High-sensitivity frequency modulation CARS with a compact and fast tunable fiber-based light source

Abstract

Frequency modulation (FM) coherent anti-Stokes Raman scattering (CARS) is presented, using a compact as well as fast and widely tunable fiber-based light source. With this light source, Raman resonances between 700 c m − 1 and 3200 c m − 1 can be addressed via wavelength tuning within only 5 ms, which allows for FM CARS measurements with frame-to-frame wavelength switching. Moreover, the functionality for high-sensitivity FM CARS measurements was integrated by means of fiber optics to keep a stable and reliable operation. The light source accomplished FM CARS measurements with a 40 times enhanced sensitivity at a lock-in amplifier (LIA) bandwidth of 1 Hz. For fast imaging with frame-to-frame wavelength switching at a LIA bandwidth of 1 MHz, an 18-fold contrast enhancement could be verified, making this light source ideal for routine and out-of-lab FM CARS measurements for medical diagnostics or environmental sensing.

Fiber-based light source with multi-color output and fast wavelength tuning

“…Due to a steep phase matching curve of the FOPO the relatively narrow tuning of the oscillator wavelength leads to a broad tuning of the FOPO wavelength from 750 nm to 950 nm. In addition to the fast wavelength-switching mechanism of the oscillator, a frequency modulation (FM) [2] scheme was implemented in the FOPO to enable pulse-to-pulse wavelength-switching of the FOPO pulses, which was verified by the measurements shown in Fig. 1(b) and (c).…”

“…1(b) and (c). The FM scheme allows for probing different signals almost simultaneously, and therefore, can be used for real-time background correction in microscopy purposes [2]. As fiber lasers typically show high noise levels, that are not compatible with sensitive microscopy applications, the relative intensity noise (RIN) was measured.…”

Frequency modulation stimulated Raman scattering scheme for real-time background correction with a single light source

“…For the implementation of FM SRS a fiber-based light source ( Fig. 1 ) that was already used for CARS [ 30 , 31 ] and SRS [ 25 ] microscopy was modified to provide the wavelength-alternating pump pulses to probe signals at different wavenumbers. The light source consisted of an ytterbium-doped ( ) fiber oscillator which was mode-locked with a saturable absorber mirror (SAM).…”

“…To cover this entire range the mirror needs to be moved by 5.6 cm as shown in Ref. [ 31 ]. If pulses with a certain and fixed wavelength difference at the output are acceptable, the free-space optical delay line could be replaced by a completely fiber-integrated mirror.…”

Multiwindow SRS Imaging Using a Rapid Widely Tunable Fiber Laser

“…Recently, Brinkmann et al developed a rapid widely tunable picosecond fiber laser based on a fiber OPO (FOPO) and deployed this laser for CARS imaging. , By engineering the dispersion and nonlinearity of the fiber, the FOPO enables broad spectral tuning, which covers C–H, C–D, and fingerprint regions. By incorporating a chirped fiber Bragg grating to match the repetition rate of the two outputs automatically, this FOPO achieves a spectral tuning speed of 5 ms between arbitrary wavenumbers, which is beyond the reach of other fiber lasers used for coherent Raman microscopy.…”