Raman Amplification Process Research Articles

Atom-Light Hybrid Quantum Gyroscope

A new type of atom-light hybrid quantum gyroscope (ALHQG) is proposed due to its high rotation sensitivity. It consists of an optical Sagnac loop to couple rotation rate and an atomic ensemble as quantum beam splitter/recombiner (QBS/C) based on atomic Raman amplification process to realize the splitting and recombination of the optical wave and the atomic spin wave. The rotation sensitivity can be enhanced by the quantum correlation between Sagnac loop and QBS/C. The optimal working condition is investigated to achieve the best sensitivity. The numerical results show that the rotation sensitivity can beat the standard quantum limit (SQL) in ideal condition. Even in the presence of the attenuation under practical condition, the best sensitivity of the ALHQG can still beat the SQL and is better than that of a fiber optic gyroscope (FOG). Such an ALHQG could be practically applied for modern inertial navigation system.

Nonlinear self-polarization of Raman amplified light in fibers

We make a theoretical analysis of the energy transfer from an elliptically polarized pump to vectorial Stokes waves in a Raman amplification process. The analysis is done for cw waves using the general vectorial relations for the Raman susceptibility. In the circular-polarization basis, a simple equation is obtained when the phase shift between circularly polarized components is large, so that the phase-dependent components can be ignored as a result of averaging over propagation length. We show that for realistic initial conditions, most of the input energy of the elliptically polarized pump can transfer to a circularly polarized Stokes wave—an effect that was recently observed in experiments with pulse breakup and broadband spectrum formation in twisted fibers. Numerical simulations were carried out for a fixed ratio of 0.3 between the orthogonal and parallel Raman gains; this is the experimentally measured value for small Raman frequency shifts in silica fibers. The simulation shows that the polarization transfer effect is more pronounced when the input Stokes wave ellipticity coincides with the pump ellipticity.

Effects of losses in the atom-light hybrid SU(1,1) interferometer.

Collective atomic excitation can be realized by the Raman scattering. Such a photon-atom interface can form an SU(1,1)-typed atom-light hybrid interferometer, where the atomic Raman amplification processes take the place of the beam splitting elements in a traditional Mach-Zehnder interferometer. We numerically calculate the phase sensitivities and the signal-to-noise ratios (SNRs) of this interferometer with the method of homodyne detection and intensity detection, and give their differences of the optimal phase points to realize the best phase sensitivities and the maximal SNRs from these two detection methods. The difference of the effects of loss of light field and atomic decoherence on measure precision is analyzed.

Silica-based fiber Raman laser at > 2.4 μm.

Raman laser generation at the long wavelength transmission edge of silica fiber is explored. Numerical simulation reveals that high efficiency 2nd Stokes Raman laser operation around 2.5μm is feasible with highly Ge-doped silica fiber as the gain medium and pulsed 2-μm fiber laser as the pump source. Based on the spontaneous cascaded Raman amplification process, 0.30-W laser at 2.43μm is obtained with an optical efficiency of 16.5% pumped at 2008nm. And 0.15-W laser at 2.48μm is achieved with an optical efficiency of 7.9% pumped at 2040nm. To the best of our knowledge, the results represent the longest wavelength operation of silica-based fiber laser.

Nonlinear performances of dual-pump amplifiers in silicon waveguides

The performances of a dual-pump parametric and Raman amplification process and the wavelength conversion in silicon waveguides are investigated. By setting the Raman contribution fraction f to be 0.043 in our analytical model, the amplification gain of the probe signal can be obtained to be over 10 dB. The pump transfer noise (PTN), the quantum noise (QN), and the total noise figure (TNF) are discussed, and the TNF has a constant value of about 4 dB in the gain bandwidth. An idler signal generated during the parametric amplification (PA) process can be used to realize the wavelength conversion in wavelength division multiplexing (WDM) systems. In addition, the pump signal parameters, the generated free carrier lifetime and effective mode area (EMA) of the waveguide are analysed for the optimization of signal gain and noise characteristics.

Nonlinear Silicon Photonics: Analytical Tools

Since the recent demonstration of chip-scale, silicon-based, photonic devices, silicon photonics provides a viable and promising platform for modern nonlinear optics. The development and improvement of such devices are helped considerably by theoretical predictions based on the solution of the underlying nonlinear propagation equations. In this paper, we review the approximate analytical tools that have been developed for analyzing active and passive silicon waveguides. These analytical tools provide the much needed physical insight that is often lost during numerical simulations. Our starting point is the coupled-amplitude equations that govern the nonlinear dynamics of two optical waves interacting inside a silicon-on-insulator waveguide. In their most general form, these equations take into account not only linear losses, dispersion, and the free-carrier and Raman effects, but also allow for the tapering of the waveguide. Employing approximations based on physical insights, we simplify the equations in a number of situations of practical interest and outline techniques that can be used to examine the influence of intricate nonlinear phenomena as light propagates through a silicon waveguide. In particular, propagation of single pulse through a waveguide of constant cross section is described with a perturbation approach. The process of Raman amplification is analyzed using both purely analytical and semianalytical methods. The former avoids the undepleted-pump approximation and provides approximate expressions that can be used to discuss intensity noise transfer from the pump to the signal in silicon Raman amplifiers. The latter utilizes a variational formalism that leads to a system of nonlinear equations that governs the evolution of signal parameters under the continuous-wave pumping. It can also be used to find an optimum tapering profile of a silicon Raman amplifier that provides the highest net gain for a given pump power.

Raman amplification and optical short pulse generation in a waveguide with periodic gain

In this paper a new configuration of amplification is proposed for Raman amplification. In this new configuration a spatial periodic Raman gain is used to amplify a CW Raman seed that generates a short optical pulse taking into account the SPM, XPM, walk-off and pulse depletion. The amplification process is accomplished by using an intense pump pulse where its energy is transferred through SRS effect. A CW weak seed is used to represent the signal to be amplified and for pump is used a Gaussian pulse. We discuss the advantages to this configuration. First, this scheme means that the Raman-gain coefficient assumes a periodic characteristic and like a modulation process, it gives possibility to control the energy transfer from the pump to Raman seed. Second, the efficient use of the parameters of the periodic Raman coefficient function, like amplitude and frequency of the modulation, can result in a more efficient control of the signal amplification. This new configuration provides some very interesting features in the Raman amplification process when compared to the standard procedure. A complete discussion of this configuration is presented.

Generation of Raman solitons with a different relative phase by optical phase jump

The phase behavior of solitons generated in the Raman amplification process has been interferometrically investigated. The phase jump by π responsible for the soliton formation was injected into the input pump pulse or the Stokes pulse. In both cases, the solitons with a different relative phase have been observed. The results of the numerical integration of the plane-wave Raman equations have shown that only in-phase or antiphase solitons are generated at zero frequency detuning from the exact Raman resonance when the phase jump is injected into the Stokes pulse or the pump pulse, respectively. Realizations with an arbitrary relative phase are possible for nonzero detunings. These detunings have the order of magnitude expected due to spectral fluctuations in the Stokes pulse and thus are one of the possible causes of the observed variations in the soliton phase.

Self-imaging silicon Raman amplifier

We propose a new type of waveguide optical amplifier. The device consists of collinearly propagating pump and amplified Stokes beams with periodic imaging of the Stokes beam due to the Talbot effect. The application of this device as an Image preamplifier for Mid Wave Infrared (MWIR) remote sensing is discussed and its performance is described. Silicon is the preferred material for this application in MWIR due to its excellent transmission properties, high thermal conductivity, high damage threshold and the mature fabrication technology. In these devices, the Raman amplification process also includes four-wave-mixing between various spatial modes of pump and Stokes signals. This phenomenon is unique to nonlinear interactions in multimode waveguides and places a limit on the maximum achievable gain, beyond which the image begins to distort. Another source of image distortion is the preferential amplification of Stokes modes that have the highest overlap with the pump. These effects introduce a tradeoff between the gain and image quality. We show that a possible solution to this trade-off is to restrict the pump into a single higher order waveguide mode.

Photonic crystal development yields breakthrough in Raman spectroscopy

PurposeTo describe a new substrate for use in surface enhanced Raman spectroscopy (SERS).Design/methodology/approachNovel SERS substrates have been produced by the nanometre‐scale patterning of gold surfaces on silicon. This forms photonic crystals which tightly control the physics of the surface enhanced Raman amplification process, producing consistent SERS signals.FindingsIt has been shown that these substrates can increase significantly the sensitivity and reproducibility of SERS.Originality/valueThese substrates will allow SERS to be used in critical applications requiring high sensitivity detection of chemical compounds such as security, forensics, healthcare and environmental monitoring.

Reaching the Nonlinear Regime of Raman Amplification of Ultrashort Laser Pulses

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.

Modeling of Photonic Crystal Fiber Raman Amplifiers

An accurate model has been developed to study photonic crystal fiber Raman amplifiers. The proposed approach calculates the Raman gain efficiency starting from the knowledge of the fiber cross-sectional characteristics, i.e., the geometric parameters and the germania concentration in the doped area. The model allows the study of the Raman amplification process in a photonic crystal fiber with any air-hole dimension and disposition, with or without a central doped area, as well as in standard single-mode fibers with an arbitrary refractive-index profile. This flexibility allows identification of the optimum values of the physical and geometrical photonic crystal fiber parameters, and thus it guarantees an efficient design for Raman amplifiers. For example, a Raman gain of 13.7 dB at 1554.8 nm has been obtained with a 6-km-long low-loss germania-doped triangular photonic crystal fiber with d//spl Lambda/=0.44 and /spl Lambda/=4.2 /spl mu/m.

Raman amplifier model in single-mode optical fiber

Equations to describe the process of Raman amplification in single-mode optical fiber and Stokes and anti-Stokes spontaneous emission generation with conservation of photon number are derived from first principles. The numerical simulation of Stokes and anti-Stokes spontaneous emission is in good agreement with experimental results. The importance of the sometimes-omitted anti-Stokes spontaneous emission terms in wavelength-division-multiplexed situations for optical signal-to-noise ratio is demonstrated by the simulation of light propagation through transmission fiber with and without Raman amplification.

Phasefront preservation in high-gain Raman amplification

Experiments were performed to determine the degree of phasefront preservation possible in a high-gain, large-Fresnel-number number Raman amplifier. Phasefront information was encoded on a Stokes seed, which was then amplified in a 10-atm vibrational hydrogen Raman amplifier pumped by a XeF excimer laser. Interferometric data on the input and amplified Stokes beam indicate that their phasefront difference is less than 0.1 wave RMS. These results are consistent with the predictions of a numerical model of the Raman amplification process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Suppression of the soliton self-frequency shift by bandwidth-limited amplification

Experiments that produced 100-fsec pulses by a single-pass Raman amplification process did not lead to the expected self-frequency shift induced by intrapulse Raman scattering. We show that under conditions in which a Stokes pulse continues to receive frequency-dependent gain from a pump, a balance can be achieved between the two competing processes. Hence the progressive self-frequency shift of a soliton along a fiber can be suppressed.

Photoacoustic Raman Spectroscopy of Gases

Abstract Photoacoustic Raman spectroscopy (PARS) [1] is a nonlinear spectroscopic technique based upon the selective population of a given energy state of a system by the process of coherent Raman amplification (stimulated Raman scattering). The necessary conditions for this process are: (1) the transition involving the initial and final energy levels must be Raman-active, i.e., the transitions must involve a change in the molecular polarizability; and (2) the frequency difference of the two incident laser beams must be adjusted to equal the frequency of this Raman-active transition. Since the occurrence of a Raman spectrum depends on a change in polarizability of the molecule and not on the presence of a transition dipole moment, Raman-active transitions can occur for molecules that have no infrared spectrum. This fact makes PARS a particularly attractive analytical technique for studying molecules which have no infrared spectrum The PARS technique was first demonstrated experimentally using

Raman amplification spectroscopy

The main experimental and theoretical developments in Raman amplification spectroscopy (RAS, common term for stimulated Raman gain and stimulated Raman loss or inverse Raman spectroscopies) are reviewed. In the theoretical section the Raman amplification process is briefly described, and the dependence of the signal-to-noise ratio on the molecular and experimental parameters is discussed. The experimental results demonstrate that RAS has become a method of great importance. Thus, using high power single mode lasers Doppler limited high resolution spectra have been obtained. The introduction of picosecond lasers has lead to a dramatic improvement in the sensitivity. RAS provides a very efficient fluorescence rejection. Exploiting its high temporal resolution capability short lived species and transient states can be studied.

Raman amplification spectroscopy using mode-locked lasers

The theory necessary for an understanding of the Raman amplification process is reviewed in order to accentuate the factors which are of significance in designing an instrument to take advantage of the sensitivity potentially available with the techniques of inverse-Raman or Raman-gain spectroscopy. The dependence of the signal intensity on the product powers of the pump-and-probe lasers leads to a need for pulsed rather than continuous-wave lasers, while the requirements for a low noise level accompanying the signal dictate the need for a rapid-pulse system capable of being utilized with fast lock-in amplifiers. These requirements are most suitably met by synchronously pumped mode-locked and/or cavity-dumped dye lasers. The detailed design of systems combining either two synchronously pumped mode-locked dye laser are described and compared. Following a discussion of the factors influencing the signal-to-noise ratio attainable, representative spectra are presented which reflect the quality of the spectra attainable with this newly developed technique.