Frequent measurement can suppress the transitions between different quantum states. This quantum physics phenomenon can play a very good role in the state control. This effect can be helpful to the laser amplification technology. Therefore, this article attempts to use this quantum effect to realize the amplification without population inversion. The article first introduce this phenomenon, and then explain what frequent measurements are. Finally, a simple model is simulated by using hydrogen atom system as a medium for optical amplification without population inversions.
Frequent measurement can suppress the transitions between different quantum states. This quantum physics phenomenon can play a very good role in the state control. This effect can be helpful to the laser amplification technology. Therefore, this article attempts to use this quantum effect to realize the amplification without population inversion. The article first introduce this phenomenon, and then explain what frequent measurements are. Finally, a simple model is simulated by using hydrogen atom system as a medium for optical amplification without population inversions.
Introduction We were able to show in Chapter 3 that a medium in which we can obtain a population inversion (i.e. a situation in which the population density in the excited state is greater than that in the fundamental level) allows for optical gain of an electromagnetic wave having a frequency near to the resonant frequency of the system. By introducing feedback of the amplified signal into the medium, the system can be made to oscillate naturally, resulting in laser oscillations . To obtain this population inversion, we must introduce at least a third (and perhaps even a fourth) energy level into the system. (We saw how a two-level system under the influence of an intense pump beam will saturate with no resulting population inversion.) The aim then of this chapter is to introduce the concepts necessary to extend our two-level system into a working model capable of illustrating the phenomenon of laser oscillation. We will not spend too much time discussing atomic transition lasers as they do not figure readily in our treatment of quantum electronic properties of semiconductors. An exception will be made, however; we brush upon the particular topics of a diode pumped laser in Complement 4.E and a quantum cascade laser in Complement 13.H. Population inversion and optical amplification Population inversion We will show how population inversion can be achieved by carrier transfer from higher lying levels to the upper level of a two-level subsystem of interest.
When an atomic system interacts with light, it can amplify the electromagnetic radiation when gain exceeds loss. This requires that transitions from the upper level to the lower level prevail over the transitions from the lower level to the upper level. If one deals with a two-level system, the only way to obtain amplification is to create a state of population inversion, i.e. the upper level must be more densely populated that the lower level. "Lasing without inversion" may be possible when part of the atoms are trapped, which removes them from the subspace of states with which the upper level can have transitions. This chapter determines to what extent population inversion is necessary to achieve amplification. Independently, amplification without inversion and lasing without inversion were predicted by Harris using a different type of atomic interference. The amplification threshold has a number of features that are reminiscent of the laser threshold.
Considerable attention has been directed recently to the study of lasing without the need of population inversion. The optical coherence and quantum interference associated with the light amplification may lead to interesting statistical properties in inversionless lasers. Here we analyze a laser model that consists of an ensemble of closed multi-level (three or four levels) atoms driven by a single coherent field. Unlike conventional lasers where population inversion is maintained and kept at the threshold value for arbitrary laser output, the multi-level laser analyzed here can start lasing from population inversion and evolves from population inversion into non-inversion with the increasing atomic pumping. In other words, the system makes transition from light amplification by population inversion into light amplification by coherence. On the other hand, the system can be also made to start lasing without population inversion in any state basis. In both cases, initial inversion and non-inversion, which are controlled by the ratio of spontaneous decay rates, the laser well above threshold is always maintained by lasing without inversion (light amplification by coherence), and the laser intensity fluctuations may drop below the shot noise limit. Two factors contribute to the noise reduction: first, the disappearance of the population inversion leads to the depleted atomic population in the upper lasing state, which reduces the spontaneous emission noises; second, the fast coherent cycling of electrons between excited states leads to the highly regulated absorption and emission processes. We found that the combination of these two mechanisms can lead to the noise reduction in the laser output more than 50% below the shot-noise limit.
We have studied optical pulse propagation in a Raman fiber amplifier doped with a three-level medium and driven by a control laser pulse. We analyze the spatial-temporal dynamics of pulse propagation for different atomic initial conditions. The propagation of an optical pulse through the amplifier can be sustained by a control laser that induces transparency via quantum coherence, which is useful for extending the distance between optical repeaters. Under certain conditions, amplification is achieved without population inversion. The results could be useful for laser control of optical pulses in amplifiers and waveguides.
This thesis is concerned with the amplification of coherent radiation from an active medium under conditions which require no population inversion between the energy levels. Gain without population inversion may be obtained by creating a sufficiently strong correlation between pairs of atomic states, usually with the help of external sources of resonant electromagnetic radiation. Our main objective is to investigate two possible selections for the active medium: a three level atom in a lambda configuration and a Raman-driven four level model. For both we identify the conditions that lead to inversionless laser action, we calculate the small signal gain and investigate the physical mechanisms which are responsible for the emergence of gain. While the ultimate cause of the gain is quantum interference, we propose attractive alternative pictures based on the dressed states of the coupled atom-driving field systems. We address the important practical issue of the effect of atomic motion and related Doppler broadening on the inversionless gain profile and we also analyze the transient response of each laser model with the help of the appropriate Maxwell-Bloch equations. Finally we develop a fully quantum mechanical description of the lambda model and find that the radiated field can display unique quantum features which manifest themselves with sub-Poissonian photon fluctuations.
POPULATION RATE EQUATIONS From the discussion in the preceding chapter, it is clear that population inversion is the basic condition for an optical gain. For any system in its normal state in thermal equilibrium, a low-energy level is always more populated than a high-energy level, hence there is no population inversion. Population inversion in a system can only be accomplished through a process called pumping by actively exciting the atoms in a low-energy level to a high-energy level. If left alone, the atoms in a system relax to thermal equilibrium. Therefore, population inversion is a nonequilibrium state that cannot be sustained without active pumping. To keep a constant optical gain, continuous pumping is required to maintain population inversion. This condition is clearly consistent with the law of conservation of energy: amplification of an optical wave leads to an increase in optical energy, which is possible only if the required energy is supplied by a source. Pumping is the process that supplies energy to the gain medium for the amplification of an optical wave. There are many different pumping techniques, including optical excitation, electric current injection, electric discharge, chemical reaction, and excitation with particle beams. The use of a specific pumping technique depends on the properties of the gain medium being pumped. The lasers and optical amplifiers of particular interest in photonic systems are made of either dielectric solid-state media doped with active ions, such as Nd:YAG and Er:glass fiber, or direct-gap semiconductors, such as GaAs and InP. For a dielectric gain medium, the most commonly used pumping technique is optical pumping using either an incoherent light source, such as a flashlamp or a light-emitting diode, or a coherent light source from another laser. A semiconductor gain medium can also be optically pumped, but it is usually pumped by electric current injection. In this section, we consider the general conditions for pumping to achieve population inversion. Detailed pumping mechanisms and physical setups are not addressed here because they depend on the specific gain medium used in a particular application. The net rate of increase of the population density in a given energy level is described by a rate equation. As we shall see below, pumping for population inversion in any practical gain medium always requires the participation of more than two energy levels.