A core‐clad fiber made of elastic, tough hydrogels is highly stretchable while guiding light. Fluorescent dyes are easily doped into the hydrogel fiber by diffusion. When stretched, the transmission spectrum of the fiber is altered, enabling the strain to be measured and also its location.

Strain sensors with high flexibility and stretchability are essential for quantifying strains generated by human activities due to the soft and curvilinear surfaces of the body. Here, we report the design and fabrication of a highly flexible and stretchable optical strain sensor that is applicable for the detection of human motions. The sensor is based on dye-doped polydimethylsiloxane optical fiber, which enables the quantitative detection of tensile strains by absorption changes of the light passing through the dye-doped fiber. Elaborate mechanical and strain tests confirm durability, reliability, and long-term stability of the sensor device. The sensor exhibits linear and repeatable responses in a large dynamic range up to 100%, far exceeding the stretchability of conventional silica fibers ($<1\%$), and the strain precision is below $+/-1\%$. We show that the sensor can be used to monitor various human activities, such as joint motion, speaking, and deep breathing in real time.

We report on the first experimental observation, to the best of our knowledge, of soliton molecules in a spatiotemporal mode-locked multimode fiber (MMF) laser. By adjusting the waveplates inside the cavity, not only the spatiotemporal mode-locking state with a stable single pulse but also soliton molecules are observed. Various soliton molecules, including soliton pairs, soliton triplets, and soliton quartets with different pulse separations, are achieved. Transition of different operation states with pump power is given. The results would be beneficial for further understanding of the nonlinear dynamics in spatiotemporal mode-locked MMF lasers.

Ultrafast fiber lasers with broad spectral coverage are in great demand for a variety of applications, such as spectroscopy, and biomedical diagnosis. Graphene is an ideal ultrawide-band saturable absorber. We report broadband (up to ~1000 nm) ultrafast pulse generation from three fiber lasers mode-locked by a single graphene saturable absorber device. The mode-locked pulses were based on Yb-, Er- and Tm:Ho-doped fiber lasers at the central wavelength of 1035, 1564, and 1908 nm, respectively. The maximum output energy is up to 16.2 nJ at 1908 nm. It is the first time that ultrafast fiber lasers covering 1, 1.5, and 2 μm spectral region are mode-locked with a single graphene device. Our results validate the intrinsic broadband operation property of graphene devices for all major fiber laser wavelengths from 1 to 2 μm.

Dissipative nonlinear wave dynamics have been investigated extensively in mode-locked lasers with single transverse mode, whereas there are few studies related to three-dimensional nonlinear dynamics within lasers. Recently, spatiotemporal mode locking (STML) was proposed in lasers with small modal (i.e., transverse-mode) dispersion, which has been considered to be critical for achieving STML in those cavities because the small dispersion can be easily balanced. Here, we demonstrate that STML can also be achieved in multimode lasers with much larger modal dispersion, where we find that the intracavity saturable absorber plays an important role for counteracting the large modal dispersion. Furthermore, we observe a new STML phenomenon of passive nonlinear autoselection of single-mode mode locking, resulting from the interaction between spatiotemporal saturable absorption and spatial gain competition. Our work significantly broadens the design possibilities for useful STML lasers thus making them much more accessible for applications, and extends the explorable parameter space of the novel dissipative spatiotemporal nonlinear dynamics that can be achieved in these lasers.

We report a new kind of polarization splitter based on dual-core photonic crystal fibers. The polarization splitter has a symmetric directional coupler configuration. Each core exhibits high birefringence, which gives rise to an adequate difference in the coupling lengths for the two orthogonal polarizations. A 1.7-mm-long splitter is obtained with the splitting ratio better than -11 dB and a bandwidth of 40 nm. The relationship between the length of the polarization splitter and the diameter of the air hole in the middle of the two cores is discussed.