Abstract:We demonstrate an optical distance sensor integrated on a silicon photonic chip with a footprint of well below 1 mm 2 . The integrated system comprises a heterodyne receiver structure with tunable power splitting ratio and on-chip photodetectors. The functionality of the device is demonstrated in a synthetic-wavelength interferometry experiment using frequency combs as optical sources. We obtain accurate and fast distance measurements with an unambiguity range of 3.75 mm, a root-mean-square error of 3.4 µm and acquisition times of 14 µs.

We demonstrate fast high-precision non-contact distance measurements to technical surfaces using a pair of dual-color electro-optic frequency combs for synthetic-wavelength interferometry (SWI). The dual-color combs are generated from continuous-wave (cw) lasers at 1300 nm and 1550 nm, which are jointly fed to a pair of high-speed dual-drive Mach-Zehnder modulators. The dual-color approach is used for continuous and dead-zone-free compensation of temperature-induced fiber drift. We achieve standard deviations below 2 µm at an acquisition time of 9.1 µs for measurements through 7 m of single-mode fiber. Despite the technical simplicity of our scheme, our concept can well compete with other comb-based distance metrology approaches, and it can maintain its accuracy even under industrial operating conditions. The viability of the concept is demonstrated by attaching the fiber-coupled sensor head to an industrial coordinate measuring machine for acquisition of surface profiles of various technical samples. Exploiting real-time signal processing along with continuous fiber drift compensation, we demonstrate the acquisition of point clouds of up to 5 million data points during continuous movement of the sensor head. The paper is structured as follows: Section 2 provides details on the experimental setup and the comb-based distance measurement principle. Section 3 is dedicated to an in-depth characterization of the system performance and to a comparison with competing concepts. In Vol. 26, No. 26 | 24 Dec 2018 | OPTICS EXPRESS 34306 Section 4, we give a detailed description of our experimental demonstrations. The appendices A-F give mathematical details of the multi-heterodyne detection scheme and of the impact of noise on the measurement accuracy. Experimental setup and measurement principle Experimental setup of measurement systemA schematic of the measurement system is depicted in Fig. 1. The optical setup is entirely based on fiber-coupled, commercially available telecom-grade equipment. The light from two cw lasers with wavelengths of cal 1300nm   and obj 1550 nm   and power levels of 15 dBm and 18 dBm respectively is split and combined by fiber couplers, feeding two Mach-Zehnder modulators (MZM1 and MZM2) for frequency comb generation. The light entering MZM2 is additionally frequency-shifted by a pair of acousto-optical modulators (AOM). The carrier at wavelength cal  is shifted by 80 MHz, the carrier at obj  by 55 MHz. The lithium-niobate MZM are driven by sinusoidal electrical signals with frequencies of 39.957 GHz for MZM1 and 40.000 GHz for MZM2. Both signal generators are referenced to a common clock signal (not depicted). The phase-modulated light shows broadband frequency comb spectra with line spacings that are precisely defined by the respective driving frequencies. By adjusting the bias voltage, the relative phase and the amplitudes of the driving signals between both arms of the modulator, spectrally flat frequency combs can be obtained [28,29].The measured spectra are depicted in Fig. 1, Inset...

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We demonstrate a synthetic-wavelength interferometry system on a silicon photonic chip, comprising an interferometer with tunable power splitting ratio and photodetectors. The system enables distance measurements with errors below 5 µm and acquisition times of 14 µs. IntroductionHigh-precision distance sensors are of great importance for a wide range of applications such as inline inspection of mechanical parts or precise referencing of translation axes. These sensors need to be compact, robust, and capable of providing absolute distance measurements with micrometer accuracy and short acquisition times. So far, miniaturization of optical sensors has mainly been achieved by emulating conventional macroscopic optical configurations using discrete micro-optical components such as microlenses and micro-optical benches [1,2]. These schemes, however, require costly assembly and highly precise alignment of a multitude of components and are therefore not well suited for mass production. At the same time, photonic integrated circuits (PIC) have gained considerable maturity, and silicon photonics is considered a particularly attractive platform, lending itself to largescale photonic-electronic integration on the basis of mature high-yield CMOS processes that are offered by widely available foundry services [3]. Photonic integration is currently mainly driven by data-and telecommunication applications such as high-speed optical interconnects, but it is indisputable that there is also large application potential for PIC in optical sensing and metrology. However, while early demonstrations of PIC interferometers made use of GaAs/AlGaAs and Lithium-Niobate (LiNbO 3 ) as integration platform [4][5][6], there are only very few examples [7,8] where large-scale silicon photonic integration has been exploited for applications in optical metrology. In particular, an integrated silicon photonic distance measurement system has not yet been demonstrated. Here we show that silicon photonics is also a viable platform for high-precision distance metrology. We demonstrate a silicon PIC for distance measurement by synthetic-wavelength interferometry. The PIC is fed by externally generated frequency combs (FC) and comprises passive components such as beam splitters and waveguides as well as phase shifters and photodetectors. Using acquisition times of only 14 µs, our first proof-of-principle demonstrator already permits measurements with standard deviations of less than 5 µm. The PIC relies entirely on the standard device portfolio offered by a silicon photonic foundry and does not require any customized components or fabrication steps. Measurement principle, experimental setup and chip layoutThe distance measurement system is based on synthetic-wavelength interferometry and multi-heterodyne detection using two FC with slightly detuned center frequencies and line spacings, see Fig. 1(a). One frequency comb (FC 2) is split into two parts, one of which is directly fed to the reference photodiode (PD) whereas the other propagates through the fr...