See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/269072826

Fiber Bragg Grating vibration sensor with DFB diode

Conference Paper in Proceedings of SPIE - The International Society for Optical Engineering · December 2012 DOI: 10.1117/12.2010467

CITATIONS READS 6 237

8 authors, including:

Jakub Cubik Stanislav Kepak VŠB-Technical University of Ostrava University of Strathclyde

45 PUBLICATIONS 204 CITATIONS 53 PUBLICATIONS 335 CITATIONS

SEE PROFILE SEE PROFILE

Petr Koudelka Jan Latal Czech Telecommunication Office VŠB-Technical University of Ostrava

84 PUBLICATIONS 494 CITATIONS 113 PUBLICATIONS 383 CITATIONS

SEE PROFILE SEE PROFILE

Some of the authors of this publication are also working on these related projects:

Advances in Fetal Monitoring View project

TA ČR GAMA: PRE SEED VŠB-Technical University of Ostrava fund - BroadbandLIGHT View project

All content following this page was uploaded by Jan Latal on 16 April 2018.

The user has requested enhancement of the downloaded file. Fiber Bragg Grating vibration sensor with DFB laser diode

Petr Siska*, Martin Brozovic, Jakub Cubik, Stanislav Kepak, Jan Vitasek, Petr Koudelka, Jan Latal, Vladimir Vasinek

*VSB-Technical University of Ostrava, Faculty of Electrical Engineering and Computer Science, Department of Telecommunications, 17. listopadu 15, Ostrava, 708 33, Czech Republic *[email protected]; phone +420 596991417; fax +420 597321650; http://kat440.vsb.cz/optice

ABSTRACT

The Fiber Bragg Grating (FBG) sensors are nowadays used in many applications. Thanks to its quite big sensitivity to a surrounding environment, they can be used for sensing of temperature, strain, vibration or pressure. A fiber Bragg grating vibration sensor, which is interrogated by a distributed feedback laser diode (DFB) is demonstrated in this article. The system is based on the intensity modulation of the narrow spectral bandwidth of the DFB laser, when the spectrum of the FBG sensor is shifted due to the strain that is applied on it in form of vibrations caused by acoustic wave pressure from loud speaker. The sensor’s response in frequency domain and strain is measured; also the factor of sensor pre-strain impact on its sensitivity is discussed. Keywords: Fiber Bragg Grating, temperature, sensor, vibrations, strain, DFB, laser diode, .

1. INTRODUCTION Fiber Bragg Grating (FBG) can be described as periodical variation of refractive indices along the core. That variation in causes the disintegration of the light in the basic traveling mode. The variations are based on the photosensitivity property of the SiO2 optical fibers. The photosensitivity was discovered in 1978 by Ken Hill at Canadian Communications Research Center [1]. For almost eleven years there was practically no development in the field of photosensitive optical fibers, but with the discovery of side holographic writing of gratings into the optical fiber core using ultra violet (UV) light in 1989, the technological advances in that field made a huge impact on the field of telecommunications and sensing. For example optical fiber amplifiers would not be possible without FBG, or almost every semiconductor laser diodes have a FBG in their structure, also FBG can be used as compensators, in wave division multiplex (WDM) systems for channel selection etc., narrow band filters and many more applications. FBGs can be also used in biomedical applications such as in-body sensing or tumor detection. But the mayor area where FBGs are used nowadays is sensing, with their low price, electro-magnetic interference (EMI) immunity, small size and multiplexing properties. The big industrial companies are using FBG based sensors for oil and gas exploration, mine safety monitoring, temperature sensing and for many others [2], [3].

1.1 Theory behind fiber Bragg grating The theory behind the principle of operation of the fiber Bragg gratings can be simplified such that FBG is a periodic modulation of a refractive index within the core of an optical fiber. So if we let optical power Pin into the fiber core as a forward propagation mode, the periodic modulation within the fiber core reflects a certain wavelength λ from that forward propagation core mode, assuming the single mode operation conditions are met, to a backward core mode. Reflected wavelength λ is called Bragg wavelength λB and it’s given by equation:

B  2neff  , (1) where λB is the reflected wavelength, neff is an effective refractive index, Λ is the grating period. Any intrusion such as temperature or strain that changes the neff or the grating period Λ also changes the reflected wavelength λB. Based on that, FBG’s can be thought of as intrinsic fiber detectors, which are changing their reflected spectrum of light. This FBG’s are called short period gratings, they support counter propagating interaction within the fiber. They are the most used type of FBG nowadays.

Fig. 1: Transmission and reflection spectra of FBG.

1.2 Distributed Feedback Laser DFB are heterostructure semiconductor lasers with active region consisting of MQWs, they emit only a single wavelength of light with a spectral width around 0.1 nm, DFB lasers are mostly used for optical communications on wavelength λ = 1310 nm or 1550 nm [5]. DFB laser has on the top of its active layer (see Fig.2) layer that is usually called guiding layer and works as a Bragg reflector. Thanks to that layer DFB lasers do not use a two mirror system as a positive feedback like conventional FP LD. Instead of that DFB lasers have on one side anti-reflexive coating and high reflectivity on other side so the forms a distributed mirror on the anti-reflexive side selectively reflecting only those satisfying the condition for reflected wavelengths according to equation (2). Operation of a DFB laser can be described like that, while the radiation is fed from the active region to the guiding layer, which reflects only a narrow band of wavelengths, the left and right traveling waves can only coherently coupled to set up a mode if their frequency is related to the diffraction period Λ [6]. These modes are not given exactly by Bragg condition but they are symmetrically placed around the Bragg wavelength λb. 2     b m 1, (2) m b 2nL where m is a mode integer, 0,1,2,... and L is the effective length of the diffraction grating [6]. Since L is far larger than the diffraction period Λ the second term in the equation is then very small and the emitted wavelength λm is very close to λb making the emitted spectral width very small. Semiconductor basis of a DFB laser makes the laser easily tunable to a different wavelength λ only by changing the temperature and provided drive current. The emitted wavelength λ can range by ± 20 nm easily. The DFB lasers usually have a set operating point, in that point the laser operates in the manufactured wavelength λ for example 1550 nm.

Fig. 2: DFB laser structure [6].

Fig. 3: Operation of a DFB laser, selectively reflecting only λm wavelength [6].

2. FIBER BRAGG GRATING FOR VIBRATION SENSING Sensing with FBG is based on the sensitivity of a FBG to its environment as the material properties of glass are a function of temperature, strain, pressure and vibration. All these variables are influencing the Bragg wavelength of a FBG. It can be seen from equation one, the periodicity of a grating Λ changes and the refractive index neff is also dependent on this variables. Therefore FBG sensors are not appropriate for applications requiring stability of the reflected wavelength. When measuring strain it is necessary to compensate for the temperature effect on the FBG or when measuring temperature it is necessary to compensate for the wavelength change that appears due to strain. Many methods had been shown how to compensate for the effect of other variables while measuring the one main quantity. The aim of this article was to use FBG’s as vibration sensor. The typical way of the vibration sensing is measuring in the spectral domain and following the change in the wavelength caused by vibration applied through some source of vibrations. The second approach, used in this experiment, is based on modulation of the intensity of DFB laser diode by the change of the FBG reflection spectra caused by strain due to applied vibration [4].

Fig. 4: Working principle of the vibration sensing method.

Fig. 5: Spectral characteristic of used DFB laser SPL1550-1-9PD measured by multi-wavelength meter.

Fig. 6: Spectral characteristic of used FBG measured by multi-wavelength meter. On the figure 6 can be seen the measured spectra of used Bragg grating illuminated by the ELED at 1550 nm. This figure shows the real shape of Bragg grating as it was manufactured by the producer. The energy fluctuations in spectra are caused directly by the manufacturing technology of given grating. The real shape is subsequently generalized and idealized on the shape with Gaussian course, as you can see in figures 1 and 4.

3. EXPERIMENTAL RESULTS 3.1 Measurement of a possible tune-ability of a DFB laser The first step that was necessary to do in this measurement was to measure possible tune-ability of DFB laser diode (SPL1550-1-9-PD), in other words to find the range of the possible radiated wavelengths that this diode can provide. The LD was meant to be radiating wavelength λ=1550 nm while in the working point (26 mA and T = 300K) but as first measurement shown that it is not exactly true, the radiated wavelength λ from LD was moved more to the higher wavelength about 0,3 nm. Because of this finding it was necessary to perform this measurement. The LD was mounted into a LD mount from ThorLabs Company (TCLDM9), which can stabilize the LD around the working point of LD in our case the mount was able to stabilize the diode in range from 19 C up to 35 C. The working current range of this diode is from 6.9 mA to 50 mA. There were made four independent measurement for three different temperatures each (20 C, 30 C and 34.5 C) and whole current range of the diode to find out range of the possible radiated wavelengths. This diode showed these results, for temperature of 20 C can radiate light in range from 1550.01 nm at 6.9 mA up to 1551.32 nm at 50 mA. For temperature of 30 C can radiate light in range from 1550.55 nm at 6.9 mA up to 1551.74 nm at 50 mA and finally for temperature of 34.5 C can radiate light in range from 1550.91 nm at 6.9 mA up to 1552.64 nm at 50 mA. All of these possible wavelengths were too high for our experiment, because the crucial point of this set up is to set the DFB laser diode wavelength into the middle of FBG reflectivity leading or falling edge. To satisfy of this condition was necessary to add alternative cooling procedure of LD. Peltier’s module placed on the passive cooler was used for this purpose. While cooling the LD with Peltier’s module, which was able to cool down diode to the 0 C, the wavelengths below the 1550 nm of central wavelength λ were easily reached and placing the working point of the system to a FBG leading edge was achieved. 3.2 Measurement of the reflected power on the leading edge of used FBG The second step of this measurement was to measure the reflected power and wavelength on the leading edge of the FBG and to choose a working point that is most suitable for the experiment.

Fig. 7: Block scheme of the leading edge measurement. The optical power from the LD goes into the port one of the circulator and leaves at port two, where the power is reflected by FBG and goes back into port two. The power then goes into ports two and three. In port two the reflected power does not affect the LD, because it is not let through by an isolator. In port three is the reflected optical power split into two parts by a coupler. Ten percent of the power is then forwarded into the EXFO WA- 7400 MWM, where the power and wavelength is measured. Ninety percent of the reflected power is forwarded into the SPU (Signal Processing Unit), where the signal is analyzed. LD is then stabilized in the maximum reflectivity point in λB, and the reflected power is converted to voltage and its peak to peak character is measured in the SPU. LD is then cooled by the Peltier’s module linearly by providing a current to the module. The change in the current provided to the module was with step of 100 mA. The current change was applied until the lowest point on the leading edge was found. Sensing FBG was just placed on the wooden desk and was not strained by force. The optical power was let into the FBG sensor just from one end other end was covered. In this part of measurement the range of leading edge was measured in wavelength domain. The range was 1.14 nm. Based on this measurement the working point for the experiment was chosen at 1549.32 nm, which is in the center of the leading edge without applied strain, so there was enough space to go higher or lower in intensity modulation while applying vibrations to the FBG sensor system. 4. MEASUREMENT OF THE FREQUENCY SENSITIVITY OF FBG The main goal in this measurement was to measure the sensitivity of the FBG sensor. The FBG sensor was pre- strained by given force to obtain repeatable results. We were also interested in finding of the minimum and maximum frequency, to which is the FBG sensor still sensitive and finding of a vibration frequency that is the FBG sensor sensitive the most. At the figure 8 is shown experiment setup for this measurement

Fig. 8: Block scheme of the FBG frequency sensitivity measurement. The one end of the sensing FBG was placed and fixed onto a wooden desk and the second end to a force meter, which was attached to a stable construction. By applying force the FBG was pre-strained and this force was measured and controlled by the force meter. The acoustic pressure was applied through loud speaker and this allowed measuring the frequency sensitivity of the FBG. LD was stabilized at 1549.32 nm, which was a working point without applied strain. If the strain was applied on the FBG its whole characteristic shifts to higher wavelengths. So there is a need to find the new working point after the FBG is pre-strained. That was done by finding the “new” wavelength range of the FBG leading edge (abbreviation LE in Table 1), which was almost the same during all measurements. The minimal frequency was also affected by the 17 Hz high-pass filter at the SPU. The response threshold limit value at 5 V rms was set on the evaluation device from National Instruments. Frequencies exceeding this limit value were considered as a frequency range measurable area. In this area was then searched the frequency with the highest response for every used pre-strain, see figures 11 and 12. Measurement results in form of the table and pictures from software LabView Signal Express, which was used as SPU, are shown below. The strain calculated in the last column was based on equation 3 and measured values.

Table 1: Measurement results.

Strain was calculated using equation:

B  CS  B (3) Where ΔλB is change in Bragg wavelength due to applied strain. λB is the Bragg wavelength before application −7 −1 of strain. CS is the strain sensitivity of the fiber and is equal to CS = 7.8∗10 μϵ and Δϵ is the strain. This formula is valid in range of the Hooke’s law validity, which is saying that extension of optical fiber is directly proportional to the applied strain. Linearity verification was performed indirectly by the five times repeated measurement, when the optical fiber regresses to its original length in case of all repeated measurements.

Fig. 9: Graph of strain dependency on applied force.

Fig. 10: Graph of maximal sensitivity of FBG in strain dependency.

The trend equation of polynomial regresion of the second order shown in the figure 10 is representing maximal frequency sensitivity dependence on different pre-strain values of FBG and it is defined by following formula: 6 2 y nd  3,54210  x 0,0107  x  32,0868; R  0,9864. 2 ,

The trend equation of polynomial regression of the third order shown in the figure 10 is representing maximal frequency sensitivity dependence on different pre-strain values of FBG and it is defined by following formula: 8 3 5 2 y rd  1,168310  x  3,4989 10  x  0,0369  x  38,7898; R  0,9916. 3 ,

Fig. 11: Measurement example - highest sensitivity of FBG on applied pre-strain of 0,7 N.

Fig. 12: Measurement example - highest sensitivity of FBG on applied pre-strain of 1,0 N.

5. DISCUSSION This experiment goal and motivation was to be performed as a testing measurement against to interferometer sensory measurement performed in project GUARDSENSE - The modern structure of photonic sensors and new innovative principles for intrusion detection systems, integrity and protection of critical infrastructure. Where for detection is used the interferometer sensory system. This system is very sensitive but also quite expensive and for this reason the FBGs were used as another possibility for cheaper detection system. In view of the fact that FBGs are less sensitive then interferometers all mentioned measurements were performed to verify its properties to be enough sufficiently sensitive elements of detection system. Our interests laid mainly in lower frequencies, since movement of human intruder is represented by low frequency spectral characteristic. Low frequencies are also typical for example for earthquakes. The team of authors was focused on measuring the sensitivity of FBG sensor. Finding the minimum and maximum frequency, to which the FBG sensor is still sensitive and also find a frequency of vibration that is the FBG sensor sensitive the most. And also finding if it is possible to change this frequency and adjust FBG best sensitivity. This was done thanks to pre-straining of the FBG by selected strains. The range was found experimentally, where the frequency was generated in the frequency generator that was connected to a loud speaker, while changing the vibration frequency the voltage response was continuously checked on the SPU. Minimum frequency was unfortunately influenced by 17 Hz high pass filter in the SPU. The minimum frequencies were then 17 Hz for all pre-strains. Maximum frequencies were also practically the same for all pre-strains. They were found at range from 45 to 47 Hz. However the FBG sensor showed ability to change sensitivity to vibration frequencies with increasing pre-strain. With increasing pre-strain the frequency at which the sensor was sensitive the most was decreasing.

6. CONCLUSION In this paper the team of authors was dealing with an experiment of vibration sensor system based on FBG, which was interrogated by a DFB laser. Three differently housed FBGs were tested during preparation of this experiment and for its purposes the bare fiber FBG was selected as it showed best frequency properties. Also the measurement of LD tune-ability was performed, because it was necessary to set the working point on the leading edge of the FBG central lobe. On the grounds of that measurement it was needed to introduce a different type of cooling mechanism for the DFB laser diode. The mechanism was introduced using a Peltier’s module that was able to cool the diode down to 0 C and moved then radiated wavelength way below 1549 nm. Then the measurement of the FBG frequency sensitivity was performed according to setup showed in figure 8. The final results showed that it is possible to change FBGs frequency that is sensitive the most by applying pre-strain, which caused decreasing of the frequency at which the sensor was sensitive the most. Applied pre-strain was in range from 0,2 to 1 N only, because strain of 1 N moved λB of the FBG for about 1,6 nm higher and reaching of this wavelength meant to heat LD over 50 C. The future plans for this vibration detection system are to measure the last experiment more times to get more precise results of the frequency range, strain and sensibility to vibration frequency. Next step in the experiment would be to test the FBG on a different surface and to calculate and work with the temperature change of the reflection spectrum of the FBG. Applying the vibration from a different speaker, that would be able to produce lower frequencies more precisely. Construct new filter at the SPU that would be designed as a low-pass filter instead of the high-pass filter at 17 Hz. Also to test higher pre- strains values then 1 N, for which we will, need find LD with radiation wavelength close to 1552 nm. ACKNOWLEDGEMENTS This article was supported by project VG20102015053 - The modern structure of photonic sensors and new innovative principles for intrusion detection systems, integrity and protection of critical infrastructure – GUARDSENSE. This work was supported also by the Ministry of Education of the Czech Republic within the project no. SP2012/165 of the VSB-Technical University of Ostrava. The research has been partially supported by the GACR (Czech Science Foundation) GAP108/11/1057 -Synthesis, structure and properties of nanocomposites conducting polymer/phyllosilicate.

REFERENCES

[1] Kashyap, R. Fiber Bragg gratings. 2nd ed. Burlington, MA: Academic Press, c2010, 614 p. ISBN 01-237-2579-8.

[2] Gemzicky, E., Mullerova, J. "Analysis of Simulated Reflection Characteristics of Uniform and Apodized Fiber Bragg Gratings," Advances in Electrical and Electronic Engineering 7(1-2), 21-24 (2008). [3] Righini, G. C., Tajani, A., Cutolo, A. An introduction to optoelectronic sensors. Hackensack, NJ: World Scientific, c2009, 570 p. Series in optics and photonics, v. 7. ISBN 98-128-3412- 5. [4] Chang, J., Wang, Q., Zhang, X., Huo, D., Ma, L., Liu, X., Liu, T., Wang, C. "A fiber bragg grating acceleration sensor interrogated by a DFB laser diode," Laser Physics 19(1), 134-137 (2009). [5] Kasap, S. O. Optoelectronics and Photonics. Optoelectronics and photonics: principles and practices. Internat. ed. Upper Saddle River, NJ: Pearson, Prentice Hall, 2001. ISBN 03-211- 9046-7. [6] Svelto, O. Principles of lasers. 5th ed. New York: Springer, 2010. ISBN 14-419-1301-7.

View publication stats