B. L. Danielson

Absolute optical ranging using low coherence interferometry

We describe a method for measuring submicrometer distances with an asymmetric fiber Michelson interferometer having an LED as a source of radiation. By measuring the phase slope of the Fourier components in the frequency domain, it is possible to locate the position of reflections with nanometer precision even in the presence of sample dispersion. The method is compatible with time domain sampling at the Nyquist rate which assures efficiency in data acquisition and processing.

Guided-wave reflectometry with micrometer resolution.

We describe a new type of optical reflectometry which is useful in testing single-mode lightguide systems. This technique uses a scanning Michelson interferometer in conjunction with a broadband illuminating source and cross-correlation detection. High resolution is achieved through the limited coherence of the backscattered radiation. With this approach it is possible to distinguish scattering centers separated by only a few micrometers. In some cases loss may be estimated for components in the transmission path of a test lightguide. The basic principles of this diagnostic technique, along with some performance characteristics, are illustrated for an all-fiber reflectometer. We also discuss several laboratory applications which serve to demonstrate the resolution capabilities of this measurement concept.

Accurate frequencies of molecular transitions used in laser stabilization: the 3.39‐μm transition in CH4 and the 9.33‐ and 10.18‐μm transitions in CO2

The frequencies of three lasers stabilized to molecular absorptions were measured with an infrared‐frequency synthesis chain extending upwards from the cesium frequency standard. The measured values are 29.442 483 315 (25) THz for the 10.18‐μm R(30) transition in CO2, 32.134 266 891 (24) THz for the 9.33‐μm R(10) transition in CO2, and 88.376 181 627 (50) THz for the 3.39‐μm P(7) transition in CH4. The frequency of methane, when multiplied by the measured wavelength reported in the following letter, yields 299 792 456.2(1.1) m/sec for the speed of light.

Optical time-domain reflectometer specifications and performance testing

From a researchers as well as a users point of view, it is highly desirable to adopt a common basis for specifying optical time-domain reflectometer performance parameters. This paper proposes some procedures and test methods which permit these devices to be characterized in a consistent way. Passive test fixtures are also described which may facilitate measurements of dynamic range and other reflectometer properties.

Josephson junctions at 45 times the energy‐gap frequency

Superconductive Nb–Nb point contacts have been studied with 9.5‐μm radiation from CO2 lasers. Two models are considered to explain the experiments: one is Werthamers Josephson junction model and the other is a thermally modulated Josephson junction. The evidence favors Werthamers model but is not conclusive.

Accurate Rotational Constants, Frequencies, and Wavelengths from 12C16O2 Lasers Stabilized by Saturated Absorption

New experimental measurements of the frequency separations of 30 pairs of 12C16O2 laser lines in the 10.4 µm band and 26 pairs in the 9.4 µm band have been made with lasers stabilized to Lamb-dip-type resonances observed in the 4.3 µm fluorescent radiation. The use of a Josephson junction as the frequency mixing element simplified the measurements. Uncertainties in existing rotational constants for the laser vibrational levels were reduced 20 to 30 times and an additional rotational constant, Hv was determined for the first time. Frequency and wavelength tables with estimated uncertainties are calculated for both bands with the new constants.

Interlaboratory measurement comparison to determine the attenuation and bandwidth of graded-index optical fibers

An interlaboratory measurement comparison was conducted by the National Bureau of Standards in cooperation with the Electronic Industries Association. Participants included NBS and nine optical fiber and cable manufacturers. Four graded-index fibers having lengths of 2, 2, 2, and 0.9 km were used. Measurements of attenuation at 850 nm, using both beam optics and mode filter approaches to achieve a restricted launch, gave one standard deviation spreads of 0.24, 0.12, 0.12, and 0.43 dB/km for an overall average of 0.23 dB/km. Best measurement agreement was obtained for a fiber having little differential mode attenuation. Measurements of -3-dB bandwidth from time domain acquired data at 90 nm gave an average one standard deviation spread of 12% with poorer agreement on the higher frequency portion of the frequency response.

Precise length measurements in multimode optical fibers

Selective optical excitation permits both the group index and the group delay of on-axis modes of multimode fibers to be determined with high precision. The group index of several types of fiber was measured at 1310 nm in a fiber Michelson interferometer, and the values were tabulated. Group delays were obtained from the transit time of short-duration optical pulses. From these data the length of reference fibers approximately 2 km long was calculated. Length-measurement accuracy was limited by group-index uncertainties to approximately 0.04%. Also, a technique that uses these reference fibers to minimize uncertainties in distance measurements made with multimode optical-time-domain reflectometers is described.

Optical fiber phase discriminator

Phase discriminators are devices widely used at rf and microwave frequencies to convert phase, or frequency, changes to amplitude changes. They find widespread use in generating audio feedback signals for frequency stabilization of oscillators and in angle demodulation applications. This paper demonstrates that similar devices, with similar functions, can be constructed in the visible region using optical fibers as delay-line elements. The operating principles of an optical-fiber delay-line phase discriminator are discussed. The sensitivity is shown to be proportional to the fiber propagation-delay time. A device working at 0.6328 microm is described and compared with predictions.

Low-coherence optical reflectometry of laser diode waveguides

Laser diode waveguides are probed using low coherence optical reflectometry. Reflections from the launch optics, front facet, and rear facet are located with a resolution of approximately 10 micrometers . Diodes mounted in pigtailed packages and on chip carriers have been studied.