Optical Time Domain Reflectometry System Research Articles

Coherent optical time domain reflectometry by logarithmic detection and timed random frequency hopping

A scheme using timed random frequency hopping and signal logarithmic mean method is proposed and experimentally demonstrated in a coherent optical time domain reflectometry (OTDR) system to reduce the fading noise of the OTDR trace and simplify the signal processing procedure. The timed random frequency hopping is realized by randomly changing the driving current of the laser at certain time points. By this method, the fading noise of OTDR trace can be reduced to be 1/5 of that without using it. Also, a radio frequency power detector (RFPD), whose output voltage has linear relationship with the input logarithmic RF power, is used to extract the power of the RF signals from the balanced photodetector. Then, a data acquisition card directly captures and adds the digital voltage signals from the RFPD to reduce the fading noise and improve the measurement dynamic range. Compared with synchronous and asynchronous frequency hopping scheme, the proposed method is of high efficiency.

Fault monitoring in passive optical network using code division multiplex-based dual-OTDR traces comparison

A dual optical time domain reflectometry (OTDR) system, which employs two different continuous waves at the optical line terminal and a pair of fiber Bragg gratings at the end of each optical network unit, is proposed in a time-division multiplexing passive optical network (PON). The proposed scheme accomplishes the fiber fault monitoring by comparing the different wavelength’s testing curves. Complete complementary code is utilized to measure multiple wavelength signals simultaneously with only one receiver and to improve the dynamic range of this system. The PON system consisting of 20 km feeding fiber and a 1:16 splitter is investigated by the experiments. The experimental results show that the faulty branch can be successfully identified by using our scheme. What is more, we also demonstrate that our scheme can be applied to the multi-stage PON.

Distributed Optical Fiber Radiation Sensing in a Mixed-Field Radiation Environment at CERN

In paper we present the results from distributed optical fiber radiation sensing measurements at the “Cern High energy AcceleRator Mixed field facility”, CHARM. The measurements have been carried out on a multimode (MM) radiation sensitive P-doped optical fiber. The distributed measurements have then been carried out by means of an Optical Time Domain Reflectometry (OTDR) system using different interrogation and acquisition parameters to understand their influence on the results. We demonstrate the possible localization of the total dose with meter scale spatial resolutions and dose resolution down to 10–15 Gy. The obtained distributed dose profiles are validated by the benchmark with RadFET dosimeters and appear very promising for high energy accelerators and physics experiments as well as for industrial applications.

Enhanced ν -optical time domain reflectometry using gigahertz sinusoidally gated InGaAs/InP single-photon avalanche detector

Optical time domain reflectometry (OTDR) is one of the most successful diagnostic tools for nondestructive attenuation measurement of a fiber link. To achieve better sensitivity, spatial resolution, and avoid dead-zone in conversional OTDR, a single-photon detector has been introduced to form the photon-counting OTDR ( ν -OTDR). We have proposed a ν -OTDR system using a gigahertz sinusoidally gated InGaAs/InP single-photon avalanche detector (SPAD). Benefiting from the superior performance of a sinusoidal gated SPAD on dark count probability, gating frequency, and gate duration, our ν -OTDR system has achieved a dynamic range (DR) of 33.4 dB with 1 μ s probe pulse width after an equivalent measurement time of 51 s. This obtainable DR corresponds to a sensing length over 150 km. Our system has also obtained a spatial resolution of 5 cm at the end of a 5-km standard single-mode fiber. By employing a sinusoidal gating technique, we have improved the ν -OTDR spatial resolution and significantly reduced the measurement time.

A hybrid Φ/B-OTDR for simultaneous vibration and strain measurement

A hybrid phase-sensitive optical time domain reflectometry (Φ-OTDR) and Brillouin optical time domain reflectometry (B-OTDR) system which can realize simultaneous measurement of both dynamic vibration and static strain is proposed. Because the Rayleigh scattering light and spontaneous Brilliouin scattering light are naturally frequency-multiplexed, the heterodyne asynchronous demodulation of frequency shift keying (FSK) in optical fiber communications is utilized, and the demodulations of the two scattering signals are synchronized. In addition, the forward Raman amplification is introduced to the system, which not only makes up for the deficiency of spontaneous Brilliouin scattering based distributed fiber sensor, but also has the merit of the single end measurement of B-OTDR. The designed Φ/B-OTDR hybrid system has the sensing range of 49 km with 10 m spatial resolution. The vibration and strain experiments show that this hybrid system has great potential for use in long-distance structural health monitoring.

Temporal depolarization suppressed POTDR system for quasi-distributed instantaneous intrusion sensing and vibration frequency measurement

Theoretical investigation on the mechanism of temporal depolarization effect in a polarization optical time-domain reflectometry (POTDR) system is conducted. First, the relationship between the dynamic range of the incomplete POTDR system and the degree of polarization (DOP) of the backscattered optical signal is derived. Second, a temporal depolarization suppressed POTDR system is proposed by introducing quasi-distributed Fresnel reflections. Third, the DOP distribution of the proposed system is demonstrated by numerical simulations and experiments. It is shown that the temporal depolarization effect can be significantly suppressed to maintain sufficient dynamic range for fast and accurate polarimetric measurement. The signal-to-noise ratio (SNR) of the received optical signal is also improved with the help of Fresnel reflections. With high DOP and improved SNR, instantaneous intrusion sensing and accurate vibration frequency measurement along the fiber link have been achieved without performing any data averaging.

Fundamentals of Optical Fiber Sensing Schemes Based on Coherent Optical Time Domain Reflectometry: Signal Model Under Static Fiber Conditions

The paper develops a statistical model for the signals received in phase-sensitive optical time domain reflectometry (OTDR) probed by highly coherent sources. The backscattering process is modelled by a set of discrete scatterers with properly chosen parameters. Explicit equations for calculating the amplitude and the phase of the backscattered signal are obtained. The developed model predicts spectral and autocorrelation characteristics of the amplitude signals that are validated by experimental results. Characteristics of the phase signals, practicable for studying the sensing applications of the OTDR system, are presented and studied as well, demonstrating good correspondence with experiment. A more detailed modelling of distributed vibration sensing systems and their response to disturbances along an optical fiber will be possible as an extension of the developed formalism.

The Theoretical Analysis and Design of Coding BOTDR System With APD Detector

The signal-to-noise ratio and the coding gain of Brillouin optical time domain reflectometry system with APD detector are analyzed. In coding distributed optical fiber sensing system, there are a large number of multipath interferences (MPIs), which are proportional to the code length. The random shot noise caused by MPI will be generated before decoding and cannot be eliminated by decoding process. In addition, the shot noise power is proportional to the square of the code length; the thermal noise power increases linearly with the code length, then, the increase of the speed of the coding gain will be decreased with the increase of the code length. When the shot noise is much larger than the thermal noise, the coding gain will not be increased with the increase of the code length, but will maintain a constant value. This conclusion is in contradiction with the traditional conclusion. At the same time, the shot noise depends on the pulsewidth and power, but the thermal noise is independent on the pulsewidth and the power, so there exists an optimum code length for specific system.

Relationship between temporal and spatial reflectogram scales in fiber optical time domain reflectometry systems

Correspondence between time instant of the reflectogram and position of the optical fiber sector at which the scattering occurred is analyzed for the longitudinally inhomogeneous optical fiber and the external perturbation of the general kind. Conditions on the rate of the refractive index and external perturbation dependence on the longitudinal coordinate are stated that imply explicit formulae for shifts of scattering segments of various position with respect to the external perturbation. The effects are numerically estimated in the cases of geometric elongation and heating of the fiber.

Development of an Intrusion Sensor Based on a Polarization-OTDR System

In this paper, we demonstrate the possibility of using a polarization optical time domain reflectometry (POTDR) system as an intrusion sensor. This system measures the evolution of the state of polarization (SOP) along the fiber length. In the typical application of fence monitoring, an intrusion leads to a modification of the SOP, which appears, after a temporal averaging, as a detectable flattening of the POTDR trace starting from the intrusion position. Experiments show that an intrusion leading to a 1.5 cm fiber displacement can be detected, using a 5 s averaging and 50 ns pulses with an error on the intrusion localization of the order of 5 m.

Amplified OTDR Systems for Multipoint Corrosion Monitoring

We present two configurations of an amplified fiber-optic-based corrosion sensor using the optical time domain reflectometry (OTDR) technique as the interrogation method. The sensor system is multipoint, self-referenced, has no moving parts and can measure the corrosion rate several kilometers away from the OTDR equipment. The first OTDR monitoring system employs a remotely pumped in-line EDFA and it is used to evaluate the increase in system reach compared to a non-amplified configuration. The other amplified monitoring system uses an EDFA in booster configuration and we perform corrosion measurements and evaluations of system sensitivity to amplifier gain variations. Our experimental results obtained under controlled laboratory conditions show the advantages of the amplified system in terms of longer system reach with better spatial resolution, and also that the corrosion measurements obtained from our system are not sensitive to 3 dB gain variations.

Determination of thermal residual strain in cabled optical fiber with high spatial resolution by Brillouin optical time-domain reflectometry

The thermal residual strain induced in the cabled optical fiber is a very important factor for evaluating the reliability of optical fiber cables. In order to determine the distributed thermal residual strain in cabled optical fiber, a measurement method based on Brillouin optical time-domain reflectometry (BOTDR) system is proposed in the article. Thermal characteristics of residual strain along cabled optical fibers are investigated theoretically and experimentally based on Brillouin frequency shift (BFS) detection. The thermal residual strain along the cabled fiber, if any, can be determined with a high spatial resolution that is equal to that of the BOTDR system and can be less than a few meters. A double-coated fiber in loose optical cable was used as the test cabled optical fiber, and the experimental results were in good agreement with those predicted from the theory. It has been found that the fiber residual strain increases linearly with decreasing temperature in the range from 50 to −50 °C.

Distributed Vibration Sensor Based on Coherent Detection of Phase-OTDR

We developed a distributed vibration sensor by using heterodyne detection and signal processing of moving averaging and moving differential for the phase optical time domain reflectometry system. The broadband acoustic frequency components generated by pencil-break vibration have been measured and identified in location by our distributed vibration sensor for the first time. Pencil break measurement is a standard technique to emulate the acoustic emission of cracks in concrete or steel bridges for early crack identification. The spatial resolution is 5m and the highest frequency response is 1 kHz, which is limited by the trigger frequency of data acquisition card. This new sensing system can be used for vibration detection of health monitoring of various civil structures as well as any dynamic monitoring requirement.

Novel Technique to Improve Spatial Resolution in Brillouin Optical Time-Domain Reflectometry

A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e., the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system.

New sensing mechanisms using an optical time domain reflectometry with fiber Bragg gratings

A new optical time domain reflectometry (OTDR) sensing mechanism combined with fiber Bragg gratings (FBGs) is presented. The intensity-based optical fiber sensor is implemented between a pair of FBGs in an OTDR system. Based on the ratio from two reflecting intensities of the reference and the sensing FBGs, the appropriate measurable parameters associated with the intensity variations can be detected. By applying the Fresnel reflection signals caused by FBGs as the measuring indicator, the dynamic range and signal-to-noise ratio of an OTDR FBGs system are improved in comparison to the traditional method. Besides, by adapting the system using a double-light-source scheme, the spatial resolution limited by the ghost zone in a traditional OTDR system is also resolved. The sensors in an OTDR FBGs system could be configured for multiplexing and immunity to the variations from the light source and the lead-in/out fibers caused by the external loads. The new method using single-light-source and double-light-source schemes has been validated by fiber bending loss and refractive index measurements.

15 μm photon-counting optical time-domain reflectometry with a single-photon detector based on upconversion in a periodically poled lithium niobate waveguide

Optical time-domain reflectometry (OTDR) is one of the most powerful tools in the characterization of optical fiber links. We demonstrate a photon-counting OTDR system at 1.5 microm with a single-photon detector, which combines frequency upconversion in a periodically poled lithium niobate waveguide and a silicon avalanche photodiode. The system exhibits high sensitivity, good spatial resolution, and short measurement time.

Extended-range optical time domain–reflectometry system at 165??µm based on delayed Raman amplification

We describe both theoretical and experimental results obtained in an investigation of a new technique for increasing the dynamic range of 1.65-µm optical time domain–reflectometry (OTDR) systems. The technique utilizes delayed Raman amplification of a 1.65-µm signal pulse by a 1.53-µm pump pulse. Amplification occurs when the two pulses overlap, and this position is determined by the initial delay between the pulses and the fiber dispersion. An increase in dynamic range of 17.5??dB has been observed, and the OTDR backscattered Rayleigh signal was detected up to 100??km. No significant noise penalty is introduced as a result of the directionality of the Raman gain.

Input Power Conditions for Distributed Fiber Optic Temperature Sensors in Raman Optical Time-Domain Reflectometry

The critical power of the input pulse in which spontaneous Raman scattering is not seriously disturbed by stimulated Raman scattering in a fiber optic temperature sensor has been investigated experimentally and theoretically. The critical power determines the critical distance, which is nearly equal to the distance where optical time-domain reflectometry (OTDR) signal of the stimulated Raman scattering becomes maximum. From this fact, a new method to determine the critical input power with the OTDR system has been proposed, which may be applicable to most distributed fiber optic temperature sensors. It has been determined that the critical power of the input pulse is 4 watts for an infinitely long silica fiber.

Characterization of silica-based waveguides with an interferometric optical time-domain reflectometry system using a 13-μm-wavelength superluminescent diode

Backscattering of silica-based glass waveguides is characterized for the first time to our knowledge by using an interferometric optical time-domain reflectometry system. High spatial resolution, as short as 15 microm, is obtained by using a newly developed 1.3-microm-wavelength superluminescent diode. Scattering centers produced by waveguide irregularities are clearly observed in glass optical waveguides. Waveguide loss and bend loss in the curved regions are estimated from the backscattered light intensity distribution.

Optical reflectometry with micrometer resolution for the investigation of integrated optical devices

An optical time-domain reflectometry (OTDR) system using an infrared subpicosecond pulse source in conjunction with balanced heterodyne detection is discussed. Experimental results show a density of more than 100 dB and a resolution of 60 mu m in air. Taking advantage of the large tuning range of the laser system, it is possible to improve the resolution to less than 10 mu m. The applicability of the OTDR system for the diagnostics of integrated optical devices is demonstrated for a simple GaAs waveguide structure. >