The following tutorial will serve as an outline of the fundamental elements involved in the

Transcription

1 ACOUSTO-OPTICS TUTORIAL G. Ivtsenkov, Ph.D. & V. Pachkov, Ph.D. Light Manageent Group Inc. Research & Developent Burlington, Ontario, Canada INTRODUCTION THE ACOUSTO-OPTIC SWITCH PHYSICAL FUNDAMENTALS OF AN ACOUSTO-OPTIC CELL APPLICATIONS OF THE ACOUSTO-OPTIC DEVICE CONCLUSION REFERENCE. INTRODUCTION The following tutorial will serve as an outline of the fundaental eleents involved in the developent of Acousto-Optic switching technology. Acousto-Optic switching technology is based on the anipulation of light through the use of sound waves and a crystal. The benefits of such technology are nuerous and include switching speed, energy efficiency, output capacity and negligible aintenance. LMG has invented and developed four proprietary products, which are its Acousto-Optic (AO) Switch; its Multi-Channel AO Deflector; its Ultraviolet (UV) Scanner; and its Red-Green-Blue (RGB) Laser Projection Syste. Scientists at LMG s Research & Developent facility are continually developing these products further.

2 . THE ACOUSTO-OPTIC SWITCH Currently, research is being conducted in the field of developing the acousto-optic crossbar switch [reference ]. This switching idea is described in [refs.,]. The essence of the idea is thus: in a 4 4 configuration optical switch, light is delivered to the switch by input ports (fibers) arranged in a linear array (4 fibers). Light fro each input fiber is colliated along the optical axis. Each colliated bea is incident on one channel of a ulti-channel acousto-optical cell (4 channels). The acoustic wave deflects off the incident light at an angle that is proportional to the addressing frequency. The radio frequency (rf) signal is tuned to provide the proper deflection angle. Light reaches the output fiber (one of 4 fibers) by passing through an optical syste. The optical syste then directs the bea fro each vertical fiber position onto the horizontal output axis. It is point-to-point architecture the rf signals are supplied to each channel by a single prograable rf source. This approach provides a connection for each input fiber with each output fiber by eans of a prograable rf. An advantage of this switch is the possibility of connecting several input channels (in this case 4) with the sae nuber of output channels at the sae tie. There is, however, a liitation with this switch. It is probleatic to increase the quantity of input channels and output channels in this configuration. It is defined ostly by the configuration of the acousto-optic device and its optical schee. Fro our standpoint, the idea of using an acousto-optic cell for creating the switch (as in the case described above) is ore attractive. The difference is that a colliated light bea is deflected on two coordinates by eans of two acousto-optic cells. This approach has the sae advantages as entioned above, as well as having the ability to increase the quantity of output channels. This approach will now be considered. 3. PHYSICAL FUNDAMENTALS OF AN ACOUSTO-OPTIC CELL

3 An elastic-optical effect is a refraction index variation under a echanical stress action. This effect is well known and described in any publications. In our description we follow [ref. 3]. The echanical stress is created by eans of an acoustic wave. The refraction index variation of the aterial occurs under the wave action. The refraction index value odulation is defined with the acoustic power supplied. The photo-elastic phenoena are in all aggregate states of atter, including a crystal state at any crystal syetry class (crystal lattice). The atter acquires the property of an optical phase grating for a period equal to the acoustic wavelength. Acoustic wave propagating in an optical transparent substance causes a periodical variation of the refraction index n: n( y, t) = n + n sin( Ky Ωt) The diffraction period grating is equal to: Λ- is the ultrasonic wavelength. Λ = π K If a light bea with aperture a uch greater than Λ is coing through the substance, the diffraction pattern is created on output: (,, ) E x y t = E C ( x) exp j[ ω t ( k = x x + k y y) n( y, t)], C (x) - is a relative wave aplitude of the th axiu, ω - is the wave frequency; k x + k y = k = ω / c. The aplitudes are described with a differential equation syste: 3

4 dc dx C exp( jη x) q + C exp( jη ) () = q + x q = nk k ; = k + k n /, x Also, the conditions are observed for any : ω + η () (, x, x ) = ω + Ω; nk +, y = nk, y + K (3) Equation (3) defines the light wave frequency and its propagation direction for every axiu of diffraction. Usually, ω >> Ω, k >> K; thus the diffraction angles are sall and Equation () looks like: q πλ n = q ; η πλ [ θ i + ( + / ) λ / nλ], θ i - is the light wave incident angle on the plate x =, λ - is the light wavelength in a vacuu. The value n of light with a polarization vector d α is associated with a stress aplitude u in the acoustic wave with equation: 3 n = / n b b αα are the indices deterined fro the analytical equations that describe a photo-elastic effect. αα u, The indices are the eleents of the 4 th rank elastic-optical tensor that associates the stress tensor coponents with the substance refraction index indicatrix. A kind of elastic optical tensor for all syetrical classes of isotropic aterials is presented in the reference literature. The kind of light diffraction on acoustic waves depends on the paraeter: 4

5 Q = λ l / Λ There is the Raan-Nath diffraction of Q with any diffraction axius. There is the Bragg diffraction in the opposite case of Q. With this condition, the light incident angle ust be close to the Bragg angle θ B : sinθ v - is the acoustic wave velocity, f - is the acoustic wave frequency. n = λf / nv = λ / nλ B, At the Bragg diffraction, the pattern consists of two axius: the first and second orders of the diffraction. The light intensity in the first order of the diffraction is defined with the equation: q l I = I sin q + η. (4) q + η If η =, i.e. θ i = θ B, then Equation (4) ay be transfored to:. I π = I sin M P ac λ l l (5) M / ρv 6 3 = bαα nα - is the acousto-optic quality of the substance; ρ - is the substance density; l and l - are the ultrasonic bea cross-section diensions; P ac - is the acoustic radiation power. There is a possibility of diffracted light controlled by eans of the acousto-optic power variation fro the last expression. Equation (5) shows the odulation characteristics of the acoustic light aplitude odulator. A switching tie is liited with sound propagation through a light bea aperture τ 5

6 = a/v. Because of the tie required, the new acoustic wave aplitude size is established on all bea apertures. As follows fro this discussion, the acousto optic cell is a unique eleent of the scanning light bea. According to Equation (3), the th diffraction axiu direction θ is defined with the equation: sinθ λ / nλ = λ / nv =. The axiu diffraction is scanned after the angle at the ultrasound frequency variation. Usually, the first order of diffraction is used for scanning purposes. At the frequency variation in band f, the diffracted bea is deflected on an angle: θ d = λ f / nv The Bragg diffraction is used for scanning; however, it is selective. At the fixed incident angle θ I, the diffraction occurs in the liited band f near the frequency f = nvλ sinθ. i The band f ay be calculated fro Equation (4), assuing that the diffraction efficiency is sall, i.e. I /I <<, thus, Equation (4) is changed to appear: I I q 4 sin [( πl / Λ)( θ θ )] i B = l (6) [( πl / Λ)( θ i θ B )]. The incident angle deviation tolerance at that intensity varies no ore than 3 db, equal to θ I - θ B =.89Λ/λ. Hence, the axiu scanning band is / f =.78nv λlf. The diffraction presented concerns the substance index refraction variation and describes only isotropy diffraction. There is another kind of diffraction in anisotropy aterials that is related to the axes rotation of the light polarization in the substance under the acoustic wave action. This effect allows an increase in the scanning band. At the anisotropy diffraction, the light wave ay be represented with Equation () in the interaction region. In this case, however, all even diffraction axius have a 6

7 polarization coponent that coincides with the incident wave polarization; all odd diffraction axius have an orthogonal polarization. Siilarly, the isotropy diffraction frequencies of the wave are diffracted and satisfy the equation: However, a propagation direction is different: n ω = ω + Ω, + α k +, y = nβ ky + K,. : α,β =, и α β. The difference between n and n is sall. However, this difference significantly changes the light and acoustic wave interaction geoetry. The diffraction equations deterining C coincide with Equation (), q and η ay be found fro the equation: q k n n = bu ; η = nβ k +, x nα k, k n + n, x and b ay be calculated fro equations that describe the photo-elastic effect. The incident angle θ i and the diffraction angle θ d ay be deterined by eans of a wave vector diagra. It ay be shown that: sinθ i λ v = f + ( n ) i nd n iv fλ (7) sinθ d λ v = f ( n ) i nd nd v fλ. At the isotropy diffraction n i = n d, so Equation (7) is transferred, thus: sinθ = sinθ = λf i d / nv. At the anisotropy diffraction n i n d ; coonly, θ i θ d. 7

8 In reality, the interaction zone width l is always liited and the sound wave has a finite divergence. Because of this divergence, it is possible to define light scanning in the finite band as f. At the isotropy, light diffraction on a sound wave with a divergence ϕs, the frequency deviation axiu f at the Bragg diffraction is observed; therefore, the scanning angle θd and the resolution N are equal: For a unifor bea with a rectangular cross-section: f = nλ - vϕs; θd = ϕs; N = ϕs/ϕl. ϕ s =.89Λ/l =.89v/lf. The resolution at the Bragg anisotropy diffraction is inversely proportional to the ultrasound frequency. Hence, the resolution axiu ay be achieved in a diffraction area between the Raan- Nath and Bragg diffractions. At the anisotropy diffraction, the acousto-optic interaction geoetry is possible with significantly ore wide scanning bands being achieved. This eans that θd and N ay be increased. This is caused by a different dependence of θ i and θ d against f, copared to the isotropy diffraction. At a defined condition (near the incident angle iniu θ i and an ultrasound wave propagation direction orthogonal to the optical axis of the one-axis crystal), a negligible acoustic bea divergence ϕs ay provide scanning after the angle θd >> ϕs. The dependence between the scanning angle and the ultrasound width at the anisotropy diffraction looks like: θ d.7 λ / nl. The resolution advantage at the anisotropy diffraction copared to the isotropy diffraction is equal to: N an / N is =.5 f λl / nv. The advantage is that the higher the ultrasound frequency, the longer the interaction zone. Deflector resolution also depends on input light bea aperture a. This value is deterined with a syste rate. If a high scanning speed is not required, this value depends on optical atter quality and the ultrasound attenuation. The attenuation causes the diffracted light intensity to decrease and the light 8

9 divergence to increase. The higher the ultrasound frequency, the ore light divergence. This is because the absorption index is usually proportional to f. The absorption tolerance is near 4 db at the aperture length. Deflectors ay carry out light bea scanning according to any rule. The iniu switching tie that is necessary for the bea to switch fro one position to another is τ = a/v. Thus, the deflector is capable of scanning (B = τ - ) eleents in a rando scanning ode. However, at the linear ultrasound frequency variation, a higher scanning rate ay be achieved. The advantage is achieved at the expense of rando ode scanning loss, and in this case, the scanning ode is linear. At the linear scanning ode, the light bea switches to the next position for a tie: t = ( στ ), σ = f / T -is the ultrasound frequency rate variation; T - is the frequency period odulation. The size of t is στ less than tie τ. A one-tie scanning string is equal to T, and the less tie the light scans at linear ode, the faster scanning occurs. However, after every discrete frequency jup there is a transitional process that has tie τ in the syste; thus scanning is accoplished within part of the frequency period only. The axiu quantity of pixels that are scanned per unit of tie is achieved at T = τ and is equal to B ax = f/4. It is N/4 ties ore than in the sae deflector with an arbitrary scanning ode. The ain requireent of the crystal atter is the crystal s capability to create a regular structure of the variable refraction index. For this reason, the ost iportant characteristics of the atter are the quality, M, and the ultrasound attenuation. The grating created in the crystal provides: uch larger deflection angles; a wider band of refraction index variations; and a wider ultrasound frequency band peritted by a crystal (and uch better crystal quality). Also, an attribute of crystal quality is the tolerance to high density of input power radiation during a long period. 9

10 The reliability of an acousto-optic device can be investigated here; initially, with regard to scanners assigned for high-power laser radiation control. The acousto-optic substance ost attractive is paratellurit (TeO). The highest value of M ay be achieved thanks to the low speed of the transverse ultrasound waves. However, the absorption index is significant at the frequency MHz, at 3 db/s. Therefore, another cut of crystal with a longitudinal acoustic wave on the z axis is preferable. The basic schee of the acousto-optic device will now be considered (shown in Figure ). Usually, a crystal is used as the basic eleent of an acousto-optic device such as the deflector, scanner and switch. The crystal atter is optially selected and processed for a required radiation wavelength (ultraviolet, visible or infrared band). The physics of a light bea and an acoustic wave interaction have been described above. Ultrasound is injected into the crystal by eans of a special eleent (transducer) fixed on the crystal surface. The role of the transducer is to transfor a periodical electrical signal (radio frequency signal rf) into a sound signal. The sound absorber is fixed on the opposite surface of the crystal to avoid interference fro the reflection of a sound wave. This basic idea allows us to design devices that use colliated light bea control such as switches and scanners. - input bea; - acousto-optic crystal; 3 - regular structure created with ultrasonic wave; 4 - transducer; 5 - deflected bea; 6 - output bea passed without transforation (zero order); 7 - ultrasound absorber Figure.

11 In the switch used for fiber optic applications, optical signals are redirected fro one or several input fibers into another output fiber. The output fibers are usually joined in a bundle. In general, the switch uses the optical connection of input and output fibers that are controlled by a special progra that controls the switching of the light bea. The scanner uses a scanning light bea in the sae anner. - fiber; - input foring optical syste; 3 - st AO crystal (x-diension); 4,6 - transducers; 5 - nd AO crystal (y-diension) 7 - output foring optical syste; 8 - output atrix of the fibers Figure. Figure shows a configuration of the acousto-optic device that can be a basic design for a switch or a scanner. In the scanner, instead of the output bundle (8), a target ay be used and an optical receiving channel ay be added for analyzing a target pattern.

12 The iportant characteristic of the switch is the paraeter that defines the optical losses. If the device is optially designed and adjusted, these losses are due to reflection losses and losses at the light bea injection into an output fiber. The discrepancies between the light bea and the output fiber apertures cause the losses. Our research with the switch prototype shows reflection losses of 4 db. The losses attributed to the light bea injection into an output fiber cannot be decreased to less than.5 db using this prototype 4. APPLICATIONS OF THE ACOUSTO-OPTIC DEVICE There are significant requireents arising in inforation exchange in recent years. Based on these needs, the rate and volue of transitting data have increased significantly. Fiber optic systes now play a significant role in fulfilling these needs. Re-direction of the data strea is the ost iportant proble that ust be solved when developing the fiber optic syste for data transission. Optoelectronic and optical switches ay be used as the apparatae that allow re-direction of the optical data strea. Aong the any types of optical switches, the acousto-optical switch is preferable on several paraeters. Such advantages include its switching tie and the ability to design the device with, outputs and ore than one input. These switches ay be applied to telecounications and coputer networks. As previously entioned, the basic configuration in Figure is the basis for the scanner designed as visible at the UV band. Scanners of this kind are used in the fields of printing and designing, as well as surface inspection including icrochips and printed plates. UV scanners can be used for any diverse applications. An apparatus of this kind ay be applied in areas such as photolithography, biotechnology, icroscopy, icroachining and nanotechnology using laser bea control. Application of UV lasers in photolithography is due to the developing aspiration of increasing the icrochip degree integration. The analysis conducted shows that iproving the photolithography apparatus follows these ain directions: iproveent of project optical systes in the field of iage quality; increasing the nuerical aperture; application of new substances, etc.; developent of UV lasers and applications with a shorter

13 wavelength and accopanying optic developent; and developent of systes with non-axis optics. There are other directions that involve increasing the quantity of processed surfaces. As the analysis shows, the scanner ay be applied to bea scanning in an optical projection syste instead of coplicated optical echanical equipent of the bea foring. However, the scanner ust provide: the required accuracy of bea positioning (3-5 n); the necessary bea size and the power density spread after a cross-section (unifority is desirable); and the saple square (required covered) and the exposition required in every point of the saple. At the sae tie, the scanner ust provide the syste output (no ore than 46 surfaces processed per hour a iniu provided by existing systes). The next area for application of the UV scanner concerns the different aterials surface processing including glass, quartz and different kinds of polyers used for edical purposes. Usually with all technologies including icrodrilling, the approach is siilar to photolithography, (i.e. the application of different kinds of foring optical systes called bea delivering which asks the patterns that for on the surface). The processing of deep sets by the exposition and a pattern configuration are the results of changing asks. This operation is executed by coputer control. Clearly these technologies are coplicated and expensive; use of the UV scanner could ake the uch ore affordable. 5. CONCLUSION The study of optical switches shows that there are a nuber of switches, each one based on different fundaentals of physics. The ost advanced direction is a switch based on icro-irrors. However, this kind of switch has liitations caused by using echanical parts. With the exception of the acousto-optical switch, ost switches are in the initial stages of developent and their paraeters are uch worse, (i.e. either poor switching speed or the sall nuber of input/output channels). Acousto-optical switches are all-optical and have several advantages that include: 3

14 No oving echanical parts Fluent electronic control High switching speed Reliability Sall optical losses Many input channels and output channels 6. REFERENCES. Glosselln, S., Sapriel, J.. Systèe de Coutation Optique Spatiale à Dèflecteur Acousto- Optique Multicanaux. European Patent EP A.. El Dahdah, N.. Optiisation des Coposants d un Coutateur Acousto-Optique pour Telecounications. D.E.A. Optique et Photonique, Universitè Paris XI, Orsay, Stage de Novebre 999 à Juin : 3. de Zwart, G., Van den Brink, M., George, R., et al Perforance of a Step and Scan Syste for DUV Lithography. SPIE Syposiu on Optical Microlithography. Santa Clara, CA. 4. Pressley, R.J. Ed. 97. Handbook of Lasers With Selected Data on Optical Technology. Cheical Rubber Co. Cleveland. 5. Stephens, W.E., Huang, P.C., Banwell, T.C., Reith, L.A., Cheng, S.S. 99. Deonstration of a Photonic Space Switch Utilizing Acousto-Optic Eleent. Optical Engineering. 9(3): Harris, D.O. 99. Multi-Channel Acousto-Optic Crossbar Switch. Applied Optics. 3(9): McCarthy, D.C.. Photonic Switches: Fast, but Functional? Photonics Spectra. March :4. 8. Harris, D.O. and VanderLugt A. Acousto-optic Photonics Switch. Optics Letters, 989, Vol. 4, No. :