Lecture 2 Basic technological lasers 1 СО

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Lecture 2 Basic technological lasers

1. СО 2 -laser ( 0 = 9, 6 10, 6 µm) Active media: СО 2: N 2: He at p 0, 1 10 atm. Excitation: electrical and HF discharge. Upper level of CO 2 molecule (U) is populated by resonant collisions with excited N 2 molecule. It is metastable. Lower level (L) has small life time. Lower intermediate (IL) level is effectively emptied by collision of CO 2 and He molecules. This helium function can be also fulfilled by H 2 O vapor. Besides, He simplifies ignition and maintenance of glow discharge. Gas mixture cooling is necessary to avoid thermal population of IL level. Gas flow is essential to remove gas dissociation products and supply of “fresh” molecules.

Longitudinal flow Transverse gas flow Reflective optics – metals (Cu, Al, …). Transparent materials – Na. Cl, KCl, Ba. F 2, Zn. Se, diamond. Efficiency – theoretical 30% - real 5 -10% Working regimes of СО 2 -lasers: -CW Р 1 МW (in technology Р 30 KW) -ТЕА (Е 10 KJ) -High repetition rate (Р 100 KW)

Gas dynamic lasers Phase I Gas heating (up to 1300 -1400 К), e. g. by fuel combustion Phase II Supersonic gas expansion from nozzle and cooling for time r<< U, r>> L. Then upper level (U) population practically does not change and remains equal to that of hot gas, while lower level (L) population drops. L U

2. Neodymium lasers ( 0=1, 06µm) Different materials can be easily doped with Nd+ (glasses up to 10% and crystals up to 1 %). Nd: YAG (matrix Y 3 Al 5 O 12) – most widespread U 0, 2 -0, 7 ms (metastable level) L << U, 0, 05 e. V >> к. Т Pumping with lamps efficiency up to 5 -10% and laser diodes (efficiency up to 40 -60%). Active elements: cylinders, plates, fibers. Operating regimes: 1) CW (Р 5 k. W) 2) high repetition (Р 10 k. W, f 100 MHz) 3) pulsed ( Е 1 MJ, 100 fs – 10 ms) Harmonics: 0, 0/2, 0/3, 0/4

3. Semiconductor laser Principle: 1) pumping of electrons into conduction band 2) radiative transision to valance band Way of pumping – electric current. Advantages: 1) High probability of radiative transitions in direct band gap semiconductors 2) Pumping into broad zones (in other solid state lasers pumping into bands) As a result light amplification coefficient 104 cm-1 and possibility to miniaturize lasers. High efficiency. Practical realization – injection of carriers into p-n junction. Thickness of active volume does not exceed few micrometers. . Classical example Ga. As ( = 0, 84µm). Nowadays - heterostructures. Power of single laser diode (Р 1 W). Matrix of diodes – way to increase laser power. Working regimes – CW (Р 1 к. W). - high repetition rate. Solid state lasers with diode pumping.

5. Argon laser Source of radiation – exited argon ions. Inversion is a two-step process: creation of ion and its excitation. Way of excitation – electrical capillary discharge with high current density (~100 -1000 А/сm 2) Efficiency 0, 1% Operation: CW (Р 100 W) high repetition rate Basic wavelengths (obtained by narrow-band mirrors): 514, 5 и 488 mm for (Ar+)* 363, 8 и 351, 1 nm for (Ar++)* Second harmonic generation is also possible. High quality of radiation (single mode)

4. Eximer lasers. Unexcited gas consists of atoms. Active medium – exited molecules Xe 2*, Ar. F* and etc. Life time of upper level ~ 10 ns. Lower (basic) level of molecule is unstable (zero population of lower level – characteristic feature of eximer lasers). Regimes: pulsed ( 10 -100 ns) and pulse repetition rate (f 10 k. Hz), efficiency 13%. Most widely used active molecules: F 2* ( =157 nm ), Ar. F* ( =193 nm, gas mixture Ar: F 2: He), Kr. F* ( =248 nm), Xe. Cl* ( =308 nm). Peculiarity – nigh light amplification coefficients, super luminescence. Problem: operation with halogen gases (safety, materials stability).

Multi wavelength Gas Laser (for Model 1701) Ar. F Kr. F Xe. Cl N 2 Xe. F HF CO 2 , nm 193 248 308 337 351 2600 3000 10600 E, m. J 80 200 120 5 80 250 550 , % 0, 9 1, 7 1, 2 0, 04 0, 7 2, 4 5 , ns 15 20 15 5 20 400 1. 8. 105 Double wavelength HF-N 2 CO 2 -N 2 1 = 3 µm… 30 m. J 1 = 10. 6 µ… 60 m. J 2 = 0. 337 µm… 0, 1 m. J 2 = 0. 337 µ… 0, 5 m. J Medical Excimer Laser Model 1730 Wavelength, nm - 308 Pulse duration, ns - 200 Repetition rate, Hz - 50 Energy on the distal end of optical fiber, m. J - 40 Size, mm -1500 x 700 Weight, kg - 400

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