Chapter 5. Summary

The general objective of the work was to increase the level of integration of pulsed time-of-flight laser radar devices. One specific aim was to construct a laser radar module which can be used for different measurement tasks by means of an external fibre-connected measurement head customised for the application. Another purpose of the module construction was to verify the functionality of the receiver ASICs developed here, which are necessary when increasing the integration level of the system. The ultimate goal of the work was to pave the way to a single-chip realisation which would include parallel customised photodetectors with receiver channels and a multi-channel TDC.

Three receiver channels were developed: one with a leading edge discriminator that achieves a compensated walk error of ±35 mm in an input signal dynamic range of 1:4000, one with a high-pass timing discriminator that achieves a walk error of ±4 mm with an input signal dynamic range of 1:620 and one in which the high-pass timing discriminator is combined with time-to-amplitude conversion that achieves a walk error of ±0.5 mm with an input signal dynamic range of 1:21. The level of integration is highest in the latter receiver, as time interval measurement is located in the same chip.

The usability of the receiver channels was verified by constructing laser radar modules which can be also used as such for measurement purposes. The device achieves mm-level measurement accuracy with passive targets over a range from 4 to 34 metres. The measurement range can be modified for each application by designing a customised optomechanical measurement head. The laser radar technology developed here can be used for a wide variety of measurements, including positioning of tools and vehicles, velocity measurement, anti-collision radars and proximity sensors, for example.

Integration of the photodetector into the same chip as the rest of the receiver channel electronics was also addressed. The responsivity of the photodiode is about 0.3 A/W, but the noise level of the receiver is reduced by a factor of about three because of the absence of parasitic capacitances and inductances caused by photodiode and receiver channel packages, PCB wiring, bond wires, the I/O cell and ESD protection structures. The receiver structure can be multiplied, as was demonstrated by designing and testing a laser radar chip which includes four receiver channels with photodiodes and a three-channel time-to-digital converter that can be used to measure distances in three directions with a single optical pulse. The laser pulse transmitter and the optics are external components of the system.

The scientific contribution of this work, in the addition to the circuits and devices that were developed, lies in the analysis of walk error in the leading edge discriminator, which proves the significance of the receiver bandwidth and rise time of the optical pulse, the totally new receiver channel structure combining timing discrimination with time interval measurement and the demonstration of a laser radar chip which includes four receiver channels with photodiodes and a three-channel time-to-digital converter. All of these contributions improve the level of integration of the pulsed time-of-flight laser rangefinding module.