We present a microwave photonic RF comb generator based on a quartz-stabilized, ultra-low phase noise fs-pulse laser in combination with high-speed RF waveguide coupled photodetectors. With 325 GHz, the cutoff frequency of the subsystem exceeds the state of the art and enables broadband microwave comb generation with 1 GHz spacing in the D- and J-band, when changing the photodetector from a WR6.5 to a WR3.4 device. We generate microwave tones at 270 GHz with a phase noise of -81 dBc/Hz at 1 kHz frequency offset and below -130 dBc/Hz at 1 MHz offset while achieving a spectral width below 1.7 Hz in the electrical domain

Ultrafast solid-state glass lasers are frequently mode-locked using a semiconductor saturable absorber mirror (SESAM), which can provide reliable laser self-start and stable mode-locked operation. However, state-of-the-art GaAs-based SESAMs for the 1.55 µm wavelength range require highly strained InGaAs quantum well absorbers with a lattice-mismatch of ≥ 2% relative to the GaAs substrate, which leads to defect formation, reduced damage threshold, and limited design freedom. Here, we present the first fully strain-free SESAMs for solid-state glass lasers at 1.55 µm wavelength, which overcome these constraints. Our devices are grown on InP with a very low residual lattice-mismatch of < 0.1% and incorporate an iron-doped InGaAs bulk absorber. This enables continuous tunability of the SESAM modulation depth and precise control over the ultrafast SESAM recovery time - an unprecedented degree of design freedom. Furthermore, the SESAM structure combines an anti-resonant design with a highly reflective InAlAs/InGaAlAs bottom DBR and a TiO2/SiO2 top DBR, which results in record-low non-saturable losses for InP-based SESAMs of 0.6% for a modulation depth of 0.6%. With this approach, we demonstrate the first stable continuous-wave mode-locked operation of a solid-state Er,Yb:glass laser using an InP-based SESAM. We achieve 224 fs pulse duration at a maximum output power of 102.5 mW and a 79.1 MHz repetition rate, in combination with excellent noise properties of 0.005% integrated RIN over [100 Hz, 1 MHz] and 3 fs integrated timing jitter over [1 kHz, 1 MHz].

A co-packaged 76 GHz InP-based Mach-Zehnder modulator and a 224 GBaud-class linear differential EML driver was demonstrated, achieving 180 GBaud PAM4 back-to-back in an IM/DD transmission for O-Band operation.

50 GHz colliding-pulse mode-locked lasers with an integrated front-side spot-size converter have been investigated. These devices achieve sub-400 fs pulse durations after external compression, timing jitters as low as 135 fs, and optical peak power up to 5 W. The integrated spot-size converter provides a circular, narrow far field for efficient fiber coupling. The colliding-pulse mode-locked lasers exhibit superior characteristics, achieving an RF linewidth of 3.5 kHz and a Lorentzian optical linewidth of 740 kHz. To evaluate the impact of the cavity design, a direct comparison is performed with a conventional mode-locked laser featuring two separate gain sections. Both devices have identical InGaAsP multi-quantum well active layer structures, and both have electrically isolated gain sections for fair comparison. A threshold current of 8 mA and a time-averaged output power of 110 mW are obtained in both cases, even though the colliding-pulse configuration has double the cavity length.

We analyze the weather-related link availabilities of high-capacity THz-wireless transmission systems, focusing on fiber-integrated point-to-point links in the lower THz frequency range around 300 GHz. First, we discuss latest component technologies for electronic and photonic THz generation, showing that there is a good basis for the implementation of wideband THz-wireless links. Then, we review the application of a THz-wireless fiber extender and identify challenges for its integration into future 6G optical networks, being mainly the linear optical-to-THz conversion and the weather-dependent THz link loss. Both aspects will be addressed in the following: We describe the concept and implementation of a fiber-integrated THz outdoor unit prototype based on electronic THz components and provide link budget calculations for a data rate of 100 Gbit/s. Then, we estimate the THz link loss from theoretical models and real weather data, showing that high link availabilities above 99.999% are possible for link distances of 500 m with state-of-the-art components. Estimates for a longer distance of 1 km are given as well. Finally, we describe our outdoor testbed built with our prototypes and present measured data on the link attenuation over 500 m in comparison to the theoretical expectations, achieving a good correlation.

We present an InGaAsP/InP SPAD module with a DCR below 100 Hz for 22.5 % PDE at 229 K and 1 kHz for 35.5 % at 233 K. The NEP is fivefold lower than the state-of-the-art. The QBER estimation is 0.5 % over 100 km.

Accurate characterization of thin semiconductor layers is crucial for quality control in the production of semiconductor devices. Traditional methods, like the four-point probe technique, require direct contact and are thus destructive. This study explores terahertz time-domain spectroscopy (THz TDS) as a promising non-contact alternative for measuring the electrical properties of semiconductor layers. We investigated 1 μm to 2 μm thick highly doped indium gallium arsenide (InGaAs) layers on indium phosphide (InP) substrates, with electrical sheet conductance between 10-7 and 10-1 S/sq. Utilizing a fiber-coupled THz-TDS system, we performed measurements in both reflection and transmission geometry. For the first time, we apply the self-referenced method to thin semiconductor layers as well as systematically compare self-referenced and referenced based methods to extract the electrical sheet conductance of thin semiconductor layers. In reflection geometry, self-referencing improves accuracy by minimizing operator error and eliminating frequent re-referencing. Agreement with contact-based four-point probes validates THz-TDS for nondestructive evaluation of thin semiconductor layers and extends its measurable conductance range, enabling reference-free deployments.

Photomixers, which convert optical signals into high-frequency electrical signals, are promising sources and detectors for terahertz (THz) wireless communications due to their broad tunability, high bandwidth, and easy integration with fiber-optic networks. Photodiode (PD)-based THz emitters are already the state-of-the-art for highest data rate THz wireless links. Photonic THz receivers, such as photoconductive antennas (PCAs), have the same benefits of high THz bandwidth and potentially the same very low phase-noise as PD emitters. However, PCAs have not yet demonstrated competitive receiver performance compared to electronic mixers. This limitation arises from the restricted conversion gain and intermediate frequency (IF) bandwidth of the top-illuminated PCAs used in current systems. In this work, we present a novel photomixing heterodyne THz receiver based on waveguide-integrated (win) PCAs, which offers a 25 dB increase in conversion gain due to benefits arising from the optical waveguide coupling. We design and optimize a high-frequency package for the win-PCAs, achieving a record 3- and 6-dB IF bandwidth of 25 and 40 GHz, respectively. With this receiver, we now attain gross data rates of up to 84 Gbit/s, which is a new record for photonic wireless links with PCA receivers. At the same time, we demonstrate the ultra-broadband operation capabilities of the win-PCA, enabling data transmission at carrier frequencies from 100 to 600 GHz with the same receiver.

A new monolithically integrated self-injection locking scheme, based on a ring resonator and a reflector, is demonstrated for InP lasers. We compare three different configurations, focusing particularly on the Lorentzian linewidth: an SGDBR with a Sagnac reflector, a DFB with a Sagnac reflector and a DFB with an MMI reflector. Of the three, the MMI DFB configuration is found to be superior for FMCW applications. It achieved the narrowest Lorentzian linewidth of 33 kHz and direct frequency modulation shows a chirp range of up to 6.8 GHz for this laser configuration. The Sagnac SGDBR has a larger tuning range of 30 nm and a slightly wider Lorentzian linewidth of 41 kHz. Integrated photodiodes allow electrical stabilization to minimum linewidth operation.

Fiber-coupled terahertz (THz) time-domain spectroscopy (TDS) systems often employ photoconductive antennas (PCAs) as THz emitters due to their high IR-to-THz conversion efficiency. However, their THz power has been constrained to about 1 mW, as fiber-based pulse delivery limits the excitation power to under 60 mW at typical fiber laser repetition rates around 100 MHz. Here, we report the first THz TDS setup that overcomes this limitation by operating at an elevated repetition rate of 1 GHz. We show that the conversion efficiency of InGaAs:Rh-based PCAs is preserved at this one-order-of-magnitude higher repetition rate, resulting in a record emitted THz power of 1.58 ± 0.08 mW for fiber-coupled THz emitters. This significant improvement is enabled by an ultrafast dual-comb optical parametric oscillator operating at 1 GHz repetition rate, delivering two trains of 200 fs pulses centered at 1.55 μm wavelength with up to 650 mW average power. Further pulse compression to 86 fs is achieved using a 2.6 m long fiber delivery to the PCAs, combining dispersion-compensating and standard polarization-maintaining fibers. Using this setup, we demonstrate THz TDS utilizing asynchronous optical sampling at a scan rate of 414 Hz with a peak dynamic range of up to 92 dB at 20 GHz frequency resolution, and 79 dB at 1.1 GHz frequency resolution in 113 seconds integration time.

We present a photonic integrated circuit (PIC) designed for high-speed terahertz (THz) spectroscopy, operating at kilohertz repetition rates. This PIC integrates all components to generate and modulate the optical beating signals, enabling continuous wave THz generation and coherent detection. We implement an innovative sparse sensing approach with a discrete frequency sweep across 735 GHz in less than 1 ms and achieve a peak dynamic range exceeding 90 dB within a measurement time of less than 1 second. We apply this approach to a multi-layer thickness evaluation and validate the acquired amplitude and phase information, yielding a standard deviation below 2 µm across all three layers. Our experimental evaluation is the first demonstration of spectroscopic THz measurements utilizing a PIC operated at kilohertz repetition rates. This innovative spectrometer is ideally suited for industrial applications, including in-line monitoring and material characterization, showcasing the potential of photonic integration to advance THz spectroscopy.

An InP-based arrayed waveguide grating (AWG) demultiplexer photodetector PIC for LAN WDM is presented. The AWG has a flat-top transmission with an overall crosstalk below -30 dB and a 3 dB channel bandwidth of 3.2 nm for both polarizations. The 3 dB bandwidth of the photodiodes of the PIC is 62 GHz, being suitable for 100 GBd PAM 4 transmission. (c) 2025 The Author(s)

An adapted wavefront matching (WFM) method for the optimization of high-index contrast waveguides is presented. Amending the WFM method by introducing intermediate refractive index values enables the method to be used for the design of waveguide structures in indium phosphide. As an example, the design of a TE0 to TE1 mode converter is presented.

Compact single-photon detectors are essential components for field-deployable quantum technology systems, such as fiber-based quantum key distribution (QKD) receivers or time-of-flight sensors for ranging applications. In contrast to cryogenic superconducting nanowires, InGaAs/InP semiconductor-based single-photon avalanche diodes (SPADs) have significant advantages in terms of size, weight, power, and cost (SWaP-C) as they are only moderately cooled by thermoelectric coolers (TECs). In addition, the non-cryogenic temperatures provide a better reliability of the device and allow its use in non-academic applications. We present a significantly improved chip generation compared to previous results [1], [2] with a system application oriented housing (fiber in-plane with PCB), by packaging the chip in a hermetic butterfly housing with fiber-pigtail and integrated TEC for cryo-free operation at temperatures down to −50°C, see Fig. 1.

Optical frequency comb generators (OFCGs) play a key role in applications like spectroscopy and microwave photonics [1]. While requirements vary by application, compact OFCG development is essential for widespread industrial adoption. Photonic integrated circuits (PICs) enable this compactness and, when fabricated on a generic foundry platform, offer cost efficiency and easy integration of optical functionalities. Tunability both in wavelength and free spectral range (FSR) is critical in spectroscopy and THz/mmW generation [1]. Modulation-based methods, known for high tunability, are employed in this work to address these requirements effectively.

The increasing bandwidth demands in data centers require optical transceivers that can provide aggregated capacities of 1.6 Tb/s and beyond. In spatial-division-multiplexing (SDM) for optical transceivers, multi-core fibers (MCF) enable the transport of different spatially separated channels on a shallow footprint. MCF cores are usually arranged in multiple layers, which poses a challenge for interfacing with conventional photonic integrated circuits (PICs) with only one waveguide propagation layer. To address this challenge, a novel 3D photonic integrated interposer structure is proposed in this work for coupling the different layers of MCF cores with arrays of conventional 2D electro-absorption modulated lasers (EML) and photodetectors (PD). The interposer, developed on Fraunhofer HHI's polymer-based hybrid integration platform, contains 1x1 3D multimode interference (MMI) couplers. These novel 3D MMIs can interconnect a vertical pitch of 22.5 mu m to match the large vertical distance of the MCF cores. MMI losses of 3 dB (best case) were measured for the fabricated MMIs at a wavelength of 1310 nm. This enables the connection of five MCFs with 8 cores each in an array to two arrays of 4 EMLs and two arrays of 4 PDs. We will discuss the design, and characterization of this 3D photonic integrated interposer in more detail.

We present a terahertz (THz) frequency-domain spectrometer with a spectral coverage of up to 5 THz featuring two modes of operation: a fast sweeping frequency-modulated continuous wave (FMCW) mode with 1 GHz resolution, and a novel high-resolution mode that achieves a nominal frequency resolution of 20 MHz. This 50-fold increase in spectral resolution is achieved by smaller laser sweep steps combined with a different measurement scheme. We demonstrate the capabilities of this system by measuring water vapor absorption lines and the transfer function of a silicon wafer. Consequently, this frequency-domain spectrometer is the first to integrate both fast and broadband as well as high-resolution THz measurement capabilities.