Measurements of the linewidth of a continuous-wave distributed feedback quantum cascade laser

Two-sample (Allan) variance with a modified algorithm was applied to the determination of the experimental linewidth of a thermoelectrically-cooled continuous-wave distributed feedback quantum cascade laser at a wavelength of 4.333 μm. From successive laser transmittance scans over the CO₂ ν₃, (01¹1 − 01¹0) P(33) absorption line at 2307.653 cm^− 1, two-sample variances were calculated for the laser frequency difference between different points on the sides of the absorption peak. From the minimum two-sample variance of the laser frequency difference between two adjacent points (5 μs between the points) on the same side of the absorption line the experimental laser linewidth was estimated to be less than 36(7) kHz at a laser power of ~ 25 mW, a laser current of 976 mA and a laser temperature of + 19.5 °C.     

Room-temperature continuous-wave operation of an external-cavity quantum cascade laser

Room-temperature, continuous-wave operation of an external-cavity quantum cascade laser (EC-QCL) is reported. Single-mode tuning range of 120 cm(-1) was achieved, from 7.96 to 8.84 microm. The gain chips utilized are based on the bound to continuum design and were fabricated as buried heterostructure lasers. Gap-free tuning (mode hops only on the external-cavity modes) is demonstrated for an antireflection-coated laser, just by grating rotation. The EC-QCL was implemented in a Littrow setup and an average power of 1.5 mW was obtained at 20 degrees C, while a peak power of 20 mW was obtained for a modified Littrow setup with the back extraction of light.     

Design of surface emitting distributed feedback quantum cascade laser with single-lobe far-field pattern and high outcoupling efficiency

A 7.8-$\mu $m surface emitting second-order distributed feedback quantum cascade laser (DFB QCL) structure with metallized surface grating is studied. The modal property of this structure is described by utilizing coupled-mode theory where the coupling coefficients are derived from exact Floquet--Bloch solutions of infinite periodic structure. Based on this theory, the influence of waveguide structure and grating topography as well as device length on the laser performance is numerically investigated. The optimized surface emitting second-order DFB QCL structure design exhibits a high surface outcoupling efficiency of 22{\%} and a low threshold gain of 10~cm$^{ - 1}$. Using a {$\pi $} phase-shift in the centre of the grating, a high-quality single-lobe far-field radiation pattern is obtained.     

Doppler-free saturated-absorption spectroscopy of CO2 at 4.3 microm by means of a distributed feedback quantum cascade laser

We report the application of a cw distributed feedback quantum cascade laser to Lamb-dip spectroscopy of CO2 at 4.3 microm. With the laser operating in the free-running mode, we observed the sub-Doppler profile of the P(28) line of the (0,1(1),0)->(0,1(1),1) hot band by implementing a pump-probe scheme and using wavelength modulation spectroscopy for highly sensitive detection of saturated absorption signals. We investigated the main limitations to the observation of a narrow resonance, with particular attention to the effect of the laser current noise. We determined the intrinsic laser emission width, which was found to be approximately 3.4 MHz (FWHM) for an observation time of approximately 200 ms.     

Continuous wave,distributed feedback diode laser based sensor for trace-gas detection of ethane

The development of a continuous wave (CW), thermoelectrically cooled (TEC), distributed feedback (DFB) laser diode based spectroscopic trace-gas sensor for ultra-sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable diode laser absorption spectroscopy (TDLAS) based on a 2f wavelength modulation (WM) detection technique. TDLAS was performed with a 100 m optical path length astigmatic Herriott cell. For an interference free C₂H₆ absorption line located at 2976.8 cm⁻¹ a 1σ minimum detection limit of 240 pptv (part per trillion by volume) with a 1 second lock-in amplifier time constant was achieved. In addition, reliable and long-term sensor performance was obtained when operating the sensor in an absorption line locked mode.     

Ultra-short distributed feedback fiber laser with sub-kilohertz linewidth for sensing applications

A low-noise, ultra-short linear-cavity distributed feedback fiber laser (DFB-FL) with extremely narrow linewidth is presented. The FL has a total length of 17 mm, which is, to our knowledge, the shortest DFB-FL being reported. It has a measured linewidth of merely 250 Hz without active stabilization. It has a polarization beat frequency of 101 MHz, which is several times lower than that of most FLs. The relaxation oscillation frequency and relative peak are 110 kHz and −76.5 dB/Hz, respectively. The FL exhibited low-noise characteristics, with an intensity noise of −107 dB/Hz at 1 MHz. Due to low dopant concentration of the EDF and low splice loss with ordinary single-mode fibers, the net insertion loss amounts to only 0.45 dB. Such low pump power loss greatly enhances the capability of multiplexing a large number of FL sensors. Thus, these ultra-short DFB-FLs open up new opportunities for the development of compact-sized point sensor array systems for large-scale high sensitivity sensing applications.     

Short distributed feedback fiber laser with unidirectional output for sensing applications

A short distributed feedback fiber laser with a nearly unidirectional output is fabricated and tested. The short fiber laser is made of a polarization-dependent phase-shift grating fabricated with a vertically polarized 244-nm ultraviolet (UV) laser. A single ∏ phase-shift is introduced to a 2-cm grating at a specified position by directly moving the phase mask during UV beam scanning. Test results show that the laser has a single polarization longitudinal mode with 2.6-mW pump threshold. The backward-to-forward output power ratio is approximately 30:1. The relative intensity noise is -88 dB/Hz, and the linewidth is approximately 10 kHz at 75-mW 980-nm pumping. The unidirectional output and short dimension of this short fiber laser make it very useful in sensing applications, especially in multiplexed sensing applications.     

Coherent spectroscopy with a distributed feedback dye laser

A simple and flexible distributed feedback dye laser source pumped by the harmonic of a mode-locked YAG laser is described. Measurements of dephasing times of isotopic and hot bands in CS₂ liquid exploiting both the frequency and time-resolution of this source are reported.     

基于量子级联激光器的气体检测系统的发展与应用

量子级联激光器(QCLs)的快速、灵敏和选择性气体检测等独特优点,使其成为2.5～25μm波长范围内中红外气体检测的理想光源。中红外QCLs在气体检测方面的应用已成为世界各国的研究焦点。尤其是在大气环境监测、太空探索和反恐防恐等领域的应用表现出了其他红外检测系统无可比拟的优势。该文重点介绍了基于量子级联激光器的气体检测系统工作原理、应用及发展趋势。     

Spectroscopic trace-gas sensor with rapidly scanned wavelengths of a pulsed quantum cascade laser for in situ NO monitoring of industrial exhaust systems

Development of a pulsed quantum cascade laser (QCL)-based spectroscopic trace-gas sensor for sub-part-per-million detection of nitric oxide (NO) and capable of monitoring other molecular species such as CO₂, H₂O, and NH₃ in industrial combustion exhaust systems is reported. Rapid frequency modulation is applied to the QCL to minimize the influence of fluctuating non-selective absorption. A novel method utilizes only a few laser pulses within a single wavelength scan to probe an absorption spectrum at precisely selected optical frequencies. A high-temperature gas cell was used for laboratory evaluation of the NO sensor performance. A noise-equivalent sensitivity (1) of 100 ppb × m/ at room temperature and 200 ppb × m/ at 630 K was achieved by measuring the NO R(6.5) absorption doublet at 1900.075 cm^–1.     

Large-signal modulation in distributed feedback quantum cascade lasers for coherent multiharmonic signal generation

In this Letter, an experimental analysis for coherent multiharmonic signal generation in quantum cascade lasers is presented. The underlying principle relies on the direct modulation of the laser with a large sinusoidal wave to drive the laser above and below threshold. This generates a multiharmonic optical signal in the frequency domain as the device is forced to respond in a nonlinear fashion. The impact of the modulation parameters is assessed in the resultant optical spectrum of a continuous-wave distributed feedback quantum cascade laser. Despite not creating pulses due to the ultrafast dynamics of the laser, the results reveal that a comb structure with uniform line spacing and high phase correlation between teeth can be straightforwardly generated.     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46 μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of 3 × 10(-9)/cm for 1 s averaging of spectra composed by 100 independent points. At 4.46 μm, this yields a detection limit of 35 parts in 10(12) by volume for N(2)O at 50 mbar, corresponding to 4 × 10(7) molecules/cm(3), or still to 1 fmol in the sample volume.     

Two-level model of a superradiant laser with distributed feedback

Numerical one-dimensional simulation of the superradiance phenomenon in a sample of a two-level medium with cw pumping under the conditions of resonant Bragg backscattering of counterpropagating waves has been performed. A range of parameters is determined in which a superradiant laser generating a chaotic sequence of ultrashort high-power pulses can be implemented, and the properties of these pulses are studied.     

Intracavity sensing via compliance voltage in an external cavity quantum cascade laser

We demonstrate a technique for gas phase spectroscopy and sensing by detecting changes in compliance voltage of an external cavity quantum cascade laser due to intracavity absorption. The technique is characterized and used to measure the absorption spectrum of water vapor and Freon-134a.     

A distributed model for continuous-wave erbium-doped fiber laser

A distributed model of a continuous-wave erbium-doped fiber laser is discussed. The model is based on two contra-propagated traveling laser waves, and includes inhomogeneous pumping, excited state absorption at the pump and the laser wavelengths, amplified spontaneous emission and radial distribution of populations of erbium levels. It is shown that excited state absorption is a main limiting factor to the laser's efficiency. Moreover, consideration of radial distributions of erbium levels' populations in the model reduces laser efficiency and decreases optimal reflection of the laser output coupler. The modeling results are in excellent agreement with the experimental study on the EDFL efficiency.     

基于中红外分布反馈量子级联激光器的光声光谱技术用于痕量甲烷气体检测

由于工业监控和环境检测的需要,甲烷气体检测日益得到人们的关注。研究了基于中红外分布反馈量子级联激光器(DFB-QCL)的光声光谱技术,并应用于痕量甲烷的检测。自主研发的DFB-QCL室温工作时的激射波长在7.6μm附近,覆盖了甲烷的特征吸收谱线1 316.83cm-1。待测甲烷气体充入亥姆霍兹光声谐振腔中,DFB-QCL的工作频率为234Hz、室温脉冲工作时峰值功率为80mW。中红外光经过甲烷吸收后,产生的声波信号经麦克风检测,由锁相放大器对信号进行采集并输入计算机进行处理。按信噪比为1计算,得到甲烷的探测极限为189nmol.mol-1。     

Highly efficient single-pass frequency doubling of a continuous-wave distributed feedback laser diode using a PPLN waveguide crystal at 488 nm

A continuous-wave distributed feedback diode laser emitting at 976 nm was frequency doubled by the use of a periodically poled lithium niobate waveguide crystal with a channel size of 3 microm x 5 microm and an interaction length of 10 mm. A laser to waveguide coupling efficiency of 75% could be achieved resulting in 304 mW of incident infrared light inside the waveguide. Blue laser light emission of 159 mW at 488 nm has been generated, which equals to a conversion efficiency of 52%. The resulting wall plug efficiency was 7.4%.     

Generation of tunable far-infrared radiation with a quantum cascade laser

We demonstrate the generation of cw tunable far-infrared radiation by mixing a quantum cascade laser and a CO>(2) laser in a W-Ni metal-insulator-metal diode. The first known spectroscopic application to the recording of an H(79)Br transition near 4.47 THz is reported.