Fluorescence lifetime imaging can reveal the high-resolution structure of various biophysical and chemical parameters in a microenvironment quantitatively. However, the depth of imaging is generally limited to hundreds of micrometers due to aberration and light scattering in biological tissues. This paper introduces an iterative multi-photon adaptive compensation technique (IMPACT) into a two-photon fluorescence lifetime microscopy system to successfully overcome aberrations and multiple scattering problems in deep tissues. It shows that 400 correction modes can be achieved within 5 min, which was mainly limited by the frame rate of a spatial light modulator. This system was used for high-resolution imaging of mice brain tissue and live zebrafish, further verifying its superior performance in imaging quality and photon accumulation speed.

Optical fiber technology has changed the world by enabling extraordinary growth in world-wide communications and sensing. The rapid development and wide deployment of optical fiber sensors are driven by their excellent sensing performance with outstanding flexibility, functionality, and versatility. Notably, the research on specialty optical fibers is playing a critical role in enabling and proliferating the optical fiber sensing applications. This paper overviews recent developments in specialty optical fibers and their sensing applications. The specialty optical fibers are reviewed based on their innovations in special structures, special materials, and technologies to realize lab in/on a fiber. An overview of sensing applications in various fields is presented. The prospects and emerging research areas of specialty optical fibers are also discussed.

We demonstrate the fabrication of single-mode helical Bragg grating waveguides (HBGWs) in a multimode coreless fiber by using a femtosecond laser direct writing technique. This approach provides a single-step method for creating Bragg grating waveguides. Specifically, the unique helical structure in such an HBGW serves as a depressed cladding waveguide and also generates strong Bragg resonance due to its periodicity. Effects of pulse energy, helical diameter, and helical pitch used for fabricating HBGWs were studied, and a single-mode HBGW with a narrow bandwidth of 0.43 nm and a Bragg wavelength of 1546.50 nm was achieved by using appropriate parameters, including a diameter of 10 μm and a helical pitch of 1.07 μm. The measured cross-sectional refractive index profile indicates that a depressed cladding waveguide has been created in this single-mode HBGW. Moreover, five single-mode HBGWs with various Bragg wavelengths were successfully fabricated by controlling the helical pitch, and this technique could be used for achieving a wavelength-division-multiplexed HBGW array. Then, the temperature and strain responses of the fabricated single-mode HBGW were tested, exhibiting a temperature sensitivity of 11.65 pm/°C and a strain sensitivity of 1.29 pm/με, respectively. In addition, the thermal stability of the single-mode HBGW was also studied by annealing at a high temperature of 700°C for 15 h. The degeneration of the single-mode waveguide into a multimode waveguide was observed during the isothermal annealing process, and the peak reflection and the Bragg wavelength of the fundamental mode exhibited a decrease of ∼7dB and a “blue” shift of 0.36 nm. Hence, such a femtosecond laser directly written single-mode HBGW could be used in many applications, such as sapphire fiber sensors, photonic integrated circuits, and monolithic waveguide lasers.