Advanced fibre circuitry for all-optical signal processing
This thesis presents results of new types of fibre lasers and oscillators as well as a new all-fibre nonlinear modulator with nearly instantaneous response time. The development of a simple and stable, passively mode-locked source of picosecond pulses is described in chapter 2. Here the mode locking of the laser was obtained by using the combined effect of frequency-shifted feedback and a nonlinear amplifying loop mirror. The new cavity configuration allowed tuning of the laser continuously over 25 nm of the erbium gain bandwidth by using a bulk diffraction grating. The shortest reported pulse width (1.2 ps) from this type of laser has been demonstrated. A complete characterisation of the laser, including its mode locking build-up time, is presented. Cascaded nonlinearity in quadratic nonlinear materials is the basis in realising nonlinearity free optical amplifiers. Experimental results on cascaded nonlinearity in a periodically poled lithium niobate sample are presented in chapter 3. A nonlinear phase shift of more than 1.5p was obtained from only a 4 mm long sample due to its large effective nonlinear refractive index coefficient (~1 x 10-13 cm2/W). Experiments on the nonlinear phase shift compensation in an optical fibre and amplifier are presented in chapter 4. Phase shift compensation of more than 1.5p has been successfully demonstrated. Chapter 5 presents theoretical investigations on the modulation of optical signals using the stimulated Raman scattering (SRS) process in silica fibre. Numerical results reveal that to modulate signals in this scheme it is necessary to consider other competing nonlinear effects such as cascaded SRS, modulation instability etc. actively. Signal modulation as fast as 250 - 300 GHz can be realised using this intensity modulator. Chapter 6 describes the experimental results on the modulation of optical signal using SRS in optical fibre. With this Raman intensity modulator, bit-by-bit modulation of 10 Gbit/s simulated data stream has been demonstrated. An extinction ratio (modulation depth) of more than 15 dB was realised. The proposed intensity modulator can also be used as a time domain scalpel and can create a dark pulse in a bright background.