NPE mode-locking polarization-maintaining large-mode-field fiber laser with enhanced self-starting capability

Abstract

The utility model provides an NPE mode-locking polarization-maintaining large-mode-field fiber laser with enhanced self-starting capability, and relates to the technical field of fiber lasers, the fiber laser comprises a pumping source and a laser oscillation cavity, the pumping source is connected with a gain fiber through a fiber beam combiner, and the laser oscillation cavity is connected with the gain fiber through a fiber beam combiner. The gain fiber is connected with a spatial light path of the laser oscillation cavity part through the photonic crystal fiber, a polarization beam splitter is arranged in the laser oscillation cavity, and a phase shifter is arranged between the polarization beam splitter and a spatial light path of the pumping source part and used for realizing nonlinear polarization rotation (NPE) mode locking; the polarization beam splitter is arranged at the position where two orthogonal polarization states transmitted in the optical fiber interfere, dual-port output of the laser is provided, vertical polarized light is output from a reflection port, horizontal polarized light is output from a transmission port, and the mode locking self-starting capacity and stability of the laser are improved.

Description

Technical Field

[0001] The utility model relates to the technical field of fiber lasers, and particularly relates to an NPE mode-locked polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability. Background Technique

[0002] Mode-locking technology is one of the most commonly used methods for realizing the output of ultra-short laser pulses at present, including active mode-locking and passive mode-locking. Active mode-locking is achieved by adding an artificial modulator in the cavity. It is often limited by the response time of the modulator, and the pulse width of its output can only reach the order of nanoseconds (ns) and picoseconds (ps), and it is difficult to obtain ultra-short pulses. Passive mode-locking uses the saturable absorber effect and realizes mode-locking by using the structure of the resonant cavity itself rather than external modulation. Its response speed is fast, and the pulse width of the output can generally achieve picoseconds or even femtosecond-level pulses. In addition, passive mode-locking has many advantages such as relatively simple structure, low cost, and good stability, and is more widely used in many mode-locked oscillators.

[0003] The passive mode-locking mechanism often uses a saturable absorber to achieve mode-locking. The existing relatively mature passive mode-locking mechanisms include real saturable absorber mode-locking and artificial saturable absorber mode-locking. Among them, the recovery time of the real saturable absorber is between hundreds of femtoseconds and nanoseconds, which limits the generation of ultra-short pulses. The artificial saturable absorber only needs a few femtoseconds and belongs to a fast-saturating type of absorber, which is more likely to generate ultra-short pulses with high peak values. Its two common forms are the non-linear polarization rotation evolution (NPE) mode-locking technology and the non-linear loop mirror (NALM) mode-locking technology.

[0004] Fiber lasers based on mode-locking technology can easily obtain the output of nanosecond and femtosecond pulses. Common fiber lasers use non-polarization-maintaining fibers as fiber components. In the case of long-term operation of the laser, the mode-locking is unstable due to environmental influence, which affects the output stability of the laser. This has also become a stumbling block for the commercialization of NPE mode-locked fiber lasers. Polarization-maintaining fibers are not sensitive to the external environment. Replacing non-polarization-maintaining fibers with polarization-maintaining fibers can avoid the modulation instability caused by the weak birefringence effect of non-polarization-maintaining fibers, and then improve the overall environmental stability of the laser cavity. It is an essential element for realizing the commercialization of fiber lasers.

[0005] The true saturable absorber mode-locking technology is a common method to achieve mode-locking of polarization-maintaining fiber lasers. Common devices include semiconductor saturable absorber mirrors (SESAMs), graphene, carbon nanotubes, black phosphorus, etc. However, since the true saturable absorber may face problems such as performance degradation after long-term operation and needs to be replaced regularly to maintain the performance of the laser, there are problems such as difficult post-maintenance when only using the true saturable absorber to construct a polarization-maintaining fiber laser. The artificial saturable absorber mode-locking technology can ensure long-term stability and avoid post-maintenance problems. Among them, the NALM mode-locking technology is naturally suitable for polarization-maintaining fiber lasers. However, the traditional "8"-shaped oscillation cavity needs to accumulate a certain amount of nonlinear phase shift to achieve mode-locking. Therefore, the cavity length cannot be too short, and it is impossible to achieve high-repetition-rate pulse output, and the output energy is also limited. The NPE mode-locking technology can achieve high-repetition-rate and high-power pulse output. However, due to the polarization evolution characteristics of the NPE technology, it is mainly used for non-polarization-maintaining fibers. If it is used in polarization-maintaining fibers, the polarization evolution will fail due to time delay.

[0006] To achieve high stability, high repetition rate, and high-power output of polarization-maintaining fiber lasers, most of the previous reported works were usually polarization-maintaining fiber lasers with a ring cavity structure. To compensate for the problems of pulse walk-off and group velocity mismatch, it was necessary to angle-fuse multiple sections of fiber to multiple sections of fiber, and the length of each section of fiber needed to be precisely controlled proportionally. This was a difficult problem at the technical level. Therefore, in experiments, the phenomenon of irregular output spectra would occur. In extreme cases, pulse splitting would also occur, and stable pulse output could not be achieved. Due to technical and other problems, the above solutions could not be mass-produced, which was not conducive to the commercial development of fiber lasers. In addition, so far, most of the research on mode-locked large-mode-field fiber oscillators has focused on ytterbium-doped fibers operating in the 11m region. Although extensive research has been carried out on traditional erbium-doped ultrafast fiber oscillators operating around 1.551m, the development of erbium-doped large-mode-field fiber oscillators lags behind that of their ytterbium-doped fiber counterparts. Erbium-doped large-mode-field fibers are usually co-doped with Yb ions to increase pump absorption and reduce upconversion, and co-doped with a large amount of phosphorus to enhance pump transmission. However, phosphorus increases the core refractive index, making it challenging to fabricate erbium-ytterbium co-doped large-mode-field fibers with a low numerical aperture. Therefore, Er / Yb-doped large-mode-field step-index fibers usually contain some high-order modes, which are harmful to mode locking. So far, even with high-order mode suppression, there have been only a few reports on mode locking of Er / Yb-doped high-order mode fibers. It has been reported that the highest pulse energy of a mode-locked Er / Yb-doped large-mode-field fiber oscillator is 20nJ, but the corresponding pulse duration (20ps) is relatively long. If a customized multi-core erbium-ytterbium co-doped large-mode-field single-mode fiber is used as the gain medium, the fiber oscillator directly generates soliton pulses with a duration of 1.6ps and an energy of 9.1nJ. However, these Er / Yb-doped large-mode-field fiber oscillators are mode-locked by a material-saturable absorber and are based on non-polarization-maintaining large-mode-field fibers, which is not conducive to long-term stability. Not to mention that the perturbation of high-order modes may further deteriorate the mode-locking stability. In terms of technical scalability and large-scale reproducibility, it is necessary to overcome these limitations and seek a widely applicable solution. Summary of the Invention

[0007] In view of this, the present invention proposes an NPE mode-locked polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability. By introducing a Faraday rotator and a total reflector into the laser cavity, adaptive compensation for group velocity delay and linear phase bias is achieved, and high-order modes are suppressed by photonic crystal fiber, improving the mode-locking self-starting ability and stability of the laser, and solving the problems of group velocity mismatch, poor mode-locking stability, and high self-starting threshold existing in the prior art by an adaptive compensation method.

[0008] The present invention is implemented by the following technical solutions:

[0009] In a first aspect, the present utility model provides an NPE mode-locked large mode field fiber laser with enhanced self-starting ability, comprising a pump source and a laser oscillation cavity. The pump source is connected to a gain fiber through a fiber combiner, and the gain fiber is connected to the spatial optical path of the laser oscillation cavity through a photonic crystal fiber. A polarization beam splitter is provided in the laser oscillation cavity, and a phase shifter is provided between the polarization beam splitter and a part of the spatial optical path of the pump source to achieve non-linear polarization rotation (NPE) mode locking. The polarization beam splitter is arranged at the position where two orthogonal polarization states transmitted in the fiber interfere, providing dual-port output of the laser. Vertically polarized light is output from the reflection port, and horizontally polarized light is output from the transmission port.

[0010] As a further solution of the present utility model, the spatial optical paths of the pump source part and the laser oscillation cavity part include Faraday rotators and reflectors arranged at both ends of the linear cavity.

[0011] As a further solution of the present utility model, the fiber combiner is a pump light-signal light combiner for combining pump light and signal light and then coupling them into the gain fiber, and the pump source is used to provide 976 nm pump light.

[0012] As a further solution of the present utility model, the gain fiber is a 1.6 m long double-clad polarization-maintaining erbium-ytterbium co-doped large mode field fiber with a core diameter of 251 m and a numerical aperture of 0.09, which is used to amplify the signal light.

[0013] As a further solution of the present utility model, the gain fiber is pumped by a 976 nm multimode laser diode through a pump light-signal light combiner based on passive PM-LMA fiber. The corresponding core diameter of the fiber is 251 m, the numerical aperture (Numerical Aperture, NA) is 0.08, and the length is 60 cm.

[0014] As a further solution of the present utility model, the photonic crystal fiber includes a first photonic crystal fiber and a second photonic crystal fiber respectively arranged at the output ends of the gain fiber and the passive fiber. In the NPE mode-locked large mode field fiber laser with enhanced self-starting ability, the two-end photonic crystal fibers are respectively fusion-spliced to one side of the output ends of the gain fiber and the passive large mode field fiber, and the remaining free ends are polished at an angle of 8° to eliminate parasitic oscillation and increase the environmental stability of the laser cavity.

[0015] As a further solution of the present utility model, the gain fiber has a group velocity dispersion of -28.6 ps / (km), and the gain fiber is placed in a cooling device, and a layer of high refractive index glue is covered at the fusion joints of the photonic crystal fiber, the gain fiber and the passive large mode field fiber, which is used as a pump light stripper to prevent overheating damage under high pump power and slightly improve the self-starting ability.

[0016] As a further solution of the present utility model, an optical fiber connector is connected to the free end of the photonic crystal fiber, and a collimating lens is provided between the optical fiber connector and the Faraday rotator of the spatial optical path.

[0017] As a further solution of the present utility model, a first optical fiber connector is connected to the free end of the first photonic crystal fiber, a first collimating lens is provided between the first optical fiber connector and the first Faraday rotator, a second optical fiber connector is connected to the free end of the second photonic crystal fiber, and a second collimating lens is provided between the second optical fiber connector and the second Faraday rotator.

[0018] As a further solution of the present utility model, the spatial optical path of the pump source part includes a first Faraday rotator and a first mirror, which are used to rotate the main axis of the elliptically polarized light by 90°, and compensate for the group velocity delay and linear phase bias.

[0019] As a further solution of the present utility model, the pump light of the pump source enters the large-mode field gain fiber through the pump light-signal light combiner, the generated signal light is transmitted bidirectionally in the first photonic crystal fiber and enters the spatial optical path of the pump source part, the main axis of the elliptically polarized light is rotated by 90° through the first Faraday rotator and the first mirror, and then returns to the linear cavity. The spatial optical path of the pump source part is used to rotate the polarization state of the light beam and return it to the linear cavity, compensating for the group velocity delay and linear phase bias.

[0020] As a further solution of the present utility model, the spatial optical path of the laser oscillation cavity part includes a second Faraday rotator and a second mirror. The phase shifter is composed of a second Faraday rotator, a half-wave plate and a first quarter-wave plate, which is used to realize non-linear polarization rotation mode locking. A second quarter-wave plate is arranged between the polarization beam splitter and the second mirror. The spatial optical path of the laser oscillation cavity part is used to receive the signal light returned by the first mirror and amplified by the gain fiber in the spatial optical path of the pump source part.

[0021] As a further solution of the present utility model, the polarization beam splitter cooperates with the second quarter-wave plate and the second mirror to realize the pulse output of the dual ports. The vertically polarized light is output from the reflection port of the polarization beam splitter, and the horizontally polarized light is output from the other reflection port of the polarization beam splitter after passing through the second mirror.

[0022] As a further solution of the present utility model, a true saturable absorber mode locking is also provided in the NPE mode-locked large mode field fiber laser with enhanced self-starting ability, which is arranged in the laser cavity; the true saturable absorber mode locking includes a third collimating lens and a semiconductor saturable absorber mirror, the third collimating lens is an aspherical lens, and the semiconductor saturable absorber mirror in the true saturable absorber mode locking replaces the second reflector in the spatial optical path of a part of the laser oscillation cavity; the third collimating lens in the true saturable absorber mode locking replaces the second quarter-wave plate in the spatial optical path of a part of the laser oscillation cavity; the true saturable absorber mode locking is used to reduce the mode-locking self-starting threshold of the laser, improve the self-starting ability of the laser under low pump power, and the mode-locking mechanism is still dominated by NPE.

[0023] Compared with the prior art, the NPE mode-locked large mode field fiber laser with enhanced self-starting ability provided by the present utility model has the following beneficial effects:

[0024] 1. Effectively compensates for group velocity mismatch and evolution pulse time delay. The present utility model introduces a Faraday rotator and a total reflector at one end of the online cavity, and realizes perfect compensation of group velocity delay and linear phase bias through the exchange of the optical path. This design avoids the complex operation of precisely controlling the fusion angle of the optical fiber segment in the traditional method, and greatly improves the practicability and stability of the system. Through adaptive compensation, the technical bottleneck caused by group velocity mismatch and time delay evolution in the application of NPE mode-locking technology in polarization-maintaining optical fiber is solved.

[0025] 2. Suppresses high-order modes and improves the mode-locking self-starting ability. The present utility model fuses a small section of endlessly single-mode photonic crystal fiber (PCF) on both sides of the gain optical fiber and the pump light-signal optical fiber combiner, and effectively suppresses the influence of high-order modes by using the unique characteristics of PCF. Through this design, the laser can maintain a stable mode-locked state, significantly improve the mode-locking self-starting ability, enhance the stability and consistency of the laser mode, and the beam quality of the laser output is also improved due to the endless single-mode characteristic of PCF.

[0026] 3. Simplifies the optical fiber structure and improves the environmental stability. The gain optical fiber and the passive large mode field optical fiber in the present utility model are both double-clad polarization-maintaining optical fibers. Although non-polarization-maintaining optical fibers (such as photonic crystal fibers) are used in the optical path, since their lengths are short, the entire laser cavity is still mainly composed of polarization-maintaining optical fibers, thus ensuring the nonlinear pulse shaping and environmental stability of the system. Combined with the application of the cooling water tank, overheating damage of the optical fiber fusion point under high pump power is further prevented, ensuring the long-term reliable operation of the system.

[0027] 4. An SESAM is introduced to assist in mode locking, significantly reducing the mode-locking threshold. The present utility model proposes to replace the second mirror in the original optical path with an SESAM structure, which greatly reduces the pump power required to initiate mode locking. Moreover, the introduction of the SESAM only serves to assist in mode locking, without changing the mode-locking state, and at the same time, it also plays a role in smoothing the spectrum.

[0028] 5. The present utility model designs an optical path including elements such as a first Faraday rotator, a second Faraday rotator, a half-wave plate, a quarter-wave plate, and a polarization beam splitter. This design not only realizes NPE mode locking but also, through the beam splitting effect of the second quarter-wave plate, enables the horizontally polarized light transmitted from the polarization beam splitter to be reflected from the other side of the polarization beam splitter after being reflected by a mirror, thereby obtaining a dual-port pulse output.

[0029] In summary, the present utility model provides an NPE mode-locked polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability. Through the comprehensive optimization of the fiber structure, mode-locking mechanism, phase compensation, and beam quality, the performance and operation convenience of the laser are significantly improved, enabling high-quality pulse output under low power consumption and high stability conditions, and having broad application prospects. The design of the present utility model also fully considers the operation convenience of actual operation and the adaptability of the system. By introducing the adaptive compensation method and photonic crystal fiber, not only the problems of complex operation and difficulty in implementation of traditional solutions are solved, but also the stability and efficiency of the system in different environments are ensured, making the laser show stronger practicality in various application scenarios.

[0030] These aspects or other aspects of the present utility model will be more clearly understood in the following description of the embodiments. It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present utility model. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] To more clearly illustrate the technical solutions in the embodiments of the present utility model or related technologies, the following will briefly introduce the drawings required for describing the exemplary embodiments or related technologies. The drawings are used to provide a further understanding of the present utility model and constitute a part of the specification, and are used together with the embodiments of the present utility model to explain the present utility model, and do not constitute a limitation to the present utility model. In the drawings:

[0032] Figure 1 is a structural diagram of an NPE mode-locked polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability according to an embodiment of the present utility model;

[0033] Figure 2 is a structural diagram of mode locking using a real saturable absorber in an NPE mode-locked polarization-maintaining large-mode-field fiber laser according to an embodiment of the present utility model;

[0034] Figure 3 Spectra and autocorrelation curves of two output ports when the pump power is 2.38W in the NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to the embodiment of the present invention;

[0035] Figure 4 Schematic diagram of the beam quality corresponding to two output ports when the pump power is 2.38W in the NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to the embodiment of the present invention;

[0036] Figure 5 Spectra of two output ports in the NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to the embodiment of the present invention after the mirror is replaced by SESAM;

[0037] Figure 6 Schematic diagram of the beam quality corresponding to two output ports in the NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to the embodiment of the present invention after the mirror is replaced by SESAM;

[0038] Figure 7 Schematic diagram of the autocorrelation curve corresponding to two output ports in the NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to the embodiment of the present invention after the mirror is replaced by SESAM. Detailed implementation manners

[0039] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0040] In some processes described in the specification and claims of the present invention and the above-mentioned drawings, a plurality of operations appear in a specific order. However, it should be clearly understood that these operations may not be executed in the order in which they appear herein or may be executed in parallel. The operation numbers such as 101, 102, etc. are only used to distinguish different operations, and the numbers themselves do not represent any execution order. In addition, these processes may include more or fewer operations, and these operations may be executed in sequence or in parallel. It should be noted that the descriptions such as "first" and "second" in this article are used to distinguish different messages, devices, modules, etc., and do not represent a sequence, nor do they limit that "first" and "second" are of different types.

[0041] The technical solutions in the exemplary embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the exemplary embodiments of the present invention. Obviously, the described exemplary embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts belong to the protection scope of the present invention.

[0042] When solving the problems that the evolution pulse time delay cannot be compensated and the group velocity mismatch occur when the NPE mode-locking technology is applied to polarization-maintaining fibers, the existing solutions compensate for the time delay caused by transmission in polarization-maintaining fibers by controlling the fusion angle of the fiber segments. However, this solution is complex to operate and has low practicability. Therefore, the solution proposed by the NPE mode-locking polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability provided by the present invention is to add a Faraday rotator and a total reflector at one end of the linear cavity, and achieve perfect compensation of group velocity delay and linear phase bias by exchanging the optical paths of the round-trip process, and solve this technical problem by an adaptive compensation method.

[0043] When solving the problem that the high-order modes introduced by using a large-mode-field area gain fiber affect the mode locking of the laser, the present invention respectively fuses a small section of photonic crystal fiber (PCF) on one side of the gain fiber and on one side of the pump light-signal optical combiner, and uses the non-cutoff single-mode transmission characteristic of the PCF to suppress the high-order modes.

[0044] The technical solutions of the present invention will be further described below with reference to specific embodiments:

[0045] Refer to Figure 1 As shown, the embodiment of the present invention provides an NPE mode-locking polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability. The fiber laser is composed of two parts: a pump source 1 and a laser oscillation cavity 22. The pump source 1 is connected to a gain fiber 3 through an optical fiber combiner. The gain fiber 3 bi-directionally transmits the pump light through the photonic crystal fiber and respectively enters the spatial optical paths of the pump source 1 part and the laser oscillation cavity 22 part. The spatial optical paths include a Faraday rotator and a reflector provided at both ends of the linear cavity. A polarization beam splitter 15 is provided in the laser oscillation cavity 22. A phase shifter 18 for realizing NPE mode locking by performing phase bias is provided between the polarization beam splitter 15 and the spatial optical path of the pump source 1 part. The polarization beam splitter 15 is arranged at the position where two orthogonally polarized states transmitted in the optical fiber interfere, and is used to separate the optical signal into two orthogonally polarized states and provide a dual-port output of the laser. Among them, the vertically polarized light is output from the reflection port (R port), and the horizontally polarized light is output from the transmission port (T port).

[0046] In this embodiment, the gain fiber 3 is a 1.6 m long double-clad polarization-maintaining erbium-ytterbium co-doped large mode field fiber (Nufern, PLMA-EYDF-25P / 300-HE), with a core diameter of 251 μm, a numerical aperture of 0.09, and is used to amplify the signal light. The gain fiber 3 is pumped by a 976 nm multimode laser diode through a pump light-signal light combiner 2 based on a passive PM-LMA fiber (iXblue, IXF-2CF-PAS-PM-25-300-0.08). The corresponding fiber has a core diameter of 251 μm, a numerical aperture (Numerical Aperture, NA) of 0.08, and a length of 60 cm.

[0047] Among them, NA is the numerical aperture, and the calculation formula of the numerical aperture (NA) is:

[0048]

[0049] Among them, n1 is the refractive index of the fiber core layer; n2 is the refractive index of the fiber cladding layer. The 0.08 in NA = 0.08 is a dimensionless pure numerical value representing the size of the numerical aperture of the fiber. The value of the numerical aperture determines the angle at which the fiber can accept or emit light. The larger the value, the larger the incident angle of the light that the fiber can accept, and the stronger the light-gathering ability of the fiber.

[0050] The gain fiber 3 has a group velocity dispersion of -28.6 ps / (km), and a layer of high refractive index glue is covered at the fusion point of the photonic crystal fiber, the gain fiber 3, and the passive large mode field fiber, which is used as a pump light stripper to prevent overheating damage under high pump power and slightly improve the self-starting ability. The fiber combiner is the pump light-signal light combiner 2, which is used to combine the pump light emitted by the pump source 1 with the signal light and then couple it into the gain fiber 3. The pump source 1 is used to provide 976 nm pump light.

[0051] It can be calculated from the parameters of the gain fiber that this large mode field fiber supports the transmission of 4 transverse modes. In order to suppress the high-order modes and enhance the mode-locking self-starting ability of the laser cavity, two sections of about 20 cm long endlessly single-mode large mode field photonic crystal fibers 4 (LMA-PCF; NKT, LMA-25) are respectively fusion-spliced to one side of the output ends of the gain fiber 3 and the passive large mode field fiber. In this embodiment, the photonic crystal fiber includes a first photonic crystal fiber 4 and a second photonic crystal fiber 9 respectively arranged at the output ends of the gain fiber 3 and the passive fiber; in the NPE mode-locking polarization-maintaining large mode field fiber laser with enhanced self-starting ability, the two ends of the photonic crystal fiber are respectively fusion-spliced to one side of the output ends of the gain fiber 3 and the passive large mode field fiber, and the remaining free ends are polished at an angle of 8° to eliminate parasitic oscillations and increase the environmental stability of the laser cavity.

[0052] The gain fiber 3 is placed in a cooling device, that is, the gain fiber 3 is entirely placed in a cooling water tank to prevent the fusion joint of the gain fiber and the passive large-mode-field fiber from being damaged by overheating under high pump power. At the same time, a high-refractive-index glue is coated on the fusion joint of the first photonic crystal fiber 4 and the second photonic crystal fiber 9 as a pump light stripper, which can slightly improve the self-starting ability.

[0053] In this embodiment, an optical fiber connector is connected to the free end of the photonic crystal fiber, and a collimating lens is provided between the optical fiber connector and the Faraday rotator in the spatial optical path. An optical fiber connector 5 is connected to the free end of the first photonic crystal fiber 4, and a first collimating lens 6 is provided between the optical fiber connector 5 and the first Faraday rotator 7. An optical fiber connector 10 is connected to the free end of the second photonic crystal fiber 9, and a second collimating lens 11 is provided between the optical fiber connector 10 and the second Faraday rotator 12.

[0054] Among them, the spatial optical path of the pump source 1 part includes a first Faraday rotator 7 and a first mirror 8. The pump light of the pump source 1 enters the large-mode-field gain fiber 3 through the pump light-signal light combiner 2. The generated signal light is transmitted bidirectionally in the first photonic crystal fiber 4 and enters the spatial optical path of the pump source 1 part. The main axis of the elliptical polarized light is rotated by 90° through the first Faraday rotator 7 and the first mirror 8, and then returns to the linear cavity. The spatial optical path of the pump source 1 part is used to rotate the polarization state of the light beam and return it to the linear cavity to compensate for the group velocity delay and the linear phase bias.

[0055] In this embodiment, the spatial optical path of the laser oscillation cavity 22 part includes a second Faraday rotator 12 and a second mirror 17. The phase shifter 18 is composed of a second Faraday rotator 12, a half-wave plate 13, and a first quarter-wave plate 14, and is used to realize nonlinear polarization rotation mode locking. A second quarter-wave plate 16 is provided between the polarization beam splitter 15 and the second mirror 17. The spatial optical path of the laser oscillation cavity 22 part is used to receive the signal light that returns through the first mirror 8 and is amplified by the gain fiber 3 in the spatial optical path of the pump source 1 part.

[0056] The polarization beam splitter 15, in cooperation with the second quarter-wave plate 16 and the second mirror 17, realizes the pulsed output of two ports. The vertically polarized light is output from the reflection port of the polarization beam splitter 15, and the horizontally polarized light is output from the other reflection port of the polarization beam splitter 15 after passing through the second mirror 17.

[0057] Therefore, in the NPE mode-locked large-mode-field fiber laser with enhanced self-starting ability of the present utility model, the overall optical path direction is as follows: The pump light of the 976 nm pump source 1 enters the large-mode-field gain fiber 3 through the pump light-signal light combiner 2, and the generated 1.5 μm signal light is transmitted bidirectionally in the fiber (the first photonic crystal fiber 4) and enters the spatial optical path. As Figure 1 The right-side spatial optical path part shown in the figure includes a first Faraday rotator 7 (FR, Faraday rotator) and a first mirror 8 (M, gold mirror), which are used to rotate the major axis of the elliptical polarized light by 90°, and then return to the linear cavity. The combined setting of the FR (the first Faraday rotator 7) and the M (the first mirror 8) solves the difficulties of angular splicing and precise control of fiber length in the all-fiber structure on the one hand, and can completely cancel the linear phase delay on the other hand, enabling the non-linear phase shift to accumulate continuously, thereby eliminating the spatial hole burning effect in the cavity, reducing the mode-locking self-starting threshold, and solving the problem of group velocity mismatch at the same time. The 1.5 μm signal light returns to the gain fiber 3 through the right-side mirror (the first mirror 8) for amplification, and then enters the left-side spatial optical path, and the phase is offset through the phase shifter 18 composed of the second Faraday rotator 12, the half-wave plate 13, and the first quarter-wave plate 14, so as to realize NPE mode-locking. The polarization beam splitter 15 (PBS, polarizing beam splitter) is the position where the two orthogonal polarization states transmitted in the fiber interfere, and also provides an output port for the laser. The vertically polarized light is directly output from the reflection end (Reflection port, hereinafter referred to as the R port) of the PBS (polarization beam splitter 15), and the second quarter-wave plate 16 is used for beam splitting, so that the horizontally polarized light passing through the PBS (polarization beam splitter 15) can be output from another reflection port (Transmission port, hereinafter referred to as the T port) of the PBS (polarization beam splitter 15) after passing through the mirror (the second mirror 17), thereby obtaining a pulsed output with dual ports.

[0058] See Figure 3 As shown in the figure, when the pump power reaches 7.07 W, the laser operates in a multi-pulse state and has continuous light. When the pump power is reduced to 2.38 W, stable single-pulse mode-locking can be achieved, and its basic repetition frequency is 34.47 MHz. At this time, the powers of the T port and the R port are measured to be 40 mW and 8.6 mW respectively, corresponding to pulse energies of 1.15 nJ and 0.25 nJ respectively. The spectra of the T and R ports are as Figure 3As shown in (a) and (b), the central wavelength is 1566 nm, and the 10 dB bandwidths are 19.95 nm and 21.17 nm respectively. The presence of the Kelly sidebands in the spectrum indicates that the output is a soliton-like pulse, and the net dispersion in the cavity can be calculated to be -0.145 ps based on the sideband positions. 2 ; The autocorrelation curve is as Figure 3 shown in (c) and (d). Since the spectrum modulation at the R port is complex and the pulse quality is low, the PICASO algorithm is used to restore the true pulse shape (as shown in the inset). The true pulse width at the T port can be measured to be 325 fs. In addition, it can be found that the pulse quality at the T port is higher than that at the R port. The difference in the spectral shape and pulse quality between the two output ports is caused by the cross-phase modulation between the two orthogonal polarization states in the polarization-maintaining fiber.

[0059] See Figure 4 shown. The M 2 values of each output port can be measured using a beam analyzer, which reflects the beam quality of the output of the large-mode-field fiber laser. The measurement results are as Figure 4 shown. The average M 2 value at the T port is 1.17, and the average M 2 value at the R port is 1.10. The slight difference between the two ports is due to the aberration of the spatial optical device in the cavity or the coupling not being optimal. Nevertheless, these M 2 values are close to the M 2 value (~1.105) of the self-made all-fiber carbon nanotube mode-locked laser based on single-mode fiber, which proves that the use of PCF effectively suppresses the high-order modes and significantly improves the beam quality.

[0060] In an embodiment of the present invention, see Figure 1 and Figure 2 shown. The NPE mode-locked polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability further includes a real saturable absorber mode-locking 21, which is arranged in the laser cavity; the real saturable absorber mode-locking 21 includes a third collimating lens 19 and a semiconductor saturable absorber mirror 20. The third collimating lens 19 is an aspherical lens. The semiconductor saturable absorber mirror 20 in the real saturable absorber mode-locking 21 replaces the second reflecting mirror 17 in part of the spatial optical path of the laser oscillation cavity 22; the third collimating lens 19 in the real saturable absorber mode-locking 21 replaces the second quarter-wave plate 16 in part of the spatial optical path of the laser oscillation cavity 22; the real saturable absorber mode-locking 21 is used to reduce the mode-locking self-starting threshold of the laser and improve the self-starting ability of the laser at low pump power, and the mode-locking mechanism is still dominated by NPE.

[0061] In this embodiment, after replacement, the mode-locking self-starting threshold of the laser is significantly reduced from a pump power of 7.07 W to 2.5 W. At this power, mode-locking self-starts in a single-pulse state with continuous light. The first quarter-wave plate can be tuned within a range of 10° without causing loss of lock, indicating that SESAM significantly improves the self-starting ability of the laser. And it is verified that mode-locking can only be achieved when the wave plate angle is set correctly, indicating that the mode-locking mechanism is still dominated by NPE, and SESAM only assists in mode-locking self-starting. When the pump power is further reduced to 1.91 W, stable single-pulse mode-locking can be achieved, corresponding to a repetition frequency of 33.8 MHz, the output power at the T port is 14 mW, the output power at the R port is 3 mW, and the pulse energies corresponding to the two ports are 0.4 nJ and 0.09 nJ. The corresponding spectrum and pulse are as shown in Figure 5 . It can be found that the addition of SESAM has little effect on the spectral characteristics of the output pulses of the laser, and the modulation phenomenon at the top of the spectrum at the T port weakens, indicating that the influence of some remaining high-order modes in the cavity on the spectrum becomes smaller. To more intuitively observe the influence of high-order modes, we also measured the beam quality M 2 of the two ports. The results are as shown in Figure 6 . The beam quality at the T port has a slight improvement, which may be caused by the slight spatial filtering of the high-order modes by the lens in front of SESAM, while the beam quality at the R port has a certain degree of decline, which may be related to its very low output power. Figure 7 are the autocorrelation curves of the two ports, and their shapes are also similar to those without using SESAM. However, due to the relatively long recovery time of SESAM, the pulse width becomes wider as a whole. Generally speaking, the introduction of SESAM does not significantly change the soliton-like output characteristics of the polarization-maintaining large-mode-field fiber laser, but significantly reduces the mode-locking threshold (~65%) and significantly improves the mode-locking self-starting ability.

[0062] An NPE mode-locking polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability of the present utility model has natural high stability, which is conducive to commercial application. With the continuous development of fiber preparation technology, polarization-maintaining rare-earth-doped fibers have gradually become commercial. Since polarization-maintaining fibers will cause problems of pulse delay mismatch and cannot achieve polarization evolution, it is difficult to apply NPE mode-locking technology to polarization-maintaining fibers. The traditional method used is to use the angle splicing technology in the ring cavity structure. However, on the one hand, this technology requires precise control of the splicing angle, which reduces the preparation efficiency. On the other hand, it requires precise control of the fiber length, which is a challenging task.

[0063] The large-mode-area fiber laser with a linear cavity proposed by the present utility model uses a combination scheme of FR and M to compensate for the pulse time delay, which not only avoids the difficulties of angular fusion splicing and length control, but also has a relatively simple linear cavity structure, low coupling difficulty of spatial light, and does not require too much maintenance cost in the later stage, which is conducive to realizing commercial development. In addition, SESAM is introduced to assist mode locking, which greatly reduces the mode-locking threshold and improves the mode-locking self-starting ability. At the same time, the present utility model also effectively solves the problem that the perturbation of high-order modes often leads to poor mode-locking self-starting ability by fusing a small section of PCF on the fiber output side. In addition, the present utility model is also the first to apply the large-mode-area polarization-maintaining fiber to the NPE mode-locked linear cavity fiber laser in the 1.5 μm band, and the output pulse energy is increased from 0.1 - 0.2 nJ of the single-mode fiber laser with a similar structure to 1.15 nJ, achieving a maximum increase of more than 10 times. A potentially attractive application of such a high-power laser is to use frequency doubling to generate an output of approximately 800 nm, which is the preferred wavelength for many current applications in ultrafast optics.

[0064] In application, from the emergence of the first solid-state laser to the development of fiber lasers today, the laser pulse width has achieved a breakthrough from the nanosecond level to the femtosecond level, and the output performance of the laser has also developed in a higher and better direction. Compared with solid-state lasers, ultrafast fiber lasers have important applications in military, scientific research, industrial, and medical fields because of their advantages such as good beam quality, high thermal management efficiency, flexible and compact structure, and low maintenance cost. By changing the doping ion species of the gain fiber in the laser oscillation cavity, ultrashort pulses of different wavelengths can be output, covering the working wavelength band of the fiber laser from the visible light band to the mid-infrared and far-infrared bands to meet the different needs of different fields. For example, ytterbium-doped fiber lasers can generate lasers in the 1.01μm band and have important applications in the industrial field (such as cutting and welding); erbium-doped fiber lasers can generate lasers in the 1.51μm band, which is a low-loss communication window in the optical communication band, and the loss during transmission in the fiber is usually only 0.2dB / km. Studying stable light sources in the 1.51μm band plays an important role in fiber optic communication; at the same time, the light in the 1.51μm band is in the eye-safe band, and the contrast between this band and many targets (such as vehicles, ships, cement buildings, etc.) and the background is relatively large, which is very attractive in military applications such as lidar and target recognition. Another example is that thulium-doped or holmium-doped fiber lasers can generate ultrashort laser pulses in the 2μm band in the molecular "fingerprint" region. This band covers the absorption spectra of molecules such as CO2, H2O, and NO2, and can constitute a highly sensitive gas sensor for long-range atmospheric remote sensing. Also, since more than 75% of human tissue is composed of water, ultrashort laser pulses in the 2μm band are also widely used in the medical field (such as laser scalpels, tissue resection, etc.); at the same time, using ultrashort pulses in the 2μm band as the seed source of the laser, the laser wavelength can be easily extended to the mid-infrared and far-infrared bands above 10μm by using optical nonlinear conversion effects (such as difference frequency, supercontinuum generation, etc.). With the development of fiber preparation technology and related fields, fiber lasers with better and more excellent performance are continuously being explored. Polarization-maintaining fiber lasers with high stability and easy mode-locking improve the overall environmental stability of the laser in applications, making important contributions to the development of fiber lasers and promoting the development of fiber lasers.

[0065] In the present utility model, a Faraday rotator and a total reflector are introduced at one end of the online cavity, and perfect compensation of group velocity delay and linear phase bias is achieved by using the exchange of the optical path. This design avoids the complex operation of precisely controlling the fusion angle of the fiber segment in the traditional method and greatly improves the practicability and stability of the system. Through adaptive compensation, the technical bottleneck caused by group velocity mismatch and time-delay evolution during the application of the NPE mode-locking technology in polarization-maintaining fibers is solved.

[0066] In this utility model, a small section of endlessly single-mode photonic crystal fiber (PCF) is fusion spliced on both sides of the gain fiber 3 and the pump light-signal light combiner 2 respectively. The unique characteristics of the PCF are utilized to effectively suppress the influence of high-order modes. Through this design, the laser can maintain a stable mode-locked state, significantly improve the mode-locking self-starting ability, enhance the stability and consistency of the laser mode, and due to the endless single-mode characteristic of the PCF, the beam quality of the laser output is also improved.

[0067] Both the gain fiber 3 and the passive large mode field fiber in this utility model are double-clad polarization-maintaining fibers. Although non-polarization-maintaining fibers such as photonic crystal fibers are used in the optical path, due to their short length, the entire laser cavity is still mainly composed of polarization-maintaining fibers, thus ensuring the nonlinear pulse shaping and environmental stability of the system. Combined with the application of the cooling water tank, overheating damage of the fiber fusion splice point under high pump power is further prevented, ensuring the long-term reliable operation of the system.

[0068] This utility model designs an optical path including elements such as the first Faraday rotator 7, the second Faraday rotator 12, a half-wave plate 13, a quarter-wave plate, and a polarization beam splitter 15. This design not only realizes NPE mode-locking but also, through the beam splitting effect of the second quarter-wave plate, enables the horizontally polarized light transmitted from the polarization beam splitter to be reflected from the other side of the polarization beam splitter after being reflected by a mirror, thereby obtaining a dual-port pulse output.

[0069] In summary, this utility model provides an optimized NPE mode-locked polarization-maintaining large mode field fiber laser with enhanced self-starting ability. Through the comprehensive optimization of the fiber structure, mode-locking mechanism, phase compensation, and beam quality, the performance and operation convenience of the laser are significantly improved, enabling it to achieve high-quality pulse output under the conditions of low power consumption and high stability, and having a wide range of application prospects. The design of this utility model also fully considers the operation convenience of actual operation and the adaptability of the system. Through the introduction of the adaptive compensation method and photonic crystal fiber, not only the problems of complex operation and difficulty in implementation of the traditional scheme are solved, but also the stability and efficiency of the system in different environments are ensured, making the laser show stronger practicability in various application scenarios.

[0070] The above description is only a preferred embodiment of the present utility model and is not intended to limit the present utility model. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present utility model shall be included in the protection scope of the present utility model.

Claims

1. An NPE mode-locked large mode area fiber laser with enhanced self-starting ability, characterized in that, It includes a pump source (1) and a laser oscillation cavity (22). The pump source (1) is connected to a gain fiber (3) through an optical fiber combiner. The gain fiber (3) is spatially optically connected to the laser oscillation cavity (22) through a photonic crystal fiber. A polarization beam splitter (15) is provided in the laser oscillation cavity (22), and a phase shifter (18) is provided between the polarization beam splitter (15) and a part of the spatial optical path of the pump source (1); the polarization beam splitter (15) is arranged at the position where two orthogonally polarized states transmitted in the optical fiber interfere, and provides a dual-port output of the laser. The vertically polarized light is output from the reflection port, and the horizontally polarized light is output from the transmission port.

2. The NPE mode-locked large-mode-area fiber laser with enhanced self-starting ability according to claim 1, wherein The spatial optical path of the pump source (1) part and the laser oscillation cavity (22) part includes a Faraday rotator and a mirror arranged at both ends of the linear cavity.

3. The NPE mode-locked large mode area fiber laser with enhanced self-starting ability according to claim 2, characterized in that The optical fiber combiner is a pump light-signal light combiner (2) for combining the pump light emitted by the pump source (1) with the signal light and then coupling them into the gain fiber (3). The pump source (1) is used to provide pump light.

4. The NPE mode-locked large-mode-field fiber laser with enhanced self-starting ability according to claim 3, wherein, The gain fiber (3) is a double-clad polarization-maintaining erbium-ytterbium co-doped large-mode-field fiber, and the gain fiber (3) is pumped by a multimode laser diode through a pump light-signal light combiner (2) based on a passive PM-LMA optical fiber.

5. The NPE mode-locked large-mode-area fiber laser with enhanced self-starting ability as described in claim 4, wherein The photonic crystal fiber includes a first photonic crystal fiber (4) and a second photonic crystal fiber (9) respectively arranged at the output ends of the gain fiber (3) and the passive optical fiber; in the NPE mode-locked polarization-maintaining large-mode-field fiber laser with enhanced self-starting ability, the two-end photonic crystal fibers are respectively fusion-spliced to one side of the output ends of the gain fiber (3) and the passive large-mode-field optical fiber, and the remaining free ends are polished.

6. The NPE mode-locked large-mode-area fiber laser with enhanced self-starting ability as described in claim 5, characterized in that, The free end of the photonic crystal fiber is connected with an optical fiber connector, and a collimating lens is provided between the optical fiber connector and the Faraday rotator of the spatial optical path. The free end of the first photonic crystal fiber (4) is connected with a first optical fiber connector (5), and a first collimating lens (6) is provided between the first optical fiber connector (5) and the first Faraday rotator (7). The free end of the second photonic crystal fiber (9) is connected with a second optical fiber connector (10), and a second collimating lens (11) is provided between the second optical fiber connector (10) and the second Faraday rotator (12).

7. The NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to claim 6, characterized in that The spatial optical path of the pump source (1) part includes a first Faraday rotator (7) and a first mirror (8). Among them, the pump light of the pump source (1) enters the large-mode-field gain fiber (3) through the pump light-signal light combiner (2). The generated signal light is transmitted bidirectionally in the first photonic crystal fiber (4) and enters the spatial optical path of the pump source (1) part. The main axis of the elliptically polarized light is rotated by 90° through the first Faraday rotator (7) and the first mirror (8), and then returns to the linear cavity.

8. The NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to claim 7, characterized in that, The spatial optical path of the laser oscillation cavity (22) portion includes a second Faraday rotator (12) and a second mirror (17). The phase shifter (18) is composed of a second Faraday rotator (12), a half-wave plate (13), and a first quarter-wave plate (14), and is used to achieve non-linear polarization rotation mode locking. A second quarter-wave plate (16) is arranged between the polarization beam splitter (15) and the second mirror (17). The spatial optical path of the laser oscillation cavity (22) portion is used to receive the signal light that returns through the first mirror (8) and is amplified by the gain fiber (3) in the spatial optical path of the pump source (1) portion.

9. The NPE mode-locked large mode area fiber laser with enhanced self-starting ability according to claim 8, characterized in that, The polarization beam splitter (15) cooperates with the second quarter-wave plate (16) and the second mirror (17) to achieve dual-port pulse output. The vertically polarized light is output from the reflection port of the polarization beam splitter (15), and the horizontally polarized light is output from the other reflection port of the polarization beam splitter (15) after passing through the second mirror (17).

10. The NPE mode-locked large mode field fiber laser with enhanced self-starting ability according to claim 9, characterized in that, The fiber laser also has a real saturable absorber mode locking (21), which is arranged in the laser cavity. The real saturable absorber mode locking (21) includes a third collimating lens (19) and a semiconductor saturable absorber mirror (20). The third collimating lens (19) is an aspherical lens. The semiconductor saturable absorber mirror (20) in the real saturable absorber mode locking (21) replaces the second mirror (17) in the spatial optical path of the laser oscillation cavity (22) portion. The third collimating lens (19) in the real saturable absorber mode locking (21) replaces the second quarter-wave plate (16) in the spatial optical path of the laser oscillation cavity (22) portion.

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