Sebastian Dulme

In this paper, we report on waveguide-type modified uni-traveling-carrier photodiodes (MUTC-PDs) providing a record high output power level for non-resonant photodiodes in the WR3.4 band. Indium phosphide (InP) based waveguide-type 1.55 µm MUTC-PDs have been fabricated and characterized thoroughly. Maximum output powers of −0.6 dBm and −2.7 dBm were achieved at 240 GHz and 280 GHz, respectively. This has been accomplished by an optimized layer structure and doping profile design that takes transient carrier dynamics into account. An energy-balance model has been developed to study and optimize carrier transport at high optical input intensities. The advantageous THz capabilities of the optimized MUTC layer structure are confirmed by experiments revealing a transit time limited cutoff frequency of 249 GHz and a saturation photocurrent beyond 20 mA in the WR3.4 band. The responsivity for a 16 µm long waveguide-type THz MUTC-PD is found to be 0.25 A/W. In addition, bow-tie antenna integrated waveguide-type MUTC-PDs are fabricated and reported to operate up to 0.7 THz above a received power of −40 dBm.

An indium phosphide (InP)-based E-plane transition for monolithically integrating terahertz photodiodes with standard rectangular waveguide (WR)-outputs is presented for all standard WR-frequency bands from 0.22 THz to 2.2 THz, i.e., from WR3 to WR0.51. The integration concept comprises a modified uni-travelling carrier photodiode (MUTC-PD) chip, an E-plane transition and a stepped impedance low-pass filter (LPF), which are all monolithically integrated on an InP substrate. The E-plane transition converts the quasi-TEM coplanar waveguide (CPW) mode of the MUTC-PD output to the dominant TE ₁₀ mode of the WR. To our knowledge, this is the first frequency-scalable monolithic integration concept that enables packaging of photodiodes with standard WR-outputs up to 2.2 THz. The recommended thickness of the InP substrate and the proposed E-plane transitions’ design parameters are investigated by numerical analysis to achieve minimum insertion loss (IL) and a wide operational bandwidth (BW). The presented optimized transitions exhibit a maximum IL of 1.4 dB, a return loss (RL) better than 10 dB and a minimum 1 dB IL BW of 92.23% for all WR-bands up to 2.2 THz. To prove the proposed monolithic integration concept, a MUTC-PD is integrated with a CPW-to-WR3 E-plane transition (220-320 GHz) on a 95 µm-thick InP substrate. At 300 GHz, the maximum achieved RF output power of the fabricated MUTC-PD chip is −12.4 dBm at a photocurrent of 18.5 mA. For experimental characterization, the MUTC-PD chip with the integrated E-plane transition has been mounted on a 1 mm-thick soda-lime glass substrate as carrier and it has been manually aligned within a WR3 together with an adjustable back-short. Due to non-perfect alignment of the chip and the back-short as well as the additional losses and substrate modes due to the thick glass carrier, the calculated average IL is increased to 5.3 dB. Experimentally, an average IL of 8.6 dB is measured within the WR3-band from 220 GHz to 320 GHz. Integration of the chip in a real package without misalignment and without the glass carrier is expected to improve the IL by ∼4.8 dB.

For mobile THz applications, integrated beam steering THz transmitters are essential. Beam steering approaches using leaky-wave antennas (LWAs) are quite attractive in that regard since they do not require complex feeding control circuits because beam steering is simply accomplished by sweeping the operating frequency. To date, only a few THz LWAs have been reported. These LWAs are based on polymer or graphene substrates and thus it is quite impossible to monolithically integrate these antennas with state-of-the-art indium phosphide (InP) based photonic or electronic THz sources and receivers. Therefore, in this paper, we report on an InP-based THz LWA for the first time. The developed and fabricated THz LWA consists of a periodic leaking microstrip line integrated with a grounded coplanar waveguide to microstrip line (GCPW-MSL) transition for future integration with InP-based photodiodes. For fabrication, a substrate-transfer process using silicon as carrier substrate for a 50 m thin InP THz antenna chip has been established. By changing the operating frequency from 230 GHz to 330 GHz, the fabricated antenna allows to sweep the beam direction quasi-linearly from -46 to 42, i.e. the total scanning angle is 88. The measured average realized gain and 3 dB beam width of a 1.5 mm wide InP LWA are ~11 dBi and 10. The paper furthermore discusses the use of the fabricated LWA for THz interconnects.

In this paper, we demonstrate a phase-sensitive photonic terahertz imaging system, based on two-tone square-law detection with a record-low phase noise. The system comprises a high-frequency photodiode (PD) for THz generation and a square-law detector (SLD) for THz detection. Two terahertz of approximately 300 GHz tones, separated by an intermediate frequency (IF) (7 GHz–15 GHz), are generated in the PD by optical heterodyning and radiated into free-space. After transmission through a device-under-test, the two-tones are self-mixed inside the SLD. The mixing results in an IF-signal, which still contains the phase information of the terahertz tones. To achieve ultra-low phase-noise, we developed a new mixing scheme using a reference PD and a low-frequency electrical local oscillator (LO) to get rid of additional phase-noise terms. In combination with a second reference PD, the output signal of the SLD can be down-converted to the kHz region to realize lock-in detection with ultra-low phase noise. The evaluation of the phase-noise shows the to-date lowest reported value of phase deviation in a frequency domain photonic terahertz imaging and spectroscopy system of 0.034°. Consequently, we also attain a low minimum detectable path difference of 2 µm for a terahertz difference frequency of 15 GHz. This is in the same range as in coherent single-tone THz systems. At the same time, it lacks their complexity and restrictions caused by the necessary optical LOs, photoconductive antennas, temperature control and delay lines.

A novel 3-D multilayer packaging technology for integrating an array of indium phosphide (InP)-based terahertz photodiodes (THz-PDs) with a rectangular waveguide power combiner (WR-PC) is proposed. The packaging concept is based on a vertical integration of an InP THz-PDs array with a multilayered WR-PC made of metallized glass-reinforced epoxy FR4 laminates using direct wafer bonding. The key motivation of this work is to develop a low-cost packaging technology for coherent power combining in the THz regime. The proposed multilayered packaging technology is generic, i.e., in principle, it would allow the integration of different and multiple planar arrays featuring photonic or electronic devices. To our knowledge, this is the first 3-D packaging concept for the THz frequency range that enables the integration of 2-D arrays of photonic or electronic devices. As a proof of concept, we here report on the design, fabrication, and experimental characterization of a straight hollow rectangular waveguide (WR-waveguide) and a 2 x 1 WR-PC for the WR3-band (220-320 GHz). Both packages feature standard cross-sectional hollow WR3-waveguides (862 μm x 431 μm) and corresponding standard UG-387/U-M flanges. They are fabricated using a stack of vertically bonded unit cells, each consisting of a 50-μm-thick glass-reinforced epoxy FR4 laminate with a 23-μm-thick top and bottom metallization. The size of a single quadratic FR4 unit cell is 24 mm x 24 mm. The fabricated straight WR3-waveguide consists of a stack of 57 FR4 unit cells of a total length of 5.4 mm. The measured transmission loss is less than 0.3 dB/mm and the return loss (RL) is less than 10 dB, within the entire WR3-band. Next, a multilayered FR4 2 x 1 WR3-PC is reported. Its design is based on that of a T-junction in H-plane where the inputs are modified to be on the same plane for facilitating subsequent integration with the planar InP chips of THz-PDs. The length of the WR3-PC is intentionally reduced for compactness and its impedance is gradually matched to achieve a low insertion loss (IL) over the entire WR3-band. The fabricated multilayered WR3-PC consists of 41 FR4 unit-cells resulting in a total thickness of only 3.9 mm. For the frequency range from 240 to 320 GHz, the simulated IL, minimum isolation, and RL are 0.16, 4.2, and 6.5 dB, respectively. For the experimental characterization using a WR3-band vector network analyzer (VNA), two multilayered FR4 2 x 1 WR3-PCs are connected back-to-back (B2B). The measured average IL and RL within the frequency range from 240 to 320 GHz are found to be 3.6 dB and below 5.2 dB, respectively. The measured average IL for a single WR3-PC is 1.8 dB, which is higher than expected because of imperfect fabrication and misalignment losses during the integration. An improved fabrication and packaging process will allow mitigating the losses in future runs.

In this paper, we report a phase-sensitive photonic THz two-tone self-mixing imaging system, comprising a self-developed vertical illuminated triple transit region photodiode (TTR-PD) and a commercial square law Schottky barrier diode as THz emitter and THz detector, respectively. Using the two-tone self-mixing approach, the phase information of a device-under-test can be extracted by down-mixing two THz signals inside the SBD, whereas the phase information remains in the output signal. The THz tones are generated by two free-running lasers and the TTR-PD, while one optical signal is externally modulated for double sideband carrier suppression. By means of the self-mixing, the phase noises of the free running lasers are canceled out. Using a second PD for the trigger, lock-in detection allows fast imaging speed, only limited by the integration constant. Beside the imaging system, we present the characteristics of the used vertically illuminated terahertz triple transit region photodiodes with thin depletion zone and small active areas. Numerical analysis by energy-balance model based TCAD simulations show transit-time limitations over 200 GHz, due to the electron field management within the active photodiode layers and the resulting high electron velocities. Therefore, the fabricated TTR-PDs show a flat ± 2dB frequency response within the frequency range from 225 GHz to 305 GHz. By employing the proposed photonic two-tone imaging system with the fabricated TTR-PDs, amplitude and phase difference vector images are taken at 299.5 GHz and 300.5 GHz. Metal and acrylic glass items inside a paper envelope are clearly visible, which demonstrates the potential of the system.

In this paper, we report on the integration of a photonic beam steering transmitter for the 5G26 GHz band. The transmitter, designed for fiber-to-the-antenna applications, is based on an optical polarization diversity receiver and a PCB leaky-wave antenna. Therefore, an integrated photoreceiver with polarization dependent couplers and high frequency waveguide photodiodes provides optoelectronic conversion of an incoming radio-over-fiber (RoF) signal. The antenna is fabricated from low-cost PCB processes and includes a bias-tee to provide an integration platform for the photodiode. The leaky-wave antenna further operates in the frequency range from 24 GHz to 33 GHz, where it scans its beam angle with changing the radio frequency. Thus, the integrated transmitter provides photonic beam steering via changing the beat frequency of the optical RoF signal due to the LWA frequency scanning behavior. A prototype transmitter has been successfully integrated, with which the photonic beam steering properties are experimentally demonstrated.

This paper discusses the use of advantageous photonic integrated chips for beam switching antennas and beam steering antennas in the millimeter-wave and THz frequency ranges. Planar directive (up 15.4 dBi) frequency scanning leaky wave antennas (LWAs) connected to photodiodes for 5G beam steering (26 GHz and 60 GHz) with scanning angles up to 110° are reported. One and two dimensional (1D/2D) beam steering for wireless communication with multiple users as well as mobile terminal localization is demonstrated. Also, THz beam switching using lensed-assisted THz antenna arrays as well as THz beam steering using THz LWA and novel photonic integrated circuits (PICs) providing phase shift and true time delay (TTD) based optical beam forming networks are presented.

Compact planar laminate-based transitions for integrating high-frequency photodiodes (PDs) with rectangular waveguides (WRs) are presented for the WR15 to WR1 standard waveguide bands, i.e. from 0.05 THz up to 1.1 THz. The transitions couple the optically generated (e.g. via heterodyning) millimeter-wave or terahertz signals from the grounded coplanar waveguide (GCPW) output of the PD chip to the WR. To our knowledge, this is the first scalable integration concept that enables hermetic photodiode packaging up to the terahertz frequency range. For the WR15-WR6 bands, all transitions are designed on ultra-thin microfiber reinforced PTFE composites (127 μm Rogers RT/duroid 5880 laminate). For the WR5-WR2.2 and the WR1.5-WR1 bands, liquid crystalline polymer ULTRALAM 3850 laminates are used with a thickness of 50 μm and 25 μm, respectively. The proposed GCPW-WR transition design is based on a double-slot antenna structure and enables the development of fully-hermetic photonic packages, which is required to improve the durability of the PD chip. The transition designs are optimized by systematic EM-wave propagation modelling for achieving a wide operational bandwidth of up to 30% of the respective center frequency for each WR band. The optimized transitions exhibit an average insertion loss (IL) of about 1.5 dB and a return loss (RL) of about 10 dB for all waveguide bands (WR15-WR1). Based upon the systematic numerical modeling, generic guidelines are developed that allow designing a specific transition for any given waveguide band to 1.1 THz. In addition to the numerical analysis, GCPW-WR12 transitions for E-band (60-90 GHz) operation are fabricated and integrated with InP-based balanced-PD (BPD) chips and GaAs HEMT MMIC medium power amplifiers in a fully-hermetic package. Furthermore, hermetic WR12 coherent photonic mixer (CPX) modules are developed. The packaged WR12-type CPX allows direct optical-to-wireless conversion of optical baseband or IF-band signals e.g. for radio-over-fiber (RoF) fronthauling in mobile communications. The WR12-CPX modules provide RF output power of up to +15 dBm at 77.5 GHz.

This manuscript discusses photonic assisted beam steering and beam switching millimeter-wave and THz antennas. Planar frequency scanning 60 GHz band leaky wave antennas (LWA) providing up to 15.4 dBi gain and scanning angles up to 110° are reported next to 300 GHz band LWAs with a max. directivity of 16 dBi and 72° scanning angle. Furthermore, photonic beam forming networks (OBFN) for true time delay (TTD) and phased array wideband (275-350 GHz) THz antennas are described. Finally, multiple beam antenna arrays for 1D and 2D beam switching are shown. Experimentally, 1D THz beam switching of 18° is demonstrated. Simulation results for 2D beam switching show a scanning angle of 70°.