Anwenderberichte
Erzeugung von frequenzstabiler, durchstimmbarer Dauerstrich-Terahertz-Strahlung Präzisionsspektroskopie Optische Mikroresonatoren
Erzeugung von frequenzstabiler, durchstimmbarer Dauerstrich-Terahertz-Strahlung
Eine Methode zur Erzegung von THz-Strahlung ist die optische Überlagerung von zwei kontinuierlichen Laserfeldern in einem Photomischer. Der Vorteil einer cw-THz-Quelle im Vergleich zu gepulsten Quellen ist, dass eine Messung über lange Zeit ohne Unterbrechung mit einer beliebigen, festen Frequenz erfolgen kann. Dies ermöglicht z. B. hochauflösende Spektroskopie und interferometrische Abstands- oder Brechzahlmessungen.
Eine Voraussetzung für hochpräzise cw-THz-Messungen ist die exakte Bestimmung und vorzugsweise auch die Regelung der THz-Frequenz.
Weitere Information: Th. Kinder, Th. Müller-Wirts, A. Deninger: Precision Frequency Metrology and Stabilization For Continuous Wave (cw) THz Sources Based on Two-Color Laser Mixing Poster THz-Forum Kaiserlautern 2010 Anwendungsbericht
A. Deninger, Th. Kinder, Th. Müller-Wirts and F. Lison: High-Power Dual-Color Diode Laser System with Precise Frequency Control for CW-THz Generation.Optical Society of America, 2007
A. Deninger, Th. Göbel, D. Schönherr, Th. Kinder, A. Roggenbuck, M. Köberle, F. Lison, Th. Müller-Wirts and P. Meissner: Precisely tunable continuous-wave terahertz source with interferometric frequency control. REVIEW OF SCIENTIFIC INSTRUMENTS 79, 044702 (2008)
Francis Hindle, Chun Yang, Arnaud Cuisset, Robin Bocquet, Gael Mouret: A compact CW-THz spectrometer for applications to gas phase identification and quantification of multiple species.
C. Yang, J. Buldyreva, I.E. Gordon, F. Rohart, A.Cuisset, G. Mouret, R. Bocquet, F. Hindle:“Oxygen,nitrogen and air broadening of HCN spectral lines at terahertz frequencies“.Journal of Quantitative Spectroscopy & Radiative Transfer 109(2008) 2857-2868
„A continuous-wave terahertz (CW-THz) spectrometer was constructed using a photomixing radiation source, Fig. 1. The instrument can be divided into the following functional units: a dual-frequency optical source, the photomixer element, a THz beam propagation path including the sample cell and a detector (bolometer). The optical source contains two (Toptica DL- 100) extended cavity diode lasers (ECDL) operating at 780nm. The lasers are spatially mixed using a beamsplitter to create a beat note in the THz frequency range. In order to optimize the spectral resolution of the source, a frequency stabilization scheme was applied to each laser. The first laser was frequency locked to a saturated absorption feature of the rubidium D2- line, using commercially available apparatus (TEM Messtechnik, CoSy). The second laser was stabilized using a low- contrast Fabry–Perot interferometer system (TEM Messtechnik, iScan). Unlike many Fabry–Perot systems that can only provide information at the resonant frequency, the advantage of this low-contrast interferometer is that it is capable of providing a stabilization signal at any frequency. An error signal generated by the difference between an arbitrary set-point and the Fabry–Perot is applied to the piezo electric element of the laser. Hence the laser can be frequency scanned across its gain profile with an active frequency correction being provided by the interferometer. In the particular case of an ECDL, the fine adjustment of the grating alignment using the piezoelectric allows a continuous tuning range of around 10GHz to be routinely obtained. Larger frequency steps are realized by manual adjustments of the grating alignment along with current and temperature of the laser diode.“
Präzisionsspektroskopie
Beispiel: Rydberg-Spektroscopie an Rubidium
Bruno Sanguinetti: BUILDING A MODERN MICROMASER : ATOMS AND CAVITIES Dissertation, Universität Leeds, 2009.
„The One Atom Maser, or Micromaser is a cavity Q.E.D. experiment consisting of a high-Q superconducting microwave cavity which interacts with a sequence of single 2-level Rydberg atoms. […] In particular I develop a 3-step laser excitation system for Rubidium Rydberg states, which I present together with the most accurate Rydberg spectroscopy to date of Rubidium P states with principal quantum number 36 to 63, from which quantum defects and an accurate value of the ionisation energy of Rubidium is derived. Work on locking multiple lasers to a single frequency comb, thus performing accurate multi-step spectroscopy is presented.“
Five different diode lasers are used in the micromaser experiment: […] The velocity-selecting 780 nm laser is locked to a specific offet frequency relative to the first laser, employing a commercial cavity (iScan).
This system, together with a temperature controller, a PID and a laser controller is commercially available in the form of the iScan from TEM Messtechnik. We used this apparatus to lock a laser, reliably and linearly scan it over tens of GHz and have it rapidly jump from one frequency to another within 8 GHz. When optimally set up, the frequency resolution is on the order of 300 kHz, mainly limited by the resolution of the photodiodes and electronics. The Allan deviation of this system was measured by beating a Toptica DL 100 laser locked to the iScan with a line of a frequency comb (see Section 3.3) and recording the resulting beat-note. The Allan deviation was observed to stay below 2 MHz for approximately 30 minutes, and the thermal drift rate was measured to be approximately 2 MHz/hour.“
Optische Mikroresonatoren
Beispiel: Kopplung zweier optischer Mikroresonatoren ultrahoher Güte über eine makroskopische Distanz
Diplomarbeit von Andreas Vogler, Universität Mainz
iScan-stabilisiertes Lasersystem zur Anregung von Flaschenmoden in Mikroresonatoren (mit freundlicher Genehmigung von A. Vogler)
„Die spektrale Vermessung der Resonatormoden geschieht mit einem DFB-Diodenlaser […]. Um während der Messung ein Driften des Lasers auszuschließen und um den Laser definiert verstimmen zu können, befindet sich direkt hinter dem Laser ein Stabilisierungsinterferometer (Firma TEM-Messtechnik, iScan). Es besteht aus einem Messkopf und einer Mikrocontrollereinheit. Der Messkopf enthält ein Referenzinterferometer, das den zeitlichen Frequenzverlauf des Laserslichts ermittelt. Er liefert ein Ist-Signal an den Mikrocontroller, der es mit einem Referenzsignal aus einer externen Quelle vergleicht und den Laser entsprechend nachregelt. […] Die interessanten Frequenzbereiche wurden mit dem iScan angewählt und die beiden Flaschenmoden so weit durchgestimmt, dass sie im Transmissionsspektrum wenige hundert MHz getrennt liegen.“
