Abstract

Modulation instability (MI) in the normal dispersion regime has been achieved by pumping a high birefringence single mode fibre at 45° to the principal axes of the fibre with high intensity pulses either from a mode-locked cavity-dumped (MLCD) Ar-laser^[1] or from a MLCD dye laser.^[2] The MI laser (see Figure 1 ) utilises part of the output of the single mode fibre to enhance this four wave mixing effect by providing a seed pulse. Lasing is observed even in the case where no single pass MI is detectable. The threshold in this case is at about 30 W where as for a single pass 0.5 kW peak power is needed. While the beat frequency changes only slightly with the amount of feedback the spectral width of the sidebands changes dramatically from about 0.1 nm to 0.5 nm by increasing the back coupling from about 0.5% to 5%. Using low feedback to achieve modulation over a long period of time the auto correlation function (using background free SHG) of both pump and the upshifted sideband was measured. Figure 2 shows this trace both on a long and on a short time scale. The autocorrelation function of just one of the sidebands was also measured but no modulation can be seen on these traces. The beat frequency of 2.1 THz in Figure 3b corresponds to a separation in wavelength of about 2 nm as can be seen in Figure 2. The MI laser gives a much more stable output than can be obtained by single pass MI experiments and the Raman gain on the downshifted sideband is greatly reduced due to the lower peak power. Noise characteristics measured on the intensity difference between the upshifted and the downshifted sideband indicate a strong correlation of the photons generated by this four wave mixing process.^[3]

© 1992 IQEC