KR102519766B1 - Device of Stimulated Brillioun Scattering Suppression in Narrow Linewidth High Power Large Mode Area Fiber Laser and Method thereof - Google Patents

Abstract

The present invention includes a narrowband seed light source unit 30, a preamplifier unit 40 and a main amplifier unit 50 which are double optical fiber cladding, an SBS effect suppression unit 60 made of an optical fiber identical in size to the optical fiber of the main amplification unit, and The present invention relates to a narrowband, high-output, large-diameter fiber laser device including an output stage optical unit 70, and the SBS effect suppression unit having a first CLS and a second CLS disposed at both ends of a long period grating (LPG) filter.

Description

Narrowband high power large diameter fiber laser device and method for suppressing stimulated Brillouin scattering

The present invention relates to an apparatus and method for suppressing Stimulated Brillioun Scattering (SBS) generated from a high power narrowband fiber laser using an optical fiber grating.

Compared with solid-state lasers and gas lasers, fiber lasers have high structural stability, excellent beam quality and output stability.

A beam combining method for combining high-power fiber lasers has been reported to increase the power of a single resonator or amplifier-based laser. Among the beam combining methods, there are typically a wavelength-controlled beam combining method and a phase-controlled beam combining method, and research on lasers with an output of several tens of kW or more is being conducted using these methods. A key requirement in high-power laser research using beam-combining technology is high-power narrowband fiber laser generation technology that can be beam-combined.

At this time, there is Stimulated Brillouin scattering (SBS) light as a major limiting factor in terms of increasing the output of the fiber laser. SBS light has a characteristic of traveling in the opposite direction to the laser, and factors related to the SBS effect include laser power, fiber length, and fiber structure.

Methods reported to suppress the SBS effect generated in the optical fiber include adjusting the length of the transmission fiber, adjusting the physical properties of the optical fiber, temperature gradient, tension distribution, and input beam width modulation. In the case of adjusting the length of the transmission fiber, the most effective way to suppress the SBS light in the narrowband high-power fiber laser, but the fiber laser for the wavelength-controlled beam-combining laser includes a transmission fiber to transmit the high-power laser to the beam combining device. SBS light suppression by adjusting the length of the transmission fiber is limited. In addition, the SBS suppression method through adjusting the physical properties of the optical fiber can be effective, but there are difficulties in securing the performance of the optical fiber due to the change in physical properties. The method of suppressing the SBS effect generated in the optical fiber using the optical fiber temperature purchase and tension distribution method is disadvantageous in terms of miniaturization and weight reduction because an additional structure is inserted into the optical fiber laser to increase the size of the optical fiber laser. The SBS effect is related to the line width of the input beam, and a line width modulated input beam having a line width of several to several tens of GHz is used to suppress the SBS effect.

These fiber lasers are applied in industrial, medical, and defense fields, and as the application fields increase, the demand for increasing the output of the fiber laser is further increasing. As the application field of the fiber laser expands, the demand for increasing the power of the fiber laser increases, so additional SBS light suppression structures and methods are required.

Korean Registered Patent Publication No. 10-2271640 B1 (2021.6.25)

A technical problem to be achieved by the present invention is a long period grating-based optical filter (Long period grating, LPG) or tilted fiber Bragg grating (TFBG), in order to suppress Stimulated Brillioun Scattering (SBS) generated in a high-power narrowband fiber laser. ) filter to suppress the SBS light of the fiber laser by selectively removing only the SBS light.

In the present invention, the narrowband high-output large-diameter fiber laser device includes a narrowband seed light source unit 30, a preamplifier unit 40 and a main amplifier unit 50, which are double fiber cladding, and an optical fiber of the same size as the optical fiber of the main amplifier unit. It includes an SBS effect suppression unit 60 and an output stage optical unit 70, wherein the SBS effect suppression unit has a first CLS and a second CLS disposed at both ends of a long period grating (LPG) filter. to be

In addition, the distance between the first CLS and a long period grating (LPG) filter is within 20 cm, and SBS suppression in the SBS effect suppression unit generates more than 40 dB of SBS light loss by an optical fiber grating-based optical filter, The core beam is converted to a clad beam by the optical filter, the beam converted to the clad beam is guided to the optical fiber cladding, and the clad beam is removed by the first CLS and the second CLS.

The present invention is a method for suppressing Stimulated Brillioun Scattering (SBS) by a narrow-band, high-power, large-diameter fiber laser device, comprising the steps of turning on a seed light source (S10) and setting the center wavelength of the seed light source (S20). , the step of turning on the preamplifier (S30), the step of setting the applied current of the main amplifier (S40), the step of measuring the final output of the amplifier and the SBS monitoring output (S50), the output analysis step, the SBS output Including, analyzing the ratio of the reverse output (S62), comparing and analyzing the Rayleigh and SBS optical spectrum (S64), determining whether the SBS acceptance standard is exceeded (S70), when the SBS acceptance standard is exceeded, Determining whether to suppress SBS (S80), if SBS suppression does not occur, turn off the main amplifier and the preamplifier (S82), and then reset the center wavelength of the seed light source (S20). do. In addition, a step of deriving the grating spacing of the optical fiber grating may be included.

In addition, SBS suppression generates higher-order modes higher than LP02 by the long-period grating, and the TFBG central wavelength is greater than the laser oscillation wavelength.

In addition, the step of coupling the higher-order modes generated by the optical fiber grating-based optical filter to the core, the step of generating the higher-order mode loss in the core by coiling the transmission fiber after LPG (or TFBG), and the step of higher-order mode loss by fiber coiling It may include a step in which a higher order mode is generated and coupled to a clad, and a step in which a loss occurs by CLS.

The SBS light suppression structure and method to be proposed in the present invention is a structure that suppresses the SBS effect by intentionally generating the loss of SBS light generated in a narrowband high power fiber laser, and effectively suppresses the SBS light to increase the output of the narrowband high power fiber laser. way to increase When using the present invention, it is determined that it is possible to implement a fiber laser having a transmission fiber length of 2 m or more. The fiber optic grating-based SBS light suppression structure has the advantage of being applicable to fiber lasers along with previously reported SBS light suppression schemes. By applying the proposed SBS suppression structure and method, SBS light is suppressed to increase laser output and secure operation stability. The increase in the output of the fiber laser is related to the reduction in the number of fiber lasers required for the beam-combining laser, making it possible to reduce the size and weight of the laser.

1 is a conceptual diagram of an optical filter-based SBS suppression scheme.
2 is a conceptual diagram illustrating the operation principle of a long-period grating-based optical filter.
3 is a conceptual diagram of an operating principle of a TFBG grating-based optical filter.
4 is a configuration diagram of a narrowband, high-power, and large-area fiber laser including an optical fiber-based guided Brillouin scattering suppression structure.
5 is a flowchart of a method for optimizing guided Brillouin scattering suppression.
6 is a conceptual diagram illustrating the optimization of stimulated Brillouin scattering suppression.
7 is a graph of laser power analysis according to suppression of stimulated Brillouin scattering.

The present invention is a detailed description of a SBS (Stimulated Brillioun Scattering) suppression structure generated in a high-power narrowband fiber laser, and a high-power narrowband optical fiber having a transmission fiber length of 2 m or more and suppressing SBS light to implement a wavelength-controlled beam-combining laser. The structure of the laser is explained.

When a narrowband high-power fiber laser is projected, there are Rayleigh scattered light and SBS (Stimulated Brillioun Scattering) light generated from the fiber end face as light traveling in the opposite direction. Characteristics of Rayleigh scattered light and SBS (Stimulated Brillioun Scattering) light, Rayleigh scattered light has the same wavelength characteristics as the laser output wavelength band, and the center wavelength of SBS light is shifted by about 16 GHz compared to the laser oscillation wavelength.

The present invention uses a long period grating (LPG) or a tilted fiber Bragg grating (TFBG) filter to suppress SBS light of a fiber laser by selectively removing only SBS light.

First, FIG. 1 is a conceptual diagram of an optical filter-based SBS suppression method, which is a conceptual diagram of a nonlinear characteristic suppression method using wavelength shift characteristics of SBS light to be proposed in the present invention. Since the center wavelength of SBS light is shifted by 16 GHz compared to Rayleigh scattered light, it is difficult to selectively remove only SBS light using a general bandpass filter.

In addition, commercial bandwidth filters have limitations in application of narrowband high power fiber lasers having various center wavelengths. An advantage of an optical fiber grating-based filter is that it is possible to adjust the filter bandwidth center wavelength by adjusting the grating spacing. Figure 1 (a) shows each wavelength of Rayleigh scattered light and SBS (Stimulated Brillioun Scattering) light, and shows that it is shifted to 16 GHz compared to Rayleigh scattered light. , is a conceptual diagram in which Stimulated Brillioun Scattering (SBS) light is removed and only Rayleigh scattered light exists in FIG. 1(c).

2 is a conceptual diagram of an optical filter operation principle based on a large-area optical fiber long-period grating. The sizes of the core 10 and the clad 20 of the optical fiber used in the narrowband high-power fiber laser are 20 to 30 μm and 400 μm, respectively, and the core NA is 0.065. In the core of the large-area optical fiber, not only single mode but also high-order mode (LP02) mode can be waveguided. The higher-order mode generated by the long-period grating must be higher than the LP02 mode, and the lattice spacing of the long-period grating must be selected in consideration of the effective refractive index of the higher-order mode.

When a higher-order mode is generated at a corresponding wavelength by the long-period grating, the higher-order mode is cladding coupled and guided. Since the SBS light coupled to the clad and guided is not coupled to the core, it does not affect the high-power laser coupled to the core, and the loss of SBS light can suppress the SBS effect of the narrowband fiber laser. A long period grating (LPG) based bandwidth filter has a relatively wide bandwidth characteristic, and a small difference in wavelength between signal light and SBS light may cause loss in some of the signal light. Therefore, it is necessary to optimize the ratio of SBS light and laser signal light. In addition, LPG has the advantage of being relatively simple compared to FBG in terms of grid fabrication with a grid spacing of mm.

3 is a conceptual diagram illustrating the operation principle of a tilted fiber Bragg grating (TFBG) filter among optical fiber grating-based bandwidth filters. A general fiber Bragg grating (FBG) filter is an element that reflects only a specific wavelength (λ _Bragg ) guided to an optical fiber core. When FBG is applied to a high-power fiber laser, it is possible to suppress SBS light, but laser feedback occurs due to reflection of signal light, resulting in laser damage. Therefore, there is a need for a TFBG capable of suppressing back-reflection of signal light by inclining the arrangement of the grating and enabling spectral components of a specific wavelength band to be coupled to the clad. TFBG has been applied to FBG-based sensors and its performance has been optically verified.

TFBG can also reflect a specific wavelength (λ _Bragg ) and can be used as a specific wavelength filter because clad mode coupling by an inclined grating is possible. In particular, since the spectral component removed by the generation of the clad mode is not waveguided into the core mode, unlike FBG, the laser feedback effect does not occur. Due to the nature of the SBS light, the difference between the laser signal light and the central wavelength is small, so it is necessary to optimize the ratio of the SBS light and the laser signal light.

4 is a conceptual diagram of a narrowband high-power fiber laser based on a large-area fiber grating in which SBS light is suppressed. The high power narrowband fiber laser of FIG. 4 is composed of a narrowband seed light source 30, a preamplifier 40, a main amplifier 50, an SBS effect suppressor 60, and an optical output stage 70. .

The light source unit 30 is a laser diode and is a distributed feed back type laser diode (DFB-LD). DFB-LD is a laser diode that has a gain by injecting current into a portion with a diffraction grating (Bragg grating). Electro-optic modulators (EOMs) may be provided in the light source unit. The SBS effect suppression unit 60 includes a first clad light stripper (CLS), a fiber bragg grating based optical filter (FBG filter), and a second CLS.

That is, the narrowband seed light source 30 is composed of a line width modulation based seed beam light source, and the amplification section is composed of a two-stage amplifier. The gain fiber of the preamplifier unit 40 is composed of a 10 μm/125 μm double fiber cladding. The main amplifier 50 is composed of a double optical fiber clad having a core/clad diameter of 20 to 30 μm/400 μm.

The SBS effect suppression unit 60 is composed of an optical fiber having the same size as the optical fiber of the main amplifying unit and is a Germanium doped fiber (GDF). The SBS effect suppression unit 60 arranges two clad light strippers (CLS) at both ends of LPG to eliminate clad mode caused by LPG and TFGB and damage to devices by clad light. The distance between the first CLS and the optical fiber grating-based optical filter should be minimized (within 20 cm, an optical fiber splicing length). The reason is to suppress the SBS light generated by the length of the fiber by minimizing the length of the GDF connected after the gain fiber. The second CLS should be fastened after an appropriate optical fiber length after an optical filter, and if necessary, an optical fiber length should be secured so that high-order mode loss can be induced by bending or coiling a transmission optical fiber. In the case of a high-order mode guided to the optical fiber core, it is coupled to the cladding and guided by bending or coiling the optical fiber, which can be removed by the second CLS.

The optical fiber grating-based optical filter generates more than 40 dB of SBS light loss, the optical filter converts the core beam into a clad beam, the converted beam into a clad beam is guided to the optical fiber cladding, and the clad beam is removed by the CLS. .

In addition, in a structure including a long-period grating, higher order modes of LP02 or higher may be converted by the long-period grating, or in a structure including TFBG, the TFBG central wavelength is greater than the laser oscillation wavelength, and thus SBS light loss may occur.

In addition, higher-order modes generated by an optical fiber grating-based optical filter are coupled to the core, high-order mode loss occurs in the core by coiling of transmission fibers after LPG (or TFBG), and higher-order mode loss is generated by optical fiber coiling. Higher-order modes are coupled to the clad, and at this time, the coiling diameter is less than 10 cm, and the clad-coupled higher-order modes may be lost by the CLS.

5 is a flowchart showing a method of suppressing the SBS effect according to the present invention.

First, the step of turning on the seed light source (S10), the step of setting the center wavelength of the seed light source (S20), the step of turning on the preamplifier (S30), the applied current of the main amplifier Setting step (S40), measuring the amplifier final output and SBS monitoring output (S50), analyzing the ratio of reverse output including SBS output as an output analysis step (S62) and Rayleigh and SBS optical The step of comparing and analyzing the spectrum (S64), the step of determining whether or not the SBS tolerance level is exceeded (S70), and if the SBS tolerance level is not exceeded, the process returns to the step of resetting the applied current of the main amplifier (S40), allowing SBS. When the reference value is exceeded, the step (S80) of determining whether to optimize the SBS suppression effect proceeds. Optimization is determined when the same SBS characteristics are displayed after optimization of the SBS suppression effect and the laser output is increased by 10% or more.

If the SBS suppression effect is not optimized in S80, the center wavelength of the seed light source is reset by returning to the step of turning off the main amplifier and the preamplifier (S82) and setting the center wavelength of the seed light source (S20). When the SBS suppression effect is optimized in S80, measuring the laser performance (S90), deriving the optimized fiber grating spacing (S100), manufacturing the fiber grating and applying it to the fiber laser (S120), laser performance It proceeds to the measuring step (S120).

That is, after optimizing the SBS effect by adjusting the central wavelength of the narrowband seed light source and increasing the laser output through optimizing the SBS suppression effect, the optimum lattice spacing of the optical fiber grating can be derived.

More specifically, generating an SBS light loss of 40 dB or more by an optical fiber grating-based optical filter; converting the core beam into a clad beam by an optical filter; guiding a beam converted into a clad beam into an optical fiber clad; removing the clad beam by CLS; generating laser signal light and SBS light loss; In the order of, SBS generation is suppressed.

In addition, in a structure including a long-period grating, higher-order modes of LP02 or higher are converted by the long-period grating, or in a structure including TFBG, the TFBG center wavelength is greater than the laser oscillation wavelength and SBS optical loss occurs; through which SBS is generated. are suppressed

In addition, the step of coupling the higher order mode generated by the optical fiber grating-based optical filter to the core; generating high-order mode loss in the core by transmission fiber coiling after LPG (or TFBG); coupling higher-order modes to the clad by generating higher-order mode loss by optical fiber coiling; At this time, the coiling diameter is less than 10 cm, and the clad-coupled higher-order mode is lost by the CLS, and the SBS generation is suppressed.

In FIG. 6, the Rayleigh scattered light has the same wavelength characteristics as the laser output wavelength band, and the center wavelength of the SBS light has a wavelength shift of about 16 GHz compared to the laser oscillation wavelength. Since the difference between the wavelengths of the laser signal light and the SBS light is 16 GHz, which is a very small value, the center wavelength of the laser signal light is adjusted in consideration of the bandwidth of the optical filter and the ratio of the SBS light to the signal light.

Since some of the signal light may be lost by the optical filter, the SBS suppression effect can be optimized by analyzing the SBS light ratio.

When optimizing the SBS suppression effect, the power of the amplifier must be turned off to prevent laser burnout. Compared to the conventional SBS effect suppression method, the SBS effect can be suppressed only by changing the optical characteristics of the laser while the structure of the laser is the same, which has an advantage in securing high power laser output performance. In the future, it is possible to manufacture an optical filter element optimized for the laser oscillation wavelength by optimizing the lattice spacing of LPG and TFBG based on the results of the SBS suppression effect.

7 is a laser output simulation result according to the SBS suppression effect. The output ratio of retro-reflected light to signal light was analyzed while the optical fiber length of the output optical part was fixed. The pump LD output of the main amplifier was increased considering the retroreflection light ratio (SBS light reference value). As a result, it was confirmed that the laser power increased as the SBS optical loss increased. Finally, it was confirmed that the laser power increased by about 13%, which is expected to reduce the number of laser channels required due to the increased laser power when manufacturing a narrowband high-power fiber laser for beam combining. Reducing the number of laser channels to achieve the target output of the beam-coupled laser has an advantage in terms of miniaturization and weight reduction of the beam-coupled laser.

Claims (8)

delete delete delete Turning on a seed light source (S10);
Setting the center wavelength of the seed light source (S20);
Turning on the preamplifier (S30);
Setting the applied current of the main amplifier (S40);
Measuring the amplifier final output and SBS monitoring output (S50);
As an output analysis step,
Analyzing the ratio of the reverse output including the SBS output (S62), and comparatively analyzing the Rayleigh and SBS light spectrum (S64);
Determining whether or not the SBS acceptance criterion is exceeded (S70);
When the SBS acceptance criterion is exceeded, determining whether SBS is suppressed (S80);
If SBS suppression is not achieved, after turning off the main amplifier and the preamplifier (S82), resetting the center wavelength of the seed light source (S20),
The step of determining whether the SBS suppression (S80) is that higher order modes of LP02 or higher are generated by the long period grating, or the TFBG center wavelength is greater than the laser oscillation wavelength in the structure including TFBG, and SBS optical loss is determined through characterized by
Stimulated Brillioun Scattering (SBS) Suppression Method by Narrowband High Power Large Diameter Fiber Laser Device According to claim 4,
Further comprising: deriving the grating spacing of the optical fiber grating;
Stimulated Brillioun Scattering (SBS) Suppression Method by Narrowband High Power Large Diameter Fiber Laser Device delete delete According to claim 4,
SBS suppression is
coupling a higher order mode generated by an optical fiber grating-based optical filter to a core; generating high-order mode loss in the core by transmission fiber coiling after LPG (or TFBG);
coupling higher-order modes to the clad by generating higher-order mode loss by optical fiber coiling; and generating a loss by CLS; characterized in that
Stimulated Brillioun Scattering (SBS) Suppression Method by Narrowband High Power Large Diameter Fiber Laser Device

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