The fundamental vibrational-rotational absorption signature of almost all the chemical compounds lies in the Mid- IR spectrum (λ=3-15μm) thus offering superior light-analyte interaction in this regime. In particular, the successful inscription of infrared-spectroscopy in a multi-pass cell has significantly boosted its use mainly in the gas-sensing application at the sub-ppm/ppb level. However, the requirement of bulky, alignment sensitive, and need of expertise-hands makes it inappropriate for many fields especially in portable applications like stand-alone environment monitoring, detection of chemical-warfare-agent in the battlefield, Astro-biological applications, etc. An external disruption-free handheld device (i.e., unaffected from any external vibration, physical stress, and thermal variations) with high specificity and selectivity are still prerequisites for such in-situ applications. The advancements in photonics have shown enormous possibilities to miniaturize all spectroscopic components to a single chip. In this context, the slow light-assisted engineered photonic structure on a QCL/QCD (quantum confined laser and detector) is most promising to replace bulky multi-pass cell optics. In principle, it slows down the light with several folds to enhance the light-analyte interaction and thus open an avenue for an on-chip sensing platform. Most efficient QCLs demonstration explored in the InP platform, also a selection of InP-InGaAs eliminates the requirement of the costly wafer-bonding process. In this paper, we consider slow-light assisted and wavelength-tunable periodic photonic structures. The device is designed such that it supports transverse magnetically polarized mode directly emitted from QCLs. It eliminates the use of any additional polarization-rotator (conversion from TM to TE mode) which reduces fabrication complexity and additional space on the chip.
Over the past decade, in the near-infrared spectral region, optical phased array (OPA) technology has advanced from basic concepts to demonstrations in efficient, high-resolution, wide-angle systems. Extension to the mid-infrared spectral region has only recently begun. In this work, at λ = 4.6 μm, on an InGaAs/InP platform, we demonstrate the operation of a 32-channel OPA beam steering device. Through thermo-optic tuning we steer the beam laterally within a ±11.5° field-of-view.
Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and Sub-wavelength waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ=6.15µm in the low index contrast InGaAs-InP platform. We experimentally detect 5 ppm ammonia in 1 mm long suspended HPCW and 3 mm long suspended SWW with propagation loss of 39.1 dB/cm and 4.1 dB/cm, respectively. This result emphasizes the advantage of SWWs which can utilize the longer devices by virtue of its low propagation loss to increase the gas sensing sensitivity compare to HPCWs.
With the mid-infrared spectral region (3 to 20 µm) home to the fundamental absorption peaks of most molecules, the region has come to be known as the “molecular-fingerprint” region and various technologies have evolved to perform the detection and quantification task. Popular technologies include cavity ring-down spectroscopy, tunable diode laser absorption spectroscopy and Fourier transform infrared spectroscopy; and parts-per-billion sensitivity is commonly achieved. But the size, weight and sensitive optics of these technologies tends to limit their use to the laboratory. Field applications, particularly airborne and handheld ones, demand compact on-chip technologies. Now, with the ability of quantum cascade lasers (QCLs) to provide narrow-band tunable room-temperature emission across the majority of the mid-infrared region, the move to on-chip technologies is enabled. In this work we seek to demonstrate the monolithic integration of QCL, quantum cascade detector (QCD) and passive components for an on-chip gas sensor around λ = 4.6 μm (an absorption peak of carbon monoxide). Since most efficient QCLs have been demonstrated in InP-based material, we use a lattice-matched InGaAs/InP platform to avoid the low-yield lossy costs of bonding, and work with a single-growth epitaxial structure. Light is coupled from the QCL active region downward to the passive InGaAs waveguide structure by a coupling taper; interaction with the analyte occurs on or near the passive structure and is subsequently passed to the QCD. Variations in design are investigated to compare sensitivity.
Gas sensing based on infrared absorption spectroscopy has attracted considerable attention owing to the rovibrational signatures of compounds of interest in the molecular fingerprint region. On-chip spectrometers are promising devices that unlike their on-chip commercially available counterparts offer sensing in portable applications. Carbon monoxide as one of the major air pollutants is dangerous even at very low concentrations. For real-time and precise detection of trace amounts of this gas we need a compact highly sensitive and selective sensor. In this paper, we design a grating array device in the InP/InGaAs platform for trace detection of carbon monoxide. The passive device consists of an InGaAs strip waveguide with InP as cladding, split-cascaded into an equally spaced array of 32 separate optical paths by y-junctions. At the output of each waveguide shallow etched subwavelength emitter gratings couple the light out. Light-analyte interaction occurs on top of the gratings as well as in free space. The device is optimized to operate at mid-IR wavelength of λ=4.6μm where the absorption peak of carbon monoxide is located. Using this structure, gas sensing is experimentally demonstrated down to a concentration of 10ppm. Feasibility of achieving lower limit of detection will be shown by design modifications. This device, can be simply integrated with QCL/QCD and be used for portable high sensitivity gas sensors.
Chemicals are best recognized by their unique wavelength specific optical absorption signatures in the molecular fingerprint region from λ=3-15μm. In recent years, photonic devices on chips are increasingly being used for chemical and biological sensing. Silicon has been the material of choice of the photonics industry over the last decade due to its easy integration with silicon electronics as well as its optical transparency in the near-infrared telecom wavelengths. Silicon is optically transparent from 1.1 μm to 8 μm with research from several groups in the mid-IR. However, intrinsic material losses in silicon exceed 2dB/cm after λ~7μm (~0.25dB/cm at λ=6μm). In addition to the waveguiding core, an appropriate transparent cladding is also required. Available core-cladding choices such as Ge-GaAs, GaAs-AlGaAs, InGaAs-InP would need suspended membrane photonic crystal waveguide geometries. However, since the most efficient QCLs demonstrated are in the InP platform, the choice of InGaAs-InP eliminates need for wafer bonding versus other choices. The InGaAs-InP material platform can also potentially cover the entire molecular fingerprint region from λ=3-15μm. At long wavelengths, in monolithic architectures integrating lasers, detectors and passive sensor photonic components without wafer bonding, compact passive photonic integrated circuit (PIC) components are desirable to reduce expensive epi material loss in passive PIC etched areas. In this paper, we consider miniaturization of waveguide bends and polarization rotators. We experimentally demonstrate suspended membrane subwavelength waveguide bends with compact sub-50μm bend radius and compact sub-300μm long polarization rotators in the InGaAs/InP material system. Measurements are centered at λ=6.15μm for sensing ammonia.
Mid-infrared trace gas sensing is a rapidly developing field with wide range of applications. Although CRDS, TDLAS, FTIR and others, can provide parts per billion and in some cases, parts per trillion sensitivities, these systems require bulky and expensive optical elements and, furthermore, are very sensitive to beam alignment and have significant size and weight that place constrains on their applications in the field, particularly for airborne or handheld platforms. Monolithic integration of light sources and detectors with an optically transparent passive photonics platform is required to enable a compact trace gas sensing system that is robust to vibrations and physical stress. Since the most efficient quantum cascade lasers (QCLs) demonstrated are in the InP platform, the choice of InGaAs-InP for passive photonics eliminates the need for costly wafer bonding versus silicon, germanium of GaAs, that would require optically absorbing bonding interfaces. The InGaAs-InP material platform can potentially cover the entire λ=3-15μm molecular fingerprint region. In this paper, we experimentally demonstrate monolithic integration of QCL, quantum cascade detector (QCD) and suspended membrane sub-wavelength waveguides in a fully monolithic InGaAs/InP material system. The transverse magnetic polarized QCL emission is efficiently coupled into an underlying InGaAs suspended membrane subwavelength waveguide. In addition to low-loss compact waveguide bends, the suspended membrane architecture offers a high analyte overlap integral with the analyte. The propagating light is absorbed at the peak absorbance wavelength of the selected analyte gas and the transduced signal is detected by the integrated QCD. Gas sensing will be demonstrated.
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