A bidirectional output pulse laser based on thulium-doped fiber mode locking

Abstract

The application discloses a bidirectional output pulse laser based on thulium-doped fiber mode locking, and belongs to the technical field of ultrafast pulse laser. The laser system structure is mainly composed of a pump source (1), a wavelength division multiplexer (2), a thulium-doped fiber (3) used as a gain medium, a 2*2 90 / 10 optical fiber coupler (4), two 1*2 50 / 50 optical fiber couplers (5) and (6), a polarization controller (7), two tunable optical attenuators (8) and (11), two thulium-doped fibers (9) and (12) used as saturable absorbers, and two polarization-independent optical fiber isolators (10) and (13). A clockwise (CW) and counterclockwise (CCW) bidirectional propagation path is constructed in the same ring cavity, and by adjusting the loss of the tunable optical attenuator and the state of the polarization controller, stable mode-locked pulse output in two directions is realized. In addition, the thulium-doped fiber is used as a saturable absorber, the modulation depth of which can be flexibly adjusted through the fiber length, so as to effectively match the mode-locked condition of the bidirectional cavity and improve the overall stability of the system. The bidirectional output pulse laser has the advantages of compact structure, all-fiber integration, strong noise suppression capability, adjustable bidirectional output repetition frequency difference and the like, and can provide a stable and reliable light source for single-cavity dual-optical comb, laser interferometry and fiber sensing and the like application scenarios.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast pulsed laser technology, specifically a bidirectional output pulsed laser based on thulium-doped fiber mode-locking. Background Technology

[0002] Bidirectional mode-locked fiber lasers have become an important research direction in the field of ultrafast lasers in recent years, attracting widespread attention due to their unique structural characteristics and application potential. These lasers can achieve bidirectional pulse output within the same resonant cavity, possessing not only excellent coherence but also effectively suppressing common-mode noise, thereby significantly improving system stability and measurement accuracy. With these advantages, bidirectional mode-locked structures show broad application prospects in fields such as laser gyroscopes, asynchronous sampling, and dual optical combs.

[0003] The selection of a saturable absorber is crucial for the stable operation of bidirectional mode-locked fiber lasers. However, existing saturable absorbers still have limitations to varying degrees. For example, while semiconductor saturable absorber mirrors (SESAMs) are mature in performance, their fabrication process is complex and costly, and their coupling methods with fiber optic systems are limited, making it difficult to achieve true all-fiber integration and high integration. Saturable absorbers based on two-dimensional materials such as graphene, carbon nanotubes (CNTs), and transition metal sulfides (TMDs) offer advantages such as broadband response and fast recovery, but generally suffer from high insertion loss, difficulty in packaging, short device lifetime, and insufficient long-term stability. Furthermore, passive mode-locking methods relying on nonlinear polarization rotation are susceptible to polarization drift in bidirectional cavity structures, resulting in poor system stability and making mode-locking operation difficult. These shortcomings, to some extent, restrict the further development and application of bidirectional mode-locked fiber lasers. Therefore, there is an urgent need for a saturable absorber that can meet the requirements of all-fiber integration, low loss, and high stability to realize the construction of bidirectional mode-locked fiber lasers. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes using thulium-doped fiber as a saturable absorber to achieve bidirectional laser pulse output. Thulium-doped fiber exhibits good structural compatibility with standard single-mode fiber, enabling low-loss all-fiber integration through fusion welding, effectively reducing insertion loss and coupling instability associated with traditional saturable absorbers. In terms of optical performance, thulium-doped fiber demonstrates excellent saturable absorption characteristics, and its modulation depth can be adjusted by the fiber length, thus meeting the mode-locking requirements of a bidirectional resonant cavity. Simultaneously, thulium-doped fiber possesses a high optical damage threshold and good robustness, which is beneficial for the long-term stable operation of the system. Its inherent fiber-like structural advantages also facilitate mass production and engineering applications. Therefore, using thulium-doped fiber as a saturable absorber effectively overcomes the shortcomings of traditional saturable absorber materials, and utilizing it for mode-locking to construct a high-performance bidirectional mode-locked fiber laser is an excellent solution.

[0005] The first aspect of the present invention proposes a bidirectional output pulsed fiber laser, the bidirectional output pulsed fiber laser comprising: a pump source (1), a wavelength division multiplexer (2), an erbium-doped fiber (3) used as a gain medium, a 2×2 90 / 10 fiber coupler (4), two 1×2 50 / 50 fiber couplers (5) and (6), a polarization controller (7), two tunable optical attenuators (8) and (11), two thulium-doped fibers (9) and (12) used as saturable absorbers, and two polarization-independent fiber isolators (10) and (13); the bidirectional output pulsed laser is capable of pulsed laser output in both clockwise (CW) and counterclockwise (CCW) directions. The clockwise propagating pulse light is transmitted sequentially through the erbium-doped fiber (3), the 2×2 90 / 10 fiber coupler (4), the 1×2 50 / 50 fiber coupler (5), the tunable optical attenuator (11), the thulium-doped fiber (12), the polarization-independent fiber isolator (13), the 1×2 50 / 50 fiber coupler (6), the polarization controller (7), and the wavelength division multiplexer (2), and is then amplified in the cavity to form a clockwise pulse output. The counterclockwise propagating pulsed light is transmitted sequentially through the erbium-doped fiber (3), wavelength division multiplexer (2), polarization controller (7), 1×2 50 / 50 fiber coupler (6), tunable optical attenuator (8), thulium-doped fiber (9), polarization-independent fiber isolator (10), 1×2 50 / 50 fiber coupler (5), and 2×2 90 / 10 fiber coupler (4), and after being amplified in the cavity, it forms a counterclockwise pulse output; in the bidirectional output pulsed laser, each fiber segment, including the erbium-doped fiber and the thulium-doped fiber, is fused with standard single-mode fiber to the optical devices to ensure low-loss transmission and stable coupling of the optical signal in the cavity.

[0006] In the bidirectional output pulsed fiber laser described in the first aspect of the present invention, the pump source is a semiconductor laser diode with an output wavelength of 980 nm and an output power range of 0~1200 mW.

[0007] In the bidirectional output pulsed fiber laser described in the first aspect of the present invention, the wavelength division multiplexer is selected as a 980 / 1550 nm model.

[0008] In the bidirectional output pulsed fiber laser described in the first aspect of the present invention, the erbium-doped fiber has a group velocity dispersion coefficient of -26 ps / nm / km at 1550 nm and a length of 2.4 m.

[0009] In the bidirectional output pulsed fiber laser described in the first aspect of the present invention, the adjustable optical attenuator loss is adjustable in the range of 0~30 dB.

[0010] In the bidirectional output pulsed fiber laser described in the first aspect of the present invention, the splice loss of each pigtail is controlled within 0.1 dB.

[0011] A second aspect of the present invention provides a saturable absorber device based on thulium-doped fiber mode-locking, wherein the thulium-doped fiber has a group velocity dispersion coefficient of 9.36 ps / nm / km at 1550 nm and a length of 14 cm; the modulation depth of the saturable absorber can be changed by adjusting the length of the thulium-doped fiber.

[0012] The beneficial effects of this invention are as follows: The bidirectional output pulsed laser of this invention establishes a counter-clockwise and counter-clockwise pulse propagation mechanism within the same ring cavity. The two pulses share a portion of the fiber path within the cavity and propagate independently on the remaining paths, thereby achieving stable output of bidirectional mode-locked pulses. This structure can obtain a bidirectional pulse pair with good coherence within the same system, and has advantages such as common-mode noise suppression, compact structure, and flexible adjustment of repetition frequency difference, demonstrating good application potential in the construction of single-cavity dual-comb lasers.

[0013] The thulium-doped fiber used in this invention as a saturable absorber device exhibits excellent saturable absorption effect, and the modulation depth can be flexibly adjusted by the fiber length, meeting the requirements for stable pulse generation in bidirectional cavities. Furthermore, thulium-doped fiber possesses a high optical damage threshold and good robustness, enabling long-term stable operation of the laser at high pump power. The fiber-type saturable absorber structure significantly reduces insertion loss, improves system energy utilization efficiency, and facilitates the high integration and engineering applications of fiber lasers. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the bidirectional output pulsed laser structure based on thulium-doped fiber mode-locking according to the present invention; Figure 2 The graph shows the modulation depth test results of the thulium-doped optical fiber used as a saturable absorber in this invention. Figure 3 (a) shows the pulse spectral characteristics of the present invention in the clockwise direction with a pump power of 150 mW; Figure 3 (b) shows the pulse time-domain characteristics of the present invention in the clockwise direction with a pump power of 150 mW; Figure 3 (c) is a single-pulse magnification profile of the present invention with a pump power of 150 mW in the clockwise direction; Figure 3 (d) shows the pulse radio frequency characteristics of the present invention in the clockwise direction with a pump power of 150 mW; Figure 4(a) shows the pulse spectral characteristics of the present invention in the counterclockwise direction with a pump power of 220 mW; Figure 4 (b) shows the pulse time-domain characteristics of the present invention in the counterclockwise direction with a pump power of 220 mW; Figure 4 (c) is a magnified profile of a single pulse with a pump power of 220 mW in the counterclockwise direction of the present invention; Figure 4 (d) shows the pulse radio frequency characteristics of the present invention in the counterclockwise direction with a pump power of 220 mW; Detailed Implementation

[0015] The invention will be further described in detail below with reference to the laser system structure and experimental results. The embodiments described are only for illustrating specific implementations of the invention and do not constitute a limitation thereof; unless otherwise specified, the configurations and devices used in the embodiments are solutions and devices that are conventionally available and achievable by those skilled in the art.

[0016] The first aspect of the present invention provides a bidirectional output pulsed laser.

[0017] Figure 1This is a schematic diagram of the bidirectional output pulsed laser structure based on thulium-doped fiber mode-locking according to the present invention. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking includes: a pump source (1), a wavelength division multiplexer (2), an erbium-doped fiber (3) used as a gain medium, a 2×2 90 / 10 fiber coupler (4), a 1×2 50 / 50 fiber coupler (5) and (6), a three-ring polarization controller (7), tunable optical attenuators (8) and (11), thulium-doped fibers (9) and (12) used as saturable absorbers, and polarization-independent fiber isolators (10) and (13); the bidirectional output pulsed laser can achieve pulsed laser output in both clockwise (CW) and counterclockwise (CCW) directions. The clockwise propagating pulse light is transmitted sequentially through the erbium-doped fiber (3), the 2×2 90 / 10 fiber coupler (4), the 1×2 50 / 50 fiber coupler (5), the tunable optical attenuator (11), the thulium-doped fiber (12), the polarization-independent fiber isolator (13), the 1×2 50 / 50 fiber coupler (6), the three-ring polarization controller (7), and the wavelength division multiplexer (2), and is then amplified in the cavity to form a clockwise pulse output. The counterclockwise propagating pulsed light is transmitted sequentially through the erbium-doped fiber, wavelength division multiplexer (2), polarization controller (7), 1×2 50 / 50 fiber coupler (6), tunable optical attenuator (8), thulium-doped fiber (9), polarization-independent fiber isolator (10), 1×2 50 / 50 fiber coupler (5), and 2×2 90 / 10 fiber coupler (4), and after intracavity cyclic amplification, forms a counterclockwise pulse output; in the bidirectional output pulsed laser, each fiber segment, including the erbium-doped fiber and the thulium-doped fiber, is fused with standard single-mode fiber to the optical devices to ensure low-loss transmission and stable coupling of the intracavity optical signal.

[0018] In this embodiment of the invention, the pump source is a semiconductor laser diode with an output wavelength of 980nm and an output power range of 0~1200 mW.

[0019] In this embodiment of the invention, the erbium-doped fiber has a group velocity dispersion coefficient of -26 ps / nm / km at 1550 nm and a length of 2.4 m.

[0020] In this embodiment of the invention, the wavelength division multiplexer is selected as a 980 / 1550 nm model.

[0021] In this embodiment of the invention, the polarization controller is a three-ring polarization controller.

[0022] In this embodiment of the invention, the adjustable optical attenuator loss is adjustable in the range of 0~30 dB.

[0023] In this embodiment of the invention, the fiber optic splicing loss is controlled to be within 0.1 dB.

[0024] A second aspect of the present invention provides a saturable absorber device based on thulium-doped fiber mode-locking.

[0025] Figure 2 The figure shows the modulation depth test results of the thulium-doped fiber used in this invention as a saturable absorber. For a thulium-doped fiber with a length of 14 cm, the measured modulation depth is 15.2%. As the length of the thulium-doped fiber decreases, the modulation depth also decreases: when the fiber length is 13 cm, 12 cm, 11 cm, and 10 cm, the modulation depths are 14.0%, 12.9%, 11.7%, and 10.8%, respectively. Therefore, the modulation depth of the saturable absorber can be effectively controlled by adjusting the length of the thulium-doped fiber.

[0026] In an embodiment of the present invention, the thulium-doped fiber has a group velocity dispersion coefficient of 9.36 ps / nm / km at 1550 nm.

[0027] Figure 3 This invention describes the clockwise pulse output characteristics. When the pump power increases to 150mW, by reducing the loss of the tunable optical attenuator (11), increasing the loss of the tunable optical attenuator (8), and carefully adjusting the state of the polarization controller (7), a stable mode-locked pulse output can be obtained in the clockwise direction. Figure 3 As shown in the spectrum in (a), the pulse center wavelength is 1559.35 nm, the 3 dB bandwidth is 0.35 nm, and there are symmetrically distributed Kelly bands on both sides of the spectrum, indicating that this pulse is a conventional soliton. Figure 3 (b) shows the pulse time-domain characteristics, with a pulse period of 77.8 ns and a repetition frequency of 12.85 MHz. This invention amplifies the time-domain pulse signal to obtain single-pulse characteristics, such as... Figure 3 As shown in (c), the pulse width is 1.3 ns. Figure 3 (d) shows that the pulse fundamental frequency signal-to-noise ratio reaches 55dB, indicating that the clockwise pulse is in a stable mode-locked state.

[0028] Figure 4 This invention describes the counterclockwise pulse output characteristics. By further increasing the pump power to 220mW, reducing the loss of the tunable optical attenuator (8), increasing the loss of the tunable optical attenuator (11), and carefully adjusting the state of the polarization controller (7), a stable mode-locked pulse output can be obtained in the counterclockwise direction. Figure 4 As shown in the spectrum in (a), the pulse center wavelength is 1559.81 nm, the 3 dB bandwidth is 0.30 nm, and there are still symmetrically distributed Kelly lateral bands on both sides of the spectrum. Figure 4(b) shows the pulse time-domain characteristics, with a pulse period of 81.32 ns and a repetition frequency of 12.29 MHz. The single-pulse amplification profile is as follows. Figure 4 As shown in (c), the pulse width is 1.5 ns. Figure 4 (d) shows that the pulse fundamental frequency signal-to-noise ratio reaches 56dB, indicating that the pulse in the counterclockwise direction is in a stable mode-locked state.

[0029] As can be seen from the above embodiments, the bidirectional output pulsed laser based on thulium-doped fiber mode-locking disclosed in this invention achieves stable bidirectional pulse output while maintaining a compact structure and all-fiber architecture. By utilizing thulium-doped fiber as a saturable absorber and adjusting its length and intracavity loss, the mode-locking conditions can be flexibly controlled. This solution combines advantages such as high stability, low loss, and high integration, demonstrating good practicality and engineering application potential.

[0030] The above embodiments are for illustrative purposes only and do not constitute a limitation thereof. With a full understanding of this invention, those skilled in the art can make various modifications and optimizations. As long as these modifications and optimizations do not deviate from the innovative ideas and core principles of this invention, they should be considered to fall within the scope of the claims of this invention.

Claims

1. A bidirectional output pulsed laser based on thulium-doped fiber mode-locking, characterized in that, include: The pump source (1), wavelength division multiplexer (2), erbium-doped fiber (3) used as gain medium, 2×2 90 / 10 fiber coupler (4), 1×2 50 / 50 fiber coupler (5) and (6), polarization controller (7), tunable optical attenuator (8) and (11), thulium-doped fiber (9) and (12) used as saturable absorber, polarization-independent fiber isolator (10) and (13); the bidirectional output pulsed laser is capable of pulsed laser output in both clockwise (CW) and counterclockwise (CCW) directions. The clockwise propagating pulse light is transmitted sequentially through the erbium-doped fiber (3), the 2×2 90 / 10 fiber coupler (4), the 1×2 50 / 50 fiber coupler (5), the tunable optical attenuator (11), the thulium-doped fiber (12), the polarization-independent fiber isolator (13), the 1×2 50 / 50 fiber coupler (6), the polarization controller (7), and the wavelength division multiplexer (2). After being amplified in the cavity, it forms a clockwise pulse output. The counterclockwise propagating pulse light is transmitted sequentially through the erbium-doped fiber, the wavelength division multiplexer (2), the polarization controller (7), the 1×2 50 / 50 fiber coupler (6), the tunable optical attenuator (8), the thulium-doped fiber (9), the polarization-independent fiber isolator (10), the 1×2 50 / 50 fiber coupler (5), and the 2×2 90 / 10 fiber coupler (4). After being amplified in the cavity, it forms a counterclockwise pulse output. In the bidirectional output pulsed laser, each fiber segment, including erbium-doped fiber and thulium-doped fiber, is fused with standard single-mode fiber to optical devices to ensure low-loss transmission and stable coupling of optical signals within the cavity.

2. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The pump source is a semiconductor laser diode with an output wavelength of 980 nm and an output power range of 0~1200 mW.

3. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The wavelength division multiplexer selected is model 980 / 1550.

4. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The erbium-doped fiber has a group velocity dispersion coefficient of -26 ps / nm / km at 1550 nm and a length of 2.4 m.

5. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The thulium-doped fiber has a group velocity dispersion coefficient of 9.36 ps / nm / km at 1550 nm and a length of 14 cm.

6. The thulium-doped optical fiber according to claim 5, characterized in that, The modulation depth of the saturable absorber can be changed by altering the length of the thulium-doped fiber.

7. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The polarization controller can be either a three-ring type or a squeeze type polarization controller.

8. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The adjustable optical attenuator has an adjustable loss range of 0~30 dB and is continuously adjustable.

9. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, The splicing loss between each optical fiber device is controlled within 0.1 dB.

10. The bidirectional output pulsed laser based on thulium-doped fiber mode-locking according to claim 1, characterized in that, By changing the optical path length between two 1×2 50 / 50 fiber couplers in the clockwise and counterclockwise directions, the repetition frequency difference of the laser pulses in the two directions can be flexibly adjusted.

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