Abstract
For speckle pattern-based wavemeters or spectrometers, the intermodal and the chromatic dispersion of the diffusion waveguide are key factors in determining the wavelength resolution. In this study, we propose a new mechanism to modulate the fiber speckles aside from the dispersion related effect. The polarization modulation is introduced in a rectangular core fiber (RCF) by using an in-line polarization rotator (IPR). The IPR can rotate the polarization angle at different wavelengths. Interestingly, it is observed that the modulated speckle patterns appear periodically similar, and they present more variations when compared to unmodulated ones. The theoretical simulation indicates that the polarization exerts an influence on the fiber speckles by modulating the mode coupling coefficients when light is coupled into the RCF. In the demonstration experiment, we first built a precise laser frequency tuning system based on an electro-optic modulator and a microwave source. By comparing the recorded speckles with and without polarization modulation, the former speckles have lower correlations, which are evaluated using the Arc Cosine Similarity algorithm. Reconstructing the spectrum of speckles, the smallest detectable wavelength that can be detected with polarization modulation is 0.2 fm. The experimental results prove that the polarization modulation is a convenient and effective method to enhance the speckle’s wavelength sensitivity. In addition, this study paves a new way to design high-resolution wavemeters with shorter, more compact optical waveguides.
Highlights
When coherent light travels through an optical multimode waveguide, a granular speckle pattern can be observed at the output end face
A new polarization modulation mechanism to improve the wavemeter wavelength resolution was introduced in this study
Further theoretical simulations indicate that the mode coupling coefficients of the rectangular core fiber (RCF) vary with the polarization angle and modulate the speckle patterns
Summary
When coherent light travels through an optical multimode waveguide, a granular speckle pattern can be observed at the output end face. Goodman found that the nature of the speckle depends on the frequency of the light.. According to the subtle differences between patterns, the wavelength or spectrum of the light can be measured precisely by processing the image data of the pattern.. Cao was first to report a fiber speckle spectrometer using a piece of multimode fiber (MMF) as the diffusing media.. With the development of machine learning, many researchers are using speckle patterns for imaging and sensing by using deep learning algorithms.. One of the essential conditions for the generation of speckles is the existence of intermodal dispersion in the multimode waveguide.
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