This work presents a novel simultaneous optical data transmission and sensing concept. For this combined system, neither additional components nor spectral segregation is required. In contrary to other combined systems, the complexity does not increase in the presented approach. The sensor system is a code-division multiplex (CDM)-wavelength-division multiplex (WDM) system that is able to interrogate thousands of serial fiber-Bragg gratings (FBGs) within a single optical fiber. Simultaneously, this fiber is also used as the data transmission medium. In the proposed system, the performance limitations due to the shared broadband light source are mitigated by using different post-equalization schemes. The proof of concept is verified using an experimental setup, which demonstrates the interrogation of 1000 sensors and concurrent data transmission at a bit rate of 1 Gbps using a 1.6 km FBG sensor network employing a unified broadband light source.

Multiple-input and multiple-output (MIMO) technology have attracted a lot of research due to the rising demands of higher capacity and data rates. To actively address these challenges in the area of optical fiber communication, the spatial division multiplexing techniques with an optical MIMO system by using multi-mode fibers are applied. While restraining the transmitter's complexity and allowing multi-level signaling, a linear pre- and post-equalization (PPE) is proposed for an optical MIMO system. A numerical optimization solver provides an aid to jointly optimize the proposed PPE scheme according to the maximum power constraint. In contrast to existing research, the joint-PPE scheme is implemented for a multi-mode fiber system, which is adversely affected by the modal dispersion. In the analyzed scenario, the experimental measurement results confirm that the joint-PPE scheme is advantageous with multi-mode fiber links. While using the proposed joint-PPE scheme, the required optical received power to reach a bit-error rate of 10(-3) is reduced by 3.8 % and 2.7 % in comparison to the zero-forcing and the minimum mean square error post equalizers respectively.

Structural health monitoring and other smart structures gain an increased attention which can be satisfied by the quasi-distributed optical sensing approach. Serial fiber optic sensors, such as fiber-Bragg gratings (FBGs) provide among others small size, immunity to electro-magnetic interference, an accurate sensing accuracy and a high multiplexing capability to increase the amount of sensing points in an optical sensor network. Different multiplexing approaches demonstrated a limited number of FBGs, such as time-division multiplex, optical frequency domain refractometry or frequency shifted interferometry. This work introduces a code-division multiplex (CDM) - wavelength-division multiplex (WDM) interrogator for massive serial FBG sensor networks. The interrogation of 4000 serial sensors in a network with a length of 113m, 200 identical WDM sections over a length of 200m and a network with 1000 sensors and a length of 1.6km show a massive multiplexing capability of up to 16000 sensors and possible network lengths of several kilometers. Strain measurements with FBGs in rear sections prove the sensing applicability of the CDM-WDM scheme.

This contribution evaluates the potential of a code-division multiplex (CDM) interrogated fiber optical sensor network, namely a fiber-Bragg grating (FBG) sensor network. The basic principle is briefly outlined and a detailed analysis on trigger schemes is performed. It focuses on the impact on the autocorrelation function of a predetermined orthogonal code. For limit assessments, a modified signal to multi user interference (MUI) ratio $$\textrm{mSMUI}$$ is introduced which can be applied to a CDM interrogated sensing network, as well as to a wavelengthdivision multiplex (WDM) interrogation. A trigger scheme which realizes a code delay with rotated codes and adds a zero padding to fill the integration time without truncating codes turns out of to be the best suitable trigger scheme in terms of $$\textrm{mSMUI}$$ . Based on typical hardware parameters, a theoretical capacity limit of a CDM interrogation system is estimated to 376 sensors with overlapping spectra. A $$\textrm{mSMUI}$$ measurement in a sensor network testbed proves the practical operation of the system. A comparison of CDM and WDM in terms of the $$\textrm{mSMUI}$$ shows ratios of 21.41 dB for CDM in contrary to 12.43 dB for WDM, even when the practical peak height of sensors in the spectrum reached only up to 70% of the theoretical height. At the end, a hybrid scheme containing a combination of CDM and WDM is shown which is able to interrogate 2000 serial FBGs.

This work evaluates the accuracy of a code-division multiplex (CDM) - wavelength-division multiplex (WDM) inter-rogation system for massive serial fiber optical sensing using fiber-Bragg gratings. The standard deviation of wavelength detection dependent on a signal-to-noise-ratio, referred to as static deviations, is measured to be less than 3 pm. Two consecutive correlation steps are subtracted. Dynamic influences occur due to a wavelength shift between the correlation steps. They are investigated using a gradient from a linear regression. Dynamic self interference ranges from 0 pm/pm (picometer deviation per picometer wavelength shift) for the maximum peak to -3.47 pm/pm at 20 % of the maximum peak height. Dynamic multi-user interference, caused by spectral overlapping sensors, starts at -0.5 pm/pm down to -7.27 pm/pm, which makes this system well competitive to other multiplexing techniques.

This contribution demonstrates a massive hybrid code-division multiplexing (CDM)-wavelength-division multiplexing (WDM) scheme that interrogates 2×2000 serial fiber-Bragg gratings (FBGs). The overlapping FBG spectra lead to maximum deviation errors of only 8 pm at a reflectance of 1 %.

To increase the multiplexing capability of code-division multiplexing (CDM) applied in optical sensor networks, a system based code evaluation is required. This contribution analyses evaluation criteria for sequences applied in CDM systems. A comparison of an optical sensor application and a single user data transmission system is presented. While a detection signal-to-noise ratio and the bit error rate are used to evaluate data transmission systems, the proposed optical sensor application uses a modified signal-to-multiuser-interference ratio (mSMUI). The main difference exists in the handling of interference. In contrary to data transmission, the mSMUI requires a separation of positive and negative interferences. Both applications are simulated for different binary sequences. While the Legendre sequence with a length of 503 chips achieves the over all best results for the optical sensor application, the single user data transmission simulation shows no significant sequence influence.

This article presents a new interrogation technique for serial fiber optical sensor networks. A hybrid code-division multiplexing (CDM)-wavelength-division multiplexing (WDM) scheme allows interrogating 2000 serial fiber-Bragg grating (FBG) sensors in a network with a length of 50 m. Furthermore, an interrogation of 477 sensors over a length of 150 m is shown. The work focuses on influences given by the above-mentioned sensor networks, such as reflectance, multiple reflections, and overlapping FBG spectra, as well as on specifications of used equipment. Since CDM relies on orthogonal codes, an autocorrelation function of a code is presented that suits for the interrogation of the previous mentioned sensor networks. A simulation of the interrogation scheme allowed for the study of the overall sensor network performance and the impact of its different components. The promising results were reproduced in a testbed. Furthermore, strain calibration measurements showed a high level of accuracy. The proposed hybrid CDM-WDM interrogation scheme can deal with sensor network lengths from a few meters to the range of a few kilometers with massive numbers of freely distributable sensors in the fiber. It is a promising approach in the field of FBG based optical sensing.

This contribution shows the interrogation of 4000 serial fiber-Bragg gratings (FBGs) by means of a hybrid code-division multiplex (CDM) - wavelength-division multiplex (WDM) scheme. A strain measurement shows no influence of a strained WDM-section to another non-strained WDM-section. Polarization effects are diminished by the implementation of a semiconductor optical amplifier.

Massive optical sensor networks gained a lot of attention in recent years. They offer new advances in the fields of smart structures and health monitoring. All serial optical sensor networks rely on multiplexing techniques that provide huge amounts of sensors in a single optical fiber. Wavelength-division multiplex (WDM) which has been established in many applications, is restricted to the spectral width of the used light source that needs to be shared by several non-overlapping fiber-Bragg-grating (FBG) spectra. Time-division multiplex (TDM) uses short impulses and relies on different sensor round trip delays to distinguish each single FBG. These short impulses and long round trip times lead to a low signal-to-noise ratio (SNR). Optical frequency-domain reflectometry (OFDR) offers a high spatial resolution of FBGs but only within a short fiber length. This contribution deals with a code-division multiplex (CDM) interrogation technique that provides numerous sensors in a single optical fiber, a better SNR, and a long range of distributed sensing points. It requires codes with good autocorrelation behavior which is characterized by certain criteria. The detectable criteria are limited which narrows significantly a search for best possible codes for the interrogation system. In this contribution, practical implementation limits such as the trigger timing and the achievable SNR are studied. Based on the introduced SNR definitions for CDM and WDM systems, a direct comparison is possible and it shows the superiority of the proposed CDM scheme. A network with 25 sensors operating at the same wavelength can provide a 2.67 dB improvement compared to WDM

Massive optical sensor networks gained a lot of attention in recent years. They offer new advances in the fields of smart structures and health monitoring. All serial optical sensor networks rely on multiplexing techniques that provide huge amounts of sensors in a single optical fiber. Wavelength-division Multiplex (WDM) which has been established in many applications, is restricted to the spectral width of the used light source that needs to be shared by several non-overlapping Fiber-Bragg-Grating (FBG) spectra. Time-division Multiplex (TDM) uses short impulses and a different optical path length between all optical sensors, to distinguish each single FBG. Short impulses lead to a low Signal-to-Noise Ratio (SNR). Optical Frequency Domain Reflectometry (OFDR) offers a high spatial resolution of FBGs within a short fiber length. This contribution deals with a Code-division Multiplex (CDM) interrogation technique that provides numerous sensors in a single optical fiber, a better SNR, and a long range of distributed sensing points. It requires codes with good autocorrelation behavior which is characterized by certain criteria. The detectable criteria are limited which narrows significantly a search for best possible codes for the interrogation system.

Massive optical sensor networks applied in smart structures or for health monitoring have attracted a lot of attention within recent years. Different multiplexing techniques for interrogation have been investigated in order to increase the number of sensors within a network. Whereas optical frequency-domain reflectometry (OFDR) offers a high resolution at short distances, time-division multiplex (TDM) provides long distance sensing at the cost of lower resolution. In this contribution, a code-division multiplex (CDM) interrogation scheme is analysed which overcomes these restrictions. A massive enlargement of fiber-Bragg-grating (FBG) sensors in an optical fiber is obtained allowing spectral overlapping. While TDM makes use of extreme short impulses, CDM profits from several light pulses that form a code. This leads to an improved signal-to-noise ration (SNR) of the measurement. Optical CDM, applied in this work, is realized with the aid of a transmitter and a receiver modulator. Those provide the required optical correlation. In this paper the non-ideal optical correlation is studied when imperfect modulators are utilized. Despite of non-ideal components our theoretical approach, computer simulations and measurements outline that CDM interrogation of spectrally overlapping FBGs is robust and applicable. Scaling effects based on the extinction of both modulators and an error introduced particularly by an offset of the transmitter modulator are pointed out.

Citation Götten, Marek (2022). Massive Quasi-Distributed FBG Sensor Networks Interrogated with a CDM-WDM System. Thesis (Doctoral), E.T.S.I. y Sistemas de Telecomunicación (UPM). https://doi.org/10.20868/UPM.thesis.70352.