Tandem Pumping Scheme Research Articles

The design and optimization methods towards 10 kW high beam quality fiber laser based on counter tandem pumping scheme

The design and optimization methods towards 10 kW high beam quality fiber laser based on counter tandem pumping scheme

First Demonstration of a Bidirectional Tandem-Pumped High-Brightness 8 kW Level Confined-Doped Fiber Amplifier

In this paper, we demonstrated a high-brightness bidirectional tandem-pumped confined-doped fiber amplifier with nearly 8 kW output power. The technical difficulties for realizing the bidirectional tandem-pumping scheme are comprehensively analyzed, according to which, the homemade high signal-isolation side-coupled pump and signal combiner as well as stable 1018 nm pump sources were developed and applied in the experimental study, successfully enabling the first demonstration of the bidirectional tandem pumping scheme. Moreover, benefiting from the confined-doped fiber design and the bidirectional tandem pumping technique, the transverse mode instability was successfully mitigated and the beam quality was well preserved during the power scaling process. Eventually, an output power of 7.88 kW was obtained with the beam quality factor of ∼1.97 and the optical-to-optical efficiency of ∼84.76%. This work provides a good reference for realizing large-mode-area fiber lasers with high output power and good beam quality by utilizing the confined-doped fiber design and bidirectional tandem-pumping technique.

More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme.

We demonstrate the first all-fiber monolithic bidirectional tandem pumping amplifier, to the best of our knowledge, based on a 30/250 µm conventional ytterbium-doped double-clad fiber. By optimizing the bidirectional pumping power distribution, an output power of 6.22 kW is obtained with near single-mode beam quality (M2=1.53), and no transverse mode instability is observed. This work could provide an excellent reference for high-power, higher-brightness fiber lasers.

10 kW Fiber Amplifier Seeded by Random Fiber Laser With Suppression of Spectral Broadening and SRS

A 10 kW fiber amplifier seeded by the random fiber laser (RFL) with 1018 nm tandem-pumping scheme is reported in this letter. The amplified RFL centered at 1085 nm is seeded by the RFL with a full-open cavity. An array of chirped and tilted fiber Bragg gratings placed before the main amplifier prevent the stimulated Raman scattering with a suppression ratio of 55.8 dB. Control experiments prove that the fiber amplifier seeded by the RFL can suppress the spectral broadening during the amplification process effectively. We demonstrate experimentally that this phenomenon is due to the better temporal stability of the RFL compared with that of the fixed-cavity fiber laser. This fiber amplifier seeded by the RFL is the highest output power record for the amplified RFL to the best of our knowledge, and the spectral evolution suggests the potential for further power scaling-up.

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.

More than 5 kW counter tandem pumped fiber amplifier with near single-mode beam quality

We report on what we believe is the first demonstration of a monolithic counter tandem pumped ytterbium-doped fiber amplifier based on conventional double-clad fiber. At the maximum power of 5.03 kW, the amplifier delivered a near single-mode beam quality (M2 = 1.54), without stimulated Raman scattering effect or mode instability. In addition, we theoretically and experimentally investigated the impact of the 1080 nm signal laser from the amplifier on the 1018 nm fiber lasers used as pump sources. It revealed that even a small proportion of signal light coupled into a 1018 nm fiber laser cavity could cause a considerable power decline of the pump laser. Possible solutions to reduce the influence of signal light on pump sources were extensively discussed. Moreover, we experimentally confirmed the advantage in nonlinear effects mitigation of counter-pumping compared to co-pumping. This work proves the feasibility and superiority of the all-fiberized counter tandem pumping scheme, which could provide inspiration for future power scalability.

More Than 8 kW Fiber Laser Amplifier with High Beam Quality Enabled by Counter Tandem Pumping Scheme

More Than 8 kW Fiber Laser Amplifier with High Beam Quality Enabled by Counter Tandem Pumping Scheme

Comprehensive investigations on the tandem pumping scheme employing the pump fiber laser operating at an extremely short wavelength.

In this work, with the aim of improving the nonlinearity threshold in tandem-pumped fiber amplifiers for higher output power, theoretical and experimental work was carried out to enhance the pump absorption and thereby decrease the required length of ytterbium-doped fiber by employing shorter-wavelength fiber lasers as the pump sources. Systematical simulations were first carried out to optimize the cavity parameters of a short-wavelength fiber oscillator at 1007 nm, and subsequently, the performance of the 1007 nm fiber laser in tandem pumping was simulated and compared with that of the 1018 nm fiber laser pumped results. Considerable absorption increment and efficiency improvement could be realized in the 1007 nm fiber laser pumped fiber amplifier relative to the 1018 nm fiber laser pumped one. Furthermore, according to the simulation results, a fiber laser operating at 1007.7 nm with the output power of ∼170 W and a slope efficiency of ∼72.90% was experimentally demonstrated. By applying this fiber laser in tandem pumping a 1080 nm fiber amplifier with different gain fiber lengths, improved performance was acquired in comparison with the 1018.6 nm tandem pumping scheme, the experimental results of which were coherent with the simulation results. This work could provide an effective approach for improving the nonlinearity threshold of tandem-pumped fiber amplifiers and paving the way for higher output power.

High absorption large-mode area step-index fiber for tandem-pumped high-brightness high-power lasers

A short absorption length ytterbium (Yb)-doped large-mode area (LMA) fiber is presented as a step forward to mitigate the stern problem of nonlinear scatterings in a tandem pumping scheme adopted for high-power fiber laser. The short absorption length was realized by incorporating high Yb concentration in the fiber core. Furthermore, by replacing the inherent silica cladding with a Ge-doped cladding, we were able to obtain low core numerical aperture (NA) and negate the detrimental effect of index-raising by high Yb concentrations. This overcomes the long-standing limitation in step-index Yb-doped fibers (YDFs) where high cladding absorption inevitably results in high NA, thus hampering single-mode operation. We report an LMA ( ∼ 575 μm 2 ) YDF with NA of 0.04 and absorption of 27 dB/m at 976 nm—both traits promote power scaling of single-mode tandem pumped fiber lasers. To our knowledge, this is the highest cladding absorption attained in a low-NA step-index fiber to date. An all-fiber tandem-pumped amplifier was built using only ∼ 14 m of the YDF. The amplifier delivered a near-Gaussian beam ( M 2 ∼ 1.27 ) at 836 W output power (pump power limited) with a high slope efficiency of ∼ 83 % . Thanks to the short length and the tandem pumping, no indication of limiting factors such as stimulated Raman scattering, photodarkening, and transverse mode instability was observed.

An Efficient 4-kW Level Random Fiber Laser Based on a Tandem-Pumping Scheme

In this letter, we presented a monolithic high-power random fiber laser (RFL) with a tandem-pumping scheme. The RFL seed centered at 1064 nm was pumped by a 1018-nm fiber laser and reached an output power of 8.66 W. After two amplifiers, the seed was amplified to 4020 W, and the 3-dB width of laser spectrum was broadened from 0.78 nm to 0.99 nm. To increase the efficiency of the 4-kW laser, a specially designed high absorption coefficient (0.6 dB/m @ 1018 nm) 30/250- $\mu \text{m}$ Yb-doped fiber and high-brightness 1018-nm pump laser were adopted. The optical-to-optical efficiency could reach as high as 88.5%. No obvious mode instability was found during the experiment. Furthermore, with the same amplifier stage, we have also built another 4-kW level fiber laser seeded by an oscillator consisting of a pair of fiber Bragg gratings. The comparison of the experimental results showed that the high-power laser based on the RFL seed would have a better control on spectral broadening.

Overlapped Pulsed Pumping of Tandem Pumped Fiber Amplifiers to Increase Achievable Pulse Energy

It has been reported previously that in the regime appropriate for amplifying femtosecond pulses using the chirped pulse amplification technique in Yb-fiber sources that sub-micro-second pulsed tandem pumping not only provides the thermal benefits of c.w. tandem pumping but also enables strong suppression of ASE. In that case, the pump pulse preceded the signal pulse train. Here we propose a tandem pumping scheme in rare-earth-doped fiber amplifiers, where a train of signal pulses is amplified by a pump pulse which is almost exactly temporally overlapped. Simulations demonstrate that this can be used to create uniform gain across the signal pulse train, even at very high total pulse energies where there would be significant gain shaping in the previous case. In addition the pump is absorbed in a much shorter length, which increases the threshold for nonlinear effects and gain of greater than 26 dB is shown to be readily achievable in an amplifier as short as 1.5 m. This results in increased extractable energy before reaching the threshold for limiting nonlinear effects such as stimulated Raman scattering. These attributes should be attractive for high energy, high average power, ultrashort pulse, coherently combined fiber laser systems.

High-power fiber lasers based on tandem pumping

Power scaling of fiber lasers is challenged by several factors, such as the brightness of the pump source, the nonlinear effect, modal instability, and so on. Pumping active fibers with high-brightness fiber lasers instead of laser diodes is a promising solution for the brightness limitation and modal instability. In this paper, for the first time to our knowledge, we present a general review on the achievements of various kinds of high-power fiber lasers based on the tandem pumping scheme in the past few years. The requirements for tandem pumping ytterbium (Yb), erbium (Er), thulium (Tm), and holmium (Ho)-doped fibers are analyzed, and corresponding achievements are summarized. Hundreds of watts of fiber lasers at ∼1020, ∼1500, and 1900 nm and hundred-watt-level fiber lasers at ∼1150 and ∼1180 nm have been successfully achieved. Then, these powerful fiber lasers with high brightness can be employed as pump sources for Yb-, Er-, Tm- and Ho-doped fibers. Moreover, a recent experimental result of a 3.5 kW Yb-doped fiber amplifier in an all-fiber format is reported in addition to previous typical achievements. The underlying challenges for further power scaling, including the nonlinear effect suppression and special fiber design, are briefly discussed. Exploring the tandem pumping scheme in novel application fields is discussed as well.

Sub-Microsecond Pulsed Pumping as a Means of Suppressing Amplified Spontaneous Emission in Tandem Pumped Fiber Amplifiers

We propose a new tandem pumping scheme in three-level rare-earth-doped optical fibers, where energy storage in a pump fiber laser enables much shorter ( $\sim 500$ ns) pump pulses than is possible with pulsed diode laser pumping. This has the effect of reducing the time during which the final amplifier has high inversion and, hence, reduces the amplified spontaneous emission (ASE) when compared with continuous wave (CW) tandem pumping. We simulate the gain and ASE dynamics in a pulse-pumped Yb-doped fiber amplifier and show that the ratio of amplified signal to ASE background can be improved by up to two orders of magnitude over a large range of pulse energies compared with CW tandem pumping. Sub-microsecond pulsed tandem pumping offers a relatively straightforward, as yet experimentally untried, way to substantially reduce undesirable ASE compared with the levels in diode-pumped fiber amplifiers.