High-efficiency erbium-ytterbium co-doped fiber laser for long-wavelength peak shifting pumping

Abstract

The invention discloses a high-efficiency erbium-ytterbium co-doped fiber laser for long-wavelength peak shifting pumping. The laser is characterized in that pump light enters an active fiber through a pump coupling device; the active optical fiber is doped with erbium ions and ytterbium ions, part of the pump light entering the active optical fiber is absorbed by the ytterbium ions, and part of the pump light entering the active optical fiber is absorbed by the erbium ions; ytterbium ions are transited from a ground state 2F7 / 2 to an excited state 2F5 / 2 energy level, cross relaxation occurs between the ytterbium ions and ground state erbium ions nearby the ytterbium ions, energy is transferred to the erbium ions, and the erbium ions are relaxed to a 4I13 / 2 energy level after transition from 4I15 / 2 to 4I11 / 2 is completed; erbium ions absorb pump light, transition from a ground state 4I15 / 2 to a 4I11 / 2 energy level and then relaxation to a 4I13 / 2 energy level are carried out, the erbium ions of the 4I13 / 2 energy level generate laser gain of 1.5 microns, and when the gain exceeds loss of a laser resonant cavity formed by the first resonant cavity feedback device and the second resonant cavity feedback device, 1.5 microns-band laser oscillation is formed and is output through the second resonant cavity feedback device. The influence of amplified spontaneous emission of ytterbium ions and excited state absorption of erbium ions on the efficiency of the laser is weakened, and the efficiency of the laser is improved.

Description

Technical Field

[0001] The present invention relates to the field of lasers, and in particular to a high-efficiency erbium-ytterbium co-doped fiber laser with long-wavelength peak-shifting pumping. Background Art

[0002] Based on erbium ions 4 I 13 / 2 → 4 I 15 / 2 Erbium-doped fiber lasers with luminescent transitions are one of the main technical approaches to generate 1.5μm band lasers. Erbium-doped fiber lasers can be pumped by mature and economical 980nm semiconductor lasers (LDs), that is, from the ground state 4 I 15 / 2 Absorbing pump photons and transitioning to excited states 4 I 11 / 2 , followed by nonradiative relaxation to the upper laser level 4 I 13 / 2 , forming laser gain [1] There are two main factors that limit the performance of erbium-doped fiber lasers: erbium ions 4 I 15 / 2 → 4 I 11 / 2 The stimulated absorption cross section of the pump absorption transition is very small, and tens of meters of active optical fiber are often required to achieve relatively sufficient pump absorption, which seriously limits the efficiency, cost and compactness of the laser.

[0003] To solve the problem of low pump absorption of erbium-doped fiber, the current 1.5μm band fiber lasers mostly use erbium-ytterbium co-doped fiber as the active fiber: ytterbium ions have a high stimulated absorption cross section for pump light with a peak absorption wavelength of 976nm, and only a short fiber is needed to fully absorb the pump light, from the ground state 2 F 7 / 2 Transition to excited state 2 F 5 / 2 energy level, and then the energy is transferred from the excited state to the 2 F 5 / 2 The ytterbium ion is transferred to the ground state 4 I 15 / 2 of erbium ions, so that the erbium ions complete 4 I 15 / 2 → 4 I 11 / 2 Relaxation after the transition 4 I 13 / 2energy level to generate laser gain in the 1.5μm band. This method greatly simplifies the structure of 1.5μm band fiber lasers and is currently the most commonly used method. However, there are also obvious problems with erbium-ytterbium co-doped fiber lasers. On the one hand, when the pump power is high, there are many ytterbium ions in the excited state, and the energy transfer rate to the erbium ions is limited, making it difficult to quickly deplete the excited ytterbium ions. The excited ytterbium ions will produce strong 1μm band amplified spontaneous emission (ASE), and even form lasing in severe cases, resulting in a waste of pump energy and hindering further improvement of the laser power. On the other hand, erbium ions exist at a wavelength of 970nm. 4 I 11 / 2 → 4 F 7 / 2 The excited state absorption (ESA) peak is still quite significant for 976nm pump light, resulting in wasted pump energy. Due to these factors, the efficiency of 976nm pumped Er / Yb co-doped fiber lasers is mostly below 40%, and the output power can only reach around 300W.

[0004] To solve the above-mentioned problems of ytterbium ion ASE and lasing, researchers have reported erbium-ytterbium co-doped fiber lasers pumped by 940nm and 915nm equal absorption sub-peaks. [2,3] . By adopting a staggered pump wavelength with a relatively low absorption coefficient, the absorption of pump light by ytterbium ions is no longer too concentrated, and the ASE and lasing of ytterbium ions in the 1μm band are effectively suppressed, which improves the utilization rate of pump light energy. The slope efficiency of the 1.5μm band laser reaches 50.5% and 56%, respectively, which is a significant improvement over the traditional 976nm pumping method. A large proportion of the current commercial erbium-ytterbium co-doped fiber lasers have adopted the 940nm pumping scheme. However, 915nm and 940nm pumping are obviously not the best pumping methods for erbium-ytterbium co-doped fiber lasers. Due to the short pump wavelength, the large quantum loss limits the efficiency of the laser. The excitation of erbium ions is completely dependent on ytterbium ions. 2 F 5 / 2 to erbium ions 4 I 11 / 2 The non-100% quantum efficiency of the cross-relaxation process also has a negative impact on the efficiency of the laser.

[0005] References

[0006] [1]H.Lin, et al, "656W Er-doped, Yb-free large-core fiber laser," Opt.Lett.43(13):3080-3083, 2018.

[0007] [2] D. Creeden, et al, "Single frequency 1560nm Er:Yb fiber amplifier with 207W output power and 50.5% slope efficiency," Proc.SPIE 9728,97282L, 2016.

[0008] [3] T.Matniyaz, et al, "302W single-mode power from an Er / Yb fiberMOPA," Opt.Lett.45(10):2910-2913, 2020. Summary of the Invention

[0009] The present invention provides a high-efficiency erbium-ytterbium co-doped fiber laser with long-wavelength peak-shifted pumping. The present invention utilizes pump light with a wavelength longer than the 976nm absorption peak of ytterbium ions to pump the erbium-ytterbium co-doped fiber laser, thereby avoiding ASE and lasing in the 1μm band caused by excessive absorption of the pump light by the ytterbium ions. Simultaneously, the lower quantum defect of the long pump wavelength also helps improve the efficiency of the laser. Furthermore, part of the pump light in this wavelength band is also directly absorbed by the erbium ions, thereby increasing both pump absorption and pump quantum efficiency. The longer-wavelength pump light also avoids the excited-state absorption peak of the erbium ions, reducing the impact of excited-state absorption on the laser efficiency and improving the laser efficiency. Details are described below:

[0010] A high-efficiency erbium-ytterbium co-doped fiber laser with long-wavelength peak-shifting pumping, the laser comprising:

[0011] The pump light enters the active optical fiber through the pump coupling device; the active optical fiber is doped with erbium ions and ytterbium ions, the pump light entering the active optical fiber is absorbed by the ytterbium ions and partially absorbed by the erbium ions; the ytterbium ions are absorbed from the ground state 2 F 7 / 2 Transition excited state 2 F 5 / 2 energy level, cross-relaxes with the ground state erbium ions nearby, transferring energy to the erbium ions, allowing the erbium ions to complete 4 I 15 / 2 → 4 I 11 / 2 After the transition, it relaxes to the upper energy level of the 1.5μm laser 4 I 13 / 2 ;

[0012] The erbium ion absorbs the pump light and changes from the ground state to 4 I 15 / 2 Jump to 4 I 11 / 2 After relaxing to 4I 13 / 2 , 4 I 13 / 2 The erbium ions at the energy level generate laser gain in the 1.5μm band. When the gain exceeds the loss of the laser resonant cavity composed of the first resonant cavity feedback device and the second resonant cavity feedback device, 1.5μm band laser oscillation is formed and output through the second resonant cavity feedback device.

[0013] The pump source emits pump light with a wavelength of more than 980 nm, which is significantly longer than the ytterbium ion absorption peak of 976 nm. The pump light enters the active optical fiber through the pump coupling device.

[0014] The first resonant cavity feedback device is highly transparent to pump light and highly reflective to 1.5 μm band laser light; the second resonant cavity feedback device is partially transparent to 1.5 μm band laser light.

[0015] Furthermore, the pump source is a semiconductor laser, or a fiber laser, or a solid-state laser, the laser mode is a fundamental transverse mode, or a multi-transverse mode, and the emission wavelength is longer than the ytterbium ion absorption peak of 976nm, and is within the absorption band of ytterbium ions and erbium ions, and can be absorbed by both to generate laser gain.

[0016] Wherein, the erbium-ytterbium co-doped active optical fiber is a single-mode optical fiber, or a multi-mode optical fiber, or a single-clad optical fiber, or a double-clad optical fiber.

[0017] Furthermore, the first resonant cavity feedback device and the second resonant cavity feedback device are fiber gratings, or coated reflective mirrors and diffraction gratings or volume gratings.

[0018] The pump coupling device is a fiber wavelength division multiplexer or a signal pump combiner, which adopts direct fusion coupling or spatial optical path focusing coupling.

[0019] The beneficial effects of the technical solution provided by the present invention are:

[0020] 1) Peak-shifted pumping is used for Er-Yb co-doped fiber to avoid the problem of ytterbium ions absorbing the pump light too strongly, which leads to the accumulation of upper-level ytterbium ions and triggers ASE and lasing in the 1μm band. This improves the signal-to-noise ratio, improves the conversion efficiency of the laser, and increases the upper limit of the laser output power, which is limited by ASE and lasing factors in the 1μm band.

[0021] 2) The pump is simultaneously within the absorption bands of erbium and ytterbium ions. Peak-shifted pumping weakens the absorption of ytterbium ions, allowing part of the pump light to be directly absorbed by erbium ions, which excite them to the upper laser energy level to generate laser gain without going through the ytterbium ion absorption and cross-relaxation process, thereby improving the quantum efficiency of the pump process.

[0022] 3) Under the long-wavelength staggered pumping mode, the pump light deviates further from the ESA peak of erbium ions, weakening the ESA process, avoiding the waste of pump energy, and improving the pump quantum efficiency;

[0023] 4) Long-wavelength off-peak pumping has smaller quantum loss than sub-peak pumping such as 940nm and 915nm, which helps improve the efficiency of the laser;

[0024] 5) Thanks to the improvement of quantum efficiency and the reduction of quantum loss, the laser is more efficient and generates less heat, which helps to achieve higher laser output power. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the energy levels and pumping process of erbium and ytterbium ions in erbium-ytterbium co-doped optical fiber;

[0026] Figure 2 Schematic diagram of the optical path of a high-efficiency Er-Yb co-doped fiber laser with long-wavelength peak-shifted pumping;

[0027] Figure 3 Schematic diagram of the ground state absorption of ytterbium ions and the ground state absorption and excited state absorption spectra of erbium ions in erbium-ytterbium co-doped optical fiber;

[0028] Figure 4 Another optical path schematic diagram of a high-efficiency erbium-ytterbium co-doped fiber laser with long-wavelength peak-shifting pumping.

[0029] In the accompanying drawings, the components represented by the reference numerals are as follows:

[0030] 1: First pump source; 2: Pump coupling device;

[0031] 3: First resonant cavity feedback device; 4: First Erbium-Ytterbium co-doped active optical fiber;

[0032] 5: Second resonant cavity feedback device; 6: First cladding mode filter;

[0033] 7: seed source; 8: isolator;

[0034] 9: Second pump source; 10: Signal / pump combiner;

[0035] 11: Second Erbium-Ytterbium co-doped active optical fiber; 12: Second cladding mode filter. DETAILED DESCRIPTION

[0036] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0037] Example 1

[0038] The embodiment of the present invention provides a high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifting pumping, such as Figure 2 As shown, the laser adopts an oscillator structure, including: a first pump source 1, a pump coupling device 2, a first resonant cavity feedback device 3, a first erbium-ytterbium co-doped active fiber 4, a second resonant cavity feedback device 5, and a first cladding mode filter 6;

[0039] The first pump source 1 is a wavelength-stabilized semiconductor laser with a wavelength of 981 nm, fiber-coupled output, a pigtail core diameter of 105 μm, and a single pump source maximum power of 140 W; the pump coupling device 2 is a fiber-pumped combiner; the first resonant cavity feedback device 3 is a high-reflection fiber Bragg grating with a reflectivity of >99% at a wavelength of 1560 nm; the first erbium-ytterbium co-doped active fiber 4 has a core and inner cladding diameter of 25 μm / 400 μm respectively (hereinafter referred to as "25 / 400" for this size fiber). The invention relates to a double-clad erbium-ytterbium co-doped optical fiber ("optical fiber"), with a core erbium ion doping concentration of 0.2 atomic percent and a ytterbium ion doping concentration of 2 atomic percent, and a length of 5 meters. The second resonant cavity feedback device 5 is a low-reflection fiber Bragg grating (FBG) with a reflectivity of 30% (transmittance of ∼70%) at a wavelength of 1560 nm. The fiber specifications of the pump coupling device 2, the first resonant cavity feedback device 3, the second resonant cavity feedback device 5, and the first cladding mode filter 6 all match those of the first erbium-ytterbium co-doped active optical fiber 4. The above-mentioned devices are fusion-connected to each other.

[0040] The 981nm pump light emitted by the first pump source 1 is coupled into the cladding of the first erbium-ytterbium co-doped active optical fiber 4 through the pump coupling device 2, and is transmitted in the cladding. When passing through the fiber core, it is absorbed by the ytterbium ions and erbium ions therein; the pump light absorbed by the ytterbium ions excites the ytterbium ions to 2 F 5 / 2 energy level, and then through cross relaxation, the erbium ion is excited to 4 I 11 / 2 After relaxation 4 I 13 / 2 The pump light absorbed by the erbium ions directly excites the erbium ions to the upper laser level. 4 I 11 / 2 After relaxation 4 I 13 / 2 ;Laser upper energy level 4 I 13 / 2 The erbium ions generate laser gain in the 1.5μm band, and under the feedback of the first resonant cavity feedback device 3 and the second resonant cavity feedback device 5, a 1560nm wavelength laser oscillation is formed, which is output through the second resonant cavity feedback device 5, and the residual pump light is filtered out by the first cladding mode filter 6.

[0041] Since the 981nm pump wavelength deviates from the absorption peak of ytterbium ions, the absorption coefficient decreases, such as Figure 3As shown, the absorption coefficient of ytterbium ions at 981nm is only about half that of 976nm. This avoids the ASE and lasing issues in the 1μm band caused by upper-level ytterbium ion accumulation, improves the output light signal-to-noise ratio, and simultaneously increases laser efficiency while avoiding the power limitation of lasing in the 1μm band. Because some of the 981nm pump light is directly absorbed by the erbium ions, the cross-relaxation process is omitted, improving the pump quantum efficiency. At the same time, the 981nm pump light deviates from the ESA absorption peak of the erbium ions, and the ESA absorption cross section decreases by ~30% relative to the absorption peak. This weakens the pump light ESA, further improving the pump quantum efficiency and contributing to improved laser efficiency. Compared to other off-peak pump wavelengths such as 915nm and 940nm, the 981nm pump light has longer photon energy, lower photon energy, and smaller quantum loss, which also contributes to improved laser efficiency. Under the parameters of this embodiment, the laser oscillator can generate 151 W of 1560 nm laser output at a pump power of 270 W, and the slope efficiency reaches 58%, which is close to the efficiency upper limit of 62.8% determined by quantum defect.

[0042] The first resonant cavity feedback device 3 and the second resonant cavity feedback device 5 can be fiber gratings, or other feedback devices such as coated reflectors, diffraction gratings, or volume gratings, as long as they can provide feedback to the laser.

[0043] Among them, the pump coupling device 2 can be a fiber wavelength division multiplexer (WDM) or a signal pump combiner. It can also adopt a direct fusion coupling method or a spatial optical path focusing coupling method. The corresponding coupling method and device are selected according to the form and transverse mode of the pump source. The laser can be an all-fiber structure or a spatial optical path structure.

[0044] Example 2

[0045] The embodiment of the present invention provides a high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifting pumping, such as Figure 4 As shown, the laser adopts a master oscillator-power amplifier (MOPA) structure, including: a seed source 7, an isolator 8, a second pump source 9, a signal / pump combiner 10, a second erbium-ytterbium co-doped active fiber 11, and a second cladding mode filter 12;

[0046] The seed source 7 is a pre-amplified single-longitudinal-mode fiber laser with a wavelength of 1550 nm and a power of 5 W. After passing through the isolator 8 and the signal / pump combiner 10, it enters the core of the second erbium-ytterbium co-doped active fiber 11. The second pump source 9 is a wavelength-stabilized semiconductor laser with a wavelength of 981 nm, fiber-coupled output, a pigtail core diameter of 105 μm, and a maximum power of 140 W per pump source. The emitted pump light passes through the signal / pump combiner 10 and enters the cladding of the second erbium-ytterbium co-doped active fiber 11, pumping it. The pigtail specifications of the seed source 7, isolator 8, second pump source 9, signal / pump combiner 10, second erbium-ytterbium co-doped active fiber 11, and second cladding mode filter 12 are all matched. These components are fusion-connected.

[0047] The ytterbium ions and erbium ions in the second erbium-ytterbium co-doped active fiber 11 absorb the pump light, and the erbium ions are excited to the laser upper energy level. 4 I 13 / 2 , generating laser gain, which is extracted by the fiber core seed light, resulting in high-power single-longitudinal-mode 1550nm laser output. Due to the advantages of the aforementioned staggered pumping method in terms of ytterbium ion ASE and lasing, erbium ion ground-state absorption and excited-state absorption, and quantum loss, the laser's efficiency and signal-to-noise ratio are effectively improved. At 140W pump power, it can generate 78W of signal light output, with a slope efficiency of 55%. The optical signal-to-noise ratio relative to 1μm band ASE is improved by 9dB and 4dB compared to 976nm and 940nm pumping, respectively.

[0048] In the above embodiment, the second pump source 9 can be a semiconductor laser, or a fiber laser, a solid-state laser, or other forms of laser, and its laser mode can be a fundamental transverse mode or a multi-transverse mode. The pump wavelength can be 981 nm, or other wavelengths within the absorption bands of ytterbium ions and erbium ions, as long as its emission wavelength is significantly longer than the ytterbium ion absorption peak of 976 nm and can be absorbed by both to generate laser gain.

[0049] The second erbium-ytterbium co-doped active optical fiber 11 can be a single-mode optical fiber or a multi-mode optical fiber, a single-clad optical fiber or a double-clad optical fiber. The type of optical fiber and the corresponding pumping method are selected according to the required power, as long as it can provide 1.5μm band laser gain and form laser output.

[0050] Unless otherwise specified, the models of the components in the embodiments of the present invention are not limited to the models and specifications of other components, including the size, numerical aperture, length, and doping concentration of the optical fiber. Any component that can perform the above functions may be used.

[0051] Those skilled in the art will understand that the accompanying drawings are only a schematic diagram of a preferred embodiment, and the serial numbers of the embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0052] 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 in the scope of protection of the present invention.

Claims

1. A high-efficiency Erbium-Ytterbium co-doped fiber laser with long-wavelength peak-shifted pumping, characterized in that: The laser comprises: The pump light enters the active optical fiber through the pump coupling device; the active optical fiber is doped with erbium ions and ytterbium ions, and the pump light entering the active optical fiber is partially absorbed by the ytterbium ions and partially absorbed by the erbium ions; the ytterbium ions are absorbed from the ground state 2 F 7 / 2 Transition to excited state 2 F 5 / 2 energy level, cross-relaxes with the ground state erbium ions nearby, transferring energy to the erbium ions, allowing the erbium ions to complete 4 I 15 / 2 → 4 I 11 / 2 Relaxation after the transition 4 I 13 / 2 energy level; The erbium ion absorbs the pump light and changes from the ground state to 4 I 15 / 2 Jump to 4 I 11 / 2 After relaxing to 4 I 13 / 2 energy level, 4 I 13 / 2 The erbium ions at the energy level generate laser gain in the 1.5μm band. When the gain exceeds the loss of the laser resonant cavity composed of the first resonant cavity feedback device and the second resonant cavity feedback device, 1.5μm band laser oscillation is formed and output through the second resonant cavity feedback device.

2. The high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifted pumping according to claim 1, characterized in that: The pump source emits pump light with a wavelength of more than 980 nm and significantly longer than the ytterbium ion absorption peak of 976 nm. The pump light enters the active optical fiber through the pump coupling device.

3. The high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifted pumping according to claim 1, characterized in that: The first resonant cavity feedback device is highly transparent to pump light and highly reflective to 1.5 μm band laser light; the second resonant cavity feedback device is partially transparent to 1.5 μm band laser light.

4. The high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifted pumping according to claim 1, characterized in that: The pump source is a semiconductor laser, or a fiber laser, or a solid laser. The laser mode is a fundamental transverse mode, or multiple transverse modes. The emission wavelength is longer than the ytterbium ion absorption peak of 976nm and is within the absorption band of ytterbium ions and erbium ions.

5. The high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifted pumping according to claim 1, characterized in that: The erbium-ytterbium co-doped active optical fiber is a single-mode optical fiber, or a multi-mode optical fiber, or a single-clad optical fiber, or a double-clad optical fiber.

6. The high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifted pumping according to claim 1, characterized in that: The first resonant cavity feedback device and the second resonant cavity feedback device are fiber gratings, or coated reflective mirrors and diffraction gratings or volume gratings.

7. The high-efficiency Er-Yb co-doped fiber laser with long wavelength peak-shifted pumping according to claim 1, characterized in that: The pump coupling device is an optical fiber wavelength division multiplexer or a signal pump combiner, which adopts direct fusion coupling or spatial optical path focusing coupling.

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