Lect11 - RenewableEnergyEngineering422 -2021.pptx

REE422

1. Optics and Photonics ( REE 422 ) Spring 2021 Lecture 11 Dr. Salem Hegazy Email: [email protected] Lasers

2. Optical Power Transmission  Power by light (also power over fiber ) is especially advantageous for systems located in remote or critical locations where a conventional power supply based on copper wiring is not feasible or difficult to install. Laser

3. Absorption, spontaneous (random photon) emission, and stimulated emission. Atoms initially in the ground state If a photon has the energy the atom can absorb this photon and does discrete energy transition from to If the photon has higher or lower energy, no interaction takes place.   The electron falls down in energy from level to and emits a photon of energy Any propagation direction Any polarization After a period life time ( )   An incoming photon of energy stimulates the emission process by inducing the electron at to transit down to . The two photons have: Same polarization and propagation direction Same energy: The same phase (in phase) L ight A mplification by S timulated E mission of R adiation ( LASER )   Ground state   Excited state   (1) (2) (2’)

4. A: It depends on the number of ions in the ground level and upper level. As a result, Naturally, if a photon strike a material, absorption is more likely than stimulated emission for 2-levels system. Effective stimulated emission and optical amplification require more than 2 levels!   Boltzmann statistics:   In thermal equilibrium (no-excitation and isolation) , Probability that the atom resides in the level :   Atoms concentration:   Q: If a photon of energy passes such medium with , which process is more probable: absorption or stimulated emission?   Boltzmann constant, absolute temperature   Naturally, physical systems like atoms tend to be in the lower energy level

5. Population inversion & stimulated emission in Three level system: Ruby Laser < 694 nm optical pumping Cr 3+ ions Fast decay ( ps ) by emitting phonons (Lattice vibrations or heat)   Population inversion Lasing: avalanche of stimulated emissions Long-lived state Q : Stimulated emission at 694 nm, what can be the frequency of the pump source??

6. A more realistic energy diagram for Ruby crystal Electric fields of neighboring ions in the crystal split the Energy Levels to energy bands of many narrowly spaced energy states (this is called Stark effect). Absorption bands of Ruby crystal ( ) and ( ) are in green and blue regions, therefore Ruby is red , or pink for low concentrations of Cr . (White light is composite of red; green, blue)   Energy diagram: for the Cr 3+ ion in the ruby crystal (Al 2 O 3 ),

7. Laser ; L ight A mplification by S timulated E mission of R adiation

8. Travelling wave : The sinusoidal function along is dephased by .   Q: What is needed to convert it to “standing” (stationary) wave? A wave reflector. Standing wave: two counter propagating waves The sinusoidal function along is stationary.   Optical Cavity

9. Interference of all of these wave is constructive only for well-defined wavelengths (resonance wavelength) also named cavity modes with This encourages stimulated emissions by bouncing photons of wavelength along the normal direction to two-mirrors planes.   Optical Cavity In optical cavity , infinite number of counterpropagating waves, in principle.

10. Optical Resonator Over multiple roundtrips in the cavity, waves that survive (called cavity modes) can constructively interfere every roundtrip. Therefore over , there is integer no. of wavelengths Allowed wavelengths thus satisfy the cavity length: Cavity stores radiation at certain wavelengths obtained by its dimensions  

11. Optical Resonator Fabry-Perot Optical Cavity Schematic illustration of the Fabry -Perot optical cavity and its properties. (a) Reflected waves interfere . (b) Only standing EM waves, modes, of certain wavelengths are allowed in the cavity. (c) Intensity vs. frequency for various modes. R is mirror reflectance and lower R means higher loss from the cavity. Note: The two curves are sketched so that the maximum intensity is unity. However there is some energy escapes from cavity due to partial reflection  

12. Optical Resonator Fabry-Perot Optical Cavity E cavity = A + B +  = + r 2 exp( - j 2 kL ) + r 4 exp( - j 4 kL ) + r 6 exp( - j 6 kL ) +    Maxima at k m L = m p m = 1,2,3,…integer q sin q p 2 p 3 p p /2 A + B =   For two waves:       Cavity modes, other wavelength are suppressed

13. Optical Resonator Fabry-Perot Optical Cavity Maxima at k m L = m p m = 1,2,3,…integer m (  m / 2 ) = L (2  /  m ) L = m p   q sin q p 2 p 3 p p /2 Cavity modes (at and ), other wavelength are suppressed   As expected,

14. The condition physically means waves after each roundtrip should be in phase. The resonance wavelengths are at , is the refractive index and is wavelength in free space ( ). Equally, the frequencies are The free spectral range (frequency difference) is . FWHM is full width at half maximum  

15. Practical examples for : Semiconductor laser: Medium: GaAs ; , , emission wavelength . HeNe laser: Medium: Helium Neon gas at 1 torr ; , , emission wavelength .  

16. Geometrical solutions for efficient pumping Elliptical reflector: has the ruby crystal at one focus and the pump light at the other focus. Helical flash lamb.

17. T. H. Maiman , Stimulated optical radiation in ruby lasers”, Nature , 187 , 493, 1960 Ruby Laser (the 1 st made laser!) Charles Townes , Arthur Schawlow, and first maser!

18. Question : Ruby laser is 3-level laser system, Do you see any privilege in 4-level system?? Answer : Population inversion can be easily and quickly achieved with the lower level other than ground level. Nd 3+ ions doped in a Y 3 Al 5 O 11 (yttrium aluminate garnate , YAG) crystal 4-level laser: Nd:YAG (~ ps ) (~ ps ) (~ 0.25ms) S.O. Kasap , Optoelectronics and photonics: Principles and Practices, Second Edition, 2013 Pearson Education Question : While Ruby laser is always pulsed, Nd:YAG laser may by CW or pulsed. Answer : Population inversion in Nd:YAG can survive during lasing. For Ruby, lasing kills the population inversion 1064 nm