2.1.

Design of Asymmetric 1 × 2 Optical Waveguide for Optical Oxygen Gas-Sensor Probe

Figure 1 shows the overall structure of the proposed optical oxygen sensor including the asymmetric $1\times 2$ optical waveguide probe module consisting of two waveguides with different sizes. The optical oxygen gas sensor comprises an optical source, photodetector, oxygen-sensing probe module including the oxygen-sensitive fluorescent layer, an electronic driver circuit, and data signal-processing unit. The optical source is the energy source to excite the fluorescent dye at a specific wavelength. When the fluorescent sensing layer absorbs excitation light of a specific wavelength from the optical source, it emits the fluorescence. The emitted fluorescent intensity is dependent on the concentration of oxygen present in the measurement environment. Therefore, the oxygen sensor can detect the oxygen concentration by measuring the variation of emitted fluorescence intensity according to the gas concentration. The photodetector is a device that detects the fluorescence signal discharged by the reaction with oxygen. The photodetector is configured to detect only the fluorescence signal by the bandpass filter.

Fig. 1

Structure of optical oxygen sensor with asymmetric $1\times 2$ beam optical waveguide.

In order to transmit both the power of the optical source and the fluorescence signal at the same time efficiently, the probe module for the optical oxygen sensor comprises an asymmetric $1\times 2$ optical waveguide that branches into channel 1 and channel 2 with different core sizes, as shown in Fig. 2(a). The optical source enters through input channel 1 and passes through channel 2. The oxygen-sensitive fluorescent layer is coated on the end face of channel 2. The fluorescence emitted by the fluorescent sensing layer passes through channel 3.

Fig. 2

Design of asymmetric $1\times 2$ optical waveguide with different core sizes waveguides: (a) structure of asymmetric $1\times 2$ optical waveguide and (b) optical transmission efficiency according to branching angle.

We conducted a optic simulation based on beam propagation method using the design parameters listed in Table 1. We designed the asymmetric $1\times 2$ optical waveguide structure for efficiently transmitting the power of the optical source (port 1 → port 2) and fluorescence signal (port 2 → port 3) simultaneously. The core size of channel 2 and channel 3 in the asymmetric $1\times 2$ optical waveguide is $2000\mu \mathrm{m}\times 2000\mu \mathrm{m}$ and the core size of channel 1 for optical source transmission is $200\mu \mathrm{m}$. The refractive indices of the core and cladding were 1.49 and 1.42, respectively. Because the transmission efficiency of the optical source and fluorescence signal depends on the branch angle of channel 1 and channel 2, we respectively simulated the transmission efficiency of the optical source and the fluorescence signal according to the branch angle.

Table 1

Optical design parameters for asymmetric 1×2 optical waveguide.

Figure 2(b) shows the transmission efficiency of the optical source and fluorescence signal with the branch angle varied from 1 deg to 5 deg at intervals of 0.2 deg. The transmission efficiency of the optical source decreased significantly at a branch angle of $\sim 3\mathrm{deg}$, with the fluorescence signal efficiency increasing relatively slightly as the branch angle increased.

We designed the optimum branch angle of 3 deg considering the transmission efficiency and manufacturing tolerance. The optical source and fluorescence signal efficiency on the waveguide at a branch angle of 3 deg was $\sim 89\%$ and 92% for the optical source and fluorescence signal transmission, respectively.

2.2.

Fabrication of Asymmetric 1 × 2 Optical Waveguide Device

Figure 3 shows the fabrication process of the asymmetric $1\times 2$ optical waveguide for the optical oxygen sensor probe. The fabrication process includes four steps: (1) original metallic mold fabrication, (2) fabrication of a polydimethyl siloxane (PDMS) stamp using double-casting technology, (3) formation of the core layer using UV imprint lithography, and (4) lamination process.

Fig. 3

Schematic view of fabrication process of asymmetric $1\times 2$ optical waveguide for oxygen sensor probe.

The original metallic mold with the asymmetric $1\times 2$ optical waveguide pattern was fabricated with an ultraprecision machine to reduce the optical propagation loss caused by the surface roughness. Then, we fabricated the PDMS stamp using PDMS double-casting technology to retain the same shape as the original mold. PDMS double-casting consists of two replication steps, where a structure obtained in the first replica molding becomes a master for the second replica molding step. However, PDMS double-casting may be problematic, because the first replicated PDMS structure strongly adheres to the second PDMS structure. Therefore, the surface modification of the first PDMS master is required. We modified the surface of PDMS by using thermal aging of the PDMS surface.

First, we precisely replicated the first PDMS master by using the original fabricated metallic mold. The PDMS solution was made by mixing the precursor and curing agent (Sylgard™ 184, Dow Corning Co.) in a weight ratio of 10:1 followed by sufficient stabilization time. The mixtured PDMS solution was carefully poured onto the original mold and cured at 70°C for 30 min. We could fabricate the first PDMS master with positive type by peeling off from the original mold. The surface of the first PDMS master was modified by thermal aging at 120°C for 24 h. Then, we poured the PDMS solution on the first PDMS master and cured it at room temperature and 70°C for 1 h each. Through the PDMS double-casting process, the PDMS stamp in the same shape as the initial original master could be obtained, as shown in Fig. 4(a).

Fig. 4

Fabricated $1\times 2$ optical waveguide with asymmetric structure and different core sizes: (a) SEM image of PDMS stamp patterns, (b) SEM image of core layer, (c) image of asymmetric $1\times 2$ optical waveguide with different sizes, and (d) optical insertion loss of fabricated asymmetric $1\times 2$ optical waveguide.

Afterward, the core resin (Chem Optics Co., FOWG 116) was filled in the patterns of asymmetric $1\times 2$ optical waveguides on a PMMA sheet to be coated with the underclad resin (Chem Optics Co., FOWG 506). The core UV resin was cured by UV exposure of $110\mathrm{mJ}/{\mathrm{cm}}^2$ in a nitrogen atmosphere for 15 min. Figure 4(b) shows the fabricated core pattern of asymmetric $1\times 2$ optical waveguides. We also formed the upper cladding layer in the lamination process to fabricate the optical waveguides, as shown in Fig. 4(c). Then, we cut the cross-section of the end of the channels and polished the end face to measure the characteristics of the asymmetric $1\times 2$ optical waveguide. We measured the insertion loss of the fabricated asymmetric $1\times 2$ optical waveguide. Figure 4(d) shows the insertion losses of the optical source unit and fluorescence signal unit. The insertion loss of each channel was measured as 0.97 and 0.86 dB. The optical oxygen-sensor probe transmitted the optical source power and the fluorescence signal with 80% and 82% efficiency, respectively.

2.3.

Fabrication of Fluorescent Oxygen Gas Sensor Probe Module

In this work, the fluorescent ruthenium complex ((tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II), ${{\mathrm{RuDpp}}_3}^{2+}$) was synthesized as the oxygen-sensing dye because of its long lifetime and strong absorption in the blue-green region of the spectrum, which is compatible with recently available high-brightness blue light-emitting diodes (LEDs). The ${{\mathrm{RuDpp}}_3}^{2+}$ was purchased from GFS Chemicals, Inc. and purified as described in the literature.¹⁴ The ${{\mathrm{RuDpp}}_3}^{2+}$ displays fluorescence quenching effects to differing levels of oxygen concentration.^15,16

We analyzed the luminescence intensity at the fluorescent layer according to ${{\mathrm{RuDpp}}_3}^{2+}$ concentration in the sol–gel solution. For the analysis, the sol–gel solution was prepared with a composition ratio of 3.79 g: 1.45 g: 0.72 g: 21.24 mg for MTMS (Methyltrimethoxysylane, Sigma-Aldrich Chemical Co.), EtOH, ${\mathrm{H}}_2\mathrm{O}$, and HCl (35%), respectively. The mixtured solution was left under stirring at room temperature for 2 h for forming sol–gel. We mixed the ${{\mathrm{RuDpp}}_3}^{2+}$ to be attached on the silica particles with a diameter of $4\mu \mathrm{m}$ in the sol–gel solution with concentrations of 3, 5, 10, and $20\mathrm{mg}/\mathrm{ml}$. Each sol–gel solution mixed with ${{\mathrm{RuDpp}}_3}^{2+}$ was coated on the UV-curable layer using the spray coating method and was dried at room temperature for 24 h and finally heated at 60°C for 24 h.

The excitation source is a high-brightness blue LED whose spectral output peaks at 470 nm. The emission spectrum of the LED has good overlap with the absorption spectrum of the ruthenium complex. A silicon photodiode with built-in OP-AMP as the photodetector was used as the photodetector for sensing the fluorescence signal. The bandpass filter (bandwidth 20 nm, centered at 600 nm) at the front of the photodetector was packaged.

Figure 5 shows the fluorescence intensity at 0% and 25% standard oxygen gas according to ${{\mathrm{RuDpp}}_3}^{2+}$ concentrations. The difference in fluorescence intensity decreased at a specific concentration or higher. The UV-curable layer to be coated with a concentration of $5\mathrm{mg}/\mathrm{ml}$ showed the highest fluorescence intensity variation between 0% and 25% standard oxygen. We coated the sol–gel solution with an optimized concentration of $5\mathrm{mg}/\mathrm{ml}$ on the end of the $2000\text{-}\mu \mathrm{m}$ channel in the asymmetric $1\times 2$ optical waveguide using the spray coating method. The thickness to be coated with sol–gel solution is about $10\mu \mathrm{m}$. The $200\text{-}\mu \mathrm{m}$ channel was bonded to the $100\text{-}\mu \mathrm{m}$ optical fiber connected to the 470-nm LED optical source.

Fig. 5

Fluorescence intensity variation according to ruthenium concentration at 0% and 25% standard oxygen gas.

Figure 6 shows the channel image of the fabricated asymmetric $1\times 2$ optical waveguide and 600-nm fluorescence emitted when the 450-nm optical source is inputted.

Fig. 6

Oxygen optical sensor probe module: (a) image of coated fluorescence layer on end face of optical waveguide and (b) beam propagation image and 600-nm fluorescence image.