Bretislav Mikel

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.

White-light fringe detection based on a novel light source and colour CCD camera

We describe in this paper a pilot experiment of optimization of a white-light source for a low-coherence interferometry. The white-light source combines the light beams generated with colour LEDs. By modelling the white-light spectra, the contrast of a white-light interference fringe could be changed and set to the maximal value. The second part of this paper is a description of a white-light fringe analysis ensured with a low-cost colour CCD camera. The used detection technique employs a phase-crossing algorithm which identifies a zero optical path difference as the point where the phase difference between the red, green and blue parts of the white-light interference fringe becomes equal to zero. The optimized white-light source is designed to be a crucial part of an experimental setup for the surface diagnostics and automatic calibration of gauge blocks.

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.

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.

Precise monitoring of ultra low expansion Fabry-Perot cavity length by the use of a stabilized optical frequency comb

The use of an ultra low expansion cavity plays a crucial role in laser stabilization, and it is essential in atomic or ion clocks. In this work we present a method of mirror distance monitoring in an evacuated Fabry-Perot cavity (FPC) made from an ultra low expansion material (Zerodur expansion class 0). The FPC was placed into a temperature-stabilized stainless steel chamber and the FPC temperature was set to remain close to the point where the expansion coefficient of the cavity material is the lowest. The precise distance of the FPC mirrors in vacuum was indirectly proportional to the optical frequency of a laser. One of the frequency comb component chosen by FBG grating was locked to the FPC of 400 MHz free spectral range. The 100 MHz repetition frequency of the femtosecond laser comb was monitored with counter referenced to a crystal oscillator with short term stability < 10−12 and long term stability locked to a GPS.

Two axis micropositioning system with piezoelectric actuators and digital control

The scanning table, which is controlled by a PZT control system that has been constructed, is presented. It is used for a fine manipulation of a probe in measurement of the profile of the laser beam focused by high NA lens e.g.. The whole system has been calibrated by a two-axis laser interferometer. Where the PZT control system and the interferometer were connected to the CAN network and the actual X-Y position was saved to the PC synchronously with the CAN network time stamps generated by the PZT control system. Precision about 300nm at the positioning system was reached.

Calibration of elongation of fiber Bragg gratings by laser interferometer

We present method of measurement of fiber Bragg gratings (FBG) stretching by laser interferometer. We are oriented to research, development and to methodology of the production and application of FBGs. The elongation of FBGs can calculated from frequency drift of the Bragg resonance wavelength and from the change of the grating period during stretching. The inhomogeneity of optical fibers and of the FBGs can induce change of the grating period (Bragg resonance wavelength) of FBG which can be nonlinear with elongation of the optical fiber with FBG. The calibration of FBGs will be used to preparation and to the realization of the technology of measurement of shape deformation of concrete buildings.

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.

Stabilization of semiconductor lasers by fiber Bragg gratings

We present methods of improvement of wavelength stability and tuneability of semiconductor laser diodes in fiber laser interferometers by fiber Bragg gratings (FBGs). We developed simulation method to calculation of arbitrary fiber grating (apodized, chirp etc.) with high precision by combination of methods based on layered dielectric media (LDM) and transfer matrix. On the basis of our simulations and measurements of the commercially available fiber gratings we designed a special 100 mm long fiber Bragg grating with apodization. We expect the application of the FBG to improvement of the linewidth and mode-hop free tuning range of semiconductor lasers at the wavelength 760 nm to increase resolution of fiber laser interferometer based on these diodes. We built the absolute fiber laser interferometer with Vertical Cavity Surface Emitting Laser (VCSEL) to easy employ FBG to stabilize wavelength and control the tuning range. First set up is presented.