Cross-phase modulation-induced spectral broadening in silicon waveguides

Area of Science:

• Nonlinear optics
• Photonics
• Materials science

Background:

• Cross-phase modulation (XPM) is a key nonlinear optical phenomenon.
• In pure Kerr media, the spectral broadening ratio of XPM to self-phase modulation (SPM) is theoretically two.
• Silicon photonics is a rapidly growing field for integrated optical devices.

Purpose of the Study:

• Investigate the effects of two-photon absorption (TPA) and free carriers on XPM in silicon waveguides.
• Quantify the spectral broadening ratio of XPM to SPM under these conditions.
• Understand the underlying physical mechanisms causing deviations from ideal Kerr behavior.

Main Methods:

• Analytical modeling of nonlinear optical effects in silicon.
• Experimental investigation using pump-probe measurements.
• Fabrication and characterization of silicon photonic crystal waveguides.

Main Results:

• The spectral broadening ratio of XPM to SPM deviates from two in silicon waveguides with TPA or free carriers.
• Two-photon absorption causes differential pulse attenuation, slightly modifying pulse shape.
• Free carriers introduce asymmetric absorption and dispersion, competing with Kerr-induced phase shifts.

Conclusions:

• Nonlinear absorption and free-carrier effects significantly alter XPM dynamics in silicon waveguides.
• The observed deviations are attributed to TPA-induced differential absorption and free-carrier absorption/dispersion.
• Experimental results confirm the theoretical analysis, highlighting the importance of these effects in silicon photonics.