Novel In-Line Fiber-Optic Fabry-Perot Sensors Based on Etched Erbium- and Boron-Doped Optical Fibers

Yun-Jiang Rao, Bing Xu, Zeng-Ling Ran, and Yuan Gong Key Lab of Broadband Optical Fiber Transmission & Communication Networks (Education Ministry of China),

University of Electronic Science & Technology of China, Chengdu, 610054, China E-mail: yjrao@uestc.edu.cn

Abstract—Novel in-line fiber-optic extrinsic Fabry-Perot interferometers (EFPIs), with cavity lengths of up to ~9μm, are fabricated by chemically etching the Erbium- and Boron-doped optical fibers and splicing the etched optical fiber to a single-mode fiber, respectively. The strain and temperature responses of the EFPI sensors are investigated by experiments. Both good linearity and high sensitivity are achieved. Such types of EFPI sensors are very cost-effective and suitable for mass production, offering great potentials for wide applications where the sensor cost is essential.

I. INTRODUCTION Fiber-optic sensors (FOS) have found numerous industrial,

military, and civil applications in recent years as they have a number of outstanding advantages over conventional sensors such as immunity to electromagnetic interference, capability of responding to a wide variety of measurands, high accuracy, small size, etc. Fabry-Perot interferometric (FPI) fiber-optic sensor [1-13], including both extrinsic FPI (EFPI) sensor and intrinsic FPI (IFPI) sensor, is an important branch of FOS. Recently, the laser micro-machining [12] and chemical etching [13] techniques were employed to fabricate micro EFPI (MEFPI) sensors, which had many advantages over conventional EFPI sensors. The laser micro-machining technique was well-controlled and can form a MEFPI at one step. However, the laser micro-machining system was expensive and the relative positions of the laser beam, the mask and the fiber to be processed needed to be precisely adjusted. The chemical etching method is low-cost and has great potential for mass production. In this paper a MEFPI sensor is fabricated by chemically etching the Erbium and Boron doped fibers and splicing the etched fiber end to a cleaved single-mode fiber (SMF). It is cost-effective and suitable for mass production. The strain and temperature characteristics of the MEFPI sensor are investigated. Experimental results show good linearity and high sensitivity.

II. SENSOR FABRICATION

Common fibers have a Ge-doped core with pure silica cladding. The fiber core can be etched by a suitable buffered

acid solution (40% w.t. NH4F; 50% w.t. HF; H2O) either slower or faster than the fiber cadding. With volume ratio of 1.7 : 1 : 1 between NH4F : HF : H2O, the etching speed of the cladding equals that of the core [14]. With NH4F to HF volume ratio below 1.7 : 1, the core is etched faster than the cladding. By chemical etching and fusion splicing, MEFPI sensors are fabricated.

This method was used to etch multi-mode fibers (MMF) [13], and formed a MEFPI with a cavity length of approximately 16μm as a high-temperature pressure sensor [15]. However, the remained cladding of MMF after chemical etching was very thin, making it difficult to form an MEFPI by splicing the etched MMF to other fibers. Generally the reflection spectrum of a MMF-based MEFPI was worse than that of the single-mode fiber (SMF) based MEFPI. However, when etching a common SMF, the etched depth in the core area was very small, on the order of 1-2μm [16], due to the slight etching rate difference between the Ge-doped core and pure silica cladding. Such a short-length air-gap formed by splicing the etched SMF to an unetched fiber end was difficult to be directly applied as a FP sensor, although the air-gap can serve as a reflector for a FP cavity.

On the other hand, the doping concentration of Erbium and Boron doped SMFs are much higher than that of Ge in common SMFs. Therefore, MEFPI with larger cavity lengths can be fabricated as the etching rate difference between the fiber core and cladding increases. In the following MEFPI sensors based on Erbium doped fiber (EDF) and Boron doped fiber (BDF) are fabricated by chemical etching and fusion splicing. This eliminates the complicated multicavity scheme [16] and avoids the precise cleaving manipulation under an optical microscope [13], [15].

In our experiment EDF from Nufern Inc. and BDF from Fibercore Inc., with high doping concentrations of Er and B, respectively, are etched. Buffered hydrogen fluoride (BHF), being composed of 20 mg NH4F, 30mL 40% w.t. HF and

This work was supported by the Key Project of National Natural Science Foundation of China under Grant 60537040

978-1-4244-5335-1/09/$26.00 ©2009 IEEE 861 IEEE SENSORS 2009 Conference

54mL deionized water, is used for etching the fiber core faster than the cladding. EDF and BDF are etched about 45 and 40 minutes, respectively. Corresponding three dimensional Surface profiles are shown in Fig. 1(a) and 1(b). The depths of the fiber dips are about 9 μm. The etched EDF and BDF are then spliced to a SMF (Corning, SMF-28) to form a MEFPI sensor. A typical MEFPI sensor based on etched EDF is shown in Fig. 1(c). The outside diameter of the EDF after chemical etching is about 90μm, measured by an optical microscope.

Figure 1. Three dimensional Surface profiles of (a) etched EDF and (b)

BDF. A typical MEFPI sensor is shown in (c).

III. CHARACTERISTICS OF MEFPI

A. Strain responses of MEFPI The experimental setup employed for testing the strain

responses of MEFPI is shown in Fig. 2(a). Light from the sweeping laser in the optical spectrum analyzer (OSA) (Si720, Micron Optics, USA) is lunched into the MEFPI

sensor via a 50:50 coupler. The sweeping laser has a wavelength scanning range of 1510nm-1590nm and the OSA can precisely measure wavelength with a resolution of 2.5 pm. Typical reflection spectra of the MEFPI sensor are shown in Fig. 2(b). An excellent visibility of ~20 dB is achieved by the EDF based MEFPI sensor, compared with that of ~5 dB by BDF based MEFPI sensor.

1510 1530 1550 1570 1590-50

-45

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-38BDF

EDF

Am

plitu

de /d

Bm

Ampl

itude

/dBm

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Figure 2. (a) Schematic diagram of experimental setup for measuring the strain responses of MEFPI and (b) typical reflection spectra.

The strain can be expressed as [17]

.Lk kL

λελ

Δ Δ= = (1)

Where L is the cavity length, λ is the optical wavelength, ΔL is the change of the cavity length and Δλ is the wavelength shift. As shown in Fig. 2(a), two high-accuracy translation stages are used for applying strains to the MEFPI sensor. Figure 3 gives the strain responses of the EDF and BDF based MEFPI sensors at room temperature. Good linearity of 0.9993 and 0.9987 are obtained for the strain responses of EDF and BDF based MEFPI sensors. The corresponding wavelength-strain sensitivities are 1.67 pm/με and 1.34 pm/με, respectively. The strain resolution limit for both MEFPI sensors is about 2 με, as the wavelength resolution of the OSA is 2.5 pm.

(a)

(b)

(c)

(a)

862

0 200 400 600 800 10001525.5

1525.8

1526.1

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1526.7

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Experimental data Linear fit

Wav

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gth

/μm

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0 200 400 600 800 10001525.8

1526.1

1526.4

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Experimental data Linear fit

Wav

elen

gth

/μm

Strain /με Figure 3. Strain responses of (a) EDF and (b) BDF based MEFPIs.

B. Temperature responses of MEFPI The experimental setup employed for testing the

temperature responses of MEFPI is similar to that given in Fig. 2(a), except that the MEFPI sensor is located in temperature-controlled water. As the MEFPI sensors used in this section are different from that used to examine the strain responses in the last section, the initial wavelengths of the reflection spectrum of these MEFPI sensors are quite different, as shown in Fig. 3(a) and 4(a). The temperature responses of the EDF and BDF based MEFPI sensors are shown in Fig. 4. Good linearity of 0.9991 and 0.9986 are obtained for the temperature responses of EDF and BDF based MEFPI sensors. The corresponding wavelength-temperature sensitivities are 3.88 pm/°C and 3.92 pm/°C, respectively.

20 30 40 50 60 70 80

1553.30

1553.35

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1553.45

1553.50

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Experimental data Linear fit

(a)

10 20 30 40 50 60 70 801536.30

1536.35

1536.40

1536.45

1536.50

1536.55

(b)Wav

elen

gth

/μm

Temperature /°C

Experimental data Linear fit

Figure 4. Temperature responses of (a) EDF and (b) BDF based MEFPIs.

IV. CONCLUSION Micro in-line fiber-optic Fabry-Perot interferometric

sensors have been developed by chemically etching Erbium- and Boron-doped fibers and then splicing the etched fiber dip to a single-mode fiber. The strain and temperature responses of the MEFPI sensors have been investigated by experiment. Experimental results show that these cost-effective MEFPI sensors with high sensitivity could be widely used due to their very low-cost and potential for mass-production.

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