Spiral-structured fiber Bragg grating for contact force sensing through direct power measurement

A high-sensitivity fiber Bragg grating (FBG) force sensor based on direct optical power measurement is presented. The approach utilizes a novel structure where the FBG is mounted on a thin tube-like fixture spirally. Contact force measurement is achieved through direct measurement of the FBG reflection power at a single wavelength using a power meter. The measuring system in our approach is simple and does not require processing of massive amount of spectral data, enabling real-time contact force monitoring. When force is applied to the FBG sensor, the unique spiral structure leads to FBG chirping and reflection spectrum broadening. A proportional relationship and linear fit are found between the force applied (up to 1.55 N) and the optical power reflected by the proposed FBG sensor. An average sensitivity of 11.16 dB/N is experimentally achieved. This design significantly reduces system complexity and improves data processing speed, which has great practical value in real-time FBG sensing applications. ©2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors, (060.3735) Fiber Bragg gratings, (050.1590)


Introduction
Fiber Bragg grating (FBG) based sensor has been well studied and widely used in different applications due to its unique advantages such as small size, robustness and immunity to electromagnetic interference [1].Different approaches have been reported to achieve force measurement, the most conventional force sensors have a simple structure in which the FBG is placed in parallel with the target object [2,3].An applied force causes a peak wavelength shift of the FBG reflection spectrum.Thus, by measuring the wavelength shift using an optical spectrum analyzer (OSA) and broadband source (BBS), the amount of applied force is determined.Although the structure is simple, this design requires a complex measuring system to determine the peak wavelength shift.Large amounts of data points over a wide range of spectrum must be continually analyzed in high resolution, requiring the use of a bulky and expensive OSA and resulting in a rather slow processing speed.Charge-coupled device (CCD) array based interrogation systems are reported for the peak measurement and can be as fast as 1000 measurements per second [4], but expensive high-resolution CCD and signal processing algorithm are required.Therefore, these conventional systems are not suitable for real-time low-cost applications and simplifying the entire FBG sensing system has great practical value.
Several approaches have been proposed to simplify the measuring system [5], examples include the use of an edge filter [6], tunable filter [7], and interferometric scanning method [8].It has been shown that transducing a measured wavelength shift of the FBG reflection spectrum to direct optical power measurement is a desired method due to the compactness and simplicity offered by direct power measurement.However, response time in the above wavelength-to-power conversion approaches are limited by the tuning or scanning speed, making it impractical for real-time applications.To overcome this issue, an intensityreferenced curvature sensor [9] is proposed which requires two chirped fiber Bragg gratings (chirped FBG) that are embedded on two opposite sides of a composite laminate, such that wavelength changes can be directly transduced to optical power variations.A direct intensity measurement approach based on induced sinusoidal chirp in bending FBG is reported [10], the measured optical power change is 0.6 dB with a displacement of 400 µm.Another force measurement approach based on bandwidth modulation and optical-power detection technique is proposed [11], which uses a bending cantilever beam and a photodiode to directly measure the total reflection power.The maximum measured optical power change is about 3 dB with an applied force of 12 N, resulting in a sensitivity of about 0.25 dB/N.The approach requires the measurement of total reflection power of the FBG, which has relatively low measurement sensitivity due to the small change in total reflection power.
In this paper, we demonstrate a compact force sensing system through direct optical power measurement at one particular wavelength such that the measurement sensitivity is significantly enhanced.Our approach uses a standard FBG that is spirally fixed onto a catheter, while only a laser source and a power meter are used for force measurement, which significantly reduces system complexity and cost.We focus on the contact force sensing that is in the F z direction of the fixture, as illustrated in Fig. 1(a).The measurement of contact force in the F z direction with a tiny sensor probe is particularly important for minimally application [3].The measured optical power change is 17.3 dB over an applied force ranged from 0 to 1.55 N, resulting in an average sensitivity of 11.16 dB/N.A proportional relationship and linear fit are observed between the applied force and the reflected optical power from the FBG.Due to the direct relationship between the measured optical power and the applied force, there is no need to go through complicated process for determining the amount of peak wavelength shift and converting it back to the amount of force applied, enabling real-time measurement and a simple measuring system.This design has been tested as an electromagnetic immune magnetic resonance imaging (MRI)-conditional contact force catheter sensor in electrophysiological (EP) therapy [12], and can be used in various types of real-time high precision sensing applications.In principle, instead of fixing the sensor on a catheter, the tiny FBG can be directly fixed on any cylindrical tube-like fixture for force measurement, which is well suited for minimally applications.

Principle and experimental setup
The reflected Bragg wavelength λ B is given by the following expression: where n eff and Λ are the effective refractive index and the length of the grating pitch, respectively.Bragg wavelength λ B shift of the FBG in response to an applied force mainly arises from the length change of grating pitch Λ [11].In the conventional force sensor where the FBG sensor area is placed in parallel with the target object, only the peak reflection wavelength λ B is shifted significantly under different applied force, while the reflection bandwidth is remained unchanged.To determine the strength of force applied to the FBG, the amount of λ B shift has to be measured, which typically involve the use of OSA and BBS.In our approach, a standard FBG is used for force measurement, which is spirally fixed onto a thin tube (e.g. a catheter) as illustrated in Fig. 1(a).With the spiral structure, chirp effect is introduced to the FBG when force is applied, resulting in a broadening in spectral shape and reflection bandwidth.Compare with a parallel structure, the spiral structure enhances the bending of FBG and resulting in a more significant chirping effect.It has been shown that the chirping can be induced to a standard FBG and the reflection spectrum bandwidth can be adjusted by displacing one end of FBG in axial direction with the other end fixed [10] or by fixing the FBG onto a support beam with slanted direction and changing the beam curvature [13].When force is applied onto the catheter tip, it is effectively transferred to the bending of the FBG sensor area due to the elastic property of the catheter.Since the FBG is mounted spirally onto the catheter, bending of the catheter results in uneven changes in each of the grating pitch length Λ at different positions, most are stretched in different extends and some are kept unchanged, as illustrated in Fig. 1(b).Stretched gratings with longer pitch length Λ lead the λ B shift to longer wavelength direction, as a result, the non-uniform stretching of grating pitches lead to FBG chirping, providing a broadened reflection spectrum towards longer wavelength direction and slower ramp edge at shorter wavelength side, as illustrated in Fig. 2(a).Due to the spiral structure, the stretching of FBG is non-uniform, most of the gratings are stretched in different extends and contributes to the broaden reflection spectrum at the longer wavelength as well as a stronger reflection.Only a small portion of gratings keep unchanged, resulting in the slow ramp profile at shorter wavelengths and a weaker reflection, similar results can be found in previous research [11,13].
Instead of measuring the reflection wavelength shift, our approach makes use of the change in spectral shape of the FBG and performs direct power measurement.As indicate in Fig. 2(a), the slope of the slow ramp edge at shorter wavelength changes as the applied force increases.Thus, the amount of applied force can be determined by measuring the reflection power at a particular wavelength at the slow ramp edge.This is achieved by launching a single wavelength laser light (at λ w ) into the FBG through a circulator and measure the reflection power using an optical power meter, as shown in Fig. 2(b).The basic system consists of a laser source, an optical circulator, and an optical power meter, while the part inside the dotted box is for temperature effect mitigation.To result in high sensitivity, λ w is picked such that the difference in reflection power is the largest under different amount of applied force, as illustrated by the red dotted line in Fig. 2(a).In the experiment a non-magnetic EP catheter with a radius of 4 mm is used as the fixture, which is fabricated in a way that it is slightly bent in one particular direction.Thus, when force is applied to the sensor in F z direction, the catheter always bends to the same particular direction.Moreover, one end of the catheter is fixed so that the bending position is also the same when force is applied.A 35 mm long FBG is fixed in the middle of the catheter (the bending position), with 12 mm spiral pitch gap.In order to prevent breakage and to reduce bending loss of the FBG, the grating area is wrapped around the tube at an angle of about 40° with respect to the catheter.A DFB laser (ILX Lightwave 79800) at 1549.9 nm with + 5 dBm power is used as the laser beam for launching into the spiral FBG sensor, and an optical power meter (EXFO EPM-50) is used to measure the reflected optical power.Compare with conventional sensing systems based on OSA and BBS, our approach does not require capturing the entire spectrum for determining the wavelength shift.Thus, our approach is simple, compact, consumes less computation power, and has low latency.

Results and discussion
Figure 3(a) is the measured reflection spectra of our proposed spiral-structure FBG sensor.The spectra are measured under different contact forces ranging from 0 to 1.55 N, with 0.15 N force differences for each curve.An amplified spontaneous emission BBS is used as the input light and an OSA with a resolution of 0.8 pm (APEX AP2040A) is used for capturing the spectra.This setup is for comparing the change in spectrum and is not necessary for force measurement in the spiral-structure FBG sensor.
As shown in Fig. 3(a), peak reflection wavelength of the spiral structure FBG is shifted by ~1.5 nm and significant broadening in reflection bandwidth is observed when force is applied to the catheter.There is a slight decrease in the maximum power at the spectrum peak, but the total reflection power is increased due to the broadened spectrum.The entire reflection spectrum moves toward the longer wavelength direction, at the same time, the ramp profile at the shorter wavelength gradually gets broader as the strength of applied force increases, while the longer wavelength side stays steep.The slow ramp profile is caused by the non-uniform stretching of most grating pitches in different extends and only a small portion of gratings keep unchanged, thus, the change of reflection power at different wavelength is uneven.Figure 3(b) shows the measured reflection bandwidth under different amount of applied force, which is determined from the original spectra in Fig. 3(a).With an increasing applied force, the reflection bandwidth gradually increases by ~1.7 nm, i.e. from 0.5 to 2.2 nm.The significant change in the slow ramp profile shown in Fig. 3(a) becomes the ideal region for discriminating changes in the FBG spectrum under different amounts of applied force, because the optical power reflected at a particular wavelength is different under different amounts of applied force.In general, the peak reflection wavelength λ B of the original reflection spectrum (at 0 N, indicated by the red dashed line in Fig. 3(a)) is the optimal working wavelength λ w .At this wavelength, the measured power is at maximum value when no force is applied and a monotonically decreasing curve can be obtained under increasing applied force.It also has the largest power change for discriminating the amount of applied force and resulting in highest sensitivity.Figure 4 is the measured power response against different amounts of applied contact force, ranging from 0 to 1.55 N. The DFB laser has very good stability in terms of power and wavelength, corresponding fluctuations are less than 0.01 dB and 0.001nm, respectively.The reflected insertion loss of the whole system (most from the spiral FBG) is 4.7 dB, remains unchanged during measurement.Under an applied force from 0 to 1.55 N, the reflection power is decreased from 0.3 to −17 dBm, resulting in an average sensitivity of 11.16 dB/N.The sensitivity is significantly better than previous power measurement approaches.A good discrimination of applied force with a step interval of 0.025 N is experimentally achieved among the whole test range, resulting in a measurement resolution of 0.025 N. As shown in Fig. 4, a proportional relationship is found between the applied force and reflection power, with a linear fit R 2 of 0.98216 in dBm unit.The proportional relationship and linear fit allow simple conversion from reflected optical power to the amount of applied force, enabling fast signal processing speed and real-time measurement.The sensor meets it's limitation at 1.55 N due to the over-bending of catheter under a strong contact force, with a maximum bending radius of ~5 cm.The reflection power is back to 0 dBm when the applied force is removed and the measurement system has a good repeatability.Temperature induced spectral shift is an undesired behavior and has general influence to all FBG sensors, which would significantly affect sensing accuracy.Figures 5(a) and 5(b) show the reflection spectra of the spiral FBG at different temperature (from 20 to 60 °C with an interval of 10 °C) under a fixed force of 0 and 1.0 N, respectively.An increase in temperature results in spectral shift to the longer wavelength direction, while the spectral shape and reflection power is kept unchanged.Compare Fig. 5(a) and 5(b), the amount of temperature induced wavelength shift is almost the same (~0.4nm) when different amount of force is applied to the spiral-structured FBG.In our approach, temperature influence can be compensated using a similar FBG as a reference grating, which has the same original Bragg wavelength as the spiral FBG.The temperature compensation scheme is illustrated in the dotted box in Fig. 2(b).The reference grating is fixed near the catheter but no force is applied onto it.This reference grating shares the same laser source by splitting the laser output with an optical coupler.Temperature change shifts the reflection wavelength of both the spiral FBG and the reference FBG by the same amount.Thus, the wavelength shift resulted from temperature fluctuation can be reflected by power measurement at the reference grating.The power information of the reference grating can be used to tune the laser wavelength to be aligned to the reflection peak of the reference grating as well as the spiral FBG, which is the new optimized working wavelength resulted from temperature change.Figure 5(c) is results of the spiral structure FBG sensing system measured at different temperature ranging from 20 °C to 60 °C with temperature compensation.The power vs. force curves at different temperature settings are almost the same, proofing that the temperature effect is successfully compensated.

Conclusion
In summary, a high-sensitivity FBG contact force sensor is proposed and experimentally demonstrated.The measurement system requires only a laser source and an optical power meter.With a novel spiral FBG structure, we achieve a simple, compact, and high-sensitivity force measurement design through direct optical power measurement.A proportional relationship and linear fit are found between optical power and applied force up to 1.55 N, and an average sensitivity of 11.16 dB/N is experimentally achieved.Temperature influence has been studied and a reference grating is proposed for temperature effect mitigation.This design significantly reduces the system complexity and latency, as well as improves data processing speed, which has a great practical value in real-time FBG sensing applications.

Fig. 1 .
Fig. 1.(a) Illustration of the spiral-structured FBG force sensor mounted on a catheter.(b) Corresponding change in grating pitches under the influence of applied force.

Fig. 2 .
Fig. 2. (a) Illustration of the change in spectral shape under different amount of applied force.(b) Measurement system for the spiral-structured FBG force sensor.

Fig. 3 .
Fig. 3. (a) Measured FBG reflection spectra under different amount of applied force.(b) Reflection bandwidth of the FBG under different amount of applied force.

Fig. 4 .
Fig. 4. Measured reflection optical power of the spirally structured FBG at different amount of applied force.

Fig. 5 .
Fig. 5. (a)(b) Spiral FBG reflection spectra measured at different temperature under fixed force of 0 and 1.0 N. (c) Power measurement test at different temperature with temperature compensation.