  Light carrying spatiotemporal orbital angular momentum (ST-OAM) makes
possible new types of optical vortices arising from transverse OAM. ST-OAM
pulses exhibit novel properties during propagation, transmission, refraction,
diffraction, and nonlinear conversion, attracting growing experimental and
theoretical interest and studies. However, one major challenge is the lack of a
simple and straightforward method for characterizing ultrafast ST-OAM pulses.
Using spatially resolved spectral interferometry, we demonstrate a simple,
stationary, single-frame method to quantitatively characterize ultrashort light
pulses carrying ST-OAM. Using our method, the presence of an ST-OAM pulse,
including its main characteristics such as topological charge numbers and OAM
helicity, can be identified easily from the unique and unambiguous features
directly seen on the raw data--without any need for a full analysis of the
data. After processing and reconstructions, other exquisite features, including
pulse dispersion and beam divergence, can also be fully characterized. Our fast
characterization method allows high-throughput and quick feedback during the
generation and optical alignment processes of ST-OAM pulses. It is
straightforward to extend our method to single-shot measurement by using a
high-speed camera that matches the pulse repetition rate. This new method can
help advance the field of spatially and temporally structured light and its
applications in advanced metrologies.
