Microscopy Used to Profile Refractive Index

Michael A. Greenwood

A microscopy technique that measures the refractive index profile of an optical waveguide has been devised by researchers in Taiwan.

An accurate profile is used, in turn, to determine the mode properties of the waveguide, which are important in several applications, including optical fibers used for communication and in integrated optical devices.

An existing method for refractive index profiling uses an objective lens and a CCD camera. This approach, however, has several shortcomings. Most importantly, the high-frequency noise in the measurement makes it nearly impossible to take the mode’s first and second derivatives, which are necessary to reconstruct a profile accurately.

The measured output of the near-field intensity of a single-mode fiber is shown at left (a). The measured image of the first derivative of output field intensity is shown in the center (b). The image on the right (c) shows the measurement of the second derivative of output field intensity. Reprinted with permission of Applied Physics Letters.

Attempts to solve the problem have relied on spatial smoothing of the measured intensity. Although this method is generally successful with one-dimensional index profiling, errors can result when the technique is applied to two-dimensional profiles, which are used to develop new optical waveguides.

Researchers from National Taiwan University and from Academia Sinica, both in Taipei, tested an alternative technique that uses differential near-field scanning optical microscopy to overcome the profiling problem.

Lead researcher Pei-Kuen Wei of Academia Sinica said that the optical setup was built around the controller of an atomic force microscope that was modified with a tapered fiber probe, which was connected to a Hamamatsu photomultiplier tube. A Melles Griot 632.8-nm-wavelength helium-neon laser was coupled into the optical waveguide, and the signal was read by three lock-in amplifiers. A computer received the amplitude and phase signals and rendered the near-field images.

The technique measured simultaneously the guiding mode distribution and the first and second derivatives of the optical waveguide. The refractive index profile was directly reconstructed by an inverse calculation algorithm. No fitting functions or filters were used in the calculations. The tapered fiber probe allowed also for subwavelength optical resolution.

The research team said that the experimental results show that the measured optical field distribution and refractive index profile closely match the calculated mode and the known index profile. The microscopy technique could allow for simpler and more feasible comparisons of various waveguide shapes; for instance, the many optical waveguides in integrated optical devices. Even small changes in the way these are manufactured could affect a waveguide’s size and distribution. A tool is needed that allows a range of manufacturing parameters to be tested.

Wei said that cost was one hurdle to creating the optical setup the team used to measure the index profiles. However, by using precise stages, the sample-probe distance could be maintained at less than 100 nm, eliminating the need for an atomic force microscope to regulate the distance and thus reducing expenses.

Applied Physics Letters, Aug. 6, 2007, Vol. 91, 061123.