Terahertz Frequency Combs
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Terahertz Frequency Combs Theoretical Aspects and Applications Heiko Fuser ¨ · Mark Bieler
Received: 30 September 2013 / Accepted: 11 November 2013 © Springer Science+Business Media New York 2013
Abstract We review recent work on THz frequency combs generated by femtosecond lasers. An overview about the underlying theory and application examples for the analysis of THz signals are given. Using different optoelectronic detection methods, spatially resolved measurements of frequency, phase, and amplitude of GHz and THz emitters are presented. The measurement setup allows for high-precision measurements of the frequency (accuracy of 9 · 10−14 within a 10 s measurement interval), relative amplitude (standard deviation of the mean 0.1 %) and phase (standard deviation of the mean 0.2◦ ). Moreover, the accuracy and invasiveness of different optoelectronic techniques are compared. The large spectral coverage provided by THz frequency combs is demonstrated by simultaneously measuring signals at 16 GHz and 28 THz. Keywords THz frequency comb · THz metrology · Antenna pattern · Photoconductive sampling · Electro-optic sampling
1 Introduction Due to the technical progress of the last decades, the precise knowledge of time and frequency affects our day-to-day life in a more and more significant way. Various electronic applications providing fundamental backbones of today’s society are based on processes which take place on ever-short time scales. The technical evolution from first radio-frequency devices in the early 20th century up to modern optical data transmission techniques or global-positioning applications spans several orders of
H. F¨user () · M. Bieler Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany e-mail: [email protected]
J Infrared Milli Terahz Waves
magnitude in carrier frequency and bandwidth. Additionally, urgent scientific questions such as tests of relativity or a deeper understanding of physical constants and units can only be approached via high-precision frequency measurements. Driven by such needs frequency has become the physical quantity which can be measured with by far the highest precision and accuracy. Utilizing devices like cesium atomic clocks, it is possible to measure frequencies with an uncertainty of 10−16 [1]. The remarkable high accuracy of these atomic clocks is nowadays used to define the second within the International System of Units (SI). By this, a very precise time-base standard is established, which can easily be transferred into physics laboratories to act as a reference signal for measurement equipment. Thus, in daily scientific work, high frequency measurement precision is achievable for a wide range of technical devices. This aspect requires a brief discussion of the accessibility of the time-base standard in different regions of the electromagnetic spectrum. As the corresponding transition frequency of the cesium atom lies in the microwave range, it is well suitable to act as a reference signal for electronic devices and measurement e
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