phase shift method for distance measurements (original) (raw)

Definition: a method for optical distance measurements

Categories: optical metrology, methods

• distance measurements with lasers
  • phase shift method for distance measurements
  • time-of-flight measurements
  • triangulation

Related: distance measurements with laserslaser rangefindersinterferometerstime-of-flight measurements

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Contents

What is the Phase Shift Method?

Laser rangefinders are often based on the phase shift method, a technique for measuring distances in the following way. A laser beam with sinusoidally modulated optical power is sent to a target. Some reflected light (from diffuse or specular reflections) is monitored, and that will exhibit a power modulation with the same frequency. The phase of that modulation is then compared with that of the sent light. The phase shift obtained is 2($\pi$) times the time of flight times the modulation frequency. This shows that higher modulation frequencies can result in a higher spatial resolution.

Although the phase shift is directly proportional to the time of flight, the term time-of-flight method should be reserved to cases where one really measures a delay time more directly, e.g. with light pulses.

As for an optical interferometer, the phase shift method involves an ambiguity regarding the measured distance because with increasing distance the phase will vary periodically. However, the periodicity is much larger than in an interferometer, since the modulation frequency is much lower than the optical frequency. Also, the ambiguity can easily be removed, e.g. by using measurement results with different modulation frequencies.

Compared with interferometers, devices based on the phase shift technique are less accurate, but they allow unambiguous measurements over larger distances. Also, they are suitable for targets with diffuse reflection (scattering) from a rough surface.

Different Realizations

The power modulation may be obtained with an electro-optic modulator, acting on a continuous-wave laser beam. Modulation frequencies of many megahertz or even multiple gigahertz are easily obtained. A resonant type of modulator can be operated with relatively low input voltage, but only with a small range of modulation frequencies, making the removal of the mentioned ambiguity more difficult.

Some kinds of lasers, particularly laser diodes, can be modulated directly via the drive current at frequencies in the megahertz domain or even higher.

A special kind of power modulation is achieved by using a mode-locked laser. Advantages are the high modulation frequency (allowing for high accuracy) and (for passive mode locking) that no optical modulator is required.

The use of a laser beam allows one to realize a laser radar, where an image is formed by scanning the laser beam direction in two dimensions. However, imaging systems can also be made with one or several current-modulated light-emitting diodes (LEDs) illuminating the whole object area. The spatial resolution is then obtained via imaging detection. There are photonic integrated circuits with two-dimensional sensor arrays, being able to measure the phase shift for each pixel.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is the phase shift method for distance measurement?

The phase shift method is a technique used in devices like laser rangefinders for measuring distances. It involves sending out a laser beam with a sinusoidally modulated optical power and then measuring the phase shift of the modulation in the light reflected from a target.

What are the main advantages of the phase shift method compared to interferometry?

Compared to optical interferometry, the phase shift method allows for unambiguous measurements over larger distances and is suitable for targets with rough, diffusely scattering surfaces, although it is generally less accurate.

How is the ambiguity in distance measurement handled in the phase shift method?

The measured phase varies periodically with distance, which creates an ambiguity. This ambiguity can be removed by performing measurements with several different modulation frequencies.

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