scholarly journals Conical diffraction: observations and theory

2006 ◽  
Vol 462 (2070) ◽  
pp. 1629-1642 ◽  
Author(s):  
M.V Berry ◽  
M.R Jeffrey ◽  
J.G Lunney
Keyword(s):  
Laser Beam ◽  
Dimensionless Parameter ◽  
Incident Beam ◽  
Asymptotic Limit ◽  
Biaxial Crystal ◽  
Conical Refraction ◽  
Dark Ring ◽  
Internal Conical Refraction

Conical refraction was produced by a transparent biaxial crystal of KGd(WO 4 ) 2 illuminated by a laser beam. The ring patterns at different distances from the crystal were magnified and projected onto a screen, giving rings whose diameter was 265 mm. Comparison with theory revealed all predicted geometrical and diffraction features: close to the crystal, there are two bright rings of internal conical refraction, separated by the Poggendorff dark ring; secondary diffraction rings decorate the inner bright ring; as the distance from the crystal increases, the inner bright ring condenses onto an axial spot surrounded by diffraction rings. The scales of these features were measured and agreed well with paraxial theory; this involves a single dimensionless parameter ρ 0 , defined as the radius of the rings emerging from the crystal divided by the width of the incident beam. The different features emerge clearly in the asymptotic limit ρ 0 ≫1; in these experiments, ρ 0 =60.

Cherenkov ray cones in crystalline media

1987 ◽  
Vol 411 (1840) ◽  
pp. 35-47
Keyword(s):  
Photographic Plate ◽  
Uniaxial Crystal ◽  
Biaxial Crystal ◽  
Normal Cones ◽  
Conical Refraction ◽  
Internal Conical Refraction ◽  
To Come ◽  
A Charge ◽  
Wave Normal ◽  
Do So

The rings of light on the photographic plate in Cherenkov emission in crystalline media are shown to come from sections of the Cherenkov ‘ray cones’ rather than the ‘wave-normal cones’. Equations are derived for the ordinary and extraordinary ray cones emitted by a charge moving in an arbitrary direction in a uniaxial crystal. It is shown that, for a range of angles between the direction of motion of the charge and the optic axis, the ray cones intersect one another, whereas the wave-normal cones do not do so. A method is given to obtain sections of the ray cones on the photographic plate from corresponding sections of the wave-normal cones emitted by a charge moving along a principal axis of a biaxial crystal. It is applied to study the Cherenkov analogue of internal conical refraction that occurs for a critical velocity of the charge such that a Cherenkov-wave normal coincides with a binormal of the crystal.

A high-speed-modulated retro-reflector for lasers using an acousto-optic modulator

2003 ◽  
Vol 81 (4) ◽  
pp. 625-638 ◽  
Author(s):  
G Spirou ◽  
I Yavin ◽  
M Weel ◽  
A Vorozcovs ◽  
A Kumarakrishnan ◽  
...  
Keyword(s):  
Laser Beam ◽  
Acoustic Waves ◽  
High Speed ◽  
Incident Beam ◽  
Bragg Diffraction ◽  
Heterodyne Detection ◽  
Incident Laser Beam ◽  
Signal To Noise ◽  
Incident Laser ◽  
Acousto Optic Modulator

We have used an acousto-optic modulator (AOM) to impose a frequency-modulated signal on an incident laser beam. The incident laser beam is focussed into the AOM where it undergoes Bragg diffraction and is then retro-reflected. The diffracted beam is also retro-reflected so that it is diffracted again by the AOM and overlaps the incident beam. The overlapped beams are frequency shifted with respect to each other. These features allow us to detect the frequency-modulated signal with high signal-to-noise ratio using heterodyne detection. Since the optical setup is simple and can be made very compact, this device may be ideal for certain forms of high-speed, free-space optical communication. We demonstrate a 1 MHz data transmission rate in the Bragg regime. We measured the acceptance angle of the device and find that it is limited only by the divergence of the focussed laser beam and the divergence of the acoustic waves in the AOM crystal. We have also studied the range of acoustic frequencies and drive power of the AOM, for which the retro-reflected beam can be detected with adequate signal to noise. PACS Nos.: 42.60.–V, 42.62.Cf, 42.62.Fi, 42.79.Sz, 42.79.Hp

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