Fiber Bragg Gratings

In a FBG, the refractive index of the core is periodically modulated along the fiber’s main axis. The period of the modulation ranges typically from a few hundred nanometers to a few microns. When light is launched into a FBG, it experiences a certain amount of scattering at each grating plane. Most of the scattered light becomes more and more out-of-phase and eventually decays due to deconstructive interference. If the Bragg condition is satisfied for one of the colors of the input light, then a sharp reflected peak is observed in the backward direction with a center wavelength determined by the grating’s parameters. 

Full FBG Design Control

FBGs can be successfully manufactured with Newport’s femtoFBG using point-by-point and line-by-line methods. In both cases, the operator has full design control allowing for the writing of uniform, chirped, apodized, and sampled FBGs. By adjusting the Bragg period and the laser energy per pulse, the femtoFBG is capable of producing FBGs with tailored Bragg wavelengths and spectral characteristics. For example, Bragg gratings with rejection bands of more than -30 dB can be formed at telecommunications wavelengths with bandwidths smaller than 0.5 nm.

Point-by-Point FBG Fabrication

A common way to write FBGs with the Point-by-Point (PbP) method is to use each laser pulse to create a grating plane. To accomplish this, the fiber is moved along its main axis at a speed vtrans so that the grating period ΛFBG is now defined by the ratio vtrans/fr where fr is the laser repetition rate.  This method of writing FBGs is not only rapid (typical translational stage speeds are on the order of millimeters per second), but it provides great design flexibility. For example, changing the speed of the stage creates Bragg effects at different wavelengths. Furthermore, chirped gratings can easily be manufactured by varying the translational speed during the writing process.

(a) Transmission (blue) and reflection (red) spectra of FBG manufactured with the femtoFBG using the point-by-point writing method. (b) Image of FBG using femtoFBG

Line-by-Line FBG Fabrication

In Line-by-Line (LbL) writing, the fiber is translated by two high-resolution stages following a symmetric raster scan pathway. The grating period (ΛFBG) is defined by the spacing between the lines in the raster scan. The laser exposure is synchronized with the stages motion to ensure laser exposure only along the horizontal lines (x direction) of the raster scan.

(a) Transmission (blue) and reflection (red) spectra of FBG manufactured with the femtoFBG using line-by-line method. (b) Image of FBG using femtoFBG

Optimized for Spectra-Physics Lasers

The optics within the femtoFBG can be configured for the output of two types of lasers. In one case, reflective and transmissive optical components capable of working at both 520 nm and 1040 nm and possessing high energy damage thresholds are used in order to accommodate the emission of Spectra-Physics Spirit laser. In the other case, NIR optics with minimal group velocity dispersion are used with the outputs of Spectra-Physics Spitfire or Solstice lasers.

Granite Composite Frame

The framework of the femtoFBG is made from a single piece of granite composite manufactured by mineral casting. Mineral castings offer significant weight and thermal mass to provide excellent thermal and vibrational stability. Producing the femtoFBG framework using mineral casting technology creates a compact, but stable and functional design.

PC and Application Software is Included

The femtoFBG is delivered with a personal computer loaded with proprietary software developed by the engineers and scientists in the Technology and Applications Center at Newport. With this tool, the operator has full control of all experimental conditions from irradiation parameters to complex motion within an easy to use customer inspired GUI.