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Spectrometric techniques for frequency comb spectroscopy. a. Direct frequency comb spectroscopy with the example of two--photon Doppler--free excitation in a standing wave and detection of fluorescence of the sample. b. Ramsey--comb spectroscopy also with the example of two--photon Doppler--free excitation in a standing wave and detection of fluorescence of the sample. c. Frequency--comb spectrometry with a disperser for absorption measurements. Here a simple grating and a detector array are represented. d. Frequency--comb Fourier transform spectroscopy with a scanning Michelson interferometer and an absorbing sample. e. Dual--comb spectroscopy with one comb interrogating the sample and the other acting as a local oscillator. The absorption and the dispersion of the sample are measured.
Source publication
A laser frequency comb is a broad spectrum composed of equidistant narrow lines. Initially invented for frequency metrology, such combs enable new approaches to spectroscopy over broad spectral bandwidths, of particular relevance to molecules. The performance of existing spectrometers — such as crossed dispersers employing, for example, virtual ima...
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... frequency comb spectroscopy 8 (Fig. 3a) is the simplest approach to linear or nonlinear frequency comb spectroscopy. For linear spectroscopy 20,47 , a single comb line is resonant with a transition and all the other lines should ideally be detuned from Fig.4a), which can be as efficient as with a continuous--wave laser of the same average power. The excitation can even be ...
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... nonlinear excitation in the vacuum ultraviolet, reaching sufficient power with high--harmonic comb generators of large line spacing is a major challenge 46 . 3.2 Ramsey--comb spectroscopy Ramsey--comb spectroscopy 51 is a related time--domain technique (Fig.3b) that measures the interference between the excitations of an atomic or molecular sample by two time--delayed intense pulses derived from a frequency comb. ...
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... already demonstrated in the deep ultraviolet with two--photon transitions of H2 ( Fig.5b) around 202 nm (1,485 THz) 52 , Ramsey--comb spectroscopy holds particular promise, because the pairs of phase--coherent infrared pulses amplified to the millijoule level allow efficient frequency conversion. 3.3 Spectroscopy using a dispersive spectrometer Dispersive spectrographs (Fig.3c) provide simple and robust tools for multichannel approaches to broadband spectroscopy with frequency combs. ...
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... detector signal comprises this interference pattern, resulting from the down-- conversion of the optical frequencies mostly to the audio--range. When a frequency comb is used as a light source in front of the interferometer (Fig.3d), the frequency of each comb line, reflected in the moving arm of the interferometer, is shifted (Fig.4b) by a factor --2(n frep + f0) v/c, which gives the frequencies of the acoustic comb generated at the detector. ...
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... frequency--comb Fourier transform spectroscopy has shown remarkable results for disentangling complex spectra (Fig.5c) of heavy molecules 58, 59 . 3.5 Dual--comb spectroscopy Dual--comb spectroscopy (Fig.3e) is a comb--enabled approach to Fourier transform interferometry without moving parts 60 . This instrumental scheme of frequency comb spectroscopy is currently that which attracts the highest interest. ...
Citations
... High-precision detection of greenhouse gases is primarily required for achieving emission reduction. The traditional optical methods for detecting the greenhouse gases include non-dispersive infrared spectroscopy (NDIR) [1], cavity ring-down spectroscopy (CRDS) [2], Fourier transform spectroscopy (FTIR) [3], tunable diode laser absorption spectroscopy (TDLAS) [4,5], direct frequency comb spectroscopy (DFCS) [6], and photoacoustic spectroscopy (PAS) [7,8]. However, the detection sensitivity of sensors based on NDIR, CRDS, or TDLAS basically relies on stable optical paths and sensitive photodetectors. ...
A high-precision low-cost mid-infrared photoacoustic sensor for greenhouse composite gases based on aspherical beam shaping is proposed and demonstrated. The assembled optical source module and luminous characteristics of infrared source are innovatively investigated and analyzed with aspherical beam shaping. The proposed aspherical-beam-shaping-technique could effectively reduce optical loss and enhance system sensitivity, achieving an effective power utilization ratio of a radiation source of 91% and sidewall noise ratio of 8.9%. Experiments verify the 1.7 times improvement in responsivity and 50% enhancement in minimum detection limit (MDL) on average. In terms of comprehensive greenhouse gas composites and with short integration time of 1 s, MDLs of CO2, CH4, N2O, NF3, SF6, PFC-14, and HFC-134a are 73 ppb, 267 ppb, 72 ppb, 81 ppb, 14 ppb, 9 ppb and 115 ppb, respectively. Furthermore, a 48 h continuous monitoring of H2O, CO2 and CH4 in the atmosphere is conducted and verifies the performance of the gas sensor. The developed sensor allows for the rapid route of low-cost and high-precision detection of multiple greenhouse gases.
... O wing to the widespread applications in both scientific research and industries such as frequency combbased spectroscopy, [1][2][3] laser processing, 4,5) and bio-imaging, 6) high repetition rate fiber lasers have been developed intensively and many breakthroughs have been achieved. Techniques such as active mode-locking, 7) passively harmonic mode-locking, 8,9) dissipative four-wave mixing 10) and mode filtering 11) could achieve a very high repetition rate, however, they exhibit a higher instability in terms of output performance compared to fundamentally passive mode-locking. ...
We demonstrate a 783 MHz fundamental repetition rate mode-locked Er-doped all-fiber ring laser with a pulse width of 623 fs. By using carbon nanotubes saturable absorber, a relatively low self-starting pump threshold of 108 mW is achieved. The laser has a very compact footprint less than 10 cm × 10 cm, benefiting from the all-active-fiber cavity design. The robust mode-locking is confirmed by the low relative intensity noise and a long-term stability test. We propose a new scheme for generating high repetition rate femtosecond optical pulses from a compact and stable all-active-fiber ring oscillator.
... The high-peak-power ultrashort optical pulses play a crucial role in a wide range of applications, such as microwave photonics [1], atomic clocks [2], spectroscopy [3], optical communication [4] and optical wave synthesis [5]. Conventionally, high-peakpower ultrashort optical pulses are generated using table-top pulse lasers; however, this increases the complexity, volume, and cost of the systems [6]. ...
We demonstrate a circulator-free thin-film lithium niobate (TFLN) dispersion compensator based on the cascading 2 × 2 multimode interferometer (MMI) and two identical chirped Bragg gratings (CBGs). The cascaded MMI-CBG structure provides a dispersion value of 920 ps/nm/m over a 20 nm bandwidth covering 1537 to 1557 nm, featuring a compact footprint of 1 mm × 0.7 mm. Utilizing this device within a TFLN electro-optic time-lens system, we successfully generate 863-fs pulses at a 37 GHz repetition rate. Our compact, scalable, low-loss, and circulator-free dispersion compensator is the building block for the efficient generation of high-peak-power femtosecond laser pulses.
... 53 An alternative detection method coupled to the USF, and potentially quite promising, is that of direct frequency comb spectroscopy (DFCS). 54 It is still a broadband spectroscopic method and able to potentially monitor more than one species simultaneously, but now simply uses direct absorption spectroscopy via the Beer-Lambert Law, without having the same issues as chirpedpulse microwave spectroscopy with the high gas density. DFCS has been used successfully in a variety of gas-phase reaction kinetics studies, primarily in cavity-enhanced configurations surrounding flow cells or high temperature environments. ...
We present the development of a new astrochemical research tool HILTRAC, the Highly Instrumented Low Temperature ReAction Chamber. The instrument is based on a pulsed form of the CRESU (Cin\'etique de R\'eaction en \'Ecoulement Supersonique Uniforme, meaning reaction kinetics in a uniform supersonic flow) apparatus, with the aim of collecting kinetics and spectroscopic information on gas phase chemical reactions important in interstellar space or planetary atmospheres. We discuss the apparatus design and its flexibility, the implementation of pulsed laser photolysis followed by laser induced fluorescence (PLP-LIF), and the first implementation of direct infrared frequency comb spectroscopy (DFCS) coupled to the uniform supersonic flow. Achievable flow temperatures range from 32(3) - 111(9) K, characterising a total of five Laval nozzles for use with N2 and Ar buffer gases by pressure impact measurements. These results were further validated using LIF and DFCS measurements of the CH radical and OCS, respectively. Spectroscopic constants and linelists for OCS are reported for the 1001 band near $2890 - 2940 cm^{-1}$ for both $OC^{32}S$ and $OC^{34}S$, measured using DFCS. Additional peaks in the spectrum are tentatively assigned to the OCS-Ar complex. The first reaction rate coefficients for the CH + OCS reaction measured between 32(3) K and 58(5) K are reported. The reaction rate coefficient at 32(3) K was measured to be $3.9(4) \times 10^{10} cm^3 molecule^{-1} s^{-1}$ and the reaction was found to exhibit no observable temperature dependence over this low temperature range.
... 5 In particular, owing to their high coherence, high-frequency resolution, and broadband spectra, OFCs are considered to be an ideal tool/light source for molecular spectroscopy. [6][7][8] This technique, called direct frequency comb spectroscopy, has enabled groundbreaking results in fields as diverse as time-resolved spectroscopy, 9,10 human breath analysis, 11 and real-time sensing. 12 Conventionally, Fourier transform spectrometers (FTS), 13,14 gratingbased spectrometers, and virtually imaged phase arrays (VIPAs) 15 are used as spectrometers. ...
We present a compact, reliable, and robust free-running all-polarization-maintaining erbium (Er) single-cavity dual-comb laser generated via polarization multiplexing with gain sharing. Polarization multiplexing exploits the fast and slow axes of the fiber, while modelocking is achieved through a nonlinear amplifying loop mirror scheme using readily available components. The laser operates at a repetition rate of around 74.74 MHz with a tuning capability in the difference in repetition rates from 500 Hz to 200 kHz. This tunability makes the system more flexible for dual-comb spectroscopy experiments. Consequently, using this laser, we demonstrated a proof-of-principle dual-comb spectroscopy of carbon monoxide (CO), operating without any active stabilization.
... From the scaling relation Δ = 0 Δ / 0 we determine the frequency FSR Δ ≅ 215 MHz. Microresonators with such small FSRs are crucial for generating OFCs with low repetition rates (~100 MHz), which are important for applications like high-resolution spectroscopy [40,41] as well as for the fabrication of miniature tunable optical delay lines [42] Next, we demonstrate the tunability of the parabolic SNAP microresonator shown in Fig. 2(b) and repeatability of our system by varying the separation between the plates from 1 = 4.35 mm to 2 = 6.35 mm (a 2 mm range ) in cycles. In each cycle, we incrementally adjust in steps of 10 m, moving from 1 to 2 and then back from 2 to 1 . ...
Surface Nanoscale Axial Photonic (SNAP) microresonators are fabricated on silica optical fibers, leveraging silica's outstanding material and mechanical properties. These properties allow for precise control over the microresonator dimension, shape, and mode structure, a key feature for reconfigurable photonic circuits. Such circuits find applications in high-speed communications, optical computing, and optical frequency combs (OFC). However, consistently producing SNAP microresonators with equally spaced eigenmodes has remained challenging. In this study, we introduce a method to create a SNAP microresonator with a parabolic profile. We accomplish this by bending a silica optical fiber in a controlled manner using two linear stages. This approach achieves a uniform free spectral range (FSR) as narrow as 1 pm across more than 45 modes. We further demonstrate that the FSR of the SNAP microresonator can be continuously adjusted over a range nearly as wide as one FSR itself, specifically from 1.09 pm to 1.72 pm, with a precision of +/-0.01 pm and high repeatability. Given its compact size and tuning capability, this SNAP microresonator is highly promising for various applications, including the generation of tunable low-repetition-rate OFC and delay lines.
... Over the past two decades, the field of optical frequency combs has witnessed significant advancements, leading to the development of various types of frequency combs with distinct characteristics and applications [3][4][5][6] . The development of broad and stable combs has led to their integration in various fields, notably playing vital roles in the establishment of ultra-stable frequency references for metrology applications, high-capacity coherent telecommunication, and high-precision spectroscopy [7][8][9][10] . ...
... where the quantities in Eq. (8) have been replaced by their scalar-valued equivalents. ...
Performing noise characterizations of lasers and optical frequency combs on sampled and digitized data offers numerous advantages compared to analog measurement techniques. One of the main advantages is that the measurement setup is greatly simplified. Only a balanced detector followed by an analog-to-digital converter is needed, allowing all the complexity to be moved to the digital domain. Secondly, near-optimal phase estimators are efficiently implementable, providing accurate phase noise estimation in the presence of the measurement noise. Finally, joint processing of multiple comb lines is feasible, enabling computation of phase noise correlation matrix, which includes all information about the phase noise of the optical frequency comb. This tutorial introduces a framework based on digital signal processing for phase noise characterization of lasers and optical frequency combs. The framework is based on the extended Kalman filter (EKF) and automatic differentiation. The EKF is a near-optimal estimator of the optical phase in the presence of measurement noise, making it very suitable for phase noise measurements. Automatic differentiation is key to efficiently optimizing many parameters entering the EKF framework. More specifically, the combination of EKF and automatic differentiation enables the efficient optimization of phase noise measurement for optical frequency combs with arbitrarily complex noise dynamics that may include many free parameters. We show the framework's efficacy through simulations and experimental data, showcasing its application across various comb types and in dual-comb measurements, highlighting its accuracy and versatility. Finally, we discuss its capability for digital phase noise compensation, which is highly relevant to free-running dual-comb spectroscopy applications.
... Frequency combs serve as high-precision rulers for frequency and time measurement, playing a pivotal role in a wide variety of modern science and technologies [1][2][3][4][5], including optical clocks, LIDAR, spectroscopy, arbitrary waveform generation, and optical neural networks. Over the past two decades, integrated combs have garnered significant research interests [5][6][7][8][9][10][11][12][13][14][15][16][17][18], leading to miniaturized and chip-based photonic systems [7]. ...
... = 0 + , where 0 denotes the carrier offset frequency, represents the repetition frequency, and m stands for the mode number [1][2][3][4][5]. The extracted mode frequencies are shown in Fig. 1d, align perfectly with fitting to the comb formula. ...
... To examine the coherence state of the emitted signal, we firstly proved that each individual mode has a stable phase using the heterodyne detection technique (Supplementary Figure 2). One direct consequence of phase-coherent comb is the generation of pulses in the time domain [1][2][3][4][5]. Figure 2a displays a comb spectrum, and its corresponding time-dependent waveform is shown in Figs. ...
Frequency combs, specialized laser sources emitting multiple equidistant frequency lines, have revolutionized science and technology with unprecedented precision and versatility. Recently, integrated frequency combs are emerging as scalable solutions for on-chip photonics. Here, we demonstrate a fully integrated superconducting microcomb that is easy to manufacture, simple to operate, and consumes ultra-low power. Our turnkey apparatus comprises a basic nonlinear superconducting device, a Josephson junction, directly coupled to a superconducting microstrip resonator. We showcase coherent comb generation through self-started mode-locking. Therefore, comb emission is initiated solely by activating a DC bias source, with power consumption as low as tens of picowatts. The resulting comb spectrum resides in the microwave domain and spans multiple octaves. The linewidths of all comb lines can be narrowed down to 1 Hz through a unique coherent injection-locking technique. Our work represents a critical step towards fully integrated microwave photonics and offers the potential for integrated quantum processors.
... Therefore, QCLs are an effective technology for highly sensitive and selective molecular sensing by means of MIR spectroscopy. [3,4] However, free-running QCLs exhibit typical linewidths around half a MHz, [5,6] which limits applications requiring high spectral purity, hence restricting the impact and use of QCL-combs without a proper stabilization scheme. For example, cavityenhanced spectroscopy [7] with QCL-combs could boost the sensitivity to unprecedented levels. ...
... [8][9][10] Also there, tight stabilization could lead to higher sensitivity through continuous and long-term coherent averaging. [3] While the full stabilization of MIR QCLcombs has already been reported, [11] several aspects still need to be addressed. First, the actuation of the drive current allows the stabilization of one degree of freedom, [12] but as it exploits the same thermal effects as the noise process, [13,14] the spectral purity is limited. ...
Frequency combs are powerful tools for many applications and high performances are achieved by stabilizing these lasers. For operation in the mid‐infrared, quantum cascade lasers (QCL) are ideal candidates as they present numerous advantages. However, stabilized QCL‐combs lack a detailed characterization of their noise properties due to the sensitivity limits of current analyzing techniques. To overcome these challenges, what is believed to be the first tightly locked dual QCL‐comb system is developed. Its use is twofold. First, phase noise analysis of the dual‐comb signal shows residual phase noise below 600 mrad for all comb lines, and the comb coherence as well as the performances of the repetition frequency locking mechanism is characterized. Second, coherent averaging with a 7 × 10⁵ Hz1/2 figure‐of‐merit system is demonstrated, which is compatible with high‐precision spectroscopy.
... Accurate dual comb spectroscopy (DCS) relies on the precise control or knowledge of the phase, timing and amplitude relation between pulses in pairs coming from the two combs [1][2][3][4]. Any effect altering these parameters can lead to measurement errors. ...
Systematic errors are observed in dual comb spectroscopy when pulses from the two sources travel in a common fiber before interrogating the sample of interest. When sounding a molecular gas, these errors distort both the line shapes and retrieved concentrations. Simulations of dual comb interferograms based on a generalized nonlinear Schrodinger equation highlight two processes for these systematic errors. Self-phase modulation changes the spectral content of the field interrogating the molecular response but affects the recorded spectral baseline and absorption features differently, leading to line intensity errors. Cross-phase modulation modifies the relative inter-pulse delay, thus introducing interferogram sampling errors and creating a characteristic asymmetric distortion on spectral lines. Simulations capture the shape and amplitude of experimental errors which are around 0.1% on spectral transmittance residuals for 10 mW of total average power in 10 meters of common fiber, scaling up to above 0.6% for 20 mW and 60 m.
