Ari Kilpelä

Precise pulsed time-of-flight laser range finder for industrial distance measurements

A pulsed time-of-flight laser range finder with a 1 GHz avalanche photo diode (APD) receiver and a laser pulser with ∼35 ps pulse width has been developed and tested. The receiver channel is constructed using a silicon ASIC chip and a commercially available silicon APD placed on a hybrid ceramic susbstrate. The laser pulser utilizes a single heterostructure laser operating in Q-switching mode. It is shown that the single-shot precision of the complete laser range finder is ∼2.1 mm (σ value) at best. The nonaccuracy in the distance range of 0.5–34.5 m was ∼±2 mm excluding errors caused by the statistical variations and long-term instability. The single-shot precision is clearly better than the single-shot precision of the earlier laser range finders with ∼100–200 MHz bandwidths. Also, two types of optics, coaxial and paraxial, were tested. The linearity of the coaxial optics was better, especially with a long (4 m) receiver fiber. Some possible applications of the laser range finder utilizing ps level puls...

A sub-ns time-gated CMOS single photon avalanche diode detector for Raman spectroscopy

A time-gated single photon avalanche diode (SPAD) has been designed and fabricated in a standard high voltage 0.35 μm CMOS technology for Raman spectroscopy. The sub-ns time gating window is used to suppress the fluorescence background typical of Raman studies, and also to minimize the dark count rate in order to maximize the signal-to-noise ratio of the Raman signal. The proposed time-gating technique is applied for measuring the Raman spectra of olive oil with a gate window of 300 ps, and shows significant fluorescence suppression.

Acquisition Of Three-Dimensional Image Data By A Scanning Laser Range Finder

We describe a 3-D vision system designed and constructed at the Technical Research Centre of Finland in cooperation with the University of Oulu. The system was developed chiefly for geometric measurements of large objects. The system has been operative for about one year, and its performance has been extensively tested. The system consists of three main units: the range finder, the scanner, and the computer. The range finder is based on the direct measurement of the time-of-flight of a laser pulse. The scanner consists of two mirrors driven by moving iron galvanometers; this unit is controlled by servo amplifiers. The computer controls the scanner, transforms the measured coordinates into a Cartesian coordinate system, and serves as a user interface and postprocessing environment.

Laser pulser for a time-of-flight laser radar

A laser pulser for a pulsed time-of-flight laser radar is presented. The pulser is constructed using a single avalanche transistor in order to keep the schematic simple and to avoid the problems encountered when connecting several avalanche transistors in parallel. The schematic of the laser pulser was optimized and it was noticed that the optical peak pulse power of the laser can be increased significantly by adding a parallel capacitor to the laser. The measured increase of the optical power was up to 26%. The simulations show that the parallel capacitor and also the serial inductances of the components in the laser pulser play a significant role in adjusting the shape of the current pulse. In order to find the transistor, which gives the highest current, the properties of several transistor types were compared. It was noticed that there can be a great difference between the avalanche properties of a group of transistors even if they are of the same type.

Timing discriminator for pulsed time-of-flight laser rangefinding measurements

A time-pickoff circuit based on the constant fraction discriminator (CFD) timing principle has been developed for pulsed time-of-flight laser rangefinding with pulse lengths of 5–10 ns. It is based on detection of the crossing point of the trailing edge of the original timing pulse and the leading edge of its delayed replica with a fast emitter coupled logic comparator. A simplified theory is presented here for its walk error. Three types of comparator were tested in the CFD. It is shown that the dominant source of walk error is that produced by the limited gain-bandwidth product of the comparator and that walk error can be reduced to ±1 mm in a 1:10 dynamic range of input pulses by adding an external offset voltage between the input nodes of the comparator.

Multistreamer regime of GaAs thyristor switching

GaAs bipolar thyristors have been used to obtain current pulses of over 100 A with rise times less than 600 ps and load resistance of approximately 0.4 /spl Omega/. The maximum voltage has been shown to exceeded 500 V in some cases. To interpret the experimental results a multichannel switch regime is proposed. Analysis of the experimental data suggests the possibility of a further increase in the maximum amplitude of the current pulse. >

Internal Q-switching in semiconductor lasers: high intensity pulses of the picosecond range and the spectral peculiarities

Optical pulses of /spl sim/100 ps duration, and /spl sim/10/sup 2/ W power were obtained from the industrial single heterostructure lasers with a standard pulse generation power of /spl sim/10 W in the internal Q-switching mode. Temporal and spectral analyses allow three components to be distinguished in the laser optical pulses: ordinary delayed pulses of large duration at energies considerably lower than the energy gap, short optical pulses caused by the gain-switching effect at higher energies, and short optical pulses at the end of the current pulse (Q-switching mode) at the highest energies. A model is proposed involving band tail states as a saturable absorber causing large delays. >

Pulsed time-of-flight radar for fiber-optic strain sensing

This article describes a fiber-optic interrogation device based on the pulsed time-of-flight technique. The apparatus is capable of measuring time delays between wideband reflectors, such as connectors, along a fiber path with a precision of about 280 fs (rms value) and a spatial resolution of about 3 ns (0.30 m) in a measurement time of 25 ms. Potential application areas include measuring integral strain and its derivatives such as cracks, deflections, and displacements, particularly in large civil engineering and composite structures. The operation and basic blocks of the measurement system are presented in detail together with measurement results obtained in laboratory and field conditions. It is shown that by using a fiber loop sensor with a reference fiber, it is possible to achieve a strain precision below 1 microstrain and a measurement frequency of 4 Hz. System performance proved adequate for the study of both static and dynamic phenomena in a bridge deck.

A high-power picosecond near-infrared laser transmitter module

A compact laser diode based transmitter was designed and tested for laser radar and various laboratory applications. Single optical pulses with a peak power of up to 200 W, 23–65 ps pulse duration, and a repetition rate of up to 50 kHz were measured. Transient mode spectral filtering suppressed after pulsing modes by a factor of 104–105 with respect to the peak power. A control module was developed which provided a jitter value between electrical triggering and the optical pulses as low as 14 ps. Averaging of 103 events allows 1.5 ps stability between the triggering and the optical pulses to be achieved within a delay range from 5 to 250 ns.

A novel compact 35 A/150 ps current pulse generator for a new generation of the laser radars

A current pulse generator is presented which allows a 35 A current amplitude with a rise time of 150 ps to be achieved across a low-ohmic load. The generator is based on an avalanche transistor matrix with intrinsic switching synchronisation. The front of the pulse is further sharpened by employing sub-nanosecond diode recovery in the first stage and super-fast switching of another diode under pulsed reverse biasing in the second stage. An application of this current pulse for the feeding of a high-power gain-switched semiconductor laser is demonstrated.