 20% of the ports are used for laser output and 80% of the ports are fed back into the resonant cavity.

[0019] In one embodiment of the present invention, the splice loss between the gain fiber and the first wavelength division multiplexer is less than 0.02 dB; the splice loss between the gain fiber and the second wavelength division multiplexer is less than 0.02 dB.

[0020] The present invention also provides an optical communication system, including the O-band harmonic mode-locked laser based on dual-pump nonlinear polarization evolution.

[0021] The technical solution of the present invention has the following advantages compared with the prior art:

[0022] This invention constructs an all-fiber harmonic mode-locked laser suitable for the O-band by combining a bidirectional pump structure with a nonlinear polarization evolution mode-locking structure. Dual pumping, with simultaneous injection from both ends of the gain fiber, allows for nearly twice the total pump power within the resonant cavity compared to a single pump, thus providing the necessary high-gain conditions for achieving high-order harmonic mode-locking. Simultaneously, the employed nonlinear polarization evolution structure, as an all-fiber artificial saturable absorber, possesses inherently high damage thresholds and is not wavelength-limited, enabling it to withstand the high power introduced by dual pumping and achieve stable mode-locking initiation and maintenance. The synergistic effect of these two structures successfully overcomes the bottleneck in existing O-band lasers where the use of low-damage-threshold absorbers or complex structures makes it difficult to achieve both high power and high repetition rate. This structure significantly increases the harmonic mode-locking order from a maximum of 3rd order under single pumping to 33rd order, correspondingly increasing the repetition rate from 10.2 MHz to 112.2 MHz, breaking through the previous upper limit of approximately 10 MHz for all-fiber mode-locked lasers in this band. This invention features a compact structure and primarily employs general-purpose fiber optic devices that are easy to fusion-integrate. It is low in cost and highly stable, providing a reliable all-fiber solution for achieving high-efficiency, high-repetition-frequency O-band ultrafast laser output. Attached Figure Description

[0023] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0024] Figure 1 This is a schematic diagram of the ultrafast fiber laser resonator structure of the single-pump nonlinear polarization evolution structure in an embodiment of the present invention;

[0025] Figure 2 This is a schematic diagram of the ultrafast fiber laser resonator structure of the dual-pump nonlinear polarization evolution structure in an embodiment of the present invention;

[0026] Figure 3 This is a schematic diagram of experimental measurement results under different mode-locking states in a single-pump structure according to an embodiment of the present invention;

[0027] (a1)–(a3) are spectra, which correspond to the output spectra under the fundamental mode-locked, second harmonic mode-locked and third harmonic mode-locked states, respectively, showing spectral characteristics such as center wavelength and 3 dB bandwidth.

[0028] (b1)–(b3) are pulse sequence diagrams, which respectively show the time-domain pulse sequences under the above three states, and intuitively reflect the pulse interval and repetition frequency;

[0029] (c1)–(c3) are radio frequency spectrum diagrams, which show the fundamental frequency signal and its signal-to-noise ratio and side-mode rejection ratio under three mode-locked states, respectively;

[0030] (d1)–(d3) are autocorrelation curves, which correspond to the pulse autocorrelation traces of the three states, respectively, and are used to calculate the pulse width;

[0031] (e1)–(e3) are curves showing the output power as a function of pump power, respectively demonstrating the mode-locking threshold and slope efficiency for the three states.

[0032] Figure 4 This is a schematic diagram of the experimental measurement results of achieving high-order harmonic mode-locking under the dual-pump structure in an embodiment of the present invention;

[0033] Where (a1)–(a2) are spectra, which correspond to the output spectra under the fourth-order harmonic mode-locked state and the thirty-third-order harmonic mode-locked state, respectively;

[0034] (b1)–(b2) are pulse sequence diagrams, showing the time-domain pulse sequences and high repetition frequency characteristics under fourth-order and thirty-third-order harmonic mode-locking, respectively;

[0035] (c1)–(c2) are radio frequency spectrum diagrams, which respectively show the signal-to-noise ratio and side-mode rejection ratio of the two higher-order mode-locked states;

[0036] (d1)–(d2) are autocorrelation curves, corresponding to the pulse width measurement results of fourth-order and thirty-third-order mode-locked circuits, respectively;

[0037] (e1)–(e2) are output power curves, showing the threshold and efficiency characteristics of the two higher-order mode-locked states.

[0038] Explanation of reference numerals in the accompanying drawings: 1. Pump source; 2. Wavelength division multiplexer; 3. Gain fiber; 4. First polarization controller; 5. Polarization correlation isolator; 6. Second polarization controller; 7. Coupler; 11. First pump source; 12. Second pump source; 21. First wavelength division multiplexer; 22. Second wavelength division multiplexer. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0040] Example 1:

[0041] This invention provides an O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution, comprising:

[0042] First pump source, second pump source, first wavelength division multiplexer, second wavelength division multiplexer, gain fiber, first polarization controller, polarization-dependent isolator, second polarization controller, and coupler;

[0043] The first pump source is connected to one end of the gain fiber through the first wavelength division multiplexer, and the second pump source is connected to the other end of the gain fiber through the second wavelength division multiplexer, forming a bidirectional pumped ring laser resonator. The first pump source and the second pump source inject pump light into the ring laser resonator through the first wavelength division multiplexer and the second wavelength division multiplexer, respectively, so that the total pump power injected into the ring laser resonator is the sum of the pump power of the first pump source and the second pump source.

[0044] The output end of the gain fiber is connected to the first polarization controller, the polarization-dependent isolator, the second polarization controller, and the coupler; the output port of the coupler is fed back to the first wavelength division multiplexer, so that the optical path is closed to form the ring laser resonant cavity;

[0045] The first polarization controller and the second polarization controller are respectively disposed on both sides of the polarization-dependent isolator; the first polarization controller, the polarization-dependent isolator and the second polarization controller together constitute a nonlinear polarization evolution mode-locking structure.

[0046] This invention provides an O-band harmonic mode-locked laser based on dual-pump nonlinear polarization evolution (NPE). The core of its technical solution lies in the innovative integration of a bidirectional pump structure and a nonlinear polarization evolution mode-locking structure within the same ring resonant cavity. Pump light is simultaneously injected from both ends of the gain fiber via two wavelength division multiplexers from a first and a second pump source, respectively, achieving effective superposition of pump power within the cavity. Simultaneously, a first polarization controller located on either side of a polarization-dependent isolator, the isolator itself, and a second polarization controller together constitute an all-fiber NPE mode-locking unit, forming an artificial saturable absorption effect through precise control of the intracavity polarization state. The dual-pump structure multiplies the total pump power injected into the resonant cavity, providing the necessary high-gain conditions for forming and maintaining high-order harmonic mode-locking. The NPE mode-locking structure has the advantages of a high damage threshold and wavelength independence, enabling it to withstand high pump power and achieve stable mode-locking self-starting and pulse shaping. Working together, the two technologies have solved the key technical bottleneck of existing O-band all-fiber mode-locked lasers, which are difficult to achieve high repetition rate output in the hundreds of megahertz range due to insufficient gain or limitations in the performance of saturable absorbers. This provides a brand-new all-fiber solution for realizing compact, stable, and efficient high repetition rate O-band ultrafast laser sources.

[0047] In this embodiment of the invention, a fully fiber-based ring laser resonator is first constructed. The core gain medium is a 50-meter-long bismuth-doped phosphate silica fiber, which provides effective luminescence gain in the O-band (1260-1360 nm). A fiber fusion splicer is used to perform low-loss splicing of this gain fiber with all other fiber optic devices, with the loss at each splice controlled below 0.02 dB to minimize intracavity loss.

[0048] like Figure 1 As shown, the initial laser structure is an ultrafast fiber laser resonator with a single-pump nonlinear polarization evolution structure. In this structure, a semiconductor laser with a center wavelength of 1240 nm is used as the pump source 1, with a maximum output power of approximately 550 mW. The pump light is injected into the ring cavity through a 1240 / 1310 nm wavelength division multiplexer 2. A coupler 7 with a splitting ratio of 80:20 is placed in the resonator, where 20% of the ports are used for laser output to connect to subsequent measurement equipment for characterizing laser performance; the other 80% of the ports feed the light back into the resonator, forming a closed-loop oscillation in the optical path, which is fundamental to achieving stable laser operation. By adjusting the power of the pump source 1, a stable continuous laser output with a center wavelength in the O-band can be obtained.

[0049] To transition from continuous-wave laser operation to ultrashort-pulse laser operation, a mode-locking mechanism must be introduced within the ultrafast fiber laser resonator. This invention employs nonlinear polarization evolution technology to achieve passive mode-locking. Specifically, an optical device assembly consisting of two polarization controllers and a polarization-dependent isolator 5 is introduced into the cavity. The first polarization controller 4 and the second polarization controller 6 are located on opposite sides of the polarization-dependent isolator 5, forming a nonlinear polarization evolution mode-locking structure. The working principle of the nonlinear polarization evolution mode-locking structure is a dynamic polarization state modulation and selection process: light propagating randomly within the resonator first passes through the polarization-dependent isolator 5. The isolator 5 acts as a polarizer, allowing only linearly polarized light of a specific direction to pass efficiently, while light of other polarization states is attenuated, thus outputting linearly polarized light of a defined direction. This linearly polarized light then passes through the first polarization controller 4 and is adjusted to elliptically polarized light. Elliptically polarized light can be decomposed into two orthogonal polarization components.

[0050] When these two orthogonal polarization components propagate through the subsequent single-mode fiber (including gain fiber 3 and other transmission fibers within the cavity), they are affected by the fiber Kerr effect, resulting in a nonlinear phase shift, primarily self-phase modulation. Since the magnitude of the nonlinear phase shift is proportional to the light intensity, the intensity of the peak portion of an optical pulse is much higher than that of the two flanks (background). Therefore, the accumulated nonlinear phase shift at the pulse peak is significantly greater than that at the flanks. This intensity-dependent, unevenly distributed nonlinear phase shift alters the polarization state of the light, causing a separation between the polarization state evolution trajectories of the pulse peak and the pulse flanks. The light continues to propagate within the cavity, passing through the second polarization controller 6. By finely adjusting the state of the second polarization controller 6, the polarization state of the light after undergoing the nonlinear phase shift can be further fine-tuned or compensated.

[0051] The arrangement of placing two polarization controllers on either side of the polarization-dependent isolator 5 is key to achieving stable and efficient nonlinear polarization evolution mode-locking. The core reason is that this structure provides the physical basis for "segmented" and "closed-loop" control of the polarization state. The first polarization controller 4, located before the isolator, primarily shapes the initial polarization state of the light about to enter the nonlinear transmission stage, setting it to a specific elliptic polarization state—a prerequisite for subsequent intensity-dependent polarization evolution. The second polarization controller 6, located after the isolator, serves for feedback and final state optimization. After the light pulse completes its entire loop journey within the cavity, including gain and nonlinear phase shift accumulation, its polarization state has undergone complex changes. The second polarization controller 6 precisely fine-tunes the final polarization state of the light about to re-enter the isolator after completing this complete journey.

[0052] If both polarization controllers are placed on the same side of the isolator, the system cannot independently and precisely control the polarization state of the light after it has completed the entire loop transmission; it can only adjust the initial polarization state of the light before it enters the loop. In actual laser resonant cavities, fiber optics are subject to unpredictable and slowly drifting polarization disturbances due to complex environmental factors such as bending and stress. By adopting a structural layout that places the polarization controllers on opposite sides of the isolator, the first polarization controller 4 can set a robust initial polarization condition, while the second polarization controller 6 can dynamically compensate for the impact of the aforementioned environmental disturbances on the final polarization state of the light after one cycle of transmission. This ensures that the pulse peak portion is aligned with the isolator in the optimal polarization direction each time, achieving efficient passage. Conversely, without this compensation mechanism, the mode-locked state becomes extremely unstable, highly sensitive to external environmental interference, leading to a significant increase in the mode-locking threshold, difficulty in maintaining stable mode-locking, and an inability to achieve high-order harmonic mode-locking under high pump power conditions. Therefore, the structure adopted in this invention essentially constructs a polarization control closed-loop system that combines feedforward setting and feedback correction functions. This structure not only significantly enhances the robustness and self-starting reliability of the mode-locking process, but also allows the nonlinear polarization evolution structure to fully utilize its high damage threshold characteristics and withstand the high-power pump light introduced by dual pumping.

[0053] When the light recycles and reaches the polarization selection element, the polarization-dependent isolator 5, its polarization state has changed compared to its initial state. By precisely adjusting the states of the two polarization controllers, an optimized polarization configuration can be achieved: the polarization state of the pulse peak portion is highly aligned with the specific polarization direction allowed to pass through by the isolator, thus passing through with extremely low loss; while the polarization state of the lower energy flanks of the pulse deviates significantly from this transmission direction, thus undergoing strong attenuation. This characteristic allows high-intensity light (pulse center) to experience low loss, while low-intensity light (noise and pulse edges) experiences high loss, thus accurately simulating the function of a saturable absorber. Each time the light completes a cycle within the resonant cavity, the energy of the pulse flanks is partially suppressed due to high loss, while the energy of the pulse peak is retained and enhanced due to low loss. After multiple cycles, the pulse width is continuously compressed, eventually forming and maintaining a stable ultrashort pulse sequence, achieving fundamental frequency mode-locking. At this point, only a single pulse circulates within the resonant cavity, and its repetition frequency is determined by the total optical length of the laser resonant cavity, numerically equal to the reciprocal of the time required for the light to circulate once within the cavity.

[0054] As the pump power is increased, the laser gain also increases. If the gain is too high, a single pulse cannot carry all the energy, and nonlinear effects within the cavity will cause a single pulse to split into multiple identical pulses. Under the constraints and management of an effective mode-locking mechanism (i.e., the aforementioned nonlinear polarization evolution structure), these multiple pulses will not move randomly, but will automatically find and occupy equally spaced positions within the cavity, forming a new dynamic equilibrium state of periodic arrangement and stable coexistence in the time domain. This is harmonic mode-locking. In harmonic mode-locking, the output pulse repetition frequency is an integer multiple of the fundamental frequency repetition frequency; this integer is the order of harmonic mode-locking. The formation and stability of harmonic mode-locking are highly dependent on the gain provided by a sufficiently high pump power and the effective control capability of the mode-locking structure over the multi-pulse state.

[0055] like Figure 2 As shown, based on the experimental foundation and principle of the single-pump structure described above, in order to break through the upper limit of the repetition frequency of the O-band bismuth-doped fiber laser, this invention further proposes a bidirectional pump structure.

[0056] Specifically, based on the single-pump structure, a second semiconductor laser with the same performance parameters (i.e., a center wavelength of 1240 nm) is introduced as the second pump source 12. The pump light from the second pump source 12 is injected into the resonant cavity from the other end of the gain fiber 3 through another 1240 / 1310 nm wavelength division multiplexer. In this way, the first pump source 11 and the second pump source 12 simultaneously and bidirectionally inject pump light into the ring laser resonant cavity from both ends of the gain fiber 3 through the first wavelength division multiplexer 21 and the second wavelength division multiplexer 22, respectively.

[0057] The key advantage of this structure is that when the first pump source 11 and the second pump source 12 operate simultaneously, the total pump power injected into the resonant cavity is equivalent to the superposition of the powers of the two pump sources. If the powers of the two pump sources are comparable, the total pump power can reach twice that of a single-pump structure. This power multiplication effect can significantly improve the population inversion density and stimulated emission gain in the gain fiber 3, providing the necessary high-energy conditions for the formation and maintenance of a larger number of mode-locked pulses (i.e., higher-order harmonic mode-locking) within the cavity.

[0058] After constructing the bidirectional pumping structure, by jointly increasing the output power of both pump lasers (e.g., to a total pump power of approximately 1 watt) and coordinating and finely adjusting the overall intracavity polarization state determined by the first polarization controller 4 and the second polarization controller 6, higher-order harmonic mode-locking states can be excited and stabilized. Experimental results show that, compared to the single-pump structure which can only achieve a maximum of third-order harmonic mode-locking, the bidirectional pumping structure can significantly increase the stable harmonic mode-locking order to thirty-third. The corresponding output pulse repetition frequency jumps from 10.2 MHz under single pumping to 112.2 MHz. This repetition frequency successfully breaks through the technical bottleneck of approximately 10 MHz for the highest repetition frequency of O-band all-fiber mode-locked ultrafast lasers previously reported, improving it by an order of magnitude. This significant effect verifies the technical superiority of the bidirectional pumping structure in providing high gain, combined with the high-damage-threshold mode-locking structure of nonlinear polarization evolution.

[0059] Based on the structural construction and working principle described in Embodiment 1 above, the specific implementation of the present invention is further verified and explained through experimental data and details of the embodiments.

[0060] In practical implementation, the first step is to Figure 1 The ultrafast fiber laser resonator with the single-pumped nonlinear polarization evolution structure shown was systematically measured. The measurement data includes spectra, pulse sequences, frequency spectra, autocorrelation curves, and output power, corresponding to three states: fundamental mode-locked, second-harmonic mode-locked, and third-harmonic mode-locked, respectively. Figure 3 As shown. Figure 3 (a1)–(a3) are spectra obtained by a spectrometer, showing that the center wavelengths (λc) of the fundamental, second-order and third-order mode-locked frequencies are 1335.4 nm, 1317.9 nm and 1321.1 nm, respectively, all located in the O band; their 3 dB bandwidths (Δλ) are 26.4 nm, 27.3 nm and 26.3 nm, respectively. Figure 3 Figures (b1)–(b3) show the pulse sequence observed by an oscilloscope, showing that the fundamental frequency mode-locked pulse interval is 0.29 microseconds, corresponding to a repetition frequency of 3.4 MHz; the second-order and third-order harmonic mode-locked repetition frequencies are 6.8 MHz and 10.2 MHz, respectively, with uniform pulse intervals. Figure 3 (c1)–(c3) are spectrum diagrams analyzed by a spectrum analyzer, showing that the signal-to-noise ratios of the fundamental frequency, second-order and third-order mode-locked frequencies are 52.9 dB, 48.2 dB and 43 dB, respectively, and the side-mode rejection ratios are 14.3 dB and 20.7 dB, respectively, under second-order and third-order mode-locked frequencies. Figure 3 In the diagram (d1)–(d3), the autocorrelation curves were measured and fitted by an autocorrelation instrument, and the pulse widths of the fundamental frequency, second-order and third-order mode-locked pulses were found to be 68.9 femtoseconds, 106.8 femtoseconds and 120 femtoseconds, respectively. Figure 3Figures (e1)–(e3) show the output power graphs recorded by the power meter, indicating that the pump powers for fundamental, second-order, and third-order mode-locking are approximately 300 mW, 210 mW, and 250 mW, respectively, with laser slope efficiencies of 0.49%, 0.92%, and 0.99% in the mode-locked regions. At the highest single-pump power of 550 mW, the highest harmonic mode-locking order achievable by adjusting the two polarization controllers is third-order.

[0061] Subsequently, based on Figure 2 Experiments were conducted on the dual-pump nonlinear polarization evolution structure shown. With the total pump power increased to approximately 1 watt, higher-order harmonic mode-locking was achieved through fine-tuning of the intracavity polarization state. The measurement data are as follows: Figure 4 As shown. Figure 4 The (a1)–(a2) spectra show that the center wavelengths of the fourth and thirty-third mode locks are 1326.5 nm and 1344.2 nm, respectively, and the 3 dB bandwidths are 37.2 nm and 46.7 nm, respectively. Figure 4 (b1)–(b2) are pulse sequence diagrams, showing that the fundamental frequency repetition frequency is still about 3.4 MHz, and the repetition frequencies corresponding to the fourth and thirty-third harmonic mode locking are 13.6 MHz and 112.2 MHz, respectively, with the pulses arranged uniformly. Figure 4 (c1)–(c2) are spectrograms, showing that the signal-to-noise ratios of the fourth-order and thirty-third-order mode-locked modes are 44.4 dB and 38.7 dB, respectively, and the side-mode rejection ratios are 25.1 dB and 15.7 dB, respectively. Figure 4 The graph (d1)–(d2) shows the autocorrelation curves, and the pulse widths for the fourth-order and thirty-third-order mode-locked modes are 66.5 femtoseconds and 75.1 femtoseconds, respectively. Figure 4 Figures (e1)–(e2) show the output power, revealing that the mode-locking threshold pump powers for fourth-order and thirty-third-order mode-locking are approximately 210 mW and 160 mW, respectively, with slope efficiencies of 0.7% and 2.8% in the mode-locking region. The 112.2 MHz repetition frequency corresponding to the thirty-third-order harmonic mode-locking significantly surpasses the previously reported upper limit of approximately 10 MHz for the highest repetition frequency of O-band all-fiber mode-locked ultrafast lasers, representing an improvement of an order of magnitude.

[0062] In the specific implementation process, all single-mode fibers and optical components within the resonant cavity are spliced ​​using a fiber optic fusion splicer (model: Fujikura 87C+), with the loss at each splice strictly controlled below 0.02 dB to minimize intracavity loss. Ultrafast laser output measurement employs angled physical contact fiber optic connectors to reduce losses introduced by Fresnel reflection. The implementation of this invention verifies the effectiveness of combining a dual-pump structure with a nonlinear polarization evolution mode-locking mechanism. By providing high gain conditions approaching double the pump power and utilizing the high damage threshold and excellent pulse management capabilities of the NPE structure, stable, high-repetition-frequency harmonic mode-locked ultrafast laser output in the O-band is successfully achieved, providing a compact and reliable all-fiber solution for related applications.

[0063] Example 2:

[0064] This invention also provides an optical communication system comprising an O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution as described in Embodiment 1. This laser, serving as the system's light source, provides ultrashort pulse sequences with a center wavelength in the O-band (1260 nm-1360 nm) and a repetition rate on the order of hundreds of megahertz (e.g., 112.2 MHz). Due to the all-fiber structure combining bidirectional pumping and nonlinear polarization evolution mode-locking, the laser offers advantages such as compact structure, high stability, low cost, and ease of system integration. This optical communication system can utilize the high repetition rate, ultrashort pulse light source generated by the laser to achieve high-speed, high-capacity optical signal modulation and transmission within the O-band communication window, effectively improving the system's communication rate and bandwidth performance. It is suitable for applications such as modern fiber optic communication networks, data center interconnects, and future high-speed optical access networks.

[0065] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution, characterized in that, include: First pump source, second pump source, first wavelength division multiplexer, second wavelength division multiplexer, gain fiber, first polarization controller, polarization-dependent isolator, second polarization controller, and coupler; The first pump source is connected to one end of the gain fiber through the first wavelength division multiplexer, and the second pump source is connected to the other end of the gain fiber through the second wavelength division multiplexer, forming a bidirectional pumped ring laser resonator. The first pump source and the second pump source inject pump light into the ring laser resonator through the first wavelength division multiplexer and the second wavelength division multiplexer, respectively, so that the total pump power injected into the ring laser resonator is the sum of the pump power of the first pump source and the second pump source. The output end of the gain fiber is connected to the first polarization controller, the polarization-dependent isolator, the second polarization controller, and the coupler; the output port of the coupler is fed back to the first wavelength division multiplexer, so that the optical path is closed to form the ring laser resonant cavity; The first polarization controller and the second polarization controller are respectively disposed on both sides of the polarization-dependent isolator; the first polarization controller, the polarization-dependent isolator and the second polarization controller together constitute a nonlinear polarization evolution mode-locking structure.

2. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: The output power of the first pump source and the second pump source is increased, and the polarization state inside the ring laser resonator is adjusted by the first polarization controller and the second polarization controller to achieve the switching of the operating state from fundamental frequency mode-locking to multi-harmonic mode-locking.

3. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 2, characterized in that: The highest order of the multi-harmonic mode-locking is 33.

4. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: Both the first pump source and the second pump source are semiconductor lasers with a wavelength of 1240 nanometers.

5. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: The gain fiber is a bismuth-doped phosphate quartz fiber.

6. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: The length of the gain fiber is 50 meters.

7. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: Both the first and second wavelength division multiplexers are 1240 / 1310 nm wavelength division multiplexers.

8. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: The coupler has a splitting ratio of 80:20, with 20% of the ports used for laser output and 80% of the ports fed back into the resonant cavity.

9. The O-band harmonic mode-locked laser based on dual-pumped nonlinear polarization evolution according to claim 1, characterized in that: The splice loss between the gain fiber and the first wavelength division multiplexer is less than 0.02 dB; the splice loss between the gain fiber and the second wavelength division multiplexer is less than 0.02 dB.

10. An optical communication system, characterized in that, Includes an O-band harmonic mode-locked laser based on dual-pump nonlinear polarization evolution as described in any one of claims 1 to 9.

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