All-polarization-maintaining fiber femtosecond laser based on mode-locking start facilitating structure

Abstract

The application relates to a full-polarization-maintaining fiber femtosecond laser based on a mode-locking starting promotion structure. The laser comprises a nonlinear amplification ring mirror with a double-path gain fiber, can realize continuous adjustment of a phase difference, introduces a nonlinear phase shift related to laser pulse intensity, reduces a mode-locking threshold of the nonlinear amplification ring mirror laser, improves self-starting mode-locking capability of the laser, and outputs high-power and high-stability ultrashort pulses; a non-reciprocal phase shift element introduces a linear phase shift bias for clockwise and counterclockwise propagation pulses; a linear arm introduces an optical element with a dispersion compensation function, ensures compactness of the oscillator, is used for outputting pulses, simultaneously provides seed pulse pre-chirp management for subsequent higher-power amplification, and realizes flat wide spectrum. The laser adopts a full-polarization-maintaining fiber structure, has good self-starting mode-locking characteristics, is beneficial to pulse filtering and shaping, and simultaneously has the advantages of compact structure, good stability, high damage threshold, high output power and low noise.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of femtosecond fiber lasers, and more particularly relates to a full-polarization fiber ultra-short pulse laser based on a mode-locked start promoting structure and adopting a double-pumped gain ring cavity "9-shaped" structure. BACKGROUND

[0002] Femtosecond lasers have the advantages of extremely short time scale and extremely high peak power, and can provide advantageous conditions for various applications, and are thus widely used in the fields of material processing, biomedical imaging, microscopic spectroscopy, etc. With the in-depth research on the amplification of ultrafast laser pulses, fiber lasers have gradually become one of the research hotspots in the field of high-power lasers, and have obvious advantages in beam quality, laser structure, energy conversion efficiency, internal thermal management, service life, etc. It can be predicted that the full-polarization fiber femtosecond laser with compact structure, high efficiency and strong stability has broad application prospects in scientific research and industrial production.

[0003] In order to realize a high-efficiency high-power femtosecond laser amplification system, a high-performance mode-locked fiber laser oscillator is particularly important. At present, there are mainly three kinds of mode-locked mechanisms for femtosecond fiber lasers: saturable absorber passive mode-locking, nonlinear polarization rotation mode-locking based on additive pulse mode-locking technology, and nonlinear optical ring mirror mode-locking. The saturable absorber is generally a material with fine structure such as semiconductor or graphene, carbon nanotube, etc. Due to its low damage threshold, easy degradation, large intrinsic noise, poor stability and other characteristics, it is not suitable for long-term use and use in some complex environments. The state of nonlinear polarization rotation mode-locking lacks determinacy, and the state after mode-locking changes with the combination of wave plates, and is easily affected by external pressure and temperature changes, which destroys the original polarization state, has low repeatability, is highly dependent on polarization, and has poor mechanical stability. In comparison, in the nonlinear optical ring mirror mode-locked laser, the counter-propagating pulses in the ring mirror experience asymmetric amplification in the fiber and thus accumulate different additional phase shifts, and meet again at the coupler, interfere constructively or destructively. In the case of a pulse with intensity distribution, if the intensity of the pulse is close to the maximum of the transmission rate, the wings of the pulse are reflected more, while the sharp peaks are transmitted, and the pulse is continuously narrowed to achieve mode-locking. Its advantages are fast response speed, low phase noise and jitter, high signal-to-noise ratio, and the mode-locking mechanism can realize full-polarization fiber, the mode-locked state is not disturbed by the external environment, has strong repeatability and good long-term stability.

[0004] The typical structure for realizing mode-locked laser by using nonlinear optical ring mirror mechanism is the "8-shaped" structure proposed by Richard. It relies on the nonlinear phase shift difference in the fiber loop and the interference effect in the beam splitter. However, in order to realize the saturable absorption effect, the laser needs to increase the additional pump and use the relatively long fiber to accumulate enough nonlinear phase shift to improve the self-starting ability, which is easy to produce multiple pulses and has low output power, greatly limiting its practical application. In addition, the "8-shaped" structure needs to use two loops, and the length of the fiber is long, which is not conducive to high repetition frequency. In addition, due to the lack of end mirrors, it is difficult to realize the frequency adjustment and frequency stability.

[0005] In order to overcome the defects of the "8-shaped" structure, the German Menlo company first developed and realized the "9-shaped" structure. By using the reflection of the loop, two ring cavities are changed into one loop mirror and one linear arm. By inserting a non-reciprocal phase shift element to increase the phase bias, a certain linear phase bias can be added to the two counter-propagating linearly polarized lights, thereby reducing the self-starting mode-locking threshold. However, the non-reciprocal phase shift element introduced in this structure needs to include two Faraday rotators and at least one wave plate, and the artificial phase bias is discrete, such as using an eighth wave plate or a quarter wave plate to provide a non-continuous phase difference of ±π / 4, ±π / 2. It strictly depends on the selection of the wave plate, and the selection range is small. At the same time, the laser does not work in the corresponding optimal solution. In addition, since the mode-locking mechanism in the cavity has not been fully understood, it is difficult to quantify the nonlinear phase shift accumulated in the fiber loop, so the wave plate providing the optimal phase difference and its optimal angle need to be obtained by trial and error method, which has large uncertainty and is complicated to operate. It greatly limits the reduction of the self-starting mode-locking threshold and further improves the performance of the laser.

[0006] In addition, although the insertion of the non-reciprocal phase shift element can improve the self-starting characteristics of the laser, most of the "9-shaped" laser cavity structures have complex spatial structures, which are not easy to integrate and difficult to realize a compact all-fiber structure. SUMMARY

[0007] In order to overcome the above-mentioned defects of the existing femtosecond fiber laser "9-shaped" laser cavity structure, the present application proposes a new full polarization-maintaining fiber femtosecond laser based on a mode-locked start promoting structure. By using the nonlinear amplification ring mirror of the double-path gain fiber, the phase difference is continuously adjustable, which promotes the self-starting mode-locking of the laser, and ensures that the laser works in the optimal solution region of the mode-locking. At the same time, the laser of the present application also has the characteristics of low mode-locking threshold, good stability, high damage threshold, compact structure, supporting high-power output, high signal-to-noise ratio, etc.

[0008] In order to achieve the above-mentioned purpose, the application provides a full polarization maintaining fiber femtosecond laser based on a mode-locked start promoting structure, which comprises: a nonlinear amplifying ring mirror with a double-path gain fiber, which can realize continuous adjustment of phase difference, introduce nonlinear phase shift related to laser pulse intensity, reduce the mode-locked threshold of the nonlinear amplifying ring mirror laser, improve the self-starting mode-locked capability of the laser, and output high-power and high-stability ultrashort pulses; the nonlinear amplifying ring mirror with the double-path gain fiber can also be used for tuning of output power and can realize high-power output; a non-reciprocal phase shift element, which introduces linear phase shift bias for clockwise and counterclockwise propagation pulses; a linear arm, which introduces optical elements with dispersion compensation function to ensure the compactness of the oscillator and is used for output of pulses and provides seed pulse pre-chirp management for subsequent higher power amplification, so as to realize flat wide spectrum.

[0009] Specifically, the laser comprises the following components: a nonlinear amplifying ring mirror and a linear arm. The nonlinear amplifying ring mirror and the linear arm jointly constitute a complete "9-shaped" structure of a laser resonant cavity, so that the laser is started and amplified in the cavity. All the fiber elements contained in the nonlinear amplifying ring mirror and the linear arm are polarization maintaining devices, so as to facilitate transmission and output of ultrashort laser pulses with specific polarization states and enhance system stability.

[0010] In the laser, the nonlinear amplifying ring mirror is composed of a coupler, a double-path gain fiber, a wavelength division multiplexer, a non-reciprocal phase shift element and other components, and the components are connected into a ring, which has the beneficial effect of promoting self-starting mode-locking of the laser and making the laser work in a stable mode-locked state.

[0011] Further, the double-path gain fiber in the nonlinear amplifying ring mirror precisely controls the nonlinear phase shift difference accumulated by the counter-propagating pulses in the ring cavity by adjusting the difference between the pump powers of the two, which has the beneficial effect of enabling the phase difference obtained by the counter-propagating pulses in the ring to be precisely and continuously adjustable, enhancing the adjustability of the laser, easily finding the optimal solution of the working region of the laser, improving the mode-locking characteristics, improving the stability of the laser, and expanding the application range of the laser.

[0012] Further, the gain fiber can adopt active optical fibers with different doping concentrations or special optical fibers such as high nonlinear optical fibers according to the need for accumulating nonlinear phase shift difference during the pulse mode-locking process.

[0013] Further, the doping element of the gain fiber can be erbium, ytterbium, thulium, neodymium, multiple rare earth ions co-doped or other rare earth elements, which has the beneficial effect of expanding the laser to other wavebands.

[0014] In the laser, the non-reciprocal phase shift element can be arranged in the nonlinear amplification ring mirror or in the linear arm to add a fixed artificial linear phase bias to the two counter-propagating polarized lights, and the linear phase bias makes the laser transmittance curve have a basic offset.

[0015] Further, the non-reciprocal phase shift element can be any phase bias of π / 16, π / 8, π / 4, π / 2, etc. according to different linear phase shifts required by the laser self-starting.

[0016] Further, the non-reciprocal phase shift element can be a transmission structure or a reflection structure.

[0017] Further, the non-reciprocal phase shift element can be integrated into a closed structure, which has the beneficial effects of compact laser structure, reduced environmental interference, and improved laser stability.

[0018] Further, the splitting ratio of the coupler in the nonlinear amplification ring mirror can be 50:50, 40:60, 30:70, 20:80, 10:90, or other ratios, which has the beneficial effects of facilitating the change of the asymmetry of the nonlinear amplification ring mirror, more easily realizing the laser mode locking, and changing the output characteristics according to actual needs. In addition, a polarized light beam combiner can also be used in the laser to realize different splitting ratio outputs by adjusting the polarization state of the input light.

[0019] Preferably, the splitting ratio of the coupler in the nonlinear amplification ring mirror is 50:50, so that the two counter-propagating lights in the ring cavity have the same splitting ratio, which has the beneficial effects of ensuring the maximum mode locking modulation depth and obtaining high-energy and stable single pulse output.

[0020] Further, if the non-reciprocal phase shift element is placed in the linear arm, the polarization state of the light in the non-reciprocal phase shift element can be controlled to change the splitting ratio into the nonlinear amplification ring mirror, affect the asymmetry of the ring mirror, and change the modulation depth.

[0021] In the laser, the linear arm is composed of a polarization maintaining fiber and an optical element with dispersion compensation connected by one port of the coupler, which has the beneficial effects of ensuring the compactness of the oscillator, outputting pulses, and compensating dispersion to provide a high-quality seed source for subsequent higher power amplification.

[0022] Further, the dispersion compensation fiber can also be directly connected with the non-reciprocal phase shift element and arranged between the two gain fibers of the nonlinear amplification ring mirror or in the linear arm.

[0023] Further, the dispersion compensation optical element can be a chirped fiber grating, a prism pair, a Treacy grating pair, a dispersion compensation fiber, etc. according to the dispersion requirement in the laser resonant cavity, and the beneficial effect is that the dispersion distribution in the fiber laser determines the laser pulse evolution mechanism: soliton pulses in the negative dispersion region, dispersion management solitons in the dispersion management region, and self-similar pulses in the positive dispersion region, and the dispersion compensation condition corresponding to different working regions of the laser and the structure of the laser itself.

[0024] Further preferably, the output end of the laser can be directly outputted after the dispersion compensation device, and simultaneously plays the three roles of the dispersion compensation device, the end mirror and the output end.

[0025] In summary, compared with the prior art, the all-polarization-maintaining fiber femtosecond laser based on the mode-locked starting promotion structure according to the above concept and design can achieve the following beneficial effects:

[0026] (1) The laser adopts an all-polarization-maintaining fiber structure, ensures that the laser beam is in a specific polarization state during the propagation in the cavity, outputs femtosecond laser pulses in a specific polarization state, and the mode-locked state is not disturbed by the external environment, thereby improving the stability of the system and facilitating the integration of the laser.

[0027] (2) Compared with the "9-shaped" laser of Menlo Company, the nonlinear amplification ring mirror of the laser adopts a double-gain fiber, so that the nonlinear phase difference is continuously adjustable, overcomes the defects of small linear bias adjustment range, separated adjustment interval and difficulty in obtaining the optimal solution of mode locking introduced by the non-reciprocal phase shift element, improves the optimization space and performance parameters of the stable mode-locked state, and increases the adjustability of the system; at the same time, it can promote the mode-locked starting process and reduce the self-starting mode-locked threshold of the laser.

[0028] (3) Compared with the existing "8-shaped" laser, the laser changes two ring cavities into one ring cavity and one linear arm, greatly shortens the cavity length, is beneficial to improve the repetition frequency of the laser, is easier to realize reliable repetition frequency stability, and the direct output port of the ring cavity can be used to monitor the pulse evolution characteristics in the cavity in real time.

[0029] (4) The laser introduces a flexible control dispersion compensation device, which can control the net dispersion and dispersion distribution in the laser, and pre-chirp manage the output pulse, which is beneficial to subsequent higher power amplification and realization of flat wide spectrum pulse, and effectively overcomes the gain narrowing effect.

[0030] (5) The non-reciprocal phase shift element and the dispersion compensation device of the laser can be designed into a closed structure of a fiber integrated module, which is beneficial to the miniaturization and integration of the laser.

[0031] (6) According to the difference of the working wavelength of the laser, the laser can be designed by using gain optical fibers with different doping elements, so that the application scenarios of the laser are widened. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings used in the description of the embodiments of the present application are briefly introduced as follows. Obviously, the following drawings are only some embodiments described in the present application, and other drawings can also be obtained by those skilled in the art without creative labor on the premise of not departing from the spirit of the present application.

[0033] Figure 1 The full-polarization fiber femtosecond laser structure design schematic diagram based on the mode-locking start promotion structure provided for the first embodiment of the present application.

[0034] Figure 2 The full-polarization fiber femtosecond laser structure design schematic diagram based on the mode-locking start promotion structure provided for the second embodiment of the present application.

[0035] Figure 3 The full-polarization fiber femtosecond laser structure design schematic diagram based on the mode-locking start promotion structure provided for the third embodiment of the present application.

[0036] The drawings are identified as follows: 1-passive polarization maintaining optical fiber; 2-first gain optical fiber; 3-second gain optical fiber; 4-first semiconductor laser; 5-second semiconductor laser; 6-first wavelength division multiplexer; 7-second wavelength division multiplexer; 8-non-reciprocal phase shift element; 9-coupler; 10-dispersion compensation device; 11-linear arm output end; 12-ring cavity output end; 13-reflector. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present application more clear, the technical solutions of the present application will be described clearly and completely in the following combined with specific embodiments and corresponding drawings. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments, and the present application can also be implemented or applied by using different specific embodiments, and each detail in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application.

[0038] At the same time, it should be understood that the protection scope of the present application is not limited to the following specific specific embodiments; it should also be understood that the terms used in the embodiments of the present application are for describing the specific specific embodiments, not for limiting the protection scope of the present application.

[0039] Embodiment 1: First full-polarization fiber femtosecond laser based on mode-locking start promotion structure

[0040] Embodiment 1: First full-polarization fiber femtosecond laser based on mode-locking start promotion structureFigure 1 is a full polarization maintaining fiber femtosecond laser structure based on a mode-locking start promoting structure provided by the present application, using a ytterbium-doped fiber as a gain medium. A "9" shaped cavity structure is adopted, and a passive polarization maintaining single mode fiber 1 is used to connect the fiber device part. The pulses are divided into two beams of equal intensity at the coupler 9, and the two pulses propagate in opposite directions in the ring in clockwise and counterclockwise directions, respectively, pass through two gain fibers 2 and 3, and the semiconductor lasers 4 and 5 are connected with the wavelength division multiplexer 6 and 7 to provide pumping for the gain fiber to realize population inversion. By adjusting the difference between the pumping powers of the two, the two beams of light accumulate different nonlinear phase shifts in the ring-shaped structure fiber, and the nonlinear phase shift difference is continuously adjustable. Thereafter, the clockwise and counterclockwise propagating pulses enter the non-reciprocal phase shift element 8 to produce a linear phase bias of π / 2. When the two beams of light are retransmitted to the coupler, there is an intensity-dependent phase difference. Since the pulse center corresponding to the intensity is also large, the reflectivity of the coupler 9 and the phase difference are proportional within a certain range, so the intensity of the pulse center is close to the maximum value of the reflectivity, and more of it can be reflected back into the cavity to achieve continuous growth, while the two wings of the pulse will be weakened more, so the shaped pulse can achieve compression in the time domain, playing the role of pulse mode-locking. In addition, since the non-reciprocal phase shift element 8 introduces an initial phase shift, it shifts the transmission curve of the nonlinear amplification ring mirror, so that the transmission value and slope are not zero at the initial stage of mode-locking. The adjustable double-path pumping further manages the nonlinear phase shift difference, making it easy for the laser to reach the mode-locking region, reducing the mode-locking threshold, and achieving stable and reliable self-starting mode-locking. Finally, the laser is compensated for the dispersion in the laser resonant cavity by the dispersion compensation device 10 and constitutes a closed loop of the laser, and the pulses are output from the linear arm output end 11 and the ring cavity output end 12.

[0041] Example 2: A second full polarization maintaining fiber femtosecond laser based on a mode-locking start promoting structure

[0042] As shown in Figure 2 , in the second embodiment of the present application, a full polarization maintaining fiber femtosecond laser based on a mode-locking start promoting structure is provided, using a ytterbium-doped fiber as a gain medium. It is basically the same as in Example 1, and the laser is composed of a nonlinear amplification ring mirror and a linear arm provided with a reflector 13, wherein the non-reciprocal phase shift element 8 and the dispersion compensation device 10 are both arranged in the nonlinear amplification ring mirror, and the stable mode-locking of the laser is realized by the mode-locking mechanism of the nonlinear amplification ring mirror, and the non-reciprocal phase shift element 8 introduces an initial phase shift to assist mode-locking start. When the reflector 13 has a certain transmissivity, the laser outputs pulses through the reflector 13 and the ring cavity output end 12.

[0043] Example 3: A third full polarization maintaining fiber femtosecond laser based on a mode-locking start promoting structure

[0044] AsFigure 3 As shown in the third embodiment of the present application, a full polarization maintaining fiber femtosecond laser based on a mode-lock start promoting structure is provided, which uses a ytterbium-doped fiber as a gain medium. The laser consists of a nonlinear amplifying ring mirror composed of ytterbium-doped gain fibers 2, 3 and a polarization maintaining common single-mode fiber, a non-reciprocal phase shift element 8 and a dispersion compensation device 10. After the input light enters the non-reciprocal phase shift element 8 to obtain a fixed phase bias, it is split into mutually perpendicular polarization components, enters the nonlinear amplifying ring mirror to form counterclockwise and clockwise counter-propagating pulses, respectively passes through two gain fibers 2, 3, and by adjusting the difference between the pump powers of the two, the two beams accumulate different nonlinear phase shifts in the ring structure fiber, and the nonlinear phase shift difference is continuously adjustable. When the nonlinear phase shift difference causes the transmittance curve of the laser to have a non-zero value and a positive slope under low power conditions, self-starting mode locking is achieved. The laser outputs pulses through the linear arm output end 11 and the ring cavity output end 12.

[0045] The above is only the preferred embodiment of the present application, and does not limit the present application in any form. Any skilled person in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application to obtain equivalent embodiments with equivalent changes. Any simple modification, equivalent change and modification of the above embodiments made according to the technical essence of the present application should be included in the protection scope of the claims of the present application.

Claims

1. A full-polarization maintaining fiber femtosecond laser based on a mode-lock start promoting structure, characterized in that, The full-polarization fiber femtosecond laser comprises a passive polarization maintaining fiber (1), a first gain fiber (2), a second gain fiber (3), a first semiconductor laser (4), a second semiconductor laser (5), a first wavelength division multiplexer (6), a second wavelength division multiplexer (7), a non-reciprocal phase shift element (8), a coupler (9), a dispersion compensation device (10), a linear arm output end (11) and a ring cavity output end (12), wherein: The passive polarization maintaining fiber (1) is used as a component of a laser resonant cavity, for transmitting laser and keeping a specific polarization state during propagation and output. The first gain fiber (2) and the second gain fiber (3) are used as components of a nonlinear amplification ring mirror, for providing gain for laser generation and realizing continuous adjustment of nonlinear phase shift difference in the nonlinear amplification ring mirror, thereby improving the self-starting ability of the laser and realizing tuning of laser output power. The first semiconductor laser (4) and the second semiconductor laser (5) are connected with the second gain fiber (3) and the first gain fiber (2) through the second wavelength division multiplexer (7) and the first wavelength division multiplexer (6) respectively, for generating pump light to cause particle inversion in the gain medium and provide preset amplitude gain for the transmitted laser pulse. The non-reciprocal phase shift element (8) is used as a component of the laser resonant cavity, for generating linear phase bias. The coupler (9) is used for composing the nonlinear amplification ring mirror and connecting with the linear arm. The dispersion compensation device (10) is used for compensating dispersion in the laser resonant cavity and constituting a closed loop of the laser, for outputting pulses with corresponding shapes according to required dispersion conditions and realizing stable mode-locked operation of the laser. The linear arm output end (11) and the ring cavity output end (12) are used for outputting pulses and monitoring pulse evolution characteristics in the laser resonant cavity in real time.

2. The full polarization maintaining fiber femtosecond laser based on the mode-lock start facilitating structure according to claim 1, characterized in that, The splitting ratio of the coupler (9) is 50:50, so that two beams of light transmitted in opposite directions in the ring cavity have the same splitting ratio, thereby ensuring maximum mode-locked modulation depth and symmetry.

3. The mode-locking start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 1, wherein, The first gain fiber (2) and the second gain fiber (3) are selected according to the requirement of nonlinear phase difference introduced by mode locking, and are active doped optical fibers or special optical fibers with different doping concentrations.

4. The mode-locking start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 3, wherein, The doping elements of the first gain fiber (2) and the second gain fiber (3) can be erbium, ytterbium, thulium, neodymium or other rare earth elements according to different operating wavelengths.

5. The mode-lock start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 1, wherein, The first gain fiber (2) and the second gain fiber (3) can be positive dispersion optical fibers, negative dispersion optical fibers or near-zero dispersion optical fibers with corresponding lengths according to the sign and size of dispersion in the laser resonant cavity.

6. The mode-lock start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 1, wherein, The non-reciprocal phase shift element (8) is selected according to the linear phase bias introduced by mode locking, and is π / 16, π / 8, π / 4, π / 2 or other phase biases. The non-reciprocal phase shift element (8) is arranged between the first gain fiber (2) and the second gain fiber (3) in the nonlinear amplification ring mirror or in the linear arm.

7. The mode-lock start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 1, wherein, The non-reciprocal phase shift element (8) is integrated together by an adhesive or a mechanical method to form a closed structure.

8. The mode-lock start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 1, wherein, The dispersion compensation device (10) is selected from a chirped fiber grating, a prism pair, a Treacy grating pair or a dispersion compensation fiber according to the need of dispersion compensation in a laser resonant cavity. The dispersion compensation device (10) is arranged in a nonlinear amplification ring mirror or in a linear arm.

9. The mode-lock start facilitating structure based full polarization maintaining fiber femtosecond laser of claim 1, wherein, The full-polarization-maintaining fiber femtosecond laser further comprises a reflector (13) arranged at an output end of the linear arm, the reflector (13) having a certain transmittance, and the full-polarization-maintaining fiber femtosecond laser outputs pulses through the reflector (13) and the ring cavity output end (12).

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