Generation and propagation of ultrashort laser pulses using nonlinear waveguides

Abstract

The main objectives in this research project related to the generation of ultrashort laser pulses using a KC1:T1 colour-centre laser and the study of their propagation in optical waveguides. Coupled-cavity mode-locking of the KC1:T1 colour-centre laser using either monomode optical fibre or passive AlGaAs waveguides as the nonlinear element in the control-cavity have been investigated. With the optical fibre as the nonlinear element, pulses as short as 63 fs have been generated. The large nonlinearity of the AlGaAs waveguides illuminated near the half- bandgap energy has been confirmed through the propagation of ultrashort laser pulses, and nonlinear phase shift in excess of 2mu has been observed. By undertaking the studies described here, the measurements have indicated that the waveguides used have a linear loss of 0.74 cm<super> -1</super>, a two-photon-absorption coefficient of about 0.1 cm/GW, and a nonlinear refractive index of 0.8 x 10<super>-13</super> cm2/W. Coupled-cavity mode-locking with passive AlGaAs waveguides as the control-cavity nonlinear element has been achieved, and two different guiding geometries of the AlGaAs waveguides have been used. With a straight waveguide geometry, pulses having duration of ~ 230 fs have been generated. When a curved waveguide geometry was utilised and appropriate dispersion compensation applied then pulses as short as 160 fs have been produced. By employing waveguides having reduced lengths (down to 1.2 mm), some further shortening of the output pulses was achieved and pulses as short as 150 fs have been recorded. By using a gold coating on rear facet of the waveguide, successful coupled-cavity mode-locking has been achieved at an output power level as low as 4 mW. Some applications using the coupled-cavity mode-locked KC1:T1 colour-centre laser have been performed. Femtosecond pulses at 1.3-mum spectral region have been produced through the process of self-phase-modulation mediated four-wave-mixing in an erbium-doped fibre. Measurement on the group-velocity-dispersion of the AlGaAs waveguide has also been made, and a value of D = -1100 ps/nm/km has been deduced.