C.C. Chan

Enhancement of measurement accuracy in fiber Bragg grating sensors by using digital signal processing

Wavelength detection accuracy in fiber Bragg grating (FBG) sensors can be increased by use of digital signal processing after photo-detection. Finite impulse response and infinite impulse response algorithms were implemented and used to improve the wavelength detection accuracy of peak detection and time-zero-crossing techniques. The wavelength detection resolution can be improved by an order of magnitude compared with that obtained directly from an optical spectrum analyzer/tunable laser.

Enhancement of wavelength detection accuracy in fiber Bragg grating sensors by using a spectrum correlation technique

A digital post-processing technique based on spectrum correlation is developed and applied to determining the Bragg wavelength shift accuracy in fiber Bragg grating (FBG) sensors. Theoretical analysis shows that the enhancement factor is proportional to the square root of the number of the independent sampling points within the 3d B bandwidth of the FBG. A wavelength detection accuracy of better than 1 pm in terms of standard deviation was experimentally demonstrated, representing an enhancement factor of 7.3 over the conventional peak detection technique. This technique may also be used for applications such as white light interferometer sensors where the interferometric pattern shifts with applied measurand while the shape of the pattern remains unchanged.

Cantilever optical vibrometer using fiber Bragg grating

A cantilever optical vibrometer using a fiber Bragg grating (FBG) is developed. The sensor head is made by attaching an FBG to a triangular beam to provide position-independent strain for the purpose of acceleration measurement. The vibrometer demonstrates a noise-limited acceleration resolution of 2.8×10 –4 √Hz with repeatability error of less than 2.4%.

Multiplexing of optical fiber gas sensors with a frequency-modulated continuous-wave technique

We report on the use of a frequency-modulated continuous-wave technique for multiplexing optical fiber gas sensors. The sensor network is of a ladder topology and is interrogated by a tunable laser. The system performance in terms of detection sensitivity and cross talk between sensors was investigated and found to be limited by coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation-scanning coupled with low-pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000 parts in 10(6) for each sensor may be realized. A two-sensor acetylene detection system was experimentally demonstrated that had a detection sensitivity of 165 parts in 10(6) for 2.5-cm gas cells (or a minimum detectable absorbance of 2.1 x 10(-4)) and a cross talk of -25 dB.

Investigation of unwanted interferometric signals in a fiber Bragg grating sensor using a tunable laser and a first derivative interrogation technique

The effect of unwanted interferometric signals on the wavelength detection accuracy in a fiber Bragg grating sensor using a tunable laser and a first derivative interrogation technique are investigated. The wavelength detection error can be reduced significantly by proper modulating the laser wavelength.

Fiber Bragg grating current sensor using linear magnetic actuator

We report a novel, to our knowledge, current sensor based on a fiber Bragg grating (FBG). Using a cantilever beam and a linear magnetic actuator, the change of current is transferred to the Bragg wavelength shift of the FBG with good linearity and repeatability. The current sensor thus gains the merits of FBG sensors, and thus is immune to light intensity fluctuation, multiplexing capability, etc., and can be adopted for real industrial monitoring.

Intrinsic crosstalk analysis of a serial TDM FGB sensor array by using a tunable laser

Fiber Bragg gratings (FBGs), with their wavelength-encoded nature and their capacity for been demonstrated for a number of sensing applications, especially for strain measurements in smart structures. Recently, there has been interest in using tunable lasers for interrogating FBG sensors in serial topology for improving signal to noise ratio and for multiplexing a large number of sensors using TDM or a combination of WDM and TDM techniques in a single optical fiber. However, there are some drawbacks in using a serial topology because the reflected light signals from FBGs separated in time but overlapping in wavelength can experience crosstalk, termed spectral shadowing and multiple reflection. These kinds of crosstalks are intrinsic and would limit the accuracy of the wavelength detection and the number of sensors in the array. We report the results of our recent theoretical investigation and computer simulations.

Characterization of crosstalk of a TDM FBG sensor array using a laser source

The crosstalk characteristics of a time-division multiplexing fiber Bragg grating sensor array that uses a laser source are examined. The system performance is found to be limited by the extinction ratio of the input optical pulse modulation and can be improved significantly by properly modulating the laser wavelength.

Time division multiplexed strain sensing system by the use of dual-wavelength fiber Bragg gratings

The dual-wavelength fiber Bragg gratings (DWFBGs) accompanying with dual-wavelength pulsed laser source for strain measurement in a time division multiplexed (TDM) sensing system have been demonstrated. The accuracy up to 2.3 μe in a ±50 μe range has been achieved.

Simultaneous interrogation of multiple FBG sensors for dynamic measurands

A fiber Bragg grating (FBG) sensor system for multi point, dynamic strain or temperature measurement is reported. The system uses a combination of an unbalanced fiber interferometer and phase locked loops and is capable of measuring dynamic signals with frequencies ranging from 100Hz to10kHz. Strain measurements with parallel FBG sensor arrays demonstrate strain resolution of better than (available in paper).