We study the instantaneous nonlinear index change in Al,,,Gac,sAs waveguides below the twophoton absorption edge and find S-phase shifts with 80 pJ, 0.4 ps pulses at wavelengths near 1.6 pm. These large phase shifts are obtained with less than 1 dB of loss from multiphoton absorption. Our results indicate that AlGaAs waveguides, which have a mature fabrication technology, can be used as compact nonlinear elements in switching and quantum optics applications in the near-infrared. Further optimization of the waveguide geometry should result in useful nonlinear phase shifts and low losses for pulse energies approaching a picojoule.
We study the nonlinear properties of bulk AlGaAs and GaAs/AlGaAs multiple quantum wells (MQW) below the half-band-gap energy using subpicosecond pulses between 1.65 and 1.7 μm. In the bulk material we find a value for the nonlinear index n2 = +3.6× 10−14 cm2/W and a two-photon absorption coefficient β = 0.26 × 10−4 cm/MW. In the MQW we measure an n2 up to 2.4 times larger, and we attribute this enhancement to a stronger 1S-exciton intermediate state. The β value is up to 25 times larger in the MQW. This larger value may result from midgap states that resonantly enhance the virtual intermediate state in two-photon absorption and act as a real transition in a two-step absorption process. The resulting figure of merit (2n2/βλ) for the bulk (MQW) material is 17 (1.6), which means that these semiconductors below half band gap are appropriate for all-optical switching and quantum optics applications. We confirm that n2 is instantaneous on the 300 fs time scale of our pulses from self-phase-modulation spectra as well as time-resolved pump-probe measurements. However, we find an intriguing exchange of energy between the two orthogonal axes as evidenced by the signal along the probe axis following the negative derivative of the pump intensity. This result may be explained by self-phase modulation of the pump combined with a low-frequency Raman process that couples the modes along orthogonal axes.
Group-delay ripple (GDR) introduced by systematic and random errors in chirped fiber Bragg grating fabrication is the most significant impediment to application of these devices in optical communication systems. We suggest and demonstrate a novel iterative procedure for GDR correction by subsequent UV exposure by use of a simple solution of the inverse problem for the coupled-wave equation. Our method is partly based but does not fully rely on the accuracy of this solution. In the experiment we achieved substantial reduction of the low-frequency group-delay ripple, from +/- 15 to +/- 2 ps, which resulted in dramatic improvement of the optical signal-to-noise-ratio system penalty, from 7 to less than 1 dB, for a chirped fiber Bragg grating used as a dispersion compensator in a 40-Gbit/s carrier-suppressed return-to-zero system.
We actively mode lock a high-frequency GaInAsP laser at a rate of 16 GHz to obtain nearly transform-limited hyperbolic secant pulses with a pulse width of 0.58 ps. This is the shortest pulse width yet demonstrated for either passively or actively mode-locked semiconductor lasers.
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