The Laser Metrology and Optic Active Control (LM&OAC) program has been carried out under ESA contract with the purpose to design and validate a laser metrology system and an actuation mechanism to monitor and control at microarcsec level the stability of the Basic Angle (angle between the lines of sight of the two telescopes) of GAIA satellite. As part of the program, a breadboard (including some EQM elements) of the laser metrology and control system has been built and submitted to functional, performance and environmental tests. In the followings we describe the mission requirements, the system architecture, the breadboard design, and finally the performed validation tests. Conclusion and appraisals from this experience are also reported.

Lasers from five national metrological institutes (NMIs) were brought to the BIPM in November 2006 as part of the BIPM.L-K11 ongoing key comparison initiated by the Comité Consultative des Longueurs (CCL) 11th meeting in 2003. The absolute frequency of the f component of the R(127) 11-5 transition was measured for these lasers following the Protocol for BIPM.L-K11. The results of these measurements are compiled in the present paper. The comparison reports, as communicated by each participant, are included as Appendices. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

An interferometric force balance (Nanobalance) for space micro-thruster qualification is presented. A new generation of space micro-thrusters is under development aiming at thrust resolution and accuracy below one micronewton. Nanobalance has been conceived and developed for ground calibration of thrust time profile and noise with sub-micronewton accuracy.

Subnanometer distance stabilization and positioning of large optical structures is a key active control issue for future space telescopes needing picoradian precision. We present solutions and results of a leading experiment, called COSI (control optics structure interaction), aimed at investigating Fabry-Perot cavities as active optical links, capable of stiffening large and massive structures operating in a vacuum. The optical length of each cavity is actuated by a piezoelectric translator (PZT)driven mechanism and the cavity ensemble is controlled by a model-based digital control unit. The first experiment stabilized the relative tip-tilt motion of two 6.9 kg annular plates, 0.5 m distant. A residual error, lower than 3 tau (-1/2)pm(1 sigma), tau >0.1 s, is achieved in the presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating the feasibility of the COSI concept and technology. (C) 2001 Society of Photo-Optical instrumentation Engineers.

We report preliminary results of the frequency stabilization of DBR-diode lasers to saturated absorption of Cs D-line by means of the modulation transfer spectroscopy method. From the experimental analysis of the signal to noise ratio, a relative frequency stability of the order of /spl ap/10/sup -12//spl middot//spl tau//sup - 1/2 / should be achieved.

A digital control application to frequency stabilization of optical frequency standards is presented. Optical frequency standards usually consist of a gas or monolithic laser source locked to a molecular (or atomic) frequency reference. Traditionally laser frequency is locked to reference through analog loops. In this paper theory and preliminary experimental results of a digital control unit (DCU) which has been designed for improving and facilitating stability performance are presented. The frequency error signal between laser and reference is elaborated at 10 kHz by the DCU so as to coordinate A set of three frequency actuators (temperature servo, piezo-electric ceramics, acousto-optic modulator) capable of compensating frequency drifts below 1 part per 10/sup 12/ at 1s under normal environmental conditions. The main concern of the paper is with modelling, design and test of the digital frequency loop. Its effects on the metrology performance of the stabilized laser source will be briefly mentioned.

The accurate measurement of the position of celestial objects is a fundamental step for several astrophysical investigations.For ground based instruments, the atmosphere is considered the basic limiting factor; in space, the knowledge of the instrumental parameters and/or of their stability define the performance limits, but CCD cameras operated in time delay integration may take advantage of their operating mode to reduce significantly the calibration problem.We implemented a low-cost laboratory experiment aimed at assessing the precision achievable in the location determination with a CCD camera, by evaluating the measurement repeatability throughout a set of images of a simulated stellar field.Our experiment provides an initial location dispersion of the order of 1/100 of the CCD pixel, with clear evidence of dominant common mode effects.After removing such terms with straightforward numerical procedures, we achieve a final location precision of 1/700 pixel on individual images, or 1/1300 pixel on co-added images.The scaling of precision with target magnitude is in quite good agreement with theoretical expectations.The initial common mode systematics appear to be induced by the thermal control of the CCD camera head, which degrades the structural stability.In actual implementations, such problems can be greatly reduced by proper design.Finally, our results show that residual effects, which could hamper the final astrometric accuracy, can be calibrated out with simple procedures.