Large-Scale Absolute Distance Ranging Through Comb-Based Nonlinear Asynchronous Optical Sampling

Large-scale absolute distance ranging,with high resolution and rapid update rate,has always been attractive for metrology and industry.As developing requirements of scientific researches and manufacturing cannot be satisfied with laser interferometers which are based on incremental measurement,absolute distance ranging with a resolution of better than 1 ?m and an update rate of less than 1 s will benefit fundamental scientific researches and high-end equipment manufacturing undoubtedly.Traditional absolute distance ranging can be categorized by two methods: synthetic wavelength interferometry and time-of-flight measurement.As an estimated distance and several wavelengths serve as the prerequisites for the establishment of an effective synthetic wavelength chain,the synthetic wavelength interferometry is not suitable for rapid measurement over an unknown scale.On the contrary,time-of-flight measurement satisfies this condition.However,limited by the best available electric instruments,a resolution of several picoseconds,corresponding to a distance of ~1 mm,is the nominal limit for this scheme.To overcome this limitation,an incoherent time-of-flight scheme based on asynchronous optical sampling and nonlinear effects is proposed to enhance the time resolution to better than 1 fs,corresponding to a distance of less than 300 nm,which satisfies the requirements of large-scale absolute distance ranging.The absolute distance ranging system is built using the optical frequency comb.With the difference in repetition rates,asynchronous optical sampling is conducted to scan pulses step by step.The overlap is quantified with type II second harmonic generation.The maximum represents entire overlap,so that it is used to illustrate temporal positions of light pulses.With optical sampling,the limitation of traditional time-of-flight method is overcame,and a time resolution of better than 1 fs is achieved.Moreover,the inherent non-ambiguity range,limited by the repetition rate,is extended using coupled pulse trains with different repetition rates.Simultaneous measurement with two different repetition rates,instead of sequential measurement,makes integer determination immune to target drifts during the change of the repetition rate and guarantees reliable non-ambiguity range extension.As the proposed scheme is based on nonlinear effects,laser power is saved using electric reference to substantiate optical reference.Thus,laser power of reference arm can be diverted to measurement arm and promises a longer range.As for measurement in open air,refractive index is measured with simultaneous distance ranging using fundamental pulses and second harmonic pulses.The dual-color method completes refractive index compensation in approximate dry air.Temporal positions of fundamental pulses are determined through second harmonic generation,while those of second harmonic pulses are determined through sum frequency generation.A refractive index compensation of 10-6 is achieved over a distance of 12.3 m.For extremely large-scale applications in universe,an active laser ranging scheme is proposed using the frequency comb.Pulses from two ends where distance is measured are sent mutually to serve as time tags for time-of-flight absolute distance ranging.Meanwhile,frequency transfer is conducted between the two ends by observing time intervals among second harmonic pulses.The frequency transfer is designed to correct time error introduced by two clocks used at the two ends and ensures a common time standard for the entire time-of-flight system.Results show that residuals are less than 400 nm over 23 m.For frequency transfer,the Allan deviation for 1 s is 1.3×10-16,ensuring accurate synchronization between the two ends.