Speaker
Description
We report on results of irradiation experiments with ring modulators and Mach-Zehnder modulators of our current silicon photonic transmitter chip COTTONTAIL. Ex-situ experiments on ring modulators show a significant degradation from a total ionizing dose of more than 3 MGy and a difference in low and high frequency behavior. Forward bias annealing can mostly restore the pre-irradiation characteristics, but can leave a small penalty of up to 3 dB, even after extended annealing. In-situ experiments for continuous measurements while irradiating are currently prepared for ring modulators as well as for Mach-Zehnder modulators.
Summary (500 words)
We develop silicon photonic transmitters for use in future detectors. Especially in particle physics, their radiation tolerance is an important figure of merit.
Now we performed a first irradiation campaign on ring modulators of our most recent COTTONTAIL chip. This chip features several silicon photonic transmitter components, like ring modulators (RM) and Mach-Zehnder modulators (MZM) of different lengths in standard and radiation hardened versions, fabricated in the imec iSiPP50G process. With these, ex-situ irradiation experiments were performed using a 3 kW X-ray tube with a tungsten anode, producing up to 60 keV photons. Three RMs were irradiated unbiased without signal for a certain dose and then characterized in a fiberoptic characterization setup. Then the next irradiation dose was applied and so on. Characterization included a wavelength sweep to measure wavelength shifts of the resonance frequencies and electrooptical frequency response measurements between 100 MHz and 40 GHz.
All RMs show a consistent behavior. The low frequency response remains rather stable for an accumulated dose of up to 3 MGy, while for high frequencies a drop of up to 6 dB can be observed from 0 MGy to 3 MGy (see attached figure, dotted lines show pre-irradiation reference levels). A further increase of the total dose results in a drop of more than 20 dB in the electrooptical response at 7.5 MGy. Even higher doses do not change the frequency response any further.
To reactivate the modulators, a forward-bias annealing was performed, first with 50 µA forward current and up to 450 s. After 20 s for low frequencies and 150 s for high frequencies, no further improvement could be seen, but the response is on average 2.5 dB lower than pre-irradiation. Increasing the forward bias current to 200 µA shows a further improvement and finally the pre-irradiation state could be achieved for two out of three modulators, while one still showed a frequency response 3 dB lower than the pre-irradiation state.
To achieve a better radiation dose resolution, we currently prepare in-situ irradiation experiments for continuous measurements while irradiating. The experiments are set up for ring modulators as well as for Mach-Zehnder modulators. Their results will also be presented at the conference and compared to former results as well as results from other groups.