Photonics & Fiber Bragg Gratings

Since the breakthrough of lasers in the 1960s, photonics has been moving out of the laboratory and into industry, starting with the revolution that fiber optics brought to telecommunications and the Internet. Whether in manufacturing, communications, sensing, or defense, photonics is going to be a driving technology of the 21st century, and the demand for the powerful solutions it can deliver has never been stronger.

The Core of Photonic Technology is Based on Fiber Bragg Grating (FBG).

What is A FBG?

By Technical Definition, a Fiber Bragg Grating is a periodic or aperiodic perturbation of the effective refractive index in the core of an optical fiber, or a fiber device containing such a perturbation. A Fiber Bragg Grating (FBG) is made by periodically changing the refraction index in the glass core of the fiber. The refraction changes are made by exposing the fiber to UV-light with a fixed pattern.

What Does This Mean?

Translated into “laymen terms” after exposing the optical fiber to the special high-intensity laser light, the Fiber Bragg Gratings (FBGs) remain permanently embedded into the core of the fiber.  These FBGs themselves can then become unique, addressable sensors, or they can be used to divide the fiber into sections, called sensing zones.  Spools of fiber with FBGs, many kilometer long are then incorporated into rugged fiber optic cables.  Once assembled into an appropriate cable based upon the application, the optical fiber is fully protected from damage.

How Does The USSI System Work?

The Figure below represents a simplified schematic of our standard fiber optic sensing system.  The electronics includes a laser source with a phase modulator/pulse generator for launching light down the fiber, as well as the receiver electronics for demodulating the reflected signals and translating them into a digital electronic signal.   When light traveling down the fiber encounters a Fiber Bragg Grating (FBG), a portion of the energy is reflected back towards the source.  For one pulse out, there will be N return pulses, where N equals the number of FBGs.  The FBGs provide the separation between individual sensor sections.   The fiber between the FBGs is the actual sensor. 

Referring to the Figure above, the CW light from the laser is sent to the pulse generator which actually performs two functions, it imparts a high frequency carrier (modulation signal) onto the light and also serves as an optical on/off switch to generate the pulses.  A light pulse signal is sent to the array of sensors, if there is no acoustic disturbance along a length of fiber between a given pair of FBGs, the reflected modulated light pulse will be unchanged.  However, if an acoustic disturbance is present, it will strain the fiber, causing a phase shift in the reflected light signal.  (For example, the acoustic sensitivity of a fiber optic hydrophone is given as degrees of phase shift per micro pascal of the applied pressure signal.) 

The phase shifted signal is compared to an unchanging reference, and the difference between the two generates an interference pattern (hence interferometer).  The demodulator translates the interference signal into digital (electronic) acoustic data.  The remotely deployed fiber sensor array (Sensor Cable) contains no electronics.  All of the electronics resides in a standard electronics cabinet at a remote location.  The digital output of the demodulator is sent to the acoustic processor and display.