Periodic Reporting for period 1 - GRAPH-X (GRAphene PHotonic frequency miXer)
Reporting period: 2022-10-01 to 2024-03-31
GRAPH-X addresses the distribution and detection of multi Gbit/s radio signals at sub-THz carrier frequencies. This will be essential to sustain traffic growth in the next generation 6G technology, and enable short-range sub-THz RADAR technologies. GRAPH-X uses an innovative approach based on graphene photonic integrated circuit (GPIC) to solve the current technical challenges of sub-THz wireless communication, i.e. the high phase noise, the carrier frequency stability and tunability. The main objective is the development of a monolithic platform based on GPIC on SiGe BiCMOS electronics for ultra-high speed and scalable sub-THz wireless links (110-250GHz). This technology will be a basic building block for high-speed radio back haul links, multi beam forming antennas for massive MIMO, short distance high resolution RADAR sensing. GRAPH-X approach is compatible with fiber optical core links, enabling seamless integration to fiber back-bones and towards optical/radio convergence. The key element is a graphene photonic sub-THz up/down converter with low power consumption (<50mW), phase noise reduction (<96dBc/Hz@1MHz), carrier frequency stability (<50ppm), broad band and tunable operating frequency covering the D-band (110-170GHz) and H-band (170-260GHz).
In the first period, we defined the applications for the two use cases based on a radio transceiver operating above 100GHz: mobile transport network and radar applications for automotive. The focus is on: future 5G/6G networks, CRAN approach and backhauling/fronthauling connections; in-cabin radar monitoring and external short-range radars. The architecture of the sub-THz transceiver was defined, and the TX and RX specifications delivered. The graphene photonic frequency mixer (GPFM) was co-designed with the amplifying electronics. The GPFM was optimized in terms of linearity and conversion efficiency, maximizing the expected responsivity. We simulated an improved conversion efficiency up to -35dB using an optical local oscillator power >13dBm. The electronics was designed to optimize the impedance matching and the amplification of the D-band RF signal in both RX and TX paths. The technology is the IHP 130nm SiGe BiCMOS SG13G2, featuring high speed HBT with fT/fMAX of 350/450GHz. The design features low noise figure (9dB and <12dB for TX and RX), high gain (~25dB for TX and ~20dB for RX) and high linearity (IOP3 >22dBm) with a dynamic range of 7dB. We designed the dual wavelength laser source for the generation of a low phase noise (<96dBc/Hz) high power (>16dBm) LO in the D and H band. The prototype is based on an optical phased loop (OPLL) circuit where an ultra-low linewidth master laser is used to generate a comb through a phase modulator, and two DFB slave lasers locked to the comb lines at the desired sub-THz frequency spacing. The design of the prototype assembly started. The designed package shall provide ePIC (electronic photonic integrated circuit) with proper electrical/ optical interfaces, as well as reliable mechanical support and thermal dissipation. The designed module is based on the use of an aluminium case, including E-plane probe and milled sub-THz waveguide as transitions from the ePIC pads to the WR6 waveguide. We developed high-mobility graphene (HMG) stacks and their transfer on SiN waveguides for the fabrication of the photonic mixer. We developed HMGs featuring mobility >10000cm2/(Vs) and wafer scale growth of hexagonal boron nitride (hBN). IIT optimized HMG stacks with both monolayer (stack size > 50μm) and 30° twisted bilayer (TB) (stack size > 30μm) graphene, reaching remarkable RT mobilities of 50000cm2/(Vs) and 70000cm2/(Vs), respectively. UCAM worked on the synthesis of large-area single layer graphene and hBN continuous films on copper by CVD. GPFM were fabricated by using the developed HMGs on custom SiN passive waveguides fabricated for the realization of the photonic mixer circuit. The monolithic integration of the SiN PIC platform on the final RFIC is under development. The work is focused on the individual steps for the integration of the SiN waveguides in the back-end of line of the IHP’s 0.13µm BiCMOS technology. The first batch of EICs without SiN waveguides was fabricated through an MPW run in the IHP 130nm SiGe BiCMOS SG13G2 technology. TB GPFM were characterized in terms of DC and RF performance. We measured a maximum responsivity at 1550 nm >0.06 A/W. Compared to previously reported GPFMs, this device shows higher responsivity and linearity. The TB GPFM showed a flat conversion efficiency of -43dB up to 180GHz, validating the large bandwidth of the device. The GPFM was tested as a D-band source and transmitter. We transmitted coherent data signals up to 4Gb/s QAM-16 and 10Gb/s QPSK at 150GHz. The GPFM based on the wafer scale HMG showed responsivity of 0.04 mA/W at 1550nm. Tested as sub-THz receiver, it successfully demonstrated for the first time the downconversion of a 100 GHz signal (W-band) with a conversion efficiency of -45 dB. The electronic amplifiers were tested showing flat bandwidth in the D-Band with a net gain of 20dB. Project GRAPH-X is proceeding according to the expected timeline. We successfully developed the fundamental technologies towards the goals of the project. The effort is now devoted to the demonstration of an operating prototype.
GRAPH-X achieved remarkable results beyond the state of the art, such as the transmission of 4Gb/s QAM-16 and 10Gb/s QPSK signals at 150 GHz, and, for the first time, the down conversion and reception of signals at 100 GHz. These are key results towards the next step, i.e. the monolithic integration of the GPFM on the IHP BiCMOS electronics to build the sub-THz TX/RX prototype. These results and the expected achievements at the end of the project will contribute to strengthen European industry and R&I. The greatest challenge of GRAPH-X is the fusion of the photonics with the radio world, to widen the application range of radio/radar towards sub-THz and THz frequencies. This will open new market opportunities in communication and automotive industries. A wide range of use cases for short-range radio and radar systems is an integral part of the development plan of the 5G ecosystem but also of the 6G.