Mmyshev oscillator based on large-bandwidth filter and high nonlinear arm

Abstract

The invention discloses a compact Mmyshev oscillator designed based on a large-band wide-band pass filter and a high-nonlinearity passive arm, and belongs to the field of ultrafast laser and nonlinear optics. Ultra-short pulse generation of the oscillator is based on spectral broadening caused by self-phase modulation and the spectral filtering effect of the band-pass filter. The optical fiber oscillator mainly comprises two parts which are respectively a gain arm and a high nonlinear passive arm formed by a nonlinear optical fiber. The use of the large-band wide-band pass filter improves the pulse energy after filtering, and the passive arm is combined to realize effective spectrum broadening under the structure of a single gain arm, and reduce the mode locking threshold. The large-band wide-band-pass filter allows a shorter passive arm length to reduce the loss in a resonant cavity, enhance the tolerance to nonlinear phase shift and improve the output performance of the oscillator. The Mmyshev ultrashort pulse fiber laser oscillator provided by the invention is lower in cost, more compact in structure, excellent in performance and wider in applicable wavelength range.

Description

Technical Field

[0001] The invention patent belongs to the field of ultrafast laser and nonlinear optics technology, and specifically relates to a Mamyshev type ultrashort pulse fiber oscillator with a single gain arm structure. Background Art

[0002] Ultrafast fiber lasers have been widely used in optical communications, environmental monitoring, biomedical imaging and other fields due to their excellent pulse performance and beam quality. Passively mode-locked fiber lasers based on natural saturable absorbers and artificial saturable absorbers are currently a common method for generating ultrafast pulsed lasers. However, the low tolerance to nonlinear phase shifts of the above technologies makes it difficult to output high-energy mode-locked pulses. Mamyshev oscillators have been proven to be an effective solution for generating high-energy ultrashort pulses. The mode-locking mechanism of the Mamyshev oscillator is based on the spectral broadening caused by self-phase modulation and the spectral filtering effect of the intracavity bandpass filter. Benefiting from its unique mode-locking mechanism, the Mamyshev oscillator can suppress continuous light while also having extremely high tolerance to nonlinear effects in the cavity, and can output high-energy ultrafast mode-locked pulses.

[0003] The traditional Mamyshev oscillator adopts a dual-gain arm structure, and the pulse can undergo two amplification processes in the cavity, thereby outputting high-energy pulses. However, this dual-gain arm structure also has its limitations: compared with traditional passively mode-locked fiber lasers, the dual-gain arm design not only increases the complexity of the system, but also increases its cost.

[0004] In response to the problem of complex structure and high cost of Mamyshev oscillator, scientists proposed a Mamyshev oscillator based on a single gain arm ring cavity structure. By replacing a section of gain fiber in the traditional dual gain arm Mamyshev oscillator with a single-mode transmission fiber, the system complexity can be reduced while having excellent output performance. For example, in the single gain arm MO design adopted in the literature (Optics Letters, 2024, 49(22): 6397-6400.), a small bandwidth filter with a bandwidth of 3nm is connected after the gain fiber. The smaller filter bandwidth will cause the pulse energy in the cavity to be significantly reduced after passing through the filter, so the nonlinear effect is not strong enough, and it is difficult to obtain sufficient nonlinear spectrum broadening in the subsequent passive fiber. Therefore, this design uses a 58-meter-long nonlinear fiber to obtain the necessary pulse broadening. However, the overly long high nonlinear fiber not only reduces the tolerance of MO to nonlinear phase shift, but also increases the intracavity loss, resulting in poor output performance (pulse width 2.8ps, pulse energy 0.56 nJ, peak power 0.23 kW). An important innovation of the present invention is the use of a large bandwidth filter (bandwidth 19nm) design connected between the gain fiber and the nonlinear fiber to ensure that the pulse can still maintain sufficient pulse energy after passing through the filter, thereby effectively achieving spectral broadening. In addition, the introduction of the large bandwidth filter enables us to use a shorter high nonlinear fiber (7m) in Example 1, while reducing the intracavity loss and improving the tolerance to nonlinear phase shift, significantly improving the output performance (pulse width 113 fs, energy 2.7 nJ, peak power 20.3 kW).

[0005] The work in the literature (Optics Letters, 2022, 47(4): 762-765.) uses a small bandwidth filter, but through the design of double-clad ytterbium-doped fiber with high pump power, the spectrum of the 1 μm band can still be broadened in the passive arm. Due to the anomalous dispersion characteristics of double-clad erbium-doped fiber, it is not suitable as a gain medium for a 1.5 μm single-gain arm Mamyshev oscillator. The innovative design of the large bandwidth bandpass filter adopted by the present invention can also be applied to their work. Using single-mode gain fiber, the Mamyshev oscillator can also obtain sufficient spectral broadening at lower pump power and show better output performance. Their double-clad fiber can also be replaced with a 1-micron single-mode gain fiber, that is, the present invention can be extended to other bands and improve performance, but their method cannot be extended to 1.5 μm.

[0006] The work shown in the literature (Optics Letters, 2020, 45(12): 3317-3320.) has good performance. However, in the linear cavity Mamyshev oscillator, the pulse undergoes two amplification processes in the cavity, so it can still be classified as a traditional dual-gain arm Mamyshev oscillator. The study used two circulators (the present invention does not require any circulators) and a 1480nm light source (while the present invention can be pumped with a cheap 976nm light source), which significantly increased the system cost. The innovation of using a large bandwidth filter in our work can also be extended to their structure, that is, introducing a large bandwidth filter design to replace a narrowband filter in the linear cavity, which can not only shorten the length of the gain fiber, but also achieve sufficient spectral broadening, thereby further improving the output performance of the system.

[0007] The above-mentioned single-gain-arm Mamyshev oscillator uses two narrow-linewidth bandpass filters, which makes it difficult for the pulse to obtain sufficient spectral broadening in the passive arm. This results in the pump power being too high when the single-gain-arm Mamyshev oscillator is working, and it is impossible to obtain mode-locked pulses at lower pump powers, making it difficult to obtain widespread application. By introducing a large-bandwidth filter design, the patent of this invention allows the use of shorter nonlinear optical fibers in single-arm Mamyshev oscillators, which not only reduces intracavity losses, but also improves tolerance to nonlinear phase shifts, significantly improving output performance. Previous work has used narrowband filters for both 1 micron and 1.5 microns.

[0008] With regard to the problem of excessively high pump power of a single-gain-arm Mamyshev oscillator in the related art, no effective solution has been proposed so far. Summary of the invention

[0009] In view of the problem that the pump power threshold of a single-gain-arm Mamyshev oscillator in the related art is too high, the present invention provides a compact Mamyshev oscillator design based on a large-bandwidth bandpass filter and a highly nonlinear passive arm to solve the above problem.

[0010] In order to solve the above technical problems, the present invention puts a section of high nonlinear optical fiber into the passive arm of the Mamyshev oscillator to form a high nonlinear passive arm, so as to achieve sufficient spectral broadening between the two filters. At the same time, a large bandwidth bandpass filter is set after the gain arm to allow the pulse to still have a sufficiently high pulse energy when propagating in the high nonlinear passive arm after passing through the filter to achieve sufficient spectral broadening, and reduce the system's mode-locking threshold to achieve mode-locked pulse output under lower pump power. In the traditional dual-gain arm Mamyshev oscillator, the bandpass filter placed after the gain arm usually has a smaller filtering bandwidth (2-5 nm). When the pulse passes through the above bandpass filter, the pulse energy is significantly reduced, making it difficult to obtain sufficient broadening to pass through the second filter. Therefore, in the traditional dual-gain arm Mamyshev oscillator, the second gain arm is used to increase the pulse energy to obtain sufficient spectral broadening. The present invention adopts the design of a large bandwidth bandpass filter and a high nonlinear passive arm, while making the structure more compact, while increasing the energy of the pulse after passing through the filter, ensuring that the pulse obtains sufficient broadening in the subsequent passive arm, and reducing the pump power required to maintain intracavity oscillation.

[0011] In one embodiment, the oscillator comprises: a forward pump laser, a reverse pump laser, an ultrashort pulse seed laser, a gain arm and a passive arm; the tunable fiber bandpass filter (11), the seed input coupler (13), the first wavelength division multiplexer (2), the gain fiber (3), the second wavelength division multiplexer (4), the fiber isolator (6), and the output coupler (7) are sequentially connected to form the gain arm, the pump end of the first wavelength division multiplexer (2) is connected to the output end of the forward pump laser (1), and the pump end of the second wavelength division multiplexer (4) is connected to the output end of the reverse pump laser (5); the wide bandwidth bandpass filter (8), the polarization controller (9), and the high nonlinear fiber (10) are sequentially connected to form the passive arm; the output end of the seed injection coupler (13) is connected to the output end of the reverse pump laser (5); the high bandwidth bandpass filter (8), the polarization controller (9), and the high nonlinear fiber (10) are sequentially connected to form the passive arm; the output end of the seed injection coupler (13) is connected to the output end of the reverse pump laser (5); the high bandwidth bandpass filter (8), the polarization controller (9), and the high nonlinear fiber (10) are sequentially connected to form the passive arm; the output end of the seed injection coupler (13) is connected to the output end of the reverse pump laser (5); the high nonlinear fiber (10 ... The input end is connected to the output end of the ultrashort pulse seed laser (12); the output end of the output coupler (7) is connected to the input end of the large bandwidth bandpass filter (8); the output end of the high nonlinear optical fiber (10) is connected to the input end of the tunable optical fiber bandpass filter (11), forming a closed loop; wherein, after the forward pump laser (1), the reverse pump laser (5) and the ultrashort pulse seed laser (12) are connected, the ultrashort pulse seed laser (12) generates an initial ultrashort seed pulse, the initial ultrashort seed pulse enters the gain arm through the seed injection coupler (13), the pulse enters the passive arm through the large bandwidth bandpass filter (8) after being amplified in the gain arm, and the pulse enters the gain arm again through the seed injection coupler (13) after propagating in the passive arm. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a schematic diagram of the structure of the single-gain-arm Mamyshev ultrashort pulse fiber oscillator proposed in the present invention.

[0013] Figure 2 (a) is a schematic diagram of the output pulse spectrum of Example 1 of the present invention.

[0014] Figure 2 (b) is a schematic diagram of the output pulse time series of Example 1 of the present invention.

[0015] Figure 2 (c) is a schematic diagram of the output pulse spectrum of Example 1 of the present invention.

[0016] Figure 2 (d) is a schematic diagram of the output pulse width of Example 1 of the present invention. DETAILED DESCRIPTION

[0017] The present invention is further described below through embodiments in conjunction with the accompanying drawings. Example 1

[0018] The present invention proposes a single-gain-arm Mamyshev ultrashort pulse fiber oscillator based on a large-bandwidth bandpass filter design, the components of which include: a forward pump laser, a reverse pump laser, an ultrashort pulse seed laser, a gain arm and a passive arm; the tunable fiber bandpass filter (11), a seed input coupler (13), a first wavelength division multiplexer (2), a gain fiber (3), a second wavelength division multiplexer (4), a fiber isolator (6) and an output coupler (7) are sequentially connected to form the gain arm, the pump end of the first wavelength division multiplexer (2) is connected to the output end of the forward pump laser (1), and the pump end of the second wavelength division multiplexer (4) is connected to the output end of the reverse pump laser (5); the large-bandwidth bandpass filter (8), a polarization controller (9) and a high nonlinear fiber (10) are sequentially connected to form the passive arm; the seed injection The input end of the coupler (13) is connected to the output end of the ultrashort pulse seed laser (12); the output end of the output coupler (7) is connected to the input end of the large bandwidth bandpass filter (8); the output end of the high nonlinear optical fiber (10) is connected to the input end of the tunable optical fiber bandpass filter (11), forming a closed loop; wherein, after the forward pump laser (1), the reverse pump laser (5) and the ultrashort pulse seed laser (12) are connected, the ultrashort pulse seed laser (12) generates an initial ultrashort seed pulse, the initial ultrashort seed pulse enters the gain arm through the seed injection coupler (13), the pulse enters the passive arm through the large bandwidth bandpass filter (8) after being amplified in the gain arm, the pulse enters the gain arm again through the seed injection coupler (13) after propagating in the passive arm, and a resonant pulse is formed after multiple cycles.

[0019] In this embodiment, the forward pump laser (1) and the reverse pump laser (5) have a central wavelength of 976nm and a maximum output power of 600mW; the first wavelength division multiplexer (2) and the second wavelength division multiplexer (4) are 980 / 1550nm wavelength division multiplexers respectively; the gain fiber (3) is an erbium-doped fiber, model EDFC-980HP, and the gain fiber length is 10m; the output coupler (7) and the seed input coupler (13) have a splitting ratio of 50:50 respectively; the wide bandwidth bandpass filter (8) has a central wavelength of 1570nm and a 3dB bandwidth of 19nm; the high nonlinear fiber (10) is model UHNA4, and the fiber length is 7m; the tunable fiber bandpass filter has a central wavelength of 1557.8nm and a 3dB bandwidth of 3nm; the ultrashort pulse seed laser (12) is a self-made NPR-based soliton fiber mode-locked laser, with an output pulse width of 500fs and a spectrum central wavelength of 1565nm.

[0020] In this embodiment, when the output powers of the forward pump laser (1) and the reverse pump laser (5) are 120 mW respectively, the ultrashort pulse seed laser (12) is turned on, the initial pulse is injected into the cavity through the seed input coupler (13), and the polarization controller (9) is adjusted to achieve a stable mode-locked pulse with a 10 dB spectrum width of 70.4 nm, a repetition frequency of 7.36 MHz, a signal-to-noise ratio of 68 dB, a pulse width of 113 fs, an average output power of 16.68 mW, and a single pulse energy of 2.27 nJ, thus having excellent output performance.

[0021] The wide bandwidth bandpass filter and high nonlinear passive arm design adopted by the present invention allow sufficient spectrum broadening to be achieved at lower pump power, showing better performance. The design can also be extended to other wavelengths, showing wider applicability. The compact Mamyshev oscillator design based on the wide bandwidth filter and the high nonlinear arm proposed by the present invention can also be applied to the linear cavity Mamyshev oscillator structure. Replacing one of the narrow bandwidth filters with the wide bandwidth bandpass filter can not only shorten the gain fiber length, but also reduce the cost of the system and further improve the performance of the system.

[0022] By introducing a compact Mamyshev oscillator design based on a large bandwidth filter and a highly nonlinear arm, our invention allows the use of shorter nonlinear fibers in the Mamyshev oscillator, reduces cavity losses while increasing the system's tolerance to nonlinear phase shifts, significantly improving output performance.

Claims

1. A Mamyshev oscillator based on a large bandwidth filter and a highly nonlinear arm, characterized in that: The oscillator comprises: a forward pump laser, a reverse pump laser, an ultrashort pulse seed laser, a wide bandwidth bandpass filter, a high nonlinear optical fiber, a gain optical fiber, a bandpass filter, an isolator, a seed injection coupler and an output coupler. The forward pump laser, the reverse pump laser, the bandpass filter and the gain medium constitute a gain arm; the wide bandwidth bandpass filter and the high nonlinear optical fiber constitute a high nonlinear passive arm. The seed injection coupler is used to inject an ultrashort pulse seed laser to start the pulse operation; the output coupler is used to output the laser energy in the oscillator.

2. The Mamyshev oscillator according to claim 1, characterized in that The wide bandwidth bandpass filter is located between the gain fiber and the high nonlinear fiber, so that the resonant pulse still has enough energy to obtain sufficient spectrum broadening in the high nonlinear fiber after being amplified by the gain fiber and passing through the wide bandwidth bandpass filter.

3. The Mamyshev oscillator according to claim 1, characterized in that The ultrashort pulse seed laser is first in a light-transmitting state when the pump is turned on, and after the ultrashort pulse resonance is established in the oscillator cavity, the ultrashort pulse seed laser is in a closed state.

4. The Mamyshev oscillator according to claim 1, characterized in that The transmission wavelength range of the wide bandwidth bandpass filter is within the gain range of the gain optical fiber; the transmission wavelength range of the bandpass filter is within the gain range of the gain optical fiber; the transmission wavelength region of the bandpass filter does not overlap with the transmission wavelength region of the wide bandwidth bandpass filter, so as to suppress the generation of continuous laser.

5. The Mamyshev oscillator according to claim 1, characterized in that The gain optical fiber includes one of the following: ytterbium-doped optical fiber, erbium-doped optical fiber, and thulium-doped optical fiber.

6. The Mamyshev oscillator according to claim 1, characterized in that The large bandwidth bandpass filter can allow a shorter length of the highly nonlinear optical fiber, improve the tolerance to nonlinear phase shift, and thus improve the laser output performance.

7. The Mamyshev oscillator according to claim 1, characterized in that The design of the wide bandwidth bandpass filter allows sufficient spectrum broadening to be achieved under low pump power conditions, thereby improving the output performance of the oscillator.

8. The Mamyshev oscillator according to claim 1, characterized in that The large bandwidth bandpass filter design is suitable for a ring cavity structure and a linear cavity structure.

9. The Mamyshev oscillator according to claim 1, characterized in that The large bandwidth bandpass filter design can be extended to other bands and improve output performance.

10. The Mamyshev oscillator according to claim 1, characterized in that The wide bandwidth filter design allows the introduction of a narrowband filter in a replacement linear cavity to shorten the length of the gain fiber while achieving sufficient spectrum broadening, thereby further improving the output performance of the system.

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