High-power flat mid-infrared supercontinuum spectral light source (Newsletter)

Objective Supercontinuum (SC) sources spanning the near-infrared (NIR) and mid-infrared (MIR) regions have garnered significant interest and found widespread applications in tissue imaging, sensing, spectroscopy, defense, and environmental monitoring. While numerous SC laser systems with impressive performance have been reported globally, challenges such as low damage thresholds and susceptibility to deliquescence in air have persisted with fluoride nonlinear fibers (e.g., ZBLAN and InF₃ fibers), even with end cap protection during prolonged high-power operation. Hence, the imperative need arises to develop mid-infrared fibers characterized by higher damage resistance thresholds, enhanced nonlinear coefficients, and improved water resistance to elevate the overall performance of SC lasers.

Methods  A 1 550 nm ns pulsed fiber laser was used as the seed source, and the seed pulse was first amplified and spectrally pre-broadened by a two-stage erbium-ytterbium doped fiber amplifier (EYDFA) and a section of single-mode fiber, respectively. The high-power flat 1.9-2.7 μm SC laser was obtained by a thulium-doped fiber amplifier (TDFA) with power amplification and spectral broadening again. The 1.9-2.7 μm SC laser was then coupled into a 60-cm-long fluorotellurite fiber (TBY) with a core diameter of 10 μm. The TBY fiber exhibited a nonlinear coefficient of 22.4 W⁻¹/km and a measured loss of 0.094 dB/m at 2 μm. The schematic of the SC laser setup is depicted in Fig.1.

Results and Discussions The SC spectrum characterization system comprised of a power meter (Thorlabs) for power measurement, an optical spectrum analyzer (Yokogawa, AQ630D) with a measurement range of 600-1700 nm to capture SC spectra at shorter wavelengths (<1.5 µm), and a Mid-IR/IR spinning grating spectrometer (A.P.E, waveScan) with a measurement range of 1500-6500 nm for recording spectral distribution at longer wavelengths (>1.5 µm). As depicted in Fig.2, the SC laser output spectrum exhibited gradual expansion and enhanced flatness with increasing pump power, reaching a spectrum range of 1.25 to 4.05 μm at an output power of 10.19 W. Notably, the exceptional damage and OH⁻ resistance of the TBY fiber contributed to the laser's excellent power stability over extended durations.

Conclusions This study successfully demonstrated an all-fiber ultra-flat SC source utilizing TBY fiber, spanning from 1.25 to 4.05 μm and yielding an output power of 10.19 W. Leveraging a flat pump source and the high non-linearity coefficient of the TBY fiber facilitated the achievement of an ultra-flat SC spectrum with impressive 3 dB and 10 dB bandwidths of 1290 nm and 2390 nm, respectively. The measured power fluctuations of the two SC sources stood at 0.78% (RMS), indicative of remarkable power stability. The ultra-flat spectrum and outstanding power stability showcased by these SC sources highlight their potential for real-world applications.