Question

A LASIK vision correction system uses a laser that emits 10 ns -long pulses of light, each with 2.5 mJ of energy. The laser is focused to a 0.75 mm -diameter circle.

Part A- What is the average power of each laser pulse?

Part B-What is the electric field strength of the laser light at the focus point?

Express your answers using two significant figures.

Answer #1

Solution:

Part A: Average power of a laser pulse depends on the pulse energy and the laser repetition rate.

=> Pavg = Energy * frequency

= energy / time

Energy = 2.5 mJ = 2.5 e-3 J

t = 10ns= 10 e-9 s

=> Pavg = (2.5 e-3) / (10 e-9)

= **2.5 e+5 watts**

Part B:

Diameter = d = 0.75 mm

radius = r = 0.75/2 = 0.375 mm =0.375 e-3 m

Power / Area = Intensity (avg)

I avg = (2.5 e5 ) / (pi*0.375^2 e-6)

= 5.66 e11 W

I peak = 2 * I avg = 2 * (5.66 e11)

= 1.13 e12 W

The electric field strength depends on the intensity and the impedence of free space.

=> sqrt(I peak Zo) = E

Zo= sqrt ( / = 376.73 ohms

Electric field strength = **E**= sqrt [ (1.13
e12)*(376.73) ]

= **2.06 e7 V/m**

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