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Systém pro stabilizaci femtosekundových laserů
Čížek, Martin
V principu je optický hřeben (comb) založen na femtosekundovém (fs) laseru se synchronizací módů. Výstupem fs laseru je sled pulzů o délce řádově od desítek fs po 1 ps. Opakovací frekvence těchto pulzů fr bývá nejčastěji od desítek po stovky MHz. Z pohledu teorie zpracování signálů se na výstup fs laseru můžeme dívat jako na optickou nosnou vlnu amplitudově modulovanou se 100% hloubkou modulace pulzním signálem o výše zmíněné frekvenci a šířce pulzu. Fourierovou transformací takového signálu získáme spektrum tvořené diskrétními komponentami, jejichž rozestup je roven repetiční frekvenci fr. Spektrum je rozprostřeno okolo určité centrální vlnové délky. V případě Er+ dopovaného vláknového infračerveného laseru je výstupní spektrum cca 200 nm široké a centrální vlnová délka je cca 1 550 nm. Spektrum lze jednoduše matematicky popsat vztahem, kde νi je optická frekvence i-té spektrální komponenty, fceo je offsetová frekvence a frep je repetiční frekvence fs laseru.
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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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The femtosecond optical frequency comb based tool for spectral analysis of molecular absorbtion gases
Lešundák, Adam ; Šmíd, Radek ; Buchta, Zdeněk ; Mikel, Břetislav ; Lazar, Josef ; Randula, A. ; Číp, Ondřej
The spectral analysis of absorption of molecular gases belongs to very powerful methods for detection of concentration of gases in a tested sample. In case of the human breath it consists of hundreds of molecules of gasses and their concentrations are known as biomarkers. An increased concentration of more of them indicates certain diseases or symptoms. The presenting work covers a pilot experiment where femtosecond pulse probe laser and a detection chain based on cross-correlation makes an analysis of the specimen of the absorption gas. In the pilot experimental set-up the detection part utilizes a Michelson interferometer with movable measuring mirror. We developed a novel digital processing method for precise stabilization of the speed of the mirror during measuring process. The caught optical signals are calculated by the fast Fourier transform from the time to wavelength domain. The first caught spectra of the femtosecond beam passing trough the specimens show absorption lines of one isotope of acetylene which was tested as the first sample. The work proofed positive the possibility of using the femtosecond laser for detection of broadband absorption spectra in the infrared region.
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