低相干干涉仪及相应光纤传感器的研究

Research on Lower Coherence Interferometer and

corresponding optic fiber sensor

Cheng-yu Hong Jian-hua Yin

Department of Urban and Civil Engineering Department of Civil and Structural Engineering Shenzhen Graduate School, Harbin Institute of Technology Hong Kong Polytechnic University

Shenzhen, P. R. China, 518055 Hong Kong. P. R. China

E-mail：joeyhcy@ E-mail：cejhyin@.hk

You-hua Fan Chao Wang

Department of Urban and Civil Engineering

Shenzhen Graduate School, Harbin Institute of Technology Bookham company

Shenzhen, P. R. China, 518055 Shenzhen, P. R. China, 518055

E-mail：yhfan@ E-mail：wangchao_prc@

Abstract

Applying lower coherence interferometer executes scan of optic fiber sensor under different environmental state. Factors influence accuracy of results can be obtained through comparison between above tests, which are shown in details in this paper. Repeating test of optic fibers is completed as a part of calibration, an accuracy of 4um can be achieved as well. Data acquisition and data analysis are accomplished by the software of Labview. Installing reference point on signal arm to eliminate the deviation of initial scan time and reduce the error of lower coherence system.

Keywords：optic fiber sensor，repeating test，Labview，reference point

1. Introduction

Lower coherence interferometer is widely used in civil engineering area[1], and its principal is similar to Mach–Zehnder interferometer [2], meanwhile, the strain sensing array has been produced [2]. New apparatus is produced according to Michelson interference principal as is shown in Figure 1. Wide spectrum light split into signal arm from laser source, generate signal R1 and R2 by the sensors with different reflectivity along signal arm, as is shown in Figure 1. An optical path difference b generated. Signal R1 and R2 reflected passes through the 2×2 coupler with power ratio 50: 50, divided into two branches, one is to the reference arm, and the other is to the mirror. R1 and R2 will be reflected both by reference arm and mirror on the electromotor, split into coupler again to achieve PIN[3].Interference will

Figure 1: Working principal of optic fiber sensor based on Michelson white light interferometer

be generated as soon as light path differences satisfy equation 2-1:

T c 0102n L n L L +−≤[4] (2-1)

0n is the refractive index, c L is the lowest length of interference. Position of the highest peak, which

locates in the center of the interference peak, corresponds to the exact optical path matching of these two arms. That is T 0102n L n L +=[5]. Light path difference between two arms will change along with scan continuing.

Light power output satisfies the following equation:

{}

I R P +P (t)-(t)]ref dut ref dut φφ=[6]

(2-2)

As is shown in equation 2-2, I is power of interference signal finally. R is responsibility of diode; P ref is power of reference light reflected, P dut is the power of the light reflected by mirror, (t)ref φ is phase

of reference light reflected, (t)dut φis phase of light reflected by mirror.

Speed of electromotor is invariable; meanwhile interval of peaks generated can be recorded by computer, thus changing in displacement of the section can be detected through the product of interval and electromotor speed.

2. Repeating Test

Scanning is carried out on fibers under different environmental conditions: first one is encapsulated in aluminum slot with epoxy[2] as is shown in figure 2, material for encapsulation is epoxy; the other one sticks to desk directly with adhesive tape, no force applied on both of these two conditions.

Figure 2: Fiber encapsulating into aluminum slot

Based on the working principal of lower coherence Michelson interferometer, calibration test is carried out in lab. Continue scanning on the same fiber under no force; and execute data acquisition by Labview software. Comparison work can be carried out in this way. Keep repeating scan for five times on a fiber with five sensors which have no encapsulation, therefore, three differences of distance between two

Fiber encapsulated with epoxy in aluminum slot

Optic Fiber