Topological insulator mode-locked laser and pulse width compression method thereof

Abstract

The invention provides a topological insulator mode-locked laser and a pulse width compression method thereof, the laser comprises an annular cavity structure, the annular cavity structure comprises a pumping source, a one-stage or two-stage hybrid mode-locked structure and a coupler, the two-stage hybrid mode-locked structure comprises a first-stage hybrid mode-locked structure and a second-stage hybrid mode-locked structure which are connected in series, each hybrid mode-locking structure comprises a saturable absorber and a nonlinear polarization rotation unit, and the saturable absorber of the first-stage hybrid mode-locking structure adopts an II-type semiconductor topological insulator heterojunction; the saturable absorbers of the first-stage hybrid mode-locking structure and the second-stage hybrid mode-locking structure respectively adopt II-type semiconductor topological insulator heterojunction and graphene; the pumping source emits a laser pulse signal; the first-stage hybrid mode locking structure carries out low-intensity noise suppression, high-intensity pulse screening, polarization state regulation and control and dynamic dispersion compensation on laser pulse signals, and stable ultra-short mode locking pulse signals are output through a coupler. The pulse width of the ultra-short mode-locked pulse signal can reach femtosecond magnitude.

Description

Technical Field

[0001] The present invention relates to the technical field of mode-locked lasers, and in particular to a topological insulator mode-locked laser and a pulse width compression method thereof. Background Art

[0002] With the rapid development of optical communications, precision machining, biomedicine and other fields, the performance requirements for mode-locked lasers are increasing, especially the ability to generate ultrashort pulses. Mode-locked lasers, through the modulation of saturable absorbers (SAs), can generate femtosecond ultrashort pulses, which have important applications in high-speed optical communications, high-precision materials processing, ultrafast spectroscopy and other fields.

[0003] As a new type of saturable absorber material, topological insulators (such as Bi2Se3 and bismuth selenide) have become an ideal choice for mode-locked lasers due to their advantages such as broadband response, high modulation depth (up to 95% or more), high damage threshold and low saturation power. However, this type of topological insulator has the problems of long surface state carrier lifetime and slow nonlinear absorption dynamics, which makes it difficult to significantly improve the nonlinear optical response, resulting in pulse broadening (too wide) of the output of the mode-locked laser, limiting its application in ultrashort pulses. For example, the output pulse width of traditional Bi2Se3-based mode-locked lasers is usually in the order of picoseconds. Although it is better than some semiconductor materials, it still cannot meet the requirements of femtosecond ultrashort pulses for precision machining, ultrafast spectroscopy, etc., and is sensitive to light intensity fluctuations, easily affected by environmental interference, and has poor stability, which is not conducive to pulse chirp formation and efficient compression. Summary of the Invention

[0004] In view of this, an embodiment of the present invention provides a topological insulator mode-locked laser and a pulse width compression method thereof to eliminate or improve one or more defects existing in the prior art.

[0005] A first aspect of the present invention provides a topological insulator mode-locked laser, comprising: a ring cavity structure, the ring cavity structure comprising a pump source, a first-stage hybrid mode-locked structure or a two-stage hybrid mode-locked structure, and a coupler, the two-stage hybrid mode-locked structure comprising a first-stage hybrid mode-locked structure and a second-stage hybrid mode-locked structure connected in series, the hybrid mode-locked structure comprising a saturable absorber and a nonlinear polarization rotation unit, the saturable absorber in the first-stage hybrid mode-locked structure employing a type II semiconductor topological insulator heterojunction, and the saturable absorbers in the first-stage hybrid mode-locked structure and the second-stage hybrid mode-locked structure employing a type II semiconductor topological insulator heterojunction and graphene, respectively;

[0006] The pump source emits a laser pulse signal;

[0007] The first-stage hybrid mode-locked structure performs low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation on the laser pulse signal, and outputs a stable ultrashort mode-locked pulse signal through the coupler;

[0008] The first-stage hybrid mode-locking structure mode-locks and compresses the laser pulse signal and outputs a pre-compressed mode-locked pulse signal. The second-stage hybrid mode-locking structure further compresses the pre-compressed mode-locked pulse signal and induces nonlinear phase modulation, and outputs a stable ultrashort mode-locked pulse signal through the coupler.

[0009] In some embodiments of the present invention, the ring cavity structure further includes a wavelength division multiplexer and an erbium-doped fiber. The wavelength division multiplexer is connected to the erbium-doped fiber. The wavelength division multiplexer transmits the laser pulse signal emitted by the pump source or the laser pulse signal after gain amplification by the erbium-doped fiber to the first-stage hybrid mode-locked structure or the two-stage hybrid mode-locked structure. The erbium-doped fiber amplifies the mode-locked pulse signal output by the first-stage hybrid mode-locked structure or the two-stage hybrid mode-locked structure and compensates for loop loss.

[0010] In some embodiments of the present invention, the nonlinear polarization rotation unit in the one-stage hybrid mode-locked structure includes a polarization-dependent polarizer, a first polarization controller, and a second polarization controller;

[0011] The saturable absorber performs rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening on the laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped fiber, so as to mode lock, modulate, and compress the laser pulse signal, and cooperates with the second polarization controller to ultimately output a stable ultrashort mode-locked pulse signal;

[0012] The second polarization controller dynamically adjusts the polarization state of the mode-locked pulse signal output by the saturable absorber and transmitted through the coupler;

[0013] The polarization-dependent polarizer further mode-locks, modulates and compresses the mode-locked pulse signal output by the second polarization controller through a nonlinear effect, and enables the mode-locked pulse signal to be transmitted unidirectionally in the ring cavity;

[0014] The first polarization controller optimizes the polarization state and dynamic dispersion compensation of the mode-locked pulse signal output by the polarization-dependent polarizer and improves the gain efficiency, and transmits the mode-locked pulse signal output by the first polarization controller to the erbium-doped optical fiber.

[0015] In some embodiments of the present invention, the first-stage hybrid mode-locked structure includes a first saturable absorber and a first nonlinear polarization rotation unit, and the first nonlinear polarization rotation unit includes a first polarization-dependent polarizer and a first polarization controller;

[0016] The first polarization controller dynamically controls the polarization state of the laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped optical fiber;

[0017] The first saturable absorber performs rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening on the laser pulse signal output by the first polarization controller, so as to mode lock, modulate, and compress the laser pulse signal and output a mode-locked pulse signal;

[0018] The first polarization-dependent polarizer dynamically controls the polarization state and dynamically optimizes the dispersion compensation of the mode-locked pulse signal output by the first saturable absorber through the first polarization controller, outputs a pre-compressed mode-locked pulse signal, and enables the mode-locked pulse signal to be transmitted unidirectionally in the ring cavity.

[0019] In some embodiments of the present invention, the second-stage hybrid mode-locked structure includes a second saturable absorber and a second nonlinear polarization rotation unit, and the second nonlinear polarization rotation unit includes a second polarization-dependent polarizer, a second polarization controller, and a third polarization controller;

[0020] The second polarization controller dynamically controls the polarization state of the mode-locked pulse signal output by the first polarization-dependent polarizer;

[0021] The second saturable absorber further compresses the mode-locked pulse signal output by the second polarization controller;

[0022] The second polarization-dependent polarizer induces nonlinear phase modulation and eliminates residual tail noise on the mode-locked pulse signal output by the second saturable absorber through the second polarization controller and the third polarization controller in combination with the strong Kerr effect of the second saturable absorber;

[0023] The third polarization controller dynamically controls the polarization state of the mode-locked pulse signal output by the second polarization-dependent polarizer, and transmits the mode-locked pulse signal output by the third polarization controller to the erbium-doped optical fiber via the coupler, thereby finally outputting a stable ultrashort mode-locked pulse signal.

[0024] In some embodiments of the present invention, the type II semiconductor topological insulator heterojunction includes a type II semiconductor topological insulator heterojunction formed by Bi2Se3 and WSe2.

[0025] In some embodiments of the present invention, the thickness of the type II semiconductor topological insulator heterojunction includes 5 nm, 8 nm and 10 nm.

[0026] In some embodiments of the present invention, the coupling ratio of the coupler is 90:10 or 80:20.

[0027] A second aspect of the present invention provides a method for compressing the pulse width of a topological insulator mode-locked laser, wherein the topological insulator mode-locked laser includes the topological insulator mode-locked laser as described in the first aspect, and the method comprises the following steps:

[0028] A laser pulse signal is emitted through a pump source;

[0029] Through a first-stage hybrid mode-locked structure, the laser pulse signal is subjected to low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation, and a stable ultrashort mode-locked pulse signal is output through a coupler;

[0030] Alternatively, the laser pulse signal is mode-locked and compressed by the first-stage hybrid mode-locking structure in the two-stage hybrid mode-locking structure and a pre-compressed mode-locked pulse signal is output. The pre-compressed mode-locked pulse signal is further compressed and nonlinear phase modulation is induced by the second-stage hybrid mode-locking structure in the two-stage hybrid mode-locking structure, and a stable ultrashort mode-locked pulse signal is output through a coupler.

[0031] In some embodiments of the present invention, the method of performing low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation on a laser pulse signal through a first-stage hybrid mode-locked structure, and outputting a stable ultrashort mode-locked pulse signal through a coupler, includes:

[0032] The laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped fiber is subjected to rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening through a saturable absorber, so as to mode lock, modulate, and compress the laser pulse signal, and finally output a stable ultrashort mode-locked pulse signal in cooperation with the second polarization controller;

[0033] Dynamically controlling the polarization state of the mode-locked pulse signal output by the saturable absorber and transmitted through the coupler by a second polarization controller;

[0034] The polarization-dependent polarizer further mode-locks, modulates and compresses the mode-locked pulse signal output by the second polarization controller through nonlinear effects, and makes the mode-locked pulse signal unidirectionally transmitted in the ring cavity;

[0035] The polarization state and dynamic dispersion compensation of the mode-locked pulse signal output by the polarization-dependent polarizer are optimized by the first polarization controller, and the gain efficiency is improved, and the mode-locked pulse signal output by the first polarization controller is transmitted to the erbium-doped optical fiber.

[0036] The topological insulator mode-locked laser and its pulse compression method of the present invention aim to shorten the pulse width of existing topological insulator mode-locked lasers. Through the strategy of coordinated optimization of material innovation and optical structure design of the saturable absorber in the laser, the problems of excessively wide pulse width and poor stability of traditional topological insulator mode-locked lasers are effectively solved, the pulse quality, anti-interference and overall stability of the laser are significantly improved, and stable femtosecond ultrashort pulses can be output.

[0037] Additional advantages, objects, and features of the present invention will be set forth in part in the following description and will become apparent to those skilled in the art upon examination of the following or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained by the structures particularly pointed out in the description and drawings.

[0038] Those skilled in the art will understand that the purposes and advantages that can be achieved by the present invention are not limited to the above specific descriptions, and the above and other purposes that can be achieved by the present invention will be more clearly understood based on the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of this application, and do not constitute a limitation of the present invention.

[0040] Figure 1 Schematic diagram of the structure of a topological insulator mode-locked laser in one embodiment of the present invention;

[0041] Figure 2 Schematic diagram of the structure of a topological insulator mode-locked laser in another embodiment of the present invention;

[0042] Figure 3 Schematic diagram of the energy bands of a type II semiconductor heterojunction formed by combining Bi2Se3 and WSe2 in one embodiment of the present invention;

[0043] Figure 4 Schematic diagram of a process for compressing the pulse width of a topological insulator mode-locked laser according to an embodiment of the present invention;

[0044] Figure 5 Schematic diagram of the flow of a pulse width compression method for a topological insulator mode-locked laser in another embodiment of the present invention. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments and the accompanying drawings. Here, the exemplary embodiments of the present invention and their descriptions are used to explain the present invention, but are not intended to limit the present invention.

[0046] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, the accompanying drawings only show structures and / or processing steps closely related to the solutions according to the present invention, while other details that are not closely related to the present invention are omitted.

[0047] It should be emphasized that the term "include / comprises" when used herein refers to the existence of features, elements, steps or components, but does not exclude the existence or addition of one or more other features, elements, steps or components.

[0048] It should also be noted that, unless otherwise specified, the term "connection" herein may refer not only to a direct connection but also to an indirect connection involving an intermediate.

[0049] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the accompanying drawings, the same reference numerals represent the same or similar components, or the same or similar steps.

[0050] In order to effectively compress the overly wide pulses output by existing topological insulator mode-locked lasers and output stable femtosecond ultrashort pulses, the embodiments of the present invention propose a topological insulator mode-locked laser and a pulse compression method thereof. With the goal of shortening the pulse width of existing topological insulator mode-locked lasers, a strategy of coordinated optimization of material innovation and optical structure design of the saturable absorber in the laser is adopted to effectively break through the technical bottleneck of the limited pulse width of traditional topological insulator mode-locked lasers, and significantly improve the pulse quality and overall stability of the laser.

[0051] Figure 1 FIG. 1 is a schematic diagram of the structure of a topological insulator mode-locked laser according to an embodiment of the present invention. Figure 1 As shown, the topological insulator mode-locked laser comprises a ring cavity structure, which includes a pump source, a first-stage hybrid mode-locked structure, and a coupler. The hybrid mode-locked structure comprises a saturable absorber and a nonlinear polarization rotation unit. The saturable absorber in the first-stage hybrid mode-locked structure utilizes a type II semiconductor topological insulator heterojunction. The pump source emits a laser pulse signal; the first-stage hybrid mode-locked structure suppresses low-intensity noise, filters high-intensity pulses, modulates polarization states, and dynamically compensates for dispersion in the laser pulse signal. The coupler then outputs a stable ultrashort mode-locked pulse signal.

[0052] In some embodiments, the ring cavity structure further includes a wavelength division multiplexer and an erbium-doped fiber, the wavelength division multiplexer is connected to the erbium-doped fiber, the wavelength division multiplexer transmits the laser pulse signal emitted by the pump source or the laser pulse signal after gain amplification by the erbium-doped fiber to the first-stage hybrid mode-locked structure, and the erbium-doped fiber amplifies the mode-locked pulse signal output by the first-stage hybrid mode-locked structure and compensates for loop loss.

[0053] In some embodiments, the nonlinear polarization rotation unit in the one-stage hybrid mode-locked structure includes a polarization-dependent polarizer, a first polarization controller, and a second polarization controller;

[0054] The saturable absorber performs rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening on the laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped fiber, so as to mode lock, modulate, and compress the laser pulse signal, and cooperates with the second polarization controller to ultimately output a stable ultrashort mode-locked pulse signal;

[0055] The second polarization controller dynamically adjusts the polarization state of the mode-locked pulse signal output by the saturable absorber and transmitted through the coupler;

[0056] The polarization-dependent polarizer further mode-locks, modulates and compresses the mode-locked pulse signal output by the second polarization controller through a nonlinear effect, and enables the mode-locked pulse signal to be transmitted unidirectionally in the ring cavity;

[0057] The first polarization controller optimizes the polarization state and dynamic dispersion compensation of the mode-locked pulse signal output by the polarization-dependent polarizer and improves the gain efficiency, and transmits the mode-locked pulse signal output by the first polarization controller to the erbium-doped optical fiber.

[0058] like Figure 1As shown, the pump source can provide a laser pulse signal in the 980nm to 1600nm band as a light source, which is coupled into the ring cavity structure through a wavelength division multiplexer. For example, laser pulse signals of 980nm or 1480nm need to be amplified to the 1550nm band by erbium-doped fiber before being transmitted to the first-stage hybrid mode-locked structure. The polarization-dependent polarizer in the ring cavity structure acts as an optical isolator, allowing the light pulse to propagate unidirectionally within the ring cavity to avoid backscattering interference. On the other hand, it can assist in mode locking and further modulation and pulse width compression by inducing nonlinear effects. A type II semiconductor topological insulator heterojunction is used as the saturable absorber of the core element of passive mode locking in this embodiment. The intensity-dependent nonlinear absorption characteristics of the heterojunction rapidly initiate the mode locking process, preferentially screening higher-intensity pulses and suppressing lower-intensity noise pulses. It works together with the polarization controller 2 and forms a stable ultrashort mode-locked pulse optical signal through multiple cycles of the optical pulse signal within the ring cavity. Polarization controllers 1 and 2 are located near the polarization-dependent polarizer and are respectively set at the two ends of the polarization-dependent polarizer. Polarization controller 2 is set between the saturable absorber and the polarization-dependent polarizer to adjust the polarization state of the light pulse in the cavity, and cooperates with the saturable absorber to finally obtain a stable ultrashort mode-locked pulse light signal; polarization controller 1 is set between the polarization-dependent polarizer and the erbium-doped fiber to optimize the polarization state and dispersion compensation of the light pulse to improve the gain efficiency, further enhancing the pulse control capability. The nonlinear polarization rotation (NPR) mechanism formed by the polarization-dependent polarizer and polarization controllers 1 and 2 dynamically manipulates the polarization state of the optical pulse, inducing the Kerr effect of a saturable absorber. This enhances self-phase modulation (SPM) and cross-phase modulation (XPM) to effectively control the polarization state of the optical pulse and optimize the pulse shape, inducing stronger nonlinear effects within the cavity. Dispersion compensation further enables pulse modulation and compression. Polarization controllers 1 and 2 collaboratively adjust the polarization state and birefringence phase of the optical pulse signal within the cavity, optimizing the nonlinear polarization rotation mode-locking effect and improving pulse stability and quality. The polarization-dependent polarizer selects and locks the optical pulse signal to the target polarization direction, suppressing noise interference. Erbium-doped fiber (EDF) serves as the gain medium, absorbing the optical pulse and generating stimulated emission, amplifying the optical pulse signal and compensating for losses within the loop. The coupling ratio of the coupler in the ring cavity structure is 90:10 or 80:20. The coupler can also be connected to a detection device for detection and analysis of the optical pulse signal.

[0059] The optical pulses output by the saturable absorber are output and split through, for example, a 90:10 coupler. 10% of the optical power is output for detection and analysis by spectrometers, oscilloscopes, and radio frequency analyzers; the 90% power pulses continue to circulate within the ring cavity. This 10% pulse signal can then be split equally into two paths through a 50:50 optical coupler, connected to a spectrometer and an oscilloscope, respectively, for spectral synchronization and time-domain characterization. This ensures that the final laser pulse output from the laser has high stability, effectively shortened pulse width, and a high signal-to-noise ratio. A spectrometer measures the spectral characteristics of the ring cavity output pulses, such as bandwidth, center wavelength, and sidelobe suppression ratio, to verify the broadband spectral characteristics in the mode-locked state. An oscilloscope, coupled with a high-speed photodetector, observes the pulse time-domain waveform and analyzes pulse width, repetition rate, and stability. An radio frequency analyzer monitors the RF spectrum of the laser pulses, such as the fundamental frequency and its harmonics, to assess the long-term stability of the mode-locked state and identify the presence of multiple pulses or noise disturbances.

[0060] The mode-locked laser in this embodiment combines passive mode locking with active regulation, and achieves ultrashort mode-locked pulse laser signal output with high stability and narrow pulse width by optimizing polarization state and nonlinear phase shift.

[0061] Figure 2 FIG. 1 is a schematic diagram of the structure of a topological insulator mode-locked laser according to another embodiment of the present invention. Figure 2 As shown, the topological insulator mode-locked laser comprises a ring cavity structure, which includes a pump source, a two-stage hybrid mode-locked structure, and a coupler. The two-stage hybrid mode-locked structure comprises a first-stage hybrid mode-locked structure and a second-stage hybrid mode-locked structure connected in series. The hybrid mode-locked structure comprises a saturable absorber and a nonlinear polarization rotation unit. The saturable absorber in the first-stage hybrid mode-locked structure utilizes a type II semiconductor topological insulator heterojunction, while the saturable absorber in the second-stage hybrid mode-locked structure utilizes graphene. The pump source emits a laser pulse signal; the first-stage hybrid mode-locked structure mode-locks and compresses the laser pulse signal, outputting a pre-compressed mode-locked pulse signal; the second-stage hybrid mode-locked structure further compresses the pre-compressed mode-locked pulse signal and induces nonlinear phase modulation, and outputs a stable ultrashort mode-locked pulse signal via the coupler.

[0062] In some embodiments, the ring cavity structure further includes a wavelength division multiplexer and an erbium-doped fiber, the wavelength division multiplexer being connected to the erbium-doped fiber, the wavelength division multiplexer transmitting the laser pulse signal emitted by the pump source or after gain amplification by the erbium-doped fiber to the two-stage hybrid mode-locked structure, the erbium-doped fiber amplifying the mode-locked pulse signal output by the two-stage hybrid mode-locked structure and compensating for loop loss.

[0063] In some embodiments, the first-stage hybrid mode-locked structure includes a first saturable absorber and a first nonlinear polarization rotation unit, the second-stage hybrid mode-locked structure includes a second saturable absorber and a second nonlinear polarization rotation unit, the first nonlinear polarization rotation unit includes a first polarization-dependent polarizer and a first polarization controller, and the second nonlinear polarization rotation unit includes a second polarization-dependent polarizer, a second polarization controller, and a third polarization controller;

[0064] The first polarization controller dynamically controls the polarization state of the laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped optical fiber;

[0065] The first saturable absorber performs rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening on the laser pulse signal output by the first polarization controller, so as to mode lock, modulate, and compress the laser pulse signal and output a mode-locked pulse signal;

[0066] The first polarization-dependent polarizer dynamically controls the polarization state and dynamically optimizes the dispersion compensation of the mode-locked pulse signal output by the first saturable absorber through the first polarization controller, outputs a pre-compressed mode-locked pulse signal, and enables the mode-locked pulse signal to be transmitted unidirectionally in the ring cavity;

[0067] The second polarization controller dynamically controls the polarization state of the mode-locked pulse signal output by the first polarization-dependent polarizer;

[0068] The second saturable absorber further compresses the mode-locked pulse signal output by the second polarization controller;

[0069] The second polarization-dependent polarizer induces nonlinear phase modulation and eliminates residual tail noise on the mode-locked pulse signal output by the second saturable absorber through the second polarization controller and the third polarization controller in combination with the strong Kerr effect of the second saturable absorber;

[0070] The third polarization controller dynamically controls the polarization state of the mode-locked pulse signal output by the second polarization-dependent polarizer, and transmits the mode-locked pulse signal output by the third polarization controller to the erbium-doped optical fiber via the coupler, thereby finally outputting a stable ultrashort mode-locked pulse signal.

[0071] like Figure 2As shown, the ring cavity structure in this embodiment connects two hybrid mode-locked structures in series, achieving efficient pulse width compression and stable mode-locking effects. The pump source can provide a laser pulse signal in the 980nm to 1600nm band as a light source, which is coupled into the optical path of the ring cavity structure through a wavelength division multiplexer. For example, laser pulse signals of 980nm or 1480nm need to be amplified to the 1550nm band by erbium-doped fiber before being transmitted to the two-stage hybrid mode-locked structure. In the first-stage hybrid mode-locked structure, the optical pulse is polarized by a polarization controller 1. Then, the saturable absorber 1 of the type II semiconductor topological insulator heterojunction uses its unique transient saturated nonlinear absorption characteristics to filter out higher-intensity optical pulses and suppress lower-intensity noise. The polarization-dependent polarizer 1 dynamically adjusts the polarization state of the optical pulse through the polarization controller 1, optimizing the dispersion compensation within the ring cavity and pre-compressing the optical pulse. That is, a pre-compressed chirped pulse is obtained and transmitted to the second-stage hybrid mode-locked structure. In the second-stage hybrid mode-locked structure, the graphene saturable absorber 2 can be obtained by transferring a single layer of graphene to a silicon dioxide / silicon substrate using mechanical exfoliation or chemical vapor deposition (CVD). Graphene's ultrafast carrier relaxation time and high damage threshold make it an ideal material for saturable absorbers, further compressing the light pulse (suppressing higher-intensity noise pulses). In the nonlinear polarization rotation unit, the polarization-dependent polarizer 2 precisely controls the polarization state of the light pulse through polarization controllers 2 and 3. Combined with the strong Kerr effect of graphene, it further induces nonlinear phase modulation, thereby eliminating the residual tail noise of the light pulse output by the first-stage hybrid mode-locked structure. The relevant contents of the erbium-doped fiber and coupler are similar to those of the aforementioned embodiment and will not be repeated here. The light pulse is output through the coupler and can be transmitted to a detection device such as an oscilloscope to monitor the waveform and intensity of the pulse in real time to ensure optimal performance. Through multiple cycles of the light pulse in the ring cavity structure, a stable ultrashort mode-locked pulse light signal is eventually formed. The dual-stage hybrid mode-locked structure of this embodiment achieves collaborative operation through dynamic phase matching of the two-stage hybrid mode-locked structure. The chirped pulse output by the first stage undergoes secondary SPM action and secondary compression in the second stage. At the same time, the higher intensity noise suppression capability of the saturable absorber 2 and the lower intensity noise suppression of the saturable absorber 1 complement each other, optimizing the pulse width compression of the optical pulse. The pulse width of the output optical pulse is finally compressed to the femtosecond level, significantly improving the stability and anti-interference capability of the mode-locked laser.

[0072] The mode-locked laser in this embodiment achieves efficient pulse width compression, low noise suppression, and extremely high stability through precise polarization control and coordinated operation, which significantly improves the pulse width compression performance and stability of the topological insulator mode-locked laser.

[0073] In some embodiments, the type II semiconductor topological insulator heterojunction includes a type II semiconductor topological insulator heterojunction formed by Bi2Se3 and WSe2 (tungsten selenide).

[0074] Figure 3 The figure is a schematic diagram of the energy band of a type II semiconductor heterojunction formed by combining Bi2Se3 and WSe2 in one embodiment of the present invention. In this embodiment, a type II semiconductor topological insulator heterojunction formed by combining two-dimensional materials Bi2Se3 and WSe2 is used as a saturable absorber. Specifically, a Bi2Se3-WSe2 type II semiconductor heterojunction saturable absorber is prepared on a quartz substrate by chemical vapor deposition (CVD). Figure 3 As shown, the band structure characteristics of this type II semiconductor heterojunction enable electron-hole pairs to separate and transfer rapidly and efficiently vertically between different layers, forming stable cross-layer excitons. This cross-layer charge transfer is extremely efficient, taking only tens of femtoseconds, far faster than the recombination time of excitons within the layers of a single Bi2Se3 material. This efficient charge separation helps reduce inefficient light energy loss and improve photoelectric conversion efficiency. This enhanced charge separation efficiency reduces optical energy loss, improves photoelectric conversion efficiency, and converts more photon energy into charge. These cross-layer charges in a type II semiconductor heterojunction, also known as cross-layer excitons, are highly stable, less prone to recombination, and have a longer lifetime than traditional excitons. With this long lifetime, cross-layer excitons resist rapid recombination under high-intensity light pulses, maintaining a nonlinear absorption state and preventing significant changes in the absorption state due to rapid exciton disappearance. This ensures the stability and uniformity of the optical transmission process. The long lifetime of cross-layer excitons makes the absorption and transmission process of intense light pulses by the saturable absorber more uniform, avoiding energy loss caused by rapid recombination. More efficient energy storage and release increases the energy of individual pulses, thereby increasing the pulse power. Thus, these cross-layer excitons avoid fluctuations in nonlinear absorption properties caused by rapid exciton recombination and annihilation, accelerating the nonlinear absorption process of optical signals and enhancing pulse shaping capabilities. This provides the foundation for the stable operation of the topological insulator mode-locked laser in the present invention, thereby improving the nonlinear optical response speed, pulse compression effect, and pulse stability of the topological insulator mode-locked laser.

[0075] More importantly, for pulse width, the cross-layer excitons exhibit a large and enhanced electric dipole moment effect at the type II semiconductor heterojunction interface due to the separation of electrons and holes between different layers, as shown in the following formula:

[0076] μ=q·d

[0077] Where q is the charge and d is the distance between the charges in the layers. By increasing d, the electric dipole moment becomes larger. The larger electric dipole moment leads to an enhanced local electric field, which makes the incident light field drive the polarization more effectively. This enhanced coupling effect improves the nonlinear polarization rate. The enhancement of the specific nonlinear optical effect of the electric dipole moment effect is reflected in the enhanced self-phase modulation, which significantly improves the third-order nonlinear polarization rate X. (3) , which makes the refractive index more dependent on light intensity, as shown in the following formula:

[0078] n=n0+n2·l

[0079] Where n0 is the linear refractive index, n2 is the nonlinear refractive index, which is proportional to the third-order nonlinear polarizability, and l is the incident light intensity. Higher light intensity results in a greater change in the refractive index n, leading to a more pronounced nonlinear refractive effect (Kerr effect). Higher light intensity at the center of the pulse causes a greater localized refractive index change. This dynamic spatial and temporal variation in the refractive index causes different frequency components of the light pulse to accumulate different phases, thus forming self-phase modulation (SPM).

[0080] When a light pulse propagates, the light intensity changes with time, causing the refractive index n to change dynamically, thereby causing dynamic modulation of the pulse phase, as shown in the following formula:

[0081]

[0082] Where ω is the angular frequency, c is the speed of light, I(t) is the light intensity, and L is the fiber length. Phase changes in the optical pulse at different time points result in frequency shifts, as shown in the following equation:

[0083]

[0084] During the pulse's leading edge (where light intensity increases), the frequency shifts toward higher frequencies, while during the pulse's trailing edge (where light intensity decreases), the frequency shifts toward lower frequencies, resulting in spectral broadening. This spectral broadening intensifies during pulse propagation, enhancing the spectral broadening capability of the mode-locked laser. The pulse's temporal width Δt and spectral width Δω satisfy the Fourier transform relationship; therefore, the wider the spectrum, the shorter the pulse's temporal width. This wider spectrum allows the generation of ultrashort pulses on the order of picoseconds or even femtoseconds.

[0085] Therefore, due to the formation of stable, long-lived cross-layer excitons in the WSe2-Bi2Se3 heterojunction, the enhanced electric dipole moment effect at the heterojunction interface increases the third-order nonlinear polarizability, enhancing the self-phase modulation and cross-phase modulation effects, and providing an optical nonlinear basis for promoting pulse compression. Furthermore, the faster carrier dynamics and ultrafast relaxation characteristics of WSe2 compensate for the slower response of Bi2Se3, thereby achieving dual optimization of the nonlinear absorption coefficient and modulation depth, providing better conditions for mode locking and enhancing pulse compression capabilities. Furthermore, the WSe2-Bi2Se3 heterojunction, as a saturable absorber, can utilize its transient saturated nonlinear absorption characteristics during the mode locking process. Through the absorption of strong light pulses, mode locking can be quickly initiated, suppressing lower-intensity noise pulses and retaining only higher-intensity pulses. Therefore, the hybrid mode-locking scheme based on the WSe2-Bi2Se3 heterojunction will obtain narrower pulse widths and better stability than the hybrid mode-locking scheme based on Bi2Se3 alone, reducing the impact of environmental changes on pulse quality.

[0086] Furthermore, the present invention rationally regulates the thickness of the type II semiconductor topological insulator heterojunction to assist in optimizing and enhancing the pulse width compression capability of the topological insulator mode-locked laser. By comparing the performance of different thicknesses such as 5nm, 8nm, and 10nm, the optimal thickness of the WSe2-Bi2Se3 type II semiconductor topological insulator heterojunction is 10nm, which can effectively enhance the nonlinear absorption capability of the heterojunction, balance the nonlinear absorption intensity with the carrier dynamics, and enhance the self-phase modulation and cross-phase modulation effects. Thickness regulation and optimization not only helps to increase the modulation depth of the saturable absorber, enhance the nonlinear refractive index, and enhance the screening ability of higher-intensity pulses, but also forms a better synergistic effect with the nonlinear polarization rotation mechanism, thereby promoting pulse width compression. When the central wavelength of the light pulse in the cavity is 1554nm, the mode-locked laser of the WSe2-Bi2Se3 type II semiconductor heterojunction with a thickness of 10nm outputs ultrashort pulses with strong anti-interference performance, high stability, and high signal-to-noise ratio, which can meet the requirements of applications such as precision machining and ultrafast spectroscopy.

[0087] Figure 4 and Figure 5 Schematic diagrams of the flow of the pulse width compression method of the topological insulator mode-locked laser according to one embodiment and another embodiment of the present invention respectively. Figure 4 or Figure 5 As shown, an embodiment of the present invention further provides a pulse width compression method for a topological insulator mode-locked laser, wherein the topological insulator mode-locked laser includes the topological insulator mode-locked laser described in each of the aforementioned embodiments, and the method includes the following steps:

[0088] Step S410, emitting a laser pulse signal through a pump source;

[0089] Step S420, performing low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation on the laser pulse signal through a first-stage hybrid mode-locked structure, and outputting a stable ultrashort mode-locked pulse signal through a coupler;

[0090] Alternatively, in step S420, the laser pulse signal is mode-locked and compressed by the first-stage hybrid mode-locking structure in the two-stage hybrid mode-locking structure and a pre-compressed mode-locked pulse signal is output, and the pre-compressed mode-locked pulse signal is further compressed and nonlinear phase modulation is induced by the second-stage hybrid mode-locking structure in the two-stage hybrid mode-locking structure, and a stable ultrashort mode-locked pulse signal is output through the coupler.

[0091] In some embodiments, the method of performing low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation on a laser pulse signal through a first-stage hybrid mode-locked structure, and outputting a stable ultrashort mode-locked pulse signal through a coupler, includes the following steps:

[0092] The laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped fiber is subjected to rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening through a saturable absorber, so as to mode lock, modulate, and compress the laser pulse signal, and finally output a stable ultrashort mode-locked pulse signal in cooperation with the second polarization controller;

[0093] Dynamically controlling the polarization state of the mode-locked pulse signal output by the saturable absorber and transmitted through the coupler by a second polarization controller;

[0094] The polarization-dependent polarizer further mode-locks, modulates and compresses the mode-locked pulse signal output by the second polarization controller through nonlinear effects, and makes the mode-locked pulse signal unidirectionally transmitted in the ring cavity;

[0095] The polarization state and dynamic dispersion compensation of the mode-locked pulse signal output by the polarization-dependent polarizer are optimized by the first polarization controller, and the gain efficiency is improved, and the mode-locked pulse signal output by the first polarization controller is transmitted to the erbium-doped optical fiber.

[0096] In summary, the topological insulator mode-locked laser and its pulse compression method proposed in the embodiments of the present invention effectively address the long recovery time, low pulse compression efficiency, wide pulse width, and poor stability issues inherent in conventional topological insulator mode-locked lasers due to their use of single-material saturable absorbers. This method achieves stable output of femtosecond ultrashort pulses. This provides a new solution for the efficient and stable output of ultrafast lasers and has broad application prospects, particularly providing critical technical support in applications such as ultrafast laser technology, quantum communications, precision micromachining, bioimaging, and optical communications.

[0097] It should be understood by those skilled in the art that the various exemplary components, systems and methods described in conjunction with the embodiments disclosed herein can be implemented in hardware, software or a combination of the two. Whether it is specifically performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention. When implemented in hardware, it can be, for example, an electronic circuit, an application specific integrated circuit (ASIC), appropriate firmware, a plug-in, a function card, etc. When implemented in software, the elements of the present invention are programs or code segments that are used to perform the required tasks. The program or code segment can be stored in a machine-readable medium, or transmitted on a transmission medium or a communication link via a data signal carried in a carrier.

[0098] It should be understood that the present invention is not limited to the specific configurations and processes described above and illustrated in the figures. For the sake of brevity, a detailed description of known methods is omitted. In the above embodiments, several specific steps are described and illustrated as examples. However, the method of the present invention is not limited to the specific steps described and illustrated. Those skilled in the art may make various changes, modifications, and additions, or change the order of the steps after understanding the spirit of the present invention.

[0099] In the present invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or replace features of other embodiments.

[0100] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations to the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A topological insulator mode-locked laser, characterized in that The laser comprises: a ring cavity structure, the ring cavity structure comprising a pump source, a first-stage hybrid mode-locking structure or a two-stage hybrid mode-locking structure, and a coupler, the two-stage hybrid mode-locking structure comprising a first-stage hybrid mode-locking structure and a second-stage hybrid mode-locking structure connected in series, the hybrid mode-locking structure comprising a saturable absorber and a nonlinear polarization rotation unit, the saturable absorber in the first-stage hybrid mode-locking structure adopting a type II semiconductor topological insulator heterojunction, and the saturable absorbers in the first-stage hybrid mode-locking structure and the second-stage hybrid mode-locking structure adopting a type II semiconductor topological insulator heterojunction and graphene, respectively; The pump source emits a laser pulse signal; The first-stage hybrid mode-locked structure performs low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation on the laser pulse signal, and outputs a stable ultrashort mode-locked pulse signal through the coupler; The first-stage hybrid mode-locking structure mode-locks and compresses the laser pulse signal and outputs a pre-compressed mode-locked pulse signal. The second-stage hybrid mode-locking structure further compresses the pre-compressed mode-locked pulse signal and induces nonlinear phase modulation, and outputs a stable ultrashort mode-locked pulse signal through the coupler.

2. The topological insulator mode-locked laser according to claim 1, wherein The ring cavity structure also includes a wavelength division multiplexer and an erbium-doped optical fiber. The wavelength division multiplexer is connected to the erbium-doped optical fiber. The wavelength division multiplexer transmits the laser pulse signal emitted by the pump source or the laser pulse signal after gain amplification by the erbium-doped optical fiber to the first-stage hybrid mode-locked structure or the second-stage hybrid mode-locked structure. The erbium-doped optical fiber amplifies the mode-locked pulse signal output by the first-stage hybrid mode-locked structure or the second-stage hybrid mode-locked structure and compensates for loop loss.

3. The topological insulator mode-locked laser according to claim 2, wherein The nonlinear polarization rotation unit in the one-stage hybrid mode-locked structure includes a polarization-dependent polarizer, a first polarization controller, and a second polarization controller; The saturable absorber performs rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening on the laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped fiber, so as to mode lock, modulate, and compress the laser pulse signal, and cooperates with the second polarization controller to ultimately output a stable ultrashort mode-locked pulse signal; The second polarization controller dynamically adjusts the polarization state of the mode-locked pulse signal output by the saturable absorber and transmitted through the coupler; The polarization-dependent polarizer further mode-locks, modulates and compresses the mode-locked pulse signal output by the second polarization controller through a nonlinear effect, and enables the mode-locked pulse signal to be transmitted unidirectionally in the ring cavity; The first polarization controller optimizes the polarization state and dynamic dispersion compensation of the mode-locked pulse signal output by the polarization-dependent polarizer and improves the gain efficiency, and transmits the mode-locked pulse signal output by the first polarization controller to the erbium-doped optical fiber.

4. The topological insulator mode-locked laser according to claim 2, wherein The first-stage hybrid mode-locked structure includes a first saturable absorber and a first nonlinear polarization rotation unit, wherein the first nonlinear polarization rotation unit includes a first polarization-dependent polarizer and a first polarization controller; The first polarization controller dynamically controls the polarization state of the laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped optical fiber; The first saturable absorber performs rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening on the laser pulse signal output by the first polarization controller, so as to mode lock, modulate, and compress the laser pulse signal and output a mode-locked pulse signal; The first polarization-dependent polarizer dynamically controls the polarization state and dynamically optimizes the dispersion compensation of the mode-locked pulse signal output by the first saturable absorber through the first polarization controller, outputs a pre-compressed mode-locked pulse signal, and enables the mode-locked pulse signal to be transmitted unidirectionally in the ring cavity.

5. The topological insulator mode-locked laser according to claim 4, wherein The second-stage hybrid mode-locked structure includes a second saturable absorber and a second nonlinear polarization rotation unit, and the second nonlinear polarization rotation unit includes a second polarization-dependent polarizer, a second polarization controller, and a third polarization controller; The second polarization controller dynamically controls the polarization state of the mode-locked pulse signal output by the first polarization-dependent polarizer; The second saturable absorber further compresses the mode-locked pulse signal output by the second polarization controller; The second polarization-dependent polarizer induces nonlinear phase modulation and eliminates residual tail noise on the mode-locked pulse signal output by the second saturable absorber through the second polarization controller and the third polarization controller in combination with the strong Kerr effect of the second saturable absorber; The third polarization controller dynamically controls the polarization state of the mode-locked pulse signal output by the second polarization-dependent polarizer, and transmits the mode-locked pulse signal output by the third polarization controller to the erbium-doped optical fiber via the coupler, thereby finally outputting a stable ultrashort mode-locked pulse signal.

6. The topological insulator mode-locked laser according to claim 1, wherein The type II semiconductor topological insulator heterojunction includes a type II semiconductor topological insulator heterojunction formed by Bi2Se3 and WSe2.

7. The topological insulator mode-locked laser according to claim 6, wherein The thickness of the type II semiconductor topological insulator heterojunction includes 5nm, 8nm and 10nm.

8. The topological insulator mode-locked laser according to claim 1, wherein The coupling ratio of the coupler is 90:10 or 80:

20.

9. A method for compressing the pulse width of a topological insulator mode-locked laser, characterized in that: The topological insulator mode-locked laser comprises the topological insulator mode-locked laser according to any one of claims 1 to 8, and the method comprises: A laser pulse signal is emitted through a pump source; Through a first-stage hybrid mode-locked structure, the laser pulse signal is subjected to low-intensity noise suppression, high-intensity pulse screening, polarization state regulation, and dynamic dispersion compensation, and a stable ultrashort mode-locked pulse signal is output through a coupler; Alternatively, the laser pulse signal is mode-locked and compressed by the first-stage hybrid mode-locking structure in the two-stage hybrid mode-locking structure and a pre-compressed mode-locked pulse signal is output. The pre-compressed mode-locked pulse signal is further compressed and nonlinear phase modulation is induced by the second-stage hybrid mode-locking structure in the two-stage hybrid mode-locking structure, and a stable ultrashort mode-locked pulse signal is output through a coupler.

10. The pulse width compression method according to claim 9, characterized in that: The method uses a first-stage hybrid mode-locked structure to suppress low-intensity noise, screen high-intensity pulses, regulate polarization states, and dynamically compensate for dispersion on a laser pulse signal, and outputs a stable ultrashort mode-locked pulse signal through a coupler, including: The laser pulse signal output by the wavelength division multiplexer or the laser pulse signal after gain amplification by the erbium-doped fiber is subjected to rapid mode locking, low-intensity noise suppression, and high-intensity pulse screening through a saturable absorber, so as to mode lock, modulate, and compress the laser pulse signal, and finally output a stable ultrashort mode-locked pulse signal in cooperation with the second polarization controller; Dynamically controlling the polarization state of the mode-locked pulse signal output by the saturable absorber and transmitted through the coupler by a second polarization controller; The polarization-dependent polarizer further mode-locks, modulates and compresses the mode-locked pulse signal output by the second polarization controller through nonlinear effects, and makes the mode-locked pulse signal unidirectionally transmitted in the ring cavity; The polarization state and dynamic dispersion compensation of the mode-locked pulse signal output by the polarization-dependent polarizer are optimized by the first polarization controller, and the gain efficiency is improved, and the mode-locked pulse signal output by the first polarization controller is transmitted to the erbium-doped optical fiber.

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