Design, fabrication and characterization of infrared LVOFs for measuring gas composition

Abstract

This paper presents the design, fabrication and characterization of a linear-variable optical-filter (LVOF) that will be used in a micro-spectrometer operating in infrared (IR) for natural gas composition measurement. An LVOF is placed on top of an array of detectors and transforms the optical spectrum into a lateral intensity profile, which is recorded by the detectors. The IR LVOF was fabricated in an IC-compatible process using a photoresist reflow technique, followed by transfer etching of the photoresist into the optical resonator layer. The spectral range between 3 to 5 μm contains the absorption peaks for hydrocarbons, carbon-monoxide and carbon-dioxide. The resulting optical absorption is utilized to measure the gas concentrations in a sample volume. Two LVOF structures were designed and fabricated on silicon wafers using alternate layers of sputtered silicon and silicon-dioxide as the high-and low-refractive index materials. These filters consist of a Fabry-Pérot resonator combined with a band-pass filter designed to block out-of-band transmissions. Finally, the filters were fully characterized with an FTIR spectrometer and showed satisfactory agreement with the optical thin-film simulations. The characterization showed a spectral resolution of 100 nm, which can be further improved with signal processing algorithms. This method makes it possible to fabricate small and robust LVOFs with high resolving power in the IR spectral range directly on the detector array chip.

Compact gas cell integrated with a linear variable optical filter

“…Linear variable optical filters have previously been presented as microspectrometers in the visible [23] and mid-IR [24] with tapered silicon dioxide resonator layers. The optical requirements in LVOF design for the gas filled resonator are different from the typical LVOF design in the sense that a much longer cavity is used, resulting in a higher mode of resonance (m > 10).…”

“…Subsequently, the profile of the photoresist is transferred to the desired optical layer lying underneath by one-to-one plasma etching. This fabrication method has previously been used for tapering resonator layers in LVOFs [23,24]. In this paper, the same method is used to taper the first layer of the tapered Bragg mirror.…”

“…2(a). Several research groups have reported miniaturized light sources [18][19][20], optical filters [17,[21][22][23][24][25][26][27][28], and detectors [29,30]. However, the sample cell which is used for storing the sample gas occupies a large part of the microspectrometer.…”

CMOS-compatible mid-IR metamaterial absorbers for out-of-band suppression in optical MEMS

“…These users combined constitute a high-volume application, which favors the use of a technique such as CMOS that offers costs advantages in batch fabrication [3]. The characteristic absorption lines of the hydrocarbons are in the mid-infrared (mid-infrared) spectral range between about 3.1 μm and 3.5 μm (Free Spectral Range, FSR = 400 nm at λ o = 3300 nm) and should be measured with a Full-Width Half-Magnitude resolution, FWHM, in the range between λ FWHM = 2 and 5 nm for effective composition measurement [4].…”

Linear variable filters fabricated by ion beam etching with triangle-shaped mask and normal film coating technique

“…They also can be utilized to make up a spectrum detector with a CCD/CMOS camera, remarkably improving the simplicity, stability, and efficiency of the optical splitting system [1][2][3] . A LVOF set as the pivotal optical splitting component of a spectrometer can be applied to applications in aerospace, field detection, atmospheric monitoring, biological fluid detection, chemical sensors, spectral imaging, and so on [4][5][6] . As such, relevant research on LVOF has attracted extensive interest [7][8][9][10][11][12][13][14][15] .…”