Fig. 1. Plasmon resonance-based fiber sensors. (a) Left: schematic of the fiber biosensor based on SPR with cladding partially polished for mode overlap with the metal layer. Right: normalized transmission spectra of the SPR-based sensor under different concentrations of glucose solution. (Adapted with permission from [32]. Copyright Optical Society of America.) (b) Fluorescence emission spectrum of Rh B under different concentrations in side-polished microstructured fiber with gold coating. Inset: cross section of D-shaped microstructured fiber with gold coated on top surface. (Adapted with permission from [34]. Copyright 2011 Elsevier.) (c) Microstructured fiber with Au coated on the inner wall of the air hole filled with analyte solution. (Adapted with permission from [40]. Copyright IOP Publishing.) (d) Left: schematic of fiber biosensor based on LSPR with Au nanorods coated on the fiber core. Right: normalized excitation spectrum showing LSPR redshift as OTA concentration increases. (Adapted with permission from [44]. Copyright 2018 Elsevier.) (e) Left: LSPR with Au NPs coated on tapered fiber, demonstrating an ultralow LOD of 5 aM. Right: sensing result showing the variation of transmission spectrum as the cholesterol concentration increases. (Reprinted with permission from [50]. Copyright 2019 Elsevier.) (f) Left: LMR with either SnO2 or indium tin oxide (ITO) coated on D-shaped single-mode fiber (SMF) or unclad multimode fiber (MMF). Right: LMR shift with respect to antigen concentration for three kinds of LMR sensors. (Adapted with permission from [52]. Copyright 2018 American Chemical Society.)

Fig. 2. SERS-based fiber biosensors. (a) Left: schematic of the SERS-based fiber optic biosensor with metallic nanoparticle coated on the inner wall of PCF. Right: SERS spectra of R6G molecules with concentration of 10−6  M, comparing the cases among the direct sampling and coated LCPCFs. (Adapted from [75] with the permission of AIP Publishing.) (b) Left: schematic of the SERS-based fiber optic biosensor utilizing bioanalyte and the metallic nanoparticle solution. Right: measured SERS spectra of 0.2 μg/mL cytochrome C solution by using bulk solution and TCMMF. (Adapted with permission from [65]. Copyright 2011 American Chemical Society.) SNP is short for silver nanoparticles. CTAB is short for cetyltrimethylammonium bromide. (c) SERS spectrum of 10−10  M R6G using liquid solution to fill the core of the HCF. Inset: cross section of HCF, demonstrating LOD of 10−10  M. (Adapted with permission from [80]. Copyright Optical Society of America.) (d) SERS spectra under R6G solution with different concentrations. Inset: cross section of channel PCF achieving LOD of 50 fM. (Adapted with permission from [68]. Copyright 2016 Elsevier.) (e) Simulation results investigating how fiber length affects the Raman intensity under different NP concentrations for SERS-based fiber sensor. (Reprinted from [76] with the permission of AIP Publishing.)

Fig. 3. Bragg grating-based fiber sensors. (a) Top: schematic of grating-based fiber sensors with LPBG using GO coating to achieve ultrasensitive detection. Bottom: sensorgram showing wavelength shift normalized to SRI sensitivity under different analyte concentrations for three sensors. (Adapted with permission from [13]. Copyright 2018 Elsevier.) (b) Top: cross section of PBGF with PS/PMMA multilayer structure. Bottom: transmission spectra of PBGF during the PVB layer dissolution process. (Reprinted with permission from [89]. Copyright Optical Society of America.) (c) Top: schematic of side-polished PBGF. Bottom: the PBGF wavelength shift with respect to refractive index change for three different polishing depths (PDs). (Reprinted with permission from [91]. Copyright 2012 IEEE.)

Fig. 4. Interferometer-based optical fiber sensors. (a) Schematic of fiber with SMF-MMF-SMF structure for the interferometry-based sensor. (b) Left: schematic of interferometry-based fiber sensor with SMF-etched MMF-SMF. Right: sensing result of wavelength shift corresponding to different concentrations of goat anti-IgG from 4 mg/L to 200 mg/L. (Adapted with permission from [92]. Copyright 2018 Elsevier.) (c) Top: schematic of the fiber sensor with SMF-misaligned fiber-SMF. Bottom: sensing measurement data showing wavelength shift with respect to the analyte concentration. The measurement data is fitted by Langmuir isotherm curve. Inset: the linear fitting curve in a low analyte concentration range from 0.5 to 5 μg/mL. (Adapted with permission from [96]. Copyright 2018 Elsevier.) (d) Top: schematic of the fiber sensor SMF-NCF-SMF structure. Bottom: sensing measurement result showing the absolute wavelength shift with respect to the analyte concentration. (Adapted with permission from [14]. Copyright 2018 Elsevier.) (e) Top: schematic of the interferometry-based sensor with the FP cavity external of the optical fiber. Bottom: measured reflection spectra from the samples with different refractive indices. (Adapted with permission from [103]. Copyright Optical Society of America.)

Fig. 5. Nonlinear bioimaging techniques. (a) Schematic of a two-photon fluorescence endoscope. (b) Scanning electron microscopy (SEM) image of a DCPCF overlaid with the light output patterns: single-mode propagation in the core at 800 nm and multimode propagation in the inner cladding at 410 nm. (c) 3D visualization of the human breast cancer tissue imaged using system in (a). [(a)–(c) reprinted from [115] licensed under CC BY 4.0.] (d) Schematic of a fiber-probe based CARS imaging system. Fiber 1 was used for delivery of the excitation pulses and fiber 2 was used for detecting the CARS signal from the sample. (e) CARS image of small adipocytes of mouse ear skin. (f) CARS image of adipocytes of the subcutaneous layer of rabbit skin. Scale bar is 50 μm. [(d)–(f) reprinted with permission from [116]. Copyright Optical Society of America.)

Fig. 6. Fiber probes for acoustic bioimaging. (a) SEM images of carbon nanotubes and gold nanoparticles as the light-absorbing materials attached at the fiber tip for LGUS. (Adapted from [128] licensed under CC BY 4.0. Adapted with permission from [129]. Copyright 2014 AIP Publishing.) (b) All-optical ultrasound imaging based on fibers encased in a needle for interventional imaging. (c) The needle tip was positioned at the right atrial appendage wall with imaging depths extended more than 1 cm into the tissue. [(b) and (c) adapted from [130] licensed under CC BY 4.0.] (d) Schematic of the all-optical endoscopic imaging system with a dual clad fiber. (e) Photo of the concave cavity FP sensor probe. [(d) and (e) adapted with permission from [131]. Copyright 2011 SPIE.]

Fig. 7. Selected examples of angled and tapered fiber probes for in vivo applications. (a) Flexible transbronchial smart needle with angle fiber probe for biopsy guidance. (b) Microscopic photograph of the polished ball-lens optical probe. (Reprinted with permission from [150]. Copyright Optical Society of America.) (c) Schematics, (d) cross-sectional view, (e) integrated device of flexible tube-shaped neural probe for recording and optical stimulation of neurons at arbitrary depths. (Adapted with permission from [151]. Copyright Optical Society of America.)

Fig. 8. Three-dimensional optrode array for infrared neural stimulation. (a) SEM micrograph of the 3D optrodes array with different lengths, (b) tapered profile of the 3D optrode tip, and (c) a detailed description of the structure. (Reprinted with permission from [157]. Copyright Optical Society of America.)

Fig. 9. Ray tracing simulation of a contact focusing three-sphere microprobe. (Reprinted with permission from [162]. Copyright Optical Society of America.)