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Theses

Femto second sampler

Abstract : As the world becomes more and more connected, the need for higher data throughput becomes increasingly urgent. This implied more creativity from circuit designers to come up with higher bandwidth circuits and faster pace clocks to drive them.However, the traditional microelectronics clock circuits fail short to fulfill the ever-increasing need for higher sampling frequencies because of their inherent noise. The latter renders the sampled data altogether unusable and therefore hinders the efforts towards higher streams of data.Integrated photonics, the optics alternative to microelectronics, may bring an end to this problem. Indeed, when it comes to pulsed lasers, a not that new technology, the optical pulses clocking features a very low noise compared to its microelectronics counterpart, i.e a jitter that is five folds lower than the best literature microelectronics clock.The main idea of this thesis is to design a sampling circuit that uses the laser pulses as a clock, but samples an electronic signal. This circuit design is mainly based on the use of a Germanium photoconductor, i.e, a Germanium resistor that changes its resistance according to the value of the optical signal it receives. This photoconductor plays then the role of a clocked switch and connects the RF input signal to the hold capacitor. When the switch receives a pulse of light, its resistance drops and the signal can be copied to the capacitor, and once the pulse of light is over, the Germanium photoconductor recovers its first resistance value and the input signal node is disconnected from the capacitor that holds the sampled signal.The aim of this thesis is to study the feasibility of such a design and whether or not it allows taking advantage of the very low jitter value of the laser. Within the thesis, we tried to implement the up said design and stumbled upon many challenges. First, the values of the off switch resistance were not high enough to disconnect the hold capacitor from the input node due to the low resistivity of the used Germanium (residual doping). The Germanium implies also that the switching from one resistance value to another is not instantaneous but rather as long as the photo-generated carriers exist, which lasts for at least a nanosecond. We resolved these problems using three methods: a novel geometry of Germanium photoconductors that allows for high Roff/Ron ratios (patented geometry), a counter-doping to increase the resistivity of the material and finally, a re-sampling circuit driven by a photonic clock based on the laser pulses. We also explored the possibility of making very steep-edged clocks with customizable pulse lengths based on the laser pulses.
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Hanae Zegmout. Femto second sampler. Optics / Photonic. Université Grenoble Alpes, 2019. English. ⟨NNT : 2019GREAT084⟩. ⟨tel-02634439⟩

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