Compact gas cell integrated with a linear variable optical filter

Folded beam path architecture for highly efficient filter-based spectral sensors

This paper demonstrates a method to significantly enhance the detection efficiency of filter-based spectral sensors without the use of additional dichroic optics for spectral preselection. The fundamental principle is that light reflected from one interference filter or filter segment can be used consecutively, reducing the overall system losses. The proof-of-concept is presented using two compact optical modules. The first module uses 10 individual filters between 520 and 800 nm, and the second is capable of continuous spectrum acquisition between 450 and 825 nm using a linear variable filter (LVF) as a key element. An efficiency increase factor of up to approximately 100 compared to a common system, where the entire LVF is directly illuminated, was demonstrated.

On-Chip Non-Dispersive Infrared CO2 Sensor Based On an Integrating Cylinder

In this paper, we propose a novel, miniaturized non-dispersive infrared (NDIR) CO2 sensor implemented on a silicon chip. The sensor has a simple structure, consisting of a hollow metallic cylindrical cavity along with access waveguides. A detailed analysis of the proposed sensor is presented. Simulation with 3D ray tracing shows that an integrating cylinder with 4 mm diameter gives an equivalent optical path length of 3 . 5 cm. The sensor is fabricated using Deep Reactive Ion Etching (DRIE) and wafer bonding. The fabricated sensor was evaluated by performing a CO2 concentration measurement, showing a limit of detection of ∼100 ppm. The response time of the sensor is only ∼2.8 s, due to its small footprint. The use of DRIE-based waveguide structures enables mass fabrication, as well as the potential co-integration of flip-chip integrated midIR light-emitting diodes (LEDs) and photodetectors, resulting in a compact, low-power, and low-cost NDIR CO2 sensor.

Real-time and non-destructive gas mixture analysis using linear various filter enabled mid-infrared visualization

Real-time gas mixture analysis has been demonstrated using various linear variable filter (LVF)-enabled mid-infrared (mid-IR) visualizations. Due to the characteristic absorptions of different gases, the algorithm-enabled sensing method has the ability to detect multi-component gas mixtures noninvasively. The proposed system consisted of a broadband light source, a gas mixing and delivery chamber made by polydimethylsiloxane (PDMS), a LVF, and a real-time monitoring mid-IR camera. The system performance was evaluated by detecting CH₄ and C₂H₂ at their characteristic C-H absorptions from λ = 3.0 to 3.5 µm. A fast and accurate identification of gas samples was achieved. Therefore, our real-time and non-destructive gas analysis system enables a new visualization technology for environmental monitoring and industrial measurement.

Multiband absorbers for the long-wave infrared regime

We present the development of a long-wave infrared regime multiband absorption filter with simultaneous wavelength and intensity selectivity. The approach employs a technique we call "spectral dithering" to place single-band absorbing pixels across a unit cell such that their weighted sum describes a multiband absorption spectrum. The number of absorption bands is proportional to the number of unique pixels present. Pixel patterning controls wavelength selectivity, whereas pixel distribution controls intensity selectivity. Using the rigorous coupled-wave method for modelling, a filter with three absorption bands between 6 and 14 μm is designed using the spectral dithering technique. The device is fabricated and experimentally verified using Fourier-transform infrared spectroscopy.

Functionalizing a Tapered Microcavity as a Gas Cell for On-Chip Mid-Infrared Absorption Spectroscopy

Increasing demand for field instruments designed to measure gas composition has strongly promoted the development of robust, miniaturized and low-cost handheld absorption spectrometers in the mid-infrared. Efforts thus far have focused on miniaturizing individual components. However, the optical absorption path that the light beam travels through the sample defines the length of the gas cell and has so far limited miniaturization. Here, we present a functionally integrated linear variable optical filter and gas cell, where the sample to be measured is fed through the resonator cavity of the filter. By using multiple reflections from the mirrors on each side of the cavity, the optical absorption path is elongated from the physical μm-level to the effective mm-level. The device is batch-fabricated at the wafer level in a CMOS-compatible approach. The optical performance is analyzed using the Fizeau interferometer model and demonstrated with actual gas measurements.

Angle-tolerant linear variable color filter based on a tapered etalon

We propose and fabricate a linear variable color filter (LVCF) that possesses an enhanced angular tolerance in conjunction with a wide linear filtering range (LFR) by taking advantage of an Ag-TiO₂-Ag configuration. The TiO₂ cavity is tapered in thickness along the device so that the resonance wavelength can be continuously tuned according to the position. In addition, the metal-dielectric-metal structure is overlaid with a pre-designed graded anti-reflection coating in SiO₂ to complete the etalon, thereby maximizing the transmission efficiency across the entire device. The tapered dielectric layers in the proposed filter were fabricated via glancing angle deposition without the help of any mask or moving parts. The center wavelength was scanned from 410 nm to 566 nm, resulting in an LFR of 156 nm, and the overall spectra exhibited an approximate peak transmission of 40% and spectral bandwidth of 68 nm. The angular tolerance was as large as 45°, incurring a fractional wavelength shift below 4.2%. The resonance wavelength was verified to be linearly dependent on the position, providing a linearity beyond 99%. The proposed LVCF will thus be actively utilized in a portable micro-spectrometer and spectral scanning device.

Single-step fabrication of thin-film linear variable bandpass filters based on metal-insulator-metal geometry

A single-step fabrication method is presented for ultra-thin, linearly variable optical bandpass filters (LVBFs) based on a metal-insulator-metal arrangement using modified evaporation deposition techniques. This alternate process methodology offers reduced complexity and cost in comparison to conventional techniques for fabricating LVBFs. We are able to achieve linear variation of insulator thickness across a sample, by adjusting the geometrical parameters of a typical physical vapor deposition process. We demonstrate LVBFs with spectral selectivity from 400 to 850 nm based on Ag (25 nm) and MgF₂ (75-250 nm). Maximum spectral transmittance is measured at ∼70% with a Q-factor of ∼20.