Measuring tiny mode-field diameters allows efficient coupling

Lensed and tapered specialty fibers are designed to optimize coupling between the fiber and optical components such as edge-emitting laser diodes and the waveguides in an integrated optical device. Researchers at Photon Inc. (San Jose, CA) investigated techniques for measuring tapered fibers' very small mode-field diameters (MFDs). The most efficient coupling occurs when the MFD of the joined components is identical.

The MFD represents a measure of the transverse extent of the electromagnetic field intensity of the mode in a fiber cross section.¹ For 1310 and 1550 nm, the MFD of single-mode fiber is 9.2 and 10.5 µm, respectively-significantly larger than the MFD of most waveguide channels. In addition, as the MFD gets smaller, the loss due to small MFD mismatch increases-up to a few dB for a one micron of MFD mismatch. Therefore, as the waveguide channels get narrower, one must take the MFD of the fibers and waveguides into account to ensure that components have low (or at least acceptable) losses.

Near-field techniques used to measure MFD below 10 µm are difficult to perform, and produce questionable results because of the required sample positioning accuracy, limitations of the instrument optics, and large divergence of the light from the fiber. Instead, say Photon researchers Jeff Guttman and Derrick Peterman, a better approach for measuring MFDs smaller than 10 µm is to measure the far-field distribution pattern of the beam using a scanning goniometer. In the far field, a scanning goniometer with a radius of 8 cm or more minimizes the dependence of the results on sample position-allowing testers to use simple sample positioning schemes. Also, with a goniometer, light from the fiber is measured without optics (which introduce error).

Photon studied six commercially available tapered and/or lensed fibers to understand the issues surrounding MFD measurements of these fibers. The results were generated using a Photon Model LD 8900 HDR Far-Field Profiler with more than 60 dB of dynamic range and a scanning radius of 13.26 cm.^{2, 3} The fibers were coupled to a stable, narrow-linewidth source operating at a nominal wavelength of 1550 nm at an output power of 7 dBm (see figure). They used the direct far-field method employing the Petermann II integral (outlined in TIA/EIA FOTP Standard 191) to generate the MFD results.

The tapered/lensed fibers tended to produce highly non-Gaussian beams. This calls into question the validity of Gaussian approximations for modeling these devices. The MFD of the fibers could be reliably determined from far-field data using the Petermann II integral from the standard, as long as a wide angular scan of 120° (full angle) was used. Currently, TIA/EIA FOTP Standard 191 requires a scan angle only one third as large. The researchers continue investigating MFD measurements on other samples and geometrical forms.

For more information contact Derrick Peterman at [email protected].

Fiberoptic Test Procedure FOTP-191, Telecom. Ind. Assoc., Arlington, VA, (1998).
U.S. Patent 5,949,534, "Goniometric Scanning Radiometer," (Sept. 7, 1999).
J. L. Guttman, R. Chirita, and C. D. Palsan, Symp. Opt. Fiber Meas., Natl. Inst. of Standards and Technol., Boulder, CO (Sept. 26-28, 2000).