Distortion-free enhancement of terahertz signals measured by electro-optic sampling II Experiment

Abstract

Three methods for distortion-free enhancement of electro-optic sampling measurements of terahertz signals are tested. In the first part of this two-paper series [J. Opt. Soc. Am B 31, 904-910 (2014)], the theoretical framework for describing the signal enhancement was presented and discussed. As the applied optical bias is decreased, individual signal traces become enhanced but distorted. Here we experimentally show that nonlinear signal components that distort the terahertz electric field measurement can be removed by subtracting traces recorded with opposite optical bias values. In all three methods tested, we observe up to an order of magnitude increase in distortion-free signal enhancement, in agreement with the theory, making possible measurements of small terahertzinduced transient birefringence signals with increased signal-to-noise ratio.

Distortion-free enhancement of terahertz signals measured by electro-optic sampling I Theory

“…Although the undistorted enhancement of a transient birefringence signal was first introduced for optical heterodyne detection, this technique also works for large THz signals and arbitrary optical biases. The experimental demonstration will be given in a following paper [10].…”

Effective detection of weak terahertz pulses in electro-optic sampling at kilohertz repetition rate

“…Within these schemes, distortions of the measured THz pulse have been identified as a key limiting factor. Distortion-free amplification techniques have been reported, but the usability is limited since either two measurements are needed to reconstruct the true waveform [5] or they are not applicable to detect weak THz fields [4]. In this paper, we show how the strength of the THz field itself is a key parameter to limit amplification and distortion as hypothesized before [6].…”

“…As EOS basically is a translation of field strength into change of polarization, it is possible to take advantage of the excess energy to improve detection sensitivity with careful manipulation of the polarization in the detection process. Previous studies achieved that by biasing the input polarization with an optical retarder before [4] and/or after the optical mixing within the detection crystal [3,5]. In a comparable scheme, a series of wedges was used to control the polarization [6].…”

“…Additionally, we show how distortion-free amplification is possible at a small THz field, allowing us to detect weak signals with increased sensitivity. To this purpose, we used a simple scheme presented before by Johnson et al [5] and Brunner et al [3] and apply the Mueller formalism to propagate the polarization state through the detection. This allows us (i) to account for the imperfect polarization of the sampling pulse (also defined as gate pulse in this work) and (ii) to understand the origin and the link between distortions, THz strength, and amplification in detail.…”

A decade-spanning high-resolution asynchronous optical sampling terahertz time-domain and frequency comb spectrometer

“…17 Frequency content is extracted from the time-domain data by a fast Fourier transform (FFT) allowing THz-TDS to overcome broadband detector noise through the multiplex principle. To highlight the potential of ASOPS based THz-TDS, the capabilities of two recent ASOPS-THz-TDS instruments are listed here: a high-speed 1 GHz oscillator ASOPS-THz-TDS system with 7 THz of bandwidth that has achieved a mean frequency deviation of 142 MHz when measuring the line centers of water vapor absorptions from 0.2 to 6.3 THz, 18,19 and a high-resolution ASOPS-THz-TDS system based on two Er-doped fiber lasers achieved a mean frequency deviation of 4 MHz when measuring line centers of acetonitrile absorptions from 642.0 GHz to 643.2 GHz.…”