The high frequency generator (F = 80 MHz) generated in the strong wave acoustic wavelength Λ = C / F where the acoustic velocity is C = 4200 m / s therefore Λ = 52.5 m. The laser beam is diffracted by the ultrasonic wave. Here we are dealing with the Bragg diffraction since the diffraction angle is such that we can not neglect the diffracting three-dimensional environment (ie in inside the crystal, the deviation of a light ray is such that it will through several periods of network noise, see below **). For a maximum intensity is diffracted in the order 1, we must direct the cell to have an angle of incidence equals the angle of Bragg:
θB = λ / 2Λ = 6.0 mrad = 0.35 °
The laser beam is then deflected a corner:
2 θB = λ / Λ = 12 mrad = 0.69 °
Network ** 2D or 3D? : There are in general two types of diffraction, following a laser beam diffracted or not returned in an adjacent layer of a thick network: In the 1st case it is Bragg diffraction (3D network) in the second case, we speak of the Raman-Nath (2D network). On moves from one regime to another for a network of thickness Ec. In the experiment described in the leaflet "Optical and acoustic,the light was diffracted by a 2D network (the frequency being much lower, the period is greater: Λ / 2 ≈ 0.7 mm
Thus Ec ≈ 1m). Here against the network period is very short short (Λ / 2 = 26 m,thus Ec ≈ 1 mm): we are here in a Bragg regime.
Measuring θB:
Adjust the orientation of the cell so as to have a maximum intensity in the order 1 (use a photodetector for further details). At a distance 245 cm from the cell, separating the two beams main (order 0 and 1) is 3.0 cm. We therefore measured
θB an angle 2 = 3.0 / 245 = 12 mrad in agreement with the value calculated above.
Measuring the effectiveness of the modulator:
In the absence of ultrasonic wave is measured by such a power P = 3.30 mW.
With the ultrasonic wave, the power measured in the order 1 is P1 = 2.40 mW.
Efficiency is therefore P1 / P = 2.40 / 3.30 = 73% (max efficiency = 85% according to the notice).
Modulation:
The amplitude of the ultrasonic wave can be modulated by an external (GBF 0-5 V connected to the "mod
input generator HF) frequency fmod (15 MHz max), which has the effect of the intensity diffracted in
Order 1. In this case, the HF generator switch must be set to "OFF" (in the "cw", the HF signal is not modulated). To demonstrate an application telecoms ", the adjustment can be made from the signal output of a radio (see montage below).
• Other applications: ACOUSTO-OPTIC DEFLECTOR:
By modulating this time not the amplitude but the frequency F of the HF signal, it changes the wavelength Ultrasonic Λ therefore the angle of diffraction. This allows to control the deflection of a laser beam by a signal electric. This effect is used in scanners for example. This experience can not be achieved with our hardware because we do not currently supply a variable frequency RF or deflector acoustooptique.
Indeed, for applications to the deflection, we use acousto-optic cells that operate on a acoustic mode of low speed of propagation, which allows for a low acoustic wavelength Λ and a wide angle of deflection (typically: C = 500 m / s therefore Λ m = 6 to F = 80 MHz and Δθ = 50 mrad at λ = 633 nm for a variation of the acoustic frequency ΔF = 40 MHz). For applications to modulation, is preferred by against using a sound speed (C = 4200 m / s) because the time between the laser beam in the
acoustic wavelength Λ, which limits the high frequency performance of the modulator, with typically 160 ns per mm laser beam to the modulator, 1μs/mm against the deflector (source: AA Optoelectronic).
No comments:
Post a Comment