JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 7,JULY 2007 1797 Microgap Multicavity Fabry–Pérot Biosensor Yan Zhang, Member, OSA, Xiaopei Chen, Student Member, IEEE, Yongxin Wang, Kristie L. Cooper, Member, IEEE, and Anbo Wang, Senior Member, IEEE Abstract—This paper presents a microgap multicavity Fabry– Pérot interferometric sensor fabricated by wet etching and fusion splicing of single-mode optical fibers. The temperature depen- dence of the optical thickness measurement of self-assembled thin films can be compensated by extracting the temperature infor- mation from the multiplexed temperature sensor. Experimental results demonstrate that thin-film characteristics under tempera- ture variations can be examined accurately. The high-temperature sensitivity of the temperature sensor also enables biosensing un- der temperature variations. This greatly improves the flexibility in sample handling and provides the opportunity to investigate temperature effects in biological applications. Index Terms—Fabry–Pérot interferometers, multicavity sensor, optical fiber sensors, temperature compensation. I. I NTRODUCTION A MONG the many types of fiber-optic sensors, fiber-optic Fabry–Pérot interferometric (FFPI) sensors are distinctive because of their high sensitivity, ease of fabrication, localiza- tion, and lead insensitivity. FFPI sensors can be classified as extrinsic Fabry–Pérot interferometric (EFPI) sensors and intrin- sic Fabry–Pérot interferometric (IFPI) sensors. An EFPI sensor uses an air cavity between two cleaved fiber ends inserted into an alignment ferrule and bonded by laser welding or epoxy adhesive [1], [2]. Although the EFPI sensor is attractive in various applications, it has intrinsic disadvantages such as diffi- culty in bonding, diameter nonuniformity due to the alignment ferrule, and limitation on the cavity size due to coupling loss. The large mismatch in thermal expansion coefficients of fibers, alignment ferrules, and bonding materials will cause severe stress in sensor construction. The bonding adhesive such as epoxy undergoes viscoelastic creep and cannot survive at high temperature [3]. The geometric discontinuity will create diffi- culty in protecting and mounting the sensor in the measurement. The large coupling loss reduces the multiplexing capability. In contrast, an IFPI sensor contains the sensing element, i.e., the Fabry–Pérot (FP) cavity, inside the fiber. The fiber both guides light and experiences the perturbation of interest. IFPI sensors reduce the bonding difficulties experienced in EFPI sensor Manuscript received July 20, 2006; revised March 25, 2007. This work was supported in part by the U.S. Army Medical Research and Materiel Command under Award DAMD17-03-1-0008. Y. Zhang is with the Fitzpatrick Institute for Photonics, Duke University, Durham, NC 27708 USA (e-mail: yazhang1@duke.edu). X. Chen, Y. Wang, K. L. Cooper, and A. Wang are with the Center for Photonics Technology, Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA (e-mail: xichen5@vt.edu; ywang1@vt.edu; klcooper@vt.edu; awang@vt.edu). Digital Object Identifier 10.1109/JLT.2007.899169 fabrication and provide miniature size, continuous geometry, robust structure, and versatile installation. In tubing-based multicavity Fabry–Pérot interferometric (MFPI) sensors, cleaved fiber ends separated by air gaps can serve as reflectors for the FP cavity [4]. However, the cavity size is limited by the coupling efficiency, which thus reduces the flexibility in fabrication and multiplexing. The local reflectors or mirrors inside the fiber can also be fabricated with various methods, such as dielectric thin films [5], [6] and fiber Bragg gratings (FBGs) [7]. Single-layer [5] or multilayer [6] dielectric mirrors can be deposited onto the fiber by magnetron sput- tering. After splicing with another fiber, internal mirrors with reflectance of greater than 85% can be achieved [6]. Although this technique has shown success, it is still limited by the need for a special coating on the fiber and deterioration in film quality during splicing. Moreover, additional loss could occur due to fiber end surface roughness, cleave angle, or reflection into the cladding of the fiber [8]. Another method of producing high-finesse fiber cavities is to combine two FBGs as mirrors [7]. High-finesse values can be achieved with narrow-spectral- width FBGs [9]. Recently, chirped Bragg gratings with much wider spectral widths have been examined in FP filters [10]. Theoretical analyses predict that high finesse and wide spectral widths could be achieved with chirped Bragg gratings [11]. This technique remains expensive and complicated because of the need for chirped FBGs. In this paper, microgap FP sensors were fabricated by com- bining wet etching and fusion splicing. This process is cost effective, easy to multiplex, and suitable for batch produc- tion [12]. Multicavity sensors for thin-film applications were constructed by multiplexing individual microgap sensors. This approach not only has similar advantages to tubing-based MFPI sensors [13] but also is notable for its simple fabrication process and flexibility in cavity lengths. Furthermore, the temperature sensitivity for temperature compensation has increased by an order of magnitude from the tubing-based MFPI sensor. Thus, thin-film characteristics under temperature variations can be examined more accurately. II. SENSOR FABRICATION Chemical etching is widely used in fiber-optic probe fabrica- tion. Conical cores [14] and microwells [15], as well as nanotips [16], can be achieved with wet etching techniques. Chemical etching offers a simple and cost-effective fabrication technique in which the optical fibers are dipped into a balanced solution of hydrofluoric acid (HF) and into an ammonium fluoride (NH 4 F) buffer. The process depends on a differential etch rate between the pure silica of the fiber cladding and the germanium-doped 0733-8724/$25.00 © 2007 IEEE