Border/Perimeter Security
Hydrophone
Structural Health Monitoring
Quantum Cryptography
Narrowband Spectroscopy

 

 

Border Security Systems Using Fiber Sensing

Technology

The challenge of securing a long international boundary or strategically important installation can extend over tens to thousands of miles. Optical fiber as a sensing element is a cost-effective means of covertly detecting intruders along such lengthy boundaries. Commercial systems are now available that can detect a personnel or vehicular intruder within 5-20m at spacings of 10 m along a 50 km span of fiber buried approximately 12 inches underground. Deployment of such systems is then relatively straightforward, with the fiber monitoring equipment housed in one spot at the end of each fiber.

The technology that enables the success of these acoustic sensing systems relies on the measurement of a small change of strain in the fiber, caused by sound waves propagating through the ground. The strain change is detected as a shift in a scattering peak that results from backscattered laser light as it is pulsed down the fiber. The precise location of a given alarm condition can be determined based on the timing of the returned laser pulses.

Distributed optical fiber sensing systems become more advantageous with longer and longer reaches. The cost of camera-based surveillance systems scales linearly with border length, while the only incremental cost in fiber sensing systems is additional fiber. With fiber-based systems, simultaneous disturbances can be detected, and the systems are therefore difficult to divert or jam.

The Lepton Advantage

Lepton’s superior signal/noise performance and optical dynamic range can extend the benefits of distributed optical fiber sensing to even greater distances for next generation border sensing systems. The narrow optical bandwidth of Lepton’s sensors also offer a natural filtering advantage in detecting the smaller Brillouin-shifted signal relative to the strong Rayleigh backscattered signal. Lepton’s detectors deliver performance advantages of up to 10X in both signal/noise and dynamic range relative to currently available detectors at 1.55 µm.

References and Links

“Distributed Fiber-Optic Sensors: Principles and Applications”, by A. Hartog in “Optical Fiber Sensor Technology”, K.T.V. Grattan and B.T. Meggitt, Kluwer: Boston (2000).


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Acoustic Sensing Using Fiber Optics
Seismic and Hydrophones

Technology

Seismic

The characterization of Earth-based vibrational activity is used in both the scientific and commercial realms. Seismic data is employed extensively in the Exploration and Production of new oil and gas fields. It is also used by geophysicists to understand and model future seismic activity, in order to better predict earthquakes. Fiber optic acoustic sensing techniques present an attractive technology alternative to more commonly used electrical sensing of these vibrations, due to the higher reliability of optical fiber relative to buried electronics. Another advantage is the opportunity for distributed sensing, or the ability to measure vibrations along the entire length of the fiber cable, as opposed to networks of point sensors used in an electrical system.

An optical fiber is extremely sensitive to small changes in strain caused by Earth’s vibrations. This principle can be exploited for seismic measurement through the use of interferometric techniques. In a simple example, a sensor can be constructed using a pair of fibers at some separation. A vibration will induce a slight difference in optical path length between the fibers, which can be detected as a phase difference between returned laser pulses transmitted through the fibers. Correlation of the phase difference with strain gives a measure of the strength of the vibration.

Hydrophones

Similar technical concepts are used to detect sound waves in undersea applications. Passive acoustic sensing can be achieved through the use of towed fiber optic arrays, which are growing in popularity as they displace piezoelectric or piezoceramic arrays for harbor defense applications. The reliability of fiber as the sensing element also allows for cost-effective permanent installations of fiber arrays on the ocean floor.

The Lepton Advantage

As towed and permanent fiber arrays become larger and more complex, Lepton’s superior signal/noise and dynamic range performance will be able to maintain current optical budgets over much longer fiber spans. These longer spans will provide flexibility to current system designs and will also enable larger area coverage for permanent ocean floor fiber arrays.

References and Links

T.K. Stanton, R.G. Pridham, W.V. McCollough, M.P. Sanguinetti, J. Acous. Soc. Am., 66 (1979), 1893. J.E. Parsons, C.A. Cain, J.B. Fowlkes, J. Acous. Soc. Am., 119 (2006), 1432. G.A. Cranch, R. Crickmore, C.K. Kirkendall, A. Bautista, K. Daley, S. Motley, J. Salzano, J. Latchem, P.J. Nash, J. Acous. Soc. Am., 115 (2004), 2848.

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Structural Health Monitoring

Technology

A structural health monitoring system comprises a distributed array of sensors, embedded inside or attached to a structure, along with the hardware to transmit or analyze the data from the sensors. The purpose of the system is to continuously or periodically monitor the integrity of the structure, or to measure deformation or strain. Over the long term, the output of the process is updated information about the ability of the structure to perform its intended function in light of aging and degradation from the environment. After extreme events, such as earthquakes or blast loading, the system can be used for rapid status screening.

The applications for “smart structure” technology are widespread – including buildings, the decks and pylons of bridges, dams, storage tanks, aircraft, ships, submarines and ground vehicles. Benefits derived from the technology include improved public safety, reduced maintenance costs, improved readiness, and near real-time information on the health of the structure.

Electrical sensor technology

Many systems with electrical sensors employ piezoelectric sensors, which produce a voltage in response to deformation. The sensors are inexpensive and the associated systems have been developed. Downsides to this approach include the need for two wires (or a coaxial cable) running to each sensor, and in some cases an associated requirement for a preamplifier and signal conditioner at each sensor (also requiring wires). The wires must be shielded against electromagnetic interference (EMI). If there is a need for a large number of sensing points, the cost and complexity of the wiring becomes problematic.

A recent Navy SBIR solicitation (N091-077) described this type of technology as intrusive, heavy, susceptible to EMI, having many failure points and being overly complicated.

Optical technology

An alternative approach is to use an optical fiber for sensing. The fiber may have multiple Fiber Bragg Gratings (FBGs), each one acting as an individual sensor, or the system may use Brillouin scattering in the fiber to accomplish distributed measurement – allowing data to be gathered at any point along the length of the fiber. Fibers have little loss, and provide enhanced sensitivity when coupled with advanced demodulation techniques. There is no need for preamplifiers or signal conditioners, and the fibers are immune to EMI. The resulting system is lightweight, and can accommodate multiple wavelengths (data channels) on a single fiber.

Lepton is currently engaging partners to develop a high-performance optical system. One arrangement could be such that Lepton provides the optical sensor, a second group provides the laser/fiber/FBG portion, and a third group provides the post-sensor hardware and data management expertise to extract health information. Interest from potential partners is solicited.

References

Francis T.S. Yu and Shizhuo Yin, Fiber Optic Sensors, Marcel Dekker, 2002.




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Quantum Cryptography

Technology

Quantum cryptography, or quantum key distribution (QKD), is a highly secure technique for optical transmission of sensitive information. It involves the creation of an encryption key, which is shared by two parties prior to exchanging data that is encoded using conventional algorithms. The key is sent at single-photon light levels, where the quantum mechanical nature of parameters such as polarization is important. This feature of the key allows the parties to detect eavesdropping on transmission of the key. Upon uncompromised key transmission, secure messages may be sent over a standard communications channel.

Free-space QKD is possible, but the more common implementation is over fiber. A limiting component in the system is the optical sensor, which must be capable of single photon detection. A common choice for the sensor is an InGaAs/InP avalanche photodiode (APD). These devices have good sensitivity to the inbound photons (quantum efficiency), but suffer from high dark count rates and afterpulsing, which results in a requirement for gating and dead times on the order of microseconds. The impact on system performance is to limit the data rate and span length of the fiber. Due to the tradeoffs in dark count and quantum efficiency performance as a function of temperature, the devices are typically cooled to the vicinity of -60şC.

The Lepton Advantage

Lepton’s photon counting module has very low dark count rates (~ 100x better than APDs) and no afterpulsing. It requires no cooling. The output pulse duration in response to a single photon is about 3 ns wide, making the sensor much faster than APDs, with no gating requirement.

References and Links

Trifonov, Alexei, Darius Subacius, Audrius Berzanskis and Anton Zavriyev. “Single photon counting at telecom wavelength and quantum key distribution.” Journal of Modern Optics (2004) vol. 51, no. 9-10, 1399-1415.



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Narrowband Spectroscopy

Technology



Broadband infrared fiber spectrometers are available, and currently find application in substance identification. In these systems, laser light incident on a substance may vaporize the sample or simply excite it such that it fluoresces. Laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) systems analyze the spectrum of light emanating from the excited sample to identify the atomic or molecular makeup of the sample. In some circumstances the concentration of a gas may be determined by the fractional absorption of the laser light as it passes through the sample. In the case where O2 and CO2 detection are of interest, laser wavelengths in the 1 – 2 µm range are appropriate.

The Lepton Advantage

In the case when a particular species is of interest, such that the presence or concentration can be deduced by observation of a single (or few) spectral lines in the 1 – 2 µm range, then Lepton’s sensor can offer large performance advantages. These are manifest in the high sensitivity (up to 1000x improved), wide dynamic range, and high signal-to-noise ratio. Together these offer improved sensitivity to the species, i.e. the ability to detect levels that are orders of magnitude lower in concentration.

References and Links



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