Integrated electronic and optoelectronic circuits and devices for pulsed time-of-flight laser rangefinding

Pasi Palojärvi

Department of Electrical and Information Engineering and Infotech Oulu, University of Oulu

Abstract

The main focus of this work concerned with the development of integrated electronic and optoelectronic circuits and devices for pulsed time-of-flight laser rangefinding is on the construction of the receiver channel, system level integration aimed at realisation of the laser radar module and in integration of all the receiver functions of laser radar on one chip.

Since the timing discriminator is a very important part of a pulsed time-of-flight laser rangefinder, two timing discrimination methods are presented and verified by means of circuit implementations, a leading edge discriminator and a high-pass timing discriminator. The walk error of the high-pass timing discriminator is ±4 mm in a dynamic range of 1:620 and the uncompensatable walk error of the leading edge discriminator is ±30 mm in a dynamic range of 1:4000. Additionally a new way of combining the timing discriminator with time interval measurement is presented which achieves a walk error of ±0.5 mm in a dynamic range of 1:21.

The usability of the receiver channel chip is verified by constructing three prototypes of pulsed TOF laser radar module. The laser radar achieves mm-level accuracy in a measurement range from 4 m to 34 m with non-cooperative targets. This performance is similar to that of earlier realisations using discrete components or even better and has markedly reduced power consumption and size.

The integration level has been increased further by implementing a photodetector on the same chip as the rest of the receiver electronics. The responsivity of the photodetector is about 0.3 A/W at 850 nm wavelength and the noise of the receiver is reduced by a factor of about two relative to realisations using an external photodetector, because of the absence of parasitic capacitances and inductances caused by packages, PCB wiring, bond wires and ESD and I/O cell structures.

The functionality of a multi-channel pulsed TOF laser radar chip is demonstrated using the photodiode structure investigated here. The chip includes four photodetectors with receiver channels and a three-channel time-to-digital converter. The chip together with external optics and a laser pulse transmitter enables distances to be measured in three directions with a single optical pulse, thus showing the feasibility of implementing all the receiver functions of a pulsed time-of-flight imager on a single chip using a current semiconductor process.

List of Tables
1. Performance of the receiver channel with a high-pass timing discriminator.
2. Performance of the receiver channel with a leading edge discriminator.
3. Performance of the laser rangefinding devices.
4. Comparison of Silicon Integrated Photodiodes at NIR wavelengths.
5. Performance of the receiver for a pulsed TOF laser radar.
6. Comparison of transimpedance amplifier channels.
7. Comparison of timing discriminators.

List of Figures
1. Three phases of laser radar research at the Electronics Laboratory.
2. Principle of a pulsed time-of-flight laser rangefinder.
3. a) Walk error with a leading edge discriminator, and b) the bipolar pulse used in the high-pass timing discrimination technique.
4. a) A simple model of the receiver channel and input and output signals of the bandwidth limiting circuit with b) small and c) large signal amplitudes.
5. Delay in the timing point of the leading edge discriminator as a function of signal level.
6. a) Schematic diagram of a high-pass filter and its input and output signals, and b) change in the timing point with offset voltage.
7. Coarse schematic diagram of a high-pass timing discriminator with offset voltage.
8. Measured walk error of a high-pass timing discriminator as a function of offset voltage.
9. Diagram and photograph of the receiver circuit, comprising the start and stop channels.
10. Output signal from the amplifier channel as a function of the input signal when gain controls are used.
11. Diagram and photograph of the receiver with a leading edge discriminator.
12. Error in distance measurement after temperature and walk error compensation at different temperatures.
13. Schematic diagram of the differential pair.
14. Operation of the TAC with different slew rates of the stop signal.
15. Building blocks and signals of the receiver channel.
16. Diagram of the circuitry used to distinguish the ramp signal.
17. Measured walk error of the receiver channel.
18. Block diagram of laser rangefinder described in paper VI.
19. Optical pulse of the laser pulse transmitter described in paper VI.
20. Linearity error and amplitude of the stop signal of the rangefinding device described in paper VI.
21. Single-shot precision of the rangefinding device described in paper VI.
22. Cross-section of the pn-photodiode integrated in a 0.8 µm CMOS process.
23. Cross-section of the pn-photodiode integrated in a 1.2 µm BiCMOS process.
24. Depths within which carriers have to be collected to achieve given responsivities as a function of wavelength.
25. Coarse schematic diagram of the integrated receiver channel described in paper VIII.
26. Coarse schematic diagram of the multi-channel receiver.
27. Photograph of the multichannel receiver.
28. A photograph of the hybrid test circuit of the multichannel receiver.
29. Measured linearity error of the multichannel receiver.

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		Acknowledgements