Continuous wave THz (cw THz) systems define the state-of-the-art in terms of spectral resolution in THz spectroscopy. Hitherto, acquisition of broadband spectra in a cw THz system was always connected with slow operation. Therefore, high update rate applications like inline process monitoring and non-destructive testing are served by time domain spectroscopy (TDS) systems. However, no fundamental restriction prevents cw THz technology from achieving faster update rates and be competitive in this field. In this paper, we present a fully fiber-coupled cw THz spectrometer. Its sweep speed is two orders of magnitude higher compared to commercial state-of-the-art systems and reaches a record performance of 24 spectra per second with a bandwidth of more than 2 THz. In the single-shot mode, the same system reaches a peak dynamic range of 67 dB and exceeds a value of 100 dB with averaging of 7 min, which is among the highest values ever reported. The frequency steps can be as low as 40 MHz. Due to the fully homodyne detection, each spectrum contains full amplitude and phase information. This demonstration of THz-spectroscopy at video-rate is an essential step towards applying cw THz systems in non-destructive, in line testing.

To fulfill the functionality demands from the fast developing optical networks, a hybrid integration approach allows for combining the advantages of various material platforms. We have established a polymer-based hybrid integration platform (polyboard), which provides flexible optical input/ouptut interfaces (I/Os) that allow robust coupling of indium phosphide (InP)-based active components, passive insertion of thin-film-based optical elements, and on-chip attachment of optical fibers. This work reviews the recent progress of our polyboard platform. On the fundamental level, multi-core waveguides and polymer/silicon nitride heterogeneous waveguides have been fabricated, broadening device design possibilities and enabling 3D photonic integration. Furthermore, 40-channel optical line terminals and compact, bi-directional optical network units have been developed as highly functional, low-cost devices for the wavelength division multiplexed passive optical network. On a larger scale, thermo-optic elements, thin-film elements and an InP gain chip have been integrated on the polyboard to realize a colorless, dual-polarization optical 90° hybrid as the frontend of a coherent receiver. For high-end applications, a wavelength tunable 100Gbaud transmitter module has been demonstrated, manifesting the joint contribution from the polyboard technology, high speed polymer electro-optic modulator, InP driver electronics and ceramic electronic interconnects.

This paper describes a fabrication process for realizing Indium-Phosphide-based photonic-integrated circuits (PICs) with a high level of integration to target a wide variety of optical applications. To show the diversity in PICs achievable with our open-access foundry process, we illustrate two examples: a fully-integrated 20 Gb/s dual-polarization electro-absorption-modulated laser, and a balanced detector composed of avalanche photodiodes for detection of 28 Gb/s optical signals. On another note, datacenters are increasingly relying on hybrid integration of PICs from different technology platforms to increase transmission capacity, while simultaneously lowering cost, size, and power consumption. Several technology platforms require surface coupling rather than the traditional edge coupling to couple the light from one PIC to another. To accommodate the surface-coupling approach in our integration platform, we have developed a strategy to transfer the following optical Input/Output devices into our fabrication process: grating couplers, and vertical mirrors. In addition, we introduced etched facets into the process to improve the usability of our edge-coupling elements. We believe that the additional flexibility in Input/Output interfacing combined with the integration of multiple devices onto one PIC to reduce the number of PIC-to-PIC alignments can contribute significantly to the development of compact, low-cost, and high-performance datacenter modules.

We present a novel array of electroabsorption modulated lasers as compact and low cost single-chip solution for future 200Gb/s transmitters. The array is designed for high optical output power with semiconductor optical amplifiers at the front side of the chip. A common InGaAlAs-MQW active layer structure allows for simple and cost effective monolithic integration. On chip RF transmission lines are implemented to bring all electrical contacts to the rear side of the array-chip which supports packaging with short wire bonds. The array operates at four different wavelengths spanning over 7.5nm in the L-band. Uniformity of each wavelength channel is experimentally proven regarding modulation bandwidth >30GHz, extinction ratio >7dB and output power up to 8 dBm. The influence of the semiconducting optical amplifiers on signal quality is investigated by back to back bit error ratio measurements. In transmission experiments over standard single mode fiber links the array's performance at 4x56GBd NRZ and 4x28GBd PAM4 is demonstrated and the arrays capability for up to 7km transmission in case of PAM4 signaling is shown.

Continuous wave (cw) terahertz (THz) radiation has a wide array of applications, ranging from sensing to next-generation wireless communication links. Industrial applications frequently require THz systems that are broadband, highly efficient, and compact. Photomixer-based solutions hold promise in meeting these demands, offering extremely broadband operation and the potential for miniaturization through photonic integration. However, current photoconductive antenna (PCA) receivers used in these systems are top-illuminated, which strongly limits their efficiency and renders them incompatible with photonic integration. To overcome these limitations, we developed optical waveguide-integrated photoconductive antennas (win-PCAs) for cw-THz detection. These antennas not only facilitate integration into photonic integrated chips (PICs) but also allow us to explore new device geometries to optimize the PCA’s responsivity. By optimizing the absorber geometry of the win-PCAs, we achieve a 22-fold increase in photoresponse, a 500-fold improvement in THz responsivity, and a 4.7-fold reduction in noise-equivalent power compared to state-of-the-art top-illuminated PCAs. In a coherent cw-THz spectrometer, these improvements enable measurements with a peak dynamic range of 123 dB for 300 ms averaging, which is 11 dB higher than what is achievable with comparable top-illuminated receivers. The presented win-PCAs represent a significant step toward fully integrated, high-performing photonic cw-THz systems for both spectroscopy and high-capacity wireless links.

We demonstrate InP PICs with avalanche photodiodes (APD) consisting of InP and InAlAs multiplication regions and investigate these devices regarding their DC and RF linearity characteristics. The optical input dependent gain, bandwidth and output power are characterized and compared to electric field simulations. The space-charge induced electric field reduction is considered to determine the measured properties of the APDs. The APDs with an InAlAs multiplication region exhibit a high bandwidth of 27 GHz in combination with high sensitivity for optical input powers low as 34 µW at a responsivity of 0.65 A/W and a dynamic range of 34 dB. Although the APD with an InP multiplication region shows lower bandwidths of maximum 6 GHz, it demonstrates a high sensitivity for input powers low as 26 µW at a responsivity of 0.85 A/W and a dynamic range of 30 dB making it very attractive for sensing applications. Further, the bias and optical input dependent electric field help to conclude the linearity limitations of the presented devices.