Question

3- Consider a HeNe laser radiating at spatial TEM mode, wavelength à = 632 nm and spectral line-width AA = 0.1nm. Beam diameter at z, =0 is w, =1.0 mm. Laser resonator consists of two plane mirrors of separation L=1.0 m. a) What is the beam diameter at at 1Km. b) What is divergence angle of beam. c) What is the coherence length of this laser. d) What is the longitudinal frequency separation. (assume refractive index n=1 inside the cavity) e) How many longitudinal modes exist under the gain curve of this laser.

Answer 1

ginen - wavelength, 2= 632om line width, at = .1 nm Beam diameter, wool mm = 10t6 am reparation, L=1m beam diametre at I kn, = Tortos (2) = 1 wo I wo 103x632x10-9 3.148 10-3 z T (132 636x19 = 2.01x10-1 had . 6) divergence angle, total spread, 20= 42 22 wo 2 two 2x632 3.14 X 186 = 4.02 810-G rad. so divergence angle, o = 20 10=2.01 X10 4 rad - c- hell of light Cherence length, L=12 = 've r- time vo-frequency - 22 2 3.99 x 10 m & longitudinal Eroquaincy separation OU ZAL 270x1 3X10k - = 1.5x108 Hz

e) no of logitudinal mode, n=2. L = n2 n= 22 2x1 632x10-9 n = 3.168106

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