Requirements for Laser Oscillation

If a sample of molecules with inverted populations is placed in an optical cavity which may consist, for instance, of two suitably aligned mirrors (see next section), induced radiation can change the densities N and N of Eq. (1). The interaction of a system with population inversion with a radiation field of the right frequency v can be described in the so-called rate equation approximation. The simplified rate equations for the laser are obtained as follows if two states Ni, Na are considered with the energy difference AE =hvia connected by a radiational transition (13). 

A balance equation for the quantum density q in the laser may be written with regard to the processes shown in (13). 

While ANi describes the total spontaneous emission rate. A N refers to that part of it that remains in the cavity and contributes to the photon density q. A is related to the spontaneous emission coefficient A in a more complicated fashion which involves consideration of the resonator modes and the bandwidth of the transition. This will not be discussed in detail here. 2 25 The term Sq describes the output with the coupling coefficient 

Thus the first term in Eq. (15) describes the contribution by spontaneous emission which is important only at the beginning of the oscillation and may be ignored lateron. The second term is the rate of the stimulated processes, 

The above rate equation for the photons may be supplemented in the same way by equations describing the rate of change of the upper and lower state densities N i, N%. The notation here follows from (14) and (15). As 

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Contents Population Inversion and Molecular Amplification. Energy-partitioning in Elementary Chemical Reactions Vibrational Relaxation. Requirements for Laser Oscillation. Design Parameters of Pulsed Chemical Lasers. Specific Chemical Laser Systems. Future Chemical Lasers. Present Perspectives of High-Power Chemical Lasers. Kinetic Information through Chemical Laser Studies. 

This article deals with a field of research on the borderline between physical chemistry and laser physics. As it is intended to combine aspects of both areas, molecular amplifiers based on partial or total vibrational inversion are first characterized in general, after which the generation, storage, distribution, and transfer of vibrational energy in chemical processes is reviewed. There is a brief discussion of the experimental requirements for laser oscillation and associated hardware problems. Experimental results for specific chemical laser systems are then surveyed and the prospects for high-power chemical laser operation considered. The concluding sections are devoted to the contribution of chemical lasers to reaction kinetics and their other uses in chemistry. 

Lick Observatory. The success of the LLNL/AVLIS demonstration led to the deployment of a pulsed dye laser / AO system on the Lick Observatory 3-m telescope (Friedman et al., 1995). LGS system (Fig. 14). The dye cells are pumped by 4 70 W, frequency-doubled, flashlamp-pumped, solid-state Nd YAG lasers. Each laser dissipates 8 kW, which is removed by watercooling. The YAG lasers, oscillator, dye pumps and control system are located in a room in the Observatory basement to isolate heat production and vibrations from the telescope. A grazing incidence dye master oscillator (DMO) provides a single frequency 589.2 nm pulse, 100-150 ns in length at an 11 kHz repetition rate. The pulse width is a compromise between the requirements for Na excitation and the need for efficient conversion in the dye, for which shorter pulses are optimum. The laser utilizes a custom designed laser dye, R-2 perchlorate, that lasts for 1-2 years of use before replacement is required. 

Up to now/ the dimer laser system has been described alone in terms of population inversion between suitable energy levels/ and for this description the condition S2 > A 2 is indeed the only necessary condition for cw laser oscillation/ as long as the thermal population density in the lower laser level remains negligibly low. However/ as this optically pumped laser system is a coherently excited three level system/ the coherent emission can also be described as stimulated Raman scattering/ which is resonantly enhanced by the common level 3 of the pump and laser transitions. This coupled two photon or Raman process does not require a population inversion between levels 3 and 2 and introduces qualitatively new aspects which appreciably influence and change the normal laser behaviour. For a detailed and deeper description of the coherently excited three level dimer... 

We have used a femtosecond-written Nd YAG ceramic optical waveguide as an active media to achieve continuous wave 1.06 pm laser operation. We have obtained output laser power of 40 mW and with a laser slope efficiency in excess of 40%. Single mode and stable laser oscillation have been achieved by using the natural Fresnel reflection for optical feedback without the requirement of any kind of mirror or reflective component. 

The macro time clock can be started by an external experiment trigger or by a start-measurement command from the operating software. In some TCSPC modules the clock signal source of the macro time clock can be selected. The macro time clock can be an internal quartz oscillator, an external clock source, or the reference signal from the laser. Triggering and external clock synchronisation are absolute requirements for multimodule operation in the time-tag mode, see Sect. 5.11.3, page 189. 

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