This Small Business Innovation Research (SBIR) Phase I project aims to develop a laser source for label-free microscopy technique, in particular coherent Raman scattering (CRS) microscopy. In contrast to other techniques, CRS requires excitation with two synchronized laser pulse trains (picosecond pulse duration) with a difference frequency that can be tuned to the precision of a typical line width of Raman spectra (<1nm). The key innovation of the proposal is the realization that the difference frequency of the two major gain media used in the telecommunication industry, Erbium (Er) and Ytterbium (Yb), corresponds to the high-wavenumber region of Raman spectra, where most CRS imaging is performed. Based on recent advances in robust all-fiber design, the application proposes to develop a novel dual-color Er-Yb-laser-system based on optical synchronization of two picosecond power amplifiers using super-continuum generation. While this could provide an elegant, economical laser source for CRS, the physics associated with the required high peak powers in fibers is challenging.
The broader impact/commercial potential of this project is in the area of biological and material science research, and ultimately medical diagnostics. CRS allows microscopic imaging with chemical contrast based on intrinsic spectroscopic properties of the sample. It circumvents the issues associated with fluorescent labeling or dye staining, which can be especially problematic for imaging of molecules that are smaller than typical labels or for use in vivo in patients. Wide ranging applications include studying lipid metabolism, trans-dermal drug delivery, biomass conversion to biofuel, and tumor margin delineation during cancer surgery has been demonstrated. While laser systems have come a long way and different approaches exist at various degrees of commercialization, they are expensive (~$300,000), require experienced optics personnel for operation, and are not robust. This greatly limits the access to this exciting new technology and prevents use in medical diagnostics. The light-source proposed in this SBIR application aims to overcome these limitations, as it is based on a all-fiber design from robust and low-cost telecommunication components.
Coherent Raman scattering (CRS) microscopy allows high-resolution imaging of samples with chemical contrast derived from the intrinsic spectroscopic properties of the sample. It circumvents the issues associated with fluorescent labeling and dye staining, which can be particularly problematic for in vivo imaging of animals and man. Wide-ranging applications of CRS including studying lipid metabolism, transdermal drug delivery, biomass conversion to biofuel, and tumor margin delineation have been demonstrated. However, use of this exciting new technology has been greatly limited by the availability of inexpensive, easy-to-use and robust laser sources. Under this Small Business Innovation Research (SBIR) Phase I project, we developed a CRS laser source based on a novel all-fiber design using low-cost telecommunication components. CRS requires excitation with two synchronized laser pulse trains with a difference frequency that can be tuned to the precision of a typical Raman linewidth (<1nm). Our approach is based on the fact that the difference frequency of the two most popular fiber gain media, Erbium (Er) and Ytterbium (Yb), corresponds to the high-wavenumber region of Raman spectra, where most CRS imaging is performed. We developed a dual-color laser based on optical synchronization of two picosecond power amplifiers. Figure 1 shows the optimized output spectra. The range of difference frequencies of the tunable Stokes beam and the narrowband pump beam coincides with the targeted high-wavenumber region of Raman spectra. Figure 2 shows multi-color CRS images acquired with this novel laser source. The different types of beads, which appear identical to the bare eye, are represented in different colors corresponding to the CRS signal, which derives from the molecular make-up of the bead. We have also demonstrated that the sensitivity of our robust, inexpensive light source matches that of the current gold-stranded laser source for CRS, even at a lower average power, as is preferred for medical applications. Based on the success of the Phase 1 developments, we have submitted a Phase 2 proposal to develop and commercialize a fully-integrated CRS microscope based on this novel laser source. The INVENIO IMAGING team thanks the National Science Foundation (NSF) for the support of this SBIR project.