An all-fiber positive-dispersion bidirectionally operating mode-locked laser

Abstract

The present invention belongs to the field of optics, and specifically relates to an all-fiber positive dispersion bidirectionally operating mode-locked laser. The present invention provides an all-fiber positive dispersion bidirectionally operating mode-locked laser that can realize bidirectional propagation of dissipative soliton mode-locked dual-combs in the optical cavity of an all-fiber device: First, due to the removal of the spatial optical device part by all-fiberization, the loss is reduced, so that the pump power required for bidirectional mode locking is reduced, thereby reducing the noise caused by the pump. On the basis of achieving the miniaturization of the laser, the noise performance, stability, and resistance to physical vibration interference of the laser are greatly improved, and bidirectional self-starting can be achieved; secondly, compared with the positive dispersion bidirectionally operating mode-locked laser of spatial light, it has a lower implementation cost. The laser output pulse energy of the present invention can reach the nJ level, and can output a 20nm overlapping flat-top spectrum in both directions. It has great application prospects and the possibility of practical engineering use in the field of dual-comb spectroscopy measurement.

Description

Technical Field

[0001] The invention belongs to the field of optics and relates to an optical fiber mode-locked laser, in particular to an all-optical positive dispersion bidirectionally operating mode-locked laser. Background Art

[0002] Single-cavity dual-comb lasers are a highly sought-after research area in the field of optical frequency combs, and their development is in a state of continuous evolution. Positive-dispersion bidirectionally operating fiber mode-locked lasers can simultaneously generate two independently operating optical frequency combs within a single optical cavity. Their high mode-locked pulse energy offers numerous applications, such as dual-comb spectroscopy, dual-comb ranging, and fiber-optic sensing, and have become a hot topic in the optical field.

[0003] Research on single-cavity dual-comb light sources begins with the powerful tool of dual-comb spectroscopy. Compared to other spectroscopic detection techniques, it offers rapid data acquisition and, due to the inherent properties of the optical frequency comb, fine spectral resolution and high signal-to-noise ratio. Recent advances in dual-comb spectroscopy, coupled with its potential for rapid and simultaneous detection of multiple samples, have made this powerful tool a promising tool in numerous markets, such as optical sensors, breath gas detection, and atmospheric monitoring.

[0004] Due to its high output pulse energy (over 8 nJ) and inherent common-mode noise suppression, the positive-dispersion bidirectionally mode-locked laser achieves long-term stability in the repetition rate difference compared to two completely independent optical combs without the need for additional servoing. Furthermore, the spectral overlap of the two generated optical frequency combs is very wide, typically exceeding 20 nm, resulting in spectral measurement bandwidth, dual-comb spectral resolution, and dual-comb spectral measurement sensitivity unmatched by other dual-comb optical frequency comb sources.

[0005] However, current positive-dispersion bidirectionally mode-locked lasers still have several drawbacks. First, their further miniaturization is limited by the volume of spatial optical components used. However, miniaturizing high-quality, highly stable dual-comb sources is a practical engineering requirement. Second, spatial optical components incur significant inherent losses, requiring higher pump powers to achieve bidirectional mode-locking, which in turn results in increased pump noise. Summary of the Invention

[0006] In view of the above-mentioned problems or shortcomings, in order to solve the problems of further miniaturization, cost reduction and noise optimization of the current spatial light positive dispersion bidirectional operation mode-locked laser, the present invention provides an all-fiber positive dispersion bidirectional operation mode-locked laser, which utilizes the birefringence characteristics of polarization-maintaining fiber in combination with a fiber polarization beam splitter and a polarization controller to form a fiber lyot filter and a passive mode-locking mechanism of nonlinear polarization rotation (NPR) to realize an all-fiber small and compact positive dispersion dual-comb laser, which can generate bidirectional output pulse energy reaching the nJ level, realize bidirectional output of overlapping spectra of more than 20nm, and realize miniaturized packaging of single-cavity dual-comb laser.

[0007] The present invention adopts the following technical solutions:

[0008] An all-fiber positive dispersion bidirectional mode-locked laser comprises a 980nm single-frequency laser, a coupling device, a rare earth ion-doped fiber, a passive fiber, a squeezed polarization controller, a fiber polarization beam splitter and a polarization-maintaining fiber.

[0009] The rare earth ion doped optical fiber is connected to two passive optical fibers respectively, the two passive optical fibers are connected to a fiber polarization beam splitter respectively, and the two fiber polarization beam splitters are connected through a polarization maintaining optical fiber to form an optical cavity of the loop.

[0010] The 980nm single-frequency laser used as the pump source generates pump light with a wavelength fluctuation of no more than 5%. Any wavelength within this wavelength range can serve as the pump source for the laser, such as a 976nm single-frequency laser source. Depending on the required energy, the number of pump lasers can be one or more, and the type of pump laser can be single-mode or multi-mode.

[0011] The coupling device couples the pump light into the optical cavity using a beam combiner or a wavelength division multiplexer.

[0012] The rare earth ion doped optical fiber converts the pump light into corresponding higher wavelength light (self-excited amplified radiation ASE) and serves as a gain medium.

[0013] There are two passive optical fibers, one end of which is connected to the two ends of the rare earth ion-doped optical fiber respectively, and the other end of the passive optical fiber is connected to an optical fiber polarization beam splitter respectively.

[0014] There are two fiber polarization beam splitters in total, which are four-port or three-port devices. On the one hand, they form a passive mode-locking mechanism based on nonlinear polarization rotation with a squeezed polarization controller, and on the other hand, they form a fiber lyot filter with the polarization-maintaining fiber. The two fiber polarization beam splitters also serve as the two output ends of the entire positive dispersion bidirectionally operating mode-locked laser.

[0015] The polarization-maintaining fiber consists of three segments, which are fused sequentially at offset angles. The central segment is fused to the two side segments at offset angles of 30-60° to achieve filtering and thus mode locking. Together with a fiber polarization beam splitter, the three segments form a fiber lyot filter. The filtering effect can be modulated by adjusting the length of the central segment.

[0016] The squeezed polarization controller is arranged in the non-fiber Lyot filter region of the entire optical cavity to control the polarization of the pulse running in the optical cavity. The squeezed polarization controller and the fiber polarization beam splitter combine to form a passive mode-locking mechanism based on nonlinear polarization rotation. The squeezed polarization controller controls the polarization of the light beam running in the optical cavity. The fiber polarization beam splitter brings about polarization-dependent beam splitting, thereby realizing the saturable absorption effect of nonlinear polarization rotation. The birefringence effect of the polarization-maintaining fiber and the polarization-dependent beam splitting of the fiber polarization beam splitter form a sinusoidal spectral filter based on the Lyot filter principle. Ultimately, the combination of the squeezed polarization controller, the fiber polarization beam splitter, and the polarization-maintaining fiber can simultaneously achieve the saturable absorption effect of nonlinear polarization rotation and the spectral filtering effect.

[0017] Furthermore, the rare earth ion-doped optical fiber is one or more of ytterbium-doped optical fiber, erbium-doped optical fiber, thulium-doped optical fiber, neodymium-doped optical fiber and holmium-doped optical fiber, and the dispersion is normal dispersion or anomalous dispersion.

[0018] Furthermore, the passive optical fiber is a single-mode optical fiber or a multi-mode optical fiber, and the dispersion is normal dispersion or anomalous dispersion.

[0019] Furthermore, the deviation angles of the polarization-maintaining optical fibers fused together are all 45°, so as to realize a Lyot filter with a maximum extinction ratio.

[0020] Furthermore, the squeezed polarization controller controls the polarization of the pulse running in the optical cavity, wherein the number of the squeezed polarization controllers can be one, two or more.

[0021] Furthermore, the all-fiber positive dispersion bidirectionally operating mode-locked laser adjusts the mode-locked state by controlling the polarization state in the optical cavity or changing the pump energy:

[0022] By changing the pump light power entering the optical cavity, the laser can be operated in different mode-locked states.

[0023] Alternatively, the laser can be operated in different mode-locked states by adjusting the squeezed polarization controller.

[0024] Furthermore, the mode-locked state includes a unidirectional mode-locked state, a bidirectional single-wavelength mode-locked state, or a bidirectional dual-wavelength mode-locked state.

[0025] Furthermore, the all-fiber positive dispersion bidirectionally operating mode-locked laser adjusts the repetition frequency difference of the solitons running in the two directions by controlling the polarization state in the optical cavity or changing the pump energy:

[0026] The difference between the counterclockwise repetition rate of the counterclockwise pulse train and the clockwise repetition rate of the clockwise pulse train is changed by changing the power of the pump light entering the optical cavity.

[0027] Alternatively, the difference between the counterclockwise repetition rate of the counterclockwise pulse train and the clockwise repetition rate of the clockwise pulse train can be changed by adjusting one or both intracavity polarization controllers.

[0028] The present invention uses a 980nm single-frequency laser as a pump source, coupled into an optical cavity via a coupling device. In this cavity, a rare-earth ion-doped fiber serves as the gain medium. The 980nm single-frequency laser is converted to higher-wavelength light within the gain fiber. Two passive optical fibers are coupled to the doped fiber at either end. A squeezed polarization controller is used to control the polarization of the pulses operating within the cavity. This squeezed polarization controller, combined with a fiber polarization beam splitter, forms a passive mode-locking mechanism based on nonlinear polarization rotation. A fiber Lyot filter is constructed using a combination of polarization-maintaining fiber, an offset-angle fusion splice between the polarization-maintaining fibers, and a polarization beam splitter. These components together form an optical ring cavity design. Due to the balance between positive dispersion within the cavity, spectral filtering, and nonlinear effects, stable positive dispersion mode-locked dissipative solitons can be generated simultaneously in both directions. These bidirectional positive dispersion mode-locked dissipative solitons can be of the same or different wavelengths, depending on the mode-locking state. By controlling the intracavity polarization or varying the pump energy, the mode-locking state of this all-fiber positive dispersion bidirectionally mode-locked laser, as well as the repetition frequency difference between the solitons operating in the two directions, can be adjusted.

[0029] The optical cavity of the present invention is designed as a ring, and the two directions of the ring cavity (clockwise and counterclockwise) are reused to generate two relatively independent optical combs running in different directions. Because the dual optical combs reuse the same ring cavity, the optical pulse repetition frequencies can be highly similar, with the normalized difference between the clockwise and counterclockwise repetition rates being ≤10%.

[0030] In summary, the present invention provides an all-fiber positive-dispersion bidirectional mode-locked laser that can achieve bidirectional propagation of dissipative soliton mode-locked dual-combs within an all-fiber optical cavity. This solves the pain points of existing positive-dispersion bidirectional mode-locked lasers: First, because the all-fiber design removes the spatial optical component, losses are reduced, reducing the pump power required for bidirectional mode-locking and thus reducing pump noise. This significantly improves the laser's noise performance, stability, and resistance to physical vibration interference, while also enabling bidirectional self-starting. Second, compared to spatial-light positive-dispersion bidirectional mode-locked lasers, the present invention has a lower implementation cost. The laser output pulse energy can reach the nanojoule level and can bidirectionally output a 20nm overlapping flat-top spectrum. This laser has great application prospects and practical engineering applications in the field of dual-comb spectroscopy. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic block diagram of the overall structure of the embodiment;

[0032] Figure 2 Schematic diagram of the filtering effect of the optical fiber lyot filter in the embodiment;

[0033] Figure 3 Schematic diagram of the spectrum in two directions under two mode-locked states of the embodiment, and the radio frequency spectrum detected by a photodetector after the two directions are combined;

[0034] Figure 4 The relative intensity noise (RIN) characterization of two different mode-locked states of the all-fiber positive dispersion bidirectional mode-locked laser in the embodiment is shown;

[0035] Figure 5 The phase noise and accumulated time jitter of two different mode-locked states of the all-fiber positive dispersion bidirectional mode-locked laser in the embodiment are compared with those of a spatial light positive dispersion bidirectional mode-locked laser;

[0036] Figure 6 The jitter characterization of the repetition frequency and repetition frequency difference in two different locking states of the embodiment;

[0037] Figure 7 Schematic diagram of the self-start characterization of two mode-locked states of the all-fiber positive dispersion bidirectional mode-locked laser in the embodiment. DETAILED DESCRIPTION

[0038] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0039] The structure of the all-fiber positive dispersion bidirectionally operating mode-locked laser provided in this embodiment is as follows: Figure 1 shown.

[0040] The 980nm single-frequency laser is a multi-mode 980nm pump laser with a maximum output of 3W. When the pump light source driving module provides an output current of 3A, the output is 2W (33dBm).

[0041] The multimode fiber output of the pump source is coupled into the optical cavity through a beam combiner (coupling device). The pump input of the beam combiner is also a multimode fiber. The cavity of the beam combiner is divided into double-clad fibers. The gain fiber is a double-clad ytterbium-doped fiber, and the remaining passive fibers are Hi1060 fibers.

[0042] This embodiment uses two squeezed polarization controllers set at two passive optical fibers (Hi1060 optical fibers) and fused with a fiber polarization beam splitter and a polarization-maintaining optical fiber at a 45° deviation angle to form a nonlinear polarization rotation mode-locking mechanism, as well as a 20nm filtering period Lyot filter. The filter's filtering effect on the self-excited amplified radiation (ASE) generated by the ytterbium-doped optical fiber is shown in the following spectrum: Figure 2 shown.

[0043] The mode-locked laser of the embodiment can be realized as follows by adjusting the polarization controller: Figure 3 The mode-locked spectrum shown is Figure 3 Two typical mode-locked states and the combined radio frequency spectrum are demonstrated, and a slight adjustment of the polarization controller can achieve switching between the two mode-locked states without losing mode locking.

[0044] Figures 4 to 6 The noise performance of the all-fiber positive dispersion bidirectionally mode-locked laser of the embodiment is characterized and compared with the positive dispersion bidirectionally mode-locked laser of spatial light, which further proves the excellent noise performance of the all-fiber positive dispersion bidirectionally mode-locked laser of this embodiment. Figure 4 The relative intensity noise (RIN) of the single-wavelength bidirectional mode-locked state (a) and the dual-wavelength bidirectional mode-locked state are shown respectively. Figure 5 (a) shows the phase noise and accumulated time jitter of the single-wavelength bidirectional mode-locked state of this embodiment and the comparison with the spatial light positive dispersion bidirectional mode-locked laser. Figure 5 (b) shows the phase noise and accumulated time jitter of the dual-wavelength bidirectional mode-locked state of this embodiment. Figure 6 (a) shows the jitter characteristics of the repetition frequency and repetition frequency difference in the single-wavelength bidirectional mode-locked state of this embodiment. Figure 6 (b) shows the jitter characterization of the repetition frequency and repetition frequency difference in the dual-wavelength bidirectional mode-locked state of this embodiment.

[0045] Figure 7This figure illustrates the self-startup performance of an all-fiber positive-dispersion bidirectionally mode-locked laser operating in both mode-locked states. This figure demonstrates the stable self-startup performance of the all-fiber positive-dispersion bidirectionally mode-locked laser. A 10s period, 50% duty cycle square wave was used to control the pumping operation. The real-time changes in the optical spectrum output from the laser in both directions were measured. This demonstrates that the laser demonstrated in this example can achieve stable bidirectional self-startup in both mode-locked states using both switching pumps.

[0046] Due to the balance between positive dispersion within the optical cavity and spectral filtering and nonlinear effects, the present invention can simultaneously generate stable positive dispersion mode-locked dissipative solitons in both directions. These bidirectionally operating positive dispersion mode-locked dissipative solitons can have the same wavelength or different wavelengths, depending on the mode-locked state. By controlling the polarization state within the optical cavity or varying the pump energy, the mode-locked state of the all-fiber positive dispersion bidirectionally mode-locked laser, as well as the repetition frequency difference between the solitons operating in the two directions, can be adjusted.

[0047] As can be seen from the above examples, the present invention realizes an all-fiber positive-dispersion bidirectional mode-locked laser. It is capable of high-energy bidirectional pulse energy output and a high bidirectional coincidence flat-top spectrum. Compared to positive-dispersion bidirectional mode-locked lasers implemented with spatial optical devices, this achieves a bidirectional dual-wavelength mode-locked state, which is not available in these devices, while being miniaturized. This significantly improves the laser's noise performance, stability, and resistance to physical vibration interference, and enables bidirectional stable self-starting in both mode-locked states.

Claims

1. An all-fiber positive-dispersion bidirectionally mode-locked laser, characterized by: Including 980nm single-frequency laser, coupling devices, rare earth ion doped fiber, passive fiber, squeezed polarization controller, fiber polarization beam splitter and polarization-maintaining fiber; The rare earth ion doped optical fiber is connected to two passive optical fibers respectively, and the two passive optical fibers are connected to a fiber polarization beam splitter respectively. The two fiber polarization beam splitters are connected through a polarization maintaining optical fiber to form an optical cavity of the loop; The 980nm single-frequency laser is used as a pump source to generate pump light, and its wavelength fluctuation is no more than 5%; The coupling device couples the pump light into the optical cavity; The rare earth ion doped optical fiber converts the pump light into corresponding higher wavelength light and serves as a gain medium; There are two passive optical fibers, one end of which is connected to the two ends of the rare earth ion-doped optical fiber, and the other end of the passive optical fiber is connected to a fiber polarization beam splitter; There are two fiber polarization beam splitters in total, which are four-port or three-port devices. On the one hand, they form a passive mode-locking mechanism based on nonlinear polarization rotation with a squeezed polarization controller, and on the other hand, they form a fiber lyot filter with the polarization-maintaining fiber. The two fiber polarization beam splitters also serve as the two output ends of the entire positive dispersion bidirectionally operating mode-locked laser. The polarization-maintaining optical fiber comprises three sections, which are fused sequentially at deviation angles. The middle section is fused with the two side sections at deviation angles of 30-60 degrees to achieve a filtering effect and thus mode locking. The three sections of polarization-maintaining optical fiber and the optical fiber polarization beam splitter together constitute an optical fiber Lyot filter. The squeezed polarization controller is arranged in the non-fiber lyot filter area of ​​the entire optical cavity to control the polarization of the pulse running in the optical cavity. The squeezed polarization controller is combined with the fiber polarization beam splitter to form a passive mode locking mechanism based on nonlinear polarization rotation.

2. The all-fiber positive-dispersion bidirectionally mode-locked laser according to claim 1, characterized in that: The rare earth ion-doped optical fiber is one or more of ytterbium-doped optical fiber, erbium-doped optical fiber, thulium-doped optical fiber, neodymium-doped optical fiber and holmium-doped optical fiber, and the dispersion is normal dispersion or anomalous dispersion.

3. The all-fiber positive dispersion bidirectionally mode-locked laser according to claim 1, characterized in that: The deviation angles of the polarization-maintaining optical fibers during fusion splicing are all 45°.

4. The all-fiber positive dispersion bidirectionally mode-locked laser according to claim 1, characterized in that: The 980nm single-frequency laser is a pump laser, and there are one or more pump lasers. The pump laser type is single-mode or multi-mode pumping.

5. The all-fiber positive dispersion bidirectionally mode-locked laser according to claim 1, characterized in that: The coupling device is a beam combiner or a wavelength division multiplexer.

6. The all-fiber positive dispersion bidirectionally operating mode-locked laser according to claim 1, characterized in that: The passive optical fiber is a single-mode optical fiber or a multi-mode optical fiber, and the dispersion is normal dispersion or anomalous dispersion.

7. The all-fiber positive dispersion bidirectionally operated mode-locked laser according to claim 1, characterized in that: The middle section of polarization-maintaining optical fiber can realize the regulation of the filtering effect of the optical fiber lyot filter by adjusting its length.

8. The all-fiber positive dispersion bidirectionally operating mode-locked laser according to claim 1, characterized in that: Clamping state is adjustable: By changing the pump light power entering the optical cavity, the laser can be operated in different mode-locked states; Alternatively, the laser can be operated in different mode-locked states by adjusting the squeezed polarization controller.

9. The all-fiber positive dispersion bidirectionally operating mode-locked laser according to claim 8, characterized in that: The mode-locked state is a unidirectional mode-locked state, a bidirectional single-wavelength mode-locked state, or a bidirectional dual-wavelength mode-locked state.

10. The all-fiber positive dispersion bidirectionally operating mode-locked laser according to claim 1, characterized in that: By controlling the polarization state in the optical cavity or changing the pump energy, the repetition frequency difference of the solitons running in the two directions can be adjusted: The difference between the counterclockwise repetition rate of the counterclockwise pulse and the clockwise repetition rate of the clockwise pulse is changed by changing the pump light power entering the optical cavity; Alternatively, the difference between the counterclockwise repetition rate of the counterclockwise pulse train and the clockwise repetition rate of the clockwise pulse train is changed by adjusting the squeezed polarization controller.

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