Distributed Temperature Sensing (DTS)

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Brillouin based sensors have emerged as an optical fiber sensing technology that is growing in importance for distributed sensing of temperature and strain. The numerous applications in industrial temperature sensing, structural health monitoring and pipeline or slope monitoring place increasing demands on the performance of the Brillouin Optical Time Domain Analysis (BOTDA).

Selecting Electro-Optic Modulators and Fibers for BOTDA Systems

Electro-Optic Modulators (EOM) are the most critical components in the optical sub-systems of BOTDA systems and the selection of EOM with the right specifications and operating conditions greatly affects the overall performance, consistency and reliability.

The sensing element, ie the fiber, must also be carefully selected depending of the targeted temperature range and the presence of radiation.

Recent Advances in Brillouin Optical Time Domain Reflectometry

▲ Botda diagram

There are 2 EOMs in a conventional BOTDA system: EOM1 for pulse generation (Pump signal) and EOM2 for generating a seed signal shifted by the Brillouin Frequency shift of the sensing fiber (relative to the Pump Optical Frequency). Under normal conditions, The Brillouin Frequency shift is 10.820 GHz for SMF-28. By seeding a probe signal into the opposite end of the sensing fiber, Brillouin Backscattering becomes a stimulated process with higher SNR.

More info on BOTDR Measurement Techniques and Brillouin Backscatter Characteristics of Corning Single-Mode Optical Fibers

 

The important features of a BOTDA system are the Spatial Resolution, Temperature/Strain Resolution, Maximum Measurement Range and the Speed of Measurement.

Spatial resolution is the minimum resolvable distance between 2 data points in a distributed fiber sensor. Generally, this is dependent on the pulse duration and reducing the pulse duration will increase the spatial resolution. For conventional BOTDA systems, the typical spatial resolution is ~1 m.  As the pulse duration is reduced below 10 ns, it becomes increasingly difficult to measure the peak and determine Brillouin Frequency Shift (BFS) as the backscattered signal is broadened. However, improvements to the conventional BOTDA such as the Double Pulse or DP-BOTDA can achieve sub-meter or 20cm Spatial Resolution with pulse duration and separation of 2 ns and 5 ns respectively.

The figure shows the principle of the use of the MTF of an Intensity Modulator for generating optical pulses. For BOTDA applications with pulse duration of the order of ~1ns, an EOM bandwidth of 10 GHz with rise/fall time of less than 100 ps matches the requirements. It is also critically important for the EOM to be able to generate and maintain High Extinction Ratio (ER) for BOTDA systems.

When the CW laser is carved into short pulses, the finite ER of the EOM will cause CW leakage. The pulse & leakage is amplified by an optical amplifier and launched into the sensing fiber. ER > 32dB is required for long-range BOTDA to mitigate depletion effects.

Effects of pump pulse extinction ratio in Brillouin optical time-domain analysis sensors

The Extinction Ratio (ER) of the pump pulses directly affects the Signal-to-Noise-Ratio of the entire BOTDA measurement systems. A High Extinction Ratio EOM therefore directly impacts the maximum measurement range and the speed of measurement of the BOTDA system by reducing averaging.

The MXER-LN series of intensity modulators is a family of high-performance modulators exhibiting superior Extinction Ratio. Their specific design relies on iXblue “Magic Junction” (patent n° US2008193077). MXER-LN series intensity modulators are key devices in BOTDA applications where a combination of high extinction and high bandwidth is required.

 

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Extinction Ratio 30 dB, 35 dB or 40 dB
Vπ @ 10 GHz Max. 7 V
Input Optical Power Max. 20 dBm (100mW)
Insertion Loss 3.5 dB
Wavelength 1530-1625 nm
 
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Extinction Ratio 30 dB, 35 dB or 40 dB
Vπ @ 10 GHz Max. 7 V
Input Optical Power Max. 20 dBm (100mW)
Insertion Loss 3.5 dB
Wavelength 1530-1625 nm
 

The DR-VE-10-MO RF drivers are amplifier modules designed to drive LiNbO3 optical modulators for pulse, analog or digital applications. In this case, they are used to generate undistorted optical pulses.

In BOTDA systems, electrical pulsed signals have long duty cycles. In order to generate clean optical pulses with sharp edges, sustained high and low levels and no overshoot while maintaining high ER, BOTDA pulsed signals require specific RF amplifiers with the suitable characteristics.

The DR-VE-10-MO driver is optimized for low and high Pulse Repetition Frequency (PRF) signals from 10 Hz to 1 GHz. The bandwidth up to 12 GHz accommodates 70 ps narrow pulse width with short rise and fall time (down to 24 ps) and can withstand longer pulses up to 300 ns.

The DR-VE-10-MO drivers come in compact connectorized modules that match directly with iXblue’s EOM. They use a single voltage power supply for ease and safety of use and feature a Graphical User Interface which integrates output voltage control for maximum flexibility.

In operations, the generation of pulses with repeatable characteristic for reliable BOTDA systems for industrial sensing applications such as structural health monitoring and pipeline monitoring requires stable operation of the optical hardware over extended periods and robustness against perturbations.

The Mach-Zehnder interferometer in the EOM is subject to bias drift caused by thermal changes, thermal inhomogeneity, aging, photo refractive effects, static electrical charge accumulation and other environmental effects. The bias drift causes the transfer function to deviate the operating point for the modulation signal. As a result, the output pulse’s characteristics such as the ER and pulse duration could be disturbed.

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▲ Stable reastable high extinction rate pulse generation against bias drift.

 

The drift of the DC bias can be actively monitored and controlled using the iXblue’s MBC (Modulator Bias controller) solutions: the bench-top instrument MBC-DG-LAB or MBC-DG-board locks the operating point of Mach-Zehnder modulators for BOTDA applications.

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▲ Set up basic principle of the DSB-SC generation with Intensity Modulator Driver Amplifier MBC

 

In conventional BOTDA systems, a second EOM is required to generate the seed signal with the optical frequency that is locked to the Pump pulses’ optical frequency but shifted by the Brillouin frequency shift (BFS), typically 10.8 GHz for SMF sensing fibers. The frequency of the seed signal is also scanned or tuned by steps to detect changes in the Brillouin Gain Spectrum (BGS) around the sensing fiber’s BFS. Typically, frequency steps of 500 kHz, 1 MHz or 5 MHz are used depending on the required resolution.

There are several options to generate of this tunable optical frequency shifted using EOM, 2 of the most commonly used are the Double Sideband- Suppressed Carrier Modulation (DSB-SC) with Optical filtering and the Carrier-Suppressed Single Sideband (CS-SSB) Modulation.

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Electro-optic Bandwidth 12 GHz typical
Vπ @ 10 GHz Max. 7 V
Input Optical Power Max. 20 dBm (100 mW)
Insertion Loss 3.5 dBm
Wavelength 1530-1625 nm
 
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Electro-optic Bandwidth 12 GHz typical
Vπ @ 10 GHz Max. 7 V
Input Optical Power Max. 20 dBm (100 mW)
Insertion Loss 3.5 dBm
Wavelength 1530-1625 nm
 
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Cut-off frequency 11 GHz typical
Output voltage 12,5 V typical
Gain 30 dB
Satured Output Power Min. 26 dBm
Noise Figure 3 dB
 
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Cut-off frequency 11 GHz typical
Output voltage 12,5 V typical
Gain 30 dB
Satured Output Power Min. 26 dBm
Noise Figure 3 dB
 

The highly versatile MX-LN-10 series of intensity modulators has a X-cut design resulting in unmatched stability in a wide range of operational conditions, as well as a zero-chirp performance. iXblue’s proprietary waveguide design offers a low insertion loss combined with a high contrast. The MX-LN-10 is ideally suited for DSB-SC for BOTDA systems due to the high bandwidth, stability and low insertion loss.

The DR-AN-10-HO is a wideband RF amplifier module designed for analog applications at frequencies up to 11 GHz. The DR-AN-10-HO is characterized by a low Noise Figure and a linear transfer function whose 1 dB compression point is above 23 dBm. It exhibits flat Group Delay and Gain curves with reduced ripple over the entire bandwidth. This amplifier module is ideally suited to drive MX-LN-10 for DSB-SC.

The long-term stability against bias drift can be ensured using the MBC-DG-LAB and MBC-DG-board that can lock the operating point of Mach-Zehnder modulators.

Other components for the BOTDA system include the Polarization Scrambler and FBG Sideband Filter.

Polarization Scrambler: Brillouin Scattering is sensitive to polarization. This problem can be mitigated by polarization scrambling the SOP of the signals. Polarization scrambling improves SNR, reduces measurement time resulting in improve accuracy, resolution and measurement range. The PSC-LN are a compact and high speed electro-optic Polarization Scramblers. These integrated-optic devices feature a low-loss single-mode waveguide and can modulate the polarization at frequencies ranging from DC to more than 10 GHz.  They operate over a broad optical bandwidth of more than 100 nm.

Download the iXblue Polarization SCrambler (PSCLN) presentation (PDF):

FBG Sideband Filter: Fiber Bragg Grating filters can be used as DSB-CS sideband filters for BOTDA systems. The IXC-FBG is a Fiber Bragg Grating UV-printed into an optical fiber. As a result of know-how acquired over many years, iXblue can supply highly customized FBGs, that can be packaged into environmentally stable optical filters for removing the unwanted DSB-CS sideband in a BOTDA system.

▲ iXblue Packaged Fiber Bragg Grating filter for BOTDA DSB-SC sideband filtering

Sensing Fiber: In basic cases, standard SMF fiber would be enough; but as soon as temperature exceeds 85°C, the standard coating of fibers will not withstand such high temperature and would melt. iXblue offers a full set of fibers with acylate high temperature coating , up to 130°C, polyimide coating up to 300°C or even aluminium coating up to 400°C. In radiative environment, as space or nuclear, specific fibers with guaranteed Radiation Induced Attenuation (RIA) would be needed.

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Fiber Coating Type Max. Temperature
Acrylate (Telecom Type) 85°C
Acrylate High Temperature 150°C
Polyimide 300°C
Aluminium 400°C
  More info
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Fiber Coating Type Max. Temperature
Acrylate (Telecom Type) 85°C
Acrylate High Temperature 150°C
Polyimide 300°C
Aluminium 400°C
  More info

Dedicated products

Pulse Generation

Product Specification Datasheet
O-Band
MXER1300-LN-10-PD-P-P-FA-FA-30dB 1310 nm band
10 GHz High Extinction Ratio Intensity Modulator
PDF More info
ANalog Drivers
DR-VE-10-MO 12 GHz VErsatile RF Amplifier PDF More info
DiGital MZ Modulator Bias Controller
MBC-DG-LAB-A1 PDF More info

Frequency Generation

Product Specification Datasheet
ANalog Drivers
DR-AN-10-HO 10 GHz Analog High Output Voltage Driver Module PDF More info

Other Components

Product Specification Datasheet
Polarization switches and scramblers
PSC-LN-0.1-P-S-FA-FA C-band low frequencies Polarization Scrambler PDF More info
Optical Filters
IXC-FBG Other filter type More info

Sensing Fibers

Product Specification Datasheet
Rad Hard Fibers
IXF-SM-1550-80-0.19-PI Low radiation level
Polyimide coating
PDF More info
IXF-SM-1550-80-0.24-PI Low radiation level
Polyimide coating
PDF More info
IXF-SM-1550-125-0.12-PI Low radiation level
Polyimide coating
PDF More info
IXF-SM-1550-125-0.14-PI Low radiation level
Polyimide coating
PDF More info
IXF-SM-1550-125-0.28-PI Low radiation level
Polyimide coating
PDF More info
IXF-SM-1550-125-0.14-HT Low radiation level
Acrylate coating
PDF More info
IXF-RAD-SM-1550-0.14-HT High radiation level
Acrylate coating
PDF More info
IXF-RAD-SM-1550-0.14-PI High radiation level
Polyimide coating
PDF More info
IXF-RAD-SM-1550-0.14-AL High radiation level
Aluminum coating
PDF More info
Passive Fibers
IXF-MMF-105-125-0.22-RAD 105 µm core diameter
Rad Hard Space Grade
PDF More info

Publications

  • Recent Advances in Brillouin Optical Time Domain Reflectometry

    Q. Bai, Q. Wang, D. Wang, Y. Wand, Y. Gao, H. Zhang (Key Lab of Advanced Transducers and Intelligent Control Systems); M. Zhang (College of Physics & Optoelectronics); B. Jin ( State Key Lab of Coal and CBM Co-mining)

    MDPI Sensors Vol 19 – Issue 8 – 18th April 2019

    In the past two decades Brillouin-based sensors have emerged as a newly-developed optical fiber sensing technology for distributed temperature and strain measurements…

    Read more
  • Effects of pump pulse extinction ratio in Brillouin optical time-domain analysis sensors

    H. Iribas, J. Mariñelarena, J. Urricelqui, A. Loayssa (nstitute of Smart Cities); C. Feng, T Schneider (Institut für Hochfrequenztechnik)

    Optics Express Vol 25 – No 22 – 30 th October 2017

    We report on two previously unknown non-local effects that have been found to impair Brillouin optical time-domain analysis (BOTDA) sensors that deploy limited extinction ratio (ER) pump pulses…

    Read more
  • Combined effect of radiation and temperature: towards optical fibers suited to distributed sensing in extreme radiation environments

    G. Mélin, A. Barnini, A. Morana, S. Girard, P. Guitton & R. Montron

    30th Conference on Radiation and its Effects on Components and Systems (RADECS 2019) – September 2019

    Combined effect of radiation and temperature on the response of polyimide coated radiation hardened single-mode fibers is investigated in the context of distributed monitoring of large nuclear infrastructures. Radiation induced attenuation (RIA) is evaluated for doses ranging from 1 to 10 MGy(SiO2) and temperatures up to ~250 °C.

    Learn more
  • Radiation resistant single-mode fiber with different coatings for sensing in high dose environments

    G. Mélin, P. Guitton, R. Montron, T. Gotter, T. Robin, B. Overton, A. Morana, S. Rizzolo & S. Girard

    IEEE Transactions on Nuclear Science – 10 December 2018

    A radiation single-mode optical fiber has been specifically developed for distributed sensing in harsh environments associated with MGy(SiO2) dose radiation. Different types of coating have been used: acrylate, polyimide, aluminum that allow extending the range of accessible temperatures up to 400°C…

    Learn more

Application Notes

BOTDR Measurement Techniques and Brillouin Backscatter Characteristics of Corning Single-Mode Optical Fibers

(396 KB)
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iXblue Polarization Scrambler

(1001 KB)
Download

AN – Temperature Sensor – BOTDA

(899 KB)
Download
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