2.1

Antwerp Space EPFCV2: A Q/V band Electro-Photonic Frequency Convertor Module

Figure 2-1 presents AWS Q/V band frequency convertor design. The golden box inside the module is the PPIC. A specific electronic design is made to control PIC components inside the gold box. A Q/V band LNA is integrated to compensate for the optical insertion losses. Polarization maintaining fibers are attached to the output and input of Tx & Rx PICs, respectively. The fiber optic connection to Tx and Rx dies not only enable RF-over-Fiber distribution but also provide the possibility to add an extra amplification stage between Tx and Rx dies. Positioning an amplification stage between the Tx and Rx PICs is a risk mitigation approach in case of high optical insertion losses or low out power of integrated DFB laser.

Figure 2-1

Antwerp Space Q/V band Frequency Convertor Module. The gold package inside the module is the PPIC. One of the optical fibers is connected to the Tx while the other enable the optical connection to the Rx.

Table 2-1

EPFCV2 Technical specification

2.2

EPFCV2 packaging requirements: A Hermetically Sealed Space Qualified PIC

A hermetically sealed space-qualified package was developed during EPFCV2 project which is shown in Figure 2-2. The package contains Tx and Rx dies, Thermal Control element (TEC) and an optical bench. Although Tx and Rx are separate dies they are placed inside a single package and use a single TEC (for SWaP reduction). All materials and joints developed for this product are space qualified and has followed space qualified packaging methods & techniques¹. Main technical specifications of PPIC is presented in Table 2-2.

Figure 2-2

(a) PPIC assembly without lid (by Alter UK) (b) Hermetically sealed PIC-based electro-photonic frequency convertor: DC pins are designed to DC bias PIC components inside the package, Optical Fibers are attached to TX and Rx PICs. : The PPIC assembly is designed and fabricated by Alter UK.

Table 2-2

PPIC Technical Specifications

On PPIC optical performance, coupling efficiency of fibers to the optical chips both on Tx and Rx sides have been evaluated. Optical modelling showed 73% and 80% coupling efficiency for Tx and Rx, respectively.

To ensure PPIC is robust w.r.t stresses caused by thermal loading, contraction or expansion various design and simulations have been performed to ensure desired thermal, RF, and optical performance. For thermal performance design, thermal simulations have been done on PPIC and on different join levels, i.e., TEC to package, TEC to optical bench, etc. Based on thermal simulation results this package meet a great mechanical integrity when PPIC temperature range is set between -65°C to +15°C. On the PPIC RF performance, three discrete RF circuits, i.e., 50 GHz RF, 30 GHz LO, and 20 GHz IF have been designed starting from their connector down to connections to the PIC interfaces. Target metrics were 50 Ohm impedance, <0.5 dB insertion loss, and <20 dB return loss.

We have now presented the EPFCV2 module as well as PPIC position and details. In the next section, we focus on the integrated microwave photonic link inside the PPIC following by measurement results on each of Tx/Rx dies.

2.3

EPFCV2 Tx/Rx PIC: An integrated Microwave Photonic link

Figure 2-3 shows the microwave photonic link (with integrated Tx and Rx dies) which is the baseline of the frequency down-conversion in EPFCV2 project. On the Tx side, a DFB laser is integrated with a high-speed IQM modulator on InP photonic integration platform and on the Rx side an SOA is integrated with a high-speed PD. Spot Size Converters (SSC) are considered for coupling the light outside the Tx PIC and inside the Rx PIC, respectively. Space-qualified Mini

AVIM connectors are used on both Tx and Rx together with a single-mode polarization maintaining optical fiber. TEC’s are foreseen on both Tx and Rx side in order to keep the PIC temperature at 20°C.

In theory there are multiple photonics-based approaches for frequency down-conversion like a mode-locked laser integrated with a Mach-Zehnder modulator [4], an integrated dual-polarization dual-parallel MZM with tunable optoelectronic oscillator (OEO) and phase shift [5] or a dual-drive MZM [6]. In our case we considered an IQM which has two child-MZMs in each arm. This design enables us to apply RF and LO signals on each arm of IQM while at the same time gives the flexibility to DC bias either each of the IQM arms or child-MZMs.

Figure 2-3

A Microwave photonic link with integrated Tx and Rx photonic chips as a baseline technology of EPFCV2 frequency down-conversion. A DFB laser is integrated with a high-speed IQM on Tx side. On the Rx side an SOA is integrated with a high-speed Photo-detector (PD). SSC stands for Spot Size convertor. TIA could be integrated into the PD or placed outside of the PIC.

We summarize this section by presenting the main design metrics of Tx/Rx dies in Table 2-3 and a general view on Tx/Rx layouts in Figure 2-4.

Table 2-3

Main technical specifications of EPFCV2 Tx/Rx dies.

Figure 2-4

Laser integrated with IQM on Tx side (top) and SOA integrated with PD on Rx side (bottom).

2.4

Tx/Rx Main Measurement Results

In this section we present main finding of measurements from Tx and Rx dies. On Tx, laser characteristics have been measured by the foundry (HHI, DE) the result of which is shown in Figure 2-5. These results are in accordance with specified metrics in Table 2-3. The 3dB electro-optic response of IQM is shown in Figure 2-6. As it is shown, there is a slight difference with the intended 50.2 GHz 3dB bandwidth. It has to be noted that this value corresponds to both child-MZMs in IQM design.

Figure 2-5

Measurement results of laser characteristics at 110 mA of laser current (a) RIN, (b) linewidth (blue curve as measured, red 10 pts adjacent average) and (c) SMSR

Figure 2-6

Measured 48.6 GHz 3dB electro-optic response of the IQM modulator on Tx die

On the Rx side a 36.5 GHz bandwidth has been measured for the photo-detector. On the same wafer run and design, the integrated SOA showed larger than 20dB of gain. The details of these results are shown in Figure 2-7.

Figure 2-7

Measured (a) 36.5 GHz 3dB bandwidth of the photo-diode on Rx dies and (b) SOA Gain on Rx dies. This measurement comes from the same wafer run.

Responsivity of PDs on the same wafer run showed a good compliance to specification of 0.8 A/W as shown in Figure 2-8.

Figure 2-8

Measured responsivity of photodiodes on 3 different HHI wafers

Once the PICs are integrated into the final packaging assembly the system metrics like Gain, NF, SFDR, and power consumption will be measured.

3.1

Antwerp Space PIC-based Photonic Lantern Receiver

The PIC-based PLR is designed to solve main challenges of current LCT terminals. Important LCT requirements are: I) increasing the surface of receiving area, II) integration of a non-mechanical solution with the photo-detector, III) high data rate, and IV) reduced AIT & cost. We present main design metrics and technical specifications of the Antwerp Space PIC-based lantern in Table 3-1.

Table 3-1

Main technical specifications of PIC-based PLR

PLR hardware and test strategy is presented in Figure 3-1. As it can be seen, a PIC-based lantern is positioned on a Thorlab cage system to enable proper positioning of incoming laser light converged using an optical lens system. The Photo-diodes on PIC are wire-bonded to Trans-Impedance Amplifiers (TIAs) to amplify the signal. A dedicated control board is designed to enable the 4Q-sesnor functionality and to transmit the electrical signals to BER tester.

This experiment is design to simulate auto-detection of incoming laser light on a PLR receiver inside a LCT terminal. The idea is to steer the laser light over the PLR receiving area which is 200μm × 200μm and to automatically detect it at BER tester using the 4Q sensor approach and at 1.2 Gbps data-rate. In the next sections we present design and characterization of the main receiving components inside the PIC i.e., GCs and PDs.

Figure 3-1

Envisioned PLR hardware and test strategy.

3.2

Grating Couplers: Passive Receiving components for PIC-based PLR

The PLRPIC-based approach is a non-mechanical solution which is based on passive Grating Coupler (GC) arrays. GC arrays (14 × 12) are designed in an end-fire/back-fire approach covering an area of 200μm × 200μm, as presented in Figure 3-2. The incident angle of incoming laser light is 10° which is typical for silicon photonics [10]. Each of these GCs are connected via optical waveguide to a single PD with a responsivity better than 0.8 A/W and a bandwidth larger than 5 GHz. The PICs are designed and fabricated at IMEC (BE) and on IMEC silicon photonics platform.

Figure 3-2

Grating couplers covering an area of 200μm x 200μm in an end-fire back-fire design configuration (by IMEC) [11].

Coupling efficiencies of GCs are calculated using both a 10 μm source and a 20 μm source and is shown in Figure 3-3. The 10 μm source is used at early test stage because it was available at IMEC, while the 20 μm source will be used later during testing of the PLR.

Figure 3-3

Calculated coupling efficiencies of end-fire (EF) and back-fire (BF) Grating Couplers (GC) (by IMEC) [11].

Coupling efficiencies of the fabricated PICs have been calculated not only on single GCs but also over the whole chip surface area. The simulation results for a 10 μm and a 20 μm source are shown in Figure 3-4. For a larger mode-field diameter of 20 μm the simulation results which is presented in Figure 3-4 (b) shows higher coupling efficiency of minimum 10.5 dB for 95% of the whole receiving area [11]. It should be noted here that the PDs are also measured at IMEC and have shown responsivity of larger than 1.1 A/W for reverse biased ranging from -1 V to -3 V. These results are in accordance with technical specification of PIC-based PLR which was previously presented in Table 3-1.

Figure 3-4

Simulation results of coupling efficiency over 200μm x 200μm design area for a 14x12 array using (a) 10 μm diameter source and (b) using a 20 μm diameter source [11].

3.3

Control electronics for PIC-based PLR

The control electronics for the PIC-based lantern also enables the 4Q sensor functionality. The initial schematic of the design is shown in Figure 3-5. Each quadrant of the PIC is connected to a single TIA via the PDs. Later each TIA will be wire-bonded to a total number of 42 PDs. The microcontroller enables switching functionality and the board has a single output for BER measurement purposes.

Figure 3-5

Initial schematic of the control electronics for photonic lantern receiver. A 4Q sensor functionality is envisioned for automatic detection of laser light while sweeping over the area of 200 μm x 200 μm.