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.

Abstract This thesis describes the development of integrated structures and circuit implementations for the receiver channel of portable pulsed time-of-flight laser rangefinders for industrial measurement applications where the measurement range is from ∼1 m to ∼100 m to noncooperative targets and the required measurement accuracy is from a few millimetres to a few centimetres. The receiver channel is used to convert the current pulse from a photodetector to a voltage pulse, amplify it, discriminate the timing point and produce an accurately timed logic-level pulse for a time-to-digital converter. Since the length of the laser pulse, typically 5 ns, is large compared to the required accuracy, a specific point in the pulses has to be discriminated. The amplitude of the input pulses varies widely as a function of measurement range and the reflectivity of the target, typically from 1 to 100 ... 1000, so that the gain of the amplifier channel needs to be controlled and the discrimination scheme should be insensitive to the amplitude variation of the input signal. Furthermore, the amplifier channel should have low noise in order to minimize timing jitter. Alternative circuit structures are discussed, the treatment concentrating on the preamplifier, gain control circuitry and timing discriminator, which are the key circuit blocks from the performance point of view. New circuit techniques and structures, such as a fully differential transimpedance preamplifier and a current mode gain control scheme, have been developed. Several circuit implementations for different applications are presented together with experimental results, one of them being a differential BiCMOS receiver channel with a bandwidth of 170 MHz, input referred noise of 6 pA/√Hz and maximum transimpedance of 260 kW. It has an accuracy of about +/- 7 mm (average of 10000 measurements), taking into account walk error with an input signal range of 1:624 and jitter (3s). The achievable performance level using integrated circuit technology is comparable or superior to that of the previously developed commercially available discrete component implementations, and the significantly reduced size and power consumption open up new application areas.

Abstract This thesis describes the development of high bandwidth (~1 GHz) TOF (time-of-flight) laser range finder techniques for industrial measurement applications in the measurement range of zero to a few dozen metres to diffusely reflecting targets. The main goal has been to improve single-shot precision to mm-level in order to shorten the measurement result acquisition time. A TOF laser range finder consists of a laser transmitter, one or two receivers and timing discriminators, and a time measuring unit. In order to improve single-shot precision the slew-rate of the measurement pulse should be increased, so the optical pulse of the laser transmitter should be narrower and more powerful and the bandwidth of the receiver should be higher without increasing the noise level too much. In the transmitter usually avalanche transistors are used for generating the short (3–10 ns) and powerful (20–100 A) current pulses for the semiconductor laser. Several avalanche transistor types were compared and the optimization of the switching circuit was studied. It was shown that as high as 130 A current pulses are achievable using commercially available surface mount avalanche transistors. The timing discriminator was noticed to give the minimum walk error, when high slew rate measurement pulses and a high bandwidth comparator were used. A walk error of less than +/- 1 mm in an input amplitude dynamic range higher than 1:10 can be achieved with a high bandwidth receiver channel. Adding an external offset voltage between the input nodes of the comparator additionally minimized the walk error. A prototype ~1 GHz laser range finder constructed in the thesis consists of a laser pulser and two integrated ASIC receiver channels with silicon APDs (avalanche photodiodes), crossover timing discriminators and Gilbert cell attenuators. The laser pulser utilizes an internal Q-switching mode of a commercially available SH-laser and produces optical pulses with a pulse peak power and FWHM (full-width-at-half-maximum) of 44 W and 74 ps, respectively. Using single-axis optics and 1 m long multimode fibres between the optics and receivers a total accuracy of +/-2 mm in the measurement range of 0.5–34.5 m was measured. The single-shot precision (σ-value) was 14 ps–34 ps (2–5 mm) in the measurement range. The single-shot precision agrees well with the simulations and is better with a factor of about 3-5 as compared to earlier published pulsed TOF laser radars in comparable measuring conditions.

Abstract In this thesis integrated receiver channel techniques and circuit implementations for a pulsed time-of-flight (TOF) laser rangefinder are developed with the aim to achieve centimetre level accuracy within the dynamic range of > 1:10 000 of the input pulse amplitudes. The receiver channel converts the input current pulses produced by the photo detector to voltage pulses and produces a logic-level timing pulse for the time interval measurement. In addition to the minimization of noise, the main design challenge is the minimization of the timing walk error resulting from the varying amplitude of the received optical echo. In automotive perception laser radar application, which was the target application of this work, the input amplitude of the received echo varies in a range of 1:10 000 or even more due to changes in the measured distance and reflectivity and orientation of the target. Two receiver channel and timing discriminator architectures were developed and realized as integrated circuits in 0.35 μm BiCMOS technology, and finally verified by measurements. One of the receiver channels is based on the detection of the zero-crossing of the timing pulse produced with a unipolar-to-bipolar conversion at the input of the receiver. It achieved a timing walk error of ±8 mm in a dynamic range of 1:3000. Another receiver channel is based on the leading edge timing discrimination, in which the timing walk error is being compensated for in time domain by measuring the width of the timing pulse simultaneously with its leading edge time position. An important feature of this technique, suggested in this thesis, is that it is operative also beyond the linear range of the receiver channel, which is typically limited to < 1:100. The receiver channel with leading edge detection and pulse width compensation achieved a compensated walk error of ± 2–3 mm in a dynamic range of ~ 1:100 000. The bandwidth and input referred current noise of the channel were 230 MHz and <100 nArms, respectively. The single-shot timing precision was 120 ps (20 mm in distance) at the SNR of 10. The feasibility of the receiver electronics was verified by two laser radar prototypes. An accuracy of < ± 5 mm was measured in a measurement range from 1 to 55 m, which corresponds to the receiver dynamic range of > 1:10 000 taking into consideration the varying reflectivity of the target materials used. / Tiivistelmä Väitöskirjatyössä on suunniteltu integroituja vastaanotintekniikoita ja –piirejä valopulssin kulkuaikamittaustekniikkaan perustuvaan laseretäisyysmittaukseen. Tavoitteena on ollut saavuttaa senttimetriluokan tarkkuus laajalla tulopulssin amplitudin dynaamisella alueella > 1:10 000. Vastaanotinkanava muuntaa valoilmaisimelta saadun tulovirtapulssin jännitepulssiksi ja muodostaa siitä logiikkatasoisen ajoituspulssin aikavälimittauspiirille. Kohinan minimoimisen lisäksi toinen suuri suunnitteluhaaste on minimoida ajoitusvirhe, jota syntyy vastaanotetun optisen tulosignaalin amplitudin vaihdellessa laajalla alueella. Työssä kehitettyjen vastaanotinkanavien yksi sovelluskohdetavoitteista on ollut autoteollisuudessa käytettävät etäisyysmittarit. Näissä tulosignaalin taso vaihtelee erittäin laajalla dynaamisella alueella, joka voi olla > 1:10 000, johtuen laajasta etäisyysmittausalueesta sekä kohteen heijastavuuden ja orientaation vaihteluista. Väitöskirjatyössä kehitettiin ja valmistettiin kaksi vastaanotin- ja ajoitusilmaisurakennetta. Piirit valmistettiin 0,35 μm BiCMOS- teknologialla, ja niiden toiminta varmistettiin mittauksilla. Ensimmäinen vastaanotinkanava-arkkitehtuuri perustuu kanavan tulossa tapahtuvaan unipolaari-bipolaari muutokseen ja sen jälkeiseen nollaylityskohdan ilmaisuun. Piirillä saavutettiin ±8 mm ajoitusvirhe 1:3000 dynaamisella alueella. Toinen vastaanotinkanava-arkkitehtuuri perustuu etureunanilmaisuun, jossa ajoitusvirhe korjataan aikatasossa mittaamalla samanaikaisesti ajoituspulssin paikka ja leveys. Ajoitusvirheenkorjausmenetelmän tärkeä ominaisuus on, että se toimii laajemmalla kuin vastaanottimen lineaarisella alueella (< 1:100). Etureunanilmaisuun ja pulssinleveyden korjaukseen perustuvalla vastaanotinkanavalla saavutettiin korjattu ajoitusvirhe ± 2–3 mm 1:100 000 dynaamisella alueella. Kanavan kaistanleveys oli 230 MHz ja tulon redusoitu virtakohina < 100 nArms. Signaalikohinasuhteella 10 laseretäisyysmittauksen kertamittaustarkkuudeksi mitattiin 120 ps (20 mm etäisyydessä). Väitöskirjatyön yhteydessä valmistettiin lisäksi kaksi prototyyppilasertutkaa, joilla varmistettiin vastaanotinelektroniikan toiminta laajalla > 1:10 000 dynaamisella tulopulssin amplitudin vaihtelualueella. Lasertutkan ajoitusvirheeksi mitattiin < ± 5 mm 1–55 m:n mittausalueella.

Abstract The main aim of this work was to design CMOS receiver channels for the integrated receiver chip of a pulsed time-of-flight (TOF) laser rangefinder. The chip includes both the receiver channel and the time-to-digital converter (TDC) in a single die, thus increasing the level of integration of the system, with the corresponding advantages of a cheaper price and lower power consumption, for example. Receiver channels with both linear and leading edge timing discriminator schemes were investigated. In general the receiver channel consists of a preamplifier, a postamplifier and a timing comparator. Since a large systematic timing error may occur due to high variation in the amplitude of the received echo, a leading edge timing discriminator scheme with time domain walk error compensation is proposed here, making use of the TDC already available in the chip to measure the slew rate of the pulse and using that information to evaluate the timing error. This compensation scheme benefits from the fact that compensation can be continued even though the signal is clipped in the amplitude domain, because the slew rate continues to increase even then. The receiver channel with leading edge detection and time domain walk error compensation achieved a compensated timing walk error of ±4.5 mm within a dynamic range of more than 1:10000. The standard deviation in single shot precision was less than 25 mm with an SNR of more than 20. The usability of the receiver channel in pulsed TOF laser rangefinders was verified by making actual time-of-flight measurements on a calibrated measurement track. The linearity of the receiver chip was better than ±5 mm in a measurement range from 3 m to 21 m, with the dynamic range of the receiver channel reaching more than 1:2000. An integrated CMOS laser diode pulser was also demonstrated to prove its functionality for generating ampere-scale peak current pulses through a low ohmic load and a laser diode. The CMOS pulser achieved a peak current pulse with the amplitude of ~1 A, an optical pulse width of ~2.5 ns and a rise time of ~1 ns with a 5 V power supply. / Tiivistelmä Työn ensisijaisena tavoitteena oli suunnitella CMOS-vastaanottimia valopulssin kulkuajan mittaukseen perustuvan lasertutkan integroituun vastaanotinpiiriin. Vastaanotinpiiri sisältää sekä vastaanotinkanavan että aika-digitaalimuuntimen yhdellä integroidulla sirulla. Tällöin systeemin integrointiastetta saadaan kasvatettua, mikä merkitsee esimerkiksi halvempaa hintaa ja pienempää tehon kulutusta. Työssä on tutkittu vastaanotinkanavia, jotka käyttävät joko lineaariseen ilmaisuun tai etureunailmaisuun perustuvaa ajoitusilmaisutekniikkaa. Yleisesti vastaanotinkanava sisältää esivahvistimen, jälkivahvistimen ja ajoituskomparaattorin. Vastaanotetun signaalin tason voimakas vaihtelu saattaa aiheuttaa suuren systemaattisen virheen etureunailmaisuun perustuvassa ajoitusilmaisussa. Tässä työssä on esitetty etureunailmaisua käyttävä ajoitusilmaisin, jossa syntyvää ajoitusvirhettä voidaan korjata mittaamalla pulssin nousunopeutta aika-digitaalimuuntimella, joka on integroitu samalle sirulle. Aikatasossa tapahtuvan virheenkorjauksen etuna on mahdollisuus jatkaa virheenkorjausta amplituditasossa tapahtuvan signaalin leikkautumisen jälkeenkin, koska signaalin nousunopeus kasvaa leikkaantumisesta huolimatta. Etureunailmaisua käyttävällä vastaanotinkanavalla, jossa ajoitusvirhettä korjattiin pulssin nousunopeutta mittaamalla, saavutettiin ±4,5 mm ajoitusvirhe 1:10000 dynaamisella alueella. Kertamittaustarkkuuden keskihajonta oli vähemmän kuin 25 mm, kun signaalikohinasuhde oli enemmän kuin 20. Vastaanotinkanavan käytettävyys osana lasertutkaa todettiin tekemällä tutkamittauksia kalibroidulla mittaradalla. Mittauksissa saavutettu lineaarisuus oli ±5 mm mittausalueen vaihdellessa 3 metristä 21 metriin ja signaalin dynamiikan ollessa enemmän kuin 1:2000. Lisäksi työssä esitellään integroitu CMOS-pulssitin, joka pystyy tuottamaan ampeeri-luokan virtapulsseja laserdiodiin. CMOS-pulssittimella voitiin tuottaa 5 V käyttöjännitteellä ~1 A virtapulsseja optisen pulssin leveyden ja nousuajan ollessa ~2,5 ns ja ~1 ns.

Abstract In this thesis, a new type of laser diode transmitter using enhanced gain-switching suitable for use with a single photon avalanche diode (SPAD) detector was developed and tested in the pulsed time-of-flight laser range finding (lidar) application. Several laser diode versions were tested and the driving electronics were developed. The driving electronics improvements enabled a pulsing frequency of up to 1 MHz, while the maximum laser output power was about 5–40 W depending on the laser diode dimensions. The large output power is advantageous especially in conditions of strong photon noise emerging from ambient light outdoors. The length of the laser pulse matches the jitter of a typical SPAD detector providing several advantages. The new laser pulser structure enables a compact rangefinder for 50 m distance measurement outdoors in sunny conditions with sub-centimeter precision (σ-value) at a valid distance measurement rate of more than 10 kHz, for example. Single photon range finding techniques were also shown to enable a char bed level measurement of a recovery boiler containing highly attenuating and dispersing flue gas. In addition, gated single photon detector techniques were shown to provide a rejection of fluorescent photons in a Raman spectroscope leading to a greatly improved signal-to-noise ratio. Photonic effects were also studied in the case of a pulsed time-of-flight laser rangefinder utilizing a linear photodetector. It was shown that signal photon noise has an effect on the optimum detector configuration, and that pulse detection jitter can be minimized with an appropriate timing discriminator. / Tiivistelmä Tässä työssä kehitettiin uudentyyppinen, tehostettua "gain-switchingiä" hyödyntävä laserdiodilähetin käytettäväksi yksittäisten fotonien avalanche-ilmaisimien (SPAD) kanssa, ja sitä testattiin pulssin lentoaikaan perustuvassa laseretäisyysmittaussovelluksessa. Useita laserdiodiversioita testattiin ja ohjauselektroniikkaa kehitettiin. Ohjauselektroniikan parannukset mahdollistivat jopa 1 MHz pulssitustaajuuden, kun taas laserin maksimiteho oli noin 5–40 W riippuen laserdiodin dimensioista. Suuri lähtöteho on edullinen varsinkin vahvoissa taustafotoniolosuhteissa ulkona. Laserpulssin pituus vastaa tyypillisen SPAD-ilmaisimen jitteriä tarjoten useita etuja. Uusi laserpulssitinrakenne mahdollistaa esimerkiksi kompaktin etäisyysmittarin 50 m mittausetäisyydelle ulkona aurinkoisessa olosuhteessa mm–cm -mittaustarkkuudella (σ-arvo) yli 10 kHz mittaustahdilla. Yksittäisten fotonien lentoaikamittaustekniikan osoitettiin myös mahdollistavan soodakattilan keon korkeuden mittauksen, jossa on voimakkaasti vaimentavaa ja dispersoivaa savukaasua. Lisäksi portitetun yksittäisten fotonien ilmaisutekniikan osoitettiin hylkäävän fluoresenssin synnyttämiä fotoneita Raman-spektroskoopissa, joka johtaa selvästi parempaan signaali-kohinasuhteeseen. Fotoni-ilmiöitä tutkittiin myös lineaarista valoilmaisinta hyödyntävän pulssin kulkuaikamittaukseen perustuvan lasertutkan tapauksessa. Osoitettiin, että signaalin fotonikohina vaikuttaa optimaaliseen ilmaisinkonfiguraatioon, ja että pulssin ilmaisujitteri voidaan minimoida sopivalla ajoitusdiskriminaattorilla.