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INCREASED ACCURACY OF DISTANCE MEASUREMENT OF SELF-MIXING WITH FREQUENCY CHIRPING

IP.com Disclosure Number: IPCOM000241473D
Publication Date: 2015-May-04
Document File: 5 page(s) / 301K

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The IP.com Prior Art Database

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2014ID04153

Increased accuracy of distance measurement of self-mixing with frequency chirping

    With self-mixing the distance towards an object can be measured [1]. Due to non- linear effects a linear current ramp is translated into a non-linear frequency ramp. This non- linearity limits the accuracy of the distance measurement. By applying a compensated current ramp the frequency ramp can be made linear and an increased distance resolution.

A semiconductor laser, a VCSEL or edge emitter, is used as a light source, with a lens the light is focussed onto an object at constant distance. A laser driver supplies the laser with a DC + sawtooth current modulation. Some of the scattered light of the object ends back-up in the laser and will interfere. The intensity in the laser will change depending on the phase of the back scattered light, this interference modulation is called the self-mixing signal. In case a linear frequency is applied to the light the self-mixing frequency should be a constant frequency depending on the distance object.

Distance calculation

Figure 1: set-up

    The modulation of the laser light is detected with a photodiode. The signal is after sufficient amplification sampled and processed to extract the frequency.

When a saw tooth modulation is applied to the laser current, the self-mixing is sampled, via a fft the frequency of the self-mixing signal can be determined. In figure 2 the signal is divided up into small parts and by means of these parts the frequency is determined. Apart from some outliers, a sawtooth structure is visible in the frequency content. This indicates a deviation from a linear frequency ramp as in that case the frequency would be constant. Due to this spreading out over frequency of the signal the distance accuracy is impaired.

Furthermore it is visible that the deviation from constant frequency seems to be a linear variation. Leading to the assumption that using a quadratic correction to the current ramp, could help reduce the frequency spread.

Photo detector

Laser driver

object

lens

laser


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frequency (arb.un.)

Time (arb.un.)

Figure 2: frequency of self-mixing signal, during a saw tooth modulation

    In figure 3 and figure 4, the frequency spectra is shown obtained after doing a fourier transform on samples taking from one flank of the saw tooth modulation. In figure 3 a linear current ramp is used in figure 4 the current ramp is compensated wi...