Widely-Tunable All-Fiber Laser Source for Coherent Raman Scattering Microscopy

Development of fiber laser sources has been a focus of research for several decades, driven by the increasing demand for numerous applications, including Coherent Raman Scattering (CRS) microscopy. CRS stands out among microscopy techniques due to its advantages of label-free imaging and high resolution. In this talk, I will present the development of three wavelength-tunable laser sources based on a fixed wavelength oscillator, which enables measurements in both fingerprint and C-H stretch Raman regions. Access to the fingerprint region is particularly valuable, as it facilitates improved chemosensitivity assessment, an area that remains poorly addressed by current state-of-the-art fiber light sources.

The first dual-wavelength Stimulated Raman Scattering (SRS) light source allows measurements in the fingerprint Raman region, covering the range from 950 cm⁻¹ to 1600 cm⁻¹ . The tunable output for the pump and Stokes beams ranges from 913 nm to 930 nm, and from 1020 nm to 1070 nm, respectively, with an average power of 40 mW for each beam. The tunability of the Stokes beam is achieved by using a novel method of spectral broadening through Self-Phase Modulation (SPM) of chirped pulses in an all-fiber configuration. To validate our light source, SRS imaging of leukemic cells was performed.

The second light was designed to perform imaging in both the fingerprint and C-H stretch Raman regions. For this, a fiber optical parametric oscillator (FOPO) was developed based on the Four-Wave Mixing (FWM) nonlinear effect. The system is in an all-fiber configuration and built around the Yb-doped fiber oscillator with a fixed central wavelength at 1030 nm, serving as a seed beam. The pump tunability was achieved by exploiting a novel method involving spectral broadening of chirped pulses, followed by spectral filtering and amplification similar to the approach used in the first light source. The obtained FWM signals are tunable from 730 nm up to 940 nm. While considering the second beam at 1030 nm, this can enable Raman measurements in the range from 929 cm⁻¹ to 3990 cm⁻¹.

The third light source was focused on the development of a novel method for tuning FWM sidebands based on the SPM of chirped pulses. This method involves varying the chirp of the pump pulses at a fixed central wavelength that are next injected into the concatenation of standard fiber and a photonic crystal fiber. The outermost redshifted SPM peak exhibits significant spectral power density to act as a tunable pump for the D-FWM process. When combined with a 1030 nm beam, it allows measurements in the fingerprint and C-H stretch Raman regions spanning from 873 cm⁻¹ and 3738 cm⁻¹. Coherent Anti-Stokes Raman Scattering imaging of polystyrene beads and droplet paraffin was conducted to validate this light source.