G.D.Emmerson, C.B.E.Gawith, R.B.Williams and P.G.R.Smith

1. Ultra-wide planar Bragg grating detuning and 2d channel waveguide integration through direct grating writing G.D.Emmerson, C.B.E.Gawith, R.B.Williams and P.G.R.Smith Optoelectronics Research Centre, University of Southampton, SO17 1BJ, United Kingdom S.G.McMeekin, J.R.Bonar and R.I.Laming AVANEX Livingston, Starlow Park, Livingston, EH54 8SF

2. Outline • Goals of the work • Direct UV writing • How structures are defined • Direct Grating Writing • Comparison with existing techniques • Method of single-step processing • Grating detuning • Ultra-wide results • Unique features • Conclusions

3. Motivation • Goal • To devise a way of writing high quality Bragg gratings in planar waveguides • Issues: • 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!

4. Approach • Builds on two key techniques: • Direct UV writing into silica – particularly the work by Mikael Svalgaard, COM, Technical University of Denmark • Work in the ORC (and elsewhere) on writing of Bragg gratings in fibre using the phase mask stepping technique

5. Direct UV writing • UV 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

6. Planar Bragg gratings • Traditionally planar and fibre Bragg gratings fabricated in two steps: • Channel waveguide fabrication • Superimposed grating modulation through exposure to a UV interference pattern

7. Writing spot formed by crossing two focused 244nm beams Resultant spot has an inherent interference pattern Channels waveguides defined by translation with laser constantly on Modulating the laser during translation results in grating structure defined at the same time as a channel waveguide Direct Grating Writing

8. The writing spot contains the grating structure Can define channels or channels and Bragg gratings with the same neff Can use the maximum grating contrast possible Small writing spot allows rapid variation of grating parameters Single-step characteristics

9. Samples • Flame Hydrolysis Deposition (FHD) silica-on-silicon samples produced by Alcatel Optronics UK • Core layer co-doped with germanium to produce intrinsic photosensitivity • Samples Deuterium or Hydrogen loaded @150Bar for 1 week

10. Waveguide results • Illustrated: three cross-coupler structures written using DGW • ‘Strong’ waveguides as visible as etched structures to the naked eye • NA @633nm = 0.17±0.02 from far field imaging • Fibre-fibre loss @ 1.55µm ~2.5dB for a 30mm long channel waveguide

11. DGW structures ~11μm x 12μm Mode Profile @1.55μm Bragg grating spectral response as expected

12. Grating Response Control 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.

13. Detuned Grating Formation Example writing spot, 2.5µm wide with a Λ=500nm interference pattern • Beam modulated every 600nm (100nm different from interference pattern) • Grating with 600nm period built up, with reduced contrast • Detuning range inversely proportional to the number of grating plains in the spot

14. 0%span Detuning range

15. 2-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.

16. Wavelength Detuning All gratings written with a writing spot period of 532nm Grating period controlled only through software

17. Wavelength Detuning All gratings written with a writing spot period of 532nm Grating period controlled only through software

18. Unique features of Direct Grating Writing

19. Material Insight The 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

20. 1.4570 -2 34.4 KJcm 1.4565 -2 17.2 KJcm -2 8.6 KJcm 1.4560 1.4555 1.4550 1.4545 1.4540 1.4535 0 100 200 300 400 500 600 700 Temperature / °C Thermal properties • Gratings allow assessment of material thermal characteristics and stability Thermal annealing of grating (30 minutes per anneal step) Effective index Thermal tuning of grating

21. Dispersion measurement • Ultrawide detuning gives a powerful technique for measuring waveguide dispersion • Arguably ‘non-intrusive’?

22. Thermal locking • One problem with Direct UV writing is H2/D2 out-diffusion • Thermal locking (rapid heat treatment – 1200 to 1400C for a few seconds) locks the photosensitivity into the glass – lasts > 6months • Much 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.

23. Conclusions • Simultaneous writing provides a large control over the writing conditions for the Bragg grating structures • Grating parameters can be varied to give responses >30dB with a range of bandwidths • Small writing spot allows for almost unparralled flexibility in the grating period defined • The 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 setup • Gratings provide a power material characterisation tool