Direct Comb Spectroscopy Research Articles

Sub-terahertz-repetition-rate frequency comb generated by filter-induced instabilities in passive driven fiber resonators

Ultrahigh-repetition-rate frequency comb generation exhibits great potential in applications of optical waveform synthesis, direct comb spectroscopy, and high capacity telecommunications. Here we present the theoretical investigations of a filter-induced instability mechanism in passive driven fiber resonators with a wide range of cavity dispersion regimes. In this novel concept of modulation instability, coherent frequency combs are demonstrated numerically with rates up to sub-terahertz level. Floquet stability analysis based on the Ikeda map is utilized to understand the physical origin of the filter-induced instability. Comparison with the well-known Benjamin–Feir instability and parametric instability is performed, revealing the intrinsic distinction in the family of modulation instabilities. Our investigations might benefit the development of ultrahigh-repetition-rate frequency comb generation, providing an alternative method for the microresonators.

260 kHz mode-spacing optical frequency combs for scan-free high-resolution direct-comb spectroscopy

For scan-free high-resolution direct-comb spectroscopy, mode spacing of an optical frequency comb is reduced down to 260 kHz by phase modulation. It turns out that time-domain signal is degraded by averaging because of slow optical path length fluctuations and fast optical pulse timing jitter. In this study, compensation of these effects is introduced, and signal degradation by averaging is avoided. With demonstrations of direct-comb spectroscopy with the narrow-mode-spacing optical frequency comb, Doppler-limited absorption spectrum of methane and reflection spectrum from an optical ring cavity are observed. As a result, detailed resonance spectral line profile of 8 MHz linewidth for the optical ring cavity is obtained in 50 ms measurement time.

Optical frequency comb-based cavity-enhanced Fourier-transform spectroscopy: Application to collisional line-shape study

Direct-comb spectroscopy techniques uses optical frequency combs (OFCs) as spectroscopic light source. They deliver high sensitivity, high frequency resolution and precision in a broad spectral range. Due to these features, the field has burgeoned in recent years. In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the near-infrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar. The highly sensitive measurements spanned the wavenumber range from 6270 cm−1 to 6410 cm−1, which covered both P and R branch of the second overtone band of CO. The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length, successfully removing ringing and broadening effects caused by instrumental line shape function. The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile. We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC. The results demonstrate that OFC-based cavity-enhanced Fourier-transform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.

Mode density multiplication of an optical frequency comb by N2 with phase modulation.

We introduce a simple scheme for mode-density multiplication of an optical frequency comb (OFC) by a factor of square of an arbitrary integer N using phase modulation. This scheme is employed to multiply the mode density of an erbium-doped fiber laser OFC (repetition rate of 66.87 MHz) by factors of 42, 82, and 3 · 42 using an electro-optic phase modulator. The OFC multiplied by 42 is applied to direct-comb spectroscopy of methane with a spectral resolution of 4.18 MHz.

Coherent, directional supercontinuum generation.

We demonstrate a novel approach to producing coherent, directional supercontinuum and cascaded dispersive waves using dispersion engineering in waveguides. By pumping in the normal group-velocity dispersion (GVD) regime, with two zero-GVD points to one side of the pump, pulse compression of the first dispersive wave generated in the anomalous GVD region results in the generation of a second dispersive wave beyond the second zero-GVD point in the normal GVD regime. As a result, we achieve an octave-spanning supercontinuum generated primarily to one side of the pump spectrum. We theoretically investigate the dynamics and show that the generated spectrum is highly coherent. We experimentally confirm this dynamical behavior and the coherence properties in silicon nitride waveguides by performing direct detection of the carrier-envelope-offset frequency of our femtosecond pump source using an f-2f interferometer. Our technique offers a path towards a stabilized, high-power, integrated supercontinuum source with low noise and high coherence, with applications including direct comb spectroscopy.

Complex direct comb spectroscopy with a virtually imaged phased array.

We demonstrate a simple interferometric technique to directly measure the complex optical transmittance over a large spectral range using a frequency-comb spectrometer based on a virtually imaged phased array. A Michelson interferometer encodes the phase deviations induced by a sample contained in one of its arms into an interferogram image. When combined with an additional image taken from each arm separately, along with a frequency-calibration image, this allows full reconstruction of the sample's optical transfer function. We demonstrate the technique with a vapor cell containing H13C14N, producing transmittance and phase spectra spanning 2.9 THz (∼23 nm) with ∼1 GHz resolution.

Coherent radio-frequency detection for narrowband direct comb spectroscopy.

We demonstrate a scheme for coherent narrowband direct optical frequency comb spectroscopy. An extended cavity diode laser is injection locked to a single mode of an optical frequency comb, frequency shifted, and used as a local oscillator to optically down-mix the interrogating comb on a fast photodetector. The high spectral coherence of the injection lock generates a microwave frequency comb at the output of the photodiode with very narrow features, enabling spectral information to be further down-mixed to RF frequencies, allowing optical transmittance and phase to be obtained using electronics commonly found in the lab. We demonstrate two methods for achieving this step: a serial mode-by-mode approach and a parallel dual-comb approach, with the Cs D1 transition at 894 nm as a test case.

Tracing part-per-billion line shifts with direct-frequency-comb Vernier spectroscopy

Accurate frequency measurements of molecular transitions around $2 \ensuremath{\mu}\mathrm{m}$ are performed by using a direct-frequency-comb spectroscopy approach that combines an ${\mathrm{Er}}^{+}$ frequency-comb oscillator at $1.5 \ensuremath{\mu}\mathrm{m}$, a Tm-Ho fiber amplifier, and a Fabry-Perot-filter, high-resolution dispersive spectrometer optical multiplex-detection system. This apparatus has unique performances in terms of a wide dynamic range to integrate the intensity per comb mode, which allows one to measure molecular absorption profiles with high precision. Spectroscopic information about transition frequencies and linewidths is very accurately determined. Relative frequency uncertainties of the order of a few parts in ${10}^{\ensuremath{-}9}$ are achieved for rovibrational transitions of the ${\mathrm{CO}}_{2}$ molecule around $5100 {\mathrm{cm}}^{\ensuremath{-}1}$. Moreover, tiny frequency shifts due to molecular collisions and interacting laser power using direct comb spectroscopy are investigated in a systematic way.

Phase-locked, chip-based, cascaded stimulated Brillouin scattering

Compact optical frequency comb sources with gigahertz repetition rates are desirable for a number of important applications including arbitrary optical waveform generation and direct comb spectroscopy. We report the generation of phase-locked, gigahertz repetition rate optical frequency combs in a chalcogenide photonic chip. The combs are formed via the interplay of stimulated Brillouin scattering and Kerr-nonlinear four-wave mixing in an on-chip Fabry–Perot waveguide resonator incorporating a Bragg grating. Phase-locking of the comb is confirmed with real-time measurements, and a chirp of the comb repetition rate within the pump pulse was observed. These results represent a significant step towards the realization of integrated optical frequency comb sources with gigahertz repetition rates.

Mid-infrared frequency comb for broadband high precision and sensitivity molecular spectroscopy.

We report on the experimental demonstration of the metrological and spectroscopic performances of a mid-infrared comb generated by a nonlinear downconversion process from a Ti:sapphire-based near-infrared comb. A quantum cascade laser at 4330 nm was phase-locked to a single tooth of this mid-infrared comb and its frequency-noise power spectral density was measured. The mid-infrared comb itself was also used as a multifrequency highly coherent source to perform ambient air direct comb spectroscopy with the Vernier technique, by demultiplexing it with a high-finesse Fabry-Perot cavity.

Universal formation dynamics and noise of Kerr-frequency combs in microresonators

Optical frequency combs allow for the precise measurement of optical frequencies and are used in a growing number of applications. The new class of Kerr-frequency comb sources, based on parametric frequency conversion in optical microresonators, can complement conventional systems in applications requiring high repetition rates such as direct comb spectroscopy, spectrometer calibration, arbitrary optical waveform generation and advanced telecommunications. However, a severe limitation in experiments working towards practical systems is phase noise, observed in the form of linewidth broadening, multiple repetition-rate beat notes and loss of temporal coherence. These phenomena are not explained by the current theory of Kerr comb formation, yet understanding this is crucial to the maturation of Kerr comb technology. Here, based on observations in crystalline MgF2 and planar Si3N4 microresonators, we reveal the universal, platform-independent dynamics of Kerr comb formation, allowing the explanation of a wide range of phenomena not previously understood, as well as identifying the condition for, and transition to, low-phase-noise performance. Based on observations in crystalline MgF2 and planar Si3N4 microresonators, scientists reveal that the existence of multiple and broad-beat notes in a Kerr-frequency comb is due to the formation dynamics of the comb itself. This work identifies the conditions requires for low-phase-noise performance and also helps to elucidate a number of yet-unexplained phenomena.

Spectroscopy of the methaneν3band with an accurate midinfrared coherent dual-comb spectrometer

We demonstrate a high-accuracy dual-comb spectrometer centered at 3.4 $\ensuremath{\mu}$m. The amplitude and phase spectra of the $P$, $Q$, and partial $R$ branches of the methane ${\ensuremath{\nu}}_{3}$ band are measured at 25 to 100 MHz point spacing with resolution under 10 kHz and a signal-to-noise ratio of up to 3500. A fit of the absorbance and phase spectra yields the center frequency of 132 rovibrational lines. The systematic uncertainty is estimated to be 300 kHz, which is ${10}^{\ensuremath{-}3}$ of the Doppler width and a 10-fold improvement over Fourier transform spectroscopy. These data quantify the accuracy and resolution achievable with direct comb spectroscopy in the midinfrared.