3.1

Photonic frequency generator unit (PFGU)

The PFGU is an optical transmitter based on external modulation that converts an electrical LO to the optical domain, integrating also optical amplification (OA) for power conditioning, as shown in Figure 3-2 (left). The module comprises also the control and biasing electronics required to monitor and adjust parameters such as operation wavelength, temperature, output power, etc. In OPTIMA, particular effort has been devoted to the miniaturization of photonic modules and CORDON Electronics has been responsible for developing a co-packaged Laser-Modulator, shown in Figure 3-2 (center). The device has already been fabricated and comprises a DFB laser chip, a LiNbO3 modulator chip, monitoring photodiodes chips, built-in TEC and coupling and mounting benches. The package is hermetically sealed. The measured RF response Figure 3-2 (right) confirms that operation up to 40 GHz band is possible.

Figure 3-2

Block diagram of photonic frequency generation unit (left), picture of the co-packaged laser modulator device (center) and simulated and measured characteristic curves of the PFGU Tx module (right).

3.2

Photonic multi-frequency converter (PMFC)

This assembly mixes the photonic LO with an RF signal. The mixing process is performed by an optical modulator that is in charge of modulating the RF signal on the wavelengths generated by the PFGU. This assembly also integrates optical amplification (EDFA fabricated by DAS Photonics) for power conditioning. The block diagram of the PMFC, as well as the optical spectrum at the different interfaces is shown in Figure 3-3. The optical carrier modulated by the LO signal (shown in red, the one corresponding to the laser at λ1 and the two LO side-bands) is modulated by the RF signal. The LO carrier with more power (laser) generates one side-band with the RF signal information at an offset equal to the input frequency. The secondary LO carriers also generate the same RF side-band at the same offset frequency, but offset with respect to the laser carrier is incremented by the LO frequency in one case, and decremented by this same quantity in the other case. The same process applies also for all the LO carriers generated by the PFGU. As described in Section 3.3, this optical spectrum generates up and down conversions once a homodyne mixing is performed in the OEM.

Figure 3-3

Block diagram of photonic multi-frequency converter (PMFC)

3.3

Optical to electrical module (OEM)

The OEM is composed of a 40 GHz Linear InGaAs PIN Photodetector and the associated circuitry. The coplanar waveguide photodiode design optimizes speed and sensitivity for the 1250 nm through 1650 nm wavelength range, and assures a 30 GHz frequency response necessary for OPTIMA specifications. As shown in Figure 3-4, the optical signal with the multiple mixing products generated in the PMFC is converted from optical to electrical in a homodyne detection process, which mathematically is described as:

Where, I_PD is the photo detected current, R is the responsivity (the capacity to convert optical power into current) and E_OPT is the optical field arriving to the photodiode. The square of the modulus of the optical field generate mixing products of the EOPT by itself, as well as other mixing products generated by the different LO carriers with the RF side bands. The IF electrical spectrum after the photodetector is shown in Figure 3-4, composed of harmonics of the f_LO frequency (n·LO, n=1,2…), replica of the input RF signal and mixing products of the input RF signal with the LO at f=f_RF ± n f_LO. The desired mixing product is selected by an IF filter at the OEM output prior to be amplified and fed to the antenna. As this filter is in the IF domain, the photonic payload can be used at any RF, LO and IF frequency.

Figure 3-4

IF spectrum after optoelectronic conversion

4.1

OPTIMA objective at switch matrix level

The optical switch is at the core of the photonic payload. It enables the routing of signals from any input port to any output port providing a high flexibility to the payload. It is fully transparent and can handle 1.55 μm signals independently of their wavelengths, intensity and modulations.

In OPTIMA project, Sodern and HUBER+SUHNER Polatis are developing a fully space qualified product by leveraging the state of the art DirectLight® terrestrial technology from Polatis.

The current Polatis optical switch matrix addresses the terrestrial telecom and datacenter markets, with a core technology currently supporting up to 384x384 ports. This is the only identified solution for addressing a large number of ports while reaching 1dB of insertion loss for space applications. The principle of the DirectLight® is shown in Figure 4-1.

Figure 4-1

DirectLight® core technology with mirror reflection based configuration

Each input fiber is terminated by an optical collimator to generate a parallel beam. The collimator orientation is piloted by a 2D piezoelectric actuator. The position is controlled by a local closed loop using an integrated position sensor in order to cancel piezo hysteresis, creep and drift. The collimator is then pointed to the desired output oriented collimator. The output collimator focuses the beam on the fiber end, and injects the light in the fiber. The link between two ports is established when input collimator and output collimator are aligned facing each other.

At optical switch level the main objectives in the frame of OPTIMA project were:

4.2

Materials and process of DirectLight® technology

Sodern with support from Polatis has performed an in-depth analysis of DirectLight® core technology materials and processes. About 15 materials, 5 mechanical parts and 20 processes, of the manufacturing line (machines, tools, test benches, monitoring too), electronic assemblies, soldering, gluing and welding processes have been successfully reviewed to determine their compatibility with space standards.

4.3

Switch ASIC radiation evaluation

The latest, most compact, optical switch products of Polatis uses a high-voltage CMOS ASIC to drive piezo-actuators that has been specifically designed for the terrestrial communication market switch [5]. IMEC with the support of Sodern has performed first evaluation of the capacity of the ASIC to withstand radiations as shown in Figure 4-2. The aim was to evaluate the possibility to reuse this ASIC which was not specifically designed for use in space environment.

First, Total Ionizing Dose tests have been performed. There was no degradation up to 27 krad, however, a full loss of functionality has been observed at 59 krad. In depth shielding analysis has demonstrated that the use of a 3.5 mm thick packaging would be sufficient to make the ASIC compatible with the GEO environment.

Second, the sensitivity to single event effects induced by heavy ions has been tested. Failure of samples has been observed at low energy (LET of 5.7 MeV.cm²/mg) and flux (1E5 particles/cm²) at nearly the same spot despite the use of an external de-latching circuit protection. The failure mechanism has been identified as a Single Event latch-up or a Single Event Burnout. This means that the ASIC cannot be used as is in the GEO environment due to its high sensitivity to heavy ions.

In order to better understand the exact origin of the internal failure, a laser spot scanning test of the ASIC has been performed. The conclusion is that several circuits of the ASIC are sensitive to injection of charges. As a consequence a local modification of the terrestrial ASIC will not be sufficient to improve its robustness with regards to Heavy Ions.

IMEC, Polatis and Sodern are now investigating different options for the redesign of a new ASIC for use in orbit. Most issues can be addressed by well-known design and layout techniques, however, the high-voltage part makes the new design particularly challenging, possibly requiring a change in ASIC technology.

Figure 4-2

ASIC radiation tests

4.4

48-port switch breadboard design

Sodern has performed an extensive vibration test campaign on Polatis switch core technology to characterize its dynamic mechanical behavior and the cause of vibration failures observed at relatively low input level compared to the vibration environment induced by rocket launch.

In parallel Finite Element Modeling Analysis has been performed to establish a correlation with experimental results and numerical test improvement solutions. Simulations have shown that several vibration modes were the reason for the failure. The local stiffening of some elements (via a change of material and design), and the use of mechanical dampers have been identified as solutions to dramatically enhance the robustness with respect to vibration and shock. Based on these results a 48-port switch breadboard has been designed and manufactured jointly by Sodern and Polatis as shown in Figure 4-3.

Figure 4-3

Picture of the OPTIMA 48-port switch breadboard

Full optical switching performances of the upgraded breadboard of the optical switch matrix have been measured and are compliant with the expected results. The breadboard was then subjected to 20g sine vibrations, 16g rms random vibration, -40 to +70 °C thermal vacuum cycling and shock up to 1300g. Insertion loss measurements of the optical switch matrix have confirmed its performance integrity and stability Pre and post environmental tests mentioned above.

5.1

Payload demonstrator architecture

The photonic payload system to be designed, developed and tested to TRL6 in OPTIMA is shown in Figure 3-1.

Final mechanical designs for the PFGU, PMFC, OSM and OEM, including size, power consumption and mass estimation is shown in Figure 5-1. Each unit will have its own DC power and TM/TC interfaces as well as the specific optical and RF ports representative of a real implementation (e.g. mini-AVIM optical connectors). The demonstration will comprise of three laser wavelengths set in ITU channels centered at 1558.17 nm, 1549.32 nm, and 1541.35, and the electrical LO frequencies selected for the demonstrator are 9, 10.5 and 12 GHz. A fully functional validation will be carried out, measuring performance parameters such as system conversion gain, SFDR, noise figure, etc, as well as performing a fully functional dynamic validation demonstrating the flexibility of the photonic payload in terms of channel selection, routing and allocation. The TT&C system will be validated as well.

Figure 5-1

Mechanical designs of the OPTIMA modules, from left to right: PFGU, PMFC, OSM and OEM.

OPTIMA is aiming to reduce the SWaP (Size, Weight and Power) of the payload by optimizing the photonic modules integration and using optical fibre instead of coaxial or waveguide transmission lines in a distributed architecture to facilitate the adoption of the photonic technology for space applications. Some of the key photonic components have previous heritage or are rad-hard parts, and others are being developed within the project to achieve TRL 6. On average, the goal of the activity is to demonstrate a reduction at payload level for HTS applications in size and weight of 50%, and a 10% power consumption reduction compared with traditional RF implementations.

Building blocks of OPTIMA will reach TRL6 by early part of 2019. In-orbit Demonstration and Verification (IOD/IOV) as early as 2021 can be envisaged as shown in Figure 5-2.

Figure 5-2

OPTIMA development and IOD/IOV timeline