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In this paper, a 6 kW continuous-wave all-fiber laser by direct 975 nm diode pumping is reported. The unique part of this system is the usage of non-wavelength-stabilized laser diodes in a bidirectional pumping configuration. The bidirectional pumping configuration not only increases the pumping power, but also shortens the fiber length and thus, restricts the nonlinear Raman effect. The fiber laser emits a maximum output power of 6020 W at a wavelength of 1070.1 nm without reaching the Raman threshold. With a corresponding optical-to-optical efficiency of 76.5%, ASE and Raman suppression of 25 dB could be reached. No transverse mode instability occurred during the experiment, and the power stability was measured to be within 2.5%@5260 W during a 1 h continuous operation test.

Single fiber imaging has evolved into a powerful method for detecting minute objects in narrow spaces. However, existing systems are not conducive to imaging dynamic objects at depth due to their bulky probes, time-consuming scanning acquisition methods, and transmissive illumination mode. Minimally invasive reflection mode imaging with high spatial and temporal resolution remains an open challenge. Here, a precise and high-speed imaging scheme without scanning is proposed. Multimode fiber imaging technology is incorporated into an all-fiber aberration-free precision detection system. High temporal resolution (5000 fps) detection of tiny natural scenes is experimentally realized by optimizing the approximation of the inverse transmission matrix in a simple and compact setup. The system can display the detected screen in real-time and the computational imaging with a large depth of field (1 mm) is enabled by jointly learning. The recovery results are superior compared to typical deep neural networks. The demonstrated scheme offers a new possibility for many applications, for example, microendoscopy, all-optical computing, and remote high-speed video transmission.

This paper presents an approach that combines the generalized multimode nonlinear Schrodinger equation with a transmission model to analyze spatiotemporal characteristics of multimode interference in single mode/large mode area fiber–graded-index multimode fiber–single mode fiber (SMF/LMA-GIMF-SMF) structures for the first time. Approximated self-imaging (ASIM) behavior in GIMF and the study of the latter structure used in spatiotemporal mode-locked fiber lasers are first demonstrated. Simulations show that these structures can work as saturable absorbers enabling high-energy pulse output due to nonlinear intermodal interactions and intensity-dependent multimode interference. Otherwise, underlying ASIM is proven that it can perturb the transmission of SMF/LMA-GIMF-SMF, causing instability of their saturable-absorption characteristics. This paper provides a theoretical guide for many applications, such as beam shaping, mode conversion, and high-energy ultrafast fiber laser.

Detection of dynamical scenes at ultrafast speeds serves as a foundation for modern engineering, chemistry, material science and biomedicine, et al. For biomedical applications, in vivo microscopic imaging is often required, which has led to the development of fiber-probe-based endoscopy. However, the combination of ultrafast image acquiring with fiber endoscopy has not yet been achieved, which is vital for exploration of transient biomedical phenomena. Here, we propose a scheme of all-fiber image detection at an ultrafast speed without any free-space optical elements. Image detection is achieved based on the transformation of two-dimensional spatial information into one-dimensional temporal pulsed signal streams by leveraging the high intermodal dispersion in a multimode fiber. Deep learning algorithms are subsequently deployed to reconstruct the images detected by the fiber probe from the temporal waveforms acquired at the other end of the fiber. The fiber probe can directly detect micron-scale objects without any bulk objective, and image detection has been experimentally realized with a high frame rate (15 Mfps), a large frame depth (104) and extremely short shutter time (30 ps) simultaneously. This ultrafast detection scheme, combined with high mechanical flexibility and high level of integration, can stimulate future research on exploring various in-vivo ultrafast phenomena.