Laser continuous operation

Population inversion is difficult not only to achieve but also to maintain. Indeed, for many laser systems there is no method of pumping which will maintain a population inversion continuously. For such systems inversion can be brought about only by means of a pumping source which delivers short, high-energy pulses. The result is a pulsed laser as opposed to a continuous wave, or CW, laser which operates continuously. 

The light source for excitation of Nd YAG lasers may be a pulsed flashlamp for pulsed operation, a continuous-arc lamp for continuous operation, or a semiconductor laser diode, for either pulsed or continuous operation. The use of semiconductor laser diodes as the pump source for sohd-state lasers became common in the early 1990s. A variety of commercial diode-pumped lasers are available. One possible configuration is shown in Figure 8. The output of the diode is adjusted by composition and temperature to be near 810 nm, ie, near the peak of the neodymium absorption. The diode lasers are themselves relatively efficient and the output is absorbed better by the Nd YAG than the light from flashlamps or arc lamps. Thus diode-pumped sohd-state lasers have much higher efficiency than conventionally pumped devices. Correspondingly, there is less heat to remove. Thus diode-pumped sohd-state lasers represent a laser class that is much more compact and efficient than eadier devices. 

Table 1 gives wavelengths and output powers for some important laser types operated in a continuous-wave (cw) or pulsed mode. The pulsed lasers normally have much higher peak powers but there are technical or theoretical limitations of the maximum repetition frequency, which means that their time-averaged intensity is often below that of the cw lasers. 

The efforts to obtain increasing output powers in pulsed and continuous operation, stimulated mainly by military needs but eminently useful for basic spectroscopic research, e. g. in nonlinear optics or in plasma physics, has already resulted in pulsed output powers of 10 watts (neodymium laser) and continuous powers of 30 Kw (CO2 lasers). 

Since the chromium-neodymium-exchange time is much longer than the neodymium-decay time, the threshold for pulsed-laser oscillation will not be significantly improved. However, for continuous operation the threshold should drop. Such was found to be the case. 

The most common lasers are continuously operating, gas (Ar+, Kr+, or He-Ne) lasers. Typical characteristics of the lasers commonly used are summarized in Table VII. 

In this method, two pulsed lasers are used, both usually in the nanosecond regime. One (the burn laser) is operated at high power, and is scanned across the absorption spectrum. It excites molecules (or clusters) from the particular vibrational level (usually the i = 0 level) to an electronically excited state. The upper state relaxes (radiatively or otherwise) back to the ground state, but not necessarily to i = 0. Thus, depletion in the population of this species is achieved. A second, low-power laser (the probe laser) is fired after a suitable time delay (to allow complete decay of the emission induced by the pump laser). It is tuned to one of the excitation spectrum vibronic bands of the system, and the fluorescence induced by it (the signal ) is continuously monitored. Whenever the frequency of the bum laser corresponds to excitation of the species giving rise to the absorption of the probe laser, the signal is reduced. This reduction appears as a hole that is burned in the spectrum—hence the name of the method. If a different species is excited (another molecule or a different vibrational level) no change in fluorescence intensity is incurred. 

If a film is heated by the continuous-operated or oscillated lasers, temperature overshoot takes place in ftie films of smaller values of CoX /a within a very short period of time. The effect of ftie laser heat source on ftie temperature distribution in the film becomes larger in the thin film. In other words, if ftie absorption coefficient, b, of the laser increases, ftie temperature is more dependent on the laser heat source in a ftiin film thMi in a ftiick film. Overshoot and oscillation of thermal wave depend on the frequency (o of the heat source time characteristics. 

Population inversion is often achieved by a multi-level atomic or molecular system in which the excitation process, called pumping, is accomplished by electrical means, by optical methods, or by chemical reactions. In some cases, the population inversion can be sustained to produce a continuous wave (CW) output beam that is continuous with respect to time. In other cases, the lasing action is self terminating, so that the laser is operated in a pulsed mode to produce a repetitive pulse train or a single-shot action. ... 

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