Abstract
The characterisation of loss in optical waveguides is essential in understanding the performance of these devices and their limitations. Whilst interferometric-based methods generally provide the best results for low-loss waveguides, they are almost exclusively used to provide characterization in cases where the waveguide is spatially single-mode. Here, we introduce a Fabry-Pérot-based scheme to estimate the losses of a nonlinear (birefringent or quasi-phase matched) waveguide at a wavelength where it is multi-mode. The method involves measuring the generated second harmonic power as the pump wavelength is scanned over the phase matching region. Furthermore, it is shown that this method allows one to infer the losses of different second harmonic spatial modes by scanning the pump field over the separated phase matching spectra. By fitting the measured phase matching spectra from different titanium indiffused lithium niobate waveguides to the model presented in this paper, it is shown that one can estimate the second harmonic losses of a single spatial-mode, at wavelengths where the waveguides are spatially multi-mode.
Highlights
Optical waveguides have enabled the expansion of optical networks in a very short time
We apply the described measurement technique in order to retrieve the losses of a 31.2mm long 7μm-wide titanium indiffused waveguide quasi-phase matched for type 0 second harmonic generation in the TM00 spatial mode when pumped with a fundamental field at 1525nm
In order to check the validity of the fit, in particular the performance of the chosen minimisation routine, we show the variation of the mean squared error (MSE) for both of these fits as the second harmonic losses are varied, holding all other parameters constant
Summary
Optical waveguides have enabled the expansion of optical networks in a very short time. This technology is advancing in order to include, for example, high power, high efficiency and/or quantum applications [1, 2]. The reliable characterisation of these losses is a critical issue. A number of methods for loss characterization and variants of these methods exist These methods can be categorised into a few broad schemes: cut-back methods, fluorescence/scatter imaging, resonance techniques and optical transmission measures. These various methods perform differently under given circumstances. Interferometric methods tend to have greater precision as the losses decrease and so are more suited for characterisation of low loss waveguides [5, 6, 7]
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