5 kW-level single-mode fiber amplifier based on low-numerical-aperture fiber

A low-numerical-aperture(NA)concept enables large-mode-area fiber with better single-mode operation ability,which is beneficial for transverse mode instability and nonlinear effects suppression.In this contribution,we reported a high-power fiber amplifier based on a piece of self-developed large-mode-area low-NA fiber with a core NA of 0.049 and a core/inner cladding diameter of 25/400μm.The influence of the pump wavelength and fiber length on the power scaling potential of the fiber amplifier is systematically investigated.As a result,an output of 4.80 kW and a beam quality factor of~1.33 were finally obtained,which is the highest output power ever reported in a fiber amplifier exploiting the low-NA fiber.The results reveal that low-NA fibers have superiority in power scaling and beam quality maintenance at high power levels.

Six kilowatt record all-fiberized and narrow-linewidth fiber amplifier with near-diffraction-limited beam quality

In this work,an all-fiberized and narrow-linewidth fiber amplifier with record output power and near-diffraction-limited beam quality is presented.Up to 6.12 kW fiber laser with the conversion efficiency of approximately 78.8%is achieved through the fiber amplifier based on a conventional step-index active fiber.At the maximum output power,the 3 dB spectral linewidth is approximately 0.86 nm and the beam quality factor is M_(x)^(2)=1.43,M_(y)^(2)=1.36.We have also measured and compared the output properties of the fiber amplifier employing different pumping schemes.Notably,the practical power limit of the fiber amplifier could be estimated through the maximum output powers of the fiber amplifier employing unidirectional pumping schemes.Overall,this work could provide a good reference for the optimal design and potential exploration of high-power narrow-linewidth fiber laser systems.

Near-infrared small molecular fluorescent dyes for photothermal therapy

Near-infrared(NIR)fluorescent dyes based on small organic molecules are characterized with low cytotoxicity,good biocompatibility and minimum interference from auto-fluorescence background,which are widely used in tumor diagnosis.Intensive research on molecular properties and photothermal properties of fluorescent dyes have been explored for non-invasive photothermal treatment of cancer.In this review,we focus on the development of imaging-induced photothermal therapy of small molecules and classification according to the structures of organic molecules including cyanines,phthalocyanines,rhodamine analogues and BODIPYs.

Photodynamic therapy based on organic small molecular fluorescent dyes

Photodynamic therapy(PDT) has shown promise as an effective treatment modality for cancer and other localized diseases due to its noninvasive properties and spatiotemporal selectivity. Near-infrared(NIR)fluorescent dyes based on organic small molecules are characterized with low cytotoxicity, good biocompatibility and excellent phototoxicity, which are widely used in PDT. In this review, we attempt to summarize the development of imaging-induced PDT based on organic small molecules and classify it according to the structures of dyes including cyanines, 4,4-difluoro-4-bora-3 a,4 a-diaza-s-indacene(BODIPY) analogues, phthalocyanine and other agents such as rhodamine analogues.

Effective suppression of the photodarkening effect in high-power Yb-doped fiber amplifiers by H2 loading

The radical suppression of the photodarkening effect and laser performance deterioration via H2 loading were demonstrated in high-power Yb-doped fiber(YDF)amplifiers.The photodarkening loss at equilibrium was114.4 d B/m at 702 nm in the pristine fiber,while it vanished in the H2-loaded fiber.To obtain a deeper understanding of the impact of photodarkening on laser properties,the evolution of the mode instability threshold and output power in fiber amplifiers was investigated.After pumping for 300 min,the mode instability threshold of the pristine fiber dropped from 770 to 612 W,and the periodic fluctuation of the output power became intense,finally reaching 100 W.To address the detrimental effects originating from photodarkening,H2 loading was applied in contrast experiments.The output power remained stable,and no sign of mode instability was observed in the H2-loaded fiber.Moreover,the transmittance at 638 nm confirmed the absence of the photodarkening effect.The results pave the way for the further development of high-power fiber lasers.

Spectrally U-shaped profile of beam propagation factor in all-solid photonic bandgap fiber

We found the beam quality factor M^(2)of the fundamental mode as a function of wavelength is U-shaped in the working photonic bandgap(PBG) of an all-solid PBG fiber(AS-PBGF) for the first time,to the best of our knowledge,and our simulation results also match well with the phenomenon.The normal band that is near the high-frequency edge of the third PBG integrates the lowest M^(2)and single-mode operation simultaneously,while the other two edge regions suffer from anomalous variation of M^(2)versus wavelength.The general applicability of this finding can be further extended to other PBGs and also other representative structures in the AS-PBGF field.