Yves Salvadé

Frequency-comb-referenced two-wavelength source for absolute distance measurement

We propose a new tunable laser source concept for multiple-wavelength interferometry, offering an unprecedented large choice of synthetic wavelengths with a relative uncertainty better than 10(-11) in vacuum. Two lasers are frequency stabilized over a wide range of frequency intervals defined by the frequency comb generated by a mode-locked fiber laser. In addition, we present experimental results demonstrating the generation of a 90 mum synthetic wavelength calibrated with an accuracy better than 0.2 parts in 10(6). With this synthetic wavelength we can resolve one optical wavelength, which opens the way to absolute distance measurement with nanometer accuracy.

High-accuracy absolute distance measurement using frequency comb referenced multiwavelength source

We propose a new approach to multiple-wavelength interferometry, targeted to high bandwidth absolute distance measurement, with nanometer accuracy over long distances. Two cw lasers are stabilized over a wide range of frequency intervals defined by an optical frequency comb, thus offering an unprecedented large choice of synthetic wavelengths. By applying a superheterodyne detection technique, we demonstrated experimentally an accuracy of 8 nm over 800 mm for target velocities up to 50 mm/s.

A proposal for a new moving-coil experiment

A new type of moving-coil watt balance is under construction at the Swiss Federal Office of Metrology (OFMET). The aim of the experiment is the monitoring of the kilogram by means of the electrical quantum standards with a relative uncertainty of /spl les/10/sup -8/. The paper presents the main features of the proposed instrument.

Stabilized three-wavelength source calibrated by electronic means for high-accuracy absolute distance measurement

Multiple-wavelength interferometry offers great f lexibility in working distance and sensitivity by permitting an appropriate choice of the different wavelengths used, and it can be operated on rough surfaces. The accuracy of distance measurement with this approach depends on the stability and the calibration of the different wavelengths. A novel concept of a multiple-wavelength source that uses laser diodes has been developed. It allows one to obtain an absolute calibration of the synthetic wavelengths by the use of electronic beat-frequency measurements. Experimental results show that a calibration of the synthetic wavelength in the millimeter range with an accuracy of better than 10(-5) is feasible.

Limitations of interferometry due to the flicker noise of laser diodes

Interferometry with laser diodes is a cost-effective way to perform displacement measurement. The tunability of laser diodes is also of great interest in multiple-wavelength interferometry. However, the additional flicker noise in the frequency-noise spectrum of semiconductor lasers may become a limiting factor. Investigations on the limitations due to the 1/f noise of laser diodes are presented for both classical and multiple-wavelength interferometry. Measurements at the limit of the coherence length of laser diodes with the corresponding phase fluctuations are reported. The theoretical results are verified experimentally.

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.

Toward nanometer accuracy laser metrology for phase-referenced interferometry with the VLTI

The PRIMA laser metrology system is being developed to monitor optical path differences and optical path fluctuations encountered by two stellar objects inside the VLTI during phased-referenced observations. This system, which will operate at the scale of the VLTI, has an accuracy goal of a few nanometers. After an introduction to its design, based on heterodyne interferometry, this paper presents the results of sub-system characterization and prototyping as well as experimental results obtained during full-scale testing at the Paranal Observatory.

High-precision velocimetry: optimization of a fabry-perot interferometer.

We present the optimization of a Fabry-Perot velocimeter designed to measure speed at a few millimeters per second with a relative uncertainty of 10(-8). We focus on the accuracy and the optimization of the Fabry-Perot, with a review of the uncertainties related to the geometry, the beam shape, and the Doppler frequency measurement. These errors are quantified to ensure that the required accuracy is reached. We then describe the practical implementation and show the results.

Superheterodyne configuration for two-wavelength interferometry applied to absolute distance measurement

We present a new superheterodyne technique for long-distance measurements by two-wavelength interferometry (TWI). While conventional systems use two acousto-optic modulators to generate two different heterodyne frequencies, here the two frequencies result from synchronized sweeps of optical and radio frequencies. A distributed feedback laser source is injected in an intensity modulator that is driven at the half-wave voltage mode. A radio-frequency signal is applied to this intensity modulator to generate two optical sidebands around the optical carrier. This applied radio frequency consists of a digital ramp between 13 and 15 GHz, with 1 ms duration and with an accuracy of better than 1 ppm. Simultaneously, the laser source is frequency modulated by a current modulation that is synchronized on the radio-frequency ramp as well as on a triangle waveform. These two frequency-swept optical signals at the output of the modulator illuminate a Michelson interferometer and create two distinct distance-dependent heterodyne frequencies on the photodetector. The superheterodyne signal is then detected and bandpass filtered to retrieve the absolute distance measurement. Experiments between 1 and 15 m confirm the validity of this new concept, leading to a distance accuracy of +/- 50 microm for a 1 ms acquisition time.

Sub-ppm absolute distance measurements using an optical frequency comb generated by a conventional dual-drive Mach-Zehnder modulator

A simple technique to generate an optical frequency comb, based on a conventional dual-drive Mach-Zehnder intensity modulator, has been used as optical source for a high accuracy distance measurement in an interferometric set-up. The modulator has been driven by a direct-digital synthesizer that is able to deliver a pure ramp in frequency between 13 GHz and 14 GHz. We have obtained about 15 modes, corresponding to a spectral span of 200 GHz. This optical signal, launched in a Michelson interferometric set-up, allowed performing absolute distance measurement by sweeping the radio-frequency of the direct digital synthesizer. Measurements have been compared to a standard, which was a mode-locked femtosecond laser along with a counting interferometer. Absolute distance measurements over a range of 1 to 24 meters gave an accuracy of about ± 10 microns, with a repeatability of ± 5 microns, corresponding to a sub-ppm absolute distance measurement.