Laser Physics EAL 501

1. Laser Physics EAL 501 Lecture 6 Power & Frequency

2. We said that there are two types of cavities • 1- Ring cavities

5. Standing wave cavities • 1- Longitudenal modes due to standing waves Do ring lasers have longitudenal modes?

7. 2-Transversal modes [Gaussian beams] The electric field distribution in the x-y plane Satisfy the wave eq. The Fresnel-Kirchoff’s theory states that “if we know the field distribution at a plan 1 (x1,y1,z1) then the distribution at any plane (x,y,z) is given by We can write Where The solution is

8. Higher order modes are given by

9. Gaussian Beams The solution of wave equation can be plane waves But it is not real q(z) is the complex beam parameter of the beam

11. Amplitude factor Longitudenal phase factor Transverse phase factor Rayleigh range Beam waist Radius of curvature

14. Gain Saturation We drive the rate equations

15. The steady state solution of the rate equation in the case I=0 gives us the threshold condition • The steady state solution of the rate equation in the case after the threshold can be shown to be

16. Steady state under threshold solution is In case of 3 level laser In case of 4 level laser

17. Solution above threshold in the case of two levels without pumping

18. Solution above threshold in the case of 3levels with pumping Since g(ν)=σ(v)(N2-N1) then we can write Problem : find small signal gain and saturation intensity for 4 level laser

19. Hole burning Spatial hole burning Spectral hole burning 1- Spatial hole burning in the case of standing wave cavity there are points of zero intensity and points of max. intensity which depletes the gain much more rapidly 2- Spectral hole burning in the case of inhomogeneous broadening the atoms that feed the laser modes will be depleted much more rapidly

20. Output power Usually only one mirror produces output and if the mirror reflection is high The condition of steady state lasing Where s is the scattering losses

21. It is easy to find the value of transmission t to get the maximum output So the optimum output intensity is