Abstract
In this manuscript, we studied the thermal properties of hundred-watt fiber laser oscillator by real-time
in-situ
distributed temperature measurement. Optical frequency domain reflectometry ...(OFDR) was introduced to measure the temperature distribution of gain fiber core. The fiber laser oscillator operated at 1080 nm and the wavelength of detecting signal from OFDR was ~1550 nm. The maximum output power of this fiber oscillator was 100 W. The fiber core temperature distributions in experiment agree well with our theoretical simulation. The temperature measurement of gain fiber core in oscillator has always been a problem because the backward laser from the oscillator may reduce the signal-to-noise ratio in OFDR. To the best of our knowledge, this is the first temperature distribution measurement of fiber core in hundred-watt oscillator. By the experimental measurement and theoretical model, we also analyzed the thermal properties of laser oscillator respectively pumped by 915 nm and 976 nm LD sources. We found fiber laser oscillator pumped by 976 nm LD sources experienced not only higher maximum thermal load but also higher average thermal load than that pumped by 915 nm LD sources at the same level output power. We also analyzed the fiber core temperature of other components in system, such as combiners and fiber Bragg gratings (FBG). These results are meaningful for us to improve the thermal design and management in fiber lasers.
We present an investigation of the effect of pumping wavelength on the threshold of transverse mode instability (TMI) in high-power ytterbium-doped fiber laser. By using a fiber laser amplifier with ...a bi-directional pumping structure, we obtain the TMI thresholds under wavelength-stabilized and non-wavelength-stabilized 976 nm laser diodes (LDs) pumping scheme. The results show that the non-wavelength-stabilized LD pump has a higher TMI threshold compared to the wavelength-stabilized LD pump. By optimizing the operating current of the non-wavelength-stabilized 976 nm LD, a ∼4kW output laser with near single-mode beam quality (M 2 ∼1.3) is achieved, with a slope efficiency of ∼84.4%. The analysis yields that when the pumping wavelength deviates from the 976 nm peak, the TMI threshold can be increased. This reveals that optimizing pump source wavelength has great potential for suppressing TMI effect in the power scaling of fiber lasers.
A novel high-power fiber laser oscillator employing a saddle-shaped core ytterbium-doped fiber (SSCYDF) is proposed and demonstrated experimentally. The SSCYDF is designed and fabricated with a ...long-tapering core (diameter of ~ 30 µm at both ends and ~ 23 µm in the middle) and a constant inner cladding (diameter of ~ 400 µm) in longitudinal dimension. On the one hand, the small core section of the fiber can only support less than two modes, which is helpful for the mitigation of the transverse mode instability. On the other hand, the large core section provides a large mode area for suppression of stimulated Raman scattering. Therefore, this type of laser oscillator holds the potential advantages for both mitigation of transverse mode instability and suppression of stimulated Raman scattering. Based on the homemade SSCYDF, an all-fiber laser oscillator is constructed and investigated by pumping with laser diodes at wavelength of 976 nm and 915 nm, respectively. The maximum output power of 1312 W is achieved in the case of co-pumping at 915 nm. This is the first work, to the best of our knowledge, to validate the feasibility of using saddle-shaped core ytterbium-doped fiber with constant cladding for high-power fiber lasers.
A novel bidirectional output oscillating-amplifying integrated fiber laser which combines the advantages of the bidirectional output fiber laser oscillator and the oscillating-amplifying integrated ...fiber laser is proposed and demonstrated experimentally. The influences of the reflectivities and center wavelengths of FBGs on the laser output power and efficiency were studied theoretically. The characteristics of the output laser were studied in detail in experiment. Based on this structure, we finally demonstrated a bidirectional laser output of 2 × 2-kW level with an active fiber length of 6 m for the oscillating section and 7 m for the amplifying section at both ends. The optical-to-optical conversion efficiency is 80.4%. The beam qualities of both ends are <inline-formula><tex-math notation="LaTeX">{M}_{A}^{2}\sim 1.3</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{M}_{B}^{2}\sim 1.4</tex-math></inline-formula>, which indicates a near-single-mode output. To the best of our knowledge, it was the first time that this structure has been proposed, and its feasibility was demonstrated experimentally.
We studied the transverse mode instability (TMI) characteristics of few-mode fiber laser amplifier employing ytterbium-doped fiber (YDF) with core/cladding diameter of 30/400 μm under different ...bending diameters. The gain fiber is coiled in ellipse shape with minimum bending diameter changed from 9 cm, 10 cm, 11 cm to 12 cm in the bending part, and the experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 772 W, 1149 W and 1458 W to 1641 W when the bending diameter of YDF increase from 9 cm, 10 cm, 11 cm to 12 cm, accompanied by the deterioration of beam quality from 1.60, 1.67, 1.87 to 1.95. This is mainly because tightly bending in few-mode fiber will promote the overlap between modes and lead to mode coupling, which may trigger TMI. Besides, the bending losses of HOMs decrease while bending diameter increasing, so that there is higher output efficiency and deteriorated beam quality when bending diameter increase. Although the phenomenon is not agreed with common TMI with near single-mode fiber laser, the result will be helpful for the design and power scaling of few-mode fiber lasers.
Tapered Yb-doped fiber (T-YDF) can balance the suppression of transverse mode instability (TMI) and stimulated Raman scattering (SRS) in high power fiber lasers. In this work, we have constructed an ...all-fiberized master oscillator power amplifier emitting at the central wavelength of 1050 nm based on a T-YDF and investigated the laser performance in respective co-pumped and counter-pumped configurations. The results show that the T-YDF can effectively mitigate SRS effect in short-wavelength fiber lasers. A nearly single mode beam quality (M2∼1.20) is obtained at output power of 3 kW-level. In particular, the SRS suppression ratio in the co-pump and counter-pump scheme is about ∼33.2 dB at ∼3039 W and ∼46.6 dB at ∼2818 W, respectively. By further optimizing the structure of the tapered active fiber, it is promising to further improve the output power and beam quality in short-wavelength fiber lasers.
The main factors limiting the power scaling of high-peak-power high-brightness quasi-continuous wave (QCW) fiber laser oscillator and a possible solution are analyzed in this paper. The impacts of ...fiber length and grating parameters on the output power are studied by rate equations. Under the optimized parameters, a high-brightness QCW fiber laser oscillator with a peak power of 7.3 kW is presented experimentally by using a 24 meter long spindle-shaped Yb-doped fiber with a constant core cladding ratio, and that is the best performance of peak power and brightness in near-single-mode QCW fiber lasers at present to the best of our knowledge. The corresponding pulse energy is 0.57 J and the beam quality M 2 factor is about 1.43 under the condition of repetition frequency of 1 kHz and pulse width of 100 μs. The details are analyzed, and also the limiting factors of further power scaling, strategies of suppressing nonlinear effects and the principle of optical device parameter optimization.
In this paper, we construct a high power monolithic fiber laser oscillator based on a commercial ytterbium-doped fiber with core/inner cladding diameter of 25/400 μ m. The output performances of the ...fiber oscillator are experimentally investigated in the conditions of either with or without an external feedback. In the presence of external feedback, a very strong stimulated Raman scattering (SRS) (∼9 dB below the signal light) emerges at the output power of ∼3.56 kW, where the onset of the transverse mode instability (TMI) limits further power scaling. By large angle cleaving the signal arm of the forward combiner to avoid external feedback, the maximum output power of 5.07 kW is achieved with no sign of TMI. The Raman stokes light is ∼35 dB below the signal light and the M 2 factor is measured to be ∼1.6. To directly characterize its power stability and engineering capability, the fiber oscillator is tested for continuous 5-h operation at ∼5.07 kW with power fluctuation less than 0.42%. During the test process, the output power, spectrum, beam quality, and temporal signal perform excellent stability. The experimental results reveal that thresholds of SRS and TMI strongly depend on the external feedback in high power fiber oscillators.
Fiber laser has been developed to the point where the average power scaling can reach several kilowatts readily for oscillators and more for amplifiers. In the meanwhile, the thermal effects inside ...the fiber laser also become prominent. Temperature rise of the fiber core caused by thermal effects has a conspicuous impact on laser performance. In this paper, the spectral properties of superfluorescent fiber sources at low temperature have been studied experimentally and theoretically. We observe a significant flattening of the arc top of the spectra and a broadening to the short-wave direction as the gain fiber temperature decreases from 25 °C to −95 °C. The corresponding 10 dB bandwidth and 20 dB bandwidth increased by 7.216 and 4.004 nm, respectively. The 10 dB center position and the 20 dB center position of the spectrum also move 6.1 nm and 4.2 nm towards short-wave direction, respectively. The absorption and emission cross-sections at different temperatures, calculated based on Lorenz fitting theory, are used to simulate the experimental phenomena, and the results are consistent with the experiment. Additionally, the small signal gain coefficient at low temperature is calculated to help explain the observed phenomena in the experiment.
Single-stage high-power narrow-linewidth fiber laser has been investigated intensively recently because of its simple and robust configuration and great potential in spectral/coherent beam ...combination. In this work, a 6 kW narrow-linewidth fiber amplifier was experimentally achieved based on a fiber oscillator seed. By employing a few-mode ytterbium-doped fiber, the spectral broadening and SRS effects are both significantly mitigated. Combined with a wavelength-stabilized 981 nm pump source, the threshold of transverse mode instability is improved, then, a maximum output power of 6020 W at the central wavelength 1080 nm was achieved with 3-dB bandwidth of ∼0.37 nm and optical-to-optical efficiency of ∼85.6%. The mode instability and nonlinear effects were balanced well. The measured beam quality and the signal to Raman ratio were M 2 ∼2.7 and ∼27 dB, respectively. This work shows the great potential of such amplification structure for the power scaling of high-power narrow-linewidth fiber lasers.
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