Development of fiber based nonlinear endoscopes is of significant interest in the bio-imaging community for its ability to perform in-depth, label-free and optically sectioned in-vivo imaging. Conventional fiber-based endoscopes typically use a single mode fiber in conjunction with miniaturized objective lenses for the delivery and focusing of an ultrashort light pulse onto the sample and mechanical scanning elements at the distal end to perform imaging. In order to be minimally invasive, a high degree of miniaturization is desired which is usually limited by these mechanical elements. And there have been only very few reports of nonlinear imaging through fibers with no distal mechanical elements.

In their work, Morales et al. have demonstrated a key step towards the development of two-photon endoscopes based on multimode fibers (MMF). In stark contrast to propagation in single mode fibers, an ultrashort pulse in a multimode fiber undergoes modal dispersion and arrives at the distal end as a speckle which is also significantly longer in the time domain. While digital phase conjugation techniques, i.e. a time reversal of the propagation direction and phase of the electromagnetic fields, have been earlier demonstrated to generate a focal spot through multimode fibers, the problem of pulse stretching has not been addressed in this framework. Building on their earlier work on digital phase conjugation, the authors demonstrate selective propagation of only a subset of the fiber modes with similar propagation constants with a combination of time-gated interferometry and digital holography. This in combination with compensation of the material dispersion ensures that the original short pulse lengths can be retained (117 fs) and this is a crucial aspect for two-photon imaging. Focusing and scanning is now performed by recording and displaying a series of time-gated phase conjugation holograms on a spatial light modulator that sequentially generates the focal spots on a rectangular grid akin to raster scanning.

Morales et al. further demonstrate two-photon imaging of test samples such as fluorescent beads through a 20 cm long fiber probe consisting of a graded index (GRIN) multimode fiber with a miniature GRIN lens attached to its end. This results in a probe of 350 µm diameter making it one of the smallest reported nonlinear endoscopes. They experimentally achieve a best two-photon lateral resolution of 1.05 µm, and an axial resolution of 10 µm. In addition, they demonstrate a useful field of view of 80 µm in the transverse plane along with a 100 µm axial scan range. These features are compatible with existing demands for applications in bio-imaging.

In summary, the authors have made an important demonstration of a multimode fiber based two-photon endoscope. The future will see other nonlinear contrast mechanisms in multimode fiber endoscopes, paving way for multimodal imaging. Challenges still remain over speed, length and flexibility to offer imaging capabilities over and above conventional microscopes. The rapid advances being made in spatio-temporal control of light and fiber technology are very promising and the arrival of miniaturized high resolution endoscopes will probably be sooner rather than later.

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