Compared with ordinary optical fiber, polyimide fiber has the characteristics of high temperature resistance and high strength, which has important application in the field of optical fiber sensing. The common methods of polyimide coating stripping were introduced in this paper, including high temperature stripping, chemical stripping and arc ablation. In order to meet the requirements of FBG writing technology, a method using argon ion laser ablation coating was proposed. The method can precisely control the stripping length of the coating and completely does not affect the tensile strength of the optical fiber. According to the experiment, the fabrication process of polyimide FBG is stripping-hydrogen loadingwriting. Under the same conditions, 10 FBG samples were fabricated with good uniformity of wavelength bandwidth and reflectivity. UV laser ablation of polyimide coating has been proved to be a safe, reliable and efficient method.
For single longitudinal-mode lasers, the phase and frequency noises are very important parameters for their applications. The passive self-homodyne technique with an unbalanced Michelson interferometer is demonstrated to measure the phase and frequency noise characteristics. The Michelson interferometer for measurements is composed of a 3×3 optical fiber coupler and two Faraday rotator mirrors. The measurement performance is derived and discussed strictly from the transmission matrix of the coupler. The influence to the demodulation resulting from the asymmetry of the 3×3 optical fiber coupler is eliminated through the derived relationship between the differential phase fluctuation and the interferometer fringes. The technique is utilized to measure the phase and frequency noise features of a distributed feedback fiber laser. Based on the measured frequency noise spectrum, the lineshape, linewidth, and phase-error variance related to the laser coherence are able to be calculated and discussed well.
An intensity demodulated strain sensing scheme based on multiple phase-shifted fiber Bragg grating (FBG) is investigated. Wavelength shifts related to the external strain signal will result in various power change received by the photodetector. The reflected spectrum of the sensing multiple phase-shifted FBG are designed using the transfer matrix method to and fabricated by dithering phase mask method. The strain sensing scheme is analyzed numerically by relate the power change to reflectivity spectrum, thus help us optimize the design and fabrication of multiple phase-shifted FBG to achieve maximum sensitivity. The experimental results show that the strain sensitivity can be significantly enhanced by over 10dB by using phase-shifted FBG with many phase shifts compared to a normal FBG.
A dual-wavelength distributed feedback fiber laser with two symmetrical π phase shifts is fabricated and investigated. The stable operation in single polarization and dual wavelengths with 20 pm separation is achieved. It shows good performances including low relative intensity and narrow linewidth.
A distributed feedback (DFB) fiber laser with a ratio of backward to forward output power of 1:100 was composed by a 45mm length asymmetrical phase-shifted fiber grating fabricated on 50mm erbium-doped photosensitive fiber. Forward output laser was amplified using a certain length of Nufern EDFL980-Hp erbium-doped fiber to absorb surplus pump power after the active phase-shifted fiber grating and get population inversion. Using OptiSystem software, the best fiber length of the EDFL to get the highest gain was simulated. In order to keep the amplified laser with narrow line-width and low noise, a narrow-band light filter consisted of a FBG with the same Bragg wavelength as the laser and an optical circulator was used to filter the ASE noise of the out-cavity erbium-doped fiber. The designed laser structure sufficiently utilized the pump power, a DFB fiber laser of 32.5mW output power, 11.5 kHz line width, and -87dB/Hz relative intensity noise (RIN) at 300mW of 980 nm pump power was brought out.
A dual-π-phase shift distributed fiber laser (DFB FL) with symmetric structure is investigated as a strain sensor. The
sensing performances under both monolithic and local axial strain are discussed by use of transfer matrix method. While
the two lasing wavelengths of dual-π-phase shift DFB FL experience the same change as that of single-η-phase shift
DFB FL under monolithic strain, their wavelength interval changes with local strain. And the strain sensitivity of beat
frequency is calculated to be 70.6 MHz/με while one phase shift area is insensitive.
A novel heterodyne continuous wave lidar system based on single-mode fiber (SMF) components and instruments is
reported. In order to improve the signal-to-noise ratio (SNR) of heterodyne lidar system, the four causes producing carrier
feed-through are presented, including: (1) the return loss of optical antenna; (2) the direction of fiber circulator; (3) the
extinction ratio of acousto-optic frequency shifter (AOFS); (4) close object's reflection. Then theoretical analysis and
experimental study for the methods of eliminating carrier feed-through are conducted. The results demonstrate that
carrier feed-through mainly arises from the non-ideality of optical components. By improving the traditional heterodyne
optical structure and enhancing the performance of optical components, the carrier feed-through power can be decreased
by more than 20dB.