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Annual Meeting Optical Society of America

Raman gain suppression in D2

Michael D. Duncan, John F. Reintjes, R. Mahon, Lawrence L. Tankersley

Published Jan 1, 1986• 0 citations
Publisher

Abstract

We have confirmed, for the first time to our knowledge, the nonexponential character of Raman amplification at the phase matching angle ϕ for which 2kL = ks + kA. The experiments reported here used spatially filtered pump and probe beams at 532 and 633 nm, respectively, of 30-ps duration interacting in a one-half meter cell filled with deuterium at a pressure of 600 psi. The pump and probe beams were crossed at various small angles in the cell and the gain experienced by the probe pulse was measured. At angles below 5 mrad the Stokes amplification was exponential in character and reached levels of 107. At an angle of 7 mrad, the phase matching angle for our conditions, there was no observed amplification of the Stokes pulse. At yet larger angles (~9 mrad) the gain was again exponential in character and reached levels of 107. We have also observed a double ring pattern in the anti-Stokes radiation produced in our experiment. This pattern was predicted by Bloembergen1 with the explanation that a single anti-Stokes ring is split by a region of gain suppression at the phase matching angle. We have verified that the diameter of the dark band in the anti-Stokes emission grows as the square root of the pressure, exactly as would be predicted assuming that the dark region is caused by gain suppression at phase matching.

DOI
10.1364/oam.1986.tum2

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In this paper, we demonstrate that, by continuous single beam excitation, one can generate an abnormal anti-Stokes Raman emission (AASRE) whose properties are similar to a coherent anti-Stokes Raman scattering (CARS). The effect has been observed in materials which possess intrinsically nonlinear properties (LiNbO3 and CdS), which have the electric susceptibility of third order different from zero, χ(3) ≠ 0, as well as in materials that become nonlinear under resonant optical excitation. In the latter case, we used poly-3,4-ethylendioxythiophene (PEDOT) in its undoped state deposited electrochemically on Au support. Raman studies corroborated with images of optical microscopy demonstrate that the production of AASRE is conditioned by the existence of a particular morphology of the sample able to ensure efficient transport of the light inside the sample through a multiple light scattering mechanism. In this context, it was found that LiNbO3 and CdS in powder form as well as the PEDOT films layered on a rough Au substrate are suitable morphological forms. We explain AASRE as resulting from a wave-mixing mechanism of the incident laser light ωl with a Stokes-shifted Raman light ωS produced by a spontaneous Raman light scattering process, both strongly scattered inside the sample. As a CARS process, AASRE is conditioned by the achievement of phase-matching requirements, which makes the difference between the wave vectors of mixing light close to zero, Δk =/2kl − kS − kCARS/≈ 0. In condensed media, the small dispersion of the refractive index makes Δk ≈ 0 so that the formation of a favourable phase-matching geometry may be accomplished even at a crossing angle θ of travelling scattered light ωl and ωS. For tightly focused beams, the requirement of phase matching relaxes; it is no longer sensitive to the Raman shift, so that a wide intense anti-Stokes Raman spectrum is observed at an angle larger than the Stokes Raman spectrum.

Journal of Physics B Atomic Molecular and Optical Physics•Apr 19, 2011•1 citation
We have confirmed, for the first time to our knowledge, the nonexponential character of Raman amplification at the phase matching angle ϕ for which 2kL = ks + kA. The experiments reported here used spatially filtered pump and probe beams at 532 and 633 nm, respectively, of 30-ps duration interacting in a one-half meter cell filled with deuterium at a pressure of 600 psi. The pump and probe beams were crossed at various small angles in the cell and the gain experienced by the probe pulse was measured. At angles below 5 mrad the Stokes amplification was exponential in character and reached levels of 107. At an angle of 7 mrad, the phase matching angle for our conditions, there was no observed amplification of the Stokes pulse. At yet larger angles (~9 mrad) the gain was again exponential in character and reached levels of 107. We have also observed a double ring pattern in the anti-Stokes radiation produced in our experiment. This pattern was predicted by Bloembergen1 with the explanation that a single anti-Stokes ring is split by a region of gain suppression at the phase matching angle. We have verified that the diameter of the dark band in the anti-Stokes emission grows as the square root of the pressure, exactly as would be predicted assuming that the dark region is caused by gain suppression at phase matching.
We have previously reported experimental results of Raman amplification of a low-light-level seed-Stokes beam in the transient limit.1 Here we present results of measurements which used a 10-nsec seed-injected Nd:YAG laser. The 532-nm frequency-doubled output had a duration of 8.5 ns and was used both to generate a 4.5-ns duration seed-Stokes pulse at 683 nm and to pump the 1-m long Raman amplifier. The seed-Stokes beam was spatially filtered, apertured at the Airy minimum, and collimated before the amplifier at a diameter of 5.2 mm. Likewise, the spatial mode of the pump was optimized to be as smooth as possible and was collimated into the Raman amplifier at a diameter of 3.4 mm. With no seed Stokes incident in the amplifier, random spatial patterns characteristic of the quantum-noise initiated, self­generated Stokes light were observed over a spatially gain-narrowed beam size supporting ~ 12 spatial modes. For the Raman gains of ~105 used in these experiments the number of temporal modes was 1.8. The seed-Stokes light was used to illuminate a bar chart of line-width 0.315 mm, and either a direct image of the bar chart, or its Fourier transform, was amplified in the Raman amplifier. The original bar chart image was then observed using an intensified camera. As the level of seed-Stokes was reduced to that approaching one photon per spatial-temporal mode, we were able to observe intensity modulations approaching 100% within the amplified bar chart images. Modulation levels of a few percent persist out to input levels of ~103 photons per mode. This observation is consistent with an interpretation of interference between electric field of the seed-Stokes beam and the vacuum fluctuations present in the amplifier but is inconsistent with a photon number description where the maximum predicted modulation would be only 50%.
We have studied the phase pulling and locking of an injected Stokes pulse in a transient Raman amplifier both experimentally and theoretically. In addition, we have investigated the pulse energy statistics and related spatial modes of a high gain amplified Stokes pulse. Experiments reported here show that in the unsaturated case, the phase of the amplified Stokes is pulled almost instantaneously to that of the pump, as soon as integrated gain levels of slightly greater than unity are reached. For higher input seed-Stokes levels, such that the amplified Stokes energy is saturated, the phase locking is completed early, and the phase of the amplified Stokes then follows the phase excursion of the pump. The phase pulling has been modelled theoretically for the specific parameters of our experiment. We have also used the pulse energy statistics to study the seed-Stokes control of a high gain Raman amplifier. For an input seed-Stokes beam to control a transient, high gain Raman amplifier, operated near threshold, it must dominate the self-generated Raman which builds from quantum noise. For a near-unity-Fresnel-number Raman amplifier we have measured the number of input seed-Stokes photons needed to produce an unchanged statistical signature on the amplified Stokes pulse and have found that a surprisingly large number (109) were needed. For a larger Fresnel number Raman amplifier, a near-field camera was used to study the level at which the seed-Stokes pulse controlled the spatial output of the amplifier. Control of the spatial modes seems to have a sharp threshold with input Stokes energy and was fully established with an input of 2x103 photons per mode.
A theory of nonstationary stimulated Raman scattering is developed, allowing for the coupling between the Stokes and anti-Stokes waves. The main equations are solved in the asymptotic approximation, which is valid for large values of the gain. The dependences of the gain on the phase mismatch Δ k , pulse duration τ p , and cell length l are discussed and the optimal angle for the observation of the anti-Stokes rings is calculated. Moreover, the spectral distributions of the Stokes and anti-Stokes waves are considered.
An experimental study of normal and stimulated Raman emission in diamond is presented, including measurements of linewidth, intensity, and angular distribution of radiation having a frequency shift of 1332.0 ${\mathrm{cm}}^{\ensuremath{-}1}$. The diamond crystal formed a Raman resonator; excitation was by a giant-pulse ruby laser. The dependence of normal Stokes emission on laser intensity and the threshold for oscillation were investigated. A value for the Raman gain ($g=6.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ per MW/cm2{\mathrm{cm}}^{2}cm2) was calculated from the measured linewidth ($\ensuremath{\Delta}\ensuremath{\nu}=2.04$ ${\mathrm{cm}}^{\ensuremath{-}1}$) and cross section for scattering, and found to be in good agreement with a value determined from the threshold for oscillation. The angular distributions of anti-Stokes and Stokes emission and absorption for parallel and convergent laser light are also in good agreement with theory: The emission cone angles are strongly dependent on the angle of convergence of the incident light, and it is established that the preferred directions for emission are those making use of the most intense first-order Stokes radiation which is peaked in the forward direction within the Raman resonator.
In this paper, we demonstrate that, by continuous single beam excitation, one can generate an abnormal anti-Stokes Raman emission (AASRE) whose properties are similar to a coherent anti-Stokes Raman scattering (CARS). The effect has been observed in materials which possess intrinsically nonlinear properties (LiNbO3 and CdS), which have the electric susceptibility of third order different from zero, χ(3) ≠ 0, as well as in materials that become nonlinear under resonant optical excitation. In the latter case, we used poly-3,4-ethylendioxythiophene (PEDOT) in its undoped state deposited electrochemically on Au support. Raman studies corroborated with images of optical microscopy demonstrate that the production of AASRE is conditioned by the existence of a particular morphology of the sample able to ensure efficient transport of the light inside the sample through a multiple light scattering mechanism. In this context, it was found that LiNbO3 and CdS in powder form as well as the PEDOT films layered on a rough Au substrate are suitable morphological forms. We explain AASRE as resulting from a wave-mixing mechanism of the incident laser light ωl with a Stokes-shifted Raman light ωS produced by a spontaneous Raman light scattering process, both strongly scattered inside the sample. As a CARS process, AASRE is conditioned by the achievement of phase-matching requirements, which makes the difference between the wave vectors of mixing light close to zero, Δk =/2kl − kS − kCARS/≈ 0. In condensed media, the small dispersion of the refractive index makes Δk ≈ 0 so that the formation of a favourable phase-matching geometry may be accomplished even at a crossing angle θ of travelling scattered light ωl and ωS. For tightly focused beams, the requirement of phase matching relaxes; it is no longer sensitive to the Raman shift, so that a wide intense anti-Stokes Raman spectrum is observed at an angle larger than the Stokes Raman spectrum.