S. E. Mechels

Accurate Measurements of the Zero-Dispersion Wavelength in Optical Fibers

We have developed a frequency-domain phase shift system for measuring the zero-dispersion wavelength and the dispersion slope of single-mode optical fibers. A differential phase shift method and nonlinear four-wave mixing technique were also investigated. The frequency-domain phase shift method is used to produce Standard Reference Materials that have their zero-dispersion wavelengths characterized with an expanded uncertainty (k = 2) of ± 0.060 nm.

High-resolution differential-mode delay measurements in optical fibers using a frequency-domain phase-shift technique

A frequency-domain phase-shift technique, with a temporal resolution of 0.2 ps, is used to obtain differential mode delay measurements in graded-index multimode fibers. This resolution is a significant improvement over previously reported time-domain methods. As a consequence, useful results can be obtained from fibers as short as 15 m. Measurements performed at 850 nm, on 62.5-/spl mu/m core diameter fibers from several different manufacturers, indicate a rich variety of mode delay profiles. Measurements on lengths ranging from 3 to 500 m, indicate that delay profiles are established in the first few meters of fiber, and the general characteristics are retained over long distances.

Optical Fiber Geometry: Accurate Measurement of Cladding Diameter

We have developed three instruments for accurate measurement of optieal fiber cladding diameter: a contact micrometer, a scanning confocal microscope, and a white-light interference microscope. Each instrument has an estimated uncertainty (3 standard deviations) of 50 nm or less, but the confocal microscope may display a 20 nm systematic error as well. The micrometer is used to generate Standard Reference Materials that are commercially available.

Video microscope with submicrometer resolution

We have constructed and evaluated a video microscope with a 150- x 150-microm field of view for performing measurements of optical fiber geometry. The microscope consists of a frame transfer video camera, condensing and filtering optics, a 40x, 0.65 N.A. microscope objective, and frame digitizing electronics. Using simple digital algorithms, we measure distance with a random uncertainty of approximately 40 nm across the full field of view, but width measurements suffer from a systematic error between 0.1 and 0.2 microm.

High resolution differential mode delay measurements and bandwidth of multimode fibers

Summary form only given. There is growing interest in the use of multimode fibers for high speed data networks. According to present industry test procedures, multimode fiber bandwidth is characterized using overfilled launching conditions. Such a launch is achieved by uniformly exciting the core and launching with a numerical aperture (NA) which exceeds the fiber NA. For many multimode fibers, the bandwidth increases when the launching conditions are restricted; the specific behavior, however, depends on the differential mode delay (DMD) characteristics of the refractive index profile. We have developed a frequency domain phase shift technique capable of acquiring DMD profiles with a temporal resolution less than 0.2 ps. DMD measurements are now possible on fiber lengths as short as 15 m.

Scanning confocal microscope for precise measurement of optical fiber diameter

We have constructed and evaluated a scanning confocal microscope for the precise measurement of optical fiber cladding diameter. The system measures the fiber endface directly and differs from conventional microscopes in that there is no systematic error due to partial coherence. The results obtained with the scanning confocal microscope are checked by comparison with those obtained from a contact micrometer and by measuring a chrome-on- glass standard reference material provided by NIST, Gaithersburg, Maryland. Fiber diameters can be measured with a random uncertainty of 40 nm and a systematic error estimated to be 40 nm.

Accurate zero-dispersion wavelength measurements in single-mode fibers: two frequency-domain methods

Accurate determination of the zero-dispersion wavelength (/spl lambda//sub 0/) is crucial for high bandwidth performance in single-mode fiber systems. We examine two dispersion measurement systems, based on the frequency-domain phase shift and differential phase shift techniques. Both systems are capable of measuring /spl lambda//sub 0/, with repeatabilities (precisions) of /spl plusmn/0.1 nm; however, their ultimate accuracies have yet to be determined. By comparing the two systems, we get an estimate of potential systematic errors. The systems are used to determine /spl lambda//sub 0/ in standard reference fibers.

Scanning confocal microscope for accurate dimensional measurement

We have improved and evaluated a scanning confocal microscope for the precise measurement of optical fiber cladding diameter. In particular, we have studied the systematic error that results from a finite detector aperture and concluded that the diameter of that aperture must be less than one-half the radius of the Airy disk in the detector plane. We compared our measurements with a chrome-on-glass standard reference material provided by NIST- Gaithersburg and with optical fibers that were measured with a contact micrometer. We estimate the overall uncertainty of our measurements to be around +/- 50 nm.