A microwave photonic filter(MPF) with variable coefficient is proposed and demonstrated, which is constructed by a multi-wavelength fiber laser and Mach-Zehnder interferometer(MZI). Through changing the slope characteristics of Mach-Zehnder interference spectrum adjusted by optical variable delay line(OVDL), the conversion from phase modulation(PM) to intensity modulation(IM) is realized. The multi-wavelength fiber laser with Lyot-Sagnac optical filter has variable wavelength spacing. So the designed filter has a variable number of taps and tap weights. As a result, the tunable range of passband center frequency is 2.6 GHz. The reconfigurability of MPF can be also realized by adjusting the output of fiber laser.
A novel switchable dual-wavelength erbium-doped fiber(EDF)laser is demonstrated.The wavelength selection element consists of two fiber Bragg gratings(FBGs),a polarization controller(PC)and a 3 dB coupler forming a Sagnac loop inserted with two FBGs.We study the effect of coupling ratio on filtering performance in this paper.By adjusting PC,we can change the wavelength-dependent loss,and then using nonlinear polarization rotation effect to suppress the mode competition caused by the homogeneous broadening of EDF,we obtain single-and double-wavelength laser outputs.At room temperature,under 200 mW pump power,dual-wavelength laser is achieved,and the center wavelengths are 1545.34 nm and 1548.20 nm,respectively.The peak power values are–13.36 dBm and–14.58 dBm,and side mode suppression ratios(SMSRs)are 41.10 dB and 39.88 dB,respectively.Within two hours,the maximum fluctuation of peak power is less than 0.7 dB,which shows that the demonstrated fiber laser is stable.Moreover,by adjusting PC,singel-wavelength laser output is obtained,the peak power is–6.27 dBm or–5.45 dBm,and SMSR is40.03 dB or 39.96 dB at 1545.34 nm or 1548.20 nm,respectively.
An all-fiber sensor based on a cascaded optical fiber device is proposed and demonstrated, and its sensor head is composed of a core-offset Mach-Zehnder interferometer(MZI) and a long-period fiber grating(LPFG). In the experiment, two dips shaped by the intermodulation between the interference fringe of MZI and the resonant wavelength of LPFG are monitored. Experimental results show that temperature sensitivities of two dips are 0.060 7 nm/°C and 0.056 3 pm/°C, and the refractive index(RI) sensitivities are –18.025 nm/RIU and –55.06 nm/RIU, respectively. The simultaneous measurement of the temperature and external RI is demonstrated based on the sensitive matrix. Its low fabrication cost, simple configuration and high sensitivity make this sensor have potential applications in chemical and biological sensing.
An alternative solution for the simultaneous measurement of temperature and refractive index is presented. A local micro-structured fiber Bragg grating （LMSFBG） is formed as the sensing head, in which a standard grating is etched by HF. According to the phase shift theory, the main spectral change of the LMSFBG is the formation of a narrow allowed band, which is strongly dependent on the etching features and the surrounding refractive index. As such, the temperature and refractive index measurements can be achieved by the shifts of the double peaks and narrow allowed band, and their fitting linearity coefficients are 0.996 and 0.994, respectively. Thus, the reflection and transmission peaks of the LMSFBG have a good linear relationshi~ with tem^era,ture and refractive index.
A highly birefringent index-guiding photonic crystal fiber (PCF) with flattened dispersion and low confinement loss is proposed by introducing two small air holes with the same diameter in the core area. The fundamental mode field, birefringence, confinement loss, effective mode area and dispersion characteristic of the fibers are studied by the full-vector finite element method (FEM). Simulation results show that a high birefringence with the order of 10 -3 and a low confinement loss of 0.001 dB/km are obtained at 1550 nm. Furthermore, flattened chromatic dispersion from 1450 nm to 1590 nm is obtained.
