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Optical and Quantum Electronics

Investigation of ASE contribution to copper bromide amplifier output signal

Nikolai A. Vasnev, Maxim V. Trigub

Published Dec 14, 2023• 4 citations
Publisher

Abstract

Abstract The paper is devoted to research the single-pass operating mode of active element on copper bromide vapors and time dependence of ratio «signal/ASE». The results show how the single-pass radiation power depends on time delay between ASE pulse and moment when the input signal enters to active element. Light waveforms (ASE, input signal, output signal) are recorded at each time delay. The minimal time delay was 5.2 ns, power radiation in this case was 3.47 W. The radiation power was reducing as the time delay was increased in increments of about 2 ns. The waveforms prove that the active element output signal contains two components: the first – ASE (amplified spontaneous emission), which is formed as result of the active substance atom excitation and their further spontaneous transitions; the second – single-pass radiation amplified by means of inverse population of the active media. The waveforms allow to estimate the useful signal to noise ratio or other words «signal/ASE». The highest gain is provided with the lowest time delay due to the population inversion is maximum in this case.

DOI
10.1007/s11082-023-05724-6

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Abstract The paper is devoted to research the single-pass operating mode of active element on copper bromide vapors and time dependence of ratio «signal/ASE». The results show how the single-pass radiation power depends on time delay between ASE pulse and moment when the input signal enters to active element. Light waveforms (ASE, input signal, output signal) are recorded at each time delay. The minimal time delay was 5.2 ns, power radiation in this case was 3.47 W. The radiation power was reducing as the time delay was increased in increments of about 2 ns. The waveforms prove that the active element output signal contains two components: the first – ASE (amplified spontaneous emission), which is formed as result of the active substance atom excitation and their further spontaneous transitions; the second – single-pass radiation amplified by means of inverse population of the active media. The waveforms allow to estimate the useful signal to noise ratio or other words «signal/ASE». The highest gain is provided with the lowest time delay due to the population inversion is maximum in this case.
We report the characteristics of GHz bandwidth amplified spontaneous emission (ASE) from a hot Cs atom vapor cell, where the optical feedback was inhibited. When pumped by an 852 nm laser, both forward and backward ASE output near 894 nm showed a nonlinear increase in its power without a pump power threshold. A continual decrease in spectral width down to 4.7 GHz was experimentally observed as the ASE output power increased. Using the same vapor cell, we injected a 1mW signal to configure a single-pass optical amplifier, and we monitored the forward output both in temporal and spectral domains. We found the signal laser efficiently suppressed the ASE and obtained a large amplification factor over 700 at the pump power of 1.2 W.
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 widely used technique for determining optical gain and absorption from single-pass, amplified spontaneous emission (ASE) is analyzed with particular attention given to active-volume saturation and three-dimensional effects. The results for homogeneously broadened media show that the population inversion and hence the gain in the active region is diminished by high levels of spontaneous and ASE. The externally measured gain is further reduced for large output collection apertures that admit incompletely amplified off-axis rays. Criteria are given for the excitation and detection geometries required to ensure accurate (∼10-percent) gain measurement. The simultaneous measurement of the equilibrium absorption coefficient can be made under less restrictive conditions.
The use of metal atom active media allows to convert optical signals with the transferring of the adjusted contrast. Due to the opportunities of metal vapor active media it makes possible to increase the intensity of the signal in the narrow spectral range. As a result, the signal-to-noise ratio can be dramatically increased. It makes possible to build active optical systems for high speed imaging of processes which are hidden by high intensive radiation. In the systems is available the image active filtration due to induced transition on metal atoms. Active optical systems with metal vapor brightness amplifiers (which is called Laser monitor), have shown the high efficiency for reducing the background radiation effect on the observing processes in the real-time mode. Moreover, the use of different active media makes it possible to change the spectral contest of the input signal with the increasing of the intensity. In the work the result of the research such systems are presented. The method of transformation IR images to VIS images is discussed. The work presents the results of the using of different active optical systems for high speed imaging.
We have observed, for the first time to our knowledge, stimulated emission in atomic-copper vapor that is excited by a resonant tunable laser beam. One of the important and interesting results obtained in this experiment is that excitation of the (2)P(1/2) level of the copper atoms generates strong amplified spontaneous emission (ASE) for both(2)P(1/2)-(2)D(3/2) and (2)P(3/2)-(2)D(5/2) transitions. This is the first reported direct experimental evidence observed for collisional mixing between the (2)P(1/2) and (2)P(3/2) levels in a copper-vapor laser. Excitation of the (2)P(3/2) level induces substantially weaker ASE for the (2)P(1/2)-(2)D(3/2) transition. In addition, we observed collision-induced ASE for both transitions over a wide range of detuning of the pump frequency. The preliminary results of the experiment are presented, and the implications of the results for high-pressure copper-vapor lasers are discussed.