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.
Infrared laser spectroscopy allows the sensitive detection of gaseous substances. We present a novel gas sensing approach based on photothermal common-path interferometry that offers parts-per-billion sensitivity in microliter volumes: The thermal lens distortion pattern imprinted onto a near-infrared probe beam by the absorption of an infrared pump laser serves as signal for the sensing of low trace substance concentrations in the ppb range. Next to high specific sensitivity, additional focus lies on a tiny measurement volume and a compact overall setup. The advantages of this approach are discussed at the example of real-time measurements of breath N2O.
Nonlinear interferometers based on correlated photon pairs allow mid-infrared spectroscopy by measuring only near-infrared photons with a silicon-based detector, which offers higher detectivities than mid-infrared detectors. Here, we use the nonlinear interferometer not only to determine the transmission of a sample but also for spectral analysis – analogue to classical Fourier transform infrared (FTIR) spectrometers. The Quantum FTIR analyzes the absorption of gas mixtures with high spectral resolution over a broad mid-infrared wavelength range. Improved emission rates of the photon pair source yield sensitivities similar to a classical FTIR despite an extremely low light exposure on the sample.
Researchers around the world focus on the search for the perfect quantum system suitable for applications of ‘quantum technology 2.0’. Those range from quantum sensing, via communication to computing using qubits. Even though these applications are based on quantum effects, their diversity brings with them a variety of different requirements on the ideal quantum systems. A prominent class of quantum systems are defect centers in solid-state materials. They have been found to represent quantum systems that are precisely measurable and controllable, while being almost unaffected by their environment. Characterization of the different systems at various ambient conditions using photoluminescence excitation (PLE) spectroscopy techniques helps identifying the right quantum system for the respective target application. In this context we discuss the design and unique performance characteristics of widely tunable CW optical parametric oscillators as novel laser light sources for PLE.
Digital holography enables high-precision quality control in machining production and has already been introduced to several multi-axis systems1, 2. To meet the demanding measurement tasks in the quality control of complex components, accuracies in the sub-micrometer range with measurement ranges larger than several centimeters are required. Previous measurements have shown the potential of multiwavelength digital holography to allow unambiguous ranges of few millimeters3 . We present multiwavelength digital holographic measurements using synthetic wavelengths with two meters down to a few micrometers, potentially enabling measurements with meter-scale unambiguity at sub-micrometer accuracy. Measurements on a 10 cm step-height sample have been conducted using the compact digital-holographic sensor HoloTop NX for various multi-axis systems, supplied by an Ondax LMFC single frequency diode laser at 632.852 nm and the tunable laser Hübner C-Wave used in the wavelength range of 480.786 nm – 632.852 nm. The latter offers a frequency stability of 150 MHz on a time scale of several hours. The maximum laser drift during data acquisition was observed to be 0.02 pm. Thus, at the 2 m synthetic wavelength, this results in a maximum synthetic wavelength error of 200 mm. Random noise of 20 mm at the largest used synthetic wavelength of 2 m requires multiple synthetic wavelengths to get down to micrometer precision: Eight nested synthetic wavelengths from 2 µm to 2 m and numerical refocusing of the hologram were used to evaluate a milled sample with multiple step heights, machined on a Hermle C32U machine tool. Ten repetitive measurements confirm a machining uncertainty of 9 µm for this sample at its maximum step height of 10 cm.
KEYWORDS: Spectroscopy, Mid-IR, Signal to noise ratio, Time metrology, Optical parametric oscillators, Spectrometer engineering, Photodetectors, Modulation, Gases, Frequency combs
A dual frequency comb spectrometer is realized by electro-optic modulation of a 1550 nm laser and subsequent conversion to the mid-infrared by difference-frequency generation (DFG). Using an optical parametric oscillator for the DFG the combs can be tuned from 3 μm to 4.7 μm with 440 comb modes covering 220 GHz (< 6 cm-1). Trace gas detection of nitrous oxide, carbon dioxide and methane is demonstrated with a 7.2-m-multi-pass cell while a sufficiently low noise-equivalent absorbance is reached in already 1 s. The bandwidth normalized noise-equivalent-absorption coefficient is consistently below 2.8 × 10-6 Hz-1/2 cm-1 while the precision of the determined concentrations is better 2 % Hz-1/2.
Nonlinear interferometers based on non-degenerate spontaneous parametric down-conversion (SPDC) create a link between separate spectral ranges. This allows for measurements in remote spectral regions while detecting light in easily accessible wavelengths. In our work, we use periodically poled lithium niobate to create correlated signal (visible or near-infrared) and idler (mid-infrared) photon pairs. Using a nonlinear interferometer in Michelson geometry, we obtain broadband mid-infrared spectra from light detected with a silicon avalanche photodiode. Combining the nonlinear interferometer with a measurement scheme in close analogy to classical Fourier-transform infrared spectroscopy allows for sub-wavenumber spectral resolution, which opens up possibilities for applications such as precise spectroscopic gas analysis.
Mid-infrared spectroscopy is one of the most important techniques in chemical analysis. However, the detectors for the mid-infrared range suffer from lower specific detectivities in comparison to their visible counterparts, cost more and often require cryogenic cooling. Nonlinear interferometers allow measuring mid-infrared spectra by detecting only visible light using the induced coherence effect. In our work, we realize a nonlinear interferometer designed for broadband mid-infrared spectra with high resolution, which is easily tunable, and in analogy to classical Fourier transform infrared (FTIR) spectrometers requires no additional spectral selection.
The thermo-optic coefficient of lithium niobate (LiNbO3) has been measured in the temperature range from 10 to 160 °C
using an interferometric setup. Undoped and magnesium-doped congruently melting LiNbO3 and undoped stoichiometric
LiNbO3 were studied over a wide wavelength range in the visible and near infrared (450 – 600 nm and 900 – 1130 nm)
using a frequency-doubled cw optical parametric oscillator. Experimental results for congruently grown lithium niobate
were aggregated using a Schott equation to describe the wavelength and temperature dependence of the thermo-optic
coefficient.
We demonstrate the continuous-wave operation of a cascade that has been successfully applied so far only for
picosecond systems: A doubly-resonant optical-parametric oscillator (OPO) based on lithium niobate generates
signal and idler waves close to degeneracy. Subsequently, these two light fields are converted to a terahertz wave
via difference frequency mixing in an orientation-patterned gallium arsenide crystal placed inside the OPO cavity.
Using this scheme, we achieved tunability from 1 to 4:5 THz frequency, a linewidth smaller than 10 MHz, and
a Gaussian beam profile. The output power is of the order of tens of μW, with a scalability into the milliwatt
regime.
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