Stokes Generation Research Articles

Investigation of spontaneous Brillouin scattering generation based on non-adiabatic microfibres

Brillouin Stokes and anti-Stokes generation is successfully demonstrated in backward direction using a non-adiabatic microfibre as the gain medium. The Stokes light wavelength is up-shifted by 0.088 nm (10 GHz) from the BP wavelength as monitored by using an optical spectrum analyzer. The Brillouin scattering can also be enhanced by employing a microfibre based inline Mach–Zehnder interferometer (IMZI) as the gain medium due to its stronger multimode interference effect. It is shown that the microfibre geometry plays an important role in the spontaneous Brillouin scattering generation and gain bandwidth broadening due to its effect on irritation of the acoustic modes inside the microfibre.

Stimulated Raman scattering spectroscopy of Y0.4Gd0.6VO4 crystals with partly disordered zircon-type structure—observation of new χ(3)-nonlinear optical interactions

This letter presents the results of stimulated Raman scattering (SRS) spectroscopy of tetragonal Y0.4Gd0.6VO4 vanadate crystals having a partly disordered zircon-type structure. Picosecond laser pumping revealed previously unexplored χ(3)-nonlinear interactions comprising the generation of an almost two-octave spanning Stokes and anti-Stokes frequency comb, self-third harmonic generation as well as cross-cascaded processes involving four different Raman-active modes. The recorded spectral lines have been assigned to the participating molecular vibrational modes of the (VO4)3− structural units of the studied crystal. In addition, this work provides a brief review of all known SRS-active tetragonal and monoclinic vanadates of the form REVO4 including their χ(3)-nonlinear properties and steady-state Raman gain coefficients for first Stokes generation.

Monoclinic m-LaVO4—a novel high Raman gain SRS-active crystal

Monoclinic m-LaVO4, having a monazite-type structure and being known as a host crystal for Nd3+-lasants, is an attractive χ(3)-nonlinear optical material for Raman laser converters. This paper presents the stimulated Raman scattering (SRS) spectroscopy of m-LaVO4 in the visible and near-IR spectral range using picosecond laser excitation. High-order SRS and Raman-induced four-wave parametric mixing processes result in an almost two-octave-wide Stokes and anti-Stokes frequency comb. All the measured spectral lines were assigned to the participating molecular vibration modes of the studied vanadate. Finally, an estimation of the steady-state Raman gain coefficient for first Stokes generation in near-IR was performed.

Trigonal LaF3: a novel SRS-active crystal

Trigonal fluoride LaF3, widely known as a host crystal for Ln3+-lasants, was found to be an attractive many-phonon Raman material and a subject for the investigation of different χ(3)-nonlinear optical effects. We present the manifestation of photon-phonon interactions related to stimulated Raman scattering (SRS) and Raman-induced four-wave mixing (RFWM) processes, initiated by picosecond exсitation at room temperature. Sesqui-octave-spanning Stokes and anti-Stokes frequency comb generation as well as many-step cascaded and cross-cascaded up-conversion χ(3)-nonlinear processes have been observed. The recorded spectral lines originated by SRS and RFWM are identified and attributed to the three observed SRS-promoting phonon modes. The lower limit of the steady-state Raman gain coefficient for near-IR first Stokes generation was estimated. Moreover, a brief review of known Ln3+ : LaF3 laser crystals and SRS-active fluorides is given.

Higher order processes in random Raman lasing.

Random Raman lasers offer a unique opportunity to study many exciting dynamics of light propagation in turbid media. One of the most notable features observed to exist in the recently discovered random Raman laser are the presence of higher order stimulated Raman scattering (SRS) processes. The higher order Stokes generation likely comes from photons that have the longest pathlengths, thus have the most gain. This makes these photons particularly likely to offer interesting insight into wave propagation effects such as coherent backscattering and optical Anderson localization. In this work, we use Monte Carlo simulations to investigate how these higher order processes occur and what properties they are expected to exhibit when considering only transport equation dynamics. This knowledge will allow us to look for deviations from this theory in future experiments to determine if wavelike properties play an active role in random Raman lasing.

Tetragonal SrGdGa3O7 and SrLaGa3O7—novel SRS-active crystals

We report on the first observation of stimulated Raman scattering (SRS) effects in tetragonal crystals of SrGdGa3O7 and SrLaGa3O7, known as host crystals for Ln3+-lasant ions. All recorded Raman-induced spectral lines were identified and attributed to the promoting χ(3)-vibrational modes of these gallates. Steady-state Raman gain coefficients for first Stokes generation were estimated. A short review of known simultaneous SRS- and Ln3+-laser active melilite-type crystals is given as well.

Cascaded Raman generation of the N’th Stokes in single mode-fibers

The evolution of Stokes generation up to the n’th stage and the analytical solutions of commonly used Raman equations including numerical simulation and experimental results is reported. For the experimental work, a 1-km un-doped single-mode fiber was pumped with an ytterbium-doped fiber laser system (FL) in CW regime at 1064 nm in a free running configuration. We showed that it is possible to obtain up to the N’th power thresholds and maximum power for each Stokes by using compact analytical solutions as a first approximation in an arguably simple, quick process.

Tetragonal LiLuF4 - A novel crystal simultaneously active for SRS- and Ln3+ -lasing

In this paper, the first investigation of stimulated Raman scattering (SRS) effects in the tetragonal crystal LiLuF4 is presented. It is known as a promising host-material for trivalent lasant lanthanides (Ln3+). We discovered its three “original” and one “combined” SRS-promoting modes at room temperature under picosecond laser excitation in the visible and near-IR spectral range. Apart from multi-phonon Stokes and anti-Stokes generation, we observed cross-cascaded χ(3) ↔ χ(3) processes involving different pairs of SRS-active phonons. Furthermore, a comparative estimation of the stead-state Raman gain coefficient for first Stokes generation in LiLuF4 related to the most active SRS-phonon mode with ωSRS1 ≈ 323 cm−1 has been carried out for both the visible and near-IR. A brief review of the pioneering publications on laser action in Ln3+-ions of all known fluoride SRS-active single crystals is given in tabular form as well.

Alignment and temperature effects in liquid-crystal-based active polarimetry

It is well known that in liquid crystal (LC)-based active polarimetry, alignment and temperature effects impact polarimeter performance. Practically speaking, when constructing a polarimetric measurement system from LC variable retarders (LCVRs), unavoidable alignment and temperature uncertainties will occur, leading to systematic error that propagates to the Mueller matrix. Typical calibration methods use only a single metric to assess polarimeter performance (the condition number) and often ignore the relationship between systematic error and specific Mueller matrix elements. Here we explore alignment and temperature effects in a Stokes generator and polarimeter, each consisting of two LCVRs, through a series of simulations to calibrate the polarimeter and measure the Mueller matrix of air. We achieve this by modifying an existing LCVR model to incorporate alignment and temperature effects. This new approach offers insight into employing LCVRs individually and associating particular Mueller matrix element error with specific LCVR effects.

Temperature control of power distribution on the Stokes components of a Raman fiber laser

Temperature changes applied to a short ytterbium-doped fiber that is part of an Yb-enhanced Raman fiber laser have resulted in the capability for redistributing power between pump and Stokes components. The red shifting of the amplified spontaneous emission (ASE) at higher temperatures has also resulted in an additional efficiency enhancement for first Stokes generation. As a consequence of this, an earlier second Stokes emission takes place, further enhancing the already ASE-enhanced laser. An increment from 20 to 110 °C of a weak first Stokes resulted in an energy transfer from pump to Stokes of 20%. More characterizations, results and possibilities are discussed in this work.

Low‐temperature stimulated Raman scattering spectroscopy of tetragonal GdVO4 single crystals

We report on the results of low‐temperature stimulated Raman scattering (SRS)‐spectroscopy of tetragonal GdVO4 single crystals. Measurements at T ≈ 9 K and picosecond excitation in the visible and near‐infrared range revealed new manifestations of χ(3)‐nonlinear photon–phonon interactions. Besides many‐phonon Stokes and anti‐Stokes octave spanning SRS generation we discovered a new fundamental χ(3)‐active phonon mode of the crystal. Furthermore, we received data on combined SRS‐active vibrational modes. The lower limits of the steady‐state Raman gain coefficients for first Stokes generation have been estimated together with the temperature dependence of these values in the range from T ≈ 9 and 273 K. A short review of SRS‐promoting vibrational modes and manifestations of χ(3)‐nonlinear interactions of all known SRS‐active tetragonal rare‐earth REVO4 vanadates is given as well.

Cascaded and cross-cascaded χ(3) nonlinear optical effects in a new SRS-active YAlO3 crystal

Stimulated Raman scattering (SRS) is discovered under picosecond laser excitation in a YAlO3 orthorhombic crystal, and its cascaded and cross-cascaded χ(3) nonlinear optical interactions are studied. Investigations revealed three SRS-active phonon modes of the crystal and identified all its recorded Stokes and anti-Stokes generation lines.

Integrated liquid-core optical fibers for ultra-efficient nonlinear liquid photonics

We have developed a novel integrated platform for liquid photonics based on liquid core optical fiber (LCOF). The platform is created by fusion splicing liquid core optical fiber to standard single-mode optical fiber making it fully integrated and practical - a major challenge that has greatly hindered progress in liquid-photonic applications. As an example, we report here the realization of ultralow threshold Raman generation using an integrated CS₂ filled LCOF pumped with sub-nanosecond pulses at 532 nm and 1064 nm. The measured energy threshold for the Stokes generation is 1nJ, about three orders of magnitude lower than previously reported values in the literature for hydrogen gas, a popular Raman medium. The integrated LCOF platform opens up new possibilities for ultralow power nonlinear optics such as efficient white light generation for displays, mid-IR generation, slow light generation, parametric amplification, all-optical switching and wavelength conversion using liquids that have orders of magnitude larger optical nonlinearities compared with silica glass.

Enhancement of Brillouin Stokes generation in the S-band region using a combination S-band Depressed Cladding Erbium Doped Fiber and Semiconductor Optical Amplifier

A hybrid Depressed-Cladding Erbium Doped Fiber (DC-EDF) together with a broadband Semi-conductor Optical Amplifier (SOA) to enhance the generation of Stokes and anti-Stokes in a 7.7 km long Dispersion Compensating Fiber (DCF) is proposed and demonstrated. The performance of the proposed system is compared against that of a conventional system with only the DC-EDF as the gain medium, and the incorporation of the SOA can be seen to enhance the generation of Brillouin Stokes significantly. The proposed system is able to generate an output comb with a flat-top and more than 29 Stokes and 13 anti-Stokes lines, an increase of 14 and 8 lines respectively over the conventional system and covering the entire S-band region. The output comb generated is highly stable over time.

Cavity enhanced stimulated Brillouin scattering in an optical chip for multiorder Stokes generation

We report the first demonstration of on-chip cascaded stimulated Brillouin scattering (SBS). Cascaded SBS is characterized in a 4 cm long chalcogenide (As₂S₃) rib waveguide where the end facet reflections provide a monolithic Fabry-Perot (FP) resonator. The presence of the FP cavity reduces the Brillouin gain threshold, which enables observation of cascaded SBS at reduced pump powers. We observe up to three orders of Stokes waves in the backscattered signal at a coupled peak power of 1.34 W. Anti-Stokes waves due to four-wave mixing between the pump and the Stokes wave were observed in the forward spectrum.

Stimulated Raman scattering in natural crystals of SrSO4, BaSO4 and PbSO4: High-order Stokes and anti-Stokes generation with single-wavelength UV, visible, and near-IR excitation, as well as cascaded up-conversion nonlinear χ (3)↔χ (3) lasing effects under dual-wavelength picosecond collinear coherent pumping

Orthorhombic crystals of SrSO4, BaSO4, and PbSO4, known as natural crystals celestine, barite, and anglesite, were found to be attractive χ(3)-active nonlinear optical materials. High-order Stokes and anti-Stokes picosecond generation that spans almost two octaves has been recorded with single-wavelength laser excitation in the UV, visible, and near-IR ranges. All recorded Raman induced lasing components were identified and attributed to the SRS-promoting vibration modes of the studied crystals (ωSRS≈999 cm−1 for SrSO4,ωSRS≈985 cm−1 for BaSO4 and ωSRS≈977 cm−1 for PbSO4). Under dual-wavelength (λf1=1.06415 μm + λf2=0.53207 μm) collinear coherent picosecond pumping several new manifestations of cascaded χ(3)↔χ(3) nonlinear up-conversion lasing effects were observed in BaSO4 and SrSO4 crystals. We classify all three studied sulfate crystals as promising SRS-active materials for Raman laser frequency converters and as efficient χ(3)-crystals that efficiently generate Stokes and anti-Stokes frequency combs, which can enable experiments of ultra-short pulse syntheses.

Gigantic stimulated Raman scattering in one-dimensional Mott–Hubbard insulators: A possible THz source

One-dimensional Mott–Hubbard insulators like Sr 2CuO 3, halogen-bridged Nickel chain compounds have orders-of-magnitude nonlinear optical properties compared to other one-dimensional organic or inorganic compounds. We show theoretically, that the stimulated Raman scattering susceptibility for such insulators could be order(s)-of-magnitude larger even compared to other nonlinear optical susceptibilities. The lowest two-photon state is at lower energy than the lowest one-photon state in some of these insulators. This leads to a potential for strong Stokes generation in the THz regime from these compounds. Our results and conclusions are based on exact numerical solution of finite size two-band extended Hubbard model.

Multiple Brillouin Stokes generation with bismuth‐based erbium‐doped fiber

AbstractA multiple Brillouin Stokes generation is demonstrated at extended L‐band region using the bismuth‐based erbium‐doped fiber (Bi‐EDF) in the linear cavity. A 2.15‐m long Bi‐EDF is used to provide both nonlinear and linear gains to generate a stimulated Brillouin scattering (SBS) and to amplify the generated SBS, respectively. The Brillouin erbium fiber laser operates at 1615 nm region with more than 40 Stokes and anti‐Stokes lines, and the line spacing is measured to be ∼0.09 nm. Based on the four‐wave mixing signals spacing, the dispersion coefficient of the Bi‐EDF is estimated to be ∼95 ps/nm/km. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1416–1418, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25207

Self-mode locking at multiple Stokes generation in the Raman laser

A self-mode-locking at multiple Stokes generation in Raman lasers is reported. Trains of pulses with durations of down to 50 ps each have been obtained. The reasons for the self-mode-locking have been investigated numerically. Way to the further decreasing of pulse durations was formulated.

All-fiber CARS microscopy of live cells

Using an all-fiber laser system consisting of a femtosecond Er/Yb fiber oscillator as the pump and an ultra-highly nonlinear fiber for Stokes generation, we demonstrate multimodal (TPF+SHG+CARS) non-linear optical microscopy of both tissue samples and live cells. Multimodal imaging was successfully performed with pixel dwell times as short as 4 micros at low laser powers (< 40 mW total).