Compact Wavelength- and Pulse-Duration-Tunable Ultrafast Laser System for Coherent Raman Microscopy

Abstract

We demonstrated a three-dimensional nonlinear microscopy of a diamond film and brain tissue using temporally and spectrally shaped ultrashort laser pulses, delivered by a oscillator-only laser system, which integrates a photonic-crystal-fiber and a nonlinear crystals. After the demonstration of unique properties and excellent performance of photonic-crystal fibers (PCFs) as frequency converters and elements of advanced fiber lasers, waveguides of this class find growing applications as components of optical devices for a broad variety of applications (Knight, J Opt Soc Am B 24(8):1661, 2007). In optical imaging, PCFs have been advantageously integrated into the apparatus for coherent Raman microscopy (Andresen et al., Opt Lett 31(9):1328, 2006; Sidorov-Biryukov et al., Opt Lett 31(15):2323, 2006), radically simplifying the design of a multicolor optical source required for this method of imaging. Here, we demonstrate that soliton self-frequency shift (SSFS) in a PCF pumped by an increased-energy output of a long-cavity mode-locked Cr: forsterite laser can be combined with second-harmonic generation (SHG) in a nonlinear crystal to provide a broad wavelength and pulse-width tunability of ultrashort light pulses in the range of wavelengths from 680 to 1,800 nm at the 20-MHz repetition rate. We show that LBO crystals can support phase matching for short-pulse SHG within the entire tunability range of the nanojoule-level PCF output, stretching from 1.35 to 1. 80 μm (Fig. 53.1 left) (Lanin et al., Opt Lett 37:1508–1510, 2012). Thin LBO crystals are employed in our experiments to generate broadly tunable sub-100-fs second-harmonic pulses in the range of wavelengths from 680 to 900 nm. Thicker nonlinear crystals with properly adapted beam focusing are shown to provide up-to-40 %-efficiency frequency doubling of the PCF output with simultaneous spectral narrowing, yielding ultrashort light pulses with an average power up to 10 mW ideally suited for coherent Raman microspectroscopy. Simultaneous wavelength and pulse-width tunability provided by a combination of the SSFS and SHG technologies is especially helpful for nonlinear Raman spectroscopy and microscopy, where the wavelength tunability is needed for a selective excitation of Raman modes, while the pulse-width tunability is instrumental in reducing the coherent background, related to nonresonant four-wave mixing, in the overall coherent Raman response. In Fig. 53.1 we present typical images provided by coherent anti-Stokes Raman scattering (CARS) which were obtained for synthetic diamond film and a slice of brain (Doronina-Amitonova et al., Appl Phys Lett 99:231109, 2011). The pump field in these experiments had a fixed wavelength, λ p = 623 nm, and was provided by the Cr: forsterite laser output frequency-doubled in a periodically poled lithium niobate (PPLN) waveguide. The Stokes field with a tunable wavelength λ s was delivered by PCF-based SSFS combined with SHG as described above.