Locking the frequency of lasers to an optical cavity at the 1.6×10−17 relative instability level

We stabilized the frequencies of two independent Nd:YAG lasers to two adjacent longitudinal modes of a high-finesse Fabry–Pérot resonator and obtained a beat frequency instability of 6.3 mHz at an integration time of 40 s. Referred to a single laser, this is 1.6×10^?17 relative to the laser frequency, and 1.3×10^?6 relative to the full width at half maximum of the cavity resonance. The amplitude spectrum of the beat signal had a FWHM of 7.8 mHz. This stable frequency locking is of importance for next-generation optical clock interrogation lasers and fundamental physics tests.     

Highly efficient continuous-wave intracavity frequency-doubled Nd:YVO4-LBO laser at 457 nm under diode pumping into the emitting level 4F3/2

The continuous-wave high efficiency laser emission of Nd:YVO₄ at the fundamental wavelength of 914 nm and its 457 nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 880 nm into emitting level ⁴F_3/2. 6.5 W at 457 nm with M ²=1.8 was obtained from a 5-mm-thick 0.4 at.% Nd:YVO₄ laser medium and a 15-mm-long LBO nonlinear crystal in a Z-type cavity for 18.6 W absorbed pump power. An optical-to-optical efficiency with respect to the absorbed pump power was 0.35. Comparative results obtained for the pump with diode laser at 808 nm, into the highly-absorbing level ⁴F_5/2, are given in order to prove the advantages of the 880 nm wavelength pumping.     

A laser setup for rubidium cooling dedicated to space applications

We present the complete characterization of a laser setup for rubidium cooling dedicated to space applications. The experimental setup is realized with commercial off-the-shelf fiber components suitable for space applications. By frequency doubling two fiber laser diodes at 1560 nm, we produce the two optical frequencies at 780 nm required for atomic cooling of ⁸⁷Rb. The first laser is locked on saturated absorption signal and long-term frequency drift has been canceled using a digital integrator. The optical frequency of the second laser is controlled relatively to the first one by a frequency comparison method. A full characterization of the setup, including frequency stability evaluation and frequency noise measurement, has been performed. The optical frequency doubling module has been submitted to environmental tests to verify its compatibility with space applications.     

Diode Laser‐Excited Optogalvanic and Absorption Measurements of Uranium in a Hollow Cathode Discharge

Optogalvanic spectra for fifty two transition lines of uranium in the wavelength ranges of 662–683, 774–792, and 834–862 nm were measured by using external‐cavity diode lasers. Among these transitions, 860.795 nm and 682.691 nm were chosen for a detailed investigation of the detection limit for uranium by wavelength modulation spectroscopy due to its stronger signal magnitudes. A detection limit of about 2 × 10^? 5 absorbance achieved at 860.795 nm is more sensitive than that obtained at 682.691 nm, but the absorption spectrum at 682.691 nm is preferable to determine the isotope ratio due to the narrower hyperfine structure as well as the larger isotope shift. A preliminary result for an isotope ratio determination in a depleted sample is presented.     

Diode-Side-Pumped Passively Q-Switched Mode-Locked 532 nm Laser with a Nd:YAG/Cr4+:YAG/YAG Composite Crystal

We present a diode-side-pumped Q-switched mode-locked (QML) laser operating at 532 nm for the first time employing a composite crystal Nd:YAG/Cr⁴⁺:YAG/YAG and a KTP. The experimental results show that, using a suitable cavity and crystals, one can obtain high-quality QML laser output at 532 nm with a side-pumped system. We measure and analyze the QML performance of the fundamental frequency laser and the green laser. Under a pump power of 127 W, we obtain a QML laser operating at 532 nm with an average power of 4.97 W and a repetition rate of 0.2 GHz for the mode-locked pulses; the corresponding depth of modulation is close to 100%.     

High-resolution laser spectroscopy of ultracold ytterbium atoms using spin-forbidden electric quadrupole transition

We have successfully observed high-resolution spectra of spin-forbidden electric quadrupole transition (¹ S ₀→³ D ₂) in ytterbium (¹⁷⁴Yb) atoms. The differential light shifts between the ¹ S ₀ and the ³ D ₂ states in a far-off resonant trap at 532 nm are also measured. For the spectroscopy, we developed simple, narrow-linewidth, and long-term frequency stabilized violet diode laser systems. Long-term drifts of the excitation laser (404 nm) is suppressed by locking the laser to a length stabilized optical cavity. The optical path length of the cavity is stabilized to another diode laser whose frequency is locked to a strong ¹ S ₀→¹ P ₁ transition (399 nm) of Yb. Both lasers are standard extended-cavity diode lasers (ECDLs) in the Littrow configuration. Since the linewidth of a violet ECDL (～10 MHz) is broader than a typical value of a red or near infra-red ECDL (<1 MHz), we employ optical feedback from a narrow-band Fabry–Perot cavity to reduce the linewidth. The linewidth is expected to be <20 kHz for 1 ms averaging time, and the long-term frequency stability is estimated to be ～200 kHz/h.     

An ultra-fast all-optical RS flip-flop based on nonlinear photonic crystal structures

In this paper, an all-optical RS flip-flop was proposed using nonlinear Kerr effect in photonic crystals. The proposed structure is composed of a core section and two optical switches. The core section consists of two cross-connected resonant cavities whose resonant mode are at wavelengths 1586 and 1620 nm. The cavities were designed such that the resonance of one cavity prevents the signal coupling through the other one. For designing the switch sections, a bias port was used to keep data when there is no input for the flip-flop. Therefore, when both input ports are inactive, the previous state of the flip-flop will be kept. Total footprint and maximum frequency of the proposed structure are obtained 361 μm² and 320 GHz, respectively.     

A compact resonant Π-shaped photoacoustic cell with low window background for gas sensing

A resonant photoacoustic cell capable of detecting the traces of gases at an amplitude-modulation regime is represented. The cell is designed so as to minimize the window background for the cell operation at a selected acoustic resonance. A compact prototype cell (the volume of acoustic cavity of ~0.2 cm³, total cell weight of 3.5 g) adapted to the narrow diffraction-limited beam of near-infrared laser is produced and examined experimentally. The noise-associated measurement error and laser-initiated signals are studied as functions of modulation frequency. The background signal and useful response to light absorption by the gas are analyzed in measurements of absorption for ammonia traces in nitrogen flow with the help of a pigtailed DFB laser diode operated near a wavelength of 1.53 µm. The performance of absorption detection and gas-leak sensing for the prototype operated at the second longitudinal acoustic resonance (the resonance frequency of ~4.38 kHz, Q-factor of ~13.9) is estimated. The noise-equivalent absorption normalized to laser-beam power, and detection bandwidth is ~1.44 × 10^?9 cm^?1 W Hz^?1/2. The amplitude of the window-background signal is equivalent to an absorption coefficient of ~2.82 × 10^?7 cm^?1.     

Ultra-stable microwave generation with a diode-pumped solid-state laser in the 1.5-μm range

We demonstrate the first ultra-stable microwave generation based on a 1.5-μm diode-pumped solid-state laser (DPSSL) frequency comb. Our system relies on optical-to-microwave frequency division from a planar-waveguide external cavity laser referenced to an ultra-stable Fabry–Perot cavity. The evaluation of the microwave signal at ~10 GHz uses the transportable ultra-low-instability signal source ULISS^®, which employs a cryo-cooled sapphire oscillator. With the DPSSL comb, we measured ?125 dBc/Hz phase noise at 1 kHz offset frequency, likely limited by the photo-detection shot-noise or by the noise floor of the reference cryo-cooled sapphire oscillator. For comparison, we also generated low-noise microwave using a commercial Er:fiber comb stabilized in similar conditions and observed >20 dB lower phase noise in the microwave generated from the DPSSL comb. Our results confirm the high potential of the DPSSL technology for low-noise comb applications.     

Active laser frequency stabilization using neutral praseodymium (Pr)

We present a new possibility for the active frequency stabilization of a laser using transitions in neutral praseodymium. Because of its five outer electrons, this element shows a high density of energy levels leading to an extremely line-rich excitation spectrum with more than 25?000 known spectral lines ranging from the UV to the infrared. We demonstrate the active frequency stabilization of a diode laser on several praseodymium lines between 1105 and 1123 nm. The excitation signals were recorded in a hollow cathode lamp and observed via laser-induced fluorescence. These signals are strong enough to lock the diode laser onto most of the lines by using standard laser locking techniques. In this way, the frequency drifts of the unlocked laser of more than 30 MHz/h were eliminated and the laser frequency stabilized to within 1.4(1) MHz for averaging times >0.2 s. Frequency quadrupling the stabilized diode laser can produce frequency-stable UV-light in the range from 276 to 281 nm. In particular, using a strong hyperfine component of the praseodymium excitation line E=16?502.616_7/2 cm $^{-1}\rightarrow E'=25\,442.742^{\mathrm{o}}_{9/2}$  cm^?1 at λ=1118.5397(4) nm makes it possible—after frequency quadruplication—to produce laser radiation at λ/4=279.6349(1) nm, which can be used to excite the D₂ line in Mg⁺.     

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared application to trace detection of hydrogen peroxide

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high-power Yb:fiber laser, a high-finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.76 μm is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e. the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4×10^?9 cm^?1?Hz^?1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9×10^?11 cm^?1?Hz^?1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8 % of water the detection limit is 130 ppb. Spectra of acetylene, methane, and nitrous oxide at atmospheric pressure are also presented, and a line-shape model is developed to simulate the experimental data.     

A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM₀₀ cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH₄ and N₂O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm^?1. A minimum detectable absorption coefficient of 5.5×10^?8 cm^?1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10^?9 cm^?1?Hz^?1/2 (1σ).     

Using a DS-DBR laser for widely tunable near-infrared cavity ring-down spectroscopy

Studies into the suitability of a novel, widely tunable telecom L-band (1,563–1,613 nm) digital supermode distributed Bragg reflector (DS-DBR) laser for cavity ring-down spectroscopy (CRDS) are presented. The spectrometer comprised of a 36.6?cm long linear cavity with ring-down times varying between 19–26 μs across the 50 nm DS-DBR wavelength range due to changes in the cavity mirror reflectivities with wavelength. The potential of such a broadband, high-resolution CRD spectrometer was illustrated by investigating several transitions of CO₂ in air, a 5 % calibrated mixture and breath samples. Allan variance measurements at a single wavelength indicated an optimal minimum detectable absorption coefficient (α _min) of 3 × 10^?10 cm^?1 over 20 s.     

Development of Very-Low-Temperature Millimeter-Wave Electron-Spin-Resonance Measurement System

We report the development of a millimeter-wave electron-spin-resonance (ESR) measurement system at the University of Fukui using a ³He/⁴He dilution refrigerator to reach temperatures below 1 K. The system operates in the frequency range of 125–130 GHz, with a homodyne detection. A nuclear-magnetic-resonance (NMR) measurement system was also developed in this system as the extension for millimeter-wave ESR/NMR double magnetic-resonance (DoMR) experiments. Several types of Fabry–Pérot-type resonators (FPR) have been developed: A piezo actuator attached to an FPR enables an electric tuning of cavity frequency. A flat mirror of an FPR has been fabricated using a gold thin film aiming for DoMR. ESR signal was measured down to 0.09 K. Results of ESR measurements of an organic radical crystal and phosphorous-doped silicon are presented. The NMR signal from ¹H contained in the resonator is also detected successfully as a test for DoMR.     

Solid-state laser system for laser cooling of sodium

We demonstrate a frequency-stabilized, all-solid laser source at 589 nm with up to 800 mW output power. The laser relies on sum-frequency generation from two laser sources at 1064 nm and 1319 nm through a PPKTP crystal in a doubly resonant cavity. We obtain conversion efficiencies as high as 2 W/W² after careful optimization of the cavity parameters. The output wavelength is tunable over 60 GHz, which is sufficient to lock on the sodium D₂ line. The robustness, beam quality, spectral narrowness and tunability of our source make it an alternative to dye lasers for atomic physics experiments with sodium atoms.     

Cavity-enhanced optical feedback-assisted photo-acoustic spectroscopy with a 10.4 μm external cavity quantum cascade laser

An ultra-sensitive photo-acoustic spectrometer using a 10.4 μm broadly tunable mid-IR external cavity quantum cascade laser (EC-QCL) coupled with optical feedback to an optical power buildup cavity with high reflectivity mirrors was developed and tested. A laser optical power buildup factor of 181 was achieved, which corresponds to an intra-cavity power of 9.6 W at a wavelength of 10.4 μm. With a photo-acoustic resonance cell placed inside the cavity this resulted in the noise-equivalent absorption coefficient of 1.9 × 10^?10 cm^?1 Hz^?1/2, and a normalized noise-equivalent absorption of 1.1 × 10^?11 cm^?1 W Hz^?1/2. A novel photo-acoustic signal normalization technique makes the photo-acoustic spectrometer’s response immune to changes and drifts in the EC-QCL excitation power, EC-QCL to cavity coupling efficiency and cavity mirrors aging and contamination. An automatic lock of the EC-QCL to the cavity and optical feedback phase optimization permitted long wavelength scans within the entire EC-QCL spectral tuning range.     

Synthesis and optical properties of pencil-like and shuttle-like ZnO microrods

The pencil-like and shuttle-like ZnO microrods have been fabricated on Si (100) substrates by chemical vapor deposition. Structure characterization results show that the microrods are perfect single crystals with the wurtzite structure along the [0001] growth direction and have diameters ranging from 100 nm to 2 μm and lengths up to 10 μm. Room-temperature photoluminescence measurements of ZnO microstructures exhibit an intensive ultraviolet peak at 390 nm and a broad peak centered at about 526 nm, which can be attributed to the free exciton emission and the deep level emission, respectively. Cathodoluminescence measurements show the same ultraviolet and green emissions as seen in the photoluminescence results. A possible growth mechanism of ZnO microrods is finally proposed.     

Precision spectroscopy technique for dipole-allowed transitions in laser-cooled ions

In this paper , we present a technique for the precise measurement of electric dipole-allowed transitions in trapped ions. By applying a probe and a cooling laser in quick succession, the full transition can be probed without causing distortion from heating the ion. In addition, two probes can be utilized to measure a dispersion-like signal, which is well suited to stabilizing the laser to the transition. We have fully characterized the parameters for the measurement and find that it is possible to measure the line center to better than 100 kHz with an interrogation time of 30 s. The long-term stability of the spectroscopy signal is determined by employing two independent ion trap systems. The first ion trap is used to stabilize the spectroscopy laser. The second ion trap is then employed to measure the stability by continuously probing the transition at two frequencies. From the Allan variance, we obtained a frequency instability of \(1\cdot 10^{-10}\) for an interrogation time of 1,000 s.     

A picosecond near-infrared laser source based on a self-seeded optical parametric generator

We report on the design and development of a new type of near-IR laser source. The source comprises of an optical parametric generator (OPG) and a second harmonic generator (SHG) pumped by an 80-MHz, 1064-nm, 7-ps Nd:YVO₄ laser. The OPG is self-seeded with a fraction of its own signal output, which significantly enhances its conversion efficiency. The SHG doubles the frequency of OPG signal to produce a coherent output. The final output beam has a tunable wavelength near 800 nm, an average power of over 1 W, and a pulse duration around 5 ps. The M²-factor of the output beam can reach 1.1 after spatial filtering. With the new laser source, we have successfully demonstrated coherent anti-Stokes Raman scattering microscopy on 1 μm polystyrene beads, which shows that it has the potential to be a substitute for a picosecond optical parametric oscillator in certain microscopy or spectroscopy applications.     

Idler-resonant optical parametric oscillator based on KTiOAsO4

An idler-resonant KTiOAsO₄ (KTA) optical parametric oscillator is demonstrated within a diode-end-pumped acousto-optically Q-switched Nd:YAG laser. With an X-cut KTA crystal, idler wave at 3467 nm and signal wave at 1535 nm are generated. Under an incident diode pump power of 15.4 W, the idler output power of 105 mW and signal power of 720 mW are obtained at a pulse repetition rate of 40 kHz. The pulse widths of the idler and signal waves are 7.2 and 3.1 ns, respectively. The beam quality factors (M²) of the idler wave are within 1.2 in both horizontal and vertical directions.