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

Abstract

We experimentally demonstrate that a femtosecond frequency comb laser can be applied as a tool for longdistance 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 m (4 ϫ 10 −8 at 50 m). © 2009 Optical Society of America OCIS codes: 320.7100, 320.2250, 320.1590 Traditional techniques for long-distance measurements are often based on optical interferometry when the demands on accuracy rise. Most of these interferometric techniques rely on incremental measurements of phase accumulation. A priori knowledge of the distance to be measured is required or a complex multiwavelength system may be needed. In 2004, Ye [1] proposed a simple scheme for measuring long distances in space with a stabilized femtosecond frequency comb. The scheme is based on a Michelsontype interferometry with optical interference between individual pulses. The technique proposed by Ye has been demonstrated for interferometric measurement of short displacement [2,3]. The main advantage of applying a frequency comb for distance measurement is the large range of nonambiguity, which is determined by the cavity length of the pulsed laser, ranging from about 30 cm to 3 m. It is thus not necessary to rely on incremental measurement of the optical phase. The ambiguity is easily overcome by, e.g., a laser distance meter. The stabilized frequency comb has been applied as a source in various distance measurement schemes [4,5]. In this Letter, we demonstrate distance measurements of up to 50 m in air by analyzing the cross correlation between pulses emitted from a stabilized frequency comb source. We have implemented a model of pulse propagation in air to account for the effect of air dispersion on the measured cross-correlation functions. The measurement results obtained with the frequency comb and a conventional counting laser interferometer are compared.A mode-locked Ti:sapphire laser is the frequency comb source, with both the repetition frequency and the carrier-envelop offset (CEO) frequency referenced to a cesium atomic clock (Fig. 1). The pulse duration is 40 fs, and the repetition rate f r is locked at approximately 1 GHz, corresponding to a pulse to pulse distance l pp = c / ͑n g f r ͒ of 30 cm. Here c is the speed of light in vacuum and n g is the group refractive index at the center wavelength. The CEO frequency f 0 is fixed at 180 MHz. The center wavelength of the pulses is 815 nm, with an FWHM of about 20 nm. After collimation the beam is sent to a Michelson interferometer. One part of the beam is reflected by a hollow corner cube mounted on a piezoelectric transducer (PZT) along the short reference arm. The length of the short arm can be scanned by a translation stage. The other part of the beam is reflected by two mirrors and propagates along...

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