Miniaturized semiconductor laser modules capable of delivering high output power in pulsed operation are of great interest for applications such as lidar and spectroscopy. Until today, conventional systems often rely on bulky and power inefficient solid-state lasers like Nd:YAG, due to the limited availability of compact high-power semiconductor solutions. In this work, we present tapered semiconductor amplifier (TA) modules specifically optimized for pulsed operation at center wavelengths of 780 nm and 828 nm. The TAs are integrated into hermetically sealed 14-pin butterfly packages, featuring polarization-maintaining fiber input and collimated free-space output beam. Particular attention is given to managing the drive current-dependent beam astigmatism, which requires dynamic optimization of the collimation optics across different operating points. Beam profile characterization and power measurements were conducted under various pulsed drive conditions. The amplifier modules are seeded by distributed feedback (DFB) lasers operated in continuous-wave mode and driven with microsecond pulses at kilohertz repetition rates. This results in a duty cycle of 1 % with peak powers up to 10 W. This pulse regime supports compact and highly power-efficient operation at wavelengths near 780 nm and 828 nm. The TAs are ideally suited for demanding environments where small size, power efficiency, and reliability are critical. This development paves the way for very compact and power efficient high power laser sources for high resolution lidar applications.

We investigate terahertz time-domain spectroscopy using a low-noise dual-frequency-comb laser based on a single spatially multiplexed laser cavity. The laser cavity includes a reflective biprism, which enables generation of a pair of modelocked output pulse trains with slightly different repetition rates and highly correlated noise characteristics. These two pulse trains are used to generate the THz waves and detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominal repetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We perform THz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1 mu m lasers for the first time, to our knowledge, and obtain THz signal currents approximately as strong as those from reference measurements at 1.55 mu m and 80 MHz. We investigate the influence of the laser's timing noise properties on THz measurements, showing that the laser's timing jitter is quantitatively explained by power-dependent shifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power and show up to 20 dB suppression in noise by the combination of shared pumping and shared cavity architecture. The laser's ultra-low-noise properties enable averaging of the THz waveform for repetition rate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55 dB when operating at 1 kHz and averaging for 2 s. We show that the obtained dynamic range is competitive and can be well explained by accounting for the measured optical delay range, integration time, as well as the measurement bandwidth dependence of the noise from transimpedance amplification. These results will help enable a new approach to high-resolution THzTDS enabled by low-noise gigahertz dual-comb lasers. (c) 2024 Optica Publishing Group under the terms of the Optica

We report on an external cavity diode laser (ECDL) in a hermetically sealed 14-pin butterfly package with a collimated output beam. The laser emits more than 50 mW at a drive current of 165 mA and the emission wavelength can be tuned between 460.74 nm and 460.97 nm. The maximum mode-hop-free tuning range measures 26 GHz. The laser consists of a GaN-based gain chip, which is collimated by an aspheric lens. Behind the lens, a volume Bragg grating (VBG) stabilizes the laser emission to the target wavelength of around 460.8 nm, which can therefore be used for laser cooling or trapping of strontium without the need for frequency-doubling. Inside the butterfly package, the laser diode is soldered on a ceramic submount and mounted on an optical work bench together with the aspheric lens and the VBG. The optical work bench is temperature stabilized by a thermoelectric cooler (TEC) in combination with a thermistor. In frequency noise measurements of the packaged laser we measured a white noise level of 1.29 MHz. This demonstrates a reasonable stabilization of the emission wavelength of the gain chip via the VBG. By further improvements of the ECDL we expect that a linewidth < 1 MHz can be achieved. This development paves the way for compact, blue laser diodes with a narrow linewidth for quantum technology, spectroscopy, and sensing.

For compact spectrometers with high resolution and especially for quantum technology (QT) of the second generation, compact, portable, and long-term stable systems are a prerequisite for the breakthrough of the technology. In this paper, we demonstrate a miniaturized external-cavity diode laser (miniECL), which is frequency stabilized with a volume Bragg grating (VBG), and a miniaturized fiber-coupled tapered amplifier (miniTA). Both devices feature a collimated output beam and are integrated in a hermetically sealed 14-pin butterfly module. Due to the external cavity design and a robust packaging process, the miniECL has a linewidth below 350 kHz with an output power > 80 mW at 670 nm, 770 nm, 780 nm, 852 nm, and 894 nm. These wavelengths correspond to the D1 and D2 transitions of Li, the D1 transition of K, the D2 transition of Rb and the D2 and D1 transition of Cs, respectively. Hence, the miniECL is perfectly suited to excite atomic transitions. In addition, the miniECL can be used as a seed laser for the miniTA, which amplifies its output power to 1.5 W - 3.0 W with excellent beam quality (M-2 between 1.3 and 1.7 and beam divergence < 3 mrad), while maintaining the narrow linewidth of the seed laser. The amplification bandwidth of the miniTA matches with the emission wavelength of the miniECL, which enables extremely compact master oscillator power amplifier (MOPA) setups.

After the discovery of quantum mechanics in the early 20th century, lots of progress was made in using quantum effects. Many applications became part of everyday's life. Today, we are at the beginning of the second quantum revolution, that will not only use quantum effects indirectly, but aims at controlling quantum states and using this for a variety of new applications such as quantum computing, information processing, sensing and metrology. The laser, a tool from the first quantum revolution, will play a major role. Many quantum systems based on optical transitions, such as single atoms, ions or artificial atoms require lasers for the initialization and manipulation. Many of them demand "high-quality photons", that is a laser with stable frequency, low linewidth, high beam quality and single-mode frequency tunability. External cavity diode lasers (ECDL) meet these requirements. In addition, each quantum system requires a specific combination of wavelengths and power levels. TOPTICA Photonics offers the broadest wavelength coverage from 190 nm to 4 mu m based on direct diode lasers and filling the optical gap with frequency converted diode lasers. For a variety of applications, the output power of an ECDL in the range of a few mW up to approximately 150 mW is not sufficient. Using tapered amplifier chips in a master oscillator power amplifier (MOPA) configuration, commercially available output powers up to 4 W can be achieved while maintaining the properties of the master oscillator, i.e. single-frequency and single-spatial mode operation. R&D results of even higher output power of "high-quality photons" are presented.

Dual-comb generation from a single laser cavity provides a simple and high-performance solution to time sampling applications. We demonstrate a compact single-cavity dual-comb laser operating at gigahertz repetition rates and high repetition rate differences up to more than 100 kHz with sub-100 fs pulse duration. The single cavity approach leads to passive common noise suppression resulting in ultra-low relative timing jitter and fully resolvable comb lines in free-running operation. We showcase the laser performance with two application demonstrations: (a) time-domain spectroscopy of acetylene in the near-infrared via computational comb line tracking and (b) free-space THz time domain spectroscopy and thickness-measurements via adaptive sampling. For (b) we use efficient state-of the art iron-doped InGaAs photoconductive antennas to generate and detect the THz light. Here we operate these devices with an efficient Yb-based gigahertz repetition rate laser for the first time. One optical comb generates the THz light, while the other probes it via equivalent time sampling. We obtain signal strengths comparable to reference measurements with MHz repetition rate Er-based laser systems while achieving close to 1 GHz spectral resolution (defined by the comb line spacing) and generating THz frequencies up to 3 THz. By carrying out a careful investigation of the noise properties of the laser we confirm that the free-running gigahertz dual-comb oscillator provides a rapid yet highly precise optical delay sweep from a simple setup. Therefore, our approach will be beneficial for high-update rate time sampling and time-domain spectroscopy applications.

We demonstrate THz time-domain spectroscopy (TDS) with a single-cavity dual-comb laser operated at 1 GHz repetition rate. The compact single laser emits two pulse trains with slightly different repetition rates. Due to this condition very rapid and precise optical delay sweeps between two pulse trains are obtained. We show THz-TDS measurements with 1 kHz and 22 kHz delay-sweep rates. Further data processing allows to perform triggerless signal averaging leading to 55 dB dynamic range in the THz spectrum with 2 GHz spectral resolution for a total measurement time of 2 seconds.

We present an optoelectronic mixer for the terahertz (THz) frequency-domain based on an iron-doped InGaAs layer integrated in a plasmonic microcavity. We show that this structure, under 1550-nm-wavelength illumination, allows for more than 70% absorption efficiency in a 220 nm-thin InGaAs absorber and very high Roff/Ron >1000. It leads to THz mixers driven by 1550-nm lasers showing conversion loss as low as ∼30 dB at 300 GHz. Therefore, this design is very promising for application as receivers in high-data-rate wireless telecom, in cw-THz spectrometers, or in photonics-enabled THz spectrum analyzers.

The wavelength range around 780 nm and 850 nm is extensively used for both high resolution spectroscopy and quantum technology (QT). For example, the D2 transition of cesium (Cs) and the D2 transition of rubidium (Rb) lie within these wavelength ranges. [1], [2] For compact spectrometers with high resolution and especially for QT of the second generation, compact and long-term stable systems are a prerequisite for the breakthrough of the technology. [3] In this paper we demonstrate a miniaturized fiber-coupled tapered amplifier (mini TA), which combines high output power, excellent beam properties $(\mathrm{M}^{2} < 2)$ and high coherence in 14-pin butterfly package with collimated output beam. To the best of our knowledge this is the first time demonstration of a fiber-coupled TA in this power range with a collimated output beam in a hermetically sealed package.

A novel photonic terahertz measurement system based on a frequency‐modulated continuous‐wave (FMCW) radar approach is presented. In previous works, fast frequency modulation has been demonstrated in connection with a continuous wave terahertz spectroscopy setup based on the photomixing principle. In this paper, a terahertz radar based on both a photomixing transmitter and a photomixing receiver, in contrast to the rigid spectroscopy approach, is reported. Hereby, frequency modulation bandwidths of more than 1.65 THz in radar operation is achieved. This corresponds to an order of magnitude more than what is previously achieved by terahertz radar systems. At the same time, measurement rates can be achieved that are comparable with radar systems based on Monolithic Microwave Integrated Circuits (MMICs) according to the current state of the art. Within the scope of the work, two operating modes are realized, one with a measurement rate of about 560 Hz at 600 GHz modulation bandwidth and one with 200 Hz at 1.65 THz modulation bandwidth, which can be set within the spectrum from 50 GHz to about 4.5 THz. The possibility to adjust the operating range of the radar without necessary hardware adaptations is another unique feature of the presented system, which allows the operator to choose a suitable frequency band that corresponds best to a certain measurement scheme via software settings. Besides the potential for multi‐layer thickness inspections, the capabilities of this technique for terahertz imaging applications are presented.

Material characterisation and imaging applications using terahertz radiation have gained interest in the past few years due to their enormous potential for industrial applications. The availability of fast terahertz spectrometers or multi-pixel terahertz cameras has accelerated research in this domain. In this work, we present a novel vector-based implementation of the gradient descent algorithm to fit the measured transmission and reflection coefficients of multilayered objects to a scattering parameter-based model, without requiring any analytical formulation of the error function. We thereby extract thicknesses and refractive indices of the layers within a maximum 2% error margin. Using the precise thickness estimates, we further image a 50 nm-thick Siemens star deposited on a silicon substrate using wavelengths larger than 300 µm. The vector-based algorithm heuristically finds the error minimum where the optimisation problem cannot be analytically formulated, which can be utilised also for applications outside the terahertz domain.

We demonstrate synthetic aperture terahertz imaging with an optoelectronic system based on continuous-wave lasers. A frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing. The system is setup in a monostatic transceiver configuration in connection with a X-Y scanning stage. While the transceiver provides frequency tunability to up to 4 THz, it is operated with a frequency modulation bandwidth of 600 GHz within the lower terahertz regime (< 1 THz) in order to achieve a reasonable dynamic range at high measurement rates, allowing to do imaging with synthetic apertures and in the future real time measurements with MIMO systems

Photoconductive antennas (PCAs) are well established as optoelectronic terahertz receivers. Size-reduction of emitter and receiver is key to address future applications like wireless communication, imaging and radar with optoelectronic continuous-wave (cw) terahertz systems. At the same time, directivity optimization and detailed knowledge of the radiation pattern is required. In this paper, we present a miniaturized PCA receiver and compare it to a state-of-the-art receiver. In a free-space scenario, where no additional terahertz optics are used, the miniaturized receivers perform up to + 10 dB beyond state-of-the-art. Radiation pattern measurements of both receivers are carried out between 100GHz and 1 THz, showing a 300 GHz extended main lobe with the miniaturized device.

We describe the assembly of a 5G transceiver leveraging photonics for the generation, emission and detection of THz wireless signals. The transceiver and all associated control electronics and power supplies are designed for mounting in a mobile aerial unit. A photonics motherboard concept that brings together polymer, III-V and SiN-based photonic platforms and provides optical fiber connectivity is used for the assembly. In addition, scalable integration of 3D components, in this case an antenna rod or rod array, is demonstrated. Thermal considerations arising from the dense integration of photonic and electronic components and the resulting concentrated heat load are also discussed.

An all-photonic THz-receiver PIC comprising an on-chip frequency stabilization scheme and a novel InP-based photoconductive antenna is presented. Characterization of the key photonic building blocks shows the functionality of the PIC. © 2022 The Authors

For wireless networks beyond 5G, directivity and reconfigurability of antennas are highly relevant. Therefore, we propose a linear antenna array based on photodiodes operating at 300 GHz, and an optical phased array based on polymer waveguides to orchestrate the antennas. Due to its low thermal conductivity and high thermo-optical coefficient, the polymer chip enables highly efficient and crosstalk-free phase shifting. With these, we demonstrate purely photonic-controlled beam steering across 20°. Compared to a single emitter, the 3-dB beam width is reduced by 8.5° to 22.5° and the output power is >10 dB higher. Employing Snell's law for coupling into air, we can precisely predict the radiation patterns.

We present sheet resistance imaging of silver (Ag) thin films on a $\sim 50 \ \mu \mathrm{m} -$thick dielectric layer on a plane steel substrate. THz imaging avoids damaging these films through contacting and allows for a high spatial resolution. We use a commercial THz time-domain spectroscopy system in reflection geometry. Our measurement data are interpreted by a least squares fit to a simple model of the frequency response of the layer stack based on Fresnel equations and the Drude model. The analysis reveals the thickness of the dielectric and the Ag-layer as well as the dielectric function of the Ag-layer. By raster-scanning the sample, spatially-resolved sheet resistance maps are non-destructively generated.

The increasing demand for high-capacity wireless communication requires data links at millimeter waves and terahertz frequencies, respectively. At those frequencies, electronic and photonic technologies compete to prove powerful transmitters and receivers. In this work, we demonstrate a wireless link at 300 GHz using a fiber-coupled PIN photodiode as the transmitter. Thus, the whole emitter side is based on components and techniques from standard fiber-optical communication, which inherently enable broadband data channels. We investigated two antenna designs with amplitude modulated and coherent data signals. Despite similar characteristics in terms of output power and carrier bandwidth, the quality of the data signals differed significantly. In addition, we found that the bit-error ratio (BER) scales non-monotonically with the optical input power of the photodiode, which is proportional to the terahertz output power. Depending on the modulation format and the symbol rate, we identified the optimal driving conditions of the photodiode. For amplitude modulation at 5 Gbit/s, we achieved error-free transmission with a BER of 7.5 × 10 −13 . QPSK modulation was error-free up to 64 Gbit/s. The highest line rate of 160 Gbit/s was achieved with 32QAM modulation. This corresponds to 133 Gbit/s net data rate after forward-error correction with 20% overhead. The highest spectral efficiency was achieved with 64QAM at 8 GBaud, i.e., 48 Gbit/s line rate. The presented results highlight the high bandwidth of photonic wireless THz links. Furthermore, the carefully analysis helps to improve the quality of future wireless links in the 300 GHz band.

Beam steering is advantageous for continuous-wave terahertz applications in sensing, imaging, and wireless communication. While wide spread in lower frequency bands, in the terahertz region complex emitter and receiver structures are hardly feasible with discrete devices, due to the shorter wavelengths. Therefore, we propose and demonstrate the use of photonic integration: A 1x4 antenna array based on photodiodes is driven by an optical phased array based on polymer. The packaged emitter features increased directivity and enables beam steering across 30 °.

Semiconductor saturable absorber mirrors (SESAMs) are key devices for passive mode locking of numerous laser types and have been implemented for a variety of operational wavelengths ranging from 800 nm to 2400 nm. However, for 1560 nm the fabrication of SESAMs based on the standard AlAs/GaAs material system requires highly strained InGaAs absorber layers, which reduce the device efficiency and compromise fragile long-term performance. Here, we present SESAMs for ultrashort pulse generation at 1560 nm that are grown entirely lattice-matched to InP and thus have the potential for less structural defects and a higher operational lifetime. A highly reflective InGaAlAs-InAlAs Bragg mirror is capped with a heavily iron doped InGaAs:Fe absorber layer, which facilitates an unprecedented combination of sub-picosecond carrier lifetime and high optical quality. Therefore, the presented SESAMs show ultrafast response (τA < 1 ps), low non-saturable losses and high effective modulation depth (ΔReff ≥ 5.8%). Moreover, a nearly anti-resonant SESAM design provides high saturation and roll-over fluence (Fsat ≥ 17  µJ/cm2, F2 ≥ 21 mJ/cm2). With these SESAMs, we show self-starting and stable mode locking of an erbium doped fiber laser at 80 MHz repetition rate, providing ultrashort optical pulses at 17.5  mW average power.