Photonic crystal fiber (PCF) can provide both large mode area and low numerical aperture, meanwhile, offer heavy doping rate, good beam quality, and effective dispersion control. Coherent combining is an effective way to further elevate output power of a fiber laser. It is hence very beneficial to develop high-power fiber lasers using coherent combining through a multi-core PCF. In this work, by using multi-core PCF design and coherent combining technique, we studied theoretically and experimentally the multi-beam coherent combining in PCFs. We adopted a symmetrical structure to arrange multiple fiber cores, and have air holes surround them uniformly forming the cladding. Between these cores are solid glass filled. With an appropriate design of PCF structure, the coherent combining of multiple laser beams can bring higher output power up to kilowatts, while maintain good beam quality. Based on the evanescent-wave coupling theory, the modal coupling among seven cores and nineteen cores was studied. It is shown that the evanescent coupling is much stronger than the diffractive coupling if the cores are close enough. In the experiment, we fabricated rare-earth doped double-clad seven-core and nineteen-core PCFs using a stacking-capillary method. The near-field and far-field images were adopted to observe the mode coupling. Since all laser beams pass nearly the same optical length in one PCF, the phase matching can be easily realized. The theoretical and experimental results were compared, which shows that this kind of integrated multi-core PCFs can be very good candidates to achieve high-power coherent beam combining.
High-birefringence photonic crystal fiber (PCF) has many applications in fiber lasers, optical fiber communications and sensors, and is usually made by arranging asymmetrical air holes in its cross section. Though this design can obtain high birefringence in the fiber, it may lose the structural symmetry of mode field, and increase difficulty when connected to other fiber-optic devices. In this work, we propose a new design with a capability of having both high birefringence and good structural symmetry. Our design is based on a photonic quasi-crystal (PQC) fiber structure, in which air holes are arranged in an aperiodic order in the cladding. This kind of PQC fiber can introduce controllable birefringence into the fiber core and maintain certain symmetry of mode field. Using the full vector finite element method, we studied the mode field, birefringence, loss and nonlinear effect of the proposed PCF. It is shown that a birefringence coefficient of glt;1.5x10<sup>-2</sup> can be obtained at 1.55 μm.
Total-internal-reflection (TIR) typed fiber sensors based on photonic crystal fibers (PCFs) are made by filling samples into PCF cladding holes, where the interaction of light wave occurs between the evanescent wave of fiber core and the filled samples. This can avoid common transmission losses caused by fiber surface roughness. The interaction region of the evanescent wave in a PCF and the filled samples are almost coincident, so increasing fiber length can enhance the light-matter interaction and enable accurate detection of tiny changes of samples. However, it is difficult to inject sample materials into stomatal cladding of TIR-typed PCF sensors due to small volume of pores. In addition, the energy utilization rates of TIR-typed sensors are relatively low, only about 6%. A main influencing factor on the sensitivity of TIR-typed PCF sensors is the power fraction of pores in the whole cross section. In order to improve the sensitivity, one can elevate the power fraction of PCF pores. Based on the above considerations, a novel three-core double-clad PCF is designed, where samples are injected into the middle hole and two Yd-doped cores are arranged on its two sides for active excitation. Our theoretical calculation and experimental test show that this kind of structure can not only increase the coupling efficiency of the evanescent wave into the air holes effectively, but also gain higher detection sensitivity of trace samples.