As is well known, the phenomenon of surface plasmon resonance (SPR) in thin metal films is highly sensitive to the optical and structural properties of the metal interface. In recent years there have been many sensors proposed using Kretschmann’s configuration1 where the index of refraction of the prism increases the wave vector of the incoming light and the phenomenon of the surface plasmons are excited at a certain angle for a specific wavelength. Owing to well-known advantages over the conventional sensors of Kretschmann’s configuration, optical fiber sensors have attracted much attention in the past two decades. In 1993, Jorgenson et al.; proposed an optical fiber for SPR sensing.2 Afterward, optical fiber sensors with SPR have attracted considerable interest. Many researchers have proposed various configurations of optical fiber sensors including uncladded multimode fibers,3 D-shaped fibers,4 and single-mode taped fibers.5 In spite of their configurations, the claddings of the fibers were removed and coated with a thin layer of metal.
Recently, Chiu et al.; presented a paper6 on a fiber optical liquid refractometer where the sensing device was a D-type single mode optical fiber with a half-polished core. In this letter we develop an alternative configuration for an optical fiber sensor (OFS) based on Kretschmann’s SPR theory. The D-type OFS is still used as the sensing device, but a thin film layer of gold is deposited on the flat side of the sensor. The D-type OFS is different from the D-shaped5 fiber sensor. The core of the D-type OFS was half-polished, but Sorin et al.; proposed a D-shaped fiber sensor for which the cladding was only partially polished. However, the D-type OFS is a new type of fiber sensor. From the results of simulations, we find that the sensor has the best sensitivity only if we choose suitable parameters, e.g., the thickness of the thin film layer of gold and the length of the sensor.
The D-type OFS configuration is shown in Fig. 1(a). Figure 1(b) shows that a beam with P polarization (P-Pol.) is coupled in and out of the fiber using two objective lenses (L1,L2). The D-type OFS consists of a D-type fiber (D) with two transmission fiber sections. The sensing section is polished as a D-type probe (D), and the core is coated with a thin film layer of gold. From Fig. 1(a), the number of attenuated total reflections (ATRs) is given usingn1=1.47, L=4 mm and h=4 μm. As an example, for the special incident angle θi=86.45 deg, the number m of ATRs was calculated to be about 31, by substituting the values of L, h, and θi into Eq. (1). Because the SPR phenomenon does not occur when the incident beam has an s-polarization (S-Pol.), a polarization beam splitter (PBS; the transmitted light from PBS is P-Pol. and the reflected light from PBS is S-Pol.) and a polarizer (PL, extinction ratio: 1/105) were placed in front of the objective lens L1 to allow the p-polarization light (P-Pol.) to only pass through the D-type OFS. In order to match the scheme of the D-type OFS, we used the linear photodetector as the detector, which is orthogonal to the plane of the sensing surface as shown in Fig. 1(b). The normalized transmission power P trans in the D-type OFS is7 8 Rp m(θ) is the intensity reflectance of the p-polarization light at some incident angle θ in the case of m=L/(2h×tan θ), θi,min =86.1 deg and n1 is the index of refraction of the core.
Simulation and Experimental Results
In order to demonstrate its feasibility, we measured the transmission powers for various alcohol concentrations. As listed in Table 1, the refractive indices n3 of alcohol with variable concentrations are measured by an Abbe refractometer (Model 2WAJ, Milton Roy Co.). As shown in Fig. 2, these experimental and theoretical results are in good correspondence for d2=15 nm and L=4 mm at a constant wavelength λ=632.8 nm.
|Refractive indices for various alcohol concentrations.|
Conclusion and Discussion
As shown in Fig. 3, we have designed a D-type OFS. The fiber sensor is designed as D-type configuration in order to increase the sensitivity of the sensor and coat the film easily. It does not reduce the feasibility of the sensor because the NA value of the single fiber is very low. From Fig. 2, the normalized transmission power P trans can reach above 50 for d2=15 nm and L=4 mm at a constant wavelength λ=632.8 nm. The sensitivity S could be defined asΔP trans is an estimation performed by considering the resolution of practical normalized transmitted power P trans variation measurement (ΔP trans ∼0.1) and δP trans /δn3 is the slope of P trans variation versus the change in the refractive index n3. Therefore, the sensitivity can be given as 2×10−4 refractive index unit (RIU) at least. Owing to the merits of the sensor, e.g., small size, less costly, smaller sample volume, easy measurement, and suitability for in vivo testing, etc., the D-type OFS is valuable for chemical, biological, and biochemical sensing.