Abstract-Self-normalizing phase measurement in multimode terahertz spectroscopy based on photomixing of

K. Thirunavukkuarasu, M. Langenbach, A. Roggenbuck, E. Vidal, H. Schmitz, J. Hemberger, M. Grüninger

http://arxiv-web3.library.cornell.edu/abs/1410.0648

(Submitted on 2 Oct 2014)

Photomixing of two near-infrared lasers is well established for continuous-wave terahertz spectroscopy. Photomixing of three lasers allows us to measure at three terahertz frequencies simultaneously. Similar to Fourier spectroscopy, the spectral information is contained in an nterferogram, which is equivalent to the waveform in time-domain spectroscopy. We use one fixed terahertz frequency \nu_ref to monitor temporal drifts of the setup, i.e., of the optical path-length difference. The other two frequencies are scanned for broadband high-resolution spectroscopy. The frequency dependence of the phase is obtained with high accuracy by normalizing it to the data obtained at \nu_ref, which eliminates drifts of the optical path-length difference. We achieve an accuracy of about 1-2 microns or 10^{-8} of the optical path length. This method is particularly suitable for applications in nonideal environmental conditions outside of an air-conditioned laboratory.

Abstract-Enhancing the stability of a continuous-wave terahertz system by photocurrent normalization

Roggenbuck, A ; Langenbach, M ; Thirunavukkuarasu, K ; Schmitz, H ; Deninger, A ; Mayorga, I Camara ; Güsten, R ; Hemberger, J ; Grüninger, M

http://cds.cern.ch/record/1516814?ln=en

In a continuous-wave terahertz system based on photomixing, the measured amplitude of the terahertz signal shows an uncertainty due to drifts of the responsivities of the photomixers and of the optical power illuminating the photomixers. We report on a simple method to substantially reduce this uncertainty. By normalizing the amplitude to the DC photocurrents in both the transmitter and receiver photomixers, we achieve a significant increase of the stability. If, e.g., the optical power of one laser is reduced by 10%, the normalized signal is expected to change by only 0.3%, i.e., less than the typical uncertainty due to short-term fluctuations. This stabilization can be particularly valuable for terahertz applications in non-ideal environmental conditions outside of a temperature-stabilized laboratory.