Jul 14 2008

How do FBG sensor interrogators work?

Published by under Instruments

Here are the basics. Light is sent into a fiber and reflects back from the FBG. The reflected light travels back to the instrument’s photo detectors and is compared to wavelength reference artifacts so that the instrument can determine the position of the center wavelength of the FBG. Wavelength information is converted to engineering units, e.g., 1.2 picometers of wavelength shift could correspond to 1 microstrain. The actual translation is given by the gage factor supplied with the FBG sensor.

When more than one FBG is present on a fiber (this is often the case), the instrument will use one of two schemes to discriminate between one FBG and the next. Time division multiplexing (TDM) systems use the known speed of light in the fiber to discern which signal is reflected from which FBG along the fiber path. Theoretically, 100 or more FBGs can be on the same fiber at the same nominal center wavelength.

The most utilized scheme is wavelength division multiplexing, or WDM. WDM FBGs are at distinctly different nominal center wavelengths from their neighbors, and the interrogator uses these unique FBG wavelengths to keep track of which sensor is which. Sensor capacity on each fiber is determined by the range that each sensor will measure and the total spectral range of the instrument. WDM ranges are now very large and can also accommodate more than 100 sensors per fiber.

We (Micron Optics) have chosen the WDM approach for a few reasons.

1) Scanning speed is not a function of the number of sensors, and the scan speed can be up to 2kHz.

2) Swept laser sources are powerful enough to split into four fibers for simultaneous measurement of >300 sensors.

3) FBGs can be highly reflective. This coupled with the high dynamic range of the instrument make the system much more flexible for measurements over tens of kilometers of fiber.

4) WDM is compatible with Micron Optics fast, narrow line, wide range lasers that are stable over time and temperature, and are mechanically robust in environments where they will encounter shock and vibration.

There are other approaches, but they have significant drawbacks:

a) Broadband source, Dispersive element, Diode Array
Limitations: This method cannot achieve the required wavelength measurement repeatability and resolution with commercially available diode arrays. Low broadband source power limits the ultimate needed combination of channel count/sensor capacity and dynamic range/distance to sensors.

b) Broadband source, Optical Spectrum Analyzer/Multi-line wavelength meter
Limitations: Laboratory OSAs are large, slow, expensive, and do not have a wide operating temperature range. Multi-line wavelength meters acquire data at slow speeds only, and are not mechanically robust. Low broadband source power limits the ultimate needed combination of channel count/sensor capacity and dynamic range/distance to sensors.

c. OTDR/TDM systems
Limitations: Low loss budget precludes a solution with the required number of sensors and/or channels, and data acquisition rates scale down with increasing sensor counts. Minimum physical grating spacing limits some applications.

d. External Cavity Tunable Laser, Power Meter, Wavelength Meter
Limitations: External cavity tunable lasers are slow, expensive, and do not have a wide operating temperature range or the required mechanical robustness. The addition of power meters and wavelength meters add to the bulk, complexity, and cost, as well as reduce reliability and speed. Polarization properties of the narrow line lasers may not be an ideal match for all sensing applications.

Tom Graver
Director, Optical Sensing

3 responses so far

Jul 03 2008

How do FBG sensors work?

Published by under Sensors

FBGs are essentially reflectors built inside the core of an optical fiber. The reflectors are made by permanently altering the refractive index of the core. FBGs are off-the-shelf items today, and can also be made to order to fit particular applications (e.g., many FBGs in an array with irregular spacing on a single fiber).

Each FBG reflects a certain narrow slice of spectrum. Such slices typically have a smooth Gaussian shape. The center (i.e., top) of the reflected Gaussian peak is what is used to make measurements (aka, the “center wavelength”). The center wavelength is a digitally encoded zero point for each sensor — one that doesn’t change with time. This is one fundamental characteristic of FBG sensors that make the technology so valuable for long term monitoring of structures.

A FBG’s peak shifts (to a higher or lower center wavelength) when either the fiber is strained or its temperature changes. When the strain or temperature change is returned to the zero point, so does the sensor reading, i.e., there is no hysteresis.

FBGs are specially packaged to isolate the measurement property of interest. For example, the Micron Optics os3100 strain gage uses a steel carrier to transfer strain from the structure to the FBG, whereas the os4100 temperature gage isolates the FBG from strain. Used together, one can easily collect temperature-compensated strain measurements.

If you’re buying FBGs for your application, consider these specifications:

* FBG length of 10mm, >80% reflectivity, 3dB bandwidth of 0.25nm, 15dB isolation (that’s the “clean” region of the FBG peak), and a tensile strength of 150kpsi. This strength is equivalent to a proof strain of 15,000ue. Stronger and shorter FBGs are available, but you trade other properties to improve these characteristics.

* Also consider fiber coatings. Most fibers and FBGs are coated with acrylate. This dates back to FBG’s telecom roots. Polyimide coatings, however, have a higher temperature tolerance (250 C vs. 80 C for acrylate), and are far superior in transferring strain to the fiber core.

Tom Graver
Director, Optical Sensing

10 responses so far

Jun 13 2008

What are the Specific Advantages of FBG Sensors?

Published by under Sensors

I often ask a prospective customer: Why are you considering using optical sensors? Sometimes it’s simply that they are enamored with new technology. Granted it is cool but what are the real problems that they need to solve? Here are examples of problems that I look for and the associated benefits provided by fiber Bragg grating sensors:

We need a passive Class 1, Div 1 sensor for an explosive environment.
Benefit: FBG sensors are passive.

Lightning destroyed our electrical gages.
Benefit: Optical fibers are non conductive, so lightning will not destroy FBG sensors with an electrical surge.

The EMI (electromagnetic interference) is so strong, we get more noise than signal with our thermocouples.
Benefit: FBG sensors are immune to EMI.

Our bridge is 5 kilometers long and we need 720 sensors.
Benefit: Fiber sensor instruments (aka interrogators) have a range of well over 30 km and a capacity for more than 80 sensors per fiber and 16 fibers. That’s a total of >1280 sensors per demodulation instrument.

We need to study the temperature transients that happen in a few milliseconds.
Benefit: FBGs respond quickly to even slight temperature variations.

Fifty sensors must fit inside a 0.5 meter long tube that’s only 2mm in diameter.
Benefit: FBGs can be spaced at 1 cm intervals along a fiber that is only 155 microns in diameter.

Salt air corrodes our foil strain gages in just a few weeks.
Benefit: FBG sensors are made of silica (i.e., glass). They do not corrode.

Conventional gages take too long to install and to home run all of the wires.
Benefit: Multiplexing dozens of FBGs in series in one fiber saves the cost of a home run lead to each sensor. Also, varying FBG sensor lead lengths does not impact sensor calibration.

We spend too much time calibrating sensors and instruments.
Benefit: Micron Optics sensor interrogation instruments have built-in calibration artifacts that last for the life of the instrument. The FBG sensors each have a digitally encoded identity that does not change. So once a system is installed and sensor zero points are recorded, no further calibration is required. Ever.

Our conventional gages fail after about one million cycles.
Benefit: Optical fiber is amazingly robust. Our FBG gages have been tested to >100 million cycles of +/-3,000 microstrain with no degradation of the measurement.

We need to measure strain ranges of more than 6,000 microstrain.
Benefit: Some FBG strain gages can measure up to ~30,000 microstrain (i.e., 3% elongation).

The wire for the foil gages is too heavy and cumbersome.
Benefit: Again, multiplexing is the key. A single, small fiber can connect 10s of gages to the interrogator.

We need one versatile system that can measure strain, temperature and pressure.
Benefit: FBGs measure directly strain and temperature. Tranducer packaging around FBGs makes measurement of other properties possible – like pressure, acceleration, displacement, chemical presence, etc. All of these sensors, no matter what they measure, are measured by the same interrogator.

We need to embed sensors into our composite structure.
Benefit: Because fibers are so small, they can be embedded in structures built with carbon fibers, glass fibers, concrete and steel, etc.

We need gages that will last for a decade or even longer.
Benefit: Optical components like the FBGs themselves and those used to build the interrogators, are Telcordia qualified for a >25 year lifetime. Telcordia is a set of standards established by the telecom industry for critical equipment deployed in harsh field applications.

We need to measure extreme temperatures (e.g., as low as -200°C or up to +750°C)
Benefit: Commercial quality FBG-based temperature sensors are available now for the -200°C to 300°C range, and promising prototypes have been shown to operate in 1,000 hour tests at 750°C. Materials like sapphire FBGs are underdevelopment for even higher temperatures.

Do any of these FBG advantages address your needs? Feel free to contact us to discuss your application and to see if FBG sensors might hold at least some of the answers you seek.

Tom Graver
Director, Optical Sensing

Comments Off

Jun 02 2008

What Types of Fiber Optic Sensing Technologies are Available?

Published by under OS Technology

There are many technologies, but commercial solutions really boil down to two main categories: point sensing for which the active portion of the fiber is <= 1cm, and distributed sensing where the entire fiber, perhaps tens of kilometers long, is the sensor.

Fiber optic distributed sensors measure temperature only (Raman Optical Time Domain Reflectometry -- ROTDR) or both strain and temperature (Brillioun Optical Time Domain Reflectometry -- BOTDR). Spatial resolution is typically one meter or more and strain and temperature resolution are reported at about one microstrain and one degree C respectively, with sampling rates of a few seconds per measurement. The beauty of these approaches is that standard (i.e., inexpensive) telecom fiber is the sensor. The fiber is usually packaged in a tough outer jacket for deployment. Instrumentation is often US$100,000 or more, however. But still the value is very good for long range (>2 km) applications such as pipelines, tunnels, power transmission lines.

Fiber optic point sensors are found in two basic types: fiber Bragg grating (FBG) sensors and Fabry-Perot (FP) sensors. FP sensors have found an important niche in measuring strain, temperature, and particularly pressure for medical applications. They are very small (especially the pressure sensors), but only one sensor can be used per fiber.

FBG sensors for strain and temperature are also very small – as short as 2mm in a 150 micron fiber diameter or as long as a few meters for long gage strain measurements. Other properties like pressure, acceleration, displacement, humidity, and chemical presence, are measured by using a transducer to relate strain to pressure or strain to acceleration, for example. A key advantage of FBG sensors is that dozens, or even a hundred, can be used in series on a single fiber — even if they are measuring different physical properties.

Fiber Bragg grating technology is by far the most widely used fiber optic sensor technology. The versatility of the technology and relatively low cost make it a winner for many applications. At Micron Optics, well over 90% of our sensing customers use FBG based sensors. Whether they’re examining a cancer patient, monitoring a bridge, flying an airplane or pumping oil, they need the information that Micron Optics technology can glean from fundamental measurements of FBGs

These applications, and the physics of how FBG sensors work, will be included as future blog topics.

Comments Off

« Prev - Next »