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High-power continuous wave fiber lasers with narrow linewidths are required for spectral beam combining systems. A 2.9 kW Yb-doped fiber laser with an output linewidth as narrow as 0.31 nm employing a narrow-linewidth multilongitudinal-mode oscillator is experimentally demonstrated. The numerical simulations reveal that the spectral broadening of the bandwidth grows almost linearly with injected mode number, which provides a potential way to further suppress the nonlinear spectral broadening in high-power fiber lasers.

The use of shorter pulses is a practical way to improve the signal in two-photon excitation fluorescence microscopy. We report on the theoretical and experimental results of pulse compression in a two-photon excitation fluorescence microscope by using approximately 100-fs Ti:Sapphire laser and highly nonlinear photonic crystal fiber. Effects of the fiber parameters, transmitted power, and group-delay dispersion provided by the gratings have been investigated to optimize the compressor performance. By using a 20-mm-long photonic crystal fiber with a zero dispersion wavelength of 850 nm, a compressed pulse of 23.6 fs starting from 94 fs at 790 nm is experimentally demonstrated as a verification of our simulations. By integrating the compressor with a two-photon excitation fluorescence microscope, 5.6 times increase in autofluorescence intensity of NAD(P)H in Nasopharyngeal carcinoma cells is demonstrated, showing its potential in enhanced imaging and sensing for disease diagnosis.

In nonlinear optical microendoscope (NOME), a fiber with excellent optical characteristics and a miniature scanning mechanism at the distal end are two key components. Double-clad fibers (DCFs) and double-clad photonic crystal fibers (DCPCFs) have shown great optical characteristics but limited vibration amplitude due to large diameter. Besides reducing the damping of fiber cantilever, optimizing the structural of the actuator for lower energy dissipation also contributes to better driving capability. This paper presented an optimized actuator for driving a particular fiber cantilever in the view point of energy. Firstly, deformation energy of a bending fiber cantilever operating in resonant mode is investigated. Secondly, strain and stress analyses revealed that the four-plate actuator achieved lower energy dissipation. Then, finite-element simulations showed that the large-diameter fiber yielded an adequate vibration amplitude driven by a four-plate actuator, which was confirmed by experiments of our home-made four-plate actuator prototypes. Additionally, a NOME based on a DCPCF with a diameter of 350 μm driven by four-plate piezoelectric actuator has been developed. The NOME can excite and collect intrinsic second-harmonic and two-photon fluorescence signals with the excitation power of 10-30 mW and an adequate field of view of 200 μm, which suggest great potential applications in neuroscience and clinical diagnoses.