1.2. Content and contributions of the work

The content of this thesis is as follows. The operating principle of the pulsed TOF laser rangefinder is described in Chapter 2. As the technique is based on measuring the time interval between transmitting and receiving an optical pulse, a logic level timing mark for the time-to-digital converter (TDC) has to be generated accurately from the pulse even when its amplitude varies. This is achieved with a timing discriminator, which is a critical block in a pulsed TOF laser radar from the performance point of view. Two ways of detecting the timing point from the pulse signal, a high-pass timing discriminator and a leading edge discriminator, are presented, together with their benefits and drawbacks. The origin of the walk error, a change in the timing mark when the amplitude of the signal changes, is also explained for both methods.

The development of pulsed TOF laser rangefinding techniques and structures suitable for a laser radar module is described in Chapter 3 with the aid of the original papers included at the end of the thesis. The chapter begins by presenting two integrated receiver channel realisations: the first with a high-pass timing discriminator (paper I) and the second with a leading edge discriminator (paper II). After that a new method for avoiding walk error by combining the timing discrimination and time interval measurement by means of linear signal processing is presented (paper III). The operation of the developed integrated receiver channel is verified by constructing prototypes of a portable laser rangefinder, which are presented with their performance in papers IV–VI.

In order to further increase the level of integration and reduce the size and mechanical complexity of the laser radar, the possibility of integrating a photodetector into the same chip as the rest of the electronics was investigated. The properties of photodiodes implemented in standard CMOS/BiCMOS processes without any modifications were studied (paper VII), and finally two receiver chips with integrated photodiodes are described, one for “normal” distance measurement purposes (paper VIII) and the other for measuring distances from three separate targets with a single optical pulse. The latter chip includes four receiver channels with integrated photodetectors and a three-channel time-to-digital converter.

The results of the work are discussed and compared with research published in the literature in Chapter 4 and a summary of the work is given in Chapter 5.

The scientific contribution of the thesis lies in the development of integrated receiver channel realisations for a pulsed TOF laser radar and the construction of high-performance laser radar modules based on the use of these receivers. In this connection the origin of the walk error observed in the leading edge discriminator was also analysed. In addition, a totally new receiver channel structure based on combination of the timing discriminator with time interval measurement was developed and integration of the photodiode into the same chip as the rest of the receiver and the feasibility of an optoelectronic laser radar chip capable of measuring distances from three points with a single optical pulse were demonstrated.