High Precision Laser Range Finder

(NLC Special Projects)

Expected Performance:

Principal of operation

The range finder operates by modulating a laser with RF. The RF phase of the laser reflected from the target is compared with the driving RF phase. The RF frequency is swept, and the phase Vs frequency is measured to derive the distance to the target. A pulsed laser is used to allow the same circuitry to measure the out-going and returning RF phase, eliminating systematic due to component variations and spurious optical reflections.

Remote steering of the transmitter head is used to allow ranging to multiple targets. A corner cube must be mounted on each target to provide a return signal.

Possible Applications:

The immediate application for the low resolution system is to measure the length of the ATF ring. A measurement long term stability of about 500 microns is required.

Other system variants could be used for general purpose surveying and alignment.

Range finder schematic. CanvasFormat.cv5

System Operation: The frequency of the synthesizer is swept while the mixer output for the forward signal from the laser and return signal from the target   are measured. Note that these signals occur at different times. The long (200 Meter) fiber delay causes the output phase of the forward signal to go through a zero every approximately 1MHz. This allows an accurate estimate of the output phase from the laser at the frequencies where the target phase is zero. Measuring the differences between the zero's of the target return phase (corrected for the laser output phase changes) allows a measurement of the target distance.

NOTE: A commercial unit by Lecia is avaialble with approximately 50uM resolution, 35M range, at a cost of approximately $200K.

Links to: the NLC Home Page,  the SLAC and KEK Institutional Home Pages.

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Page by:  Josef Frisch