A mid-infrared 4μm and 3μm dual-wavelength all-fiber laser

Abstract

The present invention relates to a fiber laser, and in particular to a mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser, which is used to solve the problem that an erbium-doped zirconium fluoride all-fiber laser pumped by a 980nm semiconductor laser and a 1970nm / 970nm dual-wavelength cascade pump can only obtain a single output in the 3μm band, and that an 888nm semiconductor laser pumped heavily doped holmium ion indium fluoride fiber laser can only obtain a single output in the 4μm band. At the same time, a spatial optical device makes the 4μm The mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser has the disadvantages of difficult pump coupling adjustment and low pump coupling efficiency, poor laser structure stability, and great difficulty in engineering application. The mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser includes a semiconductor laser pump source, a gain fiber, a 4μm band fiber Bragg grating pair and a 3μm band fiber Bragg grating pair; the present invention adopts an 885-890nm semiconductor laser to pump holmium-praseodymium co-doped indium fluoride fiber to achieve cascade output of 4μm band laser and 3μm band laser.

Description

Technical Field

[0001] The present invention relates to a fiber laser, in particular to a mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser. Background Art

[0002] In recent years, mid-infrared laser sources in the 3-5μm band, located within the atmospheric transmission window, have shown promising applications in laser medicine, infrared pumping, spectroscopy, and infrared countermeasures, and have become a research hotspot both domestically and internationally. Compared to other mid-infrared laser generation methods in the 3-5μm band, fiber lasers offer significant advantages, including high brightness, tunability, excellent beam quality, high conversion efficiency, excellent stability, compact structure, and ease of miniaturization. Therefore, the development of all-fiber lasers in the 3-5μm band has significant scientific significance and application value. In particular, all-fiber lasers capable of simultaneously generating laser output in the 3μm and 4μm bands have important applications in military and scientific research. Due to the limitations of the phonon energy and transmittance of silica-based optical fiber materials, fluoride optical fiber materials with lower phonon energy and longer transmission spectra are generally used to generate lasers above 2.5μm.

[0003] Currently, fiber lasers generating 3μm wavelengths are primarily based on zirconium fluoride (ZBLAN) fibers incorporating erbium or holmium ions. Using femtosecond laser inscription of fluoride fiber Bragg gratings and quartz fiber-fluoride fiber fusion splicing, researchers have achieved maximum output powers of 30.5W and 5.6W at 2.94μm and 3.55μm, respectively, using 980nm semiconductor laser pumping and 1970nm / 970nm dual-wavelength cascade pumping. However, these lasers can only produce a single output at 3μm.

[0004] Compared with zirconium fluoride fiber, indium fluoride fiber has lower phonon energy and a wider infrared transmission spectrum, making 4μm band fiber lasers possible. In 2018, researchers at the University of Lava used an 888nm semiconductor laser as a pump source, heavily doped holmium ion indium fluoride fiber as a gain medium, and two dichroic mirrors as a resonant cavity to achieve the longest wavelength 3.92μm laser output at room temperature. This is also the longest operating wavelength of rare earth ion-doped fiber lasers reported so far, with an average output power of 200mW and a slope efficiency of about 10% (2018, Volume 5, "Optica", "Room-temperature fiber laser at 3.92μm"). However, this technology can only obtain a single wavelength output in the 4μm band. At the same time, the laser pumping method uses a lens for free-space pump coupling, and the resonant cavity uses a dichroic mirror to bond with the optical fiber. These spatial optical devices make the 4μm wavelength laser difficult to adjust the pump coupling and the pump coupling efficiency low, the laser structure stability is poor, and the engineering application is difficult, which limits its application in the field of scientific research. Summary of the Invention

[0005] The purpose of the present invention is to solve the shortcomings that an erbium-doped zirconium fluoride all-fiber laser pumped by a 980nm semiconductor laser and a 1970nm / 970nm dual-wavelength cascade pump can only obtain a single output in the 3μm band, and that an 888nm semiconductor laser pumped heavily doped holmium ion indium fluoride fiber laser can only obtain a single output in the 4μm band. At the same time, spatial optical devices make the 4μm band laser difficult to adjust pump coupling and have low pump coupling efficiency, poor laser structural stability, and great difficulty in engineering application. The present invention provides a mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser.

[0006] In order to solve the deficiencies of the above-mentioned prior art, the present invention provides the following technical solutions:

[0007] A mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser, which is special in that it includes a semiconductor laser pump source, a gain fiber connected to the output end of the semiconductor laser pump source, and a 4μm band fiber Bragg grating pair and a 3μm band fiber Bragg grating pair arranged on the gain fiber;

[0008] The semiconductor laser pump source is a semiconductor laser with a wavelength in the range of 885 to 890 nm; the gain fiber is a holmium-praseodymium co-doped indium fluoride fiber (Ho 3+ / Pr 3+ :InF);

[0009] The 4μm-band fiber Bragg grating pair includes a first fluoride fiber Bragg grating and a fourth fluoride fiber Bragg grating; the first fluoride fiber Bragg grating has a reflectivity of greater than 95% for 4μm-band lasers and a transmittance of greater than 95% for 885-890nm lasers; the fourth fluoride fiber Bragg grating has a reflectivity of 80-96% for 4μm-band lasers and a transmittance of greater than 95% for 3μm-band lasers;

[0010] The 3μm-band fiber Bragg grating pair includes a second fluoride fiber Bragg grating and a third fluoride fiber Bragg grating; the reflectivity of the second fluoride fiber Bragg grating to 3μm-band laser is greater than 95%, and the transmittance to 885-890nm laser is greater than 95%; the reflectivity of the third fluoride fiber Bragg grating to 3μm-band laser is 4%-96%, and the transmittance to 4μm-band laser is greater than 95%;

[0011] The 4 μm band has a wavelength of 3.85 to 3.98 μm, and the 3 μm band has a wavelength of 2.82 to 3.01 μm;

[0012] The first fluoride fiber grating and the second fluoride fiber grating are sequentially arranged along the optical path near the laser incident end of the gain fiber, and the third fluoride fiber grating and the fourth fluoride fiber grating are sequentially arranged along the optical path near the laser emitting end of the gain fiber.

[0013] Furthermore, the doping concentration of holmium ions in the gain optical fiber is 1 to 5 mol%, and the doping concentration of praseodymium ions is 0.05 to 0.5 mol%.

[0014] Furthermore, the first fluoride fiber grating, the second fluoride fiber grating, the third fluoride fiber grating, and the fourth fluoride fiber grating are all fiber Bragg gratings directly written on the gain fiber.

[0015] Furthermore, the output end of the semiconductor laser pump source and one end of the gain optical fiber are connected by end-face fusion splicing or end-face direct butt connection.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) The present invention provides a mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser, comprising a semiconductor laser pump source, a gain fiber, a 4μm band fiber Bragg grating pair, and a 3μm band fiber Bragg grating pair; the present invention uses an 885-890nm semiconductor laser to pump holmium-praseodymium co-doped indium fluoride fiber, and the 3μm band (2.82-3.01μm) laser generates a laser that helps holmium ions 5 I5 energy level and 5 The number of particles between the I6 energy levels is reversed, which improves the laser efficiency in the 4μm band (3.85~3.98μm). 5 The holmium ion at the I7 energy level helps the holmium ion to 5 I6 energy level and 5 The particle number inversion between the I7 energy levels improves the laser efficiency in the 3μm band (2.82~3.01μm), realizes the cascade output of the 4μm band (3.85~3.98μm) laser and the 3μm band (2.82~3.01μm) laser, and then simultaneously obtains the 4μm band and 3μm band laser output through one structure.

[0018] (2) In the mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser of the present invention, the gain fiber is connected to the output end of the semiconductor laser pump source, and the first fluoride fiber grating, the second fluoride fiber grating, the third fluoride fiber grating, and the fourth fluoride fiber grating are all directly inscribed on the gain fiber, so that the present invention realizes an all-fiber structure with the advantages of simple structure, compact size, good beam quality, stable and reliable performance, and easy to obtain high-power output.

[0019] (3) In the mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser of the present invention, the first fluoride fiber grating, the second fluoride fiber grating, the third fluoride fiber grating, and the fourth fluoride fiber grating are all directly inscribed on the gain fiber, without the need to add fiber components and fiber fusion points or connection points, so that the semiconductor laser pump source loss and the oscillation loss in the resonant cavity are lower, the laser efficiency is higher, and the structure is simpler. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of a mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser of the present invention;

[0021] Figure 2 Schematic diagram of the ion energy level transition process corresponding to the 4μm band and 3μm band laser generation process of the present invention.

[0022] The reference numerals are as follows: 1- semiconductor laser pump source; 2- first fluoride fiber Bragg grating; 3- second fluoride fiber Bragg grating; 4- gain fiber; 5- third fluoride fiber Bragg grating; 6- fourth fluoride fiber Bragg grating; 7- holmium ion ground state 5 I8 energy level; 8-holmium ion 5 I7 energy level; 9-holmium ion 5 I6 energy level; 10-holmium ion 5 I5 energy level; 11- 5 I8→ 5 I5; 12-4μm stimulated emission; 13-3μm stimulated emission; 14-energy transfer process; 15-praseodymium ion 3 F2 energy level; 16-multiphonon relaxation; 17-praseodymium ion 3 H4 energy level. DETAILED DESCRIPTION

[0023] The present invention will be further described below with reference to the accompanying drawings and exemplary embodiments.

[0024] Reference Figure 1 A mid-infrared 4μm and 3μm dual-wavelength all-fiber laser includes a semiconductor laser pump source 1, a gain fiber 4 connected to the output end of the semiconductor laser pump source 1, and a 4μm-band fiber Bragg grating pair and a 3μm-band fiber Bragg grating pair arranged on the gain fiber 4.

[0025] The semiconductor laser pump source 1 is a semiconductor laser with a pigtail output and a wavelength of 886 nm.

[0026] The gain fiber 4 is a holmium-praseodymium co-doped indium fluoride fiber, in which the holmium ion doping concentration is 3 mol%, the praseodymium ion doping concentration is 0.25 mol%, the core diameter is 17 μm, the inner cladding diameter is 100 μm, and the outer cladding diameter is 250 μm.

[0027] The 4μm-band fiber Bragg grating pair includes a first fluoride fiber Bragg grating 2 and a fourth fluoride fiber Bragg grating 6. The first fluoride fiber Bragg grating 2 has a reflectivity of greater than 95% for 4μm-band laser and a transmittance of greater than 95% for 885-890nm laser. The fourth fluoride fiber Bragg grating 6 has a reflectivity of 80-96% for 4μm-band laser and a transmittance of greater than 95% for 3μm-band laser. The 3μm-band fiber Bragg grating pair includes a second fluoride and a third fluoride. The second fluoride has a reflectivity of greater than 95% for 3μm-band laser and a transmittance of greater than 95% for 885-890nm laser. The third fluoride has a reflectivity of 4%-96% for 3μm-band laser and a transmittance of greater than 95% for 4μm-band laser.

[0028] The 4 μm band has a wavelength of 3.85 to 3.98 μm, and the 3 μm band has a wavelength of 2.82 to 3.01 μm.

[0029] The first fluoride fiber Bragg grating 2, the second fluoride fiber Bragg grating 3, the third fluoride fiber Bragg grating 5, and the fourth fluoride fiber Bragg grating 6 are all fiber Bragg gratings directly inscribed on the gain fiber 4. The first fluoride fiber Bragg grating 2 and the second fluoride fiber Bragg grating 3 are sequentially arranged along the optical path near the laser incident end of the gain fiber 4, and the third fluoride fiber Bragg grating 5 and the fourth fluoride fiber Bragg grating 6 are sequentially arranged along the optical path near the laser exit end of the gain fiber 4.

[0030] The output end of the semiconductor laser pump source 1 is connected to one end of the holmium-praseodymium co-doped indium fluoride optical fiber by end-face fusion splicing or end-face direct butt connection.

[0031] The first fluoride fiber Bragg grating 2 and the fourth fluoride fiber Bragg grating 6 form a first resonant cavity for generating 4 μm band laser, and the second fluoride fiber Bragg grating 3 and the third fluoride fiber Bragg grating 5 of holmium-praseodymium co-doped indium fluoride fiber form a second resonant cavity for generating 3 μm band laser, generating 4 μm band and 3 μm band laser respectively.

[0032] The ion energy level transition process corresponding to the above 4μm band and 3μm band laser generation process is as follows: Figure 2 As shown. Under 886nm laser pumping, the holmium ion in the holmium-praseodymium co-doped indium fluoride fiber is in the ground state. 5 The holmium ion at I8 energy level 7 absorbs 886nm pump laser and transitions to the holmium ion 5I5 energy level 10, that is, through the ground state absorption process 5 I8→ 5 I511, holmium ion 5 The number of particles at the I5 energy level 10 continues to accumulate. 5 When the number of particles on I5 energy level 10 reaches a certain level, holmium ions are realized. 5 I5 energy level 10 and holmium ion 5 The number of particles on the I6 energy level 9 is inverted, resulting in 4μm stimulated radiation 12 and 4μm band laser, but the holmium ion 5 The lifetime of particles at energy level 10 is shorter than that of holmium ions. 5 The particle lifetime at I6 energy level 9 is the reason why the laser radiation in the 4μm band usually terminates itself. 5 The holmium ion at I6 energy level 9 further transitions downward to the holmium ion 5 I7 energy level 8, when the holmium ion 5 When the number of particles on I6 energy level 9 reaches a certain level, it will be realized 5 I6 energy level 9 and 5 The number of particles on the I7 energy level 8 is reversed, resulting in 3μm stimulated radiation 13 and 3μm band laser. In this process, due to the holmium ion 5 The lifetime of particles at energy level 9 of I6 is shorter than that of holmium ions. 5 The particle lifetime at I7 energy level 8 is large, so the self-termination phenomenon of 3μm laser radiation usually occurs. 3 Praseodymium ions at F2 energy level 15 absorb holmium ions 5 The energy of the holmium ion at I7 energy level 8, that is, the energy transfer process 14 occurs, and the praseodymium ion 3 The praseodymium ion at F2 level 15 undergoes multiphonon relaxation 16 and transitions downward to the praseodymium ion 3 H4 energy level 17, while in the holmium ion 5 The holmium ion at energy level 8 consumes the holmium ion through the energy transfer process 14. 5 I7 energy level 8 particles, and then continue to obtain holmium ions 5 I6 energy level 9 and holmium ion 5 The particle number inversion on I7 energy level 8 solves the self-termination problem of 3μm laser radiation and realizes efficient generation of 3μm laser. Furthermore, the generation of 3μm laser consumes a large amount of holmium ions. 5 The number of particles on I7 energy level 8 is very conducive to the realization of holmium ions 5 I5 energy level 10 and holmium ion 5 The particle number inversion at I6 energy level 9 solves the self-termination problem of 4μm band laser radiation, and ultimately realizes the simultaneous output of high-efficiency all-fiber 4μm band and 3μm band lasers.

[0033] In other embodiments, the semiconductor laser pump source 1 is a semiconductor laser with a wavelength in the range of 885 to 890 nm, that is, the ground state of holmium ions 5 I8 energy level 7 absorption transition to holmium ion 5 Within the absorption spectrum range required by the I5 energy level 10, the gain fiber 4 satisfies the holmium ion doping concentration of 1-5 mol% and the praseodymium ion doping concentration of 0.05-0.5 mol%, that is, the simultaneous output of 4 μm and 3 μm lasers can be achieved.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. For ordinary professional and technical personnel in this field, the specific technical solutions recorded in the above embodiments can be modified, or some of the technical features therein can be replaced by equivalents. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions protected by the present invention.

Claims

1. A mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser, characterized by: It comprises a semiconductor laser pump source (1), a gain optical fiber (4) connected to the output end of the semiconductor laser pump source (1), and a 4μm-band fiber Bragg grating pair and a 3μm-band fiber Bragg grating pair arranged on the gain optical fiber (4); The semiconductor laser pump source (1) is a semiconductor laser with a wavelength in the range of 885 to 890 nm; the gain fiber (4) is a holmium-praseodymium co-doped indium fluoride fiber, wherein the holmium ion doping concentration is 1 to 5 mol% and the praseodymium ion doping concentration is 0.05 to 0.5 mol%; The 4 μm-band fiber Bragg grating pair comprises a first fluoride fiber Bragg grating (2) and a fourth fluoride fiber Bragg grating (6); the reflectivity of the first fluoride fiber Bragg grating (2) to 4 μm-band laser is greater than 95%, and the transmittance to 885-890 nm laser is greater than 95%; the reflectivity of the fourth fluoride fiber Bragg grating (6) to 4 μm-band laser is 80-96%, and the transmittance to 3 μm-band laser is greater than 95%; The 3 μm-band fiber Bragg grating pair comprises a second fluoride fiber Bragg grating (3) and a third fluoride fiber Bragg grating (5); the reflectivity of the second fluoride fiber Bragg grating (3) to 3 μm-band laser is greater than 95%, and the transmittance to 885-890 nm laser is greater than 95%; the reflectivity of the third fluoride fiber Bragg grating (5) to 3 μm-band laser is 4%-96%, and the transmittance to 4 μm-band laser is greater than 95%; The 4 μm band has a wavelength of 3.85 to 3.98 μm, and the 3 μm band has a wavelength of 2.82 to 3.01 μm; The first fluoride fiber Bragg grating (2) and the second fluoride fiber Bragg grating (3) are sequentially arranged along the optical path near the laser incident end of the gain fiber (4), and the third fluoride fiber Bragg grating (5) and the fourth fluoride fiber Bragg grating (6) are sequentially arranged along the optical path near the laser exit end of the gain fiber (4).

2. The mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser according to claim 1, characterized in that: The first fluoride fiber Bragg grating (2), the second fluoride fiber Bragg grating (3), the third fluoride fiber Bragg grating (5), and the fourth fluoride fiber Bragg grating (6) are all fiber Bragg gratings directly inscribed on the gain fiber (4).

3. A mid-infrared 4μm band and 3μm band dual-wavelength all-fiber laser according to claim 1 or 2, characterized in that: The output end of the semiconductor laser pump source (1) and one end of the gain optical fiber (4) are connected by end-face fusion or end-face direct butt connection.

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