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Journal of the Optical Society of America

Investigation of light amplification by enhanced internal reflection Part II Experimental determination of the single-pass reflectance of an optically pumped gain region

Mark P. Silverman, R. F. Cybulski

Published Dec 1, 1983• 10 citations
Publisher

Abstract

We have measured as a function of incident angle, for a range of critical angles between 88° and 89° and of characteristic gain depths between 41 and 96 probe wavelengths, the single-pass reflectance of a He–Ne probe beam reflected from a transverse optically pumped solution of rhodamine B. In contrast to previously reported experiments, no anomalously high reflectances were observed. The theoretical formulas for the exponentially nonuniform-gain model reproduce well the observed reflectance curves including the locations, slopes, and heights of the amplification maximum in the vicinity of critical angle. Peak amplifications of 200–300% were predicted and observed for the given experimental conditions.

DOI
10.1364/josa.73.001739

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We have measured as a function of incident angle, for a range of critical angles between 88° and 89° and of characteristic gain depths between 41 and 96 probe wavelengths, the single-pass reflectance of a He–Ne probe beam reflected from a transverse optically pumped solution of rhodamine B. In contrast to previously reported experiments, no anomalously high reflectances were observed. The theoretical formulas for the exponentially nonuniform-gain model reproduce well the observed reflectance curves including the locations, slopes, and heights of the amplification maximum in the vicinity of critical angle. Peak amplifications of 200–300% were predicted and observed for the given experimental conditions.
We investigate the reflection of plane waves of s and p polarization from the interface between a homogeneous transparent medium and an amplifying medium with a gain profile decreasing exponentially with depth of penetration. For angles of incidence less than critical angle, the reflectance curves are essentially the same as those deduced from the usual Fresnel formulas for two transparent media. In the vicinity of critical angle, the reflection coefficients are virtually the same for both polarizations and are greater than unity. Beyond critical angle, the reflectances fall toward unity as grazing incidence is approached. Both the reflectance and the far-field transmittance peak in the immediate vicinity of critical angle. For angles less than critical angle, the transmittance can be 2 orders of magnitude greater than the reflectance. Certain values of gain parameters and incident angle lead to singularities in the theoretical reflectance. At these points the usual neglect of the effect of the transmitted wave on the population inversion is no longer valid.
An experimental investigation was made of the influence of the reflection conditions and of the characteristics of the incident radiation on the amplification of light in the course of its reflection by an excited dye solution. A strong dependence of the reflection coefficient on the angle of incidence was observed and its maximum occurred near the critical angle of total reflection. The amplitude and width of the reflection peak depended on the parameters of the ambient media, as well as on the spectral composition and energy of the incident radiation.
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 studied the optical signal amplification properties of coumarin 460 (C460) in solution. An optical signal amplification experiment was performed using a side-pumping setup. Amplification of more than 7 dB was achieved over a 13-nm spectral width in the signal wavelength range of 448-461 nm with gains above 10 dB at 448 nm and in the 455-461-nm range. The signal-to-noise ratio (SNR) of the entire band was 5.8-11.1 dB. When the solution concentration and pump energy were 0.001 M (mol/L) and 117 μJ, respectively, the optical gain reached a maximum of 12.25 dB. Thus, the gain and SNR were measured by adjusting the pump energy and solution concentration to determine the optimal concentration of coumarin 460 in methanol solution in the amplifier. Additionally, amplification of the spontaneous emission was found to seriously reduce the SNR, adversely affecting the optical gain.
Enhancement of the diffusive albedo from a random amplifying medium is discussed. A pump-probe setup is used to study the amplification of the reflected probe pulse light outside the backscattering cone. Strong super-reflection is measured and its dependence on the disorder strength is studied. An optimal disorder strength is observed at high excitation energy wherein the super-reflection is maximal. The origins of this behavior are traced to the interplay between the out-coupling and amplification from a disordered amplifying medium with an inhomogeneous distribution of gain.