Tunable two-color lasers for THz generation

Achieved results and methods

In the previous phases, photonic THz transmitters and receivers and first photonic integrated circuits for 1D and 2D THz beam steering were developed, validated, and successfully demonstrated in application-oriented scenarios. The outstanding results including record output power photonic THz transmitter and world first demonstration of photonic assisted 2D beam steering were published in 19 well-known high IF journals and presented in 23 international conferences (until the end of April 2024).

THz Transmitters

In the 2^nd phase, C07 has successfully fabricated individual photonic THz transmitters as well as transmitter arrays using Modified Uni-Travelling Carrier PhotoDiodes (MUTC-PDs) in the UDE cleanroom. The experimentally determined saturation THz output power level ranges from ~500 µW to ~5 µW when increasing the frequency from 0.3 THz to 1 THz [19]. C07 also developed MUTC-PDs with monolithically integrated full-band E-plane transitions for all WR bands up to 2.2 THz. As an example, the experimentally determined maximum RF saturation output power at 0.3 THz and the 3 dB bandwidth are ~100 µW and >100 GHz, respectively. For increasing the optical coupling efficiency and simplifying optical end-fire coupling of the InP MUTC-PDs with the SiN OBFN chips, C07 developed an InP-based optical spot-size converter that is monolithically integrated below the waveguide MUTC-PDs structure. The spot-size converter transforms the Mode Field Diameter (MFD) from 10 µm down to 2 µm, thus the 1dB alignment tolerances were increased from +/-0.5 µm to +/-2.3 µm. Finally, C07 successfully investigated and produced various power-combiner prototypes in the UDE cleanrooms for high output power photonic THz transmitter. This includes multi-port planar antennas fed by an array of MUTC-PDs and rectangular-waveguide type power combiner [20]. 

Optical beam steering PICs

At the beginning of the 2^nd phase of C07, UDE successfully demonstrated 1D THz beam steering using frequency scanning Leaky-Wave Antenna (LWA) technology invented in the 1^st phase [11]. According to a recent review, the MARIE C07 beam steering solution provides the highest steering angle compared to other technologies [21]. For validation, the fabricated beam steering PICs have been successfully employed in mobile scenarios [10] and for generating multiple individual steerable beams [22]. In the following, a 2D THz beam steering PIC consisting of an InP-based beamforming network monolithically integrated with an LWA antenna array has been fabricated in the UDE cleanroom (see inset of Fig 2(b)). Using this PIC, C07 demonstrated, to our knowledge, the world’s first photonic-assisted 2D beam steering chip [7]. In detail, the Coherently Radiating Periodic Structures BeamForming Network (CORPS-BFN) is fabricated on a 50µm thin InP-substrate. It is based on a concatenation of Wilkinson power dividers and combiners forming a phase distribution network for the WR3 frequency band [23, 24]. The concept of the beam steering PIC and a photo of the fabricated chip are shown in Fig. 2. Experimentally, the beam steering PIC generates a THz beam in the 300 GHz band that can be scanned between −12° and 33° in elevation and −19° to +20° in azimuth as can be seen from Fig. 2. The 2D THz beam steering PIC chip is furthermore validated in an application-oriented system experiment [25].

Furthermore, C07 has fabricated multiple OBFN PICs based on SiN/SiO₂ technology (see Fig. 2). The OBFN PICs consist of multiple Optical Phase Shifters (OPSs) and TTD [26]. The PICs have been successfully employed for continuous beam steering at 0.3 THz with a maximum steering angle of 62° [7]. Each OPS is realized by two cascaded identical ORRs to reach a phase shift of 2π with an ultra-low steering power consumption of only 0.058 W/π. It should be noted that the OBFN PIC allows a wide tuning range of the optical TTD from fs to µs region.

Receivers

TUDa has designed a PIC OBFN with phase tuning sections and distribution network in the optical domain. We take advantage of the fact that the THz phase is given by  where  are the phases of the two laser signals that are to be heterodyned. By adding an optical phase shifter in one of the two laser paths prior to their combination a phase shift in the optical domain directly translates to a THz phase shift of the same amount. This severely simplifies the setup as no true time delays covering several 100 µm are required any more, a sub-micron phase shift in the optical domain is sufficient. Fig. 3a shows a photograph of the PIC for 16 array elements fabricated by Lionix. Its size is only 1x2 cm².

Fig. 3b shows the developed 4x4 ErAs:InGaAs photoconductor receiver array fabricated in-house at TUDa. It consists of slot antennas spaced by 232 µm for a design frequency of 300 GHz, corresponding to an inter-device spacing of 0.79l_eff if a silicon lens is applied on top of the array. The generally considered optimum spacing of l_eff /2 was violated on purpose as a silicon lens would not image the side lobes appearing at larger angles. Further, for a receiver, side lobes are not reducing the performance as opposed to sources where power coupled to side lobes is lost. We remark that the next resonance appears around 1 THz so we expect that the array can be used for both target frequencies with the compromise of higher side lobe levels at 1 THz. Due to a severe delay by the PIC manufacturer, the individual components are currently being tested but not yet completely assembled. We expect first results of the PIC-integrated receiver array in June 2024, still in line with the goals of the 2^nd phase.

Lasers

In the 2^nd phase, RUB has developed and characterized laser diode systems and has partially implemented them in THz systems. First, a Y-shaped monolithic two-color laser operating at about 1 THz difference frequency was implemented into a CW THz homodyne system.  By current tuning, the difference frequency can be altered by about 45 GHz, and thus, sampling of the THz signal could be achieved by frequency tuning rather than by mechanical scanning of a delay line. When changing the injection current, the frequency tuning has a slow (kHz) and a fast (MHz) component due to temperature and carrier density induced changes of the refractive index in the laser, respectively. The corresponding tuning ranges are 3 GHz and 1 GHz, respectively. In the proof of principle studies shown in Fig. 4, we demonstrated fast THz thickness measurements by current tuning with this laser system [5]. Data acquisition time was ~100 milliseconds per measurement including 50 times averaging. For two reference ABS samples with nominal thicknesses of 200 and 300 µm, the THz system measures a thickness of 247 µm for sample I and 361 µm for sample II while a confocal achromatic sensor provides reference real thicknesses of 263 µm and 336 µm for the samples. Both measurements agree within a range of 8% deviation.

In a second approach, we have designed a PIC containing two tunable Distributed FeedBack (DFB) lasers and phase control sections. The PIC was designed by us, fabricated by the HHI foundry service, and then characterized in our laboratories. Fig. 5 shows a microscope image of the PIC. Difference frequency tuning from 350 GHz to 1.15 THz was achieved, and the PIC was successfully implemented and operated in a CW THz homodyne system.

Overall, in comparison to the Y-shaped two-color lasers, the PIC chips are the most promising laser realization for integrated THz systems. While the Y-shaped two-color laser enables fast but limited tunability, the PIC-based laser chips enable large tuning width and the potential of also integrating THz transmitters on the PIC as planned for the next phase of this project.

In a third approach, we characterized monolithically mode-locked diode laser chips. The first device was an integrated external-cavity mode-locked laser chip designed for sub-GHz repetition rate. Though mode-locking was achieved, the obtained pulse widths and spectral widths were not competitive for THz applications [6]. Other, more promising chips were delivered by our partner HHI in Berlin. The two-segment chips were designed for passive mode-locking operation and available with Quantum Dot (QD) and Quantum Well (QW) active regions. The lasers provide frequency combs with about 50 GHz frequency spacing and widths exceeding 1 THz. Thus, individual selected comb lines could serve for CW THz systems operating between 0 and 1 THz. Mode-locked operation was observed to be imperfect in passive mode-locking regime [27]. In contrast, self-mode-locking via four wave mixing [28] turned out to be successful, in particular for the QW laser diodes when subsequent chirp compensation by a dispersive fiber was implemented. Fig. 6 shows a train of pulses with a duration below 500 fs and the corresponding spectrum with a 3 dB bandwidth of 1.4 THz. Thus, these chips are promising sources also for THz Time Domain Spectroscopy (TDS) systems. Moreover, the repetition frequency depends on the injection current and can be tuned for ASynchronous OPtical Sampling (ASOPS) operation [29] with two lasers. Recently, we have also realized sub-picosecond pulse trains at much lower 1 GHz repetition rate with external cavity operation.

THz Imaging and additional system-level applications

In C07, UDE has validated the fabricated generic THz transmitter and THz beam steering PICs for various applications including THz imaging and wireless communications. In [30, 31], the photonic THz transmitter coupled to Dielectric Rod Waveguides (DRW) were utilized for high-resolution THz imaging. In [30], C07 reported a multi-spectral photonic frequency domain THz imaging system operating from 60 GHz to beyond 0.3 THz. The multi-spectral imaging system allows to adjust the spatial resolution by optical means. The scanned 2D image of a 1951 USAF shown in Fig. 7 prove a spatial resolution of 500 µm but also structures with dimensions below 500 µm were resolved.

Also in [32], the photonic THz transmitter featuring a DRW antenna was used for imaging. A scanned THz image of a leaf taken at 0.3 THz reveals a spatial resolution of 500 µm. The C07 beam steering PIC technologies were also exploited for steering multiple THz beams. Results include 1D and 2D steerable single THz beams with bandwidths of several 10 GHz [22, 33]. In [22], also steerable multi THz beams are reported for the first time, to our knowledge.

In order to bridge the gap caused by the delay by the PIC manufacturing of the receiver OFBN, TUDa focused on a 3^rd phase-relevant application: A key MARIE theme is to localize persons in a hazardous environment. Of particular concern is to find unconscious persons. We successfully developed a technique based on a homodyne and a heterodyne photomixing system, similar to a photonic radar, to detect the breathing motion and breathing rate of a human being remotely and contactless as shown in Fig. 8. A study with seven test subjects was carried out under rest conditions as well after physical exertion. No fixtures or similar were used to emulate a realistic MARIE scenario. The test subjects sat in front of the single pixel setup. The data were compared to a standard on-body breath rate monitor showing excellent agreement. The results were published in [9]. In the 3^rd phase, we will use PIC-integrated receiver arrays instead of a single pixel detector in order to be able to scan for the best position to capture the breathing motion and to enable synchronous referencing. This will also allow for a dynamic environment, e.g., a moving robot or drone, going beyond the simplified static laboratory experiment. With THz homodyne systems based on either two commercial tunable DFB lasers or our tunable two-color Y-laser [5] the RUB group successfully demonstrated THz applications in analysis of heavy metal contamination of plants  [34, 35] as well as fast material thickness measurements [36]. In addition, we have suggested and verified a new concept for THz sampling without moving parts based on dispersive mirrors [37].

Selected project-related publications

[1] S. Makhlouf, O. Cojocari, M. Hofmann, T. Nagatsuma, S. Preu, N. Weimann, H.-W. Hübers, and A. Stöhr, "Terahertz Sources and Receivers:  From the Past to the Future," IEEE Journal of Microwaves, July 2023, [DOI: 10.1109/JMW.2023.3282875]

[2] S. Makhlouf, S. Dülme, M. Grzeslo, J. L. F. Estévez, V. Rymanov, J. Lackmann, and A. Stöhr, "Monolithically Integrated THz Photodiodes With CPW-to-WR3 E-Plane Transitions for Photodiodes Packages With WR3-Outputs," Journal of Lightwave Technology, Sep. 2021, [DOI: 10.1109/JLT.2021.3115469]

[3] S. Makhlouf, J. Martinez-Gil, M. Grzeslo, D. Moro-Melgar, O. Cojocari, and A. Stöhr, "High-power UTC-photodiodes for an optically pumped subharmonic terahertz receiver," Optics Express, Nov. 2022, [DOI: 10.1364/OE.470375]

[4] T. Vogel, S. Mansourzadeh, U. Nandi, J. Norman, S. Preu, and C.J. Saraceno, "Performance of Photoconductive Receivers at 1030 nm Excited by High Average Power THz Pulses," IEEE Transactions on Terahertz Science and Technology, Jan. 2024, [DOI: 10.1109/TTHZ.2024.3358616]

[5] N. Surkamp, A. Gerling, J. O'Gorman, M. Honsberg, S. Schmidtmann, U. Nandi, S. Preu, J. Sacher, C. Brenner, and M. R. Hofmann "Current tuned slotted Y‐branch laser for wafer thickness measurements with THz radiation," Electronics letters, Sep. 2021, [DOI: 10.1049/ell2.12314]

[6] M. Ali Alloush, M. van Delden, A. Bassal, N. Kleemann, C. Brenner, M.-C. Lo, L. Augustin, R. Guzmán, T. Musch, G. Carpintero, M. R. Hofmann, "RF Analysis of a Sub-GHz InP-Based 1550 nm Monolithic Mode-Locked Laser Chip," IEEE Photonics Technology Letters, May 2021, [DOI: 10.1109/LPT.2021.3083096]

[7] T. Haddad, C. Biurrun-Quel, P. Lu, J. Tebart, B. Sievert, S. Makhlouf, M. Grzeslo, J. Teniente, C. Del-Río, and A. Stöhr, "Photonic-assisted 2-D terahertz beam steering enabled by a LWA array monolithically integrated with a BFN," Optics Express, Oct. 2022, [DOI: 10.1364/OE.468200]

[8] P. Lu, T. Haddad, J. Tebart, C. Roeloffzen, and A. Stöhr, "Photonic Integrated Circuit for Optical Phase Control of 1× 4 Terahertz Phased Arrays," Photonics, Nov. 2022, [DOI: 10.3390/photonics9120902]

[9] C. Hoog Antink, R. Schulz, M. Rohr, K. Wenzel, L. Liebermeister, R. Kohlhaas, and S. Preu, "Estimating Thoracic Movement with High-Sampling Rate THz Technology," Sensors, May 2023, [DOI: 10.3390/s23115233]

[10] P. Lu, T. Haddad, J. Tebart, M. Steeg, B. Sievert, J. Lackmann, A. Rennings, and A. Stöhr, "Mobile THz communications using photonic assisted beam steering leaky-wave antennas," Optics Express, Jul. 2021, [DOI: 10.1364/OE.427575]