Tunable dual-wavelength two-picosecond light source for coherent Raman scattering microscopy

Abstract

In narrow-bandwidth coherent Raman scattering (CRS) microscopy, efficient signal generation is accomplished with two-color laser sources providing synchronized picosecond pulses whose frequency difference and spectral widths match the molecular Raman frequency and bandwidth, respectively. With vibrational bandwidths of typically 10 cm -1 , the optimum laser pulse durations thus correspond to about 2 ps. Here, we present a new light source consisting of an amplified Yb-fiber oscillator providing 2-ps pulses at 1031 nm and a synchronously green-pumped optical parametric oscillator (OPO). The OPO slightly shortens the pulses to < 2 ps while maintaining a bandwidth of 10 cm -1 . Output power levels of 1 W in both the 1031-nm and the OPO-branch with continuously tunable frequency differences between the two beams covering a broad range from 700 to 4500 cm -1 are achieved. In addition to CARS microscopy, this light source allows for SRS imaging via an integrated electro-optical modulation of the 1031-nm beam at 20 MHz with a depth of >95%, locked to the laser repetition rate of 80 MHz. The OPO noise at 20 MHz was found to be only 60% above the combined detector and laser noise of a conventional Nd:YVO pump source. This represents a significant reduction in laser noise when compared to other fiber-based laser sources previously proposed for SRS microscopy. When SRS imaging with this new light is compared with a Nd:YVO pumped OPO (delivering 7 ps and 5 ps pulses, respectively), a 5-to 6-fold increase in SRS signal strength and signal-to-noise ratio has been achieved. Video-rate SRS and the capability of multi-spectral SRS imaging are demonstrated.

Background-suppressed SRS fingerprint imaging with a fully integrated system using a single optical parametric oscillator

“…We have developed a compact and integrated system for SRGOLD excitation, which can be coupled to any OPO source pumped by a frequency doubled Ytterbium [27] or Neodymium laser (1031 nm, 1040 nm or 1064 nm). This system is designed for the control of spatially overlapped, collimated beams that are split each in one of the three paths, according to their wavelength (signal < 1000 nm, idler >1100 nm and ω 0 beam).…”

Developments in spontaneous and coherent Raman scattering microscopic imaging for biomedical applications

“…74 OPOs pumped by Yb fiber lasers are commercially available. 75 Instead of using a fiber laser to pump an OPO, alternatively passive optical fibers can be used to substitute the OPO for frequency conversion. Here, nonlinear frequency conversion processes in these fibers are employed in order to generate spectrally separated emission wavelengths, which are inherently synchronized to the pump laser.…”

“…90 Since the spectral separation between the pump laser and the generated signal and idler can be much smaller than in conventional OPOs, a larger part of the Raman spectrum can be covered by such a source without the need for a second OPO for imaging resonances below 700 cm À1 . 71,73,75 Before presenting all-fiber based light sources for CRS, another compact alternative will be briefly introduced.…”

Origin and suppression of parasitic signals in Kagomé lattice hollow core fibers used for SRS microscopy and endoscopy

“…Finally, the minimal fractional modulation which is possible to detect with the A.P.E LIA module is around 5 • 10 −7 for the conditions used in the experiment (50 mW on the LIA detector, 20 µs integration time) which corresponds to the shot noise limit. Using the picoEmerald™ S, which is shot noise limited [32], the KL-HCF affects the SRS ultimate sensitivity by a factor of 10 only (for this particular integration time). From Fig.…”