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Modal instabilities in high power fiber laser oscillators
摘要： Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.
关键词： Transverse mode instability,inversion effects,thermal effects,high-power fiber lasers,mode competition
更新于2025-09-23 15:19:57
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Pump-Power-Noise Influence on Mode Instabilities in High-Power Fiber Laser Systems
摘要： Transverse mode instability (TMI) is still the most limiting effect for the further average-power scaling of fiber lasers and amplifiers with excellent beam quality. For the modal energy transfer to happen, a phase shift between the thermally-induced refractive index grating and the modal interference pattern in the fiber is essential. In former experimental and theoretical studies it has been shown that such a phase shift can be induced by changes of the pump power. Thus, the investigation of the influence of pump-power noise on the TMI threshold is of high relevance since this noise could act as the main trigger for TMI. To experimentally examine this behavior, white noise with a bandwidth of 2 kHz was generated and transferred via an arbitrary waveform generator to the pump-diode driver. The connected laser diode pumped a 1.1 m long Large Pitch Fiber (LPF) with an active core of ~65 μm from the counter-propagating direction. The TMI threshold of the free-running system was measured according to [5] to be 256 W. The above mentioned frequency range of the white noise was chosen because it was found that only frequency components below 2 kHz have an significant impact on the TMI threshold of the particular system. To investigate the influence of the pump-power noise on the TMI threshold we varied the amplitude of the generated pump noise and measured the latter by sending a partial reflex of the pump beam onto a photodiode. The relative intensity noise (RIN) serves as a measure of the pump-noise amplitude and was calculated by integrating the power spectral density of the measured noise trace. Afterwards, the TMI threshold was determined according to [5] for different RINs, i.e., pump-noise amplitudes. The results are illustrated in Fig. 1. As can be seen, the fiber laser system started with a TMI threshold of 256 W (no artificial noise, RIN = 0.209 %, close to the measured dark noise) and could be decreased by almost a factor of three to 89 W (RIN = 2.927 %) by steadily increasing the pump-noise amplitude. Thus, the experiments have revealed that pump-power noise significantly influences the TMI threshold of fiber laser systems and, thus, likely acts as the main trigger for TMI. The nonlinear decrease of the TMI threshold with increasing pump-power noise (i.e., RIN), as shown in Fig. 1, can be explained with the nonlinearity with which the strength of the energy transfer grows with increasing average power. This correlation together with the current investigations on seed-power noise will also be discussed at the conference. From the experiments it can be concluded that developing low-noise pump sources and drivers could increase the TMI threshold of fiber lasers and amplifiers and, thus, it has the potential to enable a further average-power scaling of these systems with excellent beam quality.
关键词： Transverse mode instability,pump-power noise,fiber lasers,TMI,RIN
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Pulsed Yb-Doped Phospho-Silicate Fiber MOPA Source with 25kW Peak Power and Excellent Beam Quality
摘要： With ytterbium doped fiber (YDF) lasers and amplifiers have reached continuous wave output power of multi-kW with direct diode pumping [1] and 10kW with tandem pumping [2] in a good beam quality, today fiber lasers are becoming the laser choice for many industrial applications and processes, defense, and scientific research. One of the challenges of high average power fiber laser is to maintain a long-term stability of the output power. In particular, photodarkening (PD) is seen as a power loss in YDF gain medium that can significantly influence the operation lifetime of the device. Moreover, it was observed that the PD is related to the transverse mode instability (TMI) that can limit the output power of the laser [3]. In silica host, co-dopant phosphorous (P) is more effective than aluminium (Al) to suppress the PD effect in YDF. In addition, Yb-doped phosphosilicate (Yb-PS) fiber is found suitable for high power laser operation below 1020nm [4]. Such lasers are used as pumps in tandem pumping to reduce the heat load associated with the quantum defects in YDF lasers operating at 10kW level [4]. However, Yb-PS fiber are generally considered difficult to fabricate due to evaporation of P2O5 during the preform fabrication process, resulting in a central dip in the core refractive index profile that has detrimental effect on the output beam quality of fiber lasers. Other disadvantage of P co-doping is its smaller Yb - absorption and emission cross-sections compared to Al counterpart. To compensate for the cross-sections, higher concentrations of Yb and P (to prevent Yb ions from clustering) are needed in fiber. This contributes to a high core NA and poses a challenge to fabricate a large mode area Yb-PS fiber. Here we report an efficient Yb-PS high power laser fiber fabricated using an optimized MCVD (modified chemical vapor deposition) and all-vapor-phase chelate precursor doping technique. Double-clad fiber with a 150μm quasi-octagonal inner cladding and a 12μm core diameter was drawn with a low index polymer outer cladding. The core NA was 0.1, Fig.1 (a). The small signal absorption at the pump wavelength of ~976nm was measured as 2.5 dB/m. Initially the fiber was tested in a 4%-4% laser cavity. An output power of >100W (limited by the available pump power) and the slope efficiency of >85% were obtained. The laser emission was centered at 1066nm. We then tested the same fiber in a pico-second MOPA configuration. Fig. 1(b) shows the experimental schematic. A 4m long Yb-PS fiber was used in the final stage of the MOPA. A gain-switched diode operating at ~1035nm with 180ps pulse width and 2.95MHz repetition rate was used as a seed laser. Fig. 1(c) shows the average signal output power with respect to the absorbed pump power in the final stage of the amplifier. The output power reached 13.3W and the slope efficiency was 74%. The corresponding pulse energy and peak power was 4.5μJ and 25kW respectively. The measured beam quality (M2) was ~ 1.13, as shown in inset of Fig. 1(c). The spectra of the seed laser and at the maximum signal output power are shown in Fig. 1(d). The 3dB spectral bandwidth at the maximum output power was measured as 0.29nm compared to 0.03nm of the seed laser. The magnitude of the stimulated Raman scattering (SRS) appeared at a wavelength of ~1080nm was 30dB lower than the output signal – see Fig. 1(d). In conclusion, we have demonstrated that an all-vapor-phase chelate doping technique holds great potential to fabricate LMA Yb-doped phosphosilicate fibers with diffraction limited beam for high power lasers.
关键词： high power laser,Yb-doped phosphosilicate fiber,photodarkening,all-vapor-phase chelate precursor doping,transverse mode instability,MCVD
更新于2025-09-11 14:15:04