TEMoo mode

Telnic bronze, 24 426 Telo-inertinite, 6 707t Telomeres, 17 610, 2 814 Telomer formation, 11 865 Telomerization, 2 261 butadiene, 4 374-375 TEMoo mode, 14 683. See also Gaussian mode (TEMoo)... 

With the assumption of a Gaussian temperature distribution (TEMoo-mode) on the laser affected zone, the temperature increase AT can be estimated by the following equation [63] ... 

The experimental setup is shown in Fig. 1.18. The laser pulses are coupled into the resonator by carefully designed mode-matching optics, which ensure that only the TEMoo modes of the cavity are excited. Diffraction losses are minimized by spherical mirrors, which also form the end windows of the absorption cell. If the absorbing species are in a molecular beam inside the cavity, the mirrors form the windows of the vacuum chamber. For a sufficiently short input pulse (Tp < 7r), the output consists of a sequence of pulses with a time separation Tr and with exponentially decreasing intensities, which are detected with a boxcar integrator. For longer pulses (Tp > 7r), these pulses overlap in time and one observes a quasi-continuous exponential decay of the transmitted intensity. Instead of input pulses, the resonator can also be illuminated with cw radiation, which is suddenly switched off at f = 0. 

The experimental high pressure process provides cw power up to 270 mW. The beam from a one watt, TEMoo mode, cw Nd-YAG laser (emission wavelength 1.064 m) is passed through a polarizer, a Linconix laser power stabilizer, a variable neutral density filter, a beam expander, and focused with a 10 cm focal length lens into the fiber growth reactor [2] [12]. Laser power ranges from 0 to 200 mW with a stability of one mW, as measured outside the reaction chamber. 

The propagation of heat in the woikpiece is encountered in all laser materials processing. The heating of a workpiece with multimode laser beam is different from the heating with a TEMOO mode beam. This is because the multimode beam produces multiple localized hot spots, whereas the TEMOO mode beams generate a single central hot spot. 

Figure 5.10 Radial intensity distribution of the fundamental TEMoo mode...

In Fig. 5.34, the ratio yio/yoo of the diffraction losses for the TEMio and the TEMoo modes in a symmetric resonator with gi = gi = g is plotted for different values of g as a function of the Fresnel number Np. From this diagram one can obtain, for any given resonator, the diameter 2a of an aperture that suppresses the TEMio mode but still has sufficiently small losses for the fundamental TEMoo mode with beam radius w. In gas lasers, the diameter 2a of the discharge tube generally forms the limiting aperture. One has to choose the resonator parameters in such a way that a 3w/2 because this assures that the fundamental mode nearly fills the whole active medium, but still suffers less than 1 % diffraction losses (Sect. 5.2.6). 

An argon laser osdUating at A = 488 nm with resonator length d = 100 cm and two mirrors with radius R = oo and R2 = 400 cm has an intracavity circular aperture close to the spherical mirror to prevent oscillation on transversal modes. Estimate the maximnm diameter of the aperture that introduces losses yaiffr < 1 % for the TEMoo mode, but prevents oscillation of higher transverse modes, which without the aperture have a net gain of 10 %. 

Fig. 5.11. Phase fronts and intensity profiles of the fundamental TEMoo mode at several locations z in a confocal resonator with the mirrors at z = J/2...

In the previous seetions we have seen that without specific manipulation a laser generally oscillates in many modes, for which the gain exceeds the total losses. In order to select a single wanted mode, one has to suppress all others by increasing their losses to such an amount that they do not reach the oscillation threshold. The suppression of higher-order transverse TEMm modes demands actions other than the selection of a single longitudinal mode out of many other TEMoo modes. 

Fig. 5.34. Ratio yio/yoo of diffraction losses for the TEMio and TEMoo modes in symmetric resonators as a function of the Fresnel number for different resonator parameters g= l—d/R...

Figure 10 Spectral and spatial modes of a laser. (A) Atomic line enclosing several longitudinal resonator modes. (B) Transverse spatial modes of a laser beam. The fundamental TEMoo mode is the Gaussian laser beam profile. The arrows indicate the direction of the beam s electric field at a given instant of time.

Fig.6.10. Ratio yio/yOO of the diffraction losses for the TEMio to that of the TEMoo mode, as a function of the Fresnel number N for a resonator with two mirrors of equal curvatures where = 92 = 9...

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