Dynamic strain response of a &#x03C0;-phase-shifted FBG sensor with phase-sensitive detection

Bhargab Das; Deepa Srivastava; Umesh Kumar Tiwari; B. C. Choudhary

doi:10.1364/OSAC.1.001172

1. Introduction

High resolution measurement of dynamic strain modulation is of utmost importance in many research applications encompassing underwater acoustic array detectors such as hydrophones; structural health monitoring of civil and aerospace edifices through acoustic emission (AE) studies; ultrasonic detectors for medical sensing etc. Fiber Bragg grating (FBG) based sensors were extensively explored for this purpose owing to their well-known advantages such as small size, immunity to electromagnetic interference and the ability of remote & multiplexed sensing [1,2]. The measurement resolution or the detection limit for an FBG sensor is predominantly governed by the bandwidth (FWHM: Full-Width at Half Maximum) of the Bragg reflected light signal. This is because the Bragg wavelength shift measurement resolution is highly dependent on the spectral linewidth of the grating sensor. Unfortunately, the spectral linewidth of a reasonable length (e.g. 2 cm) FBG sensor is relatively wide with typical bandwidth (FWHM) being greater than 200 pm. This limits the usage of FBG sensors for applications requiring dynamic strain measurement down to pico-strain levels $(p ϵ = 10^{- 12} ϵ)$ .

Towards this goal, a special type of fiber grating sensor known as π-phase shifted fiber Bragg grating (or πFBG) have drawn significant attention [3–12]. A πFBG sensor, as shown schematically in Fig. 1(a)

Fig. 1 (a) Schematic of a π-phase-shifted fiber Bragg grating. Λ is the grating pitch. (b) Reflection spectrum of a simulated πFBG. The grating is of length L = 25 mm, refractive index modulation $Δ n = 1.5 \times 10^{- 4}$ with raised cosine apodization profile.

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, introduces a π-phase-shift region at the center of an otherwise periodic FBG. Thus, effectively, a πFBG sensor is equivalent to two identical gratings separated by half grating period. This phase jump leads to a narrow spectral transmission resonance at the center of the complete transmission spectrum of the grating. Figure 1(b) shows the reflection spectrum of a simulated πFBG of length 25 mm, refractive index modulation of

Δ n = 1.5 \times 10^{- 4}

with raised cosine apodization profile. The π-phase-shift region is exactly at the center of the grating. The reflection spectrum is characterized by a very narrow reflection notch at the center. Figure 2(a)

Fig. 2 (a) Transmission spectrum of the πFBG. (b) Enlarged view of the narrow transmission resonance (FWHM of this transmission peak is ~10 pm).

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shows the transmission spectrum of the same grating with the appearance of an extremely narrow transmission peak. As shown in Fig. 2(b), the FWHM of the transmission peak is ~10 pm, which is significantly smaller than the FWHM of reflection spectrum of a regular FBG.

The position of this transmission peak formed in a π-FBG, known as Bragg wavelength ( $λ_{B}$ ), is also governed by the grating equation, $λ_{B} = 2 n_{e f f} Λ$ . Here, $n_{e f f}$ is the effective refractive index of the optical mode propagating along the fiber and $Λ$ is the grating period. Dynamic strain perturbations acting on the πFBG sensor modulates the grating period as well as the effective refractive index through elasto-optic effect, causing a dynamic shift of the Bragg wavelength (i. e. the narrow transmission peak) around the mean position. The magnitude of the strain perturbation is estimated from the Bragg wavelength shift measurements. However, the main challenge comes during the measurement of wavelength-shift of this significantly narrow transmission resonance especially for small amplitude dynamic perturbation sensing. This is because the conventional angular interrogation scheme which use a dispersive optical element (e. g. phase grating) and a 1D or 2D detector array does not provide enough wavelength shift measurement resolution for this type of sensors. Thus, the considerably narrow reflection notch or transmission peak of a πFBG sensor demands for special approaches for the measurement of their Bragg wavelength shift.

2. Principle of πFBG sensor interrogation

As shown in the simulated plots presented in the previous section, spectral bandwidth of the narrow transmission peak of a πFBG is almost an order of magnitude smaller than a regular FBG of similar length. The commercially available FBG sensor interrogation systems working on the principle of spectral dispersion require this spectral bandwidth to be > 0.15 nm for successful detection and measurement. Hence, these FBG sensor interrogators are not suitable for πFBG sensor applications. Alternatively, this narrow transmission peak can be easily monitored in a high-resolution optical spectrum analyzer (OSA). However, because of the inherent limitations of low-frequency operation, extremely high cost, and bulkiness, an OSA cannot be used for high frequency dynamic signal sensing. At present, there are two known methods to perform this measurement. The first technique involves the measurement of reflection of an extremely narrow linewidth (~0.1 pm) tunable laser source (TLS) whose wavelength is set at the center of linear region of the reflection notch or transmission peak [4,8,9]. The reflected power is measured with a high-speed photodetector. In a slightly different configuration, two cascaded πFBGs, where one πFBG is used as sensor and the other as a filter has been used [6]. In the second type, the Pound-Drever-Hall (PDH) method based on a narrow-linewidth TLS is employed [3,10,11]. The laser frequency is locked to the center of the πFBG resonance spectrum. The PDH error signal, which reflects the frequency detuning between the laser frequency and the πFBG resonance, gives an estimate of the applied dynamic perturbations. PDH method is usually adopted for laser frequency stabilization as well as gravitational wave signal extraction [13–15].

Out of the above two techniques, only PDH method have been demonstrated to offer measurement sensitivity down to pico-strain ( $p ϵ$ ) levels. Guo and Yang reported a PDH based πFBG sensing system for ultrasonic detection with noise limited detectable strain of 8.7 $p ϵ$ [11]. However, it must be realized that this improved detection capability comes at the cost of increased experimental complexity of the PDH technique, which require an extremely narrow linewidth TLS and a high frequency phase modulator. Furthermore, a tight wavelength tracking method is also needed to lock the TLS frequency against the πFBG resonance center. This is essential to suppress the long-term laser frequency drift and environmental noise.

In order to circumvent the complexity associated with PDH technique and also to retain the capability of pico-level dynamic strain detection, we have recently proposed an alternative and relatively simpler approach based on phase-sensitive interrogation approach to perform high-resolution dynamic strain measurement using a πFBG sensor [12]. The Bragg wavelength shift $(Δ λ_{B})$ measurements of the narrow transmission resonance is performed with the help of a fiber interferometer, wherein, the interferometer converts the wavelength-shift into a corresponding phase-shift [16–18]. For dynamic strain-induced modulation in the Bragg wavelength $Δ λ_{B} \sin ω t$ , the corresponding phase modulation can be expressed as [2,12]:

Δ ϕ (t) = - \frac{2 π O P D}{λ_{B}^{2}} Δ λ_{B} \sin ω t

where,

O P D = n_{e f f} d

is the optical path difference between the two arms and d is the physical length imbalance.

ω

is the angular frequency of strain-induced Bragg wavelength modulation.

The output intensity of a fiber interferometer e. g. Mach-Zehnder interferometer can be expressed as [2]:

I (λ_{B}) = A [1 + k \cos Δ ϕ (t)]

where, A is proportional to the input intensity as well as system loses and k is the interference fringe visibility.

It is clear from Eq. (1) that the wavelength-shift to phase-shift responsivity is directly proportional to the OPD of the interferometer. Thus, the extremely narrow transmission resonance of a πFBG sensor gives the leverage to maintain a longer OPD. This fundamental concept is exploited to demonstrate a phase-sensitive detection technique for πFBG sensor which enhances the dynamic strain measurement resolution capability. Utilizing the relation suggested by Weiss et al. [19], the OPD and the spectral bandwidth of the light input to the fiber interferometer can be expressed as $O P D \times Δ k = 2.355$ . Here $Δ k$ is spectral bandwidth expressed in wavenumber units. Corresponding to the linewidth of the transmission peak shown in Fig. 2(b), this OPD is approximately ~8.0 cm, which is significantly longer than a regular FBG sensor [20].

However, there is a small trick in realizing the above mentioned phase sensitive detection principle for πFBG sensor. In case of a regular FBG sensor, the reflected light signal acts as a source input for the fiber interferometer. In contrast, the reflection spectrum of πFBG lacks any well-defined peak [Fig. 1(b)] that can be used as source input to an interferometer. Consequently, for a πFBG sensor, it is essential to exploit the transmission spectrum. Hence, we have recently proposed an innovative as well as simple concept wherein the πFBG transmission spectrum is optically filtered using a specially designed fiber grating. The design parameters of the fiber grating filter i. e. bandwidth, center wavelength are chosen in such a way that the reflected light retains only the central narrow spectral peak. This removes the undesired spectral content of πFBG transmission spectrum and the central peak is used for the fiber interferometer. The details of the experimental setup and measurement of dynamic signals are presented in the next few sections.

3. Experimental setup design

Figure 3(a)

Fig. 3 (a) Transmission spectrum of a 2.5 cm long πFBG sensor. (b) Reflection spectrum of a 10 cm long fabricated FBG filter. The fiber grating has a flat top reflection profile designed to avoid any intensity variation of the πFBG transmission peak for small amplitude external perturbations.

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shows the transmission spectrum of a commercially (Ascentta, Inc. USA) acquired 2.5 cm long πFBG as recorded by an optical spectrum analyzer (OSA) [Yokogawa Model No. AQ6370C] operated at a resolution of 20 pm. For our phase-sensitive interferometric interrogation purpose, we are interested only in the central part of this spectrum containing the narrow spectral peak. In order to retain only the narrow transmission peak, a fiber grating filter of length 10 cm is fabricated with our in-house facility. The reflection spectrum of this FBG filter is shown in Fig. 3(b). The πFBG sensor and the fiber grating filter are then employed in the experimental optical configuration shown by the block diagram Fig. 4(a)

Fig. 4 (a) Block diagram of experimental configuration for optically filtering the narrow transmission resonance of πFBG sensor. (b) Spectrum of the πFBG sensor output after reflection from the FBG filter. The calculated FWHM $(Δ λ)$ is ~26 pm, which is almost an order of magnitude smaller than a regular FBG sensor reflection spectrum.

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. The circulator directs the reflected light signal to the OSA and the resultant filtered light signal is shown in Fig. 4(b). The FWHM of this filtered light signal from πFBG sensor is ~26 pm. The spectrum shown in Fig. 4(b) is similar to the reflection spectrum of an FBG sensor and also governed by the grating equation as mentioned earlier. The major difference is that the FWHM is almost an order of magnitude smaller than a regular FBG sensor. Dynamic strain-induced modulation of the spectrum [Fig. 4(b)] is measured using phase-sensitive detection implemented using a fiber interferometer.

The filtered light signal from the sensor is then directed to an F-MZI. It is important to realize that in order to maintain the temporal coherence between the interfering beams travelling through the two arms of the F-MZI, the interferometer path difference must be less than the effective coherence length. Noting this, we have fabricated a fiber Mach-Zehnder interferometer (F-MZI) using two 2 × 2 3dB fiber couplers. Figure 5

Fig. 5 Power spectrum of the fabricated F-MZI. The optical path difference is estimated to be 3.6 cm. The blue rectangle shows the two consecutive peaks used to estimate the OPD.

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shows the power spectrum of the fabricated F-MZI recorded by an OSA. Knowing the wavelength separation between two consecutive peaks (shown by the blue colored box) at around 1549.9 nm, the resultant OPD of the F-MZI is calculated to be ~3.6 cm. This is smaller than the effective coherence length as well as matches very well with the theoretical optimum OPD for maximum sensitivity suggested by Weiss et al. [19] as presented earlier.

Furthermore, in order to demonstrate the dynamic sensing capability of the proposed phase-sensitive detection scheme an experimental configuration as shown in Fig. 6

Fig. 6 Experimental setup for dynamic signal sensing. The πFBG sensor is glued onto a cantilever plate. The transmitted light from the sensor goes to the FBG filter. The optically filtered narrow transmission resonance acts as the input light to the F-MZI. A photodetector and a DAQ is used for data collection.

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is realized. An Amplified Spontaneous Emission (ASE) source (Wavelength range: 1525 nm – 1565 nm) is used as the light source. The πFBG sensor is glued onto a stainless steel cantilever plate with dimensions (175 mm × 100 mm × 0.5 mm). The plate is placed under a speaker with the help of a clamp stand. Sound signal from the speaker excite dynamic vibrations in the cantilever beam thereby inducing dynamic strain modulation in the πFBG sensor. A generic desktop application is used to control the speaker for generation of sound signals of different frequencies with a frequency resolution of 1 Hz. The amplitude of the acoustic signal is adjusted in such a way that it is barely audible from a distance of approximately 1 meter. The transmitted light from the πFBG sensor is sent to the FBG filter through the circulator. Consequently, the reflected light from the fiber grating filter goes to the F-MZI. The output end of the F-MZI is connected to a high-speed photodetector. A data acquisition device (DAQ) is used to record the analog interference signal for further digital processing and analysis. In a slightly different arrangement of the above experimental setup, the FBG filter can also be placed together with the light source in a single housing. In this case, the ASE light first goes to the FBG filter through the circulator and the reflected light is then sent through the πFBG sensor. Finally, the transmitted light from the πFBG sensor acts as the input for the F-MZI. Several experimental studies are performed with the designed experimental setup and the details are presented in the next section.

4. Experimental results and discussion

4.1 Temperature response

Initially, we observed the temperature response of the πFBG sensor by placing it in a hot water bath and noting the temperature with the help of a digital thermometer as shown in the experimental setup of Fig. 7(a)

Fig. 7 Temperature response of πFBG sensor. (a) Experimental setup: The πFBG sensor is placed in a hot water bath and the temperature is measured with a digital thermometer. (b) Transmission spectrum recorded by OSA. (c) Temperature responsivity evaluated by noting the center wavelength of all the recorded transmission spectrums. The red line is the fitted data. The slope is 10.5 pm/°C which gives the temperature sensitivity.

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. The transmission spectrum of the πFBG sensor is recorded by an OSA at each 2°C rise/fall in temperature. Figure 7(b) shows a few of the recorded spectra. The center wavelength of the narrow transmission peak of all the recorded spectra are evaluated to analyze the temperature response or sensitivity. As shown in Fig. 7(c), the red line is obtained through curve fitting to the experimental data points. The slope of this fitted line gives the temperature sensitivity as 10.5 pm/°C which is similar to the temperature responsivity of an FBG sensor. The temperature sensitivity calculated using the reflected spectra of the πFBG sensor is also 10.5 pm/°C since the reflection and transmission spectra are complementary to each other. This similarity of temperature response for both πFBG sensor and a regular FBG sensor is as expected since the fiber material for both the grating types is same. This temperature response study is extremely important to understand and develop a suitable scheme for compensating any temperature induced displacement of the FBG filter centre wavelength with respect to the πFBG transmission peak. One more key observation which is worth noting is that the shape of the spectra of the πFBG sensor remains intact over a wide temperature range of 30°C as demonstrated in Fig. 7(b). Hence, a πFBG sensor can also be effectively used as a high-resolution temperature sensor. For the purpose of completeness, we have also observed the spectrum of the πFBG sensor after reflection from the FBG filter. This is shown in Fig. 8

Fig. 8 Spectrum of the πFBG sensor after reflection from the FBG filter for five temperature values.

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for five different temperature values.

4.2 Dynamic strain measurement

Figures 9(a) and 9(b)

Fig. 9 Experimental results of phase-sensitive interrogation technique for πFBG sensor. (a) Optical response for an acoustical signal of 580 Hz at the output of the fiber interferometer. The red sinusoidal curve is the fitted data. (b) Power spectral density (PSD) of the time signal. The peak-to-baseline height at 580 Hz is approximately ~43 dB. (c) Detection of dynamic strain perturbations at multiples frequencies simultaneously. Three marked peaks corresponding to applied signals at frequencies of 380 Hz, 580 Hz, and 880 Hz can be observed.

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show the response of the complete setup for a sound signal of 580 Hz. The sound signal excites vibrations in the cantilever plate thereby inducing dynamic-strain modulation. This, in effect, results in the dynamic Bragg wavelength shift of the πFBG sensor. The recorded time series interference signal at the output of F-MZI is shown in Fig. 9(a) together with a sinusoidal curve fitting onto the experimental data. The red sinusoidal curve shows the fitted data which has a period of 1.72 milliseconds corresponding to a frequency of 580 Hz. The power spectral density (PSD) of the recorded time signal is shown in Fig. 9(b) wherein the very sharp peak is for detected signal of 580 Hz. The peak-to-baseline height at 580 Hz is approximately ~43 dB. Furthermore, in order to illustrate the simultaneous detection of acoustic signals (or dynamic-strain modulation) of multiple frequencies, the speaker is operated with three different acoustic frequencies simultaneously. Figure 9(c) shows the PSD wherein three marked peaks at the modulation frequencies of 380 Hz, 580 Hz, and 880 Hz can be observed. We have also measured the frequency response of the πFBG sensor glued cantilever plate in the range from 200 Hz to 2.5 kHz which shows peak sensitivities at around the modal frequencies of the cantilever structure. These experimental results demonstrate the capability of the proposed technique for dynamic strain modulation measurement.

4.3 Strain sensitivity analysis

We have, further, carried out an experimental analysis to get an estimate of the minimum detectable dynamic strain modulation with the proposed configuration of πFBG sensor and interferometric Bragg wavelength shift measurement. For this, we have devised an indirect method. In the same experimental configuration, as described in the previous section, the πFBG sensor is now replaced with a regular FBG sensor glued onto a similar cantilever plate of exactly same dimensions. The response of the FBG sensor is now recorded with the help of a CCD spectrometer based interrogation unit (Ibsen Photonics A/S I-MON 256HS). The Bragg wavelength shift data and the corresponding FFT (fast-Fourier-transform) spectrum is collected. From the Bragg wavelength shift data, it is possible to estimate the amount of strain modulation developed in the cantilever plate for a specific acoustic signal (i.e. frequency and operating sound level). For this, we have used the empirical model given by Kersey et al. [16],

Δ ε = \frac{Δ λ_{B}}{γ}

where

Δ ε

is the strain perturbation and

γ = 1.2 p m / μ ε

at around 1550 nm for bare fiber FBG. The fiber grating sensors are fabricated in a typical single mode germanosilicate photosensitive optical fiber and hence the strain to wavelength-shift responsivity of

γ = 1.2 p m / μ ε

at around 1550 nm is considered in our study.

Figure 10(a)

Fig. 10 Estimation of minimum detectable dynamic strain. (a) Response in terms of FFT (fast-Fourier-transform) of the FBG sensor using a CCD spectrometer based interrogator for a signal at 440 Hz. The estimated dynamic strain amplitude ~28 $n ε$ . (b) PSD for the $π$ FBG sensor at similar experimental conditions. The detection limit of dynamic strain over 1 Hz measurement bandwidth is estimated to be 17 $p ε$ .

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shows the FFT amplitude spectrum for the FBG sensor for a sound signal of 440 Hz. The

Δ λ_{r m s}

value at 440 Hz is used in Eq. (3) to calculate the effective dynamic-strain modulation developed in the cantilever plate. The estimated dynamic strain amplitude is ~28

n ε

. Since, the cantilever plate characteristics are identical for both the two experimental cases, we can faithfully assume that approximately same strain modulation would be developed in case of the πFBG sensor for the same frequency and operating sound level. The PSD for the πFBG sensor at similar experimental conditions is shown in Fig. 10(b). Therefore, the peak observed at 440 Hz corresponds to a dynamic strain modulation of ~28

n ε

. Knowing this, we can further notice that the peak at 440 Hz is approximately 32 dB above the noise floor of −115 dB which sets the lower limit of strain sensing. The minimum detectable strain is defined as the strain modulation when the voltages generated by the signal and noise are equivalent. This can be estimated by using the following expression:

Δ ε_{m i n} = \frac{Estimated dynamic strain amplitude (Δ ε)}{Peak to Baseline Height}

Converting the peak to baseline height of 32 dB to linear scale results into a factor of 1584. Dividing the estimated dynamic strain amplitude of 28

n ε

with this factor yields the strain sensitivity at around 440 Hz. This minimum detectable strain over 1 Hz measurement bandwidth comes out to be 17

p ε

. This shows the potential of the proposed technique for high-resolution dynamic strain measurement. Thus the detection limit for dynamic strain sensing can be significantly improved.

4.4 Dynamic range estimation

Another important parameter to quantitatively analyze is the dynamic range or operating system range which is inversely proportional to the OPD of F-MZI. For phase-sensitive detection technique implemented with a fiber interferometer, the unambiguous measurement range is limited to ± π/2 phase change as depicted by the black colored rectangle in Fig. 11(a)

Fig. 11 Estimation of dynamic range. (a) Interferometer transfer function showing the unambiguous measurement range in the black colored rectangle which corresponds to ± π/2 phase change. (b) Strain to phase shift conversion responsivity for three different OPD i.e. 26 mm, 36 mm, and 46 mm. For ± π/2 phase change limit at 36 mm OPD, the maximum dynamic strain range is ~ ± 14 $μ ε$ shown by the dotted arrow.

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. This ± π/2 phase change restriction gives an estimate of the dynamic range. Corresponding to this, Fig. 11(b) shows strain to phase shift conversion responsivity for three different values of OPD i.e. 26 mm, 36 mm and 46 mm. This strain to phase shift responsivity is calculated by using Eq. (3) and Eq. (1). For unambiguous measurement, the maximum anticipated strain level applied onto the πFBG sensor should be such that the phase shift is less than ± π/2 radians. This yields a maximum strain range of ~ ± 14

μ ε

for 36 mm OPD of the fiber interferometer [shown by the arrow in Fig. 11(b)].

While performing the dynamic range estimation, we must also consider the fact that the movement of narrow transmission peak of the πFBG sensor is also restricted by the FBG filter. This dynamic shift of the transmission peak occurring under the influence of any external perturbation must be limited to the flat-top region of the FBG filter reflection spectrum. This is necessary to avoid any possible erroneous intensity variation [12]. Figure 12

Fig. 12 Region of interest of FBG filter reflection spectrum. The dotted vertical lines show a region of ± 10 pm around the central position. The reflected power within this region varies by a factor of only 0.23 dB.

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shows the region of interest of the FBG reflection spectrum shown earlier in Fig. 3(b). The dotted vertical lines show a region of ± 10 pm around the central position and the reflected power within this region varies by a factor of only 0.23 dB, which is very negligible. Hence, considering ± 10 pm as the maximum permissible wavelength shift and utilizing Eq. (3), the corresponding strain range can be evaluated to be ± 8.5

μ ε

. Therefore, the actual dynamic range of the system is limited by the FBG filter. At a first glance, this dynamic range of strain measurement might seem to be too small as compared to an FBG sensor. However, we must also remember the fact that the minimum detectable strain level has also gone down significantly to pico-strain (

p ε

) levels as compared to regular FBG sensors which is limited to nano-strain (

n ε

) levels. Additionally, the new πFBG sensor interrogation concepts are aimed at enhancing the measurement sensitivity so that very low-amplitude dynamic perturbations can be successfully detected. Dynamic perturbation measurement in micro-strain (

μ ε

) range can always be easily detected with a regular FBG sensor. Considering these above points, the estimated dynamic range of ± 8.5

μ ε

for the πFBG sensor with phase-sensitive interrogation principle which ranges from pico-strain (

p ε

) to micro-strain (

μ ε

) levels is more than sufficient for several applications.

5. Summary

In summary, an innovative concept for performing the Bragg wavelength shift measurements of passive πFBG sensors is experimentally demonstrated. External perturbation induced dynamic modulation of the narrow transmission peak is monitored by using an unbalanced F-MZI as a wavelength-shift to phase-shift converter. This phase-sensitive interrogation scheme is relatively simpler as compared to PDH architecture and simultaneously has the ability to detect pico-level ( $p ε$ ) dynamic strain perturbations. Leveraging the smaller bandwidth of the narrow transmission peak, which allows for a significantly longer OPD, it is experimentally estimated that dynamic strain sensing down to 17 $p ε$ over 1 $H z$ measurement bandwidth is achievable in the current experimental configuration. Moreover, the estimated dynamic range for the current experimental configuration permits the measurements of strain modulation up to ± 8.5 $μ ε$ . Thus the dynamic strain measurement capability ranges from pico-strain ( $p ε$ ) to micro-strain ( $μ ε$ ) levels. Hence, the proposed phase sensitive interrogation technique for π-phase-shifted Bragg grating sensor has potential applications in the field of high resolution dynamic strain measurement systems such as hydrophones for SONAR & medical sensing, acoustic emission studies etc. Additionally, the π–phase-shift region at the center of the grating decreases the effective length of the sensor making it extremely suitable for high frequency ultrasonic detection.

At last, it is important to understand the difference between passive πFBG and active πFBG as sensing devices. An active πFBG also known distributed feedback (DFB) fiber laser can be fabricated if the π–phase-shift fiber Bragg grating is written in a fiber amplifier (e.g. Erbium doped fiber) and pumped optically. These DFB fiber lasers have attracted significant interests as active sensing elements for various schemes [21,22]. Optical pumping leads to the generation of very narrow linewidth (< 1.0 pm) optical signal from the active πFBG structure. The shift in this lasing wavelength due to environmental perturbations are used as the sensor signal. This extremely narrow linewidth laser emission leads to ultra-high resolution dynamic sensing capability. Hence, fundamentally, it is prudent to not compare the sensing performances of passive and active πFBG structures. However, we believe that in many applications it is advantageous to use the passive πFBG sensor due to lower cost, better signal to noise ratio and its suitability for remote sensing.

Funding

Council of Scientific and Industrial Research (CSIR) under project CSIR-YSA (EMR 0001).

Acknowledgment

One of the authors (Umesh K. Tiwari) like to thank the Council of Scientific and Industrial Research (CSIR) for financial support under the project CSIR-YSA (EMR 0001).

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Dynamic strain response of a π-phase-shifted FBG sensor with phase-sensitive detection

Abstract

1. Introduction

2. Principle of πFBG sensor interrogation

3. Experimental setup design

4. Experimental results and discussion

4.1 Temperature response

4.2 Dynamic strain measurement

4.3 Strain sensitivity analysis

4.4 Dynamic range estimation

5. Summary

Funding

Acknowledgment

References

Cited By

Figures (12)

Equations (4)

OSA Continuum