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Page 1 of 4 AOTF Set Up Manual Acousto Optic Tunable Filter 1. What is an AOTF? • Brief Description Figure 1. Schematic representation of a non-collinear AOTF. An AOTF acts as an electronically tunable spectral bandpass filter. It is a solid state electro-optical device with no moving parts. It consists of a crystal in which acoustic (vibrational) waves, at radio frequencies (RF) are used to separate a single wavelength of light from a broadband or multi-color source. The wavelength of light selected is a function of the frequency of the RF applied to the crystal. Thus, by varying the frequency of the RF, the wavelength of the separated or filtered light can be varied. This wavelength is independent of device geometry. • Technical Details The most common types of AOTF which operate in the NIR region use a crystal of Tellurium Dioxide (TeO2) in a so-called non-collinear configuration - the acoustic and optical waves propagate at quite different angles through the crystal. Figure 1 is a schematic representation of a TeO2 AOTF. A transducer is bonded to one side of the TeO2 crystal. This transducer emits vibrations (acoustic waves) when RF is applied to it. The frequency of the vibrations equals the frequency of the applied RF. As these acoustic waves pass through the TeO2, they cause the crystal lattice to be alternately compressed and relaxed. The resultant refractive index variations act like a transmission diffraction grating or Bragg diffracter. Unlike a classical diffraction grating, however, the AOTF only diffracts one specific wavelength of light, so that it acts more like a filter than a diffraction grating. This is a result of the fact that the diffraction takes place over an extended volume, not just at a surface or plane, and that the diffraction pattern is moving in real time. The Brimrose Corporation of America 19 Loveton Circle, Hunt Valley Loveton Center, Sparks, MD 21152-9201 USA. Phone: +1 410 472-7070 . Fax: +1 410 472-7960 E-Mail: [email protected] . Web: http://www.brimrose.com DOC: SUMan-AOTFIntro-20081212 Page 2 of 4 AOTF Set Up Manual wavelength of light that is diffracted is determined by the "phase matching" condition as described: Where ∆n is the birefringence of the the TeO2 crystal, Vα and fα are the velocity and frequency of the acoustic wave, and α is a complex parameter depending on the design of the AOTF. The wavelength of the light that is selected by this diffraction can therefore be varied simply by changing the frequency of the applied RF. As indicated in the figure, the diffracted light intensity is directed into two first order beams, termed the (+) and (-) beams. These beams are orthogonally polarized, which is utilized in certain applications. To use the AOTF as a tunable filter, a beam stop is used to block the undiffracted, broadband light and the (+) and/or (-) monochromatic light is directed to the experiment. The angle between the beams is a function of device design, but is typically a few degrees. The bandwidth of the selected light depends on the device and the wavelength of operation, and can be as narrow as 1nm FWHM. Transmission efficiencies are high (up to 98%), with the intensity divided between the (+) and (-) beams. Another useful and unique feature of the AOTF is its ability to precisely and rapidly adjust the intensity of the diffracted (filtered) light by varying the RF power. 2. AOTF Set-Up AOTF Set-Up with a White Light Source Polarized White Light Source Collimating Optics Detector AOTF Figure 2. The input light into the AOTF must be collimated with an angle equal or smaller than the acceptance angle. The polarizers can be eliminated when the input white light acceptance angle is smaller than the acceptance angle. Brimrose Corporation of America 19 Loveton Circle, Hunt Valley Loveton Center, Sparks, MD 21152-9201 USA. Phone: +1 410 472-7070 . Fax: +1 410 472-7960 E-Mail: [email protected] . Web: http://www.brimrose.com DOC: SUMan-AOTFIntro-20081212 Page 3 of 4 AOTF Set Up Manual A typical wavelength vs. frequency curve for the fabricated AOTF is shown in Figure 1. 3. Test Data 3.1 Spectral Resolution. The AOTF device spectral resolution (Full Width Half Maximum) was measured using a linearly polarized laser light. The optical set-up for this measurement is shown in Figure 3, and test results are in Figure 5. Spectral Resolution and Diffraction Efficiency AOTF Set-Up Detector Laser AOTF Figure 3. 3.2 Optical Surface Finish. The Surface flatness is λ / 10 with a scratch and dig specification of 20 -10. 3.3 Diffraction Efficiency. The diffraction efficiency was measure using the same set-up in Figure 3. with a linearly polarized laser and is shown in Figure 6. The diffraction efficiency is defined as follow: I - Is DE (%) = * 100% Io Where I is the deflected light intensity. Is is the scatter light intensity and the Io is the input light intensity. The white light response can be found in Figure 7. Brimrose Corporation of America 19 Loveton Circle, Hunt Valley Loveton Center, Sparks, MD 21152-9201 USA. Phone: +1 410 472-7070 . Fax: +1 410 472-7960 E-Mail: [email protected] . Web: http://www.brimrose.com DOC: SUMan-AOTFIntro-20081212 AOTF Set Up Manual Page 4 of 4 3.4 Electrical Text Data. The electrical text data of the AOTF device describe the conversion efficiency from the RF source into the transducer. The following test data was measures and shown in Figure 8. • • • • SMITH CHART POLAR CHART VSWR CHART RETURN LOSS CHART Brimrose Corporation of America 19 Loveton Circle, Hunt Valley Loveton Center, Sparks, MD 21152-9201 USA. Phone: +1 410 472-7070 . Fax: +1 410 472-7960 E-Mail: [email protected] . Web: http://www.brimrose.com DOC: SUMan-AOTFIntro-20081212