Optical Frequency Comb Synthesizer Research Articles

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.

Measurement of the mercury (6s6p)P13 -state lifetime in the frequency domain from integrated absorbance data

Quantitative UV laser absorption spectroscopy is implemented to measure the strength of the $(6{s}^{2})\phantom{\rule{0.28em}{0ex}}^{1}S_{0}\ensuremath{\rightarrow}(6s6p)^{3}P_{1}$ intercombination transition of Hg atoms near the wavelength of 253.7 nm and to retrieve the lifetime of the excited state. Tunable, single-mode, coherent radiation in the deep UV is produced through a double-stage second-harmonic generation process, starting from the output radiation of an external-cavity diode laser at 1014.8 nm. This latter is referenced to an optical frequency comb synthesizer. The measurement strategy exploits the relationship between the strength of an optical transition and the spontaneous emission Einstein coefficient. The complete uncertainty budget leads to an overall relative uncertainty of 0.6%, with a statistical contribution of 0.28 ns, which is the lowest ever obtained for the $(6s6p)\phantom{\rule{0.28em}{0ex}}^{3}P_{1}$ lifetime. The new value, amounting to $125.12\ifmmode\pm\else\textpm\fi{}0.79\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$, compares well with most of the past determinations within two times the quoted uncertainties. Despite this, it does not agree with the weighted average of the 12 prior values, thus suggesting that a critical reassessment of the uncertainty of some of the past measurements is probably necessary.

Spectral Broadening in a Continuously Pumped Singly Resonant Second-Harmonic Cavity

Optical frequency comb synthesizers with a wide spectral range are an essential tool for many research areas such as spectroscopy, precision metrology, optical communication, and sensing. Recent studies have demonstrated the direct generation of frequency combs, via second-order processes, that are centered on two different spectral regions separated by an octave. Here, we present the capability of optical quadratic frequency combs for broad-bandwidth spectral emission in unexplored regimes. We consider comb formation under phase-matched conditions in a continuous-wave pumped singly resonant second-harmonic cavity, with large intracavity power and control of the detuning over several cavity linewidths. The spectral analysis reveals quite distinctive sidebands that arise far away from the pump, singularly or in a mixed regime together with narrowband frequency combs. Notably, by increasing the input power, the optical frequency lines evolve into widely spaced frequency clusters, and at maximum power, they appear in a wavelength range spanning up to 100 nm. The obtained results demonstrate the capacity of second-order nonlinearities to produce direct comb within a wide range of wavelengths.

Homogeneous quantum cascade lasers operating as terahertz frequency combs over their entire operational regime

AbstractWe report a homogeneous quantum cascade laser (QCL) emitting at terahertz (THz) frequencies, with a total spectral emission of about 0.6 THz, centered around 3.3 THz, a current density dynamic range Jdr = 1.53, and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as an optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ∼200 µW of optical power per optical mode, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of about 500 Hz, is observed over a current density range of 240 A/cm2 and even across the negative differential resistance region. We further prove that the QCL FC can be injection locked with moderate radio frequency power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological applications requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.

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.

Dispersion pre-compensation of 25.6 Tbps waveforms using an optical frequency comb synthesizer/analyzer

In this study, we propose and demonstrate a dispersion compensation system for an ultrafast 25.6 Tbps waveform using multilevel 8-ary amplitude and 32-ary phase modulation. The waveform bit period was 312.5 fs, which was controlled and compensated by a 200 GHz optical frequency comb (OFC) synthesizer with a 6.4 THz bandwidth. Dispersion spectra were measured in parallel and simultaneously (within 1 ms), based on single-shot dual-heterodyne mixing by introducing an OFC and arrayed waveguide grating to separate sidebands. An optical pulse synthesizer (OPS) can individually control the phase and amplitude spectra of the OFC. To control transmitted waveforms in a dispersed media, the OPS can be oppositely biased to feed the measured dispersion back into the source waveform when it generates an arbitrary waveform. In this study, a 25.6 Tbps dispersion-free waveform was successfully transmitted through a 10.55 km fiber.

Optical Frequency Combs in Quadratically Nonlinear Resonators.

Optical frequency combs are one of the most remarkable inventions in recent decades. Originally conceived as the spectral counterpart of the train of short pulses emitted by mode-locked lasers, frequency combs have also been subsequently generated in continuously pumped microresonators, through third-order parametric processes. Quite recently, direct generation of optical frequency combs has been demonstrated in continuous-wave laser-pumped optical resonators with a second-order nonlinear medium inside. Here, we present a concise introduction to such quadratic combs and the physical mechanism that underlies their formation. We mainly review our recent experimental and theoretical work on formation and dynamics of quadratic frequency combs. We experimentally demonstrated comb generation in two configurations: a cavity for second harmonic generation, where combs are generated both around the pump frequency and its second harmonic and a degenerate optical parametric oscillator, where combs are generated around the pump frequency and its subharmonic. The experiments have been supported by a thorough theoretical analysis, aimed at modelling the dynamics of quadratic combs, both in frequency and time domains, providing useful insights into the physics of this new class of optical frequency comb synthesizers. Quadratic combs establish a new class of efficient frequency comb synthesizers, with unique features, which could enable straightforward access to new spectral regions and stimulate novel applications.

High Dynamic Range, Heterogeneous, Terahertz Quantum Cascade Lasers Featuring Thermally Tunable Frequency Comb Operation over a Broad Current Range

We report on the engineering of broadband quantum cascade lasers (QCLs) emitting at Terahertz (THz) frequencies, which exploit a heterogeneous active region scheme and have a current density dynamic range (Jdr) of 3.2, significantly larger than the state of the art, over a 1.3 THz bandwidth. We demonstrate that the devised broadband lasers operate as THz optical frequency comb synthesizers in continuous wave, with a maximum optical output power of 4 mW (0.73 mW in the comb regime). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24), significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of 1.05 THz ans a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB. Analysis of the electrical and thermal beatnote tuning reveals a current-tuning coefficient ranging between 5 MHz/mA and 2.1 MHz/mA and a temperature-tuning coefficient of -4 MHz/K. The ability to tune the THz QCL combs over their full dynamic range by temperature and current paves the way for their use as powerful spectroscopy tool that can provide broad frequency coverage combined with high precision spectral accuracy.

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.

Optical Frequency Comb Synthesizer & Analyzer Applied to Ultrafast Optical Waveform Control and Dispersion Measurement

Optical Frequency Comb Synthesizer & Analyzer Applied to Ultrafast Optical Waveform Control and Dispersion Measurement

Frequency-comb-assisted absolute calibration and linestrength of H12C13CH ro-vibrational transitions in the 2ν3 band

We report on a precision spectroscopic study of room-temperature H12C13CH in the 6463–6520 cm−1 range, using a continuous-wave diode laser phase-locked to a near-infrared optical frequency comb synthesizer stabilized, in turn, against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several ro-vibrational transitions in the P and R branches of the 2ν3 overtone band are recorded. Accuracy levels as low as 100 kHz, 5·10−10 in fractional terms, are obtained in the determination of absolute line-center frequencies. In addition, line intensities are measured for the first time, with a best fractional uncertainty of 1%, and used to retrieve the purely vibrational transition dipole moment in conjunction with the first-order Herman–Wallis factor. Finally, the pressure broadening coefficients γself are also derived for the studied transitions.

Highly accurate intensity factors of pure CO2 lines near 2 μm.

Line intensities for carbon dioxide are measured with a novel spectroscopic approach, assisted by an optical frequency comb synthesizer for frequency calibration purposes. The main feature of the spectrometer consists in the exploitation of optical feedback from a V-shaped high-finesse optical resonator to effectively narrow a distributed feedback diode laser at the wavelength of 2 μm. Laser-gas interaction takes place inside an isothermal cell, which is placed on the transmission from the cavity. High quality, self-calibrated, absorption spectra are observed in pure CO2 samples at different gas pressures, in coincidence with three lines of the R-branch of the ν1 + 2ν2 + ν3 band. Line intensities are determined using a global fitting approach in which a manifold of spectra are simultaneously analyzed across the range of pressures between 5 and 100 Torr, sharing a restricted number of unknown parameters. Various sources of uncertainty have been identified and carefully quantified, thus leading to an overall uncertainty ranging between 0.17% and 0.23%. The measured values are in a very good agreement with recent ab initio predictions.

Absolute frequency measurements of CHF3 Doppler-free ro-vibrational transitions at 8.6 μm.

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the υ5 vibrational band are investigated at around 1158.9 cm-1 with a fractional frequency precision of 8.6·10-12 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10-11 is mainly limited by the reproducibility of the frequency measurements.

An ultra-stable optical frequency standard for telecommunication purposes based upon the 5S1/2 → 5D5/2 two-photon transition in rubidium

In this study, we report the development of a frequency standard for optical fiber communication applications based on a two-photon transition in rubidium at 385.2 THz. This standard kills two birds with one stone in the sense it is capable of providing us with two highly stable serviceable wavelengths at 778.1 and 1556.2 nm. In this system, we exploit the narrow line-width of a fiber laser emitting at 1556.2 nm in conjunction with an erbium-doped fiber amplifier to generate a sufficient second harmonic laser beam at 778.1 nm in a periodically polled lithium niobate waveguide mixer in order to probe and frequency-lock the laser to the 5S1/2 (F g = 3)–5D5/2 (F e = 5) hyperfine two-photon transition component in 85Rb. The metrological performance of the standard is evaluated with the aid of an optical frequency comb synthesizer. Allan variance measurement shows a stability of 4 × 10−12 at 1 s (limited by the comb stability), reaching a floor of 6.8 × 10−13 at 1000 s. After correction of all the major systematic frequency shifts including the light shift, the absolute frequency is found to be 385 285 142 374.0 (5.0) kHz. Moreover, the absolute frequencies of most of the hyperfine components of the 5S1/2–5D5/2 transition of the two naturally existing rubidium isotopes are measured using a femtosecond frequency comb synthesizer after stabilizing a laser on each component.

Femtosecond Optical Arbitrary Waveform Control Based on Ultrafast Optical Frequency Comb Synthesizer and Analyzer

Femtosecond Optical Arbitrary Waveform Control Based on Ultrafast Optical Frequency Comb Synthesizer and Analyzer

Comb-assisted subkilohertz linewidth quantum cascade laser for high-precision mid-infrared spectroscopy

We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250 kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1 kHz, and its frequency is stabilized with an absolute traceability of 2×10−12. This source has allowed the measurement of the absolute frequency of a CO2 molecular transition with an uncertainty of about 1 kHz.

Ultrafast optical frequency comb synthesizer and analyzer

We propose an ultrafast optical arbitrary waveform synthesizing/analyzing technique demonstrated with 2 Tbit/s waveforms. An ultrafast waveform was generated by manipulating the amplitude and phase of a 400 GHz optical frequency comb using a newly developed colorless optical synthesizer. The 400 GHz optical frequency comb was generated from a 25 GHz optical frequency comb using a colorless arrayed waveguide grating. This waveform was then analyzed on the frequency axis using a custom heterodyne-detection technique based on the dual-heterodyne mixing method. The phase and amplitude spectra can be observed in parallel using another optical frequency comb as a reference combined with an arrayed waveguide grating. This optical system, named the ultrafast optical frequency comb synthesizer and analyzer, can synthesize and analyze an arbitrary waveform in the THz frequency region.

Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers

Optical frequency comb synthesizers have represented a revolutionary approach to frequency metrology, providing a grid of frequency references for any laser emitting within their spectral coverage. Extending the metrological features of optical frequency comb synthesizers to the terahertz domain would be a major breakthrough, due to the widespread range of accessible strategic applications and the availability of stable, high-power and widely tunable sources such as quantum cascade lasers. Here we demonstrate phase-locking of a 2.5 THz quantum cascade laser to a free-space comb, generated in a LiNbO(3) waveguide and covering the 0.1-6 THz frequency range. We show that even a small fraction (<100 nW) of the radiation emitted from the quantum cascade laser is sufficient to generate a beat note suitable for phase-locking to the comb, paving the way to novel metrological-grade terahertz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and telecommunications.

Optical frequency comb assisted laser system for multiplex precision spectroscopy

A laser system composed of two lasers phase-locked onto an Optical Frequency Comb Synthesizer (OFCS), operating around 1083 nm, was developed. An absolute frequency precision of 6x10(-13) at 1s, limited by the OFCS, was measured with a residual rms phase-noise of 71 mrad and 87 mrad for the two phase-locks, respectively. Multiplex spectroscopy on 1083 nm Helium transitions with this set-up is demonstrated. Generalization of this system to a larger number of OFCS assisted laser sources for wider frequency separations, even in other spectral regions, is discussed.

Absolute measurement of the S(0) and S(1) lines in the electric quadrupole fundamental band of D2 around 3 μm

The electric quadrupole fundamental (v=1←0) band of molecular deuterium around 3 μm is accessed by cavity ring-down spectroscopy using a difference-frequency-generation source linked to the Cs-clock primary standard via an optical frequency comb synthesizer. An absolute determination of the line position and strength is reported for the first two transitions (J=2←0 and J=3←1) of the S branch. An accuracy of 6×10(-8) is achieved for the line-center frequencies, which improves by a factor 20 previous experimental results [A. R. W. McKellar and T. Oka, Can. J. Phys. 56, 1315 (1978)]. The line strength values, measured with 1% accuracy, are used to retrieve the quadrupole moment matrix elements which are found in good agreement with previous theoretical calculations [A. Birnbaum and J. D. Poll, J. Atmos. Sci. 26, 943 (1969); J. L. Hunt, J. D. Poll, and L. Wolniewicz, Can. J. Phys. 62, 1719 (1984)].