Sensitive interferometry with multimode fibres

Fibre coupling is a convenient implementation of various types of optical sensors. However, when the optical sensor is based upon interferometry, the choice of fibre is usually restricted to a single-mode fibre, limiting the & eacute;tendue of the sensor system. Here, we show that it is possible to configure an interferometric sensor using a multimode optical fibre supporting many spatial modes, thereby increasing the & eacute;tendue. By using off-axis holography, the full complex amplitude of the speckle pattern transmitted through the fibre can be recovered from a single frame of a detector array, enabling high-speed operation. Using this approach, we have demonstrated stand-off laser vibrometry where the only collection optic is the multimode fibre itself. Our vibrometer can recover vibration frequencies using a high-speed camera with power levels as low as 0.003 mu W, well below the noise floor of the camera, with an overall sensor bandwidth above 2300 Hz. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.