3D Time-of-flight distance measurement with custom - Universität ...

136 CHAPTER 5
This is only possible, since the CCD-performance of adding and storing signals in
the charge domain is nearly noise-free. Normally, one would expect a lower optical
power to lead to a worse demodulation contrast, since we know, from CTE
measurements of CCDs that are realized in the same technology, that the CTE is
better, the more charge is to be transported (c.f. Figure 3.20). This is because all
three transport mechanisms (self-induced drift, diffusion and fringing fields)
contribute to the charge transport. Self-induced drift strongly depends on the
number of integrated electrons, because a smaller number of electrons leads to a
reduced influence of self-induced drift on the overall charge transfer.
For 20 MHz modulation, the short-time integration lasts only 25 ns (50 ns
modulation period). In this period, only few electrons are generated and transported
to the integration gate. In fact, as one can see from MCD05, for 175,000
femtowatts, the highest optical power used, only 18 electrons are generated within
50ns, statistically speaking, i.e. only 9 electrons within the active sampling time. For
the lowest optical power used the conditions are much worse: one electron is
generated, statistically speaking, only every 300 th modulation period. From these
numbers, we understand that, under the above conditions, self-induced drift can
have no influence at all on the charge transfer. We really move single electrons.
Demodulation only works here with the help of statistics. This enormous
performance is a big welcome surprise.
In addition to the total optical power on one photogate, the average resulting output
voltage swing is also listed in MCD05. This can be calculated with known quantum
efficiency, wavelength, integration time, conversion capacitance and amplification
of the output stage. Also the total number of electrons generated during the
integration time is listed, as well as the number of electrons statistically generated
during only one modulation period. Since these values are smaller than one, it is
more meaningful to list the inverse value, which is the number of modulation cycles
that have to pass until one electron is optically generated by the modulated light
source, statistically speaking.