1 Short-Pulse Lasers (25 pts) a) Short pulse lasers are used in many different b
ID: 2305027 • Letter: 1
Question
1 Short-Pulse Lasers (25 pts) a) Short pulse lasers are used in many different branches of physics. A laser in use in one laboratory in Germany sends out a pulse of light whose duration is 35 fs long (1 fs = 10-15 s), whose wavelength is ?=1.8 ?m. The pulse has an energy of 900 ?J; the pulse is focused into a circle with diameter D-100 ?m. What is the total length (as in distance) of the pulse? (5 pts) b) What is the frequency of the laser pulse? For a wave train this short, how many pts) c) What is the total intensity of the pulse (intensity power/area)? Please re- member that power, energy and intensity all refer to different things. (7 pts.) d) What is the amplitude of the electric field of the pulse? The electric field required to create an electrical discharge (a spark) in air is E 3x10s V/m. How many times stronger than this is the field you just calculated? (8 pts.)Explanation / Answer
(A) L = (35 x 10^-15)(3 x 10^8) = 10.5 x 10^-6 m OR 10.5 um
(B) f = c / wavelength
f = (3 x 10^8) / (1.8 x 10^-6)
f = 1.67 x 10^14 Hz ......Ans
t = 35 x 10^-15 s
n = (35 x 10^-15)(1.67 x 10^14) = 5.8 times
(B) P = energy / time
= (900 x 10^-6) / (35 x 10^-15)
P = 2.57 x 10^10 W
I = P / A = (2.57 x 10^10)/(pi (100 x 10^-6/2)^2)
I = 3.27 x 10^18 W/m^2
(D) I = E^2 / (2 u0 c)
E = 4.965 x 10^10 V/n
n = E / (3 x 10^6) = 16551 times
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