Ondrej Cip

A scale-linearization method for precise laser interferometry

The authors discuss an increase in the precision of a laser interferometer based on the phase division of the fringe using a single-frequency laser for a distance measurement in the subnanometre range. The resolution of the interferometer is extended by using four paths of the laser beam across the measurement arm of the Michelson interferometer and by subsequent electronic division to the total resolution /2048. The technique fulfils the expectation that the influence of the fringe distortion caused by a phase difference of /2 between two quadrature signals and the difference of the intensities will be lower. A further improvement of the linearity can be achieved by using a mathematical method, which is based on the real-time measurement of parameters of the detected interferometer fringe and an optimal approximation of the quadrature signals obtained by using a conical section. The obtained parameters are transformed from the analytical form to the parametric form of the equation of the conical section and an inverse function is established. Reproducible measurements with an error of less than 0.5 nm are available now.

Conversion of stability of femtosecond mode-locked laser to optical cavity length

In this contribution we propose a scheme for a generation of precise displacements through conversion of relative stability of components of a femtosecond laser into the length of a Fabry-Perot cavity. The spacing of mirrors of a Fabry-Perot interferometer represents a mechanical length standard referenced to stable optical frequency of a femtosecond mode-locked laser. With the help of a highly selective optical filter, it is possible to get only a few discrete spectral components. By tuning and locking the Fabry-Perot cavity to a selected single component it is possible to get a mechanical length standard with the uncertainty of the repetition frequency of the femtosecond laser. To verify the method, an auxiliary single-frequency laser is locked to the resonance mode of the cavity and simultaneously it is optically mixed with an independent optical frequency standard He-Ne-I2. The stability of the beat-frequency between these 2 lasers represents the stability of the Fabry-Perot cavity length. The stability recording evaluated through Allan variances for one hour of operation is presented. The pilot experimental setup is able to generate the length standard in the order of 0.01 nm for 20 min of integration time.

Detection of Interference Phase by Digital Computation of Quadrature Signals in Homodyne Laser Interferometry

We have proposed an approach to the interference phase extraction in the homodyne laser interferometry. The method employs a series of computational steps to reconstruct the signals for quadrature detection from an interference signal from a non-polarising interferometer sampled by a simple photodetector. The complexity trade-off is the use of laser beam with frequency modulation capability. It is analytically derived and its validity and performance is experimentally verified. The method has proven to be a feasible alternative for the traditional homodyne detection since it performs with comparable accuracy, especially where the optical setup complexity is principal issue and the modulation of laser beam is not a heavy burden (e.g., in multi-axis sensor or laser diode based systems).

Highly coherent tunable semiconductor lasers in metrology of length

Laser diodes became the most widespread lasers and now are available in a broad spectrum of wavelengths ranging from infrared to the visible region. The low power ones mainly those with the quantum well structure and gain or index guided configuration perform a narrow linewidth and soon became a favourite tool for interferometry and spectroscopy. The need for continuous tuning range led to the development Extended Cavity Laser systems (ECL) and Vertical Cavity Surface Emitting Lasers (VCSEL). Both systems seem to be promising laser sources for design of optical frequency standards or interferometric distance measurement devices. We present design of these laser systems and their applications in metrology of length.

Standing Wave Interferometer with Stabilization of Wavelength on Air

Abstract We present an experimental arrangement of an interferometric system designed to operate with full compensation for varying refractive index of air in the measuring axis. The concept is based on a principle where the wavelength of the laser source is derived not from an optical frequency of the stabilized laser but from a fixed length being a base-plate or a frame of the whole measuring setup. This results into stabilization of the wavelength of the laser source in atmospheric conditions to mechanical length of suitable etalon made of a material with very low thermal expansion. The ultra-low thermal expanding glass ceramic materials available on the market perform thermal expansion coefficients on the level 10−8 which significantly exceeds the limits of uncertainty posed by indirect evaluation of refractive index of air through Edlen formula. This approach represents a contribution primarily to high-resolution and high-precision dimensional metrology in the nanoscale. Zusammenfassung Ein Interferometer wird vorgestellt, welches die vollständige Kompensation des zeitlich variierenden Brechungsindex der Luft entlang des Messpfades erreicht. Das Konzept basiert auf der Idee die Laserwellenlänge nicht optisch zu stabilisieren, sondern von einer bekannten Distanz abzuleiten, welche über ein Material mit niedrigem thermischem Ausdehnungskoeffizienten realisiert wird. Die hier eingesetzte kommerziell verfügbare Glaskeramik erreicht thermische Ausdehnungskoeffizienten in der Größenordnung von 10−8 / °C. Dies ist weit unterhalb jener Grenze die erreicht werden kann, wenn der Brechungsindex der Luft mithilfe der Beziehung von Edlen korrigiert wird. Somit ergeben sich Anwendungen in der präzisen Bestimmung von geometrischen Größen auch im Nanometerbereich.

Joint accurate time and stable frequency distribution infrastructure sharing fiber footprint with research network

Abstract. The infrastructure essentialities for accurate time and stable frequency distribution are presented. Our solution is based on sharing fibers for a research and educational network carrying live data traffic with time and frequency transfer in parallel. Accurate time and stable frequency transmission uses mainly dark channels amplified by dedicated bidirectional amplifiers with the same propagation path for both directions of transmission. This paper targets challenges related to bidirectional transmission, particularly, directional nonreciprocities.

AFM nanometrology interferometric system with the compensation of angle errors

The contribution is oriented towards measuring in the nanoscale through local probe microscopy techniques, primarily the AFM microscopy. The need to make the AFM microscope a nanometrology tool not only the positioning of the tip has to be based on precise measurements but the traceability of the measuring technique has to be ensured up to the primary standard. This leads to the engagement of laser interferometric measuring methods. We present a improved design of the six-axes dimensional interferometric measurement tool for local probe microscopy stage nanopositioning with the compensation system of angle errors. The setup is powered with the help of a single-frequency frequency-doubled Nd:YAG laser which is stabilized by thermal frequency control locked to a Doppler-broadened absorption line in iodine. The laser stabilization technique is described together with comparison of frequency stability and angle errors compensation system performance.

Stabilization of VCSEL laser source for absolute laser interferometry

We describe a new laser head with Vertical Cavity Surface Emitting Laser (VCSEL) diode for absolute laser interferometry with Michelson interferometer. The VCSEL lasers at 760 rim with mode-hop free tuning range above 1 nm are affordable now. The VCSELs are fabricated as circulary-symetric lasers; the circular aperture is of particular value as it implies a circular beam profile. The linewidth and the optical noise of the VCSEL laser diode depend on the stability of the current source controller and on the quality temperature control system. We developed a digital temperature control system with temperature stability below 1 mK and high stability current source controller.

Transfer of stable optical frequency for sensory networks via 306 km optical fiber link

Optical fiber links for distribution of highly-stable optical frequencies were experimentally tested by many metrology laboratories in the past fifteen years. But recent development of new optical sensors for industrial application puts demands on a technology transfer of this high-end technology from laboratory experiments to the real industry. The remote calibration of interrogators of Fiber Bragg Grating strain sensory networks is one of important examples. We present a 306 km long optical fiber link established in the Czech Republic where a coherent transfer of stable optical frequency has been firstly demonstrated. The link between ISI CAS Brno and CESNET Prague uses an internet communication fiber where a window 1540 to 1546 nm in DWDM grid is dedicated for the coherent transfer and 1 PPS signal. The optical frequency standard at 1540.5 nm is used for the coherent transfer where compensation of the Doppler shift induced by the optical fiber is done by an acousto-optic modulator driven by a servo loop. The closed loop action is continuously recorded. This enables to compute changes in the transport delay introduced by external influences on the optical line. A comparison with a different measuring method based on analyzing the transport delay of a 1 PPS signal transmitted via the same DWDM window is done. This comparison is a subject of results of the paper.

Short-Range Six-Axis Interferometer Controlled Positioning for Scanning Probe Microscopy

We present a design of a nanometrology measuring setup which is a part of the national standard instrumentation for nanometrology operated by the Czech Metrology Institute (CMI) in Brno, Czech Republic. The system employs a full six-axis interferometric position measurement of the sample holder consisting of six independent interferometers. Here we report on description of alignment issues and accurate adjustment of orthogonality of the measuring axes. Consequently, suppression of cosine errors and reduction of sensitivity to Abbe offset is achieved through full control in all six degrees of freedom. Due to the geometric configuration including a wide basis of the two units measuring in y-direction and the three measuring in z-direction the angle resolution of the whole setup is minimize to tens of nanoradians. Moreover, the servo-control of all six degrees of freedom allows to keep guidance errors below 100 nrad. This small range system is based on a commercial nanopositioning stage driven by piezoelectric transducers with the range (200 × 200 × 10) μm. Thermally compensated miniature interferometric units with fiber-optic light delivery and integrated homodyne detection system were developed especially for this system and serve as sensors for othogonality alignment.