1.

Introduction

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.

2.

Principle

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 using

Fig. 1

(a) Configuration of the sensor. (b) Beam coupled in and out of the D-type optical fiber biosensor (PBS, polarization beam splitter; PL, polarizer).

3.

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.

Fig. 2

Experimental and theoretical results for d₂=15 nm and L=4 mm.

Table 1

4.

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 as

Fig. 3

Photograph of the D-type optical fiber sensor.