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].

In this paper we present a confocal optical THz-scanner with real-time data acquisition at a measuring rate of 10 kHz and scanning speed of 250 mm/s. It comprises two THz modules with OAP collimation, a tip-tilt mirror and a refractive focusing lens made of silicon (diffraction limited, f = 30 mm). The scanning field dimensions are about 10 x 10 mm(2). Reflectivity is simultaneously evaluated at four freely selectable frequencies between 0.1 and 0.8 THz. To reduce Fresnel losses at the lens's plano Si-air interface we design and evaluate a moth-eye structure acting as anti-reflection coating that was manufactured in-house by an ultrafast laser ablation process.

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.

Integrated sensing and communication has been considered a focal 6G technology, enabling simultaneous high capacity transmission and mm-scale resolution. Applications such as metrology, radar or imaging which require both high resolution and high frequency stability will potentially benefit from utilizing the sub-THz frequency band. Electronics has limitations in this range of the spectrum, while optical methods using free-running independent lasers exhibit high phase noise in generated signals and high frequency drift due to lack of phase correlation between them. Optical heterodyning based on optical frequency combs can ensure phase correlation while representing a straightforward approach for sub-THz signal generation. A frequency comb centered on a sub-THz carrier frequency with a line spacing of several lines of MHz can advantage sensing applications and assist with channel estimation in the THz spectrum. We demonstrate the generation of a 50 MHz free spectral range sub-THz frequency comb centered at 126 GHz (D-Band), expanding across 3 GHz with 5 dB flatness. A cascaded modulator architecture is used to generate an electro-optical frequency comb with excellent phase correlation, and a second comb generation stage using pulse modulation achieves the free spectral range in the 10's MHz range. Finally, a waveguide integrated PIN photodiode helped to achieve the sub-THz comb in the electrical domain. We demonstrated RF linewidths in the Hz range for the sub-THz comb and the coherence between its lines. Our approach, with integration potential, enables low size, weight and power consumption characteristics, together with Hz order frequency stability and low phase noise for multiple frequencies simultaneously.

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.

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.

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.

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.

Continuous wave (CW) terahertz spectroscopy systems are a proven and convenient solution for industrial non-destructive testing and multi-layer thickness determination. For these systems to find use in real-time inline monitoring applications, a high acquisition and data evaluation rate is required. One approach that can increase and potentially enable kHz acquisition rates is to scan through only a few selected frequencies instead of a full spectrum. However, the data analysis can become a bottleneck when realizing measurement systems capable of operating at these speeds. Here we show the feasibility of extracting multi-layer thicknesses from a sparsely sampled spectrum by using a real-time evaluation scheme capable of runtimes below a millisecond with a thickness uncertainty comparable to a full spectrum evaluation. This is achieved by reducing the number of computations by three orders of magnitude. The proposed measurement scheme requires complete knowledge of the sample composition and the refractive index of each layer. Additionally, achieving these high evaluation rates assumes that individual layer thicknesses deviate by no more than 200 mu m from their nominal values. However, both conditions are often met in an industrial setting. The sparse evaluation is demonstrated on a three layer sample, the achieved standard deviation of the layer thicknesses remains below 5 mu m for each layer. These measurements confirm the effectiveness of the sparse sampling approach for THz spectroscopy and demonstrate the ability to address industrial applications with measurement rates in the kilohertz range. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We present versatile, strain-free, InP-based SESAMs with iron-doped InGaAs absorber, which facilitate all critical requirements for ultrashort pulse generation at 1560 nm: adjustable ultrafast response and modulation depth as well as low non-saturable losses.

Transition metal-doped InGaAs material has proven to be extremely efficient when used as photoconductor in photoconductive antennas excited with a 1550 nm wavelength laser. In this contribution, we apply a time-domain modeling procedure to predict the radiated power by InGaAs:Fe strip-line antennas. The simulations are verified with measurements, showing an excellent match between them. Additionally, we model the reconstructed current when such a strip-line antenna is coupled with a receiver through a Quasi-Optical (QO) link.

Spectrum analyzers and spectrometers are essential for designing sources, analyzing material properties, layer structures and fingerprinting substances. We present an ultra-wideband, continuous-wave photonic receiver with kHz-level spectral resolution in the terahertz domain for both heterodyne and homodyne detection. Employed as a spectrum analyzer front end, it records the emitted spectrum of a source under test, assessing spectral purity, spectral shape and undesired frequency components. It outperforms state-of-the-art electronic systems in terms of frequency coverage and system cost with a competitive spectral resolution and noise floor on the few aW/Hz level at room temperature. It covers the important frequencies above 1.5 THz, yet commercially inaccessible, where sources like quantum cascade lasers operate. When combined with a comb-based photonic source, we demonstrate hetero- and homodyne spectroscopy over an unprecedented frequency range from below 100 GHz to 6.5 THz and a very low noise floor. Locking the photonic system to GPS enables tracing back the measured parameters to SI units, being of key importance for metrological applications. The presented setups offer the broadest continuous-wave frequency coverage to date, combined with a sharp spectral resolution, enabling diverse applications ranging from fast non-destructive testing, astronomic high-resolution spectroscopy, to frequency-modulated RADAR.

In this article, we present an alternative sampling approach for continuous-wave terahertz homodyne systems that overcomes limitations regarding the measurement of dispersive samples of currently used techniques. The wavelength-dependent phase-delay mirrors, which were developed for this work, induce a frequency-dependent phase shift of up to pi/2. This technique allows sampling of the THz-field, by tuning the laser sources in such a way that a constant frequency difference is maintained and the center frequency is shifted. In our configuration, the phase shift between transmitter and receiver arms depends only on the center frequency of the lasers. This allows for replacing the movement of a delay stage with variation of the center frequency to capture a THz trace. Consequently, measurements are not constrained by the speed of the delay line anymore. Furthermore, this phase shift is unaffected by differences in path length within the setup and does not require phase modulators. Prior simulations show that these mirrors achieve a phase shift up to pi in the C-band for a difference frequency of 280 GHz, which could be confirmed by our measurements. We successfully demonstrated the first application by measuring sample thickness.

Photonic transmitters are a promising approach for future wireless backhauling due to their high bandwidth and compatibility with fiber-optical networks. Additionally, photonic integration allows to create complex structures, e.g. antenna arrays, on a low-footprint chip or chiplets. So far, however, mainly fixed point-to-point links are demonstrated, and point-to-multipoint concepts come with increased effort, e.g. controlling numerous phase shifters to drive optical phased arrays. Our beam-switching approach demonstrates a multi-directional sub-THz transmitter with low operation effort. In a proof-of-concept experiment, we demonstrate 6.4 Gbps net data rate across an angular coverage of 85° in the W-band, specifically at 100 GHz.