Since in the near future I will be a laboratory instructor, and in this lab there are many experiments which are not related to my research field, therefore I am experiencing difficulties to understand basic concepts.
One of the experiments is acousto-optic modulator so I want to better understand acousto-optics effect. So I have few questions on this topic:
(a)For which purposes used and may be used acousto-optic modulators?
(b)What are the properties of the material, which considered as effective acousto-optic material?
(c)What is the working principle of the acousto-optic modulators(Explain in the simple terms, please)?
Thank you in advance!
(a) A common application is beam steering.
(b) The material must alter its refractive index when its lattice is compressed/decompressed by an acoustic wave.
(c) An acoustic wave is launched into a material like lithium niobate, it increases refractive index in compressed regions and decreases refractive index in uncompressed regions. The acoustic wave indiuces a diffraction grating. The frequency of the acoustic wave determines the period of the grating. When an optical beam passes through a grating, it "splits" into multiple diffraction orders. That is, there will be discrete beams in different directions. Their direction depends on the period of the grating and the wavelength of the light. By adjusting the frequency of the acoustic wave, the beam can be steered in different directions.
(a) A common application is beam steering.
(b) The material must alter its refractive index when its lattice is compressed/decompressed by an acoustic wave.
(c) An acoustic wave is launched into a material like lithium niobate, it increases refractive index in compressed regions and decreases refractive index in uncompressed regions. The acoustic wave indiuces a diffraction grating. The frequency of the acoustic wave determines the period of the grating. When an optical beam passes through a grating, it "splits" into multiple diffraction orders. That is, there will be discrete beams in different directions. Their direction depends on the period of the grating and the wavelength of the light. By adjusting the frequency of the acoustic wave, the beam can be steered in different directions.
Thank you Raymond.
As I understand, there is a relation between scaterring and acousto-optic effect, but I not realy understand physics behind the scattering process.
And I have few additional clarifying questions:
d)What are the main differences between Raman-Nath scattering and Bragg scaterring?
e)What applications has each one of this scattering?
f)If there are other types of scattering related to acousto-optic effect?
_________________
g) What is the beam steering?
Thank you in advance!
In my experimental setup I'm using acousto-optic modulators and deflectors (that are pretty much the same). The sound wave generated by the compression and decompression of the crystal inside the AOM (or AOD) interacts with the laser wavelenght, thanks to the double nature of light (particle and wave). I 'm not a physist, so I can't answer rigourosly the questions about AOM. Anyhow I use this technique to move very fast the laser on the sample (they deflect literally the laser beam much faster than a galvo mirror). The laser can jump between two sites of the sample in tens of microsecond.
You can have a look at the methods part of my PNAS paper or to Sacconi et al. PNAS 2012.
Physics of he process:
There are two things happening.
First, an acoustic wave is a propagating vibration in the crystal lattice of the acousto-optic material. It is essentially a pressure wave. At the wave peaks it compresses the crystal and at wave valleys it stretches the crystal. When the crystal lattice is compresses or stretched, it modifies the refractive index. Since the acoustic wave is a wave, it creates a periodic modulation in the refractive index of the crystal. This is a diffraction grating. The period of the acoustic wave is controlled by the frequency. They are launched into the crystal usually using a high frequency source driving a piezo-electric device.
Second, a beam of light is applied to the acousto-optic crystal and passes through the grating. The grating "splits" the beam into multiple discrete beams. The directions of the beams depends on the period of the grating. As the period is changed by adjusting the frequency of he source driving the acoustic wave, the diffracted beams change direction. This steers the beam in whatever direction you wish. It is a very fast phenomenon.
You learn more about diffraction from gratings in Lecture 9 here:
http://emlab.utep.edu/ee5390em21.htm
Thank you.
I have few additional questions:
1)What is an Acousto-Optic Tunable Filter(AOTF)?
2)In what applications is it used?
Acousto-Optic effect simply boils down to the fact that refractive index of any material depends on the pressure.
Acoustic wave is basically wave of pressure (periodic modulation of pressure traveling with speed of sound inside material), so it produces periodic pattern of refractive index within material - diffraction grating.
From here on, please use whatsoever knowledge on, say Fourier optics to prove that diffraction grating scatters light into discreet orders.
AOTF is conceptually simple thing:
A nice link on application:
http://www.olympusconfocal.com/theory/aotfintro.html
Think about a thick grating. Angles of diffraction of different wavelength depend on the grating period (pitch). Now think of a grating that is produced by acoustic wave in a crystal. You can tune the period of this grating by tuning the frequency of your acoustic transducer. If you send your white light in, and collect light diffracted at some angle, you can tune its wavelength by choosing the frequency of your transducer.
It is a rather common technique.
Somewhat more interesting fact is that, since you acoustic wave is actually traveling and not stationary, the diffracted light is frequency shifted (Doppler) and the amount of shift actually depends on the diffraction angle.
I have additional question.
What is the physical explanation for the difference in the diffraction pattern between thin phase grating and think phase grating?
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