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We report on an optical frequency standard at λ = 612 nm, which takes the form of a He-Ne laser stabilized with respect to molecular iodine by means of frequency modulation spectroscopy. The measured frequency stability was σ0(τ) = 8,7 × 10-13 τ-1/2 for sampling times (τ) between 10-3 s and 103 s, and the estimated reproducibility was 1,2 kHz. Spectroscopy of the R(47) 9-2 transition was performed, and iodine hyperfine constants were calculated.

This paper reports the fourth set of results of a series of grouped laser comparisons from national laboratories undertaken by the Bureau International des Poids et Mesures (BIPM) at the request of the Comite Consultatif pour la Definition du Metre (CCDM; now the Consultative Committee for Length, CCL) during the period July 1993 to September 1995. The results of this comparison, involving eleven lasers from eight countries and the BIPM, again meet the goals set by the CCDM in 1992 and adopted by the Comite International des Poids et Mesures (CIPM) the same year. The standard uncertainty of the frequency of the He-Ne laser stabilized on the saturated absorption of 127I2 at λ ≈ 633 nm is reduced to a level of 12 kHz (2.5 parts in 1011) when the lasers compared meet the recommended values of the parameters. The lasers were first compared with the BIPM4 laser with the parameters set to the values normally used in each laboratory; using the BIPM4 laser as a reference the results ranged from-15.4 kHz to 36.8 kHz. After checking and readjusting the values of all the parameters, the range was reduced to-8.6 kHz to 14.0 kHz. Under the latter conditions, the average frequency difference of the group of lasers, with respect to the BIPM4 laser, was 2.7 kHz with a standard uncertainty (1 σ) of 8.1 kHz. The best relative frequency stabilities, with Allan standard deviations of about 5.5 parts in 1012 and 4.6 parts in 1013, were observed with sampling times of 1 s and 100 s, respectively.

Frequency modulation spectroscopy is used to frequency stabilize an Ar/sup +/ laser at 515 nm with respect to a commercial passive cavity. Two transducers, a slow and a fast one, are used. The fast transducer is also used to perform amplitude stabilization of a laser beam. Measured amplitude and frequency spectral noise densities are reported. A simplified scheme of the circuitry used to reduce amplitude noise is shown. Noise characteristics of the realized system are adequate to perform spectroscopy of hyperfine-structure transitions of molecular iodine.

The result of a recent comparison of iodine-stabilized He-Ne lasers at 474 THz from the IMGC and the LMM is presented. After one week of measurements, the mean frequency difference between components d, e, f and g of the R(127)11-5 transition of 127I2 was found to be νLMM-1 - νIMGC5/7 = (-4,5 ± 0,4) kHz. Parameters which affect the laser frequency were investigated on laser LMM-1.

Preliminary results on absolute frequency stabilization of Distributed Bragg Reflector (DBR) diode-lasers with emission wavelength at 852 nm will be reported. Saturated absorption D2 lines of cesium atom have been adopted as a frequency reference and the nonlinear spectroscopy method of Modulation Transfer has been used to lock the laser frequency against the resonance. From the preliminary results on the beat signal between two independent laser systems a relative frequency stability of 10-11 has been reached at an integration time of 0.1 s. The analysis on the obtained signal to noise ratio shows a frequency noise floor in the order of approximately 10-12 which should be achieved in an integration time of 1 s.

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.

The results of an international comparison between the He-Ne lasers stabilized to 127I2 at 633 nm of the IMGC (Istituto di Metrologia "G. Colonnetti", Turin, Italy) and the IFA-IFTAR (Institute of Atomic Physics-IFTAR, Department of Lasers, Bucharest, Romania) are reported. The mean frequency difference, considering the hyperfine structure components d, e, f and g of the R(127) 11-5 transition, was Δν = νRO.2 - νIMGCA4/5 = 3,8 kHz, standard uncertainty u = 5,7 kHz, or Δν/ν0 = 0,8 × 10-11, standard uncertainty u = 1,2 × 10-11. The frequency stability, expressed as the relative Allan standard deviation of the beat note between the two lasers, resulted 3,7 × 10-12 for a 100 s sampling time.

A new human leucocyte antigen (HLA)-DRB1 allele, HLA-DRB1*1149, has been identified in three members of an Italian family during routine sequence based typing. This new allele differs from HLA-DRB1*110101 only for a single nucleotide substitution at position 113 of exon 2 resulting in an amino acid change from Valine (GTG) to Alanine (GCG) at codon 38.

Deals with the experimental characterization of the frequency modulation spectroscopy method by means of a frequency-doubled Nd:YAG laser and saturated absorptions of /sup 127/I/sub 2/ at the wavelength of 532 nm. A power of 8.7 mW at 532 nm with a continuous frequency tuning range of 80 GHz has been obtained by doubling 80 mW of the infrared Nd:YAG radiation incident on a self made external cavity doubler. In order to obtain a high degree of stability and reproducibility of the 532 nm radiation locked to a saturated absorption of /sup 127/I/sub 2/, linewidth, sensitivity, and signal to noise ratio of the demodulated signals have been characterized for different phase modulation conditions, iodine vapor pressures and for different optical power levels.

The hyperfine spectroscopy of the rovibronic P(54)32-0 iodine line at 532 nm has been performed using diode-pumped frequency-doubled Nd:YAG lasers and the frequency modulation technique under saturated absorption conditions. Due to the high stability degree of the optical sources, the hyperfine frequency separations have been measured with an uncertainty lower than 100 Hz. From these measurement data, the hyperfine structure constants for a four term Hamiltonian have been estimated.

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.

The absorption properties of hydrogen iodide (HI) near 1.55 mu m are described. Absorption lines of this gas located at wavelengths ranging from 1.534 to 1.594 mu m have been identified. They are proposed as absolute wavelength references for erbium-doped fiber amplifier (EDFA) based multi-channel optical networks. >

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.

Within a Technology Research Program funded by the European Space Agency, a team led by Alenia Aerospazio has investigated and started the development of some technologies which are considered fundamental for the achievement of the scientific objectives of the future astrometric mission GAIA. The activities have been focused on the design of a two-aperture optical interferometer and of a system for the active stabilization of its configuration within few picometers. A laboratory prototype of the active stabilization system has been implemented and tested. The results achieved in the laboratory tests proved that the very challenging requirements imposed by the GAIA astrometric goal of 10 micro-arcsec accuracy can be fulfilled.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The paper presents control design and results of a leading experiment in the distance stabilization of large bodies, emulating optical mirrors, with picometer repeatability. The experiment, called COSI (Control Optics Structure Interaction), was funded by the European Space Agency (ESA) in view of future space telescopes needing picoradian precision over time scales >1 s. Distance stabilization is achieved by actively controlling the optical length of Fabry-Perot cavities in the vacuum. The first experiments stabilized three 0.5 m distances between two 7 kg plates with a residual control error better than 3 pm (1/spl sigma/), in presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating feasibility of COSI concept and technology.

We present final results from four laser frequency comparisons involving the following He-Ne/Iodine lasers operating at wavelength (lambda) equals 633 nm: the laser RO.2 from the National Institute for Physics of Lasers, Plasma and Radiation (Romania); the laser BIPM4 from the Bureau International des Poids et Mesures (France); the laser IMGC4/5 from the Institute for Metrology `Gustavo Collonetti' (Italy); the laser INVAR3 from the Institute for Scientific Instruments (Czech Republic); the laser NMS121 from the National Standards Laboratory (Norway). We report the results coming from matrix determinations on the lines d, e, f, g and then, by reducing all mean frequency differences to the same reference laser (BIPM4), we presented them as frequency traceability of a laser through that of the reference laser.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Preliminary results of an international comparison between the He-Ne lasers stabilized to 127I2 at 633 nm of the Instituto di Metrologia G. Colonnetti (IMGC) and the Institute of Atomic Physics-IFTAR (IFA-IFTAR), Department of Lasers are reported. The mean frequency difference, considering the hyperfine structure components d, e, f and g, resulted as (Delta) (nu) equals (nu) IMGC4/5 - (nu) R0.2 equals (-3.8 +/- 5.7) kHz, or as relative value reported to the laser frequency as (Delta) (nu) /(nu) o equals (0.8 +/- 1.2) X 10-11. The frequency stability, expressed as the Allan variance of the beat note between the two lasers, resulted 3.65 X 10-12 for 100 s sampling time.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The frequency of the RI(32) CO2 line has been measured with respect to a 3.39 μm He22Ne laser stabilized to the F2 (2) P(7) transition in thev 3 band of CH4. The value obtained isv=29477160862±12 kHz, in good agreement with the only value reported in the literature. Using the available data of the RI(32)-RI(30) (difference) frequency, the following value can be derived for the RI(30) line: ν=29442483320±18 kHz.

In a 127I2 stabilized 633‐nm He‐Ne laser, the effects of saturation dispersion in iodine on the inverted Lamb dips of the iodine hyperfine components are discussed. Analytical expressions for the second and third harmonic of the frequency modulated dips are derived. Dispersion is found to influence strongly the modulation dependent frequency shifts of the absorption line centers and also introduce asymmetry in the second harmonic.

Point contact MIM diodes of different materials have been tested as harmonic mixers at 29 THz and 88 THz. From the analysis of the I-V static characteristic quantitative informations have been obtained about the effectiveness of the diodes as high order mixers.

The absorptions of RuO4 coincident with12C16O2 and14N216O laser lines have been observed with sub-Doppler resolution and their frequency distances from the laser line-centers have been measured. These preliminary results suggest a possible extension of the absolute reference frequency grid up to the 10.9 μm region, based on transitions of hyperfine-structure free heavy molecules.

The stabilization of a He-Ne (Helium-Neon) laser to hyper-fine structure (HFS) components of the P(48) 11-3 transition of 127I2 at 612 nm observed in an external iodine cell, may give a frequency reproducibility of ±5 kHz or ±1 × 10-11·? and a stability of 1.4 × 1013·? for an observation time ? = 100 s. From 3He-22Ne lasers stabilized to HFS components of 127I2 observed in an intracavity cell, both at 612 and 640 nm wavelengths, a reproducibility of ±2 × 10-10· ? can be expected. Common and convenient operating conditions have been found at both wavelengths.

In November 1981, a comparison between BIPM and IMGC He-Ne lasers stabilized to saturated absorption lines of 127I2, at the wavelengths ? = 633 nm and ? = 612 nm has been made at the BIPM. At 633 nm, a repeatability and a reproducibility within +1 × 10-11 · ? have been obtained. At 612 nm, a repeatability within a few 10-11 · ? only for one laser and a reproducibility of ± 2 × 10-10 · ? have been obtained.

The progress in frequency synthesis to near IR and to the visible region was essential for the formulation of the new definition of the metre approved by the XVII CGPM (Conference Generale des Poids et Mesures) in Nov.1983 /1/.

Iodine stabilized lasers will be applied at IMGC as secondary wavelength standards in laser frequency calibrations, measurements of “g” and silicon lattice parameter determination.

In a two level system, according to the lifetime of the excited state, optical pumping (short lifetime) or coherence effects (long lifetime) can be observed in a low pressure absorption cell using parallel travelling wave beams. A theoretical analysis will be given.

The dispersion signal, observed with sub-Doppler spectroscopic techniques using a phase modulation of the probe beam, is antisymmetric and shows a remarkable slant at the central zero-crossing. Because of these characteristics, it is particularly suitable as an error signal for stabilizing the laser frequency using as a reference an atomic or molecular transition in a low-pressure gas. At IMGC the fitness of this method for the achievement of optical frequency standards has been tested using two He-Ne lasers, independently stabilized to the hyperfine structure component “b 15 ” of the P (48) 11–3 iodine transition at 612 nm.

Preliminary results for the implementation of modulation transfer technique using a He-Ne laser at 612 nm are given. The performances are presently limited by the noise of the detector, which was designed to be operated at a frequency higher than the one used for modulating the saturating beam.

Using FM spectroscopy as the detection method in an experiment of optical saturation absorption, a fractional stability (Allan variance) of 5/spl times/10/sup -14/ has been observed for an integration time of 10/sup 3/ s,and a reproducibility better than 1 part in 10/sup 11/ has been achieved. >

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.

The frequencies of the most intense RuO 4 transitions in coincidence with the emission lines of CO 2 and N 2 O lasers have been measured with respect to the CO 2 lines with an uncertainty of less than 3× 10 -9 . Difference and absolute frequency values are here reported.