A recirculating frequency-shifting loop (FSL) provides a highly flexible platform for generating coherent optically chirped light with tunable bandwidth, duration, chirp rate and repetition rate. The properties of the chirped light are controlled using low-frequency sinusoidal electronic drive signals, enabling deterministic waveform synthesis without complex high-speed electronics. We achieve chirp bandwidths of 10 GHz with a duration in the nanosecond regime, representing one of the fastest tunable laser sources to date. Using such chirped laser pulses, we demonstrate coherent FMCW LiDAR measurements over distances up to 3 m, highlighting the potential of FSL-based sources for compact, scalable and high-performance ranging systems.

Multi-wavelength digital holography enables surface-shape measurements with an exceptional dynamic range by combining interferometric resolution with synthetic wavelengths spanning multiple length scales. Although the concept promises measurement ranges of many orders of magnitude, its practical implementation is limited by the lack of light sources that allow fast, reliable, and calibration-free switching between synthetic wavelengths over a wide frequency range. Here, we present a synthetic-wavelength generator based on an electro-optic frequency comb with electronically tunable modulation frequency and a set of switchable band-pass filters. By combining discrete selection of comb-lines with continuous radio-frequency tuning, the proposed scheme merges the advantages of single-sideband modulation and filter-based comb extraction. Using only off-the-shelf components, the system provides synthetic frequencies from 0.1-220GHz, corresponding to synthetic wavelengths from meters down to millimeters in the visible. The generator achieves MHz-level frequency accuracy, side-mode suppression exceeding 40dB, and switching times below 25ms, even without active stabilization. We characterize the spectral purity and frequency agility of the source and demonstrate rapid tuning of synthetic wavelengths over 3 orders of magnitude. We apply the light source to multi-wavelength digital holography and reconstruct the surface of an industrially machined metal part featuring height variations from 0.1-100mm. The measurements achieve ten-mum-level precision using 7 single wavelengths covering synthetic wavelengths from 1.36mm to 1.874m within an acquisition time < 2s. The presented architecture combines high dynamic measurement range of 50dB, fast electronic reconfigurability, and intrinsic frequency calibration, making it a promising light source for high-speed interferometric surface metrology.

Adiabatic frequency conversion enables fast and efficient tuning of laser light by coupling it into an optical resonator whose eigenfrequency is varied on a timescale shorter than its photon lifetime. In this regime, the optical frequency follows the cavity resonance, allowing frequency shifts of several hundred gigahertz within sub-microsecond time - independent of optical power and without phase-matching constraints. While a linear dependence of the cavity resonance on a control parameter (e.g., applied voltage) suggests that arbitrary temporal signals could be linearly transferred to optical frequency changes, we show that this assumption fails near mechanical resonances of the resonator. Using a millimeter-sized lithium niobate whispering gallery resonator with a pronounced mechanical mode at a center frequency of 10.5 MHz, we observe strong deviations from linearity even when higher harmonics of the control signal coincide with this resonance. The experimental results are in excellent agreement with theoretical predictions. They demonstrate that mechanical resonances impose intrinsic limits on the linearity of adiabatic frequency conversion and other frequency control schemes based on the variation of the eigenfrequency of an optical cavity.

Interferometric shape measurement is inherently limited by its unambiguity range, which is directly determined by the wavelength. Multi-wavelength interferometry extends the unambiguity range, allowing it to better address individual samples. However, deploying such systems outside the laboratory remains difficult. Tunable or individually stabilized lasers are prone to long-term wavelength drift, which makes additional calibration equipment necessary. In this work, we demonstrate a robust light source that requires no wavelength calibration and enables flexible generation of synthetic wavelengths-from meter-scale down to 15 mm using a single non-stabilized laser in combination with single-sideband modulation. Importantly, the entire setup is built exclusively from commercially available, fiber-coupled components, many of which are field-proven in the telecommunications industry. This ensures high reliability, robustness, and ease of integration into industrial environments. As a proof of concept, we reconstruct the surface profiles of two samples made of entirely different materials and geometries, featuring height variations from 1 to 120 mm. For each sample, the synthetic wavelength is individually tailored between 375 and 15 mm. Our results show that all features are accurately resolved with the expected height-resolution. Furthermore, this resolution can be improved by an order of magnitude through the use of commercially available thin-film lithium niobate modulators. This makes the proposed scheme a highly practical solution for robust inline inspection systems across a wide range of industrial parts.

The emissions of two identical spontaneous parametric down-conversion (SPDC) sources pumped coherently, e.g. with the same beam (c.f. Fig. 1(a)), show interference in both, the signal and idler light. This phenomenon results from the indistinguishability of the photon pairs emitted by the identical sources [1]. If a medium between the two SPDC sources absorbs the idler light, the photon pairs from the first and second SPDC source become distinguishable and the interference contrast decreases for both, signal and idler light. By measuring the signal light spectrum in the visible or near-infrared (NIR) range and the corresponding interference contrast, we obtain the information about the idler light absorption of the analyte in the mid-infrared (MIR). Hereby, we enable spectroscopy in the MIR using a standard silicon detector, making detection easier and more efficient than using typical MIR detectors [2].

Changing the frequency of light is crucial for numerous technologies and can be achieved using various methods. Adiabatic frequency conversion (AFC) is a promising yet underexplored approach that enables nearly 100% conversion efficiency without requiring phase-matching, even at the single-photon level. In AFC, the light frequency is shifted by varying the optical length of a resonator, with intracavity light following eigenfrequency changes if these occur faster than the resonator's decay time [1]. Whispering gallery resonators (WGRs), with their ultra-high quality factors Q, efficient light confinement, and electro-optic tunability - particularly in lithium niobate (LN) - provide an ideal platform for implementing AFC. We previously demonstrated that AFC achieves highly linear frequency modulation of light in an FMCW-LiDAR setup [2].

Single-wavelength interferometry achieves high resolution for smooth surfaces but struggles with rough, industrially relevant ones due to limited unambiguous measuring range and speckle effects. Multi-wavelength interferometry addresses these challenges by using synthetic waveleths, enabling a balance between extended measurement range and resolution by combining several synthetic wavelengths. This approach holds immense potential for diverse industrial applications, yet it remains largely untapped due to the lack of suitable light sources. Existing solutions are constrained by limited flexibility in synthetic-wavelength generation and slow switching speeds. We demonstrate a light source for multi-wavelength interferometry based on electro-optic single-sideband modulation. It reliably generates synthetic wavelengths with arbitrary values from centimeters to meters and switching times below 30 ms. This breakthrough paves the way for dynamic, reconfigurable multi-wavelength interferometry capable of adapting to complex surfaces and operating efficiently even outside laboratory settings. These capabilities unlock the full potential of multi-wavelength interferometry, offering unprecedented flexibility and speed for industrial and technological applications.

Making the plus and minus c faces of a periodically-poled waveguide of lithium niobate conductive by suitable doping or by covering with films of sufficient conductivity may short-circuit light-induced space charge fields originating from the bulk photovoltaic effect. We estimate that this should be possible without sacrificing low propagation losses. Hence unwanted light-induced refractive index changes can be prevented, helping to increase the light powers up to which the devices can handle light and frequency conversion happens.

Barium magnesium fluoride (BMF) is a ferroelectric crystal with a transparency range far beyond the one of other optical materials. In particular, its low loss in the deep ultraviolet makes this material an unique candidate for frequency conversion in this spectral range. Due to its relatively weak second-order nonlinearity, a resonant configuration such as an optical whispering gallery would be beneficial. We show that femtosecond-laser based material processing enables the reliable fabrication of BMF whispering gallery resonators with quality factors beyond 10^7. Their resonance frequencies can be shifted linearly by applying electric fields between the +c and -c faces of the crystal. The slope of the shift is -0.8 MHz/(V/mm). It seems that the origin of this shift is piezoelectricity, while the electro-optic effect is negligible. Our results pave the way for millimeter-sized frequency converters in the deep ultraviolet. Furthermore, they indicate that a careful determination of fundamental material properties is still necessary.

Single-wavelength interferometry achieves high resolution for smooth surfaces but struggles with rough industrially relevant ones due to limited unambiguous measuring range and speckle effects. Multiwavelength interferometry addresses these challenges using synthetic wavelengths, enabling a balance between extended measurement range and resolution by combining several synthetic wavelengths. This approach holds immense potential for diverse industrial applications, yet it remains largely untapped due to the lack of suitable light sources. Existing solutions are constrained by limited flexibility in synthetic-wavelength generation and slow switching speeds. We demonstrate a light source for multiwavelength interferometry based on electro-optic single-sideband modulation. It reliably generates synthetic wavelengths with arbitrary values from centimeters to meters and switching time below 30 ms. This breakthrough paves the way for dynamic reconfigurable multiwavelength interferometry capable of adapting to complex surfaces and operating efficiently even outside laboratory settings. These capabilities unlock the full potential of multiwavelength interferometry, offering unprecedented flexibility and speed for industrial and technological applications.

Optical parametric oscillators (OPOs) emit narrow-linewidth light that is widely tunable in wavelength. In particular in the mid infrared, they are of prime interest because a single device can cover the entire wavelength range between 3 and 5 μm. Their realizations based on whispering gallery resonators (WGRs) provide a small footprint and ultra-low pump thresholds. We demonstrate millimeter-sized WGR based OPOs pumped with compact diode lasers operating at 1.55 μm wavelength. The parametric oscillation is tunable between 2.3 and 5.1 μm wavelength with milliwatt output powers. Tuning is achieved by changing the temperature of the resonator or by varying the pump wavelength. Operating the OPO at the point of degeneracy enables to generate THz-wide frequency combs centered around 3.1 μm wavelength. Our results show that whispering gallery resonators made of non-oxide materials can be the key component for realizing miniaturized wavelength-flexible sources for coherent mid infrared light.

Continuous tuning of the frequency of laser light serves as the fundamental basis for a myriad of applications spanning basic scientific research to industrial settings. These applications encompass endeavors such as the detection of gravitational waves, the development of precise optical clocks, environmental monitoring for health and ecological purposes, as well as distance measurement techniques. However, achieving a broad tuning range exceeding 100 GHz along with sub-microsecond tuning times, inherent linearity in tuning, and coherence lengths beyond 10 m presents significant challenges. Here, we demonstrate that electro-optically driven adiabatic frequency converters utilizing high-Q microresonators fabricated from lithium niobate possess the capability to convert arbitrary voltage signals into frequency chirps with temporal resolutions below 1 µs. The temporal evolution of the frequency correlates accurately with the applied voltage signal. We have achieved to generate 200-ns-long frequency chirps with deviations of less than 1 % from perfect linearity without requiring supplementary measures. The coefficient of determination is R 2 > 0.999 . Moreover, the coherence length of the emitted light exceeds 20 m. To validate these findings, we employ the linear frequency sweeps for Frequency-Modulated Continuous Wave (FMCW) LiDAR covering distances ranging from 0.5 to 10 m. Leveraging the demonstrated nanosecond-level tuning capabilities, coupled with the potential to tune the eigenfrequency of lithium-niobate-based resonators by several hundred GHz, our results show that electro-optically driven adiabatic frequency converters can be used in applications that require ultrafast and flexible continuous frequency tuning characterized by inherent linearity and substantial coherence length.

Multi-wavelength holography is a convenient method for the inline inspection of industrially machined parts. Compared with the single-wavelength approach, the unambiguity range can be extended by orders of magnitude. This is typically achieved by using multiple lasers. The smaller their wavelength difference, the larger is the unambiguity range. Reaching the centimeter or even meter range is difficult with individual lasers because of their relative wavelength drift. Here, we demonstrate multi-wavelength holography with 37.5 cm unambiguity range using only one single laser. The wavelength shifts are achieved with acousto-optic modulators driven at 200 MHz and 1 GHz. This provides unambiguity ranges of 37.5 and 7.5 cm respectively. Importantly, the perturbation caused by a possible long-term drift of the laser is significantly reduced. For a proof-of-concept demonstration, we determine the shape of a metallic object comprising height differences between 1 and 100 mm. The scheme can be extended to larger frequency shifts, i.e. better axial resolution, by using electro-optic modulators. This would enable to conveniently select the measurement range between some millimeters and meters although only one laser is used.