Twin-core rare-earth doped nonlinear fiber devices

Twin-core rare-earth doped nonlinear optical fiber devices, including optical fiberswitches, optical fiber filters and optical fiber amplifiers, are explored in this thesis.An analytical method resorting to the energy E of an oscillator with perturbednonlinear potential is proposed and demonstrated for investigating the bifurcation andthe near-separatrix-crossing of a nonlinear fiber coupler with loss and saturablenonlinearity. Two characteristic equations for bifurcation and near-separatrix-cros singare obtained, from which the critical optical power, the minimum coupling length andthe maximum coupler length can be decided. These two equations can be used as thedesign criteria of a nonlinear fiber coupler with loss and saturable nonlinearity.The techniques of fabrication and measurement of twin-core rare-earth dopednonlinear fibers are described. The nonlinear refractive index coefficients in erbiumor neodymium doped fibers are determined using two core fibers in which only onecore is doped. The nonlinearity caused by population redistribution in rare-earth dopedfibers is wavelength-dependent. Moreover, the nonlinear saturation effect is observed.The all-optical fiber switch constructed from a twin-core erbium-doped fiber isstudied theoretically and experimentally. The switching behaviour is simulatedtheoretically, and a switching fraction of 65% is obtained experimentally. Theadvantage of this switch is that it requires only a fraction of a milliwatt of input powerto switch. The transient switching experiment indicates that the switching responsetime is in the order of 10 ms.Furthermore, a compact optical switching device through the use of a twin-coreNd3+-doped fiber coupler and a laser diode is demonstrated. Steady state switching andtransient switching is obtained. The switching power is 5.6 mW and the switchingspeed is 500 p.s which is 20 times faster than that in twin-core Er^-doped fiber couplers.In addition to the passive fiber devices, an active twin-core rare-earth doped fibercoupler is proposed and studied theoretically. It is demonstrated both numerically andexperimentally that a narrow-band wavelength filter/demultiplexer with gain can beconstructed from a twin-core rare-earth-doped fiber. The experimental bandwidthobtained is 6.5 nm. In addition, the filtered signal experiences a gain of 20 dB insteadof attenuation. A preliminary study of using light to control light has been carried out.Optical signal switching through the use of a pump light is observed.Finally, a theoretical model has been proposed in that a twin-core erbium-dopedfiber can be used as an optical fiber amplifier. Its gain spectrum is flat within 2.5 dBin a bandwidth of 30 nm. This gain spectrum can be adjusted continuously by varyingthe pump power launched into one of the cores