Towards photonic integrated THz systems on a chip

Lauri Schwenson1, Florian Walter1, Milan Deumer1, Shahram Keyvaninia1, Steffen Breuer1, Simon Nellen1, Lars Liebermeister1, Martin Schell1,2 and Robert B. Kohlhaas1

1Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institute, Einsteinufer 37, 10587 Berlin, Germany
2Technische Universität Berlin, Institut für Festkörperphysik, Hardenbergstraße 36, 10623 Berlin, Germany

Over the past decade or so, the spectral bandwidth and dynamic range of commercially available terahertz (THz) spectrometers have markedly improved [1], [2], [3]. Despite these advancements, the high cost of these sophisticated measurement instruments remains a significant barrier, limiting their adoption primarily to applications where they confer substantial benefits, such as significant cost reductions or novel measurement capabilities. A notable example where THz technology is gaining traction is in the automotive industry, specifically for quality control through thickness measurements in the painting process [4], [5]. The large-scale production of automobiles justifies the investment in high-quality, albeit expensive, systems. Nevertheless, the prohibitive cost of THz systems still hinders their adoption in numerous cost-sensitive applications.
A particularly promising avenue for broadening the accessibility of THz technology is through photonic integration technology, which offers a pathway to develop extremely compact and cost-effective THz systems. In this contribution, we present the latest advancements in the development of photonic integrated THz systems. This includes an overview of system design considerations, details of the microtechnological processes involved, and empirical results from both individual components and a fully integrated continuous wave (cw) THz system.
Photonic integrated cw-THz systems rely on photomixing within ultrafast optoelectronic components to generate and detect THz radiation. This process involves the superposition of two single-frequency lasers to create an optical beat note, the frequency of which equals the difference between the frequencies of the two lasers. This optical beat note is subsequently transformed into the THz-domain by a photodiode. In the configuration of a homodyne spectrometer, the same optical beat note is used to excite a photoconductive receiver. This receiver mixes the incoming THz signal with the modulated photoconductance, resulting in a detectable receiver current. Furthermore, the phase of the THz signal can be modulated by modulating the phase of one of the laser lines. Through this modulation, both the amplitude and phase of the THz signal can be recovered, enabling a coherent measurement [6], [7].
Here, we present a dual laser source-PIC as optical backend within a terahertz spectroscopy setup. Figure 1 shows a micrograph of this fabricated PIC. Our proposed PIC contains two widely tunable sampled grating DBR lasers in combination with a waveguide network that allows for signal superposition, phase modulation and optical amplification. Thus, all optical signal generation and processing for the spectrometer is combined within a single InP-based chip on an area of 10 x 3 mm².

Figure 1: Micrograph of the photonic integrated cw-THz spectrometer fabricated at Fraunhofer HHI with two SG-DBR laser sources, a waveguide network for optical superposition, two phase modulators (PMs) and optical semiconductor amplifiers (SOAs) for signal processing.

The individual lasers are designed for a tuning range of approx. 20 nm within the C-band. By introducing an offset in the center wavelength between the two lasers, we are able to extend the combined tuning range to approximately 40 nm, which corresponds to nearly 5 THz of available tuning bandwidth for cw-THz generation. Figure 2 a) presents the frequency map of laser SG-DBR-2 when front and rear heater are adjusted. As depicted, the laser exhibits eight distinct frequency bands where it operates in single-mode. Additionally, fine-tuning of the laser frequency is possible by adjusting the phase section.
The device was packaged into a prototype housing that allows optical coupling with polarization maintaining fibers, electrical connection to all active elements via external connectors as well as a temperature control of the whole device. Figure 2 b) compares the measured THz spectra of our novel PIC-based THz system with a state-of-the-art laboratory setup employing external cavity lasers. For these measurements, state-of-the-art cw-THz antennas based on a PIN-photodiode emitter and an iron doped indium gallium arsenide (InGaAs:Fe) photomixer as receiver were used. As shown, the PIC-based spectrometer achieves >90 dB dynamic range around 100 GHz and a record spectral bandwidth of 4 THz.

Figure 2: a) Frequency map of the SG-DBR-2 laser when tuning the front and rear heater sections. b) THz spectra as recorded with the PIC-spectrometer and a lab-system with external cavity lasers (ECL). For the PIC-spectrometer a record spectral bandwidth of 4 THz is achieved.

A previous study demonstrated a photonic integrated circuit (PIC) backend integrating two distributed feedback lasers, achieving a tuning bandwidth of 570 GHz within the terahertz spectrum [8]. In contrast, the current work employs sampled grating distributed Bragg reflector lasers, which enhance the tuning bandwidth by over eightfold to nearly 5 THz, encapsulating the entire spectral range accessible to state-of-the-art continuous-wave cw-THz antennas [9]. Previous experiments with a similar PIC have shown that equal THz output power can be expected from the single PIC laser source compared to commercial external cavity lasers [10]. This paper thus presents the first demonstration of coherent terahertz measurements powered by a single PIC-backend with an expansive tuning bandwidth of 5 THz. Consequently, these findings represent a significant advancement towards the development of compact and scalable THz systems, poised to facilitate cost-sensitive industrial applications of THz spectroscopy.

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