Optical Frequency Synthesis Research Articles

Long-term Absolute Wavelength Stability of Acetylene-stabilized Reference Laser at 1533 nm

The second harmonic generation process in Periodically Poled Lithium Niobate (PPLN) has been applied in order to measure frequency of reference laser locked to acetylene absorption peak 12C2H2 (P13) (1533 nm) against optical frequency synthesizer. The measurement results have been compared to the results obtained using different techniques for the same reference laser during the past 10 years in other laboratories.

From quantum transitions to electronic motions

Laser spectroscopy and chromoscopy permit precision measurement of quantum transitions and captures atomic-scale dynamics, respectively. Frequency- and time-domain metrology ranks among the supreme laser disciplines in fundamental science. For decades, these fields evolved independently, without interaction and synergy between them. This has changed profoundly with controlling the position of the equidistant frequency spikes of a mode-locked laser oscillator. By the self-referencing technique invented by Theodor Hänsch, the comb can be coherently linked to microwaves and used for precision measurements of energy differences between quantum states. The resultant optical frequency synthesis has revolutionized precision spectroscopy. Locking the comb lines to the resonator round-trip frequency by the same approach has given rise to laser pulses with controlled field oscillations. This article reviews, from a personal perspective, how the bridge between frequency- and time-resolved metrology emerged on the turn of the millennium and how synthesized several-cycle laser fields have been instrumental in establishing the basic tools and techniques for attosecond science.

A low noise optical frequency synthesizer at 700–990 nm

Optical frequency synthesizers can generate single-frequency laser light with high precision and accuracy at any desired wavelength over a wide optical region. Here, we demonstrate such an optical frequency synthesizer, which yields coherent light at any wavelength within 700–990 nm with more than 500 mW of power. The relative fractional frequency instability and uncertainty between the output light and the reference light of the optical frequency synthesizer are 6 × 10−19 at 1 s averaging time and 2 × 10−21, respectively. This synthesis noise is two orders of magnitude better than the frequency stability and accuracy provided by optical clocks, supporting optical frequency synthesis from the most accurate optical clocks. When the optical frequency synthesizer is referenced to a cavity-stabilized laser at 1064 nm, the output of the optical frequency synthesizer is tested to have an average linewidth of 1 Hz and frequency instability of 1.5 × 10−15 at 1 s, limited by the reference laser.

Multicolor cavity soliton.

We show a new class of complex solitary wave that exists in a nonlinear optical cavity with appropriate dispersion characteristics. The cavity soliton consists of multiple soliton-like spectro-temporal components that exhibit distinctive colors but coincide in time and share a common phase, formed together via strong inter-soliton four-wave mixing and Cherenkov radiation. The multicolor cavity soliton shows intriguing spectral locking characteristics and remarkable capability of spectrum management to tailor soliton frequencies, which would be very useful for versatile generation and manipulation of multi-octave spanning phase-locked Kerr frequency combs, with great potential for applications in frequency metrology, optical frequency synthesis, and spectroscopy.

Traceability of laser frequency/wavelength calibration through the frequency comb at Inmetro

The acquisition of a femtosecond laser comb by the Optical Metrology Division of Inmetro now allows for carrying out high precision calibrations of optical frequencies for lasers which are used as standards of the length unit with gauge block interferometers. The frequency comb is operated as an optical frequency synthesizer and is presently linked to the time unit by a 10 MHz oscillator which is disciplined by GPS. Laser frequencies are determined with accuracy in the range of few parts in 1012. This measurement method now links the length unit, meter, to the SI-second attending the recommendation by the BIPM.

Counting the cycles of light using a self-referenced optical microresonator

Phase coherently linking optical to radio frequencies with femtosecond mode-locked laser frequency combs enabled counting the cycles of light and is the basis of optical clocks, absolute frequency synthesis, tests of fundamental physics, and improved spectroscopy. Using an optical microresonator frequency comb to establish a coherent link between optical and microwave frequencies will extend optical frequency synthesis and measurements to areas requiring compact form factor, on chip integration and comb line spacing in the microwave regime, including coherent telecommunications, astrophysical spectrometer calibration or microwave photonics. Here we demonstrate a microwave to optical link with a microresonator. Using a temporal dissipative single soliton state in an ultra-high Q crystalline microresonator that is broadened in highly nonlinear fiber an optical frequency comb is generated that is self-referenced, allowing to phase coherently link a 190 THz optical carrier directly to a 14 GHz microwave frequency. Our work demonstrates precision optical frequency measurements can be realized with compact high Q microresonators.

Absolute gauge block calibration using ultra-precise optical frequency synthesizer locked to a femtosecond comb.

In this paper, we report a gauge block (GB) calibration that is traceable to the SI unit of time, the second. Four ultra-stable optical telecommunication wavelengths near 1556 nm are obtained by locking a narrow-tuning-range fiber laser to a fiber-based femtosecond frequency comb. Since the GB calibration system does not operate at this region of spectrum, the superior frequency stability of the laser is transferred to the 778 nm region by using a waveguide periodically poled lithium niobate crystal. After applying the locking scheme, the stability and accuracy of the laser become better than 8×10(-12). The frequency-doubled light is sent through 30 m optical fiber to a GB interferometer, which is installed at a different laboratory in the same building. Using this calibration scheme, a GB with a nominal length of 100 mm is calibrated with an uncertainty of ±52 nm. This uncertainty value is still comparable to or even better than other metrology laboratories for a similar block length.

Ultrahigh Count Coherent WDM Channels Transmission Using Optical Parametric Comb-Based Frequency Synthesizer

A novel optical frequency synthesizer capable of generating a multitude of narrow linewidth oscillations, fully compatible with high-speed fiber-optic transmission requirements, is presented. The new device is based on an ultra-dense and wideband parametric comb capable of generating more than 1800 spectral lines with 6.25 GHz spacing within 100 nm (C + L band) of optical bandwidth and 3 dB spectral flatness. The parametric comb generation relies on the highly efficient nonlinear interaction of the ultra-short and high peak power pulse train in the precisely controlled dispersive medium. The generated comb source is used for coherent transmission of 1520 Nyquist ultra-dense wavelength division multiplexed channels in a flex-grid compatible optical access network architecture, offering up to 30 Gb/s per user over a 50 km of standard single mode fiber with estimated aggregate capacity of 32 Tb/s. The line spacing tunablity and one-to-multiple coherent cloning capability make the proposed multicarrier generator a strong contender fully capable of replacing thousands of discrete lasers in optical data communication systems.

A High Stable Optical Frequency Synthesizer and its Application

A High Stable Optical Frequency Synthesizer and its Application

Precision frequency measurement of 1S0–3P1 intercombination lines of Sr isotopes**Project supported by the National Natural Science Foundation of China (Grant No. 61127901) and the Key Project of the Chinese Academy of Sciences (Grant No. KJZD-EW-W02).

We report on frequency measurement of the intercombination (5s2)1S0–(5s5p)3P1 transition of the four natural isotopes of strontium, including 88Sr (82.58%), 87Sr (7.0%), 86Sr (9.86%), and 84Sr (0.56%). A narrow-linewidth laser that is locked to an ultra-low expansion (ULE) optical cavity with a finesse of 12000 is evaluated at a linewidth of 200 Hz with a fractional frequency drift of 2.8×10−13 at an integration time of 1 s. The fluorescence collector and detector are specially designed, based on a thermal atomic beam. Using a double-pass acousto-optic modulator (AOM) combined with a fiber and laser power stabilization configuration to detune the laser frequency enables high signal-to-noise ratios and precision saturated spectra to be obtained for the six transition lines, which allows us to determine the transition frequency precisely. The optical frequency is measured using an optical frequency synthesizer referenced to an H maser. Both the statistical values and the final values, including the corrections and uncertainties, are derived for a comparison with the values given in other works.

Laser frequency counting and frequency modulation width measurement from a fft signal

Abstract A new method of optical frequency beat counting based on fast Fourier transform (FFT) analysis is described. Signals with a worse signal-to-noise ratio can be counted correctly comparing to the conventional counting method of detecting each period separately. The systematic error of FFT counting below 10 Hz is demonstrated and can be decreased. Additionally the modulation width of a frequency-stabilized laser with high frequency modulation index can be simultaneously measured during a carrier frequency measurement against an optical frequency synthesizer or other laser.

Phase-shifting interferometer using a frequency-tunable diode laser calibrated by an optical frequency comb

We present a phase-shifting interferometer based on a frequency-tunable diode laser calibrated by an optical frequency comb and the Carré algorithm. By use of the frequency control strategies of locking the diode laser to different comb modes and scanning the repetition rate, an arbitrary single optical frequency synthesizer is obtained. The relative laser frequency uncertainty is 5.7 × 10(-12) for 1 s averaging time with tracing to an Rb clock and accurate phase steps are achieved by optical frequency tuning. The surface topography of a standard sphere is measured by this phase-shifting interferometer based on a flat reference. The phase measurement repeatability is λ/200. With this technique, phase measurement uncertainties from the laser frequency and phase steps are negligible.

Widely and continuously tunable terahertz synthesizer traceable to a microwave frequency standard

We constructed a widely and continuously tunable terahertz frequency synthesizer traceable to a hydrogen maser linked to coordinated universal time. Photomixing of two optical frequency synthesizers, linked to the hydrogen maser via dual optical frequency combs, gave this THz synthesizer frequency uncertainty of 10⁻¹². To demonstrate the potential of wide and continuous tunability in the THz synthesizer, we tuned its output frequency up to 50 GHz discretely and 1.26 GHz continuously in the F-band while maintaining the unprecedented frequency uncertainty by using a uni-traveling-carrier photodiode as a photomixer. This THz synthesizer will be a powerful tool for broadband, high-precision THz spectroscopy and THz frequency metrology.

Optical frequency comb comparison between optical clock mode and optical frequency synthesizer mode

We report on an accuracy limit test result of the optical frequency comb technique by comparing the frequencies of two optical frequency combs that operate in two different modes. These two modes are distinguished by the reference frequency for the comb frequency stabilization; an optical frequency (in an optical clock mode) or a microwave frequency (in an optical frequency synthesizer mode). The comparison of the two combs was carried out by simultaneously measuring an absolute frequency of an acetylene-stabilized diode laser with both combs. The frequencies of the two combs agreed with each other within the uncertainty of 16 Hz at 194 THz. The uncertainty is reduced by more than an order compared with a previous report.

光コムによる精度19桁級の周波数合成

光コムによる精度19桁級の周波数合成

Demonstration of an optical frequency synthesizer with zero carrier-envelope-offset frequency stabilized by the direct locking method

We developed an optical frequency synthesizer (OFS) with the carrier-envelope-offset frequency locked to 0 Hz achieved using the "direct locking method." This method differs from a conventional phaselock method in that the interference signal from a self-referencing f-2f interferometer is directly fed back to the carrier-envelope-phase control of a femtosecond laser in the time domain. A comparison of the optical frequency of the new OFS to that of a conventional OFS stabilized by a phase-lock method showed that the frequency comb of the new OFS was not different to that of the conventional OFS within an uncertainty of 5.68x10(-16). As a practical application of this OFS, we measured the absolute frequency of an acetylene-stabilized diode laser serving as an optical frequency standard in optical communications.

Discretely Tunable Optical Frequency Synthesizer Utilizing a Femtosecond Fiber Laser Injection-Locking Technique

In this letter, we propose a discretely tunable optical frequency synthesizer (DTOFS) based on distributed-feedback (DFB) lasers that can be selectively injection locked to a desired single mode of the femtosecond fiber laser comb. The methods for the injection locking, the monitoring of the lock status, and the selection of the desired comb mode are given. We demonstrate the discrete tunability of the DTOFS by constructing two frequency-stabilized DFB lasers, both of which are injection locked to a respective single mode of the fiber laser comb with the frequency difference, which is an exact multiple of the comb repetition frequency.

Precision spectroscopy of acetylene transitions using an optical frequency synthesizer

An optical frequency synthesizer is used for saturation spectroscopy of acetylene near 1540 nm. In the synthesizer, a user-specified frequency is generated from an atomic time base by phase locking the second harmonic of a cw near-IR external-cavity diode laser (ECDL) to a Ti:sapphire frequency comb. By stepping the repetition rate of the frequency comb, the ECDL frequency is swept over an acetylene transition in a saturated absorption spectroscopy setup. Hence, a spectral lineshape is measured with an absolute frequency scale. Line-center frequencies determined by fitting theoretical line profiles to the measured data are in good agreement with values measured with the ECDL stabilized to acetylene by third-harmonic locking and with the values recommended by the International Committee for Weights and Measures (CIPM).

Widely tunable, single-mode optical frequency synthesizer with a 100 kHz uncertainty

A tunable cw Ti:sapphire laser is locked to a self-referenced femtosecond frequency comb generated by an erbium-doped fiber laser thereby creating a single-frequency optical frequency synthesizer. This synthesizer has a tuning range of 760-820 nm with an absolute frequency uncertainty less than 100 kHz. We demonstrate the precision of this cw optical frequency synthesizer by measuring the optical frequency of several transitions on the D2 lines of 85Rb and 87Rb using a saturated absorption spectrometer.

Optical frequency synthesis and measurement using fibre-based femtosecond lasers

We report the synthesis and measurement of an ultra-precise and extremely stable optical frequency in the telecommunications window around 1543 nm. Using a fibre-based femtosecond frequency comb we have phase-stabilised a fibre laser at 194 THz to an optical frequency standard at 344 THz, thus transferring the properties of the optical frequency standard to another spectral region. Relative to the optical frequency standard, the synthesised frequency at 194 THz is determined to within 1 mHz and its fractional frequency instability is measured to be less than 2×10-15 at 1 s, reaching 5× 10-18 after 8000 s. We also measured the synthesised frequency against a caesium fountain clock: here the frequency comparison itself contributes less than 4 mHz (2×10-17) to the uncertainty. Our results confirm the suitability of fibre based frequency comb technology for precision measurements and frequency synthesis, and enable long-distance comparison of optical clocks by using optical fibres to transmit the frequency information.