A compact temperature sensor based on a fiber loop mirror (FLM) combined with an alcohol-filled high- birefringence photonic crystal fiber (PCF) is proposed and experimentally demonstrated. The output of the FLM is an interference spectrum with many resonant dips, of which the wavelengths are quite sensitive to the change of the refractive index of the filled alcohol for the interference of the FLM. Simulation analysis predicts a high temperature sensitivity, and experimental results show it reaches up to $6.6\mathrm{nm}/°\mathrm{C}$ for the $6.1\text{-}\mathrm{cm}$-long PCF used in the FLM.

A mode-locked all-fiber laser based on nonlinear multimode interference effect of the graded index fiber (GIMF) is proposed and demonstrated. The saturable absorber (SA) used in the laser consists of a step index multimode fiber connected by a graded index multimode fiber. Compared with the theoretically proposed SMF-GIMF-SMF structure, the introduction of the step index multimode fiber (SIMF) eliminates the restriction on the length of GIMF and provides a more flexible method of making a SA based on the NL-MMI. Through bending the device to a certain state, the modulation depth of the SA was measured to be 3.16%. The mode-locked laser output pulses have the pulse width of ~446 fs, bandwidth of 4.48 nm at the fundamental repetition rate of 11.73 MHz. This versatility and simplicity of the SA device combined with the possibility of scaling the pulse energy in large mode area double clad fiber laser make it highly attractive in ultrafast photonics.

A compact fiber polarizer is demonstrated by the filling of selected air holes of a hollow-core photonic crystal fiber (PCF) with a liquid. The liquid-filling results in an asymmetric waveguide structure, leading to a large polarization dependent loss. A $6\mathrm{mm}$ long ethanol-filled PCF exhibits a polarization extinction ratio of $\sim 18\mathrm{dB}$ over a wavelength range from $1480\mathrm{nm}$ to $1600\mathrm{nm}$.

We proposed and experimentally demonstrated a compact micro-displacement sensor with high sensitivity based on a long-period fiber grating (LPG) with an air-cavity. The sensor head is obtained by composing an air-cavity with the ends of a LPG and a single mode fiber (SMF). The wavelength shift of the LPG has a linear relationship with the length of the air gap which agrees well with the theoretical analysis. The experimental results show that the sensitivity is ~0.22 nm/µm within the micro-displacement range of 0 to 140 µm.

A novel fiber-optic surface plasmon resonance sensor for detection of $Cu^{2+}$ ions in an aquatic environment is proposed and experimentally demonstrated. The proposed sensor is fabricated by splicing no-core fiber between multimode fibers and then functionalizing the sensor surface with gold film and chitosan (CS)/poly(acrylic acid) (PAA) bilayers. The gold film is used to stimulate surface plasmon waves and make the sensor sensitive to changes in refractive index induced by $Cu^{2+}$ ions absorption on the CS/PAA bilayers. The proposed sensor has a linear relationship between the wavelength shifts and the concentrations of $Cu^{2+}$ ions from 0.2 to 50 μM and 50 to 500 μM, respectively. It is also observed that the sensor's performance is better at low concentrations, and its sensitivity decreases with the increasing concentration. Moreover, we find the proposed sensor can express the self-assembly process between CS and PAA clearly, which shows us a new and easy way to investigate the polyelectrolyte self-assembling.

An alcohol not full-filled high-birefringence photonic crystal fiber (HiBi-PCF) temperature sensor based on an optical fiber Sagnac interferometer (OFSI) is demonstrated and investigated in detail. A new phenomenon that the resonant dip wavelengths of the temperature sensor blueshift with temperature increasing is observed, which is contrary to that of the previously reported alcohol filled HiBi-PCF OFSI temperature sensor. By considering the influences of the group birefringence and the thermo expansion of alcohol, this phenomenon is explained very well. The temperature sensitivity of the proposed sensor is about $-1.17\mathrm{nm}/°\mathrm{C}$ and is only one-sixth of that of the alcohol full-filled HiBi-PCF OSFI.