Modeling of a Single-Notch Microfiber Coupler for High-Sensitivity and Low Detection-Limit Refractive Index Sensing

Abstract

A highly sensitive refractive index sensor with low detection limit based on an asymmetric optical microfiber coupler is proposed. It is composed of a silica optical microfiber and an As₂Se₃ optical microfiber. Due to the asymmetry of the microfiber materials, a single-notch transmission spectrum is demonstrated by the large refractive index difference between the two optical microfibers. Compared with the symmetric coupler, the bandwidth of the asymmetric structure is over one order of magnitude narrower than that of the former. Therefore, the asymmetric optical microfiber coupler based sensor can reach over one order of magnitude smaller detection limit, which is defined as the minimal detectable refractive index change caused by the surrounding analyte. With the advantage of large evanescent field, the results also show that a sensitivity of up to 3212 nm per refractive index unit with a bandwidth of 12 nm is achieved with the asymmetric optical microfiber coupler. Furthermore, a maximum sensitivity of 4549 nm per refractive index unit can be reached while the radii of the silica optical microfiber and As₂Se₃ optical microfiber are 0.5 μm and a 0.128 μm, respectively. This sensor component may have important potential for low detection-limit physical and biochemical sensing applications.

Keywords: asymmetric coupler; low detection limit; optical microfiber sensors.

Figure 1

The structure of the sensor component.

Figure 2

The optical microfiber (OMF) radius of phase match at λ = 1.55 μm.

Figure 3

(a) The coupling length as a function of the distance between two OMFs; (b) The power maps of the evanescent coupling between silica OMF and As₂Se₃ OMF with d = 2000, 1400, 800 nm; (c) The bandwidth of the transmission spectrum as a function of the distance between two OMFs (R₁ = 0.8 μm, R₂ = 0.147 μm, n_c = 1.318).

Figure 4

Transmission spectra of the asymmetric OMF coupler and the symmetric OMF coupler, R₁ = 0.8 μm, R₂ = 0.147 μm, d = 2000 nm.

Figure 5

(a) The sensitivity as a function of the distance between the two OMFs, R₁ = 0.8 μm, R₂ = 0.147 μm; (b) The spectrum shift when n_c varies from 1.318 to 1.328 for different distances between the two OMFs, R₁ = 0.8 μm, R₂ = 0.147 μm.

Figure 6

(a) The coupling length as a function of the radius of the silica OMF; (b) The power maps of the evanescent coupling between the silica OMF and the As₂Se₃ OMF with R₁ = 0.5, 0.7, 0.9 μm, respectively; (c) The bandwidth of the transmission spectrum as a function of the radius of the silica OMF with d = 2000 nm.

Figure 7

(a) The sensitivity as a function of the radius of the silica OMF with d = 2000 nm; (b) The sensitivity as a function of the radius of the silica OMF which ranging from 0.6 μm to 0.9 μm with d = 2000 nm.

Figure 8

The transmission spectrum with R₁ = 0.9 μm, R₂ = 0.15 μm, L_C = 406 μm, d = 2000 nm.