Continuous-wave Distributed-feedback Research Articles

Highly Sensitive Hydrogen Sensing Based on Tunable Diode Laser Absorption Spectroscopy with a 2.1 μm Diode Laser

As a new form of energy, hydrogen (H2) has clean and green features, and the detection of H2 has been a hot topic in recent years. However, the lack of suitable laser sources and the weak optical absorption of H2 limit the research concerning its detection. In this study, a continuous-wave distributed feedback (CW-DFB) diode laser was employed for sensing H2. Tunable diode laser absorption spectroscopy (TDLAS) was adopted as the detection technique. The strongest H2 absorption line, located at 4712.90 cm−1 (2121.83 nm, line strength: 3.19 × 10−26 cm−1/cm−2 × molec), was selected. We propose a H2-TDLAS sensor based on the wavelength modulation spectroscopy (WMS) technique and a Herriott multipass gas cell (HMPC) with an optical length of 10.13 m to achieve a sensitive detection. The WMS technique and second harmonic (2f) demodulation technique were utilized to suppress system noise and simplify the data processing. The 2f signal of the H2-TDLAS sensor, with respect to different H2 concentrations, was measured when the laser wavelength modulation depth was at the optimal value of 0.016 cm−1. The system’s signal-to-noise ratio (SNR) and minimum detection limit (MDL) were improved from 248.02 and 0.40% to 509.55 and 0.20%, respectively, by applying Daubechies (DB) wavelet denoising, resulting in 10 vanishing moments. The Allan variance was calculated, and the optimum MDL of 522.02 ppm was obtained when the integration time of the system was 36 s.

Extended tuning of distributed-feedback lasers in a bias-tee circuit via waveform optimization for MHz-rate absorption spectroscopy

Variations in injection-current waveform are examined using diplexed radio-frequency modulation with continuous-wave distributed-feedback (CW-DFB) lasers, with the aim to maximize the spectral tuning range and signal-to-noise ratio for MHz-rate laser absorption spectroscopy (LAS). Utilizing a bias-tee circuit, laser chirp rates are shown to increase by modulating the AC input voltage using square waves instead of sine waves and by scanning the laser below the lasing threshold during the modulation period. The effect of waveform duty cycle and leading-edge ramp rate are further examined. A spectral scan depth on the order of 1 cm−1 at a scan frequency of 1 MHz is achieved with a representative CW-DFB quantum cascade laser near 5 μm. Distortion of high-frequency optical signals due to detector bandwidth is also examined, and limitations are noted for applications with narrow spectral features and low-bandwidth detectors. Based on common detection system limitations, an optimization approach is established for a given detection bandwidth and target spectra. A representative optimization is presented for measurements of sub-atmospheric carbon monoxide spectra with a 200-MHz detection system. The methods are then demonstrated to resolve transient gas properties (pressure and temperature) via LAS at MHz rates in a detonation tube and shock tube facility. An appendix detailing a first-order model of high-speed distributed feedback laser tuning dynamics is also included to support the experimental observations of this work.

Ultra-Highly Sensitive Hydrogen Chloride Detection Based on Quartz-Enhanced Photothermal Spectroscopy.

Combining the merits of non-contact measurement and high sensitivity, the quartz-enhanced photothermal spectroscopy (QEPTS) technique is suitable for measuring acid gases such as hydrogen chloride (HCl). In this invited paper, we report, for the first time, on an ultra-highly sensitive HCl sensor based on the QEPTS technique. A continuous wave, distributed feedback (CW-DFB) fiber-coupled diode laser with emission wavelength of 1.74 µm was used as the excitation source. A certified mixture of 500 ppm HCl:N2 was adapted as the analyte. Wavelength modulation spectroscopy was used to simplify the data processing. The wavelength modulation depth was optimized. The relationships between the second harmonic (2f) amplitude of HCl-QEPTS signal and the laser power as well as HCl concentration were investigated. An Allan variance analysis was performed to prove that this sensor had good stability and high sensitivity. The proposed HCl-QEPTS sensor can achieve a minimum detection limit (MDL) of ~17 parts per billion (ppb) with an integration time of 130 s. Further improvement of such an HCl-QEPTS sensor performance was proposed.

Wavelength modulation spectroscopy near 5 $$\upmu$$m for carbon monoxide sensing in a high-pressure kerosene-fueled liquid rocket combustor

A laser absorption sensor was developed for carbon monoxide (CO) sensing in high-pressure, fuel-rich combustion gases associated with the internal conditions of hydrocarbon-fueled liquid bipropellant rockets. An absorption feature near 4.98 $$\upmu$$ m, comprised primarily of two rovibrational lines from the P-branch of the fundamental band, was selected to minimize temperature sensitivity and spectral interference with other combustion gas species at the extreme temperatures (> 3000 K) and pressures (> 50 atm) in the combustion chamber environment. A scanned wavelength modulation spectroscopy technique (1f-normalized 2f detection) is utilized to infer species concentration from CO absorption, and mitigate the influence of non-absorption transmission losses and noise associated with the harsh sooting combustor environment. To implement the sensing strategy, a continuous-wave distributed-feedback (DFB) quantum cascade laser (QCL) was coupled to a hollow-core optical fiber for remote mid-infrared light delivery to the test article, with high-bandwidth light detection by a direct-mounted photovoltaic detector. The method was demonstrated to measure time-resolved CO mole fraction over a range of oxidizer-to-fuel ratios and pressures (20–70 atm) in a single-element-injector RP-2-GOx rocket combustor.

Theorical analysis of a monolithic all-active three-section semiconductor laser

We propose a novel semiconductor laser structure. It is composed of three cascaded active sections: a Fabry-Pérot laser section sandwiched between two gain-coupled distributed feedback (DFB) laser sections. We have modeled this multi-section structure. The simulation results show that compared with index- and gain-coupled DFB lasers, a significant reduction in the longitudinal spatial-hole burning can be obtained with the proposed device, and that this leads to a stable single longitudinal mode operation at relatively high optical power with a SMSR exceeding 56dB. Full Text: PDF ReferencesL.A. Coldren, "Monolithic tunable diode lasers", IEEE J. Select. Topics Quant. Electron. 6, 988 (2000) CrossRef O. Kjebon, R. Schatz, S. Lourdudoss, S. Nilsson, B. Stalnacke, L. Backbom, "30 GHz direct modulation bandwidth in detuned loaded InGaAsP DBR lasers at 1.55 [micro sign]m wavelength", Electron. Lett. 33(6), 488 (1997). CrossRef N. Kim, J. Shin, E. Sim, C.W. Lee, D.-S. Yee, M.Y. Jeon, Y. Jang, K.H. Park, "Monolithic dual-mode distributed feedback semiconductor laser for tunable continuous-wave terahertz generation", Opt. Expr. 17(16), 13851 (2009). CrossRef M.J. Wallace, R. ORreilly Meehan, R.R Enright, F. Bello, D. Mccloskey, B. Barabadi, E.N. Wang, J.F. Donegan, "Athermal operation of multi-section slotted tunable lasers", Opt. Expr. 25(13), 14426 (2017). CrossRef J.E. Carroll, J.E.A. Whiteaway, R.G.S. Plumb, "Distributed Feedback Semiconductor Lasers", Distributed feedback semiconductor lasers (IEE and SPIE, 1998). CrossRef H. Ghafour-Shiraz, Distributed Feedback Laser Diodes and Optical Tunable Filters (Wiley, 2003). CrossRef D.D. Marcenac, Ph.D dissertation (University of Cambridge, 1993). DirectLink L.M. Zhang, J.E. Carroll, C. Tsang, "Dynamic response of the gain-coupled DFB laser", IEEE J. Quant. Electr. 29, 1722 (1993). CrossRef W. Li, W.-P. Huang, X. Li, J. Hong, "Multiwavelength gain-coupled DFB laser cascade: design modeling and simulation", IEEE J. Quant. Electro. 36(10), 1110 (2000). CrossRef B.M. Mehdi, H. Rachid, in Proc. 3rd Intern. Conf. on Embedded Systems in Telecomm. and Instrument., Annaba, Algeria (2016). DirectLinkC. Henry, "Theory of the linewidth of semiconductor lasers", IEEE J.Quant. Electr. QE-18, 259 (1982). CrossRef K. Takaki, T. Kise, K. Maruyama, N. Yamanaka, M. Funabashi, A. Kasukawa, "Reduced linewidth re-broadening by suppressing longitudinal spatial hole burning in high-power 1.55-/spl mu/m continuous-wave distributed-feedback (CW-DFB) laser diodes", IEEE J. Quant. Electr. 39, 1060 (2003) CrossRef

Room Temperature CW Operation of Mid-IR Distributed Feedback Quantum Cascade Lasers for $\hbox{CO}_{2}, \hbox{N}_{2}$O, and NO Gas Sensing

We present the design and the performance of midinfrared distributed feedback (DFB) quantum cascade lasers (QCLs) made of strain balanced Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> As material on InP substrates for sensing CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O, and NO in the middle wavelength range of mid-IR from 4 to 6 μm. We present the performances of our DFB QCLs at three different aspects: high power, low threshold power consumption, and wide wavelength coverage. We demonstrated a continuous wave (CW) DFB QCL with an output power of 220mW at 20°C, a CW DFB QCL with low threshold voltage of 8V and low-divergent far-field angles of 27.5° × 32.9° by reducing the number of quantum cascaded stages, a CW DFB QCL with a low threshold power consumption of 0.7W by reducing doping density in the active core, and DFB QCLs wide wavelength coverage of 325 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , 16.9% of center wavelength at 5.2 μm, within one wafer by changing the grating period. 12 DFB QCL chips have being aged for 3000 h under 25°C and a constant quasi-CW current of 0.37 A. No decrease in power was observed.

Reduced linewidth re-broadening by suppressing longitudinal spatial hole burning in high-power 1.55-μm continuous-wave distributed-feedback (CW-DFB) laser diodes

For the first time, the impact of longitudinal photon density distribution and longitudinal carrier density distribution on the spectral linewidth re-broadening effect in single-electrode 1.55-μm distributed feedback (DFB) laser diodes (LDs) is investigated experimentally in details. By optimizing the front-to-rear facet power ratio, the nonuniformity of the photon density distribution along the laser cavity is reduced, hence reducing the degree of longitudinal spatial hole burning (SHB). Using this optimized value of front-to-rear facet power ratio, the degree of longitudinal SHB can be further reduced through reduction of the nonuniformity of the longitudinal carrier density distribution by increasing the cavity length. As a result, the local stimulated emission is reduced, hence reducing linewidth re-broadening caused by longitudinal SHB. The outcomes of this analysis is being used fruitfully to design high-power 1.55-μm DFB LDs exhibiting very narrow spectral linewidth of approximately 1.3 MHz at an output power of 175 mW under continuous-wave operation.