Borel, Peter Ingo

Chong, H.

Frandsen, Lars Hagedorn

Kristensen, Martin

Lavrinenko, Andrei

Thorhauge, Morten

M. Thorhauge

P. Borel

L. Frandsen

M. Kristensen

A. Lavrinenko

H. Chong

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Properties of directional couplers using photonic crystal waveguides

Coupled photonic crystal waveguides have been designed and modelled with a 3D finite-difference-time-domain method, and fabricated in silicon-on-insulator material. Good agreement between modelled and measured results has been found

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Properties of directional couplers using photonic crystal waveguidesThorhauge, Morten; Borel, Peter Ingo; Frandsen, Lars Hagedorn; Kristensen, Martin; Lavrinenko, Andrei;Chong, H.Published in:Optical Fiber Communications Conference, 2003. OFC 2003Link to article, DOI:10.1109/OFC.2003.1248363Publication date:2003Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Thorhauge, M., Borel, P. I., Frandsen, L. H., Kristensen, M., Lavrinenko, A., & Chong, H. (2003). Properties ofdirectional couplers using photonic crystal waveguides. In Optical Fiber Communications Conference, 2003.OFC 2003 (pp. 494-495). IEEE. DOI: 10.1109/OFC.2003.1248363494 / VOL. 2 / OFC 2003 / THURSDAY MORNING first describe the propenies of the high index inclusions (layen or cylinders). Each high-index laver can be considered as a sten index wavewide that supports bound normal m d e s  and assoc&ted cutoff conditions for those modes. In what fol- m o d e s  d t h &  high-index layers in the cladding of the waveguide. In the case of step-index planar and cylindrical waveguides, the cutoff condition for these modes can be found analytically. At the cutoff (resonant) wavelength for a particular mode, the high-index layer becomes “transparent” and light escapes from the core resulting in min- ima in the transmission spectrum. For planar and ring s m c w e s ,  the location of the transmission minima using this merhod are the same as those derived in Ref. [5] for the ARROW waveguide using a Fabry-Perot approach. This should be expected as the resonance condition for the cascaded Fabry-Perot Sections matches the mode cutoff condition in the slab waveguide. The present approach, however has an advantage over the Fabrv-Perot analogy when considering com- considered iogeiher. 1. Planar and ringstructures (Fig. l(a),(bj) For these geomeaies the cutoffconditions (and the spectral minima) for the one-dimensional structure can be found from the following equation: and are given by A comparison of the predictions obtained from Eq. (2)  with numerical simulations using beam propagation method, are shown in figure 2 along with a schematic of modal cutoff. Along with verifying the accuracy of our predic- tions for the spectral minima, figure 2(c) indicates that it is generally higher order modes that are excited in the high index inclusions. Although any mode i s  allowed by equation 2, the light is launched into the low index region, thus Snell’s law determines the angles of the light in the high index areas Because the . index difference between these regions is large, the low angles n e c e . ~ !  1.1 exrtie rhu louen modci innnui he rcaihed Thu, rhe p r m q  mode rs:rtcd uill bu the lozest one oomblc Hieher modcr ;an c u t .  but i i  typical l&ch condittks, these modes will have little energy. 11. Twdlmensiond sfmcruw - cylinders Again we look for the location ofthe transmission dips in the MOF to be at the mode cutoff loca- tions. The natural modes of the most general cir- cular cylinder with arbitrary isotropic internal and external media are given by Stratton 161. The S ~ C N I C  shown in F k .  l(c) &pports thke-mode types: TE, TM, and hybrid EH modes. Similar to section I, we find the resonant condi- tion as follows (3) miision .sp&trum minima. Th; Incation> pre- d r t d  h) Eq ~ 3 ,  arc compared 10 those obtained usin* thr multioole mrthud and are shdum bv s&ht lines i&&3 (a).Also described in thi figure i s  the resulting modal s m c w e  (longitudi- nal pan of the Poynting vector S and cross sec- tion of s, along x axis) in the inefusions. 3. Conclusion In summary, we pmposed a simple analytical model to describe the spectral properties ofphoto- nic crystal waveguides and fibers with low-index core and high-index inclusions. This model sug- gests that the positions of spectral minima can be found by calculating the cutoffwavelength for the modes of hi@-index regions and therefore depend only on the mode structure of those inclu- sions. These predictions were compared with numerical simulations. 4. References [I] I. C. Knight, J. Broeng, T. A. Birks, and P. St. J. Russell, “Photonic band gap guidance in optical fibers,” Science 282, 1476 (1998). [Z] B. J. Eggleton, C. Kerbage, P. Westbrook, R. S. Windeler, and A. Hale, “MicrostrucNred opti- CBI fiber devices,” Optics Express 9,698 (2001). [3] T. F. Krauss, R. M. De La Rue, “Photonic crystals in the optical regime: past, present and future,” Progress in Quantum Electron. 23, 51 (1999). [4] R. Bise, R. S. Windeler, K. S. Krm, C. Kerb- age, B. 1. Eggletan, and D. 1. Trevor, OFC 2002, Paper ThK3. [SI N. M. Litchinitser, A. K. Abeeluck, C. Head- ley, B. J. Eggleton, “Antiresanant reflecting pho- tonic crystal optical waveguides,” Opt. Len. 21, 1592 (2002). [6]  1. A. Stranon, Electromagnetic theory, (McCRAW-HILL, 1941). Th15 9:45AM Properties of Directional Couplers using Photonic Crystal Waveguides M. Thorhauge, p. Borel, L. Frandsen, M. Kristensen, COM, Tech. Ulniversiry of Denmork, &s. Lyngby, Denmark; A. Lavrinenka, Belorussion Stole Universiy, Mlnsk, Belanrs; H. Chong, Univerrity of Glargow, Glasgow. United Kingdom, Email: mth@com.dm.dk. Coupled phutdnw c ~ r i a l  u m c y i d c ,  have hscn designed and mdclled uith 3 3D Finite-Differ- encc-Time-Uoman merhod. and f3hn;ated m %I. icon-on-Insulator material. Good agreement between modelled and measured results has been found. Port POlt Figure 1 Figure 2 l a  lb lntr@duetion Photonic Band Gap (PBG) materials [ I ]  are impartant building blocks for fume optoelec- tronic devices. In principle they can be used instead of usual index-guiding components, utilis- ing the original properties of PEG effects in Pho- tonic Crystals (PCs). The more confined guidance provided in PCs may be beneficial to linear and non-linear components, and the PC structure may allow novel types of components. One of the interesting components, which can be made in PC, is the wavelength selective waveguide coupler. Such a coupler may show new feawes not realisable in traditional ittdex- guided ridge waveguide couplers due to the much higher confinement of light and the fundamen- tally different guiding propenies. We have done extensive modelling of such cou- pled waveguide  structure^ in a 3D Finite Differ- ence Time Domain (FDTD) scheme, and fabricated the designed components in Silicon-on- Insulator (Sol). The modelled and experimental results are in goad apement. Modelling The software used for modelling is the Onyx-2 FDTD code [2]  modified for 3D computation. As pointed out recently by several authors [3-71 the transmission spectrum in the PBG zone is rather broad, indicating leaky mode contributions to the transmission. propagation losses are deter- mined by the coupling of leaky modes lying above the light cone with air and subsmte radia- tive modes, and can therefore only be described correctly in 3D calculations. The PC structures modelled consist of a triangular panem of air holes made in Sol. The holes were chosen to have a normalised radius of 0.375 (radiusflanice pitch), BE this giver the largest band-gap [E]. The holes in the calculations were given a dielectric constant E = 1.4111, in order to mimic the fabricated samples, which have a layer of Silica on the hole \\‘all- The ud\eguiJc\ uerc defined by r:mo\mg one I J ~  of ncarcst-nriehhour holes thercbv making a ~~ ~~~~ ~~~ line defect. The I& skchlrefor the iD~caI&- tions consisted of 1 A Silica base cladding, SI8 A Silicon core, 114 A Silica top cladding, and 314 A air, A being the lattice pitch, the centre-m-centre distance between nearest-neighbour holes. Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 03,2010 at 10:04:57 EST from IEEE Xplore.  Restrictions apply. The modelled structure had a coupling region consisting of W O  waveguides next to each other separated by a single IOW of holes. The coupling length was 27 A. AAer the coupling region one waveguide was continued for an additional 7 A, while the other channel was led away from the first via two 60-degree bends and a short interme- diate waveguide section, creating an output pon separation of 5 rows, see figure 1. The bends were optimised in order to increase transmission (one hole moved) [9]. The component has four ports, two input parts l a  and Ib, and two output ports 2 and 3. Only one input port was excited at a time, the input port not in use being blacked by three holes following the triangular panem. Experiment For the experimental part two samples were fabri- cated with structures resembling the ones modelled (figure I), one with poR l a  blocked, and the other with port Ib blacked (figure 2). To be able to defme in- and out-coupling ridge waveguides it was necessruy to separate the output ports further with a longer intermediate section in the waveguide with two sixty degree bends, so the output pom were spaced by 28 cows. E-beam lithography was used to defme the hole panem in resist on So1 wafen. The e-beam resist acted as a mask in the subsequent wnsfer of the panem to the Silicon layer by Reactive Ion Etching (HE). AAer removal of the resist, the pattern was funher msferred to the Silica base cladding by HE, using the Silicon core layer as a mask. Finally, a Silica top cladding was p w n  on the surface of the Silicon layer by thermal oxidation. The Silica top cladding sewes to make the sh'ucwe vertically symmetric and ,more robust. The lanice pitch A was chosen to 428 m. Ridge waveguides gradually tapered d o m  &om 4 pm to 1 pm at the PC smctux were used to route the light to and 6wm the ports We performed uansmission measmments using tapered lensed fibres to couple light into and out of the sample. The light sources consisted of unpo- larised light emitting diodes with centre wave- lengths at 1310 nm and 1550 nm. The spectra were recorded using an AND0 AQ-6315A optid spec- tmn analyser. Before each meamemem the mu- piing to the sample was optimised for maximum transmission. All measurements were normalised to a fibre-to-tPpered-ridg~.to-fibre measurement. Results and comparison For the coupler with input in pon la, (see figure I), we get the results plotted in figure 3. This is the straight-through result with transmission measured at port 2. As the 3D FDTD model uses a TE polarised source, and the sources in the experiment were unpolarised, the measmd results have been added 3dB to account for this, as the TM modes are expected to be highly lossy due to the lack ofband gapattheoraminedwavelenglhband[l,8]. Figure 3 There is a significant resemblance between the messured and the calculated data. The trend is the same for the WO sets of results. There is some dif- ference in absolute value. The coupling lass from ridge waveguide to PBG waveguide and from PBG waveguide to ridge waveguide is one source of loss in the experiment, which i s  not present in the mad- elled results, and is not eliminated by normalisation to a ridge waveguide. Same of the fmer features &om the measurement are missing in the computed result. This is likely caused by the difference in the output section for THURSDAY MORNING I OFC 2003 I VOL. port 3, where the arm i s  longer in the experimental sample, creating more and finer Fabry-Perot me resonances. Due to a limited bandwidth of the source, the exaerimental data eets mite noim at the sides of I .  th; spectrum. The measured results look slightly shifted along the hole radii be&een the iddelled sample and the measured sample cased by insufficient precision in the fabrication. Thhe fabrication methods are con- stantly being improved to reduce such differences. In figure 4 modelled and measured results far a coupler with input into port l b  and transmission measured at port 2 is shown. This is the coupled result. Again a good resemblance is seen, though the calculated peak around 1350 nm is com- pletely missing in the experimental data. The reason for this i s  currently not fully understood. Between 1400 nm and 1480 nm the measured data are quite noisy, which i s  due to the weak- ness afthe source in this region together with the low level of coupling. 3 dB are added to the measured data here too. Looking at figures 3 and 4 a strong dependence of coupling on frequency can be seen. Around 1560 nm the uncoupled tranrmission drops rapidly more than 20 dB, and the srmcture acts as a low-pass fil- ter. For the coupled msmission in figure 4, the spectrum peaks amund 1545 nm with a peak level around 10 dB higher than the dip at 1500 m n  Conclusion We have investigated coupled wavemides in photonic cqst~i i -with triangular .!mhrlry b) 3D FDTD madclling and in rcil life srmple\. \ \e ha,? shoun. that nc ;an h h n c a r  w n n l e s  shou- ~~~~ ~~~~ ing qualitiiively coGect uansmissiin  spectra when compared to 3D FDTD modelling results. We have also shown, we can design and fabri- cate stmctures exhibiting highly wavelength selective transmission spectrum behaviour. These promising results have provided us with valuable insight into the behavior of coupled PBG waveguides, which can be used in the design and modelling of new, efficient compo- nents utilising such structures. References 1 1. D. Joannopoulos et al, Photonic Crystals: Molding the Flaw of Light, Princeton University PWCS 110951 2 A. J. Ward, and J. B. Pendry, Comp. Phys. Com., 128 (2000), 590. 3 N. Kawai et al, Phys. Rev. Len., 86 (ZOOI), 2289. 4 Y Sugimoto et al, Appl. Phys. Len., 79 (2001), 47Rh i-?-Sugimoto et al, @tics Len., 27 (ZOOZ), 388. 6 T. Ochiai, and K. Sakoda, Phys. Rev. B, 63 (2001), 125107. 7 M. Loncar et 81, Appl. Phys. Len., 80 (2002), 1689. 8 2  Smdngaardetal,OpticsCom., 194(2001),341. 9 A. Talneau et al, Appl. Phys. Len., 80 (2002), 547. 2 1495 Th16 1O:OO AM Fabrication and Characterization OILOW Loss Heterostruetured Photonic Crystal Waveguides Y. Ohtera, K. Miura, H. Ohkubo, T. Sato, S. Kawakami, NKHe,  Tohob UniversiQ, Sendai. Japan: T. Sato, S. Kawakami, Phoronic Lanice We report a fiber bun-jointable Ta,O$30, pho- tonic crystal waveguide with a record low-lass (0.56 dBimm). The waveguide has a heterosrmc- turc and is fabricated by the autocloning technol- ogy. Details are described. 1. Introduction In recent years, a growing number of research have been carried out to establish the waveguide technologies of photonic crystals, especially the types called "line-defect waveguides," which con- ftne light by the effect of photonic bandgap. The merits of employing such photonic cryslal waveguides are the availability of strong Bragg reflections of the guided modes and sharp bends. It is expected that one needs only a few square millimeters of circuit to integrate multiple func- tions such as wavelength filters and multiplexers1 demultiplexrers. However, one of n a n ~ v i a l  diffi- culties of such waveguides is the problem how to couple totfrom optical fibers: Typically, their mode spotsize is less than one micrometer, and bun-coGplmd results In a large loss. Thnc I )  no othrr choice than to " S I .  a number of bulky l enm We hasc SJ iar m " c d  and dewloDcd J novel class of photo& 'crystal wavepllfdes, which shows low-loss light propagation and fiberPLC compatible modal fields. In this study, we rede- signed the core structure and improved the waveguide performance of loss characteristics, hiRh-density circuit intemation capability, and - . .  e63y butt-c&pling. The following is the key of this work. (1) The minimum net propagation loss is 0.56 dBimm ath=1.55 pm. (2) The waveguide consists of a heterostructured photonic crystal. The core and the claddings over and under it have flat multilayer stmuctwe On the other hand. the side claddinis have two-dimen- sional periddic S t N C h u e  (3) The size of the core is 2 pm x 2.6 pm. The in- plane index difference between the core and the side eladdims is 3.6% ~~c ~ ~ ~ ~~~ ~~~ ~ (4) The spot at the waveguide end is circular and its diameter is 4.5 w. We thus report a significant advance of the waveguide as compared to our previous work [I]. The new waveguide is of course compatible with devices such as resonators [ I ]  etc. 2. Structure of the waveguide and the principle of the operation Fig. 1 shows a schematic view of the waveguide. It consists of a heterosmctured photonic crystal with multiple regions having different lanice typesiconstants [2, 31. It is fabricated by the auto- cloning method [4]. As shown in Fig. 2, the waveguide has nine different regions in its cross section. In the horizontal direction, a flat region of the width of 2 pm acts as a core, while wavy regions with the in-plane pitch of 1. Ipm around it act as cladding. For the vertical direction, the core layers and the cladding layers have the lattice constants of 0.73 pm and 0.60 pm, respectively, and the difference causes confinement of light. The light is confined in the center region of Fig. 2 and propagates along z direction. The constituent materials of the multilayer are Ta20 (n-2 1) and SiO, (n=1.5). Each region has an eff(ectiieiefrac- tive;ndex as shown in Fig. 3 for the horizontally polarired light of A=1.55 ptn. We make use of the effect that the flat multilayer has a larger effective index than that of wavy multilayers for horizon- tally polarired lipht. In this desip,  the relative index difference [A) between the core and the Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 03,2010 at 10:04:57 EST from IEEE Xplore.  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Properties of directional couplers using photonic crystal waveguides

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