外文翻譯--兩個(gè)單模光纖之間的包層腐蝕細(xì)芯光纖高靈敏度折射率傳感器（英文）

1、CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 094203Highly Sensitive Refractive Index Sensor Based on a Cladding-Etched Thin-Core Fiber Sandwiched between Two Single-Mode Fibers *XU Ben(徐賁)**, LI Yi(李裔), DONG Xin-Yong(董新永), JI

2、N Shang-Zhong(金尚忠), ZHANG Zai-Xuan(張?jiān)谛?College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018(Received 14 February 2012)A refractive index (RI) sensor based on a cladding-etched thin-core

3、 single-mode fiber (TCSMF) sandwiched between two single-mode fibers is demonstrated. The experimental results show that the sensitivity, within the RI range of 1.333–1.340, is enhanced at least 6 times by etching. It in

4、creases with the surrounding RI and reaches 857.5 nm/RIU at RI of 1.3684, and it can be expected to be higher with the decrease of the cladding diameter of the TCSMF.PACS: 42.81.Pa, 07.60.Hv, 07.60.Ly DOI: 10.1088/0256-3

5、07X/29/9/094203Optical refractometers based on waveguide tech- nology are being increasingly applied in biomedi- cal, chemical and industrial applications and other fields.[1?4] In recent years, many types of opti- cal f

6、iber refractive index (RI) sensors have been reported based on various configurations such as long-period fiber gratings (LPFGs),[5?8] fiber Bragg gratings (FBGs),[11] surface Plasmon resonance (SPR),[12,13] core-offset

7、attenuators,[14] hetero-core attenuators,[15?19] and so on.[20?23] Among them, hetero-core attenuators, which are fabricated by fusion splicing directly two or more kinds of fibers with dif- ferent core diameters, have b

8、een studied due to their advantages of robustness, high sensitivity and sim- ple fabrication. For example, Wu et al. presented a kind of RI sensor based on a single-mode–thin-core single-mode–single-mode fiber (SMF-TCSMF

9、-SMF) structure.[15,16] It is found that the sensors are only sensitive to the external RI and insensitive to temper- ature. In this Letter, we try to improve the sensitivity of this kind of RI sensors by at least 6 time

10、s through decreasing the cladding diameter of the TCSMF with a chemical etching process. Figure 1 shows the schematic of the proposed RI sensor. A short segment of cladding-etched TCSMF is sandwiched between two conventi

11、onal SMFs by using a fusion splicer, so that an in-line Mach–Zehnder in- terferometer (MZI) is formed. Many cladding modes are excited at the splicing point between the SMF and the TCSMF, but usually only one or several

12、of them are dominant in power. If we consider only one cladding mode, the interference signal reaches its min- imum when the phase difference between the cladding and core modes satisfies the following condition:[2]2

13、0587;[𝑛co eff(𝜆) ? 𝑛cl,𝑗 eff (𝜆, 𝑛ext)] 𝐿𝜆D = (2𝑘 + 1)𝜋, (1)where 𝑛ext is the RI of the surrounding medium, 𝐿 is the length

14、of the TCSMF, 𝜆D is the wavelength ofthe transmission dip, and 𝑘 is an integer, 𝑛cl,𝑗 eff (𝜆, 𝑛ext)and 𝑛co eff(𝜆) are the effective RIs of the 𝑗th o

15、rder cladding mode and the core mode, respectively. Since 𝑛cl,𝑗 eff (𝜆, 𝑛ext) is sensitive to the surrounding refractive index (SRI) 𝑛ext and 𝑛co eff(𝜆) is not, the

16、 transmission dip will shift with 𝑛ext. The sensitivity of the trans- mission dip to the change of SRI can be deduced from Eq. (1) as𝑆 = 𝑑𝜆D𝑑𝑛ext = ? 2𝐿2𝑘 +

17、 1𝜕𝑛cl,𝑗 eff𝜕𝑛ext· [? 1 ? 2𝐿2𝑘 + 1(?𝜕𝑛co eff𝜕𝜆 ? 𝜕𝑛cl,𝑗 eff𝜕𝜆)?]? . (2)In this study, t

18、he middle of TCSMF is etched with a chemical process and the diameter of its cladding decreases. Thus the sensitivity of the cladding modes to SRI, that is, 𝜕𝑛cl,𝑗 eff /𝜕𝑛ext, is e

19、nhanced, and then the sensitivity of the sensor to SRI is also increased. The similar simulation result has been reported in Ref. [16]. In the experiment, the SMF is a conventional fiber used in optical communications, a

20、nd the TC- SMF (405-HP, Nufern) has a core diameter of 3.0 µm, and its cutoff wavelength is of 370±20 nm. Firstly, a 62-mm-long TCSMF without coating is sandwiched between two SMFs to form a fiber in-line MZI u

21、sing a fusion splicer (Fujikura, FSM-60 S). The main pa- rameters of the arc splicer used in our experiments in- cludes arc-power of standard (the absolute arc-power was not displayed), arc duration of 1500 ms, pre fusio

22、n power of standard and pre fusion time of 180 ms. The insertion loss of the TCSMF is 7.11 dB at 1550 nm. Then we use the etching method reported in Refs. [15,25] to decrease the cladding diameter of the TCSMF. The MZI i

23、s held by clamps across a hydroflu- oric acid (HF)-resistant polythene slice. Drop several drops of 49% HF (by weight) onto the slice, a big HF droplet will be formed and stand out from the slice be- cause of the polythe

24、ne slice’s hydrophobicity. Then, the slice is raised by a translation stage to immerse the mid TCSMF into the droplet to initiate the etch- ing. During the etching, HF flows from the droplet*Supported by Zhejiang Provinc

25、ial Natural Science Foundation of China under Grant Nos Y5090150 and Y6100244, and the National Natural Science Foundation of China under Grant No 61007051. **Corresponding author. Email: xubenfiles@163.com © 2012 C

26、hinese Physical Society and IOP Publishing Ltd094203-1CHIN. PHYS. LETT. Vol. 29, No. 9 (2012) 09420369.3 nm/RIU. It is also noted that the sensitivity is related to the transmission dip being selected. For ex- ample, for

27、 the dip around 1474 nm, flagged as dip A in Fig. 2, the sensitivity is 493.9 nm/RIU, as shown in Fig. 3. Another set of salt solutions with a broader range of concentrations of 0%, 1.98%, 3.95%, 6.06%, 7.62%, 9.42%, 11.

28、17%, 12.88%, 14.56%, 16.19%, 17.90%, and 19.36% (by weight) are also used in the measurements. The corresponding RIs are 1.3330, 1.3366, 1.3401, 1.3441, 1.3469, 1.3502, 1.3534, 1.3566, 1.3596, 1.3626, 1.3658, and 1.3684,

29、 respectively. The interference spectra show an obvious red shift with the increas- ing of SRI. With tracking the dip B in the fringes, the response is obtained, as shown in Fig. 4. For a 0.0354 SRI change, a 23.22 nm sh

30、ift is observed. A poly- nomial fitting gives the varying sensitivity depending on the SRI, as shown in the insert in Fig. 4. Obvi- ously, the sensitivity increases with the SRI, which includes the simulation result in R

31、ef. [16]. It reaches 857.53 nm/RIU at the SRI of 1.3684. Assuming that the resolution of 1 pm is achieved in a refined op- tical spectrum analyzer (OSA), a detection limit of 1.17 × 10?6 RIU is attainable.1.330 1.33

32、5 1.340 1.345 1.350 1.355 1.360 1.365 1.37005101520251.33 1.34 1.35 1.36 1.37400500600700800900Dip wavelength shift (nm)Polynomial fit of dataR efractive indexDip BRefractive indexSe nsitivity(nm/R IU )Fig. 4. Wavelength

33、 shifts of the RI sensor after etching for higher SRIs. Inset: sensitivity vs SRI.In addition, according to the trend of sensitivity as shown in Fig. 4, it can be higher for a larger SRI but no more than the RI of the fi

34、ber cladding. On the other hand, with further decrease of the fiber waist, the sensitivity will keep increasing. That is also sup- ported by the analysis reported in Refs. [16,24]. We compare our result with those of pre

35、viously reported wavelength-modulated optical fiber RI sen- sors. Table 1 lists their structures and sensitivities. It is found that the sensitivity of our proposed sensor is ～8 times higher than that of sensors based on

36、 two cascaded LPFGs with rotary RI modulation,[8] ～17 times higher than that of MZI formed by three cas- caded single-mode fiber tapers,[22] and comparable to that of non-adiabatic tapered fiber sensor fabricated by CO2

37、laser.[23] Our sensor has the advantages of easy fabrication. Finally, the mechanical strength of the sensor after etching is measured in a simple static experiment by applying an axial tension force 𝐹 to the sen

38、sor and theforce is increased gradually until fracture occurs. An ultimate strength, defined as 𝜎 = 𝐹/𝐴 with 𝐴 being the cross section area of the waist, is 5.2 GPa, which is comparable to

39、 that of a bare SMF without etching.[28]In summary, a high sensitive refractive index sen- sor has been proposed and demonstrated. It is com- posed of a short segment of cladding-etched TCSMF sandwiched between two conve

40、ntional SMFs. The ex- perimental results show that the sensitivity is about 6 times higher than that of the sensor before etching. Furthermore, the sensitivity increases with the SRI and reaches 857.5 nm/RIU at the RI of

41、 1.3684. This highly sensitive RI sensor will find wide potential ap- plications in biomedical, chemical and other industrial areas. The authors thank Dr. Dong T K for helpful dis- cussions and Sun M and Zhao X W for the

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