Laser Frequency Measurements Research Articles

Precise Laser Frequency Measurement of Weak Power With Submegahertz Linewidth

Precise Laser Frequency Measurement of Weak Power With Submegahertz Linewidth

Precise determination of characteristic laser frequencies by an Er-doped fiber optical frequency comb

Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently. Therefore, any optical frequency in visible to near-infrared region can be directly traced to a microwave frequency. As a result, the length unit “meter” is directly related to the time unit “second”. This paper validates the capability of the national wavelength standards based on a home-made Er-doped fiber femtosecond optical frequency comb to measure the laser frequencies ranging from visible to near-infrared region. Optical frequency conversion in the femtosecond optical frequency comb is achieved by combining spectral broadening in a highly nonlinear fiber with a single-point frequency-doubling scheme. The signal-to-noise ratio of the beat notes between the femtosecond optical frequency comb and the lasers at 633, 698, 729, 780, 1064, and 1542 nm is better than 30 dB. The frequency instability of the above lasers is evaluated by using a hydrogen clock signal with a instability of better than 1 × 10−13 at 1-s averaging time. The measurement is further validated by measuring the absolute optical frequency of an iodine-stabilized 532-nm laser and an acetylene-stabilized 1542-nm laser. The results are within the uncertainty range of the international recommended values. Our results demonstrate the accurate optical frequency measurement of lasers at different frequencies using the femtosecond optical frequency comb, which is not only important for the precise and accurate traceability and calibration of the laser frequencies, but also provides technical support for establishing the national wavelength standards based on the femtosecond optical frequency comb.

Laser frequency measurement in the short-wavelength region using an intermediate laser and a frequency noise cancellation method.

Optical frequency combs play a crucial role supporting optical frequency standards and cover a wide range of wavelengths (octaves). However, broadening the comb spectrum to the short-wavelength visible region (λ < 500 nm), where GaN-based blue diode lasers are available, is not an easy task. In this study, we propose a method for measuring the laser frequency in the short-wavelength region using an intermediate laser and a noise-canceling scheme. We demonstrate this method by measuring the frequency of a GaN-based laser at 399 nm and confirming that the frequency measurement is not affected by the frequency noise of the intermediate laser.

The influence of polarization misalignment for modulation transfer spectrum in atom gravimeter.

The accuracy of atom gravimeters is directly related to the Raman laser, which is used to manipulate the atomic wave packet, and the frequency of the Raman laser could be affected by temperature when the laser polarization is not along the preferred axis of the electro-optic crystal employed in the modulation transfer spectrum (MTS). This effect has been researched by modulating the laser polarization in the MTS in this work. The experimental results show that both the laser frequency and gravity measurement results have a sinusoidal dependence on temperature, and the period of the fluctuation is 0.8 °C. The systematic effect can reach 12.4 μGal when the polarization misalignment is 15°, which is a remarkable contribution to the absolute gravity measurement. The amplitude of this effect could be reduced by adjusting the laser polarization to the crystal's preferred axis. According to the result, the included angle between the laser polarization and the crystal's preferred axis should be smaller than 5° if 2 μGal accuracy is required.

Isotope-shift measurements and King-fit analysis in nickel isotopes

The isotope shifts of the $3{d}^{9}4s^{3}D_{3}\ensuremath{\rightarrow}3{d}^{9}4p^{3}P_{2}$ transition in the stable even-even nickel isotopes were measured. An improved accuracy was achieved by transforming the systematic contribution of the wavelength-meter-based laser-frequency measurement into a statistical-acting contribution by measuring the same observables at different frequency sets. A detailed King-fit analysis was performed to extract the mass-shift and field-shift parameters, which are crucial for the determination of the charge radii of short-lived isotopes from recently measured isotope shifts. A critical dependence of the achievable charge-radius accuracy on the choice of the reference isotope in the King-fit analysis was observed and is discussed.

ON POTENTIAL OF USING WIND TURBINES WITH COAXIAL WIND ROTORS FOR AUTONOMOUS POWER SUPPLY

The paper presents the experimental research results for the horizontal-axis wind turbine with coaxial wind rotors. It is assumed that such coaxial layout of the wind turbine can be used for designing of the wind energy systems with relatively low capacity and limited location area since the coaxial systems have advantages in overall dimensions and maximum using of the swept area. Possibility of coaxial horizontal-axis wind turbines usage is determined by positive or negative effect of turbines on each other. Literature review shows that closely spaced wind turbines can generally improve flow characteristics under certain conditions and consequently increase wind energy system efficiency. We have carried out the experiments in T-5 wind tunnel with two coaxial model two-bladed wind turbines which rotate in opposite directions. The generator of the first turbine and first turbine itself are located on the same shaft in the test section of wind tunnel. The second generator is in a lower compartment of the experimental setup and is connected by the transmission. We have measured the dynamic, energy and frequency characteristics of wind energy systems based on created experimental setup. A Pitot tube and automatic metering devises have measured the dynamic parameters and energy performance respectively. A frequency counter has saved all of the data obtained with the laser frequency measurement technique. The experiment has some specific technical features so the data received need to be corrected. The coaxial wind turbine power has decreased in comparison to isolated wind turbine at low wind speed. The return flows reinforce turbulence so wind speed falls. If wind speed increases, the impact of the return flows decreases, the coaxial wind turbine capacity significantly grows and exceeds isolated turbine capacity. The possibility of using wind turbines with coaxial wind rotors for autonomous power supply is shown. Such wind turbines are perspective and require more detailed analysis.

Unambiguous real-time terahertz frequency metrology using dual 10 GHz femtosecond frequency combs

Terahertz frequency metrology by radio frequency downconversion using femtosecond optical sampling relies on the harmonic factor retrieval between the terahertz frequency and the optical sampling rate. At typical femtosecond laser repetition rates, this imposes an ambiguity for frequency metrology. We report on a dual-comb sampling system for the unambiguous frequency measurement of terahertz quantum cascade lasers with hertz-level precision. Two Ti:sapphire oscillators with 10 GHz repetition rate are used for the electro-optic sampling of terahertz radiation at 2.5 THz emitted by actively mode-locked terahertz quantum cascade lasers with 9.7 GHz and 19.6 GHz repetition rates. By coherent downconversion, the emitted terahertz waveforms are measured in the radio frequency domain. The terahertz frequency comb is stabilized by employing a phase-locked loop on a radio frequency beat-note signal. A second infrared sampling comb is used to measure the absolute frequencies of the terahertz radiation. This method, which is based on the detuning of the sampling repetition rates, allows the direct retrieval of the quantum cascade laser’s absolute frequency in real time without using additional optical frequency references for calibration. In order to demonstrate the feasibility of the stabilization and readout technique, a high-resolution spectroscopy measurement on gaseous methanol is presented.

All-optically stabilized frequency comb

We present an all-optically stabilized, erbium-doped mode-locked fiber laser with an optically pumped ytterbium-doped fiber. The mode-locked fiber laser has two frequency-control actuators that are pump laser powers for erbium-doped and ytterbium-doped fibers. We investigate the frequency-control characteristics of the mode-locked laser and find that the fixed points for the two actuators are sufficiently apart from each other, realizing the simultaneous phase locking of the repetition and carrier envelope offset frequencies. We describe a long-term frequency measurement of an acetylene-stabilized laser at 1542 nm using an all-optically stabilized frequency comb.

Continuously tunable wavelength output from an Er-doped fiber femtosecond optical frequency comb with single-point frequency-doubling technique

Femtosecond optical frequency combs (FOFCs) with wavelengths covering the visible range have potential applications in the absolute frequency measurement of iodine-stabilized lasers and optical clock lasers. In this paper, an Er-FOFC with a tunable wavelength output from 689 to 813 nm based on the single-point frequency-doubling technique is demonstrated. Meanwhile, a beat frequency signal between the Er-FOFC and a tested laser at 729 nm with a signal-to-noise ratio of 30 dB at a resolution bandwidth of 100 kHz is obtained.

Assignment of far-infrared laser lines of O-17 methanol by synchrotron FTIR spectroscopy and laser frequency measurements

Analysis of synchrotron FTIR spectra has revealed new assignments for a number of optically pumped far-infrared laser lines from the CH317OH isotopologue of methanol, with definitive confirmation provided by recent accurate measurements of the laser frequencies. In this Note the lasing energy level systems are discussed, and the spectroscopic evidence for the assignments is presented.

Fibre optics wavemeters calibration using a self-referenced optical frequency comb.

Self-referenced optical frequency combs enable the measurement of optical frequencies with a very high accuracy, achieving uncertainties close to the atomic clock used as reference (<10(-13) s). In this paper, we present the technique for the measurement of laser frequencies for optical communications followed at IO-CSIC and its application to the calibration of two wavemeters in the 1.5 μm optical communication window. Calibration uncertainties down to 12 MHz and 59 MHz were obtained, respectively, for each of the devices. Furthermore, the long-term behaviour of the higher resolution wavemeter was studied during a 750 h period of sustained operation, exhibiting a dispersion in the measurements of 7.72 MHz. Temperature dependence of the device was analysed, enabling to further reduce dispersion down to a 2.15 MHz range, with no significant temporal deviations.

Measurement of Laser Frequencies from CD3OH and CD3OD up to 8.6 THz

Twenty two laser frequencies, whose values range from 1.6 to 8.6 THz, have been measured for the first time using heterodyne techniques. These laser emissions were generated by an optically pumped molecular laser that used either CD3OH or CD3OD as its lasing medium. At least three of the observed laser emissions generated by CD3OH were discovered during this investigation and the first laser frequencies measured for CD3OH above 8 THz are reported. The laser frequencies were measured with fractional uncertainties up to ± 2 × 10−7, of sufficient accuracy to confirm two proposed far-infrared laser assignments. The offset frequency of the CO2 pump laser with respect to its center frequency was also measured for nearly all laser emissions generated by CD3OH.

Precise frequency measurement and characterization of a continuous scanning single-mode laser with an optical frequency comb.

We developed a high-precision frequency-measurement system for continuously scanning single-mode cw lasers. The high precision for wide frequency range is realized by employing an optical frequency comb. The developed system operates as a strong tool for analyzing the spectral profile and scan characteristics of lasers, which is suitable for high-resolution molecular spectroscopy. We demonstrated the ability of our system with Doppler-free spectroscopy of molecular iodine.

Transition frequencies of oxygen B-band lines measured with optical frequency comb assisted cavity ring-down spectroscopy

Transition frequencies of weak B-band lines of O2 were measured with low uncertainties. These results were compared to data available in the literature. In order to achieve reported accuracy we took into account in the data analysis several subtle line-shape effects, such as speed dependence of collisional broadening and Dicke narrowing. We found that the speed-dependent Nelkin-Ghatak profile can properly describe experimental line shapes in the investigated pressure range of up to 22 Torr. Our experimental setup consists of the Pound-Drever-Hall locked frequency-stabilized cavity ring-down spectrometer. This spectrometer provides high-resolution and very high signal-to-noise ratio of measured spectra. It was recently linked to the optical frequency comb working in the visible region of spectrum. This upgrade allows for accurate determination of the absolute frequency axis of the CRD spectrum by direct measurement of the probe laser frequency.

Erratum to: New Fir Laser Lines and Frequency Measurements from Optically pumped 13CD3OH3

Erratum to: New Fir Laser Lines and Frequency Measurements from Optically pumped 13CD3OH3

Frequency measurement of dual frequency He-Ne laser based on a femtosecond optical frequency comb

A frequency measurement system for dual frequency He-Ne laser is set up based on a fiber femtosecond optical frequency comb and an external cavity diode laser. Using a Rb clock as a frequency standard, the diode laser that is locked to the optical frequency comb is traced to the Rb clock, and then the frequencies of the orthogonal polarized lasers are measured by beating with the locked diode laser at the same time. Locking the diode laser to the 1894449 th comb mode, the absolute frequency of the diode laser is 473612190000.0 ± 2.7 kHz, with a relative frequency uncertainty of 5.7×10-12. A commercial dual frequency He-Ne laser is measured to test the system, and the results show that the mean absolute frequencies of the horizontal polarized laser and the vertical polarized laser are 473612229934 kHz and 473612232111 kHz, respectively, with a relative Allan deviation of 5.2×10-11 at 1024 s, and the mean split frequency is 2.177 MHz with a standard deviation of 2 kHz.

Absolute frequency measurement of a water-stabilized diode laser at 1.384 μm by means of a fiber frequency comb

We report on the absolute frequency measurement of an extended-cavity diode laser stabilized against a Doppler-free vibration–rotation transition of the H218O isotopologue near 1.384 μm. Absolute determination is performed by comparing the water-stabilized laser frequency with respect to a GPS-disciplined Rb microwave standard by means of a self-referenced fiber-based optical frequency comb. The line center frequency of the 22,1→22,0 transition of the H218O ν1+ν3 combination band is found to be 216 519 045 955(13) kHz.

Terahertz frequency measurement of far-infrared laser with an improvement of Martin–Puplett interferometer

In this paper, we present an improvement of Martin–Puplett interferometer (MPI) for terahertz frequency measurement, in which flat mirrors replace the roof ones. To understand the function of the improved MPI, its basic principle is described and the characteristics of electric field are analyzed. With the improved MPI, the operation frequency of a far-infrared laser is conveniently measured, and the experimental result is in good agreement with that of the far-infrared laser theoretical reported.

A 350MHz Ti:sapphire laser comb based on monolithic scheme and absolute frequency measurement of 729nm laser

We realized a novel frequency comb based on femtosecond Ti:sapphire laser with 350 MHz repetition rate and monolithic scheme for carrier-envelope-offset frequency generation. Long-term stabilization of more than 9 hours was demonstrated by simultaneously locking repetition rate (f(rep)) and carrier-envelope-offset frequency (f(ceo)) to an atomic clock. The Allan deviations of f(ceo) and f(rep) are 2.3 x 10(-13) and 1.5 x 10(-15) at 100 s averaging time, respectively, and the reproducibility of f(ceo) with high accurate performance is also verified by comparison of the locking results recorded in 3 different days. By avoiding the extra noise induced by photonic crystal fiber, we obtained the accumulated phase error increase from in-loop to out-of-loop as small as 12 mrad [1 Hz, 100 kHz]. Furthermore, the absolute optical frequency of a stabilized 729 nm Ti:sapphire laser which is referenced to a ultra-low expansion (ULE) cavity is measured by the comb and determined to be 411041985391583+/-10 Hz.

Exact frequency comb mode number determination in precision optical frequency measurements

We demonstrate a method to determine the absolute mode number of a frequency comb when it is used for high-precision laser frequency measurement, without the help of wavemeters. Our technique involves changing the repetition rate of the frequency comb in a two-step process. Guidelines for choosing the correct repetition rates for different laser linewidths are given. As a demonstration, the absolute frequencies of two lasers with different linewidths are measured with our method.