# Spectral Shaping of a Superluminescent Diode for Terahertz Cross-Correlation Spectroscopy
## 1 Introduction  
Since terahertz cross-correlation spectroscopy (THz-CCS) was introduced in 2000, the researchers dedicated to this research field have made significant progress towards reasonable performance improvements for practical applications. The first step towards compact THz-CCS systems was presented by [PERSON] et al. (2000), using a commercially available multimode laser diode (MMLD) and photoconductive antennas (PCAs) [1]. This was the first time that a MMLD was used for broadband terahertz generation instead of an ultrafast mode-locked laser to create terahertz radiation. However, the free-space transmission in the optical path makes this system not suitable for mobile applications. Inserting single-mode fiber and a fiber-coupled 50:50 beam splitter into the optical path instead of a free space 50:50 beam splitter improved the signal-to-noise ratio (SNR) and the compactness for THz-CCS systems [2, 3]. This technique for terahertz generation was later extended and renamed to terahertz quasi time-domain spectroscopy (THz-QTDS) [4]. THz-QTDS has the disadvantage of discrete frequency components, much lower bandwidth, and lower dynamic range compared to state-of-the-art terahertz time-domain spectroscopy (THz-TDS) systems. Within the last ten years researchers tried to overcome these disadvantages with different methods. One of these methods was the implementation of a spatial light filter in the optical path to increase the continuous signal components of the terahertz spectrum by additional mode filtering. This further enhanced the SNR towards practical relevance [5]. Another solution to get a continuous terahertz spectrum using two spectrally overlapping semiconductor laser sources was presented by [PERSON] et al. (2011). The group used two modulation techniquesin combination, which resulted in a continuous terahertz spectrum [6]. Another issue that led to advances in THz-QTDS systems was the use of 1550 nm communications technology components. Fiber-coupled components from optical communications engineering can be easily implemented in THz-QTDS systems with a center wavelength of 1550 nm. A fully fiber-coupled THz-QTDS system has been demonstrated by [PERSON] et al. (2017) with a bandwidth of 1.8 THz and a single shot peak dynamic range of 60 dB. This was a huge step towards compact and mobile terahertz systems and had a bandwidth improvement of 50% [7]. All these systems used a MMLD as proposed by [PERSON] et al. in 2000. The first use of monolithic-mode-locked laser diodes (MLLD) for THz-TDS as a light source was presented by [PERSON] et al. in 2017 [8]. This light source creates periodic pulse trains in the time-domain and discrete components in the frequency-domain. The terahertz frequency components have the same free spectral range as the laser modes. This system achieved a bandwidth of 0.6 THz with a peak dynamic range of 45 dB. A method for further enhancing the performance of THz-QTDS systems regarding the bandwidth is the realization of a low duty cycle of the injection current of the MMLD. Pulsing the injection current showed an increase of the bandwidth [9]. The possibility of optical feedback of the laser diode was presented one year later by [PERSON] et al. (2019). The group could raise the bandwidth of the system from 1.8 THz to 2.5 THz with optical feedback [10]. These improvements bring the performance level of modern THz-QTDS systems to practical relevance and therefore the research towards more compact systems matured as well. [PERSON] et al. (2019) presented a compact and inexpensive THz-QTDS system for mobile scenarios with a bandwidth of \(\sim\)0.8 THz [11]. Until recently, it was always a laser source that illuminates the PCAs or the photomixers of a terahertz system. In 2019, [PERSON] et al. showed that THz-QTDS systems can also be driven by an incoherent light source. The used light source was a superluminescent diode (SLD), which is normally used in optical coherence tomography because of its broad spectrum and short coherence length.