  Coherent Raman scattering microscopy is a fast, label-free and chemically
specific imaging technique that has a high potential for future in-vivo optical
histology. However, its imaging depth into tissues is limited to the
sub-millimeter range by absorption and scattering. Performing coherent Raman
imaging in a fiber endoscope system is a crucial step to image deep inside
living tissues and provide the information inaccessible with current microscopy
tools. However the development of coherent Raman endoscopy has been hampered by
several issues in the fiber delivery of the excitation pulses and signal
collection. Here, we present a flexible, compact, and multimodal nonlinear
endoscope (4.2 mm outer diameter, 71 mm rigid length) based on a resonantly
scanned hollow-core Kagom\'e-lattice double-clad fiber. The fiber design allows
distortion-less, background-free delivery of femtosecond excitation pulses and
the back-collection of nonlinear signals through the same fiber. Sub-micron
spatial resolution together with large field of view is made possible by the
combination of a miniature objective lens together with a silica microsphere
lens inserted into the fiber core. We demonstrate coherent anti-Stokes Raman
scattering, 2-photon fluorescence and second harmonic generation imaging with
0.8 {\mu}m resolution over a field of view up to 320 {\mu}m and at a rate of
0.8 frames/s. These results pave the way for intra-operative label-free imaging
applied to real-time histopathology diagnosis and surgery guidance.
