Mid‐infrared Frequency‐Swept Laser at 50 MScans/s Via Frequency Down‐Conversion of Time‐Stretched Near‐Infrared Pulses

Abstract Increasing the sweep rate of mid‐infrared (MIR) frequency‐swept sources offers significant potential for various high‐speed spectroscopy‐based applications. While continuous‐wave frequency‐swept lasers have achieved sweep rates up to 1 MHz, a recently demonstrated time‐stretched ultrashort pulsed laser has reached a significantly higher sweep rate, up to tens of MHz. However, the previous system relied on a bulky femtosecond optical parametric oscillator and produced only ≈30 discrete spectral elements due to the use of a free‐space time stretcher. This study presents a frequency‐swept MIR source that utilizes the frequency down‐conversion of time‐stretched near‐infrared pulses, employing a compact mode‐locked fiber laser and telecommunication fiber. As a proof‐of‐concept demonstration, MIR spectroscopy of methane gas ≈3.4 µm is performed at a rate of 50 MSpectra s −1 , capturing 175 spectral elements over a range of 15.1 cm −1 . This compact and robust high‐speed MIR frequency‐swept laser system holds the potential for deployment in field applications.