Path Length Control Research Articles

High accuracy laser accelerometer

A high accuracy laser accelerometer has a differential pathlength element having an outer annulus and a combined acceleration sensing proof mass with a laser mirror, a closed cavity including a cathode, anode and a movable pathlength control mirror, a lens prismatic readout, a photodetector and associated electronics. When the laser accelerometer is subjected to linear acceleration, the acceleration sensing proof mass moves relative to the outer annulus of the differential pathlength element causing a differential pathlength change in the cavity which is readily detected by the photodetector.

A New IR Detector Cell with Optimized Optical Path Length for High-Temperature SEC of Polyolefins

A commercially available infrared detector cell suitable for high-temperature size exclusion chromatography was improved by new cell windows, reduction of the cell volume, temperature control and optimization of the optical path length. The theory for the optimization of the optical path length is presented, as well as the results of test measurements with solutions of polyethylene and polypropylene at different optical path lengths. The results obtained are in good agreement with the calculations which proves that the resolution of chromatograms can be improved and the detectable range of molar mass slightly extended by correct selection of the optical path length.

Temperature controlled multiple pass absorption cell for gas phase chemical kinetics studies

The application of a Herriott-type optical multiple pass cell for absorption detection of transient species in temperature controlled laser pump-probe kinetics experiments is described. Using reaction initiation by laser photolysis in combination with reaction monitoring by absorption of a multiple pass laser allows confinement of the probed reaction volume to the temperature controlled region of a slow flow reactor. For transient measurements, this apparatus provides enhanced sensitivity from increased path length and accurate temperature control by limiting the pump-probe interaction volume. In addition, for a polarized probe laser, a simple arrangement using a polarizing beam splitter and a λ/4 plate allows doubling of the path length.

Continuously tunable narrow-band operation of an injection-seeded ring-cavity optical parametric oscillator: spectroscopic applications

A pulsed beta-barium borate optical parametric oscillator is injection seeded by cw diode lasers, yielding continuously tunable narrow-band output at its idler wavelength. A notable feature of this optical parametric oscillator is that it comprises a simple passive ring cavity with no active control of path length as the seed wavelength is scanned. We demonstrate this by recording Doppler-limited photoacoustic absorption spectra of acetylene gas and Doppler-free two-photon fluorescence spectra of rubidium vapor.

Low-Volume Diffuse-Transmittance Fiber-Optic Near-IR Spectrophotometer

A simple, inexpensive near-IR spectrophotometer is described that employs single fiber-optic strands in a diffuse-transmission geometry. The instrument can examine unusually small sample volumes and offers sufficient throughput to perform multicomponent analyses on diffusely transmitting powdered samples. Control of the effective optical pathlength is found to be critical in obtaining reproducible results when scattering samples are to be analyzed. A pseudo-double-beam arrangement is employed to reduce instrument noise and to set the optical pathlength independently for each sample. Sample sizes of only a few tens of milligrams are required for analysis. The resulting performance is nearly equivalent to that of a commercial reflectance instrument. An average standard error of prediction of 2.77% w/w was obtained for four-component mixtures of benzoic acid derivatives. The single-fiber transmission geometry is shown to be useful also in the near-IR analysis of microliter volumes of liquids. A standard error of cross-validation of 17 mM sodium chloride dissolved in water was achieved.

Piezoelectric ceramics for high reliability ring laser gyros

Abstract Piezoelectric ceramic micropositioning transducers and dithering motors have been used extensively in ring laser gyros (RLG) supplied to DOD and Commercial Avionics customers. The three applications for these elements are path length control (PLC) transducers, dither motor piezoelectric bender elements and dither motor sensors. This paper will review the operation of the RLG and the unique piezoelectric ceramics used, and some of the design considerations which are necessary to apply these ceramics to the RLG components.

Implementation Of Optical Path Length And Tilt Control In A Phased Array System

A description is presented of a system designed to control the path length and tilt components of the optical wavefronts in a multiple-aperture transmitting system. The system has been implemented in the three-telescope Phasar testbed at the Air Force Weapons Laboratory. The system hardware and software are described, and measurements of the system performance are presented. The measurements indicate that under laboratory conditions the current system can maintain an optical path length difference between pairs of telescopes of approximately X/15 and a tilt control error on each telescope of about 40 nrads. The band-widths of the path length control and tilt control subsystems have been measured at 130 Hz and 950 Hz, respectively.

Improved Lyman-alpha hygrometer for small-scale atmospheric turbulence measurements. Part I: Miniaturizing the sampling volume

An improved Lyman-alpha hygrometer has been developed in order to measure high-frequency small-scale fluctuations of water vapor concentration in the lower atmosphere and in tunnels (e.g., IMST large air–sea interaction simulation tunnel). The sampling volume is reduced to 3 mm in diameter by aerodynamic shaping; the signal-to-noise ratio is largely increased by designing special UV sources and using a photomultiplier connected to a low-noise electrometer; parasitic sensitivities are reduced by a precise control of the variable path length and by filtering the UV beam with an oxygen cell. This article describes the problems and improvements related to reducing the sampling volume and presents some characteristic spectral functions obtained in IMST tunnel, up to 200 Hz. A further article will describe the O2 filtering technique, the control of sensitivity, and the data reduction procedures.

Optimal Guidance for the Space Shuttle Transition

A guidance method for the space shuttle's transition from hypersonic entry to subsonic cruising flight is presented. The method evolves from a numerical trajectory optimization technique in which kinetic energy and total energy (per unit weight) replace velocity and time in the dynamic equations. This allows the open end-time problem to be transformed to one of fixed terminal energy. In its ultimate form, E-Guidance obtains energy balance (including dynamic-pressure-rate damping) and path length control by angle-of-attack modulation and cross-range control by roll angle modulation. The guidance functions also form the basis for a pilot display of instantaneous maneuver limits and destination. Numerical results illustrate the E-Guidance concept and the optimal trajectories on which it is based.

Sensitive Acoustic Vibration Measurements by Optical Heterodyne Detection of Scattered Laser Light

A sensitive method for measurement of small vibrations at acoustical frequencies has been devised using optical heterodyne spectroscopy to detect the Doppler shifts of laser light diffusely scattered from the vibration object. Beginning with the geometry of a Michelson interferometer, we replace one mirror by the scatterer and mix the reference and scattered beam on a small-area photomultiplier cathode. With only simple optical alignment, acoustic vibration spectra can be measured without perturbations even on very small-size objects, since no attachment such as an interferometer mirror is required. Vibration amplitudes as low as 0.01 nm and up to ∼100 nm can be detected with a precision of a few percent in a few seconds averaging time by phase sensitive detection of the optical beat note at the excitation frequency. Application to mechanically unstable subjects such as the auditory organs of living biological specimens is facilitated by incorporation of low-frequency feedback control of the optical path length and provision for continuous amplitude calibration. The complete system has been used in measurements of tympanal membrane movements of acoustic receptor organs. [Supported by NSF and NIH].