Since the foundation of the University of Gdańsk in1970, a research group in the Institute of Physics hasstarted to arrange laboratories, and one of them isthe ultrasonic laboratory, where acousto-optic (AO)methods were implemented. At that time acousto-optics, already having more than 40 years history(since Brillouin’s (1921) [1], (1933) [2] theoreticaland Debye and Sears’s (1932) [3], and Lucas and Bi-quard’s (1932) [4] experimental independent papers),outlived its bloom after the invention of the laser. Be-fore that a lot in acousto-optics was already discov-ered and done. However, laser techniques requiredefficient methods for processing coherent lightbeams, and AO devices (deflectors, modulators, cor-relators, etc.) were found to be very useful tools forthat purpose. Intense growth of research and appli-
cation development in acousto-optics has followed.The situation inspired us to undertake our own in-vestigations in the field, too. Previously during the7th International Congress on Acoustics (ICA) in Bu-dapest in 1971 our group presented a paper on AOinteractions in fluctuating media [5], and therewas an opportunity to meet some interesting peoplefrom around the world and to start mutual coopera-tion in the field of acousto-optics. We should mentionsuch names as Robert Mertens from the University ofGent and Oswald Leroy from the University ofLeuven-Campus Kortrijk, Belgium; Ranier Reiboldfrom ITB, Germany; Daniel Sette and Adriano Alippifrom the University of Rome, Italy; Walter Mayerfrom Georgetown University, Washington, DC,USA; Sergey N. Rżewkin and Vladimir A. Krasilni-kov from Lomonosov University, Moscow; RaymondW. B. Stephens from Imperial College, London;and others. Cooperation occurred on a more or lesspermanent and fruitful basis after that. There were
several international meetings such as the next ICACongresses, Ultrasonics International, the Interna-tional World Congresses on Ultrasonics, and otherswhere mutual links could be developed more andmore, and new, younger scientists joined in the coop-eration. So, it was not strange that an idea to orga-nize special international meetings as SpringSchools on Acousto-Optics and Applications devel-oped, and we decided to start such meetings. Sincethe 1st Spring School organized by Gdańsk Univer-sity in 1980 the international links among the acous-to-optics (AOs) community have become stronger andstronger and have tightened the bonds of friendship.The enthusiastic group of acousto-opticians at the
Gdańsk University (Maria Borysewicz, Marek Kos-mol (✝), Piotr Kwiek, Bogumil Linde, Anna Markie-wicz, Iwona Wojciechowska, Anna Sikorska from thebeginning, later Grzegorz Gondek, Tomasz Katkows-ki and recently Ireneusz Grulkowski, Dawid Jan-kowski) thanks those Schools (next regularlyorganized every three years) had the opportunityto exchange scientific experience and to continue or-ganizing the events until now (the Jubilee 10thSchool, Gdańsk, 2008) and to engage in mutual suc-cessful international cooperation with several AOsgroups and centers.The themes of the schools covered several topics
from fundamental theories to achievements in ex-periment and technological applications as well asdesigning AO devices. During the 28 year historyof the Schools as permanent scientific meetings,many contributions were provided to the solutionof fundamental problems of ultrasonic light diffrac-tion (ULD) phenomena in the Raman–Nath, Bragg,and intermediate regimes and to such topics as spa-tial and temporal modulation of light by continuousand pulse ultrasound, AO applications in optical ho-lography, AO steering devices for signal processing,AO lattice dynamics and superlattices, materialsstudies, and others were developed; also, photoacous-tics and AO and acoustic microscopy were repre-sented at the Schools.
2. Cooperation with the Belgium Group
In 1975 at the Symposium on Acoustics and Spectro-scopy, which took place in Gdańsk for the OpeningDay of the new physics building at the Universityof Gdańsk, among others Leroy [6] presented his pa-per on the theory of diffraction of light by adjacentultrasonic beams. These theoretical predictionshad so much potential that our experimental AOgroup decided to start with adequate experimentsto confirm the theory. We already were acquaintedwith AO achievements of the well-known Mertens’team in Gent working on the theoretical backgroundof the ULD phenomena since 1946, and Leroy be-longed to the team. Leroy initiated and has contin-ued research on UL interaction at the KortrijkCampus of the University of Leuven since it waserected in 1965. He created his own group there,and among others developed the theory of ULD by
two ultrasonic beams, studying the topic againstthe wide frequency and intensity range of ultraso-nics. His results predicted new phenomena such asdependence of light intensity in diffraction orderson the phase shift between the beams (see below);this looked to us to be an interesting invention wait-ing for experimental confirmation and for presenta-tion of perspectives for possible applications. So, ourdecision in 1975 to start with the verification of Ler-oy’s theories in our laboratory has turned out to be agood choice. Since that time mutual cooperation hasbeen continued, and soon after a dozen or so commonpapers appeared confirming the theory, of which themost historically interesting were [7–9]. Figure 1shows the original record (presented by Leroy atthe Gaithersburg symposium in 1978 [8]) of the þ1diffraction order of light intensity dependence onphase shift δ between two adjacent progressing ultra-sonic beams. Figure 2 exemplifies one such confirma-tion, which is the dependence of light intensity in þ1and −1 orders of diffraction by two ultrasonic beamspropagating in opposite (antiparallel) directions onperiodic phase shift δ ¼ Ωt between the two beams[9,10]. Later, we used the phenomena for a very niceapplication, i.e., for laser light modulation (Fig. 3)and for controlling laser action in its resonator cavity(Fig. 4). These pioneer results have had a noticeableinfluence on further fundamental research [11–15],applications of AOs in signal processing [16], as wellas ultrasonic imaging, and have been inspirations foreven quite recent research [17–19].
Members of our AO group visited Kortrijk manytimes to work together on ULD phenomena, includ-ing optical near field and polarization effects in dif-fracted light (a number of common publications) orattending the Symposia, e.g., on Physical Acousticsat Kortrijk (1990) or on Advances in Acousto-Opticsin Brugge (2000). The Belgian group took part sys-tematically in our Spring Schools since its originin 1980. Figure 5 presents a photo of that time.
Fig. 1. Original historical (1978) recording of the light intensityin Jþ1 diffraction order after passing through two ultrasonicbeams of frequency ratio n ¼ 1∶2 (1 and 2MHz, respectively) inthe first order versus the phase difference δ. After Leroy et al. [8].
Mertens (who passed away two years ago) is seenthere. It is worth mentioning two publications, oneby Mertens [12] on 50 years of AOs and one by LeroyandMertens [13] on theoretical AOs achievements inBelgium, where one can also find information aboutour mutual cooperation.Leroy’s contribution to the cooperation between
Kortrijk and Gdańsk has been recognized as so sig-nificant that in 1991 he was honored with the Doc-torate Honoris Causa of the University of Gdańsk(Figs. 6 and 7). It was our pleasure and privilegeto host Leroy and his co-workers Erik Blomme andNico Declercq during the 10th Jubilee Schoolat Sopot.
3. Cooperation with German Groups: Physikalisch-Technische Bundesanschtalt (PTB) Braunschweig andUniversity of Bremen—University of Gdańsk
The cooperation with PTB (Reibold and his co-workers) started in 1980 and later became moreand more efficient when Kwiek and Reibold were
doing common research during many shorter or long-er of Kwiek’s stays in Reibold’s laboratory (more than30 common papers were published). The common re-search in Braunschweig covered topics of AO thatcould not be experimented on in Gdańsk and forwhich Reibold’s laboratory was better equipped. Ingeneral the topics were rather fundamental for ex-plaining the nature of the ULD phenomenon [20].Among others such questions as its dependence onRaman–Nath parameter υ and the Klein–Cook para-meter Q in the entire range covering three regimes,Raman–Nath (Q ≪ 1), intermediate (for Q ≅ 1), andBragg (Q ≫ 1), distinguished by the determinedvalues of the Klein–Cook parameter Q—see below.Among others such features as the light’s “initial
Fig. 2. (a) Diffraction of light by two adjacent ultrasonic beams offrequency ratio n ¼ f 1=f 2, Raman–Nath parameter ratioα ¼ μ1=μ2, and the phase shift δ. (b) Example of the theoretical(—) and experimental (...) time dependence of light intensity inþ1 and −1 orders of light diffraction by two antiparallel ultrasonicbeams against δ ¼ Ωt for frequency ratio 1∶2, the Raman–Nathparameter ratio μ1=μ2 ¼ 1:2, f 1 ¼ 0:8MHz, and f 2 ¼ 1:6MHz.After Blomme et al. [10], reproduced from Proceedings of Ultraso-nics International, 1985, with the kind permission of Elsevier.
Fig. 3. Principle of operation of an AO modulator using two ad-jacent ultrasonic beams with the phase shift δ; G1, G2 are signalgenerators; Gδ is a phase shifting generator. After Kwiek andŚliwiński [11], reproduced from Opt. Eng., 1992, with the kind per-mission of SPIE.
Fig. 4. Application of the AO modulator using two ultrasonicbeams for controlling laser action in its resonator cavity. AfterKwiek and Śliwiński [11] reproduced from Opt. Eng., 1992, withthe kind permission of SPIE.
phase shift” (IPS), “ν-dependence phase shift” (ν-DPS), and rediffraction of light to the 0 order wereoriginally introduced, widely interpreted, andconfirmed [17,20,21]. Though the two parameters νand Q are perfectly known for acousto-opticians itis useful to remind them how they were used forIPS and νDPS definitions; namely, the Raman–Nathparameter ν ¼ 2πμL=λ and the Klein–Cook para-meter Q ¼ 2πLλ=noΛ2, where the amplitude μ of var-iations of refractive index n induced by ultrasound isproportional to acoustic pressure, and other quanti-ties are the width L of the ultrasonic beam, the wa-velength λ of light, the light refractive index no for anondisturbed medium, and the Λ- wavelength ofultrasound. The IPS for a given light diffraction or-der n at normal incidence has been expressed byφi;n ¼ Q
12 ð2n2 þ nÞ. It is seen that IPS is proportionalto Q and also that it is equal to zero for n ¼ 0. So, forthe Raman–Nath regime (Q ≪ 1) it is negligiblysmall. However, for the larger Q (for Q ≅ 1 and
>1) for the orders n ≥ 1 the value of IPS is essential.The ν-DPS or additional phase shift φadd [20,21] ap-pears in ULD (for Q ≠ 0 and Q ¼ const) as an addi-tional phase shift in respect to the case takingplace in the Raman–Nath regime (Q ≈ 0), in whichυ presents optical phase retardation relative to thenondisturbed medium, appearing due to the pre-sence of ultrasonics.
These examinations performed by Kwiek and Re-ibold were indispensable for being able to enlarge therange of appropriateness of optical tomography forcorrect reconstruction of ultrasonic fields [22–25].The results had an essential impact on the develop-ment and improvement of acousto-optical methods,which has become a supporting tool for a numberof biomedical applications. Recently, optical coher-ence tomography with AO modulation has appearedas a very useful technique for high-resolution biome-dical imaging [26–29].
The last common work with Reibold [19] (beforehis unexpected death in 2005) was connected withexperimental verification of Mertens’ theory on ULDin the case of the two overlapped ultrasonic beams inthe intermediate regime (Q ≅ 1) of ULD. (It is a pitythe publication appeared few months after the deathof Mertens in 2006).
The cooperation with Reibold has been highly ap-preciated by the University of Gdańsk authorities. InOctober 2004 Reibold was awarded with the GdańskUniversity Medal of Merit by the Senatus of the Uni-versity. The medal and the certificate were handed toReibold by Kwiek during the ceremony of his retire-ment from PTB in Braunschweig (Figs 8 and 9).
The cooperation with the University of Bremen(S. Boseck and his group) started at the 4th Schoolon Acousto-Optics in 1989. Many interesting theore-tical and experimental papers on acoustic micro-scopy and applications were presented in the nextSchools until Boseck’s retirement in 1999.
4. Cooperation with Italian Groups
The Italian AOs groups were D. Sette’s group at theUniversity La Sapienza in Rome, Alippi’s group atthe Istituto de Corbino in Rome, and Gabrielli at theUniversity of Trieste. The cooperation with Sette’sgroup in common research resulted in publicationson liquid crystals [30,31]. Scientific contacts withthe Alippi’s group enabled frequent exchange visitsand attendance at several conferences such as thePhysics Schools at Erice [14] and the 16th ICA Con-gress in Rome (2001). Alippi attended our SpringSchools several times. The most efficient cooperationtook place with Gabrielli (Fig. 5), who attended theSchools several times, contributing theory and ex-perimental results on spatial and temporal lightmodulation by ultrasound. In the 1980s Gabrielli in-vented the original procedure of phase vector analy-sis of AO arrangement and explained sophisticatedultrasonic light modulating systems by the phasevector representation that he attractively illustratedduring the 1st, 2nd, and 4th Schools. Gabrielli also
Fig. 5. (Color online) 1st School on Acousto-Optics and Applica-tions, Gdańsk-Wieżyca, 1980, in front of the Institute of PhysicsBuilding, University of Gdańsk. From the left: B. Linde, I. Gabriel-li, A. Śliwiński, P. Kwiek; from A. Śliwiński’s archives.
Fig. 6. (Color online) University of Gdańsk, 1991, Honoris CausaDoctoris ceremony for Prof. Oswald Leroy (in the middle) of Catho-lic University of Leuven, Campus Kortrijk. On the left: Prof. A. Ko-walski, Deputy Dean of the Faculty of Mathematics, Physics andChemistry. On the right: Prof. A. Śliwiński, Promotor, before pre-senting the ”Laudationem.” From A. Śliwiński’s archives.
spent some time in Kwiek’s laboratory doing commonresearch that resulted in a few common publications.Original results obtained together with Gabrielliconcerned peculiar ULD patterns and their modula-tion against the phase shift δ for two adjacent beamsof different frequencies ratios n∶m (n,m are integers)[16]. It occurred that in the case when f 1 ¼ nf ,f 2 ¼ mf , n ≠ 1, m ≠ 1, and n ≠ m there appeared ad-ditional (combinational) orders in the diffraction
spectrum on spots between those that originally ex-isted for each ultrasonic beam independently. Theseadditional orders have not appeared in the case offrequency ratio 1∶n. They appear as the combination(between n and m harmonics) of orders of indicesl ¼ pnþ qm, where l, p, and q are integers. Figure 10shows an example for n∶m ¼ 2∶3. The diffractionlines of combination frequencies are visible in theresulting spectrum formed by two ultrasonic beams
Fig. 7. (Color online) Copy of Oswald Leroy’s Doctorate Honoris Causa certification, reproduced with grateful appreciation to Prof. Leroyfor his kind permission.
interacting with the light beam. The phase modula-tion factor for the light against phase shift δ in thelowest combination orders (l ¼ −1 for p ¼ 1, q ¼ −1,and l ¼ þ1 for p ¼ −1, q ¼ þ1) has reached 100%.
5. Cooperation on Acousto-Optics with RussianGroups
The Russian AO groups attended very actively all theSpring Schools since the first one. They wereV. N. Parygin and his co-workers, V. I. Balakshi’sand V. B. Voloshinov’s groups from Moscow StateUniversity, V. V. Proklov’s group from the Instituteof Radio Engineering and Electronics of the RussianAcademy of Sciences, Moscow, S. V. Kulakov’s groupfrom the Institute of Aviation Instrument Construc-tions, St. Petersburg, and V. V. Petrov’s group fromSaratov State University and of several scientistsfrom other institutions. The Gdańsk AO group vis-ited laboratories in Moscow and attended in sympo-sia organized in St. Petersburg. Several commonpapers [32,33] were published and some experimentsperformed in the Gdańsk AO laboratory that usedthe AO cells constructed by V. Ya. Molchanov fromthe Institute of Computer Technology and Informa-tion, Moscow.Contributions of Russian AO groups to all Schools
on Acousto-Optics and Applications were importantand valuable [34–42]. Essential achievements weredelivered by V. N. Parygin (who unfortunately passed
away in 2005) and his co-workers on a strongcollinear and quasi-collinear interaction of three-dimensional beams and applications in AO filteringand image processing.
V. B. Voloshinov and his group presented many in-teresting results on peculiarities of ULD observed inacoustically anisotropic media (e.g. thelurium oxidecrystal) where the phenomena have taken place witha positive or a negative obliquity angle [40]. Manyresults were successfully applied [42] in AO filteringfor improving image resolution, etc. V. I. Balakshiand his co-workers contributed a lot to AO signal pro-cessing and imaging as well as to designing AO de-vices (e.g., an original laserlike AO generator [41].Figure 11 presents a photo of several participantsof the 7th School in Gdańsk-Jurata.
6. Other Cooperation and Contributions to SpringSchools on Acousto-Optics and Applications
From many other AO cooperating centers that per-manently contributed to the Spring Schools [34–42] organized by the University of Gdańsk we shouldrecollect AO groups from Canada, H. W. Jones, Uni-versity of Halifax; France, J. Sapiel, CNET, Paris;Germany, R. Millner (passed away in 2005), H. J.Hein, E. Rosenfeld, University of Halle/S; Lithuania,D. Ciplys, University of Vilnius; Switzerland, A. Ku-lik, Institute de Genie Atomique; the UK, R. W. B.Stephens, Imperial College, London, R. C. Chievers(passed away in 2004), University of Surrey; andfrom the USA, L. Adler, the University of Ohioand Adler Consultants; M. A. Breazeale, Universityof Mississippi; P. Banerjee, University of Alabama;
Fig. 8. (Color online) Rainer Reibold (on the right) and PiotrKwiek displaying the certificate of the Gdańsk University Medalof Merit that was awarded to Reibold, PTB Braunschweig, 10 May2004; from P. Kwiek’s archives.
Fig. 9. (Color online) Piotr Kwiek handing the Gdańsk UniversityMedal of Merit to Rainier Reibold (on the right), PTB Braunsch-weig, 10 May, 2004; from P. Kwiek’s archives.
B. D. Cook, University of Houston; P. K. Das, Rensse-laer Polytechnic Institute, Troy, New York; A. Korpel,University of Iowa; D. M. Litynski, U.S. MilitaryAcademy, West Point, New York; K. Lewis, Univer-sity of California, Livermore; W.G. Mayer, George-town University, Washington, DC; T.-C. Poon,Virginia Polytechnic Institute and State University,Blacksburg, Virginia; B. R. Tittmann, PennsylvaniaState University, State College, Pennsylvania; C.Tsai, University of California, Irvine. All the scien-tists listed above contributed essentially to our AOSchools, as evidenced by their papers on severaltopics published in the consecutive proceedings[34–42].In 1995, B. D. Cook (Fig. 12)—the co-author of the
so-called Klein–Cook parameter Q (already definedabove)— attended the 6th School [39] a year before
his unexpected death. He presented a paper on thenear field of ultrasonic transducers and how it per-tains to acousto-optics. Also, he chaired and moder-ated a very interesting round-table discussionending the meeting. During the discussion Cook re-called some facts from the 1950s to the 1960s whenacousto-optics was founded at Michigan State Uni-versity in the Laboratory of Egon Hiedemann. (Later,around 1963–1964, Bill Cook went to HoustonUniversity to establish his own AO laboratory.) Many
Fig. 10. Diffraction patterns for light interacting with the firstbeam of frequency f 1 ¼ 1:048MHz (a), with the second beam offrequency f 2 ¼ 1:572MHz (b), and with both beams of frequencyratio f 1∶f 2 ¼ 2∶3 (c); after Gabrielli et al. [16]. Reproduced fromAcustica, 1988, with the kind permission of S. Hirzel VerlagGmbH.
Fig. 11. (Color online) 7th School on Acousto-Optics and Applica-tions, Gdańsk—Jurata 1998, near the Mathematics and PhysicsBuilding, University of Gdańsk. From the left: I. Gabrielli, P. Bane-rjee, V. I. Balakshy, V. B. Voloshinov, V. N. Parygin, P. Kwiek, A.Śliwiński, G. Gondek, T. Katkowski, and V. Ya. Molchanov. FromA Śliwiński’s archives.
Fig. 12. Bill D. Cook during the 6th School at Jurata 1995. After[40], reproduced from Proc. SPIE 3581, 1998, with the kindpermission of SPIE.
interesting results were obtained and published bythe group that had been working there (Adler, Har-grove, Breazeale, Mayer, Klein, Cook, and others).TheQ parameter, so useful now in acousto-optics, ori-ginated there and was established by Klein and Cookin their paper published in JASA in 1967. The 7thSchool [40] was dedicated to Cook and the first plen-ary session in commemoration of Cook was chairedby A. Korpel (Fig 13). Several of Cook’s friends deliv-ered their commemoration words and among themwere Leroy (Fig. 14) and Breazeale (Fig. 15) who par-ticipated in the commemoration.
In the framework of the cooperation with the cen-ters in the USA the second author of this paper wasinvited in 1993 for a round-trip visit arranged andsupported by the courtesy of Dr. L. Adler, Universityof Ohio, to eight of these centers and presented sev-eral invited lectures there in each of them. Recollect-ing that, I would like again to express my thanks toW. Mayer, P. Banerjee, T.-C. Poon, M. Breazeale,C. Tsai, K. Lewis, and A. Korpel for their hospitalitythen.
7. Final Remarks and Conclusions
Some of the above-reported features of the interna-tional scientific cooperation between the GdańskUniversity AO group and several centers over theworld, mainly expressed by the list of commonpublications and by some historical facts illustrating
Fig. 13. A. Korpel chairing the session dedicated to the memoryof Bill D. Cook during the 7th School. After [40], reproduced fromProc. SPIE 3581, 1998, with the kind permission of SPIE.
Fig. 14. Leroy remembering his common work with B. D. Cook.After [40], reproduced from Proc. SPIE 3581, 1998, with the kindpermission of SPIE.
Fig. 15. M. A. Breazeale, a very close friend of B. D. Cook. After[40], reproduced from Proc. SPIE 3581, 1998, with the kind per-mission of SPIE.
research activity in the field of acousto-optics in ourInstitute, have shown that this branch of physics andtechnology has been a subject of interest of manyscientists.The more than 35 long years of cooperation and 28
years of the history of the AO Schools have been a bigscientific adventure for the authors, and we are hap-py to experience now, during the 10th Jubilee School,continuing development of AO research and applica-tions. New ideas for applying AO devices in modernscience and technology for controlling optical beams,e.g., in laser technology, in optical processing of infor-mation, in material and biomedical imaging, andmany other areas have shown that the field is stillattractive.The Spring Schools on Acousto-Optics and Appli-
cation have integrated the AO community, and wehope the meetings will be continued in the future.The 10th School, which took place in Sopot in 2008(see the papers in this issue), evidently showed thatmany topics of acousto-optics are still important andinteresting to study and to use in practice. We had agreat honor and pleasure to meet and to welcome oldparticipants as well as many young people cultivat-ing the nice field. Great thanks for all who contrib-uted to acousto-optics during the whole period ofcooperation. Special thanks, cordial congratulations,and best wishes for further long life are directed toAdrian Korpel to whom this feature issue of AppliedOptics is dedicated on the occasion of his 75 birthday.The significance of his contributions to our AOSchools cannot be overestimated. The photo of Kor-pel in Fig. 16 was taken during the 8th School andexpresses the splendid atmosphere that was asso-ciated with the meeting.
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