What is Optical Cross-Phase Modulation (XPM)?

Optical cross phase modulation (XPM) is as a non-linear optical process where the wavelength of one light source affects the phase of another wave. Optical cross phase modulation is brought about by the optical Kerr effect - a non-linear optical phenomenon in which the refractive indices of non-linear materials change when an ultra short pulse propagates through them.

The pulse phase is modulated spatially and time wise. Self-modulation and self focusing of the wave is done by the pulse itself. Cross-phase modulation occurs due to the phase modulations that are generated by the co-propagating waves. The following sections will discuss the working, experimental set up and applications of the cross-phase modulation phenomenon.

Experimental Set Up

The following is an example of an experimental set up to demonstrate cross phase modulation.

The light source is a mode-locked Nd:YAG laser with a second harmonic crystal. Infrared pulses were produced at a pulse rate of 33 psec, and green pulses at a rate of 25 psec. These two pulses were directed down different pathways of an interferometer. The energy of the two pulses was modified using neutral density fibers.

The non-linear medium was a single-mode optical fiber of 1m length. The spectra of the green pulses with and without the infrared pulses in the proximity were measured. The green pulse showed a blue and a green shift while co-propagating with the infrared pulse. Hence it was observed that there was a shift in the phase of a carrier wave when co-propagating with another wave.

Principle

When two waves having different wavelengths co-propagate through a non-linear medium the phase frequency of one wave is influenced by the other wave, due to the effect on the refractive index of the propagating medium. The refractive index is influenced by the intensity of the electric field and also the co-propagating wave.

Cross-phase modulation occurs in non-linear medium and the phase shift is more prominent in materials having 2^nd and 3^rd order non-linearity.

Applications

Some of the chief application areas of XPM are listed below:

• Spectral broadening
• Frequency tuning and multiplexing
• Spatial modulation of ultrafast pulses
• Dense wavelength division multiplexing.

References

• Instability due to cross-phase modulation in the normal-dispersion regime – Rochester University
• Cross-phase modulation and induced focusing due to optical nonlinearities in optical fibers and bulk materials – Rochester University
• Optical phase-modulated systems: numerical estimation and experimental measurement of phase jitter – Georgia Tech