Burghoff, J.

Nolte, S.

Tünnermann, A.

Will, M.

English

Repositorium für Naturwissenschaften und Technik  (TIB Hannover)

Femtosecond laser writing of waveguides in glass: Α new way to3D integrated opticsS. Nolte, M. Will, J. Burghoff, A. TünnermannFriedrich-Schiller-University Jena, Institute of Applied Physics,Max-Wien-Platz 1, 07743 Jena, GermanyIntroductionModern communication technology is based on integrated optical devices, e.g.Splitters, connectors, filters, Switches, active and passive waveguides, to control thepropagation of light in all-optical networks. Today these elements are usuallyproduced by ionic exchange or diffusion into a transparent Substrate or bylithographic methods. Although these technologies are well established andsuccessfui, their use is in general limited to the fabrication of planar structures.In the past years a novel technique based on the use of ultrashort laser pulses forthe fabrication of integrated optical devices has been demonstrated.^'^ When theseintense pulses are focused into the bulk of a transparent material nonlinearabsorption in the focal volume takes place. At sufficient high intensities this will leadto optical breakdown and the formation of a micro-plasma and, in consequence, topermanent microstructural and refractive index modifications. By moving the samplewith respect to the focal spot buried refractive index profiles can be generated whichallow to guide light. Although the physical effects leading to the refractive indexchanges are still under discussion, several integrated optical devices have beendemonstrated in various types of glasses^'' and also crystalline media.^In this presentation an overview of the recent advances in this field will be given.The fabrication of true three-dimensional integrated optical elements with details ofthe optical properties as well as the dependence on the production parameters willbe discussed.ExperimentalOptical waveguides are fabricated using a commercial Ti:sapphire femtosecondlaser System (Spectra-Physics, Spitfire), which produces 50-fs pulses with a repetItion rate of 1 kHz at a wavelength of 800 nm. To generate the refractive indexmodifications pulses with an energy of 1 pJ are focused approximately 200 pmbelow the surface of the polished Substrate using a 20x microscope objective (NA  0.45), which is corrected for a focal depth of 170 pm. The sample can be movedperpendicular to the laser beam axis by a computer-controlied two-axis positioningSystem (Physik Instrumente, M-126.DG). Three-dimensional optical devices can befabricated by varying the focal depth inside the sample using another computer-controlied translation axis. Α schematic of the principle set-up is shown in Fig. 1.Fig. 1: Schematic of the experimental setup.The produced devices are characterized with respect to their optical properties bypolishing the end-surfaces of the samples and coupling light at differentwavelengths via a single-mode fiber into the structures. By imaging the exit surfaceonto a CCD camera the near-field intensity distribution of the guided light isobtained. This is compared to simulations using the commercial programBeamPROP 5.0 (Rsoft, Inc.). The caiculations are based on measurements of therefractive index profiles of the written structures by a refracted-near-field (RNF) profilometer (Rinck Elektronik, Germany). While these measurements directly yieldthe refractive index profiles, absolute values of the refractive index are obtained byrepeating measurements with a reference sample and an immersion fluid with well-known refractive indices. ~ =Results and discussionFigure 2 shows the near-field intensity distribution at a wavelength of 800 nm of a straight waveguide fabricated in fused silica by using a pulse energy of 1 pJ and a writing speed of 100 pm/s. The damping losses are determined to < 0.8 dB/cm. Atpresent, we are evaluating how much of these losses can be attributed to theimperfections of the positioning S y s t e m (vibrations, smoothness of travel, etc.).Β Ο0 1 2 3  4 5 6 7 8 X [μιη]Fig. 2: Near-field intensity distribution at a wavelength of 800 nm at the end of a straight waveguide.In addition to the single-mode waveguide visible in Fig. 2 also multi-modewaveguides with a defined number of guided modes can be fabricated simply byvarying the writing speed, i.e. the spatial overlap of successive pulses and, thus, theenergy deposited.^As a true three-dimensional photonic device we have fabricated a 1x3 Splitter infused silica. The total device length is 10 mm and the exits are separated by100 pm. The split arms are written under angles of 120°, which means that none ofthe exit ports is lying in the same plane as the entrance arm. In order to minimizethe losses the Single arms are formed as so-calied S-bends, i.e. with a smoothcurvature with minimal bending radius and without discontinuities. With these Sbends lateral displacements of more than 100 pm can be achieved after a length of10 mm without significant losses, which has been proven experimentally.Figure 3a shows the near-field intensity distribution at the exit of the 1x3 splitter at a wavelength of 1.05 pm. The Splitting ratio is almost equal. The Splitting lossesamount to 6 dB in this case, which have been determined by measuring the overalltransmission losses and comparing these data with a straight waveguide writtenwith the same parameter.--Fig. 3:(a) Near-field intensity distribution at 1.05 μηι measured at the exit of theSplitter. The Splitting ratio is 32:33:35. (b) Caiculated near-field intensitydistribution.Based on the measured refractive index distribution of one arm of the splitter (Fig. 4)the near-field intensity distribution was caiculated. As Fig. 3b shows, theexperimental data are in good agreement with the simulations. Additionalsimulations, where artificial positioning errors were introduced, revealed that thepositioning S y s t e m must have a positioning accuracy better than 1 pm in order tofabricate such devices. These requirements can be met by commercial Systems.An im 0.5 1.0 1.5Fig. 4: Measured refractive index distribution of one arm of the 1x3 splitter.Conclusion and OutlookIn conclusion buried optical waveguides have been fabricated inside fused silicausing femtosecond laser pulses. Single- and multimode waveguides have beenproduced with damping losses below 0.8 dB/cm. However, this technique does notonly allow for the fabrication of straight waveguides. It offers the possibility toproduce true three-dimensional photonic devices. As an example, a threedimensional 1x3 splitter with a total length of 10 mm and exits separated by 100 pmhas been fabricated in fused silica.While all devices discussed above are based on fused silica several other glasses,doped and undoped, as well as crystalline media can be structured using ultrashortlaser pulses. The combination of different materials with the flexibility of thefemtosecond direct writing technique allows for new complex high-density integratedoptical elements. As an example, Fig. 5 shows a phosphate glass sample (SchottIOG-1) with an undoped area (left) and a part doped with ErA b (right). The samplewas produced by bonding the two phosphate glasses together.^ Using thefemtosecond laser a waveguide has been written through the bond. Identical writingParameters were used for both sides. In Fig. 5 only the fluorescence of light in thedoped part of the glass is visible, while the pump light at 800 nm is coupied into theproduced waveguide in the undoped region. In combination with the 3D structuringpossibilities this demonstrates the potential for the realization of complex devicesusing the femtosecond direct writing technique.Fig. 5: Femtosecond written waveguide in a bonded glass consisting of a doped(Er/Yb) and an undoped region. Pump light at 800 nm is coupied into thewaveguide in the undoped region. The waveguide was successfully writtenthrough the bond, as the fluorescence from the doped region demonstrates.One of the main drawbacks so far, the limited writing speed in the ränge of 1 mm/s,can be overcome by using higher repetition rate lasers. New developments inultrafast laser sources allow pulse energies in the order of 1 pJ at repetition rates inthe MHz r ä n g e . I n a first experiment with a fiber-based laser System producing1 pJ, 300 fs pulses at a repetition rate of 2 MHz we were able to write waveguides infused silica at a speed of 100 mm/s limited only by the positioning System. The-' waveguides showed a refractive index increase of 10"^, and preliminarymeasurements of the damping losses yielded values below 0.5 dB/cm. This enablesflexible structuring at high speed, which makes this technology interesting even formass production.AcknowledgementsWe thank M. Brinkmann from SCHOTT Glas, Germany, and J.S. Hayden fromSCHOTT Glass Technologies, Inc. USA, for usefui discussions and the Provision ofthe bonded glass samples.References1. K. M. Davies, K. Miura, N. Sugimoto, and K. Hirao, Opt. Lett. 21, 1729 (1996).2. K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, and K. Hirao, Appl. Phys. Lett. 71,3329 (1997).3. D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, Opt. Lett.24, 1311 (1999).4. C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, Opt. Lett. 26, 93 (2001).5. K. Minoshima, A. M. Kowalevicz, I. HartI, E. P. Ippen, and J.G. Fujimoto, Opt.Lett. 26, 1516(2001).6. A. M. Streltsov and N. F. Borrelli, Opt. Lett. 26, 42 (2001).7. M. Will, S. Nolte, B.N. Chichkov, A. Tünnermann, Appl. Opt. 41 , 4360 (2002)8. T. Gorelik. M. Will, S. Nolte, A. Tünnermann, U. Glatzel, Appl. Phys. Α 76, 309(2003)9. S.D. Conzone, J.S. Hayden, D.S. Funk, A. Roshko, D.L. Veasey, Opt. Lett. 26,509 (2001)10. E. Innerhofer, T. Südmeyer, F. Brunner, R. Häring, A. Aschwanden, R.Paschotta, C. Hönninger, M. Kumkar, U. Keller, Opt. Lett. 28, 367 (2003)11. J. Limpert, T. Clausnitzer, A. Liem, T. Schreiber, H.-J. Fuchs, H. Zellmer, E.-B.Kley, A. Tünnermann, „High average power femtosecond fiber CPA System",Opt. Lett. (accepted)

Femtosecond laser writing of waveguides in glass: Α new way to 3D integrated optics

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Femtosecond laser writing of waveguides in glass: Α new way to 3D integrated optics

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