Replay at Optical Communications Wavelengths of Holographic Gratings Recorded in the Visible

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1 Dublin Institute of Technology Confeence Papes Cente fo Industial and Engineeing Optics Replay at Optical Communications Wavelengths of Hologaphic Gatings Recoded in the Visible Vincent Toal Dublin Institute of Technology, Mauice Whelan Institute fo Health and Consume Potection, Joint Reseach Cente, Albeto Volcan Optics Goup, Institute fo Health and Consume Potection, Joint Reseach Cente, 2102 Ispa, Italy Izabela Naydenova Dublin Institute of Technology, Suzanne Matin Dublin Institute of Technology, Follow this and additional woks at: Pat of the Optics Commons Recommended Citation Toal, V. et al. (2006) Replay at optical communications wavelengths of hologaphic gatings ecoded in the visible. SPIE poceedings of the Intenational Confeence on Hologaphy, Optical Recoding and Pocessing of Infomation, Vol. 6252, pp This Confeence Pape is bought to you fo fee and open access by the Cente fo Industial and Engineeing Optics at It has been accepted fo inclusion in Confeence Papes by an authoized administato of Fo moe infomation, please contact This wok is licensed unde a Ceative Commons Attibution- Noncommecial-Shae Alike 3.0 License

2 Dublin Institute of Technology Aticles Cente fo Industial and Engineeing Optics Replay at Optical Communications Wavelengths of Hologaphic Gatings Recoded in the Visible Vincent Toal Dublin Institute of Technology, Mauice Whelan Institute fo Health and Consume Potection, Joint Reseach Cente, Albeto Volcan Optics Goup, Institute fo Health and Consume Potection, Joint Reseach Cente, 2102 Ispa, Italy Izabela Naydenova Dublin Institute of Technology, Suzanne Matin Dublin Institute of Technology, Recommended Citation Toal, V., Whelan, M., Volcan, A., Naydenova, I., Matin, S.: Replay at optical communications wavelengths of hologaphic gatings ecoded in the visible. SPIE poceedings of the Intenational Confeence on Hologaphy, Optical Recoding and Pocessing of Infomation, Vol. 6252, pp This Confeence Pape is bought to you fo fee and open access by the Cente fo Industial and Engineeing Optics at It has been accepted fo inclusion in Aticles by an authoized administato of Fo moe infomation, please contact

3 Replay at optical communications wavelengths of hologaphic gatings ecoded in the visible Vincent Toal, Mauice Whelan*, Albeto Volcan*, Izabela Naydenova and Suzanne Matin Cente fo Industial and Engineeing Optics, Dublin Institute of Technology, Dublin 8, Ieland *Optics Goup, Institute fo Health and Consume Potection, Joint Reseach Cente, 2102 Ispa, (Va), Italy Keywods: hologaphy, diffaction gatings, communication wavelengths Abstact In this pape we epot on hologaphic diffaction gatings ecoded at visible light wavelength, which can be pobed at telecommunication wavelengths. The ecoding mateial is an easily pepaed, self-pocessing photopolyme, all of whose components ae wate soluble. Tansmission gatings of vaious types, namely unslanted, slanted, totally intenally eflecting and Bagg gatings wee all fabicated. Diffaction efficiencies at telecommunications wavelengths compae favouably with those obtained in visible light. 1. Intoduction Thee is gowing inteest in the development of low-cost, easily fabicated optical devices. To take one example, in-fibe gating fabication fo vaious applications, including telecommunications and sensing involves the use of UV lases and diect witing into the fibe mateial by means of phase masks of fixed spatial fequency. It would be advantageous to make use of visible lase light which is easie to contol, and cheape, moe vesatile ecoding mateials. Bagg gatings fo wavelength division multiplexing have aleady been epoted in doped and patially polymeized polymethylmethacylate Photopolymes Photopolymes have long been in use fo hologaphy 2, hologaphic applications including gatings and othe hologaphic optical elements (HOE) 3,4 as well as hologaphic intefeomety and ecently electonic speckle patten intefeomety 5,6. Typically a monome, cosslinking monome, a dye sensitize and an electon dono ae dispesed in a binde such as polyvinyl alcohol. Layes up to 150 µm thick ae deposited on glass substates by gavity settling, dip coating o the use of Maye bas. Afte about 24 hous the mateial is eady fo use. Tansmission hologaphic gatings having up to 95% diffaction efficiency have been obtained using exposues of 90 mj cm -2 in dy layes of up to 125 µm in thickness. No post ecoding pocessing is equied and the gating is fixed by exposue to actinic light. In ode to measue the diffaction efficiency duing ecoding, gatings ae simultaneously illuminated at a wavelength to which the dye is insensitive, so that fo example a ecoding at 532 nm may be illuminated by a low powe He-Ne lase, fist ensuing that this lase is incident at the coect (Bagg) angle. The ability to ecod a gating at one wavelength and pobe it at anothe suggests a numbe of inteesting possibilities, one of which is the use of a hologam as a beamsplitte and combine in an electonic speckle patten intefeomete (ESPI) 6. A tansmission hologam of the object unde investigation was ecoded at 532 nm and the image econstucted using a lase diode of wavelength 780 nm by alteing the angle of illumination. This image was supeimposed on the image of the object in a simple, nea common path, out-of-plane sensitive ESPI system. Diect modulation of the lase diode wavelength allows digital phase shifting fo finge analysis, to be implemented. To extend gating illumination to telecommunication wavelengths would allow fo futhe applications of the photopolyme.

4 2. Recoding and illumination geomety (eflection gatings) In ode to ecod a gating using 532 nm light, which can be illuminated in a diection nomal to the finge planes and stongly eflect at 1550 nm, a ecoded finge spacing of about 500 nm is needed, so that the angle between the ecoding beams must be about 40 0 within the photopolyme laye of efactive index 1.5. Fig. 1 shows a numbe of possible aangements. The ecoding beams ae of wavelength and the eading beam has wavelength. All of these wee attempted but in all cases the diffaction efficiency of the gatings poved to be only 1% o less, even at the ecoding wavelength. a) b) c) d) e) Fig. 1 Aangements a) to d) pemit illumination of the gating at 1550 nm without emoving the pism. c) enables a gating to be ecoded with the finge planes paallel to the laye suface. The angle of incidence of the witing beam is 39 0 to the pism nomal so that it is waveguided in the laye, the two esulting beams tavelling at 40 0 to each othe. d) also ecods a gating vecto nomal to the laye suface. 1e) shows an aangement equiing emoval of the pisms in ode to pobe the gating at 1550 nm.

5 It is pobable that the pism couples used wee inappopiate as they wee obtained by cleaving an old cube beamsplitte along its diagonal and optical coatings on the esulting pism hypotenuse faces effectively pevented light fom being tansmitted into the photosensitive layes. 3. Recoding and illumination geomety (tansmission gatings) Thee ae a numbe of possible appoaches. These ae: 1. unslanted tansmission gatings 2. slanted gatings 3. gatings which totally intenally eflect at 1550 nm 4. edge-illuminated gatings 3.1 Unslanted tansmission gatings The simplest appoach is to ecod an unslanted tansmission gating at 532 nm and illuminate it at 1550nm at the appopiate angle (fig. 2). The condition that must be met is sin θ '/ sinθ = ' / whee dashed symbols efe to the illuminating beam (~1550nm). Λ, the ecoded finge spacing is given by /(2sinθ) with the gating ecoding wavelength θ Λ Fig. 2 Reflection of the pobe beam fom gating planes. In the exteme case of the 1550nm beam at gazing incidence, the witing beams must be incident at angles of not moe than 20 0 to the nomal fo a efactive index of 1.5. This because the angles of efaction of these beams ae both 13 0 and the value of θ fo the 1550 nm beam is then , the citical angle. The aangement fo ecoding gatings is shown in fig. 3. lase photopolyme laye Fig. 3 Gating ecoding

6 Bagg cuves obtained at 1550 nm fom unslanted gatings ecoded at 532 nm ae shown in Fig. 4. An EOS tunable lase was used at 1550 nm. Its output was coupled into a single mode fibe with a collimating lens fitted to the fibe output end. The gatings wee mounted on a venie otation stage. Fig. 4a Bagg cuve at 1550 nm fo a gating ecoded at 532 nm. whee the 532 nm beams ae incident at 15 0 and the 1550 nm beam at 49 0 to the nomal to the photopolyme laye. The efficiency of the gating at 1550 nm is 48% compaed with 55.5% at 532 nm. 500 diffacted powe (micowatt) angle (deg.) diffacted powe (micowatt) angle (deg.) Fig. 4b as 4a. The efficiency of this gating at 1550 nm is 54.5% compaed with 61.8% at 532 nm. These esults show that the efactive index modulation of the gatings at 1550 nm compaes favouably with the modulation at 532 nm. We have not taken Fesnel losses into account in obtaining diffaction efficiency values. The diffeence between the diffaction efficiency at 1550 nm and that at 532 nm can by accounted fo by highe Fesnel losses at the longe wavelength and highe angle of incidence.

7 3.2 Slanted gatings It has been demonstated that slanted gatings may be ecoded in the photopolyme system with acceptably low shinkage of aound 2% 7, which enables us to tace the ay path of a 1550 nm beam illuminating a slanted gating ecoded at 532 nm. Initially the 532 nm beams wee incident on the laye at equal angles, θ, on eithe side of the nomal. The laye was then otated by a small angle φ befoe ecoding so that the ecoded finges would be inclined at appoximately 2φ/3 to the suface nomal. By choosing appopiate values of θ and φ, the 1550 nm beam can be made to emege at vey lage angles to the laye nomal o undego total intenal eflection at the substate/ai bounday. The angle between the ecoding beams at the ecoding plane was The laye was otated by 10 0 befoe making the ecoding and the 1550 nm beam incident diection was chosen to ensue total intenal eflection (fig. 5). Fig. 5 Pobe beam wavelength coupled into photopolyme slanted gating ecoded at wavelength This was obseved on an IR senso cad at the edge of the glass substate. Because the diffacted beam was scatteed though a substantial angle it was not possible to measue the powe. Instead the powe in the diectly tansmitted beam was monitoed. Fig. 6 shows the esult.

8 TIR gating, tilt 10 deg. powe in 0-odebeam (micowatt) angle Fig. 6a Reduction in zeo ode powe at Bagg angle. The efficiency at 1550 nm is 16% TIR gating, tilt 14 deg powe in 0-ode (micowatt) angle (deg) Fig 6b Reduction in zeo ode powe at Bagg angle. Efficiency is 12.5 % at 1550 nm. Thus a slanted gating seves to couple an incoming light beam into what is effectively a slab waveguide. It would be advantageous to have the 1550 nm beam nomally incident and this was implemented using a lage tilt angle. Fig. 7 is an example of the esult obtained. powe in zeo ode at 1550 (micowatt) angle Fig. 7 Reduction in zeo ode powe at Bagg angle fo nomally incident 1550 nm light. The efficiency is 6% at 1550 nm (47 % at 532 nm). The tilt angle is

9 3.3 Edge illuminated gatings. In this geomety unslanted gatings ae ecoded, whose finge spacing is compatible with Bagg diffaction fo a 1550 nm beam diected nomally to the finge planes. In this way the gating becomes a eflection gating at 1550 nm. The intebeam angle fo the witing beams is 62 0 in ai so that the spatial fequency of the gating is aound 2000 lines/mm which matches the midband wavelength of a boadband souce (AFC BBS 1550) coupled to a 3dB multimode fibe couple. The unused pot was immesed in index matching gel. The light eflected back into the fibe fom the gating was passed to an optical spectum analyse (ANDO AQ 6315B). Fig. 8 shows the aangement with some esults in Fig. 9. BB souce OSA Fig. 8 Edge illumination of a tansmission gating, BB boadband souce, OSA optical spectum analyse. The gating is mounted on a 3 axis tanslation/otation stage. 3.00E-05 efected powe(w) 2.50E E E E E E a)

10 4.00E E-05 eflected powe(w) 3.00E E E E E E-06 eflected powe(w) 0.00E E E E E E E E E E E E b) c) Fig. 9 Back eflected powe fom edge illuminated gating plotted against pobe wavelength. a) typical cuve b) a second gating peaking at 1532 nm c) ove-modulated gating The method used was to cut though the photopolyme laye using a shap knife afte ecoding the gating. The gating was then manipulated into contact with the tip of the illuminating fibe. To achieve this the gating was mounted on a 3- axis tanslato combined with a 3-axis otato. The gating edge and pobe fibe tip wee viewed though a binocula micoscope. At close appoach, Faby-Peot oscillations wee seen in the spectum and, when the fibe came into contact with the gating, the back eflected powe fell due to the absence of Fesnel eflection, until coupling to the eflection gating was achieved. Well defined Bagg cuves wee obtained in seveal cases. As the fibe was futhe pushed into the gating, the peak of the Bagg cuve moved steadily towads shote wavelengths, indicating compession of the gating. When the fibe diection was evesed, the peak movement also evesed. Ove modulation effects wee also obseved (fig. 9c)

11 5. Discussion and conclusion We have demonstated that hologaphically ecoded gatings in acylamide based photopolyme, using visible light of wavelength 532 nm, can function as Bagg gatings at telecommunications wavelengths. Diffaction efficiencies of simple tansmission gatings ae as high at 1550 nm as at 532 nm. We have also demonstated coupling of 1550nm light into a slab waveguide by using slanted gatings ecoded in the photopolyme. Finally we have fabicated edge-lit Bagg gatings with good diffaction efficiency at 1550 nm. The photopolyme mateial is low cost and easy to pepae. In the case of gatings used as tansmission gatings at aound 1550 nm, the Bagg cuves all show angula widths of appoximately One can compae this value with that pedicted by the coupled wave theoy fo thick hologams. Using the equation fo the off-bagg paamete χ 8 χ = θ 2πd sinθ 0 / 0 one can calculate the value of θ at which the nomalised diffaction efficiency becomes zeo ( χ in the ange 2-3). d is the gating thickness, typically 100 µm, 0 the wavelength in ai and θ 0, the Bagg angle in the photopolyme. Using the value of 49 o (Fig. 4a) a value of is obtained fo the full angula width of the gating in ai, which agees with the expeimental angula width. We may also obtain an estimate of the spectal bandwidth of the gating using the equation whee is the wavelength shift at which the nomalised diffaction efficiency becomes zeo ( χ again in the ange 2- χ = 3). We obtain a spectal bandwidth of about 40 nm πd sin θ cosθ / n In the case of edge illuminated gatings we can use the measued spectal bandwidth to detemine the effective thickness of the gating used in eflection geomety. Using the appopiate fom of the off-bagg paamete fo the eflection case χ = 2 2πn d cosθ / 0 0 and a value of χ of 3.5, we obtain a value of ~1 mm fo d. Thus only a vey small length of tansmission gating, ecoded at 532 nm in this mateial, is needed in edge illuminated geomety at telecommunications wavelengths. Acknowledegements: We thank Entepise Ieland fo the povision of an Intenational Collaboation gant in suppot of this eseach. Refeences 1. O. Beye, I. Nee, F. Havemeye, and K. Buse, Hologaphic Recoding of Bagg Gatings fo Wavelength Division Multiplexing in Doped and Patially Polymeized Polymethylmethacylate, Appl. Opt. 42, (2003) 2. S. Calixto, Dy polyme fo hologaphic ecoding, App. Opt., 26, ,1987

12 3. S. Habaken, Y. Renotte, E. Stijns and Y. Lion, Polaizing hologaphic optical elements in DuPont omnidex-tm films, SPIE poceedings, Vol. 2532, pp. 9-14, M. Whelan, C. Fono, S. Matin, F. O Neill and V. Toal, "Illumination systems using photopolyme gatings fo speckle intefeomety" Intefeomety '99 Intenational confeence on optical metology, techniques and technologies, Wasaw, Poland, 20th to 23d Septembe, S. R. Guntaka, B. Bowe, V.Toal and S. Matin, Hologaphic optical elements fo combined hologaphic and digital speckle patten intefeomety Speckle Metology 2003, Tondheim, Noway, Poc. SPIE vol. 4933, eds. K. Gastinge, O.J.Lokbeg & S. Winthe, ,18-20 June S.R.Guntaka, V.Toal, and S.Matin, Hologaphic and Electonic Speckle-Patten Intefeomety using a Photopolyme Recoding Mateial, Stain, 40, 2,79-81, S. Matin PhD Thesis, Univesity of Dublin, P. Haihaan, Optical Hologaphy, Pinciples, techniques and applications Cambidge Univesity Pess, 2 nd edn. 1996