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Micro/Nano Lithography

Improved waveguide characterization using UV-written Bragg gratings

Using specialized UV irradiation, high-quality Bragg gratings can be used to characterize and optimize the effective index, birefringence, and performance of planar waveguide circuits.
11 July 2006, SPIE Newsroom. DOI: 10.1117/2.1200605.0178

Bragg grating optical filters photoinduced in waveguides are cost-effective devices for applications in optical networks, waveguide lasers, and optical sensors. Bragg gratings in Ge-doped silica planar waveguides increase the versatility of planar lightwave circuits (PLCs), which already have the advantage of being compatible with well-established semiconductor processing technologies. PLC devices—such as wavelength division multiplexers (WDMs), photonic beam-formers, external-cavity lasers and optical sensors that measure surrounding refractive index—all benefit from the incorporation of Bragg grating technology. However PLCs suffer from an intrinsic birefringence that make Bragg grating resonances sensitive to the polarization of the signal light. This results in polarization-dependent signal errors. The challenge for PLC devices with integrated Bragg gratings is the repeatability of their fabrication, as the induced refractive index profile is sensitive to variations in the physical dimensions of the waveguide and UV laser exposure conditions. Accurate monitoring and control of small changes in waveguide effective index, birefringence, and size are important.

Fabrication methods that minimize waveguide birefringence are often complex. Measuring waveguide effective index neff and birefringence can also be a challenge. Methods such as the prism coupler technique cannot be used for strip waveguides with cross-sectional dimensions of a few microns because of alignment issues. Atomic force and scanning electron microscopy (AFM and SEM) have been used to measure small dimensional variations of waveguides, but these methods are time consuming to implement. In our lab, UV irradiation methods have been developed for waveguide neff and birefringence measurement as a function of waveguide dimension and the rapid compensation of high intrinsic PLC birefringence.1,2

UV-induced birefringence, resulting from UV irradiation during Bragg grating formation, is effective for compensation of intrinsic waveguide birefringence. In planar waveguide structures, the resonant Bragg wavelength λb of a grating is given by λb = 2Λ neff, where Λ is the grating period. neff is a function of the waveguide dimension (core width and height), temperature, and stress. Generally, the shift of λ b is given by Δλb = 2Λ Δn. For a fixed Λ, small variations in dimension, temperature, or stress of the planar structure will cause a change of neff. The polarization-dependent Bragg wavelength shift is given as λb(TE) – λb (TM) where λ b(TE), λb (TM) are the Bragg wavelengths for TM and TE modes, respectively.

Large UV-induced birefringence is generated with UV ArF excimer laser irradiation polarized normal to the waveguide axis and is further enhanced through hydrogen loading of the waveguide. Planar waveguides made using plasma-enhanced chemical vapor deposition (PECVD) have an intrinsic birefringence (6.3 × 10-4). It is possible to produce polarization-insensitive Bragg gratings (PIBGs) in these waveguides by performing a low fluence blanket UV polarized exposure after the grating inscription, thus UV-trimming the birefringence to zero (see Figure 1).


Figure 1. Transmission spectra for TM (solid line) and TE (dashed line) modes of the Bragg grating written in a hydrogen-loaded waveguide with UV s-polarization exposure.
 

Changes of waveguide neff and birefringence are obtained by monitoring shifts of λb(TE) and λb(TM) with UV exposure for different waveguide dimensions (see Figure 2). Initial neff and birefringence are determined by extrapolating the curves to zero exposure time. Repeatability of the waveguide fabrication process is investigated by writing gratings simultaneously on ten adjacent identical waveguides and correlating the variation in initial neff or Δλb with a waveguide dimensional variance. A Δλb = ±0.1nm corresponds to ±0.1μm dimensional variance of the ridge waveguides tested.


Figure 2. Bragg wavelength shift for TE (square) and TM (triangle) modes as a function of UV exposure time for different waveguide dimensions. A, B, C, D and E denote waveguides with a height of 5.6μm and widths w of 8.8, 7.7, 6.6, 5.7 and 4.6μm respectively.
 

Open-top ridge waveguides (see Figure 3) provide increased surface area for sensing the external environment refractive index nex. A 50pm shift of λb corresponds to a 3 × 10-4 change of nex. With PIBGs, the measurement instability due to Bragg grating polarization dependence is improved.3 Coupling of the open-top ridge waveguide and the PIBG offers many advantages over previously proposed waveguide sensor technology, including enhanced sensitivity, better polarization stability, and a simpler fabrication process.


Figure 3. The open-top ridge waveguide Bragg grating refractometer.
Conclusion

Birefringence induced by polarized UV irradiation has been used to fabricate PIBGs in PLCs that improve the quality of WDM and biosensor devices. High repeatability in PIBG device fabrication requires control of UV irradiation to obtain an overlap of the TE and TM Bragg modes and maintain the specific Bragg spectrum. This technique can be used to evaluate the waveguide dimensional control of photolithographic and reactive ion etch processes.


Authors
Xiaoli Dai, Stephen Mihailov
Communications Research Centre Canada
Ottawa, Canada
Xiaoli Dai received her BEng and MEng degrees from the Department of Optical Instrument Engineering of Tianjing University of China, and her PhD degree from the Department of Electrical and Electronic Engineering of Niigata University of Japan. From 1996 to 1999, she was a research fellow at National Research Laboratory of Metrology in Japan. In 2001, she joined the Communications Research Centre Canada as a research scientist. Her research interests include fiber Bragg gratings, planar waveguides and optical measurements.
Stephen Mihailov received his BSc degree in physics from Carleton University, Ottawa, Canada, in 1986, and his PhD in physics from York University, Toronto, in 1992. He was with the Natural Sciences and Engineering Research Council of Canada (NSERC) from 1992 to 1994 where he conducted research on laser-material processing of polymers and glasses at the L'Université de Bordeaux I, France, and at Exitech Ltd., Oxford, United Kingdom. Between 1994 and 1996, he was with JDS-Fitel Incorporated, Nepean, Canada, where he worked on the development of passive optical component technology and was the project leader for the development of fiber Bragg grating technology. In 1996, he joined the Communications Research Centre, Canada, where he is now the Research Program Manager, conducting research on fundamentals and fiber Bragg gratings.