Measurement of Nonlinear Optical Constants

Different techniques, such as two-photon absorption spectroscopy [131], degenerate four-wave mixing (DFWM), Z-scan [132, 133], third-harmonic generation (THG), optical Kerr-shutter (OKS) [134], and self-phase modulation (SPM), have been used to measure the nonlinear refractive index, as well as the nonlinear absorption coeffcient, of chalcogenide glasses. The nonlinear refractive index n₂ of As₂S₃ glass fiber was first measured directly by a spectrum-broadening experiment at a wavelength of 1.3 μm [135]. The n₂ value was also measured by a Kerr-shutter (Fig. 4.1) experiment at 1.3 μm [136]. In the optical Kerr effect, the nonlinear phase shift induced by an intense, high-power, pump beam is used to change the transmission of a weak signal through a nonlinear medium. The operating principle of a Kerrshutter can be explained in the following way. The pump and signal beams are linearly polarized at the fiber input with a 45° angle between their directions of polarization. A gate pulse induces a phase-shift difference between two signal components whose polarization is parallel and perpendicular to that of the gate. When the phase-shift difference reaches π, the polarization of the signal is switched by 90°. The n₂ values obtained were 1.7 × 10⁻¹⁴ cm²W⁻¹ and 4 × 10⁻¹⁴ cm²W⁻¹, respectively. The difference may be due to an increase in the transmission loss caused by a higher average power in the spectrumbroadening experiment.