Single mode 4.3 kW output power from a diode-pumped Yb-doped fiber amplifier

5 kW power-level 1050 nm narrow-linewidth fiber amplifier enabled by biconical-tapered active fiber

Effective wavelength extension is vital in the applications of high-power narrow-linewidth fiber lasers. In this work, we demonstrate a 5-kW power-level narrow-linewidth fiber amplifier at 1050 nm utilizing a homemade biconical-tapered Yb-doped fiber (BT-YDF). Up to ∼4.96 kW fiber laser is achieved with a 3 dB linewidth of ∼0.54 nm and a beam quality factor of Mx 2 = 1.46, My 2 = 1.6. The experimental comparisons reveal that BT-YDF has the advantages of improving a stimulated Raman scattering threshold and balancing transverse mode instability suppression in the fiber amplifier. This work could provide a good reference for extending the operating wavelength of high-power fiber amplifiers.

2 × 4 kW near-single-mode laser output assisted by an optimized bidirectional oscillating-amplifying integrated fiber laser configuration

Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) can achieve the two-ports laser amplification based on a single cavity, showcasing a promising prospect. In order to improve both the laser power and beam quality, we first simulate and optimize the stimulated Raman scattering (SRS) effect in the B-OAIFL. The simulation results show the SRS effect can be suppressed by optimizing the diameter as well as the length of the active fiber at different locations. With the guidance of theoretical and experimental analysis for the combined suppression of SRS and transverse mode instability (TMI), a near-single-mode B-OAIFL with 2 × 4 kW was demonstrated. Based on this foundation, we further devoted ourselves to the pursuit of the optimization of the structure and performance. The necessity of the configuration of side pump, which was initially introduced for its exceptional performance in stabilizing temporal chaos, was reevaluated in detail. With its negative impacts on efficiency improvement and SRS suppression were analyzed and verified, we removed this configuration and finally demonstrated a more simplified design with superior performance. A total bidirectional output of 8105 W was achieved, with an O-O efficiency of 79.6% and a near-single-mode beam quality of M A 2∼1.36,M B 2∼1.63. No signs of TMI were observed, and the signal-to-SRS suppression ratio was over 38 dB. The results still demonstrate a promising potential for power scaling based on this configuration and parameters.

Simplified expression for transverse mode instability threshold in high power fiber lasers

In this work, we propose an analytical expression for calculating the transverse mode instability (TMI) threshold power, which clearly shows the role of various fiber parameters and system parameters. The TMI threshold expression is obtained by solving the heat conduction equation and the nonlinear coupling equation using the fundamental mode fitted by Gaussian functions. The calculation results of the proposed TMI threshold expression are consistent with the experimental phenomena and simulation results from the well-recognized theoretical model. The influence of some special parameters on the TMI threshold and the power scaling is also investigated. This work will be helpful for fiber design and TMI mitigation of high-power fiber lasers.

Laser cooling ytterbium doped silica by 67 K from ambient temperature

Laser cooling of a 5 cm long, 1 mm diameter ytterbium doped (6.56×1025 ions/m3) silica rod by 67 K from room temperature was achieved. For the pump source, a 100 W level ytterbium fiber amplifier was constructed along with a 1032 nm fiber Bragg grating seed laser. Experiments were done in vacuum and monitored with the non-contact differential luminescence thermometry method. Direct measurements of the absorption spectrum as a function of temperature were made, to avoid any possible ambiguities from site-selectivity and deviations from McCumber theory at low temperature. This allowed direct computation of the cooling efficiency versus temperature at the pump wavelength, permitting an estimated heat lift of 1.42 W/m as the sample cooled from ambient temperature to an absolute temperature of 229 K.

Demonstration of 3kW × 2 ports bidirectional output oscillating-amplifying integrated fiber laser employing chirped and tilted fiber Bragg gratings for co-SRS suppression

Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) is a newly developed configuration with many advantages like compactness and good reliability. In this work, a B-OAIFL with a low time-stabilized threshold was constructed by employing a pair of side pump/signal combiner in the oscillating section, which demonstrates smooth temporal characteristics with no pulse detected by the photodetector at the output power level of only a few of tens Watts. We investigated the effect of side pumping on the Raman Stokes light and verified its contribution to mitigating the temporal-chaos-induced stimulated Raman scattering (SRS). The phenomenon of co-SRS caused by the mutual excitation of backward Stokes light from two amplifying sections under bidirectional pumping was first reported and studied. A pair of chirped and tilted fiber Bragg gratings (CTFBGs) were applied between the oscillating and amplifying sections to suppress the co-SRS, and the effect of the number of CTFBGs on the suppression of co-SRS was studied in detail experimentally. Finally, we successfully suppressed the co-SRS, and achieved a 3kW × 2 ports laser output, with a near-single-mode beam quality of M A 2∼1.3,M B 2∼1.4. In contrast, without the use of CTFBG, only a 2 kW-level output was obtained from each port, limited by co-SRS (with an SRS suppression ratio of less than 15 dB). The maximum output power of end A and end B is 3133 W and 3213 W, with the SRS suppression ratio of about 27.6 dB and 28.1 dB, respectively. No TMI features were observed under bidirectional pumping. The results demonstrate a significant potential for further power scaling based on this configuration. To the best of our knowledge, it is the highest output power achieved based on the B-OAIFL.

TMI and polarization static energy transfer in Yb-doped low-NA PM fibers

In this work, we conduct experimental investigations of transverse mode instabilities (TMI) in a large mode area ultra-low numerical aperture polarization maintaining fiber amplifier. This fiber is few mode in the slow-axis (conventional operation mode), but single mode in the fast-axis. We test the stability of the output beam by changing the input polarization angle and systematically investigate the transverse mode instability threshold in the two principal polarization axes. The lowest TMI threshold at 300 W was found when the input polarization angle was aligned parallel to the slow-axis. Detuning the input polarization angle from the slow-axis led to increased TMI thresholds. For input polarization angle of 90° (parallel to the fast-axis), the output signal was stable up to 475 W and further scaling was limited by the available pump power. However, for fast-axis operation a lower polarization ratio compared to slow-axis operation was observed as well as an unexpected static energy transfer from the fast-axis into the slow-axis above 400 W.

Femtosecond laser fabrication of chirped and tilted fiber Bragg gratings for stimulated Raman scattering suppression in kilowatt-level fiber lasers

Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are ∼25 dB and ∼12 nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7 kW fiber amplifier, and an SRS suppression ratio of ∼4 dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.

Selection principle of seed power in high-power narrow linewidth fiber amplifier seeded by a FBGs-based fiber oscillator

Here, we have experimentally demonstrated the selection principle of the seed power in a narrow linewidth fiber amplifier seeded by fiber oscillator based on a pair of fiber Bragg gratings. During the study on the selection of seed power, the spectral instability of the amplifier is found when a low power seed with bad temporal characteristics is amplified. This phenomenon is thoroughly analyzed from seed itself and the influence of the amplifier. Increasing the seed power or isolating the backward light of amplifier could effectively eliminate the spectral instability. Based on this point, we optimize the seed power and utilize a band pass filter circulator to isolate the backward light and filter the Raman noise. Finally, a 4.2 kW narrow linewidth output power is achieved with signal to noise ratio of 35 dB, which has exceeded the value under the highest output power reported in this type of narrow linewidth fiber amplifiers. This work provides a solution for high power and high signal to noise ratio narrow-linewidth fiber amplifiers seeded by FBGs-based fiber oscillator.

Potential of ytterbium doped silica glass for solid-state optical refrigeration to below 200 K

We report on the optical refrigeration of ytterbium doped silica glass by >40 K starting at room temperature, which represents more than a two-fold improvement over the previous state-of-the-art. A spectroscopic investigation of the steady-state and time-dependent fluorescence was carried out over the temperature range 80 K to 400 K. The calculated minimum achievable temperature for our Yb³⁺ doped silica sample is ≈150 K, implying the potential for utilizing ytterbium doped silica for solid-state optical refrigeration below temperatures commonly achieved by standard Peltier devices.

High brightness in-band pumped fiber MOPA with output power scaling to >4.6k W

An in-band pumping technique that employs low numerical aperture (NA) and high brightness fiber lasers emitting at 1018 nm enables outstanding power scaling performance by increasing pump power handling capacity of fiber components, reducing thermal load and quantum defect. Here, we present design criteria for an in-band pumped fiber master oscillator power amplifier (MOPA) structure, pumped with 1018 nm fiber lasers, as well as mitigation strategies for nonlinear effects for scalable single-mode laser operation at multi-kilowatt power levels. In addition, we report experimental demonstration of an in-band pumped MOPA with an output power scaling up to 4.63 kW and a slope efficiency of ∼88%. The MOPA system is pumped by 18×264W high brightness fiber lasers operating at 1017.8±0.3n m with an NA of <0.075. The laser emits at 1080.4 nm wavelength with a stimulated Raman scattering suppression of >36.8d B and has a near-diffraction-limited beam quality of M ²∼1.61. To the best of our knowledge, our laser has the highest brightness of 153G W⋅c m ^-2⋅s r ^-1 reported at 1080 nm in co-pumping configuration.

Laser cooling experiments to measure the quantum efficiency of Yb-doped silica fibers

A detailed investigation into the wavelength-dependent cooling efficiencies of two ultra-pure large core diameter ytterbium-doped silica fibers is carried out by means of the laser-induced thermal modulation spectroscopy (LITMoS) method. From these measurements, an external quantum efficiency of 0.99 is obtained for both fibers. Optimal cooling is seen for pump wavelengths between 1032 and 1035 nm. The crossover wavelength from heating to cooling is identified to be between 1018 and 1021 nm. The fiber with higher Yb³⁺ ion density exhibits better cooling, seen by the input power normalized temperature differential.

Demonstration of constant-cladding tapered-core Yb-doped fiber for mitigating thermally-induced mode instability in high-power monolithic fiber amplifiers

In this work, a large-mode-area (LMA) step-index constant-cladding tapered-core (CCTC) Yb-doped fiber with a cladding diameter of ∼600 µm is successfully fabricated. The CCTC fiber has a small-core region (diameter of ∼20 µm) at both ends and a large-core region (diameter of ∼36 µm) in the middle. To prove the laser performance of the CCTC fiber, a detailed comparison experiment with conventional uniform fiber with the same effective core diameter is carried out in a multi-kW all-fiber MOPA configuration. The experimental results show that employing the CCTC fiber can effectively mitigate the thermally-induced transverse mode instability (TMI) in both co-pump and counter-pump schemes, and realize high slope efficiency and single-mode beam quality (M²∼1.30). Under the counter-pump scheme, the TMI threshold of the CCTC fiber is observed at ∼2.49 kW with a slope efficiency of 86.2%, while the uniform fiber amplifier exhibits a TMI threshold of ∼2.05 kW. The theoretical analysis based on a semi-analytical model indicates this CCTC fiber can effectively improve the TMI threshold owing to a stronger gain saturation. Our results verify the great potential of such an LMA CCTC fiber to mitigate thermal-induced TMI effect and achieve single-mode operation without sacrifice of laser efficiency in high power monolithic fiber lasers, and the further power scaling is expected by optimizing the fiber design.

Bidirectional tandem-pumped high-brightness 6 kW level narrow-linewidth confined-doped fiber amplifier exploiting the side-coupled technique

In this work, a bidirectional tandem-pumped high-power narrow-linewidth confined-doped ytterbium fiber amplifier is demonstrated based on side-coupled combiners. Benefiting from the large-mode-area design of the confined-doped fiber, the nonlinear effects, including stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), are effectively suppressed. While the transverse mode instability (TMI) effect is also mitigated through the combination of confined-doped fiber design and the bidirectional tandem pumping scheme. As a result, narrow-linewidth fiber laser with 5.96 kW output power is obtained, the slope efficiency and the 3-dB linewidth of which are ∼81.7% and 0.42 nm, respectively. The beam quality is well maintained during the power scaling process, being around M²= 1.6 before the TMI occurs, and is well kept (M²= 2.0 at 5.96 kW) even after the onset of TMI. No SRS or SBS is observed at the maximum output power, and the signal-to-noise ratio reaches as high as ∼61.4 dB. To the best of our knowledge, this is the record power ever reported in narrow-linewidth fiber lasers. This work could provide a good reference for realizing high-power high-brightness narrow-linewidth fiber lasers.

Experimental study on the impact of signal bandwidth on the transverse mode instability threshold of fiber amplifiers

In this work, we conduct a detailed experimental study on the impact of signal bandwidth on the TMI threshold of fiber amplifiers. Both the filtered superfluorescent fiber sources and the phase-modulated single-frequency lasers are employed to construct seed lasers with different 3 dB spectral linewidths ranging from 0.19 nm to 7.97 nm. The TMI threshold of the fiber amplifier employing those seed lasers are estimated through the intensity evolution of the signal laser, and different criteria have been utilized to characterize the spectral linewidth of the seed lasers. Notably, the experimental results reveal that the TMI threshold of fiber amplifiers grows, keeps constant, and further grows as a function of spectral linewidth of seed lasers. Our experimental results could provide a well reference to understand the mechanism of the TMI effect and optimize the TMI effect in high-power fiber amplifiers.

Investigation on the thermal blooming effect in a high power spectral beam combining fiber laser system

We demonstrate the influence of the thermal blooming effect on the far-field beam quality in a seven-channel spectral beam combining system. Stimulated Raman scattering in the incident narrow-linewidth fiber amplifier is verified to be the dominant factor that induces thermal blooming in the beam combining system. When the power density of Raman light reaches only 180W/cm², the peak intensity of the far-field beam reduces severely and the beam distribution profile spreads. We reveal that H₂O content in the atmosphere has a positive relationship with the thermal blooming effect and study the influence of the humidity on the thermal blooming effect. The influence of the optical path length on the thermal blooming effect is also revealed. The result shows that the focusing property of the far-field beam degrades gradually as the optical path length increases from 100 to 450 mm. The results are conducive to optimize the beam quality of spectral beam combining.

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

Fiber laser technology has been demonstrated as a versatile and reliable approach to laser source manufacturing with a wide range of applicability in various fields ranging from science to industry. The power/energy scaling of single-fiber laser systems has faced several fundamental limitations. To overcome them and to boost the power/energy level even further, combining the output powers of multiple lasers has become the primary approach. Among various combining techniques, the coherent beam combining of fiber amplification channels is the most promising approach, instrumenting ultra-high-power/energy lasers with near-diffraction-limited beam quality. This paper provides a comprehensive review of the progress of coherent beam combining for both continuous-wave and ultrafast fiber lasers. The concept of coherent beam combining from basic notions to specific details of methods, requirements, and challenges is discussed, along with reporting some practical architectures for both continuous and ultrafast fiber lasers.

Laser cooling of a Yb doped silica fiber by 18 Kelvin from room temperature

An ytterbium doped silica optical fiber with a core diameter of 900µm has been cooled by 18.4 K below ambient temperature by pumping with 20 W of 1035 nm light in vacuum. In air, cooling by 3.6 K below ambient was observed with the same 20 W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be 1.2±0.1%. The core of the fiber was codoped with Al³⁺ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.

Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 µm, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.

Coherent amplification of high-power laser radiation in multicore fibers from a rectangular array of cores

The coherent propagation and amplification of high-power laser radiation in a multicore fiber consisting of a square array of weakly bound cores are studied. Exact stable analytical solutions are found for the out-of-phase mode, which describes the coherent propagation of wave beams in such fibers. The analytical results are confirmed by direct numerical simulation of the wave equation. The stability conditions of the out-of-phase mode in the active medium are found.

Suppression of stimulated Brillouin scattering in optical fibers by tilted fiber Bragg gratings

Stimulated Brillouin scattering (SBS) has significant influence on optical fiber communication (OFC), optical fiber sensing (OFS), and narrow-linewidth fiber laser (NLFL) systems. How to effectively suppress it has always been a challenge. In this Letter, we propose and demonstrate a versatile solution, for the first time, to the best of our knowledge, by using tilted fiber Bragg gratings (TFBGs). A specially designed and fabricated TFBG can be used as an ultra-narrow spectral filter, precisely matching with the operation laser wavelength and the tiny frequency shift due to SBS. Experimental results show that the backward Stokes can be strongly rejected with a filtering ratio of >10dB; meanwhile, an obvious increasing of SBS threshold is observed with a maximum value of 1.7 times that without the TFBG, which enhances the effective transmission power by 33%. The operation stability of this method also is validated. This work opens new opportunities for SBS suppression in OFC, OFS, and high-power NLFL systems.

Ultra-low NA step-index large mode area Yb-doped fiber with a germanium doped cladding for high power pulse amplification

High concentration rare earth doped, large mode area (LMA) step-index fibers, which feature a very high cladding absorption per unit length at the pump wavelength, high efficiency, and excellent beam quality, are ideal for high power pulsed fiber lasers/amplifiers where large effective mode areas and short device lengths are crucial in order to reduce detrimental nonlinear effects associated with high peak power operation. In this Letter, we realize low numerical aperture (NA) high absorption fibers, simply by employing a germanium (Ge)-doped cladding rather than a pure silica cladding to offset the high refractive index associated with using a high concentration of ytterbium (Yb) in the core. This approach allows us to separate the two inter-linked fiber design parameters of pump absorption and NA in a step-index fiber. Using a conventional modified chemical vapor deposition process combined with solution doping, a low NA (0.04), LMA (475µm²) silica fiber is fabricated with a cladding absorption value of >20dB/m, which is the highest value among LMA step-index fibers with NA<0.06 so far reported to the best of our knowledge. The fabricated Yb-doped fiber was tested in a high-power picosecond amplifier system and enabled the generation of 190 ps laser pulses with a 101 µJ pulse energy and 0.5 MW peak power at an average power of 150 W.

High brightness 1018 nm monolithic fiber laser with power scaling to >500 W

We have studied 1018 nm high power monolithic fiber lasers to be used as pump sources for multi-kilowatt (kW)-level in-band master oscillator power amplifier (MOPA) systems. The performance of the cavity in terms of wavelength selectivity, optical efficiency, and robustness against amplified spontaneous emission (ASE)-induced parasitic lasing is analytically investigated with respect to doped fiber length and cavity parameters. Experimentally, we have demonstrated a 1018 nm fiber laser with an output power of 502 W from a 15/130 µm Yb³⁺-doped double-cladded fiber, enabling, to our knowledge, the highest reported brightness and radiance density as well. The laser cavity has a slope efficiency of ∼76%, ASE suppression of >50dB, and a near-diffraction-limited beam quality of M²∼1.15. Owing to high brightness with a small core size (i.e., high radiant density), the proposed laser is promising to enable high-power in-band pump integration via fused combiners for multi-kW in-band MOPA systems.

High-power cladding pumped Raman fiber amplifier with a record beam quality

In this Letter, a high-power, high-brightness all-fiberized Raman amplifier based on a cladding-pumping scheme is presented for the first time, to the best of our knowledge. The triple-clad passive fiber is employed as Raman gain fiber in the laser system. The maximum output power is 762.6 W emitting at 1130 nm. To the best of our knowledge, this is the highest power in the fields of cladding-pumped Raman amplifiers. Through a cladding-pumping process, the beam quality parameter ${{\rm M}^2}$M² improves from 6.12 of seed laser to 2.24 at maximum output power of 762.6 W, while the best ${{\rm M}^2}$M² is 1.9 at 267.2 W. It is also the best beam quality of Raman laser with brightness enhancement in any kind of configuration (graded-index fiber or multi-clad fiber, laser or amplifier, all-fiber or free-space configuration) with power of over 100 W.

Kilowatt-level ytterbium-Raman fiber amplifier with a narrow-linewidth and near-diffraction-limited beam quality

By focusing on a typical emitting wavelength of 1120 nm as an example, we present the first, to the best of our knowledge, demonstration of a high-efficiency, narrow-linewidth kilowatt-level all-fiber amplifier based on hybrid ytterbium-Raman (Yb-Raman) gains. Notably, two temporally stable, phase-modulated single-frequency lasers operating at 1064 nm and 1120 nm, respectively, were applied in the fiber amplifier, to simultaneously alleviate the spectral broadening of the 1120 nm signal laser and suppress the stimulated Brillouin scattering effect. An over 1 kW narrow-linewidth 1120 nm signal laser was obtained with slope efficiency of ${\sim}{77}\% $∼77% and beam quality of ${\rm M}_x^2\sim {1.25}$Mx2∼1.25, ${\rm M}_y^2 \sim {1.17}$My2∼1.17. The amplified spontaneous emission (ASE) noise in the fiber amplifier was effectively suppressed by incorporating an ASE-filtering system between the seed laser and the main amplifier. Furthermore, the experimental results demonstrate that the spectral linewidth broadening effect is tightly related to the injected power ratios between the two seed lasers. Overall, this setup could provide a reference on obtaining and optimizing high-power narrow-linewidth fiber lasers operating in the long wavelength extreme of the Yb gain spectrum.

Diode-pumped triple-clad fiber MOPA with an output power scaling up to 4.67 kW

In this Letter, we have demonstrated a triple-cladded fiber (TCF)-based master oscillator power amplifier (MOPA), with an output power scaling up to 4.67 kW, an optical-to-optical efficiency of 78%, and a beam quality factor ${{\rm M}^2}$M² of 1.57. The MOPA output power was limited by stimulated Raman scattering (SRS) of which our design yielded a 31.2 dB suppression ratio at the 4.67 kW output power. Such a unique design of a TCF-based structure allows a wide range of flexibility over fiber parameters, mitigation of nonlinear effects, low-loss splice integration, reliable high-power pump guiding in turn, and an ease in overall thermal management at multi-kW output power levels. Together with direct diode-pumping configuration, TCF-based designs promise thermally and mechanically robust, compact, and highly efficient MOPA systems of a superior beam quality.

Novel method for the angular chirp compensation of passively CEP-stable few-cycle pulses

We demonstrate a novel, energy-efficient, cost-effective simple method for seeding CEP-stable OPCPAs. We couple the CEP-stable idler of a broadband OPCPA into a hollow core Kagome fiber thus compensating for the angular chirp. We obtain either relatively narrow bandwidths with ∼36% coupling efficiency or quarter-octave spanning bandwidths with ∼2.2% coupling efficiency. We demonstrate spectral compressibility, good beam quality and CEP stability. Our source is an ideal seed for high-energy, high-average power, CEP-stable few-cycle OPCPA pulses around 2 µm, which can drive the generation of coherent soft X-ray radiation in the water window spectral region via HHG.

Impact of the heat load on the laser performance of chirally-coupled-core fibers

As the heat load within the central core of chirally-coupled-core (CCC) fibers will change the pre-designed refractive index profile through the thermo-optic effect, its impact on the laser performance of CCC fibers is investigated. Analysis and simulation results on two typical CCC fibers show that the effects of the heat load include the modal loss reduction and the transmission spectrum drift. The former comes from the thermal lensing effect in the central core, and the latter is caused by the change in the refractive index difference between the central core and the side core. Considering the non-uniform axial heat distribution in the actual laser operation, the overall laser performance of CCC fibers with different pump power is simulated. It is found that, because of the high pre-designed high-order mode loss, the single-mode operation of CCC fibers will be maintained but the slope efficiency may reduce dramatically if the fundamental mode loss is strongly dependent on the heat load.

Mitigation of mode instability in laser oscillators based on deuterium loading

Ytterbium-doped fiber (YDF) loaded with deuterium is used herein to mitigate mode instability. Experimental results reveal that this method can increase the mode instability threshold in a laser oscillator. Specifically, when the YDF was loaded with deuterium over two- and four-week periods, the mode instability threshold power increased from ∼459 W to ∼533 W (16%) and to ∼622 W (35%), respectively, but the respective laser efficiencies were almost unaffected (71.5% vs. 72.9% and 75.4%). In conclusion, deuterium loading is effective in the mitigation of mode instability. It is envisaged to be applied in the power scaling of high-power fiber lasers.

2.05 kW all-fiber high-beam-quality fiber amplifier with stimulated Brillouin scattering suppression incorporating a narrow-linewidth fiber-Bragg-grating-stabilized laser diode seed source

We experimentally demonstrate an all-fiber high-power fiber amplifier with high beam quality and a slope efficiency of 81.8%, using a fiber-Bragg-grating-stabilized laser diode as a narrow spectral linewidth (0.08 nm) seed source. During amplification, the spectral linewidth of the laser output is broadened from 0.08 to 0.24 nm due to nonlinear phenomena. To the best of our knowledge, we report the first experimental observation of the suppression of stimulated Brillouin scattering (SBS), with increased output power. In addition, we investigated the SBS suppression by simultaneously measuring the optical backscattered power, backscattered spectrum, and output spectrum at different values of output power. The beam quality, M², was measured to be ∼1.28 at the maximum output power of 2.05 kW, and modal instability was not observed.

Narrow-linewidth fiber amplifier for gravitational-wave detectors

We report on the design and noise performance of a narrow-linewidth Yb-doped fiber amplifier emitting up to 178 W at 1064 nm for possible use in gravitational-wave (GW) interferometric detectors. The novel design utilizes a specialty large-mode-area gain fiber with confined-core doping and depressed cladding, followed by a smaller-core passive fiber to improve output beam quality. We show that the free-running noise of the system is equal to or better than current Advanced LIGO noise requirements. Finally, we discuss potential improvements for long-term use in GW detectors.

Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier

Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.

Transverse mode instability, thermal lensing and power scaling in Yb3+-doped high-power fiber amplifiers

Transverse mode instability (TMI) is compared to thermal lensing (TL) power threshold and used to derive power scaling limits in high-power fiber amplifiers. The TMI power threshold is shown to be ~65% of the TL one and dominates power scaling. In addition to commonly used limiting effects, we introduce a bend-induced mechanical reliability criterion, which limits the maximum allowable cladding diameter to ~600μm. This also results in the introduction of a critical pump brightness, the minimum required pump brightness at which the maximum signal power is achieved. The maximum achievable power depends primarily on the choice of pumping wavelength, amplifier gain and heat coefficient. Maximum signal powers of ~28kW to ~38kW, for diode pumping (λ_p = 976nm), and ~35kW to ~52kW, for tandem pumping (λ_p = 1018nm), are predicted for single-mode fiber amplifiers operating at signal wavelength λ_s = 1070nm, when the amplifier gain is increased from 10dB to 20dB. For an amplifier gain of 10dB, the maximum achievable signal power varies from 85kW to 25kW for tandem pumping and 35kW to 20kW for diode pumping, when the heat coefficient varies from 1% to 15% and 5.5% to 20%, respectively. The corresponding critical pump brightness varies from ~0.50 W/(μm² sr) to ~0.14 W/(μm² sr) for tandem pumping and ~0.25 W/(μm² sr) to ~0.13 W/(μm² sr) for diode pumping.

Ring-up-doped fiber for the generation of more than 600 W single-mode narrow-band output at 1018 nm

We present the amplification of a narrow-bandwidth signal at a wavelength of 1018 nm to a power exceeding 600 W with a stable output polarization state. The beam showed an excellent, nearly diffraction-limited beam quality. The high-power output could be realized using an in-house designed and fabricated fiber with a core-cladding diameter ratio of 32/260, ultra-low NA of 0.041, and ring-up doping. A seed source with high amplified spontaneous emission (ASE) suppression was also required, which was realized by a double-pass pre-amplifier with 13 W output power.

Suppression of stimulated Brillouin scattering in high power fibers using nonlinear phase demodulation

We demonstrate a modulation approach that relaxes the limitations imposed by stimulated Brillouin scattering (SBS) on amplification and propagation of narrow-linewidth light in fibers. By imposing synchronous amplitude and phase modulation on an input field, the optical spectrum after high-power fiber transmission is compressed using nonlinear self-phase modulation. This effectively reduces the SBS interaction length and increases the SBS threshold, enabling narrower linewidths. Using this technique, we demonstrate >2 × increase in SBS-limited spectral brightness from a kW-class amplifier. We show that SBS suppression becomes more effective for higher powers and longer fibers.

Narrow-linewidth all-fiber amplifier with up to 3.01 kW output power based on commercial 20/400 μm active fiber and counterpumped configuration

A multikilowatt all-fiber amplifier with narrow spectral linewidth is investigated by using commercial 20/400 μm active fiber due to its more extensive application prospects. Stimulated Brillouin scattering (SBS) is suppressed effectively by combining multiple approaches including optimizing linewidth-broadened seed, shortening fiber length, and a counterpumped configuration. Consequently, the output power is pushed up to 3.01 kW with M²=1.17, and the linewidth is maintained at 48 GHz. The detailed characteristics of SBS are presented and analyzed in the frequency and time domains as output power is gradually raised. A high optical-to-optical efficiency is obtained with 85.6% and amplified spontaneous emission is reduced to 40 dB with respect to a laser signal. The proposed architecture reaches a relatively high output power with excellent beam quality under the condition of the equivalent linewidth for the commercial 20/400 μm active fibers in robustly all-fiberized structures.

3.7 kW monolithic narrow linewidth single mode fiber laser through simultaneously suppressing nonlinear effects and mode instability

In this paper, we report a 3.7 kW all fiber narrow linewidth single mode fiber laser. The full width at half-maximum is about 0.30 nm, and the beam quality is Mx2=1.358, My2=1.202 at maximum output power. The laser is achieved by simultaneously suppressing nonlinear effects and mode instability (MI). Different seeds are injected into the main amplifier to study stimulated Raman scattering (SRS) effect. The results show that the phase modulated single frequency seed is benefit to suppress the SRS effect. For the phase modulated single frequency seed, inserting a filter in preamplifier will suppress amplified spontaneous emission (ASE) and decrease the backward power. By optimizing the coiling of active fiber, the MI effect is suppressed.

High power monolithic tapered ytterbium-doped fiber laser oscillator

In the power scaling of monolithic fiber lasers, the fiber nonlinear effects and transverse mode instability are main limitations. The tapered gain fiber has a longitudinally varying mode area, which has the advantage of mitigating fiber nonlinear effects. However, the transverse mode instability (TMI) was seldom reported in the tapered fiber lasers at high average power levels. In this work, we have constructed a monolithic tapered ytterbium-doped fiber laser oscillator and investigated the laser oscillator performance with respective 976 nm and 915 nm pump, especially on the aspects of the TMI. The double cladding tapered ytterbium-doped fiber has a narrow end of ~20/400 μm and a wide end of ~30/600 μm. Fiber Bragg gratings (FBG) are respectively inscribed on double cladding fibers with core/inner cladding diameter of 20/400 μm and 30/400 μm to match with the narrow and wide end of the tapered ytterbium-doped fiber. When 915 nm pump is employed, the TMI occurs at the output power of ~1350 W. The output power is further scaled to a maximum of 1720 W. The M² factor of the output laser is ~2.1 and the full width at half maximum (FWHM) of the signal laser is ~3.6 nm. To the best of our knowledge, this is the highest average power for the tapered ytterbium-doped fiber lasers.

Ultrafast thulium fiber laser system emitting more than 1 kW of average power

In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.

Fabrication and Characteristics of Yb-Doped Silica Fibers Produced by the Sol-Gel Based Granulated Silica Method

Combining the sol-gel method for fiber material production with the granulated silica method for preform assembly results in a robust method that offers a high degree of freedom regarding both the composition and the geometry of the produced fiber. Using this method, two types of Yb-doped silica glass composition, that feature an excess in P concentration with respect to Al, have been prepared. The elemental distributions in a fiber core were analyzed by scanning transmission electron microscopy (STEM). The elemental mapping shows a similar localization of Al, P and Yb through the microstructure. In addition, the influence of the variation in the co-dopant concentration, with respect to Yb, on the fiber properties has been investigated. The results show an increase in the refractive index step and in the fiber’s transmission loss as the excess concentration of P increases. A significant contribution to the losses can be assigned to the existence of impurities such as iron, which was detected in our samples by mass spectrometer. Single exponential fluorescence decays with lifetimes of around 0.88 ms were measured for the two compositions. Finally, pumping at 976 nm a laser slope efficiency of 67% at 1031 nm was achieved for one of the fiber compositions.

Theoretical analysis of the SRS-induced mode distortion in large-mode area fiber amplifiers

In this paper, the spectral evolution properties of different transverse modes with the stimulated Raman scattering (SRS) effect are analyzed in large-mode-area (LMA) fiber amplifiers for the first time. Both the ratios of laser power in Raman Stokes light and high order modes (HOMs) can be calculated through the comprehensive analysis of transverse mode competition and nonlinear transverse mode coupling processes. The theoretical study reveals that SRS-induced inter-modal wave mixing (IM-WM) effect would transfer power from signal light in LP₀₁ mode to Raman Stokes light in LP₁₁ mode and lead to the onset of the mode distortion phenomenon in high-power LMA fiber amplifiers. Different from the traditional thermal-induced mode instability (MI) phenomenon, the SRS-induced mode distortion could occur by just with the contribution of quantum noise.

Low-loss 'crystalline-core/crystalline-clad' (C4) fibers for highly power scalable high efficiency fiber lasers

We report the latest progress in fabrication and laser performance of the fully crystalline double-clad 'Yb:YAG-core/undoped-YAG-clad' fibers grown by the hybrid crystal growth method. The single-crystalline ytterbium (Yb) doped yttrium aluminum garnet (YAG) fiber cores were grown by the laser heated pedestal growth (LHPG) method, and the single-crystalline undoped YAG claddings were grown by the liquid phase epitaxy (LPE) technique, in which the single-crystalline Yb:YAG cores were used as the growth seeds. The key parameters of the hybrid-grown 'crystalline core/crystalline clad' (C4) fibers, including material composition, crystal structure, and fiber propagation loss, were characterized. The results confirmed that the grown C4 fibers, indeed, have both the single-crystalline fiber core and single-crystalline fiber clad. By utilizing a double-clad low-loss C4 fiber as a diode-cladding-pumped laser gain medium, we realized a fiber laser with the optical-to-optical conversion efficiency of 68.7% versus the incident pump power.

Efficient 240W single-mode 1018nm laser from an Ytterbium-doped 50/400µm all-solid photonic bandgap fiber

Lowering the quantum defect by tandem pumping with fiber lasers at 1018nm was critical for achieving the record 10kW single-mode ytterbium fiber laser. Here we report the demonstration of an efficient directly-diode-pumped single-mode ytterbium fiber laser with 240W at 1018nm. The key for the combination of high efficiency, high power and single-mode at 1018nm is an ytterbium-doped 50μm/400μm all-solid photonic bandgap fiber, which has a practical all-solid design and a pump cladding much larger than those used in previous demonstrations of single-mode 1018nm ytterbium fiber lasers, enabling higher pump powers. Efficient high-power single-mode 1018nm fiber laser is critical for further power scaling of fiber lasers and the all-solid photonic bandgap fiber can potentially be a significant enabling technology.