We demonstrate high power ultrashort pulse delivery through a commercially available multicore fiber (MCF) and a multimode graded-index fiber (GRINF) for imaging and laser ablation. Lensless focusing and digital scanning of ultrashort pulses through the optical fibers is realized using wavefront shaping. We compare the performance of the two systems in terms of focusing efficiency and peak power delivery. Furthermore, we investigate the limitations that nonlinearities induce when high peak power ultrashort pulses are launched in MCFs and GRIN fibers. Proximally-only controlled two-photon fluorescence imaging and laser ablation are demonstrated through both investigated systems.
The need for ultrathin fiber-based devices that can deliver light to confined places in order to perform imaging and/or laser ablation of a desired target has been a research area of significant interest. The current endoscopic devices are based on distal optics and scanning mechanisms to focus and scan the light in the end of the fiber. The distal components are limiting factors for decreasing the size of the device. However, using wavefront shaping techniques, lensless focusing and scanning of a laser focus spot through the fiber can be achieved, enabling a smaller endoscopic tool. In our case, a high power focus spot is created by wavefront shaping of the light through a multicore fiber (MCF), providing the possibility of two-photon fluorescence (TPF) imaging. Femtosecond laser ablation through the endoscopic device can be also a powerful tool for a range of applications. Therefore, we investigate limitations in the maximum peak power that can be delivered through the MCF due to nonlinear effects induced in the fiber cores in the ablation peak power regime. After characterizing the capabilities of our system, we demonstrate that femtosecond pulsed laser ablation can be performed through the MCF.
It is possible to focus coherent light through a multicore fiber with a resolution finer than the core-to-core spacing using digital phase conjugation. In this paper we use the memory effect and show that the displacement of the focus spot can be realized simply by tilting the beam incident on the multicore fiber. We demonstrate high-resolution imaging and pattern projection with a calibration procedure consisting of the acquisition of a single digital hologram.
Optical-resolution photoacoustic microscopy offers exquisite and specific contrast to optical absorption. Conventional approaches generally involves raster scanning a focused spot over the sample. Here, we demonstrate that a full-field illumination approach with multiple speckle illumination can also provide diffraction-limited optical-resolution photoacoustic images. Two different proof-of-concepts are demonstrated with micro-structured test samples. The first approach follows the principle of correlation/ghost imaging,1, 2 and is based on cross-correlating photoacoustic signals under multiple speckle illumination with known speckle patterns measured during a calibration step. The second approach is a speckle scanning microscopy technique, which adapts the technique proposed in fluorescence microscopy by Bertolotti and al.:3 in our work, spatially unresolved photoacoustic measurements are performed for various translations of unknown speckle patterns. A phase-retrieval algorithm is used to reconstruct the object from the knowledge of the modulus of its Fourier Transform yielded by the measurements. Because speckle patterns naturally appear in many various situations, including propagation through biological tissue or multi-mode fibers (for which focusing light is either very demanding if not impossible), speckle-illumination-based photoacoustic microscopy provides a powerful framework for the development of novel reconstruction approaches, well-suited to compressed sensing approaches.2
We performed near-diffraction limited two-photon fluorescence (TPF) imaging through a lensless, multicore-fiber (MCF) endoscope utilizing digital phase conjugation. The phase conjugation technique is compatible with commercially available MCFs with high core density. We demonstrate focusing of ultrashort pulses through an MCF and show that the method allows for resolution that is not limited by the MCF core spacing. We constructed TPF images of fluorescent beads and cells by digital scanning of the phase-conjugated focus on the target object and collection of the emitted fluorescence through the MCF.
Endoscopy can be used to obtain high-resolution images at large depths in biological tissues. Usually endoscopic devices have a diameter ranging from 1 to few millimeters. Using digital phase conjugation, it is possible to adapt ultrathin multimode fibers to endoscopic purposes. Recently, we demonstrated that a 330 μm diameter, water-filled silica capillary waveguide can guide high frequency ultrasound waves through a 3 cm thick fat layer, allowing optical resolution photoacoustic imaging. Here we demonstrate that using digital phase conjugation, the same water-filled capillary waveguide (3 cm long) can be used as an endoscopic probe to obtain both fluorescence and optical resolution photoacoustic imaging, with no optical or acoustic elements at the tip of the waveguide. We study the consequences of using digital phase conjugation combined with a capillary waveguide and we conclude with possible future improvements of our endoscopic approach.
We present near diffraction limited two photon fluorescence (TPF) imaging through a lensless, multi-core fiber (MCF) endoscope utilizing digital phase conjugation. The ultra-small size of MCFs make them desirable tools for imaging deep into the body. TPF imaging enables optical sectioning and is widely used in brain and biological imaging and is a desired modality for fiber endoscopes. Previous implementations of TPF imaging through MCFs focus and scan the light from individual cores for image formation. In such systems the resolution is limited by the MCF core spacing, although a lens may be used to improve the resolution at the expense of the field of view. Other, more recent work has improved the resolution limitation using custom built MCFs for focusing and scanning of ultrafast pulses using wavefront shaping. Here we present digital phase conjugation for ultrafast pulse focusing through a MCF for an imaging resolution independent of the MCF core spacing. Furthermore, the phase conjugation technique does not require the use of a lens at the fiber end for focus formation and is compatible with commercially available MCFs with a large number of cores. Here, we present a 3000 core MCF endoscope and demonstrate ultrafast pulse focusing with sufficient focus spot contrast and power for TPF endoscope imaging. We construct TPF images by digital scanning of the phase conjugated focus on the target object and collection of the emitted fluorescence through the MCF. This work demonstrates the viability of digital conjugation combined with commercially available MCFs for higher resolution lensless, two photon endoscopy.