See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/269072826 Fiber Bragg Grating vibration sensor with DFB laser 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 reflection 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, wavelength. 1. INTRODUCTION Fiber Bragg Grating (FBG) can be described as periodical variation of refractive indices along the optical fiber core. That variation in refractive index 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 dispersion 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 2neff , (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 lasers 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) diffraction 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 diffraction grating forms a distributed mirror on the anti-reflexive side selectively reflecting only those wavelengths 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.