Mid-infrared 4.5-μm all-fiber laser

Abstract

The present invention discloses a mid-infrared 4.5 μm band all-fiber laser, which includes a first laser pump source, a second laser pump source, a fluoro-tellurite soft glass fiber, a pump combiner, a rare-earth ion doped fiber, a first fiber Bragg grating pair and a second fiber Bragg grating pair. The first laser pump source provides a first pump laser. The seed laser output by the second laser pump source generates a second pump laser after experiencing the first-order Raman frequency shift effect of the fluoro-tellurite soft glass fiber. The two pump lasers are coupled to the rare-earth ion doped fiber through the pump combiner. The rare-earth ion doped fiber generates mid-infrared laser in the 4.5 μm band under the cascaded pumping action of the first pump light and the second pump light. Based on the all-fiber structure, the present invention realizes the high-efficiency output of mid-infrared 4.5 μm band laser by using the dual-wavelength cascaded pumping method.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber lasers, and particularly to a mid-infrared 4.5 μm band all-fiber laser. Background Art

[0002] Laser emission sources in the mid-infrared (MIR) spectral region have increasingly advanced applications in various fields. In particular, the spectral range of 3 - 5 μm has attracted great interest because it encompasses several key vibrational absorption characteristics of CH and CO molecules, providing unique potential for spectroscopic and material processing applications. In addition, this window contains regions with extremely high atmospheric transmission, making light sources in this band potentially useful for optical detection, ranging, or infrared countermeasures. To meet the requirements of these applications for laser sources, rare-earth ion-doped fiber lasers have become promising candidate devices. Fiber-based systems can provide high brightness in a compact package and, through appropriate design, have the potential to efficiently generate high output power.

[0003] Optical fibers made of fluorozirconate glass (ZBLAN) have an infrared transparency range up to 4 μm and relatively low phonon energy. To date, the longest wavelength of ZBLAN fiber lasers is based on holmium (Ho 3+ ) ion-doped fibers, with an emission wavelength of 3.9 μm, although it requires cryogenic cooling. Based on fiber Bragg grating (FBG), single-mode fusion splicing technology, and semiconductor pumping technology, an Er 3+ :ZBLAN fiber laser achieved a continuous optical output of 30 W at 2.94 μm. In an Er 3+ :ZBLAN fiber laser, using dual-wavelength pumping technology, the laser output power at 3.5 μm can be increased by two orders of magnitude, reaching 5.6 W.

[0004] Since the non-radiative transition rate increases exponentially with the increase of the laser wavelength and the maximum phonon energy of the fiber material, and the loss above 3.8 μm wavelength increases exponentially, the emission wavelength of ZBLAN glass cannot be further extended. Therefore, the focus is required to shift from ZBLAN to matrix materials with further reduced phonon energy. Chalcogenide-based glass optical fibers with phonon energy as low as 300 cm -1 have shown promise, although to date, due to multi-phonon relaxation of impurities, high background losses, and the inability to support high rare-earth doping concentrations, laser action in chalcogenide fibers in the MIR has proven difficult to achieve.

[0005] Fluoride glass with indium fluoride (InF3) as the main glass former is a promising MIR material. Indium fluoride optical fibers have a high doping concentration and lower phonon energy (~509 cm -1), a wider transparent window with low loss in the mid-infrared band can be introduced into low-loss optical fibers. Therefore, for laser emission above 4 μm, indium fluoride glass is a suitable optical fiber matrix material.

[0006] In traditional mid-infrared fiber lasers, a single-wavelength pump light is used to pump a rare-earth ion-doped fluoride fiber. However, in the energy levels of rare-earth ions, the lifetimes of the lower energy levels corresponding to mid-infrared laser transitions in the 3-5 μm region are often longer than those of the upper energy levels. It is difficult to achieve population inversion at room temperature, which causes the laser transition at the corresponding wavelength to self-terminate and makes it difficult to achieve laser output with a wavelength greater than 4 μm. At the same time, laser sources doped with rare-earth ions have many disadvantages in the mid-infrared region, such as excited-state absorption, energy-transfer upconversion and other transition processes, which increase the pump threshold power of the laser, resulting in low laser efficiency and small output power, greatly limiting the practical application of mid-infrared fiber lasers. Summary of the Invention

[0007] In view of the above problems, the present invention provides a mid-infrared all-fiber laser system with dual-wavelength cascaded pumping that can obtain laser output in the 4.5 μm band at room temperature.

[0008] The present invention solves the technical problems by adopting the following technical solutions:

[0009] The present invention discloses a mid-infrared all-fiber laser in the 4.5 μm band, including a first laser pump source, a second laser pump source, a fluorotellurite soft glass fiber, a pump combiner, and a rare-earth ion-doped fiber. The first laser pump source provides a first pump laser, and the seed laser output by the second laser pump source generates a second pump laser after the first-order Raman frequency shift effect of the fluorotellurite soft glass fiber. The two pump lasers are coupled to the rare-earth ion-doped fiber through the pump combiner, and the rare-earth ion-doped fiber absorbs the first pump light and the second pump light and generates a laser in the 4.5 μm band.

[0010] Specifically:

[0011] The mid-infrared fiber laser is based on an all-fiber structure and includes a first laser pump source, a second laser pump source, a pump combiner, a fluorotellurite soft glass fiber, and a rare-earth ion-doped fiber.

[0012] The first laser pump source of the mid-infrared fiber laser provides a first pump laser, and the seed laser output by the second laser pump source generates a second pump laser after the first-order Raman frequency shift effect of the fluorotellurite soft glass fiber. The two pump lasers are coupled into the core of the rare-earth ion-doped fiber through the pump combiner, and the first pump laser and the second pump laser are used for dual-wavelength cascaded pumping of the rare-earth ion-doped fiber to generate a mid-infrared laser in the 4.5 μm band.

[0013] Further, the first laser pump source is a 980 nm semiconductor laser, which outputs first pump laser with a wavelength of 980 nm; the second laser pump source is a 1550 nm fiber laser, which outputs seed laser with a wavelength of 1550 nm.

[0014] Further, the Raman peak frequency shift of the fluoro-tellurite (TeO2-Bi2O3-ZnF2) soft glass fiber is 740-760 cm -1 , and the 1550 nm seed laser output by the second laser pump source generates second pump laser with a wavelength of 1700 nm through the first-order Raman frequency shift effect of the fluoro-tellurite soft glass fiber.

[0015] Further, the rare earth ion-doped fiber is a dysprosium-doped indium fluoride (Dy 3+ :InF3) fiber, which generates signal laser in the 4.5 μm band under the cascaded pumping of 980 nm and 1700 nm lasers.

[0016] Further, the output end of the second laser pump source is fusion-spliced to the input end of the fluoro-tellurite soft glass fiber, the output ends of the first laser pump source and the fluoro-tellurite soft glass fiber are respectively fusion-spliced to the two input ports of the pump combiner, the output end of the pump combiner is fusion-spliced to the input end of the rare earth ion-doped fiber, and the first pump laser and the second pump laser are coupled to the core of the rare earth ion-doped fiber through the pump combiner.

[0017] Further, a first fiber grating pair is inscribed at both ends of the fluoro-tellurite soft glass fiber, and its central reflection wavelength is 1700 nm, forming a resonant cavity for the generation of 1700 nm second pump laser; a second fiber grating pair is inscribed at both ends of the rare earth ion-doped fiber, and its central reflection wavelength is 4.5 μm, forming a resonant cavity for the generation of signal laser in the 4.5 μm band.

[0018] In the present invention, under the cascaded pumping mode of 980 nm and 1700 nm lasers, the generation mechanism of the signal laser in the 4.5 μm band is as follows: dysprosium (Dy 3+ :InF3) ions in the ground state 6 H 15 / 2 energy level in the fiber are pumped to the 3+ H 6 energy level by the 1700 nm second pump laser, and the ions at the 11 / 2 energy level will jump to the 6 H 11 / 2 energy level due to strong multi-phonon relaxation (MPR), and then under the pumping action of the 980 nm first pump laser, in the 6 H 13 / 2 energy level, and then under the pumping of the 980 nm first pump laser, in the 6 H 13 / 2Ions at the energy level further transition to 6 F 3 / 2 energy level. Meanwhile, the excited state absorption process of the 1700 nm second pump laser will further cause 6 H 13 / 2 ions at the energy level to transition to 6 F 9 / 2 , 6 H 7 / 2 energy level; ions excited to 6 F 3 / 2 energy level and 6 F 9 / 2 , 6 H 7 / 2 energy level respectively undergo multi-phonon relaxation processes to transition to the upper laser energy level 6 H 11 / 2 energy level; with the increase of the pump power, the cascaded pumping process of the 980 nm first pump laser and the 1700 nm second pump laser significantly consumes 6 H 15 / 2 energy level and 6 H 13 / 2 energy level of ions, and transfer the ions to the upper laser energy level 6 H 11 / 2 energy level through the above process. When the population inversion condition is satisfied, 6 H 11 / 2 ions at the energy level will release the ions to 6 H 13 / 2 energy level through stimulated emission transition process, and at the same time generate signal laser in the 4.5 μm band.

[0019] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0020] 1. The lifetime of the lower laser energy level corresponding to the 4.5 μm band laser generated by dysprosium (Dy 3+ ) ions is much higher than that of the upper energy level. It is difficult to achieve population inversion at room temperature, so it is easy to cause the self-termination of the corresponding 4.5 μm band laser transition. Compared with the single-wavelength pumping method, the present invention uses the dual-wavelength cascaded pumping method of 980 nm and 1700 nm, which can effectively overcome this self-termination phenomenon and improve the output efficiency of the 4.5 μm band laser.

[0021] 2. The present invention uses a first-order Raman shift effect of fluorotellurite soft glass fiber with high purity, high damage threshold, high nonlinearity, large Raman shift, wide gain bandwidth, wide transmission wavelength range, and low loss to generate the 1700 nm second pump laser; the Raman peak shift amount of the fluorotellurite soft glass fiber is 740 - 760 cm -1 , and the transmission wavelength range is 0.5 - 4 μm.

[0022] 3. Compared with dysprosium (Dy 3+ )-ion-doped fibers with zirconium fluoride glass (ZBLAN) as the matrix material, the present invention uses dysprosium-doped indium fluoride (Dy 3+ :InF3) fibers as the gain fibers. The indium fluoride (InF3) matrix material has a lower phonon energy and lower losses in the 4.5-μm band.

[0023] 4. Compared with a free-space-structured laser system, the present invention adopts an all-fiber structure, greatly simplifies the system structure, and at the same time has lower losses and higher efficiency. Description of the Drawings

[0024] Figure 1 It is a schematic diagram of the device of the present invention.

[0025] Figure 2 It is a schematic diagram of the ion energy level transition process involved in the present invention.

[0026] Reference numerals in the figure: 1 is the first laser pump source; 2 is the second laser pump source; 3 is the fluoro-tellurite soft glass fiber; 4 is the first fiber grating pair; 5 is the pump combiner; 6 is the rare-earth-ion-doped fiber; 7 is the second fiber grating pair; Detailed Embodiments

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings.

[0028] The laser lower-level lifetime corresponding to the 4.5-μm band laser generated by dysprosium (Dy 3+ ) ions is much higher than the upper-level lifetime. It is difficult to achieve population inversion at room temperature, which easily causes the corresponding 4.5-μm band laser transition to self-terminate. Compared with the single-wavelength pumping method, the present invention uses a 980-nm and 1700-nm dual-wavelength cascaded pumping method, which can effectively overcome this self-termination phenomenon and improve the output efficiency of the 4.5-μm band laser.

[0029] As Figure 1 shown, the laser structure includes the first laser pump source 1, the second laser pump source 2, the fluoro-tellurite soft glass fiber 3, the first fiber grating pair 4, the pump combiner 5, the rare-earth-ion-doped fiber 6, and the second fiber grating pair 7.

[0030] The first laser pump source 1 is a 980 nm semiconductor laser, which outputs first pump laser with a wavelength of 980 nm; the second laser pump source 2 is a 1550 nm fiber laser, the output end of which is fusion spliced with the input end of a fluoro-tellurite (TeO2-Bi2O3-ZnF2) soft glass fiber 3, and fiber Bragg gratings 4 with a central reflection wavelength of 1700 nm are written at both ends of the fluoro-tellurite soft glass fiber 3 to form a resonant cavity, so that the 1550 nm seed laser generates second pump laser with a wavelength of 1700 nm through the first-order Raman frequency shift (peak frequency shift 740-760 cm -1 ) effect;

[0031] The rare-earth ion-doped fiber 6 is a dysprosium-doped indium fluoride (Dy 3+ :InF3) fiber; the output end of the 980 nm first laser pump source 1 and the output end of the fluoro-tellurite soft glass fiber 3 are respectively fusion spliced with two input ports of a pump combiner 5, and the output end of the pump combiner 5 is fusion spliced with the input end of the Dy 3+ :InF3 fiber. The 980 nm first pump laser and the 1700 nm second pump laser are coupled to the core of the Dy 3+ :InF3 fiber through the pump combiner 5; fiber Bragg gratings 7 with a central reflection wavelength of 4.5 μm are written at both ends of the Dy 3 + :InF3 fiber to form a resonant cavity, so that the Dy 3+ :InF3 fiber generates signal laser in the 4.5 μm band under the action of double-wavelength cascaded pumping of 980 nm and 1700 nm.

[0032] The ion energy level transition process corresponding to the above 4.5 μm signal laser generation process is as Figure 2 shown. Ions in the ground state 6 H 15 / 2 energy level absorb 1700 nm pump laser and are pumped to the 6 H 15 / 2 → 6 H 11 / 2 energy level transition process, and ions at the 6 H 11 / 2 energy level will jump to the 6 H 11 / 2 energy level due to strong multi-phonon relaxation (MPR); then, under the pumping action of the 980 nm pump laser, ions at the 6 H 13 / 2 energy level absorb 980 nm laser and are pumped to the 6 H 13 / 2 energy level through the 6 H 13 / 2 → 6 F 3 / 2 energy level transition process, and are pumped to the 6F 3 / 2 Energy level, and meanwhile, the excited state absorption process of the 1700 nm pump laser causes 6 H 13 / 2 Some of the ions at the energy level to transition to 6 F 9 / 2 , 6 H 7 / 2 Energy level; The ions excited to 6 F 3 / 2 Energy level and 6 F 9 / 2 , 6 H 7 / 2 Energy level respectively transition to the upper laser energy level 6 H 11 / 2 Energy level through the multi-phonon relaxation process; With the increase of the pump power, the cascaded pumping process of the 980 nm and 1700 nm pump lasers will significantly consume the ions at the lower laser energy level 6 H 13 / 2 Energy level, and transfer the ions to the upper laser energy level 6 H 11 / 2 Energy level through the above process, thereby effectively suppressing the occurrence of self-termination of the 4.5 μm band laser transition; When 6 H 11 / 2 Energy level and 6 H 13 / 2 Energy level satisfy the population inversion condition, the ions at 6 H 11 / 2 Energy level will pass through 6 H 11 / 2 → 6 H 13 / 2 Stimulated emission transition process to release the ions to 6 H 13 / 2 Energy level, and meanwhile generate the signal laser in the 4.5 μm band.

[0033] The specific embodiments described above further elaborate on the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Mid-infrared 4.5μm band all-fiber laser, characterized in that: The mid-infrared 4.5-μm all-fiber laser is based on an all-fiber structure and includes a first laser pump source (1), a second laser pump source (2), a fluoro-tellurite soft glass fiber (3), a pump combiner (5), and a rare-earth ion-doped fiber (6). The first laser pump source (1) provides a first pump laser; the seed laser output by the second laser pump source (2) generates a second pump laser after experiencing a first-order Raman frequency shift effect in the fluoro-tellurite soft glass fiber (3); the two pump lasers are coupled to the rare-earth ion-doped fiber (6) through the pump combiner (5), and a mid-infrared laser in the 4.5-μm band is generated by double-wavelength cascaded pumping of the rare-earth ion-doped fiber with the first pump laser and the second pump laser. The first laser pump source (1) is a 980-nm semiconductor laser that outputs pump laser with a wavelength of 980 nm; the second laser pump source (2) is a 1550-nm fiber laser that outputs seed laser with a wavelength of 1550 nm. The Raman peak frequency shift amount of the fluoro-tellurite soft glass optical fiber (3) is 740-760 cm -1 , and the 1550 nm seed laser output by the second laser pump source generates a second pump laser with a wavelength of 1700 nm through the first-order Raman frequency shift effect of the fluoro-tellurite soft glass optical fiber (3); The rare-earth ion-doped fiber (6) is a dysprosium-doped indium fluoride fiber that outputs signal laser in the 4.5-μm band under the cascaded pumping of 980-nm and 1700-nm lasers. The output end of the second laser pump source (2) is fusion-spliced to the input end of the fluoro-tellurite soft glass fiber (3); the output end of the first laser pump source (1) and the output end of the fluoro-tellurite soft glass fiber (3) are respectively fusion-spliced to the two input ports of the pump combiner (5); the output end of the pump combiner (5) is fusion-spliced to the input end of the rare-earth ion-doped fiber (6); the first pump laser and the second pump laser are coupled into the core of the rare-earth ion-doped fiber (6) through the pump combiner (5). A first fiber Bragg grating pair (4) is inscribed at both ends of the fluoro-tellurite soft glass fiber (3), and its central reflection wavelength is 1700 nm, which forms a resonant cavity for the generation of the 1700-nm second pump laser; a second fiber Bragg grating pair (7) is inscribed at both ends of the rare-earth ion-doped fiber (6), and its central reflection wavelength is 4.5 μm, which forms a resonant cavity for the generation of the 4.5-μm band signal laser.

2. The mid-infrared 4.5-μm all-fiber laser according to claim 1, wherein: Under the cascaded pumping mode of 980 nm and 1700 nm lasers, the generation mechanism of the signal laser in the 4.5 μm band is as follows: Dysprosium ions in the ground state 6 H 15 / 2 energy level in the indium fluoride dysprosium-doped fiber are pumped to 6 H 11 / 2 energy level by the second 1700 nm pumping laser, and 6 H 11 / 2 level ions will undergo strong multi-phonon relaxation and transition to 6 H 13 / 2 energy level. Then, under the pumping action of the first 980 nm pumping laser, the ions in 6 H 13 / 2 energy level further transition to 6 F 3 / 2 energy level. At the same time, the excited state absorption process of the second 1700 nm pumping laser will further cause some ions in 6 H 13 / 2 energy level to transition to 6 F 9 / 2 , 6 H 7 / 2 energy level; the ions excited to 6 F 3 / 2 energy level and 6 F 9 / 2 , 6 H 7 / 2 energy level respectively undergo multi-phonon relaxation processes and transition to the laser upper level 6 H 11 / 2 energy level; as the pumping power increases, the cascaded pumping process of the first 980 nm pumping laser and the second 1700 nm pumping laser significantly consumes 6 H 15 / 2 energy level and 6 H 13 / 2 energy level ions, and transfer the ions to the laser upper level 6 H 11 / 2 energy level through the above process. When the population inversion condition is satisfied, 6 H 11 / 2 energy level ions will release ions through stimulated emission transition process to 6 H 13 / 2 energy level, and at the same time generate signal laser in the 4.5 μm band.

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