CN116131077A

CN116131077A - Ultrashort pulse fiber laser

Info

Publication number: CN116131077A
Application number: CN202310248108.1A
Authority: CN
Inventors: 胡凯淋; 曹涛; 陈堃; 刘少臻; 徐旗; 李舟; 鄢心怡; 杨乐言; 陆开一; 彭家晖
Original assignee: Wuxi Femtosecond Aurora Technology Co ltd; Huazhong University of Science and Technology
Current assignee: Wuxi Femtosecond Aurora Technology Co ltd; Huazhong University of Science and Technology
Priority date: 2023-03-15
Filing date: 2023-03-15
Publication date: 2023-05-16

Abstract

The invention discloses an ultrashort pulse fiber laser. Comprising the following steps: a nonlinear amplifying ring mirror, which enables pulses transmitted in opposite directions in the ring to accumulate different phase shifts so as to form an intensity modulation mechanism for laser pulses in the cavity; the linear arm is composed of an optical fiber device and a space optical element and is used for forming a resonant cavity and improving the self-locking mode characteristic of the laser; and an output terminal for directly outputting the pulse. The ultra-fast fiber laser provided by the invention is easy to package, good in mode locking self-starting characteristic, not easy to damage a saturable absorber, high in laser damage threshold, small in size, simple and reliable in system and high in stability, and has potential to be developed into a new generation of all-fiber ultra-short laser oscillators with excellent performance.

Description

Ultrashort pulse fiber laser

Technical Field

The invention belongs to the technical field of laser, and particularly relates to an ultra-short pulse fiber laser based on nonlinear amplification loop mode locking, which improves the mode locking self-starting capability and reliability by adding nonlinear phase shift.

Background

The ultra-fast laser has the advantages of extremely short time scale, extremely high peak power and the like, and is widely applied to the fields of material processing, biomedical imaging, micro-spectrum and the like. In order to obtain higher power ultrafast lasers, almost exclusively multi-stage amplified systems with a master oscillator (or laser seed source) are used, which requires a higher quality of the master oscillator, including higher output power, better power stability, faster response speed, etc. The invention aims to improve the reliability and stability of the laser seed source and reduce the complexity and cost of the system.

At present, the mode locking mode in the ultrafast fiber laser oscillator is mainly a passive mode locking technology, and the implementation mode of the passive mode locking mainly comprises three modes of saturated absorber mode locking, nonlinear polarization rotation mode locking and nonlinear optical ring mirror mode locking.

A saturable absorber (SESAM) is a material in which the absorption of incident light decreases with increasing light intensity. When the laser pulse in the resonant cavity passes through the saturable absorber, the transmittance (or reflectivity) of the peak part with higher light intensity is larger, and the transmittance (or reflectivity) of the two wing parts with lower light intensity is smaller. Thus, the mode locking process can be started by utilizing the intensity modulation effect of SESAM, and stable mode locking can be realized. However, the damage threshold of the saturable absorber of the semiconductor material is low, so that the SESAM is easily damaged by high-intensity Q-switching pulses in the Q-switching mode locking stage at the initial mode locking stage, which is one of the important problems of picosecond laser seed sources in the current industry.

The nonlinear polarization rotation (or nonlinear polarization evolution) mode locking technology is generally applied to a non-polarization maintaining system, so that the system stability is poor, and in general, the self-starting of the system needs to try different polarization directions for many times, so that a proper mode locking interval is difficult to find to realize stable mode locking, and the development of an industrial laser is not facilitated.

Compared with the two mode locking technologies, the nonlinear optical loop mode locking technology has the advantages of high response speed, high signal to noise ratio, low cost, high system stability, high bearable power, convenience in integration, small interference by external environment and the like, and therefore, the nonlinear loop mode locking-based laser is more suitable for being used in the industrial field with high stability requirements.

At present, most mode-locking lasers using a nonlinear optical ring mechanism adopt an 8-shaped structure (U.S. Pat. No. 1,786,84 B2), but because the lasers are of a full-closed-loop structure, the cavity length is not adjustable, and the physical quantity in the cavity and the influence of the physical quantity on the mode-locking process are difficult to measure, so that the improvement of the performance and the range of practical application of the mode-locking lasers are limited. The German Menlo company (Chinese patent CN 103311780B and U.S. Pat. No. 3, 5359612A) breaks through the structure, develops and realizes a 9-shaped laser cavity structure, namely an oscillation annular cavity is opened to be changed into a linear cavity on the basis of an 8-shaped structure, and one end of the linear cavity is provided with an end mirror with adjustable position, so that the cavity length is adjustable. In the 9-shaped laser structure, in order to realize self-starting mode locking, a non-dissimilarity space element is inserted into the annular cavity, so that the additional phase of a pulse is related to the propagation direction, and the optimal self-starting mode locking effect is obtained by changing the phase shift of a wave plate in the element.

However, in application, the skilled person finds that, although the mode of improving the self-starting characteristic of the laser by inserting the nonreciprocal space element is used by people, the problems of difficult integration, easy environmental interference of system stability, low output power, slow mode locking self-starting, high system cost and the like still exist due to the fact that the space elements in most of the 9-shaped laser cavity structures are more.

Disclosure of Invention

Aiming at the defects of the prior art and the requirements of actual production, the invention provides a 9-shaped cavity all-fiber structure ultrashort pulse laser based on nonlinear amplification ring mirror mode locking. The invention aims to construct an ultrafast laser with high stability, fast self-starting mode locking, difficult damage and lower cost, and provides an integrated laser scheme capable of resisting the interference of a ring mirror, so that a series of problems caused by the limitations and defects of the prior art are solved to a certain extent.

Other features and advantages of the present disclosure will be apparent from the following detailed description, or may be learned in part by the practice of the disclosure.

In order to achieve the above object, the present invention provides an ultrashort pulse fiber laser, comprising: a nonlinear amplifying ring lens 1, a linear arm 2 and an output end 3. The nonlinear amplifying ring mirror and the linear arm form a complete laser resonant cavity together, so that laser oscillates and amplifies in the cavity in a reciprocating mode.

The nonlinear amplification ring mirror 1 is used as a first equivalent cavity mirror of a laser, is of an all-fiber structure and comprises 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 is used as a pumping source, is connected with the gain optical fiber 7 through the wavelength division multiplexer 8, and forms an optical fiber loop through a first port 17 and a second port 18 which are positioned on the same side of the coupler 6; the nonlinear amplifying ring lens 1 is respectively connected with the linear arm 2 and the output end 3 through a third port 15 and a fourth port 16 which are positioned on the other side of the coupler 6; the linear arm 2 is used as a second equivalent cavity mirror of the laser and comprises a first lens 10, a polarization beam splitter 20 and a reflecting mirror 11; the output 3 comprises an isolator 9 and a pigtail 13.

The transfer function of the ultra-short pulse fiber laser is modulated by a nonlinear phase shift difference between two light beams in the nonlinear amplifying ring mirror 1.

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

Further, all optical fiber devices in the nonlinear amplification ring mirror 1 in the ultra-short pulse optical fiber laser are non-polarization maintaining optical fibers; the first laser pulse of the nonlinear amplification loop mirror 1 is incident from the first port 17 of the coupler 6, passes through the nonlinear amplification loop mirror 1 and then is emitted from the second port 18 of the coupler 6; while the second laser pulse of the nonlinear amplification loop 1 is incident from the second port 18 of the coupler 6, passes through the nonlinear amplification loop 1 and exits from the first port 17 of the coupler 6.

The polarization states of the first laser pulse and the second laser pulse are changed in the nonlinear amplification ring mirror 1; the polarization state changes of the first laser pulse and the second laser pulse are jointly caused by the optical fiber polarization manager 19 and the pump power; the change in polarization state of the first laser pulse and the second laser pulse is related to a nonlinear phase shift difference between the first laser pulse and the second laser pulse.

Meanwhile, in the ultra-short pulse fiber laser, the shape of the ultra-short laser pulse output from the fourth port 16 of the coupler 6 can be modulated by adjusting the phase shift of the nonlinear amplifying ring mirror 1, and multiple pulse shapes such as hyperbolic secant and parabola are output.

Because of the incomplete symmetry of the nonlinear amplifying ring (the spectral ratio of the coupler is not exactly equal to 50:50, and the gain fibers are not exactly distributed in the middle of the ring, etc.), the first laser pulse and the second laser pulse respectively incident through the two ports of the coupler accumulate different nonlinear phase shifts after passing through all the optical paths in the ring, and when they meet again, interference occurs, thus affecting the transfer function of the laser system, forming an intensity modulation mechanism, and the transfer function of the system is shown in fig. 5 (without bias). However, the mere nonlinear amplifying ring mirror cannot complete the self-starting of the mode locking of the laser, because when the laser starts pumping, the first laser pulse and the second laser pulse have very small phase shift difference, the slope of the transmission curve is zero, and the modulation of the laser is almost zero.

In order to facilitate the mode locking and self-starting of the laser, a nonreciprocal element is inserted into the cavity, which has the function of increasing the initial bias of the transfer function of the system, such as the bias curve shown in fig. 5, and the slope of the transfer function is larger when the laser is started, so that the modulation of the laser is larger and the self-starting is easier. Preferably, the non-reciprocal element may consist of a lambda/8 wave plate, a lambda/4 wave plate, a lambda/2 wave plate, a faraday rotator, etc. (note that although the non-reciprocal element can perform some phase shift, the non-reciprocal element is not a phase shifter).

According to the invention, the coupler divides the laser in the linear arm into a first laser pulse and a second laser pulse, the first laser pulse and the second laser pulse enter the nonlinear amplifying ring mirror, and the two laser pulses meet at the coupler after passing through the nonlinear amplifying ring mirror. However, due to the change of the polarization states, the polarization states of the first laser pulse and the second laser pulse are not identical, so that only partial interference occurs at the coupler, which has the advantage of having a time domain shaping effect on the laser pulses. The first laser pulse and the second laser pulse are collimated by the lens and then pass through the nonreciprocal element, and then partially penetrate the polarization beam splitter at the polarization beam splitter so as to be remained in the cavity, and partially reflect outside the cavity by the polarization beam splitter, so that loss is formed. Because the change of the polarization state in the cavity is related to the change of the nonlinear phase shift difference, the transmittance and the reflectivity of the polarization beam splitter also change along with the nonlinear phase shift difference, so that the transfer function of the system is subjected to nonlinear phase shift modulation in the nonlinear amplifying ring mirror, and the nonreciprocal element provides additional phase shift between the first laser pulse and the second laser pulse, thereby changing the initial position of the transfer function of the system and facilitating the self-starting of the mode locking process of the laser.

Specifically, the linear arm 2 of the ultra-short pulse fiber laser also comprises a non-reciprocal element, and the non-reciprocal element comprises a Faraday rotator and at least one wave plate.

Preferably, the nonreciprocal elements in the above-described ultrashort pulse fiber laser include a λ/4 plate 5, a λ/2 plate 12, and a faraday rotator 14.

The laser pulse in the optical fiber loop of the nonlinear amplification ring mirror 1 is emitted from the third port 15 of the coupler, collimated by the first lens 10, then sequentially passed through the lambda/4 wave plate 5, the lambda/2 wave plate 12, the Faraday rotator 14 and the polarization beam splitter 20, finally reflected by the reflecting mirror 11, and the reflected light firstly passes through the polarization beam splitter 20, then sequentially passed through the Faraday rotator 14, the lambda/2 wave plate 12 and the lambda/4 wave plate 5, finally coupled by the first lens 10 and then enters the third port 15 of the coupler.

Furthermore, the ultra-short pulse fiber laser can optimize the output spectrum and the output power by adjusting the angles of the lambda/4 wave plate 5 and the lambda/2 wave plate 12 and simultaneously matching with the rotation of the fiber polarization manager 19, finally, the positions of all the nonreciprocal elements are fixed, so that stable mode locking pulse output can be obtained, and the self-starting mode locking is realized when the laser is started again.

In order to be able to adjust the position of the system transmission curve, i.e. the value and slope of the laser start point (phase shift difference zero), the wave plate in the nonreciprocal element can be adjusted according to the specific situation. Preferably, the system has a maximum modulation depth when the fast axis angle of the λ/4 plate in the non-reciprocal element is at an angle of 0 ° to the polarization direction of the transmitted light of the polarizing beam splitter. Preferably, rotation of the lambda/2 wave plate in the non-reciprocal element acts as an adjustment element for the system transmission curve bias.

Preferably, the change of the polarization states of the first laser pulse and the second laser pulse transmitted in the nonlinear amplification loop mirror in opposite directions can be performed through a non-polarization maintaining optical fiber and a polarization manager, and the polarization states in the optical fiber can be controlled by using a mode scrambler or an electrically controlled polarization controller and other devices due to the unstable polarization state mode of the laser pulse in the non-polarization maintaining optical fiber. The polarization evolution in the ring mirror can be provided with various choices by using the non-polarization-maintaining optical fiber and the polarization manager, so that the system is easier to lock the mode.

Meanwhile, in order to adjust the position of the transmission curve of the system, namely, the value and the slope of the starting point (the position where the phase shift difference is zero) of the laser, the wave plate in the nonreciprocal element can be adjusted according to specific conditions, and the wave plate is fixed after being adjusted to a proper position, so that pulse output which is easy to lock the mode and stably works can be provided. Compared with the traditional 9-shaped cavity laser, the system transmission curve of the invention has a larger slope when the nonlinear phase shift difference is 0, so the laser has faster and more stable self-starting performance (shown in fig. 5 and embodiment one).

Compared with the traditional 9-shaped cavity laser, the non-polarization-maintaining nonlinear ring mirror and the polarization manager have the function of improving the self-starting characteristic, and the use of a special coupler (such as a polarization beam combiner) or a saturable absorber is avoided, so that the laser integration performance is improved, and the cost is reduced. Compared with a traditional Nonlinear Polarization Evolution (NPE) mode-locked laser, the nonlinear amplification loop enables mode locking to be more stable and reliable, and once all parameters are set correctly, the state of all elements is not required to be changed, so that the startup self-starting can be realized.

Alternatively, the mirror 11 in the linear arm 2 of the ultra-short pulse fiber laser of the present invention may be replaced by a saturable absorber element, which is a semiconductor saturable absorber, a two-dimensional material saturable absorber, or a combination of kerr medium and slit.

Furthermore, the linear arm of the ultra-short pulse fiber laser provided by the invention has the function of adjusting the cavity length, so that the laser can conveniently adjust the repetition frequency. Preferably, the repetition rate of the laser is adjustable by movement of the mirror 11 in the linear arm and stabilized by means of negative feedback control.

Further, in order to realize the function of accumulating different phase shifts in the cavity by different polarization states, besides controlling the polarization states by using a non-polarization-maintaining optical fiber and a polarization manager, an optical fiber with specific birefringence characteristics can be used in an optical fiber loop of the nonlinear amplifying ring mirror. The optical fiber can be used for forming an ultrafast optical fiber laser system with simpler structure and more stable performance.

According to the invention, mode locking self-starting is realized by a nonlinear optical loop technology, besides the initial bias provided for a system transfer function by using a nonreciprocal element, the self-starting performance can be improved by adopting a mode of adding other modulations, and in this case, the nonreciprocal element becomes an unnecessary structure. For example, the intensity modulation of the saturable absorber can be used for screening out pulses with certain light intensity or generating initial laser pulses in an electric modulation mode, so that the defect that a 9-shaped cavity without a non-reciprocal element cannot be started automatically is avoided, and the pulses are further compressed in the modulation process of the nonlinear amplifying ring mirror to form narrower and more stable ultrafast laser pulses. Compared with a mode-locked laser which only uses a saturable absorber for mode locking, the mode locking mode of mixed modulation combining a nonlinear amplifying ring mirror and a saturable absorber (hereinafter, the mode locking technology of mixed modulation refers to the scheme) has the following advantages: (1) The filtering effect provided by the nonlinear amplifying ring lens can eliminate the front and back edges of the pulse, so that the pulse contrast is higher (the time domain pulse is narrower); (2) The saturable absorber is only used as a mode locking starting element, focus adjustment is not needed, and the mode locking process of the hybrid mode locking system is faster due to the combined action of the nonlinear amplifying ring mirror and the saturable absorber, so that the damage rate of the saturable absorber can be reduced; (3) The system has wide application range, almost all gain optical fibers can use the scheme, and for the total positive dispersion optical fiber structure, only one filter is added to limit the continuous broadening of the spectrum in the cavity, so that the mode locking and the self-starting can be realized, and dispersion compensation devices such as chirped fiber gratings or grating systems are not needed.

Compared with the mode of realizing self-starting by using a non-reciprocal element, the realization scheme of the hybrid modulation mode locking technology has the advantages of simple structure, lower cost and good stability.

According to the invention, for gain optical fibers and single-mode optical fibers with different doping elements, the net chromatic dispersion in the cavity is different, and for an all-positive chromatic dispersion cavity, the laser compensates chromatic dispersion by the chromatic dispersion compensation module, so that the self-starting characteristic and stability of the system are improved, and the noise level is reduced. The total positive dispersion referred to in the present invention means that the net intra-cavity dispersion (or total dispersion) of the laser is normal dispersion. Typically, the dispersion compensation module includes a dispersion compensation fiber, a chirped fiber grating, a bulk grating, a prism, a chirped mirror, etc., and these dispersion compensation systems are suitable for use in the laser system of the present invention.

Preferably, the linear arm 2 of the ultra-short pulse fiber laser further comprises a dispersion compensation module, wherein the dispersion compensation module is positioned between the polarization beam splitter 20 and the reflecting mirror 11 and consists of a grating 21, a second lens 10 'and a displacement table 22, the grating 21 is positioned behind the polarization beam splitter 20, and the second lens 10' is positioned in front of the reflecting mirror 11 and is placed on the displacement table 22; the dispersion compensation module is used for compensating the intra-cavity dispersion of the laser resonant cavity, so that the laser is easy to lock the mode.

In the mixed modulation mode locking scheme, a filter is used for limiting the continuous spectrum broadening of pulses in the full positive dispersion structure, so that the full positive dispersion structure can realize mode locking self-starting without dispersion compensation; when the net dispersion in the cavity is negative, the filter becomes an unnecessary device, and the effect of the filter is not required anymore when self-priming is achieved.

Drawings

In order to more clearly illustrate the technical scheme of the embodiment of the invention, the drawings which are needed to be used in the description of the embodiment are briefly introduced below. It is apparent that the drawings described in the following description are only some, but not all, embodiments of the invention, and that other drawings may be derived from these drawings by a person skilled in the art without inventive effort.

Fig. 1 is a schematic diagram of a laser structure according to a first embodiment of the present invention;

fig. 2 is a schematic diagram of a laser structure according to a second embodiment of the present invention;

FIG. 3 is a schematic diagram of a laser structure according to a third embodiment of the present invention;

fig. 4 is a schematic diagram of a laser structure according to a fourth embodiment of the present invention;

FIG. 5 is a graph of the system transfer function of a "9" cavity laser ("reflectivity of a nonlinear ring mirror in a 9" cavity versus nonlinear phase shift modulation curve);

FIG. 6 is a graph of output spectral data according to a first embodiment of the present invention;

fig. 7 is a graph of output spectral data according to a fourth embodiment of the present invention.

Reference numerals: 1. a nonlinear magnifying ring; 2. a linear arm; 3. an output end; 4. a laser diode; 5. a lambda/4 wave plate; 6. a coupler; 7. a gain fiber; 8. a wavelength division multiplexer; 9. an isolator; 10. a first lens; 10', a second lens; 11. a reflecting mirror; 12. a lambda/2 wave plate; 13. tail fiber; 14. a Faraday rotator; 15. a coupler third port; 16. a coupler fourth port; 17. a coupler first port; 18. a coupler second port; 19. an optical fiber polarization manager; 20. a polarizing beam splitter; 21. a grating; 22. a displacement table; 23. an all-fiber device; 24. chirped fiber gratings.

Detailed Description

The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be replaced with each other as long as they do not collide with each other. In the present invention, the terms "first," "second," and the like (if any) are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In addition to the specific methods, devices, materials used in the embodiments, any methods, devices, and materials of the prior art similar or equivalent to those described in the embodiments of the present invention may be used to practice the present invention according to the knowledge of one skilled in the art and the description of the present invention. In the present invention, unless otherwise indicated, all numbers expressing quantities of parts, devices, and materials used are to be understood as being modified in all instances by the term "about" in the claims.

As shown in fig. 1, in a first embodiment of the present invention, an ultrashort pulse fiber laser design structure provided in the present invention includes: a nonlinear amplifying ring lens 1, a linear arm 2 and an output end 3. In this embodiment, all the optical fibers are non-polarization maintaining optical fibers, wherein the nonlinear amplifying ring lens comprises the following components: the optical fiber polarization manager 19, the laser diode 4 is used as a pumping source, the central wavelength of which is 980nm, is connected with the ytterbium-doped gain optical fiber 7 through the wavelength division multiplexer 8, and the first port 17 and the second port 18 which are positioned on the same side of the coupler 6 are connected with the devices to form an optical fiber loop. The coupling coefficient of the coupler 6 is 0.5, the third port 15 positioned at the other side of the coupler is coupled with a space element in the linear arm, laser pulses in the optical fiber are emitted from the third port 15 of the coupler, collimated by the first lens 10, then sequentially pass through the lambda/4 wave plate 5, the lambda/2 wave plate 12, the Faraday rotator 14, the polarization beam splitter 20, finally reflected by the reflecting mirror 11, coupled by the elements and enter the third port 15 of the coupler.

The adjustment and optimization method of the first embodiment is as follows: with the light transmission direction in the spatial light path as the z-axis, the direction parallel to the paper surface is upward as the y-axis (the polarization direction of the transmitted light of the polarization beam splitter 20 is parallel to the y-axis), the direction perpendicular to the paper surface is outward as the x-axis, the angle between the fast axis of the λ/4 plate 5 and the x-axis direction is 45 °, the angle between the fast axis of the λ/2 plate 12 and the x-axis direction is 22.5 °, and the optical rotation angle of the faraday rotator is 45 °. At this time, the pumping threshold of the laser emitted by the system is the lowest, the pumping power is increased to enable the system to emit continuous laser, and then the optical fiber polarization manager 19 is rotated to lock the system mode, at this time, compared with the conventional 9-shaped cavity laser, the transmission curve of the system has a larger slope when the nonlinear phase shift difference is 0, so that the laser has faster and more stable self-starting performance. Through constantly fine setting the angle of lambda/2 wave plate 12 and lambda/4 wave plate 5, cooperate the meticulous rotation of optic fibre polarization manager 19 simultaneously, can further optimize output spectrum and output, at last will all device's position is fixed, will obtain stable mode locking pulse output, and can realize the self-starting mode locking when starting up again.

In the first embodiment, the light passing through the polarization beam splitter 20 is transmitted through the nonreciprocal element in this order: the faraday rotator 14, the λ/2 plate 12, the λ/4 plate 5 are coupled into an optical fiber through the first lens 10 (in this case, the light pulse is elliptically polarized light), and then divided into a first laser pulse and a second laser pulse by a coupler. Because the optical fiber and the optical fiber device in the nonlinear amplification ring lens are both non-polarization-maintaining devices, the first laser pulse and the second laser pulse are elliptical polarized light, and the polarization states of the two pulses are influenced by the control of the optical fiber polarization manager 19, the power of pumping and the birefringence of the optical fiber, when the state of the optical fiber polarization manager 19 and the power of pumping are changed, the polarization states (elliptical polarization and long axis direction) of the first laser pulse and the second laser pulse are changed in the transmission process of the non-polarization-maintaining optical fiber. And because the first laser pulse and the second laser pulse accumulate different nonlinear phase shifts in the nonlinear loop, when the two meet again at the coupler 6, partial interference occurs, so that the pulse intensity is modulated by the nonlinear phase shift. After passing through the coupler 6, the first laser pulse and the second laser pulse are combined into an elliptical polarized light pulse, and then pass through the non-reciprocal element again to reach the polarizing beam splitter 20, wherein the elliptical polarization is influenced by the non-reciprocal element, the nonlinear phase shift, the non-polarization maintaining fiber birefringence, the pumping power and the fiber polarization manager, so that the transmitted light (p polarized light) passing through the polarizing beam splitter 20 is subjected to a complicated intensity adjustment mechanism. The output spectrum of the first embodiment is shown in fig. 6.

As shown in fig. 2, in the second embodiment of the present invention, the design structure of the ultrashort pulse fiber laser provided by the present invention is substantially the same as that of the first embodiment, and the second embodiment inserts the grating 21 and the second lens 10' at the position in front of the laser mirror 11, and places them on the displacement table 22, so as to form a dispersion compensation module.

As shown in fig. 3, in the third embodiment of the present invention, the design structure of the ultrashort pulse fiber laser provided by the present invention is substantially the same as that of the first embodiment, and the third embodiment integrates and assembles the nonreciprocal elements (5, 12, 14) in the linear arm, the reflecting mirror 11, the first lens 10, the polarization beam splitter 20, etc., so that the nonreciprocal elements become an all-fiber device 23, so that the system structure is simpler and more stable, and is more suitable for industrial production.

As shown in fig. 4, in a fourth embodiment of the present invention, the design structure of the ultrashort pulse fiber laser provided in the present invention mainly includes: wavelength division multiplexer 8, chirped fiber grating 24, gain fiber 7, and linear arm 2. In this embodiment, the linear arm is similar to the first embodiment. The nonlinear amplifying ring mirror in the first embodiment comprises a chirped fiber grating 24, a special birefringent fiber, a nonlinear phase shift difference derived from the birefringent properties of the polarization maintaining fiber, and two polarized light beams having different gain coefficients, and thus a phase shift difference between them, comprising a first lens 10, a faraday rotator 14, a lambda/4 wave plate 5, and a mirror 11. In the fourth embodiment, all the optical fiber devices are polarization maintaining devices, and have no fast axis or slow axis cut-off characteristics. The output spectrum of the fourth embodiment is shown in fig. 7.

While the preferred embodiments and examples of the present invention have been described in detail, the present invention is not limited to the above-described embodiments and examples, and various changes may be made within the knowledge of those skilled in the art without departing from the spirit of the present invention.

Claims

1. An ultrashort pulse fiber laser, characterized in that the ultrashort pulse fiber laser comprises the following components: a nonlinear amplifying ring mirror (1), a linear arm (2) and an output end (3); wherein:

the nonlinear amplifying ring mirror (1) and the linear arm (2) together form a laser resonant cavity;

the nonlinear amplification loop mirror (1) is used as a first equivalent cavity mirror of the laser, is of an all-fiber structure, and comprises 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) is used as a pumping source, is connected with the gain optical fiber (7) through the wavelength division multiplexer (8), and forms an optical fiber loop through a first port (17) and a second port (18) which are positioned on the same side of the coupler (6); the nonlinear amplifying ring mirror (1) is respectively connected with the linear arm (2) and the output end (3) through a third port (15) and a fourth port (16) which are positioned at the other side of the coupler (6);

the linear arm (2) is used as a second equivalent cavity mirror of the laser and comprises a first lens (10), a polarization beam splitter (20) and a reflecting mirror (11);

the output end (3) comprises an isolator (9) and a tail fiber (13);

the transfer function of the ultra-short pulse fiber laser is modulated by a nonlinear phase shift difference between two light beams in the nonlinear amplifying ring mirror (1).

2. Ultrashort pulse fiber laser according to claim 1, wherein all fiber devices in the nonlinear amplification ring mirror (1) are non-polarization maintaining fibers; the first laser pulse of the nonlinear amplification ring mirror (1) is incident from a first port (17) of the coupler (6), passes through the nonlinear amplification ring mirror (1) and then is emitted from a second port (18) of the coupler (6); simultaneously, a second laser pulse of the nonlinear amplification ring mirror (1) is incident from a second port (18) of the coupler (6) and is emitted from a first port (17) of the coupler (6) after passing through the nonlinear amplification ring mirror (1);

the polarization states of the first laser pulse and the second laser pulse are changed in the nonlinear amplifying ring mirror (1); the polarization state changes of the first laser pulse and the second laser pulse are jointly caused by an optical fiber polarization manager (19) and pump power; the change in polarization state of the first laser pulse and the second laser pulse is related to a nonlinear phase shift difference between the first laser pulse and the second laser pulse.

3. Ultrashort pulse fiber laser according to claim 1 or 2, wherein the laser diode (4) has a center wavelength of 980nm; the gain fiber (7) is erbium-doped, ytterbium-doped or thulium-doped; the coupling coefficient of the coupler (6) is 0.5.

4. Ultrashort pulse fiber laser according to claim 1 or 2, characterized in that the fiber ring in the nonlinear amplification ring mirror (1) is composed of fibers with birefringence properties such that two mutually perpendicular polarization directions of light within the fibers obtain different phase shifts.

5. Ultrashort pulse fiber laser according to claim 1 or 2, characterized in that the linear arm (2) further comprises a nonreciprocal element comprising a faraday rotator and at least one wave plate.

6. The ultra-short pulse fiber laser of claim 5 wherein the non-reciprocal element comprises a λ/4 plate (5), a λ/2 plate (12) and a faraday rotator (14);

the laser pulse in the optical fiber loop of the nonlinear amplifying ring mirror (1) is emitted from a third port (15) of the coupler, collimated by a first lens (10), sequentially passes through a lambda/4 wave plate (5), a lambda/2 wave plate (12), a Faraday rotator (14) and a polarization beam splitter (20), finally is reflected by a reflecting mirror (11), and the reflected light sequentially passes through the polarization beam splitter (20), then sequentially passes through the Faraday rotator (14), the lambda/2 wave plate (12) and the lambda/4 wave plate (5), and finally is coupled by the first lens (10) and then enters the third port (15) of the coupler.

7. The ultra-short pulse fiber laser according to claim 6, wherein the stable mode-locking pulse output can be obtained by adjusting the angles of the lambda/4 wave plate (5) and the lambda/2 wave plate (12) and simultaneously optimizing the output spectrum and the output power in cooperation with the rotation of the fiber polarization manager (19), and finally fixing the positions of all the nonreciprocal elements, and the self-starting mode locking is realized when the laser is restarted.

8. Ultrashort pulse fiber laser according to claim 1 or 2, wherein the mirror (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.

9. Ultrashort pulse fiber laser according to claim 1 or 2, characterized in that the linear arm (2) further comprises a dispersion compensation module, which is located between the polarization beam splitter (20) and the mirror (11), consisting of a grating (21), a second lens (10 ') and a displacement stage (22), the grating (21) being located after the polarization beam splitter (20), the second lens (10') being located before the mirror (11) and being placed on the displacement stage (22); the dispersion compensation module is used for compensating the intra-cavity dispersion of the laser resonant cavity, so that the laser is easy to lock the mode.

10. Ultrashort pulse fiber laser according to claim 1 or 2, characterized in that the repetition rate of the ultrashort pulse fiber laser is adjusted by the movement of a mirror (11) in the linear arm (2) and stabilized by means of negative feedback control.