Embedded microstructures such as waveguides and gratings can be written in glass using focused femtosecond laser pulses. Because such structures consist only of variations in refractive index, they are not easy to characterize. Researchers at the University of Texas (Austin, TX) have come up with a differential phase-measuring instrument for measuring embedded microstructures that is rugged, requires no calibration, and can provide quantitative depth data as well as lateral data.1
The instrument, an optical low-coherence reflectometer (OLCR), is a dual-channel interferometer, with each channel an orthogonal polarization mode in a birefringent optical fiber. The light source is broadband, with a center wavelength of 1310 nm and a bandwidth of 60 nm. A 2 ¥ 2 polarization-maintaining coupler splits the two channels into reference and measurement paths. In the measurement path, a Wollaston prism and associated free-space optics create two parallel, spatially separated converging beams focused into the sample. The reference path contains an electro-optic phase modulator driven with a ramp waveform. The phase of each channel is detected and the phase difference evaluated as the two focused spots are scanned within the sample (basing the measurements on phase difference eliminates the common-mode phase noise).
In the experimental setup, the two 12-µm-diameter focused spots within the sample were separated laterally by 500 µm. For test, a linear index-altered microstructure was fabricated within an ordinary microscope slide. The two spots were focused onto the back surface of the slide and scanned at a constant velocity over a 500-µm distance. The scan was repeated 24 times in a raster fashion, with each scan displaced from the last by 200 µm. The resulting data were used to create a two-dimensional differential phase profile (see figure).
An optical low-coherence reflectometer (OLCR) obtains a two-dimensional phase-differential phase profile of a waveguide written in glass, seen here in false color (upper left), grayscale (upper right), and as a contour map (lower left). Another phase-measuring device, a differential-interference-contrast (DIC) microscope, also captured the image (lower right). Although the DIC microscope has higher resolution, the OLCR has other advantages, such as the ability to measure three-dimensional refractive-index distribution (not shown).
Even more interesting is the instrument's capacity for mapping the three-dimensional (3-D) refractive-index distribution of a glass volume. Such a map is obtained by scanning the sample in two different directions in the same plane, rather than one, according to Digant Davé, one of the researchers. The interferometer's broadband source and the resulting short coherence length makes this capability possible. Davé explains that in the first experiment, the sample dimension and surface quality of the glass slide were not suitable for scanning an entire cross-sectional plane at once. "We are in the process of constructing 3-D refractive-index maps of a number of photoinduced microstructures in glass," he says.
In addition to the 3-D scanning capability, advantages of OLCR over other index-profiling techniques such as differential-interference-contrast microscopy include the ability to profile microstructures in scattering or birefringent samples, the ease at handling phase changes of more than 2π, and the independence of the signal from variations in amplitude or modulation frequency.
REFERENCE
- D. P. Davé and T. E.Milner, Appl. Opt. (April 1, 2002).