Frequency-comb-referenced two-wavelength source for absolute distance measurement

Abstract

We propose a new tunable laser source concept for multiple-wavelength interferometry, offering an unprecedented large choice of synthetic wavelengths with a relative uncertainty better than 10 −11 in vacuum. Two lasers are frequency stabilized over a wide range of frequency intervals defined by the frequency comb generated by a mode-locked fiber laser. In addition, we present experimental results demonstrating the generation of a 90 m synthetic wavelength calibrated with an accuracy better than 0.2 parts in 10 6 . With this synthetic wavelength we can resolve one optical wavelength, which opens the way to absolute distance measurement with nanometer accuracy. Two-wavelength interferometry allows absolute distance measurements 1,2 over a range defined by the synthetic wavelength 1 ⌳ = 1 2 / ͑ 1 − 2 ͒ = c / ⌬, where 1 and 2 are the individual wavelengths. Starting from a long synthetic wavelength (e.g., 1 m), a nanometer-level accuracy can be ultimately achieved by gradually decreasing ⌳ until single-wavelength interferometry is performed. This requires the capability to resolve an optical wavelength; thus the accuracy obtained with the smallest synthetic wavelength must be better than / 4. The measurement accuracy at large distances may be limited by the instability and uncertainty of the synthetic and optical wavelengths. We propose a new concept of a tunable twowavelength laser source stabilized with high accuracy and over large frequency ranges that replaces the previously used frequency comb of a Fabry-Perot resonator 3 with the highly accurate frequency comb of a femtosecond (fs) laser. The use of an optical frequency comb has been recently proposed for gauge block length calibration. 4 However, the reported detection technique is based on consecutive measurements at different wavelengths and therefore requires high mechanical stability during the measurement time. We present in this Letter a tunable two-wavelength source with which dynamic distance measurements with nanometer accuracy can be achieved by means of superheterodyne interferometry. 4 An active frequency comb is based on a fs modelocked laser, 5 whose repetition rate, f rep , defines exactly the frequency separation between two adjacent modes of its frequency spectrum. Several cw lasers can be locked to different modes of the comb by beat frequency measurements and electronic phase locked loops. The stability of the laser frequency separation is determined entirely by the relative stability of the frequency reference used to control the repetition rate of the fs laser. To cancel the frequency drift of the comb, either the comb can be self-referenced 6 or one of the lasers can be locked onto a molecular transition and the comb locked to that laser through control of the comb offset.The tunable two-wavelength source (Fig. 1) consists of a Nd:YAG laser ( 1 = 1.319 m, Lightwave Model 125), an external cavity laser diode (ECLD, 2 ϳ 1.3 m, Thorlabs INTUN 1300), and finally a mode-locked fiber laser (Menlo Systems TC-1500). A 10 MHz frequency reference ...

Absolute distance measurements using two-mode laser telemetry

“…By comparison, the superheterodyne interferometry method [4] has recently reached an accuracy of 160nm, but at the expense of a considerably more complex set-up [5].…”

Phase-Referenced Interferometry and Narrow-Angle Astrometry With Susi

“…Conventionally, two laser wavelengths are used to obtain a long synthetic optical wavelength by means of heterodyne interferometry (Daendliker et al, 1988;Schuhler et al, 2006) and extend the NAR of a single wavelength metrology. However, instead of using the heterodyne detection technique which requires specialized optical elements and hence has higher cost, the dual-wavelength metrology described here employs a simple homodyne fringe counting detection scheme together with a (relatively) less precise stepper motor open-loop position control system to extend the range of distance the metrology can accurately measure.…”

Advanced Optical Metrology Using Ultrashort Pulse Lasers

“…4,9,10) The ultrashort pulse laser provides an optical frequency comb, which is used as the wavelength ruler after being stabilized to an Rb clock of frequency standard. A temporal scheme of multi-wavelength interferometry is then demonstrated by tuning an external cavity laser diode consecutively to the pre-selected light modes of the optical comb.…”