Abstract
We introduce a broadly tunable robust source for fingerprint (170 - 1620 cm-1) Raman spectroscopy. A cw thulium-doped fiber laser seeds an optical parametric amplifier, which is pumped by a 7-W, 450-fs Yb:KGW bulk mode-locked oscillator with 41 MHz repetition rate. The output radiation is frequency doubled in a MgO:PPLN crystal and generates 0.7 - 1.3-ps-long narrowband pump pulses that are tunable between 885 and 1015 nm with >80 mW average power. The Stokes beam is delivered by a part of the oscillator output, which is sent through an etalon to create pulses with 1.7 ps duration. We demonstrate a stimulated Raman gain measurement of toluene in the fingerprint spectral range. The cw seeding intrinsically ensures low spectral drift.
Highlights
Coherent Raman imaging methods are emerging tools in life sciences due to their large potential in label-free, high speed imaging with applications such as medical diagnostics, the minimally invasive study of cellular metabolism, and drug delivery [1]
We introduce a broadly tunable robust source for fingerprint (170 – 1620 cm−1) Raman spectroscopy
A cw thulium-doped fiber laser seeds an optical parametric amplifier, which is pumped by a 7-W, 450-fs Yb:KGW bulk mode-locked oscillator with 41 MHz repetition rate
Summary
Coherent Raman imaging methods are emerging tools in life sciences due to their large potential in label-free, high speed imaging with applications such as medical diagnostics, the minimally invasive study of cellular metabolism, and drug delivery [1]. The two spectral regions that are of major importance for coherent Raman spectroscopy (CRS) span from 500 – 1700 cm−1 (“fingerprint”) and 2700 – 3300 cm−1 (“CH-stretch”) The latter provides strong vibrational signals, which has enabled video-rate CARS and SRS imaging of biological tissue [8]. The parametric single-pass gain for 1-μm-pumped systems is lower in this spectral region, due to group velocity mismatch [18]. We overcome these issues by using a Tm-doped fiber laser to provide the tunable cw-seed in the 1770 – 2030 nm range and an oscillator-pumped double-pass OPA [19,20,21] for power scaling. A step-by-step optimization is possible; it offers easy handling and troubleshooting
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