The iScan system is an universal tool for static and dynamic testing of the mode properties of tunable lasers. But it allows for even more:
iScan employs a patented interferometer setup with four independent photo detectors. The detectors receive several interference signals with a phase difference of approx. 90° (quadrature signals), allowing for monitoring of the tuning behavior and detection of mode hops.
Displaying the quadrature signals on a 2-channel oscilloscope in X-Y mode yields characteristic figures (Lissajous figures), which correspond to the properties of the laser.
iScan processes information given in polar coordinates: The phase is a linear function of the optical frequency (and in good approximation for the wavelength, too).The radius is a measure of the mode purity of the laser.
Single-mode-Scan:
The quadrature signals describe a circle withfixed radius
Mode hop:
Sudden jump acrossthe circle
Multi-mode-scan:
Circle with a significantlysmaller and non-constantradius
Frequency deviation of an iScan-stabilized DLpro (Toptica Photonics AG) from an optical frequency comb, measured by the group of Th. Hänsch at MPQ Garching (J. Brachmann et al., APPLIED OPTICS Vol. 51, No. 23 (2012), http://arxiv.org/pdf/1208.2375v1.pdf). The iScan system included CoSy-Rb (rubidium reference at 780nm/384THz, vertical line) and linearization package. Different colors symbolize measurements on different days in the same lab.
Generation of continuous-wave terahertz radiation
One method of generating THz radiation is optical heterodyning of two continuous laser fields on a semiconductor photomixer. The advantage of a cw THz source compared to pulsed sources is the fact, that measurements can take place at arbitrarily chosen, fixed or variable THz frequencies for unlimited and uninterrupted time intervals. This allows e.g. for high resolution spectroscopy, or for interferometric distance or a refractive index measurements.
A prerequisite for high precision cw THz measurement is the knowledge and preferably the stabilization of the THz frequency.
Read on: 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
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.
“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.”
Precision spectroscopy
Example: Rydberg spectroscopy with Rubidium
Bruno Sanguinetti: BUILDING A MODERN MICROMASER : ATOMS AND CAVITIESDissertation, 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.”
Optical micro-resonators
Example: Kopplung zweier optischer Mikroresonatoren ultrahoher Güte über eine makroskopische Distanz Diploma thesis, Andreas Vogler, Universität Mainz
iScan-stabilzed laser system for the excitation of bottle modes in microresonators (courtesy of 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.“
Pricing (EU-Countries)
8 % surcharge for non-European countries plus shipment and possible import duties/taxes in the destination country.