## 1 Introduction  
Over the last three decades, THz time-domain spectroscopy (TDS) has matured from proof of principle demonstrations [1] to a widely utilized tool in basic research and industrial applications [2]. Especially for industrial applications in real-world scenarios, it is mandatory to find compact and cost-effective solutions for the required ultrafast laser source, which drives the THz TDS system. While first experiments were performed with dye lasers, more user-friendly and reliable Ti-sapphire lasers later enabled the first THz TDS images [3]. A big step towards user-friendly and more compact systems was the employment of erbium-doped ultrafast fiber lasers [4]. With these systems it was possible to demonstrate several real-world related applications including non-destructive testing [5; 6; 7], food quality control [8; 9; 10], and applications in archeology [11; 12; 13].  
However, fiber-lasers are still not the ideal solution for ultra-compact and cost-effective systems. They must be pumped optically and still make up 50% of the total costs of a THz TDS system [14]. From a system engineer's point of view, a semiconductor laser diode is the ideal light source. It can be pumped electrically, has a very small footprint, and can be inexpensively mass-produced by the semiconductor industry. Despite these advantages, no commercially available THz TDS system is driven by a monolithic laser diode. The utter lack of ultrafast monolithic laser diodes stems from the complex semiconductor physics, which prohibit the direct generation of ultrashort laser pulses [15; 16; 17].  
One method to overcome this problem is the use of inexpensive multimode laser diodes without any locking. The photomixer generates multiple THz frequencies by mixing the longitudinal laser modes. This technique requires no specific phase relation of the modes. Furthermore, short-term fluctuations caused by mode competition average out during a measurement due to temporal integration at each time step. Initially, this technique was called THz cross-correlation spectroscopy (CCS) [18; 19] and was later established as THz quasi time-domain spectroscopy (QTDS) [20; 21]. Recent publications demonstrate the feasibility at telecom wavelengths with a bandwidth of up to 2 THz [22; 23]. However, the performance of a mode-locked laser diode in THz TDS should be superior to CCS/QTDS since the peak power and, hence, the generated THz power is higher, compared to a continuous wave laser.  
So far only one demonstration for THz TDS using a monolithic laser diode exists [24], wherein a research grade laser diode was used to drive a standard THz TDS system. The bandwidth was limited to 800 GHz with a peak dynamic range of 45 dB. The poor performance, compared to recent CCS/QTDS results, originated from rather broad pulses (\(\sim\)900 fs), low average optical power (5 mW) at the photoconductive antennas (PCA), and an emitter PCA carrier lifetime of 30 ps, which is long compared to the 25 ps period of the laser pulses.  
In this paper, a commercially available laser diode was used to drive state-of-the-art ErAs:In(Al)GaAs PCAs with carrier lifetimes around 2 ps (source: \(2.1\pm 0.4\) ps, receiver: \(1.55\pm 0.1\) ps). The intrinsic linear chirp of the fiber-coupled laser diode can be compensated by the dispersion of a standard single mode fiber. In the first step, its essential to find a stable operation mode and the optimal fiber length for minimal pulse duration. Then, a classic THz TDS setup was used to evaluate the capability to generate and detect THz radiation. The results are compared to a state-of-the-art pulsed fiber laser.