A compact low-repetition high-energy all-fiber femtosecond laser and a method for realizing the same

Abstract

This invention discloses a compact, low-repetition-rate, high-energy all-fiber femtosecond laser and its implementation method. Based on a large-mode-field fiber structure, it achieves high-energy, broadband, tunable femtosecond output. Unlike conventional solutions that involve complex processes and large systems such as mode-locking, frequency selection, multi-stage amplification, and grating compression to achieve low repetition rate and high energy, this invention provides a more integrated solution for clinical applications in nonlinear optical imaging. It achieves both high compactness and wide-band high-performance output by relying solely on two modules: a low-repetition-rate, high-energy pulse and a frequency-shifting fiber platform. Specifically, a large-mode-field fiber dual-gain-arm Mamyshev oscillator is constructed to achieve a 4.87 MHz low-repetition-rate, 10 nJ high-energy seed output. Combined with large-mode-field fiber Raman amplification and soliton self-frequency shifting technology, a wide-range continuously tunable femtosecond output of 1600–1900 nm is achieved, with a maximum single-pulse energy of 103.7 nJ. The frequency doubling module, based on a fan-shaped periodically polarized lithium niobate crystal, can be extended to the 800–950 nm band.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast fiber laser technology, specifically relating to an ultra-compact near-infrared II-zone all-fiber femtosecond laser system with low repetition rate, high pulse energy, and wide-band tunable characteristics. In particular, it relates to a low-repetition-rate, high-energy all-fiber femtosecond laser generation device and method based on a large-mode-field fiber Mamyshev oscillator seed source combined with a large-mode-field fiber Raman amplification / soliton self-frequency shifting platform. Background Technology

[0002] In recent years, ultrafast fiber lasers have been widely used in multiphoton microscopy, nonlinear optical processing, and biomedical imaging. In particular, the near-infrared IIb and IIc window bands of 1600–1880 nm have the advantages of low tissue scattering and large penetration depth.

[0003] Existing traditional methods for realizing femtosecond lasers in this band include four-wave mixing, supercontinuum broadening, thulium-doped fiber amplification, and soliton self-frequency shifting. However: 1. Four-wave mixing has high requirements for phase matching and time synchronization, making the system complex; 2. Supercontinuum light sources have insufficient coherence and extremely low power density; 3. The thulium-doped system has insufficient gain near 1.7 μm, limiting its tunability; 4. Existing soliton self-frequency shifting systems mostly operate at high repetition frequencies of tens of MHz, which requires increasing the average power to obtain sufficient pulse energy, thereby causing photothermal damage to biological samples.

[0004] Therefore, as shown in Table 1, for low-repetition-rate, high-energy femtosecond light sources in this band, commercial solutions (such as Thorlabs, Class 5 Photonics, and Calmar Laser) typically employ extremely complex solid-state optical parametric amplification systems, with the wavelength fixed only at a selected wavelength within the 1600–1700 nm range. Traditional low-repetition-rate fiber laser solutions, on the other hand, involve extremely complex implementation processes and massive system structures, including mode-locking, frequency selection, multi-stage amplification, and grating compression. All of these laser systems directly contradict the compact and portable requirements of clinical applications.

[0005] Therefore, achieving a low-repetition-rate, high-energy, wide-band tunable ultracompact near-infrared II all-fiber femtosecond laser system for clinical applications remains a significant technical challenge. Consequently, there is an urgent need to develop a novel approach that simultaneously achieves a compact and portable structure, high system stability, and comprehensive and excellent output performance. Table 1 Comparison between this work and traditional solutions type Technical solution System Structure Output wavelength Single pulse energy Pulse width repetition frequency Summarize Soleber Commercial Products Nonlinear crystal optical parametric amplification Multi-stage amplification + semi-solid laser 1600 nm – 1700 nm (single fixed wavelength) 500 nJ 50 fs 1–4 MHz Extremely complex high energy Supercontinuum Nonlinear excitation All-fiber + multi-stage amplification 1000 nm – 2300 nm 20 nJ (extremely low power density) 850 fs 50 MHz More complex low energy Thulium-doped system Rare earth ion gain All-fiber + multi-stage amplification + pulse width compression 1760 nm – 1790 nm (small range) 100 nJ 100 fs 20–60MHz More complex high energy Orphan self-shift Rod-shaped photonic crystal fiber Non-all-fiber + frequency selection + multi-stage amplification 1620 nm 73 / 75 nJ 73 / 70 fs 1 MHz Complex high energy Orphan self-shift High-frequency seed + 17 μm core fiber All-fiber multi-stage amplification + pulse width compression 1600 nm – 2040 nm 6 nJ 80 fs 80.96 MHz More complex low energy This paper's work Low-frequency seed + 25 μm core fiber All-fiber single-stage amplification 1600 nm – 1900 nm 103.7 nJ 145 fs 4.87 MHz Ultra-compact high-energy Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a femtosecond laser system that is compact, all-fiber, low repetition frequency (<5 MHz), high pulse energy (>100 nJ), wide-band tunable (1600–1900 nm), and can be extended to 800–950 nm by frequency doubling, so as to solve the problems of high phototoxicity risk, high system complexity and limited tuning range caused by high repetition frequency in the prior art.

[0007] To address the aforementioned technical problems, this invention proposes an all-fiber femtosecond laser system comprising a low-repetition-rate Mamyshev oscillator based on a large-mode-field fiber, a large-mode-field Raman amplification module, and a soliton self-frequency shifting module. This system reconstructs the Mamyshev oscillator cavity structure and utilizes the high nonlinear tolerance of the large-mode-field fiber to extend the cavity length, achieving a low repetition-rate output on the order of MHz while ensuring stable pulse evolution within the cavity. Subsequently, leveraging the high nonlinear tolerance of the large-mode-field fiber, stimulated Raman scattering is triggered during amplification to form Raman solitons, achieving a low repetition rate, higher energy, and continuously tunable soliton self-frequency shifting output in the near-infrared II region.

[0008] In terms of specific structure, the laser system of this invention includes a dual-gain-arm Mamyshev oscillator module based on a large-mode-area fiber and a large-mode-area Raman amplification and soliton self-frequency shifting module. The Mamyshev oscillator employs a dual erbium-doped gain-arm structure, with erbium-doped fiber used as the gain medium in each gain arm, and pump light injected through a 980 nm wavelength division multiplexer. Two bandpass filters with misaligned center wavelengths are placed within the cavity to form an equivalent saturable absorption mechanism for generating mode-locked pulses. A self-made, simple Q-switched fiber laser is used as a seed for initiation; a small amount of power injected into the oscillator generates sufficient disturbance to assist in the self-starting of mode-locked oscillation. The transmission medium is a large-mode-area fiber segment with a core diameter greater than 25 μm to significantly improve the cavity's nonlinear tolerance and achieve cavity length extension and repetition frequency reduction. By rationally designing the cavity length to approximately 40–50 m, a fundamental frequency repetition rate of approximately 4–5 MHz can be obtained. In a stable mode-locked state, the oscillator can output femtosecond pulses with a center wavelength of approximately 1555 nm, a pulse width of < 300 fs, an average output power of > 50 mW, and a single pulse energy of over 10 nJ, providing sufficient peak power for the subsequent nonlinear frequency shift process.

[0009] The large-mode-field fiber Raman amplification and soliton self-frequency shifting module includes Er / Yb co-doped large-mode-field active fiber and a matching large-mode-field passive fiber. The passive fiber segment is coiled in an inward spiral, with gradually increasing curvature helping to compress the core mode area, thus slowly increasing the equivalent nonlinear effect to counteract premature frequency shifting and achieve a larger frequency shift. The seed pulse is directly injected into the large-mode-field active fiber after passing through a polarization controller without any complex modulation, and achieves power amplification under the action of a 915 nm pump light. As the peak pulse power increases, strong self-phase modulation and strong stimulated Raman scattering effects are generated in the fiber, the spectrum expands towards longer wavelengths, and Raman solitons are formed. These solitons then continue to evolve in the passive large-mode-field fiber, achieving soliton self-frequency shift and obtaining continuously tunable all-fiber femtosecond output in the range of 1600–1900 nm. In a preferred embodiment, the system can obtain single-pulse energy exceeding 100 nJ, pulse width < 150 fs, and repetition frequency maintained in the range of < 5 MHz.

[0010] Furthermore, this invention can also incorporate a continuously tunable frequency doubling module at the output of an all-fiber femtosecond laser to extend its wavelength coverage. The frequency doubling module preferably employs a periodically polarized lithium niobate crystal with a fan-shaped periodic structure. By adjusting the crystal polarization period to match the incident wavelength, second harmonic conversion of the 1600–1900 nm fundamental frequency light is achieved, thereby obtaining femtosecond output in the 800–950 nm range. This structure eliminates the need for optical path realignment; wavelength tuning can be achieved simply by adjusting the pump power or crystal position, resulting in high system integration.

[0011] Compared with existing technologies, this invention has the following significant technical advantages. First, it directly achieves low repetition rate, high-energy seed output using a large-mode-field fiber Mamyshev oscillator, eliminating the need for pulse selection or multi-stage pre-amplification structures, significantly reducing system complexity. Second, the large-mode-field fiber structure effectively suppresses nonlinear instability, enabling stable high single-pulse energy output even at low repetition rates. Third, while maintaining an all-fiber structure, the system achieves continuously tunable low repetition rate, high-energy output of 1600–1900 nm, covering the near-infrared II band, suitable for nonlinear optical imaging applications such as novel deep multiphoton microscopy. It can also be frequency-doubled to extend to the near-infrared I band, covering the excitation windows commonly used in traditional multiphoton microscopy. Compared to high-repetition-rate sources (e.g., 50 MHz), the reduced repetition rate increases the single-pulse peak power by an order of magnitude, significantly lowering the average power required to achieve the same excitation efficiency, thereby effectively reducing the risk of phototoxicity and thermal damage to biological samples.

[0012] In summary, this invention achieves comprehensive optimization in terms of system compactness, stability, tunability, and low repetition rate with high energy output, and has good prospects for engineering applications and clinical translation potential, especially suitable for deep multiphoton fluorescence imaging and related biophotonics applications. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of the femtosecond laser system light source proposed in this invention, which features a compact structure, all-fiber optics, low repetition frequency, high pulse energy, and wide-band tunability.

[0014] Figure 2 This is a schematic diagram of a continuously tunable frequency doubling system. WP: waveplate; fan-out PPLN: periodically polarized lithium niobate crystal with a fan-shaped structure; L1 and L2: lenses; DM: dichroic mirror; BPF: bandpass filter; gray squares represent displacement stages.

[0015] Figure 3 The output parameters of the dual-gain arm Mamyshev oscillator based on large-mode-field fiber are as follows: (a) is the output spectrum with a center wavelength of 1555 nm; (b) is the pulse sequence; (c) is the RF spectrum with a pulse signal-to-noise ratio of 70.1 dB and a repetition frequency of 4.875 MHz; and (d) is the pulse width, which is approximately 266 fs.

[0016] Figure 4 The output spectrum and corresponding pulse width of the soliton self-frequency shift after large-mode Raman amplification and soliton self-frequency shift module are shown at different pump powers. The tuning range is 1600–1900 nm and the pulse width is about 150 fs.

[0017] Figure 5 The output characteristics after large-mode Raman amplification and soliton self-frequency shifting are shown in the following figures: (a) Output wavelength and output power curves under different pump powers, with a maximum single pulse energy of 103.7 nJ; (b) RF spectrum of the soliton self-frequency shift pulse, with a signal-to-noise ratio of 60.2 dB and a repetition frequency of 4.875 MHz; (c) Spectral stability of the soliton self-frequency shift over 2 hours, with the spectrum recorded every 30 minutes; (d) Maximum output power stability test obtained by continuous measurement over 500 minutes, with an RMS of approximately 1.08%.

[0018] Figure 6 The output characteristics after frequency doubling are shown in (a) and (b) are the tunable frequency doubling light spectrum, with a tunable range of 800–950 nm; the pulse width is approximately 219 fs. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments. Example 1

[0020] This invention proposes an ultra-compact, all-fiber, low-repetition-rate, high-pulse-energy, wide-band tunable femtosecond laser. It comprises a bandpass fiber filter 1 with a center wavelength of 1560 nm, a 976 nm pump source 2, a fiber wavelength division multiplexer 3, an erbium-doped gain fiber 4, a fiber isolator 5, a wavelength-tunable bandpass fiber filter 6, a fiber combiner 7, a 1550 nm Q-switched pulse source 8, a large mode area passive fiber 9 with a core of 25 μm, a 976 nm pump source 10, a fiber wavelength division multiplexer 11, an erbium-doped gain fiber 12, a fiber isolator 13, a polarization controller 14, and a fiber output coupler 15, all directly fused together to form a large mode area fiber Mamyshev oscillator module. By adjusting the wavelength spacing and polarization state of the two bandpass filters, opening the Q-switched seed pulse, and starting the pump source, low-repetition-rate, high-energy mode-locked pulse self-starting can be achieved.

[0021] At the output end of the fiber optic output coupler 15, a fiber optic isolator 16 and a polarization controller 17 are fused together to block the return light from the next module and flexibly adjust the laser polarization state. The large-mode-area fiber amplification and soliton self-frequency shifting module is directly fused together with a 915nm pump source 18, a high-power fiber combiner 19, a 25 μm core erbium-ytterbium co-doped large-mode-area gain fiber 20, and a 25 μm core large-mode-area passive fiber 21. Finally, the input end of the high-power fiber combiner 19 is directly fused to the output end of the polarization controller 17, thus realizing an all-fiber integrated structure for the two modules. At this point, by starting and adjusting the power of the 915 nm pump source from 5.1 W to 8.9 W, a broadband, continuously tunable, low-repetition-frequency, high-pulse-energy all-fiber femtosecond laser can be directly output.

Claims

1. A compact, low-repetition-rate, high-energy all-fiber femtosecond laser, characterized in that, include: The combination of a dual-gain arm Mamyshev oscillator module based on large mode area fiber and a Raman amplification and soliton self-frequency shifting module based on large mode area fiber achieves a balance between high compactness, wide wavelength tunability, and high energy output.

2. The compact low-repetition-rate high-energy all-fiber femtosecond laser according to claim 1, characterized in that, The dual-gain arm Mamyshev oscillator module of the large mode area fiber uses a large mode area fiber with a core diameter of 25–30 μm as the transmission fiber. Structurally, it adopts a dual-gain arm form with a total cavity length of more than 40 m. It can achieve all-fiber single-mode direct output with a repetition frequency of < 5 MHz, a single pulse energy of >10 nJ, and a pulse width of < 300 fs.

3. The compact low-repetition-rate high-energy all-fiber femtosecond laser according to claim 1, characterized in that, The Raman amplification and soliton self-frequency shifting module of the large mode area fiber uses erbium-ytterbium co-doped large mode area gain fiber with a core diameter of 25–30 μm as the amplifier gain medium. Then, a matching large mode area transmission fiber with a core diameter of 25–30 μm is directly fused together as the Raman gain medium. This transmission fiber adopts an inward spiral winding method and has a total length greater than 20 m. It is directly fused and spliced ​​with the Mamyshev oscillator, which can achieve continuous tunability of 1600–1900 nm, pulse repetition frequency < 5 MHz, single pulse energy greater than 100 nJ, and pulse width < 150 fs all-fiber single-mode direct output.

4. A method for realizing the compact low repetition rate high energy all-fiber femtosecond laser according to claim 1, characterized in that, include: Step 1: Construct an all-fiber low-repetition-rate Mamyshev oscillator based on a long-mode-field fiber. By adjusting the difference in the passband wavelengths of the two filters and adjusting the pump power of the gain arm to 600 mW, a low-repetition-rate, high-energy single-pulse mode-locked laser in the communication band is achieved. Step 2: The high-energy laser output from Step 1 is directly injected into an all-fiber Raman amplification and soliton self-frequency shifting platform built on a large-mode-field fiber. By gradually increasing the pump power from 5.1 W to 8.9 W, a broadband, continuously tunable low-repetition-rate, high-energy femtosecond output in the 1600–1900 nm range is achieved.

5. The implementation method according to claim 4 can be further extended to a third step, characterized in that: A tunable frequency doubling module is constructed using a periodically polarized lithium niobate crystal with a fan-shaped periodic structure. The crystal length is < 2 mm and the polarization period range is 21–35 μm. By matching the incident laser wavelength with the fan-shaped period through an electric displacement stage, continuous tunable frequency doubling in the 800–1200 nm band can be achieved. Coupled into the aforementioned all-fiber femtosecond laser into this frequency doubling module, continuous tunable low repetition rate high energy femtosecond laser output in the 800–950 nm band can be realized.

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