Project C06 - Compact Optoelectronic THz Spectroscopy System

Principal Investigators: Dr. Carsten Brenner, RUB; Prof. Dr. Andreas Stöhr, UDE

Time Domain Spectroscopy (TDS) is the gold standard for material characterization in the frequency range of 100 GHz to a few THz. However, typical TDS systems are complex and operate in a laboratory environment only. For the long-term vision of MARIE, a compact and portable TDS system is necessary.

This project aims at developing such a compact and portable THz system for material inspection by taking a Quasi TDS (QTDS) approach. The main difference in our system is the multimode operation of the diode lasers in contrast to the more demanding modelocked operation of solid-state or fibre lasers. During the project we will investigate the complex interplay of multimode diode lasers and optoelectronic components for THz generation and detection. We will then proceed to develop a quasi time domain spectroscopy system which incorporates all these findings making use of well-established telecommunication diode laser technology at 1550 nm. This QTDS system will be used to efficiently measure spectral fingerprints up to frequencies around 1 THz and therefore make a valuable contribution to the 2nd and 3rd challenge of MARIE.

Besides the technological development of the THz transmitter and receiver, an additional task is the collection of THz spectroscopy data of previously defined relevant materials as a basis for later material recognition. As such, this project will analyze the potential and give directions towards the development of a compact and portable optoelectronic THz QTDS system for material recognition, especially for solid materials showing spectral fingerprints without narrowband features.

In summary, C06 will develop a multi-frequency continuous wave laser system and highpower THz photomixers integrated with tailored broadband antennas to analyze its potential for material recognition to contribute to the vision of MARIE.

Achieved Results in the 1st Phase

To achieve the key objective, C06 aimed at developing Quasi Time Domain Spectroscopy (QTDS) THz system architectures that do neither require mechanical delay lines nor suffer from additional phase noise. The methodology was (1) to develop THz photonic sources for generating evenly distributed optical spectral lines with frequency spacing in the THz domain and (2) to develop a system architecture and modulation schemes for sensitive THz vector measurements. Finally, the potential of such systems for THz spectroscopy and their ability to shape the THz spectrum by the superposition of multiple single frequency optical carriers was investigated.

In the 1st phase of C06, UDE and RUB developed two-tone and multi-tone continuous-wave FD vector THz system architectures utilizing photonic self-mixing detection [F, I, 1]. In these approaches, multiple THz tones are generated which are simultaneously detected using a square-law detection scheme employing a THz Schottky Barrier Diode (SBD). The developed system does no more require a delay stage or sharing the THz reference signal between the transmitter and detector for measuring the phase response. Despite the use of envelope detection, phase information is preserved thanks to the self-mixing of the multiple THz signals in the SBD. The approach also avoids additional phase noise resulting from free-running lasers, which is an issue in heterodyne detection. In that context, UDE also developed and fabricated InP-based THz photodiodes required for the generation of the multiple THz signals. The set-up of the developed THz system is shown in Fig. 2.

Two-tone system: Two free-running wavelength-Tunable external cavity Laser Diodes (TLDs) are used to generate two optical signals (around 1.55 μm) with a difference frequency in the THz domain, e.g. 0.3 THz. By using a Mach-Zehnder-Modulator (MZM) operated at its minimum transmission point the second optical carrier is suppressed and two optical sidebands are generated. By coupling the optical signal to one of UDEs THz photodiodes (see below), two THz tones with a difference frequency of 2fmod are generated. After interacting with the DUT, the two THz signals are self-mixed in the SBD. Note that the resulting IF-output signal at 2fmod contains the THz phase information. An additional low phase noise and low-frequency LO, oscillating at fLO, is used to enable integration with a lock-in amplifier. Note that THz difference frequency 2fmod is independent of the lock-in amplifier LO signal fLO

Fig. 1: System architecture of the two-tone spectroscopy THz system (a) and photograph of THz free-space set-up.

The advantages of the developed two-tone approach are twofold: (i) the phase noises of the two free running lasers are cancelled out due to the self-mixing approach (ii) no additional optical LOs or delay lines are needed for THz vector measurements. Due to the cancelling of the laser noises and the low frequency LO, we could achieve a superior phase deviation of 0.034° at 2fmod = 15 GHz, which is twenty time less than previous works. To the best of our knowledge, this is the lowest reported phase deviation in a photonic THz continuous wave FD spectroscopy or imaging system until now. Fig. 2 shows a THz phase image of several devices taken at 2fmod = 7.5 GHz. Further, by exploiting the dependency of the detected phase w.r.t. the thickness of the DUT, the system allows refractive index measurements. For example, for acrylic glass / PMMA we measured a refractive index of 1.61, which is in good agreement with values reported in the literature.

Fig. 2: Several devices-under-test placed on double-sided adhesive tape (a) and the corresponding terahertz twotone.

For designing THz photodiodes used in the set-up in Fig. 1, a new Energy Balance (EB) model has been developed in a simulation program to model the carrier transport within the semiconductor layers with a superior physical accuracy as compared to previous methods (e.g., drift diffusion model). The developed model was first validated by comparing numerical results with previously reported experimental reports for cut-off frequencies and saturation currents. Then the layer structure and topology of the vertically illuminated THz photodiode (THz PD) developed in the 1st phase (Fig. 4) was designed and optimized using the new EB model. As a result, the layer structure consists of a partially depleted absorber design, with a graded highly p-doped and an undoped InGaAs layer. To ease the injection of photo-generated electrons into the InP layer, the energy band discontinuities at the absorber/collector interface were smoothed using quaternary InGaAsP spacers. Additional spacer layers at the anode side of the InGaAs layers furthermore decrease hole trapping at the InP/InGaAs heterojunction. In addition, a cliff layer was introduced to control the electric field inside the absorber region for achieving a high electron velocity due to transient velocity overshoot. In addition, planar impedance-matched transitions and antennas were realized in cooperation with C07 and C05. A galvanic step was implemented in the photodiode fabrication process for improving the thermal management of the high-power THz photodiodes.

Fabricated THz-PDs with a coplanar waveguide output reached a 3dB-cutoff frequency above 140 GHz. At 300 GHz, a RF-output power of -18 dBm was achieved at a photocurrent of 5 mA. As can be seen from Fig. 4. the frequency response in the WR3 band between 225 GHz and 305 GHz is flat, with a ±2 dB deviation. Furthermore, with antenna-integrated PDs, frequencies above 600 GHz were reached. Moreover, a two-port slot bowtie antenna with two monolithically integrated photodiodes was developed to combine the THz output powers of both photodiodes. With this THz power combiner, a power enhancement around 3 dB was achieved in the whole J-band (220 GHz to 330 GHz). The developed photodiodes were integrated and used for the above described THz spectroscopy and imaging investigations. Additional utilization of the photodiodes in THz communication set-ups is planned in other projects.

Fig. 3: Photograph of the THz photodiode fabricated by UDE (left) and frequency response of the fabricated

During the 1st phase in the C06 project, UDE also developed photonic modules featuring coaxial, rectangular-waveguide or quasi-optical outputs for operational frequencies up to 1 THz. This is based upon development of matched-transitions and bias-T operating up to and above 1 THz [5]. UDE has also developed a new multilayer THz module technology that enables the integration of photonic 2D THz transmitter arrays [J].

Multi-tone system: Besides the realization of the described two-tone system, RUB implemented two additional multi-tone systems to accomplish QTDS with capabilities to modify the amplitude shape of the resulting THz spectrum. The key component is a Wavelength Selective Switch (WSS), which enables mixing of the selected wavelengths of a monolithic multimode laser as well as the combination of several cost-efficient fixed frequency diode lasers. Both approaches eliminate the requirement of a tunable laser source for spectroscopy applications. As the resulting THz spectrum is determined by the available difference frequencies of the laser source(s), the selection of the lasers is crucial. To optimize the distribution of fixed frequencies over the system bandwidth, we used a Golomb ruler configuration with 5 lasers. This way, we could demonstrate that ten frequencies between 50GHz and up to 1170GHz can be accessed by this method. This approach is easily scalable, while the resulting THz spectrum can be tailored to the target application [B].

Fig. 4: QTDS system based on fixed frequency lasers. Combination of 5 lasers results in 10 difference frequencies.

If only a certain THz frequency band is relevant, the laser system can be reduced to a single multimode diode laser. By using the WSS, the optical spectrum, which is used for THz generation, can be tailored to the specific needs. Although the fundamental mode distance is given by the cavity modes of the diode laser, we can choose an arbitrary frequency band inside the system THz bandwidth [A].

Fig. 5: QTDS System based on multimode laser. Target optical frequencies are selected by WSS

Selected Project-related publications

For all project-related publications please click here and scroll to the C06 section.

[A] A. Gerling, K. Tybussek, Q. Gaimard, K. Merghem, A. Ramdane, M. R. Hofmann, C. BrennerJ. C. Balzer, “Monolithic Mode-Locked Laser Diode for THz Communication”, 44th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Paris, France, Sep. 1-6 2019, DOI: 10.1109/IRMMW-THz.2019.8874373.

[B] A. Gerling, L. Becke, S. Tonder, M.R. Hofmann, J.C. Balzer, C. Brenner, “Golomb Ruler Based Discrete Frequency Multimodal Continuous Wave THz Spectroscopy System”, 2nd International Workshop on Mobile THz Systems, Bad Neuenahr, Germany, July 1-3 2019, DOI: 10.1109/IWMTS.2019.8823651

[C] A. Gerling, S. Dülme, N. Schrinski, A. Stöhr, M. R. Hofmann, C. Brenner, "Continuous Wave Multimode Amplitude THz Spectroscopy", IRMMW, Nagoya, Japan, Sep. 9-14 2018, DOI: 10.1109/IRMMW-THz.2018.8510486.

[D] A. Stöhr, “Integrated Microwave-Photonics (iMWP) for Mobile Terahertz Systems”, 43rd International Conference on Infrared, IRMMW-THz, Nagoya, Japan, Sep. 9-14 2018.

[E] A. Gerling, M. Hofmann, C. Brenner, “High speed single point THz phase measurement based on dual channel lock-in technique”, 1st Int. Workhsop on Mobile THz Systems (IWMTS 2018), Velen, Germany, July 2018, DOI: 10.1109/IWMTS.2018.8454689.

[F] S. Dülme, N. Schrinski, M. Steeg, P. Lu, B. Khani, C. Brenner, M. R. Hofmann, and A. Stöhr, “Phase Delay of Terahertz Fabry-Perot Resonator characterized by a Photonic Two-Tone Spectroscopy System with Self-Heterodyne Receiver,” Int. Conf. Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Nagoya, Japan, 9-14 Sept. 2018.

[G] C. Brenner, Y. Hu, J. Gwaro, N. Surkamp, B. Döpke, M. Hofmann, B. Kani, A. Stöhr, B. Sumpf, A. Klehr, and J. Fricke, "Near Infrared Diode Laser THz Systems", Adv. Radio Sci. 16, April 2018, DOI:

[H] V. Rymanov, P. Lu, S. Dülme, A. Stöhr, “Lens-assisted quasi-optical THz transmitter employing antenna-integrated triple transit region photodiodes”, IEEE International Topical Meeting on Microwave Photonics, Beijing, China, Oct. 2017, DOI: 10.1109/MWP.2017.8168703.

[I] S. Dülme, M. Grzeslo, J. Morgan, M. Steeg, M. Lange, J. Tebart, N. Schrinski, I. Mohammad, T. Neerfeld, P. Lu, A. Beling, A.Stöhr, “300 GHz Photonic Self-Mixing Imaging-System with vertical illuminated Triple-Transit-Region Photodiode Terahertz Emitters”, International Topical Meeting on Microwave Photonics (MWP 2019), Ottawa, Canada, October 7-10, 2019 (best paper award).

[J] S. Dülme, N. Schrinski, B. Khani, P. Lu, V. Rymanov, C. Brenner, M. R. HofmannA. Stöhr, "Compact Optoelectronic THz Frequency Domain Spectroscopy System for Refractive Index Determination based on Fabry-Perot Effect," 1st Int. Workhsop on Mobile THz Systems (IWMTS 2018), Velen, Germany, July 2018. [DOI: 10.1109/IWMTS.2018.8454695]

[K] S. Dülme, B. Khani, V. Rymanov, P. Lu, A. Stöhr, "Terahertz Near Field Coupling for Integrating III-V Photodiodes on Silicon," German Microwave Conference, GeMiC, Freiburg, Germany, 12-14 March 2018, pp. 375-278, [DOI: GEMIC.2018.8335083]