An ultra-short pulse fiber laser

Abstract

The application discloses an ultrashort pulse fiber laser. The ultrashort pulse fiber laser comprises a nonlinear amplification ring mirror, which makes opposite transmission pulses in the ring accumulate different phase shifts, so as to form an intensity modulation mechanism for laser pulses in the cavity; a linear arm composed of a fiber device and a spatial optical element, which is used for forming a resonant cavity and improving self-locking mode characteristics of the laser; and an output end, which is used for directly outputting pulses. The ultrashort pulse fiber laser provided by the application is easy to package, has good self-starting characteristics of mode locking, is not easy to damage a saturable absorber, has a high laser damage threshold, and has the characteristics of small volume, simple and reliable system, high stability, and the like, and has potential to develop into a new generation of all-fiber ultrashort laser oscillator with excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and more specifically, relates to an ultrashort pulse fiber laser based on nonlinear amplifying ring mirror mode-locking, which improves the mode-locking self-starting capability and reliability by increasing nonlinear phase shift. Background Technology

[0002] Ultrafast lasers, due to their extremely short timescales and extremely high peak power, are widely used in materials processing, biomedical imaging, and microspectroscopy. To obtain ultrafast lasers with even higher power, it is almost always necessary to use a system with a master oscillator (or laser seed source) amplified through multiple stages. This requires the master oscillator to have high quality, including higher output power, better power stability, and faster response speed. This invention aims to improve the reliability and stability of the laser seed source while reducing the complexity and cost of the system.

[0003] Currently, the mode-locking method in ultrafast fiber laser oscillators is mainly passive mode-locking technology, and there are three main ways to achieve passive mode-locking: saturable absorber mode-locking, nonlinear polarization rotation mode-locking, and nonlinear optical ring mirror mode-locking.

[0004] A saturable absorber (SESAM) is a material whose absorptivity decreases with increasing light intensity. When a laser pulse passes through a saturable absorber in a resonant cavity, the peak portion with higher intensity has higher transmittance (or reflectivity), while the portions with lower intensity have lower transmittance (or reflectivity). Therefore, the intensity modulation effect of SESAM can be used to initiate the mode-locking process and achieve stable mode-locking. However, saturable absorbers made of semiconductor materials generally have a low damage threshold. In the initial Q-switching stage of mode-locking, SESAM is easily damaged by high-intensity Q-switching pulses, which is one of the important problems existing in the seed sources of picosecond lasers in the industry.

[0005] Nonlinear polarization rotation (or nonlinear polarization evolution) mode-locking technology is usually applied to non-polarization-maintaining systems. Therefore, the system stability is poor, and its self-starting usually requires multiple attempts at different polarization directions, making it difficult to find a suitable mode-locking interval to achieve stable mode-locking, which is not conducive to the development of industrial lasers.

[0006] Compared with the two mode-locking technologies mentioned above, nonlinear optical ring mirror mode-locking technology has advantages such as faster response speed, higher signal-to-noise ratio, lower cost, higher system stability, higher power tolerance, easier integration, and less susceptibility to external environmental interference. Therefore, lasers based on nonlinear ring mirror mode-locking are more suitable for industrial applications with high stability requirements.

[0007] Currently, most mode-locked lasers utilizing nonlinear optical ring mirror mechanisms employ an "8"-shaped structure (US Patent 7817684 B2). However, because this type of laser is a fully closed-loop structure, the cavity length is not adjustable, and the physical quantities within the cavity and their influence on the mode-locking process are difficult to measure, thus limiting its performance improvement and practical application range. Menlo GmbH of Germany (Chinese Patent CN 103311780 B and US Patent US 5359612 A) pioneered a breakthrough in this structure, developing and realizing a "9"-shaped laser cavity structure. This involves opening the oscillating ring cavity within the "8"-shaped structure to create a linear cavity, with an adjustable end mirror mounted at one end, thus making the cavity length adjustable. In the "9"-shaped laser structure, to achieve self-starting mode-locking, a non-dissimilarity spatial element is inserted within the ring cavity, making the pulse's added phase correlated with the propagation direction. The optimal self-starting mode-locking effect is obtained by changing the phase shift of the waveplate within this element.

[0008] However, in practice, technicians have found that although the method of inserting non-reciprocal space elements to improve the self-starting characteristics of lasers has been used, most figure-9 laser cavity structures still have problems such as difficulty in integration, susceptibility of system stability to environmental interference, low output power, slow mode-locking self-starting, and high system cost due to the large number of space elements in most of them. Summary of the Invention

[0009] To address the shortcomings of existing technologies and the demands of practical production, this invention provides a figure-9 cavity all-fiber structure ultrashort pulse laser based on nonlinear amplifying ring mirror mode-locking. The purpose of this invention is to construct an ultrafast laser with high stability, rapid self-starting mode-locking, low damage resistance, and low cost, providing an integrated laser solution resistant to ring mirror interference, thereby solving to some extent a series of problems caused by the limitations and shortcomings of current technologies.

[0010] Other features and advantages of this disclosure will become apparent from the following detailed description, or may be learned in part from practice of this disclosure.

[0011] To achieve the above objectives, the present invention provides an ultrashort pulse fiber laser, comprising: a nonlinear amplifying ring mirror 1, a linear arm 2, and an output end 3. The nonlinear amplifying ring mirror and the linear arm together constitute a complete laser resonant cavity, enabling the laser to oscillate and amplify within the cavity.

[0012] The nonlinear amplifying ring mirror 1 serves as the first equivalent cavity mirror of the laser. It is an all-fiber structure, including a laser diode 4, a gain fiber 7, a wavelength division multiplexer 8, a coupler 6, and a fiber polarization manager 19. The laser diode 4 serves as a pump source and is connected to the gain fiber 7 through the wavelength division multiplexer 8. It forms a fiber loop through the first port 17 and the second port 18 located on the same side of the coupler 6. The nonlinear amplifying ring mirror 1 is connected to the linear arm 2 and the output end 3 through the third port 15 and the fourth port 16 located on the other side of the coupler 6, respectively. The linear arm 2 serves as the second equivalent cavity mirror of the laser and includes a first lens 10, a polarization beam splitter 20, and a reflector 11. The output end 3 includes an isolator 9 and a pigtail 13.

[0013] The transmission function of this ultrashort pulse fiber laser is modulated by the nonlinear phase shift difference between the two beams in the nonlinear amplifying ring 1.

[0014] Preferably, in the ultrashort pulse fiber laser of the present invention, the center wavelength of the laser diode 4 is 980 nm; the gain fiber 7 is an erbium-doped, ytterbium-doped, or thulium-doped gain fiber; and the coupling coefficient of the coupler 6 is 0.5.

[0015] Furthermore, all fiber devices in the nonlinear amplifying ring 1 of the ultrashort pulse fiber laser of the present invention are non-polarization-maintaining fibers; the first laser pulse of the nonlinear amplifying ring 1 is incident from the first port 17 of the coupler 6, and after passing through the nonlinear amplifying ring 1, it is emitted from the second port 18 of the coupler 6; at the same time, the second laser pulse of the nonlinear amplifying ring 1 is incident from the second port 18 of the coupler 6, and after passing through the nonlinear amplifying ring 1, it is emitted from the first port 17 of the coupler 6.

[0016] The polarization states of the first and second laser pulses change in the nonlinear amplification ring 1; the change in polarization states of the first and second laser pulses is caused by the fiber polarization manager 19 and the pump power; the change in polarization states of the first and second laser pulses is related to the nonlinear phase shift difference between the first and second laser pulses.

[0017] Meanwhile, the shape of the ultrashort laser pulse output from the fourth port 16 of the coupler 6 in the ultrashort pulse fiber laser of the present invention can be modulated by adjusting the phase shift of the nonlinear amplification ring mirror 1, and outputting various pulse shapes such as hyperbolic secant and parabolic.

[0018] Due to the imperfect symmetry of the nonlinear amplification ring (caused by the coupler's splitting ratio not being strictly 50:50 and the gain fiber not being strictly distributed in the center of the ring), the first and second laser pulses incident through the two ports of the coupler will accumulate different nonlinear phase shifts after passing through all optical paths within the ring. When they meet again, interference will occur, affecting the laser system's transfer function and forming an intensity modulation mechanism. The system transfer function is as follows: Figure 5 (Unbiased) As shown. However, a nonlinear amplifying ring lens alone cannot complete the self-starting of laser mode-locking. The reason is that when the laser starts pumping, the first laser pulse and the second laser pulse have a very small phase shift difference. At this time, the slope of the transmission curve is zero, and the modulation of the laser is close to zero.

[0019] To facilitate mode-locking and self-starting of the laser, we inserted a non-reciprocal element into the cavity. Its function is to add an initial bias to the system's transfer function, such as... Figure 5 The bias curve shown in the figure indicates that the transfer function slope is relatively large when the laser is started, resulting in greater modulation of the laser and making it easier to self-start. Preferably, the non-reciprocal element can be composed of λ / 8 waveplate, λ / 4 waveplate, λ / 2 waveplate, Faraday rotator, etc. (It should be noted that although the non-reciprocal element can play a certain phase shifting role, the non-reciprocal element is not a phase shifter).

[0020] According to the present invention, the coupler splits the laser in the linear arm into a first laser pulse and a second laser pulse, which then enter a nonlinear amplifying ring mirror. The two laser pulses meet at the coupler after passing through the nonlinear amplifying ring mirror. However, due to the change in polarization state, the polarization states of the first and second laser pulses are not completely identical, resulting in only partial interference at the coupler. This has the beneficial effect of time-domain shaping of the laser pulses. The first and second laser pulses are collimated by a lens, and then pass through a non-reciprocal element. At the polarization beam splitter, part of the pulse is retained within the cavity, while the rest is reflected out of the cavity, resulting in losses. Since the change in the intracavity polarization state is related to the change in the nonlinear phase shift difference, the transmittance and reflectance of the polarization beam splitter also change with the nonlinear phase shift difference. This causes the system's transfer function to be modulated by the nonlinear phase shift in the nonlinear amplifying ring mirror. The non-reciprocal element provides an additional phase shift between the first and second laser pulses, thereby changing the initial position of the system's transfer function and facilitating the self-starting of the laser's mode-locking process.

[0021] Specifically, the linear arm 2 of the ultrashort pulse fiber laser of the present invention also includes a non-reciprocal element, which includes a Faraday rotator and at least one waveplate.

[0022] Preferably, the non-reciprocal elements in the above-mentioned ultrashort pulse fiber laser include a λ / 4 waveplate 5, a λ / 2 waveplate 12, and a Faraday rotator 14.

[0023] The laser pulse in the fiber optic loop of the nonlinear amplification ring mirror 1 is emitted from the third port 15 of the coupler, collimated by the first lens 10, and then passes sequentially through the λ / 4 waveplate 5, the λ / 2 waveplate 12, the Faraday rotator 14 and the polarization beam splitter 20. Finally, it is reflected by the mirror 11. The reflected light first passes through the polarization beam splitter 20, then sequentially through the Faraday rotator 14, the λ / 2 waveplate 12 and the λ / 4 waveplate 5, and finally enters the third port 15 of the coupler after being coupled by the first lens 10.

[0024] Furthermore, the aforementioned ultrashort pulse fiber laser can optimize the output spectrum and output power by adjusting the angles of the λ / 4 waveplate 5 and the λ / 2 waveplate 12, while simultaneously rotating the fiber polarization manager 19. Finally, by fixing the positions of all non-reciprocal components, a stable mode-locked pulse output can be obtained, and self-starting mode-locking can be achieved upon restarting.

[0025] To adjust the position of the system transmission curve, i.e., to adjust the value and slope of the laser start-up point (phase shift difference is zero), the waveplate in the non-reciprocal element can be adjusted according to specific circumstances. Preferably, the system has the maximum modulation depth when the fast axis angle of the λ / 4 waveplate in the non-reciprocal element forms a 0° angle with the polarization direction of the transmitted light from the polarization beam splitter. Preferably, the rotation of the λ / 2 waveplate in the non-reciprocal element serves as an adjustment element for the bias of the system transmission curve.

[0026] Preferably, the change in polarization state of the first and second laser pulses propagating in opposite directions within the nonlinear amplifying ring mirror can be achieved using non-polarization-maintaining fiber and a polarization manager. Since the polarization mode of the laser pulses in the non-polarization-maintaining fiber is unstable, devices such as mode scramblers or electrically controlled polarization controllers can be used to control the polarization state within the fiber. Using non-polarization-maintaining fiber and a polarization manager allows for multiple options in the polarization evolution within the ring mirror, making mode-locking of the system easier.

[0027] Simultaneously, to adjust the position of the system transmission curve, i.e., to adjust the value and slope of the laser start-up point (where the phase shift difference is zero), the waveplate in the non-reciprocal element can be adjusted according to specific circumstances. Once adjusted to a suitable position and fixed, it can provide an easily mode-locked and stable pulse output. Compared to traditional figure-9 cavity lasers, the system transmission curve of this invention has a larger slope when the nonlinear phase shift difference is 0. Therefore, the laser of this invention has faster and more stable self-starting performance (e.g., ...). Figure 5 (as shown in Example 1).

[0028] Compared to traditional figure-9 cavity lasers, the non-polarization-maintaining nonlinear ring mirror and polarization manager enhance self-starting characteristics and avoid the use of special couplers (such as polarization combiners) or saturable absorbers, resulting in improved laser integration performance and reduced costs. Compared to traditional nonlinear polarization evolution (NPE) mode-locked lasers, the nonlinear amplifying ring mirror makes mode-locking more stable and reliable. Once all parameters are set correctly, self-starting can be achieved without changing the state of any components.

[0029] Alternatively, the mirror 11 in the linear arm 2 of the ultrashort pulse fiber laser of the present invention can be replaced by a saturable absorber element, which can be a semiconductor saturable absorber, a two-dimensional material saturable absorber, or a combination of Kerr medium and slit.

[0030] Furthermore, the linear arm of the ultrashort pulse fiber laser of the present invention provides a function for adjusting the cavity length, which facilitates the adjustment of the laser's repetition rate. Preferably, the repetition rate of the laser can be adjusted by moving the mirror 11 in the linear arm, and the repetition rate of the laser is stabilized by negative feedback control.

[0031] Furthermore, to achieve the function of accumulating different phase shifts within the cavity for different polarization states, in addition to using non-polarization-maintaining fibers and polarization managers to control the polarization states, fibers with specific birefringence characteristics can also be used in the fiber loop of the nonlinear amplifying ring. The advantage of this type of fiber is that it allows two mutually perpendicular polarization states within the fiber to obtain different gain effects, thus resulting in different light intensities and consequently, different phase shifts. Using this type of fiber, a simpler and more stable ultrafast fiber laser system can be constructed.

[0032] According to the present invention, mode-locked self-starting is achieved through nonlinear optical ring mirror technology. Besides using non-reciprocal elements to provide an initial bias for the system's transfer function, other modulation methods can be added to improve self-starting performance. In this case, the non-reciprocal elements become unnecessary. For example, pulses with a certain intensity can be selected by intensity modulation of a saturable absorber, or an initial laser pulse can be generated through electrical modulation. This avoids the drawback of a figure-9 cavity without non-reciprocal elements being unable to self-start. The pulse is further compressed during modulation by the nonlinear amplifying ring mirror, forming a narrower and more stable ultrafast laser pulse. Compared with mode-locked lasers that only use saturable absorbers for mode-locking, the advantages of the hybrid modulation mode-locking method that combines nonlinear amplifying ring mirrors and saturable absorbers (hybrid modulation mode-locking technology in the following text specifically refers to this scheme) are: (1) The filtering effect provided by the nonlinear amplifying ring mirror can eliminate the leading and trailing edges of the pulse, making the pulse contrast higher (the time-domain pulse narrower); (2) The saturable absorber is only used as a mode-locking initiation element and does not require focusing adjustment. Moreover, thanks to the combined effect of the nonlinear amplifying ring mirror and the saturable absorber, the hybrid mode-locking system has a faster mode-locking process and can reduce the damage rate of the saturable absorber; (3) The system has a wide range of applications. Almost all gain fibers can use this scheme. For fully positive dispersion fiber structures, only one filter needs to be added to limit the continuous broadening of the intracavity spectrum to achieve mode-locking and self-starting. No more dispersion compensation devices such as chirped fiber gratings or grating systems are needed.

[0033] Compared with the method of achieving self-starting using non-reciprocal components, the hybrid modulation mode-locking technology has a simpler structure, lower cost, and better stability.

[0034] According to the present invention, the net dispersion within the cavity differs for gain fibers and single-mode fibers with different doping elements. For a fully positive dispersion cavity, the laser relies on a dispersion compensation module to compensate for dispersion, thereby improving the system's self-starting characteristics, stability, and reducing noise levels. In this invention, "fully positive dispersion" refers to the laser's net dispersion (or total dispersion) within the cavity being normal. Typically, dispersion compensation modules include dispersion compensation fibers, chirped fiber gratings, volume gratings, prisms, chirped mirrors, etc., and these dispersion compensation systems are all applicable to the laser system of the present invention.

[0035] Preferably, the linear arm 2 of the ultrashort pulse fiber laser of the present invention further includes a dispersion compensation module. The dispersion compensation module is located between the polarization beam splitter 20 and the reflector 11, and consists of a grating 21, a second lens 10' and a displacement stage 22. The grating 21 is located after the polarization beam splitter 20, and the second lens 10' is located in front of the reflector 11 and placed on the displacement stage 22. The dispersion compensation module is used to compensate for the intracavity dispersion of the laser resonant cavity, making the laser easier to mode-lock.

[0036] In the hybrid modulation mode-locking scheme, a filter is used to limit the continuous spectral broadening of the pulse in the all-positive dispersion structure, so that the all-positive dispersion structure can achieve mode-locking self-starting without dispersion compensation; when the net dispersion in the cavity is negative, the filter becomes an unnecessary device, and the function of the filter is no longer needed to achieve self-starting. Attached Figure Description

[0037] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments are briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the present invention, and not all embodiments. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the laser structure provided in the first embodiment of the present invention;

[0039] Figure 2 This is a schematic diagram of a laser structure provided in the second embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of a laser structure provided in the third embodiment of the present invention;

[0041] Figure 4 This is a schematic diagram of the laser structure provided in the fourth embodiment of the present invention;

[0042] Figure 5 The system transfer function curve of a figure-9 cavity laser (the reflectivity of the nonlinear ring mirror in the figure-9 cavity as a function of nonlinear phase shift modulation);

[0043] Figure 6 This is the output spectral data diagram of the first embodiment of the present invention;

[0044] Figure 7 This is the output spectral data diagram of the fourth embodiment of the present invention.

[0045] Reference numerals: 1. Nonlinear amplifying ring mirror; 2. Linear arm; 3. Output end; 4. Laser diode; 5. λ / 4 waveplate; 6. Coupler; 7. Gain fiber; 8. Wavelength division multiplexer; 9. Isolator; 10. First lens; 10'. Second lens; 11. Mirror; 12. λ / 2 waveplate; 13. Pigtail; 14. Faraday rotator; 15. Coupler third port; 16. Coupler fourth port; 17. Coupler first port; 18. Coupler second port; 19. Fiber polarization manager; 20. Polarization beam splitter; 21. Grating; 22. Displacement stage; 23. All-fiber device; 24. Chirped fiber grating. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the 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 merely illustrative of the invention and are not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of the invention described below can be substituted for each other as long as they do not conflict with each other. In this invention, terms such as "first," "second," etc. (if present) are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0047] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by those skilled in the art. In addition to the specific methods, apparatuses, and materials used in the embodiments, this invention can be implemented using any prior art methods, apparatuses, and materials similar to or equivalent to those described, used, or made by those skilled in the art and as described in this invention, based on their knowledge of the prior art and the description of this invention. In this invention, unless otherwise specified, all components, apparatuses, and materials involved are commercially available or manufactured using conventional methods in the art.

[0048] like Figure 1 As shown, in the first embodiment of the present invention, the ultrashort pulse fiber laser design structure provided by the present invention includes: a nonlinear amplifying ring 1, a linear arm 2, and an output end 3. In this embodiment, all optical fibers are non-polarization-maintaining fibers. The nonlinear amplifying ring includes the following devices: a fiber polarization manager 19; a laser diode 4 as a pump source with a center wavelength of 980 nm, connected to a ytterbium-doped gain fiber 7 via a wavelength division multiplexer 8; and the first port 17 and the second port 18 on the same side of the coupler 6 connecting the above devices to form a fiber loop. The coupling coefficient of the coupler 6 is 0.5. The third port 15 on the other side of the coupler is coupled to a spatial element in the linear arm. The laser pulse in the fiber exits from the third port 15 of the coupler, is collimated by the first lens 10, and then sequentially passes through a λ / 4 waveplate 5, a λ / 2 waveplate 12, a Faraday rotator 14, a polarization beam splitter 20, and finally is reflected by a mirror 11. After being coupled again by the above elements, it enters the third port 15 of the coupler.

[0049] The adjustment and optimization method of the first embodiment is as follows: Taking the light transmission direction in the spatial optical path as the z-axis, the direction parallel to the paper plane upwards as the y-axis (the polarization direction of the transmitted light from the polarization beam splitter 20 is parallel to the y-axis), and the direction perpendicular to the paper plane outwards as the x-axis, in the first embodiment, the angle between the fast axis of the λ / 4 waveplate 5 and the x-axis is set to 45°, the angle between the fast axis of the λ / 2 waveplate 12 and the x-axis is set to 22.5°, and the rotation angle of the Faraday rotator is set to 45°. At this time, the pump threshold of the system's emitted laser is at its lowest. Increasing the pump power makes the system emit continuous laser light. Then, rotating the fiber polarization manager 19 causes the system to mode-lock. At this time, the transmission curve of the system is as follows: Figure 5 As shown in Embodiment 1, compared to traditional figure-9 cavity lasers, the transmission curve of the system of this invention has a larger slope when the nonlinear phase shift difference is 0. Therefore, the laser of this invention has faster and more stable self-starting performance. By continuously fine-tuning the angles of the λ / 2 waveplate 12 and the λ / 4 waveplate 5, and simultaneously coordinating the precise rotation of the fiber polarization manager 19, the output spectrum and output power can be further optimized. Finally, by fixing the positions of all devices, a stable mode-locked pulse output will be obtained, and self-starting mode-locking can be achieved upon restarting.

[0050] In the first embodiment, the light passing through the polarization beam splitter 20 passes through non-reciprocal elements: Faraday rotator 14, λ / 2 waveplate 12, and λ / 4 waveplate 5, and is coupled into the optical fiber through the first lens 10 (at this time, the light pulse is elliptically polarized). It is then split into a first laser pulse and a second laser pulse by the coupler. Since the optical fiber and fiber devices in the nonlinear amplifying ring mirror are all non-polarization-maintaining devices, both the first and second laser pulses are elliptically polarized. The polarization states of both pulses are controlled by the fiber polarization manager 19, the pump power, and the fiber birefringence. When the state of the fiber polarization manager 19 and the pump power are changed, the polarization states (ellipsoidal deviation and major axis direction) of the first and second laser pulses will change during their transmission in the non-polarization-maintaining fiber. Furthermore, because the first and second laser pulses accumulate different nonlinear phase shifts in the nonlinear ring mirror, when they meet again at the coupler 6, partial interference occurs, causing the pulse intensity to be modulated by the nonlinear phase shift. After passing through coupler 6, the first laser pulse and the second laser pulse are combined into an elliptically polarized light pulse. This pulse then passes through a non-reciprocal element again before reaching the polarization beam splitter 20. Its ellipticity is affected by the non-reciprocal element, nonlinear phase shift, non-polarization-maintaining fiber birefringence, pump power, and the fiber polarization manager. Therefore, the transmitted light (p-polarized light) after passing through the polarization beam splitter 20 undergoes a complex intensity modulation mechanism. The output spectrum of the first embodiment is as follows: Figure 6 As shown.

[0051] like Figure 2As shown, in the second embodiment of the present invention, the design structure of the ultrashort pulse fiber laser provided by the present invention is basically the same as that of the first embodiment. In the second embodiment, a grating 21 and a second lens 10' are inserted in front of the laser reflector 11 and placed on the displacement stage 22 to form a dispersion compensation module.

[0052] like Figure 3 As shown, in the third embodiment of the present invention, the design structure of the ultrashort pulse fiber laser provided by the present invention is basically the same as that of the first embodiment. The third embodiment integrates the non-reciprocal elements (5, 12, 14) in the linear arm with the reflector 11, the first lens 10, the polarization beam splitter 20, etc., to make it an all-fiber device 23, which makes the system structure simpler and more stable, and more suitable for industrial production.

[0053] like Figure 4 As shown, in the fourth embodiment of the present invention, the ultrashort pulse fiber laser design structure provided by the present invention mainly includes: a wavelength division multiplexer 8, a chirped fiber grating 24, a gain fiber 7, and a linear arm 2. In this embodiment, the linear arm is similar to that in the first embodiment. It includes a first lens 10, a Faraday rotator 14, a λ / 4 waveplate 5, and a reflector 11. In the first embodiment, the nonlinear amplification ring is composed of a chirped fiber grating 24 and a special birefringent fiber. The nonlinear phase shift difference originates from the birefringence characteristics of the polarization-maintaining fiber. The two polarization states of the beam have different gain coefficients, thus there is a phase shift difference between them. In the fourth embodiment, all fiber devices are polarization-maintaining devices and have no fast-axis or slow-axis cutoff characteristics. The output spectrum of the fourth embodiment is as follows: Figure 7 As shown.

[0054] The preferred embodiments and examples of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments and examples. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the concept of the present invention.

Claims

1. An ultrashort pulse fiber laser, characterized in that... The ultrashort pulse fiber laser includes the following components: Nonlinear amplifying ring lens (1), linear arm (2), output terminal (3); where: The nonlinear amplifying ring mirror (1) and the linear arm (2) together constitute a laser resonant cavity; The nonlinear amplifying ring mirror (1) serves as the first equivalent cavity mirror of the laser. It is an all-fiber structure, including a laser diode (4), a gain fiber (7), a wavelength division multiplexer (8), a coupler (6), and a fiber polarization manager (19). The laser diode (4) serves as a pump source and is connected to the gain fiber (7) through the wavelength division multiplexer (8). It forms a fiber loop through the first port (17) and the second port (18) located on the same side of the coupler (6). The nonlinear amplifying ring mirror (1) is connected to the linear arm (2) and the output end (3) through the third port (15) and the fourth port (16) located on the other side of the coupler (6), respectively. All fiber optic devices in the nonlinear amplifying ring mirror (1) are non-polarization-maintaining fibers; the first laser pulse of the nonlinear amplifying ring mirror (1) is incident from the first port (17) of the coupler (6), and after passing through the nonlinear amplifying ring mirror (1), it is emitted from the second port (18) of the coupler (6); at the same time, the second laser pulse of the nonlinear amplifying ring mirror (1) is incident from the second port (18) of the coupler (6), and after passing through the nonlinear amplifying ring mirror (1), it is emitted from the first port (17) of the coupler (6); The polarization states of the first laser pulse and the second laser pulse change in the nonlinear amplifying ring (1); the change in polarization states of the first laser pulse and the second laser pulse is caused by the fiber polarization manager (19) and the pump power; the change in polarization states of the first laser pulse and the second laser pulse is related to the nonlinear phase shift difference between the first laser pulse and the second laser pulse. The fiber loop in the nonlinear amplifying ring mirror (1) is composed of optical fibers with birefringence characteristics, so that the light in two mutually perpendicular polarization directions in the fiber obtains different phase shifts. The linear arm (2) serves as the second equivalent cavity mirror of the laser, and includes a first lens (10), a polarizing beam splitter (20), and a reflector (11). The output terminal (3) includes an isolator (9) and a pigtail (13); The transmission function of the ultrashort pulse fiber laser is modulated by the nonlinear phase shift difference between the two beams in the nonlinear amplifying ring (1).

2. The ultrashort pulse fiber laser according to claim 1, characterized in that... The gain fiber (7) is an erbium-doped, ytterbium-doped, or thulium-doped gain fiber; the coupling coefficient of the coupler (6) is 0.

5.

3. The ultrashort pulse fiber laser according to claim 1, characterized in that... The linear arm (2) also includes a non-reciprocal element, which includes a Faraday rotator and at least one waveplate.

4. The ultrashort pulse fiber laser according to claim 3, characterized in that... The non-reciprocal elements include a λ / 4 waveplate (5), a λ / 2 waveplate (12), and a Faraday rotator (14). The laser pulse in the fiber optic loop of the nonlinear amplification ring mirror (1) is emitted from the third port (15) of the coupler, collimated by the first lens (10), and then passes through the λ / 4 waveplate (5), λ / 2 waveplate (12), Faraday rotator (14) and polarization beam splitter (20) in sequence. Finally, it is reflected by the mirror (11). The reflected light first passes through the polarization beam splitter (20), then passes through the Faraday rotator (14), λ / 2 waveplate (12) and λ / 4 waveplate (5) in sequence, and finally enters the third port (15) of the coupler after being coupled by the first lens (10).

5. The ultrashort pulse fiber laser according to claim 4, characterized in that... By adjusting the angles of the λ / 4 waveplate (5) and the λ / 2 waveplate (12), and simultaneously rotating the fiber polarization manager (19), the output spectrum and output power are optimized. Finally, the positions of all non-reciprocal components are fixed, and a stable mode-locked pulse output can be obtained, and self-starting mode-locking can be achieved when the power is turned on again.

6. The ultrashort pulse fiber laser according to claim 1, characterized in that... The reflector (11) in the linear arm (2) is replaced by a saturable absorber element, which is a combination of a semiconductor saturable absorber, a two-dimensional material saturable absorber, a Kerr medium and a slit.

7. The ultrashort pulse fiber laser according to claim 1, characterized in that... The linear arm (2) also includes a dispersion compensation module, which is located between the polarization beam splitter (20) and the mirror (11). It consists of a grating (21), a second lens (10'), and a displacement stage (22). The grating (21) is located after the polarization beam splitter (20), and the second lens (10') is located in front of the mirror (11) and placed on the displacement stage (22). The dispersion compensation module is used to compensate for the intracavity dispersion of the laser resonant cavity, making the laser easier to mode lock.

8. The ultrashort pulse fiber laser according to claim 1, characterized in that... The repetition frequency of the ultrashort pulse fiber laser is adjusted by moving the mirror (11) in the linear arm (2), and the repetition frequency of the laser is stabilized by negative feedback control.

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