Fiber-coupled terahertz spectroscopy system
Most terahertz time domain spectroscopy (THz TDS) systems are pumped by a beam propagating in free space. By the use of suitable fiber optics which transmits the infrared laser pulses several meters away a decoupled setup of the optical pump from the terahertz beam path can be reached. Thereby the system becomes more compact and robust against disturbing external influences.
The fiber optics transmisssion technology known from the optical communication technology cannot be applied here directly. The linear and non-linear fiber properties show much more affects on the propagation of a broadband, ultrashort and high peak-power femtosecond pulse. The transmission is based on an elaborated adaptive dispersion management.
The light pulses of the laser source are guided by highly reflective dielectric mirrors into a pulse stretcher, before they are coupled into the quartz fiber optics. By the use the same two optical griatings one reaches a dispersion adjustable pulse stretcher by the variation of the grating distance d. By that the pulses are stretched so far (prechirping) that they show a similar short pulse length like at the beginning after propagating through a certain fiber length.
To be able to reach also badly accessible measuring places with a narrow space, the THz measuring heads were built up compactly. They containan optical fiber connection, an electric interface and a lens optics. A bandwidth of about 4 THz range was reached, even with a transmission length of 15 m through a fiber.
References:
[1] "Compact fiber-coupled terahertz spectroscopy system pumped at 800 nm wavelength", F. Ellrich, T. Weinland, D. Molter, J. Jonuscheit, and R. Beigang, Rev. Sci. Instrum., Vol 82, pp. 053102 (2011)
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[2] "Terahertz-Wellen – ein neuer Spektralbereich für die industrielle Messtechnik (Terahertz Waves – A New Spectral Band for Industrial Measurement Techniques)“, F. Ellrich, T. Weinland, J. Klier, J. Jonuscheit, and R. Beigang, tm-Technisches Messen, Vol. 77, No. 9, pp. 452-461 (2010)
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[3] "Fasergekoppeltes Terahertz-Spektroskopiesystem (Fiber-coupled Terahertz Spectroscopy System)", F. Ellrich, T. Weinland, M. Theuer, J. Jonuscheit, and R. Beigang, tm-Technisches Messen, Vol. 75, No. 1, p. 14-22 (2008)
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[4] "Terahertz-Bildgebung in industriellen Anwendungen (Terahertz Imaging in Industrial Applications)", M. Theuer, G. Torosyan, F. Ellrich, J. Jonuscheit, and R. Beigang, tm-Technisches Messen, Vol. 75, No. 1, p. 64-70 (2008)
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[5] Hecht, E.: 3. Auflage: Optik. Oldenbourg Wissenschaftsverlag GmbH, München, 2001
[6] Agrawal, G. P.: Nonlinear Fiber Optics, (3rd Ed.). Academic Press, 2001
[7] Agrawal, G. P.: Fiber-Optic Communication Systems, (2nd Ed.). John Wiley & Sons, Inc., 1997
