9-shaped hybrid mode-locked fiber laser

Abstract

The invention belongs to the technical field of lasers, and discloses a 9-shaped hybrid mode-locked fiber laser, which comprises a pumping source, a 2 * 2 polarization-maintaining fiber coupler, a nonlinear amplification loop mirror loop connected with the first side of the polarization-maintaining fiber coupler, and a chirp fiber Bragg grating connected with the second side of the polarization-maintaining fiber coupler, the polarization maintaining optical fiber coupler is located at the connecting position of the nonlinear amplification annular mirror loop and a linear arm of the laser resonant cavity and used for coupling laser of the nonlinear amplification annular mirror loop to the linear arm, and after being reflected by the chirp fiber bragg grating, the laser is output to the nonlinear amplification annular mirror loop through the chirp fiber bragg grating. And then the laser on the linear arm is split into two beams of light which are transmitted clockwise and anticlockwise in a loop of the nonlinear amplification annular mirror. Two passive mode locking technologies are combined, rapid and stable mode locking can be achieved, the influence of the environment on the operation state is small, the signal-to-noise ratio of output laser pulses can be increased, and ultra-short pulses with narrower pulse width can be achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of lasers, and in particular relates to a 9-shaped hybrid mode-locked optical fiber laser. Background Art

[0002] Passive mode-locking technology is an important means of generating ultrashort fiber pulses. Saturable absorbers, as the core components of passive mode-locked fiber lasers, are usually divided into real saturable absorbers and virtual saturable absorbers. Real saturable absorbers mainly include semiconductor saturable absorber mirrors, graphene, carbon nanotubes and other materials; virtual saturable absorbers mostly use technologies such as nonlinear polarization rotation and nonlinear amplifying ring mirrors.

[0003] In recent years, with the rapid development of two-dimensional materials and the continuous progress of saturable absorber preparation technology, real saturable absorbers made of new nanomaterials have been widely used in mode-locked fiber lasers. Real saturable absorbers have good self-starting mode-locking characteristics and a wide wavelength coverage range, and have become a relatively mature saturable absorption device. However, the damage threshold of real saturable absorbers is usually low, and their performance is prone to degradation or even loss of lock after long-term operation, and they need to be replaced regularly to maintain the performance of the laser. Therefore, relying solely on real saturable absorbers to construct mode-locked fiber lasers will increase the difficulty of later maintenance and make it difficult to increase pulse energy.

[0004] In contrast, virtual saturable absorbers have a higher damage threshold and flexible structural design, making them the preferred solution for mode-locked fiber lasers in achieving high-power, high-energy, and ultrashort pulse output. In particular, the 9-shaped fiber laser based on nonlinear amplifying ring mirror mode locking has attracted much attention due to its compact structure, high damage threshold, and good stability. However, the non-reciprocal phase shifter in the 9-shaped fiber laser usually provides a fixed phase shift difference, resulting in the self-starting problem of the laser still being difficult to solve. Therefore, further improving the performance of the 9-shaped fiber laser is of great significance to the development of ultrafast fiber lasers.

[0005] At present, most ultrafast fiber lasers use a single real saturable absorber or a single virtual saturable absorber to achieve ultrafast laser output. The mode-locked state of the real saturable absorber is less affected by the environment and has good self-starting performance, but its damage threshold is usually low, and its performance is prone to degradation or even loss of lock after long-term operation. The virtual saturable absorber has a higher damage threshold, flexible structural design, and high integration, but its self-starting performance and pulse stability are poor. In addition, since the fiber laser adopts a fully polarization-maintaining structure as a whole, the adjustable degree of freedom is not high. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of a single mode-locking technology and provide a figure-9 hybrid mode-locked fiber laser.

[0007] The present invention provides a figure-9 hybrid mode-locked fiber laser, comprising a pump source and a 2×2 polarization-maintaining fiber coupler, a nonlinear amplifying ring mirror loop connected to a first side of the 2×2 polarization-maintaining fiber coupler, and a chirped fiber Bragg grating connected to a second side of the 2×2 polarization-maintaining fiber coupler;

[0008] The 2×2 polarization-maintaining fiber coupler is located at the connection position of the nonlinear amplifying ring mirror loop and the linear arm of the laser resonant cavity, and is used to couple the laser of the nonlinear amplifying ring mirror loop to the linear arm, and then split the laser on the linear arm into two beams of light that are transmitted clockwise and counterclockwise in the nonlinear amplifying ring mirror loop after being reflected by the chirped fiber Bragg grating.

[0009] Furthermore, the nonlinear amplifying loop mirror loop is a loop formed by sequentially connecting the 2×2 polarization-maintaining fiber coupler, the doped fiber, the wavelength division multiplexer, the real saturable absorber mode-locking device and the non-reciprocal phase shifter; wherein,

[0010] The first end of the wavelength division multiplexer is connected to the pump source to couple the pump light generated by the pump source into the laser resonant cavity, the second end of the wavelength division multiplexer is connected to the doped optical fiber, and the third end of the wavelength division multiplexer is connected to the real saturable absorber mode-locked device;

[0011] The doped optical fiber is connected to the first end of the 2×2 polarization-maintaining optical fiber coupler, and the doped optical fiber is excited by the pump light to generate laser signal light;

[0012] The 2×2 polarization-maintaining fiber coupler is used to couple the clockwise propagating light and the counterclockwise propagating light in the nonlinear amplifying ring mirror loop;

[0013] The non-reciprocal phase shifter is used to provide a fixed linear phase difference between the light propagating clockwise and the light propagating counterclockwise in the nonlinear amplifying loop mirror loop;

[0014] The real saturable absorber mode-locking device is used to maintain the self-starting and stable mode-locking of the optical fiber laser.

[0015] Furthermore, the doped optical fiber is a polarization-maintaining doped optical fiber, the wavelength division multiplexer is a polarization-maintaining wavelength division multiplexer, and the non-reciprocal phase shifter is a polarization-maintaining non-reciprocal phase shifter.

[0016] Furthermore, the doped optical fiber is a gain optical fiber, and the gain optical fiber is arranged close to the 2×2 polarization-maintaining optical fiber coupler.

[0017] Furthermore, the real saturable absorber device includes two jumper heads and a film located between the two jumper heads.

[0018] Furthermore, the film is a semiconductor saturable absorber mirror, graphene, or carbon nanotube.

[0019] Furthermore, the 2×2 polarization-maintaining fiber coupler is a tunable 2×2 polarization-maintaining fiber coupler.

[0020] Furthermore, the 2×2 polarization-maintaining fiber coupler divides the light into two paths for propagation in the nonlinear amplifying ring mirror loop, the two paths of light being light propagating clockwise and light propagating counterclockwise respectively. The two paths of light interfere with each other when returning to the 2×2 polarization-maintaining fiber coupler, wherein a portion of the light continues to be transmitted along the linear arm, is reflected back to the 2×2 polarization-maintaining fiber coupler through the chirped fiber Bragg grating, and is re-divided into two beams of light with opposite propagation directions and enter the nonlinear amplifying ring mirror loop; the other portion of the light is transmitted by the 2×2 polarization-maintaining fiber coupler as laser output.

[0021] Furthermore, the linear arm of the laser resonant cavity is formed by connecting the third end of the 2×2 polarization-maintaining fiber coupler with the chirped fiber Bragg grating; the chirped fiber Bragg grating is used to reflect the laser beam and perform dispersion compensation to balance the dispersion in the laser resonant cavity.

[0022] Furthermore, the dispersion parameters of the chirped fiber Bragg grating can be selected to compensate for the overall dispersion in the laser resonant cavity.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The fiber laser provided by the present invention combines two passive mode locking technologies, which can not only achieve fast and stable mode locking, and the operating state is less affected by the environment, but also improve the signal-to-noise ratio of the output laser pulse and achieve ultra-short pulses with narrower pulse width. It has the characteristics of compact optical path structure and high degree of integration.

[0025] (2) The present invention utilizes a tunable 2×2 polarization-maintaining fiber coupler to achieve real-time adjustment of the transmittance of the fiber laser and the modulation depth of the virtual saturable absorber, so that it can better serve as a nonlinear amplifying ring mirror loop under different parameter configurations, enrich the adjustment freedom of the fiber laser, and greatly improve the application compatibility of the fiber laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1A schematic structural diagram of a figure-9 hybrid mode-locked fiber laser provided in an embodiment of the present invention.

[0028] The meanings of the symbols in the accompanying drawings are:

[0029] 1- Pump source, 2- Wavelength division multiplexer, 3- Real saturable absorber mode-locked device, 4- Non-reciprocal phase shifter, 5- Doped fiber, 6- 2×2 polarization-maintaining fiber coupler, 7- Chirped fiber Bragg grating. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] For a 9-shaped fiber laser, the optical signal is input from a certain input port at one end of the coupler, and the coupler performs power distribution and enters the loop to be divided into two light fields that are transmitted in opposite directions. The total phase shift of the two light beams transmitted in opposite directions is φ 总 =φ NL +φ L , where φ L is the non-reciprocal linear phase shift, φ NL is the amount of nonlinear phase shift accumulated under the induction of high-power pulses. Due to the asymmetric amplification experienced by the counter-propagating pulses in the nonlinear amplifying ring mirror loop and the bias phase shift provided by the nonreciprocal phase shifter, the nonlinear phase shift accumulated by the counter-propagating pulses in the nonlinear amplifying ring mirror is different. Compared with the two wings of the pulse, the energy in the center of the pulse is higher and the nonlinear phase shift difference is greater. If interference occurs at the coupler, part of the energy of the incident light field is reflected and part is transmitted, the transmittance of the center of the pulse is higher and the transmittance of the two wings is lower, then the nonlinear amplifying ring mirror is equivalent to a saturable absorber, and the laser can achieve stable mode-locked operation.

[0032] According to actual needs, the present invention combines different mode-locking methods to design a 9-shaped mode-locked fiber laser with real saturable absorber assisted nonlinear amplifying ring mirror mode-locking. The present invention introduces a hybrid mode-locking method combining real saturable absorber and nonlinear amplifying ring mirror mode-locking technology, and introduces a 2×2 polarization-maintaining fiber coupler with tunable splitting ratio, so as to facilitate real-time adjustment of the transmittance of the nonlinear amplifying ring mirror loop and the modulation depth of the virtual saturable absorber. The problem of poor self-starting performance of the 9-shaped fiber laser is solved, the self-starting ability of the mode-locking is enhanced, and fast mode-locking is achieved. The signal-to-noise ratio of the output laser ultrashort pulse is further improved, and its output width is reduced, thereby improving the overall stability of the mode-locked fiber laser. The present invention combines two passive mode-locking technologies, which can give full play to their respective advantages and form a better hybrid mode-locking method.

[0033] See also Figure 1 , a figure-9 hybrid mode-locked fiber laser provided in an embodiment of the present invention comprises a pump source 1, a 2×2 polarization-maintaining fiber coupler 6, a nonlinear amplifying ring mirror loop connected to a first side of the polarization-maintaining fiber coupler 6, and a chirped fiber Bragg grating 7 connected to a second side of the polarization-maintaining fiber coupler 6;

[0034] The 2×2 polarization-maintaining fiber coupler 6 is located at the connection position between the nonlinear amplifying ring mirror loop and the linear arm of the laser resonant cavity, and is used to couple the laser of the nonlinear amplifying ring mirror loop to the linear arm, and after being reflected by the chirped fiber Bragg grating 7, the laser on the linear arm is split into two beams of light that are transmitted clockwise and counterclockwise in the nonlinear amplifying ring mirror loop.

[0035] The nonlinear amplifying ring mirror loop is a loop formed by sequentially connecting a 2×2 polarization-maintaining fiber coupler 6, a doped fiber 5, a wavelength division multiplexer 2, a real saturable absorber mode-locked device 3 and a non-reciprocal phase shifter 4.

[0036] Among them, the first end of the wavelength division multiplexer 2 is connected to the pump source 1, and the pump light generated by the pump source 1 is coupled into the laser resonant cavity. The second end of the wavelength division multiplexer 2 is connected to the doped optical fiber 5. The third end of the wavelength division multiplexer 2 is connected to the real saturable absorber mode-locked device 3. The wavelength division multiplexer 2 is used for combining the pump light and the signal light.

[0037] The doped optical fiber 5 is connected to the first end of the 2×2 polarization-maintaining optical fiber coupler 6 , and the doped optical fiber 5 is excited by the pump light to generate laser signal light.

[0038] The 2×2 polarization-maintaining fiber coupler 6 is used to couple the clockwise propagating light and the counterclockwise propagating light in the nonlinear amplifying ring mirror loop.

[0039] The non-reciprocal phase shifter 4 is used to provide a fixed linear phase difference between the clockwise light and the counterclockwise light in the nonlinear amplifying ring mirror loop. In this embodiment, the fixed linear phase difference provided is π / 2; in other embodiments, it can also be selected according to specific needs.

[0040] The real saturable absorber mode-locking device 3 is used to maintain the self-starting and stable mode-locking of the optical fiber laser.

[0041] The linear arm of the laser resonant cavity is formed by connecting the third end of the 2×2 polarization-maintaining fiber coupler 6 with the chirped fiber Bragg grating 7; the chirped fiber Bragg grating 7 is used as a cavity mirror to reflect the laser beam and perform dispersion compensation to balance the dispersion in the laser resonant cavity.

[0042] In this embodiment, the nonlinear amplifying ring mirror loop adopts a full polarization-maintaining fiber device: the doped fiber 5 is a polarization-maintaining doped fiber, the wavelength division multiplexer 2 is a polarization-maintaining wavelength division multiplexer, and the non-reciprocal phase shifter 4 is a polarization-maintaining non-reciprocal phase shifter. The full polarization-maintaining fiber device is less affected by the environment, which further ensures the environmental adaptability and reliability of the laser.

[0043] In this embodiment, the 2×2 polarization-maintaining fiber coupler uses a tunable 2×2 polarization-maintaining fiber coupler, which can realize real-time adjustment of the transmittance of the fiber laser and the modulation depth of the virtual saturable absorber, so that it can better serve as a nonlinear amplifying ring mirror loop under different parameter configurations, enrich the adjustment freedom of the fiber laser, and greatly improve the application compatibility of the fiber laser.

[0044] In this embodiment, the real saturable absorber mode-locking device 3 includes two jumper heads and a film located between the two jumper heads, wherein the film includes but is not limited to materials having nonlinear response to light such as semiconductor saturable absorber mirrors, graphene, or carbon nanotubes.

[0045] The present invention places a real saturable absorber mode-locked device 3 in a nonlinear amplifying ring mirror loop, the formation of the mode-locked pulse is dominated by the nonlinear amplifying ring mirror, and the real saturable absorber mode-locked device 3 maintains the self-starting and stable mode-locking of the fiber laser. The 2×2 polarization-maintaining fiber coupler 6 divides the light into two paths to propagate in the nonlinear amplifying ring mirror loop, the two paths of light are clockwise propagating light and counterclockwise propagating light, and the two paths of light return to the 2×2 polarization-maintaining fiber coupler 6 to interfere, wherein a part of the light continues to be transmitted along the linear arm, and is reflected back to the 2×2 polarization-maintaining fiber coupler 6 through the chirped fiber Bragg grating 7 and is re-divided into two beams of light with opposite propagation directions to enter the nonlinear amplifying ring mirror loop; the other part of the light is transmitted by the 2×2 polarization-maintaining fiber coupler 6 as the output of the laser. The 9-shaped fiber laser provided by the present invention combines the advantages of nonlinear amplifying ring mode-locking and real saturable absorber mode-locking, which can not only improve the mode-locking stability of the system, but also have the self-starting capability.

[0046] In this embodiment, the doped optical fiber 5 can reasonably select gain optical fibers with different doping elements according to the needs. In addition, in the nonlinear amplifying ring mirror loop, the doped optical fiber 5 is arranged close to the 2×2 polarization-maintaining optical fiber coupler 6; this is to make the nonlinear phase shifts accumulated by the two paths of light in the nonlinear amplifying ring mirror loop different, which is more conducive to mode locking.

[0047] In this embodiment, the chirped fiber Bragg grating 7 can select dispersion parameters according to requirements to compensate for the overall dispersion in the laser resonant cavity.

[0048] After the two beams of light transmitted clockwise and counterclockwise pass through the polarization-maintaining non-reciprocal phase shifter 4, a non-reciprocal linear phase shift difference will be generated between the two beams of light. Further, the two beams of light transmitted clockwise and counterclockwise undergo a saturable absorption process after passing through the real saturable absorber mode-locking device 3 and continue to propagate clockwise and counterclockwise along the nonlinear amplifying ring mirror ring, which will help the nonlinear accumulation of the nonlinear amplifying ring mirror. Subsequently, the two beams of light transmitted clockwise and counterclockwise return to the tunable 2×2 polarization-maintaining fiber coupler 6 and interfere. A part of the light continues to be transmitted along the linear arm, and the chirped fiber Bragg grating 7 will produce a dispersion opposite to that of the front fiber. After dispersion compensation, the signal pulse is compressed and reflected back to the tunable 2×2 polarization-maintaining fiber coupler 6, which is re-divided into two beams of light with opposite propagation directions and enters the nonlinear amplifying ring mirror loop to continue the above process; the other part of the light is transmitted through the tunable 2×2 polarization-maintaining fiber coupler 6 as the output of the laser.

[0049] The present invention combines different mode-locking methods according to specific needs, which can give full play to their respective advantages and form a better hybrid mode-locking method. In summary, the present invention introduces a hybrid mode-locking method that combines a real saturable absorber and a nonlinear amplifying ring mirror mode-locking technology to further enhance the self-starting ability of the mode-locking, improve the signal-to-noise ratio of the ultrashort pulse, and reduce its output width, thereby improving the overall stability of the mode-locked fiber laser. At the same time, a tunable 2×2 polarization-maintaining fiber coupler is introduced to further enrich the degree of freedom of the fiber laser, and the transmittance and modulation depth of the nonlinear amplifying ring mirror loop can be adjusted in real time, so that it can better serve as a nonlinear amplifying ring mirror loop under different parameter configurations, which can greatly improve the application compatibility of the fiber laser.

[0050] A 9-shaped hybrid mode-locked fiber laser disclosed in an embodiment of the present invention is introduced in detail above. Specific examples are used in this article to illustrate the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A 9-shaped hybrid mode-locked fiber laser, characterized in that: It includes a pump source and a 2×2 polarization-maintaining fiber coupler, a nonlinear amplifying ring mirror loop connected to a first side of the 2×2 polarization-maintaining fiber coupler, and a chirped fiber Bragg grating connected to a second side of the 2×2 polarization-maintaining fiber coupler; The 2×2 polarization-maintaining fiber coupler is located at the connection position of the nonlinear amplifying ring mirror loop and the linear arm of the laser resonant cavity, and is used to couple the laser of the nonlinear amplifying ring mirror loop to the linear arm, and then split the laser on the linear arm into two beams of light that are transmitted clockwise and counterclockwise in the nonlinear amplifying ring mirror loop after being reflected by the chirped fiber Bragg grating.

2. The figure-9 hybrid mode-locked fiber laser according to claim 1, characterized in that: The nonlinear amplifying loop mirror loop is a loop formed by sequentially connecting the 2×2 polarization-maintaining fiber coupler, the doped fiber, the wavelength division multiplexer, the real saturable absorber mode-locking device and the non-reciprocal phase shifter; wherein, The first end of the wavelength division multiplexer is connected to the pump source to couple the pump light generated by the pump source into the laser resonant cavity, the second end of the wavelength division multiplexer is connected to the doped optical fiber, and the third end of the wavelength division multiplexer is connected to the real saturable absorber mode-locked device; The doped optical fiber is connected to the first end of the 2×2 polarization-maintaining optical fiber coupler, and the doped optical fiber is excited by the pump light to generate laser signal light; The 2×2 polarization-maintaining fiber coupler is used to couple the clockwise propagating light and the counterclockwise propagating light in the nonlinear amplifying ring mirror loop; The non-reciprocal phase shifter is used to provide a fixed linear phase difference between the light propagating clockwise and the light propagating counterclockwise in the nonlinear amplifying loop mirror loop; The real saturable absorber mode-locking device is used to maintain the self-starting and stable mode-locking of the optical fiber laser.

3. The 9-shaped hybrid mode-locked fiber laser according to claim 2, characterized in that: The doped optical fiber is a polarization-maintaining doped optical fiber, the wavelength division multiplexer is a polarization-maintaining wavelength division multiplexer, and the non-reciprocal phase shifter is a polarization-maintaining non-reciprocal phase shifter.

4. The 9-shaped hybrid mode-locked fiber laser according to claim 3, characterized in that: The doped optical fiber is a gain optical fiber, and the gain optical fiber is arranged close to the 2×2 polarization-maintaining optical fiber coupler.

5. The 9-shaped hybrid mode-locked fiber laser according to claim 2, characterized in that: The real saturable absorber device includes two jumper heads and a thin film located between the two jumper heads.

6. The figure-9 hybrid mode-locked fiber laser according to claim 5, characterized in that: The film is a semiconductor saturable absorber mirror, graphene, or carbon nanotube.

7. The figure-9 hybrid mode-locked fiber laser according to claim 1, characterized in that: The 2×2 polarization-maintaining fiber coupler is a tunable 2×2 polarization-maintaining fiber coupler.

8. The figure-9 hybrid mode-locked fiber laser according to claim 1, characterized in that: The 2×2 polarization-maintaining fiber coupler divides the light into two paths for propagation in the nonlinear amplifying ring mirror loop, the two paths of light being the light propagating clockwise and the light propagating counterclockwise respectively. The two paths of light interfere with each other when returning to the 2×2 polarization-maintaining fiber coupler, wherein a part of the light continues to be transmitted along the linear arm, is reflected back to the 2×2 polarization-maintaining fiber coupler through the chirped fiber Bragg grating, and is re-divided into two beams of light with opposite propagation directions and enters the nonlinear amplifying ring mirror loop; the other part of the light is transmitted by the 2×2 polarization-maintaining fiber coupler as the output of the laser.

9. The 9-shaped hybrid mode-locked fiber laser according to claim 1, characterized in that: The linear arm of the laser resonant cavity is formed by connecting the third end of the 2×2 polarization-maintaining fiber coupler with the chirped fiber Bragg grating; the chirped fiber Bragg grating is used to reflect the laser beam and perform dispersion compensation to balance the dispersion in the laser resonant cavity.

10. The figure-9 hybrid mode-locked fiber laser according to claim 1, characterized in that: The dispersion parameters of the chirped fiber Bragg grating can be selected to compensate for the overall dispersion in the laser resonant cavity.

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