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Highly sensitive measurement of optical nonlinearity

In the "century of the photon", integrated optical components play an increasingly important role. Some applications, such as optical communication or high power pulse delivery systems, require photonic components with linear characteristics, whereas nonlinearities as high as possible are desired in cases like optical switches or optical memory elements. At PTB, a novel method has been developed to measure weak nonlinearities using only moderate laser intensities.

Measurements of optical nonlinearities are usually afflicted with large uncertainties, due to dependence on the excitation parameters. Therefore, methods independent of the laser and waveguide parameters are desirable. The method developed at PTB (within the framework of the German Research Foundation Collaborative Research Centre 407) performs the measurements of optical nonlinearities by comparison with well-known reference samples, thus ruling out the dependence on excitation parameters (for instance, laser pulse properties such as duration or chirp) and of the mode area of the waveguide.

To this end, the light from a mode-locked erbium fibre laser (λ≅ 1.55 µm) at the exit face of the measurement object is 1:1 imaged into a thin fused silica plate acting as reference. The nonlinear signal generated in both samples is superimposed onto a coherent frequency-shifted auxiliary field and the resulting beat note is detected with an InGaAs photodiode. The contribution from the reference plate can be switched on and off by movement of the plate in propagation direction without changing the beam geometry or other excitation parameters.

Owing to the high sensitivity of this heterodyne scheme, it was possible to measure the nonlinearity of a hollow-core photonic-crystal fibre only 21 mm long, which was found to be more than 1000 times weaker than that of standard telecommunication fibre. This is due to the fact that the dominant part of the light propagates in the air or vacuum core of this novel type of fibre. In addition to the nonlinear refractive index, the method allows the measurement of nonlinear absorption. It is not limited to waveguides but can be extended to measure optical nonlinearities in solids (e.g., polymers), in liquids (e.g., nanoparticle suspensions), of surfaces (e.g. due to plasmonic resonances), and in gas samples.

Photonic fibre and part of the parabolic broadband imaging system

Contact at PTB:

Phone: +49-531-592-0