In this manuscript, I describe an overview of special issues for new development of fiber laser technology. The topic of this issue covers development of fiber lasers, high power fiber laser technology, optical frequency comb based on fiber laser, MIR (Mid infrared) fiber lasers, specialty optical fibers, and biomedical imaging using nonlinear microscopy and optical coherence tomography.

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This paper reviews the advances of ultra-short-pulse fiber lasers. First, saturable absorbers (SAs) essential for passively mode-locked fiber lasers are classified to real SAs and artificial SAs, and investigated in detail. Next, operations of mode-locked fiber lasers are classified depending on amount of intracavity dispersion. Finally, some examples of mode-locked fiber lasers are presented.

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Fiber laser is now widely recognized as the most reliable high average power, high brightness source. However, the huge nonlinearities due to its small mode area and long interaction length make high peak-power/high energy radiation so difficult. For peak-power/energy scaling, spatial beam combining, in which multiple fiber lasers are phase-locked and their beams are combined, is the key architecture and is extensively investigated in the world. We have been exploring phase-locked multicore fiber lasers and are focusing here. A pioneering beam combining architecture based on reinforcement learning is also introduced. Another approach, temporal beam combining, has also recently been extensively investigated by researchers. Temporally separated pulses, with each peak power below the nonlinearity limit of fiber laser/amplifier, are stacked coherently and can form one giant pulse with a higher peak power. In this paper our recent results are mainly presented and the subjects and future prospects will be discussed.

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Dual-comb spectroscopy is an attractive tool for high-speed, high-resolution, and broadband spectroscopy. However, based on two mode-locked lasers and a complex servo system with an auxiliary CW laser, the prohibitive cost of a conventional dual-comb laser system prevents further applicability in broad fields. To overcome this problem, we develop a dual-comb fiber laser that simultaneously generates two high-coherence ultra-broadband frequency combs with slightly different repetition rates.

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Mid-infrared fiber laser technology has advanced dramatically over the last 20 years. In this review, we describe high-power mid-infrared lasers and mid-infrared ultrashort pulse lasers using fluoride-glass fiber gain media. Diode-pumped Er 3+-doped fiber lasers with a wavelength of around 2.8 μm have reached continuous-wave output power that exceeds 40 W. Mid-infrared fiber lasers using Ho 3+ and Dy 3+ ions have also achieved considerably higher output. Ultrashort pulse laser technology with fluoride- glass fiber gain media has been created within the decade and is developing into various research based on nonlinear optics. These laser technologies are paving the way for the development of powerful mid-IR sources for cutting-edge fields in industry, medicine, and science.

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This paper describes a new prospect for broadband mid-infrared highly coherent supercontinuum generation. Tellurite and chalcogenide glass with high transparency up to the mid-infrared range are used as fiber materials. We successfully develop all solid hybrid microstructured optical fibers made of tellurite and chalcogenide glass to control chromatic dispersion and demonstrate for the first time to our best knowledge that highly nonlinear soft glass microstructured optical fibers are promising media for broadband mid-infrared highly coherent supercontinuum generation.

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We introduce stimulated Raman scattering (SRS) microscopy, which is a powerful molecular-vibrational imaging method for biology and medicine, and discuss its requirements for pulsed lasers. After reviewing the performance of several previously reported laser systems, we describe our developed high-speed wavelength-tunable laser source, composed of a mode-locked Yb fiber laser in a figure-nine configuration and a high-speed tunable spectral filter. This laser source has been used to demonstrate various types of high-speed multicolor SRS imaging. We also describe our efforts to achieve ultralow-intensity- noise mode-locked fiber lasers.

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Ultrashort pulse fiber lasers are practical, maintenance-free, and useful light sources that have been applied for various laser applications. We have been investigating high-resolution optical coherence tomography using the broadband fiber laser sources. Recently, we demonstrated high resolution, deep tissue imaging in the optical coherence tomography (OCT) and optical coherence microscopy (OCM) using a fiber laser supercontinuum at λ = 1.7 μm. Detailed structures of mouse brain were successfully observed with high resolution.

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