Stimulated Raman Gain Research Articles

High-Resolution Stimulated Raman Gain Spectroscopy of Parahydrogen Crystals

High-resolution stimulated Raman gain spectroscopy of the Q1(0) transition of para-H2 crystals with varying ortho-H2 concentrations is reported. A crystal containing 0.06% of ortho-H2 showed a single sharp Raman transition with the width of less than 7 MHz. The Raman signal became broader with increasing concentration of ortho-H2. At the concentration of 2%, the spectrum splits into a doublet. The components of the doublet have different dependence on the relative polarization of the pump and probe laser radiations. Physical origin of the splitting and the polarization dependence is yet to be understood.

Observing the Stimulated Raman Gain Spectra of Solutions Using an Infrared Pump Pulse with Narrow Linewidth and a Low-Noise CW Probe Laser

Stimulated Raman gain (SRG) spectroscopy using infrared pump pulses with narrow linewidth and a low-noise cw probe infrared laser was proposed. High-resolution Raman spectra of solutions were obtained. The SRG spectra of crystal GaP, benzene, and toluene were measured to confirm the spectral resolution and sensitivity over the terahertz (THz) region. We discuss the polarization dependence of the spectral measurement of carbon tetrachloride. Our system can detect organic molecules in aqueous solutions.

Development of Femtosecond Stimulated Raman Spectroscopy: Stimulated Raman Gain via Elimination of Cross Phase Modulation

We have developed a new femtosecond probe technique by using stimulated Raman spectroscopy. The cross phase modulation in femtosecond time scale associated with off-resonant interaction was shown to be eliminated by integrating the transient gain/loss signal over the time delay between the Raman pump pulse and the continuum pulse. The stimulated Raman gain of neat cyclohexane was obtained to demonstrate the feasibility of the technique. Spectral and temporal widths of stimulated Raman spectra were controlled by using a narrow band pass filter. Femtosecond stimulated Raman spectroscopy was proposed as a highly useful probe in time-resolved vibrational spectroscopy.

Theory of Intersubband Raman Laser in Modulation-doped Asymmetric Coupled Double Quantum Wells

A microscopic theory is developed for the laser gain due to stimulated intersubband electronic Raman scattering pumped by CO 2 laser in modulation-doped GaAs/AlGaAs asymmetric coupled double quantum wells (ACDQWs). Based on the charge-density-excitation (CDE) mechanism, the formula for electronic Raman scattering cross-section is given, taking into account the coupling between intersubband plasmon and optical phonons including GaAs confined LO phonons and interface phonons. Stimulated Raman gain factor is then derived from the cross-section. The optimization of Raman gain, the gain saturation and threshold condition are also discussed. Numerical analysis for temporal variation of stimulated Stokes photon density, subband populations and output Raman laser power is carried out by using the self-consistent conventional rate equations. The theory can predict the presence or lack of coupled modes in lasing in agreement with the recent experimental results.

Nonlinear Absorption and Raman Scattering in Silicon-on-Insulator Optical Waveguides

We study the nonlinearities in silicon-on-insulator (SOI) optical waveguides, which include two-photon absorption (TPA), free-carrier absorption, and spontaneous and stimulated Raman scattering (SRS). We show experimentally that free carriers generated by TPA in the SOI waveguides produce large optical loss at room temperature. The experimental results confirmed the presence of relative optical signal amplification from SRS, but it was found that net gain was hardly achieved because the stimulated Raman gain was less than the induced loss from TPA-generated free carriers at room temperature with continuous-wave pumping source in a SOI rib waveguide. We also experimentally investigated the temperature dependence of Raman scattering in the SOI waveguide and observed the Raman gain to be greater than TPA-generated free-carrier absorption loss at 77 K.

Gain of intersubband Raman lasing in modulation-doped asymmetric coupled double quantum wells

A microscopic theory is developed for the laser gain due to stimulated intersubband electronic Raman effect pumped by CO 2 laser in modulation-doped asymmetric coupled double quantum wells. We focus on the charge density excitation mechanism for electronic Raman scattering, taking into account the coupling between intersubband plasmons and LO phonons. Numerical simulations for the temporal variation of stimulated Stokes photon density, subband populations and output Raman laser power are carried out by using the self-consistent conventional rate equations with stimulated Raman gain coefficient.

Raman amplification in ultrasmall silicon-on-insulator wire waveguides

We measure stimulated Raman gain at 1550 nm in an ultrasmall SOI strip waveguide, cross-section of 0.098 microm2. We obtain signal amplification of up to 0.7 dB in the counter-propagating configuration for a sample length of 4.2 mm and using a diode pump at 1435 nm with powers of <30 mW. The Raman amplifier has a figure-of-merit (FOM) of 57.47 dB/cm/W. This work shows the feasibility of ultrasmall SOI waveguides for the development of SOI-based on-chip optical amplifiers and active photonic integrated circuits.

Pump-wavelength dependence of Raman gain in single-mode optical fibers

The magnitude of the stimulated Raman gain spectrum depends on the absolute wavelength of the pump laser. For many fibers, this dependence can be cast as a simple scaling of the overall gain curve according to a power-law dependence on the pump wavelength. We present three methods of measuring this scaling: estimation from Raman gain theory and known fiber parameters, a brute-force comparison of gain versus pump wavelength, and a more elegant comparison of the asymmetry in the Stokes and anti-Stokes Raman gain spectrum at a fixed pump wavelength. We discuss the advantages of the asymmetry technique, which yields the full wavelength dependence of the Raman gain with low uncertainty using only a single pump laser, a broadband source, and an optical spectrum analyzer. Finally, we report on the pump-wavelength scaling of the Raman gain for a number of standard transmission fibers.

Simulation of semiconductor intersubband Raman laser

A Raman laser [Liu et al., Appl. Phys. Lett. 78, 3580 (2001)] that operated on quantum well intersubband transitions was reported recently. Properties of this optically pumped double quantum well structure are investigated numerically in this article. The carrier lifetime of each subband in the structure is calculated and the pump-induced carrier distributions for each subband are obtained. Using the density matrix method we calculate the stimulated Raman gain and find that it does not always increase linearly with the pumping intensity as the former analysis suggested, but, rather, only when the pumping intensity is low. When the pumping intensity is high, the Raman gain saturates. The calculated results are qualitatively in agreement with the experimental ones of Liu et al.

Complete experimental characterization of the influence of parametric four-wave mixing on stimulated Raman gain.

We present what is to our knowledge the first complete measurement of the dependence of Raman gain on chromatic dispersion, fully revealing the influence of parametric four-wave mixing on stimulated Raman scattering. In particular, a threefold increase of the Raman gain is observed under phase-matching conditions, in excellent agreement with theoretical predictions. Our experiments, which were performed in a photonic crystal fiber, demonstrate that these unique fibers can be exploited to boost the performances of fiber Raman amplifiers.

Stimulated Raman gain spectroscopy of low-frequency density of states in liquids by picosecond IR laser pulses

The low-frequency spectra of simple liquids have been studied with a stimulated Raman gain technique using tunable picosecond pulses in the mid-infrared region. The obtained results on CS 2, CCl 4, C 2Cl 4 and CDCl 3 are rescaled to reduced Raman spectra, which are a representation of the low-frequency density of states of these liquids, weighted by the corresponding intermolecular polarizability. This thermal ‘bath’ is believed to accept or provide small portions of energy during vibrational relaxation processes. The technique demonstrated here has a large potential for time-resolved studies of vibrational relaxation and related phenomena.

Multiple-vibrational-mode model for fiber-optic Raman gain spectrum and response function

We describe a model for the stimulated Raman gain spectrum and the Raman response function in silica fiber using multiple vibrational modes. We base the model on previous spectroscopic data [J. Opt. Soc. Am. B1, 652 (1984) and J. Opt. Soc. Am. B6, 1159 (1989)] and extend an earlier proposed model [Appl. Opt.21, 359 (1982)] by making use of an inhomogeneous distribution of damped oscillators. The model provides a simple analytical expression for the Raman response function and an expression for the Raman gain spectrum that is easy to evaluate numerically.

Raman gain: pump-wavelength dependence in single-mode fiber

The magnitude of the stimulated Raman gain spectrum will depend on the absolute pump wavelength. Measurements of the pump-wavelength scaling of the stimulated Raman gain of several single-mode fibers are presented. The measurements were obtained by use of two techniques: a brute-force comparison of gain versus pump wavelength and a more elegant comparison of the asymmetry in the Stokes and anti-Stokes Raman gain spectrum at a fixed pump wavelength. This second asymmetry technique has the advantage that it is independent of the uncertainties typically associated with relative measurements of optical power.

Feasibility of nonlinear Raman lidar based on stimulated Raman gain spectroscopy without a tunable laser.

A novel technique of lidar for atmospheric gas detection by use of stimulated Raman gain spectroscopy without any tunable laser is proposed. Detection sensitivity and detectable range are estimated on the basis of the lidar equation for CO2, CH4, and H2 in the atmosphere. The feasibility study clearly shows that the technique has a potential for application to lidar and that, in addition, the construction of the system is simpler than those of traditional differential absorption lidars.

Observation of low-frequency Raman and Kerr effect contributions in picosecond infrared pump probe experiments

It is shown experimentally and theoretically that in nonlinear infrared spectroscopic studies applying laser pulses of ≈2 ps duration or shorter quite different resonant and nonresonant processes can give rise to significant transient intensity changes in the probe beam. Particularly stimulated Raman gain and loss due to low-lying vibrational resonances and, near-zero frequency, intermolecular interactions and librational modes are observed in liquid CCl 4, CS 2, C 2Cl 4, and a dilute solution of CHBr 3 in the latter. It is discussed that in two color pump probe experiments these effects can reach the same order of magnitude as changes due to transient excess population, particularly when dilute solutions are studied. Additionally a novel device for comprehensive single-shot polarization analysis is described, and its usefulness for the investigation of the real part of χ (3) is demonstrated.

Stimulated Raman gain in a Λ-type atomic system with doubly excited transitions

Measurements of stimulated Raman gain in rubidium vapor are made and compared with a detailed model. Amplification of a weak probe laser is observed in a \ensuremath{\Lambda}-type three-level system that is driven by a strong cw coupling laser. The intensity of the coupling laser is sufficiently strong that it both optically pumps the atomic levels, causing a population inversion in the ground-state hyperfine levels, and drives the stimulated Raman process producing gain. A theory that models all aspects of the gain mechanism is presented. The model includes all interactions between the external fields and the atomic transitions, including those processes that involve multiple fields interacting with the same transition. Numerical calculations are compared with gain measurements made at several coupling laser intensities and detunings from the $D1$ line in ${}^{85}\mathrm{Rb}.$

Raman gain and optical loss in GaP–AlGaP waveguides

The stimulated Raman gain coefficient and the optical loss in GaP/AlGaP semiconductor Raman amplifiers are measured for both double path and single path structures. For the double path structure, the highest gain per waveguide length is 3.27 dB/cm for a waveguide with a cross-sectional area of 4 μm2, under continuous wave pumping. A long waveguide with a length of 10 mm is demonstrated to have the highest gain of 2.57 dB. For a single path structure, the gains of backward scattering and forward scattering are separately measured and it is found that the gain of backward scattering is dominant. From these results, the backward gain coefficient is estimated to be 12.3×10−8 W cm−1. This value is about five times that of CS2 and LiNbO3, which is known to have the highest Raman scattering gain. However, the optical losses of the waveguides are estimated from the finesse measurement. The lowest one way loss corresponds to an effective absorption coefficient of αeff=0.28 cm−1.

Collisional effects on spectral line shape from the Doppler to the collisional regime: A rigorous test of a unified model

The paper presents high resolution Raman investigations of the Q(1) line of H2 in Ar mixture from low density (Doppler regime) to high density (collisional regime) analyzed with a unique line shape profile. Measurements are performed by stimulated Raman gain spectroscopy between 300 and 1000 K in a wide density range (from 0.2 to 11 amagat). All the observed spectral features are accurately described by a unified model recently proposed by two of the authors. This model accounts for a velocity-memory process, not restricted to the usual hard and soft limits. It also includes correlation between velocity- and phase-changing collisions. An exhaustive analysis of various possible mechanisms on the line shape is achieved. These mechanisms are the Dicke narrowing, the radiator speed dependence of the collisional broadening and shifting parameters, the collisionally induced speed-class exchange and the nonimpact effect. The present test shows the high consistency of the unified model, since it allows one to get a remarkable agreement with all the data by using a unique set of parameters. This model should be useful for optical diagnostics at moderate density, as required for combusting media or atmospheric work.

Stimulated Raman scattering and four-wave Raman mixing seeded by a supercontinuum generated in dibromomethane using picosecond and femtosecond laser sources

Stimulated Raman emission from liquid dibromomethane (vibrational Raman shift frequency, 588 cm −1) is introduced into hydrogen gas (rotational Raman shift frequency, 587 cm −1) as a seed beam, in order to generate numerous rotational lines by four-wave Raman mixing. Unexpectedly, a supercontinuum, which is generated by self-phase modulation in dibromomethane, acted as a seed beam to exclusively generate vibrational lines; the rotational lines are generated only when the supercontinuum is minimal. The former is explained by a competition between the high-gain vibrational and low-gain rotational Raman effects when strongly seeded by a supercontinuum. The latter is explained by stimulated Raman gain under the seed effect exclusively to the first-Stokes rotational line.

Detection of Trace Molecules in the Atmosphere by Nonlinear Raman Spectroscopy without a Tunable Laser.

A new technique for nonlinear Raman spectroscopy is proposed and discussed theoretically in terms of the detection of gas molecules in atmospheric pressure. In the proposed scheme, only a pulsed Nd: YAG laser is used as a pumping source, and instead of a tunable laser, a Raman shifter filled with the sametype of gas as that to be detected automatically generates the Raman shifted radiation. The detection limits of H2for SRGS (Stimulated Raman Gain Spectroscopy), PARS (Photo-Acoustic Raman Spectroscopy), and CARS (Coherent Anti-Stokes Raman Spectroscopy) are demonstrated experimentally, and the ppm order detection limit can be achieved by PARS. Although the sensitivity of CARS is smaller than that of PARS, the signal tonoise ratio in higher concentration is more effective.