The concept of the refluorescent atomic filter is shown in figure 2. Only photons at the resonant frequency of the atomic transition line are absorbed; all other photons pass through the cell without interference. If the atomic vapor is optically thick on the resonance absorption line, then photons absorbed by the atoms are trapped within the optical depth the entrance window of the cell. This trapping occurs at the same wavelength independent of the angle at which the photons arrive. In the absence of quenching. these photons undergo a random walk as they are absorbed and entitled the potassium atoms.
The concept of the refluorescent atomic filter is shown in figure 2. Only photons at the resonant frequency of the atomic transition line are absorbed; all other photons pass through the cell without interference. If the atomic vapor is optically thick on the resonance absorption line, then photons absorbed by the atoms are trapped within the optical depth the entrance window of the cell. This trapping occurs at the same wavelength independent of the angle at which the photons arrive. In the absence of quenching. these photons undergo a random walk as they are absorbed and entitled the potassium atoms.
The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.
Abstract The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.
A new resonance principle in optical filters has been utilized to simultaneously permit realization of wide field-of-view (∼2π sr) and very narrow acceptance bandwidths, approaching 0.01 Å. A light signal is transmitted through an outer bandpass filter into a resonantly absorbing atomic vapor, resulting in a fluorescence signal at a different wavelength which is transmitted through an inner bandpass filter. The outer and inner bandpass filters have no common transmission band, resulting in complete blockage of all optical signals (background noise) that are not resonantly shifted in wavelength by the intervening atomic vapor. This ’’resonance filter’’ principle is experimentally verified, and system parameters are discussed for filters utilizing potassium, rubidium, or cesium vapor at nine wavelengths between 420 and 532 nm.
A high efficiency, wide field of view imaging system which has ultra narrow linewidth for near infrared detection of missile plumes is presented in this paper. By taking advantage of the optical trapping feature of potassium vapor at its resonant wavelength, it is shown that a potassium vapor cell can be used as an imaging, narrow line width passband filter with a 2 theoretical acceptance solid angle. The passband of the filter can be a narrow as 1 GHz, and the maximum resolution is 33 lines per mm. The quantum efficiency of the refluorescence cell is measured to be>60%. I.
Abstract Since the beginning of this century we have a rough understanding about the structure of atoms. They consist of a nucleus and electrons orbiting around this nucleus. We know that electrons occupy only certain discrete energy states. When they decay from a high-energy state to a low-energy state, they emit electromagnetic radiation. Usually, the low-energy state will be the ground state. In that case, the emitted radiation is called resonance radiation.1 Since the resonance photons have an energy that corresponds to the energy of the transition, they can easily be absorbed by other ground-state atoms. A ground-state atom that absorbs the resonance photon is excited to the upper state for some time and will eventually release the photon again. This absorption/reemission process can be repeated many times, until the photon finally escapes from the vapour. This process is called ‘radiation trapping’ (also known as ‘imprisonment of resonance radiation’, ‘radiative transfer of spectral lines’, ‘line transfer’, or ‘radiation diffusion’).
Abstract Radiation from spectral lines can be absorbed and re- emitted many times in atomic vapours before it reaches the boundaries of the container encasing the vapour. This effect is known as radiation trapping. It plays an important role practically everywhere where atomic vapours occur, e.g. in spectroscopy, in gas lasers, in atomic line filters, in the determination of atomic lifetimes, in measurements of atomic interaction potentials, and in electric discharge lamps. This book for the first time assembles all the information necessary for a treatment of practical problems, emphasizing both physical insights and mathematical methods. After an introduction that reviews resonance radiation and collisional processes in atomic vapours, physical effects and mathematical methods for various types of problems (e.g. with or without saturation, particle diffusion, reflecting cell walls, etc.) are explained in detail. The last part of the book describes the applications of these methods to a variety of practical problems like cross-section measurements or the design of discharge lamps.Atoms can both absorb and emit electromagnetic radiation like visible light In an assembly of many atoms, e.g., in a vapour or in a gas, radiation emitted by one atom may be reabsorbed by one of its neighbours, just to be reemitted a little later. The game can go on for quite some time—until the photon manages somehow to escape from this prison. At first glance, quite easy to understand. On the other hand, there must be some reason for this voluminous book full of sums, integrals and matrices. Our interest in this subject originally stemmed from investigations on atomic line filters we did for the European Space Agency, ESA. We believed that radiation trapping would have some influence on the performance of these filters and were looking for a simple means to take it into account We started out the same way as probably anyone new to the field would have done when faced with the problem of trapping. We read the classical paper of Holstein, and thought that was all there was to it. We soon had to realize, however, that the assumptions made by Holstein were not fulfilled in our problem, and decided to give the whole matter a closer look. That ‘closer look’ obtained a momentum of its own, and trapping became the centre of our own research in the following years. As our overview of the subject increased. we found that trapping plays a role in a staggering variety of situations. Trapping appears to be important in most experimental setups and in most engineering applications where atomic vapours are involved.