Coherent Anti-Stokes Raman Spectroscopy based on Fiber Optics

We experimentally demonstrate compact and efficient single-pulse coherent anti-Stokes Raman spectroscopy (CARS) via spectral notch shaping implemented with a fiber Bragg grating.We show that a fiber Bragg grating can serve as a narrowband notch filtering component on a 90 nm broadband femtosecond pulsed laser without spectral distortion. Finally, we obtain CARS spectra of various samples in the fingerprint region of molecular vibrations. This scheme has potential for compact implementations of all fiber single-pulse multiplex CARS due to its compatibility with fiber optics.

We show the feasibility of an alll-fiber based single-pulse coherent anti-Stokes Raman spectroscopy. The system consists of a supercontinuum source from Erbium-doped fiber amplifier and fiber Bragg grating for notch filtering.

<References> 

[1] S.R. Oh, J.H. Park, K.-S. Kim, E.S. Lee, J.Y. Lee, S. Kim. "Investigation of fiber Bragg grating as a spectral notch shaper for single-pulse coherent anti-Stokes Raman spectroscopy." Optics Communications, 383, pp.107-112, 2017.

[2] S.R. Oh, J.H. Park, W.S. Kwon, J.H. Kim, K.-S. Kim, J.Y. Lee, S. Kim, "Single-pulse coherent anti-stokes Raman spectroscopy via fiber Bragg, grating," in: SPIE LASE, International Society for Optics and Photonics, SPIE, San Francisco, CA, 2016.

[3] S.R. Oh, D. Kang, J. Choi, J.H. Kim, H. Lee, K.-S. Kim, S. Kim, "Supercontinuum notch shaping via fiber Bragg grating for the excitation source in coherent anti-stokes Raman spectroscopy," in: Conference on Lasers and Electro-Optics/Pacific Rim, Optical Society of America, Busan, South Korea, 2015.

Fig. 1. Experimental setup of single-pulse CARS via spectral notch shaping implemented with fiber Bragg gratings. (I) Initial pulse laser. (II) Spectral notch shaping via a fiber Bragg grating. (III) Blocking short wavelength by a long-pass filter. (IV) Third-order nonlinear signal generated by a four-wave-mixing process. (V) Third-order nonlinear signal is collected by multimode fiber after the band-pass filter. FI: Faraday isolator, ND: neutral density filter, C-OBJ: coupling objective lens, FBG: fiber Bragg grating, CL: collimating lens, CM: cut mirror, LPF: long-pass filter, OBJ: objective lens, BPF: band pass filter, MMF: multimode fiber. 

Fig. 2. (a) Observed spectra of notch-shaped pulse lasers for three types of FBGs. (b) Single-shot CARS spectra measured from acetone for three notch locations. In each spectrum, a sharp dip feature is observed that corresponds to the acetone 790 cm^-1 vibrational line, and the features are shifted according to the notch position of the excitation pulse laser. (c) The CARS signals of various samples. For the notch location at 790 nm, the peak features appear at 743.5 nm (acetone), 738.5 nm (ethanol), 742 nm (isopropyl alcohol), 743.5 nm and 732.5 nm (toluene). (d) Normalized CARS signals. For the notch location at 790 nm (12658 cm^-1), CARS features appear in Fig. 5(c), which correspond to Raman frequencies of 790 cm^-1 (acetone), 880 cm^-1 (ethanol), 819 cm^-1 (isopropyl alcohol), 793 cm^-1 and 990 cm^-1 (toluene).

 <Schematic of reference [1], [2]>

Fig. 3. Spectral data of the original, amplified, supercontinuum generated, and notch filtered source. (b) Interferometric autocorrelation of the supercontinuum source.

<Schematic of reference [3]>