We demonstrated a compact tunable and switchable single/dual-wavelength erbium-doped fiber laser. The fiber laser can be tuned and switched from single-wavelength to dual-wavelength oscillation by using our recently proposed tunable comb filter. The comb filter consists of a section of multimode fiber (MMF) coiled into a polarization controller and two sections of single mode fibers (SMFs) to form a SMF/MMF/SMF structure, serving as a simple tunable all-fiber Mach-Zehnder interferometer. Due to the insertion of the MMF-based polarization controller (PC), an additional phase shift is introduced from the difference of the birefringence intensity in different dominant modes, which can be used to tune the fiber laser. In the experiment, by properly adjusting the PC, a tuning range of 9.3 nm can be achieved for the single-wavelength operation. Moreover, dual-wavelength operation with different free-spectral-ranges can be obtained. The tunable and switchable fiber lasers are of great importance for their applications in optical testing, optical fiber sensing, and signal processing.
A spectrum tunable method is presented for a high-birefringence fiber loop mirror (HiBi-FLM) comb filter with a polarization controller (PC) in the loop, where not only the wavelength of the comb filter can be continuously tuned with peak transmission remaining at unity, but the peak amplitude can be also continuously tuned without wavelength shift. By using the Jones matrix, we theoretically analyze the comb filter. The analytical expressions of the reflectivity and transmissivity are derived in terms of the orientation angles of three waveplates of the PC. The optimum conditions for wavelength and peak amplitude tunable operations are obtained. Our theoretical results show that when the two quarter-waveplate (QWP) angles are optimally set, the PC could act as a phase shifter or a polarization rotator, which empowers the comb filter to be wavelength continuously tunable or to be peak amplitude variable from 0 to 1 by only rotating the half-waveplate (HWP) of the PC. Moreover, the wavelength shift has a linear relationship with the HWP angle. When the two QWP angles are not optimized, PC has a combined effect of a phase shifter and a polarization rotator, leading to a wavelength shift accompanied by a change of the peak amplitude when tuning the HWP. Our theoretical prediction has been verified by experimental results.
We proposed an ultra-sensitive refractive index sensor by using indium-doped cadmium oxide as a plasmonic material operating in near-infrared based on Fano resonance. The proposed sensor has a hybrid multilayer waveguide structure that supports both a long-range surface plasmon polariton (LRSPP) mode and a dielectric waveguide (DWG) mode. The design strategy of the structure parameters of the inner layers is elaborated in detail through the numerical analysis of the two modes. By suitably tailoring the thickness of the coupling layer, a strong mode coupling between the two modes could be achieved, leading to a sharp asymmetric Fano resonance. With the designed optimal physical parameters, our proposed sensor could achieve a maximum intensity sensitivity of 19,909 RIU−1, a 193-fold enhancement than that of a conventional long-range SPR (LRSPR) based scheme. The proposed design can be a promising platform for biochemical sensing in the near-infrared region.
We present a new tunable all-fiber compact multimode fiber (MMF)-based filter and its applications in fiber sensors. Using the matrix optics approach, we theoretically analyze the transmission characteristics of the tunable filter. The expression of the transmission of the optical filter is the same as that of a regular Mach-Zehnder interferometer (MZI) but with an additional optical phase shift in the sinusoidal function, which makes the MMF-based filter tunable. The phase shift could be changed by properly adjusting a polarization controller where the MMF is simply coiled into. The theoretical result has been verified by our experiment. The proposed tunable filter has been employed for intensity interrogation of a fiber Bragg grating (FBG)-based temperature sensor where the MMF-based filter serves as an edge filter. With the tuning technique, we are able to set the FBG peak to the linear regime of the interference pattern to achieve optimum sensing operation range. By monitoring the optical power changes, it is feasible to obtain information that permits temperature measurement with a simple and low-cost structure.
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