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

A frequency modulation (FM) scheme for stimulated Raman scattering (SRS) is presented with a single fiber-based light source. Pulse-to-pulse wavelength-switching allows real-time subtraction of parasitic signals leaving only the resonant SRS signal with a noise reduction of up to 30 % compared to digital subtraction schemes, leading effectively to a contrast improvement by a factor of up to 8.3. The wide tuning range of the light source from 1500 cm-1 to 3000 cm-1 and the possibility to separately adjust the resonant and the nonresonant wavenumber for every specimen allow to investigate a variety of samples with high contrast and high signal-to-noise ratio, e. g., for medical diagnostics.

Diabetes disturbs functional adaptation of the remote myocardium after ischemia/reperfusion.

Diabetes mellitus type 2 is associated with adverse clinical outcome after myocardial infarction. To better understand the underlying causes we here investigated sarcomere protein function and its calcium-dependent regulation in the non-ischemic remote myocardium (RM) of diabetic mice (db/db) after transient occlusion of the left anterior descending coronary artery. Before and 24 h after surgery db/db and non-diabetic db/+ underwent magnetic resonance imaging followed by histological and biochemical analyses of heart tissue. Intracellular calcium transients and sarcomere function were measured in isolated cardiomyocytes. Active and passive force generation was assessed in skinned fibers and papillary muscle preparations. Before ischemia and reperfusion (I/R), beat-to-beat calcium cycling was depressed in diabetic cardiomyocytes. Nevertheless, contractile function was preserved owing to increased myofilament calcium sensitivity and higher responsiveness of myocardial force production to ß-adrenergic stimulation in db/db compared to db/+. In addition, protein kinase C activity was elevated in db/db hearts leading to strong phosphorylation of the titin PEVK region and increased titin-based tension of myofilaments. I/R impaired the function of whole hearts and RM sarcomeres in db/db to a larger extent than in non-diabetic db/+, and we identified several reasons. First, the amplitude and the kinetics of cardiomyocyte calcium transients were further reduced in the RM of db/db. Underlying causes involved altered expression of calcium regulatory proteins. Diabetes and I/R additively reduced phospholamban S16-phosphorylation by 80% (P < 000.1) leading to strong inhibition of the calcium ATPase SERCA2a. Second, titin stiffening was only observed in the RM of db/+, but not in the RM of db/db. Finally, db/db myofilament calcium sensitivity and force generation upon ß-adrenergic stimulation were no longer enhanced over db/+ in the RM. The findings demonstrate that impaired cardiomyocyte calcium cycling of db/db hearts is compensated by increased myofilament calcium sensitivity and increased titin-based stiffness prior to I/R. In contrast, sarcomere function of the RM 24 h after I/R is poor because both these compensatory mechanisms fail and myocyte calcium handling is further depressed.

Multi-window sparse spectral sampling stimulated Raman scattering microscopy.

Stimulated Raman scattering (SRS) is a nondestructive and rapid technique for imaging of biological and clinical specimens with label-free chemical specificity. SRS spectral imaging is typically carried out either via broadband methods, or by tuning narrowband ultrafast light sources over narrow spectral ranges thus specifically targeting vibrational frequencies. We demonstrate a multi-window sparse spectral sampling SRS (S4RS) approach where a rapidly-tunable dual-output all-fiber optical parametric oscillator is tuned into specific vibrational modes across more than 1400 cm-1 during imaging. This approach is capable of collecting SRS hyperspectral images either by scanning a full spectrum or by rapidly tuning into select target frequencies, hands-free and automatically, across the fingerprint, silent, and high wavenumber windows of the Raman spectrum. We further apply computational techniques for spectral decomposition and feature selection to identify a sparse subset of Raman frequencies capable of sample discrimination. Here we have applied this novel method to monitor spatiotemporal dynamic changes of active pharmaceutical ingredients in skin, which has particular relevance to topical drug product delivery.

Multi-color stimulated Raman scattering with a frame-to-frame wavelength-tunable fiber-based light source.

We present multi-color imaging by stimulated Raman scattering (SRS) enabled by an ultrafast fiber-based light source with integrated amplitude modulation and frame-to-frame wavelength tuning. With a relative intensity noise level of -153.7 dBc/Hz at 20.25 MHz the light source is well suited for SRS imaging and outperforms other fiber-based light source concepts for SRS imaging. The light source is tunable in under 5 ms per arbitrary wavelength step between 700 cm-1 and 3200 cm-1, which allows for addressing Raman resonances from the fingerprint to the CH-stretch region. Moreover, the compact and environmentally stable system is predestined for fast multi-color assessments of medical or rapidly evolving samples with high chemical specificity, paving the way for diagnostics and sensing outside of specialized laser laboratories.

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

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 700cm-1 and 3200cm-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.

Portable all-fiber dual-output widely tunable light source for coherent Raman imaging.

We present a rapidly tunable dual-output all-fiber light source for coherent Raman imaging, based on a dispersively matched mode-locked laser pumping a parametric oscillator. Output pump and Stokes pulses with a maximal power of 170 and 400 mW, respectively, and equal durations of 7 ps could be generated. The tuning mechanism required no mechanical delay line, enabling all-electronic arbitrary wavelength switching across more than 2700 cm - 1 in less than 5 ms. The compact setup showed a reliable operation despite mechanical shocks of more than 25 m / s 2 and is, thus, well suited for operation in a mobile cart. Imaging mouse and human skin tissue with both the portable light source and a commercial laboratory-bound reference system yielded qualitatively equal results and verified the portable light source being well suited for coherent Raman microscopy.

Rapid spectro-polarimetry to probe molecular symmetry in multiplex coherent anti-Stokes Raman scattering.

We present the simultaneous detection of the spectrum and the complete polarization state of a multiplex coherent anti-Stokes Raman scattering signal with a fast division-of-amplitude spectro-polarimeter. The spectro-polarimeter is based on a commercial imaging spectrograph, a birefringent wedge prism, and a segmented polarizer. Compared to the standard rotating-retarder fixed-analyzer spectro-polarimeter, only a single measurement is required and an up to 21-fold reduced acquisition time is shown. The measured Stokes parameters allow us to differentiate between vibrational symmetries and to determine the depolarization ratio ρ by data post-processing.

Optical parametric chirped pulse oscillation.

A concept to flexibly adjust the spectral bandwidth of the output pulses of a fiber optical parametric oscillator is presented. By adjusting the chirp of the pump pulses appropriate to the chirp of the resonant pulses, the energy of the output pulses can be transferred into a user-defined spectral bandwidth. For this concept of optical parametric chirped pulse oscillation, we present numerical simulations of a parametric oscillator, which is able to convert pump pulses with a spectral bandwidth of 3.3 nm into output pulses with an adjustable spectral bandwidth between 9 and 0.05 nm. Combined with a wavelength tunability between 1200 and 1300 nm and pulse energies of up to 100 nJ, the concept should allow to adapt a single all-fiber parametric oscillator to a variety of applications, e.g., in multimodal nonlinear microscopy.

Electronically and rapidly tunable fiber-integrable optical parametric oscillator for nonlinear microscopy.

We present a fiber-based optical parametric oscillator (FOPO) pumped by a fiber-coupled laser diode. The FOPO consisted of a photonic crystal fiber to convert the pump pulses via four-wave mixing and a dispersive resonator formed by a single-mode fiber. Via dispersion filtering, output pulses with a bandwidth of about 3 nm, a temporal duration of about 8 ps and a pulse energy of up to 22 nJ could be generated. By changing the repetition frequency of the pump laser diode by about ±1 kHz, the wavelength of the output pulses could be tuned between 1130 and 1310 nm within 8 µs, without the need to change the length of the resonator. Therewith, the FOPO should especially be suited for hyperspectral imaging, while its all-electronic control constitutes a promising approach to a turnkey and alignment-free light source.

Light source for narrow and broadband coherent Raman scattering microspectroscopy.

We present a light source that is well adapted to both narrow- and broadband coherent Raman scattering (CRS) methods. Based on a single oscillator, the light source delivers synchronized broadband pulses via supercontinuum generation and narrowband, frequency-tunable pulses via four-wave mixing in a photonic crystal fiber. Seeding the four-wave mixing with a spectrally filtered part of the supercontinuum yields high-pulse energies up to 8 nJ and the possibility of scanning a bandwidth of 2000 cm(-1) in 25 ms. All pulses are emitted with a repetition frequency of 1 MHz, which ensures efficient generation of CRS signals while avoiding significant damage of the samples. Consequently, the light source combines the performance of individual narrow- and broadband CRS light sources in one setup, thus enabling hyperspectral imaging and rapid single-resonance imaging in parallel.

Ultrafast two-color all-optical transverse mode conversion in a graded-index fiber.

We report on experimental conversion of picosecond probe pulses from the fundamental mode (LP01) to the next higher-order mode (LP11) by optically induced transient long-period gratings in a graded-index fiber using subpicosecond control pulses. In contrast to former experiments, the temporally synchronized control (1030 nm) and probe pulses (1250 nm) were easily distinguishable by their wavelengths, allowing for the first direct observation of such an ultrafast transverse mode conversion. Despite a nonperfect pulse duration ratio between control and probe pulses as well as an unavoidable excitation of unwanted higher-order modes, a clear energy conversion of 18% between the LP01-mode and the LP11-mode was observed at 2.3 times lower control pulse energies compared to previous experiments.