Mode-locking initiation assistance system and mode-locked fiber laser

Abstract

The application relates to the technical field of fiber lasers, in particular to a mode-locked starting auxiliary system and a mode-locked fiber laser. The mode-locked starting auxiliary system comprises a fiber Bragg grating arranged on an optical fiber; a piezoelectric ceramic arranged at the fiber Bragg grating; a control unit matched with the piezoelectric ceramic; the control unit is used for controlling the length change of the piezoelectric ceramic to realize the stretching of the fiber Bragg grating; and the fiber laser comprises the above mode-locked starting auxiliary system. The application adopts the mode of fixing the fiber Bragg grating in the resonant cavity of the mode-locked fiber laser on the optical fiber, then pulling the fiber Bragg grating through the piezoelectric ceramic (PZT) to introduce the modulation of the wavelength and the resonant cavity length into the laser, when the pulling frequency is equal to the base frequency of the mode-locked pulse supported by the resonant cavity, the effective modulation can be introduced, that is, the modulation with the same frequency as the interval of the laser longitudinal mode is introduced, thereby helping to lock the phase difference between the longitudinal modes, and starting the mode locking.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fiber lasers, in particular to a mode-locked start-up auxiliary system and a mode-locked fiber laser. BACKGROUND

[0002] Ultrafast lasers, that is, pulse width in picosecond and femtosecond pulse lasers, have important application value in precision machining, strong field laser physics, precision measurement, coherent communication and ultra-high speed imaging due to its high peak power, wide spectrum and high repetition frequency. In recent years, fiber lasers have become the main platform for realizing mode locking and obtaining ultrafast laser output due to their compact structure, strong anti-interference ability and high light output efficiency. At present, there are two ways to realize mode locking in fiber lasers, that is, directly adding a real saturable absorber in the cavity and using an equivalent saturable absorber based on nonlinear effects. Essentially, both ways are to filter out the part with strong light intensity in the time domain of the laser by the saturable absorption effect of the higher transmittance with the stronger light intensity, and to realize periodic modulation to lock the phase difference between each mode of the laser, so as to realize mode locking and output ultrafast laser.

[0003] However, in the process of outputting ultrafast laser in the fiber laser, how to realize the self-starting of mode locking is a key problem. When the laser starts to generate, the light intensity in the cavity will gradually increase from weak to strong, and the existence of saturable absorption will cause great loss when the light intensity is weak, thereby inhibiting the increase of light intensity and affecting the self-starting of mode locking. At present, the realization of mode locking in fiber lasers requires optimization of the parameters in the cavity, especially the related parameters of the saturable absorber or the equivalent saturable absorber, so that it has a certain transmittance at low power. However, this way will limit the performance of the ultrafast laser after mode locking. Moreover, even if the saturable absorption parameters are optimized, the mode-locked fiber lasers with small and large repetition rates (the number of pulses emitted per second) will still have the problem of difficulty in starting the fundamental mode locking. When the repetition rate is small, the length of the resonant cavity is large, and the laser will accumulate a large nonlinear phase shift when oscillating in the cavity, so that the mode-locked self-starting is often in the harmonic or noise-like state, which cannot meet the application requirements. When the repetition rate is large, the length of the resonant cavity is small, and the laser accumulates a small phase shift when oscillating in the cavity, so that a large pump power is often required to make the laser accumulate enough phase shift to realize mode locking.

[0004] Fiber Bragg gratings are often used in mode-locked fiber lasers to perform dispersion control, wavelength selection, filtering and output coupling in the laser. Fiber Bragg gratings are realized by introducing refractive index modulation on the fiber core, and the wavelengths satisfying the Bragg condition will be reflected in the grating. The length of the grating is generally in the order of several millimeters to several centimeters, and the reflection wavelength of the grating depends on the period of the refractive index modulation on the core. When the grating is stretched, the reflection wavelength becomes larger, and when the grating is compressed, the reflection wavelength becomes smaller.

[0005] In view of this, the application adopts the mode of fixing the fiber Bragg grating in the resonant cavity of the mode-locked fiber laser on the optical fiber, and then pulling the fiber Bragg grating through the piezoelectric ceramic (PZT) to introduce the modulation of the wavelength and the resonant cavity length into the laser. When the pulling frequency is equal to the fundamental frequency of the mode-locked pulse supported by the resonant cavity, effective modulation can be introduced, that is, the modulation with the same frequency as the laser longitudinal mode interval is introduced, thereby helping to lock the phase difference between the longitudinal modes, and thus starting the mode locking. SUMMARY

[0006] In view of the above problems existing in the prior art, the application provides a mode-locked start auxiliary system and a mode-locked fiber laser.

[0007] In order to solve the above technical problems, the application is solved by the following technical scheme:

[0008] In a first aspect, a mode-locked start auxiliary system is provided, which comprises,

[0009] a fiber Bragg grating arranged on an optical fiber;

[0010] a piezoelectric ceramic arranged on the fiber Bragg grating;

[0011] a control unit cooperating with the piezoelectric ceramic;

[0012] The control unit is used to control the change of the length of the piezoelectric ceramic to realize the stretching of the fiber Bragg grating by the piezoelectric ceramic.

[0013] As a preferred, the control unit comprises a piezoelectric ceramic control module and a signal generator,

[0014] The signal generator is used to output a modulation signal with the same frequency as the laser longitudinal mode interval in the mode-locked fiber laser;

[0015] One end of the piezoelectric ceramic control module is connected with the signal generator, and the other end is connected with the piezoelectric ceramic;

[0016] The piezoelectric ceramic control module is used to receive and amplify the modulation signal and output the modulation signal to the piezoelectric ceramic to realize the change of the length of the piezoelectric ceramic.

[0017] As a preferred, when the length of the piezoelectric ceramic is less than the grating area of the fiber Bragg grating, it further comprises a first substrate and / or a second substrate,

[0018] The first substrate is arranged at one end of the piezoelectric ceramic and fixed on the fiber Bragg grating to assist the stretching of the fiber Bragg grating by the piezoelectric ceramic;

[0019] The second substrate is arranged at the other end of the piezoelectric ceramic and fixed on the fiber Bragg grating to assist the piezoelectric ceramic to stretch the fiber Bragg grating.

[0020] In a second aspect, a mode-locked fiber laser is provided, comprising the mode-locked start-up assisting system, and further comprising a first wavelength division multiplexer, a first gain fiber, a first saturable absorber and a total reflection mirror connected in sequence; the first wavelength division multiplexer is connected with a first pump laser diode and the mode-locked start-up assisting system.

[0021] The first saturable absorber is a real saturable absorber or an equivalent saturable absorber.

[0022] In a third aspect, a mode-locked fiber laser is provided, comprising the mode-locked start-up assisting system, and further comprising a second wavelength division multiplexer, a second gain fiber and a second saturable absorber connected in sequence.

[0023] The second saturable absorber is connected with the second wavelength division multiplexer through a circulator.

[0024] The second wavelength division multiplexer is further connected with a second pump laser diode.

[0025] The circulator is further connected with the mode-locked start-up assisting system.

[0026] The second saturable absorber is a real saturable absorber or an equivalent saturable absorber.

[0027] In a fourth aspect, a mode-locked fiber laser is provided, comprising the mode-locked start-up assisting system, and further comprising a third wavelength division multiplexer, a third gain fiber and a phase bias connected in sequence.

[0028] The third wavelength division multiplexer is further connected with a third pump laser diode.

[0029] The phase bias is connected with the third wavelength division multiplexer through a fiber beam splitter.

[0030] The fiber beam splitter is further connected with the mode-locked start-up assisting system and a linear cavity respectively.

[0031] The present application has at least the following advantages:

[0032] The application adopts the method of fixing the fiber Bragg grating in the resonant cavity of the mode-locked fiber laser on the fiber, and then pulling the fiber Bragg grating through the piezoelectric ceramic (PZT) to introduce the modulation of wavelength and resonant cavity length into the laser. When the pulling frequency is equal to the fundamental frequency of the mode-locked pulse supported by the resonant cavity, effective modulation can be introduced, that is, the modulation with the same frequency as the longitudinal mode interval is introduced, so as to help the phase difference locking between the longitudinal modes, thereby starting the mode locking. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The schematic diagram of the mode-locked start auxiliary system for the fiber Bragg grating whose grating area is less than or equal to the length of the piezoelectric ceramic in the embodiment 1 of the application;

[0034] Figure 2 The schematic diagram of the mode-locked start auxiliary system for the fiber Bragg grating whose grating area is greater than the length of the piezoelectric ceramic in the embodiment 1 of the application;

[0035] Figure 3 The schematic diagram of the mode-locked start auxiliary system applied to the linear cavity mode-locked fiber laser in the embodiment 2 of the application;

[0036] Figure 4 The schematic diagram of the mode-locked start auxiliary system applied to the ring cavity mode-locked fiber laser in the embodiment 3 of the application;

[0037] Figure 5 The schematic diagram of the mode-locked start auxiliary system applied to the composite cavity mode-locked fiber laser combined with the linear cavity and the ring cavity in the embodiment 4 of the application.

[0038] The names of the parts referred to by the numbers in the drawings are as follows:

[0039] 100, mode-locked start auxiliary system; 110, optical fiber; 120, fiber Bragg grating; 130, piezoelectric ceramic; 140, piezoelectric ceramic control module; 150, signal generator; 210, first base plate; 220, second base plate; 310, first wavelength division multiplexer; 320, first gain optical fiber; 330, first saturable absorber; 340, total reflection cavity mirror; 350, first pump laser diode; 410, second wavelength division multiplexer; 420, second gain optical fiber; 430, second saturable absorber; 440, circulator; 450, second pump laser diode; 510, third wavelength division multiplexer; 520, third gain optical fiber; 530, phase biasing device; 540, optical fiber beam splitter; 550, third pump laser diode; 560, linear cavity. DETAILED DESCRIPTION

[0040] For further understanding of the present application, the application will be described in detail with reference to the drawings and embodiments. It should be understood that the embodiments are only used to explain the present application but not to limit the present application.

[0041] Embodiment 1

[0042] As shown in the figure, the embodiment provides a mode-locking starting auxiliary system 100, which comprises, Figures 1-2

[0043] a fiber Bragg grating 120 arranged on the optical fiber 110;

[0044] a piezoelectric ceramic 130 arranged on the fiber Bragg grating 120;

[0045] a control unit matched with the piezoelectric ceramic 130;

[0046] The control unit is used to control the length change of the piezoelectric ceramic 130 to realize the stretching of the fiber Bragg grating 120 by the piezoelectric ceramic 130.

[0047] In the embodiment, the piezoelectric ceramic 130 is annular or rectangular or cylindrical.

[0048] In the embodiment, the control unit comprises a piezoelectric ceramic control module 140 and a signal generator 150,

[0049] The signal generator 150 is used to output a modulation signal with the same frequency as the laser longitudinal mode interval in the mode-locked fiber laser;

[0050] One end of the piezoelectric ceramic control module 140 is connected with the signal generator 150, and the other end is connected with the piezoelectric ceramic 130;

[0051] The piezoelectric ceramic control module 140 is used to receive and amplify the modulation signal and output the modulation signal to the piezoelectric ceramic 130 to realize the length change of the piezoelectric ceramic 130.

[0052] In the embodiment, when the length of the piezoelectric ceramic 130 is less than the grating area of the fiber Bragg grating 120, the embodiment further comprises a first substrate 210 and / or a second substrate 220,

[0053] The first substrate 210 is arranged at one end of the piezoelectric ceramic 130 and fixed on the fiber Bragg grating 120 to assist the stretching of the fiber Bragg grating 120 by the piezoelectric ceramic 130;

[0054] ​The second substrate 220 is arranged at the other end of the piezoelectric ceramic 130 and fixed on the fiber Bragg grating 120 to assist the piezoelectric ceramic 130 to stretch the fiber Bragg grating 120.

[0055] It should be noted that the size of the grating region of the fiber Bragg grating 120 and the length of the piezoelectric ceramic 130 are not limited in the embodiment, and thus at least the following two cases exist:

[0056] 1) As shown in FIG. 1, the grating region of the fiber Bragg grating 120 is smaller than or equal to the length of the piezoelectric ceramic 130, and the fiber Bragg grating 120 is directly fixed on the piezoelectric ceramic 130; Figure 1

[0057] In use, the fiber Bragg grating 120 and the piezoelectric ceramic 130 are fixed, the signal generator 150 outputs a modulation signal with the same frequency as the laser longitudinal mode interval in the mode-locked fiber laser, the modulation signal is amplified after passing through the piezoelectric ceramic control module 140, and then is added between the positive and negative electrodes of the piezoelectric ceramic 130. The length of the piezoelectric ceramic 130 is changed by adjusting the amplitude of the modulation signal, and the piezoelectric ceramic 130 changes in length at a set frequency, and the fiber Bragg grating 120 is stretched at the same time, so that the wavelength and length of the fiber Bragg grating 120 also change periodically at a set frequency, thereby introducing the modulation of the wavelength and the resonant cavity intensity in the mode-locked fiber laser, and the modulation frequency is equal to the laser longitudinal mode interval, thereby facilitating the phase locking between the longitudinal modes and promoting the start of the mode locking.

[0058] 2) As shown in FIG. 2, the grating region of the fiber Bragg grating 120 is greater than the length of the piezoelectric ceramic 130, one end and the other end of the piezoelectric ceramic 130 are respectively adhered to the first substrate 210 and the second substrate 220, and the two ends of the fiber Bragg grating 120 are fixed on the first substrate 210 and the second substrate 220 or the piezoelectric ceramic 130; Figure 2

[0059] In use, after the modulation signal is added to the piezoelectric ceramic 130, the piezoelectric ceramic 130 stretches to change the total length of the first substrate 210, the second substrate 220 and the piezoelectric ceramic 130, and then introduces the periodic length modulation to the fiber Bragg grating 120, and the same periodic changes of the center wavelength and the length of the fiber Bragg grating 120 are achieved.

[0060] It can be understood that in the above 2), the first substrate 210 can be adhered to only one end of the piezoelectric ceramic 130 or the second substrate 220 can be adhered to only the other end of the piezoelectric ceramic 130.

[0061] Embodiment 2

[0062] ​​This embodiment illustrates the application of the mode-locking start-up assistance system 100 from Embodiment 1 in a linear cavity mode-locked fiber laser.

[0063] like Figure 3 As shown, this embodiment proposes a mode-locked fiber laser, including the mode-locking start-up auxiliary system 100 in embodiment 1, and further including a first wavelength division multiplexer 310, a first gain fiber 320, a first saturable absorber 330 and a total reflection cavity mirror 340 connected in sequence; a first pump laser diode 350 and the mode-locking start-up auxiliary system 100 are connected at the first wavelength division multiplexer 310.

[0064] In use, the pump laser emitted by the first pump laser diode 350 enters the first gain fiber 320 in the resonant cavity through the first wavelength division multiplexer 310 to provide gain. The fiber Bragg grating 120 and the total reflection cavity mirror 340 in the mode-locking start-up auxiliary system 100 provide feedback, and the first saturable absorber 330 provides saturable absorption. Combined with the mode-locking start-up auxiliary system 100, the fundamental frequency mode-locking self-start can be realized at different repetition frequencies.

[0065] Understandably, the fiber Bragg grating 120 in the mode-locked start-up assistance system 100 can also function as an output coupler, wavelength selector, filter, and dispersion modulator.

[0066] It should be noted that the first gain fiber 320 is a gain fiber doped with rare earth elements, or a highly nonlinear fiber, or a single-mode fiber; the first saturable absorber 330 is a real saturable absorber such as a semiconductor saturable absorber mirror, carbon nanotube, or graphene, or an equivalent saturable absorber such as a nonlinear polarization rotation, a nonlinear ring mirror, or a Mamyshev oscillator.

[0067] Example 3

[0068] This embodiment illustrates the application of the mode-locking start-up assistance system 100 from Embodiment 1 in a ring cavity mode-locked fiber laser.

[0069] like Figure 4 As shown, this embodiment proposes a mode-locked fiber laser, including the mode-locking start-up auxiliary system 100 in embodiment 1, and also including a second wavelength division multiplexer 410, a second gain fiber 420 and a second saturable absorber 430 connected in sequence.

[0070] The second saturable absorber 430 is connected to the second wavelength division multiplexer 410 via a circulator 440;

[0071] A second pump laser diode 450 is also connected to the second wavelength division multiplexer 410;

[0072] The circulator 440 is also connected to the mold-locking start-up auxiliary system 100.

[0073] In use, the pump laser emitted by the second pump laser diode 450 passes through the second wavelength division multiplexer 410 into the second gain fiber 420 in the resonant cavity to provide gain, and the second saturable absorber 430 provides saturable absorption, and the mode-locking start auxiliary system 100 can realize self-starting of the fundamental frequency mode-locking at different repetition frequencies.

[0074] It can be understood that the fiber Bragg grating 120 in the mode-locking start auxiliary system 100 can also play the roles of output coupler, wavelength selection, filtering, and dispersion control.

[0075] It should be noted that the second gain fiber 420 is a gain fiber doped with a rare earth element or a high nonlinear fiber or a single-mode fiber; the second saturable absorber 430 is a real saturable absorber such as a semiconductor saturable absorber mirror, carbon nanotubes, or graphene, or an equivalent saturable absorber such as a nonlinear polarization rotation, a nonlinear ring mirror, or a Mamyshev oscillator.

[0076] Embodiment 4

[0077] This embodiment is the application of the mode-locking start auxiliary system 100 in embodiment 1 in a composite cavity mode-locked fiber laser combining a linear cavity 560 and a ring cavity.

[0078] As shown in Figure 5 This embodiment proposes a mode-locked fiber laser, which includes the mode-locking start auxiliary system 100 in embodiment 1 and further includes a third wavelength division multiplexer 510, a third gain fiber 520, and a phase biasing device 530 connected in sequence.

[0079] The third wavelength division multiplexer 510 is further connected with a third pump laser diode 550.

[0080] The phase biasing device 530 is connected with the third wavelength division multiplexer 510 through a fiber beam splitter 540.

[0081] The fiber beam splitter 540 is connected with the mode-locking start auxiliary system 100 through a linear cavity 560.

[0082] In use, the pump laser emitted by the third pump laser diode 550 passes through the third wavelength division multiplexer 510 into the third gain fiber 520 in the resonant cavity to provide gain, and the ring cavity combining the fiber beam splitter 540 and the phase biasing device 530 plays the role of an equivalent saturable absorber, and the mode-locking start auxiliary system 100 can realize self-starting of the fundamental frequency mode-locking at different repetition frequencies.

[0083] It can be understood that the fiber Bragg grating 120 in the mode-locking starting auxiliary system 100 can also play the roles of output coupler, wavelength selection, filtering and dispersion regulation.

[0084] It should be noted that the third gain fiber 520 is a gain fiber doped with a rare earth element or a high nonlinear fiber or a single-mode fiber.

[0085] In summary, the above only describes the preferred embodiments of the present application, and any equivalent changes and modifications made according to the patent application scope of the present application shall be within the scope of the present application.

Claims

1. A mode-locking start-up assistance system applied to a mode-locked fiber laser, characterized in that: Comprising, a fiber Bragg grating, disposed on an optical fiber; a piezoelectric ceramic, disposed on the fiber Bragg grating; a control unit, cooperating with the piezoelectric ceramic; the control unit is used to control the length change of the piezoelectric ceramic to realize the stretching of the fiber Bragg grating by the piezoelectric ceramic; the control unit comprises a piezoelectric ceramic control module and a signal generator, the signal generator is used to output a modulation signal with the same frequency as the laser longitudinal mode spacing in the mode-locked fiber laser; one end of the piezoelectric ceramic control module is connected with the signal generator, and the other end is connected with the piezoelectric ceramic; the piezoelectric ceramic control module is used to receive and amplify the modulation signal and output the modulation signal to the piezoelectric ceramic to realize the length change of the piezoelectric ceramic, and at the same time, the fiber Bragg grating is stretched, so that the wavelength and length of the fiber Bragg grating also change periodically with a set frequency.

2. The lockout activation assist system of claim 1, wherein: when the length of the piezoelectric ceramic is less than the grating area of the fiber Bragg grating, a first substrate and / or a second substrate are further included, the first substrate is disposed at one end of the piezoelectric ceramic and fixed on the fiber Bragg grating to assist the piezoelectric ceramic in stretching the fiber Bragg grating; the second substrate is disposed at the other end of the piezoelectric ceramic and fixed on the fiber Bragg grating to assist the piezoelectric ceramic in stretching the fiber Bragg grating.

3. A mode-locked fiber laser characterized by: The mode-locked start-up auxiliary system of any one of claims 1-2 further comprises a first wavelength division multiplexer, a first gain optical fiber, a first saturable absorber and a total reflection cavity mirror connected in sequence; the first wavelength division multiplexer is connected with a first pump laser diode and the mode-locked start-up auxiliary system; the first saturable absorber is a real saturable absorber or an equivalent saturable absorber.

4. A mode-locked fiber laser characterized by: The mode-locked start-up auxiliary system of any one of claims 1-2 further comprises a second wavelength division multiplexer, a second gain optical fiber and a second saturable absorber connected in sequence; the second saturable absorber is connected with the second wavelength division multiplexer through a circulator; the second wavelength division multiplexer is further connected with a second pump laser diode; the circulator is further connected with the mode-locked start-up auxiliary system; the second saturable absorber is a real saturable absorber or an equivalent saturable absorber.

5. A mode-locked fiber laser characterized by: The mode-locked start-up auxiliary system of any one of claims 1-2 further comprises a third wavelength division multiplexer, a third gain optical fiber, a phase biasing device and an optical fiber beam splitter connected in sequence; the third wavelength division multiplexer is further connected with a third pump laser diode; the phase biasing device is connected with the third wavelength division multiplexer through the optical fiber beam splitter; the optical fiber beam splitter is connected with the mode-locked start-up auxiliary system through a linear cavity.

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