Laser pulsed 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 laser normally operates in the pulsed mode because of the necessity of the dissipation of a large amount of heat between pulses. 

The energy input into a CO2 laser is in the form of an electrical discharge through the mixture of gases. The cavity may be sealed, in which case a little water vapour must be added in order to convert back to CO2 any CO which is formed. More commonly, longitudinal or, preferably, transverse gas flow through the cavity is used. The CO2 laser can operate in a CW or pulsed mode, with power up to 1 kW possible in the CW mode. 

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. 

A commercial fs-laser (CPA-10 Clark-MXR, MI, USA) was used for ablation. The parameters used for the laser output pulses were central wavelength 775 nm pulse energy -0.5 mj pulse duration 170-200 fs and repetition rate from single pulse operation up to 10 Hz. In these experiments the laser with Gaussian beam profile was used because of the lack of commercial beam homogenizers for femtosecond lasers. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

The photolysis of Cr(CO)6 also provides evidence for the formation of both CO (69) and Cr(CO) species (91,92) in vibrationally excited states. Since CO lasers operate on vibrational transitions of CO, they are particularly sensitive method for detecting vibrationally excited CO. It is still not clear in detail how these vibrationally excited molecules are formed during uv photolysis. For Cr(CO)6 (69,92), more CO appeared to be formed in the ground state than in the first vibrational excited state, and excited CO continued to be formed after the end of the uv laser pulse. Similarly, Fe(CO) and Cr(CO) fragments were initially generated with IR absorptions that were shifted to long wavelength (75,91). This shift was apparently due to rotationally-vibrationally excited molecules which relaxed at a rate dependent on the pressure of added buffer gas. 

We consider a model for the pump-probe stimulated emission measurement in which a pumping laser pulse excites molecules in a ground vibronic manifold g to an excited vibronic manifold 11 and a probing pulse applied to the system after the excitation. The probing laser induces stimulated emission in which transitions from the manifold 11 to the ground-state manifold m take place. We assume that there is no overlap between the two optical processes and that they are separated by a time interval x. On the basis of the perturbative density operator method, we can derive an expression for the time-resolved profiles, which are associated with the imaginary part of the transient linear susceptibility, that is,... 

Because the laser is fired independently from the electric activity in the clouds, and because natural electric activity in clouds is a random process, temporal correlations between the electric events detected by the LMA and the laser operation were used as an evidence for an effect of the laser. At each location, the events synchronized (within 2 ms) with the laser have been identified. The probability async that these events may have been obtained by chance among random events, rather than being due to an effect of the laser, was estimated. async can be understood as the risk of error when concluding that the observed pulses are related with the laser pulses. This corresponding confidence level is therefore 1 — async-... 

Some lasers produce a continuous-wave (CW) beam, where the timescale of the output cycle is of the same order as the time taken to remove photons from the system. CW lasers can be modified to produce a pulsed output, whereas other lasers are inherently pulsed due to the relative rates of the pumping and emission processes. For example, if the rate of decay from the upper laser level is greater than the rate of pumping then a population inversion cannot be maintained and pulsed operation occurs. 

Tksapphire lasers can operate both in CW mode and in pulsed mode. There are two types of pulsed Tksapphire laser ... 

Semiconductor lasers can operate in the cw regime (with output powers ranging from /xwatts to tens of watts) or in the pulsed regime, with typical peak powers of tens of watts. 

Compact and stable devices are available that take advantage of the improved quality of the crystal lasers, as well as increased pump efficiencies. Hundreds of different models of Nd + based lasers have demonstrated laser action (Kaminskii, 1981). It is possible to operate these Nd + solid state lasers in the continuous regime, with output powers ranging from 1 W to 1000 W. Pulsed operation is also possible, with a pulse length from the picosecond range, via mode-locking, to tens of nanoseconds by Q-switch operation. 

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