Adaptive demodulation of dynamic signals from fiber Bragg gratings using two-wave mixing technology

Yi Qiao , Yi Zhou and Sridhar KrishnaswamyCenter for Quality Engineering and Failure Prevention

Northwestern UniversityIllinois

Applied OpticsVol.45,No 21,Pg 5132-5142

Journal Club Presentation9/18/06

Presenter: Ashwin Kumar

Outline

Motivation

Fiber Bragg grating (FBG) sensors Demodulation Schemes Two-Wave Mixing Technology

Theory

Adaptive Two-Wave Mixing Adaptive Two- Wave Mixing Wavelength Demodulator

Experiment

Experimental Configuration Wavelength demodulation Adaptivity to Quasi-Static Drift Detection of transient Impact signals Four-channel Multiplexed Two-Wave Mixing Demodulator

Conclusions

Fiber Bragg Gratings (FBG) Sensors FBG reflects a very narrow bandwidth of light centered at the Bragg wavelength within the transmission spectrum The reflected wavelength is dependent on the period of the Bragg grating and the guiding properties of the fiber Proportionally related to environmental variables (i.e. temperature, stress and pressure ) Fiber Bragg gratings operate as sensors when changes in a particular environmental variable are correlated with shifts in the reflected wavelength of the FBG.

FBG sensors are insusceptible to EMI and have no EM emission They are intrinsically safe and have unique optical multiplexing potential A large number of FBG sensors can be addressed and can be read out by a limited number of lead fibers The FBG sensors are potentially lightweight, small and can be embedded and integrated in (composite) structures Long-term stability and fatigue durability Main Application : structural health monitoring

Demodulation Schemes Spectral shifts has to be monitored

Demodulation Schemes

Scanning Type Spectrometry based Interferometry based

Only one FBG sensor can be investigatedat a given time.

Not Suitable for multiple sensor investigation

Ideal for dynamic strain monitoring

Requires electronic feedback to activelyCompensate for quasi-static drifts to keep

The interferometer at quadrature.

Cost of Multiplexing is high

Low sensitivity

Not suitable for dynamic measurementsIn case of multiple sensor array

Need for a cost-effective and parallel demodulation scheme for arrays of FBG sensors

Two-Wave Mixing Technology

A novel two-wave mixing (TWM) wavelength demodulation scheme for FBG sensors is demostrated

Ability to compensate for quasi-static drifts without the need for active stabilization

Suitable for measuring dynamic and transient strains induced by vibrations , impact, ultrasound, or acoustic emissions

This Scheme can be readily multiplexed by using wavelength division multiplexing

Demonstration of a four-channel TWM device to demodulate dynamic strain signals from four FBG sensors simultaneously

Adaptive Two-Wave MixingSignal Beam

Pump Beam

Transmitted Signal Beam

Diffracted Pump Beam<001>

<001>

Photorefractive TWM is a dynamic holographic process Two coherent beams – Pump and Signal – interact within the photorefractive crystal Wave-Mixing Process a) Creation of Intensity gratings due to coherent stationary interference of the beams b) light is absorbed and free carriers are generated in the bright regions of the intensity pattern c) Carriers diffuse and/or drift from the bright regions leaving fixed charges of opposite sign d) Free carriers can be trapped by ionized impurities at other locations ,depositing their charge there as they recombine e) Creation of an inhomogeneous space-charge distribution that modulates the refractive index of the crystal through electro-optic effect. f) This causes diffraction of the interacting beams and this process of dynamic coupling is called wave mixing,

Adaptive Two-Wave Mixing A part of the pump beam is diffracted by the index grating in the direction of the transmitted signal beam Diffracted pump beam is wavefront matched with the quasi-static signal beam Diffracted Pump beam and the transmitted signal beam interfere with each other to demodulate any phase difference between them The TWM interferometer is adaptive as the created index grating is a dynamic grating, and the crystal can adapt to any phase shift that is slower than the PRC response time by forming a new index grating. Diffracted pump beam will therefore track any quasi-static changes in the signal beam phase, resulting in no quasi-static phase difference between the diffracted pump and transmitted signal beam. Dynamic phase changes faster than photorefractive response time of the PRC will not be present in the diffracted pump beam This will result in a net phase difference between diffracted pump and the transmitted signal beam. Which can picked up by the interference of these two beams. By applying a DC field to the PRC, the interference pattern can be kept nearly in in phase with the created index grating. By adjusting the DC field and mixing angle, the TWM interferometer can be made to work at near quadrature. (the diffracted pump beam, is 90 phase shifted w.r.t transmitted signal beam.)

Adaptive Two-Wave Mixing

Adaptivity of the TWM process comes from it’s ability to stay at quadrature PRC can therefore be regarded as a high pass filter that selectively monitors any high frequency changes in the mixing beams. It is ideal for monitoring small dynamic strain signals

Transmitted Signal Beam

0 exp[ ( )]sE E i t Diffracted Pump Beam

0{exp[ ] 1}dpE E L Interference signal at the photodetector

2 2 ' '

0 { 2sin( " ) ( )}L LI E e L e t

– dynamic phase shift - Complex amplitude of the signalL – crystal length in the beam propagation direction - TWM complex gain

( )t0E

' "i

Interference signal varies linearly with dynamicPhase shift

DC level , contributed to Photodetector shot noise

/ 2

Adaptive TWM Wavelength Demodulator

Reflected light from a FBG sensor is split into two beams (signal and pump) The Two beams are made to travel unequal paths before mixing Any wavelength shift would cause a phase shift between the beams due to travel

over unequal path lengths

Spectral shift induced by strain and temperature

2

12 11 12{1 [ ( )]} ( )2effB

z NB

np p p T

– center wavelength of the FBG sensor – wavelength shift caused due to strain or temperature – effective refractive index of the fiber – components of strain optic tensor – Poisson’s ratio – strain along the fiber – thermal expansion coefficient – Thermo- optic coefficient

BB

effnijP

Z

n

For Bragg sensors at 1550nm, it has been estimatedThat 1 microstrain will lead to 1.2 pm change in Wavelength and 1 C change in temperature will lead to about 13 pm change in wavelength

Adaptive TWM Wavelength Demodulator

Phase Shift between the beams

2

2( ) B

B

dt

d is the optical path difference (OPD)

Greater the OPD , larger is the phase shift and stronger is the interference signal Using broadband light sources to illuminate FBG’s, results in the FBG reflection spectrum having a finite line width of the order of 0.1-0.4 nm Implications: coherence of the two interfering beams needs to be taken into account. Fringe visibility due to the interference of two beams of finite spectral width k

2 22exp{ }

1 16ln 2

r k dV

r

r – Intensity ratio of the two beams

Incorporating the degradation in fringe visibility due to low coherence in the Interference signal expression , we get

2 2

2exp( ' )sin( " )exp{ }

16ln 2B

B

k d dS L L

Adaptive TWM Wavelength Demodulator

Wavelength Demodulation Signal

2 2

2exp{ }

16ln 2B

B

k d dS

L = 1 cm , (TWM gain) = 0.3 cm-1

OPD =0 , no wavelength demodulation Amplitude increases with OPD to a max and then starts to decrease due to decreasing fringe visibility Narrower the linewidth, larger the optimum OPD, larger the demodulated signal

'

Trade Offs Narrower linewidth FBG are longer in length which decreases the highest frequency to which the FBG can respond Larger OPD decreases the dynamic range the FBG can measure

Experimental Configuration

Broadband Amplified spontaneous emission (ASE) source in the C band (1530 to 1570 nm)

Optical Amplifier : EDFA working at 500 mW

Both the beams enter the crystal by the [-1 1 0] face and the DC field is applied along the <001> direction

Peltier cooler is used to prevent electrical breakdown due to crystal overheating.

Two HWP are used to rotate the beam polarization to be S- polarized. (along the <110> direction)

Under the applied DC field , index grating is in phase with the interference pattern, with the TWM kept at quadrature, provides optimal demodulation of phase/ wavelength changes.

Wavelength Demodulation

Test Parameters: FBG sensor centered at 1552 nm with a line width of 0.1 nm, length of 10 mm, and reflectivity of 50% Glued to a PZT stretcher 10kHz , 10 strain onto the FBG sensor

Intermittent DC field appplied from 1 to 6 ms

Photorefractive grating builds up

strain applied to FBG from 2 to 6 ms as a tone burst

OPD=0, TWM gain =max, zero demodulated signal detected

AS OPD increases, slowly the demodulated signal starts to appear

Wavelength Demodulation Optimum OPD - 0.1 nm line width FBG = 8mm For a OPD =8mm, the wavelength to phase shift conversion sensitivity comes about 21 radians/nm wavelength shift at 1550 nm. Implies 0.0252 radian/microstrainThese changes are easily detected using TWM

TWM gain ’ experimentally is verified to be a small number (0.47 to 0.1 cm-1) for OPD (0-12mm) For a 20KHz dynamic strain amplitude the TWM demodulator output was found to vary linearly with spectral shift (strain) Minimum detectable strain with current setup ~ 0.25 microstrain corresponds to 0.3 pm spectral shift Limited by ASE and EDFA intensity noise Improved through balanced photodetection to cancel the intensity noise.

Adaptivity to Quasi-Static Drift To demonstrate the adaptivity of TWM setup to quasi-static drift

(low frequency strain or temperature drift) A frequency sweep signal from 10 Hz to 1.2 kHz with a constant amplitude of 10 microstrains was applied

Demodulator ignores the low frequency strain applied in the beginning. Starts to respond to frequencies above 600 Hz Acts like a high pass filter with a cut off frequency of 600 Hz Cut-off frequency is directly proportional to the response time of the PRC. Faster the response, higher is the cut-off frequency.

Adaptivity to Quasi-Static Drift

In principle, there is no upper limit to detectable frequency range of FBG spectral shifts from TWM process Limit occurs only from FBG sensor response and electronics bandwidth of the photodetector A high frequency dynamic strain (580kHz , 3microstrain) is successfully demodulated.

To Demonstrate adaptability to large temperature drift Temperature drift introduced to FBG through TEC module connected to temperature controller. Temperature is monitored using a thermistor 0.1 Hz sine input was supplied to controller and a temperature drift introduced was 10 C drift within 5secs (2 C /sec) An optical spectrum analyzer was used to record the reflection spectrum of FBG as the temperature drifts. 10 C change causes a wavelength change of 110 pm Close to 130pm (13pm/C)

Adaptivity to Quasi-Static Drift

NO variation due to the 0.1Hz 10 C temperature drift 110 pm wavelength shift would correspond to a 2.3 radians for this configuration This phase shift, if picked up would cause the amplitude to vary a lot No such shift demonstrates the system ability to

compensate large temperature drifts.

Detection of Transient Impact Signals

FBG sensor was covered by a 1mm

thick dry couplant made from silicon

epoxy A 3mm metal plate was placed on top of

the epoxy A 3mm ball was dropped from a height

of 5cm above the metal plate An oscilloscope was used to capture the

demodulated signal Demodulated signal shows multiple bouncing of the ball bearing Frequency of the impact signal was

about 5 KHz

Four-Channel TWM wavelength demodulator

Multiplexing possible without significant increase in cost Channels are separated using band drop filters

Multiple FBG sensors with distinct spectral reflectivities are selected

Center wavelength separation is chosen to be large enough to avoid stationary optical interference between the multiple channels

Four-Channel TWM wavelength demodulator Inside the PRC, each channel creates it’s own index grating with different index grating pitches

2sin( / 2)C

Change in index grating pitch

Angle between signal and pump beams

Center wavelength separation

A channel separation of 4nm was chosen which gave a index grating pitch shift of 76 nm at a beam angle of 3 degs 4 nm channel spacing allows 10 channels in the C band (1530nm – 1570nm) Multiple gratings can be written in a PRC with negligible cross talk if the pitches differ by 0.03 nm. Four 0.1 nm line width FBG sensors were connected in series and are centered at 1548, 1552,1556,and 1560 nm respectively. FBG Sensor 1 (1548nm) : 10kHz, 5 microstrains FBG Sensor 2 (1552nm) : 5kHz ,5 microstrains FBG Sensor 3 (1556nm) : 2kHz ,5 microstrains FBG Sensor 4 (1560nm) : 20kHz, 5 microstrains

Four-Channel TWM wavelength demodulator

a) Demodulated signal amplitude for each channel is slightly different despite same input amplitudeb) Each channel has different optical intensities (non uniform EDFA gain)c) Can be corrected by precalibration or gain-flattened EDFAd) Fourier spectra confirms there is no cross talk between the channels

Conclusions

First Adaptive wavelength demodulator for spectrally encoded FBG sensors based on two-wave mixing interferometry Optimum value of OPD for a 0.1 nm line width FBG was found out to be 8mm Spectral resolution of the TWM wavelength demodulator

is of the order of 0.3 pm Resolution limited by EDFA and source intensity noise TWM wavelength demodulator is adaptive to quasi-static drifts and temperature drifts. Well suited for detecting dynamic strains. TWM was also used to study transient impact signals Demonstrated a four channel system investigation through wavelength multiplexing.