Investigation of fiber Bragg grating as a spectral notch shaper for single-pulse coherent anti-Stokes Raman spectroscopy
Introduction
Coherent anti-Stokes Raman scattering (CARS) has received a considerable amount of attention in the fields of spectroscopy and microscopy based on molecular vibrations [1], [2]. CARS microscopy has enabled label-free measurements [1], [2], and CARS spectroscopy has found applications for the stand-off detection of hazardous agents due to its excellent sensitivity [3], [4], [5], [6]. The signal levels in CARS can be more than 5 orders of magnitude greater than those in spontaneous Raman scattering [2].
Despite the growing demands for using a CARS system in various fields, its commercialization is hampered by the complexity involved in CARS instrumentation. A typical CARS system requires spatial overlap and temporal synchronization of two pulsed lasers to create a beat frequency which makes the overall systems bulky in size, complex, and sensitive to alignment. To overcome these disadvantages, several research groups have developed novel methods for CARS systems [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].
Fiber-based light sources for CARS microscopy were recently developed to reduce size and alleviate the alignment issues [12], [13], [14]. The method used a narrow bandwidth light source, optimized to measure the C–H stretching vibrations, where it may show limitations when applied to measure the fingerprint spectral region [12], [13], [14]. Another study demonstrated fiber-based supercontinuum sources for the fingerprint region of CARS, in which it still remains alignment issues similar to those for a typical CARS system [16]. Single-pulse CARS is a novel method that uses only one pulsed laser to obtain the fingerprint region of molecular vibrations [6], [7], [8], [9]. These methods can obtain a full spectrum of the sample simultaneously that it can be used in both microscopy and spectroscopy applications. The most recent system used a resonant photonic crystal slab as a pulse shaper [17] which requires careful selection of materials and suitable nanotechnology [18].
In this work, we demonstrate a new single-pulse CARS approach with a fiber Bragg grating, which is a widely used passive pulse-shaping component [10], [19], [20], [21]. An important issue in the development of this approach is to propagate the broad bandwidth of a femtosecond pulsed laser in a fiber Bragg grating without significant spectral distortion. We found a suitable condition for using a fiber Bragg grating as a notch shaper in a femtosecond pulsed laser. Finally, we obtained CARS spectra of various samples in the fingerprint spectral region. Fiber Bragg gratings can also be easily manufactured with a narrowband and a high rejection rate such that the spectral resolution and sensitivity can be easily enhanced. Another advantage is that fiber Bragg gratings can be used as rapid pulse shaping modulators that allow for the use of lock-in detection [22], [31]. This new scheme has potential for compact implementations of single-pulse CARS because of its natural compatibility with fiber optics [14], [20].
Section snippets
Principle and concept of single-pulse CARS
In general, CARS is a four-wave-mixing (FWM) process, and it can be described as [8], [9]where PR and PNR are the resonance and non-resonance polarizations, respectively, from a third-order nonlinear process. The Ep, Es, and Epr are the electric fields of the pump, Stokes, and probe, respectively. The χNR(3) is non-resonant third-order nonlinear susceptibility, which is not related to the Raman signal and generally has a
Investigation of the spectral distortion of pulsed laser with respect to group-velocity-dispersion pre-compensation
To obtain desirable CARS signals in the single-pulse CARS scheme, transform-limited laser pulses with a broad spectral bandwidth accompanying a narrowband notch shape are required. The detectable spectral range of the CARS signal is proportional to the bandwidth of a laser pulse used, whereas the CARS spectral resolution and the contrast of resonant CARS dips, on the other hand, are primarily determined by the bandwidth and rejection rate of the notch shape in the pulsed laser, respectively.
In
Experimental results of single-pulse CARS signals
The experimental setup for single-pulse CARS via spectral notch shaping implemented using a fiber Bragg grating is illustrated in Fig. 4. The Ti:S oscillator and the fiber delivery elements were identical to those in Fig. 1(b). The FBG is a customized product from O/E Land Inc., Canada.
We tested three types of FBGs to create a notch shape in the pulsed lasers. Each FBG has a Bragg wavelength at 785 nm, 790 nm, and 795 nm. After notch filtering by the FBG, the pulsed laser was compressed by two
Conclusion
We demonstrated that fiber Bragg gratings are applicable in the single-pulse CARS scheme. In our single-pulse CARS experiment, the excitation source had 1 nm of a narrow notch band with a time duration of 21 fs. In this paper, we investigated the spectral distortion of ultra-short pulsed lasers in the optical fiber with respect to group-velocity-dispersion pre-compensation. To alleviate the spectral distortion, we compensate the positive GDD after propagating in the FBG. Finally, we obtained CARS
Acknowledgments
This paper/work was supported by the BK21+ Program.
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