Abstract
Considerations are presented for the design of GaAs traveling-wave electro-optic modulator arrays for space data-link applications. Central to the modulator design is a low loss folded optical configuration giving direct, straight-line radio frequency (RF) access at one end of the device, with all fiber-optical ports at the opposite end. This configuration is a critical enabler for the close-packed monolithic modulator arrays needed for multi-channel applications. It also leads to much more compact packaging, improved fiber handling and contributes to high modulation bandwidths with low ripple by eliminating directional change in the RF feed arrangements. Both single Mach-Zehnder (MZ) and monolithic dual-parallel (IQ) modulators have been assessed up to 70 GHz, with bandwidths around 50 GHz achieved with a low-frequency ON/OFF voltage swing (Vπ) of 4.6 V (a voltage. length product of 8.3 Vcm). The folded devices can be significantly more compact than conventional ‘straight in-line’ modulators, while a modest array of devices (e.g., ×4) can be accommodated in a package of similar dimensions to a single modulator. Design considerations for monolithic arrays of independently addressed MZ modulators (each with its own input fiber) are discussed and practical configurations proposed.
Highlights
There is increasing interest in the advancement of optical methods for managing radio frequency (RF) signals in space, with many applications for both ground-to-satellite and inter-satellite communications envisaged
IQ Modulators (IQMs) are a specialized form of x2 array; additional MMI couplers allow both MZM units to be fed from a single input and to be merged onto a single output
Several designs of these have been realized in our GaAs/AlGaAs technology, with increasing refinement in bend, MMI and RF design
Summary
There is increasing interest in the advancement of optical methods for managing RF signals in space, with many applications for both ground-to-satellite and inter-satellite communications envisaged. Electro-optic modulators are viewed as a Critical Space Technology for microwave photonic payloads, with increasing interest in frequencies of 50 GHz and higher by the European Space Agency [1]. There is interest in discrete modulators, and in arrays and composite devices for modulation formats such as single-sideband (SSB) and quadrature phase-shift key (QPSK). Both SSB and QPSK coding can be produced using a similar dual parallel IQ (In-phase/Quadrature) modulator configuration; this is essentially a ×2 modulator array (or ×4 for the dual-polarization variant) with added optical split and recombine elements.
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