A helium-neon laser emits an unpolarized pulse of light carrying an energy of 0.

Question

A helium-neon laser emits an unpolarized pulse of light carrying an energy of 0.35 Joules in 225 fs pulses (1 femtosecond = 10-15 s). Each pulse encounters three polarizers as shown in the picture (polarization angles measured with respect to the vertical axis).

(a) What is the average power emitted by the laser during each pulse?

(b) If the beam is cylindrical with a 2.0mm diameter, ï¬nd the average intensity Io of the pulse.

(c) Find angle theta such that the intensity after the third polarizer is 10% of the original intensity [the one in part (b).]

Explanation / Answer

a) power of each without going throug the polarizer=0.35/225 E-15

==> power of each after going throug the polarizers=1.55 E12*(1/2)*(Cos(40))^2*(Cos(50))^2= 1.886 E11 Â W

b) I0=Power/A=1.886 E11/pi*1 E-6=0.6006 E17 W/m^2

c) 0.1=1/2 * (Cos(40))^2*(Cos(theta-40))^2

==>Cos(theta-40)=0.875

==>theta-40=28.92

==> Theta=68.92 degrees

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