Stokes Signal Research Articles

TiO2 Nanophotonic Sensors for Efficient Integrated Evanescent Raman Spectroscopy

Waveguide-based evanescent Raman sensors are an attractive chemical-detection technology due to their compact format, increased signal over micro-Raman spectrometers, and nanoscale surface sensitivity. To improve the device’s performance, herein we experimentally demonstrate strategies to increase the efficiency of waveguide-based evanescent Raman sensors over the state-of-the-art silicon-nitride devices by more than an order of magnitude. First, we use pumping at visible wavelengths (532 nm versus 785 nm, which is commonly used) to exploit the Raman cross-section’s λ–4 dependence. Second, we use titanium dioxide (TiO2), which combines comparatively low visible background luminescence with a high refractive index to concentrate light near the waveguide’s surface. These visibly pumped TiO2 sensors display >50× more Stokes signal per input pump power over that of silicon-nitride devices. Lastly, we explore ring resonators and observe on-resonance Stokes emission with peak rates >30× higher than an equivalen...

Single Brillouin frequency shifted S-band multi-wavelength Brillouin-Raman fiber laser utilizing fiber Bragg grating and Raman amplifier in ring cavity

This paper is focusing on simulation and analyzing of S-band multi-wavelength Brillouin–Raman fiber laser performance utilizing fiber Bragg grating and Raman amplifier in ring cavity. Raman amplifier-average power model is employed for signal amplification. This laser system is operates in S-band wavelength region due to vast demanding on transmitting the information. Multi-wavelength fiber lasers based on hybrid Brillouin–Raman gain configuration supported by Raman scattering effect have attracted significant research interest due to its ability to produced multi-wavelength signals from a single light source. In multi-wavelength Brillouin–Raman fiber, single mode fiber is utilized as the nonlinear gain medium. From output results, 90% output coupling ratio has ability to provide the maximum average output power of 43 dBm at Brillouin pump power of 20 dBm and Raman pump power of 14 dBm. Furthermore, multi-wavelength Brillouin–Raman fiber laser utilizing fiber Bragg grating and Raman amplifier is capable of generated 7 Brillouin Stokes signals at 1480 nm, 1510 nm and 1530 nm.

On the widths of Stokes lines in Raman scattering from molecules adsorbed at metal surfaces and in molecular conduction junctions.

Within a generic model we analyze the Stokes linewidth in surface enhanced Raman scattering (SERS) from molecules embedded as bridges in molecular junctions. We identify four main contributions to the off-resonant Stokes signal and show that under zero voltage bias (a situation pertaining also to standard SERS experiments) and at low bias junctions only one of these contributions is pronounced. The linewidth of this component is determined by the molecular vibrational relaxation rate, which is dominated by interactions with the essentially bosonic thermal environment when the relevant molecular electronic energy is far from the metal(s) Fermi energy(ies). It increases when the molecular electronic level is close to the metal Fermi level so that an additional vibrational relaxation channel due to electron-hole (eh) exciton in the molecule opens. Other contributions to the Raman signal, of considerably broader linewidths, can become important at larger junction bias.

Optimization of Output Coupling Ratio for Multi-Wavelength Brillouin Fiber Laser Employing FBG and DCF as Gain Medium

A ring-cavity multi-wavelength Brillouin fiber laser by employing a fiber Bragg grating and dispersion compensating fiber was experimentally demonstrated. The multi-wavelength Brillouin fiber laser has been analyzed at different output coupling ratios at 1550 nm of Brillouin pump wavelength. A ring-cavity multi-wavelength Brillouin fiber laser has been designed with a 16 km long of dispersion compensating fiber which acts as gain medium for stimulated Brillouin scattering effect. Dispersion compensating fiber is designed to have small core size and has higher nonlinear coefficient. As a result, 8 Brillouin Stokes signals were produced at maximum Brillouin power of 21 dBm and output coupling ratio of 80%. The best performance of output coupling ratio was determined by 70% with the highest of signal to noise ratio at 38.39 dB.

Correlation of Brillouin Stokes Signals and Optical-Signal-to-Noise-Ratio in Multi-Wavelength Brillouin Fiber Laser with Additional Fiber Bragg Grating

We experimentally demonstrate unidirectional propagation of multi-wavelength Brillouin fiber laser system with additional fiber Bragg grating. The configuration is arranged in a ring resonator by utilizing five different Brillouin gain mediums. In this experiment, the input Brillouin pump power was varied in order to maximize the generated Brillouin Stokes signals based on the respective fiber lengths. The influence of these controllable parameters leads to the correlation between of Brillouin Stokes signals and optical-signal-to-noise-ratio. In this case, at 10 km of single mode fiber the maximum of 38 Brillouin Stokes signals were generated together with 15.07 dB of average optical-signal-to-noise-ratio when the 17 dBm Brillouin pump power was launched into the ring resonator.

High vacuum tip-enhanced Raman spectroscope based on a scanning tunneling microscope.

In this paper, we present the construction of a high-vacuum tip-enhanced Raman spectroscopy (HV-TERS) system that allows in situ sample preparation and measurement. A detailed description of the prototype instrument is presented with experimental validation of its use and novel ex situ experimental results using the HV-TERS system. The HV-TERS system includes three chambers held under a 10(-7) Pa vacuum. The three chambers are an analysis chamber, a sample preparation chamber, and a fast loading chamber. The analysis chamber is the core chamber and contains a scanning tunneling microscope (STM) and a Raman detector coupled with a 50 × 0.5 numerical aperture objective. The sample preparation chamber is used to produce single-crystalline metal and sub-monolayer molecular films by molecular beam epitaxy. The fast loading chamber allows ex situ preparation of samples for HV-TERS analysis. Atomic resolution can be achieved by the STM on highly ordered pyrolytic graphite. We demonstrate the measurement of localized temperature using the Stokes and anti-Stokes TERS signals from a monolayer of 1,2-benzenedithiol on a gold film using a gold tip. Additionally, plasmonic catalysis can be monitored label-free at the nanoscale using our device. Moreover, the HV-TERS experiments show simultaneously activated infrared and Raman vibrational modes, Fermi resonance, and some other non-linear effects that are not observed in atmospheric TERS experiments. The high spatial and spectral resolution and pure environment of high vacuum are beneficial for basic surface studies.

Multi-wavelength Brillouin Fiber Laser with Fiber Bragg Grating Employs Loop Mirror

In this paper, the development of multi-wavelength Brillouin fiber laser with fiber Bragg grating utilizing a loop mirror is successfully demonstrated. A multi-wavelength Brillouin fiber laser configuration employs 11 km length of single mode fiber as the gain medium, while the reflection of fiber Bragg grating is adopted to produce multi-wavelength signals. Based on the experimental results, it shows that the position of the fiber Bragg grating in the Brillouin fiber laser configuration plays an important role in producing the multi-wavelength signals. By using 19.0 dBm of Brillouin pump power, the highest number of Brillouin Stokes signal and tuning range obtained from this formation is about 23 and 4 nm, respectively. Furthermore, 50% output coupling ratio demonstrates the best ratio to produce Brillouin Stokes signal which gives a better performance in the Brillouin fiber laser system.

Low-threshold stimulated Brillouin scattering in high-Q whispering gallery mode tellurite microspheres

We demonstrate the first observation of stimulated Brillouin scattering (SBS) in a high-Q whispering gallery mode tellurite microsphere. Tellurite glass with composition of 70TeO₂-20ZnO-5Na₂O-5La₂O₃ (molar ratio) was prepared in-house using a melt-quenching technique. Moreover, tellurite microspheres with Q in excess of 13 millions at 1550 nm were fabricated by melting tellurite microwires using a CO₂ laser. By pumping the tellurite microspheres with a tunable single frequency laser, SBS is further realized with a threshold as low as 0.58 mW. At last, the beat notes between the pump and the Stokes signals were measured, which indicated the Brillouin frequency shift is at the 8.2 GHz band for our tellurite glass. Our results could propel significant applications utilizing SBS by employing tellurite microspheres.

Fiber Length Optimization of Ring Cavity Multi-Wavelength Brillouin Fiber Laser Utilizing Fiber Bragg Grating

In this paper, we experimentally investigated the performance of ring cavity multi-wavelength Brillouin fiber laser utilizing fiber Bragg grating which operated in the C-band wavelength region. The combination of stimulated Brillouin scattering and selective wavelengths gave a new invention in the optical fiber communication. Five different lengths of single mode fiber are used in order to get the best gain medium for stimulated Brillouin scattering effect. Up to 33 of Brillouin Stokes signals and 31 of anti-Stokes signals were obtained when 10 km fiber length was used in the laser system. The average value optical signal to noise ratio of 15 dB has been achieved. The broadening bandwidth of Brillouin Stokes signals also occurred at the center wavelength of 1550 nm based on the 3 dB bandwidth of 5 nm fiber Bragg grating.

Investigation of germanium Raman lasers for the mid-infrared.

In this paper we present a detailed theoretical investigation of integrated racetrack Raman lasers based on the germanium material system operating in the mid-infrared beyond the germanium two-photon absorption cut-off wavelength of 3.17 μm. The effective Raman gain has been estimated in waveguides based on germanium-on-silicon, germanium-on-SOI and germanium-on-Si3N4 technology platforms as a function of their crystallographic orientations. Furthermore, general design guidelines have been determined by means of a comparative analysis of Raman laser performance, i.e. the threshold power, polarization and directionality of the excited Stokes signals as a function of racetrack cavity length and directional-coupler dimensions. Finally, the emitted Raman laser power has been evaluated as a function of overall propagation losses and operative wavelengths up to 3.8 μm, while the time dynamics of Raman lasers has been simulated assuming continuous and pulse waves as input pump signals.

Broadband-tunable LP_01 mode frequency shifting by Raman coherence waves in a H_2-filled hollow-core photonic crystal fiber

When a laser pump beam of sufficient intensity is incident on a Raman-active medium such as hydrogen gas, a strong Stokes signal, red-shifted by the Raman transition frequency {\Omega}$_R$, is generated. This is accompanied by the creation of a "coherence wave" of synchronized molecular oscillations with wavevector {\Delta}{\beta} determined by the optical dispersion. Within its lifetime, this coherence wave can be used to shift by {\Omega}$_R$ the frequency of a third "mixing" signal, provided phase-matching is satisfied, i.e., {\Delta}{\beta} is matched. Conventionally this can be arranged using non-collinear beams or higher-order waveguide modes. Here we report collinear phase-matched frequency shifting of an arbitrary mixing signal using only the fundamental LP$_{01}$ modes of a hydrogen-filled hollow-core PCF. This is made possible by the S-shaped dispersion curve that occurs around the pressure-tunable zero dispersion point. Phase-matched frequency shifting by 125 THz is possible from the UV to the near-IR. Long interaction lengths and tight modal confinement reduce the peak intensities required, allowing conversion efficiencies in excess of 70%. The system is of great interest in coherent anti-Stokes Raman spectroscopy and for wavelength-conversion of broadband laser sources.

Electric field measurements in a dielectric barrier nanosecond pulse discharge with sub-nanosecond time resolution

The paper presents the results of time-resolved electric field measurements in a nanosecond discharge between two plane electrodes covered by dielectric plates, using picosecond four-wave mixing diagnostics. For absolute calibration, the IR signal was measured in hydrogen at a pressure of 440 Torr, for electrostatic electric field ranging from 0 to 8 kV cm−1. The calibration curve (i.e. the square root of IR signal intensity versus electric field) was shown to be linear. By measuring the intensities of the pump, Stokes, and IR signal beam for each laser shot during the time sweep across the high-voltage pulse, temporal evolution of the electric field in the nanosecond pulse discharge was determined with sub-nanosecond time resolution. The results are compared to kinetic modeling predictions, showing good agreement, including non-zero electric field offset before the main high voltage pulse, breakdown moment, and reduction of electric field in the plasma after breakdown. The difference between the experimental results and model predictions is likely due to non-1D structure of the discharge. Comparison with the kinetic modeling predictions shows that electric field in the nanosecond pulse discharge is controlled primarily by electron impact excitation and charge accumulation on the dielectric surfaces.

The generation of continuous-variable entanglement frequency comb.

Continuous-variable (CV) entanglement frequency comb can be produced by enhanced Raman scattering in an above-threshold optical oscillator cavity in which a hexagonally-poled LiTaO3 crystal resides as a Raman gain medium. The Stokes and anti-Stokes Raman signals are enhanced by a coupled quasi-phase-matching optical parametric process and the entanglement natures among these Raman signals and pump are demonstrated by applying a sufficient inseparability criterion for CV entanglement. Such entanglement frequency comb source with different frequencies and continuously tunable frequency interval may be very significant for the applications in quantum communication and networks.

Modulating the correlation and squeezing of phase-conjugate four-wave mixing via the polarizable dressing states.

We report the experimental observation of the intensity noise correlation and squeezing between counter propagating Stokes and anti-Stokes signals in Pr(3+):Y2SiO5 crystals. Both the degree of correlation and squeezing as well as the oscillation frequency of correlation curves are modulated by changing the polarization states and powers of the dressing fields. The double-dressed effect and the triple-dressed effect in V-type three-level, Λ-type three-level and N-type four-level systems are compared. The polarization and power dependencies in these systems are different, and the oscillation frequency of the correlation curve in the triple-dressed process is greater than that of the double-dressed process. Our results show that the correlation and squeezing of photon pairs can be controlled via polarized dark states.

Measurement of Temperature through Raman Scattering

Abstract This paper treats the theory of distributed optical fiber sensing measurement used in the monitoring of the structures civil, like tunnels, bridges, buildings, dams, pipeline… While resorting to distributed temperature sensors Raman scattering, we present a theoretical and simulation measurement of temperature. We highlight the effect of distance in temperature measurement in particular the work in this article provides the equation which shows well the relationship between the temperature and distance in optical fiber. The results indicate that the anti stokes signal is strongly dependent on the temperature; therefore the temperature varies slightly by changing the point of measurement.

Development of a Temperature Distributed Monitoring System Based On Raman Scattering in Harsh Environment

Raman Distributed Temperature Sensors (RDTSs) offer exceptional advantages to monitor the envisioned French deep geological repository for nuclear wastes, called Cigeo. Both $\gamma $ -ray and hydrogen release from nuclear wastes can strongly affect the temperature measurements made with RDTS. We present experimental studies on how the performances of RDTS evolve in harsh environments like those associated with $\gamma $ -rays or combined radiations and ${{\rm H}_2}$ release. The response of two standard and one radiation tolerant multimode fibers (MMFs) are investigated. In all fibers the differential induced attenuation between Stokes and anti-Stokes signal, ${({{\alpha _{\rm AS}} - {\alpha _{\rm S}}})}$ causes a temperature errors, up to $30^\circ {\rm C}$ with standard multimode fibers (100 m) irradiated at 10 MGy dose. This degradation mechanism that is more detrimental than the radiation induced attenuation (RIA) limiting only the sensing range. The attenuation in the [800-1600 nm] spectral range at room temperature is explored for the three fibers $\gamma $ -irradiated and/or hydrogen loaded to understand the origin of the differential RIA. We show that by adapting the characteristics of the used fiber for the sensing, we could limit its degradation but that additional hardening by system procedure is necessary to correct the T error in view of the integration of our RDTS technology in Cigeo. The current version of our correction technique allows today to limit the temperature error to $\sim 2^\circ {\rm C}$ for 10 MGy irradiated samples.

Multi-line techniques for inference of stellar magnetic fields

AbstractSpectropolarimetric studies of stellar magnetic fields usually deal with extremely weak line polarisation signatures. The amplitudes of Stokes signals, even in magnetically sensitive spectral lines, are often well below the noise level realistically achievable with the current instrumentation. Consequently, a detection of these polarisation signatures and their meaningful analysis is impossible without combining information from multiple spectral lines. Here I review basic theoretical foundations of the multi-line spectropolarimetric diagnostic methods employed in stellar magnetometry, give examples of their application, and discuss recent efforts to interpret mean polarisation profiles with the help of detailed radiative transfer calculations.

Empirical mode decomposition based dynamic error correction in SS covered 62.5/125µm optical fiber based distributed temperature sensor

Abstract The design and implementation of empirical mode decomposition (EMD) based preprocessor for backscattered spontaneous Raman anti-Stokes (AS) and Stokes (St) signals obtained from a stainless steel (SS) covered, 62.5/125 μm optical fiber based distributed temperature sensor is presented. The preprocessor dynamically minimizes the error in temperature measurement caused by the difference in attenuation to AS and St signals offered by the optical fiber. Simultaneous denoising of AS and St signals obtained by the EMD based preprocessor yields better signal to noise ratio (SNR) of these signals and allows reduced error in temperature measurement. The EMD based technique is much better than previously reported techniques in terms of simplicity and automation. Automated and dynamic self calibration of distributed temperature sensor is also possible with the proposed preprocessor in an easier way. The use of proposed preprocessor has been demonstrated to develop an optical fiber based distributed temperature sensor with an accuracy of ±2.5 °C in a temperature range of 25–105 °C over a sensing length of 90 m with a spatial resolution of 1 m. The developed system uses a rugged stainless steel (SS) covered 62.5/125 μm Multimode optical fiber. SS covering on the fiber makes it easier and safer to install the sensing fiber in critical field locations where normal sensing fiber cannot be used.

Fiber-based optical parametric oscillator for high resolution coherent anti-Stokes Raman scattering (CARS) microscopy.

Imaging based on coherent anti-Stokes Raman scattering (CARS) relies on the interaction of high peak-power, synchronized picosecond pulses with narrow bandwidths and a well-defined frequency difference. Recently a new type of fiber-based CARS laser source based on four-wave-mixing (FWM) has been developed. In order to enhance its spectral resolution and efficiency, a FWM based fiber optical parametric oscillator (FOPO) is proposed in this work. The source delivers 180 mW with 5.6 kW peak power for the CARS pump and 130 mW with 2.9 kW peak power for the Stokes signal. CARS resonances around 2850 and 2930 cm(-1) can be resolved with a resolution of 1 cm(-1) enabling high-contrast, spectrally resolved CARS imaging of biological tissue.

Inclinations of small quiet-Sun magnetic features based on a new geometric approach

High levels of horizontal magnetic flux have been reported in the quiet-Sun internetwork, often based on Stokes profile inversions. Here we introduce a new method for deducing the inclination of magnetic elements and use it to test magnetic field inclinations from inversions. We determine accurate positions of a set of small, bright magnetic elements in high spatial resolution images sampling different photospheric heights obtained by the Sunrise balloon-borne solar observatory. Together with estimates of the formation heights of the employed spectral bands, these provide us with the inclinations of the magnetic features. We also compute the magnetic inclination angle of the same magnetic features from the inversion of simultaneously recorded Stokes parameters. Our new, geometric method returns nearly vertical fields (average inclination of around 14 deg with a relatively narrow distribution having a standard deviation of 6 deg). In strong contrast to this, the traditionally used inversions give almost horizontal fields (average inclination of 75+-8 deg) for the same small magnetic features, whose linearly polarised Stokes profiles are adversely affected by noise. The almost vertical field of bright magnetic features from our geometric method is clearly incompatible with the nearly horizontal magnetic fields obtained from the inversions. This indicates that the amount of magnetic flux in horizontal fields deduced from inversions is overestimated in the presence of weak Stokes signals, in particular if Stokes Q and U are close to or under the noise level. By combining the proposed method with inversions we are not just improving the inclination, but also the field strength. This technique allows us to analyse features that are not reliably treated by inversions, thus greatly extending our capability to study the complete magnetic field of the quiet Sun.