Question

A laser used in LASIK eye surgery produces 55 pulses per second. The wavelength is 193 nm (in air), and each pulse lasts 10.0 ps. The average power emitted by the laser is 120.0 mW and the beam diameter is 0.80 mm.

(a) What is the total energy of a single pulse? mJ

(b) What is the intensity during a pulse? W/m2

Answer 1

Given that :

wavelength (in air), = 193 x 10^-9 m

diameter of beam, D = 0.8 x 10^-3 m

radius of a beam, r = 0.4 x 10^-3 m

(a) The total energy of a single pulse which will be given as :

using an equation,    P_pulse = U_pulse / t                                           { eq.1 }

where, P_pulse = average power emitted by the laser = 120 x 10^-3 W

t = time taken = 55 pulses per second

inserting thevalues in above eq.

(120 x 10^-3 W) = 55 U_pulse / (1 sec)

U_pulse = (120 x 10^-3 J) / (55)

U_pulse = 2.18 x 10^-3 J

U_pulse = 2.18 mJ

(b) The intensity during a pulse which is given as :

I_av = P_pulse / A                                                             { eq.2 }

where, A = area of the beam = r² = (3.14) (0.4 x 10^-3 m)²

inserting the values in eq.2,

I_av = (120 x 10^-3 W) / (3.14) (0.4 x 10^-3 m)²

I_av = (120 x 10^-3 W) / (1.256 x 10^-6 m²)

I_av = 95.5 x 10³ W/m²

I_av = 9.55 x 10² W/m²