An all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 microns

Abstract

The application discloses a kind of wavelength greater than 2.1 microns all-fiber short-cavity thulium-doped fiber laser oscillator, including sequentially connected fiber output 1.5 microns pulse pump source, 1.5 / 2 microns fiber wavelength division multiplexer and a section of non-polarization thulium-doped fiber, thulium-doped fiber both ends are engraved with a Bragg fiber grating, 1.5 microns pump light is generated by reverse, core pumping thulium-doped fiber reverse output dominant 2.1 microns above wavelength band laser signal, and along thulium-doped fiber positive single pass 1.8 microns broadband amplified spontaneous emission, the amplified spontaneous emission as a second reverse pump source, the gain of 2.1 microns above wavelength band is improved, and the pulse laser output greater than 2.1 microns is generated.The application structure is compact, simple, can replace holmium-doped fiber laser to realize wavelength greater than 2.1 microns high efficiency, high peak power laser output.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic information technology, and in particular to an all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers. Background Technology

[0002] The mid-infrared 2-micron laser technology is developing rapidly due to the growing demand from applications such as environmental monitoring, medical surgery, security, and military. In particular, the atmospheric transmittance at a distance of 23 kilometers within the 2.1-2.3 micron atmospheric window is higher than 90%. Therefore, applications such as remote sensing and free-space optical communication based on this atmospheric window band have a great demand for lasers with operating wavelengths greater than 2.1 microns.

[0003] Thulium-holmium co-doped and single-doped crystal lasers and fiber lasers are commonly used to generate laser output at wavelengths of 2.1 micrometers or longer. However, compared to solid-state crystal lasers, fiber lasers offer advantages such as better beam quality, more compact structure, and higher surface-to-volume ratio. In particular, within a high-phonon-energy quartz glass matrix, from Tm... 3+ Ionic 3 H4 energy level to Ho 3+ Ionic 5 The nonradiative relaxation that occurs during energy transfer at the I7 level significantly reduces the slant efficiency of thulium-holmium co-doped fiber lasers in wavelengths above 2.1 micrometers. Therefore, thulium-holmium co-doped fiber lasers are not an effective method for generating lasers in wavelengths above 2.1 micrometers. Meanwhile, the suitable pump wavelengths for single-holmium-doped fiber lasers are at 1.15 micrometers or 1.95 micrometers. Therefore, a high average power or high peak power thulium-doped fiber laser at 1.95 micrometers is a suitable pump source for holmium-doped fiber lasers.

[0004] On the other hand, thulium-doped silica fiber lasers can theoretically achieve laser output in the 2.1-2.3 micrometer wavelength range. Literature indicates that by utilizing thulium ions... 3 H6- 3 The reabsorption mechanism in the F4 transition has enabled ultralong wavelength laser output in the range of 2.1 to 2.2 micrometers in thulium-doped quartz fiber lasers [1-3]. Specifically, in the case of thulium ions... 3 H6- 3 In a two-level laser system with F4 transition, the ground state is excited by a 1.5-micron pump light. 3 The thulium ion of H6 transitions to an excited state. 3 F4, generally, is in an excited state. 3 Thulium ions in F4 return to their ground state by emitting broadband photons with a central wavelength of 1.8 μm and wavelengths ranging from 1.7 to 2.3 μm. 3H6; however, when the thulium doping concentration is high or the thulium-doped fiber length is sufficiently long, broadband photons with a center wavelength of 1.8 μm and wavelengths of 1.7-2.3 μm can be used as pumps to excite unexcited thulium ions in the fiber core to excited states. 3 F4, and continues to emit photons with longer wavelengths. Therefore, a higher thulium ion concentration and / or a longer thulium-doped fiber is beneficial for achieving laser output with a wavelength greater than 2.1 micrometers. Similarly, by using a shorter wavelength thulium-doped fiber laser near the 1.9 micrometer wavelength to cascade pump the next stage of thulium-doped fiber laser, a thulium-doped fiber laser output with a wavelength of 2.17 micrometers has also been achieved [4].

[0005] 1. ZS Sacks, Z. Schiffer, D. David, "Long wavelength operation ofdouble-clad Tm:silica fiber lasers," Proc. of SPIE Vol. 6453, 645320, (2007).

[0006] 2. J. Li, Z. Sun, H. Luo, Z. Yan, K. Zhou, Y. Liu, and L. Zhang, "Wide wavelength selectable all-fiber thulium doped fiber laser between 1925nm and 2200 nm," Opt. Express 22, 5387-5399 (2014).

[0007] 3. X. Jin,

[0008] 4. F. Liu, P. Liu, X. Feng, C. Wang, Z. Yan, and Z. Zhang, "Tandem-pumped, tunable thulium-doped fiber laser in 2.1 μm wavelength region," Opt.Express 27, 8283-8290 (2019).

[0009] The literature reported above describes continuous-output 2.1-micron ultra-long wavelength thulium-doped fiber laser oscillators. However, many applications (such as eye-safe coherent lidar, optical countermeasures, and mid-infrared nonlinear frequency conversion) require nanosecond pulse lasers with high pulse energy in the μJ-mJ range or high peak power of 100 W-10 kW. Furthermore, the 2.1-micron ultra-long wavelength thulium-doped fiber laser oscillators described above all utilize thulium-doped silica fibers with typical lengths of 5-10 m. When the peak output power of the fiber laser oscillator exceeds 100 watts, if the oscillator is based on a 5-10 meter long fiber gain medium, nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering accumulate significantly within the single-pass gain fiber, leading to a substantial decrease in laser output performance. Using a shorter thulium-doped gain fiber is an obvious solution, but achieving high laser output efficiency within a shorter single-pass gain fiber and reaching a peak laser output pulse power of several hundred watts to several thousand watts is the technical problem this invention aims to solve. Summary of the Invention

[0010] Purpose of the invention: The purpose of this invention is to provide an all-fiber short-cavity thulium-doped fiber laser oscillator that generates high peak power pulsed lasers with wavelengths greater than 2.1 micrometers.

[0011] Technical Solution: The present invention discloses an all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers, comprising a 1.5-micrometer pulse pump source with fiber output, a 1.5 / 2-micrometer fiber wavelength division multiplexer, and a section of non-polarization-maintaining thulium-doped fiber connected in sequence. A Bragg fiber grating is respectively inscribed at both ends of the thulium-doped fiber. The 1.5-micrometer pump light generates a laser signal with a reverse output dominant wavelength band of 2.1 micrometers and above by reverse-pumping the thulium-doped fiber, and generates 1.8-micrometer broadband amplified spontaneous emission in a single pass along the forward direction of the thulium-doped fiber. This amplified spontaneous emission serves as a second reverse pump source, enhancing the gain of the wavelength band above 2.1 micrometers and generating pulsed laser output with a wavelength greater than 2.1 micrometers.

[0012] Furthermore, the reflectivity of the Bragg fiber grating near the 1.5-micron pulse pump source is 20-89%, while the reflectivity of the Bragg fiber grating far from the 1.5-micron pulse pump source is 90-99.999%; the central Bragg wavelengths of the two Bragg fiber gratings coincide, located between 2100 nm and 2300 nm.

[0013] Furthermore, the 1.5-micron pump light output from the optical fiber enters the thulium-doped fiber core from the side of the low-reflectivity Bragg fiber grating via a reverse, core-pumped method.

[0014] Furthermore, the non-polarization-maintaining thulium-doped fiber has a circularly symmetric single-clad structure with a single-mode cutoff wavelength of less than 1.5 micrometers; the length of the non-polarization-maintaining thulium-doped fiber is 2-20 centimeters, and the absorption coefficient at the 1.5-micrometer pump light wavelength is 150-1000 dB / m, with the single-pass absorption of the thulium-doped fiber for the 1.5-micrometer pump light being greater than 3 dB.

[0015] Furthermore, the Bragg fiber gratings etched at both ends of the non-polarization-maintaining thulium-doped fiber are irradiated by a phase mask and a femtosecond laser. The laser irradiates the thulium-doped fiber from one side of the circularly symmetric single-clad structure, forming a non-circular, centrosymmetric photoinduced refractive index variation distribution along the radial direction of the thulium-doped fiber core, descending from the incident direction to the other side.

[0016] Furthermore, the output pulse width of the laser oscillator is smaller than the output pulse width of the 1.5-micron pulse pump source output from the optical fiber. The overall repetition frequency of the laser oscillator is the same as the repetition frequency of the nanosecond pump laser. The output laser pulse width of the laser oscillator is 1ns-20ns, and the output laser repetition frequency of the laser oscillator is 1kHz-950kHz.

[0017] Furthermore, the 1.5-micron pulse pump source output from the optical fiber has a pulse width of 10ns-950ns, a repetition frequency of 1kHz-950kHz, an average power of 0.1W-10.00W, and a laser wavelength of 1.55-1.63 microns.

[0018] Furthermore, the laser oscillator has a laser wavelength of 2.10 micrometers to 2.30 micrometers, an output laser pulse peak power of 1 W to 5000 W, and an output laser pulse single pulse energy of 0.1 μJ to 10 μJ.

[0019] Furthermore, the non-circular, centrosymmetric photoinduced refractive index variation distribution formed radially along the core of the thulium-doped fiber, descending from the incident direction to the other side, is excited by a fundamental Gaussian mode field with a centrally circularly symmetric intensity distribution at wavelengths above 2.1 micrometers after 1.5 micrometer pump light is input into the fiber resonator. This excites the fundamental mode Gaussian mode field with a centrally circularly symmetric intensity distribution at wavelengths above 2.1 micrometers. The interaction between this mode field and the non-circular, centrosymmetric photoinduced refractive index variation distribution of the fiber grating produces an approximately elliptical mode field biased towards one side of the circular core. This generates polarization selectivity with birefringence variations of different refractive indices in two orthogonal directions, thereby oscillating between two highly reflective fiber gratings to form a highly linearly polarized laser output with wavelengths above 2.1 micrometers and a polarization extinction ratio >15 dB.

[0020] Compared with the prior art, the significant advantages of this invention are as follows:

[0021] 1. This invention achieves high-efficiency, high-peak-power pulsed laser output with a wavelength greater than 2.1 micrometers in an optical fiber oscillator composed of ultrashort thulium-doped fiber, avoiding the negative impact on laser output performance caused by the accumulation of nonlinear effects in long-gain optical fibers.

[0022] 2. Compared to traditional holmium-doped fiber lasers with wavelengths greater than 2.1 micrometers, achieved by pumping with a 1.9-micrometer band thulium-doped fiber laser, this invention utilizes a 1.5-micrometer pumping method with a core-pumped, reverse-input thulium-doped fiber laser oscillator. This results in a more uniform distribution of excited thulium ion concentration along the length of the thulium-doped fiber, preventing excessive concentration of stimulated photons in the 1.8-1.95 micrometer band, where gain and output efficiency are highest. Furthermore, by employing a high-thulium-doped silica fiber and a high-reflectivity Bragg fiber grating pair, the excited thulium ions are further enhanced. The emission wavelength is further extended to the band above 2.1 micrometers; furthermore, the pump light simultaneously generates broadband amplified spontaneous emission with a center wavelength of 1.8 micrometers along the thulium-doped fiber, and serves as a second reverse pump source to enhance the gain of the band above 2.1 micrometers; in the given example, a laser output with a wavelength greater than 2.1 micrometers, an efficiency of 29%, and a peak power of nearly 400 watts was achieved, and its slant efficiency is comparable to and slightly greater than the slant efficiency (25%) of the nanosecond pulse holmium-doped fiber laser oscillator pumped by a 1.9 micrometer thulium-doped fiber laser pumped by a 1.5 micrometer laser [5].

[0023] [5] KS Wu, D. Ottaway, J. Munch, DG Lancaster, S. Bennetts, andS. D. Jackson, "Gain-switched holmium-doped fiber laser," Opt. Express 17, 20872-20877 (2009).

[0024] 3. Additionally, since the fiber grating region formed by unilateral illumination has a non-circular symmetric refractive index distribution, highly linearly polarized laser output can be achieved in an optical fiber oscillator composed of non-polarization-maintaining thulium-doped fiber. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to the present invention.

[0026] The figures are labeled as follows: 1. 1.5 μm nanosecond pump laser; 2. Fiber wavelength division multiplexer; 3. Thulium-doped gain fiber; 4a, 4b: Bragg fiber gratings inscribed at both ends of the Thulium-doped fiber (with coincident Bragg center wavelengths greater than 2.1 μm); 5a. Pulsed pump light output from the 1.5 μm pump laser; 5b. 1.5 μm pulsed pump light propagating forward into the Thulium-doped fiber laser resonator; 6a. Backward-dominant laser pulse with a wavelength greater than 2.1 μm generated within the Thulium-doped fiber laser resonator; 6b. Laser pulse with a wavelength greater than 2.1 μm obtained from the 2 μm backward-biased output of the wavelength division multiplexer; 7. Broadband amplified spontaneous emission propagating forward from a single-pass Thulium-doped fiber within the Thulium-doped fiber laser resonator, with a center wavelength of approximately 1.8 μm.

[0027] Figure 2 This is a schematic diagram of the asymmetric refractive index distribution along the fiber core radially of the fiber grating region formed in the fiber core of the all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers, formed by irradiating one side of a non-polarization-maintaining thulium-doped gain fiber with a femtosecond laser and a phase mask (see [reference]). Figure 2 (The dotted line in the above figure). When the Gaussian mode field of the fundamental mode (see...) Figure 2 When the laser beam (solid line in the above figure) propagates along a thulium-doped gain fiber, the mode field energy distributed symmetrically along the fiber cross-section and the grating region with an asymmetric refractive index distribution along the fiber core spatially overlap, forming a non-circular, centrally symmetrical, nearly elliptical laser field distribution with wavelengths above 2.1 micrometers (see...). Figure 2 (The bright area in the image below).

[0028] Figure 3The image shows the 2.113-micron reverse laser output spectrum of the all-fiber short-cavity thulium-doped fiber laser oscillator of the present invention under pumping conditions of an average pump power of 0.8 W for the absorbed 1.55-micron laser, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz.

[0029] Figure 4 The average output power and slope efficiency (29%) of the 2.113-micron reverse laser of the all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 microns in this invention are obtained under the average pump power of different absorptions of 1.55-micron laser (pump pulse width 50 nanoseconds, repetition frequency 50 kHz).

[0030] Figure 5 The image shows the pulse time-domain plot of the 2.113-micron reversed laser output of the all-fiber short-cavity thulium-doped fiber laser oscillator of the present invention, under pumping conditions of an average pump power of 0.8 W for the absorbed 1.55-micron laser, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz. Correspondingly, a 1.5-micron pump pulse is observed simultaneously at the reversed output.

[0031] Figure 6 The image shows the 2.113-micron laser output spectrum observed at the forward output of the all-fiber short-cavity thulium-doped fiber laser oscillator of this invention, under pumping conditions of an average pump power of 0.8 W for absorbed 1.55-micron laser, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz. Correspondingly, a broadband amplified spontaneous emission line of 1.8 microns is simultaneously observed at the forward output.

[0032] Figure 7 The image shows the pulse time-domain diagram of the 2.113-micron laser output from the forward output of the all-fiber short-cavity thulium-doped fiber laser oscillator of the present invention, under pumping conditions of an average pump power of 0.8 W for the absorbed 1.55-micron laser, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz. Correspondingly, a broadband amplified stimulated emission pulse of 1.8 microns is simultaneously observed at the forward output.

[0033] Figure 8 The trend of polarization extinction ratio of the 2.113-micron laser output was obtained by testing the all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 microns under different average pump power of 1.55-micron laser absorption (pump pulse width of 50 nanoseconds and repetition frequency of 50 kHz). Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention. The invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] A short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers includes a 1.5-micrometer pulsed pump source with fiber output, a 1.5 / 2-micrometer fiber wavelength division multiplexer, and a section of non-polarization-maintaining thulium-doped fiber connected in sequence. A Bragg fiber grating is inscribed at each end of the thulium-doped fiber. The 1.5-micrometer pump light generates a laser signal with a reverse output dominant wavelength above 2.1 micrometers by reverse-pumping the thulium-doped fiber. It also generates 1.8-micrometer broadband amplified spontaneous emission in a single pass along the forward direction of the thulium-doped fiber. This amplified spontaneous emission serves as a second reverse pump source, increasing the gain in the wavelength range above 2.1 micrometers and generating pulsed laser output with a wavelength greater than 2.1 micrometers.

[0036] The two Bragg fiber gratings have different reflectivities.

[0037] A 1.5-micron pump light enters the core of a thulium-doped fiber from the side of a low-reflectivity Bragg fiber grating; a 1.5-micron reverse pump light causes the excited thulium ions to be distributed more uniformly along the length of the thulium-doped fiber; the pump light simultaneously generates forward-propagating 1.8-micron broadband amplified spontaneous emission along the thulium-doped fiber, and serves as a second reverse pump source to enhance the gain in the wavelength band above 2.1 microns; ultimately, a laser output with a wavelength greater than 2.1 microns is generated.

[0038] Furthermore, the reflectivity of the Bragg fiber grating near the 1.5-micron pulse pump source is 20-89%, while the reflectivity of the Bragg fiber grating far from the 1.5-micron pulse pump source is 90-99.999%; the central Bragg wavelengths of the two Bragg fiber gratings coincide, located between 2100 nm and 2300 nm.

[0039] Furthermore, the 1.5-micron pump light output from the optical fiber enters the thulium-doped fiber core from the side of the low-reflectivity Bragg fiber grating via a reverse, core-pumped method.

[0040] Furthermore, the non-polarization-maintaining thulium-doped fiber has a circularly symmetric single-clad structure with a single-mode cutoff wavelength of less than 1.5 micrometers. The length of the non-polarization-maintaining thulium-doped fiber is 2-20 cm, and its absorption of 1.5-micrometer pump light is greater than 50%. The absorption coefficient at the 1.5-micrometer pump light wavelength is 150-1000 dB / m, and the single-pass absorption of thulium-doped fiber for 1.5-micrometer pump light is greater than 3 dB.

[0041] Furthermore, the Bragg fiber gratings etched at both ends of the non-polarization-maintaining thulium-doped fiber are irradiated by a phase mask and a femtosecond laser. The laser irradiates the thulium-doped fiber from one side of the circularly symmetric single-clad structure, forming a non-circular, centrosymmetric photoinduced refractive index variation distribution along the radial direction of the thulium-doped fiber core, descending from the incident direction to the other side.

[0042] Furthermore, the output pulse width of the laser oscillator is smaller than the output pulse width of the 1.5-micron pulse pump source output from the optical fiber. The overall repetition frequency of the laser oscillator is the same as the repetition frequency of the nanosecond pump laser. The output laser pulse width of the laser oscillator is 1ns-20ns, and the output laser repetition frequency of the laser oscillator is 1kHz-950kHz.

[0043] Furthermore, the 1.5-micron pulse pump source output from the optical fiber has a pulse width of 10ns-950ns, a repetition frequency of 1kHz-950kHz, an average power of 0.1W-10.00W, and a laser wavelength of 1.55-1.63 microns.

[0044] Furthermore, the laser oscillator has a laser wavelength of 2.10 micrometers to 2.30 micrometers, an output laser pulse peak power of 1 W to 5000 W, and an output laser pulse single pulse energy of 0.1 μJ to 10 μJ.

[0045] Furthermore, the non-circular, centrosymmetric photoinduced refractive index variation distribution formed radially along the core of the thulium-doped fiber, descending from the incident direction to the other side, is excited by a fundamental Gaussian mode field with a centrally circularly symmetric intensity distribution at wavelengths above 2.1 micrometers after 1.5 micrometer pump light is input into the fiber resonator. This excites the fundamental mode Gaussian mode field with a centrally circularly symmetric intensity distribution at wavelengths above 2.1 micrometers. The interaction between this mode field and the non-circular, centrosymmetric photoinduced refractive index variation distribution of the fiber grating produces an approximately elliptical mode field biased towards one side of the circular core. This generates polarization selectivity with birefringence variations of different refractive indices in two orthogonal directions, thereby oscillating between two highly reflective fiber gratings to form a highly linearly polarized laser output with wavelengths above 2.1 micrometers and a polarization extinction ratio >15 dB. Example 1

[0046] Reference Figure 1 It includes a 1.5-micron pulsed pump laser 1, a 1.5 / 2-micron fiber wavelength division multiplexer 2, a non-polarization-maintaining thulium-doped silica fiber 3 with a length of 2-20 cm, and fiber gratings 4a and 4b inscribed at both ends of the thulium-doped silica fiber 3.

[0047] The wavelength of the nanosecond pump laser 1 is located at 1.55 micrometers.

[0048] Among them, the pulse pump laser 1 has a maximum average output power of 2 watts, the repetition frequency can be arbitrarily adjusted in the range of 1kHz-1000kHz, and the pulse width is adjustable in the range of 10ns-1000ns. It is connected to the 1.5-micron port of the 1.5 / 2-micron fiber wavelength division multiplexer 2 by fusion splicing.

[0049] The 2-micron output end of the 1.5 / 2-micron fiber wavelength division multiplexer 2 is fused to a 2-20 cm long non-polarization-maintaining thulium-doped silica fiber 3. The end of the thulium-doped silica fiber 3 closer to the fiber wavelength division multiplexer 2 is a fiber grating 4a with lower reflectivity, and the end of the thulium-doped silica fiber 3 further away from the fiber wavelength division multiplexer 2 is a fiber grating 4b with higher reflectivity. Both the forward and reverse fiber output ends are output at an 8-degree angle.

[0050] Reference Figure 2 A schematic diagram showing the asymmetric refractive index distribution of the fiber grating region formed in the fiber core along the radial direction of the fiber core is obtained by irradiating one side of a non-polarization-maintaining thulium-doped gain fiber with a phase mask and an 800 nm femtosecond laser (see...). Figure 2 (The dotted line in the above figure). When the Gaussian mode field of the fundamental mode (see...) Figure 2 When the laser beam (solid line in the above figure) propagates along a thulium-doped gain fiber, the mode field energy distributed symmetrically along the fiber cross-section and the grating region with an asymmetric refractive index distribution along the fiber core spatially overlap, forming a non-circular, centrally symmetrical, nearly elliptical laser field distribution with wavelengths above 2.1 micrometers (see...). Figure 2 (The bright area in the image below). This non-circular, centrosymmetric, nearly elliptical laser field with wavelengths above 2.1 micrometers implies that lasers with wavelengths above 2.1 micrometers have a large birefringence coefficient and polarization selectivity in two orthogonal directions along the fiber cross-section.

[0051] Reference Figure 3 Under the pumping conditions of an average pump power of 0.8 W, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz, a 2.113 μm laser is output in reverse from a thulium-doped fiber laser oscillator. The laser has a center wavelength of 2.113 μm, an average power of 180 mW, and a single pulse energy of 3.6 μJ.

[0052] Reference Figure 4 Under the average pump power of 1.55 μm laser with different absorptions (pump pulse width of 50 nanoseconds and repetition frequency of 50 kHz), the average output power of 2.113 μm laser from the thulium-doped fiber laser oscillator in reverse direction shows a linear increase, with a slope efficiency of 29%.

[0053] Reference Figure 5Under the pumping conditions of an average pump power of 0.8 W for the absorbed 1.55 μm laser, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz, the pulse width of the 2.113 μm laser output from the thulium-doped fiber laser oscillator is 10 nanoseconds, and the corresponding peak pulse power is 360 W.

[0054] Reference Figure 6 Under the pumping conditions of an average pump power of 0.8 W, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz, the thulium-doped fiber laser oscillator outputs a 2.113 μm laser signal in the forward output spectrum, which also contains broadband amplified spontaneous emission with a center wavelength of 1.8 μm.

[0055] Reference Figure 7 Under pumping conditions of an average pump power of 0.8 W, a pump pulse width of 50 nanoseconds, and a repetition frequency of 50 kHz, the 2.113 μm laser signal in the forward output pulse signal of the thulium-doped fiber laser oscillator also contains broadband amplified spontaneous emission with a center wavelength of 1.8 μm. This is compared to a 1.5 μm pump pulse observed simultaneously at the forward output.

[0056] Reference Figure 8 The polarization extinction ratio of the 2.113-micron laser output from a thulium-doped fiber laser oscillator was analyzed under different average pump powers of a 1.55-micron laser with varying absorption (pump pulse width of 50 nanoseconds and repetition frequency of 50 kHz). The polarization extinction ratio was greater than 22 dB across the entire power level.

[0057] With the above configuration, this invention provides an all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers. The laser wavelength is 2.1-2.3 micrometers, the peak power of the output laser pulse is 1 W-5000 W, and the single-pulse energy is 0.1 μJ-10 μJ. This invention achieves high linear polarization, high peak power, and a fiber laser output with a wavelength greater than 2.1 micrometers, while simultaneously achieving a laser output efficiency approaching 30%, which is greater than or equal to the slope efficiency of 25% for a nanosecond pulsed 2.1-micrometer holmium-doped fiber laser currently pumped by a 1.5-micrometer laser-pumped 1.9-micrometer thulium-doped fiber laser.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0059] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited thereto. Various changes that can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention are all within the protection scope of the claims of the present invention.

Claims

1. A thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers, characterized in that, It includes a 1.5-micron nanosecond pulse pump source with fiber output, a 1.5 / 2-micron fiber wavelength division multiplexer, and a section of non-polarization-maintaining thulium-doped fiber connected in sequence. Non-polarization-maintaining thulium-doped optical fibers range in length from 2 to 20 centimeters and have an absorption coefficient of 150-1000 dB / m at a pump light wavelength of 1.5 micrometers. A Bragg fiber grating is inscribed at each end of the thulium-doped fiber, and the Bragg wavelengths at the center of the two Bragg fiber gratings coincide, located between 2100 nm and 2300 nm. A 1.5-micron pump light is used to generate a reverse-output dominant laser signal in the wavelength range above 2.1 microns by pumping a thulium-doped fiber in reverse and core direction. This signal then generates a 1.8-micron broadband amplified spontaneous emission along a thulium-doped fiber with a length of 2-20 cm in the forward direction. This amplified spontaneous emission serves as a second reverse pump source, enhancing the gain in the wavelength range above 2.1 microns and generating a pulsed laser output with a wavelength greater than 2.1 microns.

2. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 1, characterized in that, The reflectivity of Bragg fiber gratings near a 1.5-micron nanosecond pulse pump source is 20-89%, while that of Bragg fiber gratings far from a 1.5-micron nanosecond pulse pump source is 90-99.999%.

3. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 2, characterized in that, The 1.5-micron pump light output from the optical fiber enters the thulium-doped fiber core from the side of the low-reflectivity Bragg fiber grating via a reverse, core-pumped method.

4. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 1, characterized in that, Non-polarization-maintaining thulium-doped fiber has a circularly symmetric single-clad structure and a single-mode cutoff wavelength of less than 1.5 micrometers; the single-pass absorption of thulium-doped fiber for 1.5-micrometer pump light is greater than 3 dB.

5. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 4, characterized in that, The Bragg fiber gratings etched at both ends of a non-polarization-maintaining thulium-doped fiber are irradiated by a phase mask and a femtosecond laser. The laser irradiates the thulium-doped fiber from one side of the circularly symmetric single-clad structure, forming a non-circular, centrosymmetric photoinduced refractive index variation distribution along the radial direction of the thulium-doped fiber core, decreasing from the incident direction to the other side.

6. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 1, characterized in that, The output pulse width of the laser oscillator is smaller than the output pulse width of the 1.5-micron nanosecond pulse pump source output from the optical fiber. The overall repetition frequency of the laser oscillator is the same as that of the 1.5-micron nanosecond pulse pump source. The output laser pulse width of the laser oscillator is 1 ns-20 ns, and the output laser repetition frequency of the laser oscillator is 1 kHz-950 kHz.

7. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 6, characterized in that, The 1.5-micron nanosecond pulse pump source output from the optical fiber has a pulse width of 10ns-950ns, a repetition frequency of 1kHz-950kHz, an average power of 0.1W-10.00W, and a laser wavelength of 1.55-micron-1.63 microns.

8. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 7, characterized in that, The laser oscillator has a laser wavelength of 2.10 micrometers to 2.30 micrometers, a peak power of 1W to 5000W for the output laser pulse, and a single pulse energy of 0.1μJ to 10μJ.

9. The all-fiber short-cavity thulium-doped fiber laser oscillator with a wavelength greater than 2.1 micrometers according to claim 1, characterized in that, The non-circular, centrosymmetric photorefractive index variation distribution formed radially along the core of the thulium-doped fiber descends from the incident direction to the other side. After a 1.5-micron pump light is input into the fiber resonator, a fundamental Gaussian mode field with a centrally circularly symmetric intensity distribution and a wavelength above 2.1 microns is excited. This mode field interacts with the non-circular, centrosymmetric photorefractive index variation distribution of the fiber grating to generate an approximately elliptical mode field biased towards one side of the circular core. This produces polarization selectivity with birefringence variations of different refractive indices in two orthogonal directions, thereby oscillating between two highly reflective fiber gratings to form a highly linearly polarized laser output with a wavelength above 2.1 microns and a polarization extinction ratio >15 dB.

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