Acetylene Absorption Lines Research Articles

A Tunable Optoelectronic Oscillator with Phase-to-Amplitude Modulation Transformation via an Acetylene Reference Cell

Operation modes of the optoelectronic oscillator (OEO), based on a phase electrooptic modulator with an acetylene reference cell as a photonic filter, have been investigated. For the wideband phase-to-amplitude demodulation that was observed when the laser wavelength was tuned to one side of the acetylene absorption line, an additional tunable electronic yttrium iron garnet (YIG) filter was introduced to observe single-mode OEO generation. This configuration generated a stable monochromatic signal in the 4–12 GHz frequency range, with phase noise of −122 dBc/Hz at 10 kHz offset from the carrier frequency. In the narrowband demodulation mode (when one optical modulation sideband was tuned to the acetylene absorption line), the quasi-single-mode oscillation could be observed without additional electronic filtering. In this case, the generation frequency was controlled optically by tuning the laser wavelength.

Self-reference shot-noise-limited phase demodulator with 1530 nm acetylene absorption line

We report results on characterization of the self-reference interferometric configuration based on quantum memory of the acetylene (12C2H2) molecules at the wavelength of 1530.37 nm of the vibrational–rotational P9 absorption line. The demodulator under consideration is robust, simple, operates at room temperature with the wavefronts of high etendue of the sub-mW power scale, and needs neither phase, no frequency locking loops. Proposed original balance detection schemes enable effective suppression of the semiconductor laser phase noise, and the shot-noise-limited demodulation resolution at frequencies higher than the high-pass configuration cut-off frequency ≈0.25 GHz.

A dual-gas sensor for simultaneous detection of methane and acetylene based on time-sharing scanning assisted wavelength modulation spectroscopy.

Methane (CH4) and acetylene (C2H2) are important bioscience and chemical gases. The real-time monitoring and analysis of them have important research value in industrial process control. The time-sharing scanning assisted wavelength modulation spectroscopy (WMS) technique is developed for real-time and simultaneous detection of CH4 and C2H2. This system involves two near-infrared distributed feedback (DFB) lasers and a compact multipass cavity with an effective optical path of 52.2m. The selected strong absorption lines of methane and acetylene are located at 6046.96cm-1 and 6531.7cm-1, respectively. The experiment environment is conducted at room temperature 23°C and pressure 760Torr. The sensor performance, including the minimum detection limit (MDL) and the stability, was improved by eliminating the influence of light intensity fluctuation using the WMS-2f/SAW technique. Allan deviation analysis indicates that a MDL of 0.1ppm for CH4 and 0.2ppm for C2H2 are achieved with 1-s integration time. And the instrument response time is about 44s through the continuous analysis of standard gases. This sensitive, simple, reliable, and lowcost dual-gas sensor is very suitable for applications in the field environment, chemical process, and many other gas-phase analysis areas.

Near-Infrared Tunable Laser Absorption Spectroscopic Acetylene Sensor System Using a Novel Three Mirror-Based, Dense Pattern Gas Cell.

By contrast with the widely reported traditional two mirror-based Herriott cell, a three mirror-based dense pattern gas cell was proposed, of which the modeling and design were proven to be effective through a comparison between the simulated spot pattern and effective path length and those of the experimental results. A mechanical structure was designed to adjust the position/angle of the three mirrors for aligning the optical path. The experimentally measured reflection number was 60, resulting in an optical path length of ~11 m, which agrees well with the theoretical value of 10.95 m. Combined with a near-infrared laser with a center wavenumber located at an acetylene (C2H2) absorption line of 6521.2 cm−1, a C2H2 sensor system was established to verify the feasibility of the three mirror-based gas cell. Assisted by a data acquisition (DAQ) card, a LabVIEW platform was developed to generate the drive signal of the laser and acquire the second harmonic (2f) signal from the output of the detector. Through Allan variance analysis, the limit of detection (LoD) of the sensor system is 4.36 ppm at an average time of 0.5 s; as the average time exceeds 10 s, the LoD is <1 ppm. The proposed model and design of the three mirror-based gas cell can be used to realize similar gas cells with different absorption path lengths for gas detection based on infrared absorption spectroscopy.

Quasi-all-fiber structure CW mid-infrared laser emission from gas-filled hollow-core silica fibers

A quasi-all-fiber structure CW mid-infrared fiber lasers based on gas-filled hollow-core fibers (HCFs) is firstly reported here. Through a thermally tapered single-mode fiber, the CW tunable 1.5 μm fiber pump light is coupled into an acetylene-filled anti-resonance HCFs, and the coupling point is sealed with epoxy glue to ensure the stability and gas tightness, then the HCFs is filled with acetylene gas by a specially designed small gas cell, just like an end cap. When the pump is precisely tuned to the resonance with the absorption lines of acetylene, CW mid-infrared emission around 3.1–3.2 μm is generated at room temperature with maximum power of ~180 mW and slope efficiency of ~14%. The laser threshold is also discussed as function of acetylene pressure. This work opens new opportunities for all-fiber mid-infrared lasers of new wavelengths those unavailable for traditional rare-earth-doped fibers.

Trace Gas Detection System Based on All-Optical Quartz-Enhanced Photoacoustic Spectroscopy.

An all-optical quartz-enhanced photoacoustic spectroscopy system (QEPAS) with quadrature point stabilization for trace gas detection was reported. The extrinsic interferometry-based optical fiber Fabry-Perot sensor with quadrature point self-stabilization for detection of quartz prong vibration was used to replace the conventional one. The optimal coefficient of the modulation depth was ∼2.2 theoretically and experimentally, corresponding to the modulation depth of ∼0.1795 cm-1 at an acetylene (C2H2) absorption line of 6534.36 cm-1. Furthermore, the enhancement of QEPAS signal was obtained by using different microresonators. The minimum detectable limit of ∼580 parts per billion by volume (ppbv) was obtained. A normalized noise equivalent absorption coefficient for C2H2 of 2.95 × 10-7 cm-1·W·Hz-1/2 was obtained. The detection sensitivity was enhanced by a factor of ∼2.1 in comparison to the conventional QEPAS system. The linear correlation coefficient of the QEPAS signal response to the C2H2 concentration was 0.998 within the range from 10 parts per million by volume (ppmv) to 500 ppmv. Finally, the long-term stability of the QEPAS system was evaluated using Allan deviation analysis, and the ultimate detection limit of ∼130 ppbv was reached for an optimum averaging time of ∼108 s at atmospheric pressure and ambient temperature.

Utilizing phase memory of a two-level quantum system for adaptive homodyne detection of optical phase modulation

We propose to use phase memory of an ensemble of two-level centers, irradiated by the resonantly tuned detected signal wave, for homodyne interferometric detection of fast optical phase modulation. Collinearly propagating dipole radiation of the excited two-level centers acts as a coherent reference wave (local oscillator) necessary to transform the phase modulation into the amplitude modulation behind the resonant medium. The phase of this radiation follows the average phase of the incident detected wave with the characteristic transverse relaxation time T2 that ensures adaptive properties of this interferometric detection configuration to slowly varying environmental conditions. We present experimental demonstration with the acetylene-filled hollow-core microstructured optical fibers at a wavelength of 1530.37 nm of the acetylene P9 absorption line, which is inside the telecommunications wavelengths range. It is shown that the response is quadratic when the acetylene ∼500 MHz inhomogeneously broadened absorption line is excited in its center, but can be essentially linearized for excitation at one of the line sides.

Towards all-fiber structure pulsed mid-infrared laser by gas-filled hollow-core fibers

We report here on a diode-pumped pulsed mid-infrared laser source based on gas-filled hollow-core fibers (HCFs) towards an all-fiber structure by the tapering method. The pump laser is coupled into an acetylene-filled HCF through a tapered single-mode fiber. By precisely tuning the wavelength of the diode to match different absorption lines of acetylene near 1.5 μm, mid-infrared emission around 3.1–3.2 μm is generated. With 2 m HCFs and 3 mbar acetylene gas, a maximum average power of 130 mW is obtained with a laser slope efficiency of ∼24%. This work provides a potential scheme for all-fiber mid-infrared fiber gas lasers.

High-power tunable mid-infrared fiber gas laser source by acetylene-filled hollow-core fibers.

High-power tunable pulsed and CW mid-infrared fiber gas laser sources in acetylene-filled hollow-core fibers, to the best of our knowledge, are demonstrated for the first time. By precisely tuning the wavelength of the pump source, an amplified tunable 1.5 μm diode laser, to match different absorption lines of acetylene, the laser output is step-tunable in the range of 3.09~3.21 μm with a maximum pulse average power of ~0.3 W (~0.6 μJ pulse energy) and a maximum CW power of ~0.77 W, making this system the first watt-level tunable fiber gas laser operating at mid-infrared range. The output spectral and power characteristics are systemically studied, and the explanations about the change of the ratio of the P over R branch emission lines with the pump power and the gas pressure are given, which is useful for the investigations of mid-infrared fiber gas lasers.

Experimental and numerical comparison of probabilistically shaped 4096 QAM and a uniformly shaped 1024 QAM in all-Raman amplified 160 km transmission.

We describe an experimental and numerical comparison of a probabilistically shaped (PS) 4096 QAM signal and a uniformly shaped 1024 QAM signal. Both modulation formats have the same transmission rate and a spectral efficiency of 15.3 bit/s/Hz. In the computational simulation, we compared the generalized mutual information (GMI) of both modulation formats with bit-wise soft decision decoding and bit-wise hard decision decoding. For bit-wise hard decision decoding with an overhead of 7%, a shaping gain of 1.8 dB was attained. Then we successfully transmitted a single-channel PS-4096 QAM signal for the first time in an all-Raman amplified 160-km link, in which the transmission performance was improved compared with a uniformly shaped 1024 QAM with the same transmission rate. Transmissions with a high QAM multiplicity were achieved by using an optical phase-locked loop (OPLL) and a frequency stabilized fiber laser locked to an acetylene absorption line. Thanks to a shaping gain based on a bit-wise hard decision decoder, the 1.9-dB power margin, which agreed with the simulation result to within 0.1 dB, was increased after transmission.

Optical nutation in acetylene-filled hollow-core photonic crystal fiber

We report detailed experimental results on the optical nutation effect in acetylene-filled hollow-core photonic crystal fibers (HC-PCFs). This coherent quantum optical effect is manifested by the appearance of attenuated oscillations in the profiles of transmitted step-like coherent optical pulses. The experiments used 15 ns optical pulses with peak powers up to ∼4 W and centered at 1530.37 nm, which corresponds to the acetylene (12C2H2) vibration-rotation resonance absorption line P9. The all-fiber cell made from a HC-PCF with internal diameter ∼10.3 μm contained a gas pressure of about 0.12 Torr. The Maxwell-Bloch equations are used to analyze the influence of the homogeneous relaxation, the inhomogeneous Doppler broadening of the absorption line, and the random orientation of the acetylene molecules. By comparing the experimental results with numerical simulations, we obtain the longitudinal and the transverse relaxation times to be close to 10 ns and find that the transition dipole moment of the acetylene P9 absorption line is ∼1.36×10−32 Cm.

Measurements of new absorption lines of acetylene at 1.53 μm using a tunable diode laser absorption spectrometer

A new investigation of acetylene absorption lines between 6526.5 and 6531.5cm−1 spectral region was performed by using a long-path absorption cell based tunable diode laser absorption spectrometer. The multi-spectrum fitting procedure has been applied to these intensive absorption lines of acetylene within the spectral range of particular interest to recover the line parameters. Line intensities and line positions of total 42 lines of acetylene were reported, including 22 new lines precisely identified for the first time. The reported results will be valuable to complete the spectroscopic databases of acetylene, and also be useful for upgrading our newly developed TDLAS sensor system for industrial C2H2 gas detection.

Pulsed-induced electromagnetically induced transparency in the acetylene-filled hollow-core fibers

Experimental results on pulsed excitation of electromagnetically induced transparency (EIT) in the acetylene-filled hollow-core photonic crystal fiber (HC-PCF) at pressures 0.1–0.4 Torr are reported. The EIT was observed both in Λ and V interaction configurations with the continuous probe wave tuned to R9 (1520.08 nm) acetylene absorption line and with the control pulses tuned to P11 (1531.58 nm) and P9 (1530.37 nm) lines, respectively. The utilized control pulses were of up to 40 ns duration with <2.5 ns fronts and with maximum input power ~1 W. The maximum modulation depth of the initial probe wave absorption via EIT was up to 40 and 15% for the co- and counter-propagation of the probe and control waves, respectively, and importance of the waves polarization matching was demonstrated. For a qualitative explanation of reduction in the counter-propagation EIT efficiency a simple model of the accelerated mismatch of the two-frequency EIT resonance with deviation of the molecule thermal velocity from the resonance value was utilized. It was shown experimentally that the EIT efficiencies in both configurations do not depend on the longitudinal velocity of the molecules. The characteristic relaxation time of the of the EIT response was found to be about 9 ns, i.e., is close to the relaxation times T 1,2 of the acetylene molecules under the utilized experimental conditions.

Optical Fiber Photoacoustic Gas Sensor With Graphene Nano-Mechanical Resonator as the Acoustic Detector

We report an all-optical fiber photoacoustic gas sensor with a graphene nano-mechanical resonator as the acoustic detector. The acoustic detector is a Fabry–Perot interferometer formed by attaching a 100-nm-thick, 2.5-mm-diameter multilayer graphene diaphragm to a hollow cavity at the end of a single-mode optical fiber. By operating at one of the mechanical resonances of the diaphragm, the sensitivity for acoustic detection is enhanced and a noise equivalent minimum detectable pressure of 2.11 μPa/Hz1/2 at 10.1 kHz is demonstrated. Detection of acetylene gas is demonstrated with a distributed feedback semiconductor laser tuned to the P(9) absorption line of acetylene and a lower detection limit of 119.8 parts-per-billion (ppb) is achieved with 123.9-mW pump power. Theoretical analysis shows that by increasing the Q-factor of the resonator, which may be achieved by operating at low gas pressures, ppb level gas detection is possible. The all-fiber photoacoustic gas sensor is immune to electromagnetic interference, safe in flammable and explosive environment, and would be ideally suited for remote, space-limited applications and for multipoint detection in a multiplexed fiber optic sensor network.

Self-calibration wavelength modulation spectroscopy for acetylene detection based on tunable diode laser absorption spectroscopy**Project supported by the National Natural Science Foundation of China (Grant Nos. 61172047, 61502538, and 61501525).

The expressions of the second harmonic (2f) signal are derived on the basis of absorption spectral and lock-in theories. A parametric study indicates that the phase shift between the intensity and wavelength modulation makes a great contribution to the 2f signal. A self-calibration wavelength modulation spectroscopy (WMS) method based on tunable diode laser absorption spectroscopy (TDLAS) is applied, combining the advantages of ambient pressure, temperature suppression, and phase-shift influences elimination. Species concentration is retrieved simultaneously from selected 2f signal pairs of measured and reference WMS-2f spectra. The absorption line of acetylene (C2H2) at 1530.36 nm near-infrared is selected to detect C2H2 concentrations in the range of 0–400 ppmv. System sensitivity, detection precision and limit are markedly improved, demonstrating that the self-calibration method has better detecting performance than the conventional WMS.

On-chip multi spectral frequency standard replication by stabilizing a microring resonator to a molecular line

Stabilized laser lines are highly desired for myriad of applications ranging from precise measurements to optical communications. While stabilization can be obtained by using molecular or atomic absorption references, these are limited to specific frequencies. On the other hand, resonators can be used as wide band frequency references. Unfortunately, such resonators are unstable and inaccurate. Here, we propose and experimentally demonstrate a chip-scale multispectral frequency standard replication operating in the spectral range of the near IR. This is obtained by frequency locking a microring resonator (MRR) to an acetylene absorption line. The MRR consists of a Si3N4 waveguides with microheater on top of it. The thermo-optic effect is utilized to lock one of the MRR resonances to an acetylene line. This locked MRR is then used to stabilize other laser sources at 980 nm and 1550 nm wavelength. By beating the stabilized laser to another stabilized laser, we obtained frequency instability floor of 4×10−9 at around 100 s in terms of Allan deviation. Such stable and accurate chip scale sources are expected to serve as important building block in diverse fields such as communication and metrology.

295 mW output, frequency-stabilized erbium silica fiber laser with a linewidth of 5 kHz and a RIN of -120 dB/Hz.

We demonstrate the frequency stabilization of a high output power, erbium silica fiber laser by utilizing a (13)C2H2 (acetylene) absorption line at 1538.8 nm and a H(13)C(14)N (hydrogen cyanide) absorption line at 1549.73 nm. We introduced a novel short ring cavity configuration and pump power feedback control to suppress the intensity noise of the laser output, which is caused by the relaxation oscillation of erbium ions. As a result, we succeeded in simultaneously obtaining a stable single-frequency oscillation with an output power of over 290 mW, a linewidth of 5 kHz, and a low relative intensity noise (RIN) of -120 dB/Hz. The frequency stabilities reached 2.8 × 10(-11) and 6.9 × 10(-11) for an integration time of 1 s with a (13)C2H2 and a H(13)C(14)N absorption line, respectively.

Novel Widely Tunable Monolithically Integrated Laser Source

We report a novel type of monolithically integrated tunable semiconductor laser. The tuning is achieved by three intracavity Mach-Zehnder interferometers, realized in passive waveguides and using voltage-controlled electro-optic phase modulators requiring only four control voltages. The potential of the design is demonstrated by a realized laser system that shows an optical linewidth of 363 kHz, output power of 3 mW, and a record tuning range of 74.3 nm. Such a continuous wavelength span is in excess of any monolithic semiconductor laser reported up to date. Precision of the tuning mechanism is demonstrated by a scan over a 0.89-GHz-wide absorption line of acetylene. The laser design is suitable for a number of applications, including gas spectroscopy, telecommunication, and optical coherence tomography. The laser has been fabricated in a multi-project wafer run on an indium phosphide-based generic photonic foundry platform and demonstrates the potential of these technology platforms.

Improved gas sensor with air-core photonic bandgap fiber

The propagation loss of a fiber can be increased by coupling core mode and surface mode which will deteriorate the performance of photonic bandgap fiber (PBGF). In this paper, we presented an aircore PBGF for gas sensing applications. By designing Λ = 2.63 µm, d = 0.95 Λ, and Rcore= 1.13 Λ, where Λ is the distance between the adjacent air holes, the fiber was single-mode, no surface mode was supported with fiber, and more than 90% of the optical power was confined in the core. Furthermore, with optimizing the fiber structural parameters, at wavelength of λ = 1.55 µm that is in acetylene gas absorption line, significant relative sensitivity of 92.5%, and acceptable confinement loss of 0.09 dB/m, were simultaneously achieved.

Warm C&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; toward NGC 7538 IRS9: Grain Surface Origin

We consider models for the observed ro-vibrational absorption lines of acetylene toward NGC 7538 IRS9. The data are fit with multiple screens, each having separate column densities, rotational and vibrational excitation temperatures, and filling factors. The best fit was determined using a chi-squared minimization scheme. We find that only one screen is necessary—multiple screens gave rise to either making one of the screens transparent, or very occasionally making the two screens the same. As a result, we can place constraints on Trot, Tvib, NC2H2, and the filling factor, f. In particular we find 0.03 vib vib C2H2 = 2.4 +/- 0.6 × 1016 cm-2, or that N × f ~ 2 × 1015 cm-2. Lastly, we find 80 rot rot ~ 100 K. Physically, this can be interpreted as: (1) no vibrational excitation, (2) the warm region only fills a small fraction of the beam, (3) the C2H2 arises very near a region of 100 K. Chemically, this is in consistent with a model where the C2H2 is formed in the gas phase. It is however consistent with a scenario where the C2H2 is evaporated at 100 K from the grain surface, suggesting either a grain-surface origin or earlier origin followed by condensation. Finally, the C2H2 column density is consistent with a disk geometry.