Distributed Bragg Reflectors in Ultra Low Loss Silicon Nitride Waveguides

• optics & opto-electronics • optics & opto-electronics • optics & opto-electronics • Page 142 Page a gratingchipwithactualwaveguides. etch process, yielding preliminary test chips (Figure1)and using electronbeamlithography(EBL)andadry the DBRs modal properties grating parameters by simulating variousetchdepthsand reasonable select to was project the of objective first The equally stringentrequirementsforwavelengthselectivity be foundinothernon-telecommunication applications with to adjust forchanges in the network. This element can also spaced single mode lasers, often able to tune wavelengths the wavelength selective mirror to enable an array of tightly DBRs are a fundamental element for these lasers providing optic networks whichspantheglobeshouldnotbeunderstated. fiber the enabling sources as integration Their age. one ofthehallmark inventions driving the information Though notwidelyknown,semiconductorlasershavebeen Introduction: Abstract: NNIN REUMentor: NNIN REUPrincipalInvestigator:JohnBowers,ElectricalandComputerEngineering,UniversityofCalifornia,SantaBarbara NNIN REUSite:Nanotech,UniversityofCalifornia,SantaBarbara,CA LowLossSiliconNitrideWaveguides inUltra Reflectors Distributed Bragg Contact: [email protected], [email protected],[email protected] Electrical Engineering and Computer Science, University of Nevada, Reno Nevada, of Electrical EngineeringandComputerScience, University Issa Beekun (2005 NNINREUInternatthe (SiO reflectorBragg distributed fabricate projectto this was of focus The (SLED) withanopticalspectrumanalyzer (OSA). diode emitting light superluminescent a using accomplished was Characterization wavelengths. Bragg and After mode. the of index refractive effective the fabrication, on it depending is then nm useful 140-520 to characterize ~ the periods gratings to with determine small their very respective scattering be loss, strengths, to need features grating the as techniques fabrication precise requires grating efficient An feedback. laser conductor lasers. Such gratings have the ability to reflect select wavelengths of light as a means of producing

2 )-clad silicon nitride (Si nitride silicon )-clad parameters ofthe fabricationprocess. Figure 1: A chipusedtotestthe . The nextpartwastosuccessfullyfabricate Jock Bovington,ElectricalandComputerEngineering,UniversityofCalifornia,SantaBarbara University of Washington)University 3 N 4 ) waveguide. DBRs are used extensively in the standard operation of semi­ of operation standard the in extensively used are DBRs waveguide. ) . is notscalable for productionoflarge numbersofdevices; as comparedto standardphotolithography, andthereforeit of alarge area. EBL can require alarge allotment of time select areasratherthanthestandard maskedsingleexposure light. EBL also uses a raster scan ofthe beam to expose opposed tostandardphotolithography processes,whichuse high resolution. Electron beam lithography (EBL) isatechnique with very Fabrication: structures couldbegin. for the length and strength of the gratings, fabrication of the matrixes. Fromtheseresultsandbyselecting ideal values periods as the product of multiple sets of 2 × 2 scattering by implementing ascript representing consecutive grating and program solver mode (FDM) method difference finite Further calculations were made with the aid of aMATLAB laser. test µm 1.045 a of specifications the around designs A simple equation was examined which allowed for initial gratings andtheequationswhichgoverntheiroperation[1]. literature search wasconducted to discover more about In ordertosimulate the relevant grating parameters, a Simulation: parameters wereinferredtoaidinfuturelaserdesign. emitted at theoutput.Bythismethod, fundamental grating single DBR and detecting the spectral response of the light thefabricated test to was DBRs bycoupling light into thewaveguide containing a project of this part final The

(DBR) structures into a silicon dioxide silicon structuresa (DBR) into It uses electrons to expose the resist, as 2011 NNIN REU Research Accomplishments Research REU NNIN 2011

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• optics & opto-electronics • optics & opto-electronics • optics & opto-electronics • Page 143 While a new chip was being fabricated, testing fabricated, was being new chip a While . Coldren, L., and S. Corzine. Diode lasers and photonic integrated 1995. Print. Wiley-Interscience, circuits. References: [1] Results and Conclusion: Results occurred during the etching to the project Complications varied with structures in resulted and fabrication of portion functionality the gratings were proceeded and spectral responses from as seen in Figure obtained for comparison with simulations to recorded being are new measurements this time, At 4. expectations. compare against the simulation generated This project demonstrates a comprehensive approach to of and testing fabrication, the principles, understanding distributed Bragg reflectors. Acknowledgements: I would like to acknowledge the Bowers Optoelectronics and the staff Daryl Spencer, Research Group, particularly at and Computer Engineering Department of the Electrical at Santa Barbara, for their help the University of California during this summer internship. I would also and guidance in organizing for her help Berenstein Angela to thank like NSF for the and Program REU NNIN the and program, the their roles in making my project possible. Figure Figure 4: Transmitted power through gratings of length 55, 111, 163, 230, 380, 2000 µm from top to bottom. Each spectrum has been normalized to a straight waveguide and shifted by the adjacent spectra for ease of viewing. from 2.5 dB After confirmation, the objective lens replaced was removed with a and fiber connected to the allowing OSA spectral response to be measured. Such a response was the test of the wavelength output the by sweeping obtained nm. Outputs from grating structures laser from 960-1160 gratings without waveguide straight a to normalized were likely model failed to the simulation and the spectra. Fitting This core. the into cladding through the an over etch due to is currently under investigation. Figure 3: Test setup for measuring 3: Test Figure spectral response from the gratings. from spectral response Figure 2: A 3D AFM of the fabricated gratings and and gratings fabricated the of AFM 3D A 2: Figure section. SEM of the grating cross corresponding waveguide. The resulting profile was imaged using a 4 N 3 . scanning electron microscope (Figure 2) Testing: the by polishing was prefaced DBR gratings the Testing . Spectral mirror-like rough facets of the chip until they were on and were done by coupling measurements transmission off the chip, with cleave fibers using index-matching gel, An infrared camera a broad band SLED source with OSA. lens setup as seen in Figure 3 lens column and objective served to confirm that light was coupled to the waveguide and not any slab modes and to confirm a TE polarization. however, however, it enables the flexibility needed . create gratings with tight space wavelengths to consistently force into the resist, atomic gratings After patterning microscopy (AFM) images were microscope optical an in visible not were they as patterns taken to confirm a series of dry etches in inductively (Figure 2). Finally, the double and chrome using an oxide etcher plasma coupled of our cladding oxide the to pattern the translated hard-mask Si 2011 NNIN REU Research Accomplishments