534phys - PowerPoint PPT Presentation

Title: 534phys


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Q(1). (a) The total power emitted by a CW
laser in the TEM00 mode is 10 W. If the focused
spot 1/e2 radius is 0.2 mm calculate the
irradiance (power density) at the spot centre.
(b) The beam radius (1/e2) for a HeNe laser
(running the most common line) TEM00 mode is
0.5 mm. Determine the spot size at distances of 1
and 500 m from the waist. (c) If the spot
radius is 1.5 cm for a HeNe TEM00 mode beam
entering a lens of f 12.5
cm Calculate the focused spot size. Q(2).
(a) What are the material and laser parameters
that affect laser processing? Describe each one
briefly. (b) Mild steel is to be welded with a
multi-kilowatt CO2 laser. Using the model of
Swift- Hook and Gick, estimate the power
required to produce a weld of 2 mm width and
4.5-mm depth at 1 m/min. k 0.4 W/cm K , k
0.09 cm2/s , Tm 1809 K Q(3). (a) A
multimode optical fiber has a core n1 1.50
and a cladding refractive index
n2 1.47. Determine (i) The critical angle
at the core-cladding interface (ii) The
numerical aperture (iii) The acceptance angle
in air for the fiber. (b) Two plane waves are
incident on a plan e one normally, the other at
an angle q (see Figure below). (i) Derive
an expression for the spacing of the fringes
formed on the plane. Page 1 of 3


Dr. A. M.
Azzeer
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(ii) A holographic optical el ement is produced
in which two plane wavefronts are used to
produce a grating. The wavelength of light used
is 0.514 mm. If the resolution of the film used
is limited to 500 lines/mm, what is the maximum
angle between the two beams which can be used in
the recording?
Q(4). (a) The laser Doppler velocimetry is
based on observing of the intensity modulation of
light scattered by moving particles. The
intensity modulation results from the
interference of scattered waves with
Doppler-shifted frequencies. A dual-beam version
of the method is particularly well suited for
velocity measurements. This method can be
explained as follows If the two incident beams
are coherent, then their intersection will
result in the formation of a set of interference
fringes in the plane of the beams at the
crossing point. A particle passing through the
alternate bright and dark bands will scatter
light whose irradiance will vary with a rate
which is proportional to the particle velocity.
Derive an expression for the particle velocity.
What is the advantage of the dual-beam method?
(b) Assume that a modulation frequency of 65 kHz
is detected in a measurement of the velocity of
a particle stream. The distance between the beams
at the first lens is 50 mm and the focal length
of the lens is 0.15 m. The laser wavelength is l
632.8 nm. Calculate the stream velocity?
Tfy-44.152 LASER TECHNOLOGY OPTICS Exercise 11
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