Hybrid photoacoustic-fluorescence microendoscopy through a multimode fiber using speckle illumination

Advances in wavefront shaping technologies have enabled minimally invasive endoscopy using a single multimode fiber (MMF) and paved way to pushing the penetration depth limit inside scattering tissue. Photoacoustic imaging and fluorescence imaging are two key enabling imaging modalities that provide excellent anatomical and functional contrast, respectively. Combining their unique advantages is desirable; however, doing so in a minimally invasive endoscope has been a challenge. Here, we present an ultrathin hybrid imaging system with a 250 mm cross section based on an optical MMF and an optical fiber hydrophone and capable of combined photoacoustic and fluorescence microscopy. A reference-free calibration, using a set of prerecorded speckle intensity patterns combined with a reconstruction algorithm, enables compressive speckle imaging with optical resolution determined by the fiber numerical aperture.