Flexible pulse-stretching for a swept source at 20  μm using free-space angular-chirp-enhanced delay

Dispersive pulse-stretching at 2.0 μm has long been hindered by the high intrinsic optical loss from conventional dispersive media. Here a flexible pulse-stretching technique at 2.0 μm is demonstrated over a broad bandwidth with large-scale dispersion and low intrinsic optical loss. The technique employs the newly proposed pulse-stretching scheme, namely, free-space angular-chirp-enhanced delay. Both normal and anomalous temporal dispersion (up to ±500 ps/nm) with low intrinsic loss (<6 dB) over a spectral bandwidth of ∼84 nm at 2.0 μm is obtained with low nonlinear effects. Based on this method, an optical wavelength-swept source at 2.0 μm is realized and applied to spectrally encoded imaging at a line scan rate of ∼19 MHz, proving the potential of this pulse-stretching technique for continuous single-shot measurements at the 2.0 μm wavelength regime, particularly for optical microscopy and spectroscopy.