OMCU - Optical Multi-Frequency Conversion Unit for Broadband Transparent Analogue Repeaters

A typical photonic payload architecture basically consists in conventional microwave low-noise receive front ends and high-power amplification chains in the transmit section, and features a photonic repeater, which acts as a space/frequency switch, and thus enables to route any RF sub-band from any input access port to any output access port and to shift its frequency position accordingly. More specifically, the photonic repeater core supports the following functions:

• optical distribution of centralized, high-frequency microwave Local Oscillator (LO) signals to numerous electro-optical mixers.
• photonic frequency-conversion of RF signals by means of electro-optical mixers.
• optical cross-connection of RF signals by means of micro-optical switches, with unique capabilities such as scalability to large port counts and RF frequency independence.

Electro-optical mixing offers attractive capabilities, out of which multiple frequency-conversion (OMC) performed by feeding the electro-optical mixer with several LO’s on separate optical carriers using wavelength-division-multiplexing (WDM).

Functional applications of the OMC technique were elaborated in more detail. OMC can be used for frequency routing, with frequency-band or frequency-slot interchange and enhance the reconfiguration flexibility of the photonic repeater architectures.

Such photonic architectures are found to exceed the capabilities of microwave/RF implementations, bringing drastic mass, power and volume savings, growing up to larger scales, and/or supporting in-orbit reconfiguration.

SYSTEM ARCHTIECTURE

OMAR implementations are designed for forward and return repeater respectively, based on similar architectures all making use of OMC concept, and on common optical building blocks. Figure  below gives the general OMAR architecture considered for the forward repeater, similar architectures being  proposed for the return repeater.

The Forward photonic payload consists in a photonic frequency-generation unit (FGU), a photonic frequency-down-converter assembly, and an optical reconfiguration section based on a wavelength-selective optical switching architecture. The Return payload iss based on the same principle, and on a similar implementation. All the LO’s are generated and transferred on optical carriers within the FGU and delivered to modulator-based electro-optical mixers. The Forward photonic FGU architecture is designed to provide 5 LO’s to all output ports. The Return FGU architecture is designed to provide 5 LO’s as well.

Photonic RF payload architecture for Forward repeater in multi-beam mission

 RF signals received from up-link antenna accesses are transferred onto optical carriers at the electro-optical mixers. When the electro-optical mixer is fed by an optical LO, the input RF frequency is down-converted to an intermediate frequency (IF). Amplification, distribution and switching are performed in the optical domain by means of optical amplifiers and micro-optical switches in the wavelength-selective switching unit.

At the output of this latter stage, opto-microwave receivers convert the optical signals back into microwave ones at IF, and RF channel filtering is achieved by means of conventional RF filter technology. From estimations of complexity and budgets, it turns out that equivalent RF payload implementations would result in huge amount of hardware, and are thus considered today as almost unfeasible.