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Soviet Journal of Quantum Electronics

Compact neodymium-glass amplifier for amplification of weak coherent radiation signals

E G Bordachev, V. I. Korolev, Victor A. Serebryakov, Anatoly D. Starikov

Published Dec 31, 1989• 5 citations
Publisher

Abstract

An investigation was made of the feasibility of using cavity flashlamps to develop neodymium glass amplifiers with a high gain M per pass for amplification of weak signals. Active elements made of GLS22P glass of varying diameter were inserted alternately in the internal cavity of INK-22 and INK-12 flashlamps. It was found that an increase in the density of packing the optical pumping system with the cavity flashlamps (an increase in the ratio of the radii of the active elements to the radius or the reflector) did not increase the pumping efficiency. Monitoring the effective lifetime of the excited particles established that the main reason for the fall in the amplifier efficiency with increasing pump energy (the greater the active element diameter, the steeper the fall) was the influence of superradiance processes. A gain M = 10 3 was achieved in an amplifier excited by an INK-22 cavity flashlamp with an active element 6 mm in diameter and 300 mm long (the excited part was 200 mm long) when the ratio of the stored excitation energy to the pump energy was ~ 0.015.

DOI
10.1070/qe1989v019n12abeh009814

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Cambridge University Press eBooks•Jul 13, 1996•0 citations
An investigation was made of the feasibility of using cavity flashlamps to develop neodymium glass amplifiers with a high gain M per pass for amplification of weak signals. Active elements made of GLS22P glass of varying diameter were inserted alternately in the internal cavity of INK-22 and INK-12 flashlamps. It was found that an increase in the density of packing the optical pumping system with the cavity flashlamps (an increase in the ratio of the radii of the active elements to the radius or the reflector) did not increase the pumping efficiency. Monitoring the effective lifetime of the excited particles established that the main reason for the fall in the amplifier efficiency with increasing pump energy (the greater the active element diameter, the steeper the fall) was the influence of superradiance processes. A gain M = 10 3 was achieved in an amplifier excited by an INK-22 cavity flashlamp with an active element 6 mm in diameter and 300 mm long (the excited part was 200 mm long) when the ratio of the stored excitation energy to the pump energy was ~ 0.015.
Experimental and theoretical investigations were made of amplifying properties of an active neodymium glass fiber and a regenerative amplification regime was observed. It was found that the intensity gain of such a glass-fiber optical quantum amplifier increased exponentially on increase in the pump energy and corresponded to a regenerative regime. It was found that such a regenerative amplifier could operate on pulses above a certain minimum power. The experimental value of this minimum power was 10 mW. The maximum gain achieved in these experiments was 105 when the active length of the fiber was 40 cm.
Introduction Most of the lightwave transmission systems operating today benefit fully from extensive deployment of optical amplifiers (OA). These components perform the amplification of optical intensity directly in the optical domain, without resorting to any electrical-domain regeneration. Optical amplifiers increase the average number of output photons per unit time by several orders of magnitude compared with the incoming signal intensity. However, as with every amplifying process, optical amplification cannot be achieved without paying back for the unavoidable noise power added to the signal intensity. As we will see in detail in Chapter 6, the optical amplification process comes with optical noise generated by spontaneous emission inside the active medium. The intensity of the optical signal is represented by the flux of the signal photon population. Conversely, the photons generated by the spontaneous emission process inside the optical cavity represent the noise photon population. The fundamental phenomenon supporting optical amplification is stimulated emission. The basic property of stimulated emission, making the great difference from spontaneous emission and enabling optical amplification, is the coherent nature of the photon generation process. Accordingly, both signal and noise photons go through the same multiplication process by means of stimulated emission. Unless the optical cavity where the stimulated emission takes place (the active cavity) presents selective features for enhancing the signal photon population relative to the spontaneous one, there is no way to avoid noise photons going through stimulated emission, filling most of the cavity population. As a consequence, both photon populations increase, leading respectively to amplified signal intensity and to amplified spontaneous emission intensity. It should be apparent that every noise photon can create a new identical noise photon as a result of stimulated emission, effectively increasing the noise intensity in the active optical cavity. This is the well-known mechanism of amplified spontaneous emission (ASE) noise in optical amplification. Any improvement in the optical amplifier structure devoted to increasing stimulated over spontaneous emission will probably increase the optical signal-to-noise ratio (OSNR) at the cavity output. The spectrum of gain characteristic of the optical active medium is usually wide enough to support an efficient amplification of the broadband spontaneous emission spectrum, leading to the broadband ASE noise power spectral density.
The major purpose of the work described in this thesis is the experimental and theoretical study of the amplification of the spontaneous emission in dye amplifiers. This work falls into three distinct sections: (a) the study of high-current short-pulse flashtubes operating in (or near) the ablation regime; (b) the study of the amplification process of the spontaneous emission in a homogeneously broadened medium, such as a dye material, in the thin cell case, where there is no variation of the pumping across the cell; and, (c) the study of the amplification of the spontaneous emission in the thick cell case, where gain and index of refraction inhomogeneities exist across the cell thickness. In each case, we have developed theoretical models and compared their predictions with the experimental results. In particular, for the ablation-type flashlamps, it is shown that their behaviour is well represented by an analysis in which the discharge plasma is supposed to radiate as a blackbody at the temperature of the plasma. This model accounts for the fact that the light output from a flashlamp saturates whereas the current does not, provides a more accurate value for the plasma resistivity as a function of temperatures and gives a useful design criterion for determining the physical dimensions of an ablation-type flashlamp. With respect to the amplification of the spontaneous emission in a thin cell, it is mainly shown that (i) the spontaneous emission propagating along the gain medium is subject to narrowing along with shifting of its peak wavelength; (ii) even at moderate pumping the population of the excited singlet state, which determines the gain of the system, is non-uniform along the cell; (iii) it is possible to calculate the unsaturated gain by calculating the intensity of a particular wave-length and its spectral narrowing both as a function of the exciation and compare them with the same quantities measured experimentally; (iv) it is also possible to develop a semianalytic system independent method to measure the gain approximately. The theory developed to account for the propagation of the amplified spontaneous emission (ASE) has been partly extended to cover the case of the propagation and spectral narrowing of the ASE in a thick cell, where non-uniformities of the index of refraction and the gain are important along with the diffraction losses. In this particular case, it is shown that a model, which assumes an index step variation across the cell, and solves the Maxwell's equation for the field propagation along the cell, can account for the spatial distribution of the field across the cell and its manifestation at the far field. Most of the predictions of the model are verified experimentally. In particular, it is shown that (i) in a thick cell, part of the ASE propagates outside the gain medium, because of the lensing effect caused by heating of the lasing material; (ii) the field across the cell can exhibit stable spatial modes, which can produce a far field far from the amplifying cell axis; (iii) it is possible to improve the spectral narrowing of the ASE since saturation can be delayed to higher pumping than before and most of the narrowing occurs below saturation of the gain; (iv) it is also possible to measure the temperature and the index of refraction change due to heating by measuring the far field angle, and estimate the diffraction losses involved in the propagation of the ASE in a thick cell. Finally, a number of suggestions on further research are proposed.
Introduction Previous discussions (see Section 10.4) suggested that stimulated emission can be used to generate optical gain, that is, to amplify radiation. The reader certainly has experience in the amplification of electronic signal. For example, radio receivers capture faint radio waves and turn them into a signal that is powerful enough to drive large speakers. This electronic amplification increases the amplitude of the signal while faithfully preserving its acoustic frequencies and modulation characteristics. Similarly, optical amplification is expected to increase the amplitude of an optical signal while preserving its frequency, its modulation characteristics, and its coherence. The latter requirement is of particular significance for optical radiation, where the coherence of naturally occurring radiation rarely exceeds one micrometer. Lasers are the primary source for coherent radiation. They depend on stimulated emission for amplification and for the generation of coherent radiation (the word laser is the acronym of Light Amplification by Stimulated Emission of Radiation). For amplification, an atomic system that is part of the laser medium must be prepared with a sufficiently large number of particles in the excited state. (The term atomic system is used here to describe all microscopic systems including molecules and free electrons.) Radiation passing through that excited medium encounters multiple events of stimulated emission, each event contributing one photon that is added coherently to the propagating beam. When the number of events of stimulated emission exceed all losses by absorption or scattering, the incident radiation is amplified.