Performance assessment of low temperature-grown GaAs-based optoelectronic devices dedicated to RF sampling applications

Abstract

In a previous presentation we have demonstrated the optoelectronic sampling of RF signals using photoswitches (PSW) made from low-temperature grown (LTG) GaAs [1]. The main characteristics that are required for better sampling performances are high optical sensitivity, high bandwidth, large ON/OFF ratio and electrical linearity response . They are closely linked both to the LTG-GaAs properties and to the geometrical design of the RF waveguide . We tested a large set ofPSW with different configurations and we present here a complete set of experimental and simulation results. Specially, we point out that using the device with a shortest gap length leads to the generation of frequency harmonics while increasing RF input electrical power. Hence, the spurious free dynamic range (SFDR) is reduced when the RF power is increased up to 13 dBm. We suggest that this results from an electrical nonlinear response which is partially due to carrier velocity saturation in LTGGaAs. In accordance with [2, 3], we model it and find an electric field saturation of 14 kV/cm.The tested devices are made of LTG-GaAs layers grown at different temperatures with a gold coplanar waveguide deposited onto. The central electrode is interrupted by a gap that acts as the PSW. This latter is triggered by femtosecond pulses (60 fs FWHM, 75 MHz repetition rate) at A=800 nm delivered by a Ti:Sa laser. The sampling experiment is performed synchronously using RF sources locked in phase with the laser pulse or in a free running sampling manner. For the synchronous experiment, an optical delay line allows us to vary the phase in between the RF source and the pumping optical pulses, leading to a reconstructed RF signal [1].First, we checked the linearity of the optoelectronic sampler by varying the RF input power from -30 dBm to 13 dBm. We used a 10 GHz sinusoidal RF source locked to the laser clock and a set of calibrated RF attenuators .

High speed photoswitching: from material properties to device performances

“…This has been reported in [8] where an optoelectronic PS sampler with 1 µm inter-electrode spacing is studied. In our work, RF sinusoidal signals at 10 GHz are sampled at a 75 Ms/s rate by synchronously exciting the PS by a fs laser pulse train.…”