We investigate THz generation using tilted pulse fronts with high power, high repetition rate driving lasers. It is shown that small beam sizes limit the maximum conversion efficiency due to spatial walk-off.

Optical rectification using tilted pulse fronts in lithium niobate (LN) is currently the method of choice for the generation of strong-field THz pulses for THz-TDS. The success of this technique owes mostly to the high conversion efficiencies achievable, reaching the percent level at room temperature [1] . So far, this method has been mostly used with amplified laser systems, with repetition rates up to the kHz range and corresponding pulse energies of more than 1 mJ. To increase the DR and SNR of today’s THz sources, there is an increasing demand for high average power, high repetition rate THz sources. Recently, we have demonstrated up to 66 mW THz average power, driven by a >100-W average power mode-locked thin-disk oscillator with 13.3 MHz repetition rate and pulse energies on the 10 µJ level [2] , enabling us to achieve the highest average power of a laser-driven THz source at MHz repetition rates. Despite this promising first achievement, the conversion efficiency of 6∙10 -4 was significantly lower than the record conversion efficiencies obtained with lower repetition rates and higher pulse energies. Furthermore, the scaling laws in this unusual excitation regime remain so far unexplored. Here we present an in-depth investigation of this excitation regime using a 2+1D model including pump beam depletion. It is shown that a combination of spatial walk-off and pump beam break-up is responsible for a reduction in efficiency at small beam sizes. Furthermore, we discuss possibilities to overcome the current limitations and predict that watt-level THz sources at MHz repetition rates will become available in the very near future.

We present the first temperature-dependent refractive index measurements of Gallium Phosphide (GaP) for a broad THz bandwidth extending to 4 THz and a wide temperature range from 77 K to 500 K. Our results show no significant degradation in velocity matching at cryogenic temperatures compared to room temperature. These measurements are critical for future average power and bandwidth scaling of optical rectification in GaP using kW-class excitation.

We demonstrate the potential of state-of-the-art high-average power THz-TDS for THz imaging applications, by performing lensless imaging of a 3D-printed mixed polymer probe using a migration algorithm. Images obtained with our 10-mW average power THz-TDS show a clear enhancement of dynamic range, which results in a significantly improved image contrast compared to the same measurements performed using a state-of-the-art commercial TDS system with less than 200-μW of average power. Our result is an important first step towards reflection-mode imaging of dielectric objects with an internal structure, potentially at long distances.

Fragestellung Damit auch nach der Rehabilitationsphase ein objektives Monitoring von Bewegung und Beweglichkeit nach endoprothetischem Gelenkersatz möglich ist, bedarf es intelligenter Systeme. Diese sollten ortsunabhängig auch strukturschwache Gegenden erreichen und ebenso unabhängig von der Tageszeit sein. Das von der EU und dem Land NRW geförderte Projekt “RehaToGo” entwickelt eine intelligente Textilie, die bei hohem Tragekomfort Bewegung misst und online dem behandelnden Orthopäden und Unfallchirurgen zur Verfügung stellt. Am Beispiel dreier verschiedener Zugangswege in der Hüftendoprothetik soll dieses System erstmals erprobt werden.

We demonstrate a 1030 nm 520 fs-pulse-driven ErAs: InGaAs photoconductive receiver suitable for detecting 14 mW average THz power (~ 950 nJ pulse energy). To the knowledge of the authors, this is the highest detected pulsed THz power using photoconductive receivers reported so far. The current (field) responsivity of this receiver is in the range of $110\pm 25\mu A/\sqrt{W}$ yet with slight saturation.

We present recent advances in the demonstration of THz-TDS setups combining high average power of tens of milliwatts with high repetition rate in the Megahertz regime, based on laser excitation using >100W class ultrafast laser drivers.

The paper describes the development of passive, chipless tags for a novel indoor self-localization system operating at high mm-wave frequencies. One tag concept is based on the low-Q fundamental mode of dielectric resonators (DR) which exhibits peak scattering at its resonance frequency. As the radar cross-section (RCS) of DRs at mm-wave frequencies is far too low for the intended application, arrays of DRs and combinations with dielectric lens and corner reflectors are investigated to boost the RCS while keeping the scattering retro-directive over wide-angle incidence. Satisfactory results are demonstrated experimentally in W-band with metal corner reflectors combined with planar arrays of DRs; the tags produce a high RCS level over a moderately broad angular range and a wide frequency range where they exhibit a notch at the resonance frequency of the dielectric resonators. These designs suffer from low coding range of 3 to 6 bit, degradations of RCS in angular range, and a difficult separation of the tag response from strong clutter. Both the suppression of large clutter interference by using time gating of the tag response and a larger coding range are promised by a chipless tag concept based on multiple high-Q resonators in photonic crystal (PhC) technology. Experimental samples are characterized as transmission resonators and as retro-directive tags at the 230 GHz band. As a concept to boost the retro-directive RCS with a truly wide-angle response, the integration of PhC resonators with a Luneburg lens is discussed.

We demonstrate optical rectification in GaP crystals driven by a 100W-class thin-disk oscillator, nonlinearly compressed to sub-100 fs in a multi-pass cell. In optimized conditions, we obtain a power of 1.35mW, which is the highest average power achieved in GaP so far. Additionally, we explore the effect of temperature of the crystal down to 77K, showing that controlling the temperature of the crystal could be a key aspect for further power scaling and for handling thermal effects that become important at such high driving powers.

We demonstrate a high repetition-rate, single-cycle THz source with a maximum average power of 1.35 mW, operating at a center frequency of 2 THz. This result was obtained by optical rectification (OR) in GaP using an amplifier-free, nonlinearly compressed modelocked thin-disk oscillator based on Yb:YAG, delivering 8.4 µJ pulses with 88 fs duration at a repetition rate of 13.4 MHz, resulting in driving pulses for OR with 112 W average power and 80 MW peak power. To the best of our knowledge, our result represents the highest average power so far achieved with OR in GaP. The demonstrated performance is very attractive for improving current linear THz time-domain spectroscopy experiments, which are currently restricted by low signal-to-noise ratio and long measurement times.

We demonstrate optical rectification in GaP of a 112W, 88fs, 13.4 MHz, nonlinearly compressed thin-disk oscillator. We obtain 1.35 mW average power, which is the highest so far achieved from a single-cycle THz source at MHz repetition rate. © 2019 The Author(s)

We demonstrate THz generation by optical rectification in GaP crystals using excitation average power levels exceeding 100 W. The laser source is a state-of-the-art diode-pumped Yb:YAG modelocked thin-disk laser, capable of generating 120 W at 13.4 MHz directly from a one-box oscillator, without the need for any amplification stages. In this first demonstration, we measured a maximum average power of 78 μW at a central frequency of 0.8 THz. This first result shows that optical rectification of high average power (100 W class) ultrafast sources is within reach, and paves the way towards Watt-level, ultrafast laser pumped THz sources.

We investigate energy scaling of modelocked thin-disk lasers via modelocking regimes which are more tolerant in terms of maximum nonlinearity than soliton modelocking. Our numerical results show promise for mJ-level pulse energies without requiring vacuum.

We demonstrate terahertz (THz) generation at megahertz repetition rate by optical rectification in GaP crystals, using excitation average power levels exceeding 100 W. The laser source is a state-of-the-art diode-pumped Yb:YAG SESAM-mode-locked thin-disk laser, capable of generating 580 fs pulses at an average power up to 120 W and a repetition rate of 13.4 MHz directly from a one-box oscillator, without the need for any extra amplification stages. In this first demonstration, we measure a maximum THz average power of 78 μW at a central frequency of 0.8 THz. Our results show that optical rectification of state-of-the-art high average power ultrafast sources in nonlinear crystals is within reach and paves the way toward high average power, ultrafast laser pumped THz sources.

We investigate power-scaling of green-diode-pumped Ti:Sapphire lasers in continuous-wave (CW) and mode-locked operation. In a first configuration with a total pump power of up to 2 W incident onto the crystal, we achieved a CW power of up to 440 mW and self-starting mode-locking with up to 200 mW average power in 68-fs pulses using semiconductor saturable absorber mirror (SESAM) as saturable absorber. In a second configuration with up to 3 W of pump power incident onto the crystal, we achieved up to 650 mW in CW operation and up to 450 mW in 58-fs pulses using Kerr-lens mode-locking (KLM). The shortest pulse duration was 39 fs, which was achieved at 350 mW average power using KLM. The mode-locked laser generates a pulse train at repetition rates around 400 MHz. No complex cooling system is required: neither the SESAM nor the Ti:Sapphire crystal is actively cooled, only air cooling is applied to the pump diodes using a small fan. Because of mass production for laser displays, we expect that prices for green laser diodes will become very favorable in the near future, opening the door for low-cost Ti:Sapphire lasers. This will be highly attractive for potential mass applications such as biomedical imaging and sensing.

An Yb:YAG SESAM-modelocked thin-disk laser delivering 1.07 ps pulses with record-high pulse energy of 80 μJ at 242 W of average power is presented. Improved SESAM designs and nonlinearity limits are explored towards multi-100 μJ modelocked oscillators.

Carrier Frequency Offset Synchronization is play a very important rule in the Distributed Multi antenna system, since the system has different individual clock instead of common clock as in the centralized multi Antenna system. One of the established synchronization methode for different transmitter devices is Costas Loop. Some implementation of the synchronization for distributed multi antenna is discussed in some research works. In this paper we propose the low cost implementation of synchronization in distributed multi antenna and more simplified measurement method of synchronization in distributed multi antenna. The measurement result showed that the syncronization can compensate 0.5 kHz frequency offset experienced by the transmitters in the Distributed Multi antenna system.

We present the first Modelocked Integrated eXternal-Cavity Surface Emitting Laser (MIXSEL) based on a quantum well absorber, achieving 4.8-ps pulses at a repetition rate of 2.9 GHz and 6.8-ps pulses at 20.8 GHz.