Optical Frequency Measurements Research Articles

Red and Green Laser Powder Bed Fusion of Pure Copper in Combination with Chemical Post-Processing for RF Cavity Fabrication

Linear particle accelerators (Linacs) are primarily composed of radio frequency cavities (cavities). Compared to traditional manufacturing, Laser Powder Bed Fusion (L-PBF) holds the potential to fabricate cavities in a single piece, enhancing Linac performance and significantly reducing investment costs. However, the question of whether red or green laser PBF yields superior results for pure copper remains a subject of ongoing debate. Eight 4.2 GHz single-cell cavities (SCs) were manufactured from pure copper using both red and green PBF (SCs R and SCs G). Subsequently, the surface roughness of the SCs was reduced through a chemical post-processing method (Hirtisation) and annealed at 460 °C to maximize their quality factor (Q0). The geometric accuracy of the printed SCs was evaluated using optical methods and resonant frequency (fR) measurements. Surface conductivity was determined by measuring the quality factor (Q0) of the SCs. Laser scanning microscopy was utilized for surface roughness characterization. The impact of annealing was quantified using Energy-Dispersive X-ray Spectroscopy and Electron Backscatter Diffraction to evaluate chemical surface properties and grain size. Both the SCs R and SCs G achieved the necessary geometric accuracy and thus fR precision. The SCs R achieved a 95% Q0 after a material removal of 40 µm. The SCs G achieved an approximately 80% Q0 after maximum material removal of 160 µm. Annealing increased the Q0 by an average of about 5%. The additive manufacturing process is at least equivalent to conventional manufacturing for producing cavities in the low-gradient range. The presented cavities justify the first high-gradient tests.

Continuous dynamic measurement of frequency scanning interferometry based on motion phase synchronization compensation and calibration.

We present a continuous dynamic frequency scanning interferometry (DFSI) measurement method based on motion phase synchronization compensation and calibration. By introducing heterodyne interferometry (HI) synchronization measurement and frequency scanning interferometry (FSI) motion phase compensation, dynamic continuous measurement is achieved and effectively suppresses the distance error introduced by the Doppler effect (DE). Based on this, the influence of the initial optical frequency deviation (OFD) of the tunable laser and the OFD of the HI laser on the dynamic absolute distance measurement (DADM) is analyzed; the relationships between the error of DADM with the variation of the OFD and the target motion parameters are investigated; and the residual DE introduced by the OFD is shown as the fundamental cause of the degradation of the accuracy of DFSI. We propose an online optical frequency measurement method based on HI combined with H13C14N gas absorption cells to resolve this problem. High-precision motion phase compensation is achieved by calibrating the optical frequency (fixed frequency) of the measured HI laser and the initial frequency of the tunable laser online during measurement and then performing motion phase calibration. To verify the effectiveness of our method, an optical frequency calibration experiment, a continuous DADM experiment, and a precision evaluation experiment were conducted, and a highly accurate continuous DADM was achieved.

Optical-referenceless optical frequency counter with twelve-digit absolute accuracy

A simpler and more accurate measurement of absolute optical frequencies (AOFs) is very important for optical communications and navigation systems. To date, an optical reference has been needed for measuring AOFs with twelve-digit accuracy because of the difficulty in measuring them directly. Here, we focus on an electro-optics-modulation comb that can bridge the vast frequency gap between photonics and electronics. We demonstrate an unprecedented method that can directly measure AOFs to an accuracy of twelve digits with an RF frequency counter by simply delivering a frequency-unknown laser into an optical phase modulator. This could open up a new horizon for optical-referenceless optical frequency metrology. Our method can also simultaneously achieve a 100-fold phase-noise reduction in a conventional signal generator. This corresponds to an increase in the transmission speed of wireless communications of by about seven times.

Absolute frequency measurements with a robust, transportable 40Ca+ optical clock

We constructed a transportable 40Ca+ optical clock (with an estimated minimum systematic shift uncertainty of and a stability of ) that can operate outside the laboratory. We transported it from the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan to the National Institute of Metrology, Beijing. The absolute frequency of the 729 nm clock transition was measured for up to 35 d by tracing its frequency to the second of International System of Units. Some improvements were implemented in the measurement process, such as the increased effective up-time of 91.3% of the 40Ca+ optical clock over a 35-day-period, the reduced statistical uncertainty of the comparison between the optical clock and hydrogen maser, and the use of longer measurement times to reduce the uncertainty of the frequency traceability link. The absolute frequency measurement of the 40Ca+ optical clock yielded a value of 411 042 129 776 400.26 (13) Hz with an uncertainty of , which is reduced by a factor of 1.7 compared with our previous results. As a result of the increase in the operating rate of the optical clock, the accuracy of 35 d of absolute frequency measurement can be comparable to the best results of different institutions in the world based on different optical frequency measurements.

Intercombination line frequencies in 171Yb validated with the clock transition.

We have carried absolute frequency measurements of the (6s 2) 1 S 0-(6s6p) 3 P 1 transition in 171 Y b (intercombination line), where the spin-1/2 isotope yields two hyperfine lines. The measurements rely on sub-Doppler spectroscopy to yield a discriminator to which a 556 nm laser is locked. The frequency reference for the optical frequency measurements is a high-quality quartz oscillator steered to the GNSS time scale that is bridged with a frequency comb. The reference is validated to ∼3×10-12 by spectroscopy on the 1 S 0- 3 P 0 (clock) line in laser cooled and trapped 171 Y b atoms. From the hyperfine separation between the F=1/2 and F=3/2 levels of 3 P 1, we determine the hyperfine constant to be A(3 P 1)=3957833(28)k H z.

Analysis of Statistical Uncertainty of Optical Frequency Measurement Due to Measurement Dead Time of Hydrogen Maser

Analysis of Statistical Uncertainty of Optical Frequency Measurement Due to Measurement Dead Time of Hydrogen Maser

Improved absolute frequency measurement of the strontium ion clock using a GPS link to the SI second

We report on an improved absolute frequency measurement of the – optical transition of a single trapped strontium ion using a Global Positioning System (GPS) link to the SI second. Compared to our previous measurement, the systematic uncertainty of the optical clock has been reduced from to . The measurement campaign was performed over a two-week period in June 2017, with a total measurement time of 92 h. The traceability to the SI second through International Atomic Time was achieved through a GPS link using the Precise Point Positioning method. The dead time uncertainty of the link between the optical clock and the maser was evaluated using standard methods based on a model of the maser noise and on the optical frequency measurement uptimes. The measured frequency of the 88Sr+ ion – transition is Hz. This result is in excellent agreement with our previous measurements and the uncertainty has been reduced by almost a factor of four, from a fractional frequency uncertainty of to .

Reducing frequency fluctuation in a Brillouin random fiber laser by a random fiber grating ring resonator.

Frequency fluctuation is a major problem in high-precision metrology as real-time optical frequency measurement is not available with commercial photodetectors; alternatively, frequency-stabilized lasers as a reference are also not accessible in most laboratories. In this study, we propose and demonstrate a polarization-maintaining random fiber grating ring (PM-RFGR) resonator in a PM Brillouin random fiber laser (BRFL) to achieve sub-MHz frequency drift, which is measured by the optical beat of the random laser and the pump laser. Experimental results show that longitudinal modes are suppressed in the BRFL owing to the feedback of the RFGR resonating with one longitudinal mode of the random laser. The BRFL shows mode-hopping-free operation over 14.9 s due to the self-adjustment of random modes with small frequency difference to thermal and acoustic variations and self-injection locking through RFGR. As a result, a small frequency drift of ∼340 kHz with single-longitudinal mode is achieved in the BRFL enabled by the RFGR, which offers an all optical locking mechanism for optical frequency stabilization.

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.

Ultra-broadband Kerr microcomb through soliton spectral translation

Broadband and low-noise microresonator frequency combs (microcombs) are critical for deployable optical frequency measurements. Here we expand the bandwidth of a microcomb far beyond its anomalous dispersion region on both sides of its spectrum through spectral translation mediated by mixing of a dissipative Kerr soliton and a secondary pump. We introduce the concept of synthetic dispersion to qualitatively capture the system’s key physical behavior, in which the second pump enables spectral translation through four-wave mixing Bragg scattering. Experimentally, we pump a silicon nitride microring at 1063 nm and 1557 nm to enable soliton spectral translation, resulting in a total bandwidth of 1.6 octaves (137–407 THz). We examine the comb’s low-noise characteristics, through heterodyne beat note measurements across its spectrum, measurements of the comb tooth spacing in its primary and spectrally translated portions, and their relative noise. These ultra-broadband microcombs provide new opportunities for optical frequency synthesis, optical atomic clocks, and reaching previously unattainable wavelengths.

Spontaneous pulse formation in edgeless photonic crystal resonators

Complex systems are a proving ground for fundamental interactions between components and their collective emergent phenomena. Through intricate design, integrated photonics offers intriguing nonlinear interactions that create new patterns of light. In particular, the canonical Kerr-nonlinear resonator becomes unstable with a sufficiently intense traveling-wave excitation, yielding instead a Turing pattern composed of a few interfering waves. These resonators also support the localized soliton pulse as a separate nonlinear stationary state. Kerr solitons are remarkably versatile for applications, but they cannot emerge from constant excitation. Here, we explore an edge-less photonic-crystal resonator (PhCR) that enables spontaneous formation of a soliton pulse in place of the Turing pattern. We design a PhCR in the regime of single-azimuthal-mode engineering to re-balance Kerr-nonlinear frequency shifts in favor of the soliton state, commensurate with how group-velocity dispersion balances nonlinearity. Our experiments establish PhCR solitons as mode-locked pulses by way of ultraprecise optical-frequency measurements, and we characterize their fundamental properties. Our work shows that sub-wavelength nanophotonic design expands the palette for nonlinear engineering of light.

Joint spectral intensity of entangled photon pairs from a warm atomic ensemble via stimulated emission beat interferometry.

Photonic quantum states generated from atomic systems play prominent roles in long-distance quantum networks and scalable quantum communication, because entangled photon pairs from atomic ensembles possess a universal identity and narrow spectral bandwidth for quantum repeaters. In this study, we propose and demonstrate a novel, to the best of our knowledge, method for the joint spectral intensity measurement of narrowband continuous wave (CW)-mode photon pairs from a warm atomic ensemble using stimulated emission and beat interferometry for the first time. Our approach offers the advantage of sub-megahertz resolution, absolute optical frequency measurements with megahertz-level accuracy, fast collection time, and high signal-to-noise ratio; thus, our method can find important applications in the characterization of narrowband photon pairs generated from sources including atoms and artificially structured material.

On the performance of wavelength meters: Part 2\u2014frequency-comb based characterization for more accurate absolute wavelength determinations

Wavelength meters are widely used for frequency determinations and stabilization purposes since they cover a large wavelength range, provide a high read-out rate and have specified accuracies of up to 10^{-8}. More accurate optical frequency measurements can be achieved with frequency combs but only at the price of considerably higher costs and complexity. In the context of precise and accurate frequency determinations for high-resolution laser spectroscopy, the performance of five different wavelength meters was quantified with respect to a frequency comb. The relative precision as well as the absolute accuracy has been investigated in detail, allowing us to give a sophisticated uncertainty margin for the individual instruments. We encountered a prominent substructure on the deviation between both device types with an amplitude of a few MHz that is repeating on the GHz scale. This finally limits the precision of laser scans which are monitored and controlled with wavelength meters. While quantifying its uncertainty margins, we found a high temporal stability in the characteristics of the wavelength meters which enables the preparation of wavelength-dependent adjustment curves for wide- and short-ranged scans. With this method, the absolute accuracy of wavelength meters can be raised up to the MHz level independently from the wavelength of the reference laser used for calibrating the device. Since this technique can be universally applied, it can lead to benefits in all fields of wavelength meter applications.

Packaging and precision testing of fiber-Bragg-grating and silicon ring-resonator thermometers: current status and challenges

In recent years photonic thermometers—temperature sensors based on optical frequency measurement which exploit the thermo-optic effect to translate thermal changes into frequency shifts—are gaining popularity as a possible alternative to their electrical counterparts: platinum resistance thermometers and thermocouples. In this work, we report our results of testing photonic thermometers based on silica fiber-Bragg-grating technology supplied by a commercial company, as well as preliminary testing results of a silicon ring-resonator thermometer developed at the National Research Council of Canada. The main purpose of showing these two examples is to highlight some of the challenges that need to be addressed if photonic thermometers are to replace thermocouples or platinum resistance thermometers in metrology laboratories and other environments where high accuracy and stability are required, namely the influence of packaging on the sensor’s performance and the need for rigorous testing to be done in a temperature metrology lab.

Generation and research progress of femtosecond optical frequency combs in extreme ultraviolet

Femtosecond optical frequency combs have revolutionized the precision measurement of optical frequency and ultrafast science. Furthermore, the frequency combs expended to extreme ultraviolet (XUV) wavelength could provide an effective tool in attosecond pulse generation, nonlinear optics in ultraviolet, spectroscopy of electronic transitions and experiment of quantum electrodynamics. XUV femtosecond optical frequency combs need to be indirectly obtained by means of high-harmonic generation (HHG) drived by femtosecond pulses with high-repetition rate and extremely high peak power. In this review, firstly, the generation principle and the driving laser source requirements of femtosecond pulses generation in XUV spectral range are introduced. Basing on the requirements of driving laser sources, the several femtosecond laser amplification techniques are described, such as chirped pulse amplification (CPA), optical parametric chirped pulse amplification (OPCPA), double cladding pumped fiber amplifier and femtosecond enhancement cavity (fsEC). Meanwhile, the relative merits and applicability of which for XUV femtosecond optical frequency combs generation are compared. Secondly, in the HHG process, the XUV is generated collinearly or non-collinearly with the optical driving field. For the collinear generation process, one of the fundamental challenges is the design of a high-efficiency XUV output coupler. Here, three methods for out-coupling the XUV are expounded. Also, the theory of non-collinear XUV generation is mentioned. Finally, some typical research progress of XUV femtosecond optical frequency combs generation based on fsEC, OPCPA and femtosecond oscillators are reviewed respectively, as well as the current problems that need to be optimized are summarized.

Hyperfine constants and line separations for the S01−P13 intercombination line in neutral ytterbium with sub-Doppler resolution

Optical frequency measurements of the intercombination line $(6s^{2})\,^{1}S_{0} -(6s6p)\,^{3}P_{1}$ in the isotopes of ytterbium are carried out with the use of sub-Doppler fluorescence spectroscopy on an atomic beam. A dispersive signal is generated to which a master laser is locked, while frequency counting of an auxiliary beat signal is performed via a frequency comb referenced to a hydrogen maser. The relative separations between the lines are used to evaluate the $^{3}P_{1}$-level magnetic dipole and electric quadrupole constants for the fermionic isotopes. The center of gravity for the $^3P_1$ levels in $^{171}$Yb and $^{173}$Yb are also evaluated, where we find significant disagreement with previously reported values. These hyperfine constants provide a valuable litmus test for atomic many-body computations in ytterbium.

Real-time phase tracking for wide-band optical frequency measurements at the 20th decimal place

Optical frequency measurements are among the most precise tools available to science. With the rapid advances in optical clocks now achieving a low 10−17 stability at 1 s and averaging down to the 10−19 level in a few hundred seconds, real-time sensing of subtle phenomena becomes essential. To render possible such measurements, we introduce real-time optical phase tracking with ultra-low-noise frequency combs as a fundamental means to constantly monitor frequency offsets. This enables the characterization of optical frequency synthesis with stability and accuracy at the 20th decimal place within a measurement time of <100 s. To enable comb operation at this level of performance, dichroic heterodyne detection is used to compensate phase drifts occurring in the generation and dissemination of widely spaced optical frequencies. We qualify an example set-up by comparison with a reference system, measuring an offset between two combs of (5.4 ± 5.3) × 10−21 in one single measurement run of 1 × 105 s. By employing a Doppler cancellation technique, optical frequency synthesis is achieved with stability and accuracy in the 10−20 range within 100 s. An offset between two optical frequency combs phase-locked at 1,542 nm is obtained as 5.4 × 10−21 at 1,063 nm within 105 s.

High contrast linking six lasers to a 1 GHz Yb:fiber laser frequency comb

We demonstrate a 0.95 GHz repetition rate fully stabilized Yb:fiber frequency comb without optical amplification. Benefitted from the high mode power and high coherence, this comb achieved 35 dB to 42 dB signal to noise ratio on the direct heterodyne beat signals with at least 6 continuous wave lasers (at 580nm, 679nm, 698nm, 707nm, 813nm and 922 nm) while keeping >40 dB carrier envelop frequency signal. It can be used for the direct measurement of optical frequencies in visible and near-infrared wavelengths, and has a great potential on simultaneous comparison of multiple optical frequencies.

Optical frequency measurement comparison using fiber laser combs between NIMT and CMS

A NIMT optical frequency comb was compare with an Er-fiber laser comb with repetition frequency of 250 MHz made by the Center for Measurement Standards (CMS), Taiwan by simultaneously measuring the frequency of a 633 nm He-Ne laser. The difference of average frequency measurement is 283 Hz and corresponds to a relative difference of 6.0x10-13.

Octave-spanning optical frequency comb based on a laser-diode pumped Kerr-lens mode-locked Yb:KYW laser for optical frequency measurement.

We developed an optical frequency comb based on a Yb:KYW laser. Soft-aperture Kerr-lens mode-locking at the cavity transverse-mode degeneration enabled us to generate 360mW from a 750mW pump laser diode. This resulted in spectral broadening over one octave using just a photonic crystal fiber. We achieved a free-running linewidth of 15kHz in the carrier-envelope offset frequency by optimizing the cavity group delay dispersion, crystal position, and pump laser power, which led to a residual phase noise of 0.51rad during phase-locking. We measured the frequency drift of a cavity-stabilized laser for a clock transition in Yb171+.