Zdenek Buchta

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).

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.

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.

An Ultra-stable Generator of Absolute Length Based on Femtosecond Mode-lock Laser and Optical Resonator

In the work, we present a proposal of a system converting excellent frequency stability of components generated by the mode-locked laser into a set of discrete values of length represented by a spacing of mirrors of an optical resonator.

Evaluation of Fabry-Perot cavity length by the stabilized optical frequency comb and acetylene absorption

The ultra low expansion cavities play a crucial role in laser stabilization, and they are essential in ion clocks. We present a method to monitor the temperature distance changes in the Fabry-Perot cavity with a spacer from an ultra low expansion material with sub-nanometer resolution. The FPC was placed into a temperature-stabilized stainless steel chamber.

Interferometric nanocomparator for calibrating precision displacement sensors

Presented work deals with the description of a novel interferometric nanocomparator intended for calibrating displacement sensors with nanometer resolution used in precision engineering. The nanocomparator is based on a 633 nm laser homodyne interferometer with 2-pass measuring arm. Digital signal filtering increases the SNR and allows achieving sub-nanometer resolution of interferometric measurements. High dynamic range of the measuring mirror displacement is achieved using a two-stage positioning system formed of a linear guide way and piezoelectric actuators. A linear guide way is used for positioning over a 100 mm range with 50 nm resolution. Piezoelectric actuators linked in a closed loop locked to the interferometer value are used for fine positioning with better than 1 nm resolution over a 5 um range. Two alternative versions of the mechanical design of the coarse positioning stage were tested and compared: a design utilizing a linear guide way with ball carrier bearings and a positioning system formed of a parallelogram frame with flexible junctions. Wearing out of linear guide ways may cause angular deviations of the mirror from the ideally perpendicular position to the laser beam. Active stabilization of the mirror using piezoelectric actuators linked to a 4-quadrant light detector was developed to eliminate these deviations and other angular errors. A set of experimental calibrations of inductive and incremental rule precision displacement sensors was conducted.

Optical fiber sensors for measurement strain and vibration

We present optical fiber sensors to measurement strain and vibration. The sensors are based on fiber Bragg gratings (FBG). We prepared construction of strain sensors with respect to its implementation on the outer surface of concrete structures and with compensation of potential temperature drifts. These sensors are projected with look forward to maximal elongation and strength which can be applied to the sensor. Each sensor contains two optical fibers with FBGs. One FBG is glued into the sensor in points of fixation which are in the line with mounting holes. This FBG is prestressed to half of measurement range, than the stretching and pressing can be measured simultaneously by one FBG. The second FBG is placed inside the sensor without fixation to measure temperature drifts. The sensor can be used to structure health monitoring. The sensors to measurement vibration are based on tilted fiber Bragg grating (TFBG) with fiber taper. The sensor uses the TFBG as a cladding modes reflector and fiber taper as a bend-sensitive recoupling member. The lower cladding modes (ghost), reflected from TFBG, is recoupled back into the fiber core via tapered fiber section. We focused on optimization of TFBG tilt angle to reach maximum reflection of the ghost and taper parameters. In this article we present complete set-up, optical and mechanical parameters of both types of sensors.

Computational approach to phase detection in frequency-modulation interferometry

We present a new approach to the interference phase detection in the frequency-modulation interferometry. Using a frequency-tunable laser beam and several computational steps we show that the interference phase can be detected with a sub-nanometer resolution from a single interference signal coming from a non-polarizing interferometer sampled by a single photo-detector. This represents significant simplification of the necessary optical setup in comparison to the conventional homodyne interferometry that requires several polarizing optical elements to produce two interference signals in quadrature. The experimental results show that our method achieves comparable accuracy as the homodyne detection which makes it usable in applications where the interferometric setup simplicity is necessary.

White-light interference fringe detection using color CCD camera

We describe in this paper a pilot experiment of a white-light fringe analysis with a low-cost color CCD camera. The used detection technique employs the phase-crossing algorithm which identifies the zero optical path difference as the point where the phase difference between the red, green and blue part of the white-light interference fringe becomes equal to zero. An experimental arrangement is based on superluminescent LED diode. The experimental setup is designed to be a crucial part of the complex system for automatic contactless diagnostic and calibration of gauge blocks.

Preparation and measurement of FBG-based length, temperature, and vibration sensors

We present system of structure health measurement by optical fiber sensors based on fiber Bragg gratings. Our system is focused to additionally install to existing buildings. We prepared first set-up of the system to monitoring of the nuclear power plant containment shape deformation. The presented system can measure up to several tens of sensors simultaneously. Each sensor contains optical fiber grating to measurement of change of length and the other independed fiber grating to monitor the temperature and the other ineligible effects.