RICERCA ITALIANA     

ABSOLUTE TIME OF FLIGHT TELEMETRY WITH MODE-LOCKED PICOSECOND LASERS

Politecnico di Torino

Abstract

We propose to develop a precision measurement system of the two-way time-of-flight of optical pulses from a picosecond laser, based on the evaluation of the phase delay that it induces om the returning pulse train with respect to the outgoing one. In order to obtain micrometer spatial resolution, the time delay must be measured with an uncertainty smaller than 100 fs, a very ambitious task.
However, the target appears within reach if one considers that in the field of Time and Frequency Metrology a measurement system of ultrastable oscillators’ phase instability was developed in the 70ies and is commonly used, which can resolve a few fs. Such system is based on the expansion of the phase difference time delay to be measured with the adoption of a heterodyne method applied symmetrically to the two channels. It is expected that micrometer absolute accuracies and even better repeatability can be obtained in this way.
In order to extend the usefulness of this method to the absolute evaluation of the delay, which is a requirement in absolute telemetry, it is necessary to study all systematic errors produced in the physics and in the electronics. In particular in the research unit of the Politecnico di Torino, possible errors of the electronics will be studied, and a prototype system will be realized with consequently optimized symmetry in order to common mode as much as possible such errors.
In the reasearch unit of the Politecnico di Milano the analysis of >>>

Principal Investigator

Andrea De Marchi Politecnico di TORINO

Research Objectives

The primary aim of this research project is to develop and realize a time-of-flight optical telemetry system based on the use of a mode-locked picosecond laser and on the measurement of time delay with a dual mixer system, also with the aim of inquiring on possible limits to the accuracy and resolution of the system.

To this aim a research programme is organized which foresees the achievement of a number of intermediate benchmarks, both on the physics of the optical pulse, for which adequate analysis of source, its stabilization, and propagation medium is necessary, and on the uncertainties embedded in the behavior of devices, in particular in the medium and long term.

More specifically, the final objective is to obtain micrometer resolution on great distances (more than 10 m), with similar accuracy and a measurement speed allowing to consider as still any object moving at velocities up to a few m/s. What this means is a target resolution of 10 micron in 10 microseconds. However, the system should be capable of varying in a simple way both resolution and measurement speed, with easily made changes on the synthesizer.

Intermediate objectives can be identified for each research unit, according to its specific task within the programme.

For the Per il Politecnico di Milano the aim is to realize a pulsed laser source with the necessary characteristics of stability and pulse shape, as seen at the output od fast >>>

First Results

The proposed laser telemeter has the advantage of being robust and compact: the optical system is essential and internal in the picosecond laser, electronics is modular and composed of few devices, fast: readings have a tuneable rate with time smaller than a millisecond.

Concerning the absolute distance measurement, the system is based on the distance measurement through the optical pulse delay measurement, translated to a phase measurement. Using a commercial device with 1 ns temporal resolution, with a repetition rate of 100 MHz, corresponding to 1.5 m fringe, we would obtain a spatial resolution of 1.5 um, with a reading rate of 1 kHz. The instrument has two independent degrees of freedom, by choosing a higher harmonic the reading rate increases, whereas by increasing the ratio between the input and output frequency of the double balanced mixer increases the spatial resolution, that could reach the micrometer range, competing with a relative interferometer.
Practically, the real limitations of the proposed telemeter comes from the optical part (pulse envelope instability, repetition rate instability) and the electronics part (channel asymmetry, unwanted phase rotations, electronics noise).

The project aims at evaluating the real capability to reach the theoretical resolution reported above and the principal uncertainty causes. Preliminary calculations based on the state of the art allows us to estimate a final uncertainty in a range >>>

Timescale

24 months

National and international background

Industrial needs for absolute length measurements on great distances are continually growing, and increasingly stringent are getting proposed accuracy, resolution and speed specifications. Applications are the most various, from aerospace industy, requesting satellite control or tridimensional measurements on wing structures, to geodetics for monitoring slow small ground displacements, for example for quake prediction or for decision making e.g. in the choice of sites for waste disposal.
The ultimate accuracy limits for distance measurements in air are in any case given by the knowledge of the average refraction index along the optical path, because it implies a correction of the raw measure. Details on this can be found in the B form of the INRIM research unit.
For absolute measurements the following methods are used.
Time-of-flight method. The total optical path is derived from the time that it takes to the light to cover it, usually back and forth. With optical pulses, the ultimate limit is set by the time resolution of light detectors. For example, a time resolution of 10 ps corresponds to an uncertainty of 1.5 mm (1.5 10-5 su 100 m).
Interferometry. The total optical path difference between two different paths is given by *********, where *** is the wavelength, N is the integer number of wavelengths and **** is the extra fraction of wavelength. It offers great resolution but it doesn’t provide an absolute measurement if a single wavelength is >>>