Near-infrared external cavity lasers with high tuning rates (“swept lasers”) have come to dominate the field of nearinfrared
low-coherence imaging of biological tissues. Compared with time-domain OCT, swept-source OCT a) replaces
slow mechanical scanning of a bulky reference mirror with much faster tuning of a laser cavity filter element and b)
provides a ×N (N being the number of axial pixels per A-scan) speed advantage with no loss of SNR.
We will argue that this striking speed advantage has not yet been fully exploited within biophotonics but will next make
its effects felt in the mid-infrared. This transformation is likely to be driven by recent advances in external cavity
quantum cascade lasers, which are the mid-IR counterpart to the OCT swept-source. These mid-IR sources are rapidly
emerging in the area of infrared spectroscopy. By noting a direct analogy between time-domain OCT and Fourier
Transform Infrared (FTIR) spectroscopy we show analytically and via simulations that the mid-IR swept laser can
acquire an infrared spectrum ×N (N being the number of spectral data points) faster than an FTIR instrument, using
identical detected flux levels and identical receiver noise.
A prototype external cavity mid-IR swept laser is demonstrated, offering a comparatively low sweep rate of 400 Hz over
60 cm-1 with 2 cm-1 linewidth, but which provides evidence that sweep rates of over a 100 kHz should be readily
achievable simply by speeding up the cavity tuning element.
Translating the knowledge and experience gained in near-IR OCT into mid-IR source development may result in sources
offering significant benefits in certain spectroscopic applications.