Stokes Pulse Research Articles

Broadband stimulated Raman scattering microscopy with wavelength-scanning detection.

We introduce a high‐sensitivity broadband stimulated Raman scattering (SRS) setup featuring wide spectral coverage (up to 500 cm−1) and high‐frequency resolution (≈20 cm−1). The system combines a narrowband Stokes pulse, obtained by spectral filtering an Yb laser, with a broadband pump pulse generated by a home‐built optical parametric oscillator. A single‐channel lock‐in amplifier connected to a single‐pixel photodiode measures the stimulated Raman loss signal, whose spectrum is scanned rapidly using a galvanometric mirror after the sample. We use the in‐line balanced detection approach to suppress laser fluctuations and achieve close to shot‐noise‐limited sensitivity. The setup is capable of measuring accurately the SRS spectra of several solvents and of obtaining hyperspectral data cubes consisting in the broadband SRS microscopy images of polymer beads test samples as well as of the distribution of different biological substances within plant cell walls.

Dynamics and quantum fluctuations of light in nonstationary operation modes of a Raman fibre amplifier

We report the results of numerical simulation of the radiation dynamics in a Raman fibre amplifier taking into account quantum fluctuations of the pump fields and Stokes waves. The simulation relies on an approach based on solutions of the transport equations for complex amplitudes and on the ‘backward’ propagation method for operators describing quantum fluctuations. It is shown that there exists an optimal Raman amplifier length corresponding to the minimum level of fluctuations of the amplified Stokes pulse.

Brillouin optical time-domain analysis enhanced by forward stimulated Brillouin scattering: proof-of-concept demonstration

A distributed Brillouin optical time-domain analysis (BOTDA) sensor enhanced by forward stimulated Brillouin scattering (FSBS) based distributed Brillouin amplification (DBA) was proposed and presented. Due to co-propagating of pump and Stokes lights, the Stokes pulse is effectively amplified in front segment of fiber, resulting in flatter gain distribution. As a proof-of-concept, FSBS-DBA based BOTDA sensing with distance of ∼74.2 km, Brillouin frequency shift standard deviation of less than ∼2 MHz and spatial resolution of ∼10 m was demonstrated. This method is compatible and complementary with backward SBS-based DBA, and features better pumping efficiency than Raman amplification.

Robust and rapidly tunable light source for SRS/CARS microscopy with low-intensity noise

We present a fully automated laser system with low-intensity noise for coherent Raman scattering microscopy. The robust two-color system is pumped by a solid-state oscillator, which provides Stokes pulses fixed at 1043 nm. The tunable pump pulses of 750 to 950 nm are generated by a frequency-doubled fiber-feedback femtosecond optical parametric oscillator. The resulting pulse duration of 1.2 ps provides a viable compromise between optimal coherent Raman scattering signal and the necessary spectral resolution. Thus a spectral range of 1015 to 3695 cm − 1 with spectral resolution of

Coherent bleaching of the medium with Raman scattering under conditions of strong size quantization

We consider stimulated resonant Raman scattering on a system of discretely quantized levels of the superlattice in a quantizing magnetic field, which leads to one-soliton solutions for the Stokes pulses of scattered waves. The consideration is carried out by the method of the equation of motion of the density matrix and perturbation theory. The threshold conditions for the establishment of the Raman oscillations and the formation of stationary one-soliton solutions for the propagation of scattered radiation and possible parametric dependences of the studied processes are revealed.

Frequency Modulation Stimulated Raman Scattering Microscopy through Polarization Encoding.

Stimulated Raman scattering (SRS) microscopy is a powerful method for imaging molecular distributions based on their intrinsic vibrational contrast. However, despite a growing list of biological applications, SRS is frequently hindered by a parasitic background signal which both overpowers the signal in low-signal applications and makes the extraction of quantitative information from images challenging. Frequency modulation (FM) has been used to suppress this parasitic background. However, many FM-SRS methods require either the acquisition of multiple images or the addition of multiple optomechanical components and an extensive realignment procedure. Herein, we report a new procedure for alignment-free FM-SRS utilizing polarization encoding. We demonstrate the efficacy of this approach, along with parabolic amplification of the Stokes pulse, at removing parasitic background signals in SRS microscopy applications. We further highlight how this technique can be used to suppress Raman signals from major molecular species to unveil spectral signatures from nucleic acids in both murine brain tissue and whole blood. Due to its ease of use and demonstrated experimental capabilities, we expect this technique to see broad use in the SRS microscopy community.

Dual-wavelength intracavity Raman laser driven by a coaxially pumped dual-crystal fundamental laser.

An actively Q-switched dual-wavelength intracavity Raman laser based on a coaxially pumped dual-crystal (Nd:YAG and b-cut Nd:YAP) fundamental configuration was theoretically and experimentally investigated. Stable dual-wavelength Stokes output at 1176 nm and 1195 nm was subsequently obtained by the Raman conversion from an a-cut YVO4 crystal. A total average power of 1.8 W was produced at 10 kHz pulse repetition frequency under an incident diode pump power of 15.8 W, in which the power levels at the two Stokes wavelengths were nearly equivalent. The power proportion and the interval between the dual-wavelength Stokes pulses could be manipulated actively by changing the pump focal position or pump wavelength.

Theoretical and experimental study on a large energy potassium titanyl phosphate terahertz parametric source

The combination of a Stokes parametric oscillator and a Stokes parametric amplifier is used to generate high-energy terahertz pulses in KTiOPO4 (KTP) crystal at 5.70 THz. The Stokes parametric oscillator generates Stokes pulses first. And then they are amplified in the Stokes parametric amplifier and at the same time, high energy terahertz pulses are generated via stimulated polariton scattering. When the incident Stokes pulse energy is 36.6 mJ and the pump energy is 580.0 mJ, the amplified Stokes energy is 192.1 mJ and the obtained maximum terahertz pulse energy is 17.0 μJ. Compared with the reported terahertz pulse energy obtained by the KTiOPO4 terahertz parametric oscillators or generators, the terahertz pulse energy is greatly enhanced. In addition, considering the large angle non-collinear phase matching, pump beam depletion, terahertz wave surface-emitted structure, Gaussian pulse distribution, we make a theoretical simulation for the Stokes parametric amplifier through the coupled-wave equations. The theoretical results are in agreement with the experimental results on the whole.

SRS converter – compressor of femtosecond ytterbium laser pulses

Stimulated Raman scattering (SRS) conversion of femtosecond pulses of an ytterbium laser at a wavelength of 1.03 μm is performed in compressed hydrogen under conditions of nonlinear phase modulation. Temporal compression of frequency-chirped Stokes pulses is carried out, as a result of which 400-fs laser pulses are converted into 27-fs Stokes pulses with a wavelength of 1.8 μm. An average Stokes power of 0.6 W is implemented at an average laser power of 6.3 W and a pulse repetition rate of 20 kHz.

Femtosecond Raman frequency shifter-pulse compressor.

The conversion to the first Stokes component at a wavelength of 1.8μm of ytterbium laser radiation with a pulse duration of 270fs by stimulated Raman scattering (SRS) in hydrogen was carried out by using double-pulse pumping scheme. Simultaneously with SRS the process of nonlinear phase modulation of laser and Stokes waves was observed. The spectrally broadened chirped Stokes pulse was compressed to 35fs in fused silica optical elements at the output of a Raman cell.

Broadband hyperspectral stimulated Raman scattering microscopy with a parabolic fiber amplifier source.

Hyperspectral stimulated Raman scattering (hsSRS) microscopy has recently emerged as a powerful non-destructive technique for the label-free chemical imaging of biological samples. In most hsSRS imaging experiments, the SRS spectral range is limited by the total bandwidth of the excitation laser to ~300 cm-1 and a spectral resolution of ~25 cm-1. Here we present a novel approach for broadband hsSRS microscopy based on parabolic fiber amplification to provide linearly chirped broadened Stokes pulses. This novel hsSRS instrument provides >600 cm-1 spectral coverage and ~10 cm-1 spectral resolution. We further demonstrated broadband hsSRS imaging of the entire Raman fingerprint region for resolving the distribution of major biomolecules in fixed cells. Moreover, we applied broadband hsSRS in imaging amyloid plaques in human brain tissue with Alzheimer's disease.

Adiabatic population transfer between electronic states of LiH molecule in two picosecond laser pulses

The population transfer of stimulated Raman adiabatic passage (STIRAP) is investigated using a time-dependent quantum wave packet method. The population is transferred from an initial state to a target state through the interaction of the external laser field with the permanent dipole moment and the transition dipole moment. The laser field is composed of an infrared pulse and a near ultraviolet pulse. The two pulses with small detunings are more efficient to achieve the STIRAP transition. The process of STIRAP is sensitive to the detunings of the two pulses. When the two detunings are the same in a Λ transition or the positive detuning of the pump pulse is equal to the negative detuning of the Stokes pulse in a ladder transition, the final population in the intermediate state can be eliminated and the target population is increased.

Laser-induced damage of fused silica optics at 355 nm due to backward stimulated Brillouin scattering: experimental and theoretical results.

Forward pump pulses with nanosecond duration are able to generate an acoustic wave via electrostriction through a few centimeters of bulk silica. Part of the incident energy is then scattered back on this sound wave, creating a backward Stokes pulse. This phenomenon known as stimulated Brillouin scattering (SBS) might induce first energy-loss, variable change of the temporal waveform depending on the location in the spatial profile making accurate metrology impossible, and moreover it might also initiate front surface damage making the optics unusable. Experiments performed on thick fused silica optics at 355 nm with single longitudinal mode pulses allowed us to detect, observe and quantify these backward pulses. Experimental results are first compared to theoretical calculations in order to strengthen our confidence in metrology. On this basis a phase-modulator has been implemented on the continuous-wave seeders of the lasers leading to pulses with a wide spectrum that suppress SBS and do not exhibit temporal overshoots that also reduce Kerr effects. The developed set-ups are used to check the reduction of the backward stimulated Brillouin scattering and they allow measuring with accuracy the rear surface damage of thick fused silica optics.

Formation of nanosecond SBS-compressed pulses for pumping an ultra-high power parametric amplifier

Compression of pulsed Nd : glass laser radiation under stimulated Brillouin scattering (SBS) in perfluorooctane is investigated. Compression of 16-ns pulses at a beam diameter of 30 mm is implemented. The maximum compression coefficient is 28 in the optimal range of laser pulse energies from 2 to 4 J. The Stokes pulse power exceeds that of the initial laser pulse by a factor of about 11.5. The Stokes pulse jitter (fluctuations of the Stokes pulse exit time from the compressor) is studied. The rms spread of these fluctuations is found to be 0.85 ns.

Interplay of pulse bandwidth and spectral resolution in spectral-focusing CARS microscopy

We present a theoretical analysis of the relationship among the spectral resolution, the degree of chirp-matching, and pulse bandwidth in coherent anti-Stokes Raman scattering microscopy implementations based on spectral focusing (SF-CARS). We find that, despite allowing better ultimate spectral resolution when chirp-matching is attained, the use of the broadest bandwidth pulses can significantly worsen the spectral resolution if the pulses are not chirp-matched. We demonstrate that the bandwidth of the detected anti-Stokes signal is minimized when the pump is twice as chirped as the Stokes, meaning that—contrary to our initial presumption—a narrow anti-Stokes bandwidth does not imply good spectral resolution. We present approximate expressions that relate the bandwidths of the pump, Stokes, and anti-Stokes pulses to the degree of chirp-matching and outline how these could be used to estimate the amount of glass needed to attain chirp-matching. These expressions also permit a useful estimate of the group delay dispersion from other optics in the pump or Stokes beam path. This work aims to clarify several technical aspects of SF-CARS microscopy and provides researchers with valuable information to consider when working with SF-CARS microscopy systems.

Long-wave infrared picosecond parametric amplifier based on Raman shifter technology.

A new method for a long-wave infrared (LWIR), picosecond difference frequency generation (DFG) source using one near-infrared laser and a Raman shifter is experimentally tested and characterized. The signal seed for DFG is a Stokes pulse generated via transient stimulated Raman scattering in a nonlinear medium with a Raman frequency in the 2-20 µm range. A study of the dynamics of the transient Raman regime in liquid C6D6 has shown that the efficiency of Stokes production can be increased and the central wavelength can be controlled by chirping the pump pulse in order to compensate for chirping caused by self-phase modulation. High energy, ≥3 µJ, picosecond pulses at 10.6 µm have been generated in a GaSe crystal pumped by 1 mJ pulses of 1060 nm light from a Nd:glass laser.

All-normal-dispersion fiber laser with NALM: power scalability of the single-pulse regime

We investigate the average power scalability of all-normal-dispersion fiber integrated lasers with a nonlinear amplifying loop. This laser generates 34 ps pulses at a repetition of 8 MHz and spectral width of 30 nm when increased to the highest power. The laser operates on a single-pulse train regime and can be de-chirped as short as 200 fs. The 60 mW output power which can be maintained while operating with low Raman effect is the highest average power achieved for this type of laser in the sub-200 fs regime. The laser becomes unstable due to the strengthening of the Raman–Stokes pulse. Output pulses show great stability in testing.

Two‐frequency CARS imaging by switching fiber laser excitation

To fully exploit the power of coherent Raman imaging, techniques are needed to image more than one vibrational frequency simultaneously. We describe a method for switching between two vibrational frequencies based on a single fiber-laser source. Stokes pulses were generated by soliton self-frequency shifting in a photonic crystal fiber. Pump and Stokes pulses were stretched to enhance vibrational resolution by spectral focusing. Stokes pulses were switched between two wavelengths on the millisecond time scale by a liquid-crystal retarder. Proof-of-principle is demonstrated by coherent anti-Stokes Raman imaging of polystyrene beads embedded in a poly(methyl methacrylate) (PMMA) matrix. The Stokes shift was switched between 3,050 cm-1 , where polystyrene has a Raman transition, and 2,950 cm-1 , where both polystyrene and PMMA have Raman resonances. The method can be extended to multiple vibrational modes.

光纤中三角斯托克斯脉冲的受激布里渊散射慢光

光纤中三角斯托克斯脉冲的受激布里渊散射慢光

Spectral-temporal dynamics of high power Raman picosecond pulse using H2-filled Kagome HC-PCF.

We report on the spectral-temporal characterization of a 1.8μm wavelength and high power picosecond pulse Raman source. It is generated via frequency conversion to the first-order Stokes of a 27ps chirped pulse Yb-doped fiber laser inside a molecular hydrogen-filled Kagome hollow-core photonic crystal fiber (HC-PCF). Depending on the average power and chirp of the pump laser, the average power of this Raman source can be as high as 9.3W, and its pulse duration can be as short as ∼17 ps. In agreement with stimulated Raman scattering under the very high gain transient regime, the experimental results show the Stokes spectral structure to change following a three-stage sequence when the average pump power is increased. For a pump with a chirp corresponding to a bandwidth of 200GHz, we found that for a pump power lower than 7W, the Stokes spectrum is generated from the blue side of the pump spectrum, and then it exhibits a spectral replica of the pump spectrum for 7-14W pump power range. Finally,the Stokes spectrum is chiefly generated from the red side of the pump spectrum when the pump power is further increased. Conversely, the Stokes pulse temporal profile shows a strong dependence with the pump power. For a low pump power range, the Stokes pulse exhibits a single peak with a full width at half-maximum of ∼17 ps. For higher pump powers, the Stokes pulse presents a double-peak structure with each peak having a duration of less than 15ps. The present results can be used to develop compact and efficient frequency down-convertors to the increasingly widespread Yb-based picosecond lasers.