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We present the development and in-depth characterization of an optical reference based on a 1.5 µm laser stabilized to a cryogenic silicon optical resonator operated at 1.7 K. The closed-cycle cryostat is equipped with a cryogenic passive vibration isolation. At τ = 1 s integration time the frequency instability is 2 × 10-14, predominantly due to residual vibrations. At τ = 100 s the frequency instability is 6.2 × 10-15. The lowest instability of 3.5 × 10-16 occurs at τ = 6000 s, and is limited by the stability of the hydrogen maser used in the comparison. The mean fractional frequency drift rate over 190 days was -3.7 × 10-20/s. In conjunction with a frequency comb and a GNSS receiver this optical reference would be suitable to provide optical frequencies with accuracies at the low 10-14 level. We show that residual vibrations affect the resonator and the optical fiber delivering the laser light to it, and that laboratory temperature variations contribute to frequency instability at short and medium integration times. Mitigation of these issues might in the future allow for demonstration of the thermal-noise-limited performance of the resonator.
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The effects of scalar and pseudoscalar ultralight bosonic dark matter (UBDM) were searched for by comparing the frequency of a quartz oscillator to that of a hyperfine-structure transition in ^{87}Rb, and an electronic transition in ^{164}Dy. We constrain linear interactions between a scalar UBDM field and standard-model (SM) fields for an underlying UBDM particle mass in the range 1×10^{-17}-8.3×10^{-13} eV and quadratic interactions between a pseudoscalar UBDM field and SM fields in the range 5×10^{-18}-4.1×10^{-13} eV. Within regions of the respective ranges, our constraints on linear interactions significantly improve on results from previous, direct searches for oscillations in atomic parameters, while constraints on quadratic interactions surpass limits imposed by such direct searches as well as by astrophysical observations.
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We present a system for passive damping of vibrations along three spatial degrees of freedom for cryostats equipped with closed-cycle coolers. The system, designed to isolate a payload of 30 kg, consists of two stages of isolation for vibrations in the vertical direction. The first isolation stage incorporates a trapezoidal beryllium copper cantilever blade. The second stage is attached to the blade via a steel wire and consists of four extension springs with an extended length of 370 mm. At 1.6 K, the stages possess vertical resonance frequencies of 2.1 and 1.3 Hz, respectively. The vertical length of the setup with a cumulative length of 580 mm also acts as a pendulum with a resonance frequency of 0.65 Hz. In the frequency band from 5 to 200 Hz, the frequency-integrated acceleration decreased from 6.7 × 10-3 to 4.3 × 10-5 g along the horizontal direction and from 4.3 × 10-3 to 7.2 × 10-5 g along the vertical direction. This corresponds to a reduction in vibrations by factors of 156 and 60, respectively. Overall, we achieve a simple, robust, and cost-efficient vibration isolation system for upgrading standard-type cryostats.
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A system providing an optical frequency with instability comparable to that of a hydrogen maser is presented. It consists of a 5 cm long, vertically oriented silicon optical resonator operated at temperatures between 1.5 K and 3.6 K in a closed-cycle cryostat with a low-temperature Joule-Thomson stage. We show that with a standard cryostat, a simple cryogenic optomechanical setup, and no active or passive vibration isolation, a minimum frequency instability of 2.5 × 10-15 at τ = 1500 s integration time can be reached. The influence of pulse-tube vibrations was minimized by using a resonator designed for low acceleration sensitivity. With reduced optical laser power and interrogation duty cycle, an ultra-low fractional frequency drift of -2.6 × 10-19/s is reached. At 3.5 K, the resonator frequency exhibits a vanishing thermal sensitivity and an ultra-small temperature derivative 8.5 × 10-12/K2. These are favorable properties that should lead to high performance also in simpler cryostats not equipped with a Joule-Thomson stage.
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We demonstrate lensless imaging of three-dimensional phantoms of fluorescent nanodiamonds in solution. Magnetofluorescence imaging is employed, which relies on a dependence of the fluorescence yield on the magnetic field, and pervading the object with an inhomogeneous magnetic field. This field provides a field-free field line, which is rastered through the object. A 3D image of the object is obtained by imaging a set of 2D slices. Each 2D slice image is computed from a set of 1D projections, obtained under different projection directions, using a backprojection algorithm. Reconstructed images containing up to 36 × 36 × 8 voxels are obtained. A spatial resolution better than 2â mm is achieved in three dimensions. The approach has the potential for scalability.
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We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and frequency metrology with quantum cascade lasers (QCLs). We have implemented frequency stabilization of a QCL to an ultra-low expansion (ULE) reference cavity, via upconversion to the near-IR spectral range, at a level of 1×10(-13). The absolute frequency of the QCL is measured relative to a hydrogen maser, with instability <1×10(-13) and inaccuracy 5×10(-13), using a frequency comb phase stabilized to an independent ultra-stable laser. The QCL linewidth is determined to be 60 Hz, dominated by fiber noise. Active suppression of fiber noise could result in sub-10 Hz linewidth.
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We correct fit formulas from a previous paper [Opt. Lett.39, 3242 (2014)10.1364/OL39.005896OPLEDP0146-9592] for the coefficient of thermal expansion αreson(T).
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We present a compact and robust transportable ultra-stable laser system with minimum fractional frequency instability of 1 × 10(-15) at integration times between 1 and 10 s. The system was conceived as a prototype of a subsystem of a microwave-optical local oscillator to be used on the satellite mission Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) (http://sci.esa.int/ste-quest/). It was therefore designed to be compact, to sustain accelerations occurring during rocket launch, to exhibit low vibration sensitivity, and to reach a low frequency instability. Overall dimensions of the optical system are 40 cm × 20 cm × 30 cm. The acceleration sensitivities of the optical frequency in the three directions were measured to be 1.7 × 10(-11)/g, 8.0 × 10(-11)/g, and 3.9 × 10(-10)/g, and the absolute frequency instability was determined via a three-cornered hat measurement. Two additional cavity-stabilized lasers were used for this purpose, one of which had an instability σy < 4 × 10(-16) at 1 s integration time. The design is also appropriate and useful for terrestrial applications.
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We report on the demonstration and characterization of a silicon optical resonator for laser frequency stabilization, operating in the deep cryogenic regime at temperatures as low as 1.5 K. Robust operation was achieved, with absolute frequency drift less than 20 Hz over 1 h. This stability allowed sensitive measurements of the resonator thermal expansion coefficient (α). We found that α=4.6×10(-13) K(-1) at 1.6 K. At 16.8 K α vanishes, with a derivative equal to -6×10(-10) K(-2). The temperature of the resonator was stabilized to a level below 10 µK for averaging times longer than 20 s. The sensitivity of the resonator frequency to a variation of the laser power was also studied. The corresponding sensitivities and the expected Brownian noise indicate that this system should enable frequency stabilization of lasers at the low-10(-17) level.
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Integrated optics has distinct advantages for applications in space because it integrates many elements onto a monolithic, robust chip. As the development of different building blocks for integrated optics advances, it is of interest to answer the important question of their resistance with respect to ionizing radiation. Here we investigate effects of proton radiation on high-Q (θ(106)) silicon nitride microresonators formed by a waveguide ring. We show that the irradiation with high-energy protons has no lasting effect on the linear optical losses of the microresonators.
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We demonstrate a robust and simple method for measurement, stabilization and tuning of the frequency of cw mid-infrared (MIR) lasers, in particular of quantum cascade lasers. The proof of principle is performed with a quantum cascade laser at 5.4 µm, which is upconverted to 1.2 µm by sum-frequency generation in orientation-patterned GaAs with the output of a standard high-power cw 1.5 µm fiber laser. Both the 1.2 µm and the 1.5 µm waves are measured by a standard Er:fiber frequency comb. Frequency measurement at the 100 kHz-level, stabilization to sub-10 kHz level, controlled frequency tuning and long-term stability are demonstrated.
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A narrow-linewidth cw 5 µm source based on difference frequency generation of a 1.3 µm quantum dot external cavity diode laser and a cw Nd:YAG laser in periodically poled MgO:LiNbO(3) has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 5.09-5.13 µm range with an output power up to 0.1 mW, and in the 5.42-5.48 µm range with sub-microwatt output. The output frequency is stabilized and its value determined by measuring the frequency of the two lasers with a remotely located frequency comb. A frequency instability of less than 4 kHz for long integration times and a linewidth smaller than 700 kHz were obtained.
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Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu(3+):Y(2)SiO(5) at a very high spectral resolution. The essential parameters determining the usefulness as a macroscopic frequency reference, linewidth, temperature sensitivity, and long-term stability, were characterized using a H-maser stabilized frequency comb. Spectral holes with a linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be 5±3 mHz/s. We discuss the necessary requirements for achieving ultrahigh stability in laser frequency stabilization to these spectral holes.
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A narrow-linewidth mid-IR source based on difference-frequency generation of an amplified 1.5 microm diode laser and a cw Tm-doped fiber laser in orientation-patterned (OP) GaAs has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 7.6-8.2 microm range with an output power of 0.5 mW. The measured characteristics of the OP-GaAs sample demonstrate a high quality of the material.
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The method of laser induced reaction is used to obtain high-resolution IR spectra of H2D+ and D2H+ in collision with n-H2 at a nominal temperature of 17 K. For this purpose three cw-laser systems have been coupled to a 22-pole ion trap apparatus, two commercial diode laser systems in the ranges of 6100-6600 cm(-1) and 6760-7300 cm(-1), respectively, and a high-power optical parametric oscillator tunable in the range of 2600-3200 cm(-1). In total, 27 new overtone and combination transitions have been detected for H2D+ and D2H+, as well as a weak line in the nu1 vibrational band of H2D+ (2(20)<--1(01)) at 3164.118 cm(-1). The line positions are compared to high accuracy ab initio calculations, showing small but mode-dependent differences, being largest for three vibrational quanta in the nu2 symmetric bending of H2D+. Within the experimental accuracy, the relative values of the ab initio predicted Einstein B coefficients are confirmed.
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High-precision laser spectroscopy of ultracold hydrogen molecular ions has the potential of improving the precision of the electron-to-proton mass ratio. An accurate knowledge of the spin structure of the transition is required in order to permit precise comparison with experimental transition frequencies. We calculate with a relative accuracy of the order of O(alpha2) the hyperfine splitting of the rovibrational states of HD+ with orbital momentum L
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Spectroscopic detection of ethane in the 3-microm wavelength region was performed by means of a cw optical parametric oscillator and cavity leak-out. We achieved a minimum detectable absorption coefficient of 1.6 x 10(-10) cm 1/square root of Hz, corresponding to an ethane detection limit of 6 parts per trillion/square root of Hz. For 3-min integration time the detection limit was 0.5 parts per trillion. The levels are to our knowledge the best demonstrated so far. These frequency-tuning capabilities facilitated multigas analysis with simultaneous monitoring of ethane, methane, and water vapor in human breath.
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We report on a new test of Lorentz invariance performed by comparing the resonance frequencies of two orthogonal cryogenic optical resonators subject to Earth's rotation over approximately 1 yr. For a possible anisotropy of the speed of light c, we obtain Delta(theta)c/c(0)=(2.6+/-1.7)x10(-15). Within the Robertson-Mansouri-Sexl (RMS) test theory, this implies an isotropy violation parameter beta-delta-1 / 2=(-2.2+/-1.5)x10(-9), about 3 times lower than the best previous result. Within the general extension of the standard model of particle physics, we extract limits on seven parameters at accuracies down to 10(-15), improving the best previous result by about 2 orders of magnitude.
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We present an all solid state, transportable photoacoustic spectrometer for highly sensitive mid-infrared trace gas detection. A complete spectral coverage between 3.1 and 3.9 microm is obtained using a PPLN-based continuous-wave optical parametric oscillator pumped by a Nd:YAG laser at 1064 nm. A low threshold is achieved by resonating the pump, and spectral agility by employing a dual-cavity setup. An etalon suppresses mode-hops. Active signal cavity stabilization yields a frequency stability better than +/- 30 MHz over 45 minutes. Output idler power is 2 x 100 mW. The frequency tuning qualities of the OPO allow reliable scan over gas absorption structures. A detection limit of 110 ppt for ethane is achieved.
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We present a novel approach for the generation of higly frequency-stable, widely tunable, single-frequency cw UV light that is suitable for high-resolution spectroscopy. Sum-frequency generation (SFG) of two solid-state sources with a single cavity resonant for both fundamental waves is employed. Using a highly stable, narrow-linewidth frequency-doubled cw Nd:YAG laser as a master laser and slaving to it the SFG cavity and the other fundamental wave from a Ti:sapphire laser, we generate UV radiation of 33-mW output power around 313 nm. Alternatively, we use a diode laser instead of the Ti:sapphire laser and produce an output power of 2.1 mW at 313 nm. With both setups we obtain a continuous tunability of >15 GHz, short-term frequency fluctuations in the submegahertz range, a long-term frequency drift below 100 MHz/h, and stable operation for several hours. The theory of optimized doubly resonant SFG is also given.