阅读说明：本技术 一种基于锥形多模光纤可饱和吸收体的锁模光纤激光器 (Mode locking fiber laser based on tapered multimode fiber saturable absorber ) 是由 李环环 任行浩 徐时清 李�灿 蒋昊 于 2019-11-27 设计创作，主要内容包括：本发明公开了一种基于锥形多模光纤可饱和吸收体的锁模光纤激光器,采用全光纤结构,由泵浦源,波分复用器,增益光纤,非保偏隔离器,光偏振控制器,光纤耦合器,锥形多模光纤可饱和吸收体组成。其中,可饱和吸收体的特性由拉锥的渐变折射率多模光纤决定,由于其非线性多模干涉效应的存在,光强强的部分的光保持在基模模式,通过纤芯传播,而光强弱的部分的光以高阶模式的光偏离纤芯,进入包层而被衰减,导致锥形多模光纤具有光强相关的特性损耗关系,使得脉冲的两翼被损耗而峰值处能够通过,从而得到超短锁模脉冲。本发明采用锥形多模光纤可饱和吸收体,得到一种全光纤结构的锁模脉冲激光器,具有成本低、结构简单、损伤阈值高等优点,值得推广使用。(The invention discloses a mode-locking fiber laser based on a tapered multimode fiber saturable absorber, which adopts an all-fiber structure and consists of a pumping source, a wavelength division multiplexer, a gain fiber, a non-polarization-maintaining isolator, a light polarization controller, a fiber coupler and a tapered multimode fiber saturable absorber. The characteristic of the saturable absorber is determined by the tapered multimode graded index fiber, due to the existence of nonlinear multimode interference effect, light of a part with strong light intensity is kept in a basic mode and is transmitted through a fiber core, while light of a part with weak light intensity deviates from the fiber core with light of a high-order mode and enters a cladding to be attenuated, so that the tapered multimode fiber has a characteristic loss relation related to the light intensity, two wings of a pulse are lost, and the peak value can pass through, thereby obtaining the ultrashort mode-locked pulse. The mode-locked pulse laser with the all-fiber structure is obtained by adopting the tapered multimode fiber saturable absorber, has the advantages of low cost, simple structure, high damage threshold value and the like, and is worthy of popularization and application.)

1. A mode-locked fiber laser based on a tapered multimode fiber saturable absorber is characterized by comprising a pumping source (1), a first non-polarization-maintaining isolator (2), a fiber wavelength division multiplexer (3), a second non-polarization-maintaining isolator (4), a gain fiber (5), a light polarization controller (6), a tapered multimode fiber saturable absorber (7) and a fiber coupler (8); the pump laser source (1) is connected with a first port (201) of a first non-polarization-maintaining isolator (2), a second port (202) of the first non-polarization-maintaining isolator (2) is connected with a first port (301) of an optical fiber wavelength division multiplexer (3), a second port (303) of the optical fiber wavelength division multiplexer (3) is connected with a second non-polarization-maintaining isolator (4), the other end of the second non-polarization-maintaining isolator (4) is connected with a first port (501) of a thulium-doped optical fiber (5), a second port (502) of the thulium-doped optical fiber (5) is connected with a first port (601) of a light polarization controller (6), a second port (602) of the light polarization controller (6) is connected with a first port (701) of a saturable absorber (7) based on a tapered multimode optical fiber, a second port (702) of the saturable absorber (7) based on the tapered multimode optical fiber is connected with a first port (801) of an optical fiber coupler (8), and a second port (802) of the optical coupler is connected with a third port (303) of the optical fiber wavelength division multiplexer (3), the laser light is output from the three ports (803) of the fiber coupler (8).

2. The tapered multimode fiber-based saturable absorber (7) of claim 1, wherein the tapered multimode fiber-based saturable absorber (7) is formed by fusion splicing single mode fiber and graded index multimode fiber according to the structure of single mode fiber 1 (703) -graded index multimode fiber (705) -single mode fiber 2 (704), wherein the tapered multimode fiber (705) is fused to the single mode fiber 2 (704) and then tapered at the fusion splice point to form the tapered structure (706).

3. The tapered structure (706) of claim 2, wherein the tapered region of the tapered structure (706) has a smallest core diameter of less than 7 μm and a length of greater than 20 μm.

4. The single mode optical fiber 1 (703) and the single mode optical fiber 2 (704) of claim 2, wherein the single mode optical fiber 1 (703) and the single mode optical fiber 2 (704) are the same single mode optical fiber.

5. The GI multimode fiber (705) according to claim 2, wherein the GI multimode fiber is a multimode fiber having a core to cladding diameter ratio of 62.5 μm/125 μm.

6. The GI multimode fiber (705) of claim 5, wherein the GI multimode fiber (705) has a length greater than 15 cm.

Technical Field

The invention belongs to the field of laser technology and nonlinear optics, and particularly relates to a mode-locked fiber laser based on a conical multimode fiber saturable absorber.

Background

Saturable absorbers are the most critical optical devices in passively mode-locked ultrashort pulse fiber lasers. Such a device is capable of satisfying a specific loss relationship that its optical loss becomes smaller and transmittance increases as the pulse energy passing through the device increases. At present, there are many types of saturable absorbers that have been used, such as semiconductor saturable absorbers (SESAMs), carbon nanotubes, novel two-dimensional materials represented by graphene, and equivalent saturable absorbers based on the Nonlinear Polarization Rotation (NPR) effect. However, the response time of the SESAM is relatively short, only in the order of picoseconds, and it is difficult to achieve an output in the order of femtoseconds. The output power of the laser is limited by the poor stability of saturable absorber materials such as carbon nanotubes and graphene and the low damage threshold of the materials. The saturable absorber based on NPR also has the defect of poor stability, and is suitable for being used in a laboratory environment.

Various fiber optic devices based on multi-mode interference effects have been reported in recent years, including beam shapers, sensors, filters, and the like. Since the GI multimode fiber can reduce modal dispersion and has the mode propagation constants that are equidistant, the self-focusing length of the optical signal can be very short, even less than 1mm, and therefore, the GI multimode fiber has very rich nonlinear effects (such as four-wave mixing, self-phase modulation, cross-phase modulation, and the like). At present, in the aspect of nonlinear effect, Bessel beams, supercontinuum and other applications are realized on the basis of the graded-index multimode fiber, and the applications provide theoretical support for manufacturing a saturable absorber by using the graded-index multimode fiber. Moreover, the saturable absorber made of the multimode graded index fiber, such as two-dimensional materials such as graphene and the like and the semiconductor saturable absorber mirror, is simpler in structure and high in damage threshold, and is a very reliable material for making the saturable absorber because the material of the fiber mainly comprises quartz and is not easy to age compared with other saturable absorbers.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention designs the saturable absorber with lower cost, simple structure and good stability, and provides a manufacturing method of the saturable absorber based on the tapered multimode fiber and a mode-locked fiber laser obtained by using the saturable absorber. The invention aims to solve the technical problems of the existing saturable absorber, provides a saturable absorber device with high damage threshold and low loss, and realizes a passive mode-locked fiber laser by using the saturable absorber, wherein the laser has the characteristics of low threshold power, simple structure, good stability and the like.

The technical scheme adopted by the invention for solving the technical problem is as follows:

a mode-locked fiber laser based on a tapered multimode fiber saturable absorber is characterized by comprising a pump laser source (1), a first non-polarization-maintaining isolator (2), a fiber wavelength division multiplexer (3), a second non-polarization-maintaining isolator (4), a thulium-doped fiber (5), a light polarization controller (6), a tapered multimode fiber saturable absorber (7) and a fiber coupler (8); the pump laser source (1) is connected with a first port (201) of a first non-polarization-maintaining isolator (2), a second port (202) of the first non-polarization-maintaining isolator (2) is connected with a first port (301) of an optical fiber wavelength division multiplexer (3), a third port (303) of the optical fiber wavelength division multiplexer (3) is connected with a second non-polarization-maintaining isolator (4), the other end of the second non-polarization-maintaining isolator (4) is connected with a first port (501) of a thulium-doped optical fiber (5), a second port (502) of the thulium-doped optical fiber (5) is connected with a first port (601) of a light polarization controller (6), a second port (602) of the light polarization controller (6) is connected with a first port (701) of a saturable absorber (7) based on a tapered multimode optical fiber, a second port (702) of the saturable absorber (7) based on the tapered multimode optical fiber is connected with a first port (801) of an optical fiber coupler (8), and a second port (802) of the optical fiber coupler is connected with a second port (302) of the optical fiber wavelength division multiplexer (3), the laser light is output from the three ports (803) of the fiber coupler (8).

The saturable absorber (7) based on the tapered multimode fiber is formed by welding a single-mode fiber and a graded-index multimode fiber according to the structural modes of the single-mode fiber 1 (703), the graded-index multimode fiber (705) and the single-mode fiber 2 (704), wherein the tapered multimode fiber (705) and the single-mode fiber 2 (704) form a tapered structure (706) after being welded.

Preferably, the single mode fiber 1 (703) and the single mode fiber 2 (704) are the same single mode fiber.

Preferably, the multimode fiber (705) in the saturable absorber (7) based on the tapered multimode fiber adopts a multimode fiber with a core-to-cladding diameter ratio of 62.5 mu m/125 mu m.

Preferably, the smallest core diameter of the cone region of the cone-shaped structure (706) is less than 7 μm, and the length of the cone region is greater than 20 μm.

Preferably, the length of the graded-index multimode fiber (705) in the saturable absorber (7) based on the tapered multimode fiber is more than 15 cm.

The invention has the beneficial effects that:

1. the cone area diameter and the cone area length of the conical multimode optical fiber-based saturable absorber can be accurately controlled by adjusting specific parameters of an optical fiber fusion splicer, so that the characteristic of the saturable absorber is improved, and the saturable absorber with excellent performance is easy to manufacture;

2. the saturable absorber is simple to manufacture, has the advantages of low cost, simple structure, high efficiency, stable performance and the like, and is beneficial to popularization and application;

3. the design of the invention adopts an all-fiber structure, has the characteristics of simple manufacture, high damage threshold, good stability and the like, and is easy to obtain the ultrashort pulse mode-locked fiber laser with simple structure, lower cost, high damage threshold and good stability;

4. by reasonably designing the length of the multimode fiber, the diameter of the cone region and the length of the cone region, the saturable absorber device can realize high transmission in a wide spectral range, thereby realizing lower insertion loss.

Drawings

The invention will be further described with reference to the accompanying drawings and examples thereof:

FIG. 1 is a schematic diagram of the structure of a mode-locked fiber laser based on a tapered multimode fiber saturable absorber according to the present invention,

1 is a pump laser source; 2 is a first non-polarization-maintaining isolator; 3 is a fiber wavelength division multiplexer; 4 is a second non-polarization-maintaining isolator; 5 is thulium doped optical fiber; 6 is a light polarization controller; 7 is a saturable absorber based on a tapered multimode fiber; 8 is a fiber coupler; 201 is a first non-polarization maintaining isolator port; 202 is a second port of the first non-polarization-maintaining isolator; 301 is a port of the optical fiber wavelength division multiplexer; 302 is a second port of the optical fiber wavelength division multiplexer; 303 is a three-port of the optical fiber wavelength division multiplexer; 501 is a port of a second non-polarization-maintaining isolator; 502 is a second non-polarization maintaining isolator two-port; 601 is a port of the optical polarization controller; 602 is a second port of the optical polarization controller; 701 is a port of a saturable absorber based on nonlinear multimode interference effect; 702 is a saturable absorber two-port based on nonlinear multimode interference effect; 703 is a single mode optical fiber 1; 704 is single mode fiber 2; 705 graded index multimode fiber; 706 is a conical structure; 801 is a port of the optical fiber coupler; 802 is a fiber coupler two port; 803 is a fiber coupler three port.

FIG. 2 is a schematic diagram of a saturable absorber of a single mode fiber-tapered GI multimode fiber-single mode fiber structure.

Fig. 3 is a microscope cone image.

Fig. 4 is a saturable absorption curve.

FIG. 5 is a spectral plot of the mode-locked operational output of a laser in an embodiment of the present invention.

FIG. 6 is a pulse train diagram of the mode-locked operational output of a laser in an embodiment of the present invention.

FIG. 7 is a graph of an autocorrelation fit of the mode-locked operational output of a laser in an embodiment of the present invention.

Detailed Description

The main idea of the invention is as follows: in the adopted saturable absorber based on the tapered multimode fiber, the refractive index is changed due to nonlinear effects such as self-phase modulation and cross-phase modulation of the graded-index multimode fiber, so that the mode distribution of light with different intensities is different when the light is transmitted in the graded-index multimode fiber. Strong light will remain in the fundamental mode and propagate in the multimode fiber due to nonlinear effects, while weak light will excite higher order modes to propagate in the multimode fiber. Because the mode field distribution in the GI multimode fibers is different, the intensity of the nonlinear effect can be influenced, so that the mode field distribution of light entering the GI multimode fibers can be obviously influenced by one end of the tapered GI multimode fibers, the influence of the nonlinear effect is enhanced, the high-peak-power optical signals of laser from the GI multimode fibers to the single-mode fibers can be coupled to the fiber core of the single-mode fibers in a loss-free mode in a fundamental mode, and the low-peak-power optical signals can excite a high-order mode to enter the cladding of the single-mode fibers for attenuation. This property corresponds to a saturable absorber. As shown in fig. 1, a mode-locked fiber laser based on a tapered multimode fiber saturable absorber is characterized by comprising a pump laser source (1), a first non-polarization-maintaining isolator (2), a fiber wavelength division multiplexer (3), a second non-polarization-maintaining isolator (4), a thulium-doped fiber (5), a light polarization controller (6), a tapered multimode fiber saturable absorber (7) and a fiber coupler (8); the pump laser source (1) is connected with a first port (201) of a first non-polarization-maintaining isolator (2), a second port (202) of the first non-polarization-maintaining isolator (2) is connected with a first port (301) of an optical fiber wavelength division multiplexer (3), a second port (302) of the optical fiber wavelength division multiplexer (3) is connected with a second non-polarization-maintaining isolator (4), the other end of the second non-polarization-maintaining isolator (4) is connected with a first port (501) of a thulium-doped optical fiber (5), a second port (502) of the thulium-doped optical fiber (5) is connected with a first port (601) of a light polarization controller (6), a second port (602) of the light polarization controller (6) is connected with a first port (701) of a saturated absorber (7) based on a nonlinear multimode interference effect, a second port (702) of the saturated absorber (7) based on the nonlinear multimode interference effect is connected with a first port (801) of an optical fiber coupler (8), and a second port (802) of the optical coupler is connected with a third port (303) of the optical fiber wavelength division isolator (3), the laser light is output from the three ports (803) of the fiber coupler (8).