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 étendue 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 étendue. 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 0.003μW, well below the noise floor of the camera, with an overall sensor bandwidth above 2300 Hz.