Selecting Electro-Optic Modulators for BOTDA Systems
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).
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.
Figure 1. Block diagram of a simplified BOTDA System
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.
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.
Figure 2. Mach-Zehnder modulation transfer function (MTF) of the EOM1 for Pulse Generation
Figure 2 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.
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.
|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|
Figure 3. MXER-LN-10: Very High Extinction Ratio, High Bandwidth, High Stability, Low Vπ & Low Loss
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.
Figure 4. Generation of Low and High Pulse Duration at Low Repetition Rate with DR-PL-10-MO
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.
Figure 5. Stable, Repeatable 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.
Figure 6. Set up and basic principle of the DSB-SC generation with Intensity Modulator, Driver Amplifier and 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.
|Electro-Optic Bandwidth||12 GHz typical|
|Vπ @ 10 GHz||Max. 7 V|
|Input Optical Power||Max. 20 dBm (100 mW)|
|Insertion Loss||3.5 dB|
|Cut-off frequency||11 GHz typical|
|Output Voltage||12.5 V typical|
|Saturated Output Power||Min. 26 dBm|
|Noise Figure||3 dB|
The highly versatile MX-LN-10 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.
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.
Figure 7. iXblue Packaged Fiber Bragg Grating filter for BOTDA DSB-SC sideband filtering
Other filter type