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Heterogeneous Advancement and hybrid integration of polymer and tripLEx platform for Integrated Microwave PhoTonics

Periodic Reporting for period 2 - HAMLET (Heterogeneous Advancement and hybrid integration of polymer and tripLEx platform for Integrated Microwave PhoTonics)

Reporting period: 2017-06-01 to 2019-05-31

HAMLET combined in a unified passive, flexible and low-loss platform a silicon-nitride platform (TriPleX) and a polymer platform (PolyBoard). Graphene sheets were integrated with the PolyBoard for the fabrication of graphene-based electro-absorption modulators enabling modulation capabilities while PZT films were integrated on TriPleX for the fabrication of ultra-low power consumption PZT-based phase shifters, enabling the fabrication of a large scale optical beam forming network (OBFN) on-chip. To this end, the upgraded platforms were hybrid-integrated using 2D or 3D integration techniques tailored to the needs of the 5G networks. To this end HAMLET pusued and achieved the following objectives:
1. Design a powerful integration platform based on optical polymers and TriPleX technology for mixed-signal photonic circuits and MWP applications.
2. Develop a simple and reliable methodology for the integration of graphene on polymer platform and develop EAMs with high bandwidth and low insertion loss.
3. Develop a simple and reliable methodology for deposition of PZT layers on TriPleX platform and development of optical phase shifters with sub-μW power consumption and ns response time.
4. Develop large-scale integrated beam-forming networks for multi-element antenna arrays based on PZT-based tunable elements on TriPleX platform.
5. Develop hybrid integration engines for polymer-to-TriPleX integration with large number of interconnected waveguides and polymer-to-InP integration with long InP component arrays.
6. Develop a simple integration engine for integration and co-packaging of optical subassemblies with CMOS electronics and MIMO antennas.
7. Develop hybrid transceivers with integrated optical and wireless sections for remote antenna units in future 5G networks operating in the 28 GHz band.
8. Evaluate the performance of HAMLET transceivers in emulating 5G system environments and demonstrate record performance in terms of system flexibility and throughput.
9. Explore the range of possible application fields of HAMLET technology and prepare a solid roadmap for its commercial uptake in the post-HAMLET era.
"Requirements for the use case of 5G systems were collected and translated into functionalities and specifications of photonics components. Designs and simulations of HAMLET transceivers were generated and interfaces between the components were defined. DSP algorithms for the optical detection and configuration of the optical beam-forming network (OBFN) were developed. The design of the OBFN was based on the Blass-Matrix architecture to enable multi-beam operation. The heterogeneous / hybrid integration methodologies and the packaging methodologies were consolidated in the final prototype blueprints.
A Pulsed Laser Deposition reactor was set up and a deposition process was developed, enabling high quality PZT material deposition on TriPleX platform. Stress optic phase shifters with 2 µm thick PZT layer on top of the waveguides were designed and fabricated inside Mach-Zehnder interferometers. Static power consumption for pi-phase shift was measured and found in the µW range, while power consumption at 1kHz was found less than 4mW per device. The optical loss was below 0.1 dB/cm, and thus in line with the typical loss of TriPleX waveguides. TriPleX chips with 2x2 and 8x8 Blass matrix Optical Beam Forming Networks (OBFNs) were fabricated for final integration and packaging. The 8x8 OBFN can feed 8 antenna elements on a phased array antenna and can produce and control the direction and shape of 8 microwave beams. A 4x16 Blass matrix network on the TriPleX was designed, put into mask for fabrication.
A method for graphene sheet transfer on PolyBoard platform was developed. GP-EAMs were designed and optimized leading to improved performance and an improved fabrication process (reduced number of required masks). The GP-EAMs were successfully fabricated as an array of 8 modulators installed in two prototypes. 3D integration between the TriPlex and Polyboard platforms was successfully demonstrated. The light transition between the two platforms was realised through vertical directional couplers with coupling loss less than 0.3 dB. The methodology was utilised to fabricate a 3D module that comprised a 1x2 Binary tree based OBFN, a single GP-EAM and two PDs.

A process for PolyBoard-TriPleX integration was defined considering the large number of waveguides and the large number of electrical lines on the chips. Mechanical and thermal simulations were carried out considering the mechanical constants and thermal expansion coefficients of the different materials. Within the project duration, 5 prototypes were assembled and packaged in total. The prototypes demonstrated increased complexity and functionality: (i) ""Precursor-1"" featured a 2x2 Blass matrix network with two InP-EAMs as the first generation of GP-EAMs did not have the required performance; (ii) ""Module-1"" was fabricated in two versions featuring a 8x8 Blass matrix network and 17 PD array integrated; (iii) ""Precursor-2"" has a 8x8 Blass matrix, an 8x array of GP-EAMs, 17x array of PDs and an additional DFB laser-thin film filter-PD assembly on chip that emulates the add-drop functionality for the signals coming from the optical fronthaul. The performance of the prototypes was evaluated using the developed testbeds.

The HAMLET optical beam forming technology was demonstrated live, to representatives from two of the biggest companies in Greece and Balkans, COSMOTE (teleocm operator) and IntraCOM, one (telecom equipment provider), who showed great interest for the demonstrated technology."
A new generation of stress-optic phase shifters, integrated on TriPleX platform, was developed with a performance that makes them at least 100 times more power-efficient than the thermo-optic phase shifters in the static case and more than 70 times at 1 ms rise time.
HAMLET has developed and optimized a fabrication process to transfer graphene films into the polymer platform which resulted in the fabrication of GP-EAMs with sufficient bandwidth, and low insertion loss. The developed GP-EAMs can cover the complete transparency window of the PolyBoard platform, enabling the use of GP-EAMs in applications beyond communications that require certain wavelengths for which no integrated high-speed intensity modulators are available so far. The two powerful photonic platforms were successfully combined in a new unified platform within HAMLET, based on 2D- and 3D- integration techniques. The impact of this unified platform at the system level is significant, as it has enabled the fabrication of optical beamforming networks based on Blass matrix architecture able to feed up to 8 antenna elements of a multi-element antenna and form up to 8 independent microwave beams.
HAMLET’s wider societal implications are intertwined with the prominent effect of the 5G networks onour daily lives worldwide. HAMLET technology aims to provide a viable and low power consumption solution for gracefully scaling the capacity in the 5G networks, by using beamforming network with multi-beam capabilities. Due to the inherent broadband nature of the microwave photonics, the use of the optical-enabled beamforming networks can be extended beyond the 5G networks, enabling the control of microwave beams even in the THz regime, unlocking new frequency bands and enabling the commercial uptake of new emerging applications within these bands.
Photograph of the fabricated Module-1 Version-2 prototype accommodating a 8x8 OBFN and 8 InP-EAMs
Photograph of fabricated TriPleX optical chips with optical beam-forming networks (OBFNs)
Photograph of the fabricated Module-1 Version-1 prototype accommodating a 8x8 OBFN and 8 GP-EAMs
Artistic layout of the hybrid (TriPleX-PolyBoard) photonic integrated circuit
Photograph of the fabricated Precursor-1 prototype accommodating a 2x2 OBFN and two InP-EAMs
Micrograph of graphene modulator before the the deposition of the polymer waveguide