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Distance Measurements with Lasers

Definition: measurements of distances using laser light

German: Distanzmessung mit Lasern, Laser-Entfernungsmessung

Category: optical metrology

How to cite the article; suggest additional literature

Lasers can be used in various ways to measure distances or displacements without physical contact. In fact they allow for the most sensitive and precise length measurements, for extremely fast recordings (sometimes with a bandwidth of many megahertz), and for the largest measurement ranges, even though these qualities are usually not combined by a single technique. Depending on the specific demands, very different technical approaches can be appropriate. They find a wide range of applications, for example in architecture, inspection of fabrication halls, criminal scene investigation (CSI), and in the military.

Techniques for Distance Measurements

Some of the most important techniques used for laser distance meters are as follows:

Laser Radar

A laser radar (more precisely: LIDAR = light detection and ranging) is a device which uses one of the distance measurement techniques as described above – usually the time-of-flight method or the phase-shift method –, and scans the direction of the distance measurement in one or typically two dimensions. That allows the acquisition of an image, or more precisely a depth profile of some object, as required e.g. in robotics or for self-driving cars. For acquiring such depth profiles at a higher rate, there are sensor chips similar to CCDs (charge-coupled devices) with internal electronics to detect phase shifts, so that the distance for each pixel can be measured simultaneously. Similarly, there are sensor chips containing an array of avalanche photodiodes for the application of the time-of-flight method with single-photon detection. Such compact devices can greatly facilitate the development of laser radars with rapid three-dimensional imaging.

Compared with ultrasonic or radio and microwave frequency devices (radar), the main advantage of laser distance measurement techniques is that laser light has a much smaller wavelength, allowing one to send out a much more concentrated probe beam and thus to achieve a higher transverse spatial resolution. Another advantage that an optical bandpass filter makes it possible to very effectively remove noise influences at other optical frequencies.

Laser Safety Aspects

Range finding with lasers can raise serious laser safety issues, particularly when intense pulses from Q-switched lasers are used; that is often required for large detection distances in order not only to obtain a detectable amount of returned light, but also to avoid a dominating influence of ambient light. Then, however, inconvenient additional measures may have to be taken to ensure safety, particularly for human eyes.

Frequently, one tries to design devices to operate a laser safety class I, so that special additional laser safety measures are not required. That, however, can severely limit the optical power which can be sent to the target, and therefore the detection capabilities.

Such trade-offs can be mitigated by applying eye-safe lasers, e.g. in the 1.5-μm spectral region, where far more optical power can be safely used than e.g. in the 1-μm region. However, both the choice of lasers and photodetectors (and their performance) are then substantially limited, and the system cost may be significantly higher.

There are also other ways to improve the trade-off between hazards and detection capabilities, e.g. using improved methods to suppress influences of disturbing ambient light. For example, a photodetector may be used in conjunction with a bandpass filter having a small transmission bandwidth around the laser wavelength, so that most ambient light is strongly suppressed. Further, one can optimize the temporal structure of the used light as well as the sensitivity of the detection apparatus.

Various Issues

As essentially all other measurement techniques using lasers, laser distance measurements can be affected by laser noise – although detection noise is usually the dominant issue. Other noise-related issues can arise from stray light and speckle effects.

The targets can have very different reflection and scattering properties. Problems can arise for very low reflection or for specular reflections. In the latter case, a lot of the incident light may be reflected in directions which are not useful for the detection.

Bibliography

[1]H. Kikuta et al., “Distance measurement by the wavelength shift of laser diode light”, Appl. Opt. 25 (17), 2976 (1986)
[2]G. Beheim and K. Fritsch, “Range finding using frequency-modulated laser diode”, Appl. Opt. 25 (9), 1439 (1986)
[3]T. Bosch et al., “The physical principles of wavelength-shift interferometric laser rangefinders”, J. Opt. 23, 117 (1992)
[4]C.-M. Wu et al., “Heterodyne interferometer with subatomic periodic nonlinearity”, Appl. Opt. 38 (19), 4089 (1999)
[5]M.-C. Amann et al., “Laser ranging: a critical review of usual techniques for distance measurement”, Opt. Eng. 40 (1), 10 (2001)
[6]S. Poujouly and B. Journet, “Laser range-finding by phase-shift measurement: moving toward smart systems”, K. G. Harding, J. W. V. Miller, and B. G. Batchlor, eds., Machine Vision and Three-Dimensional Imaging Systems for Inspection and Metrology, Proc. SPIE 4189, 152, SPIE (2001)
[7]T. R. Schibli et al., “Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer”, Opt. Express 14 (13), 5984 (2006)
[8]K. Joo et al., “Distance measurements by combined method based on a femtosecond pulse laser”, Opt. Express 16 (24), 19799 (2008)
[9]I. Coddington et al., “Rapid and precise absolute distance measurements at long range”, Nature Photon. 3, 351 (2009)
[10]G. Berkovic and E. Shafir, “Optical methods for distance and displacement measurements”, Adv. in Opt. and Photon. 4 (4), 441 (2012)

(Suggest additional literature!)

See also: triangulation, time-of-flight measurements, interferometers, phase shift method for distance measurements, laser safety, laser applications
and other articles in the category optical metrology

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