Gratings in Electrooptic Polymer Devices Venkata N.P.Sivashankar 1, Edward M. McKenna 2 and Alan R.Mickelson 3 Department of Electrical and Computer Engineering, University of Colorado at Boulder, Boulder, Colorado 80309 ABSTRACT Herein we describe a process by which one can photobleach holographically generated index gratings into the volume of the guiding region of a nonlinear guided wave polymer device. Coherent superpositions of gratings with slightly different periods were written in thin, guiding, films of poly(methyl methacrylate) (PMMA) doped with disperse red 1 (DR1) dye and slabguides in DH-6 doped amorphous polycarbonate (DH-6/APC) made by Lumera Corporation. Results for the diffraction of light incident at right angles to a guiding region in PMMA/DR1 as well as for the spectrum of broadband radiation transmitted through the gratings written into DH-6/APC slabguides are presented. Keywords: gratings, polymers, electrooptic 1. INTRODUCTION Dye doped polymers generally exhibit large χ (3) nonlinear susceptibilities 1 and when poled can exhibit large χ (2) susceptibilities and electrooptic coefficient r. 2 When the dyes are those that exhibit a cis-trans isomerization resonance such as commonly used azo (for example DR1) or stilbene (for example DCM) dyes exhibit, the composite material can also be photobleached to lower the index of refraction 3 so that guiding structures can be produced by photobleaching alone. 4 As the photobleaching to transparency is generally used to define the inplane cladding, patterned photobleaching of smaller exposure time can also be used to write much more intricate structures into guiding regions themselves. It is just this process which we investigate in the present work. 2. FILM PROCESSING The PMMA/DR1 film used in this study is made by spin coating an ITO coated glass substrate with a 3 µm film of PMMA/DR1 in diglyme. The coating is then baked at 90 C for 3 hours and then baked at 120 C for 14 hours. The thickness of the film is measured with a Dektak profilometer. Bar electrodes for DC current conduction are then made on the ITO to locally heat the sample during the bleaching process. The slabguides made with DH-6/APC used in this study are fabricated by Lumera Corporation. The slab region is bounded below by a layer of epoxy thick enough to isolate the optical field from the silicon substrate. 3. WRITING COHERENT SUPERPOSITION GRATINGS The gratings are written into the films using three interfering beams generated using the 514 nm line of an Ar + ion laser. The setup used for writing the gratings into the PMMA/DR1 film is illustrated in figure 1. 1 e-mail: npvss@hotmail.com 2 e-mail: edward.mckenna@colorado.edu 3 e-mail: alan.mickelson@colorado.edu
The large angle between writing beams was approximately 21.8, and the small angle was approximately 0.6. Using the conservation of momentum between the beams the periodicity of a grating is given by, λ Λ = 2 sin(θ), (1) where Λ is the grating period, θ is the half angle between the two writing beams, and λ is the wavelength of the two interfering beams. Using equation 1, the two periods for the gratings written into the PMMA/DR1 film are approximately 1.35 µm and 22.10 µm. The intensity in each of the writing beams is set to approximately 50 mw/cm 2. Coherent Innova 300 Ar+ HeNe Photodetector M3 BS1 BS2 M5 PC 514 nm M4 Iris Lens 1 Spatial Filter Figure 1. The setup used to write a coherent superposition of two gratings into PMMA/DR1.In the figure, M1-5 are mirrors, BS1-2 are beams splitters and the photodetector response is monitored by a computer as indicated by the arrow. The setup for writing the grating in the DH- 6/APC waveguides was slightly different. In order to generate a smaller writing angle between two of the beams the output of a Mach-Zhender interferometer was used. The other arm of the interferometer is monitored with a CCD in order to measure the angle between the two beams via the interference pattern. The large angle for writing the gratings in the DH-6/APC waveguides is approximately 32.5, and the smaller angle was measured to be approximately 0.0736. Using equation 1, the periodicities for the two gratings in the medium is approximately 0.4783 µm and 200 µm. The fundamental period results in a Bragg wavelength given by, λ B = 2 nλ, (2) where λ B is the Bragg wavelength, Λ is the grating period and n is the effective index in the medium. Since the effective index of DH-6/APC waveguides is approximately 1.625, the Bragg grating formed in the waveguide is set to have a center wavelength of approximately 1.556 µm to match the spectrum of a standard 1.55 µm laser source. The beat period is set to give sidebands in the reflection spectrum that are approximately 6.5 nm apart from the fundamental. The intensity in each of the three beams used to write the grating into the DH-6/APC waveguides was set to 23 mw/cm 2. While writing the gratings in PMMA/DR1, a HeNe beam is used to monitor the development of the first order diffraction by coupling the diffracted light into a detector that is monitored by a PC. The monitor is only used for the PMMA/DR1 gratings since the HeNe laser will bleach the DH-6/APC. In order to relieve stress in the sample and to reduce the bleaching time, the PMMA/DR1 and DH-6/APC waveguides were heated during the writing process. The PMMA/DR1 films were heated by passing a DC current through the ITO coating on the substrate, while the DH-6/APC was heated indirectly by mounting the sample onto an ITO coated piece of glass which was also heated by passing an DC current through the ITO coating. The surface temperature of the films was set by changing the DC current and measuring the surface temperature of the film with an external probe. The gratings were written into the PMMA/DR1 films at a temperature of approximately 110-120 C, and 65-80 C for the DH-6/APC waveguides. The total exposure time for the PMMA/DR1 grating was approximately 4.00 hours, while for the DH-6/APC film the total exposure time was approximately 2.25 hours. M1 M2 4. COHERENT AND INCOHERENT SUPERPOSITIONS Gratings can be superimposed in a medium coherently or incoherently. A coherent superposition of two gratings is generated by using three beam interference. This generates a coherent superposition because it inherently establishes a known phase relation between the two gratings formed by the three beam interference. An incoherent superposition of two gratings can be generated by first writing one grating and then writing the second grating after the initial exposure. There would be no defined phase relation between the two gratings and thus an incoherent superposition would be formed. Other types of incoherent gratings can be formed using phase masks,
a common technique used for making fiber Bragg gratings (FBG). The normalized intensity incident on the film for the coherent and incoherent cases is illustrated in figure 2. It can be observed that there are quite pronounced N o rm a liz e d In te n s ity (a.u.) N o rm a liz e d In te n s ity (a.u ) - 0 0 0 6-0 0 0 4-0 0 0 2 0 0 0 0 0 0 0 2 0 0 0 4 0 0 0 6-0 0 0 6-0 0 0 4-0 0 0 2 0 0 0 0 0 0 0 2 0 0 0 4 0 0 0 6 (a ) (b ) N o rm a liz e d In te n s ity (a.u ) N o rm liz e d In te n s ity (a.u.) 0 0 0 1 8 0 0 0 2 4 0 0 0 3 0 0 0 0 1 8 0 0 0 2 4 0 0 0 3 0 (c ) (d ) Figure 2. The intensity distributions for gratings written both the (a)coherent and (b) incoherent cases. A magnified region of the (c) coherent and (d) incoherent cases that clearly illustrates the phase relation between the superpositions. differences in the index profiles bleached into the medium between the coherent and incoherent cases depending upon the phase difference established when the second grating is bleached in the incoherent case. The far field intensity pattern of an incoherent superposition of two gratings is illustrated in figure 3(a). The far field intensity pattern for a coherent superposition is illustrated in figure 3(b). In te n s ity (a.u.) (a ) In te n s ity (a.u.) 7 0 0 7 5 0 8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 P o s itio n (m m ) (b ) Figure 3. A theoretical plot of the far field diffraction pattern off of an (a)incoherent superposition of two gratings written into a medium and a (b) coherent superposition of two gratings written into a medium.
5. GRATING CHARACTERIZATION The gratings written into the PMMA/DR1 film were observed with a differential interference contrast(dic) microscope as illustrated in figure 4. The two periodicities are clearly visible in the image. The diffraction pattern Figure 4. Differential interference contrast image of double grating written into PMMA/DR1. The large period visible is approximately 20 µm and the small period is approximately 1.3 µm. from the gratings was then observed by illuminating the bleached region with a HeNe beam and capturing the far field diffraction pattern with a camera located approximately 165.0 mm from the film. The captured diffraction pattern is illustrated in figure 5. When compared to the theoretical plots in figure 3, it is clear that our gratings P o s itio n ( m m ) 1 8 7.5 1 2 5.0 6 2.5 P o w e r (a.u.) 0 6 2.5 0 1 2 5.0 0 P o s itio n 1 8 7.5 0 ( m m 2 5 0 ) Figure 5. (Top) CCD capture of diffraction pattern of double grating written into PMMA/DR1.(Bottom) One line from CCD capturing the diffraction of a coherent superposition of two gratings written into PMMA/DR1. are a coherent superposition. The measurement of the gratings written into the DH-6/APC waveguides were made by end-fire coupling an erbium fiber spontaneous emission source into the waveguide and measuring the output spectrum of the waveguide using an optical spectrum analyzer. The difference of the input and output spectrum is then taken to
measure the strength of the gratings written into the waveguide. The measured spectra and their difference are illustrated in figure 6. It is still clear from looking at the difference between the two spectra that the coherent 3 0 2 5 L o s s (d B ) 2 0 1 5 1 0 P o w e r (d B m ) -5 0-6 0-7 0-8 0-9 0-1 0 0 1.5 2 1.5 4 1.5 6 1.5 8 W a v e le n g th (µm ) superposition of two gratings is present in the film. Figure 6. Spectra of erbium fiber laser (EFL) source used to excite gratings written into the DH-6/APC waveguides. From the spectra after the grating it is clear that the center wavelength is located at approximately 1.565 µm and the beat results in modulation with a period of approximately 6.5 nm. 6. DISCUSSION In optical system applications such as that of wavelength division multiplexing (WDM), one desires wavelength dependent transmission that exhibits passbands and stopbands. Apodized waveguide gratings are ideal for such applications. Multiple periodicity gratings offer other advantages. By varying the spacing and depth of the individual gratings of a superposition of gratings, one can separately control the phase and group velocity in the wavelength region between the grating stopbands. Although we have not been able to go so far in this work as to demonstrate such control, we have demonstrated that we can write such structures in a material that we know exhibits strong nonlinearity. ACKNOWLEDGMENTS The authors would like to acknowledge that the slab waveguides with the DH-6 APC active layers were provided by Lumera Corporation as a part of an existing research collaboration. We would also like to thank Sharon King and Dr. Carol Cogswell of the University of Colorado at Boulder s Imaging Systems Laboratory (ISL) for providing the DIC image of the grating. REFERENCES 1. C. W. Dirk and M. G. Kuzyk, Squarillium dye-doped polymer system as quaratic electrooptic materials, Chem. Mater. 2, pp. 4 6, 1990. 2. K. D. Singer, J. E. Sohn, and S. J. Lalama, Second harmonic generation in poled polymer films, Appl. Phys. Lett. 49, pp. 248 250, 1986. 3. D. Tomić and A. Mickelson, Photobleaching for optical waveguide formation in a guest-host polyimide, Appl. Opt. 38, pp. 3893 3903, 1999. 4. W. Feng, S. Lin, R. B. Hooker, and A. R. Mickelson, Study of uv-bleached channel-waveguide performance in nonlinear optical polymer films, Appl. Opt. 34, pp. 6885 6891, 1995.