Femtosecond fiber laser based on double-nine cavity structure

Abstract

The application discloses a kind of femtosecond fiber lasers based on double nine-character cavity structure, and its pump laser is used to output resonant cavity pump light;Wave division multiplexer is used for the beam combination of pump light and signal light;Doped optical fiber is the gain medium for radiating broadband initial signal light;First polarization maintaining fiber coupler and second polarization maintaining fiber coupler are used for the light path coupling of clockwise resonant cavity and counterclockwise resonant cavity;First polarization maintaining dispersion compensation fiber and second polarization maintaining dispersion compensation fiber are used for dispersion management dispersion matching in resonant cavity;First fixed fiber phase shifter and second fixed fiber phase shifter are used for setting fixed phase shift in resonant cavity;First bandpass fiber filter and second bandpass fiber filter are used for wavelength selection of CCW resonant cavity and CW resonant cavity respectively.Based on hybrid mode-locked mechanism, the advantages of nonlinear amplification ring mirror mode-locking and saturable absorber mode-locking are combined, that is, the mode-locking stability of the system can be improved, and self-starting ability is also possessed.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast laser technology, specifically relating to a femtosecond fiber laser based on a dual-nine-cavity structure. Background Technology

[0002] Femtosecond fiber lasers are femtosecond pulsed lasers manufactured using optical fibers, including doped fiber gain media and fiber-based mode-locked resonant cavities. Femtosecond fiber lasers are considered a next-generation femtosecond laser due to their advantages such as miniaturization, portability, lack of water cooling requirements, low cost, and high-temperature stability. Femtosecond fiber lasers can be used to study various ultrafast physical phenomena, as pump sources for generating terahertz waves using ultrafast optoelectronic methods, and also as the core seed source for fiber frequency combs.

[0003] As is well known, terahertz time-domain spectroscopy systems using femtosecond fiber lasers as pump sources are among the most mature terahertz products currently available. However, current terahertz time-domain spectroscopy systems based on femtosecond fiber laser technology and mechanical delay line technology are limited in sampling rate by mechanical inertia and other issues, typically achieving sampling rates of less than 1 kHz, severely restricting the application and promotion of terahertz spectroscopy and imaging technologies. Based on this, terahertz time-domain spectroscopy systems based on the asynchronous sampling principle of dual femtosecond fiber lasers have been proposed and have gradually developed into mature products, solving the efficiency problem to some extent. However, terahertz systems based on dual femtosecond fiber lasers suffer from high cost, complex phase-locked control, and poor environmental adaptability, hindering their widespread adoption in the industry. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and to design a femtosecond fiber laser based on a dual-nine-cavity structure. By implementing the solution, asynchronous sampling can be achieved using only one laser, and a higher sampling rate can be achieved, thus solving the pain points of current asynchronous sampling technology.

[0005] To achieve the above objectives, the present invention relates to a femtosecond fiber laser based on a dual-nine-cavity structure, comprising a pump laser, a counterclockwise resonant cavity, and a clockwise resonant cavity; wherein, the counterclockwise resonant cavity comprises a wavelength division multiplexer, a doped fiber, a second polarization-maintaining fiber coupler, a first bandpass fiber filter, a third polarization-maintaining fiber coupler, a first fixed fiber phase shifter, a first polarization-maintaining dispersion compensation fiber, and a first polarization-maintaining fiber coupler sequentially connected to form a first fiber loop, and a first fiber-coupled saturable absorber; the third polarization-maintaining fiber coupler is connected to the output end of the counterclockwise resonant cavity and... The first fiber-coupled saturable absorber is connected; the clockwise resonant cavity includes a second fiber loop formed by sequentially connecting a wavelength division multiplexer, a doped fiber, a second polarization-maintaining fiber coupler, a second bandpass fiber filter, a fourth polarization-maintaining fiber coupler, a second fixed fiber phase shifter, a second polarization-maintaining dispersion compensation fiber, and the first polarization-maintaining fiber coupler, as well as the second fiber-coupled saturable absorber. The fourth polarization-maintaining fiber coupler is connected to the output end of the clockwise resonant cavity and the second fiber-coupled saturable absorber, respectively. The pump laser is connected to one port of the wavelength division multiplexer for outputting pump light to the resonant cavity.

[0006] The wavelength division multiplexer involved in this invention is used for combining pump light and signal light; the doped fiber is used as the gain medium of the resonant cavity for radiating broadband initial signal light; the first polarization-maintaining fiber coupler and the second polarization-maintaining fiber coupler are used for optical path coupling of the clockwise and counterclockwise resonant cavities; the first polarization-maintaining dispersion compensation fiber and the second polarization-maintaining dispersion compensation fiber are used for dispersion matching within the resonant cavity to achieve signal light pulse broadening; the first fixed fiber phase shifter and the second fixed fiber phase shifter are used to set a fixed phase offset within the resonant cavity; the first bandpass fiber filter and the second bandpass fiber filter are used for wavelength selection of the CCW resonant cavity and the CW resonant cavity, respectively.

[0007] The doped optical fiber involved in this invention is an erbium-ytterbium co-doped optical fiber.

[0008] The center wavelength of the first bandpass filter involved in this invention is 1560 nm.

[0009] The center wavelength of the second bandpass filter involved in this invention is set to 1490nm or 1310nm.

[0010] The third and fourth polarization-maintaining fiber couplers involved in this invention have a splitting ratio of 90:10.

[0011] Compared with the prior art, the present invention has the following advantages:

[0012] (1) Through innovative design of the resonant cavity structure, a new path is created for signal light of different wavelengths to operate simultaneously within the cavity and be mode-locked for output. Simultaneously, based on a hybrid mode-locking mechanism, the advantages of nonlinear amplifying ring mirror (NALM) mode-locking and saturable absorber (SAM) mode-locking are combined, improving both the mode-locking stability of the system and its self-starting capability. Furthermore, the scheme employs fully polarization-maintaining fiber devices, further ensuring the environmental adaptability and reliability of the laser. Furthermore, the laser oscillator with a nine-cavity structure has a shorter resonant cavity length, which is beneficial for achieving high repetition rate optical pulse output.

[0013] (2) Dual-wavelength lasers with a large difference in center wavelength have a large difference in group dispersion parameter, which directly leads to a large difference in repetition frequency of the dual-wavelength pulse sequence when forming stable laser oscillation. This is of great value for constructing a high-speed asynchronous sampling terahertz time-domain spectroscopy system. At the same time, due to the shared resonant cavity, the repetition frequency fluctuation of the dual-wavelength pulse sequence has a consistent response to environmental changes, which is very beneficial to improving the environmental adaptability of the asynchronous sampling system. In addition, the asynchronous optical sampling purpose can be achieved using only one set of lasers, which is also of great economic value. Furthermore, dual-wavelength lasers with a large repetition frequency difference have great development potential in constructing dual optical RF frequency combs and generating mid-to-low frequency optical frequency combs through heterodyne methods. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the femtosecond fiber laser structure based on the dual-nine-cavity structure involved in Example 1. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. In the present invention, "first," "second," etc., are used to distinguish components with the same structure and do not constitute a limitation on the invention.

[0016] Example 1

[0017] like Figure 1 As shown, the femtosecond fiber laser based on a dual-nine-cavity structure involved in this embodiment includes a pump laser 1, a counterclockwise (CCW) resonant cavity, and a clockwise (CW) resonant cavity;

[0018] The counterclockwise resonant cavity includes a wavelength division multiplexer 2, a doped fiber 3, a second polarization-maintaining fiber coupler 5, a first bandpass fiber filter 10, a third polarization-maintaining fiber coupler 12, a first fixed fiber phase shifter 8, a first polarization-maintaining dispersion compensation fiber 6, and a first polarization-maintaining fiber coupler 4 connected in sequence to form a first fiber loop, as well as a first fiber-coupled saturable absorber 14. The third polarization-maintaining fiber coupler 12 is connected to the output end of the counterclockwise resonant cavity and the first fiber-coupled saturable absorber 14, respectively.

[0019] The clockwise resonant cavity includes a wavelength division multiplexer 2, a doped fiber 3, a second polarization-maintaining fiber coupler 5, a second bandpass fiber filter 11, a fourth polarization-maintaining fiber coupler 13, a second fixed fiber phase shifter 9, a second polarization-maintaining dispersion compensation fiber 7, and a first polarization-maintaining fiber coupler 4 connected in sequence to form a second fiber loop, as well as a second fiber-coupled saturable absorber 15. The fourth polarization-maintaining fiber coupler 13 is connected to the output end of the clockwise resonant cavity and the second fiber-coupled saturable absorber 15, respectively.

[0020] Pump laser 1 is connected to one port of wavelength division multiplexer 2 and is used to output resonant cavity pump light, with a center wavelength typically of 980nm.

[0021] Wavelength division multiplexer 2 is used for combining pump light and signal light;

[0022] Doped fiber 3 is the gain medium of the resonant cavity. It is generally made of highly doped fiber and is used to radiate broadband initial signal light.

[0023] The first polarization-maintaining fiber coupler 4 and the second polarization-maintaining fiber coupler 5 are used for optical path coupling of the clockwise resonant cavity and the counterclockwise resonant cavity.

[0024] The first polarization-maintaining dispersion compensation fiber 6 and the second polarization-maintaining dispersion compensation fiber 7 are used for dispersion matching in the dispersion management resonant cavity to achieve signal light pulse broadening.

[0025] The first fixed fiber phase shifter 8 and the second fixed fiber phase shifter 9 are used to set a fixed phase shift within the resonant cavity.

[0026] The first bandpass fiber filter 10 and the second bandpass fiber filter 11 are used for wavelength selection of the CCW resonant cavity and the CW resonant cavity, respectively.

[0027] The doped fiber 3 involved in this embodiment uses erbium-ytterbium co-doped fiber (Er). 3+ / Yb 3+ ) fiber optic, the reason being Er 3+ / Yb 3+ Co-doping can improve Er 3+ The doping concentration of ions, and also due to Yb 3+ Sensitization effect, and Er 3+ / Yb3+ The cross-relaxation energy conversion process of ions can yield a wider gain spectrum and higher pump efficiency.

[0028] The center wavelength of the first bandpass filter 10 involved in this embodiment is 1560nm.

[0029] In order to generate laser oscillations with different center wavelengths with the CCW direction resonant cavity, the center wavelength of the second bandpass filter 11 involved in this embodiment is set to 1490nm or 1310nm.

[0030] In this embodiment, the splitting ratio of the third polarization-maintaining fiber coupler 12 and the fourth polarization-maintaining fiber coupler 13 is 90:10.

[0031] In this embodiment, the first fiber-coupled saturable absorber 14 and the second fiber-coupled saturable absorber 15 are auxiliary phase-locking devices and are not the primary means of mode-locking. Under normal circumstances, the pulse width of the light irradiating the saturable absorber is relatively large, which helps to extend the lifespan of the saturable absorber.

[0032] In this embodiment, the signal optical channels and COM ports of the first polarization-maintaining fiber coupler 4, the second polarization-maintaining fiber coupler 5, and the wavelength division multiplexer 2 are all configured with large bandwidth to ensure high transmittance of dual-wavelength signal light.

[0033] This invention constructs a dual-nine-ring cavity femtosecond fiber laser oscillator structure with a shared pump source and doped fiber. The mode-locking method is based on a hybrid mode-locking mechanism of nonlinear amplifying ring mirror and saturable absorber (SAM). The intracavity dispersion type alternates between normal and anomalous. The goal is to achieve the generation and output of dispersion-managed solitons.

[0034] The working principle of the femtosecond fiber laser based on the dual-nine-cavity structure involved in this embodiment is as follows:

[0035] like Figure 1 The pump laser 1 outputs a 980nm continuous beam, which is coupled into the erbium-ytterbium co-doped fiber 3 through the 980nm port of the wavelength division multiplexer 2. The pump light induces stimulated transitions in the erbium-ytterbium co-doped fiber 3, with upper-level ions spontaneously transitioning to lower levels and radiating a broadband initial signal light. After exiting the erbium-ytterbium co-doped fiber 3, the signal light is split into two beams by the second polarization-maintaining fiber coupler 5, which enter the CW resonant cavity and the CCW resonant cavity, respectively.

[0036] The signal light entering the CCW resonant cavity first passes through a first bandpass filter 10 with a center wavelength of 1560 nm, and then through a third polarization-maintaining fiber coupler 12 with a splitting ratio of 90:10. 90% of the split light enters the first fiber-coupled saturable absorber 14, undergoes saturable absorption in the first fiber-coupled saturable absorber 14, and is reflected back. The reflected light returns to the resonant cavity and propagates counterclockwise and clockwise along the CCW resonant cavity. Subsequently, it passes through the first fixed fiber phase shifter 8, which generates a certain additional phase shift, which helps the nonlinear accumulation of the nonlinear amplification ring mirror. The phase-shifted signal light continues to propagate, and after passing through the first polarization-maintaining dispersion compensation fiber 6, it will generate dispersion opposite to that of the preceding fiber, thereby broadening the signal light pulse. The broadened signal light is compressed to a certain extent in subsequent propagation due to the presence of anomalous dispersion, and finally continues to return to the erbium-ytterbium co-doped fiber as signal light for amplification via the first polarization-maintaining fiber coupler 4 and wavelength division multiplexer 2. 10% of the beam split from the third polarization-maintaining fiber coupler 12 will be directly used as the laser output.

[0037] Similarly, the signal light entering the clockwise CW resonant cavity will also undergo the same process, and 10% of the beam split by the fourth polarization-maintaining fiber coupler 13 will be directly used as the laser output.

Claims

1. A femtosecond fiber laser based on a double-nine cavity structure, characterized in that, The device comprises a pump laser, a counterclockwise resonant cavity and a clockwise resonant cavity; wherein the counterclockwise resonant cavity comprises a wavelength division multiplexer, a doped optical fiber, a second polarization maintaining fiber coupler, a first bandpass fiber filter, a third polarization maintaining fiber coupler, a first fixed fiber phase shifter, a first polarization maintaining dispersion compensation fiber and a first polarization maintaining fiber coupler which are sequentially connected to form a first fiber loop, and a first fiber-coupled saturable absorber, the third polarization maintaining fiber coupler is connected with the output end of the counterclockwise resonant cavity and the first fiber-coupled saturable absorber respectively; the clockwise resonant cavity comprises a wavelength division multiplexer, a doped optical fiber, a second polarization maintaining fiber coupler, a second bandpass fiber filter, a fourth polarization maintaining fiber coupler, a second fixed fiber phase shifter, a second polarization maintaining dispersion compensation fiber and a first polarization maintaining fiber coupler which are sequentially connected to form a second fiber loop, and a second fiber-coupled saturable absorber, the fourth polarization maintaining fiber coupler is connected with the output end of the clockwise resonant cavity and the second fiber-coupled saturable absorber respectively; the pump laser is connected to a port of the wavelength division multiplexer and is used for outputting resonant cavity pump light; The center wavelength of the first bandpass filter is 1560 nm; the center wavelength of the second bandpass filter is set to 1490 nm or 1310 nm.

2. The dual-nine-hole-cavity-structure-based femtosecond fiber laser of claim 1, wherein, The wavelength division multiplexer is used for beam combination of pump light and signal light; the doped optical fiber is used as a gain medium of the resonant cavity and is used for radiating broadband initial signal light; the first polarization maintaining fiber coupler and the second polarization maintaining fiber coupler are used for optical path coupling of the clockwise resonant cavity and the counterclockwise resonant cavity; the first polarization maintaining dispersion compensation fiber and the second polarization maintaining dispersion compensation fiber are used for dispersion management of dispersion matching in the resonant cavity to realize signal light pulse broadening; the first fixed fiber phase shifter and the second fixed fiber phase shifter are used for setting a fixed phase offset in the resonant cavity. The first bandpass fiber filter and the second bandpass fiber filter are respectively used for wavelength selection of the counterclockwise resonant cavity and the clockwise resonant cavity.

3. The dual-nine-hole-cavity-structure-based femtosecond fiber laser of claim 1, wherein, The doped optical fiber adopts an erbium-ytterbium co-doped optical fiber.

4. The dual-nine-hole-cavity-structure-based femtosecond fiber laser of claim 1, wherein, The splitting ratio of the third polarization maintaining fiber coupler and the fourth polarization maintaining fiber coupler is 90:

10. The splitting ratio of the third polarization maintaining fiber coupler and the fourth polarization maintaining fiber coupler is 90:10.

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