阅读说明：本技术 一种轨道角动量模式直接谐振的单纵模布里渊光纤激光器 (Single longitudinal mode Brillouin fiber laser with orbital angular momentum mode direct resonance ) 是由 甘久林 衡小波 杨中民 张智深 林巍 于 2019-07-30 设计创作，主要内容包括：本发明公开了一种轨道角动量模式直接谐振的单纵模布里渊光纤激光器,所述激光器包括基模泵浦源、光放大器、第一偏振控制器、轨道角动量模式产生器、第二偏振控制器、第一光纤环形器、第一光纤耦合器、第三偏振控制器、选频器和涡旋光纤。本发明基于涡旋光纤的模式分离和优化设计,采用涡旋光纤器件可确保特定阶轨道角动量模式在谐振腔内和输出端的产生和稳定传输。基于同阶模式泵浦机制和谐振腔滤模效应,使得腔内同阶轨道角动量模式布里渊信号光直接谐振。通过选频器和控制腔长,使得腔内只有一个纵模起振,保证单纵模激光输出。本发明基于环形腔内单一模式直接谐振机制,输出的单纵模轨道角动量模式激光具有模式纯度高和光束质量好等优点。(The invention discloses a single longitudinal mode Brillouin optical fiber laser with direct resonance in an orbital angular momentum mode. The invention is based on mode separation and optimization design of vortex optical fiber, and can ensure generation and stable transmission of a specific order orbital angular momentum mode in a resonant cavity and at an output end by adopting a vortex optical fiber device. Based on a same-order mode pumping mechanism and a resonant cavity filtering effect, the Brillouin signal light in the same-order orbital angular momentum mode in the cavity directly resonates. And only one longitudinal mode in the cavity is subjected to oscillation starting by the frequency selector and the length of the cavity, so that the single longitudinal mode laser output is ensured. Based on a single-mode direct resonance mechanism in the annular cavity, the output single-longitudinal-mode orbital angular momentum mode laser has the advantages of high mode purity, good beam quality and the like.)

1. A single longitudinal mode Brillouin optical fiber laser with direct resonance of an orbital angular momentum mode is characterized by comprising a fundamental mode pumping source, an optical amplifier, a first polarization controller, an orbital angular momentum mode generator, a second polarization controller, a first optical fiber circulator, a first optical fiber coupler, a third polarization controller, a frequency selector and a vortex optical fiber;

the orbital angular momentum mode generator has a first port and a second port;

The first fiber optic circulator has a first port, a second port, and a third port;

The first fiber coupler has a first port, a second port, and a third port;

the fundamental mode pump source is connected to an optical amplifier through a single mode fiber, the optical amplifier is connected to a first port of an orbital angular momentum mode generator through a single mode fiber, the first polarization controller is applied to the single mode fiber of the first port of the orbital angular momentum mode generator, a second port of the orbital angular momentum mode generator is connected to a first port of a first fiber circulator through a vortex fiber, the second polarization controller is applied to the vortex fiber of the first port of the first fiber circulator, a second port of the first fiber circulator is connected to the first port of a first fiber coupler through a vortex fiber, a second port of the first fiber coupler is connected to one end of a frequency selector through a vortex fiber, the third polarization controller is applied to the vortex fiber of the second port of the first fiber coupler, and the other end of the frequency selector is connected to a third port of the first fiber circulator through a vortex fiber, a third port of the first optical fiber coupler outputs single longitudinal mode orbital angular momentum mode laser;

The vortex optical fiber is an optical fiber, wherein the refractive indexes of a fiber core and a cladding meet high refractive index contrast so that the effective refractive index difference of each vector mode supported in the vortex optical fiber is larger than 1 x 10 < -4 > and further the stable transmission of an orbital angular momentum mode is supported;

The orbital angular momentum mode generator, the optical fiber circulator, the frequency selector and the optical fiber coupler are all vortex optical fiber devices and support the generation and stable transmission of the orbital angular momentum mode in the resonant cavity and at the output end;

The frequency selector is an ultra-narrow band longitudinal mode filter, so that only one longitudinal mode is arranged in the resonant cavity for oscillation;

The first optical fiber circulator, the first optical fiber coupler and the frequency selector are connected through vortex optical fibers to form an annular resonant cavity, and Brillouin signal light in the same-order orbital angular momentum mode in the cavity directly resonates based on a same-order mode pumping mechanism and a resonant cavity filter mode effect, so that high-purity single longitudinal mode orbital angular momentum mode laser output is obtained.

2. The single longitudinal mode brillouin fiber laser of orbital angular momentum mode direct resonance according to claim 1, characterized in that, the orbital angular momentum mode generator is a fusion type fiber mode selective coupler, a long period fiber grating or a chiral fiber grating, the mode conversion efficiency is greater than 70%, the mode purity is greater than 80%, the first port adopts a single mode fiber, and the second port adopts a vortex fiber.

3. The single longitudinal mode brillouin fiber laser of orbital angular momentum mode direct resonance according to claim 1, wherein the first port, the second port and the third port of the first fiber circulator all adopt vortex fibers.

4. the single longitudinal mode Brillouin fiber laser of claim 1, wherein the first fiber coupler is a 1 x 2 coupler made by fusion tapering of a vortex fiber and a vortex fiber, and the vortex fiber is used for the first port, the second port and the third port.

5. the single longitudinal mode brillouin fiber laser of orbital angular momentum mode direct resonance according to claim 1, wherein the frequency selector is one or more combination of pi phase shift grating, composite cavity, fabry-perot (F-P cavity), Sagnac (Sagnac) ring, mach-zehnder filter or saturable absorber.

6. The single longitudinal mode brillouin fiber laser of claim 1, wherein the cavity length of the resonant cavity is greater than 6 m.

7. The single longitudinal mode Brillouin fiber laser of claim 1, wherein the vortex fiber is a step index ring core fiber, a gradient index fiber or an inverse parabolic index fiber supporting stable transmission of the orbital angular momentum mode.

Technical Field

The invention relates to the technical field of laser, in particular to a single longitudinal mode Brillouin fiber laser with direct resonance in an orbital angular momentum mode.

Background

exp(ilφ) lh l φ hthe vortex beam is a beam with a spiral phase term exp (il phi) distributed in a field, each photon in the beam carries the orbital angular momentum of lh (l is the topological charge number, phi is the azimuth angle, and h is the Planckian constant), and the vortex beam has the characteristics of annular optical field distribution and spiral phase singularity. These unique properties make them widely used in the fields of material processing, optical tweezers, high resolution imaging, fiber sensing, and fiber optic communications. However, these application prospects put higher demands on the aspects of monochromaticity, coherence, mode purity, stability and the like of the vortex beam. The single-frequency fiber laser can meet the requirements due to the advantages of narrow line width, high monochromaticity, coherence and the like. Meanwhile, single-frequency fiber lasers have become important laser sources in the fields of dense wavelength division multiplexing optical communication systems, coherent optical communication systems, distributed fiber sensing systems, coherent laser radars and the like. The single-frequency vortex laser is formed by combining the single-frequency laser and the vortex laser, and the application field is wider. Therefore, the research on the vortex optical single-frequency fiber laser is of great significance.

Many researches on single-frequency fiber lasers are available, and patent CN109149330A discloses a 2 μm waveband low-noise narrow linewidth single-frequency fiber laser, which adopts an ultra-short linear cavity architecture to realize single-frequency laser output with excellent noise performance and extremely narrow linewidth. Patent CN106410599A discloses a brillouin single longitudinal mode shift frequency fiber laser, which utilizes the mode selection characteristic of a composite cavity to realize single longitudinal mode laser output. In fact, when light waves are transmitted in an optical fiber, due to the limitation of the fiber core boundary, solving the helmholtz equation of light wave transmission results in a discontinuous electromagnetic field solution, which is called a mode and includes a fundamental transverse mode and a high-order transverse mode. The mode of stable transmission in the conventional single mode fiber is the fundamental transverse mode. The vortex light in the optical fiber is formed by overlapping odd mode and even mode of high-order vector transverse mode (HE or EH) with pi/2 phase difference, and is also called orbital angular momentum mode. Obviously, the single-frequency fiber laser disclosed in the above patent outputs fundamental transverse mode single-frequency laser, which cannot directly generate vortex rotation single-frequency laser.

The addition of an orbital angular momentum mode generator or converter at the output end of a conventional single-frequency fiber laser is an easily conceivable indirect generation scheme, that is, a fundamental transverse mode single-frequency laser is converted into an orbital angular momentum mode single-frequency laser. Although the method can also generate single-frequency laser in orbital angular momentum mode, the laser performance is heavily dependent on the performance of the orbital angular momentum mode generator or converter, the power of the output laser is reduced, the mode purity is low, and the beam quality is poor. In addition, most of the fiber type orbital angular momentum mode generators or converters are made based on single mode fibers and conventional few mode fibers or multimode fibers. The traditional single-mode fiber only supports the transmission of a fundamental transverse mode, and the traditional few-mode fiber or the traditional multi-mode fiber can not meet the requirement of stable transmission of an orbital angular momentum mode in the fiber because a high-order vector transverse mode close to a propagation constant is degenerated into a linear polarization mode due to the approximation of weak waveguide. Therefore, the single-frequency laser transmission of the orbital angular momentum mode converted from the output end of the conventional single-frequency fiber laser is unstable. Therefore, it is very important to design a single-frequency fiber laser to obtain a stable and high-mode-purity single-frequency laser in orbital angular momentum mode.

disclosure of Invention

In view of the problems in the prior art, an object of the present invention is to provide a single longitudinal mode brillouin fiber laser in which an orbital angular momentum mode directly resonates. The laser optical fiber laser has the advantages of simple structure, low cost, easy integration of an optical fiber system, high purity of an output laser orbital angular momentum mode and good stability.

The purpose of the invention can be realized by the following technical scheme:

A single longitudinal mode Brillouin optical fiber laser with direct resonance of an orbital angular momentum mode comprises a fundamental mode pump source, an optical amplifier, a first polarization controller, an orbital angular momentum mode generator, a second polarization controller, a first optical fiber circulator, a first optical fiber coupler, a third polarization controller, a frequency selector and a vortex optical fiber;

The orbital angular momentum mode generator has a first port and a second port;

the first fiber optic circulator has a first port, a second port, and a third port;

The first fiber coupler has a first port, a second port, and a third port;

The fundamental mode pump source is connected to an optical amplifier through a single mode fiber, the optical amplifier is connected to a first port of an orbital angular momentum mode generator through a single mode fiber, the first polarization controller is applied to the single mode fiber of the first port of the orbital angular momentum mode generator, a second port of the orbital angular momentum mode generator is connected to a first port of a first fiber circulator through a vortex fiber, the second polarization controller is applied to the vortex fiber of the first port of the first fiber circulator, a second port of the first fiber circulator is connected to the first port of a first fiber coupler through a vortex fiber, a second port of the first fiber coupler is connected to one end of a frequency selector through a vortex fiber, the third polarization controller is applied to the vortex fiber of the second port of the first fiber coupler, and the other end of the frequency selector is connected to a third port of the first fiber circulator through a vortex fiber, the first optical fiber circulator, the first optical fiber coupler and the frequency selector are connected through vortex optical fibers to form an annular cavity, and a third port of the first optical fiber coupler outputs single longitudinal mode orbital angular momentum mode laser.

The single longitudinal mode Brillouin fiber laser with the direct resonance of the orbital angular momentum mode is of a ring cavity structure. After the power of the fundamental mode pump light output by the fundamental mode pump source is amplified by the optical amplifier, the fundamental mode pump light is converted into orbital angular momentum mode pump light with specific topological charge number through the first polarization controller and the orbital angular momentum mode generator. And the orbit angular momentum mode pumping light enters an annular cavity formed by connecting a first optical fiber circulator, a first optical fiber coupler and a frequency selector through vortex optical fibers after passing through a second polarization controller, a stimulated Brillouin scattering effect is generated, and reversely transmitted orbit angular momentum mode Brillouin signal light is generated in the vortex optical fibers. The Brillouin signal light in the orbital angular momentum mode circulates in the ring cavity through the optical fiber circulator, the frequency selector, the third polarization controller and the first optical fiber coupler. The annular cavity is formed by connecting vortex optical fibers and devices based on the vortex optical fibers. Compared with the traditional few-mode fiber or multimode fiber, the vortex fiber is designed by increasing the refractive index contrast of the fiber core and the cladding so as to break the degeneracy of vector modes with similar propagation constants, namely the effective refractive index difference of each vector mode supported by the fiber is larger than 1 multiplied by 10 < -4 >, and further the stable transmission of the orbital angular momentum mode in the resonant cavity is realized. When the pump light power exceeds the Brillouin threshold value in the annular cavity, the orbital angular momentum mode Brillouin signal light stably exists in the annular cavity and is directly amplified in resonance. By controlling the length of the annular cavity and utilizing the longitudinal mode selection characteristic of the frequency selector, only one longitudinal mode in the annular cavity starts oscillation and forms laser emission. And the third port of the first optical fiber coupler outputs single longitudinal mode orbital angular momentum mode laser.

Preferably, the orbital angular momentum mode generator is an optical fiber type device, specifically, a fusion type optical fiber mode selection coupler, a long-period optical fiber grating or a chiral optical fiber grating can be selected, the mode conversion efficiency is greater than 70%, the mode purity is greater than 80%, the first port adopts a single mode fiber, and the second port adopts a vortex fiber.

Preferably, the first port, the second port and the third port of the first fiber circulator all adopt vortex fibers.

Preferably, the first optical fiber coupler group is a 1 × 2 coupler made of a vortex optical fiber and a vortex optical fiber fused biconical taper, and the first port, the second port and the third port are all made of vortex optical fibers.

Preferably, the frequency selector is one or more of a pi-phase shift grating, a composite cavity, a fabry-perot (F-P cavity), a Sagnac (Sagnac) ring, a mach-zehnder filter, or a saturable absorber.

preferably, the cavity length of the resonant cavity is greater than 6 m.

Preferably, the vortex optical fiber is a step-index ring-core optical fiber, a gradient-index optical fiber or an inverse parabolic-index optical fiber supporting stable transmission of an orbital angular momentum mode.

Compared with the prior art, the invention has the following beneficial effects:

1. The invention uses the optical fiber type orbit angular momentum mode generator and the coupler as a pumping mode conversion device and a laser output coupling device, and has the advantages of low loss and high efficiency;

2. According to the invention, vortex optical fibers supporting stable transmission of the orbital angular momentum mode are connected to form an annular cavity, and direct resonance amplification in the orbital angular momentum mode cavity is realized based on the Brillouin nonlinear effect in the vortex optical fibers, so that the obtained laser mode has high purity and good beam quality;

3. The invention adopts an all-fiber structure, and has the advantages of simple structure, low cost, easy integration of an optical fiber system and the like.

drawings

Fig. 1 is a schematic diagram of a single longitudinal mode brillouin fiber laser in which orbital angular momentum modes directly resonate according to the present invention.

fig. 2 is a schematic diagram of a single longitudinal mode brillouin fiber laser frequency-selected by the composite cavity technique in example 1.

Fig. 3 is a schematic diagram of a single longitudinal mode brillouin fiber laser frequency-selected using a Sagnac (Sagnac) loop and saturable absorber combination technique in example 2.

in the figure, a 1-fundamental mode pump source, a 2-optical amplifier, a 3-first polarization controller, a 4-orbital angular momentum mode generator, a 401-orbital angular momentum mode generator first port, a 402-orbital angular momentum mode generator second port, a 5-second polarization controller, a 6-first fiber circulator, a 601-first fiber circulator first port, a 602-first fiber circulator second port, a 603-first fiber circulator third port, a 7-first fiber coupler, a 701-first fiber coupler first port, a 702-first fiber coupler second port, a 703-first fiber coupler third port, an 8-third polarization controller, a 9-frequency selector, a 10-vortex fiber, a 11-second fiber coupler, a 703-first fiber coupler second port, a 9-frequency selector, a 10-vortex fiber, a 11-second fiber coupler, 1101-a second fiber coupler first port, 1102-a second fiber coupler second port, 1103-a second fiber coupler third port, 1104-a second fiber coupler fourth port, 12-a fiber isolator, 13-a second fiber circulator, 1301-a second fiber circulator first port, 1302-a second fiber circulator second port, 1303-a second fiber circulator third port, 14-a third fiber coupler, 1401-a third fiber coupler first port, 1402-a third fiber coupler second port, 1403-a third fiber coupler third port, 15-an erbium-doped vortex fiber.

Detailed Description

the present invention will be described in further detail with reference to examples and drawings, but the present invention is not limited thereto.