Rotating fiber based wavelength tunable dual-color cavity ultrafast fiber laser and control method

Abstract

This invention discloses a wavelength-tunable dual-cavity ultrafast fiber laser based on rotating optical fibers and its control method, relating to the field of laser technology. The dual-cavity ultrafast fiber laser of this invention integrates a wavelength division multiplexer within a dual-ring resonant cavity, structurally integrating two wavelength channels, each containing an independent gain fiber, to achieve dual-color soliton output and reduce gain competition. By setting an adjustable rotating fiber filter, consisting of a polarization controller, rotating optical fibers, and a polarization-dependent isolator, in the multiplexing section, the center wavelength of both wavelengths is tunable. Simultaneously, the two sub-cavities share the same saturable absorber to achieve passive synchronous mode-locking, and a delay line is introduced into one of the channels to adjust the equivalent cavity length, achieving repetition frequency matching and timing alignment between the two sub-cavities, thereby obtaining stable, tunable, and synchronous dual-wavelength ultrashort pulse output.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a wavelength-tunable dual-cavity ultrafast fiber laser based on rotating optical fibers and a control method thereof. Background Technology

[0002] In terms of center wavelength tuning, existing mode-locked fiber lasers typically achieve this by introducing tunable spectral filtering mechanisms within the cavity. These include tunable bandpass filters, FBG stress or temperature-controlled drift frequency selection, and mechanical tuning based on birefringence and polarization interference. Among these, polarization interference filtering usually consists of birefringent fiber and polarization-dependent devices. By changing the polarization state or equivalent phase delay, the transmission peak is shifted, offering advantages such as all-fiber operation, relatively simple structure, and wide tuning range. However, this type of approach is sensitive to polarization state and prone to "wavelength selection-mode-locking state" coupling. When the system is expanded to a dual-cavity configuration with shared devices in a common section, a common practice is to configure a separate filter for each cavity, leading to an increase in the number of devices and losses, and a more complex structure. Alternatively, a shared filter can introduce mutual constraints between the two channels, resulting in insufficient controllability, poor tuning repeatability, or decreased stability in adjusting the center wavelength of the two channels. This makes it difficult to achieve tunable and repeatable center wavelength settings for dual channels that meet application requirements.

[0003] Furthermore, in a dual-cavity system, the arrival time difference between the two pulses at the common segment directly affects synchronization mode-locking, mutual interference, and output stability. Existing technologies typically compensate for the cavity length difference by adding a delay line or an adjustable optical path device to one of the pulses for repetition frequency fine-tuning or synchronization alignment. However, in a dual-cavity dual-channel structure, the repetition frequency not only needs to be "aligned," but in some applications, it also needs to "maintain the difference as needed." Without coordinated design with cavity length adjustment, common segment coupling, and tuning mechanisms, it is still easy to encounter problems such as difficulty in achieving stable consistency between the two cavity repetition frequencies or difficulty in accurately setting a controllable difference, accompanied by unstable timing matching, operating point drift, and difficulty in maintaining the same or different frequency states for a long time.

[0004] Therefore, although existing technologies have methods such as center wavelength tuning based on polarization interference and cavity length compensation based on delay lines, it is still difficult to simultaneously achieve the following in the same system: 1) tunable and repeatable center wavelength of dual channels; 2) adjustable control of repetition frequency of two sub-cavities, and there is still room for improvement in terms of device quantity, insertion loss, parameter tuning coupling and long-term stability.

[0005] Existing mode-locked fiber lasers still face the following key technical challenges in achieving dual-color soliton output: In single-cavity or strongly coupled dual-cavity structures, different wavelength channels often share gain and common optical paths, leading to prominent gain competition and channel crosstalk, making it difficult for dual-color pulses to coexist stably and resulting in insufficient independent controllability; at the same time, existing dual-wavelength tunable and synchronization schemes often rely on multiple sets of filtering, mode-locking, and synchronization units, resulting in a large number of devices, high insertion loss, and complex structure, and making it difficult to maintain the consistency of dual-cavity repetition frequency and timing matching stably over a long period of time, thus limiting the application of dual-wavelength lasers at the same frequency. Summary of the Invention

[0006] To address the aforementioned problems in achieving dual-color soliton output using existing mode-locked fiber lasers, this invention provides a wavelength-tunable dual-color cavity ultrafast fiber laser based on rotating fiber and a control method thereof. This laser can achieve tunable center wavelengths of both wavelengths, as well as matching and timing alignment of the repetition frequencies of the two sub-cavities, resulting in stable, tunable, and synchronous dual-wavelength ultrashort pulse output.

[0007] The present invention adopts the following technical solution: a wavelength-tunable dual-cavity ultrafast fiber laser based on rotating optical fiber, the whole being a dual-ring resonant structure using wavelength division multiplexing, including: a first sub-cavity, a second sub-cavity, and a multiplexing section located between the two sub-cavities.

[0008] The first and second sub-cavities are connected at the left and right nodes through the third wavelength division multiplexer (WDM3) and the fourth wavelength division multiplexer (WDM4) to form two physical closed loops; the first sub-cavity adjusts its cavity length through a series delay line to achieve frequency repetition matching and timing alignment with the second sub-cavity.

[0009] The multiplexing section is located between WDM3 and WDM4. A mode-locking unit, a filtering unit, and an output coupling unit are sequentially connected in series from WDM3 to WDM4 to achieve adjustable center wavelength and synchronous output at the same frequency.

[0010] Furthermore, the mode-locking unit is equipped with a saturable absorber (SA) as a passive mode-locking device; the pulses from the two sub-cavities pass through the SA together, enabling the dual channels to establish common mode-locking gating and synchronization constraints;

[0011] The filtering unit is equipped with a polarization controller (PC), a rotating fiber (SF), and a polarization-dependent isolator (PD-ISO), which together form a polarization interference type tunable filter. The polarization controller receives the optical signal output by the SA and propagates it to the output coupling unit through the rotating fiber and the polarization-dependent isolator.

[0012] The output coupling unit is equipped with a third coupler (OC3), through which the beam combining monitoring signal is output to WDM4 and returned to the first sub-cavity and the second sub-cavity.

[0013] Furthermore, the first sub-cavity includes a first gain fiber, a first wavelength division multiplexer (WDM1), a first coupler (OC1), and a delay line connected in sequence, and is provided with pump light through a first pump source (LD1);

[0014] The second sub-cavity includes a second gain fiber, a second wavelength division multiplexer (WDM2), a second coupler (OC2) connected in sequence, and a pump light provided by a second pump source (LD2).

[0015] The first gain fiber and the second gain fiber are used to provide optical amplification gain.

[0016] WDM1 and WDM2 are connected to pump sources LD1 and LD2 and their corresponding first and second gain fibers, respectively, to couple pump light into the gain fibers while allowing signal light to transmit in the operating band.

[0017] WDM4 and WDM3 are located at the left and right nodes of the multiplexing section, respectively, and are used to achieve wavelength selective routing, separation and multiplexing between the multiplexing section and the two sub-cavities.

[0018] OC1 is located in the upper branch of the first sub-cavity, OC2 is located in the lower branch of the second sub-cavity, and OC3 is located on the left side of the multiplexing section.

[0019] PC is used to simultaneously adjust the polarization state in the first and second sub-cavities to achieve center wavelength tuning of the segment filter unit.

[0020] Furthermore, in the first sub-cavity, a delay line is connected in series in the right branch of the first sub-cavity to adjust the cavity length of the first sub-cavity, thereby achieving dual-cavity repetition frequency matching and timing alignment. By adjusting the delay line, the cavity length L1 of the first sub-cavity is matched with the effective cavity length L2 of the second sub-cavity, so that the repetition frequencies of the two sub-cavities are consistent, and the arrival times of the pulses of the two channels at the multiplexing section and the output port are aligned to obtain the same-frequency dual-color soliton output.

[0021] Furthermore, the filtering unit generates a wavelength-dependent phase difference between the two orthogonal polarization components through the SF, projects the polarization difference into a transmission loss difference through the polarization-dependent isolator, forms a wavelength-selective transmission spectrum, and changes the incident polarization by adjusting the PC, causing the transmission peak position to shift and changing the point of maximum net gain in the two sub-cavities, thereby achieving center wavelength tuning.

[0022] Furthermore, OC1, OC2 and OC3 are respectively provided with output ports 1, 2 and 3, which are used to extract the single cavity output and the shared optical path output of the two ring cavities, and the coupling ratio is 9:1.

[0023] Furthermore, the closing paths of the first and second sub-cavities are as follows:

[0024] In the first sub-cavity, the pump light is input into the first sub-cavity through WDM1, passes through OC1, is coupled from WDM3 to the multiplexing section through the delay line, is output through WDM4, and is transmitted to WDM1 through the first gain fiber to form a closed loop.

[0025] In the second sub-cavity, the pump light is input into the second sub-cavity through WDM2, passes through OC2, is coupled to the common optical path through WDM3, is output through WDM4, and is transmitted to WDM2 through the second gain fiber to form a closed loop.

[0026] The present invention also provides: a control method for a wavelength-tunable dual-cavity ultrafast fiber laser based on rotating optical fiber, applied to the aforementioned laser, comprising the following steps:

[0027] Step 1: Start the first pump source and the second pump source, and couple the pumps into the first gain fiber and the second gain fiber through WDM1 and WDM2 respectively to establish the net gain of the two sub-cavities.

[0028] Step 2: Through wavelength selective routing of WDM3 and WDM4, different wavelength channels form stable loops in their respective sub-cavities;

[0029] Step 3: Adjust the PC to match the filter transmission peak of the multiplex section filter unit with the net gain condition in the cavity, and tune the center wavelengths of the short and long waves, represented by λ1 and λ2; the center wavelength ranges of λ1 and λ2 are around 1530nm and 1550nm respectively.

[0030] Step 4: Under the action of the saturable absorber SA, passive mode-locking is achieved and maintained to form a two-color pulse;

[0031] Step 5: Adjust the delay line of the first sub-cavity to make the repetition frequency of the two sub-cavities consistent and achieve timing alignment, so as to obtain dual-wavelength output at the same frequency;

[0032] Step 6: Optimize the operating point by outputting the corresponding channels of the two sub-cavities through OC1 and OC2, or by outputting the beam combining monitoring signal through OC3.

[0033] Compared with the prior art, the present invention, employing the above technical solution, has the following technical effects:

[0034] 1. This invention provides a wavelength-tunable dual-color cavity ultrafast fiber laser: a wavelength division multiplexer is integrated in a dual-ring resonant cavity, so that two wavelength channels, each containing an independent gain fiber, are structurally integrated to achieve dual-color soliton output and reduce gain competition; an adjustable rotating fiber filter composed of a polarization controller, a rotating fiber, and a polarization correlation isolator is set in the multiplexing section to achieve tunable center wavelength of the two wavelengths; at the same time, the two sub-cavities share the same saturable absorber to achieve passive synchronous mode locking, and a delay line is introduced into one of them to adjust the equivalent cavity length, so as to achieve repetition frequency matching and timing alignment of the two sub-cavities, thereby obtaining stable, adjustable, and synchronous dual-wavelength ultrashort pulse output.

[0035] 2. This invention achieves dual-wavelength center wavelength tuning and a simple structure by sharing filters through PC, SF, and PD-ISO in the multiplexing section. The adjustable rotating fiber filter composed of PC, SF, and PD-ISO causes a wavelength-dependent phase difference between the two orthogonal polarization components through SF, and PD-ISO projects the polarization difference as a transmission loss difference, thus forming a wavelength-selective transmission spectrum. By adjusting the PC to change the incident polarization, the transmission peak position shifts, and the location of maximum net gain in the cavity changes accordingly, achieving center wavelength tuning. Since the filter is located in the multiplexing section and shared by both channels, dual-channel tuning can be achieved with a single adjustable filter unit, reducing the number of components and cavity loss, and avoiding the structural complexity and insertion loss accumulation caused by configuring filters separately for each sub-cavity. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the overall structure of the wavelength-tunable dual-color cavity ultrafast fiber laser of the present invention.

[0037] Figure 2 This is a schematic diagram showing the shift in the center wavelength of the overall optical path spectrum of the present invention;

[0038] Figure 3 This is a spectrum showing the shift in the center wavelength of the total optical path in an embodiment of the present invention.

[0039] Figure 4 This is a spectral diagram of the overall optical path repetition rate adjustment process in an embodiment of the present invention;

[0040] Figure 5 This is a spectrum diagram of the total optical path repetition rate adjustment process in an embodiment of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the application will be further described in detail below with reference to the accompanying drawings. The described embodiments are only a part of the embodiments involved in this invention. All non-innovative embodiments based on these embodiments by other researchers in the art are within the protection scope of this invention. Furthermore, the step numbers in the embodiments of this invention are only set for ease of explanation and do not limit the order of the steps. The execution order of each step in the embodiments can be adaptively adjusted according to the understanding of those skilled in the art.

[0042] In one embodiment of the present invention, a wavelength-tunable dual-cavity ultrafast fiber laser based on a rotating fiber is provided, the structure of which is as follows: Figure 1 As shown, it includes:

[0043] Pump sources LD1 and LD2: provide pump light to sub-cavity 1 and sub-cavity 2, respectively.

[0044] Gain fiber 1 and gain fiber 2 are located in sub-cavity 1 and sub-cavity 2 respectively, providing optical amplification gain.

[0045] Wavelength division multiplexer 1 (WDM1) and wavelength division multiplexer 2 (WDM2): are connected to LD1, LD2 and their corresponding gain fibers respectively, and are used to couple pump light into the gain fiber while allowing signal light to be transmitted in the working band.

[0046] Wavelength division multiplexer 4 (WDM4) and wavelength division multiplexer 3 (WDM3): located at the left and right nodes of the structure respectively, used to achieve wavelength selective routing, separation and multiplexing between the multiplexing section and the two sub-cavities.

[0047] Coupler 1, Coupler 2, and Coupler 3 are used for output coupling; among them, Coupler 1 is located on the upper branch of sub-cavity 1, Coupler 2 is located on the lower branch of sub-cavity 2, and Coupler 3 is located on the left side of the multiplexing section.

[0048] Polarization controllers: Located on the composite section, they are used to adjust the polarization state in each sub-cavity to achieve center wavelength tuning of the multiplexed section filter.

[0049] Delay line: It is set in series in the right branch of sub-cavity 1 to adjust the cavity length of sub-cavity 1 and achieve dual-cavity repetition frequency and timing matching.

[0050] Polarization controller (PC), rotating fiber (SF), and polarization-dependent isolator (PD-ISO): located in the series section in the middle of the multiplexing section, together forming an adjustable rotating fiber filter.

[0051] Saturable absorber (SA): Connected in series in the multiplexing section, it serves as a passive mode-locking device to achieve dual-channel passive synchronous mode-locking.

[0052] In this embodiment, the closing paths of sub-cavity 1 and sub-cavity 2 are as follows:

[0053] (1) Closed loop of sub-cavity 1

[0054] Sub-cavity 1 includes, in sequence along the optical propagation direction: gain fiber 1 → wavelength division multiplexer 1 → coupler 1 → delay line → wavelength division multiplexer 3 → after multiplexing section, it returns to wavelength division multiplexer 4 → returns to the left branch of sub-cavity 1 to form a closed loop.

[0055] (2) Closed loop of sub-cavity 2

[0056] Sub-cavity 2 includes, in sequence along the optical propagation direction: gain fiber 2 → wavelength division multiplexer 2 → coupler 2 → wavelength division multiplexer 3 → after multiplexing section, it returns to wavelength division multiplexer 4 → returns to the left branch of sub-cavity 2 to form a closed loop.

[0057] In this embodiment, the reuse segment structure is as follows:

[0058] The multiplexing section is located between WDM4 and WDM3, and is connected in series from WDM3 to WDM4 as follows: Saturable absorber (SA) → Polarization controller (PC) → Rotating fiber (SF) → Polarization dependent isolator (PD-ISO) → Coupler 3.

[0059] The functions of each component in the multiplexing section are as follows:

[0060] SA: Provides mode-locking function; pulses from both sub-cavities pass through SA together, enabling the dual channels to naturally establish common mode-locking gating and synchronization constraints.

[0061] PC: Adjusts the polarization state of the multiplexed segment.

[0062] PD-ISO: Used to limit unidirectional propagation.

[0063] SF: Provides wavelength-dependent birefringence phase delay and polarization evolution, and together with PC and PD-ISO, forms a polarization interferometric tunable filter.

[0064] On the one hand, this embodiment achieves adjustable center wavelength for dual wavelengths and a simple structure by sharing filters in the multiplexing sections PC, SF, and PD-ISO.

[0065] An adjustable rotating fiber filter consisting of PC, SF, and PD-ISO: SF generates a wavelength-dependent phase difference between the two orthogonal polarization components, and PD-ISO projects the polarization difference as a transmission loss difference, thus forming a wavelength-selective transmission spectrum. By adjusting the PC to change the incident polarization, the position of the transmission peak shifts, and the location of maximum net gain within the cavity changes accordingly, achieving center wavelength tuning.

[0066] Since the filter is located in the multiplexing section and is shared by the two channels, dual-channel tuning can be achieved with a single adjustable filter unit, reducing the number of devices and intracavity losses, and avoiding the structural complexity and insertion loss accumulation caused by configuring filters separately for each subcavity.

[0067] On the other hand, this embodiment can achieve dual-cavity frequency repetition matching and timing alignment by introducing a delay line in sub-cavity 1.

[0068] Since the mode-locking pulse repetition frequency and cavity length approximately satisfy: ;in, Represents the speed of light. Indicates the refractive index of the optical fiber material. Indicates the effective cavity length.

[0069] Based on this, this embodiment can adjust the repetition frequency by effectively changing the cavity length L1 of sub-cavity 1 through a delay line. By adjusting the delay line to match the effective cavity length L1 of sub-cavity 1 with the effective cavity length L2 of sub-cavity 2, the repetition frequency of the two sub-cavities can be made consistent, and the arrival time of the pulses of the two channels at the multiplexing section and the output port can be aligned to obtain the same frequency dual-color soliton output.

[0070] The working method of the dual-color fiber laser in this embodiment is as follows:

[0071] 1. Start LD1 and LD2, and pump the fibers into gain fibers 1 and 2 via WDM1 and WDM2 to establish the net gain of the two sub-cavities;

[0072] 2. By using the wavelength selective routing of WDM3 and WDM4, different wavelength channels can form stable loops in their respective sub-cavities;

[0073] 3. Adjust the PC to match the transmission peaks of the multiplexed PC, SF and PD-ISO filters with the net gain conditions in the cavity, and tune the center wavelengths of λ1 and λ2.

[0074] 4. Passive mode-locking is established and maintained under the action of SA, forming a two-color pulse;

[0075] 5. Adjust the delay line of sub-cavity 1 to make the repetition frequency of the two sub-cavities consistent and achieve timing alignment, so as to obtain dual-wavelength output at the same frequency;

[0076] 6. Output the corresponding channels of the two sub-cavities through couplers 1 and 2, or output the beam-combining monitoring signal through coupler 3 to complete the operating point optimization.

[0077] Specifically, in this embodiment, WDM3 and WDM4 can be replaced with thin-film filter-type multiplexers / demultiplexers, fused conical multiplexers / demultiplexers, etc., to achieve wavelength routing; the number of channels can be expanded to ≥3 to achieve multiple wavelengths.

[0078] The PC, SF, and PD-ISO tunable rotating fiber filters in the filtering unit can be replaced with: Lyot filters, Sagnac ring filters, FBG frequency selection structures, tunable electro-optic filters, etc.; PD-ISO can be replaced with isolators + polarizers or other equivalent polarization projection devices.

[0079] CNT-SA can be replaced with graphene, topological insulators, TMD-type SA, SESAM, or passive mode-locking methods such as NPR.

[0080] The delay line can be replaced with an adjustable fiber optic delay line, a PZT fiber optic expander, a switchable fiber segment, or a temperature-controlled optical path compensation module.

[0081] The coupling ratio of this device is 10%, which can be selected within the range of 1%–50%, and can be configured as a single-output or multi-output structure.

[0082] In summary, this invention, based on a comprehensive scheme of wavelength-selective WDM dual-cavity multiplexing, multiplexing segment PC, SF and PD-ISO polarization interference filtering tuning, and introducing a delay line in one sub-cavity for cavity length adjustment, achieves stable and controllable dual-wavelength mode-locked output, adjustable center wavelength and synchronous output at the same frequency.

[0083] Specific application 1: Achieving dual-wavelength mode-locking, adjustable center wavelength, and same-frequency matching.

[0084] 1. Implementation and debugging steps

[0085] 1) Start LD1 and LD2 to establish net gain in gain fibers 1 and 2;

[0086] 2) Two-channel oscillations were formed through WDM3 and WDM4, and bicolor solitons were observed on OSA through OC3;

[0087] 3) Continue to adjust the pump and PC to bring SA into the effective saturation absorption region and establish mode lock;

[0088] 4) Adjust the PC to match the transmission peak of the multiplexed section filter with the net gain condition to obtain two-color soliton outputs with different center wavelengths;

[0089] 5) Adjust the delay line of sub-cavity 1 and observe the RF spectrum in real time as it evolves from a double peak to a single peak until the repetition frequencies of the two cavities are consistent.

[0090] 2. Testing equipment and parameters

[0091] The following instruments can be used to complete the test: OSA spectrometer, RF spectrometer, high-speed photodetector + oscilloscope, autocorrelation analyzer, and optical power meter.

[0092] 3. Measured Results

[0093] 1) Achieve stable two-color soliton mode-locked output and provide basic performance indicators.

[0094] In mode-locked operation, the dual-color fiber laser of this invention can simultaneously output pulsed laser light from two wavelength channels, exhibiting typical soliton spectral characteristics. Basic dual-wavelength mode-locked output, such as... Figure 2 As shown:

[0095] Shortwave channel center wavelength: spectral bandwidth ;

[0096] Center wavelength of long-wave channel: spectral bandwidth ;

[0097] The RF spectrum exhibits equally spaced harmonic components in the range of 0–50 MHz. The center frequencies of the untuned DL in both the shortwave and longwave channels are around 6.65 MHz, and it has a high signal-to-noise ratio.

[0098] The autocorrelation curve and sech² fitting show that the pulse width is in the picosecond range: approximately 1.651 × 0.648 ps ≈ 1.07 ps for the short-wave channel and approximately 1.433 × 0.648 ps ≈ 0.93 ps for the long-wave channel.

[0099] The above data indicates that the mode-locking repetition frequency is stable and provides an observable basis for subsequent two-cavity frequency matching.

[0100] The corresponding technical features are: the wavelength-selective wavelength division multiplexer limits the two wavelengths to evolve in two sub-cavity gain paths respectively, reducing cross saturation and crosstalk; the shared SA trigger passive mode-locking enables the two channels to establish a common mode-locking gating.

[0101] 2) Achieve adjustable center wavelength for dual channels and maintain parallel output of dual wavelengths during tuning.

[0102] By adjusting the PC, the transmission peak position of the tunable rotating fiber filter formed by the multiplex section PC, SF, and PD-ISO is shifted, thereby changing the position of the wavelength with the largest net gain in the cavity and achieving center wavelength tuning.

[0103] The center wavelength tuning process, such as Figure 3 As shown, Figure 3 The output spectra under multiple adjustment states are given. It can be seen that the dual-band spectral envelope always exists in parallel under multiple PC adjustment states, indicating that the system has the ability to adjust the center wavelength of the dual channels and the mode-locked state can be maintained during the tuning process.

[0104] The corresponding technical features are: the multiplexing section uses a filter unit composed of a single PC, SF and PD-ISO to share frequency selection and tuning between the two channels, avoiding device stacking and insertion loss accumulation caused by configuring an independent filter for each sub-cavity.

[0105] 3) By using a delay line to achieve consistent repetition frequency and timing alignment between the two sub-cavities, dual-wavelength output at the same frequency is obtained. A delay line is introduced into sub-cavity 1, and its equivalent optical path L1 is adjusted to match the repetition frequency of the two sub-cavities.

[0106] The overall optical path frequency repetition matching result in this embodiment is as follows: Figure 4 and Figure 5 As shown, the spectral and emission spectrum evolution during the delay line adjustment process in sub-cavity 1 is illustrated: the emission spectrum gradually evolves from an initial double peak to a single sharp main peak, with the final main peak located at approximately 6.647835 MHz, indicating that the repetition frequencies of the two sub-cavities are adjusted to be consistent, achieving frequency matching; simultaneously, the spectrum maintains a dual-wavelength structure, indicating that the repetition frequencies of the two sub-cavities are consistent, resulting in simultaneous dual-wavelength output. This demonstrates that the dual-wavelength output is not disrupted during the process of achieving simultaneous frequency, achieving simultaneous dual-color soliton mode-locked output.

[0107] 4) Structural simplification and improved engineering economy.

[0108] Device reuse reduces complexity and loss: the two sub-cavities share the same multiplexing section filter and mode-locking unit, reducing the number of devices and the accumulation of insertion loss in the cavity.

[0109] Reduce external synchronization dependency: Shared SA provides passive synchronization gating, and delay lines achieve repetition frequency consistency, avoiding complex external electronic synchronization systems.

[0110] More compact system integration and lower cost: Compared with multiple lasers connected in parallel or single-source splitting and homogeneous output, this invention achieves dual-wavelength, tunable and synchronous output at the same frequency with a single system, which has better engineering and application scalability.

[0111] Specific application 2: Low loss configuration and short stroke matching.

[0112] Based on Application 1, a smaller coupling ratio is selected for the coupler, and a short-stroke adjustable optical path module is used for the delay line. The remaining connection and multiplexing section structures remain unchanged.

[0113] Effects: Reduced total loss within the cavity and easier mold clamping startup; suitable for scenarios where the initial cavity lengths of the two cavities are nearly matched and only assembly errors or temperature drift need to be compensated.

[0114] Specific application three: enhancing adjustability and long-term retention capabilities.

[0115] Based on Specific Application 1, at least one enhanced configuration is adopted: the delay line is replaced with a continuously adjustable delay line or a PZT fiber optic expander to expand the cavity length adjustment range and achieve dynamic compensation; an equivalent polarization projection element is added to the multiplex section filter structure to improve filter selectivity and tuning repeatability; and a parallel port configuration of "sub-cavity output + multiplex section monitoring output" is adopted to facilitate online observation and engineering parameter adjustment.

[0116] Effects: Easier to achieve and maintain the same frequency for a long time; better center wavelength tuning repeatability, suitable for long-term stable operation and system integration.

[0117] It should be understood that the above embodiments are for illustrative purposes only and are not intended to limit the present invention. For those skilled in the art, any equivalent substitutions, combinations, or modifications made to the device structure, connection relationships, component types, and parameters without departing from the spirit and essence of the present invention and without requiring creative effort should be considered to fall within the protection scope of the present invention.

[0118] For example, saturable absorbers other than carbon nanotubes can be equivalently replaced by SESAM (semiconductor saturable absorber), graphene, transition metal chalcogenides, black phosphorus, or their composites; the pumping scheme can be adjusted from 980 nm to 1480 nm or single-pump beam splitting; the coupling ratio of the optical coupler, the type and quantity of WDM, the type of optical fiber, the number and location of the output ports, and the order of the devices can also be adjusted as needed.

[0119] Experimental apparatus, systems and their implementation methods that are constructed based on this and are substantially the same as this device in function and effect are all within the protection scope of this application.

Claims

1. A wavelength-tunable dual-color cavity ultrafast fiber laser based on a rotating fiber, which is a dual-ring resonant structure using a wavelength division multiplexer, characterized in that, include: The first sub-cavity, the second sub-cavity, and the reused section located between the two sub-cavities; The first sub-cavity and the second sub-cavity are connected at the left and right nodes through the third and fourth wavelength division multiplexers to form two physical closed loops; the first sub-cavity adjusts its cavity length through a series delay line to achieve frequency repetition matching and timing alignment with the second sub-cavity; The multiplexing section is located between the third wavelength division multiplexer and the fourth wavelength division multiplexer. A mode-locking unit, a filtering unit, and an output coupling unit are sequentially connected in series from the third wavelength division multiplexer to the fourth wavelength division multiplexer to achieve adjustable center wavelength and synchronous output at the same frequency. The mode-locking unit is equipped with a saturable absorber as a passive mode-locking device; the pulses of the two sub-cavities pass through the saturable absorber together, enabling the two channels to establish common mode-locking gating and synchronization constraints; The filtering unit is equipped with a polarization controller, a rotating optical fiber, and a polarization-dependent isolator, which together form a polarization-rotating adjustable filter. The polarization controller receives the optical signal output from the saturable absorber, propagates it through the rotating optical fiber to the polarization-dependent isolator, and finally propagates it from the polarization-dependent isolator to the output coupling unit. The output coupling unit is equipped with a third coupler, which outputs a beam combining monitoring signal and returns the remaining signal to the first and second sub-cavities through a fourth wavelength division multiplexer. The polarization controller is located in the multiplexing section and simultaneously adjusts the polarization state in the first sub-cavity and the second sub-cavity to achieve center wavelength tuning of the filter unit. The delay line is connected in series in the right branch of the first sub-cavity. The cavity length of the first sub-cavity is adjusted by the delay line so that the effective cavity lengths of the first sub-cavity and the second sub-cavity are matched, so that the repetition frequency of the two sub-cavities is consistent and the timing is aligned. The arrival time of the pulses of the two channels at the multiplexing section and the output port is aligned to obtain the same frequency dual-color soliton output. The filtering unit generates a wavelength-dependent phase difference between two orthogonal polarization components by rotating the optical fiber. The polarization difference is projected as a transmission loss difference through a polarization-dependent isolator, forming a wavelength-selective transmission spectrum. By adjusting the polarization controller to change the incident polarization, the position of the transmission peak is moved, changing the point of maximum net gain in the two sub-cavities, thus achieving center wavelength tuning.

2. The wavelength-tunable dual-cavity ultrafast fiber laser according to claim 1, characterized in that, The first sub-cavity includes a first gain fiber, a first wavelength division multiplexer, a first coupler, and a delay line connected in sequence, and is provided with pump light through a first pump source; The second sub-cavity includes a second gain fiber, a second wavelength division multiplexer, and a second coupler connected in sequence, and provides pump light through a second pump source.

3. The wavelength-tunable dual-cavity ultrafast fiber laser according to claim 2, characterized in that, The first wavelength division multiplexer and the second wavelength division multiplexer are respectively connected to the first pump source and the second pump source and the corresponding first gain fiber and the second gain fiber, which are used to couple the pump light into the gain fiber while allowing the signal light to be transmitted in the working band. The third and fourth wavelength division multiplexers are located at the right and left nodes of the multiplexing section, respectively, and are used to achieve wavelength selective routing, separation and multiplexing between the multiplexing section and the two sub-cavities.

4. The wavelength-tunable dual-cavity ultrafast fiber laser according to claim 2, characterized in that, The first, second, and third optical couplers are equipped with output ports 1, 2, and 3, respectively, for extracting the single-cavity output and the shared optical path output of the two ring cavities, with a coupling ratio of 9:1 for each.

5. The wavelength-tunable dual-cavity ultrafast fiber laser according to claim 2, characterized in that, In the first sub-cavity, the pump light is input into the first sub-cavity through the first wavelength division multiplexer, and after passing through the first coupler, it is coupled from the third wavelength division multiplexer to the multiplexing section through the delay line, output through the fourth wavelength division multiplexer, and transmitted to the first wavelength division multiplexer through the first gain fiber to form a closed loop.

6. The wavelength-tunable dual-cavity ultrafast fiber laser according to claim 2, characterized in that, In the second sub-cavity, the pump light is input into the second sub-cavity through the second wavelength division multiplexer, and after passing through the second coupler, it is coupled to the common optical path through the third wavelength division multiplexer, output through the fourth wavelength division multiplexer, and transmitted to the second wavelength division multiplexer through the second gain fiber to form a closed loop.

7. A control method for a wavelength-tunable two-color cavity ultrafast fiber laser based on rotating fiber, applied to the laser of any one of claims 2-6, characterized in that, Includes the following steps: Step 1: Start the first pump source and the second pump source, and couple the pump into the first gain fiber and the second gain fiber through the first wavelength division multiplexer and the second wavelength division multiplexer respectively to establish the net gain of the two sub-cavities. Step 2: Through wavelength selective routing of the third and fourth wavelength division multiplexers, different wavelength channels form stable loops in their respective sub-cavities. Step 3: Adjust the polarization controller to match the filtering transmission peak of the multiplex section filter unit with the net gain condition in the cavity, and tune the center wavelengths of the short and long waves. Step 4: Under the action of the saturable absorber, passive mode-locking is established and maintained to form a two-color pulse; Step 5: Adjust the delay line of the first sub-cavity to make the repetition frequency of the two sub-cavities consistent and achieve timing alignment, so as to obtain dual-wavelength output at the same frequency; Step 6: Output the corresponding channels of the two sub-cavities through the first coupler and the second coupler, or output the beam-combining monitoring signal through the third coupler to complete the operating point optimization.

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