Self-referenced Optical Frequency Comb Research Articles

Powerful 1-µm 1-GHz optical frequency comb.

A self-referenced optical frequency comb is presented based on Kerr-lens mode-locking of ytterbium-doped CALGO. The robust source delivers 3.5 W average power in 44 fs-long pulses at 1 GHz repetition rate. The residual root-mean-square timing jitter of the emitted pulse-train is 146 fs and the residual integrated phase noise of the carrier-envelope offset frequency is 107 mrad, both in a span from 1 Hz to 10 MHz. After stabilization, 2.7 W average power remains for direct application. This work represents the first multi-mode pumped Kerr-lens mode-locked optical frequency comb at gigahertz-level repetition rate.

Palm-sized, vibration-insensitive, and vacuum-free all-fiber-photonic module for 10−14-level stabilization of CW lasers and frequency combs

Compact and robust frequency-stabilized laser sources are critical for a variety of fields that require stable frequency standards, including field spectroscopy, radio astronomy, microwave generation, and geophysical monitoring. In this work, we applied a simple and compact fiber ring-resonator configuration that can stabilize both a continuous-wave laser and a self-referenced optical frequency comb to a vibration-insensitive optical fiber delay-line. We could achieve a thermal-noise-limited frequency noise level in the 10 Hz–1 kHz offset frequency range for both the continuous-wave laser and the optical frequency comb with the minimal frequency instability of 2.7 × 10−14 at 0.03-s and 2.6 × 10−14 at 0.01-s averaging time, respectively, under non-vacuum conditions. The optical fiber spool, working as a delay reference, is designed to be insensitive to external vibrations, with a vibration sensitivity of sub-10−10 (1/g) and a volume of 32 ml. Finally, the ring-resonator setup is packaged in a palm-sized aluminum case with 171-ml volume with a vibration-insensitive spool, as well as an even smaller 97-ml-volume case with an ultracompact 9-ml miniaturized fiber spool.

Broadband optical frequency comb covering spectral regions at UV, VIS, and NIR

Herein, we present a compact, robust, self-referenced optical frequency comb (OFC) ranging from ultraviolet (UV) to near-infrared (NIR) light. The OFC system employs an all-polarization-maintained fiber figure-9 fiber oscillation as a seed laser, which is separated into two branches at the same repetition rate of approximately 100 MHz. One branch is adopted to implement frequency stabilization. Herewith, the repetition rate and the carrier-envelope phase are separately stabilized while the residual phase noises are reduced to 504 μrad and 1.12 rad (ranging from 1 Hz to 3 MHz), corresponding to fractional frequency stabilities of 1.91 × 10−12 and 1.44 × 10−17, respectively. The other branch is used to implement supercontinuum generation and quadruplicate frequency nonlinear processes, thus achieving an OFC system in the ranges of 266, 532, and 600–2200 nm. The precise frequency transferring characteristics of this OFC system exhibit great potential for precision measurement applications and optical frequency synchronization.

Comb-assisted mercury spectroscopy in the deep-ultraviolet: towards the development of a new primary thermometer

We report on the development of a new primary thermometer based upon high-precision spectroscopy of mercury vapors in the deep-ultraviolet region for the practical realization of the new kelvin. The line profile of the (6s 2)1 S 0 → (6s6p)3 P 1 intercombination transition of the 200Hg bosonic isotope is observed with a high spectral fidelity using a coherent radiation source at 253.7 nm. This latter consists of a near-IR external cavity diode laser followed by a double-stage second-harmonic generation apparatus. Metrology grade UV spectroscopy is demonstrated by locking the diode laser to a self-referenced optical frequency comb synthesizer.

Modified time-delay interferometry with an optical frequency comb

Both the fluctuations of the onboard lasers and clocks in a space-based gravitational wave interferometer can significantly decrease the sensitivity of the detector to an unacceptable performance level. Implementing a self-referenced optical frequency comb (OFC) driven by the onboard laser can establish the relationship between these two phase fluctuations. This makes a modification of the time-delay interferometry (TDI) combinations, which is originally used to reduce the larger laser phase noise, to simultaneously reduce the phase noises of the onboard lasers and clocks. At present, only a few typically modified combinations with an OFC have been constructed. In this work, we develop a general method obtaining all of the modified TDI combinations, the core of which is a skillful substitution of the time-delay operator. More specifically, with a skillful substitution, one can directly derive the modified TDI combination from an arbitrary TDI combination. This method avoids studying how to reconstruct the modified TDI combination with an OFC.

Absolute frequency stabilization of a QCL at 8.6 µm by modulation transfer spectroscopy.

Modulation transfer spectroscopy is used to demonstrate absolute frequency stabilization of an 8.6-µm-wavelength quantum cascade laser against a sub-Doppler absorption of the CHF3 molecule. The obtained spectral emission properties are thoroughly characterized through a self-referenced optical frequency comb, stabilized against either a GPS-disciplined Rb clock or a 1.54-µm Er-fiber laser locked to a high-finesse ultra-low-expansion optical cavity. Fractional long-term stability and accuracy at a level of 4×10-12 (at 100 s) and 3×10-10, respectively, are demonstrated, along with an emission linewidth as narrow as 10 kHz for observation times of 0.1 s.

Optical feedback laser absorption spectroscopy of N2O at 2 µm

We present high-accuracy measurements of the spectroscopic parameters of eight nitrous oxide transitions, mainly belonging to the P branch of the 3ν1+2ν2 vibrational band, in the spectral region around 4982 cm−1. We acquired absorption spectra in pure N2O samples, at room-temperature, by means of a diode laser based spectrometer in which the laser emission width was effectively narrowed by exploiting the optical feedback from a V-shaped high-finesse cavity. The spectrometer was assisted by a self-referenced optical frequency comb synthesizer for frequency calibration purposes. Light absorption from N2O molecules occurred inside a multiple reflections cell, which was placed on the transmission from the cavity. The successful application of a global fitting approach to a manifold of spectra, involving the symmetric uncorrelated version of quadratic speed-dependent hard collision profile, allowed us to determine line intensities with an overall relative uncertainty of 0.38%. We found a satisfactory agreement with data provided by the HITRAN and NOSD-1000 databases. Self-broadening coefficients, speed dependence components of the collisional line broadening, and Dicke narrowing parameters were also determined.

Tunable optical frequency comb from a compact and robust Er:fiber laser

We report on a compact and robust self-referenced optical frequency comb with a tunable repetition rate, generated by an all-polarization-maintaining (PM) mode-locked Er-doped fiber laser. The spacing between comb teeth can be tuned above 300 kHz at a repetition rate of 101 MHz. The repetition rate and the carrier–envelope offset of the laser are stabilized separately, and the relative residual phase noises are determined to be $336~\unicode[STIX]{x03BC}\text{rad}$ and 713 mrad (1 Hz–1 MHz). The accurate frequency characteristics and the stable structure show great potential for the use of such a comb in applications of precision measurements.

Dual-laser frequency-stabilized cavity ring-down spectroscopy for water vapor density measurements

Absolute measurements of water vapor densities have been carried out using a new concept of frequency-stabilized cavity ring-down spectroscopy. Our scheme is based on the use of a pair of phase-locked extended cavity diode lasers, emitting at 1.39 m, one of them being locked to a self-referenced optical frequency comb synthesizer. An intrinsically stable high-finesse optical cavity that tracks the laser frequency scans has been used. High quality, comb-calibrated, absorption spectra have been recorded in a certified H2O/N2 gas mixture at different gas pressures, in coincidence with the 2 transition of the H2 18O + band. Water vapor mole fractions have been determined with a statistical uncertainty of 0.6. Systematic deviations have been identified and carefully quantified, thus leading to an overall uncertainty of 0.8.

Low noise, self-referenced all polarization maintaining Ytterbium fiber laser frequency comb.

We report the implementation of a self-referenced optical frequency comb generated by a passively mode-locked all polarization maintaining (PM) Yb fiber laser based on a nonlinear amplifying loop mirror (NALM). After spectral broadening the optical spectrum spans from 650 nm to 1400 nm, allowing for the generation of an optical octave and carrier envelope offset frequency (fceo) stabilization through a conventional f-2f interferometer. We demonstrate for the first time the stabilization of the fceo of such a PM Yb system with an in-loop fractional frequency stability scaled to an optical frequency of low 10-19 at 1 second averaging time, offering a great potential for applications in optical atomic clock metrology.

Self-referenced, accurate and sensitive optical frequency comb spectroscopy with a virtually imaged phased array spectrometer

We present a cavity-enhanced direct optical frequency comb spectroscopy system with a virtually imaged phased array (VIPA) spectrometer and either a dither or a Pound-Drever-Hall (PDH) locking scheme used for stable transmission of the comb through the cavity. A self-referenced scheme for frequency axis calibration is shown along with an analysis of its accuracy. A careful comparison between both locking schemes is performed based on near-IR measurements of the carbon monoxide ν=3←0 band P branch transitions in a gas sample with known composition. The noise-equivalent absorptions (NEA) for the PDH and dither schemes are 9.9×10(-10) cm(-1) and 5.3×10(-9) cm(-1), respectively.

Brillouin filtering of optical combs for narrow linewidth frequency synthesis

We report a tunable monochromatic source generation scheme, based on Brillouin filtering of a self-referenced optical frequency comb. The system benefits from the high stability and mode linewidth of the frequency comb, significantly improving the performance of the original laser source used as Brillouin pump. A synthesized frequency with stability under 4×10−11 and a linewidth under 75kHz was experimentally demonstrated for two separate pump lasers in the C-band. The proposed monochromatic source can be tuned with high precision in a very broad band by combining a coarse control with the pumping source and a fine control with the optical frequency comb references. In our experimental setup, coarse and fine tuning resolutions were 4MHz and 20Hz, respectively. Influence of Brillouin pump fluctuations in the synthesized signal stability were also analyzed for observation times up to 104s.

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.

A three-step laser stabilization scheme for excitation to Rydberg levels in 85Rb

We demonstrate a three-step laser stabilization scheme for excitation to nP and nF Rydberg states in 85Rb, with all three lasers stabilized using active feedback to independent Rb vapor cells. The setup allows stabilization to the Rydberg states 36P3/2–70P3/2 and 33F7/2–90F7/2, with the only limiting factor being the available third step laser power. We study the scheme by monitoring the three laser frequencies simultaneously against a self-referenced optical frequency comb. The third step laser, locked to the Rydberg transition, displays an Allan deviation of 30 kHz over 1 second and <80 kHz over 1 hour. The scheme is very robust and affordable, and it would be ideal for carrying out a range of quantum information experiments.

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.

Absolute frequency measurements of 85RbnF7/2 Rydberg states using purely optical detection

A three-step laser excitation scheme is used to make absolute frequency measurements of highly excited nF7/2 Rydberg states in 85Rb for principal quantum numbers n=33–100. This work demonstrates the first absolute frequency measurements of rubidium Rydberg levels using a purely optical detection scheme. The Rydberg states are excited in a heated Rb vapour cell and Doppler-free signals are detected via purely optical means. All of the frequency measurements are made using a wavemeter that is calibrated against a Global Positioning System (GPS)-disciplined self-referenced optical frequency comb. We find that the measured levels have a very high frequency stability, and are especially robust against electric fields. The apparatus has allowed measurements of the states to an accuracy of 8.0 MHz. The new measurements are analysed by extracting the modified Rydberg–Ritz series parameters.

10-GHz Self-Referenced Optical Frequency Comb

The femtosecond laser-based frequency comb has played a key role in high-precision optical frequency metrology for a decade. Although often referred to as a precise optical frequency ruler, its tick marks are in fact too densely spaced for direct observation and individual use, limiting important applications in spectroscopy, astronomy, and ultrafast electromagnetic waveform control. We report on a femtosecond laser frequency comb with a 10-gigahertz repetition rate that creates a stabilized output spectrum with coverage from 470 to 1130 nanometers. The individual modes can be directly resolved with a grating spectrometer and are visible by eye.

Measurement of acetylene-d absorption lines with a self-referenced fiber laser frequency comb

A self-referenced optical frequency comb, derived from a mode-locked fiber laser, and a cavity-enhanced diode-laser-based saturated absorption spectrometer were employed to measure the absolute frequency of 55 absorption lines of the P and R branches of the 2ν1 band of 12C2HD at 1.5 μm with a one standard deviation uncertainty of better than 2.5 kHz for all lines measured. The shift sensitivities of the P(16) line to changes in power, pressure, and modulation amplitude have been studied in detail. Improved values of the ground-state molecular constants are obtained.

A low-threshold self-referenced Ti:Sapphire optical frequency comb

We demonstrate an octave-spanning, self-referenced optical frequency comb produced with a high-repetition-rate (frep=585 MHz) femtosecond Ti:Sapphire laser that requires less than 1 W of 532 nm pump power. The frequency comb was stabilized to a CW laser as required for optical clocks and low noise frequency synthesis. These results should be relevant for applications that require more-compact and efficient frequency combs.

Direct frequency comb generation from an octave-spanning, prismless Ti:sapphire laser.

We describe a self-referenced optical frequency comb generator based on an octave-spanning, prismless Ti:sapphire laser. Dispersion compensation is provided by novel double-chirped mirror pairs and BaF2 wedges. Current versions operate at 80 and 150 MHz. The compact prismless design allows system scaling to a gigahertz repetition rate. Its carrier-envelope beat note is intrinsically stable with a signal-to-noise ratio of 30 dB in a 100-kHz bandwidth. The octave is reached at 25 dB below the average power level. The in-loop accumulated phase error is 1.4 rad (20 mHz to 1 MHz). The technique has the advantages of simplicity and stability compared with previous designs.