Vaclav Hucl

Two-Stage System Based on a Software-Defined Radio for Stabilizing of Optical Frequency Combs in Long-Term Experiments

A passive optical resonator is a special sensor used for measurement of lengths on the nanometer and sub-nanometer scale. Astabilized optical frequency comb can provide an ultimate reference for measuring the wavelength of a tunable laser locked to the optical resonator. If we lock the repetition and offset frequencies of the comb to a high-grade radiofrequency (RF) oscillator its relative frequency stability is transferred from the RF to the optical frequency domain. Experiments in the field of precise length metrology of low-expansion materials are usually of long-term nature so it is required that the optical frequency comb stay in operation for an extended period of time. The optoelectronic closed-loop systems used for stabilization of combs are usually based on traditional analog electronic circuits processing signals from photodetectors. From an experimental point of view, these setups are very complicated and sensitive to ambient conditions, especially in the optical part, therefore maintaining long-time operation is not easy. The research presented in this paper deals with a novel approach based on digital signal processing and a software-defined radio. We describe digital signal processing algorithms intended for keeping the femtosecond optical comb in a long-time stable operation. This need arose during specialized experiments involving measurements of optical frequencies of tunable continuous-wave lasers. The resulting system is capable of keeping the comb in lock for an extensive period of time (8 days or more) with the relative stability better than 1.6 × 10−11.

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.

Narrow-linewidth tunable laser working at 633 nm suitable for industrial interferometry

Semiconductor lasers found a foothold in many fields of human activities, mainly thanks to its small size, low cost and high energy efficiency. Recent methods for accurate distance measurement in industrial practice use principles of laser interferometry, which are based on lasers operating in the visible spectrum. When the laser beam is visible the alignment of the industrial interferometer makes the measuring process easier. Traditional lasers for these purposes for many decades - HeNe gas laser - have superb coherence properties but small tunable range. On the other hand laser diodes are very useful lasers but only if the active layer of the semiconductor equips with a passive selective element that will increase the quality of their own resonator and also prevents the structure of its higher longitudinal modes. The main aim of the work is a design of the laser source based on a new commercial available laser diode with Distributed Bragg Reflector structure, butterfly package and fibre coupled output. The ultra-low noise injection current source, stable temperature controller and supply electronic equipment were developed with us and experimentally tested with this laser for the best performances required of the industrial interferometry field. The work also performs a setup for frequency noise properties investigation with an unbalanced fibre based Mach-Zehnder interferometer and 10 m long fibre spool inserted in the reference arm. The work presents the way to developing the narrow-linewidth operation the DBR laser with the wide tunable range up to more than 1 nm of the operation wavelength at the same time. Both capabilities predetermine this complex setup for the industrial interferometry application as they are the long distance surveying or absolute scale interferometry.

Active Angular Alignment of Gauge Blocks in Double-Ended Interferometers

This paper presents a method implemented in a system for automatic contactless calibration of gauge blocks designed at ISI ASCR. The system combines low-coherence interferometry and laser interferometry, where the first identifies the gauge block sides position and the second one measures the gauge block length itself. A crucial part of the system is the algorithm for gauge block alignment to the measuring beam which is able to compensate the gauge block lateral and longitudinal tilt up to 0.141 mrad. The algorithm is also important for the gauge block position monitoring during its length measurement.

Automatic unit for measuring refractive index of air based on Ciddor equation and its verification using direct interferometric measurement method

In scanning probe microscopy laser interferometers are usually used for measuring the position of the probe tip with a metrological traceability. As the most of the AFM setups are designed to work under standard atmospheric conditions the changes of the refractive index of air have an influence to measured values of the length with 1.0exp(-4) relatively. In order to achieve better accuracies the refractive index of air has to be monitored continuously and its instantaneous value has to be used for compensating the lengths measured by all of the interferometric axes. In the presented work we developed a new concept of an electronic unit which is able to monitor the refractive index of air on basis of measurement of ambient atmospheric conditions: temperature, humidity, pressure of the air and the CO2 concentration. The data processing is based on Ciddor equation for calculating the refractive index of air. The important advantage of the unit is a very low power consumption of the electronics so the unit causes only negligible temperature effects to the measured environment. The accuracy of the indirect measuring method employed by the unit was verified. We tested the accuracy in comparison with a direct method of measuring refractive index of air based on an evacuatable cell placed at the measuring arm of a laser interferometer. An experimental setup used for verification is presented together with a set of measurements describing the performance. The resulting accuracy of the electronic unit falls to the 4.1 exp(-7) relatively.

Length characterization of a piezoelectric actuator travel with a mode-locked femtosecond laser

The development of absolute distance measurement methods have been enabled by new kind of lasers, special digital signal processing electronics, algorithms and new materials for optics. The phenomenon of the mode-lock of the femtosecond pulse laser increased a number of potential applications with distance surveying where that stable generator of very short and periodically repeated coherent pulses can be used. The main aim of the work is a description of precise measuring method with absolute scale which is able to determine the length of unknown distance with direct traceability to a time standard. The principle of the method is based on a passive optical cavity with mirrors keeping measured distance, in our case a piezoelectric actuator. Time spacing of short femtosecond pulses generated by mode-locked laser is optically phase locked to the cavity free spectral range. A value of the repetition frequency of the laser determines the measured distance. The exact value of the frequency/period of the femtosecond pulse train is detected by a frequency counter. The counting gate of the counter is synchronized with a highly stable oscillator disciplined by H-maser or GPS received signal from atomic clocks. The work shows methods how to overcome problems with dispersive optics in the passive cavity and a way of phase lock of the femtosecond laser repetition rate to free spectral range of the cavity. This measuring technique is demonstrated on length characterization of the piezoelectric transducer which belongs to ultra-precise positioning actuators.

Remotely manageable system for stabilizing femtosecond lasers

In the field of precise measurement of optical frequencies, laser spectroscopy and interferometric distance surveying the optical frequency synthesizers (femtosecond combs) are used as optical frequency references. They generate thousands of narrow-linewidth coherent optical frequencies at the same time. The spacing of generated components equals to the repetition frequency of femtosecond pulses of the laser. The position of the comb spectrum has a frequency offset that is derived from carrier to envelope frequency difference. The repetition frequency and mentioned frequency offset belong to main controlled parameters of the optical frequency comb. If these frequencies are electronically locked an ultrastable frequency standard (i.e. H-maser, Cs- or Rb- clock), its relative stability is transferred to the optical frequency domain. We present a complete digitally controlled signal processing chain for phase-locked loop (PLL) control of the offset frequency. The setup is able to overcome some dropouts caused by the femtosecond laser non-stabilities (temperature drifts, ripple noise and electricity spikes). It is designed as a two-stage control loop, where controlled offset frequency is permanently monitored by digital signal processing. In case of dropouts of PLL, the frequency-locked loop keeps the controlled frequency in the required limits. The presented work gives the possibility of long-time operation of femtosecond combs which is necessary when the optical frequency stability measurement of ultra-stable lasers is required. The detailed description of the modern solution of the PLL with remote management is presented.

Optical fiber sensors measurement system and special fibers improvement

We present method for the improvement of the measurement accuracy in the optical frequency spectra measurements based on tunable optical filters. The optical filter was used during the design and realization of the measurement system for the inspection of the fiber Bragg gratings. The system incorporates a reference block for the compensation of environmental influences, an interferometric verification subsystem and a PC - based control software implemented in LabView. The preliminary experimental verification of the measurement principle and the measurement system functionality were carried out on a testing rig with a specially prepared concrete console in the UJV Řež. The presented system is the laboratory version of the special nuclear power plant containment shape deformation measurement system which was installed in the power plant Temelin during last year. On the base of this research we started with preparation other optical fiber sensors to nuclear power plants measurement. These sensors will be based on the microstructured and polarization maintaining optical fibers. We started with development of new methods and techniques of the splicing and shaping optical fibers. We are able to made optical tapers from ultra-short called adiabatic with length around 400 um up to long tapers with length up to 6 millimeters. We developed new techniques of splicing standard Single Mode (SM) and Multimode (MM) optical fibers and splicing of optical fibers with different diameters in the wavelength range from 532 to 1550 nm. Together with development these techniques we prepared other techniques to splicing and shaping special optical fibers like as Polarization-Maintaining (PM) or hollow core Photonic Crystal Fiber (PCF) and theirs cross splicing methods with focus to minimalize backreflection and attenuation. The splicing special optical fibers especially PCF fibers with standard telecommunication and other SM fibers can be done by our developed techniques. Adjustment of the splicing process has to be prepared for any new optical fibers and new fibers combinations. The splicing of the same types of fibers from different manufacturers can be adjusted by several tested changes in the splicing process. We are able to splice PCF with standard telecommunication fiber with attenuation up to 2 dB. The method is also presented. Development of these new techniques and methods of the optical fibers splicing are made with respect to using these fibers to another research and development in the field of optical fibers sensors, laser frequency stabilization and laser interferometry based on optical fibers. Especially for the field of laser frequency stabilization we developed and present new techniques to closing microstructured fibers with gases inside.

Laser source for dimensional metrology: investigation of an iodine stabilized system based on narrow linewidth 633 nm DBR diode

We demonstrated that an iodine stabilized Distributed Bragg Reflector (DBR) diode based laser system lasing at a wavelength in close proximity to

Coordinate interferometric system for measuring the position of a sample with infrared telecom laser diode

\lambda = 633\,