Lasers... the optical cavity

Transcription

1 Lasers... the optical cavity history principle, intuitive aspects, characteristics 2 levels systems Ti: Helium Al2O3 - Neon model-locked laser laser VCSEL bragg mirrors cleaved facets 13

2 ptical and/or microwave frequencies Laser... population 2 levels history principle, intuitive aspects, characteristics 2 levels systems N i = e - (E i - E j ) N kt j optical frequencies (500 nm, Hz)! ΔE >> kt N j / N i = 0 radiofrequencies (1,25 cm, Hz) ΔE << kt N j / N i = 1 Only the fundamental level is populated the two levels are equally populated 25

3 Laser... population inversion history principle, intuitive aspects, characteristics 2 levels systems N j 2 N i Population inversion (also known as negative temperature) Energy Energy Population density Population density 26

4 Laser... single line operation Single line operation! Idea: Introduce some losses at the different wavelengths except at λ 2 m m m m Argon laser m m This configuration is known as the Littrow configuration... (i.e. the wavelength λ 2 is reflected back on the same axis...) 54

5 Laser... multimode operation gain curve (closely related to the line width profile) optical cavity modes We observe experimentally that the laser tends to work on several wavelengths... This is a bit paradoxical, because of the gain saturation... 55

6 Laser... multimode operation There is a saturation of the gain above the threshold... only one mode should lase! Why does the laser work on several modes? Hole burning (spatial and spectral) 56

7 Laser... spatial hole burning spatial hole burning The different modes use different parts of the active material 57

8 Laser... spectral hole burning spectral hole burning In the case of an inhomogeneous broadening 58

9 Laser... single-frequency mode 59

10 Dye lasers due to the lifetime of the triplet states, the laser does not work in CW regime... 63

11 Les lasers accordables (dye lasers) 64

12 Argon laser... single mode Argon laser, CO2 laser, He-Ne laser Semiconductor laser Excimer laser, chemical laser Nd:YAG laser, Ti:saph laser 77

13 He-Ne laser Argon laser, CO2 laser, He-Ne laser Semiconductor laser Excimer laser, chemical laser Nd:YAG laser, Ti:saph laser A 1400 V high voltage, DC power supply maintains a glow discharge or plasma in a glass tube containing an optimal mixture (typically 5:1 to 7:1) of helium and neon gas. The discharge current is limited to about 5 ma by a 91 kw ballast resistor. Energetic electrons accelerating from the cathode to the anode collide with He and Ne atoms in the laser tube, producing a large number of neutral He and Ne atoms in excited states. He and Ne atoms in excited states can deexcite and return to their ground states by spontaneously emitting light. This light makes up the bright pink-red glow of the plasma that is seen even in the absence of laser action. Energy [ev] Helium collisions Neon * * * 1.15 µm fast decay on strong visible transitions ( µm) 3.39 µm nm 78

14 Helium-Neon laser Argon laser, CO2 laser, He-Ne laser Semiconductor laser Excimer laser, chemical laser Nd:YAG laser, Ti:saph laser 79