Presentation on theme: "AVANEX Livingston, Starlow Park, Livingston, EH54 8SF"— Presentation transcript:
1AVANEX Livingston, Starlow Park, Livingston, EH54 8SF Ultra-wide planar Bragg grating detuning and 2d channel waveguide integration through direct grating writingG.D.Emmerson, C.B.E.Gawith, R.B.Williams and P.G.R.SmithOptoelectronics Research Centre, University of Southampton, SO17 1BJ, United KingdomS.G.McMeekin, J.R.Bonar and R.I.LamingAVANEX Livingston, Starlow Park, Livingston, EH54 8SF
2Outline Goals of the work Direct UV writing Direct Grating Writing How structures are definedDirect Grating WritingComparison with existing techniquesMethod of single-step processingGrating detuningUltra-wide resultsUnique featuresConclusions
3Motivation Goal Issues: To devise a way of writing high quality Bragg gratings in planar waveguidesIssues:Number of possible solutions – based around fibre Bragg grating techniques applied to planar channels.Problem is the need for core uniformity – excellent in fibre – expensive to achieve in planar.Need constant b along waveguide!
4Approach Builds on two key techniques: Direct UV writing into silica – particularly the work by Mikael Svalgaard, COM, Technical University of DenmarkWork in the ORC (and elsewhere) on writing of Bragg gratings in fibre using the phase mask stepping technique
5Direct UV writingUV laser (244nm CW) focused down to micron order writing spot.Channel waveguide structures defined through relative translation between sample and writing spot.Translation controlled via computer control, no mask or subsequent processing required
6Planar Bragg gratingsTraditionally planar and fibre Bragg gratings fabricated in two steps:Channel waveguide fabricationSuperimposed grating modulation through exposure to a UV interference pattern
7Direct Grating Writing Writing spot formed by crossing two focused 244nm beamsResultant spot has an inherent interference patternChannels waveguides defined by translation with laser constantly onModulating the laser during translation results in grating structure defined at the same time as a channel waveguide
8Single-step characteristics The writing spot contains the grating structureCan define channels or channels and Bragg gratings with the same neffCan use the maximum grating contrast possibleSmall writing spot allows rapid variation of grating parameters
9SamplesFlame Hydrolysis Deposition (FHD) silica-on-silicon samples produced by Alcatel Optronics UKCore layer co-doped with germanium to produce intrinsic photosensitivitySamples Deuterium or Hydrogen for 1 week
10Waveguide resultsIllustrated: three cross-coupler structures written using DGW‘Strong’ waveguides as visible as etched structures to the naked eye= 0.17±0.02 from far field imagingFibre-fibre 1.55µm ~2.5dB for a 30mm long channel waveguide
11Bragg grating spectral response as expected DGW structures~11μm x 12μmModeBragg grating spectral response as expected
12Grating ResponseControl of the grating parameters through the writing conditions.Low contrast and high contrast gratings can be produced with minimal effect on the effective index of the channel.
13Detuned Grating Formation Example writing spot, 2.5µm wide with a Λ=500nm interference patternBeam modulated every 600nm (100nm different from interference pattern)Grating with 600nm period built up, with reduced contrastDetuning range inversely proportional to the number of grating plains in the spot
152-d Bragg grating incorporation Entire structure written in one go, with two gratings of differing period defined through detuning.Arm separation of 200µm, 8mm long gratings with periods of 532 and 532.4nm.
16Wavelength DetuningAll gratings written with a writing spot period of 532nmGrating period controlled only through software
17Wavelength DetuningAll gratings written with a writing spot period of 532nmGrating period controlled only through software
19Material InsightThe grating response gives a direct insight into the parameters of the structures written, e.g. birefringence of 1.2x10-4.Along with the relationship between writing conditions and the strength of the waveguide
20Thermal propertiesGratings allow assessment of material thermal characteristics and stabilityThermal annealing of grating(30 minutes per anneal step)Temperature / °C1002003004005006007001.45351.45401.45451.45501.45551.45601.45651.457034.4 KJcm-217.2 KJcm8.6 KJcmEffective indexThermal tuning of grating
21Dispersion measurement Ultrawide detuning gives a powerful technique for measuring waveguide dispersionArguably ‘non-intrusive’?
22Thermal lockingOne problem with Direct UV writing is H2/D2 out-diffusionThermal locking (rapid heat treatment – 1200 to 1400C for a few seconds) locks the photosensitivity into the glass – lasts > 6monthsMuch lower fluences are required to induce a waveguide than in the freshly loaded sample.The trend for lower writing powers to produce higher index changes remains but the discrepancy becomes less at lower fluences.Unlike thermally locked samples, the loaded samples exhibit a distinct threshold effect where channels are no longer written for fluences below 10KJcm-2. This effect is not shown in the ‘locked’ samples.
23ConclusionsSimultaneous writing provides a large control over the writing conditions for the Bragg grating structuresGrating parameters can be varied to give responses >30dB with a range of bandwidthsSmall writing spot allows for almost unparralled flexibility in the grating period definedThe period of the gratings can be varied to give responses over the O, E, S, C, L and U bands without any change to the fabrication setupGratings provide a power material characterisation tool