Tunable narrow-band picosecond laser generation system and method

Abstract

This invention discloses a tunable narrow-band picosecond laser generation system and method. The system, along its optical path, sequentially includes: a broadband picosecond seed source, a polarization beam splitter, a first Faraday rotator, a bidirectional filter, a first reflector, a pulse splitter / synthesizer, a bidirectional laser amplifier, a second Faraday rotator, and a second reflector. After exiting the polarization beam splitter, the seed light undergoes bidirectional filtering via the first Faraday rotator and the bidirectional filter. The Faraday rotator rotates the polarization state by 90°, and the beam is split by the pulse splitter / synthesizer before entering the bidirectional amplifier. It is then reflected back by the second Faraday rotator and the second reflector and recombined. This invention achieves microsecond-level wavelength switching through an externally mounted fast-tuning device, and combines pulse splitting technology to reduce peak power, effectively suppressing nonlinear spectral broadening during high-power amplification. This results in high-speed tuning, high-power narrow-band output, suitable for nonlinear imaging, spectral detection, and precision machining.

Description

Technical Field

[0001] This invention relates to the field of ultrafast laser technology, and in particular to a tunable narrow-band picosecond laser generation system and method. Background Technology

[0002] Tunable picosecond lasers with narrow bandwidths have important applications in biomedical imaging, material detection, and cutting-edge scientific research. For example, narrow bandwidth picosecond lasers can be used as a source to excite the vibrational-rotational energy levels of molecules in coherent Raman scattering imaging, drive optical parametric oscillators, or directly perform material processing. Traditional implementation schemes are mainly divided into two categories: one is intracavity tuning, which directly generates tunable picosecond lasers by adding wavelength tuning devices directly to the mode-locked laser oscillator. However, intracavity tuning is prone to affecting mode-locking stability, and this type of scheme has low output power, making it difficult to directly meet the requirements of high-power applications. The other is based on a master oscillator power amplifier structure, which first generates a wide-bandwidth seed pulse, filters it through a tunable filter, and then amplifies it through a power amplifier.

[0003] External cavity tuning offers significant advantages over internal cavity tuning in terms of mode-locking stability and output power. However, existing external cavity tuning techniques have the following drawbacks: First, existing tunable filters based on temperature or stress tuning, such as fiber gratings, have slow tuning speeds, typically on the order of milliseconds to seconds, which is insufficient for high-speed spectral scanning applications. High-speed tunable filters, such as acousto-optic tunable filters, have lower power impairment thresholds. Second, during high-power amplification, the high peak power density of the narrow-band seed light in the gain fiber easily leads to spectral distortion and broadening due to nonlinear effects such as self-phase modulation, disrupting the narrow-band characteristics of the output light and limiting the final output power increase. Furthermore, the chirped pulse amplification technique commonly used in femtosecond high-power amplification, which relies on dispersion to achieve temporal broadening of the broad-band seed light, is difficult to directly apply to the narrow-band picosecond pulse field. Summary of the Invention

[0004] To address the aforementioned issues, a tunable narrowband picosecond laser generation system and method are proposed, which combines rapid tuning with split-pulse amplification to create a high-power narrowband picosecond laser system. This system enables rapid and precise tuning of the output wavelength, and through a unique amplification link design, effectively suppresses nonlinear effects and maintains superior narrowband characteristics while achieving high output power.

[0005] The technical solution of the present invention is as follows: a tunable narrowband picosecond laser generation system, comprising, in sequence along the light propagation direction: a broadband picosecond seed source module, a four-port polarization beam splitter, a first Faraday rotator, a bidirectional filter, a first reflector, a pulse splitter / synthesizer, a bidirectional laser amplifier, a second Faraday rotator, and a second reflector; The output of the broadband picosecond seed source module is coupled to the first port of the polarization beam splitter. The second port of the polarization beam splitter is connected in sequence to the first Faraday rotator, the bidirectional filter and the first reflector. The reflected light from the first reflector returns to the second port of the polarization beam splitter along the original path through the bidirectional filter and the first Faraday rotator. After two-way reflection by the first Faraday rotator and the first reflector, the polarization state of the returned light is rotated by 90° relative to the incident light. The third port of the polarization beam splitter is connected in sequence to the pulse splitter / combiner, the bidirectional laser amplifier, the second Faraday rotator, and the second mirror. The reflected light from the second mirror returns to the third port of the polarization beam splitter along the original path through the second Faraday rotator, the bidirectional laser amplifier, and the pulse splitter / combiner. After two-way reflection by the second Faraday rotator and the second mirror, the polarization state of the returned light is rotated by 90° relative to the incident light. The bidirectional filter is configured to perform rapid wavelength selection and bandwidth control on the incident light, and the optical path therein is bidirectional. The pulse splitter / synthesizer is configured to use a birefringent crystal to introduce different group velocity delays to orthogonally polarized beams, thereby splitting the incident pulse into multiple sub-pulses in the time domain to reduce peak power, and then resynthesizing them into a single pulse through the reverse process after amplification. The bidirectional laser amplifier is configured to amplify the power of the plurality of sub-pulses, and the optical path therein is bidirectional. The amplified and synthesized laser beam is output from the fourth port of the polarization beam splitter by rotating the polarization state again.

[0006] Preferably, the broadband picosecond seed source module is a passively mode-locked laser with an all-fiber structure. Its output pulse is obtained by nonlinear amplification after photonic crystal fiber supercontinuum broadening, nonlinear fiber amplification, or dispersion compensation and filtering, and the full width at half maximum (FWHM) of the spectrum is greater than 60 nm.

[0007] Preferably, the bidirectional filter is constructed based on an acousto-optic tunable filter, including a collimator, a first liquid lens, an acousto-optic tunable filter, a fixed-focal-length lens and a first reflector arranged sequentially along the optical path, and a driving module electrically connected to the acousto-optic tunable filter; the focal length of the first liquid lens is electrically adjustable to control the filtering bandwidth.

[0008] Preferably, the bidirectional filter is constructed based on a separate structure of grating and galvanometer, including a first collimator, a first lens, a galvanometer, a second lens, a third lens, a diffraction grating, a second liquid lens, and a second collimator arranged in sequence, as well as a driving module electrically connected to the galvanometer; the galvanometer and the diffraction grating are optically conjugated through the second lens and the third lens, and the focal length of the second liquid lens is electrically adjustable to control the filtering bandwidth.

[0009] Preferably, the bidirectional filter is constructed based on a combined structure of a grating and a galvanometer, including a collimator, a lens, a galvanometer, a diffraction grating, a slit, and a first reflecting mirror arranged in sequence, as well as a driving module electrically connected to the galvanometer; the deflection angle of the galvanometer is used to achieve wavelength tuning, and the width of the slit is used to control the filtering bandwidth.

[0010] Preferably, the pulse separator / synthesizer is composed of a polarization beam splitter crystal or a birefringent crystal combined with a waveplate, and has N stages, used to separate an incident single pulse into 2^N sub-pulses according to its polarization state.

[0011] Preferably, the bidirectional laser amplifier is a solid-state laser amplifier or a fiber laser amplifier; the solid-state laser amplifier includes a gain crystal, a pump source, and a dichroic mirror; the fiber laser amplifier includes a gain fiber, a pump source, and a fiber combiner.

[0012] Preferably, the system further includes at least one cascaded unit, the structure of which is the same as the optical path structure from the polarization beam splitter to the second reflector; the laser output from the fourth port of the polarization beam splitter of the previous stage unit is coupled to the input of the next stage unit after passing through an optical isolator, thereby realizing multi-stage filtering and amplification.

[0013] Preferably, the tuning speed of the bidirectional filter is better than 10 microseconds, the full width at half maximum (FWHM) of the system output laser is less than 1 nm, preferably 0.05 nm to 1 nm; the number of stages N of the pulse separator / synthesizer is 3; the bidirectional laser amplifier is a ytterbium-doped double-clad gain fiber amplifier; and the average output power of the system is greater than 50 W.

[0014] A method for generating a tunable narrowband picosecond laser includes the following steps: S1: Generate a broadband picosecond or femtosecond pulse sequence and inject it into the first port of the polarization beam splitter. After polarization beam splitting, it is output from the second port. S2: The output light sequentially passes through the first Faraday rotator and enters the bidirectional filter. After being reflected by the first mirror, it travels back along the original optical path and passes through the bidirectional filter and the first Faraday rotator again, returning to the second port of the polarization beam splitter. During this process, the bidirectional filter is driven by an electronic control signal to achieve rapid selection of the target wavelength and adjustment of the spectral bandwidth. The returned light is output from the third port of the polarization beam splitter due to a 90° rotation of the polarization state. S3: The light output from the third port is incident on the pulse splitter / synthesizer. The birefringent crystal introduces different group velocity delays to the orthogonally polarized beams, so that a single incident pulse is separated into multiple low peak power sub-pulse sequences with fixed intervals in the time domain. S4: The sub-pulse is amplified by entering the bidirectional laser amplifier. After being reflected by the second mirror, it is reversed along the original optical path and passes through the bidirectional laser amplifier and pulse splitter / synthesizer again, returning to the third port of the polarization beam splitter. During this process, the amplified sub-pulse is resynthesized into a single pulse sequence, and the return light is transmitted and output from the fourth port of the polarization beam splitter due to the polarization state being rotated by 90° again, thus obtaining a tunable high-power narrow-band picosecond laser.

[0015] The beneficial effects of this invention are as follows: This invention provides a tunable narrowband picosecond laser generation system and method. 1. High-speed wavelength tuning capability: By using a tunable filter based on acousto-optic or galvanometer and dispersive medium, the ability to quickly select narrow-band picosecond laser from broadband incident laser is realized, with speeds reaching the microsecond to millisecond level, meeting the needs of applications such as high-speed spectral scanning imaging. 2. High-power narrow-band output: The filtering and pulse amplification functions are combined on the same polarization beamsplitter using different polarization states, simultaneously completing bidirectional filtering and bidirectional amplification. Furthermore, by separating the pulses during amplification, the difficulty of dispersion broadening and compression in narrow-band lasers is effectively overcome, significantly reducing the peak power of the pulses and suppressing spectral broadening caused by nonlinear effects during amplification. This achieves an average output power greater than 50W, with optimal output power exceeding 100W. The full width at half maximum (FWHM) of the output spectrum can be controlled within the range of 0.05nm to 1nm. Combined with a high-speed tunable filter, the wavelength switching speed is better than 10μs, making it suitable for high-speed spectral scanning and nonlinear imaging applications. 3. Wideband tuning capability: The external tuning scheme adopted in this invention allows the wavelength range of the seed light to be not limited to the gain band range. It can obtain a wide spectrum far exceeding the gain band range through supercontinuum, nonlinear amplification and other methods, resulting in a wider wavelength tuning range. 4. High stability: The broadband seed source and fast tuning filter are separated, and the seed source operates in a fixed state, resulting in high stability. The tuning function is completed by an independent external electronic control device, avoiding interference with oscillator stability during the tuning process and preventing the tuning speed from being limited by the mode-locking time during the mode-locking establishment process in the cavity. This scheme can adopt an all-fiber or fiber-spatial hybrid optical path design, with a compact structure and strong environmental stability. 5. High Integration: This invention effectively integrates filtering and pulse amplification through a single polarization beamsplitter, making full use of all ports of the polarization beamsplitter. The bidirectional filtering and amplification design further improves the efficiency of filtering and amplification, enhancing the overall integration of the optical path. 6. High efficiency based on polarization: Traditional bidirectional filtering or amplification often uses a circulator as a coupling device, with light incident at port 1, filtered and reflected at port 2, and emitted at port 3. In this method, the insertion loss between the circulator ports is typically 3dB, with a total insertion loss of about 6dB, resulting in high loss and low efficiency. This invention adopts a polarization-based approach, integrating bidirectional filtering and amplification. By manipulating the polarization state, it achieves near-zero loss optical path folding while realizing filtering and amplification functions, resulting in extremely low overall loss and high coupling efficiency. 7. Excellent scalability: The core module of this invention combines high-speed filtering and high-power amplification. This module uses a polarization beam splitter for both input and output, with both input and output light being polarization-maintaining. This characteristic ensures that the output light can be cascaded through the core module for further filtering and amplification. A multi-stage scalable structure can be designed according to requirements to achieve higher power and narrower spectral band laser output. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the overall structure of the tunable narrowband picosecond laser generation system of the present invention; Figure 2A-2C These are three typical structures of the bidirectional filter of the present invention; Figure 3 This is a schematic diagram of the high-speed, high-power tunable picosecond laser system with an all-fiber structure according to Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the high-speed, high-power tunable picosecond laser system with a spatial structure according to Embodiment 2 of the present invention; Figure 5 This is a schematic diagram of the high-speed, high-power tunable picosecond laser system with a dual-stage cascaded spatial structure, as described in Embodiment 3 of the present invention. Detailed Implementation

[0017] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0018] like Figure 1 The diagram shows the overall structure of the tunable narrowband picosecond laser generation system of the present invention. The system includes: a broadband picosecond seed source module 1, a polarization beam splitter 2, a Faraday rotator 3, a bidirectional filter 4, and a reflector 5 arranged sequentially on the same optical axis; and a pulse splitter / synthesizer 6, a bidirectional laser amplifier 7, a Faraday rotator 8, and a reflector 9 arranged sequentially perpendicular to the optical axis.

[0019] The broadband picosecond seed source outputs seed light into polarization beamsplitter 2. After being split within the beamsplitter, the projected light exits through port 2 (on the same optical axis as the input), then sequentially passes through Faraday rotator 3 and bidirectional filter 4. It is then reflected by mirror 5 and returns along the same path, passing through bidirectional filter 4 and Faraday rotator 3 again, before returning to polarization beamsplitter 2 from port 2. Since the laser polarization state is reversed by 90 degrees compared to the input at port 2 (reflection), the reflected laser exits from port 3 of the polarization beamsplitter. It then sequentially passes through pulse splitter / combiner 6, bidirectional laser amplifier 7, and Faraday rotator 8, and is reflected by mirror 9, returning along the same path. It then again passes through Faraday rotator 8, bidirectional laser amplifier 7, and pulse splitter / combiner 6, returning to port 3 of the polarization beamsplitter. Since the laser polarization state is reversed by 90 degrees compared to the input at port 3, the amplified laser is transmitted and output from port 4 of the polarization beamsplitter.

[0020] The system comprises three modules: a broadband seed light, a bidirectional laser amplifier, and a laser amplifier. The broadband seed light determines the spectral tuning range of the entire system, typically covering hundreds or even thousands of nanometers through nonlinear amplification or supercontinuum broadening. A bidirectional filter (4) determines the spectral tuning speed, achieving sub-millisecond or even tens of microsecond levels through integrated acousto-optic tunable filters. A pulse splitter / synthesizer (6) determines the system's suppression of nonlinear effects; through multi-stage beam splitting, it can generate 2^N sub-pulses, significantly reducing the nonlinearity of each pulse and effectively solving the problem of dispersion broadening in narrow-band lasers. A bidirectional laser amplifier (7) determines the system's output power; through double-clad fiber amplification or solid-state amplification, the average output power can reach hundreds of watts. The tuning range, tuning speed, and output power of the entire system are determined by these three independent modules, ensuring tuning stability.

[0021] Figure 2A , 2B 2C represents three typical structures of the bidirectional filter of this invention. Solid lines represent spatial light, and dashed lines represent electrical signals. Figure 2A , 2B In 2C, Col is short for fiber optic collimator. Figure 2A , 2BBoth 2C and 2C are fiber-coupled structures. If a full-space optical path is used, Col is not needed. The implementation methods of bidirectional filter 4 can be divided into three categories: Figure 2A Implementation method based on acousto-optic tunable filter (AOTF) Figure 2B Based on the separate implementation method of grating and galvanometer, Figure 2C This implementation is based on a combination of gratings and galvanometers. In this embodiment, the specific structures and operating methods of three types of bidirectional filters will be described respectively.

[0022] Figure 2A This is a schematic diagram of a bidirectional filter structure based on AOTF. In this scheme, the bidirectional filter module includes: a collimator (Col), a liquid lens (Lens1), an acousto-optic tunable filter (AOTF), a fixed-focal-length lens (Lens2), and a reflector (M), arranged sequentially on the same optical axis. The AOTF driving module drives the acousto-optic tunable filter (AOTF). Incident light is converted into spatial light by the collimator Col, then collimated by the liquid lens Lens1 before entering the acousto-optic tunable filter AOTF. The AOTF driving module outputs an electrical signal to drive the acousto-optic tunable filter AOTF, causing the incident light to diffract to different angles according to different wavelengths under the influence of the acousto-optic effect. The fixed-focal-length lens Lens2 collimates the spatially divergent diffracted light into parallel light, which is then incident on the reflector M. After reflection by the reflector M, the light is reflected back along the original optical path and re-incidentally into the fixed-focal-length lens Lens2, the acousto-optic tunable filter AOTF, and the liquid lens Lens1. Because AOTFs achieve the highest diffraction efficiency only for light of specific wavelengths at a specific driving frequency, high-speed filtering can be achieved by manipulating the driving frequency of the AOTF. The focal length of the liquid lens Lens1 can change with the applied voltage, thereby altering the spot size and divergence angle of the light entering the AOTF, as well as the spot size returning to the Col. Since the Col typically has a specific numerical aperture and can only receive light with a specific divergence angle, the filtering bandwidth can be changed by manipulating the focal length of the liquid lens Lens1.

[0023] Based on Gaussian beam propagation theory and the momentum matching principle of acousto-optic diffraction, the filter passband bandwidth... With the divergence angle of the incident beam There is a functional relationship .in This refers to the intrinsic bandwidth of AOTF. This refers to the angular dispersion coefficient related to the crystalline material. In this embodiment, the driving voltage of the liquid lens Lens1 is adjusted. V Change its focal length f ( V ), and then according to The relationship between the beam divergence angle entering the AOTF is precisely controlled.

[0024] For example, the collimator Col has a numerical aperture NA = 0.12, corresponding to a maximum receiving half-angle of approximately 6.9°. The focal length f of the liquid lens Lens1 is adjustable from 10 to 50 mm. Narrowband filtering mode: When the voltage is adjusted to change the focal length of the liquid lens... f At larger values ​​(e.g., 40 mm), Lens1's focusing / diverging effect on the beam weakens, the beam entering the AOTF becomes nearly parallel, the spot diameter remains at approximately 2 mm, and the divergence angle... Smaller (approximately 1.5) At this point, according to the above formula, the filtering bandwidth is narrow (e.g., 0.5nm), and the reflected light is easily received by Col; wideband filtering mode: when the voltage is adjusted to make the liquid lens focal length... f When the beam size is reduced (e.g., to 15 mm), Lens1 exerts a strong converging / diverging effect on the beam, causing a drastic change in the beam spot size inside the AOTF crystal (e.g., 1 mm at the incident surface, expanding to 5 mm at the exit surface), resulting in a decrease in the effective beam divergence angle. Increase to 5 The increased divergence angle widens the wavelength range that satisfies the momentum matching condition, thus broadening the filter bandwidth to 3 nm–5 nm. In this way, the passband bandwidth can be continuously adjusted within the range of 0.5 nm–5 nm, and since the response time of a liquid lens is typically less than 10 nm… ms The system has high-speed dynamic bandwidth switching capability.

[0025] Figure 2BThis approach employs a separate implementation of the grating and galvanometer. In this scheme, the galvanometer and grating do not need to be physically bonded together; instead, they are conjugate using two lenses with fixed focal lengths, Lens3 and Lens4. The bidirectional filter module specifically includes: collimator Col1, lens Lens1, galvanometer GS, GS driver module, lens Lens3, lens Lens4, diffraction grating G, liquid lens Lens2, and collimator Col2. The incident light sequentially passes through collimator Col1 for conversion, lens Lens1 for collimation, galvanometer GS for reflection, lens Lens3, lens Lens4, and finally to diffraction grating G. The diffracted light from grating G then reverses its original path, passing sequentially through lens Lens4, lens Lens3, galvanometer GS, and liquid lens Lens2 before entering collimator Col2. The GS driver module is connected to galvanometer GS via wires for control. Incident light is converted into spatial light by collimator Col1, then collimated by lens Lens1 before entering galvanometer GS. The GS drive module outputs an electrical signal to drive galvanometer GS to rotate, causing the incident light to be reflected at a preset angle into the optical path system composed of lenses Lens3 and Lens4 and diffraction grating G. The incident light is then split by diffraction grating G, with different wavelengths diffracted to different spatial positions. The light then travels back along the original optical path through Lens4 and Lens3, and is reflected by galvanometer GS to liquid lens Lens2. After collimation by liquid lens Lens2, it is received by collimator Col2. During this process, the light passes through diffraction grating G and galvanometer GS twice, achieving bidirectional dispersion and frequency-selective filtering. Since galvanometer GS and grating G are conjugate through Lens3 and Lens4, changing the angle of the galvanometer is equivalent to changing the position of the grating. At a specific position, only light of a specific wavelength can achieve the highest diffraction efficiency. Therefore, by controlling the angle of galvanometer GS, high-speed filtering can be achieved. The focal length of the liquid lens Lens2 can be changed with the change of the applied voltage, which in turn changes the spot size and divergence angle of Col2. Since Col2 generally has a specific numerical aperture and can only receive light with a specific divergence angle, the filtering bandwidth can be changed by manipulating the focal length of the liquid lens Lens2.

[0026] Figure 2CThis approach utilizes a combination of gratings and galvanometers. In this scheme, the galvanometer and grating are spatially integrated into a single tuning structure, achieving a more compact optical path layout by reducing the number of relay lenses. The bidirectional filter module specifically includes: a collimator (Col), a lens (Lens), a galvanometer (GS), a GS driver module, a diffraction grating (G), a slit (Slit), and a reflector (M). The collimator (Col), lens (Lens), galvanometer (GS), diffraction grating (G), slit (Slit), and reflector (M) are arranged sequentially along the optical path. The GS driver module is electrically connected to the galvanometer (GS). The incident light is first converted into spatial light by the collimator (Col), then collimated by the lens (Lens) before entering the galvanometer (GS). The GS driver module outputs an electrical signal to drive the galvanometer (GS) to deflect at different angles, causing the incident light to enter the diffraction grating (G) at different angles. Incident light undergoes dispersion at the diffraction grating G, with different wavelengths diffracted to different spatial positions. The target wavelength, satisfying the spatial gating condition, passes through the slit Slit, while other wavelengths are blocked, thus achieving spectral selectivity. The light after passing through the slit Slit is incident on the mirror M, reflected by M, and propagates back along its original path, passing through the slit Slit, diffraction grating G, and galvanometer GS again, finally coupling back to the collimator Col, achieving a bidirectional filtering process. In this process, the light passes through the diffraction grating G twice, achieving two-way dispersion enhancement filtering selectivity. By adjusting the deflection angle of the galvanometer GS, the angle of the beam incident on the grating can be changed, thereby altering the wavelength passing through the slit Slit, achieving rapid wavelength tuning. Simultaneously, the width of the slit Slit determines the spectral range, so by adjusting the slit size, the filtering bandwidth can be controlled. Compared to… Figure 2B The split structure of this scheme is more compact, with fewer components, which is conducive to system miniaturization, while maintaining high-speed tuning capability and high spectral selectivity.

[0027] Figure 3 This is a schematic diagram of an all-fiber high-speed, high-power tunable picosecond laser system according to Embodiment 1 of the present invention. In this embodiment, the system is implemented using an all-fiber structure, specifically including: a semiconductor pump source LD, a wavelength division multiplexer (WDM), a ytterbium-doped gain fiber (Yb), a polarization controller (PS), an output coupler (OC), a chirped fiber Bragg grating (CFBG), an optical isolator (OI), a polarization beam splitter (PBS), a Faraday rotator (FR1), a filter, a Faraday rotator mirror (FM1), a pulse splitter (Divider), a pulse synthesizer (Combiner), a bidirectional fiber amplifier (Yb), a Faraday rotator (FR2), and a Faraday rotator mirror (FM2).

[0028] A semiconductor pump source (LD) provides pump energy to ytterbium-doped fiber (Yb) via a waveguide-derived laser (WDM), forming a passively mode-locked all-fiber seed source. After the polarization state is adjusted by the polarization controller (PS), a broadband picosecond pulse is output via the output coupler (OC). The output pulse undergoes dispersion adjustment via a chirped fiber Bragg grating (CFBG), then passes through an optical isolator (OI) to port 1 of the polarization beamsplitter (PBS), and is output from port 2 of the PBS. It then passes through a Faraday rotator (FR1) and enters a fiber-type filter (Filter). After wavelength selection in the filter, it is reflected by a Faraday rotating mirror (FM1) and returns to the PBS via the filter and Faraday rotator (FR1). Due to a 90° polarization rotation, the laser is output from port 3 of the PBS. The output pulse enters the pulse divider, where it is split into multiple sub-pulses. These sub-pulses then enter the ytterbium-doped fiber amplifier (YB) for bidirectional amplification. After being reflected by the Faraday rotator (FR2) and the Faraday rotating mirror (FM2), the pulse propagates in the opposite direction and passes through the Faraday rotator (FR2), the ytterbium-doped fiber amplifier, and the pulse combiner (Combiner) again, achieving pulse reconstruction. Since the laser polarization state is reversed by 90 degrees compared to the input at port 3, the amplified laser is transmitted through and output from port 4 of the polarization beam splitter (PBS).

[0029] This embodiment adopts an all-fiber structure, which has good environmental stability and high coupling efficiency, and is suitable for high-stability, high-repetition-frequency application scenarios.

[0030] Figure 4 This is a schematic diagram of a high-speed, high-power tunable picosecond laser system with a spatial structure, as shown in Embodiment 2 of the present invention. In this embodiment, the system adopts a free-space optical path structure, specifically including: a saturable absorber SA, a ytterbium-doped crystal Yb:YAG, an output coupler OC, an optical isolator OI, a polarization beam splitter PBS, a Faraday rotator FR1, a lens Lens1, an acousto-optic tunable filter AOTF, an AOTF driver module, a lens Lens2, a mirror M1, a pump source Pump, a pulse splitter Divider, a dichroic mirror DM, a Faraday rotator FR2, a mirror M2, and an output terminal.

[0031] A mode-locked oscillator composed of SA and Yb:YAG generates a broadband picosecond pulse. After being output by the output coupler OC, it passes through the optical isolator OI and enters port 1 of the polarization beamsplitter PBS. From port 2 of the PBS, it passes through the Faraday rotator FR1 and enters lens Lens1. After being collimated by lens Lens1, it is incident on the acousto-optic tunable filter AOTF. The AOTF driver module outputs an electrical signal to drive the AOTF, causing the incident light to diffract at different angles according to different wavelengths under the acousto-optic effect. Lens 2 collimates the spatially divergent diffracted light into parallel light, which is then incident on mirror M1. After reflection by mirror M1, the light is reflected back along the original path and incident again on lens Lens 2, the AOTF, lens Lens 1, and Faraday rotator FR1. Since only light of a specific wavelength can achieve the highest diffraction efficiency at a specific driving frequency of the AOTF, high-speed filtering can be achieved by controlling the driving frequency of the AOTF. Due to a 90° rotation of the polarization state, the laser outputs from port 3 of the polarization beamsplitter (PBS). The output pulse enters the pulse divider (Divider), where it is split into multiple sub-pulses that then enter the dichroic mirror (DM). The pump source (Pump) generates pump light to provide energy to the ytterbium-doped crystal (Yb:YAG). The DM efficiently guides the pump light into the Yb:YAG crystal while simultaneously reflecting the laser generated by the crystal into the main optical path. Together, they achieve the crucial transition from electrical pumping to laser oscillation. After reflection by the Faraday rotator (FR2) and mirror (M2), the light propagates in the opposite direction, passing through the Faraday rotator (FR2), the ytterbium-doped crystal amplifier, the DM, and the divider again for pulse reconstruction. Finally, the synthesized high-power narrow-band pulse returns to the polarization beamsplitter (PBS). Because the laser polarization state is reversed by 90 degrees compared to its input at port 3, the amplified laser is transmitted and output from port 4 of the PBS.

[0032] This embodiment employs a spatial optical path structure, making it suitable for scenarios with higher single-pulse energy and higher peak power output.

[0033] Figure 5This is a schematic diagram of Embodiment 3 of the present invention: a high-speed, high-power tunable picosecond laser system with a dual-stage cascaded spatial structure. In this embodiment, the system consists of two cascaded filtering and amplification units, each stage including a polarization beam splitter (PBS), a Faraday rotator (FR), a filtering module, and an amplification module. To ensure precise overlap and synchronous tuning of the dual-stage filtering wavelengths, this embodiment adopts a "central control + master-slave timing synchronization" strategy. Specifically, the system is configured with a central control unit and a synchronization signal generator: the central control unit sends the same radio frequency setting command to the two-stage AOTF drive modules to ensure that AOTF1 and AOTF2 output radio frequency signals of the same frequency, thereby selecting a consistent center wavelength and achieving wavelength tuning synchronization; the synchronization signal generator outputs two synchronous clock signals to provide a unified time reference for the two-stage drive modules, with the first-stage AOTF drive module as the master and the second-stage as the slave, ensuring that the timing references of the two-stage radio frequency signals are aligned. In addition, considering that there is a fixed physical transmission delay in the transmission of the optical signal from the first stage to the second-stage AOTF2, the second-stage drive module integrates a programmable electronic delay circuit. By using the RF trigger signal of the master end (AOTF1) as a reference, the time delay of the RF signal applied to the slave end (AOTF2) is precisely adjusted so that the establishment time of the acousto-optic grating inside AOTF2 is precisely matched with the time when the optical pulse arrives at the crystal. This eliminates the timing deviation caused by interstage transmission, avoids pulse energy loss and spectral distortion, and ensures that the cascade efficiency of the two-stage filtering is maximized.

[0034] The first-stage structure includes: a saturable absorber SA, a Yb gain medium, an output coupler OC, an optical isolator OI1, a polarization beam splitter PBS1, a Faraday rotator FR1, a lens Lens1, an acousto-optic tunable filter AOTF1, an AOTF driver module, a lens Lens2, a mirror M1, a pump source Pump, a pulse splitter Divider1, a dichroic mirror DM1, a ytterbium-doped crystal Yb:YAG, a Faraday rotator FR2, and a mirror M2. The second-stage structure includes: an optical isolator OI2, a polarization beam splitter PBS2, a Faraday rotator FR4, a lens Lens3, an AOTF driver module, an acousto-optic tunable filter AOTF2, a lens Lens4, a mirror M3, a pulse splitter Divider2, a dichroic mirror DM2, a pump source Pump, a Yb gain medium, a Faraday rotator FR3, and a mirror M4.

[0035] The first stage operates similarly to Example 2, achieving initial filtering and power amplification. The narrow-band laser output from the first stage serves as the input to the second stage. It passes through optical isolator OI2 and enters port 1 of the second-stage polarization beamsplitter PBS2. The output then exits from port 2 of PBS2, passes through Faraday rotator FR4, and enters lens 3. After collimation by lens 3, it is incident on the acousto-optic tunable filter AOTF2. The AOTF driver module outputs an electrical signal to drive the acousto-optic tunable filter AOTF2, causing the incident light to diffract at different angles according to different wavelengths under the acousto-optic effect. Lens 4 collimates the spatially divergent diffracted light into parallel light, which is then incident on mirror M3. After reflection by mirror M3, the light is reflected back along the original path and incident again on lens 4, acousto-optic tunable filter AOTF2, lens 3, and Faraday rotator FR4. Since only light of a specific wavelength can achieve the highest diffraction efficiency at a specific driving frequency of AOTF, high-speed filtering can be achieved by controlling the driving frequency of AOTF. Due to a 90° rotation of the polarization state, the laser outputs from port 3 of the polarization beamsplitter PBS2. The output pulse enters the pulse splitter Divider 2, where it is split into multiple sub-pulses that then enter the dichroic mirror DM2. The pump source Pump generates pump light, providing energy to the gain medium Yb. The dichroic mirror DM2 efficiently guides the pump light into the gain medium Yb while simultaneously reflecting the laser generated by the crystal into the main optical path. These two processes work together to achieve the crucial transition from electrical pumping to laser oscillation. After being reflected by the Faraday rotator FR3 and the mirror M4, the laser propagates in the opposite direction, passing through the Faraday rotator FR3, the dichroic mirror DM2, and the pulse splitter Divider 2 again to achieve pulse reconstruction. Finally, the synthesized high-power narrow-band pulse returns to the polarization beamsplitter PBS2. Because the laser polarization state is reversed by 90 degrees compared to when it was input at port 3, the amplified laser is transmitted and output from port 4 of the polarization beamsplitter PBS2.

[0036] In the second stage, the laser undergoes another bidirectional filtering and amplification process. Through further wavelength selection and pulse separation, the spectral width is effectively compressed and the output power is increased. The final output light is a high-power narrow-band picosecond laser that has undergone two stages of filtering and two stages of amplification.

[0037] This cascaded structure, through the synergistic effect of multi-stage filtering and amplification, can achieve higher power output and narrower spectral bandwidth while maintaining high-speed tuning capability, making it suitable for high-end precision machining and nonlinear optics applications. It is particularly suitable for fields requiring rapid spectral switching, such as nonlinear microscopy, spectral detection, and precision machining.

[0038] The embodiments described above merely illustrate specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A tunable narrowband picosecond laser generation system, characterized in that, Along the direction of light propagation, the system comprises, in sequence: a broadband picosecond seed source module, a four-port polarization beam splitter, a first Faraday rotator, a bidirectional filter, a first reflector, a pulse splitter / synthesizer, a bidirectional laser amplifier, a second Faraday rotator, and a second reflector; the output of the broadband picosecond seed source module is coupled to the first port of the polarization beam splitter. The second port of the polarization beam splitter is sequentially connected to the first Faraday rotator, the bidirectional filter, and the first reflector. The reflected light from the first reflector returns to the second port of the polarization beam splitter along the original path, passing through the bidirectional filter and the first Faraday rotator. After two-way reflection by the first Faraday rotator and the first reflector, the polarization state of the returned light is rotated by 90° relative to the incident light. The third port of the polarization beam splitter is sequentially connected to the pulse splitter / combiner, the bidirectional laser amplifier, the second Faraday rotator, and the second reflector. The reflected light from the second reflector returns to the polarization beam splitter along the original path, passing through the second Faraday rotator, the bidirectional laser amplifier, and the pulse splitter / combiner. The third port of the polarization beam splitter, after two-way reflection by the second Faraday rotator and the second mirror, rotates the polarization state of the returned light by 90° relative to the incident light; the bidirectional filter is configured to perform fast wavelength selection and bandwidth control on the incident light, and the optical path therein is bidirectional; the pulse splitter / synthesizer is configured to use a birefringent crystal to introduce different group velocity delays to the orthogonally polarized beams, so that the incident pulse is separated into multiple sub-pulses in the time domain to reduce the peak power, and then resynthesized into a single pulse through the reverse process after amplification; the bidirectional laser amplifier is configured to amplify the power of the multiple sub-pulses, and the optical path therein is bidirectional; the amplified and synthesized laser is output from the fourth port of the polarization beam splitter by the second rotation of the polarization state.

2. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The broadband picosecond seed source module is a passive mode-locked laser with an all-fiber structure. Its output pulse is obtained by nonlinear amplification after supercontinuum broadening, nonlinear fiber amplification, or dispersion compensation and filtering in photonic crystal fiber, and the full width at half maximum (FWHM) of the spectrum is greater than 60 nm.

3. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The bidirectional filter is constructed based on an acousto-optic tunable filter and includes a collimator, a first liquid lens, an acousto-optic tunable filter, a fixed-focal-length lens, and a first reflector arranged sequentially along the optical path, as well as a driving module electrically connected to the acousto-optic tunable filter; the focal length of the first liquid lens is electrically adjustable to control the filtering bandwidth.

4. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The bidirectional filter is constructed based on a separate structure of grating and galvanometer, including a first collimator, a first lens, a galvanometer, a second lens, a third lens, a diffraction grating, a second liquid lens, and a second collimator arranged in sequence, as well as a driving module electrically connected to the galvanometer; the galvanometer and the diffraction grating are optically conjugated through the second lens and the third lens, and the focal length of the second liquid lens is electrically adjustable to control the filtering bandwidth.

5. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The bidirectional filter is constructed based on a combination structure of a grating and a galvanometer, including a collimator, a lens, a galvanometer, a diffraction grating, a slit, and a first reflecting mirror arranged in sequence, as well as a driving module electrically connected to the galvanometer; the deflection angle of the galvanometer is used to achieve wavelength tuning, and the width of the slit is used to control the filtering bandwidth.

6. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The pulse separator / synthesizer is composed of a polarization beam splitter crystal or a birefringent crystal combined with a waveplate, and has N stages. It is used to separate an incident single pulse into 2^N sub-pulses according to its polarization state.

7. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The bidirectional laser amplifier is a solid-state laser amplifier or a fiber laser amplifier; the solid-state laser amplifier includes a gain crystal, a pump source, and a dichroic mirror; the fiber laser amplifier includes a gain fiber, a pump source, and a fiber combiner.

8. The tunable narrowband picosecond laser generation system according to any one of claims 1 to 7, characterized in that, The system also includes at least one cascaded unit, the structure of which is the same as the optical path structure from the polarization beam splitter to the second reflector; the laser output from the fourth port of the polarization beam splitter of the previous stage unit is coupled to the input of the next stage unit after passing through an optical isolator, thereby realizing multi-stage filtering and amplification.

9. The tunable narrowband picosecond laser generation system according to claim 1, characterized in that, The tuning speed of the bidirectional filter is better than 10 microseconds, and the full width at half maximum (FWHM) of the system output laser is less than 1 nm, preferably 0.05 nm to 1 nm; the number of stages N of the pulse separator / synthesizer is 3; the bidirectional laser amplifier is a ytterbium-doped double-clad gain fiber amplifier; and the average output power of the system is greater than 50 W.

10. A method for generating a tunable narrow-band picosecond laser, characterized in that, Includes the following steps: S1: Generate a broadband picosecond or femtosecond pulse sequence and inject it into the first port of the polarization beamsplitter. After polarization beam splitting, it is output from the second port. S2: The output light sequentially passes through the first Faraday rotator into the bidirectional filter, is reflected by the first mirror, and then travels back along the original optical path through the bidirectional filter and the first Faraday rotator, returning to the second port of the polarization beamsplitter. During this process, the bidirectional filter is driven by an electronic control signal to achieve rapid selection of the target wavelength and adjustment of the spectral bandwidth. The returned light is output from the third port of the polarization beamsplitter due to a 90° rotation of the polarization state. S3: The light output from the third port is incident on the pulse splitter. The splitter / combiner uses a birefringent crystal to introduce different group velocity delays to orthogonally polarized beams, causing a single incident pulse to be separated into multiple low-peak-power sub-pulse sequences with fixed intervals in the time domain; S4: the sub-pulses are amplified by a bidirectional laser amplifier, reflected by the second mirror, and then back along the original optical path through the bidirectional laser amplifier and the pulse splitter / combiner, returning to the third port of the polarization beam splitter; during this process, the amplified sub-pulses are recombined into a single-pulse sequence, and the returning light is transmitted and output from the fourth port of the polarization beam splitter due to the polarization state rotating by 90° again, thus obtaining a tunable high-power narrow-band picosecond laser.

Images