HIGH-POWER YTTERBIUM:ERBIUM (YB:ER) FIBER LASER SYSTEM WITH 1.02 - 1.06 UM CLAD PUMPING SCHEME

Abstract

A fiber laser is configured with a double clad fiber with a core doped with ions of Erbium (Er.sup.+3) and Ytterbium (Yb.sup.+3). At least two spaced apart 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 a 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.sup.+3) and Ytterbium (Yb.sup.+3), and a pump laser generating radiation at a pump wavelength in a 1.02-1.06 wavelength range, a pump laser outputting light in a 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.

Claims

1. A fiber laser comprising: a double clad fiber with a core doped with ions of Erbium (Er.sup.+) and Ytterbium (Yb.sup.+3); at least two spaced apart high and low reflection mirrors flanking the core and defining a resonant cavity therebetween; and a pump laser generating radiation at a pump wavelength in a 1.02-1.06 μm wavelength range, a pump laser outputting light in a 1.02-1.06 μm wavelength range coupled into the Yb:Er doped double clad fiber.

2. The fiber laser of claim 1, wherein the pump laser is a single mode (SM) fiber laser or multimode (MM) fiber laser and configured as a Fabry-Perrot resonator or configured as MOPFA.

3. The fiber laser of claim 1, wherein the pump laser is selected from a bulk or semiconductor laser.

4. The fiber laser of claim 1, wherein the double clad fiber is end pumped or side-pumped.

5. The laser of claim 1, wherein the core of double clad fiber is configured to support propagation of multiple transvers modes or single transverse mode.

6. A fiber amplifier comprising: a double clad fiber with a core doped with ions of Erbium (Er.sup.+) and Ytterbium (Yb.sup.+); and a pump laser generating radiation at a pump wavelength in a 1.02-1.06 μm wavelength range, a pump laser outputting light in a 1.02-1.06 μm wavelength range coupled into the Yb:Er doped double clad fiber.

7. The amplifier of one of the above claim 6, wherein the pump laser is selected from a SM fiber or MM fiber laser having a Fabry-Perrot configuration.

8. The amplifier of claim 6, wherein the pump laser is bulk or semiconductor laser.

9. The amplifier of claim 6, wherein the fiber is end pumped or side-pumped.

10. The amplifier of claim 6, wherein the core is configured to support propagation of multiple transvers modes or single transverse mode.

11. A fiber laser system comprising: a master oscillator power fiber amplifier (MOPFA) configuration including a Yb:Er fiber laser which seeds a Yb:Er fiber amplifier, at least one of or both the Yb:Er fiber laser and amplifier being based on a double clad optical fiber which is configured with a core doped with ions of Er and Yb.sup.+, and a cladding surrounding the core; and a pump laser outputting pump light in a 1.02-1.06 μm wavelength range coupled into an Yb:Er doped active fiber of at least one of master oscillator and amplifier.

12. The fiber laser system of claim 11, wherein the pump laser is a SM or MM fiber laser and configured with a Fabry-Perrot resonator or MOPFA architecture.

13. The fiber laser system of claim 11, wherein the pump laser is a bulk laser or semiconductor, the bulk laser being selected from Nd:YAG or Yb:YAG.

14. The fiber laser system of claim 11, wherein the pump fiber laser energizes both the Yb:Er fiber laser and Yb:Er amplifier.

17. The fiber laser system of claim 11, wherein the Yb:Er fiber laser and amplifier have respective pump lasers.

18. The fiber laser system of claim 11, wherein the Yb:Er fiber amplifier is unidirectionally pumped or bi-directionally pumped.

19. The fiber laser system of claim 11, wherein the active fiber is end pumped or side-pumped.

20. The fiber laser system of claim 11 further comprising a thermostat controllable to main a temperature of the Yb:Er doped active fiber above room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other features and advantages will become more apparent from the specific description accompanied by the following drawings, in which:

[0020] FIG. 1 is the known absorption and emission cross-sections of Er.sup.+ ions in Yb:Er phosphate glasses;

[0021] FIG. 2 is the known absorption and emission cross-sections of Yb.sup.+ ions in silica glasses;

[0022] FIG. 3 is an optical schematic of the known Yb:Er laser pumped at a 9xx nm pump wavelength;

[0023] FIG. 4 illustrates an example of signal and parasitic generation at respective 1.5 μm and 1 μm wavelengths in Yb:Er QCW fiber laser.

[0024] FIG. 5 illustrates gain of Yb.sup.+ ions participating in energy transfer to Er.sup.+ in the Yb:Er fiber at a 9xx nm pump wavelength of FIG. 2;

[0025] FIG. 6 illustrates gain isolated Yb.sup.+ ions in the Yb:Er fiber of the schematic of FIG. 2.

[0026] FIG. 7 illustrates is a total gain of all Yb.sup.+ ions at parasitic in at 1 μm parasitic wavelength in the schematic of FIG. 2.

[0027] FIG. 8 is an optical schematic of the disclosed laser system;

[0028] FIG. 9 is a total gain of all Yb.sup.+ ions at 1 μm parasitic wavelength in the schematic of FIG. 8.

[0029] FIG. 10 an optical schematic illustrating the inventive fiber laser system of FIG. 8 operating in a master oscillator power fiber amplifier configuration.

[0030] FIG. 11 is an optical schematic of the inventive fiber laser system of FIG. 9 pumping gain medium which is doped with ions of Tm.

[0031] FIG. 12 illustrates the dependence of lasing threshold on temperature.

SPECIFIC DESCRIPTION

[0032] FIG. 8 illustrates the inventive schematic of MM Er fiber laser or amplifier 20 based on a double clad Yb:Er doped fiber 22 which is placed in a resonant cavity defined between MM wavelength reflectors 24. In contrast to the known art, Er fiber laser is clad pumped by a pump source, such as a Fabry-Perrot Yb fiber laser operating at a 1020-1060 nm pump wavelength or neodymium (Nd) doped fiber laser at 1050-1060 nm pump wavelength, to output signal light around a 15xx nm wavelength. The pumping arrangement may be configured in accordance with a side-pumping or end pumping technique allowing unidirectional pumping in either one of opposite light propagating directions or, as shown, bidirectional pumping.

[0033] The exemplary fiber 22 of FIG. 8 is pumped by one or more MM Yb pump fiber lasers 26 side-pumping fiber 22 at a 1028 nm pump wavelength. The fiber 22 has a 50 μm MM core doped with Yb:Er ions and the length of about 10 meters. The 1 kW output at a 1570 nm signal wavelength in a QCW regime has been obtained without reaching parasitic generation of Yb.sup.+ ions in a 1 μm wavelength range. In contrast, the same exemplary schematic in the configuration of FIG. 3, operating with a 960-970 nm pump, outputs maximum 300-400 W at the 1570 nm signal wavelength after which the parasitic generation in a 1 μm wavelength range is theoretically determined. Theoretically, if the 1 kW output at the 960 nm pump wavelength was attainable by using the configuration of FIG. 3, the total amplification in the 1 μm of all Yb.sup.+ ions would exceeded 80 dB, as shown in FIG. 7. In contrast, theoretically, the inventive structure of FIG. 8 would have only a 32 dB total parasitic amplification for the same 1 kW output, as clearly seen in FIG. 9 for fiber 22 with the above as disclosed above.

[0034] FIG. 10 illustrates an optical schematic 30 utilizing the inventive QCW Yb:Er fiber laser at a 1020-1060 nm pump wavelength range in a master oscillator power fiber amplifier (MOPFA) configuration. Placed between Yb:Er fiber laser 20 and Yb:Er fiber amplifier or booster 30 is a filter 32 configured as a length of Yb-doped fiber further dealing with a parasitic signal in the 1 μm wavelength range at the output of fiber 22. The pump arrangement including an Yb fiber laser 34 is configured to pump both laser 20 and booster 30. The pumping of Yb:Er fibers is realized by providing resonant cavity of Yb pump 34 between two relatively weak wavelength reflectors 36 and 38 which allows pump light to be coupled into laser 20 and booster 30 with the pump light coupled into laser 20 being substantially weaker than that coupled into fiber booster 30. To prevent the unwanted leakage of signal light at 15xx nm wavelength from the cavity of laser 20, a combination of multiple MM strong wavelength reflectors 40 are installed along the upstream of Yb:Er fiber 22.

[0035] Turning to FIG. 11, the inventive Yb:Er fiber configuration may be utilized as a pump unit for Tm fiber laser system 42. The latter may be implemented as an individual Tm fiber laser or, as shown, in MOPFA configuration, or separate Tm fiber amplifier.

[0036] In summary, the 1020-1060 nm pump wavelength range allows reducing the gain of isolated Yb.sup.+ ions and raising the threshold of parasitic generation in the 1 μm wavelength in 2-3 times in Yb:Er phosphate fibers.

[0037] The currently disclosed approach can help optimize the configuration of Yb:Er fiber for any given task. Typically, the maximum power of laser system and quality of light are set from the beginning. With these parameters known a priori, a maximum acceptable parasitic gain in the 1 μm wavelength range is determined. Depending on the concrete application of the Yb:Er lasers system, the acceptable parasitic gain may vary. For example, if the Yb:Er fiber laser is utilized as a pump for Tm-doped fibers, then the maximum acceptable gain in the 1 wavelength range can be higher than that of the Yb:Er fiber used for thermally treating materials which have reflective surfaces. As to the quality of light signal at the output of the Yb:Er fiber laser, it mostly depends on parameters of gain media, i.e., Yb:Er fibers, such as core dimeter, fiber length, core NA and others well known to one of ordinary skill in the laser arts.

[0038] Assume that for the desired output power of Yb:Er laser at a 15xx μm wavelength, a high power pump, operating at a 1 μm wavelength, is required. Typically, the pump efficiency is assumed to be 50% due to various light losses, i.e., for example, 1 kW system output at the 1550 nm requires roughly 2 kW of pump light at a 1 μm wavelength range.

[0039] In accordance with foregoing, there are two groups of Yb ions in Yb:Er media. The first group includes isolated Yb.sup.+ ions which constitute no more than 5% of the total number of Yb.sup.+ ions and have lifetime between 1 ms and 1.4 ms. The other group includes, for example, 95% of Yb.sup.+ ions partaking in energy transfer to Er.sup.+ ions with lifetime of tens of microseconds (μs). The absorption of pump light by the ions of Yb.sup.+ is distributed as 5% to 95% with the latter being absorbed by the ions of the second energy transferring Yb.sup.+ ions.

[0040] With the assumptions disclosed above, the level of inversion population in each of the Yb.sup.+3 ion groups is determined at the 2 kW pump output at a, for example, 1020 nm wavelength for the given fiber length. Then knowing the inversion population in both groups of Yb.sup.+ ions, the respective gains of Yb.sup.+ ions in both groups are determined and summed up. If the maximum parasitic gain exceeds the maximum acceptable level, then following steps can be undertaken.

[0041] First, the doped fiber length can be altered and recalculate the resulting gain of Yb.sup.+ ions following the procedure disclosed above. However, the fiber length cannot be limitlessly increased since it can result in intolerant light losses and reduced laser efficiency.

[0042] Second, the pump wavelength is increased. For example, instead of 1020 nm wavelength use a 1030 nm wavelength. With the longer pump wavelengths, the population inversion of isolated Yb ions and therefore the gain in the unwanted wavelength range decrease. As a consequence, with the same 2 kW pump power, the population inversion of isolated Yb ions is reduced, whereas the population inversion of energy transferring Yb.sup.+ ions remains unchanged. As a result, the unwanted total Yb gain in the 1 μm wavelength range is also reduced.

[0043] With the longer pump wavelength, the configuration of the Yb:Er fiber should be also reconsidered. For example, it may be necessary to reduce the cladding diameter from, for example, 200μ to 150μ.

[0044] FIG. 12 illustrates another important factor helping to minimize the parasitic generation at a 1 μm. Active Er:Yb fibers have a temperature close to the room temperature at the initial stage of laser operation. As the laser continues to work, the temperature of the gain medium rises. The parasitic generation in a 1 μm wavelength range is present at relatively cold temperatures during a so-called cold start. Yet, as the laser continues to operate and temperature goes up, this generation practically disappears. Accordingly, to even further minimize the parasitic generation in the 1 μm wavelength range, the inventive fiber laser system shown in FIG. 8 includes a thermostat controllable to maintain the temperature of Yb:Er fibers from the very beginning in a certain temperature range. The lowest limit of the range obviously should be higher than room temperature and the highest temperature obviously should not reach a level detrimental to the fiber integrity. The range can be determined analytically or experimentally for each individual laser.

[0045] One of ordinary skill in the laser arts readily realizes that many different configurations of the disclosed individual fiber lasers can be easily implemented without departing from the intended scope of the invention. Obviously, the operational regime of the inventive structure is not limited to a QCW configuration and can be successfully used in both CW and pulsed regimes. All SM or low mode lasers, pumps and amplifiers can replace the above-disclosed MM devices. The pump arrangement, although preferably including fiber lasers, may instead include any other suitable pump. The disclosed signal light powers are only exemplary and certainly can and will be increased with optimization of pump powers and cooling arrangement.

[0046] Accordingly, it is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.