High-power ytterbium:erbium (Yb:Er) fiber laser system using a 1.02-1.06 μm cladding excitation method

Abstract

To provide a high-power efficient Yb:Er laser and amplifier characterized by low gain in the 1 μm wavelength range.SOLUTION: A fiber laser is configured with a double clad fiber with a core doped with ions of Erbium (Er+3) and Ytterbium (Yb+3). At least two spaced high and low reflection mirrors flank the core and define a resonant cavity therebetween. The fiber laser further includes a pump laser outputting light in the 1.02-1.06 μm wavelength range which is coupled into the Yb:Er doped double clad fiber. A fiber amplifier includes a double clad fiber with a core doped with ions of Erbium (Er+3) and Ytterbium (Yb+3), and a pump laser generating radiation at a pump wavelength in the 1.02-1.06 μm wavelength range and outputting light in the 1.02-1.06 μm wavelength range coupled into the Yb:Er doped double clad fiber. The disclosed fiber laser and fiber amplifier each have a significantly higher lasing threshold in the 1 μm wavelength range than the threshold of the known schematics operating at a 9xx nm pump wavelength.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to a high-power Yb:Er fiber laser system in which the onset of Yb laser oscillation is suppressed in the 1 micron nm wavelength range. More specifically, this disclosure relates to a high-power fiber oscillator and amplifier based on a clad-pumped Yb:Er doped fiber in the 1020–1060 nm wavelength range. [Background technology]

[0002] There is a demand for high-power, practical, and low-cost erbium (Er)-doped double-clad fiber laser systems operating in the 1.5–1.6 μm wavelength range. Laser operation in this wavelength range is attractive for several reasons: it excels at exciting thulium (Tm) laser systems, mid-IR parametric amplifiers, and oscillators; and it also has low fiber loss, making high-power Er lasers highly advantageous in many scientific and engineering applications.

[0003] The most common laser transitions in Er are concentrated around 1550 nm. The overwhelmingly prevalent excitation configuration for Er-based laser systems operates at an excitation wavelength of around 980 nm and is widely used in Er fiber systems. However, Er fiber devices excited in the 9xx nm wavelength range may not have sufficient power to meet the ever-increasing industrial demand. The main reason limiting the power scaling of Er fiber devices is the lack of high-power single-mode (SM) power sources. Generally, known SM power sources are diode laser-based and their power does not exceed 2-3 W. Another limitation arises from the relatively low Er ion doping concentrations for the following reasons: Firstly, at high Er concentrations, emission is produced by Er ions and OH -It can disappear due to energy transfer processes resulting from interactions between the two. Secondly, it is the well-known "concentration quenching" process, which causes Er ions to transition downwards due to electrostatic dipole-dipole interactions. This phenomenon reduces efficiency and gain. Furthermore, when high excitation power is applied, another cooperative upconversion process occurs, leading to highly undesirable photodarkening that causes fiber degradation.

[0004] W +3 Yb acts as a sensitizer. +3 Co-doping with Yb is used to avoid relatively low pump absorption in Er fibers and to increase the power scaling of Er lasers and Er amplifiers. Yb ions have a simple electronic structure with only one metastable state above the ground state, a broad absorption spectrum, and high absorption and emission cross-sections, as shown in Figure 2 for silica fibers. Unlike Er, Yb can be used at high doping concentrations, enabling the use of cladding excitation methods with high-power MM diode lasers. Excitation can, in principle, occur over a broad wavelength range of 910 nm to 1064 nm. The large absorption cross-section, particularly at 976 nm, allows for high excitation absorption, leading to relatively short fiber lengths.

[0005] Figure 3 shows a general schematic diagram of a high-power fiber system consisting of an Er laser 10 having a Yb:Er co-doped double-clad (DC) fiber 12 positioned in an optical resonator defined between high and low reflectivity mirrors 15. The Yb:Er laser is bidirectionally laterally excited at a wavelength of 9xx nm by a diode laser-based pump 14. During operation, the excitation light is emitted, confined in the inner cladding, and spatially overlaps with the core of the fiber 12. +3 The ions absorb pump photons along the entire length of fiber 12, and their energy is converted into Er +3Resonantly move it to ions. Fiber 12 is Yb due to its high emission cross-section 3+ Er 3+ It is a phosphate-silicate glass that is considered to be an excellent host for the co-doping system. The larger phonon energy of the phosphate host increases the desirable relaxation transition probability that prevents energy transfer from Er +3 to Yb +3 . Also, the spatial overlap between the Yb emission spectrum and the Er absorption spectrum is large, and the energy transfer efficiency from Yb 3+ to Er 3+ in the phosphate-silicate fiber can reach 95%.

[0006] There are several limiting factors that prevent the power scaling of laser 12 with the excitation configuration 14 of FIG. 3 operating in the 9xx nm wavelength range. One of these limitations is parasitic Yb emission in the 1 μm wavelength range, which can damage fiber 12 of the Yb:Er fiber laser irreparably due to an unwanted high gain coefficient in this wavelength range. FIG. 4 shows that parasitic oscillation occurred in the Yb:Er fiber laser 10 of FIG. 2 in the 1 μm wavelength range. The lower graph 1 shows the Er output pulse at 1570 nm. The upper graph 2 shows the superluminescence signal of Yb with a laser oscillation peak in the 1 μm wavelength range.

[0007] One factor explaining the unwanted emission in the 1 μm wavelength range is the presence of a limited amount of Yb ions separated from Er ions in the fiber core, which do not participate in energy transfer to Er ions. Generally, separated Yb ions constitute only a few percent of the total amount of Yb ions. However, as can be seen by comparing the gain spectra in Figures 5 and 6, these separated Yb ions contribute to an overall unwanted population inversion that is incomparably larger than that of the Yb ions involved in energy transfer at 9xx excitation wavelengths. The high gain of Yb ions in the 1 μm wavelength range is a common unwanted phenomenon in Yb:Er fiber laser systems.

[0008] Another factor affecting power scaling is the occurrence of parasitic oscillations in the 1 μm wavelength range. As the temperature of the active fiber increases, the absorption coefficient and the transition rate between Er and Yb ions in this wavelength range also increase. However, if the latter condition is not met, the occurrence of parasitic oscillations in the 1 μm wavelength range increases even more. [Overview of the project] [Problems that the invention aims to solve]

[0009] Based on the above, a high-power, efficient Yb:Er laser and amplifier characterized by low gain in the 1 μm wavelength range are required. [Means for solving the problem]

[0010] This need is met by the high-power Yb:Er laser / amplifier of the present invention, which implements at least one Yb fiber laser that excites a Yb:Er-doped fiber in the excitation wavelength range of 1 to 1.06 μm.

[0011] When a Yb:Er-doped fiber is excited in the excitation wavelength range of 1 to 1.06 μm, the population inversion of separated Yb ions is suppressed to approximately 2-15%, compared to 70% at the excitation wavelength of 9xx nm. At the same time, longer excitation wavelengths are also affected by Er. +3Yb contributes to energy transfer to ions +3 It does not significantly affect the ion inversion distribution. Separated Yb +3 For small-scale ion population inversion, the maximum gain coefficient in the 1 μm range at excitation wavelengths of 1020–1060 nm is considerably lower than that at excitation wavelengths of 9xx nm. As a result, the system of the present invention has a considerably higher laser lasing threshold in the 1 μm wavelength range than the threshold of known schematic diagrams operating at excitation wavelengths of 9xx nm.

[0012] More specifically, according to one aspect of this disclosure, the system of the present invention comprises a Yb:Er fiber laser. In particular, the fiber laser is based on an Er:Yb co-doped double-clad (DC) configuration excited by a laser source operating in the excitation wavelength range of 1 to 1.06 μm. The excitation light can be coupled to the excitation cladding of the DC fiber in either the forward and backward directions (with respect to the signal light propagation direction) or in both directions.

[0013] The disclosed Yb:Er DC fiber, which outputs signal light around 15xx nm wavelength, has a single-mode (SM) core and a metric core of less than 5, preferably less than 2. 2 It may consist of a low-mode (LM) core or a multimode (MM) fiber that outputs signal light in the 15xx nm range.

[0014] In yet another embodiment, the disclosed excitation laser may be either SM or MM. Furthermore, the technique for exciting the Yb:Er fiber may be either side excitation or end-face excitation. The specific excitation technique depends on the task at hand and can be used in conjunction with any or all of the features of the embodiments described above and the features of the disclosed system.

[0015] According to the following aspects of the present disclosure, a Yb:Er system may be composed of a Yb laser pump operating in the wavelength range of 1000 - 1060 nm, a seed that outputs signal light at a wavelength of 15xx - 16xx nm, and one or more amplification cascades. At least one or both of the seed and the amplifier are based on Yb:Er fiber. Together, the seed and the amplifier constitute a master oscillator power fiber amplifier (MOPFA) architecture.

[0016] In all of the above aspects, the Yb-excited fiber laser disclosed may selectively excite a seed source, or a fiber amplifier, or both the seed source and the amplifier according to all of the excitation techniques disclosed above. Furthermore, the disclosed Yb fiber excitation can be used in combination with other excitation configurations. For example, one of the seed and the amplifier operates in combination with a diode laser-based pump, while the other uses a Yb fiber laser pump.

[0017] The above-disclosed individual fiber lasers and MOPFA configurations can be used as Yb:Er excitation for variously added active media. One of the individual applications of Yb:Er excitation includes exciting an Er fiber laser or amplifier that operates at a wavelength longer than the wavelength of the Yb:Er excitation light. Yet another application of Yb:Er excitation includes outputting excitation light that is coupled to a thulium-doped (Tm) gain medium.

[0018] The Yb:Er lasers and MOPFA configurations disclosed above can operate in different operating regimes. That is, they can operate in a continuous wave (CW) regime, a quasi-continuous wave (QCW) regime, or a pure pulse regime.

[0019] The above and other features and advantages will become more apparent from the following detailed description with the accompanying drawings.

Brief Description of the Drawings

[0020] [Figure 1] This is a diagram showing the known absorption and emission cross-sections of Er+3 ions in Yb:Er phosphate glass. [Figure 2] This is a diagram showing the known absorption and emission cross-sections of the Yb+3 ion in silica glass. [Figure 3] This is an optical schematic diagram of a known Yb:Er laser excited at an excitation wavelength of 9xx nm. [Figure 4] This figure shows examples of signals and parasitic oscillation generation at 1.5 μm and 1 μm wavelengths, respectively, in a Yb:Er QCW fiber laser. [Figure 5] Figure 2 shows the gain of Yb+3 ions involved in energy transfer to Er+3 in a Yb:Er fiber at an excitation wavelength of 9xx nm. [Figure 6] Figure 2 shows the gain of separated Yb+3 ions in a Yb:Er fiber, as shown in the schematic diagram. [Figure 7] Figure 2 is a schematic diagram showing the total gain of all Yb+3 ions in parasitic oscillation at a parasitic oscillation wavelength of 1 μm. [Figure 8] This is an optical schematic diagram of the laser system being disclosed. [Figure 9] Figure 8 is a schematic diagram showing the total gain of all Yb+3 ions at a parasitic oscillation wavelength of 1 μm. [Figure 10] Figure 8 is an optical schematic diagram showing the fiber laser system of the present invention operating with a main oscillator output fiber amplifier configuration. [Figure 11] Figure 9 is an optical schematic diagram of the fiber laser system of the present invention, which excites a gain medium doped with Tm ions. [Modes for carrying out the invention]

[0021] Figure 8 shows a schematic diagram of the present invention of an MM Er fiber laser or amplifier 20 based on a double-clad Yb:Er doped fiber 22, positioned in a resonant cavity defined between MM wavelength reflectors 24. In contrast to known techniques, the Er fiber laser is clad-excited by an excitation source such as a Fabry-Perrot Yb fiber laser operating at an excitation wavelength of 1020–1060 nm, or a neodymium (Nd) doped fiber laser at an excitation wavelength of 1050–1060 nm, to output signal light around 15xx nm wavelengths. The excitation configuration may be configured according to unidirectional excitation in either of the opposite light propagation directions, or according to side excitation or end-face excitation techniques that allow bidirectional excitation as shown.

[0022] The exemplary fiber 22 in Figure 8 is excited by lateral excitation of fiber 22 by one or more MM Yb-excited fiber lasers 26 at an excitation wavelength of 1028 nm. Fiber 22 has a 50 μm MM core doped with Yb:Er ions and is approximately 10 m long. A 1 kW output at a signal wavelength of 1570 nm in the QCW region is achieved by Yb in the 1 μm wavelength range. +3 This can be achieved without reaching parasitic oscillation generation of ions. In contrast, the same exemplary schematic diagram of the configuration in Figure 3, operating with 960-970 nm excitation, outputs up to 300-400 W at a 1570 nm signal wavelength, after which parasitic oscillation generation in the 1 μm wavelength range is theoretically determined. Theoretically, if a 1 kW output at a 960 nm excitation wavelength was achievable by using the configuration in Figure 3, all Yb +3 The overall amplification of the ions at 1 μm exceeds 80 dB, as shown in Figure 7. In contrast, theoretically, the structure of the present invention shown in Figure 8, as disclosed above and clearly shown in Figure 9 for fiber 22, has an overall parasitic oscillation amplification of only 32 dB for the same 1 kW output.

[0023] Figure 10 shows a schematic optical diagram 30 of the QCW Yb:Er fiber laser of the present invention utilizing the 1020-1060 nm excitation wavelength range in a main oscillator output fiber amplifier (MOPFA) configuration. A filter 32 configured to the length of the Yb-doped fiber is placed between the Yb:Er fiber laser 20 and the Yb:Er fiber amplifier or booster 30 to further process the parasitic oscillation signal in the 1 μm wavelength range at the output of the fiber 22. The excitation configuration including the Yb fiber laser 34 is configured to excite both the laser 20 and the booster 30. The excitation of the Yb:Er fiber is achieved by providing a resonant cavity for the Yb pump 34 between two relatively weak wavelength reflectors 36 and 38, thereby making the excitation light coupled to the laser 20 substantially weaker than the excitation light coupled to the fiber booster 30, thus enabling the excitation light to be coupled to both the laser 20 and the booster 30. To prevent unwanted leakage of signal light at a wavelength of 15xx nm from the cavity of the laser 20, a combination of multiple MM strong wavelength reflectors 40 is installed along the upstream of the Yb:Er fiber 22.

[0024] Referring to Figure 11, the Yb:Er fiber configuration of the present invention may be used as an excitation unit for a Tm fiber laser system 42. The Tm fiber laser system 42 may be implemented as individual Tm fiber lasers, in a MOPFA configuration as shown, or as separate Tm fiber amplifiers.

[0025] In summary, the excitation wavelength range of 1020-1060 nm is for separated Yb +3 This allows for a reduction in ion gain and a 2-3 fold increase in the threshold for parasitic oscillation generation at a 1 μm wavelength using Yb:Er phosphate fibers.

[0026] The methods of this disclosure can help optimize the configuration of a Yb:Er fiber for a given task. Generally, the maximum power output and optical quality of the laser system are set from the outset. Using these parameters, which are known a priori, the maximum allowable parasitic oscillation gain in the 1 μm wavelength range is determined. Depending on the specific application of the Yb:Er laser system, the allowable parasitic oscillation gain may vary. For example, when a Yb:Er fiber laser is used to excite a Tm-doped fiber, the maximum allowable gain in the 1 μm wavelength range may be higher than that of a Yb:Er fiber used in a heat-treated material with a reflective surface. The quality of the optical signal at the output of the Yb:Er fiber laser is determined primarily by the parameters of the gain medium, i.e., the Yb:Er fiber, such as core diameter, fiber length, core NA, and others well known to those skilled in the art of laser technology.

[0027] Assume that high-power excitation operating at a 1 μm wavelength is required for the desired output of a 15xx μm wavelength Yb:Er laser. Generally, the excitation efficiency is assumed to be 50% due to various optical losses; that is, a system output of 1 kW at 1550 nm, for example, requires approximately 2 kW of excitation light in the 1 μm wavelength range.

[0028] According to the above, there are two groups of Yb ions in the Yb:Er medium. The first group is Yb +3 Separated Yb, which constitutes less than 5% of the total number of ions and has a lifetime of 1 ms to 1.4 ms. +3 It contains ions. Other groups include, for example, Er, which has a lifetime of tens of microseconds (μs). +3 Yb contributes to energy transfer to ions +3 Contains 95% of ions. Yb +3 Absorption of excitation light by ions is due to the latter, which is the second energy transfer of Yb +3 As ions are absorbed by other ions, they are dispersed as a 5% to 95% dispersion.

[0029] Under the assumptions disclosed above, Yb +3The level of population inversion in each ion group is determined by, for example, a 2kW excitation power at a wavelength of 1020nm for a given fiber length. In this case, Yb +3 If we can see the inversion distribution in both groups of ions, we can see the Yb distribution in both groups. +3 The gains of each ion are determined and summed. If the maximum parasitic oscillation gain exceeds the maximum allowable level, the following steps can be initiated.

[0030] Firstly, change the length of the added fiber and, following the procedure disclosed above, Yb +3 The resulting gain from ions can be recalculated. However, the fiber length cannot be increased indefinitely, as this can lead to unacceptable optical loss and a decrease in laser efficiency.

[0031] Secondly, increase the excitation wavelength. For example, use a wavelength of 1030 nm instead of 1020 nm. As the excitation wavelength becomes longer, the population inversion of the separated Yb ions, and therefore the gain in the unwanted wavelength range, decreases. As a result, with the same 2 kW excitation power, the population inversion of the separated Yb ions decreases, but the energy transfer of Yb ions decreases. +3 The ion population inversion remains unchanged. As a result, the unwanted total Yb gain in the 1 μm wavelength range is also reduced.

[0032] As the excitation wavelength increases, the configuration of the Yb:Er fiber should also be reconsidered. For example, the cladding diameter may need to be reduced, for instance, from 200 μm to 150 μm.

[0033] Figure 12 shows another important factor that helps minimize parasitic oscillations at 1 μm. The active Er:Yb fiber has a temperature close to room temperature in the initial stages of laser operation. As the laser continues to operate, the temperature of the gain medium rises. Parasitic oscillations in the 1 μm wavelength range are present at relatively low temperatures during the so-called cold start. Furthermore, as the laser continues to operate and the temperature rises, this occurrence practically disappears. Therefore, to further minimize parasitic oscillations in the 1 μm wavelength range, the fiber laser system of the present invention shown in Figure 8 includes a thermostat that can be controlled to maintain the temperature of the Yb:Er fiber within a certain temperature range from the very beginning. The lower limit of the range must obviously be higher than room temperature, and the upper limit must obviously not reach a level that is detrimental to the integrity of the fiber. The range can be determined analytically or experimentally for each individual laser.

[0034] Those skilled in the art of laser technology will readily understand that many different configurations of the individual fiber lasers disclosed can be easily implemented without departing from the intended scope of the present invention. It is clear that the operable range of the structures of the present invention is not limited to QCW configurations and can be successfully used in both CW and pulsed regions. SM or low-mode lasers, pumps, and amplifiers can all replace the MM devices disclosed above. The excitation configuration preferably includes a fiber laser, but may instead include other suitable pumps. The disclosed signal optical output is illustrative and may increase with optimization of the excitation power and cooling configuration.

[0035] Therefore, although the present invention has been described with a detailed description thereof, it should be understood that the above description is for illustrative purposes only and does not limit the scope of the invention as defined by the appended claims. Other embodiments, advantages, and modifications fall within the following claims. [Explanation of symbols]

[0036] 10 Er laser 12 Fibers 14 Diode laser-based pump 15 High / Low Reflection Mirrors 20 Yb: Er laser / amplifier 22 Fibers 24 wavelength reflector 26 Yb-excited fiber laser 30 Yb: Er Fiber Amplifier / Booster 32 filters 34 Yb fiber laser / pump 36 wavelength reflector 38 wavelength reflector 40 wavelength reflector 42 Tm Fiber Laser System

Claims

1. An excitation laser that generates excitation light having an output in the wavelength range of 1020 to 1060 nm, A thulium (Tm) laser system excited by the aforementioned excitation light, An Er:Yb fiber laser comprising at least one erbium and ytterbium co-doped (Er:Yb) phosphate fiber excited by the excitation light, wherein one Er:Yb medium has a parasitic gain in the 1 μm wavelength range and outputs a signal in the 1.5 μm wavelength range at a certain output power, A laser system composed of these elements, The excitation light excites one Er:Yb medium, thereby separating the Yb in the Er:Yb phosphate fiber within the excitation wavelength range of 1020 to 1060 nm. ＋３ A laser system characterized by its ability to lower the ion gain and increase the threshold for parasitic oscillation generation at a wavelength of 1 μm by 2 to 3 times.

2. The laser system according to claim 1, characterized in that the excitation laser is a Yb fiber laser between two relatively weak wavelength reflectors.

3. The laser system according to claim 1, characterized in that the one Er:Yb medium includes an Er:Yb double-clad fiber (Er:Yb DC fiber) oscillator that receives the excitation light from the excitation laser.

4. The laser system according to claim 3, characterized in that the Er:Yb DC fiber oscillator is configured as an MM Er:Yb fiber oscillator, and the MM Er:Yb fiber oscillator is configured together with an isolated MM wavelength reflector.

5. The laser system according to claim 1, characterized in that the Er:Yb medium is an Er:Yb double-clad fiber (Er:Yb DC fiber) amplifier.

6. The laser system according to claim 5, characterized in that the Er:Yb DC fiber amplifier is configured as an Er:Yb SM or MM fiber amplifier.

7. The laser system according to claim 3, further comprising an additional gain medium coupled to the output of the Er:YbDC fiber oscillator.

8. The laser system according to claim 7, characterized in that the additional gain medium is an Er:Yb fiber amplifier coupled to the Er:Yb DC fiber oscillator in a main oscillator output fiber amplifier (MOPFA) configuration.

9. The laser system according to claim 8, further comprising another gain medium coupled to the output of the Er:Yb fiber amplifier, wherein the gain medium is a Tm-doped gain medium.

10. The laser system according to claim 8, characterized in that the excitation laser excites both the Er:Yb DC fiber oscillator and the Er:Yb fiber amplifier.

11. The laser system according to claim 8, wherein the Er:Yb fiber laser comprises one or more Yb-excitation fiber lasers that output excitation light of 1028 nm, and is coupled to and excited each of the one or more Er:Yb fibers.

12. The laser system according to claim 11, characterized in that the Yb-excited fiber laser is coupled to the Er:YbDC fiber oscillator and the Er:Yb fiber amplifier, respectively, in a side-excitation manner, and the Er:YbDC fiber oscillator and the Er:Yb fiber amplifier are excited in one direction or bidirectionally.

13. The laser system according to claim 11, characterized in that the Yb-excited fiber laser is coupled to the Er:YbDC fiber oscillator and the Er:Yb fiber amplifier in an end-face excitation manner.

14. A method for increasing output power in a co-doped erbium-ytterbium gain medium (Er:Yb medium), the method being: A step of generating excitation light at a certain output in the wavelength range of 1020 to 1060 nm, The step of coupling the excitation light to the Er:Yb medium, thereby separating the Yb in the Er:Yb medium at an excitation wavelength range of 1020 to 1060 nm. ＋３ A method characterized by reducing the ion gain and enabling the threshold for parasitic oscillation generation at a wavelength of 1 μm to be increased by 2 to 3 times.

15. The process further includes the step of coupling the output of the Er:Yb medium to a subsequent Er:Yb medium, The method according to 14, characterized in that the Er:Yb medium and the subsequent Er:Yb medium are respective fiber oscillators and fiber amplifiers arranged in a main oscillator output fiber amplifier (MOPFA) configuration.

16. The method according to 14, further comprising the step of coupling the output of the Er:Yb medium to a thulium (Tm) doped gain medium.

17. The method according to 14, further comprising the step of maintaining the temperature of the Er:Yb medium in a controllable manner above normal room temperature.

18. The method according to 14, characterized in that the excitation light is generated by a Yb or Nd fiber laser comprising a core that supports the propagation of multiple transverse modes and spaced-apart MM wavelength reflectors.

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