CN103474868A - Thulium-doped all-fiber laser device capable of outputting high-power 2-micron linearly polarized laser - Google Patents

Abstract

A thulium-doped all-fiber laser that outputs high-power 2-micron linearly polarized laser light, including: N first high-power laser diodes with pigtail output heads, N second high-power laser diodes with pigtail output heads, and the first A fiber combiner, a second fiber combiner, the first passive fiber with Bragg grating written, the second passive fiber with Bragg grating written, the first polarization-maintaining double-clad thulium-doped fiber, the first Two sections of polarization-maintaining double-clad thulium-doped fiber, a first fiber end cap, a second fiber end cap and a single-polarized fiber. The invention has the advantages of simple and compact structure, high output power of 2-micron laser, high degree of linear polarization, etc., and is easy to build, use and commercialize because of the all-fiber structure.

Description

Thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser

technical field

The invention relates to the technical field of fiber lasers, in particular to a thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized lasers.

Background technique

In recent years, due to the wide application of 2-micron lasers in the fields of national defense, medical treatment, remote sensing technology, and biological research, the upsurge of research on its production methods has been stimulated, which in turn has made 2-micron lasers in power, beam quality, and commercialization. Great progress. Among them, thulium-doped fiber lasers, which can be used to generate 2-micron laser light, are particularly attractive. On the one hand, this is due to some excellent characteristics shared by fiber lasers, such as weak thermal effect, single-mode operation, high beam quality, and easy integration into products after full-fiber. On the other hand, it is due to some characteristics of thulium-doped silicon dioxide as a laser gain medium. First of all, thulium-doped silica has multiple absorption peaks at each wavelength, so the choice of pump source has a lot of room. In particular, the absorption peak near 790 nm has a large absorption cross section, so even with cladding pumping, the pump light in this wavelength range can be fully absorbed within a few meters, resulting in high gain. This just makes it possible to use laser diodes in this wavelength range, which are currently developing very rapidly, as pumping sources. Under the combination of the two, companies have reported to obtain a two-micron continuous light output of more than 1 kilowatt. Secondly, thulium-doped silica has a very broad emission spectrum near 2 microns, which means it has a wide wavelength tuning range. Previous experiments have shown that it can output laser light from about 1850 nm to 2050 nm. In addition, the wide emission spectrum also means that it supports ultra-short second pulse output. According to the Fourier transform relationship, it can theoretically support the generation of ultrashort pulses less than 100 femtoseconds. Furthermore, the pump light around 800 nm can trigger the cross-relaxation process between the energy levels of the trivalent cations of thulium, which greatly improves the quantum efficiency from the pump light to the laser, which is close to 200% in theory. In the experiment Slope efficiencies of about sixty percent have also been obtained. Such a high conversion efficiency makes thulium-doped lasers dominant in the field of 2-micron laser generation.

Although it has been reported that a 2-micron continuous light output of more than 1 kW has been obtained by an all-fiber thulium-doped laser, the output light is randomly polarized, which cannot meet the requirements of some applications that require linearly polarized lasers, such as lidar, nonlinear wavelength conversion, etc. Require. At present, the reported lasers that output more than 100 watts of polarized light of 2 microns have adopted the method of spatial optical coupling, which is not conducive to the use and commercialization of lasers. However, there is no report on obtaining a hundred-watt-level 2-micron polarized light output from an easy-to-use all-fiber laser. In addition, in order to realize the output of polarized light oscillation, the method of fast and slow axis wavelength matching with feedback from both ends of the resonator is generally used at present. It is difficult to obtain a high degree of linear polarization due to the influence of other factors. The degree of polarization problem also arises in lasers using the traditional "primary power oscillator amplification" configuration. Since this type of laser usually has multiple amplification stages, it means that a long polarization-maintaining fiber and multiple splicing points that need to be fused on the axis are required. These factors are obstacles to obtaining high-power, high-degree-of-linear-polarization lasers .

Contents of the invention

The present invention proposes a thulium-doped all-fiber laser that outputs a high-power 2-micron linearly polarized laser to overcome the above-mentioned difficulties in obtaining a high-power, high-degree linearly polarized 2-micron laser with an all-fiber structure.

Technical solution of the present invention is as follows:

A thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser, which is characterized in that it includes: N first high-power laser diodes with pigtail output heads, N second high-power laser diodes with pigtail output heads Laser diode, the first fiber end cap, the second fiber end cap, the first fiber combiner, the second fiber combiner, the first section of passive optical fiber with Bragg grating written, the second section of non-active fiber with Bragg grating written Source fiber, the first section of polarization-maintaining double-clad thulium-doped fiber, the second section of polarization-maintaining double-clad thulium-doped fiber and single-polarized fiber;

The connection relationship of the above components is as follows:

The output ends of the N first high-power laser diodes are respectively passed through N sections of first pumping optical fiber and the first pumping light input end, the second pumping light input end of the first fiber combiner, ..., The Nth pump light input end is connected, the signal light input end of the first fiber combiner is connected to the first optical fiber end cap, and the signal light output end of the first fiber combiner sequentially passes through the first A section of passive fiber with Bragg grating written, the first section of polarization-maintaining double-clad thulium-doped fiber, single-polarization fiber, the second section of polarization-maintaining double-clad thulium-doped fiber, and the second section of passive fiber with Bragg grating written The signal light input end of the second optical fiber combiner is connected, and the output ends of the N second high-power laser diodes are respectively passed through N sections of second pumping fibers and the first pump of the second optical fiber combiner. The pump light input end, the second pump light input end, ..., the Nth pump light input end are connected, and the signal light output end of the second optical fiber combiner is connected with the second optical fiber end cap.

The doping weight ratios of thulium trivalent cations in the cores of the first polarization-maintaining double-clad thulium-doped optical fiber and the second polarization-maintaining double-clad thulium-doped optical fiber are greater than or equal to 1%.

The light reflectance of the passive fiber with Bragg grating written in the first paragraph at the laser wavelength is greater than or equal to 95%, and the light reflectance of the second paragraph of the passive fiber with Bragg grating written at the laser wavelength is less than or equal to 20%. .

The reflection line width of the Bragg grating in the first section of the passive optical fiber with the Bragg grating written and the second section of the passive optical fiber with the Bragg grating written is less than 0.5 nanometers.

The number of pump light input ends of the first fiber combiner and the second fiber combiner is N=2, 6, 12 or 18.

The pump light output by the first high-power laser diode and the second high-power laser diode with pigtail output head is continuous light with a wavelength range of 750 nanometers to 820 nanometers.

The length of the single-polarized optical fiber is 0.1-0.5 meters.

The cross-sectional diameters of the first fiber end cap and the second fiber end cap are both greater than 1 mm, and each output surface is chamfered at an angle of 8 degrees.

The components are connected by fusion splicing to realize an all-fiber structure.

The two sides of the fusion point when the first polarization-maintaining double-clad thulium-doped fiber and the single-polarized fiber are fused, the two sides of the fusion point when the single-polarization fiber is fused with the second polarization-maintaining double-clad thulium-doped fiber, and the second polarization-maintaining double-clad thulium-doped fiber. When the partial double-clad thulium-doped fiber and the second passive fiber are fused, the slow axes on both sides of the fusion point and the slow axes are kept perpendicular to each other, and the slow axes and the slow axes on both sides of the fusion point are kept mutually perpendicular when the other components are fused. coincide.

The optical fiber connection of each component of the thulium-doped all-fiber laser of the present invention should meet the matching of numerical aperture, and the output wavelength and spectral width of the laser are determined by the Bragg gratings in the first passive optical fiber and the second passive optical fiber. , the wavelength range of the laser output is 1850 nm to 2050 nm.

Compared with prior art, improvement and innovation of the present invention include:

1) A section of single-polarized fiber is added to the laser resonator. Different polarization-maintaining fibers only maintain the polarization state of passing light, and single-polarized fibers can only pass through the slow-axis light in the selected wavelength band, while the fast-axis light is lost. Because of this feature, only light with a specific polarization direction can be oscillated and amplified in the cavity, while light in other directions is lost when passing through the single-polarized fiber, which can increase the linear polarization degree of the output polarized light. At the same time, due to the reduction of other polarization components, the energy stored in the gain medium can also be used more effectively, which improves the conversion efficiency of pump light to laser light.

2) The present invention adopts the method of outputting hundreds of watts of high-power laser directly through the oscillation stage. Compared with the traditional laser that adopts the "main power oscillation amplification" method, the direct oscillation is more efficient, and because the laser transmission distance in the optical fiber The shortening of the polarization state degeneration effect makes the output laser have a higher degree of linear polarization.

3) High-power laser diode pumping, fiber cladding pumping, beam combiner beam combining and coupling into the cavity, end pumping, fiber fusion splicing, end cap suppression of amplified spontaneous radiation, and fast and slow axis wavelength matching at both ends of the resonant cavity feedback The combination of such technologies makes it possible for the all-fiber thulium-doped laser to output a 2-micron laser with high power and high degree of linear polarization.

The invention has the advantages of simple and compact structure, high output power of 2-micron laser, high degree of linear polarization, etc., and is easy to build, use and commercialize because of the all-fiber structure.

Description of drawings

Figure 1 is a schematic structural view of a thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser light according to the present invention.

Figure 2 is a simple energy level structure diagram of trivalent cations of thulium, and the arrows represent the main physical processes involved in generating 2-micron laser light by pumping from ³ H ₆ to ³ H ₄ .

Figure 3 shows the reflection spectrum of two passive fibers with Bragg gratings and the transmission spectrum of a single-polarized fiber.

Detailed ways

Below in conjunction with embodiment and accompanying drawing, the present invention is described in detail, present embodiment implements under the premise of technical solution of the present invention, has provided detailed embodiment and specific operation process, but protection scope of the present invention is not limited to Examples described below.

Please refer to Fig. 1 first. Fig. 1 is a structural schematic diagram of a thulium-doped all-fiber laser outputting a high-power 2-micron linearly polarized laser according to the present invention. As shown in the figure, a thulium-doped all-fiber laser outputting a high-power 2-micron linearly polarized laser , which consists of: N first high-power laser diodes 1 with pigtail output heads, N second high-power laser diodes 5 with pigtail output heads, first fiber end cap 7, second fiber end cap 8 , a first fiber combiner 41, a second fiber combiner 42 and a laser cavity. The laser resonator consists of the first section of passive fiber 2 with Bragg grating written, the second section of passive fiber 3 with Bragg grating written, the first section of polarization-maintaining double-clad thulium-doped fiber 11, and the second section of polarization-maintaining double-clad Layer thulium-doped optical fiber 12 and single-polarized optical fiber 6 are composed.

The first section of polarization-maintaining double-clad thulium-doped fiber 11 and the second section of polarization-maintaining double-clad thulium-doped fiber 12 are gain media, and are stimulated to absorb pump light and spontaneously and stimulated to emit 2 micron light. The first section of passive fiber 2 with Bragg grating written and the second section of passive fiber 3 with Bragg grating written are equivalent to the front and rear cavity mirrors in traditional solid-state lasers, providing optical feedback at specific wavelengths and directions. A section of single-polarized optical fiber 6 further screens the polarization state of the generated laser light. The length of the gain medium is usually determined by the cladding absorption coefficient of the thulium-doped fiber to the pump. A doped fiber that absorbs about 90 percent of the pump light is preferred. In order to ensure that the 2-micron laser is output from the end face of the second fiber end cap 8, the reflectivity of the first segment of the passive fiber 2 with the Bragg grating written on it should be as high as possible at the laser wavelength. According to the current technology, the reflectivity is usually up to More than ninety-nine percent, and the second paragraph of the passive fiber 3 with Bragg grating engraved, due to the high gain coefficient of the thulium-doped fiber near 2 microns, usually does not need high reflectivity, generally take about 10% It is appropriate. The single-polarized optical fiber 6 does not need to be too long, but only needs to be less than half a meter. On the one hand, it is expensive, and on the other hand, the phenomenon of polarization degradation will also occur when it is too long. In addition, the part about the polarization state selection of the laser cavity will be described below in conjunction with FIG. 3 .

The pumping sources are several first high-power laser diodes 1 and second high-power laser diodes 5, and the output of the pumping light is coupled into the optical fiber through a micro lens, and then output through the optical fiber. According to the current technology, the output light of the laser diode can be efficiently coupled into the fiber with a core diameter as small as 100 microns. The wavelength selection of the pump source will be described in conjunction with Figure 2 below. The output fiber of the high-power laser diode is fused with the pump light input fiber of the fiber combiner, and the output light of several high-power laser diodes is combined and injected into the laser cavity. In order to further improve the oblique efficiency of the laser, a bidirectional end-face pumping method is adopted, that is, as shown in FIG. Inject into cavity. Fiber combiners require high power packaging and active cooling.

The two ends of the thulium-doped all-fiber laser of the present invention are respectively connected with the first fiber end cap 7 and the second fiber end cap 8 cut at an angle of 8 degrees, which are mainly used to suppress parasitic oscillation and amplified spontaneous radiation in the laser. In addition, the output end face of the fiber end cap is preferably coated with an anti-reflection coating for laser wavelength light to reduce output loss and thermal effects caused by Fresnel reflection on the end face.

Figure 2 shows a simple energy level structure diagram of the trivalent cation of thulium, and each arrow represents the main physical process involved in the generation of 2 micron laser light by using the ³ H ₆ to ³ H ₄ transition pump. The arrow p represents the stimulated absorption process of the pump light mentioned above. Arrow l represents the laser stimulated emission process of the ³ F ₄ to ³ H ₆ transition. ^r1 and ^r2 represent the spontaneous transition processes from ^3H4 to ^3H5 and _{from 3H5} to _{3F4 ,} respectively. c1 and c2 represent a pair of cross-relaxation processes excited by the p-process: ³ H ₄ to ³ F ₄ and ³ H ₆ to ³ F ₄ . ^It is this pair of cross-relaxation processes that gives us _the reason for choosing a pump wavelength around _{800 nm (corresponding to the absorption peak for the 3H6} ^{to 3H4} transition): one pump photon can excite two The ions of _{the 4} and ³ H ₆ energy levels jump to the upper laser energy level ³ F ₄ , making the quantum efficiency of the laser close to 200%, which also makes the thulium-doped fiber laser have a high slope efficiency. In addition, it should be pointed out that commercial high-power laser diodes around 800 nm usually have 793 nm and 808 nm, and we generally choose 793 nm as the pump source for the thulium-doped fiber laser, because thulium The trivalent cations have a larger absorption cross section at this wavelength.

Figure 3 is the reflection spectrum of two sections of passive fiber with Bragg gratings and the transmission spectrum of single-polarized fiber. It can be noticed that the reflection wavelengths between the high-reflectivity grating and the low-reflectivity grating do not exactly match, only the fast-axis reflection wavelength of the high-reflection grating matches the slow-axis reflection wavelength of the low-reflection grating. This is intentional when writing the grating, so that by rotating the low-reflection grating 90 degrees during splicing (as shown in Figure 1), only one axis in the polarization-maintaining thulium-doped fiber can form laser oscillation, thereby outputting linear polarization Light. At the same time, using the property that the cut-off wavelength of the single-polarized fiber is different for the fast and slow axis light as shown in the middle figure of Figure 3, it further suppresses the start-up of laser light in other polarization directions, and plays the role of "purifying" the required linearly polarized light.

Claims (10)

1. A thulium-doped all-fiber laser that outputs high-power 2-micron linearly polarized laser light, comprising: N first high-power laser diodes (1) with pigtail output heads, N output heads with pigtails The second high-power laser diode (5), the first fiber end cap (7), the second fiber end cap (8), the first fiber combiner (41), the second fiber combiner (42), the first One section of passive fiber with Bragg grating written (2), the second section of passive fiber with Bragg grating written (3), the first section of polarization-maintaining double-clad thulium-doped fiber (11), the second section of polarization-maintaining double-clad fiber Cladding thulium-doped fiber (12) and single-polarized fiber (6); The connection relationship of the above components is as follows: The output ends of the N first high-power laser diodes (1) respectively pass through N sections of first pumping optical fiber and the first pumping light input end of the first fiber combiner (41), the second pumping light The input end, ..., the Nth pump light input end are connected, the signal light input end of the first fiber combiner (41) is connected to the first fiber end cap (7), the first fiber combiner The signal light output end of (41) sequentially passes through the first section of passive optical fiber (2) with Bragg grating written, the first section of polarization-maintaining double-clad thulium-doped optical fiber (11), single-polarized optical fiber (6), The second section of polarization-maintaining double-clad thulium-doped optical fiber (12) and the second section of passive optical fiber (3) with Bragg grating written on it are connected to the signal light input end of the second optical fiber combiner (42), so The output ends of the N second high-power laser diodes (2) respectively pass through N sections of second pumping optical fibers and the first pumping light input end and the second pumping input end of the second optical fiber combiner (42). , ..., the Nth pump light input end is connected, and the signal light output end of the second optical fiber combiner (42) is connected to the second optical fiber end cap (8). 2. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, characterized in that the first polarization-maintaining double-clad thulium-doped fiber (11) and the second polarization-maintaining double The doping weight ratio of trivalent cations of thulium in the core of the cladding thulium-doped optical fiber (12) is greater than or equal to 1%. 3. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, characterized in that the light reflectance of the first section of the passive fiber (2) with Bragg grating written on it at the laser wavelength is It is greater than or equal to 95%, and the light reflectance of the second segment of the passive optical fiber (3) with Bragg gratings written on it at the laser wavelength is less than or equal to 20%. 4. the thulium-doped all-fiber laser of output high-power 2 micron linearly polarized laser according to claim 3, is characterized in that, described first section writes the passive optical fiber of Bragg grating, the second section writes Bragg grating The reflection linewidths of Bragg gratings in passive optical fibers were less than 0.5 nm. 5. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, characterized in that, the first fiber combiner (41) and the second fiber combiner (42) The number of pump light input ends is N=2, 6, 12 or 18. 6. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, characterized in that the first high-power laser diode (1) and the second high-power laser diode (1) with a pigtail output head The pump light output by the power laser diode (5) is continuous light with a wavelength range of 750 nm to 820 nm. 7. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, characterized in that the length of the single-polarized optical fiber (6) is 0.1-0.5 meters. 8. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, characterized in that the cross-sections of the first fiber end cap (7) and the second fiber end cap (8) The diameters are all greater than 1 mm, and each output face is chamfered at an angle of 8 degrees. 9. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 1, wherein said components are connected by fusion splicing to realize an all-fiber structure. 10. The thulium-doped all-fiber laser outputting high-power 2-micron linearly polarized laser according to claim 9, characterized in that, the first polarization-maintaining double-clad thulium-doped fiber (11) and single-polarized fiber (6 ) on both sides of the fusion point during fusion splicing, single-polarized optical fiber (6) and the second polarization-maintaining double-clad thulium-doped optical fiber (12) on both sides of the fusion point during fusion splicing, and the second polarization-maintaining double-clad thulium-doped optical fiber (12) When splicing with the second passive optical fiber (3), the slow axes on both sides of the splicing point and the slow axes are kept perpendicular to each other, and the slow axes and the slow axes on both sides of the splicing point keep coincident with each other when splicing other components.

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