To accomplish micro-arcsecond astrometric measurement, stellar interferometers such as SIM require the measurement of internal optical path length delay with an accuracy of ~10 picometers level. A novel common-path laser heterodyne interferometer suitable for this application was proposed and demonstrated at JPL. In this paper, we present some of the experimental results from a laboratory demonstration unit and design considerations for SIM's internal metrology beam launcher.
The Space Interferometer Mission (SIM) demands extremely precise and well-characterized laser metrology gauges (also called beam launchers) to monitor the internal and external optical delay quantities which are required for astrometric measurements. In general, any space-based sparse aperture system will require laser metrology gauges for high-bandwidth sensing of phasing errors. Lockheed Martin has aggressively pursued a technology development program for high-accuracy, space-qualified laser gauge systems. Part of this effort is focused on making compact, lightweight, low-power consumption, relatively inexpensive beam-launcher units using integrated-optics components. This paper will describe the design, laboratory implementation, performance, and error analysis for an integrated-optic based laser gauge that was constructed in FY 2000-2001 using commercially available heterodyne interferometer optics and electronics, combined with commercial fiber-optic cables and splitters. In order to provide for heterodyne mixing between the signals in the reference and measurement arms of the gauge, polarization-maintaining (PM) fiber components were used. The PM fiber lengths were matched to within 0.5 mm to avoid differential thermal effects in the measurement and reference arms. Steps were also taken to minimize the cyclic phase error due to polarization leakage, and the residual cyclic errors were measured. While not meeting the extreme picometer-level measurement accuracy requirements of SIM, the gauge can distinguish optical path differences to better than a 10 nm accuracy, which is sufficient for many space applications.
The Space Interferometry Mission (SIM), planned for launch in 2009, will measure the positions of celestial objects to an unprecedented accuracy of 4.0 microarcseconds. In order to achieve this accuracy, which represents an improvement of almost two orders of magnitude over previous astrometric measurements, a ten-meter baseline interferometer will be flown in space. NASA challenges JPL and its industrial partners, Lockheed Martin and TRW, to develop an affordable mission. This challenge will be met using a combination of existing designs and new technology. Performance and affordability must be balanced with a cost-conscious Systems Engineering approach to design and implementation trades. This paper focuses on the Lockheed Martin-led Starlight (STL) and Metrology (MET) subsystems within the main instrument of SIM. Starlight is collected by 35cm diameter telescopes to form fringes on detectors. To achieve the stated accuracy, the position of these white-light fringes must be measured to 10-9 of a wavelength of visible light. The STL Subsystem consists of siderostats, telescopes, fast steering mirrors, roof mirrors, optical delay lines and beam combiners. The MET Subsystem is used to measure very precisely the locations of the siderostats with respect to one another as well as to measure the distance traveled by starlight from the siderostat mirrors and reference corner cubes through the system to a point very close to the detectors inside the beam combiners. The MET subsystem consists of beam launchers, double and triple corner cubes, and a laser distribution system.
Visible interferometry at µarc-second accuracy requires measurement of the interferometric baseline length and orientation at picometer accuracy. The optical metrology instruments required for these interferometers must achieve accuracy on order of 1 to 10 picometers. This paper discusses the progress in the development of optical interferometers for use in distance measurement gauges with systematic errors below 100 picometers. The design is discussed as well as test methods and test results.
It has been apparent for more than a decade that the weather-forecast wind speed reaching flight crews in commercial aircraft differs by an average of +/- 15 knots from the wind speed actually experienced during the flight at cruise altitude. We recently analyzed wind-versus-altitude forecasts and found that the forecast altitude of maximum wind is also in error, by an average of +/- 4800 feet. In this era of increasing free-flight operations, we propose the use of airborne laser radar to measure winds above and below the aircraft in real time, so that a pilot can optimize the flight altitude with respect to prevailing winds. Analysis shows that such a lidar system would generate fuel savings of $LR100,000 to $LR200,000 per aircraft per year, especially for transoceanic routes. THis saving would pay for the instrument in one to two years.
NASA and the FAA have expressed interest in laser radar's capabilities to detect wind profiles at altitude. A number of programs have been addressing the technical feasibility and utility of laser radar atmospheric backscatter data to determine wind profiles and wind hazards for aircraft guidance and navigation. In addition, the U.S. Air Force is investigating the use of airborne lidar to achieve precision air drop capability, and to increase the accuracy of the AC- 130 gunship and the B-52 bomber by measuring the wind field from the aircraft to the ground. There are emerging capabilities of airborne laser radar to measure wind velocities and detect turbulence and other atmospheric disturbances out in front of an aircraft in real time. The measurement of these parameters can significantly increase fuel efficiency, flight safety, airframe lifetime, and terminal area capacity for new and existing aircraft. This is achieved through wind velocity detection, turbulence avoidance, active control utilization to alleviate gust loading, and detection of wingtip wake vortices produced by landing aircraft. This paper presents the first flight test results of an all solid-state 2-micrometers laser radar system measuring the wind field profile 1 to 2 km in front of an aircraft in real time. We find 0.7-m/s wind measurement accuracy for the system which is configured in a rugged, light weight, high- performance ARINC package.