Confocal spectroscopy assesses glass waveguide structures

During the past couple of years, interest in laser writing of waveguides in glass has blossomed as a potential enabling technology for fabrication of integrated optics, as evidenced by a plethora of papers on the subject presented at Photonics West (San Jose, CA) in January.

"This field is so new that the presentation of reproducible results by a number of different groups is significant," said Denise Krol, coauthor of a joint paper by University of California-Davis and Lawrence Livermore National Laboratory (LLNL; Livermore, CA), which was presented at the meeting by lead author James Chan.

In conjunction with adding their waveguide fabrication results to the rapidly growing literature, the Davis and LLNL researchers also used a confocal microscopy system to probe the structure of the resultant waveguides spectroscopically.¹

"When you write a waveguide, you basically create a structure that has a higher refractive index than the surrounding material," she said. "And we were interested in finding out in what way the changed material is different from the starting material on an atomic-scale structural level."

The team actually fabricated its devices by directing 130-fs, 800-nm pulses from a regeneratively amplified Ti:sapphire laser with a 1-kHz repetition rate and energies ranging from 0.1 to 2 μJ through a 10X objective to a depth of about 700 μm with polished silica-glass cubes that were moving along the beam axis at 20 μ/s. A 633-nm, continuous-wave (cw) HeNe laser beam was also directed through the same 10X objective to couple the light into the fabricated devices.²

The confocal microscopy setup that was used to perform fluorescence and Raman spectroscopy was based on a 488-nm, cw argon-ion laser beam as the excitation source, which was focused into sample along with the femtosecond pulses through a 50X microscope objective (see figure).

"We write this line and it's only a micron or so in diameter," said Krol. "So normal spectroscopy wouldn't be able to see this particular feature in the background of the surrounding material." Confocal spectroscopy allowed an atomic-scale investigation of the structure. The Raman spectroscopy provided an overall structural view that confirmed an increase in material density that had been previously hypothesized. The fluorescence spectroscopy revealed structural defects in the form of broken bonds that could reduce the effectiveness of laser writing as a production method.

The index of refraction change that makes the waveguide is primarily due to a structural rearrangement of molecular bonds because of the rapid heating and cooling caused by the femtosecond pulses. But bonds that remain broken after the writing process also may contribute to the index change, said Krol. Because those broken bonds anneal with time (and even under the illumination of the low-power excitation beam), their effects might cause the refractive index of a waveguide to also change with time.

This would obviously not be acceptable for manufactured devices, so the question arises as to how much, if any, of the index change in laser-written waveguides is caused by broken as opposed to rearranged molecular bonds. "At the moment people don't know the answer and we think we have a way to measure it," Krol said.

"The bottom line is that in order to understand what range of index changes you can make and what kinds of waveguides you can write in silica, as well as whether you can make these devices in other types of glasses, you need to understand what kind of structural rearrangement results from the femtosecond laser."

REFERENCES
1. J. W. Chan et al., Opt. Lett. 26(21), 1726 (Nov. 1, 2001).

2. J. W. Chan et al., SPIE Proc. Photon. West 2002, in press.