M. Cui

High-accuracy long-distance measurements in air with a frequency comb laser

We experimentally demonstrate that a femtosecond frequency comb laser can be applied as a tool for long-distance measurement in air. Our method is based on the measurement of cross correlation between individual pulses in a Michelson interferometer. From the position of the correlation functions, distances of up to 50 m have been measured. We have compared this measurement to a counting laser interferometer, showing an agreement with the measured distance within 2 microm (4x10(-8) at 50 m).

Long distance measurement with femtosecond pulses using a dispersive interferometer.

We experimentally demonstrate long distance measurements with a femtosecond frequency comb laser using dispersive interferometry. The distance is derived from the unwrapped spectral phase of the dispersed interferometer output and the repetition frequency of the laser. For an interferometer length of 50 m this approach has been compared to an independent phase counting laser interferometer. The obtained mutual agreement is better than 1.5 μm (3×10(-8)), with a statistical averaging of less than 200 nm. Our experiments demonstrate that dispersive interferometry with a frequency comb laser is a powerful method for accurate and non-incremental measurement of long distances.

Time-frequency distribution of interferograms from a frequency comb in dispersive media

We investigate general properties of the interferograms from a frequency comb laser in a non-linear dispersive medium. The focus is on interferograms at large delay distances and in particular on their broadening, the fringe formation and shape. It is observed that at large delay distances the interferograms spread linearly and that its shape is determined by the source spectral profile. It is also shown that each intensity point of the interferogram is formed by the contribution of one dominant stationary frequency. This stationary frequency is seen to vary as a function of the path length difference even within the interferogram. We also show that the contributing stationary frequency remains constant if the evolution of a particular fringe is followed in the successive interferograms found periodically at different path length differences. This can be used to measure very large distances in dispersive media.

Long distance measurement with sub-micrometer accuracy using a frequency comb laser

The invention of the femtosecond frequency comb (FC) laser has revolutionized the field of high-resolution spectroscopy, by providing very accurate reference frequencies in the optical domain, acting as a ‘frequency ruler’. Similarly, a frequency comb can be viewed as a ruler for distance measurement, which is based on the fact that the vacuum distance between subsequent pulses is known with the accuracy of the used time standard. We have recently demonstrated absolute distance measurements using a FC laser applying a cross-correlation technique [1], which was supported by a theoretical and a numerical study on the formation of cross-correlation in dispersive media [2,3].

Distance measurement in air with a femtosecond frequency comb

A femtosecond frequency comb is a powerful tool in a wide range of metrological applications, one of them being in the field of distance measurement. Due to the locking of the repetition rate of the laser to a time standard, the distance between successive pulses is accurately known, providing direct traceability to the SI definition of the meter. Since the interpulse distance is typically of the order of 1 m, the range of non-ambiguity is large. Such accuracy is easily obtained with other methods like time-of-flight measurements. Several schemes for distance measurement have been proposed and demonstrated [1–4]. In this paper we report on measurement of distances up to 20 m in air, that have been acquired using a scheme based on a Michelson interferometer. The interferometer consists of a long measurement arm and a short reference arm that can be adjusted such that the path length difference between both arms is a multiple of the interpulse distance. Once this is the case, a cross-correlation function can be measured by scanning the reflector of the reference arm with a piezo element. The path-length difference between both arms is then determined from the the center of the cross-correlation function and the interpulse distance, taking into account the refractive index of air.

Long distance measurement with a femtosecond laser based frequency comb

Recent advances in the field of ultra-short pulse lasers have led to the development of reliable sources of carrier envelope phase stabilized femtosecond pulses. The pulse train generated by such a source has a frequency spectrum that consists of discrete, regularly spaced lines known as a frequency comb. In this case both the frequency repetition and the carrier-envelope-offset frequency are referenced to a frequency standard, like an atomic clock. As a result the accuracy of the frequency standard is transferred to the optical domain, with the frequency comb as transfer oscillator. These unique properties allow the frequency comb to be applied as a versatile tool, not only for time and frequency metrology, but also in fundamental physics, high-precision spectroscopy, and laser noise characterization. The pulse-to-pulse phase relationship of the light emitted by the frequency comb has opened up new directions for long range highly accurate distance measurement.

New Laser System for Distance Metrology- High Accuracy Long Distance Measurements with a Frequency Comb Laser

We experimentally demonstrate that the mode-locked femtosecond pulse laser is a promising tool for long distance metrology. Distances up to 50m in air were measured and compared to a conventional counting laser interferometer. The measurements were in agreement within 1 μm.

Correlation Functions Formed by a Femtosecond Pulse Interferometer

We experimentally demonstrate that a stabilized femtosecond frequency comb can be applied as a tool for distance measurement. The scheme is based on optical interference between individual pulses in a Michelson type interferometer. The cross-correlation functions between individual pulses with a distance of around 15 meter and 30 meter are observed and analysed.