LOLIPOP will leverage the LNOI process that has been established by CSEM, and will develop LNOI sub-circuits with 8 different combinations of elements and operation wavelengths. 

Objective 1:
Establish a process for the integration of LNOI films on the TriPleX platform, and develop hybrid TriPleX PICs that can support high-speed modulation and second-order nonlinear functions on-chip

LOLIPOP will leverage the LNOI process that has been established by CSEM [5], and will develop LNOI sub-circuits with 8 different combinations of elements and operation wavelengths. These will include phase shifters, phase modulators, Mach-Zehnder modulators (MZMs) and IQ modulators (IQMs) operating at 532 and 905 nm, as well as periodically poled waveguides for second harmonic generation (SHG)-based conversion from 1064 to 532 nm, and spontaneous parametric down conversion (SPDC) from 780 to 1560 nm. The LNOI sub-circuits will be transferred on the TriPleX platform using two main methods. The first one (die-bonding) will be carried out at PIC level, using flip-chip bonding of each LNOI die on the TriPleX PIC, and sub-micron active alignment steps. A local trench on the TriPleX platform will be etched to host the die. The target coupling loss will be 0.2 dB, using evanescent coupling with spot-size converters. The second method will be a micro-transfer printing (µTP) method at wafer-level. The LNOI films in this case will be suspended from their silicon substrate using nano-tethers, and will be stamped on the TriPleX wafers without flipping. In LOLIPOP, the flip-chip bonding will be used as the main method for the preparation of hybrid LNOI-on-TriPleX PICs. The µTP process will be developed in parallel, and will be used in the second part of the project with the aim to offer the prospect of high throughput in future production lines. Simpler butt-end-coupling methods will be also used as starting point, and as a fall-back solution. 

Objective 2:
Develop a process for heterogeneous integration of semiconducting layers on TriPleX wafers and growth of Ge photodiodes with wideband operation (400-1600 nm) and high bandwidth (up to 30 GHz)

LOLIPOP will develop a disruptive process for the growth of Ge-PDs on the TriPleX platform with high detection efficiency within the spectrum of interest for LOLIPOP, which is in broad lines from 400 to 1600 nm, but for the specific needs of the system demonstrators (modules) is from 532 to 1560 nm. The process will be realized at wafer level, starting from an etching step for the creation of pockets on the TriPleX wafers. A deposition step for selective growth of Ge islands and deposition of gallium (Ga) and boron (B) layers inside these pockets will follow. The semiconducting stack inside each pocket will act as a p-n PD with responsivity >0.3 A/W at 532 nm, >0.75 A/W at 905 nm, and >0.80 A/W in the telecom bands around 1550 nm. The active area of each PD will be vertically aligned with the corresponding waveguide on the TriPleX platform, enabling its use as a side-illuminated detector with low coupling loss (<0.1 dB). Finally, the bandwidth of the Ge-PDs will be up to 30 GHz, depending on the pocket size, the composition and the thickness of the semiconducting layers and the design of the electrodes.

Objective 3:
Design active elements and develop external cavity lasers on the hybrid TriPleX platform with ultra-narrow linewidth, wide tunability and operation in the wavelength bands from 780 to 1100 nm

LOLIPOP will develop active elements based on GaAs for operation at 780, 905 and 1064 nm. These elements will be based on multi-quantum well (MQW) structures that will be optimized in terms of maximum output power (>50 mW) and wavelength tuning (up to 60 nm) around the peak of their gain spectrum. High-reflection (HR) and anti-reflection (AR) coatings will be used at their ends to allow for their use as single-port gain chips in external cavity lasers (ECLs). An additional active element with a U-shape waveguiding structure will be also developed to act as a 2-port optical amplifier at 780 nm. The active elements will be integrated on the hybrid TriPleX platform using a flip-chip bonding method. To this end, the TriPleX platform will undergo processing steps, which will include the etching of recesses and the creation of mechanical stops and gold bumps. Using this bonding process, LOLIPOP will use the active elements and will develop three ECLs at 1064, 905 and 780 nm. In all cases, the main part of the cavity will reside at the TriPleX side. It will comprise two high Q-factor micro-ring resonators (MRRs) that will act as the semi-transparent mirror of the cavity using the Vernier effect [6]. Thanks to the low optical loss and the high selectivity of this cavity, the linewidth of all ECLs will be less than 10 kHz. Their output power will remain above 50 mW in all cases. Wide wavelength tuning will be also possible thanks to the wide gain spectrum of the active elements and the use of the Vernier effect. In the case of the 905 nm laser, this tunability will be 60 nm. Tuning of the cavity will be enabled by phase shifters based on lead zirconate titanate (PZTs), which can easily support reconfiguration rates up to several hundreds of kHz with low power consumption. The main system specs of the three lasers and their intended use in LOLIPOP are summarized in Table 3. The first one will provide input to the SHG unit of the LDV module for 1064-to-532 nm conversion. The second one will provide input to a SPDC process unit in the squeezed state source for 780-to-1560 nm conversion. Finally, the third one at 905 nm will be used as the light source in the LIDAR module and the photonic neural networks of LOLIPOP. In the first case, the wavelength tunability of this laser will be used for beam scanning in the optical phased array (OPA) of the LIDAR. 

Objective 4:
Develop CMOS electronics with low power consumption and high bandwidth (up to 30 GHz)

LOLIPOP will develop a set of electronic drivers and transimpedance amplifiers (TIAs) for the operation of the LNOI modulators and the Ge-PDs on the hybrid TriPleX PICs. Different versions of these elements will be developed on a complementary metal-oxide-semiconductor (CMOS) technology aiming at low power consumption, and on a case-by-case basis at high bandwidth (up to 30 GHz), high linearity or ultra-low noise operation. In the case of the TIAs for the squeezing source (Module-3), this performance should ensure a noise-equivalent power (NEP) of less than 50 pW/(Hz)1/2, and a clearance of more than 15 dB between the shot-noise level and the detector floor. The electronic drivers and TIAs will be integrated together with the hybrid TriPleX PICs on circuit boards.

Objective 5:
Demonstrate the use of LOLIPOP technology for Laser Doppler Vibrometers at 532 nm with ultra-high detection bandwidth (6 GHz)

LOLIPOP will use the hybrid TriPleX platform, and will develop two photonic integrated LDV modules with disruptive compactness and performance. Both LDVs will operate at 532 nm using a heterodyne detection scheme. The light will be generated by an ECL at 1064 nm with linewidth less than 10 kHz, and will be upconverted to 532 nm by SHG on the LNOI die of the hybrid PIC. The frequency shift for the heterodyne detection will be 40 MHz, and will be introduced by a phase modulator that will reside on the same die. In the reception part, the coherent detection will be realized by an interferometer and a balanced detector with Ge-PDs at 532 nm. This first LDV module will be evaluated in use cases with detection range up to 10 m in typical LDV application scenarios. The second module will have the same operation principle, but the frequency shift will be 6 GHz, enabling record performance in the detection of vibrations with large amplitude times frequency products. The frequency shift will be introduced by an IQM on the LNOI die. The main use case for this module will be the investigation of the vibration profile of microwave filters based on surface acoustic wave (SAW) elements, which are extensively used in the industry of smart phones [7]. In both modules, the hybrid TriPleX PIC will be co-integrated with a printed circuit board (PCB) that will comprise the drivers, the TIAs and all electrical lines needed for the PIC. The two modules will not have a beam scanning mechanism, and will be connected to a standard LDV scan-head via fibers.

Objective 6:
Demonstrate the use of LOLIPOP technology for FMCW LIDAR modules at 905 nm with ultra-high chirp (10 GHz) and beam scanning mechanism on-chip

Using the same platform, LOLIPOP will develop a LIDAR module based on the concept of frequency modulated continuous wave (FMCW) [8]. The module will operate at 905 nm offering superior performance and robustness under non-ideal environmental conditions [9]. A narrow linewidth laser on the hybrid TriPleX PIC will operate in a wavelength sweeping mode over a span of 60 nm with 20 Hz repetition rate, and will generate the continuous wave (cw). The frequency modulation will be performed by an IQM on the LNOI film of the hybrid PIC, and will result in the introduction of a linear 10 GHz chirp. This chirp can offer 1.5 cm axial detection resolution. A 2D beam scanning mechanism will be developed on-chip using a linear OPA with 512 grating couplers. The field of view (FOV) of the beam scanning will be 12o×25o with 0.2o and 0.1o resolution, respectively. Beam scanning in the first dimension (elevation) will be based on the wavelength sweeping of the laser and the wavelength dependence of the out-coupling angle of the gratings. Beam scanning in the azimuth will be based on the other hand on the phase control of the optical antennas using phase shifters on the LNOI film of the hybrid PIC. In the reception part of the module, a bulk lens will be used off-chip for light collection and in-coupling into the PIC (using a fiber pigtail) for coherent detection. Given the use of the FMCW concept and the expected power at the output of the PIC (20 mW), it is expected that the LIDAR module will be capable of supporting long-range operation over several hundreds of meters. Also in this case, the hybrid PIC will be co-integrated with a PCB that will comprise the drivers and the TIAs, and will accommodate the routing of the electrical lines for the active element and the phase shifters on-chip. 

Objective 7:
Demonstrate the use of LOLIPOP technology for photonic integrated convolutional neural networks with ultra-high computation speed (24 TOPS) 

LOLIPOP will further leverage the hybrid TriPleX platform, and will develop a sub-circuit that can be used inside photonic neural networks as a single neuron or as a set of parallel neurons with 73 weighted inputs in total. Its operation will be coherent in the sense that the sum will derive from interference effects, using a laser source at 905 nm, an MZM on the LNOI film of the PIC, an optical switch, a group of 72 delay blocks interconnected via reconfigurable couplers, 3 high-bandwidth Ge-PDs as signal detectors, and up to 70 low-bandwidth Ge-PDs as in-line power monitors inside the interferometers. This subcircuit will be used as the basic building block of two convolutional 2-layer neural networks for image processing tasks. Each network will comprise 4 of those building blocks, and will fit in to a single hybrid PIC. The first building block will operate at the first layer of the network as a group of 3 parallel 5×5 kernels (convolutional accelerators), while the other 3 building blocks will operate at the second layer as 3 independent neurons for classification purposes. Between the two layers, Field Programmable Gated Array (FPGA) electronics will be also employed to carry out basic thresholding and data reorganization tasks. The first network will operate at 10 Gbaud offering a computation speed of 6 Tera Operations Per Second (TOPS), whereas the second one at 40 Gbaud offering a record speed of 24 TOPS. While in the second case, the operation of the network will be off-line, in the first case, it will be real-time. Most interestingly, the data to be processed in this case will be imaging frames from the LIDAR after the functional integration of the two modules.

Objective 8:
Demonstrate the use of LOLIPOP technology for integrated optical squeezed state sources

LOLIPOP will develop an optical squeezing source for operation at telecom wavelengths (1560 nm) based on a hybrid TriPleX PIC. Both two-mode and single-mode squeezed state generation will be supported. The squeezing level that will be measured on the hybrid PIC will be 6 dB. This level will be able to support the implementation of squeezed-state protocols in continuous variable quantum key distribution (CV-QKD) systems with performance higher than the current coherent state protocols, or to support the implementation of measurement-based quantum computation protocols in quantum computing systems. The hybrid circuit of the squeezed state source will include an external cavity laser at 780 nm, serving as the pump, an active element acting as a standard optical amplifier, a periodically poled waveguide on the LNOI film serving as the nonlinear element for the implementation of the broadband spontaneous parametric down-conversion (SPDC) process, an optical parametric oscillator (OPO) on the same film acting as a reconfigurable local oscillator, directional couplers in the TriPleX part acting as wavelength filters, and a coherent detector in the same part for the measurement of the squeezing level. The Ge-PDs of this detector will be directly connected to the low-noise TIAs that will be integrated on the circuit board of the source.

Objective 9:
Work on a roadmap for the consolidation of LOLIPOP integration technology and the establishment of a low-volume production line that will offer this technology as a commercial service

LOLIPOP will elaborate on a roadmap for the consolidation of the project technology at the photonic integration level and for the commercial exploitation of this technology in the post-LOLIPOP era. As a first and decisive step, this effort will focus on the LNOI-on-TriPleX integration concept based on the micro-transfer-printing method. The aim will be to establish a low-volume production line with a viable process flow that will offer hybrid LNOI-on-TriPleX PICs to industrial customers for any type of applications in the Vis and the NIR from 400 up to 1600 nm.