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Polarization-dependent four-wave mixing in distributed-feedback lasers

We present new results on the polarization-dependence of four-wave-mixing in distributed-feedback lasers, which is useful for wavelength conversion in wavelength division multiplexing applications. The lasing TE mode of the DFB laser serves as the pump beam, and the DFB laser also serves as the wave-mixing medium. An external probe beam of either TE or TM polarization was injected into the mixing laser, and the output spectra for both polarizations were measured. For a fixed pump with TE polarization, we observe not only TE conjugate output with TE probe input, but also new results with TM conjugate output using TM probe input. The spectral dependence of the TE and TM conversion efficiencies is shown for pump/probe detunings up to 200 GHz. We also present a method to calculate and measure the refractive index dispersion spectrum for each polarization. Experimental data from both polarizations are simultaneously matched to theoretical curves. We utilize these results to calculate the relative magnitude of the phase mismatch for each polarization. We find that for an external probe TE signal, the conversion efficiency of the generated TE conjugate wave is much stronger than that of the TM (probe in) to TM (probe out) configuration. The reasons are that in addition to the difference in nonlinear susceptibility, the TETE case is well phase-matched for small detunings, while the phase-matching is poor for the TM-TM case.