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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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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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Unit for communication in scientific instruments
Hucl, Václav
We made device for communication between the computer and devices connected to the communication CAN (Controller Area Network) bus in the laboratory. This equipment generates four independent branches of the bus, which can communicate with each other, but short circuit, disconnection or other disorder in one of the branches does not affect the other three branches. Using the CANHUB card we can also interface devices connected CAN bus to a computer via USB or Ethernet. The hub consists of two AMIS-42770 integrated circuits. CAN/USB and CAN/Ethernet interface functionality is provided by a pair of Freescale 56F805 microcontrollers. The maximum communicating speed is 800 kbps while the devices in all braches of the bus must be set up to the same communication speed.
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System for stabilizing femtosecond lasers
Čížek, Martin
We describe a device intended for stabilizing optical frequency combs with respect to high-grade RF oscillators. The whole designed system is modular and can be adapted to actual conditions given mainly by level, frequency and signal to noise ratio of the processed signals. The device is based on a specialized integrated circuit (IC) that contains functional blocks for direct digital synthesis, digital phase detection, limiters, programmable dividers and control logic. The specialized IC is accompanied by specially developed control electronics based on a microcontroller that allows communication via CAN bus. This allows remote management of the whole system and integrating it into complex automated experiments with optical frequency synthesizers. The device is used for stabilizing offset or repetition frequency of the optical frequency comb. To ensure best possible conditions for a fast first-order controller the setup is extended by a slower second-order controller with a high dynamic range. This second-order controller is built around a specialized 18-bit AD/DA card and a software application in LabView environment. The bandwidth of the closed control loop can be up to single MHz. The only limiting factor is the tuning speed of the femtosecond laser.
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Nano-stage with a large range of positionig for AFM microscopy
Číp, Ondřej
We present a new concept of long-range positioning stage for scanning probe microscopy. We developed the stuck planchet moving stage where X-Y axes of positioning are controlled by two incremental piezoelectric actuators. We demonstrated moving range up to 7 mm of positioning in both axes. The absolute position of the stage is measured by two optical linear encoders. The resolution of the positioning is about 10 nm.
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Active Angular Alignment of Gauge Blocks in Double-Ended Interferometers
Buchta, Zdeněk
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.
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Evaluation of lenght of the Fabry-Perot cavity with ultra-low expansion spacer with optical frequency comb
Šmíd, Radek ; Čížek, Martin ; Číp, Ondřej
The ultra-low expansion materials play a crucial role in laser stabilization and they are essential for laser stalibization. Blocks made from ultra-low expansion material are used as a base for precise interferometric measurements and in AFM microscopes. In our experiment we have monitored the free spectral range (FSR) changes and absolute length of a Fabry-Perot cavity (FPC) with a spacer made from an ultra-low expansion ceramics (Zerodur) with the expansion coefficient (CTE) of 2.8 • 10-8K-1. Experiment was carried out in the vacuum chamber at the pressure as low as 1 O'5 Pa. A fiber based femtosecond mode-locked laser with 100 MHz repetition frequency and central wavelength of 1550 nm was locked to a GPS disciplined crystal oscillator with a short term stability better than 10"12 and long term stability controlled by GPS clocks. An auxiliary tunable DFB laser diode was locked to a certain optical mode of the FPC. A beat-note signal from optical mixing between the laser and a stabilized femtosecond frequency comb is detected and processed. The absolute optical frequency of this mode is defined by saturation to acetylene cell. The FPC spacer of Zerodur material of 175 mm length was analyzed during the temperature cycling process from 22 - 35°C. Measured CTE of the spacer material was (0.069 ± 0.005) • 10-6K-1. Material exhibited hysteresis in the spacer length up to 8 nm during cycling process.
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