Technical Field

The present invention relates to optical waveguide gas sensors.

Background Art

Various gas sensing technologies are currently available on the market. They can be divided into two main groups - sensors based on electrical property variations in materials and device employing other type of response due to the gas presence. This division already indicates that electrical gas sensors are the most common and widespread technology for gas sensing. Compared to electronical sensors, optical designs can achieve higher sensitivity, selectivity, stability, as well as device performance will not be deteriorated by catalyst poisoning caused by specific gases that can significantly influence electrical sensors [1]. Also, it is important to emphasize that optical devices are immune to electromagnetic interaction [2].

From device manufacturing point of view, devices based on optical absorption or refractive index changes are simple to implement. For devices employing absorption technology, the biggest issue is laser sources. Many gases have strong absorption in mid-infrared spectrum. However, for this spectral region most of lasers have low output power and need cooling systems. Regarding waveguide devices, design can be tuned to work for different laser sources [3]. Use of interference as detection method also enhances the sensitivity as this allows detecting small changes in refractive index of cladding material. Due to this, detection through refractive index changes is probably the best choice.

Regarding devices employing refractive index changes, Mach-Zehnder interferometer (MZI) is not the only design type applicable for such detection method. Device designs employing the similar detection method has been reported using whispering gallery mode resonators [4; 5], plasmonic devices [6 - 8], Fabry-Perot [9] interferometers as well as hollow core fibres [10]. Compared to whispering gallery mode approach, MZI has lower demands for fabrication process, as it is necessary to place resonant structure close enough to waveguide to ensure efficient coupling between waveguide and resonator. In case of inefficient coupling, sensitivity of device will drastically decrease.

Most of plasmonic devices employ some type of metallic nanoparticles or metallic layers, again complicating their manufacturing.

While Fabry-Perot gas sensors are similar to proposed technology as it also employs porous polymer as gas sensitive medium, examples present in literature uses deformation and not refractive index changes due to gas absorption as well as optical fibres for light guiding, meaning that device does not have an integrated light guiding elements. While for signal sensor this might not be an essential problem, as optical fibres are widely used for optical signal coupling into optical devices, when scaling system to multiple sensors on single chip, more complicated fibres for light coupling into device would be necessary.

Lastly, hollow core fibre gas sensors typically are quite large with fibre/gas interaction length reaching order of meters. While these types of sensors are precise, they are limited by their interaction length and cannot be made small and portable.

Summarising, comparing to other gas sensors employing refractive index changes, the most important advantage of the claimed design is its relatively simple manufacturing. Most of the commercially available devices now are either based on gas sensitive semiconductors (GSS) [11; 13], or photoionization detectors (PID) [12; 14], which are mainly based on gas detection through electrical signal, making optical sensors unique in modern gas sensor market

Disclosure of the Invention

The aim of the invention is reached by providing an optical waveguide gas sensor, comprising a glass or a polymer film substrate, a light source, a light detector, as well as an asymmetric Mach-Zehnder interferometer having an input and an output, comprising a core and a cladding, wherein the Mach-Zehnder interferometer further comprises single mode waveguides, where the cladding is pours; wherein the light source is an organic solid-state laser, having emitting wavelength between 620 nm and 680 nm; wherein the light detector comprises organic thermoelectric sensor with organic light absorber optimized for wavelengths between 620 nm and 680 nm. The light source is optically coupled with optical input of the Mach-Zehnder interferometer. According to the preferred embodiment the optical input and output of the Mach-Zehnder interferometer is provided with a Bragg grating.

According to embodiment the waveguides are SU-8 photoresist waveguides and the cladding material is a 2-(4-(bis(5,5,5-triphenylpentyl)amino)benzylidene)- lH-indene-l,3(2H)-dione and polyfmethyl methacrylate) guest-host system.

The refractive index of the cladding changes due to the absorption of the gas. The output light intensity depend on the absorbed gas amount and the light intensity is detected by the thermoelectric sensor which is coupled with the optical output of the interferometer.

The invention also provides for a method for manufacturing the optical waveguide gas sensor according to any one of the preceding claims, comprising the following steps: (i) providing a substrate, comprising a glass or a polymer film; (ii) deposition of Mach-Zehnder interferometer waveguide core by optical lithography; (iii) preparation Bragg grating on optical input and output of the Mach-Zehnder interferometer by nano-imprinting; (iv) deposition of Mach- Zehnder interferometer waveguide cladding by ink-jet printing; (v) deposition of the organic solid-state laser medium designed to have emitting wavelength between 620 nm and 680 nm on the optical input of the Mach-Zehnder interferometer by ink-jet printing; the laser medium comprising red emitting laser dyes and Bragg grating and is adapted to be pumped by green light pulse source; (vi) sequent deposition of an organic light absorber, a bottom metal electrode, organic thermoelectric material, and top metal electrode on the optical output of the Mach-Zehnder interferometer by thermal evaporation in vacuum. Short Description of Drawings

Figure 1 shows structural scheme of one embodiment of the invention, where 1 is a substrate of the sensor, 2 - a Mach-Zehnder interferometer core, 3 - a Mach- Zehnder interferometer cladding, 4 - an optical input of interferometer with organic solid-state laser, 5 - an optical output of interferometer with thermoelectric sensor.

Figure 2 shows structural scheme of a cross-section of the optical input of the interferometer, where 1 is a substrate of the sensor, 2 - a Mach-Zehnder interferometer core, 2’ - a Bragg grating in Mach-Zehnder interferometer core, 6

- organic solid-state laser.

Figure 3 shows structural scheme of a cross-section of the optical output of the interferometer, where 1 is a substrate of the sensor, 2 - a Mach-Zehnder interferometer core, 2’ - a Bragg grating in Mach-Zehnder interferometer core , 7

- bottom metal electrode, 8 - organic thermoelectric material, 9 - top metal electrode, 10 - organic light absorber.

Figure 4 shows manufacturing scheme, where 1 is a substrate of the sensor, 2 - a Mach-Zehnder interferometer core, 3 - a Mach-Zehnder interferometer cladding. Figure 5 shows design of tested gas sensor structure.

Figure 6 shows example of measured phase difference between both arms in interferometer, when the sensor was exposed to pure N ₂ flow or mixture ofl N ₂ and IPA.

Example of Implementation of the Invention

When gas is applied to the device, it is partly absorbed by the device’s cladding 3 material in its pores. As part of optical mode is traveling through cladding 3, changes in refractive index of cladding 3 material will influence output intensity of the device. Due to asymmetric arms of interferometer, refractive index changes in cladding 3 material induce different phase changes in each of interferometer arms. This leads to change in interference outcome of the device. For this purpose, the device needs to have single-mode waveguides at the working wavelength. This can be achieved through waveguide dimension tuning. According to one embodiment, the waveguide of the Mach-Zehnder interferometer is made from SU-8 negative photoresist using optical lithography. Firstly, a SU-8 and gamma-Butyrolactone solution is mixed with proportions 1:1 in weight. The solution is spin-coated on glass substrate with 4000 rpm/s to create a 0.6-1.5 pm thick SU-8 photoresist layer (Figure 4 A). The device is then exposed e.g. using 365 nm laser writer μPG 101 Heidelberg Instruments. Interferometer optical input contains up to 100 pm wide and 50 pm to 1 mm long waveguide element that over a length of 5 mm transits to 1.5-3 pm waveguide core 2. Initial waveguide is split into two interferometer arms that have bend radius of 0.5 mm. Length difference of both arms is 8 mm with shortest arm’s length being 20 mm. At sensor output 1.5-3 pm waveguide core over a length of 5 mm transits to up to 100 pm wide waveguide with length of 50 pm to 1 mm (Figure 4 B). After exposure, sample is developed in mr-Dev 600 (Micro Resist Technology) and rinsed in isopropanol. To create gas sensitive coating, firstly, a solution of chromophore 2-(4-(bis(5,5,5-triphenylpentyl)amino)benzylidene)-1H-indene - l,3(2H)-dione (DMABI-Ph6) and poly(methyl methacrylate) (PMMA) and chlorbenzol is mixed. Solution is then applied on to the waveguide interferometer arms with ink-jet printing technique (Figure 4 C). Bragg grating 2’ is made on optical input and output of the interferometer by the nano-imprinting technique. Organic solid-state laser medium is made from solution of pyraniliden fragment consisting dyes with organic solvent by ink-jet printing technique. Organic laser medium fully covers optical input of the sensor and has thickness between 150 nm and 400 nm. Optical signal detection part consists of sensors based of thermoelectric (TE) phenomena. The sensor structure contains an organic light absorber layer 10, bottom electrode 7, TE active layer 8 and the top electrode 9, which is also the heatsink. The sensor structure is formed by thermal evaporation in vacuum. Firstly, 50 nm thin light absorber layer containing zinc phthalocyanines (ZnPc) is deposited on the grid at the optical output of waveguide. Then 100 nm Cu bottom electrode 7 is deposited on the absorber layer on which tetrathiotetracene layer is deposited as TE active material 8. Then 30 micrometres thick top electrode 9 is deposited on the top of TE layer. The device manufactured shown that it has different sensitivity to various solvents widely used in chemical laboratories. Device scheme can be seen in Figure 5 with following device parameters: waveguide core is 1.5 μm wide and 0.7 μm high with refractive index n= 1.589, the thickness of the cladding layer is 1.2 μm, curve radius is R=500 μm and length of interferometer arm is L _a =10 mm while excess length of interferometer arm is L=8 mm. Device sensitivity was tested using N ₂ flow mixed with different volatile solvents. An example of device sensitivity to isopropanol (I PA) is shown in Figure 6. Similar measurements were carried out for different mixtures. The study showed that sensitivity of the device depends on gas molecule size. The device being more sensitive to water vapour and less sensitive to solvents such as acetone.

Sources of information

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II. GazDetect https://www.gazdetect.com/download/pdf-anglais-infos-dg/EN- CSC300-Weh.pdf 12. Aeroqual. https: //www.aeroqual.com /voc-sensors-monitors

13. Evikon. https://www.evikon.eu/en/evikon-mci-m-l/solvent-vapors- detectrr-transmitter-e2608-voc-p- 199

14. Ion Science, https://www.ionscience.com/products/tiger-handheld-voc- detector/?gclid=CjwKCAiA9JbwBRAAEiwAnWa4Q5OXzBfxAL0M0w6Ee0c0 22ja DoQoiKacIOt4qePfFRkI9eGRLJK-YxoCEKsQAvD_BwE