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We present an electro-optic technique that generates an independent readout of optical cavity mode frequency spacing. We demonstrate a fractional frequency drift resolution of 1 ppb, sufficient to meet expected future mass change mission requirements.

We investigate inter-core crosstalk impact on DMT-modulated signals in a 7-core fiber with different sources at the same nominal wavelength. We observe beating from relative laser frequency shift leads to time-dependent interference on DMT subcarriers.

We propose a real-time laser frequency drift measurement using chirped pulse on ultra-weak FBG grating arrays via direct detection. For chirp rate of 375MHz/30ns, minimum frequency drift of ~340 kHz is detected representing 10-9 frequency resolution.

Optical nonlinearity of semiconducting organic films based on regioregular polythiophene (RR-P3HT) was explored by means of laser frequency conversion in a fiber-coupled experimental setup. Features of harmonic generation in this medium were revealed and compared with other nonlinear media. Possible enhancement of nonlinear susceptibility of RR-P3HT in high-gradient light fields is discussed as well.

Self-injection locking of a laser is an effective method for laser frequency stabilization. We performed fourparametric optimization of a diode laser self-injection locked to high-quality-factor whispering gallery mode resonator. The influence of Kerr nonlinear effect to the optimal state were analysed. The solution to reduce the effect is suggested.

A low-power, passive integrated photonic heterodyne interferometer with one meter of on-chip delay is demonstrated for laser frequency stabilization applications with 30 parts-per-billion accuracy. This technology can be integrated with future space-borne optical metrology systems.

We propose and demonstrate a novel technique for precisely and widely sweeping the frequency of a continuous-wave laser. One of the modulation sidebands of a slave laser generated with an electro-optic modulator is phase-locked to a master laser; in this situation, the slave carrier component can be swept by sweeping the modulation frequency. It does not require beat signal detection at varying and/or high frequency, thus providing a robust and reliable laser frequency sweep. Also, it requires neither a frequency comb for the sweep nor a large power loss. We successfully swept an 852 nm laser over 20 GHz; we confirmed that a second harmonic 426 nm laser could be continuously swept over 40 GHz.

We propose a compact 4 He magnetometer realizing magnetic field measurement and laser-frequency stabilization simultaneously in a single 4 He atomic cell. The frequency stabilization scheme is based on the asymmetric line shape of magnetic resonance which is induced by spatially inhomogeneous light shifts. We investigate the asymmetric line shape of the magnetic resonance signal theoretically and experimentally in laser pumped 4 He magnetometer with the magneto-optical double-resonance configuration. Notice that, due to the asymmetric line shape, the in-phase component of the magnetic resonance signal is shown to have a linear dependence with respect to the laser frequency detuning and is used to actively lock the laser frequency to the resonant point. The method reduces the complexity of the system and improves the stability of the magnetometer, making the laser-pumped 4 He magnetometer more compact and portable.

We demonstrate a method for accurately locking the frequency of a continuous-wave laser to an optical frequency comb in conditions where the signal-to-noise ratio is low, too low to accommodate other methods. Our method is typically orders of magnitude more accurate than conventional wavemeters and can considerably extend the usable wavelength range of a given optical frequency comb. We illustrate our method by applying it to the frequency control of a dipole lattice trap for an optical lattice clock, a representative case where our method provides significantly better accuracy than other methods., 13 pages, 7 figures, 24 references

Laser frequency combs (LFCs) are well on their way to becoming the next-generation calibration sources for precision astronomical spectroscopy. This development is considered key in the hunt for low-mass rocky exoplanets around solar-type stars whose discovery with the radial-velocity method requires cm/s Doppler precision. In order to prove such precise calibration with an LFC, it must be compared to another calibrator of at least the same precision. Being the best available spectrograph calibrator, this means comparing it to a second - fully independent - LFC. This test had long been pending, but our installation of two LFCs at the ultra-stable spectrograph HARPS presented the so far unique opportunity for simultaneous calibrations with two separate LFCs. Although limited in time, the test results confirm the 1 cm/s stability that has long been anticipated by the astronomical community., 22 pages, 14 figures. View the Nature Astronomy publication under https://rdcu.be/b1tMB . Nature Astronomy article available under https://www.nature.com/articles/s41550-020-1010-x

Modulated optical cavities have been proposed and demonstrated for applications in communications, laser frequency stabilization, microwave-to-optical conversion and frequency comb generation. However, most studies are restricted to the adiabatic regime, where either the maximum excursion of the modulation or the modulation frequency itself is below the linewidth of the cavity. Here, using a fiber ring resonator with an embedded electro-optic phase modulator, we investigate the nonadiabatic regime. By strongly driving the modulator at frequencies that are significantly smaller than the free-spectral range of the ring resonator, but well beyond the linewidth of the resonator, we experimentally observe counterintuitive behavior predicted in a recent theoretical study by Minkov et al. [APL Photonics 2, 076101 (2017)], such as the complete suppression of drop-port transmission even when the input laser wavelength is on resonance with the optical cavity. This can be understood as dynamical isolation of the cavity from the input light. We also show qualitative differences in the steady-state responses of the system between the adiabatic and nonadiabatic limits. Our experiments probe a seldom explored regime of operation that is promising for applications in integrated photonic systems with current state-of-the-art technology.

A resonant fiber optic gyro (RFOG) is a high-accuracy inertial rotation sensor based on the Sagnac effect. Knowledge of the laser frequency noise, especially the study of its transfer characteristics is critical to optimize the resonant frequency servo loop parameters in the RFOG. In this paper, a Mach–Zehnder interferometer (MZI) is used to measure the changes of the power spectral density (PSD) of the laser frequency noise. Three different observation points are set to investigate the transfer characteristics of the laser beam along the transmission loop of the RFOG. Through real-time observation and analysis of the PSDs of the laser frequency noise, we found that the effects of the laser frequency noise on the RFOG can be clearly determined, and the resonant frequency servo loop parameters can be optimized according to the required gyro bandwidth. We also demonstrate the significance of reducing the residual error at the lock-in frequency while ensuring a sufficient gyro bandwidth.

Infrared nonlinear optical (IR NLO) crystals are the major materials to widen the output range of solid-state lasers to mid- or far-infrared regions. The IR NLO crystals used in the middle IR region are still inadequate for high-power laser applications because of deleterious thermal effects (lensing and expansion), low laser-induced damage threshold, and two-photon absorption. Herein, the unbiased global minimum search method was used for the first time to search for IR NLO optical materials and ultimately found a new IR NLO material NaGaS2. It meets the stringent demands for IR NLO materials pumped by high-power laser with the highest thermal conductivity among common IR NLO materials able to avoid two-photon absorption, a classic nonlinear coefficient, and wide infrared transparency.

Experimental studies aimed at developing an optical frequency standard based on ultracold magnesium atoms with a relative uncertainty of Δν/ν < 10–16 are performed. The frequency of the clock laser system at a wavelength of 457 nm is stabilised to narrow optical resonances (Ramsey fringes) in time-separated laser fields interacting with cold magnesium atoms localised in a magneto-optical trap (MOT). The frequency stability of the laser system is investigated using a femtosecond comb based on a Ti:sapphire laser. Long-term frequency stability (determined by the Allan function) of ~5 × 10–15 at averaging time τ = 1000 s is obtained.

Distortion of the spectrum of the D1-line of 85Rb in optical cells with an antirelaxation coating on the inner walls of the cell is studied. The spectrum shape is found to be significantly dependent on the velocity and direction of changes in the laser frequency. A physical explanation is provided for these features, which are confirmed by numerical simulations. The effect of the magnetic field on the spectrum shape is discussed.

利用激光透过原子气室时的端面反射产生泵浦光,实现低泵浦场的饱和吸收光谱,提高饱和吸收光谱锁频方法对运动平台的适应性。 1. 在饱和吸收光谱中实现无需额外反射镜调节的泵浦光与探测光的重合方法;2. 在饱和吸收光谱中实现强探测-弱泵浦的实验方式。 1. 利用原子气室端面反射光作为泵浦光,实现低泵浦场的饱和吸收光谱装置(图1 和2);2. 研究实现低泵浦场的饱和吸收光谱所需的角度控制精度(图4);3. 研究采用此方法的激光频率锁定效果(图5 和6)。 1. 低泵浦场的饱和吸收光谱方法可以满足激光频率锁定的要求;2. 低泵浦场的饱和吸收光谱装置相比传统装置,没有多余的可调节的反射镜,装置更加简洁,锁频效果受环境影响更小,对运动平台的适应性更强。

A compact modulation transfer spectroscopy (CMTS) module with high stability in frequency and robustness is realized for a transportable 87 Rb atomic gravimeter. The module is fabricated in a 91 mm × 169 mm × 60 mm box. The frequency instability achieved with CMTS is below 1.3 × 10 − 13 / τ in the short term, not exceeding 5 × 10 − 12 until 1 × 10 5 s, which is the best performance of 87 Rb recorded so far in the literature. The achievement of high frequency stability with CMTS is discussed, analyzing the factors that impact frequency stability, with the Allan deviation measured under various conditions. This study offers a novel and efficient choice for applications employing the stabilized laser frequency and transportable system.

Small incision lenticule extraction (SMILE), a variant of refractive lenticule extraction technology is becoming increasingly popular, as a flapless and minimally invasive form of laser vision correction (LVC) for the treatment of myopia and myopic astigmatism. This review aims at summarizing the principles, surgical technique, and clinical outcomes in terms of visual and refractive results, safety, efficacy, postoperative dry eye, aberrations, and biomechanics of SMILE and its comparison with other conventional techniques of LVC, such as laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). Recent advancements in the laser frequency and energy delivery patterns, instrumentation, and surgical techniques have shown significant improvement in the visual recovery and outcomes after SMILE, compared to the initial results published by Sekundo and Shah et al. Most of the recently published literature on long-term outcomes of SMILE shows excellent stability of the procedure, especially for higher myopia. In terms of the postoperative dry eye, SMILE shows a clear advantage over LASIK as numerous studies have shown significant differences about the Schirmer's, Tear film break up time, corneal sensitivity, and corneal nerve regeneration to be better following SMILE compared to LASIK. There is some evidence that since the Bowman's membrane (BM) and the anterior lamellae remain intact after SMILE, this may be a potential advantage for corneal biomechanics over LASIK and PRK where the BM is either severed or ablated, respectively, however, the data on biomechanics are inconclusive at present. Overall, this procedure has proved to be promising, delivering equivalent, or better visual and refractive results to LASIK and providing clear advantage in terms of being a flapless, minimally invasive procedure with minimal pain and postoperative discomfort thus offering high patient satisfaction.

Oxide perovskites such as BaZrO 3 possess many significant properties which render them useful in many technological and scientific applications such as sensors, optoelectronics, laser frequency doubling and high capacity memory cells. Several methods are used to grow BaZrO 3 crystal, and organic species that may be present during growth lead to carbon contamination. We have investigated, using density-functional theory, the role of carbon impurities on the structural, electrical and electronic properties of carbon substituting of Zr in cubic barium zirconate. The local vibrational modes of the defect centre has been calculated and we suggest it is a feasible route to experimental identification

We demonstrate laser frequency stabilization using a 2-m-long iodine cell and a Nd:YAG laser. Together with laser linewidth narrowing using a fiber interferometer, we should obtain better frequency stability than that of a hydrogen maser.

We present a novel design of hollow-core photonic ciystal fiber based spectroscopic references purposed for laser frequency stabilization and spectroscopic analysis with low optical losses and mini-invasive approach of gas media filling.

We developed a model that enables both deeper understanding of the salient physical features of the self-injection locking of a laser to high-quality factor whispering-gallery mode microresonator and corresponding improvements of the laser frequency stabilization.

We developed a model that enables both a deeper understanding of the salient physical features of the self-injection locking of a laser to high-quality factor WGM microresonator and corresponding improvements of the laser frequency stabilization.

In article number https://doi.org/10.1002/advs.201900460, Hua Li, Juncheng Cao, Kai Zhang, Heping Zeng, and co‐workers report a graphene‐based approach for enhanced passive frequency comb operation and pulse generation in terahertz semiconductor lasers. The saturable absorption and group velocity dispersion compensation provided by a multi‐layer graphene sample result in a laser frequency comb operation with highly stabilized frequency lines and optical pulse generation (16 ps) in the terahertz regime.

We report on the controllable response of the lasing frequency to the cavity round-trip path change. This is achieved by modifying the dispersion of the intracavity medium in the four-wave mixing regime in Rb. We can either increase the response by at least a factor of $2.7$ or drastically reduce it. The former regime is useful for sensitive measurements tracking the cavity round trip length and the latter regime is useful for precision metrology., 4 pages; heavily revised from version 1

We report a precise measurement of the isotope shifts in the $4{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}3{\phantom{\rule{0.16em}{0ex}}}^{2}{D}_{5/2}$ electric quadrupole transition at 729 nm in $^{40\ensuremath{-}42,44,48}\mathrm{Ca}^{+}$. The measurement has been made via high-resolution laser spectroscopy of co-trapped ions, finding measured shifts of 2 771 872 467.6(7.6), 5 340 887 394.6(7.8), and 9 990 381 870.0(6.3) Hz between $^{42,44,48}\mathrm{Ca}^{+}$ and $^{40}\mathrm{Ca}^{+}$, respectively. By exciting the two isotopes simultaneously, using frequency sidebands derived from a single laser, systematic uncertainties resulting from laser frequency drifts are eliminated. This permits far greater precision than similar previously published measurements in other alkaline-earth-metal systems. The resulting measurement accuracy provides a benchmark for tests of theoretical isotope shift calculations and also offers a step towards probing new physics via isotope shift spectroscopy.

Due to the inherent difficulties in determination of the degree of branching for polymers produced in pulsed laser polymerization (PLP) experiments, the behavior of the degree of branching and backbiting reaction in high laser frequency and relatively high reaction temperatures have not been well-established. Herein, through a simulation study, the validity of different explanations on the recovery of PLP-molar mass distribution at high laser frequencies is discussed. It is shown that the reduction of the backbiting reaction rate at high laser frequency, and consequent decrease in the degree of branching, is not a necessary condition for recovering the PLP-molar mass distribution. The findings of this work provide simulation support to a previous explanation about the possibility of using high laser frequency for reliable determination of the propagation rate coefficient for acrylic monomers. Yuri Reyes acknowledges the financial support of CONACyT (Mexico). Shaghayegh Hamzehlou acknowledges the University of the Basque Country (UPV/EHU) for the "Contratacion para la especializacion de personal investigador doctor" postdoctoral grant.

Laser frequency stabilization is one of the essential technique in atomic, molecular, and optical physics, quantum optics, optical communications, and fundamental physics. Optical offset locking is the technique to stabilize optical frequency of a single-mode laser (slave laser) with respect to another well-stabilized laser (master laser). Optical phase lock loop (OPLL) [1] is one of the offset locking schemes, with which the slave laser frequency can be stabilized very precisely (less than the uncertainty of 10−18). However, especially in noisy experimental circumstances, the OPLL does not work because its capture range is not so wide that the locking fails for large amount of frequency jitters. In order to keep the locked condition, another offset locking with wider capture range is sometimes required. Examples of such locking schemes are digital processing technique, optical frequency locking to resonance frequency of a high-finesse cavity [2], offset locking with an electrical delay line [3], and offset locking with electric LC resonant circuit [4]. The offset locking with the electric LC circuit (LC locking) has been introduced by W.-Y. Cheng et al, and with employing this scheme with the OPLL the beat-note frequency can be stabilized as precise as one millihertz.

The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of 1 nm/Hz at Fourier frequencies above 100 mHz. © 2019 authors. Published by the American Physical Society.

Modern nonlinear-optical crystals can have absorption below 0.1 m−1 in the operating wavelength range. However, in the case of laser frequency conversion even low absorption of pump and generated radiation along with multiphoton absorption can lead to a considerable nonuniform heating of crystals resulting in the decrease of the conversion efficiency due to the violation of phase-matching conditions.

Lund University atomic physicist Anne L’Huillier has been at the forefront of ultrafast laser science since its inception. In 1988, she collaborated on an experiment at the French Saclay Nuclear Research Centre with a solid-state, picosecond laser system that was one of the first to generate high-order harmonics in gases. High-order harmonic generation is a nonlinear process wherein atoms exposed to intense laser radiation emit light with a frequency equal to a multiple order of the laser frequency (1). Six years later, in 1992 at the Lund High-Power Laser Facility in southern Sweden, L’Huillier worked with a titanium-sapphire laser that was one of the first high-power femtosecond systems in Europe (2). She joined Lund University in 1995, and in 2005 her group recorded what was then one of the shortest light pulses, at 170 attoseconds using high-order harmonic generation (3). L’Huillier’s research has helped foster the field of attosecond science, allowing physicists and chemists to visualize the movements of valence electrons in light-induced processes. PNAS spoke to L’Huillier, who was elected as a foreign associate of the National Academy of Sciences in 2018, about her current research. Anne L’Huillier. Image courtesy of Erika Weiland (photographer). …

Laser frequency stabilization plays an important role in high-precision spectroscopic measurements. Since high-accuracy commercial wavemeters became available, wavemeter-based frequency stabilization has found a broad application due to its convenience, flexibility, and wide applicability. However, such stabilization schemes frequently suffer from long-term drift, since the accuracy of the wavelength measurement of a wavemeter is affected by ambient temperature fluctuation. In this work, we demonstrate that such long-term drift can be suppressed by regularly calibrating the frequency of a wavemeter-locked laser utilizing an optical frequency comb, which has much better long-term stability. Under this dual-referenced locking scheme, the Allan deviation is reduced to 3.5 E-12 at 4000 s for a fiber laser operated at 548 nm, which when used in the optical Ramsey spectroscopic measurement of 7 L i + , reduces the standard deviation by as much as 40%, compared to the case when only wavemeter locking is applied.

Graphite-oxide (GO) nanofluids with enhanced thermal conductivity are successfully prepared using pulsed laser ablation in liquid (PLAL) of a graphite target in deionized water. The effects of laser frequencies are investigated on the variations of GO nanoparticles. The morphology, structure and composition of the nanoparticles are characterized using various spectroscopic techniques. During the PLAL process, graphite is oxidized to GO which is inherently hydrophilic, no surfactant is required in the preparation of nanofluid. The laser frequency significantly affects the size and morphology of the GO nanoparticles during laser ablation, leading to a profound variation in the thermophysical properties of the GO nanofluids. At the laser frequency of 10 Hz, the maximum thermal conductivity enhancement of 82% is achieved at a temperature of 50 °C while the maximum viscosity increment is recorded at a temperature of 30 °C. This study shows the great potential of the PLAL method in synthesizing GO nanofluid with anomalously enhanced thermal conductivity.

AgGaGe5Se12 is a new quaternary nonlinear optical crystal which is applied to laser frequency conversion, especially converting 1.064 µm light to mid-IR range. In this work, we have synthesized the AgGaGe5Se12 polycrystal by the two-zones temperature method. By the modified Bridgman method, we have grown high-quality AgGaGe5Se12 single crystal 30 mm in diameter and 50 mm in length. The structure refinement was carried out by the Rietveld method. Besides, the XRS results were analyzed by comparing with AgGaGeS4 and AgGaGe5S12. In addition, the non-polarized Raman spectra were recorded and the intense peaks were observed at 200 and 232 cm−1. The peaks were attributed to corner-sharing (CS) and edge-sharing (ES) clusters. At last, the crystal was characterized by a high transparency in the 0.6–15 µm spectral range and the absorptions at 3.4 and 7 µm, observed in the as-grown crystal, have been eliminated after annealing. Thus, AgGaGe5Se12 is appropriate for further optical experiments and more comprehensive applications.

SESAM passively Q-switched microchip laser is a very promising instrument to replace mode locked lasers to obtain picosecond pulses. The biggest drawback of a passively Q-switched microchip laser is its un-avoided large timing jitter, especially when the pump intensity is low, i.e. at low laser repetition rate range. In order to obtain a low timing jitter passively Q-switched picosecond microchip laser in the whole laser repetition rate range, a 1000 kHz pulsed narrow bandwidth Fiber Bragg Grating (FBG) stablized laser diode was used as the pump source. By tuning the pump intensity, we could control the output laser frequency. In this way, we achieved a very low timing jitter passively Q-switched picosecond laser at 2.13 mW, 111.1 kHz. The relative timing jitter was only 0.0315%, which was around 100 times smaller compared with a cw LD pumped microchip working at hundred kilohertz repetition rate frequency range.

We demonstrate a Cs excited state Faraday anomalous dispersion optical filter (ESFADOF) operating at 728 nm using a novel pump method, by which the pump beam and the probe beam in the ESFADOF realized here have no a common energy level. Using this method, the ESFADOF achieves a transmission of 2.39% with a bandwidth of 22.52 MHz, which can be applied to both laser frequency stabilization and future four-level active optical clocks. Under the 455 nm laser pump, in addition to 5 2 D 5 / 2 , other states such as 7 2 S 1 / 2 , 7 2 P 3 / 2 , 6 2 P 3 / 2 , 6 2 P 1 / 2 and 5 2 D 3 / 2 have also been populated effectively. Meanwhile, multiple wavelength filters exploiting atomic transitions to these states can be realized.

Raman scattering plays a key role in unraveling the quantum dynamics of graphene, perhaps the most promising material of recent times. It is crucial to correctly interpret the meaning of the spectra. It is therefore very surprising that the widely accepted understanding of Raman scattering, i.e., Kramers-Heisenberg-Dirac theory, has never been applied to graphene. Doing so here, a remarkable mechanism we term"transition sliding" is uncovered, explaining the uncommon brightness of overtones in graphene. Graphene's dispersive and fixed Raman bands, missing bands, defect density and laser frequency dependence of band intensities, widths of overtone bands, Stokes, anti-Stokes anomalies, and other known properties emerge simply and directly.

The self-comparison method is a powerful tool in the uncertainty evaluation of optical lattice clocks, but any drifts will cause a frequency offset between the two compared clock loops and thus lead to incorrect measurement result. We propose a drift-insensitive self-comparison method to remove this frequency offset by adjusting the clock detection sequence. We also experimentally demonstrate the validity of this method in a one-dimensional 87Sr optical lattice clock. As the clock laser frequency drift exists, the measured frequency difference between two identical clock loops is (240 ± 34) mHz using the traditional self-comparison method, while it is (−15 ± 16) mHz using the drift-insensitive self-comparison method, indicating that this frequency offset is cancelled within current measurement precision. We further use the drift-insensitive self-comparison technique to measure the collisional shift and the second-order Zeeman shift of our clock and the results show that the fractional collisional shift and the second-order Zeeman shift are 4.54(28) × 10−16 and 5.06(3) × 10−17, respectively.