Laser diode based THz-TDS system with 133 dB peak signal-to-noise ratio at 100 GHz

Abstract

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful and versatile tool in various scientific fields. These include—among others—imaging, material characterization, and layer thickness measurements. While THz-TDS has achieved significant success in research environments, the high cost and bulky nature of most systems have hindered widespread commercialization of this technology. Two primary factors contributing to the size and cost of these systems are the laser and the optical delay unit (ODU). Consequently, our group has focused on developing THz-TDS systems based on compact monolithic mode-locked laser diodes (MLLDs). The ultra-high repetition rate (UHRR) of the MLLD has the added benefit that it allows us to utilize shorter ODUs, thereby reducing the overall cost and size of our systems. However, achieving the necessary precision in the ODU to acquire accurate terahertz time-domain signals remains a crucial aspect. To address this issue, we have developed and enhanced an interferometric extension for UHRR-THz-TDS systems. This extension is inexpensive, compact, and easy to incorporate. In this article, we present the system setup, the extension itself, and the algorithmic procedure for reconstructing the delay axis based on the interferometric reference signal. We evaluate a dataset comprising 10,000 signal traces and report a standard deviation of the measured terahertz phase at 1.6 THz as low as 3 mrad. Additionally, we demonstrate a remaining peak-to-peak jitter of only 20 fs and a record-high peak signal-to-noise ratio of 133 dB at 100 GHz after averaging. The method presented in this paper allows for simplified THz-TDS system builds, reducing bulk and cost. As a result, it further facilitates the transition of terahertz technologies from laboratory to field applications.

A Highly Frequency-Selective 3D-Printed Dielectric Structure for the Terahertz Range

“…The bandwidth of terahertz spectrometers nowadays exceeds more than 10 THz with a dynamic range of more than 130 dB [1][2][3]. Possible applications include materials characterization [4], security applications [5], industrial non-destructive testing [6], and high-resolution 3D reconstruction of objects [7].…”

A highly frequency-selective 3D printed dielectric structure for the terahertz range

“…The bandwidth of terahertz spectrometers nowadays exceeds more than 10 THz with a high dynamic range of more than 130 dB [1][2][3]. Possible applications are the characterization of materials [4], security applications [5], industrial non-destructive testing [6] and high-resolution 3D reconstruction of objects [7].…”

Optimal Conditions for a Multimode Laser Diode with Delayed Optical Feedback in Terahertz Time-Domain Spectroscopy

“…However, in this case, similar to MMLDs, their dynamic range is limited by random phase effects. Recently, a monolithic mode-locked laser diode (MLLD) was proposed, demonstrating high signalto-noise ratio characteristics (133 dB) and high repetition rates (several tens of GHz) in THz-TDS, and it is called ultra-high repetition rate terahertz time-domain spectroscopy (UHRR-THz-TDS) [22,23]. This light source has the potential to advance THz-TDS for outdoor applications and become a standard source in this field.…”