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Phase photo masks produced by means of electron beam lithography and ion etching for Bragg gratings
Krátký, Stanislav ; Urbánek, Michal ; Kolařík, Vladimír ; Horáček, Miroslav ; Chlumská, Jana ; Matějka, Milan ; Šerý, Mojmír ; Mikel, Břetislav
Fiber Bragg grating is based on the local changes of refractive index in the core of the optical fiber. It has a wide application area, e.g. different types of filters in communications, and it may also be used in sensing of mechanical stresses. One can use different technologies to prepare this type of grating, e.g. the refractive index can be modified directly during production of an optical fiber. Further, the grating may be exposed point by point by a laser beam. The most effective way is an exposure through a phase mask, since a mask may be used for the production of hundreds of grids. This paper discusses different approaches for the preparation of phase masks in terms of impact on the quality of the exposed Bragg grating. The lattice phase mask is defined mainly by two parameters; period and depth.
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Specific forms of users interaction with holographic optical tweezers controlled by leap motion
Keša, P. ; Tomori, Z. ; Kaňka, Jan ; Jákl, Petr ; Šerý, Mojmír ; Bernatová, Silvie ; Antalík, M. ; Zemánek, Pavel
Optical tweezers allows contactless three-dimensional manipulation with electrically neutral objects of sizes from tens of nanometers to tens of micrometers. Mouse or joystick are the traditional devices to control the positions of trapping beams in holographic optical tweezers that provide optical manipulation with microobjects. Our approach is based on a modern 3D depth sensors combined with gestures and speech recognition software. The human operator controls the position of one trapping beam by a traditional pointing device like mouse or joystick. However, manipulation with more objects needs control of corresponding number of independent trapping beams generated by a spatial light modulator. We upgraded own NUI software which processes signals from sensors and sends the control commands to holographic Raman tweezers which subsequently controls the positions of trapping microparticles and the acquisition system of Raman spectra.
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Unit for stabilizing optical frequency combs
Hucl, Václav ; Čížek, Martin ; Šmíd, Radek ; Lazar, Josef ; Číp, Ondřej
Optical frequency synthesizers (femtosecond combs) are used mainly in the field of precise measurement of optical frequencies, laser spectroscopy and interferometer distance surveying. The repetition and offset frequencies belong to main controlled parameters of such synthesizer. These two parameters need to be locked to ultra-stable frequency standards i.e. H-masers, Cs- or Rb- clocks. We present the complete digital signal processing chain for phase-locked loop (PLL) control of the repetition and offset frequency. The setup is able to overcome some dropouts causes the femtosecond laser instabilities. 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 performed.
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Laser standards for interferometry and stable frequencies distribution
Hrabina, Jan
Optical frequency references - absorption cells - represent an unique tool for frequency stabilization of the laser sources. Their spectral properies define overall quality of the stabilized laser, so these properties must be precisely controlled and verified. One of the most used absorption media in the visible spectral range is molecular iodine. It offers wide and reach spectra of very narrow and strong transitions so many types of laser standards can be realized on base of this media. Testing of final properties of iodine absorption cells is traditionaly based on two methods - laser induced fluorescence method (LIF) and evaluation of absolute frequency shifts of the iodine stabilized laser (beatnote measurement). Unfortunately both of these methods have few limitations and disadvantages - in case of high-quality cells the LIF method reaches its resolution limits, the beatnote method needs quite complicated optical setup. We present a novel approach for the iodine cells quality evaluation based on measurement of hyperfine transition linewidths - the impurities in absorption media cause line broadening of the transitions. Pilot measurement shows that presented method achieves a very good sensitivity to the iodine purity and together with a need of relatively simple setup can be considered as a novel method for absorption cells quality verification.
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Study of behavior of the refractive index fluctuations to the lenght noise in displacement interferometry
Holá, Miroslava
Nanoscale measurement became one of the key challenges in the field of metrology in last ten years. The international project launched called “Nanotrace” where renowned EU laboratories including the proposer of this project were trying to break the limits of resolution in interferometry. Techniques investigated here have proven that there are chances to achieve significant improvements in resolution of laser interferometry. Measurement of position within a limited range is typical for coordinate measuring system such as nanometrology standards combining scanning probe microscopy (SPM) with precise positioning. In this contribution we evaluate the level of uncertainty associated with spatial shift of two measuring beams and spatial shift of one measuring beam (change of length the measuring beam). Consequently the nature of the fluctuations of the refractive index of air in laser interferometry is investigated anddiscussed with the focus on potential applications in coordinate measuring systems and long-range metrological scanning pro microscopy systems.
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Electronics for reducing the sideband noise of a laser diode by utilizing an unbalanced fiber interferometer
Čížek, Martin ; Šmíd, Radek ; Číp, Ondřej
Measuring length changes of optical resonators requires using of lasers with a narrow spectral width. Moreover, for tracking a whole range of possible lengths a widely-tunable laser is required. Laser sources based on DFB laser diodes have a wide optical frequency tuning range, however their spectral linewidth is up to single MHz. The usual way of narrowing the linewidth is based on stabilizing the wavelength of the laser by locking it on an etalon resonator cavity using for example a PDH setup. This of course leads to losing the tunability. We present a different approach where an unbalanced fiber Michelson interferometer with a measuring arm formed of a long optical fiber spool is used as a an optical frequency discriminator. The output electrical signal from the photodetector proportional to the optical frequency changes is used in a fast servo loop. The controller slightly detunes the laser diode by adjusting its pump current and locks optical frequency changes to a zero value. This leads to reducing the sideband noise of the laser by up to 60 dB. The whole method is independent on the actual wavelength of the laser and works in the whole tuning range of the DFB diode.
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