Rapid and precise absolute distance …

1 Rapid and precise absolute distance measurementsat long rangeI. Coddington*, , *The ability to determine absolute distance to an object is one of the most basic measurements of remote sensing. High-precision ranging has important applications in both large-scale manufacturing and in future tight formation-flying satellitemissions, where Rapid and precise measurements of absolute distance are critical for maintaining the relative pointing andposition of the individual satellites. Using two coherent broadband fibre-laser frequency comb sources, we demonstrate acoherent laser ranging system that combines the advantages of time-of-flight and interferometric approaches to provideabsolute distance measurements, simultaneously from multiple reflectors, and at low power.

2 The pulse time-of-flight yieldsa precision of Through the optical carrier phase, the precision isimproved to better than 5 nm at 60 ms, and through the radio-frequency phase the ambiguity range is extended to 30 km,potentially providing 2 parts in 1013ranging at long satellites flying in a precision formation caneffectively act as a single distributed instrument andprovide entirely new capabilities for space-based would enable higher-resolution searches for extraterres-trial planets by providing a large synthetic aperture, enable directimaging of a black hole by supporting an X-ray telescope distributedacross satellites.

3 Or enable tests of general relativity through accuratemeasurements of satellite spacing in a gravitational field1 11. Theformation acts as a single instrument only if the relative spacingand pointing of the satellites is tightly maintained, which is madepossible by comparing distance measurements between multiplereference points on the satellites and feeding back to the satelliteposition and intrasatellite ranging, and similarly in manufacturing appli-cations12, there are three critical parameters: precision/accuracy,ambiguity range and update rate.

4 High precision is particularlyimportant in maintaining the pointing; for example, coherent com-bining of 1-m sub-apertures to form a synthetic aperture of 100-mdiameter requires a relative pointing accuracy of less than(l/100 m) rad for each sub-aperture, which in turn requires dis-tance measurements at the sub-aperture edges with less thanl (1 m/100 m) accuracy, or a few nanometres at optical wave-lengths. The ambiguity range characterizes the measurementrange window; longer distances are aliased back to within the ambi-guity range.

5 Larger ambiguity range requires lessa prioridistanceknowledge. Finally, fast millisecond-scale update rates are neededfor effective feedback. Many of these requirements push or exceedthe capabilities of current stand off ranging technology, but areachievable using an optical frequency comb, as shown speaking, laser ranging is the determination of thephase shift on a signal after traversing a given distance . Crudely,shorter-wavelength signals offer greater resolution, and longer-wavelength signals offer greater ambiguity range.

6 For instance, thewidely used continuous-wave ( ) laser interferometer measuresthe phase of optical wavelengths to achieve sub-nanometre resol-ution13 15. However, measurements are limited to relative rangechanges as the ambiguity range equals half the laser , laser radar (LIDAR) measures distance through pulsedor radio-frequency (rf)-modulated waveforms. (For pulsed systems,one simply measures the time-of-flight.) These systems offer largeambiguity ranges but with 50 100mm resolution12,16 interferometry (MWI) combines measure-ments at several optical wavelengths, which effectively generates alonger synthetic wavelength , and therefore a reasonable ambiguityrange while maintaining sub-wavelength resolution19 26.

7 However,these systems are vulnerable to systematic errors from spuriousreflections, and extending the ambiguity range beyond a millimetrecan require slow scanning. Nevertheless, with extensive care inminimizing spurious reflections, the MSTAR27system has success-fully used MWI for sub-micrometre optical frequency combs offer an intriguing sol-ution to the intrasatellite ranging problem28,29. From the earlywork by Minoshima and colleagues18, combs have been incorpor-ated into precision ranging systems using the various approachesdiscussed above23 26,30 33.

8 In particular, the comb output has beenused directly in several experiments to take advantage of its coher-ence in both the rf and optical domains27,33,34. Building on thisearlier work, we demonstrate here a comb-based coherent LIDAR that provides a unique combination of precision, speed and largeambiguity this work the pulsed nature of a comb iscombined with the coherence of the carrier, allowing for a time-of-flight measurement simultaneously with an interferometricmeasurement based on carrier phase30,33,34.

9 We implement thisapproach with dual coherent frequency combs and achieve ananometre level of precision with an ambiguity range of m in60 ms at low light levels and with high immunity to spuriousreflections. The ambiguity range is easily extended to 30 , the time-resolved signal also permits measurementsbetween multiple reference planes in a single beam path. Thishost of features is unavailable in any other single approach follows the footprint of MSTAR as well as relatedspectroscopy work35 40, in that we use of a pair of stabilized femto-second laser frequency combs having pulse trains of slightly differ-ent repetition periods (TrandTr DTr).

10 In Fig. 1, we focus on thetime-domain picture. One comb serves as the signal source andNational Institute of Standards and Technology, 325 Broadway 815 Boulder, Colorado 80305, USA.*e-mail: ONLINE: 24 MAY 2009 |DOI: PHOTONICS| VOL 3 | JUNE 2009 | 2009 Macmillan Publishers Limited. All rights reserved. samples a distance path defined by reflections off a target and refer-ence plane. The second comb serves as a broadband local oscillator(LO), and recovers range information in an approach equivalent tolinear optical sampling41,42(that is, a heterodyne cross-correlationbetween the signal and LO).