Fast CW THz Spectrometer
Goebel, T.¹; Schoenherr, D¹; Sydlo, C²; Cojocari, O²; Deninger, A³; Meissner, P¹; Hartnagel, HL^1
1Technische Universität Darmstadt; ²ACST GmbH; ³Toptica Photonics

Photoconductive THz generation has become a well accepted technique for ultra broadband measurements. CW (continuous wave) THz spectroscopy provides higher resolution compared to pulsed techniques. Hence, it is advantageous to deploy CW signals for resolving very narrow characteristics in spectroscopic samples. However, accurate and fast frequency control has been a limiting factor and has impeded the application of CW signals for spectroscopic purposes.

In this paper we introduce a fast CW THZ spectrometer consisting of a tunable distributed feedback (DFB) diode laser system, a photomixer and a Schottky diode based detector. Two laser beams of adjacent wavelengths are superimposed on the photomixer, which radiates an electromagnetic wave at the difference frequency of the two lasers – in the THz range.

External wavelength stabilization via Fabry-Perot etalons permit a precise control of the THz frequency better than 5 MHz. In addition, this novel stabilization concept allows to achieve linear frequency sweeps of up to 30 GHz/s. Therefore a frequency range of 100 GHz up to 1 THz can be swept within 30 seconds. The fast tuning speed of the system requires equally fast detection techniques for precise measurements of the spectral characteristics of samples to be investigated. Since a photoconductive detector is sensitive to amplitude and phase of the transmitted signal, the phase of the signal has to be scanned at each frequency point. This requirement makes it unsuitable to exploit the enormous scanning speed of this system. In addition, many research areas, e.g. absorption spectroscopy, do not demand phase information.

For those applications, a very fast power detector has to be deployed. Here, a zero-bias Schottky diode is coupled with a frequency independent antenna and its DC current is measured. Due to the biasless operation, very low noise levels can be attained. The incident THz signal is received by the antenna and leads to a voltage at its terminals where the Schottky diode is connected. The nonlinearity of the diodes I-V curve rectifies the THz signal. The concept of a rectifier is significantly faster compared to bolometric power detection schemes. The measurement bandwidth of the Schottky-based spectrometer is virtually only limited by the transimpedance amplifier utilized for the detector.

To ensure a good signal-to-noise ratio as well as detectability of fast varying signals, the THz signal is electrically chopped at high speed. The detected signal is sampled by a computer with a ADC (analog to digital converter). Simultaneously, the control signals of the laser stabilization are sampled to track the THz difference frequency isochronously to the measurement data. This guarantees a very low frequency error.

The characterization of the developed fast CW THz spectrometer as well as the investigation of samples with different spectral characteristics are presented in the paper.