This document discusses magneto-optic and acousto-optic effects. It explains that the Faraday effect causes a rotation of the plane of polarization of light passing through a material under a magnetic field. This effect is used in optical isolators. The document also discusses using magneto-optics for computer memory storage. It then explains that the acousto-optic effect causes a change in refractive index from strain waves, creating a diffraction grating. This can diffract light in either the Raman-Nath or Bragg regimes, depending on the thickness of the modulated region.
College Call Girls Nashik Nehal 7001305949 Independent Escort Service Nashik
Β
Magneto optic devices
1. Magneto optic and Acousto optic effect
Unit-IV
Prepared by
S.Vijayakumar, AP/ECE
Ramco Institute of Technology
Academic year
(2017-2018 odd sem)
2. MAGNETO-OPTIC DEVICES
β’ The presence of magnetic field may also affect
the optical properties of some substances
thereby giving rise to a number of useful
devices
3. FARADAY EFFECT
β’ It concerns the change in refractive index of a
material s
β’ when a beam of plane polarized light passes
through a substance subjected to a magnetic
field, its plane of polarization is observed to
rotate by an amount proportional to the
magnetic field component parallel to the
direction of propagation. subjected to a
steady magnetic field
4. β’ The rotation of the plane of polarization is
given by
β’ π = πππ ββββ β(1)
β’ We can also express interms of refractive
indices π π and ππ, that is
β’ π =
π
π0
(π π β ππ)πΏ
5. β’ A Faraday rotator used in conjunction with a
pair of polarizers acts as an optical isolator
which allows a light beam to travel through it
in one direction but not in the opposite one
7. Application
β’ One potential application of magneto-optics
currently receiving attention is large capacity
computer memories. Such memories must be
capable of storing very large amounts of
information in a relatively small area and
permit very rapid readout and preferably,
random access
8. β’ Writing may be achieved by heating the
memory elements on the storage medium to a
temperature above the Curie point using a
laser beam.
β’ The element is allowed to cool down in the
presence of an external magnetic field thereby
acquiring a magnetization in a given direction.
9. β’ Magnetizations of the elements in one
direction may represent βonesβ, in the opposite
direction βzerosβ.
10. β’ To read the information the irradiance of the
laser beam is reduced and then directed to
the memory elements.
β’ The direction of the change in the
polarization of the laser beam on passing
through or being reflected from the memory
elements depends on the directions of
magnetization; therefore we can decide if a
given element is storing a βoneβ or βzeroβ.
11. ACOUSTO-OPTIC EFFECT
β’ The acousto-optic effect is the change in
refractive index of a medium caused by the
mechanical strains accompanying the passage
of surface acoustic (strain) wave along the
medium.
β’ The refractive index varies periodically with a
wavelength Ξ» equal to that of the acoustic
wave.
12.
13. β’ the acoustic wave sets up a diffraction grating
within the medium so that optical energy is
diffracted out of the incident beam into the
various orders.
β’ There are two main cases (a) The Raman-Nath
regime and (b) the Bragg regime.
14. β’ In the Raman-Nath regime the acousting
diffraction grating is so thin.
β’ The light is diffracted from a simple plane
grating such that
β’ ππ = βπ π π π π ββββ β(1)
β’ Where m=0,Β±1, Β±2, β¦ .is the order and π π is
the corresponding angle of diffraction, as
illustrated in below figure
15.
16. β’ The fraction of light removed from the zero-
order beam is
β’ π = πΌ0 β πΌ πΌ0β
β’ Where I0 is the transmitted irradiance in the
absence of the acoustic wave.
18. β’ constructive interference occurs. The
conditions to be satisfied are:
β’ Light scattered from a given grating plane
must arrive in phase at the new wavefront and
β’ Light scattered from successive grating planes
must also arrive in phase at the new
wavefront, imlying that the path difference
must be an integral number of wavelengths.
19. β’ he first of these condition is satisfied when
π π = ππ, where π π is the angle of diffraction.
The second condition requires that
β’ π π π ππ + π π π π π = ππ ββ
β’ With m=0,1,2β¦. The two conditions nare
simultaneously fulfilled when
β’ π π π ππ = π π π π π = ππ 2ββ ββ β(2)
20. β’ scattering only takes place when m=1. This is
shown in above figure. The equation called Bragg
angle ΞΈB becomes
β’ π π π π π΅ = π 2ββ βββ β(3)
β’ At the Bragg angle, Ξ· is given by
β’ Ξ· = sin2
Ο 2β βββ β(4)
β’ Where π = 2π πβ (βππ ππππ π΅β ), in which βπ is
the amplitude of the refractive index fluctuation,
L the length of the modulator