Resolution improvement in speckle wavemeters via mechanically induced multimode fiber mode-mixing method

In speckle wavemeters, the wavelength resolution is primarily determined by the speckle pattern's sensitivity to wavelength changes and its information content. While increasing the optical path length can improve the resolution, it also makes the system more susceptible to environmental perturbations. Here, we propose a new method to enhance the wavelength resolution of a speckle wavemeter that uses a multimode fiber as the waveguide. This method employs mechanical perturbations to break the fiber's cylindrical symmetry, perturb the local refractive-index profile, and induce mode coupling. As a result, the probe light excites more modes and accumulates larger modal phase delays during propagation, without introducing additional uncontrolled variables other than the applied mechanical perturbation. We present the theoretical framework of this approach and carry out numerical simulations for different perturbation geometries and strengths. On this basis, we design and build a complete wavelength measurement setup. Experiments demonstrate that the system exhibits a highly resolvable sensitivity to wavelength variations, successfully tracking current-induced wavelength modulations as small as 10 fm using a 0.5-m multimode fiber. We also achieve successful wavelength measurements near point sources at 1388, 1550, 1598, and 1658 nm, which correspond to typical gas absorption lines. Additionally, enhancing the speckle modal content increases the complexity of the speckle field and the density of wavelength-sensitive pixels per unit area. This substantially relaxes the requirements regarding detector count and sampling rate, as well as paves the way for compact, high-performance speckle wavemeters.