Dejun Feng

Tunable dual-wavelength passively mode-locked thulium-doped fiber laser using carbon nanotube

Abstract. A tunable dual-wavelength passively mode-locked thulium-doped fiber laser (TDFL) based on single-wall carbon nanotube is demonstrated. By properly tuning the pump power and the polarization controller, both single- and dual-wavelength mode-locked operation can be achieved. The repetition rates of the single- and dual-wavelength mode-locked operation are both 17.64 MHz. The duration of the ultrashort soliton pulse is about 3.7 ps. By appropriately adjusting the polarization state of the laser, the dual wavelength can be tuned from 1879.8 and 1894.5 nm to 1903.3 and 1914.1 nm.

Novel FBG vibration sensor based on matching filter demodulation

A novel fiber Bragg grating (FBG) vibration sensor is designed based on optical fiber sensing technology. The principle of vibration detection using optical fiber Bragg grating is analyzed. At the same time, both the theories of matching grating demodulation technology and optical fiber grating tuning technology based on constant strength cantilever are adopted. As a result, the sensitivity of sensor and the stability of the detecting signal are all improved by employing additionally electromagnetic damping. Good results are obtained from vibration detecting experiments using vibrator standard signal, then Fourier analysis is applied. Frequency detection ability of the vibration sensor is favorable. The frequency detection precision is ± 0.5 Hz, and frequency measuring scope is up to 200 Hz. In addition, the detecting signal has high signal to noise ratio. From shock vibration experiment, it can be seen that free vibration frequency of the sensor is about 250 Hz. Above all, the vibration sensor has extensive application prospects in practical engineering.

Development of Distributed Temperature Sensor Based on Single-mode Fiber

The distributed optical fiber temperature measurement system (DTS) is a kind of sensing system, which is applied to the real-time measurement of the temperature field in space. It is widely used in monitoring of production process: fire alarm of coal mine and fuel depots, heat detection and temperature monitor of underground cable, seepage and leakage of dam. Through analyzing temperature effect of optical fiber Raman backscattering theoretically, a distributed temperature sensor based on single-mode fiber was designed, which overcame the inadequacies of multimode fiber. The narrow pulse width laser, excellent InGaAS PIN, low noise precision difet operational amplifier and high speed data acquisition card in order to improve the stability of this system were selected. The demodulation method based on ratio of Anti-Stokes and Stokes Raman backscattering intensity was adopted. Both hardware composition and software implementation of the system were introduced in detail. It is proved that its distinguishing ability of temperature and space are 1 m and 2 m, respectively. The system response time is about 180 s, with a sensing range of 5 km and the temperature measurement range 0~100 °C.

Design of Infrared SPR Sensor Based on Bimetallic Nanowire Gratings on Plastic Optical Fiber Surface

In this paper, a bilayer metallic grating with pitch of 1 μm on the flank of a cylindrical plastic optical fiber was proposed as a novel optical sensor. Surface plasmon resonance excited by this structure for refractive index sensing is investigated by finite difference time domain methods. A theoretical sensitivity of 643.75 nm/RIU is obtained, where the wavelength of the resonant sharp dip is obviously shifted from 1.398 μm to 1.501 μm for surrounding refractive index increased from 1.33 to 1.49. Several factors which affect the accuracy of the device, such as the polarization state of the incident light, grating pitch, and refractive index of the sensing medium, are discussed. The successful fabrication of the grating indicates the feasibility of the devices. The sensor is expected to have potential applications in the chemical, biomedical, environmental safety, and food safety industries as well as in many other aspects.

Side-Hole Plastic Optical Fiber for Testing Liquid’s Refractive Index

A multimode plastic optical fiber with a side-hole fabricated by precision mechanical drilling technology is presented for refractive index (RI) sensing. A higher sensitivity of 1862.1 μW/RIU, when center wavelength of optical source is 652 nm and launched power is 1 mW, is obtained in a larger hole diameter of 350 μm, indicating that the larger hole diameter can increase the sensitivity of the sensor. Ray-optics method and finite-difference time-domain method are both utilized to theoretically evaluate the characteristics of the structure for RI testing, which is well in agreement with the experimental results. Energy field distribution is analyzed to explain the sensing mechanism of the structure, which demonstrates that the divergent effect of the side-hole like a negative lens is the key to sensing. Due to the advantages of relatively easy fabrication, low cost, and capable of continuous measurement, this side-hole-based fiber sensing structure is attractive for RI monitoring in chemical, biological, and biochemical sensing region.

Synthesis of special fiber Bragg grating filters by layer peeling algorithm

A kind of dispersionless fiber Bragg grating (FBG) filter and two FBGs with triangular spectrum will be synthesized by discrete layer peeling algorithm. Simulation results show its feasibility of this method to design the grating filters with special spectrum requirement.

Method for tuning chirp ratio of fiber gratings without central wavelength shift

Abstract. A method for tuning the chirp ratio of a fiber grating without the central wavelength shift is proposed by using a two-fixed-end compressive beam, which induces a linear strain distribution along the grating. This technique allows dynamic control of the grating’s chirp ratio by changing the displacement of the translation stage. The 3-dB bandwidth tuning range is from 6.52 to 13.5 nm when the grating is tuned in the broadening way, while the bandwidth tuning range is from 6.52 to 0.95 nm in the compressing way. The proposed method has potential applications for the dynamic dispersion compensation and the sensors of pressure and displacement, etc., by detecting the change of bandwidth information.

Tunable chirped fiber Bragg grating based on two-fixed-end compressive bar without central wavelength shift

A novel method of tuning chirped fiber Bragg gratings (CFBGs) without a center wavelength shift is proposed. The designed structure can flexibly control the chirp ratio of the CFBG. The proposed tuning technique has potential applications in the optical communication and fiber sensor field.

Study of chirped fiber Bragg gratings apodized with different apodization length ratios

Symmetrical apodization technique of the chirped fiber Bragg grating has the advantages of suppressing the sidelobes of reflection spectra and smoothing the curves of group delay. However, it shortens the bandwidth of the reflection spectrum markedly. Compared with symmetrical apodization method, the asymmetric method can increase the 3dB bandwidth by 64.08% without the change of group delay curves. The apodization simulation is implemented by using a rised cosine function with different apodization length ratios at both ends of the grating. There is a compromise between the bandwidth and the group delay ripple. The result shows that the grating with 30% apodization at the long wavelength end and 20% apodization at the short wavelength side can improve the reflection bandwidth effectively and depress the group delay ripple within the range of ±2 ps.

Design of a multi-channel mechanically induced long period fiber gratings module

A multi-channel tunable mechanically induced long-period fiber gratings (LPFGs) scheme is presented, which can induce several LPFGs with different resonance wavelengths simultaneously. LPFGs spectra characteristics are simulated to find the influence of these parameters such as grating length, tilt angle and the pressure on the fiber. The simulation results show that the transmission loss peak mainly depends on both the grating length and the pressure, while tilt angle factor dominates the resonant wavelength. The infuence of the pressure and tilt angles on the transmission spectra is experimentally studied. This multi-channel LPFGs module will have great potential applications in the fiber sensing field and flexible filter design region.