Nanoplus (Germany)

The intensity noise of a laser source represents one of the key factors limiting the ultimate sensitivity in laser-based systems for sensing and telecommunication. For advanced applications based on interferometry, the availability of a shot-noise-limited local oscillator is even more important for the effective feasibility of high-precision measurements. This is particularly crucial in quantum optics applications based on homodyne detection schemes to measure non-classical light states, such as squeezed states. This work deeply investigates and analyzes the intensity noise features of the most widely used mid-infrared semiconductor heterostructured lasers: quantum cascade and interband cascade lasers. For this purpose, a comprehensive comparison of three different continuous-wave lasers operating at room temperature around 4.5 µm wavelength is presented. First, a thorough electro-optical characterization is given, highlighting the differences and the shared common characteristics of the tested devices. Then, a detailed intensity noise analysis is reported, identifying their different noise operations with a particular reference to shot-noise-limited operations. Finally, some perspectives towards advanced applications are discussed.

This study presents the performance of a multi-gas sensor for greenhouse detection based on quartz-enhanced photoacoustic spectroscopy (QEPAS). The QEPAS sensor exploits an innovative, compact three-wavelength laser module as excitation source. The module integrates three interband cascade laser chips with a beam combining system, all enclosed in a compact metallic package with sizes of 40 × 52 × 17 mm to generate a single output beam. The multi-gas QEPAS sensor was tested in a laboratory environment for the sequential detection of two greenhouse gases, methane (CH4) and carbon dioxide (CO2), and a precursor greenhouse gas, carbon monoxide (CO). At an integration time of 100 ms, minimum detection limits of 21 ppb, 363 ppb, and 156 ppb, were estimated for CH4, CO2, and CO detection, respectively, all well below their natural abundance in air.

The detectivity enhancement in a double-pass architecture in an interband cascade infrared photodetector (ICIP) is investigated. The ICIP consists of twelve identically thick absorbers composed of an InAs/GaSb type-II superlattice. To achieve a double-pass of radiation, a multilayer backside mirror was sputter-deposited after thinning the undoped GaSb substrate. At room temperature, a differential resistance area product of R0A= 1.37 Ω cm2 was achieved and the ICIP has a cutoff wavelength of 6.5 μm. By adding a backside mirror to the device, the responsivity was increased by 47% caused by the double radiation pass through the absorber. This resulted in a room temperature zero-bias detectivity of 8.3 × 108 Jones at a wavelength of 5.27 μm. Furthermore, oscillations are observed in the detectivity due to light interference from the reflected beam at the semiconductor/metal interface, which manifests the reflected double-pass device functionality unequivocally in the device characteristics.

A quartz‐enhanced photoacoustic spectroscopy sensor architecture capable of detecting eight different air pollutants (CH4, NO2, CO2, N2O, CO, NO, SO2, and NH3) is reported. Each analyte is targeted using the same sensor components (acoustic detection module, quartz‐tuning fork, etc.) and a dedicated laser source. Both interband cascade lasers and quantum cascade lasers are employed to target a well‐resolved and isolated absorption feature from each investigated gas, in a wavelength ranging from 3.35 to 9.06 μm. The sensor is calibrated with certified concentrations of each gas species, in a wet nitrogen matrix. For each analyte, the optimal pressure is determined. Minimum detection limits of 40, 13, 800, 230, 450, 78, 18, and 5.8 ppb are obtained for CH4, NO2, CO2, N2O, CO, NO, SO2, and NH3, respectively, at 100 ms of integration time.

An interband cascade laser multiemitter with single-mode distributed feedback (DFB) emission at two wavelengths is presented. Continuous-wave laser operation is measured from 0°C to 40°C with threshold currents of around 25 mA and output powers of around 9 mW at 20°C. The ridge waveguide DFB structures are monolithically integrated with a spacing of 70 μm and each is provided with an individual metal DFB grating to select specific single-mode wavelengths of interest for absorption spectroscopy. The emission windows at 3.92 and 4.01 μm are targeting hydrogen sulfide and sulfur dioxide, which are of importance for industrial applications since both gases are reagents of the Claus process in sulfur recovery units, recovering elemental sulfur from gaseous hydrogen sulfide.

Tunable Laser Absorption Spectroscopy (TLAS) has proven to be a versatile tool for gas sensing applications with significant advantages compared to other techniques. These advantages include real time measurement, standoff detection and ruggedness of the sensor. Especially the Mid-Infrared (MIR) wavelength region from 3 to 6 microns is of great interest for industrial process control and the reduction of pollutants. In this contribution we present novel ICL devices developed to address the crucial air pollutant sulfur dioxide SO₂ at its transition around 4 μm. In general, interband cascade lasers (ICLs) have evolved into important laser sources for the MIR spectral range. Compared to quantum cascade lasers, they offer significant advantages with respect to threshold power density as well as overall power consumption. In contrast to conventional diode lasers, ICLs are able to cover the entire MIR wavelength range of interest. For application in TLAS, single-mode devices are required. In this work application-grade distributed feedback (DFB) ICL devices for addressing SO₂at the wavelength range around 4 μm are presented. A lateral metal grating, defined by electron beam lithography, is used to achieve DFB operation and hence spectrally single-mode emission. Continuous wave laser operation with threshold power consumption below 100 mW at room temperature, side mode suppression ratio of > 30 dB and wavelength tuning range up to 28 nm are demonstrated.

Tunable diode laser absorption spectroscopy (TDLAS) is an excellent analytical technique for gas sensing applications. In situ sensing of relevant hydrocarbon gases is of substantial interest for a variety of in-field scenarios including environmental monitoring and process analysis, ideally providing accurate, molecule specific, and rapid information with minimal sampling requirements. Substrate-integrated hollow waveguides (iHWGs) have demonstrated superior properties for gas sensing applications owing to minimal sample volumes required while simultaneously serving as efficient photon conduits. Interband cascade lasers (ICLs) are recently emerging as mid-infrared light sources operating at room temperature, with low power consumption, and providing excellent potential for integration. Thereby, portable and handheld mid-infrared sensing devices are facilitated. Methane (CH4) is among the most frequently occurring, and thus, highly relevant hydrocarbons requiring in situ emission monitoring by taking advantage of its distinct molecular absorption around 3 μm. Here, an efficient combination of iHWGs with ICLs is presented providing a methane sensor calibrated in the range of 100 to 2000 ppmv with a limit of detection at 38 ppmv at the current stage of development. Furthermore, a measurement precision of 0.62 ppbv during only 1 s of averaging time has been demonstrated, thereby rendering this sensor concept useful for in-line and on-site emission monitoring and process control applications.

Single mode distributed feedback (DFB) interband cascade lasers were realized by placing metal gratings laterally to dry etched ridges. A discrete tuning range of 104 nm could be realized on the same gain material by a variation of the grating period. At room temperature, a 2.4 mm long and 9.8 μm wide ridge with as-cleaved facets emitted more than 6 mW of single mode output power in continuous-wave (cw) mode at a wavelength around 3.8 μm. With typical temperature- and current-tuning rates of 0.31 nm/ °C and 0.065 nm/mA, respectively, a total tuning bandwidth of more than 10 nm could be covered with a single device.

A compact isotope ratio laser spectrometry (IRLS) instrument was developed for simultaneous measurements of the D/H, 18O/16O and 17O/16O isotope ratios in water by laser absorption spectroscopy at 2.73 μm. Special attention is paid to the spectral data processing and implementation of a Kalman adaptive filtering to improve the measurement precision. Reduction of up to 3-fold in standard deviation in isotope ratio determination was obtained by the use of a Fourier filtering to remove undulation structure from spectrum baseline. Application of Kalman filtering enables isotope ratio measurement at 1 s time intervals with a precision (<1‰) better than that obtained by conventional 30 s averaging, while maintaining a fast system response. The implementation of the filter is described in detail and its effects on the accuracy and the precision of the isotope ratio measurements are investigated.

In recent years the importance of lasers in optical gas sensing has been continuously increasing. Tunable Laser Absorption Spectroscopy (TLAS) has proven to be a versatile tool in modern environmental analysis. In the mid-infrared wavelength region between 3 and 6 µm, which is of high interest for sensing applications, Interband Cascade Lasers (ICL) can provide monomode continuous wave (CW) emission at room temperature. We present the simulation, design and manufacturing of distributed feedback (DFB) laser devices based on this concept, with focus on devices that target specific, technologically and industrially relevant, wavelengths with low energy consumption. Finally application-grade devices from 3 to 6 µm are presented. CW operation above room temperature and tuning ranges of 11 nm with Side Mode Suppression Ratios (SMSR) greater 30 dB were achieved.