TAILIEUCHUNG - Tunable lasers handbook phần 3

Họ do đó thường được sử dụng như các bộ khuếch đại chứ không phải dao động. Thải avalanche Preionized đã được sử dụng để tạo ra một plasma thống nhất. Các electron năng lượng thấp trong huyết tương có được năng lượng đủ để kích thích | 84 Charles Freed FIGURE 8 Graphic illustration of the saturation resonance observed in co fluorescence at pm. Resonant interaction occurs for V vo when k V 0 . The figure shows an internal absorption cell within the laser cavity. External cells can also be used. Reprinted with permission from SooHoo elal. 76 . 1985 IEEE. In the initial experiments a short gas cell with a total absorption path of about 3 cm was placed inside the cavity of each stable co laser 72 with a Brewster angle window separating the cell from the laser gain tube. Pure CO2 gas at various low pressures was introduced inside the sample cell. A sapphire window at the side of the sample cell allowed the observation of the Ltm spontaneous emission signal with a liquid-nitrogen-cooled InSb detector. The detector element was about cm from the path of the laser beam in the sample cell. To reduce the broadband noise caused by background radiation the detector placement was chosen to be at the center of curvature of a gold-coated spherical mirror W hich was internal to the gas absorption cell. The photograph of the laser with which the standing-wave saturation resonance W as first observed via the fluorescence signal at pm is showm in Fig. 9. More than two orders of magnitude improvements in signal-to-noise ratios SNRs W ere subsequently achieved with improved design low-pressure co stabilization cells external to the lasers 73 . One example of such improved design is schematically showm in Figure 10. In the improved design the low -pressure gas cell the LN -cooled radiation collector and the infrared IR detector are all integral parts of one evacuated housing assembly. This arrangement minimizes signal absorption by window s and eliminates all other sources of absorption. Because of the vacuum enclosure. diffusion of other gases into the low-pressure gas reference cell is almost completely eliminated therefore the time period available for continuous use of the reference gas cell is .