Polarization control based repetition rate switchable mode-locked fiber laser and control method thereof

Abstract

The application belongs to the technical field of ultrafast laser and mode-locked fiber laser, and particularly relates to a repetition frequency switchable mode-locked fiber laser based on polarization control and a control method thereof. The laser oscillation is cut off by making the cavity loss of the cut-off direction exceed the laser oscillation threshold through the cooperative regulation of the common polarization controller and the bidirectional PBS, and the repetition frequency is switched while the mode-locked output direction is switched. The application comprises: a pump source for providing pump light; a ring resonator cavity comprising a common light path section and first and second branch light paths with opposite transmission directions; a polarization controller arranged on the common light path section; a polarization beam splitter and a saturable absorber arranged on the first and second branch light paths respectively; wherein the polarization states of the light signals entering the first and second branch light paths are changed by adjusting the polarization controller, so that the polarization-dependent loss difference between the two branch light paths is generated.

Description

Technical Field

[0001] This invention pertains to ultrafast laser technology and mode-locked fiber lasers, and particularly relates to a mode-locked fiber laser with switchable repetition frequency based on polarization control and its control method. Background Technology

[0002] Mode-locked fiber lasers are one of the important light sources in the field of ultrafast optics. The repetition frequency of the laser output pulse is an important parameter affecting its application performance. Achieving controllable adjustment or switching of the repetition frequency of mode-locked fiber lasers is of great significance for expanding the adaptability of single light sources to different operating conditions, reducing system costs, and promoting the engineering application of ultrafast laser technology.

[0003] Most existing mode-locked fiber lasers operate in a fixed repetition frequency mode. Once the cavity length and internal optical parameters are determined, the repetition frequency of the output pulse is usually fixed as well. To change the repetition frequency, it is usually necessary to change the physical length of the repetition cavity, switch different optical path branches, or change the laser's operating state, which is difficult to meet the application requirements of rapid switching between multiple repetition frequencies.

[0004] Currently, the main technical solutions for adjusting or switching the repetition frequency of mode-locked fiber lasers include the following categories:

[0005] (a) Scheme based on mechanical cavity length adjustment.

[0006] This type of scheme modifies the physical length of the resonant cavity mechanically, such as by stretching the optical fiber, moving the reflector, or changing the length of the fiber delay line, thereby altering the propagation time of one cycle of the optical pulse within the cavity and achieving repetition frequency adjustment. However, this type of scheme typically requires complex mechanical adjustment structures, resulting in problems such as large size, slow response speed, and limited repeatability. Furthermore, the adjustment process can easily introduce fiber bending loss and polarization state disturbances, affecting mode-locking stability.

[0007] (ii) Scheme based on switching of multiple resonant cavities or optical branches.

[0008] This type of scheme uses multiple resonant cavity branches with different optical path lengths and switches between them using optical switches to obtain different repetition frequencies. However, this scheme is usually complex in structure and has a large number of components. The optical switches may also introduce additional insertion loss and polarization changes, which will require readjustment of the mode-locking state after switching, affecting the switching speed and stability.

[0009] (iii) Schemes based on nonlinear effects or gain modulation.

[0010] This type of approach adjusts the repetition frequency to some extent by regulating the pump power, changing the gain state, or utilizing intracavity nonlinear effects. However, this method typically has a limited adjustment range, and the adjustment process is prone to changes in parameters such as pulse width, spectral width, and output power, making it difficult to achieve stable switching between different repetition frequency states.

[0011] Furthermore, single-cavity bidirectional ring lasers have two optical transmission paths in opposite directions. In bidirectional ring resonators with different optical path lengths, different mode-locking pulse repetition frequencies can be obtained if different transmission directions can be selected to establish mode-locking states. However, existing bidirectional lasers typically operate in bidirectional simultaneous oscillation, fixed-direction mode-locking, or fixed-direction output states, lacking a technical solution that allows for active, stable, and reversible switching of mode-locking states in different directions without altering the physical structure of the resonator.

[0012] Meanwhile, when using polarization adjustment to select different transmission directions, how to ensure that one direction meets the stable mode-locking condition and effectively suppress laser oscillation in another direction, thereby reducing the residual light output in the suppressed direction, is also a problem that existing technologies need to solve. Summary of the Invention

[0013] This invention addresses the shortcomings of existing technologies by providing a polarization-controlled switchable mode-locked fiber laser and its control method. Through polarization modulation, the two branch optical paths are respectively set to a low-loss on state and a high-loss off state, achieving switching of the mode-locked output direction and corresponding repetition frequency. It also boasts advantages such as requiring no external optical switch, compact structure, and simple control.

[0014] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, it provides a polarization-controlled repetition frequency switchable mode-locked fiber laser, comprising:

[0015] Pump source, used to provide pump light;

[0016] A ring resonant cavity includes a common optical path segment and a first branch optical path and a second branch optical path with opposite transmission directions. A gain fiber and an optical coupler are disposed inside the ring resonant cavity.

[0017] A polarization controller is installed on the common optical path segment;

[0018] The first branch optical path and the second branch optical path are respectively provided with polarization correlation devices and saturable absorbers;

[0019] Specifically, by adjusting the polarization controller to change the polarization state of the light signals entering the first branch optical path and the second branch optical path, a polarization-dependent loss difference is generated between the two branch optical paths, so that one branch optical path establishes mode-locked pulse output while the other branch optical path cuts off oscillation, thereby achieving the switching of pulse output direction.

[0020] One possible implementation involves adjusting the polarization controller to a first preset polarization state, so that the first branch optical path meets the low-loss transmission condition and the second branch optical path meets the high-loss cutoff condition, establishing a passive mode-locked state in the first branch direction and oscillating cutoff in the second branch direction; and adjusting the polarization controller to a second preset polarization state, so that the second branch optical path meets the low-loss transmission condition and the first branch optical path meets the high-loss cutoff condition, establishing a passive mode-locked state in the second branch direction and oscillating cutoff in the first branch direction.

[0021] Specifically, the low-loss transmission condition is: the angle between the polarization direction of the incident light signal and the main transmission axis direction of the corresponding branch polarization-dependent device is less than or equal to 5°, and the polarization-dependent loss of the corresponding branch is less than or equal to 0.5dB; the high-loss cutoff condition is: the angle between the polarization direction of the incident light signal and the main transmission axis direction of the corresponding branch polarization-dependent device is greater than or equal to 46°, and the polarization-dependent loss of the corresponding branch is greater than or equal to 5dB.

[0022] In one possible implementation, a branch polarization controller and / or a variable optical attenuator are respectively provided on the first branch optical path and the second branch optical path; the branch polarization controller and the variable optical attenuator are used to fix the static polarization state and static insertion loss of the two branch optical paths during the initial construction and debugging phase of the laser; during the process of switching the repetition frequency by adjusting the polarization controller set on the common optical path segment, the state of the branch polarization controller and the variable optical attenuator remains unchanged.

[0023] Among them, the branch polarization controllers set on each branch optical path are used to optimize the incident polarization state matching of each branch in front of the polarization beam splitter; the variable optical attenuators set on each branch optical path are used to independently balance the intracavity static loss of the two branch optical paths.

[0024] One possible implementation is that the ring resonator is an all-fiber structure, and a single-mode fiber for dispersive modulation is also provided inside the ring resonator.

[0025] One possible implementation is that the polarization controller installed on the common optical path segment is a 3-paddle type polarization controller, an electrically controlled polarization controller, or a squeeze-type polarization controller;

[0026] The polarization-dependent device is a fiber-optic polarization beam splitter, a polarization-dependent isolator, or an online oscillator.

[0027] The saturable absorber is a single-walled carbon nanotube saturable absorber, a semiconductor saturable absorber mirror, or a saturable absorber based on two-dimensional materials.

[0028] In one possible implementation, the pump source is a 980nm laser diode, and the length of the gain fiber is 0.3m to 100m.

[0029] On the other hand, a control method for a polarization-controlled repetition frequency switchable mode-locked fiber laser is provided, which includes the following steps:

[0030] Pump light is input into the ring resonant cavity;

[0031] Adjust the polarization controller located on the common optical path segment so that the first branch optical path meets the low-loss transmission condition and the second branch optical path meets the high-loss cutoff condition, thereby establishing a passive mode-locked state in the first branch direction and cutting off laser oscillation in the second branch direction; or

[0032] Adjust the polarization controller set on the common optical path segment so that the second branch optical path meets the low-loss transmission condition and the first branch optical path meets the high-loss cutoff condition, so that the second branch direction establishes a passive mode-locked state and the laser oscillation in the first branch direction is cut off.

[0033] Preferably, the control method further includes:

[0034] After switching the pulse output direction, the polarization angle of the polarization controller set on the common optical path is finely adjusted. While maintaining one path on and one path off, the repetition frequency of the mode-locked pulse in the conduction direction is simultaneously finely adjusted.

[0035] Compared with the prior art, the present invention has the following advantages.

[0036] (1) Unlike existing bidirectional mode-locked lasers, which may still have a large residual optical power output in the other direction after switching, this invention uses the coordinated control of a common polarization controller and a bidirectional PBS to make the cavity loss in the cut-off direction exceed the laser oscillation threshold, so that the laser oscillation is cut off and the output power is zero or close to zero, forming an equivalent optical switch state of "one path on, one path off", and realizing the switching of repetition frequency while switching the mode-locked output direction.

[0037] (2) This invention does not require additional discrete switching components such as electro-optic modulators, acousto-optic modulators, or spatial optical switches. Optical path selection and switching can be achieved using the polarization controller and polarization beam splitter within the resonant cavity. The all-fiber structure has advantages such as resistance to electromagnetic interference, compact structure, and high integration.

[0038] (3) The present invention only requires adjustment of a single polarization controller on the common optical path segment to achieve bidirectional optical path switching. The control variable is singular, avoiding the complexity of multi-device coordinated control in traditional schemes. When using an electrically controlled polarization controller, rapid switching driven by electrical signals can also be achieved.

[0039] (4) Based on the realization of equivalent optical switch switching, by finely adjusting the polarization angle of the common polarization controller, the repetition frequency of the mode-locking pulse in the conduction direction can be adjusted within a small range while maintaining one path on and one path off, thereby realizing the integration of optical switch function and repetition frequency control and further expanding the working condition adaptability of the light source. Attached Figure Description

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The scope of protection of the present invention is not limited to the following description.

[0041] Figure 1 This is a schematic diagram of the repetition frequency switchable mode-locked fiber laser structure based on polarization control according to the present invention.

[0042] Figure 2 This is a schematic diagram illustrating the process of switching repetition frequencies according to the present invention;

[0043] Figure 3 The output spectrum and spectral diagram are shown in the clockwise on and counterclockwise off states.

[0044] Figure 4 The output spectrum and spectral diagram are shown in the counterclockwise on and clockwise off states.

[0045] Figure 5 This is a time-domain oscilloscope sequence of the dual-comb pulses before and after the repetition frequency switching. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] The purpose of this invention is to provide a polarization-controlled repetition frequency switchable mode-locked fiber laser and its control method.

[0048] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0049] This embodiment proposes a mode-locked fiber laser with switchable repetition frequency based on polarization control. This laser introduces a polarization control mechanism into a bidirectional ring resonator, enabling adjustable polarization-dependent loss differences between the two optical paths with opposite propagation directions. This allows for switching the mode-locked pulse output direction without the need for external optical switching devices.

[0050] Specifically, the ring resonant cavity includes a common optical path segment and a first branch optical path and a second branch optical path with opposite transmission directions. A polarization controller is set in the common optical path segment to change the polarization state of the optical signals entering the two branch optical paths by adjusting the polarization controller; polarization selection devices are set in the two branch optical paths respectively to generate different transmission losses in the two directions for optical signals with different polarization states.

[0051] When the polarization controller is adjusted to the first preset polarization state, the optical signal in the first branch optical path meets the low-loss transmission condition, while the optical signal in the second branch optical path meets the high-loss cutoff condition. Since the two directions share the same gain medium, the low-loss direction preferentially establishes laser oscillation and forms a stable mode-locked pulse output under the action of the saturable absorber, while the high-loss direction is cut off because it cannot meet the oscillation threshold condition.

[0052] When the polarization controller is adjusted to the second preset polarization state, the loss states of the two branch optical paths are reversed, causing the second branch optical path to establish a mode-locked pulse output while the first branch optical path is cut off. In this way, switching between the two-directional mode-locked pulse output states can be achieved simply by adjusting the polarization controller on the common optical path segment.

[0053] In this embodiment, the clockwise and counterclockwise branch optical paths have different optical path lengths or physical lengths, so that the laser has different mode-locked pulse repetition frequencies in the clockwise and counterclockwise conduction states. Specifically, when the first branch optical path is in a low-loss conduction state and establishes a mode-locked pulse output, the laser outputs a first repetition frequency corresponding to the optical path length in that direction; when the second branch optical path is in a low-loss conduction state and establishes a mode-locked pulse output, the laser outputs a second repetition frequency corresponding to the optical path length in that direction. Thus, by adjusting the polarization controller on the common optical path segment to switch the mode-locked pulse output direction, the switching of the mode-locked pulse repetition frequency can be achieved synchronously.

[0054] Furthermore, during the initial construction and commissioning phase of the laser, the static polarization state and static loss of each branch optical path can be optimized by setting a polarization controller and / or a variable optical attenuator on the branch optical path; when switching repetition frequencies, only the polarization controller on the common optical path segment is adjusted, and the state of the relevant devices in the branch optical path remains unchanged.

[0055] In this embodiment, as Figure 1As shown, the polarization-controlled repetition frequency switchable mode-locked fiber laser adopts a ring resonant cavity structure. The resonant cavity is composed of the following devices connected end to end to form a closed loop: pump source (980nm laser diode), wavelength division multiplexer (WDM), erbium-doped fiber (EDF), optical coupler (OC), circulator (Cir), polarization controller (PC), polarization beam splitter (PBS), single-walled carbon nanotube saturable absorber (SWCNT-SA), tunable optical attenuator (VA), and single-mode fiber (SMF).

[0056] The system comprises the following components: a pump source outputting 980nm pump light to provide excitation energy to the resonant cavity; a wavelength division multiplexer (WDM) coupling the pump light into the resonant cavity and separating the signal light from the pump light; erbium-doped fiber serving as the gain medium to absorb pump light energy and provide stimulated emission amplification of the signal light; an optical coupler employing a 90:10 splitting ratio, with 90% of the ports used to maintain intracavity optical circulation and 10% of the ports serving as the laser output; a circulator ensuring unidirectional transmission of the optical signal in the branch optical paths; a polarization controller located in the common optical path segment to adjust the polarization state of the intracavity laser, enabling polarization state adjustment in both branch optical paths; and polarization selection devices located in the first branch optical path (also known as the clockwise branch) and the second branch optical path (also known as the counterclockwise branch), selectively transmitting or reflecting the incident light based on its polarization state, serving as the primary device for achieving direction-selective loss; and a single-walled carbon nanotube saturable absorber used as a mode-locking element to achieve passive mode-locking using the saturable absorption effect. Single-mode optical fibers are used to modulate the group velocity dispersion of resonant cavities.

[0057] See Figure 1 The clockwise optical path sequence is as follows: the signal light inside the cavity passes through the left WDM, EDF, right WDM and 2×2 OC and enters port 2 of Cir2, and is output from port 3 of Cir2; then it passes through PC, PBS, SWCNT-SA and VA on the common optical path segment in sequence, and enters port 1 of Cir1, and is output from port 2 of Cir1, and returns to the left WDM via the left PC.

[0058] The counterclockwise optical path sequence is as follows: the signal light inside the cavity passes through the right WDM, EDF, left WDM and left PC and then enters port 2 of Cir1, and is output from port 3 of Cir1; then it passes through the PC, PBS and SWCNT-SA on the corresponding branch optical path in sequence, and then enters port 1 of Cir2, and is output from port 2 of Cir2, and returns to the right WDM through 2×2 OC.

[0059] The clockwise and counterclockwise branch optical paths mentioned above have different optical path lengths or physical lengths. Therefore, when mode-locked pulse outputs are established in different directions, the corresponding intracavity round-trip times are different, resulting in different mode-locked pulse repetition frequencies.

[0060] In this embodiment, Figure 1 The functions of the main components are as follows:

[0061] Pump source (LD): provides pump energy for the gain medium;

[0062] Wavelength division multiplexer (WDM): Couples the pump light into the resonant cavity while separating the signal light from the pump light;

[0063] Erbium-doped fiber (EDF): a gain medium that provides stimulated emission amplification of signal light;

[0064] Optical coupler (OC): Some ports are used as laser output terminals;

[0065] Circulator (Cir): Ensures unidirectional transmission of optical signals in branch optical paths;

[0066] Polarization controller (PC): A polarization controller located in the common optical path is used to adjust the polarization state of the laser within the cavity; a polarization controller located in the branch optical path is used to optimize the static polarization state of the corresponding branch during the initial laser construction and commissioning phase.

[0067] Polarization beam splitter (PBS): It is set in two branch optical paths respectively, and selectively transmits or reflects according to the polarization state to achieve direction-selective loss;

[0068] Saturable absorber (SWCNT-SA): Mold-locking element for achieving passive mold-locking;

[0069] Single-mode fiber (SMF): for controlling the dispersion of the resonant cavity.

[0070] Without altering the technical concept of this embodiment, related devices can be replaced with alternative structures having the same or similar functions according to actual application requirements. The following description focuses on specific device types.

[0071] The specific devices listed in the above embodiments are only one or several preferred forms for achieving the function. Any all-fiber device or passive fiber device that can achieve the same function can be used as an equivalent replacement for the corresponding device in this embodiment. Specifically:

[0072] (a) Alternative implementation methods for polarization selection devices.

[0073] In this embodiment, the polarization beam splitter (PBS) disposed on the first branch optical path and the second branch optical path is used to provide differentiated transmission loss (i.e., polarization dependent loss, PDL) for the two orthogonal polarization components according to the polarization state of the incident light, so that the optical signal of one polarization state can pass through with low loss, while the optical signal of the other polarization state is significantly attenuated.

[0074] Any fiber optic device capable of achieving the aforementioned polarization-dependent loss function can be used as an equivalent replacement for the polarization beam splitter in this embodiment, including but not limited to:

[0075] (1) In-line polarizer: By using the principles of metal nanowire grids, birefringent crystals or fiber microstructures, light in one polarization direction can pass through while light in the other polarization direction is strongly attenuated. Similarly, directional selective loss difference can be constructed in bidirectional branches.

[0076] (2) Polarization-maintaining fiber (PMF): Combined with the polarization controller at the front end to adjust the incident polarization state, the polarization-maintaining fiber itself has different propagation constants and loss characteristics in the axis and off-axis directions to achieve polarization-dependent selective transmission; in some embodiments, the polarization-maintaining fiber can replace the independent PBS device to further simplify the structure and reduce the cost.

[0077] (3) Polarizer: Fiber optic polarizers or micro-optical packaged polarizers are used to achieve polarization selection by utilizing dichroic absorption or Brewster angle reflection principles;

[0078] (4) Birefringent crystal: The birefringence effect of the crystal is used to separate light of different polarization states in space or to produce different insertion losses.

[0079] All of the above solutions can be used as optional implementations of the polarization selection device in this embodiment. As long as the device can achieve the corresponding polarization-related loss adjustment function, it can be used to achieve the technical effect of this embodiment.

[0080] (ii) Alternative implementation methods for saturable absorbers.

[0081] The single-walled carbon nanotube saturable absorber (SWCNT-SA) used in this embodiment is employed to provide a saturable absorption effect for passive mode locking. Any device capable of providing a saturable absorption effect in the corresponding gain band is considered an equivalent solution in this embodiment, including but not limited to: semiconductor saturable absorber mirrors (SESAM), saturable absorbers based on two-dimensional materials such as graphene or black phosphorus, equivalent saturable absorbers based on nonlinear polarization rotation (NPR) effects, or saturable absorbers based on doped optical fibers.

[0082] (iii) Alternative implementations of the polarization controller.

[0083] The three-paddle polarization controller used in this embodiment primarily functions to change the polarization state of the laser within the cavity in the common optical path segment. Any device capable of controlling the polarization state within the fiber can be used as an equivalent replacement, including but not limited to: electrically controlled polarization controllers (utilizing electro-optic or magneto-optic effects), squeeze-type polarization controllers (utilizing stress birefringence), or liquid crystal-based adjustable polarization controllers. Embodiments using electrically controlled polarization controllers are particularly suitable for applications requiring millisecond-level fast optical switching.

[0084] (iv) In this embodiment, the circulator (Cir) can be replaced by a combination of fiber optic isolators with unidirectional transmission function; the adjustable optical attenuator (VA) can be omitted in some simplified embodiments or replaced by an attenuating fiber with fixed loss; the splitting ratio of the optical coupler is not limited to 90:10, and those skilled in the art can select it in the range of 70:30 to 99:1 according to the balance requirements of output power and intracavity circulation efficiency.

[0085] See Figure 2 A control method for a polarization-controlled repetition frequency switchable mode-locked fiber laser includes the following steps:

[0086] Step S1: Resonant cavity construction and initial parameter configuration.

[0087] according to Figure 1 The structure shown depicts the construction of an all-fiber single-cavity bidirectional ring laser resonator. The pump source, wavelength division multiplexer, erbium-doped fiber, optical coupler, circulator, polarization controller, polarization beam splitter, saturable absorber, and single-mode fiber are sequentially connected to form a closed ring resonator. The erbium-doped fiber is 0.65m long with a dispersion of -18 ps / km / nm; the polarization-maintaining fiber is 2m long with a dispersion of 18.2 ps / km / nm; the polarization controller is a 3-paddle type; the total length of the single-mode fiber ensures that the net cavity dispersion of the resonator is in the anomalous dispersion region. After the resonator construction is completed, performance characterization equipment is configured, including an optical spectrometer, digital oscilloscope, RF spectrometer, autocorrelation analyzer, and optical power meter.

[0088] Step S2: Turn on the pump source and establish laser oscillation.

[0089] Turn on the 980nm pump diode and input pump light into the resonant cavity. Set the pump power to a suitable value (e.g., 120mW–130mW) to generate sufficient gain in the gain medium. The pump light enters the resonant cavity via a wavelength division multiplexer and is injected into the erbium-doped fiber to excite the gain. The signal light circulates in two opposite directions within the cavity.

[0090] Step S3: Adjust the common polarization controller to the first preset state to achieve clockwise conduction and counterclockwise cutoff.

[0091] The three-paddle polarization controller on the common optical path is slowly adjusted to change the polarization state of the laser entering the two branch optical paths. When the polarization controller is adjusted to the first preset polarization angle, the polarized light of the clockwise branch is aligned with the main axis of the clockwise PBS, satisfying the low-loss transmission condition; the polarized light of the counterclockwise branch is orthogonal to the main axis of the counterclockwise PBS, satisfying the high-loss cutoff condition. Because the net cavity loss is significantly lower in the clockwise direction than in the counterclockwise direction, it has an advantage in gain competition. Combined with the nonlinear saturable absorption effect of SWCNT-SA 1, the clockwise direction establishes the passive mode-locked state first, outputting a stable mode-locked pulse sequence. In the counterclockwise direction, the polarization state mismatch leads to a significant increase in cavity loss, exceeding the laser oscillation threshold, resulting in complete laser oscillation cutoff and zero or near-zero output power.

[0092] The power at both output terminals was measured using an optical power meter: the output power in the clockwise direction was in the microwatt to milliwatt range, while the output power in the counterclockwise direction was close to the nanowatt range, and the ratio between the two was greater than 10. 2 This achieves an equivalent optical switch state of "clockwise conduction and counterclockwise cutoff".

[0093] Step S4: Adjust the common polarization controller to the second preset state to achieve counterclockwise conduction and clockwise cutoff.

[0094] Keeping the pump power constant, the polarization controller on the common optical path is adjusted to the second preset polarization angle. At this point, the polarized light of the counterclockwise branch is aligned with the main axis of the counterclockwise PBS, satisfying the low-loss transmission condition; the polarized light of the clockwise branch is orthogonal to the main axis of the clockwise PBS, satisfying the high-loss cutoff condition. The counterclockwise direction dominates in gain competition, and in conjunction with the nonlinear saturable absorption effect of SWCNT-SA 2, a passive mode-locked state is established, outputting a mode-locked pulse sequence; the laser oscillation is completely cut off in the clockwise direction, and the output power is zero or close to zero.

[0095] The power of the two output terminals was measured by an optical power meter: the output power in the counterclockwise direction was in the range of microwatts to milliwatts, and the output power in the clockwise direction was less than nanowatts, realizing the equivalent optical switch state of "counterclockwise conduction and clockwise cutoff".

[0096] Because the clockwise and counterclockwise branch optical paths correspond to different optical path lengths, the intracavity round-trip time of the mode-locking pulse in the conduction direction changes synchronously when switching the mode-locking state between the clockwise and counterclockwise directions, thereby switching the output mode-locking pulse from the first repetition frequency to the second repetition frequency. This repetition frequency switching is achieved by adjusting the polarization controller on the common optical path segment, without changing the pump power.

[0097] Step S5: Verification of switch status and performance characterization.

[0098] The spectrum of the output laser in the conduction direction was recorded using an optical spectrometer, confirming that the center wavelength was located in the C-band (approximately 1560 nm) and the 3 dB spectral bandwidth met the characteristics of a mode-locked pulse. The pulse sequence in the conduction direction was observed using a digital oscilloscope, confirming it to be a periodic pulse sequence with equal intervals and stable amplitude. The fundamental frequency repetition rate was measured using an RF spectrometer, confirming a repetition rate signal-to-noise ratio greater than 60 dB. The output power in both the conduction and cutoff directions was measured using an optical power meter, confirming that the power in the cutoff direction was zero or close to zero.

[0099] Test results show that in the clockwise conduction state, the output power is 1.005mW in the clockwise direction and less than 300nW in the counterclockwise direction; in the counterclockwise conduction state, the output power is 420μW in the counterclockwise direction and less than 3.5μW in the clockwise direction. In both switching states, the center wavelength of the mode-locked pulse in the conduction direction is around 1560nm, the 3dB spectral bandwidth is approximately 1.3nm~1.8nm, and the fundamental frequency repetition rate signal-to-noise ratio is greater than 60dB.

[0100] Step S6: Optionally, the repetition frequency is fine-tuned within a small range under the selected mode-locked output direction.

[0101] In a switching state where one direction is conducting and the other is cut off, the polarization angle of the polarization controller on the common optical path can be finely adjusted without changing the mode-locked output direction, thereby allowing for a small-range adjustment of the repetition frequency of the mode-locked pulse in the conducting direction. This small-range adjustment differs from the repetition frequency state switching achieved by switching between clockwise and counterclockwise branch optical paths. This step expands the application flexibility of this embodiment in scenarios requiring fine-tuning of the repetition rate.

[0102] The following is a specific embodiment of the present invention. In this embodiment, the following method is adopted: Figure 1 The structure shown is used to construct an all-fiber single-cavity bidirectional ring laser. The erbium-doped fiber is 0.65m long with a dispersion of -18 ps / km / nm; the polarization-maintaining fiber is 2m long with a dispersion of 18.2 ps / km / nm; the polarization controller is a 3-paddle type; the single-mode fiber length ensures a net cavity dispersion of -0.3 ps², operating in the anomalous dispersion region. The pump source is a 980nm laser diode, and the pump power is set to 123mW.

[0103] First, the pump source is turned on to establish laser oscillation in the resonant cavity. At this time, optical signals are transmitted in both directions within the cavity, but mode locking has not yet been achieved.

[0104] Secondly, the 3-paddle polarization controller on the common optical path was slowly adjusted. When adjusted to the first preset polarization angle, a series of equally spaced, stable pulses appeared at the clockwise output of the oscilloscope, while no pulse signal appeared at the counterclockwise output. The spectrum analyzer showed that the center wavelength of the clockwise output spectrum was 1560.2 nm, and the 3dB bandwidth was 1.3 nm. The optical power meter measured the clockwise output power to be 650 μW, and the counterclockwise output power to be less than 300 nW. The RF spectrum analyzer measured the clockwise fundamental frequency repetition rate to be 12.53 MHz, and the signal-to-noise ratio to be greater than 60 dB. At this time, the system was in an equivalent optical switch state of "clockwise on, counterclockwise off".

[0105] Then, the common polarization controller was adjusted to the second preset polarization angle. The oscilloscope displayed a series of equally spaced, stable pulses at the counter-clockwise output, while no pulse signal appeared at the clockwise output. The spectrum analyzer showed a center wavelength of 1559.8 nm and a 3dB bandwidth of 1.8 nm for the counter-clockwise output spectrum. The optical power meter measured a counter-clockwise output power of 420 μW, while the clockwise output power was below 200 nW. The RF spectrum analyzer measured a counter-clockwise fundamental frequency repetition rate of 14.10 MHz and a signal-to-noise ratio greater than 60 dB. At this point, the system was in an equivalent optical switch state of "counter-clockwise on, clockwise off".

[0106] The test results show that when the system is in a clockwise on / off state, the fundamental frequency repetition frequency of the mode-locked pulse in the on direction is 12.53 MHz; when the system is switched to a counterclockwise on / off state, the fundamental frequency repetition frequency of the mode-locked pulse in the on direction is 14.10 MHz. Therefore, by adjusting the common polarization controller to switch the output direction of the mode-locked pulse, the laser can be reversibly switched between the two repetition frequency states of 12.53 MHz and 14.10 MHz.

[0107] The two directional selective mode-locking states and their corresponding repetition frequency states described above can be quickly and reversibly switched by adjusting the common polarization controller. During the switching process, no other devices need to be touched, the pump power needs to be changed, or an external optical switch needs to be introduced. In both states, the output power in the cut-off direction is less than 500nW, and the extinction ratio is greater than 10 compared to the output power in the conducting direction. 3 It realizes the equivalent optical switch function of "one path with light and one path without light".

[0108] Therefore, this embodiment achieves the equivalent optical switch function of "one path on, one path off" for a bidirectional mode-locked laser by setting a polarization controller in the common optical path of the all-fiber single-cavity bidirectional ring laser and setting polarization beam splitters in the two branch optical paths. Utilizing the polarization state modulation of the polarization controller and the polarization selectivity of the polarization beam splitters, combined with the intracavity gain competition effect, the bidirectional mode-locked laser can achieve this. Simultaneously, since the clockwise and counterclockwise branch optical paths have different optical path lengths or physical lengths, switching between different mode-locked pulse repetition frequencies can be achieved while switching the output direction of the mode-locked pulse. This embodiment reduces the dependence on external electro-optic / acousto-optic modulators or spatial optical switches. Optical path selection and shutdown can be achieved within the all-fiber structure using only the existing intracavity components. This embodiment offers simple control (single polarization controller, single-point control) and thorough switching (extinction ratio greater than 10). 3 With its rapid response (adaptable to an electronically controlled polarization controller for fast switching) and compact structure (all-fiber structure, resistant to electromagnetic interference), it provides technical support for the application of single-cavity bidirectional dual-comb lasers in engineering scenarios requiring precise optical path on / off control, such as optical communication routing, pump detection, and precision optical measurement.

[0109] A computer-storable medium storing a computer program, which, when executed by a processor, implements the aforementioned polarization-controlled repetition frequency switchable mode-locked fiber laser and its control method.

[0110] Embodiments of the present invention may be provided as methods, systems, or computer program products. The invention may take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the invention may take the form of a computer program product implemented on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0111] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0112] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0113] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0114] A computer device includes a memory and a processor, the memory storing a computer program, and the processor executing the computer program to implement the above-described polarization-controlled repetition frequency switchable mode-locked fiber laser and its control method.

[0115] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple; relevant parts can be referred to the method section.

[0116] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A mode-locked fiber laser with switchable repetition frequency based on polarization control, characterized in that, include: Pump source, used to provide pump light; A ring resonant cavity includes a common optical path segment and a first branch optical path and a second branch optical path with opposite transmission directions. A gain fiber and an optical coupler are disposed inside the ring resonant cavity. A polarization controller is installed on the common optical path segment; The first branch optical path and the second branch optical path are respectively provided with polarization correlation devices and saturable absorbers; Specifically, by adjusting the polarization controller to change the polarization state of the light signals entering the first branch optical path and the second branch optical path, a polarization-dependent loss difference is generated between the two branch optical paths, so that one branch optical path establishes mode-locked pulse output while the other branch optical path cuts off oscillation, thereby achieving the switching of pulse output direction.

2. The polarization-controlled repetition frequency switchable mode-locked fiber laser according to claim 1, characterized in that: By adjusting the polarization controller to a first preset polarization state, the first branch optical path meets the low-loss transmission condition and the second branch optical path meets the high-loss cutoff condition. The first branch direction establishes a passive mode-locked state, and the second branch direction oscillates and is cut off. By adjusting the polarization controller to the second preset polarization state, the second branch optical path meets the low-loss transmission condition, the first branch optical path meets the high-loss cutoff condition, the second branch direction establishes a passive mode-locked state, and the first branch direction oscillates and cuts off.

3. The polarization-controlled repetition frequency switchable mode-locked fiber laser according to claim 2, characterized in that: The low-loss transmission condition is as follows: the angle between the polarization direction of the incident light signal and the main transmission axis direction of the corresponding branch polarization-dependent device is less than or equal to 5°, and the polarization-dependent loss of the corresponding branch is less than or equal to 0.5dB. The high-loss cutoff condition is as follows: the angle between the polarization direction of the incident light signal and the main transmission axis direction of the corresponding branch polarization-dependent device is greater than or equal to 46°, and the polarization-dependent loss of the corresponding branch is greater than or equal to 5dB.

4. The polarization-controlled repetition frequency switchable mode-locked fiber laser according to claim 1, characterized in that: The first branch optical path and the second branch optical path are respectively provided with a branch polarization controller and / or a variable optical attenuator; During the repetition frequency switching process by adjusting the polarization controller set on the common optical path segment, the states of the branch polarization controller and the variable optical attenuator remain unchanged.

5. The polarization-controlled repetition frequency switchable mode-locked fiber laser according to claim 1, characterized in that: The ring resonant cavity is an all-fiber structure, and a single-mode fiber for dispersive modulation is also provided inside the ring resonant cavity.

6. The polarization-controlled repetition frequency switchable mode-locked fiber laser according to claim 1, characterized in that: The polarization controller installed on the common optical path segment is a 3-paddle polarization controller, an electrically controlled polarization controller, or a squeeze-type polarization controller. The polarization-dependent device is a fiber-optic polarization beam splitter, a polarization-dependent isolator, or an online polarizer. The saturable absorber is a single-walled carbon nanotube saturable absorber, a semiconductor saturable absorber mirror, or a saturable absorber based on two-dimensional materials.

7. The polarization-controlled repetition frequency switchable mode-locked fiber laser according to claim 1, characterized in that: The pump source is a 980nm laser diode, and the length of the gain fiber is 0.3m to 100m.

8. A control method for a polarization-controlled repetition frequency switchable mode-locked fiber laser, using the polarization-controlled repetition frequency switchable mode-locked fiber laser as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Pump light is input into the ring resonant cavity; Adjust the polarization controller set on the common optical path segment so that the first branch optical path meets the low-loss transmission condition and the second branch optical path meets the high-loss cutoff condition, so that the first branch direction establishes a passive mode-locked state and the laser oscillation in the second branch direction is cut off. or Adjust the polarization controller set on the common optical path segment so that the second branch optical path meets the low-loss transmission condition and the first branch optical path meets the high-loss cutoff condition, so that the second branch direction establishes a passive mode-locked state and the laser oscillation in the first branch direction is cut off.

9. The control method according to claim 8, characterized in that, Also includes: After switching the pulse output direction, the polarization angle of the polarization controller set on the common optical path is finely adjusted. While maintaining one path on and one path off, the repetition frequency of the mode-locked pulse in the conduction direction is simultaneously finely adjusted.

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