F. C. Jahoda

Linear birefringence effects on fiber-optic current sensors

The occurrence of linear birefringence is inevitable when dealing with fiber optics. Intrinsic birefringence can be minimized, but deploying the fiber on an experiment will introduce stress birefringence due to bending and pressure. We have studied the effects of this extraneous linear birefringence on the measurement of current-induced circular birefringence in a fiber which also has a strong twist circular birefringence bias. Orienting the analyzing polarizer by a proscribed procedure gives minimum error. Quantitative error limits on the current for given fiber coil radii and winding tensile stress are calculated. Additional restrictions on the fiber lead-in and lead-out sections are discussed.

Fracional-fringe holographic plasma interferometry.

Holographic interferometry can be applied to plasmas whose density results in shifts of less than one fringe by superposing the small shifts on an arbitrarily shaped and positioned background fringe pattern. The sensitivity of holographic interferometry is thereby increased to that of conventional interferometry, while the inherent advantages of holography are retained. The background fringes also simplify detection of the spurious phase changes that can arise in holography from motions of the apparatus.

Optical feedback, wavelength response, and interference effects of self-pumped phase conjugation in BaTiO 3

It is shown that even slight optical feedback of a self-pumped phase-conjugate signal into dye or argon-ion lasers causes laser wavelength and conjugator reflectivity changes. The wavelength dependence of the self-pumped phase-conjugate reflectivity in barium titanate is examined over most of the visible spectrum. It is also demonstrated that interference between two collinear beams incident upon the conjugator affects the behavior of the phase-conjugate mirror.

Fiber-optic heterodyne phase-shift measurement of plasma current

By combining twisted optical sensing fiber and heterodyne phase detection of circular birefringence we have (a) overcome the distortion problem caused by residual linear birefringence in the Faraday rotation method of measuring enclosed current and (b) used only a single output detector without requiring intensity normalization. Resolution of 400 ampere-turns has been obtained in the hostile electromagnetic environment of a working thermonuclear fusion research device. The fiber was simply wound around the existing machine. The measured values are in excellent agreement with those of the electrical Rogowski coil installed when the machine was built.

Current measurements by Faraday rotation in single‐mode optical fibers

Development of techniques for measuring magnetic fields and currrents by Faraday rotation in single‐mode optical fibers has continued. We summarize the results of attempts to measure the toroidal plasma current in the ZT‐40 reversed field pinch using multiturn fiber coils. The fiber response is reproducible and in accord with theory, but the amount and distribution of the stress‐induced birefringence in this case are such that prediction of the sensor response at low currents is difficult if not impossible. The low‐current difficulty can be overcome by twisting the fiber to induce a circular birefringence bias. We report the results of auxiliary experiments with a fiber that has been twisted with 15 turns per meter and then recoated to lock the twist in place.

Self-pumped phase conjugation in BaTiO 3 at 1.06 μm

We report the first observation to our knowledge of self-pumped phase conjugation in BaTiO(3) at 1.06 microm. An essential feature is the lower threshold external ring configuration rather than total internal reflection geometry. Although the product of incident power (1 W/cm(2)) and formation time (600 sec) is much larger than the corresponding values at 0.80 microm (0.15 W/cm(2); 120 sec), the effect was observed in two crystals purchased more than a year apart and poled at separate facilities.

Two-Dimensional Interferometry with a Pulsed 10.6-μm Laser

We report the use of a simply constructed CO2 TEA laser for making space resolved end-on interferograms of the refractivity due to the free electron density of a short theta pinch plasma at late times during the discharge. For 10.6-μm radiation a phase shift of 2π only requires 2 X 10 electrons/cm, a factor of 15 increase in sensitivity over interferograms made with a ruby laser as the light source. A number of materials tha t have been found useful for recording the interferograms are described. The laser was designed to have a spatially uniform output with pulse energies in excess of 1 J. An electrode configuration employing resistor limited discharges was chosen because of its simplicity and consistent operation. About 2700 0.5-W 1000-Ω carbon resistors were inserted into the holes of Vector board (2.5/5 -mm grid). Consequently seven tightly packed rows of resistive pins were formed with a width of 1 cm and a length of 180 cm. The resistors were encapsulated in Dow Corning 182 Sylgard. The laser power supply is a 0.l-μF capacitor with a triggered spark gap. The optical cavity is formed by two 10-m radius mirrors. One of these is gold coated beryllium copper, and the other is a 6 5 % reflecting germanium output mirror. Output energy has been measured with a calibrated Scientech disk calorimeter. With the capacitor charged to 40 kV and a mixture of 90% He: 7 % CO2: 3 % N2 a t local atmospheric pressure (650 Torr) the output energy is 2.4 ± 0 .1J . For a 9 2 % He: 8% CO2 mixture the output is 0.9 ± 0.05 J. The removal of nitrogen has the previously observed effect of both reducing the amplitude of the gain switched giant pulse and removing the slower (1-2 μsec) low-power secondary pulse. Since interferometry of theta pinch plasmas requires a submicrosecond pulse to prevent smearing out of the fringes, the work reported here required the use of the 0.9-J pulses without N2 in the gas mixture. These have a 0.2-μsec half-width.

Quantitative evaluation of phase-conjugate novelty filters

A Michelson interferometer whose signal arm is terminated by a self-pumped BaTiO(3) phase conjugator records phase-change distributions in times short compared with the conjugator rewrite time that correspond quantitatively to a single (not a double) traversal of the phase disturbance. The proffered explanation, which is experimentally substantiated, also places limitations on the pictorial accuracy of intensity-change novelty filters.

Application of optical phase conjugation to plasma diagnostics (invited)

Several possibilities for plasma diagnostics provided by optical phase conjugation and, in particular, self‐pumped phase conjugation in barium titanate (BaTiO3) are discussed. These include placing a plasma within a dye laser cavity equipped with a phase conjugate mirror for intracavity absorption measurements, time differential refractometry with high spatial resolution, and simplified real‐time holographic interferometry. The principles of phase conjugation with particular reference to photorefractive media and the special advantages of self‐pumped phase conjugation are reviewed prior to the discussion of the applications. Distinctions are made in the applications between those for which photorefractive conjugators are essential and those for which they only offer experimental simplification relative to other types of phase conjugators.

Multimode fiber interferometry with and without phase conjugation

It is generally assumed that fiber interferometers must be constructed from single-mode fibers. An alternative is to employ phase conjugation mirrors on the arms of a multimode fiber interferometer so that the spatial scrambling of modes is reversed, and good fringe contrast is maintained. When several interferometers all share the same self-conjugating BaTiO(3) crystal, however, the results show crosstalk between the separate fibers ranging from negligible to prohibitively large depending on the precise spatial configuration within the conjugator. A model accounting for these results is proposed. In addition it was observed in control experiments with conventional Mach-Zehnder configurations that good data could be obtained despite reduced contrast with both graded and step index multimode fibers.