L. L. Tankersley

Time-gated imaging through scattering media using stimulated Raman amplification.

We propose the use of stimulated Raman scattering for time-gated image amplification and demonstrate its use for the detection of images through a strongly scattering material. Using 30-ps pulses from a frequency-doubled Nd:YAG laser, we have amplified and detected images through a suspension of nondairy creamer with a spatial resolution of less than 300 microm and at concentrations such that the nonscattered extinction ratio was e(-33). Our time-gated image amplifier can produce images under conditions in which the scattering medium is sufficiently dense that an image cannot be seen by using multiple exposures on a streak camera or time-integrated exposures on a sensitive, low-noise CCD camera.

Transient stimulated Raman amplification in hydrogen

We describe the results of an extensive study of transient stimulated Raman scattering in hydrogen gas. Measurements of self-oscillation thresholds, conversion efficiency, and the dependence of small-signal amplification on the pump energy and gas pressure are presented. Strong dependence of the amplification on the self-phase modulation of the pump and seed-Stokes pulses, on their relative time of arrival at the Raman amplifier, and on the seed-Stokes pulse asymmetry has been observed. The experimental measurements are compared with theoretical predictions that are based on extensions of published transient theories to take into account the spatial and temporal profiles of the pulses as well as their phase modulation. Excellent agreement with the theory, especially with regard to the scaling of the small-signal amplification with pump-pulse energy, is obtained when all the specific characteristics of the pump and Stokes pulses are included in the calculations.

Parametric Raman gain suppression in D 2 and H 2

We report direct experimental evidence of the absence of exponential gain at the Stokes wavelength for Stokes/anti-Stokes phase-matched stimulated Raman scattering. The amplification of a Stokes seed pulse was measured at various propagation angles relative to the pump direction. Nonexponential growth was observed at the Stokes/ anti-Stokes phase-matching angle with an amplification that was smaller than at non-phase-matching angles by a factor of more than 10(7).

Control of transverse spatial modes in transient stimulated Raman amplification

We have studied the transverse modes of a transient stimulated Raman signal generated in unsaturated Raman amplifiers with Fresnel numbers from one to seven, both with and without a seed Stokes pulse. In the absence of a seed Stokes pulse and for a pump Fresnel number greater than 1.5, the spatial intensity pattern of the stimulated Stokes signal is highly variable from shot to shot, reflecting the random nature of the quantum-noise source. However, when a separately generated and sufficiently strong seed Stokes pulse with a Gaussian spatial profile is injected into the amplifier, the spatial mode of the output Stokes beam assumes a smooth Gaussian spatial pattern that is significantly smaller in diameter than either the pump or the seed Stokes beam. This spatial mode control, which is due to the injected Stokes signal, persists down to a level of approximately 300 seed Stokes photons per spatial mode and, surprisingly, is observed to suppress the noise-initiated signal by at least a factor of 10 in the outer regions of the amplified Stokes beam. We have also studied the Stokes pulse-energy statistics over a range of Fresnel numbers. Data taken with systems having Fresnel numbers between 4 and 7 follow the same pattern as the spatial-mode-control results in that the statistical character of the Stokes pulse energies changes rapidly from that characteristic of a quantum-noise-initiated process to one characteristic of a stable source as the level of the injected seed Stokes is increased. However, for interaction regions with Fresnel numbers of 1 or smaller, the Stokes pulse-energy statistics show a gradual change as the level of the seed Stokes signal is increased and do not become stabilized until the seed Stokes is more than 100 times the total level needed to control the spatial character of the output Stokes in the larger-Fresnel-number cases. This slow change in the statistics for the Fresnel 1 system, and the suppression of the quantum-noise-initiated signal in the larger-Fresnel-number systems, is not fully understood at this time.

Phase pulling in transient Raman amplifers

We have studied phase pulling and phase locking of an injected Stokes pulse in a transient Raman amplifier both theoretically and experimentally. Our experimental results show that the phase of the injected Stokes pulse is pulled into synchronism with the pump phase (apart from a constant offset) when either the Raman amplification is low (of the order of 1–10) or the process is driven into saturation. However, when the amplification is moderate to high (103 or greater in our experiments) but the pump is not depleted, no evidence of phase pulling is observed. Under certain conditions a rapid phase change of π is observed near the beginning of the Stokes pulse. The observations of phase pulling at low gain and under saturated conditions are in agreement with the simple plane-wave transient theories of stimulated Raman scattering. The rapid π phase changes can also be explained within the context of these theories in terms of absorption of the injected Stokes pulse, followed by growth of a Stokes pulse with the opposite sign. The failure to observe phase pulling under conditions of high, unsaturated gain is not currently explained by the simple theories and may be an indication of more complicated processes.

Rotational Raman gain suppression in H2

Abstract We report measurements of rotational Raman gain suppression in H 2 in the transient regime. Quantitative information about the dependence of the amount of the gain suppression on the pump energy and the propagation angle relative to the pump beam is obtained from angularly resolved measurements of the small signal amplification. These measurements, along with qualitative observations of the Stokes spatial distribution under gain conditions, demonstrate that the gain suppression is confined to a narrow region around the phase matching angle for collimated beams with relatively low gain per unit length. For focused beams with higher gain per unit length the suppressed region can extend to the forward direction, resulting in a two- to three-fold estimated increase in the Raman threshold.

Second Stokes generation in deuterium and hydrogen

Abstract We have investigated the transient generation of second Stokes radiation in deuterium and hydrogen as a function of gas pressure and pump energy. We have measured the threshold energies for the first and second Stokes as a function of pressure and have found different behaviors for the second Stokes threshold in the high and low pressure regimes. We postulate that the different threshold behaviors are caused by the relative importance of four-wave mixing to the total second Stokes signal in the two regimes. Further data, based on the pulse energy statistics of the first and second Stokes, is presented that supports our postulate that the second Stokes seeding is dominated by four-wave mixing at low pressures and by amplified quantum noise at high gas pressures. Measurements of the spatial profiles of the second Stokes were also made and are consistent with this interpretation.

Narrow-linewidth unstable resonator

We describe the use of an asymmetric feedback ring in an unstable resonator to obtain arbitrarily narrow-linewidth radiation across the full available aperture. We demonstrate its use with an electric-discharge XeCl laser to produce single-line radiation with a linewidth of 0.15 cm−1.

Second Stokes generation in deuterium

We have investigated the growth, the spatial mode profiles and the pulse energy statistics of the first and second Stokes generated in a transient stimulated Raman self‐generator experiment. In particular, we have observed two distinct regions of second Stokes growth. The first region is characterized by a low signal level due to 4‐wave mixing. The second region appears to be one of stimulated growth seeded by spontaneous scattering at the second Stokes wavelength, an assumption supported by the rapid growth of the second Stokes signal and the observation of pulse energy statistics indicative of growth from noise. The absolute level of the Stokes signal in the 4‐wave mixing region varied as a function of deuterium gas pressure, increasing for lower pressures as a result of stronger 4‐wave mixing. We have also measured the second Stokes threshold as a function of deuterium pressure and have observed results consistent with first Stokes threshold measurements made at high pressures. At lower gas pressures, ...

Time-gated imaging using nonlinear optical techniques applications to turbid materials

We describe the use of various nonlinear interactions based on stimulated Raman scattering for time gated imaging and their application to imaging through turbid media. Results are presented showing images obtained through solutions of non dairy creamer with attenuation of e-33 and 100 micrometers resolution, and through 6 mm of raw chicken meat, and 12 mm of human abdominal fat.