1. AVANEX Livingston, Starlow Park, Livingston, EH54 8SF
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 Direct Grating 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 Issues:
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. Direct Grating Writing
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

8. Single-step characteristics
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

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 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
= 0.17±0.02 from far field imaging
Fibre-fibre 1.55µm ~2.5dB for a 30mm long channel waveguide

11. Bragg grating spectral response as expected
DGW structures
~11μm x 12μm
Mode
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. Detuning range
0% span

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