All-fiber positive dispersion bidirectional operation mode-locked laser

Abstract

The invention belongs to the field of optics, and particularly relates to an all-fiber positive dispersion bidirectional operation mode-locked laser. The invention provides an all-fiber positive dispersion two-way operation mode-locked laser, which can realize two-way propagation dissipative soliton mode-locked double combs in an optical cavity of an all-fiber device. The loss is reduced, so that the pumping power required by bidirectional mode locking is reduced, the noise caused by pumping is reduced, the noise performance, the stability and the physical vibration interference resistance of the laser are greatly improved on the basis of realizing the miniaturization of the laser, and bidirectional self-starting can be realized; and secondly, compared with a positive dispersion bidirectional operation mode-locked laser of spatial light, the realization cost is lower. The pulse energy output by the laser can reach nJ magnitude, the overlapped flat-topped spectrum of 20 nm can be bidirectionally output, and the laser has huge application prospect and practical engineering use possibility in the field of double-comb spectrum 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 research direction that has attracted much attention in the field of optical frequency combs, and their development has been in a state of continuous evolution. Positive dispersion bidirectional fiber mode-locked lasers can simultaneously generate two relatively independent optical frequency combs in one optical cavity, and have very high mode-locked pulse energy, which has many application values, such as dual-comb spectroscopy, dual-comb ranging, and fiber-optic sensing, which has become a hot topic in the current optical field.

[0003] First of all, the research on single-cavity dual-comb light source begins with the powerful tool of dual-comb spectroscopy. Compared with other spectral detection technologies, it can provide fast data acquisition and due to the inherent characteristics of the optical frequency comb, dual-comb spectroscopy has fine spectral resolution and high signal-to-noise ratio. In recent years, dual-comb spectroscopy technology has been continuously developed, and its potential for rapid and simultaneous detection of multiple samples makes this powerful tool have huge market potential, such as optical sensors, respiratory gas detection, atmospheric detection, etc.

[0004] The positive dispersion bidirectional mode-locked laser can achieve long-term stability of the repetition frequency difference without additional servo compared to two sets of completely independent combs due to its high output pulse energy (up to 8nJ or more) and its inherent common-mode noise suppression mechanism. In addition, the spectral overlap range of the two sets of optical frequency combs generated by it is very wide, usually up to 20nm or more, which makes it have spectral measurement bandwidth, dual-comb spectral resolution and dual-comb spectral measurement sensitivity that are unmatched by other types of dual-optical frequency comb light sources.

[0005] However, the current positive dispersion bidirectional mode-locked laser still has some defects. First, the use of a large number of spatial optical devices limits its further miniaturization due to the volume of spatial optical devices, and making a miniaturized, high-quality, and highly stable dual-comb light source is a need in current engineering practice. Second, spatial optical devices will bring great inherent losses and must use a larger pump power to achieve bidirectional mode locking, which will bring greater 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 of nJ order, 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 solution:

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

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

[0010] The 980nm single-frequency laser is used as a pump source to generate pump light, and its wavelength fluctuation is no more than 5%. Any wavelength within this wavelength range can be used as the pump source of the laser, such as a 976nm single-frequency laser source. The number of pump lasers can be one or more according to the required energy, and the type of pump laser can be single-mode or multi-mode pumping.

[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 respectively connected to the two ends of the rare earth ion-doped optical fiber, and the other end of the passive optical fiber is respectively connected to an optical fiber polarization beam splitter.

[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 bidirectional mode-locked laser.

[0015] The polarization-maintaining optical fiber has three sections in total, and the three sections are fused in sequence at deviation angles, wherein the middle section of the polarization-maintaining optical fiber is fused with the two side sections of the polarization-maintaining optical fiber at a deviation angle of 30-60°, respectively, to achieve 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 modulation of the filtering effect is achieved by adjusting the length of the middle section of the polarization-maintaining optical fiber.

[0016] 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 and the fiber polarization beam splitter are combined 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 polarization-dependent beam splitting to achieve the saturable absorption effect of nonlinear polarization rotation. The polarization-maintaining fiber birefringence effect and the polarization-dependent beam splitting of the fiber polarization beam splitter form a sinusoidal spectral filter based on the Lyot filter principle. Finally, the combination of the squeezed polarization controller, the fiber polarization beam splitter, and the polarization-maintaining fiber can simultaneously form 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 abnormal dispersion.

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

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

[0021] Furthermore, the all-fiber positive dispersion bidirectionally operated 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-locking state includes a unidirectional mode-locking state, a bidirectional single-wavelength mode-locking state or a bidirectional dual-wavelength mode-locking state.

[0025] Furthermore, the all-fiber positive dispersion bidirectionally operating mode-locked laser adjusts the repetition frequency difference of solitons running in 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 varied by adjusting one or both intracavity polarization controllers.

[0028] The present invention uses a 980nm single-frequency laser as a pump source and couples it into an optical cavity through a coupling device. In the optical cavity, a rare earth ion-doped optical fiber is used as a gain medium, and the 980nm single-frequency laser is converted into a higher wavelength light in the gain optical fiber. Two passive optical fibers are coupled at both ends of the doped optical fiber, and a squeezed polarization controller is used to control the polarization of the pulse running in the cavity. The squeezed polarization controller and the optical fiber polarization beam splitter are combined to form a passive mode-locking mechanism based on nonlinear polarization rotation. A polarization-maintaining optical fiber, a combination structure of a deviation angle fusion between polarization-maintaining optical fibers and a polarization beam splitter are used to form an optical fiber lyot filter. These devices together form the design of an optical ring cavity. Due to the balance between positive dispersion in the cavity and spectral filtering and nonlinear effects, positive dispersion mode-locked dissipative solitons that run stably can be generated bidirectionally and simultaneously. The positive dispersion mode-locked dissipative solitons that run bidirectionally can be of the same wavelength or different wavelengths, depending on the mode-locking state. The mode-locking state of the all-fiber positive dispersion bidirectional mode-locked laser and the repetition frequency difference of the solitons running in two directions can be adjusted by controlling the polarization in the cavity or changing the pump energy.

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

[0030] In summary, the present invention provides an all-fiber positive dispersion bidirectional mode-locked laser, which can realize bidirectional propagation of dissipative soliton mode-locked dual combs in an all-fiber device optical cavity, solving the pain points of existing positive dispersion bidirectional mode-locked lasers: 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, and on the basis of realizing 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 bidirectional 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, which has great application prospects and practical engineering use possibilities in the field of dual-comb spectrum measurement. 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 of the embodiment;

[0033] Figure 3 The spectrum in two directions under two mode-locked states of the embodiment is a schematic diagram of the RF spectrum detected by a photodetector after the two directions are combined;

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

[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 of the embodiment are compared with the spatial light positive dispersion bidirectional mode-locked laser;

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

[0037] Figure 7 Schematic diagram of 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 is 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 multi-mode 980nm pumped, and its maximum output is 3W. When the pump light source driving module gives it an output current of 3A, its output is 2W (33dBm).

[0041] The multimode fiber output of the pump light source is coupled into the optical cavity through a beam combiner (coupling device), and the pump input end of the beam combiner is still 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 an optical fiber polarization beam splitter and a polarization-maintaining optical fiber at a 45° deviation angle to form a mode-locking mechanism for nonlinear polarization rotation, and a lyot filter with a filtering period of 20nm. The filter has a filtering effect spectrum on the self-excited amplified radiation (ASE) generated by the ytterbium-doped optical fiber as shown in FIG. Figure 2 shown.

[0043] The mode-locked laser of the embodiment can be realized by adjusting the polarization controller as follows: Figure 3 The mode-locked spectrum shown is Figure 3 Two typical mode-locked states and the merged RF 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 bidirectional mode-locked laser of the embodiment is characterized and compared with the positive dispersion bidirectional mode-locked laser of spatial light, which further proves the excellent noise performance of the all-fiber positive dispersion bidirectional mode-locked laser of the 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 characterization of the repetition frequency and repetition frequency difference of 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 of the dual-wavelength bidirectional locking state of this embodiment.

[0045] Figure 7The figure is a schematic diagram of the self-start characterization of the two locked mode states of the all-fiber positive dispersion bidirectional mode-locked laser in the embodiment, which characterizes the stable self-start performance of the all-fiber positive dispersion bidirectional mode-locked laser. The pump is switched by using a square wave with a period of 10s and a duty cycle of 50%. At this time, the real-time change information of the spectrum output in two directions of the laser is tested. It can be seen that the laser shown in the embodiment can achieve stable bidirectional self-start of the switch pump in two locked mode states.

[0046] Due to the balance between positive dispersion in the optical cavity and spectral filtering and nonlinear effects, the present invention can generate stable positive dispersion mode-locked dissipative solitons in both directions at the same time. The positive dispersion mode-locked dissipative solitons in both directions can be of the same wavelength or different wavelengths, depending on the mode-locked state. The mode-locked state of the all-fiber positive dispersion bidirectional mode-locked laser and the repetition frequency difference of the solitons running in the two directions can be adjusted by controlling the polarization state in the optical cavity or changing the pump energy.

[0047] It can be seen from the above embodiments that the present invention realizes an all-fiber positive dispersion bidirectional operation mode-locked laser. It can achieve high-energy bidirectional pulse energy output and a relatively high bidirectional coincidence flat-top spectrum. Compared with the positive dispersion bidirectional operation mode-locked laser realized by spatial optical devices, it realizes the bidirectional dual-wavelength mode-locked state that it does not have on the basis of miniaturization, greatly improving the noise performance, stability, and resistance to physical vibration interference of the laser, and can achieve bidirectional stable self-starting in two mode-locked states.

Claims

1. An all-fiber positive dispersion bidirectionally operated mode-locked laser, characterized in that: Including 980nm single-frequency laser, coupling device, 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 respectively connected to two passive optical fibers, the two passive optical fibers are respectively connected to a fiber polarization beam splitter, and the two fiber polarization beam splitters are connected through a polarization maintaining optical fiber, thereby forming an optical cavity of a loop; The 980nm single-frequency laser is used as a pump source to generate pump light, and the 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 respectively, and the other end of the passive optical fiber is connected to an optical fiber polarization beam splitter respectively; 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 a polarization-maintaining fiber. The two fiber polarization beam splitters also serve as two output ends of the entire positive dispersion bidirectional mode-locked laser. The polarization-maintaining optical fiber has three sections in total, and the three sections are fused in sequence at deviation angles, wherein the middle section of the polarization-maintaining optical fiber is fused with the two side sections of the polarization-maintaining optical fiber at a deviation angle of 30-60°, respectively, to achieve a filtering effect and thus achieve mode locking; the three sections of the 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 bidirectional mode-locked laser as claimed in 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 bidirectional mode-locked laser as claimed in 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 bidirectional mode-locked laser as claimed in claim 1, characterized in that: The number of the pump lasers is one or more, and the type of the pump laser is single-mode or multi-mode pumping.

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

6. The all-fiber positive dispersion bidirectional mode-locked laser as claimed in 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 abnormal dispersion.

7. The all-fiber positive dispersion bidirectional mode-locked laser as claimed in claim 1, characterized in that: The middle section polarization-maintaining optical fiber can achieve regulation of the filtering effect of the optical fiber lyot filter by adjusting its length.

8. The all-fiber positive dispersion bidirectionally operated mode-locked laser as claimed in claim 1, characterized in that: The 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 bidirectional mode-locked laser as claimed in 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 operated mode-locked laser according to claim 1, characterized in that: Soliton repetition frequency difference is adjustable: 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 sequence and the clockwise repetition rate of the clockwise pulse sequence is changed by adjusting the squeezed polarization controller.

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