Passively q-switched mid-infrared fiber laser based on rare-earth ion doped nanocrystal particles

Abstract

The application discloses a passive Q-switched mid-infrared fiber laser based on rare earth ion doped nanocrystal particles, which comprises a pumping light unit, a light splitting device unit, a pulse seed light generating unit and a pulse seed light amplifying unit, wherein the pumping light unit outputs quasi-parallel linearly polarized light; the light splitting device unit splits the pumping light into two beams to simultaneously pump the pulse seed light generating unit and the pulse seed light amplifying unit, and couples the seed light transmitted in the reverse direction into the pulse seed light amplifying unit after reflection; the pulse seed light generating unit generates pulse seed light by using a nanocrystal particle saturable absorber; and the pulse seed light amplifying unit absorbs the pumping light and amplifies the seed light. The laser amplifier and the laser share the pumping source, so that the structure of the high-power laser is simplified, and the rare earth ion doped nanocrystal particle saturable absorber provides a new choice for high-performance passive Q-switched mid-infrared laser pulse output.

Description

Technical Field

[0001] This invention belongs to the field of fiber lasers, and specifically relates to a passively Q-switched mid-infrared fiber laser system based on rare-earth ion-doped nanocrystal particles. Background Technology

[0002] Mid-infrared lasers, especially high-energy pulsed lasers, with pulse widths on the order of microseconds (μs) or nanoseconds (ns), have significant application value in biomedicine, materials processing, and military applications. Fiber lasers, compared to other lasers (such as solid-state lasers, quantum cascade lasers, difference-frequency lasers, sum-frequency lasers, and optical parametric oscillators), offer advantages such as high beam quality, excellent heat dissipation, high optical conversion efficiency, and small size. Furthermore, with the maturation of doped fiber fabrication technology, rare-earth ion-doped fiber lasers exhibit advantages such as high gain, wide bandwidth, high nonlinearity, and ease of miniaturization and integration. Meanwhile, high-quality matrix materials, such as fluoride glass, especially ZrF4-BaF2-LaF3-AlF3-NaF composition (ZBLAN) fiber, have become the most commonly used rare-earth ion-doped matrix materials in the mid-infrared band due to their relatively low phonon energy, excellent rare-earth ion doping capability, high mechanical strength, and reliable chemical stability.

[0003] Currently, methods for generating mid-infrared μs or ns-level laser pulses include gain modulation, active Q-switching, and passive Q-switching. Gain modulation requires a pulsed pump source, which is implemented by a complex pump drive circuit driving a semiconductor laser, thus increasing system complexity. Active Q-switching requires the addition of active modulation elements such as acousto-optic or electro-optic modulation in the resonant cavity, similarly increasing system complexity and hindering system integration. Passive Q-switching, on the other hand, obtains Q-switched pulses by adding a saturable absorber as a lossy modulation element in the resonant cavity. Passive Q-switched fiber lasers based on saturable absorbers have advantages such as simple structure, ease of integration, and low cost, making them more advantageous than the previous two modulation methods in applications. Currently, various passive Q-switched fiber laser pulse outputs based on different saturable absorbers have been achieved, including semiconductor saturable absorber mirrors (SESAM), Fe... 2+ ZnSe crystals and some broadband two-dimensional materials (graphene, topological insulators, black phosphorus, etc.).

[0004] Currently, the operating wavelengths of mid-infrared fiber lasers based on saturable absorbers are mainly in the 2μm-4μm band. However, compared to the near-infrared band, the modulation depth of existing mid-infrared saturable absorbers is generally smaller, and their stability and damage threshold need further improvement. Therefore, mid-infrared fiber lasers based on saturable absorbers still lag significantly behind near-infrared fiber pulsed lasers in terms of stability, output efficiency, and saturation threshold.

[0005] This invention discovers that rare-earth ion-doped nanocrystal particles are a promising mid-infrared saturable absorber material. This is because rare-earth ions possess diverse band structures and exhibit rich absorption and emission spectra in the mid-infrared band. Therefore, by selecting specific rare-earth ion-doped nanocrystal particles, their nonlinear absorption region can be well matched with the laser emission band in fiber lasers. By changing the rare-earth doping concentration in the particles, the modulation depth and saturation threshold of the saturable absorber can be dynamically adjusted. By preparing the nanocrystal particles into a suspension, these micro / nano particles can be directly coated onto the end face of a total reflection mirror or optical fiber. Therefore, using rare-earth ion-doped nanocrystal particles as a saturable absorber for mid-infrared fiber lasers offers advantages such as high conversion efficiency, high modulation depth, simple structure, and high compatibility with current rare-earth ion-doped mid-infrared fiber lasers.

[0006] Given the potential bleaching effect of saturable absorbers at high light intensities, further increases in pulse average power and energy are limited. This invention constructs a power amplification system based on a passively Q-switched pulse seed source. This system is cascaded with the passively Q-switched pulse seed source system via an optical rotator crystal and a coupling lens, sharing the same pump source to fully utilize its power. This pumps particles in the ground state of the gain fiber to the upper energy level of the laser, achieving population inversion. Simultaneously, the fiber end face of the gain fiber is beveled to prevent parasitic oscillations. Thus, when the pulse seed light is incident on the gain fiber of the amplification system, stimulated emission of the laser amplifies the seed light power. Summary of the Invention

[0007] To address the shortcomings of existing pulsed fiber lasers, this invention proposes a passively Q-switched mid-infrared fiber laser based on rare-earth ion-doped nanocrystal particles. The aim is to achieve efficient and stable output of mid-infrared laser pulses by combining a fiber amplifier with a fiber laser and using rare-earth ion-doped nanocrystal particles as a saturable absorber, while simplifying the device structure of high-power mid-infrared fiber lasers.

[0008] The present invention solves the technical problem by adopting the following technical solution:

[0009] This invention discloses a passively Q-switched mid-infrared fiber laser based on rare-earth ion-doped nanocrystal particles, characterized by comprising a pump light unit, a beam splitter unit, a pulse seed light generation unit, and a pulse seed light amplification unit. The pump light unit includes a pump laser and a collimating lens; the beam splitter unit includes a polarizer and an optically rotating crystal; the pulse seed light generation unit includes a first coupling lens, a partially reflecting cavity mirror, a first gain fiber, and a total internal reflection cavity mirror with a reflective surface coated with rare-earth ion-doped nanocrystal particles; the pulse seed light amplification unit includes a second coupling lens, a second gain fiber, and a bandpass filter.

[0010] Specifically:

[0011] In the pump light unit: the pump laser outputs pump light, and the pump light is linearly polarized light with full polarization, and the wavelength of the pump light is less than the wavelength of the seed light; the collimating lens collimates the linearly polarized pump light into quasi-parallel light.

[0012] In the aforementioned beam splitting unit: the polarizer is made of a total internal reflection material; the polarizer transmits light with the same polarization direction as the polarizer itself, and reflects light with a polarization direction orthogonal to the polarizer's polarization direction; the optically rotating crystal has a cross-section of a parallelogram with a vertices angle of 45°; the optically rotating crystal can rotate the polarization direction of light passing through it by 45°, regardless of the incident direction. This device can couple forward-propagating pump light into a pulse seed light generation unit, and reverse-propagating pump light and seed light, after reflection by the polarizer, are coupled into a pulse seed light amplification unit.

[0013] In the pulsed seed light generation unit: the partial reflection cavity mirror, the first gain fiber, and the total internal reflection cavity mirror with rare-earth ion-doped nanocrystal particles coated on the reflective surface constitute a laser resonant cavity. The first coupling lens couples the input pump light into the resonant cavity; the partial reflection cavity mirror reflects part of the pump light along the original optical path into the optical rotator crystal, and transmits part into the first gain fiber; under the continuous action of the pump light, the activated ions in the first gain fiber transition from the ground state energy level to the upper laser energy level, generating seed light through stimulated emission; the reflective surface of the total internal reflection cavity mirror is coated with rare-earth ion-doped nanocrystal particles, serving as a passively Q-switched saturable absorber. The nonlinear absorption band of the rare-earth ions in the saturable absorber corresponds to the seed light band, cleverly achieving saturable absorption in the mid-infrared band and realizing pulsed output of the seed light. By coating the surface of the total internal reflection cavity mirror with a suspension of rare-earth ion-doped nanocrystal particles, the device structure is optimized and the energy conversion efficiency is improved. The partial reflection cavity mirror partially reflects and partially transmits the seed light into the optical rotator crystal.

[0014] In the pulsed seed light amplification unit: the pump light and seed light reflected by the partially reflecting cavity mirror pass through the optical rotator crystal and are reflected by the polarizer into the pulsed seed light amplification unit; the second coupling lens couples the reflected pump light and the subsequently generated seed light into the second gain fiber; the second gain fiber amplifies the seed light after being excited by the pump light; the bandpass filter only allows seed light with a center wavelength in a specific mid-infrared band to pass through, filtering out short-wavelength pump light.

[0015] Specifically:

[0016] In the pulse seed light generation unit and pulse seed light amplification unit: the first gain fiber and the second gain fiber serve as gain amplification media. To improve laser output efficiency, the activating ions in the gain fiber include, but are not limited to, rare earth ions (Er). 3+ Ho 3+ Tm 3+ Dy 3+ Yb 3+ The matrix materials in the gain fiber include, but are not limited to, oxides (such as sapphire, garnet, alumina, sulfur oxides) and fluorides (such as ZBLAN, ZrF4, InF3, LiYF4, KY3F). 10 Vanadates, tellurates, or sulfides, etc.; the parameters of the first gain fiber and the second gain fiber can be the same or different.

[0017] The saturable absorbent material coated on the reflective surface of the total internal reflection cavity mirror is mainly rare earth ion-doped nanocrystal particles (the rare earth ion doping element is Er). 3+ Ho 3+ Tm 3+ Dy 3+ Yb 3+ (etc.), whose absorbable center wavelength corresponds to the seed light wavelength, and has nonlinear absorption characteristics for mid-infrared seed light, for example, Dy 3+ The ion exhibits high absorption in the 3 μm band, and its absorption spectrum is similar to that of Er. 3+ Ions or Dy 3+ The emission spectra of ion-doped fiber lasers can highly overlap. Utilizing this characteristic, Dy... 3 + Ion-doped nanocrystal particles, acting as saturable absorbers, enable high-efficiency pulse output in 3μm-band fiber lasers. The rare-earth ion-doped nanocrystal particles contain, but are not limited to, nano-oxide crystals (such as sapphire, garnet, alumina, and sulfur oxides) and nano-fluoride crystals (such as ZBLAN, ZrF4, InF3, LiYF4, and KY3F). 10The rare earth ions are one or more combinations of nano-vanadate crystals, etc.; the doping concentration of rare earth ions is 0.01 mol% to 80 mol%. The preparation methods include, but are not limited to, physical pulverization, mechanical ball milling, vapor deposition, sol-gel method, and vapor-phase combustion synthesis method, etc., with a size of 1 nm (nanometer) to 100 μm (micrometer).

[0018] To improve the light absorption efficiency of the seeds, the coating methods for the nanocrystal particles on the reflective surface of the total reflection cavity mirror include, but are not limited to, direct pick-up, suspension spin coating, adsorption, drop coating, mechanical exfoliation, photodeposition, etc., with a coating thickness of 10 nm (nanometer) to 100 μm (micrometer).

[0019] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0020] 1. This invention employs a beam splitter based on an optically rotatable crystal to cascade a pulse seed light generation unit and a pulse seed light amplification unit into a single system. Based on a polarizer, an optically rotatable crystal, and a partially reflective cavity mirror at the input end of the pulse seed light generation unit, pump light generated by a mature semiconductor laser can simultaneously pump both the seed light generation unit and the seed light amplification unit. This achieves highly efficient utilization of the pump light and eliminates the need for a complex seed light amplification system after the seed light generation unit, resulting in a simple structure that facilitates system integration.

[0021] 2. This invention uses novel rare-earth ion-doped nanocrystal particles as saturable absorbers. By utilizing the fact that the nonlinear absorption band of the nanocrystal particles coincides with the seed light band, saturable absorption in the mid-infrared band is cleverly achieved. Moreover, the nanocrystals have similar characteristics to the rare-earth ion-doped fluoride fiber materials commonly used in the mid-infrared band, exhibiting high compatibility and filling the gap in efficient Q-switched pulse output of fiber lasers in the mid-infrared band.

[0022] 3. The saturable absorber of the present invention is a rare earth ion-doped nanocrystal particle. By adjusting parameters such as the rare earth ion doping concentration, particle diameter, and film thickness in the nanocrystal particle, the absorption characteristics of the saturable absorber can be controlled, further improving the modulation index and saturation threshold of the saturable absorber, which is beneficial to achieving stable and efficient high-power laser pulses.

[0023] 4. In this invention, rare earth ion-doped nanocrystal particles are formulated into a suspension and coated onto the reflective surface of a total reflection cavity mirror as a saturable absorber, which reduces the complexity of the optical path and improves the integration and coupling efficiency of the mid-infrared fiber laser. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the passive Q-switched mid-infrared fiber laser system based on rare-earth ion-doped nanocrystal particles according to the present invention.

[0025] Figure 2 This is a schematic diagram of the optically rotating crystal of the optically dispersive device unit in this invention.

[0026] Figure 3 This is a schematic diagram of the polarizer of the beam-splitting device unit in this invention.

[0027] In the diagram, the following labels are used: I is the pump light unit; II is the beam splitter unit; III is the pulse seed light generation unit; IV is the pulse seed light amplification unit; 1 is the pump laser; 2 is the collimating lens; 3 is the polarizer; 4 is the optical rotator crystal; 5 is the first coupling lens III; 6 is the partial reflection cavity mirror; 7 is the first gain fiber; 8 is the total reflection cavity mirror; 9 is the second coupling lens; 10 is the second gain fiber; and 11 is the bandpass filter. Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1 As shown, a passively Q-switched mid-infrared pulsed fiber laser based on rare-earth ion-doped nanocrystal saturable absorbers includes a pump beam unit I, a beam splitter unit II, a pulse seed light generation unit III, and a pulse seed light amplification unit IV. Specifically: Pump beam unit I includes a pump laser 1 and a collimating lens 2; beam splitter unit II includes a polarizer 3 and an optically rotating crystal 4; pulse seed light generation unit III includes a first coupling lens 5, a partial reflection cavity mirror 6, a first gain fiber 7, and a total reflection cavity mirror 8; pulse seed light amplification unit IV includes a second coupling lens 9, a second gain fiber 10, and a bandpass filter 11. Wherein:

[0030] Pump light unit I: Pump laser 1 provides pump light, which is fully polarized linearly polarized light with a wavelength shorter than that of seed light; collimating lens 2 can collimate the pump light into quasi-parallel light.

[0031] Beam splitter unit II: Polarizer 3 transmits light with the same polarization direction as polarizer 3 and reflects light with a polarization direction orthogonal to polarizer 3; Optical rotator crystal 4 has a cross-section of a parallelogram with a vertex angle of 45°; Optical rotator crystal 4 can rotate the polarization direction of light passing through optical rotator crystal 4 by 45°, regardless of the incident direction. The pump light output from pump light unit I has the same polarization direction as polarizer 3, thus it can pass through polarizer 3 and enter optical rotator crystal 4. After passing through optical rotator crystal 4, its polarization direction is rotated by 45° and enters pulse seed light generation unit III; Partial reflection cavity mirror 6 in pulse seed light generation unit III reflects part of the pump light, causing it to be transmitted in the opposite direction into optical rotator crystal 4. At this time, the polarization direction of the reflected pump light will be rotated by 45° again, so that the polarization direction of the pump light is orthogonal to polarizer 3, and it is totally internally reflected into pulse seed light amplification unit IV. The pulse seed light generated in pulse seed light generation unit III is transmitted in the opposite direction into optical rotator crystal 4, its polarization direction is rotated by 45°. At this time, the polarization direction of the seed light is orthogonal to polarizer 3, and it is totally internally reflected into pulse seed light amplification unit IV.

[0032] Pulse Seed Light Generation Unit III: The combination of a partial reflection cavity mirror 6, a first gain fiber 7, and a total reflection cavity mirror 8 constitutes a laser resonant cavity; the first coupling lens 5 couples the pump light into the first gain fiber; the partial reflection cavity mirror 6 splits the pump light, with one part entering the pulse seed light generation unit III and the other part entering the pulse seed light amplification unit IV; the core of the gain fiber 7 is doped with a high concentration of gain ions, which have high absorption characteristics for the pump light, forming population inversion, and under certain conditions, stimulated emission generates seed light in the mid-infrared band; the total reflection cavity mirror 8, coated with a rare earth ion-doped nanocrystal particle suspension film, acts as a saturable absorber, playing a passive Q-switching role, and by absorbing the seed light released by the energy level particle transitions on the fiber gain medium, it can keep the resonant cavity in a high-loss, low-Q state. When the gain medium inversion particle number approaches its maximum value, the material's absorption of the seed light also reaches saturation, the absorption rate drops sharply, the optical cavity loss decreases significantly, the Q value increases, the Q switch opens, the threshold inversion particle number decreases, and a large number of upper energy level particles avalanche-like transition to lower energy levels, resulting in a giant pulse with extremely high power.

[0033] Pulse seed light amplification unit IV: The second coupling lens 9 couples the reflected pump light and seed light into the second gain fiber (10); After the second gain fiber (10) absorbs the pump light, it accumulates a large number of inverted particles. When the seed light passes through, due to stimulated emission, a large number of upper energy level particles transition to lower energy levels, which amplifies the seed light that passes through; The bandpass filter 11 filters out the short-wavelength pump light and only allows specific wavelength laser output in the mid-infrared band.

[0034] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A passively Q-switched mid-infrared fiber laser based on rare-earth ion-doped nanocrystal particles, characterized in that: It includes a pump light unit (I), a beam splitter unit (II), a pulse seed light generation unit (III), and a pulse seed light amplification unit (IV). The pump light unit (I) includes a pump laser (1) and a collimating lens (2); the beam splitting unit (II) includes a polarizer (3) and an optically rotating crystal (4); the pulse seed light generation unit (III) includes a first coupling lens (5), a partial reflection cavity mirror (6), a first gain fiber (7), and a total reflection cavity mirror (8); the pulse seed light amplification unit (IV) includes a second coupling lens (9), a second gain fiber (10), and a bandpass filter (11); the reflective surface of the total reflection cavity mirror (8) is coated with rare earth ion-doped nanocrystal particles, serving as a passively Q-switched saturable absorber; In the pump light unit (I): the pump laser (1) outputs pump light, and the pump light is fully polarized linearly polarized light with a wavelength smaller than that of the seed light; the collimating lens (2) collimates the pump light into quasi-parallel light; In the beam splitting unit (II): the polarizer (3) transmits light with the same polarization direction as the polarizer (3) and reflects light with the polarization direction orthogonal to the polarizer (3); the optical rotator crystal (4) has a cross-section of a parallelogram with a vertex angle of 45°; the optical rotator crystal (4) can rotate the polarization direction of light passing through the optical rotator crystal (4) by 45°, and it is independent of the incident direction; In the pulse seed light generation unit (III): the partial reflection cavity mirror (6), the first gain fiber (7), and the total reflection cavity mirror (8) are combined to form a laser resonant cavity; the first coupling lens (5) couples the input pump light into the resonant cavity; the partial reflection cavity mirror (6) reflects part of the pump light along the original optical path into the optical rotator crystal (4), and transmits part of it into the first gain fiber (7); under the continuous action of the pump light, the activated ions in the first gain fiber (7) transition from the ground state energy level to the upper laser energy level, and generate seed light through stimulated emission; in the total reflection cavity mirror (8), the nonlinear absorption band of rare earth ions in the saturable absorber corresponds to the seed light band, and has saturable absorption characteristics in the mid-infrared band, realizing the pulse output of the seed light; the partial reflection cavity mirror (6) reflects part of the seed light and transmits part of it into the optical rotator crystal (4). In the pulsed seed light amplification unit (IV): the pump light and seed light reflected by the partially reflecting cavity mirror (6) pass through the optical rotator crystal (4) and are reflected by the polarizer (3) into the pulsed seed light amplification unit (IV); the second coupling lens (9) couples the pump light and seed light into the second gain fiber (10); the second gain fiber (10) serves as the gain amplification medium to realize the amplification of the mid-infrared seed light; The light filter (11) filters out the pump light, allowing the seed light to pass through; In the pulse seed light generation unit (III) and pulse seed light amplification unit (IV): the activation ions in the first gain fiber (7) and the second gain fiber (10) are each independently selected from one or more combinations of rare earth ions, actinide ions and transition metals, and the matrix materials are each independently selected from oxides, fluorides, vanadates, tellurates or sulfides; the parameters of the first gain fiber (7) and the second gain fiber (10) can be the same or different. The saturable absorbable material coated on the reflective surface of the total reflection cavity mirror (8) is rare earth ion-doped nanocrystal particles. Its absorbable center wavelength corresponds to the seed light wavelength, and it has nonlinear absorption characteristics for mid-infrared seed light.

2. The passively Q-switched mid-infrared fiber laser based on rare-earth ion-doped nanocrystal particles according to claim 1, characterized in that: The rare earth ion-doped nanocrystal particles have a nanocrystal matrix material that is at least one of nano-oxide crystals, nano-fluoride crystals, and nano-vanadate crystals, and the doping concentration of rare earth ions is 0.01 mol% to 80 mol%; the size of the rare earth ion-doped nanocrystal particles is 1 nm to 100 µm.

3. The passively Q-switched mid-infrared fiber laser based on rare-earth ion-doped nanocrystal particles according to claim 1, characterized in that: The coating thickness of the rare earth ion-doped nanocrystal particles on the reflective surface of the total reflection cavity mirror (8) is 10 nm to 100 µm.

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