  Optical solitary waves (solitons) that interact in a nonlinear system can
bind and form a structure similar to a molecule. The rich dynamics of this
process have created a demand for rapid spectral characterization to deepen the
understanding of soliton physics with many practical implications. Here, we
demonstrate stroboscopic, two-photon imaging of soliton molecules (SM) with
completely unsynchronized lasers, where the wavelength and bandwidth
constraints are considerably eased compared to conventional imaging techniques.
Two-photon detection enables the probe and tested oscillator to operate at
completely different wavelengths, which permits mature near-infrared laser
technology to be leveraged for rapid SM studies of emerging long-wavelength
laser sources. As a demonstration, using a 1550 nm probe laser we image the
behavior of soliton singlets across the 1800-2100 nm range, and capture the
rich dynamics of evolving multiatomic SM. This technique may prove to be an
essential, easy-to-implement diagnostic tool for detecting the presence of
loosely-bound SM, which often remain unnoticed due to instrumental resolution
or bandwidth limitations.
