Fiber laser based on erbium-doped fluoride fiber, laser system

Abstract

The application discloses an erbium-doped fluoride fiber-based fiber laser and a laser system. The erbium-doped fluoride fiber-based fiber laser comprises a pump laser, a wavelength division multiplexer connected with the pump laser, a high-reflection fiber Bragg grating with a wavelength of 1690-1730 nm connected with the wavelength division multiplexer, an erbium-doped fluoride fiber connected with the high-reflection fiber Bragg grating, and a low-reflection fiber Bragg grating with a wavelength of 1690-1730 nm connected with the erbium-doped fluoride fiber. The technical scheme provided by the application can solve the technical problem of the existing technology that the structure of a laser directly generating 1.7-micron laser through an active doped fiber is too complex.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to a fiber laser and laser system based on erbium-doped fluoride fiber. Background Technology

[0002] The 1.7μm band laser has attracted widespread attention from the scientific community due to its potential applications in fields such as medicine, gas detection, materials processing, and mid-infrared laser generation.

[0003] There are two main ways to implement conventional 1.7μm wavelength lasers:

[0004] 1. Active doped optical fibers directly generate 1.7μm laser light, and their doping ions are mainly thulium (Tm). 3+ Erbium-doped fiber (EB) and bismuth (Bi) ions are used as alternatives. Both approaches have drawbacks: For thulium-doped fiber, generating a 1.7 μm laser requires a single-mode pump light with a wavelength of 1.5 μm to 1.6 μm, but semiconductor lasers cannot generate single-mode lasers with power greater than 500 mW in this wavelength range. Therefore, the pump light must be provided by an erbium-doped fiber laser-amplifier system, increasing system complexity. For bismuth-doped fiber, the manufacturing process is not yet mature, and the quality of small-batch laboratory production varies, with the ion doping concentration being too low, preventing industrialization.

[0005] 2. Utilizing the Raman effect, a 1μm or 1.5μm laser frequency is shifted to 1.7μm in a Raman fiber laser. This approach is more complex; for example, generating a 1.7μm laser from a 1μm laser requires a seventh-order Raman effect.

[0006] Currently, there is an urgent need to develop a 1.7μm band laser that can be directly pumped by a single-mode semiconductor diode laser and has a simple structure. Summary of the Invention

[0007] This application provides a fiber laser and laser system based on erbium-doped fluoride fiber, which aims to effectively solve the problem that the structure of lasers that directly generate 1.7μm lasers through active doped fiber is too complex in the prior art.

[0008] According to a first aspect of this application, this application provides a fiber laser and laser system based on erbium-doped fluoride fiber, comprising: a pump laser; a wavelength division multiplexer connected to the pump laser; a high-reflectivity fiber Bragg grating of 1690–1730 nm connected to the wavelength division multiplexer; an erbium-doped fluoride fiber connected to the high-reflectivity fiber Bragg grating; and a low-reflectivity fiber Bragg grating of 1690–1730 nm connected to the erbium-doped fluoride fiber.

[0009] Furthermore, the pump laser is a single-mode semiconductor diode laser with a wavelength of 968nm.

[0010] Furthermore, the wavelength division multiplexer is a 968nm / 1690~1730nm wavelength division multiplexer.

[0011] Furthermore, the reflectivity of the high-reflectivity fiber Bragg grating is 90% to 100%.

[0012] Furthermore, the reflectivity of the low-reflectivity fiber Bragg grating is 40% to 70%.

[0013] Furthermore, the erbium-doped fluoride optical fiber is a low-doping fiber, with an erbium ion doping concentration of less than 1 mol.% (10,000 ppm).

[0014] According to a second aspect of this application, this application also provides a laser system comprising the fiber laser based on erbium-doped fluoride fiber as described in any one of the preceding claims.

[0015] Through one or more embodiments of the above embodiments in this application, at least the following technical effects can be achieved:

[0016] In the technical solution disclosed in this application, based on erbium-doped fluoride optical fiber, using Er 3+ The long lifetime of ions in the fluoride glass matrix allows them to operate in two excited state energy levels ( 4 S 3 / 2 and 4 I 9 / 2 It generates 1.7μm laser light between two points, and the laser can directly use a semiconductor diode laser as a pump source, with a simple structure. Attached Figure Description

[0017] The technical solution and other beneficial effects of this application will become apparent from the following detailed description of specific embodiments in conjunction with the accompanying drawings.

[0018] Figure 1 A schematic diagram of the structure of a fiber laser based on erbium-doped fluoride fiber provided in an embodiment of this application;

[0019] Figure 2 A schematic diagram of a fiber laser based on erbium-doped fluoride fiber provided for an embodiment of this application;

[0020] Figure 3 Erbium ion (Er 3+ ) 4 I 15 / 2 → 4 I 11 / 2 Ground state absorption (GSA) and 4 I11 / 2 → 4 F 7 / 2 Absorption cross-section spectrum of excited-state absorption (ESA) process. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0022] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the term "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Furthermore, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0023] 1.7μm wavelength lasers have attracted widespread attention from the scientific community due to their potential applications in medicine, gas detection, materials processing, and mid-infrared laser generation. Lasers in this wavelength band have two significant characteristics:

[0024] 1. Absorption peak located at the carbon-hydrogen bond (CH);

[0025] 2. It has a high permeability to water molecules.

[0026] Due to these two characteristics, lasers in this wavelength band are very suitable for applications in the medical field. In particular, 1.7μm wavelength lasers have outstanding application value in photoacoustic imaging technology.

[0027] Photoacoustic imaging (PAI) is a novel non-invasive and non-ionizing biomedical imaging method that has emerged in recent years. (See reference 1) The principle of this technology is based on the photoacoustic effect. When a laser irradiates biological tissue, the tissue absorbs a certain proportion of the incident light energy, causing slight local heating and rapid thermoelastic expansion. This instantaneous elastic thermal expansion, under certain thermal and pressure constraints, generates pressure waves that propagate outward in the form of ultrasound (called photoacoustic signals). The photoacoustic signals generated by the biological tissue carry information about the tissue's light absorption characteristics. After being received by a high-sensitivity ultrasound detector, and then reconstructed using appropriate algorithms, the light absorption distribution of different regions of the biological tissue can be reconstructed, forming a photoacoustic image. Compared with traditional ultrasound imaging, the biggest advantage of photoacoustic imaging is that its acoustic feedback originates from the absorption of light by the biological tissue. Different types of tissues absorb light very differently (in contrast, ultrasound imaging information comes from the acoustic impedance of the biological tissue, and the difference in acoustic impedance between different tissues is much smaller than their difference in light absorption). This allows the generated acoustic waves to characterize the chemical composition of the tissue, i.e., to carry certain pathological information. This is of great significance for the treatment of diseases such as tumors and arteriosclerosis, because photoacoustic imaging can not only create images but also provide more direct pathological evidence (which usually requires invasive methods to obtain), thus aiding in disease diagnosis. Compared with imaging techniques based on ionizing radiation, such as X-rays and CT scans, photoacoustic imaging has the advantage of low hazard to the human body, and can be used to create images for some patients who are not suitable for imaging examinations (such as pregnant women). Compared with pure optical imaging (such as OCT), photoacoustic imaging, because it receives information from acoustic signals rather than light signals, has a much greater imaging depth than optical imaging (because light is scattered by human tissue, the depth of pure optical imaging is usually only 1-2 mm, and therefore can only be used for superficial tissue imaging such as ophthalmological examinations), and can achieve deep living tissue imaging at the same level as ultrasound imaging and CT.

[0028] The 1.7μm wavelength laser has significant advantages in photoacoustic imaging of lipids. (See reference 2) As mentioned earlier, this wavelength is located at the absorption peak of carbon-hydrogen bonds, and lipids have the highest carbon-hydrogen content among the various chemical components of organisms (proteins, carbohydrates, fats, etc.). Therefore, they absorb the 1.7μm wavelength laser most strongly, resulting in the strongest photoacoustic effect and enabling high-resolution imaging. The distribution of lipids in the human body has important medical significance: lipids are important markers of cardiovascular diseases such as atherosclerosis and myocardial infarction; recent studies have shown that lipid metabolism is also related to tumorigenesis and has some significance for cancer diagnosis. (See reference 3) In addition to being strongly absorbed by lipids, the 1.7μm wavelength laser is also located in the high-transmittance window of water, thus having strong penetrating power into biological tissues. Besides its applications in imaging, the 1.7μm wavelength laser has many other applications in the medical field, such as targeted skin treatment of fat (sebaceous gland surgery) and multiphoton fluorescence microscopy. These applications are all based on the high transmittance of this wavelength of laser light to biological tissues containing a large number of water molecules, and the strong absorption of this wavelength of laser light by lipids. Due to its potential for medical applications, 1.7μm wavelength lasers have become one of the research focuses both domestically and internationally in recent years, especially fiber-optic 1.7μm lasers, which have received widespread attention because they combine the various inherent advantages of fiber lasers, and there are currently a large number of related research reports.

[0029] Because the 1.7 μm wavelength is far from the dominant emission peak of any rare-earth ion, generating laser light at this wavelength in fiber optic media is extremely difficult. Currently, generating 1.7 μm laser output in fiber lasers mainly relies on two main mechanisms: 1) directly using actively doped fiber to generate laser light through transitions between doped ion energy levels (i.e., the principle of a narrow "fiber laser"); 2) converting laser light from other wavelengths into 1.7 μm laser light through the nonlinear effects of the fiber medium.

[0030] In the mechanism of directly generating 1.7 μm laser light using actively doped optical fibers, the doping ion that can directly generate 1.7 μm laser output is mainly thulium (Tm). 3+Erbium-doped fiber lasers (EDF-1) and bismuth (Bi) ions are used, but Bi is not a rare-earth ion, and its luminescence mechanism is not fully understood; both divalent and trivalent ions may be involved. For thulium ions, the 1.7 μm wavelength range is far from its dominant gain peak (located at the edge of the emission spectrum) and exhibits extremely strong reabsorption (located in the peak region of the absorption spectrum). Therefore, generating 1.7 μm lasers with thulium ions is very difficult, and typically requires a 1.5 μm–1.6 μm wavelength erbium-doped fiber laser-amplifier system for core pumping (core pumping can shorten fiber length and reduce reabsorption), resulting in a complex structure. (See reference 4). Single-mode semiconductor diode lasers have insufficient power at 1.5 μm–1.6 μm wavelengths to generate lasers, while multimode semiconductor diode lasers cannot be coupled into the core of thulium-doped fibers, preventing core pumping. To overcome the insufficient gain of thulium ions at 1.7 μm wavelength, some research teams have studied bismuth-doped fiber lasers. (See reference 5) Bismuth-doped optical fibers are very promising for generating 1.7 μm wavelength lasers because their emission peak is located near 1.7 μm and there is no reabsorption problem. However, the manufacturing process of bismuth-doped optical fibers is not yet mature and has not been industrialized. The quality of small-batch products produced in the laboratory is inconsistent, and the ion doping concentration is too low.

[0031] Another method for generating 1.7 μm laser light in fiber media is to shift the frequency of other wavelengths of laser light (typically 1 μm or 1.5 μm) to 1.7 μm through nonlinear effects. Utilizable nonlinear effects include stimulated Raman scattering, soliton self-frequency shifting, four-wave mixing, and supercontinuum generation resulting from a combination of multiple nonlinear effects.

[0032] Among these, the Raman effect, due to its relatively low threshold, is currently the most widely used mechanism. In 2011, Alexander et al. from the University of Michigan reported the first 1.7 μm fiber laser based on stimulated Raman scattering. They used a 1542 nm laser (generated from an erbium-ytterbium co-doped fiber amplifier) ​​as the fundamental pump, and a 5 km long single-mode fiber as the nonlinear gain medium. Through cascaded Raman effects, they achieved a 4 W continuous-wave laser output at 1708 nm. (See reference 6) On the other hand, when using a 1 μm laser as the Raman fundamental pump light to generate a 1.7 μm laser, the pump source is typically ytterbium-doped (Yb) fiber. 3+Fiber lasers. Shifting a 1μm wavelength laser to 1.7μm requires many levels of Raman effect (7-8 levels). This cascaded Raman effect is typically achieved using nested resonators composed of multiple pairs of fiber Bragg gratings or random Raman fiber lasers without resonators. Random Raman fiber lasers may require thousands of meters of fiber to provide sufficient frequency shift. For example, the 1.7μm Raman fiber laser reported by Grimes et al. at OFS Laboratory in 2021 uses a 1117nm wavelength ytterbium-doped fiber laser as its fundamental frequency pump source. The Raman resonator contains 7 pairs of gratings to perform multi-level Raman frequency shifts on the fundamental frequency light, making the system extremely complex. (See reference 7)

[0033] Besides the conventional Raman fiber lasers mentioned above, there is another special type of Raman laser capable of generating 1.7μm wavelength lasers: the hollow-core fiber gas Raman laser. The nonlinear gain medium of this type of laser is a gas, such as hydrogen, filled within the hollow fiber (see reference 8). Hollow-core fiber gas Raman lasers have complex structures (requiring sealed gas chambers, etc.), high costs, low robustness, and lack practical value.

[0034] To address the aforementioned issues, this application provides a 1.7μm fiber laser and laser system based on erbium-doped fluoride fiber.

[0035] Figure 1 The image shows a fiber laser based on erbium-doped zirconium fluoride fiber provided in an embodiment of this application, comprising: a pump laser 1, a wavelength division multiplexer 2, a high-reflectivity fiber Bragg grating 3, an erbium-doped fluoride fiber 4, and a low-reflectivity fiber Bragg grating 5; wherein, the wavelength division multiplexer 2 is connected to the pump laser 1; the high-reflectivity fiber Bragg grating 3 is connected to the wavelength division multiplexer 2; the erbium-doped fluoride fiber 4 is connected to the high-reflectivity fiber Bragg grating 3; and the low-reflectivity fiber Bragg grating 5 is connected to the erbium-doped fluoride fiber 4.

[0036] The pump laser 1 is a single-mode semiconductor diode laser with a wavelength of 968nm.

[0037] In this embodiment, a 968nm wavelength single-mode semiconductor diode laser is used, allowing the fiber laser based on erbium-doped fluoride fiber to be directly core-pumped by a semiconductor laser. In this embodiment, the high-reflectivity fiber Bragg grating 3 and the low-reflectivity fiber Bragg grating 5 have the same central reflection peak wavelength, both ranging from 1690 to 1730nm. In this embodiment, the central reflection peak wavelength is 1.7μm. In other embodiments, the central reflection peak wavelength can be other values, as long as the value is within the range of 1690 to 1730nm, it falls within the protection scope of this application.

[0038] In some embodiments, the reflectivity fiber Bragg grating 3 and the low reflectivity fiber Bragg grating 5 are directly inscribed on the erbium-doped fluoride fiber 4, thereby reducing the intracavity loss of the laser.

[0039] In this embodiment, the wavelength division multiplexer 2 functions to couple 968nm pump light into the erbium-doped fluoride fiber 4, thereby initiating a pumping action. The two wavelengths separated by the wavelength division multiplexer are the pump light wavelength (968nm) and the signal light wavelength. The signal light wavelength is the same as the central reflection peak wavelength of the high-reflectivity fiber Bragg grating 3 and the low-reflectivity fiber Bragg grating 5, both being 1690–1730nm. In this embodiment, the signal light wavelength is 1.7μm. In other embodiments, the signal light wavelength can be other values, as long as the value is within the range of 1690–1730nm, it falls within the protection scope of this application.

[0040] The fiber laser based on erbium-doped fluoride fiber provided in this embodiment utilizes the long lifetime of erbium ions in the fluoride glass matrix to achieve two excited state energy levels ( 4 S 3 / 2 and 4 I 9 / 2 It generates 1.7μm laser light between two points, and the laser can directly use a semiconductor diode laser as a pump source, with a simple structure.

[0041] In some embodiments, the reflectivity of the high-reflectivity fiber Bragg grating is 90% to 100%.

[0042] In some embodiments, the reflectivity of the low-reflectivity fiber Bragg grating is 40% to 70%.

[0043] The erbium-doped fluoride optical fiber provided in this embodiment is a low-doping fiber, with erbium ions (Er) in its low concentration. 3+ The doping concentration is below 1 mol.% (10000 ppm).

[0044] In this embodiment, the doping matrix of the erbium-doped fluoride optical fiber can be zirconium fluoride optical fiber (such as ZBLAN optical fiber), indium fluoride optical fiber, and aluminum fluoride optical fiber.

[0045] Therefore, the principle of the fiber laser based on erbium-doped fluoride fiber provided in this embodiment is as follows:

[0046] like Figure 2 As shown, the 968nm pump light undergoes two cascaded absorption processes (GSA and ESA; GSA, Ground State Absorption; ESA, Excited State Absorption) to absorb Er.3+ Ion pumping to 4 F 7 / 2 The energy level then transitions to the upper energy level of the 1.7 μm laser via an extremely fast multiphonon relaxation process. 2 H 11 / 2 / 4 S 3 / 2 (The energy difference between these two energy levels is extremely small, and they can be considered as a single energy level, called a thermally coupled energy level; most ions in the thermally coupled energy level are located at lower energies.) 4 S 3 / 2 Energy levels, because the ions occupying the thermally coupled energy levels follow a Boltzmann distribution, and then through 2 H 11 / 2 / 4 S 3 / 2 → 4 I 9 / 2 Stimulated emission process generates 1.7μm laser light.

[0047] because 2 H 11 / 2 / 4 S 3 / 2 The lifetime of the energy level is 530 μs, while 4 I 9 / 2 The lifetime of the energy level is 8 μs. When these two energy levels are used as the upper and lower energy levels of the laser, population inversion is easily achieved, and there is no self-termination effect. Meanwhile, 4 I 9 / 2 Erbium ions at the energy level will rapidly transition to 4 I 11 / 2 Energy level, although 4 I 11 / 2 The energy level's duration is 7.9ms, but due to... 4 I 11 / 2 The energy level is the lower energy level of the ESA process; the pump light will pass through the ESA process to... 4 I 11 / 2 Erbium ions on the energy level are rapidly pumped to 4 F 7 / 2 The energy level then transitions back to the upper energy level of the 1.7 μm laser via an extremely fast multiphonon relaxation process. 2 H 11 / 2 / 4 S 3 / 2 ( 4 F 7 / 2 The lifetime of the energy level is only 5 μs. This enables the recycling of excited-state ions. (See reference 9)

[0048] like Figure 3As shown, the absorption peak of ESA is located at 968 nm, while the absorption peak of GSA is located at 976 nm. The 968 nm pump light can simultaneously excite both GSA and ESA processes, with the ESA process (i.e.,...) being the most efficient. 4 I 11 / 2 → 4 F 7 / 2 More importantly, it maintains the form of 4 I 11 / 2 → 4 F 7 / 2 → 2 H 11 / 2 / 4 S 3 / 2 → 4 I 9 / 2 → 4 I 11 / 2 Er 3+ In the process of ion recycling, Er 3+ Ions do not need to return to the ground state energy level 4 I 15 / 2 Therefore, the wavelength of the pump light may not correspond to the absorption peak of the GSA, but it must correspond to the absorption peak of the ESA.

[0049] Therefore, the erbium-doped fluoride fiber-based fiber laser provided in this embodiment uses erbium-doped fluoride fiber as the gain medium, operates at a wavelength in the 1.7 μm band, and its emission mechanism is Er 3+ Ionic 4 S 3 / 2 → 4 I 9 / 2 Transition. This fiber laser can be pumped with a 968nm wavelength laser.

[0050] This application also provides a laser system, including a fiber laser based on erbium-doped fluoride fiber as described in any of the above embodiments.

[0051] In the above embodiments, the specific references are as follows:

[0052] [1] Zhang Rui, Yang Meng, Jiang Yuxin, Photoacoustic Imaging Technology and Its Clinical Application, Peking Union Medical College Journal, 2019, 10(4), 381-386.

[0053] [2]B.Wang,A.Karpiouk,D.Yeager,J.Amirian,S.Litovsky,R.Smalling,andS.Emellanov,“In vivo intravascular ultrasound guided photoacoustic imaging oflipid in plaques using animal model of atherosclerosis,”Ultrasound Med.Biol.,2012,38,2098-2103.

[0054] [3]M.Martin-Perez,U.Urdiroz-Urricelqui,C.Bigas,and S.Benitah,“Therole of lipids in cancer progression and metastasis,”Cell Metab.,2022,34(11),1675-1699.

[0055] [4]J.Zhang,Q.Sheng,S.Sun,C.Shi,S.Fu,W.Shi,and J.Yao,“1.7-μm thuliumfiber laser with all-fiber ring cavity,”Opt.Commun.,2020,457,124627.

[0056] [5]S.Firstov,S.Alyshev,M.Melkumov,K Riumkin,A.Shubin,and E.Dianov,“Bismuth-doped optical fibers and fiber lasers for a spectral region of 1600-1800nm,”Opt.Lett.,2014,39(24),6927-6930.

[0057] [6] V. Alexander, K. Ke, Z. Xu, M. Islam, M. Freeman, B. Pitt, M. Welsh, and J. Orringer, "Photothermolysis of sebaceous glands in human skin ex vivo with a1708nm Raman fiber laser and contact cooling," Lasers Surg. Med., 2011, 43(6), 470-480.

[0058] [7] A.Grimes, A.Hariharan, and J.Nicholson, "Progress on high power Ramanfiber lasers at 1.48and 1.7μm," Proc.SPIE, 2021, Fiber Lasers XVIII: Technologyand Systems, 116650P.

[0059] [8] H. Li, W. Huang, W. Pei, Z. Zhou, Y. Cui, M. Wang, and Z. Wang, "All-fiber gasRaman laser oscillator," Opt Lett., 2021, 46(20), 5208-5211.

[0060] [9] C.Guo, J.Lin, Z.Tang, K.Li, L.Tu, J.Wang,

[0061] In summary, although the present application has disclosed the preferred embodiments as described above, the above preferred embodiments are not intended to limit the present application. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be determined by the scope defined in the claims.

Claims

1. A fiber laser based on erbium-doped fluoride fiber, characterized in that, include: Pumped laser; A wavelength division multiplexer is connected to the pump laser; A high-reflectivity fiber Bragg grating of 1690~1730 nm is connected to the wavelength division multiplexer; Erbium-doped fluoride optical fiber is connected to the high-reflectivity fiber Bragg grating; A low-reflectivity fiber Bragg grating of 1690~1730 nm is connected to the erbium-doped fluoride fiber.

2. The fiber laser based on erbium-doped fluoride fiber as described in claim 1, characterized in that, The pump laser is a 968nm single-mode semiconductor diode laser.

3. The fiber laser based on erbium-doped fluoride fiber as described in claim 1, characterized in that, The wavelength division multiplexer is a 968 nm / 1690~1730 nm wavelength division multiplexer.

4. The fiber laser based on erbium-doped fluoride fiber as described in claim 1, characterized in that, The high-reflectivity fiber Bragg grating has a reflectivity of 90% to 100%.

5. The fiber laser based on erbium-doped fluoride fiber as described in claim 1, characterized in that, The low-reflectivity fiber Bragg grating has a reflectivity of 40% to 70%.

6. A laser system, characterized in that, Fiber lasers including erbium-doped fluoride fibers as described in any one of claims 1-5.

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