Achieving optical-resolution photoacoustic imaging can currently only be obtained using endoscopy. It usually implies a quite bulky endoscope and/or a low signal-to-noise detection. In this paper, the authors present a technique that combines wavefront shaping through a multimode fiber, to scan the focus spot, with a single-mode fiber-based ultrasound sensor to achieve a high signal-to-noise with a small footprint (250 by 125 microns).
Photoacoustic consists in exciting a medium with an intense time-varying optical field and measuring its ultrasound response generated by the thermoelastic effect. Optical resolution is achieved when the focused beam can be scanned on the sample. When the object to observe is buried deeper in a scattering sample, such as a biological tissue, light scattering usually forbids such operation. The position can then be estimated using the detection of the ultrasound waves. In that case, the resolution is set by the acoustic waves, of the order of a few hundred microns.
Endoscopy allows retrieving optical resolution deep in the body by bringing the optical system close to the region of interest. The two main challenges are then 1) how to bring light to the object and 2) how to detect the generated ultrasound waves.
To bring light to the region of interest, it is preferable to focus light and scan the focal spot to achieve a good signal to noise ratio. While multicore fibers are typically used for such task, multimode fibers can achieve an equivalent information content (number of modes) with a much smaller diameter. However, due to dispersion, and potential defects or perturbations, an input localized excitation leads to a seemingly random speckle pattern on the output. To solve this issue, the authors calibrate the system by measuring the transmission matrix of the fiber using a spatial light modulator. They can then calculate and send the wavefront that focus on any output position.
The detection of the ultrasound wave is done using a compact fiber-optic ultrasound sensor. It has the footprint of a single-mode fiber. The length of a small cavity inside the fiber is modulated by the ultrasound. Its reflectivity is detected using an interrogation laser beam coupled into the fiber.
The system combining the two fibers parts is represented in Figure 1.
To demonstrate the potential of their system, the authors reconstruct the photoacoustic image of blood cells. Moreover, by measuring the light reflected back through the multimode fiber, they also reconstruct a fluorescence image. The main results are shown in Figure 2.
Figure 2. Imaging results. Top left, bright-field image of the sample. Top-right, fluorescent, bottom-left, photoacoustic and bottom-right, composite reconstructed image. Image from [S. Mezil et al., arxiv, 2006.10856 (2020)]