Variable optical attenuator using double acousto-optic modulator

A novel variable optical attenuator device is proposed and some of the potential application is pointed out. It relies on a double acousto optic modulator set up and allows one to attain dynamic range as high as 60 dB. Such feature steams out of the well known modeling of acousto optic devices, mostly based on the coupled wave analysis. Furthermore, is shown that the proposed device may exhibits sub microseconds access time combined with very good thermal and mechanical stabilities. The proposed device is compared with early set-ups for variable laser attenuation. . Introduction The acousto-optic, AO, technology is a far mature way to optical control in signal systems applications [1]. Laser systems with Q switch, radio frequency spectrum analyzer with deflectors, video and signal processing with modulators and photometric and spectroscopy with tunable filters are examples of the wide range one can use acousto-optic devices, AOD. In this paper we concentrate the attention on the acousto-optic set ups and devices for variable optical attenuation. A general discussion about the current technologies for optical variable attenuation is initially presented. The AO set ups proposed by early authors are discussed pointing out their limits and at the end it is presented a new device for the optical variable attenuation. Variable optical attenuation play an important role in high power lasers, beam forming antenna systems, equalizing output signal of optical amplifier and WDM systems [2]. For such applications the most important parameters are the dynamic range and the time response or access time [3]. The variable optical attenuators are usually mechanics, thermo-optics and microelectromechanical, MEMS. The mechanics attenuators are commonly based on sliding plates and the response time is around seconds. The thermo-optics present a better response time but with moderate dynamic range [4]. The MEMS type overcome some limitations of the mechanics and thermo-optic attenuators but it is hard construct such attenuators with response time lower then microseconds. In recent years it was proposed the use of AO set ups for optical variable attenuation enabling 60db dynamic range and response time of the same order of MEMS devices. Acousto-optic set ups for variable optical attenuation As mentioned before it will be presented at this section the acousto-optic set ups working as variable optical attenuators. Before discuss directly the VAO it is pointed out the most important characteristics of the acoustooptic modulator. On the Figure 1 it is shown a general scheme for the acousto-optic diffraction. The acousto-optic device, AOD, is constructed from a crystal block. A piezoelectric transducer is bonded on the crystal block. When a radio frequency is applied to the transducer it vibrates and an ultrasound propagating wave is generated inside the crystal block. A laser beam propagates as shown on Figure 1 is named incident wave. Due to the acousto-optic interaction, between the incident laser beam and the ultrasonic wave, a second laser beam is generated, the diffracted beam. The angle B is defined by the material properties, the optical wave length, , and by the radio frequency, fc. The optical intensity of the diffracted laser beam is a function of the radio frequency power according the mismatch factor of the interaction becomes null [5]. It is important explain that the frequency of the incident wave,ν, suffer a shift equal to the value of the fc. If the shift is positive or negative depends on if the XXIX ENFMC Annals of Optics 2006 incident laser beam is propagates, under the B angle, toward the transducer or going away. For a positive shift the diffracted laser beam is named +1 order. For a negative shift the diffracted beam is named -1 order. The last characteristics of the AO interaction determine the AO set ups for optical beam attenuation as will be show. On the Figure 1 it is show an incident laser beam propagating toward the transducer. Figure 1: Acousto-optic modulator and interaction. Adjusting the output level of the signal generator one can control the ultrasound wave power and in its turn the diffracted laser beam intensity. That is the key idea to VOA with AOD. The first set up designed to be a variable optical attenuator is displayed on Figure 2. It is based on two AOM [6]. For the first modulator the incident laser beam is propagating toward to the transducer and for the second it propagates going away it. Figure 2: Two AOD variable optical attenuator As one can see the +fc frequency shift at the first AOD is cancelled by the -fc shift at the second. In this way the frequency of the incident laser beam do not change. The above set up, proposed early by other authors, use two Signal generator AOD Incident laser beam Diffracted laser beam, +1 order Piezoelectric transducer Crystal block θB θB XXIX ENFMC Annals of Optics 2006 lens in the way the diffracted beam position do not depends on the fc value. The authors give a insertion loss of 3.5dB for such set up and a access time equal 0.2us. On the figure 3 one can see an early proposed set up [7] based on the double diffraction as the first one but with only one AOD. The key to reduce the number of AODs is use mirrors to bring the diffracted laser beam to be a new incident laser beam. Figure 3: set up based on a unique AOD and double diffraction. The first interaction gives a diffracted laser beam with a frequency shift equal +fc and the second interaction, after de mirrors reflections, gives a –fc frequency shift. At the single device attenuator on the Figure 3 the sum x1+x2 will determine the time delay or the distance the ultrasound wave has to travel inside the AOD. For that VOA the authors measured a total insertion loss equal to 2.5 dB and a access time around to 3 s. The half wave plate, HWP, was used to reduce the polarization dependence loss, PDL. One can now consider the two early acousto-optic AOVs shown before in the sense of its limits and characteristics. The double device attenuator need lens add this will bring difficulties for the practical mounting demanding precise focusing of those lens which in its turn will bring a physical extension due to the focal distances. Those aspects will reflect also on the mechanical stability with the air turbulence and temperature variation effects. Proposed configuration To obtain a compact VOA with a reduced insertion loss and microsecond access time it is proposed a double AOD. Such device will enable also the construction of a VOA with a reduced sensibility to the environmental effects as air turbulence and temperature variation. On the Figure 4 it is displayed the double AOD. In fact the proposed device is a combination of the two devices set up and the two interactions set up. The combination in the same device brings some advantages. First, the size is reduced. The environmental effects as air turbulence on the laser free way propagation path decrease. The mechanical instabilities with deleterious effects on the optical alignment are reduced too. Second, the number of optical interfaces reduces. It is so important if one take in account the insertion loss. For the proposed device, considering the optical characteristic of the two interactions set up one obtain something around 1.2 dB. For the dynamic range with the same modulator of the two interactions set up one can obtain 60dB. XXIX ENFMC Annals of Optics 2006 Figure 4: Double AOD with Third, the access time depends on the point the laser incident beam reach the ultrasound wave inside the crystalline block. It could be as small as possible if one reduces the equivalent distance to x1+x2 at the Figure 2. With such characteristics the access time can be of the same order of the first set up, sub-microseconds. ConclusionsIt was presented the general characteristics of the VAOs. The discussion is directed to the acousto-optictechnology and two set ups proposed by other authors were discussed. A new device to overcome the limitationsof the discussed configurations was proposed and its most relevant characteristics pointed out. The proposeddevice can be used as master intensity control in high power pulsed lasers and WDM equalizing systems. References[1] Das, P.K., DeCusatis, C.M. Acousto-Optic signal Processing. Artech House, Norwood, 1992.[2] A, Q.L.,Lin A. C.H., Lyons, E. R., Lee, H. P. An Efficient All-Fiber Variable Optical Attenuator viaAcoustooptic Mode Coupling, IEEE Photonics Technology Letters, vol. 14, NO. 11, November 2002.[3] Kaman, V. Yuam, S., Zheng, X.,Pusarla, C., Helkey, R.J., Jerphanon, O., Browers, J.E., OpticalPerformance of Variable Attenuation in Large-scale 3-D MEMS-Based Photonic Cross-Connected, IEEEPhotonics Technology Letters, vol. 17, NO. 9, September 2005.[4] S.-S. Lee, Y.-S. Jin, Y.-S. Son, and T.-K. Yoo, “Polymeric tunable optical attenuator with an opticalmonitoring tap for WDM transmission network,” IEEE Photon. Technol. Lett., vol. 11, pp. 590–592, May1999.[5] Molchanov, V. Y. , Maglidich, L.N., Acoustooptic devices and their applications, Gordon &Breach;publishers, New York, 1989.[6] Riza, N. , Yaqoob Z. Submicrosecond Speed Variable Optical Using Acoustooptics, IEEE PhotonicsTechnology Letters, vol. 13, NO. 7, July 2001.[7] Mughal, M. J., Riza,N. Compact Acoustooptic High-Speed Variable Attenuator for High-PowerApplications, IEEE Photonics Technology Letters, vol. 14, NO. 4, April 2002.[8] Sapriel, J., Charissoux, D. Voloshinov, V, Molchanov, V.Y., Tunable acousto-optic filters and equalizersfor WDM applications. Journal of Light Technology, Vol. 20, No. 5, May 2002.Incident laser beam