the double-cald fiber bragg grating is fabricated by writing gratings in doule clad fiber,the intruduction of double clad is as the following:
Double-clad fiber (DCF) is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices.

There are two different kinds of double-clad fibers.
The first was developed early in optical fiber history with the purpose of engineering the dispersion of optical fibers. In these fibers, the core carries the majority of the light, and the inner and outer cladding alter the waveguide dispersion of the core-guided signal.
The second kind of fiber was developed in the late 1980s for use with high power fiber amplifiers and fiber lasers. In these fibers, the core is doped with active dopant material; it both guides and amplifies the signal light. The inner cladding and core together guide the pump light, which provides the energy needed to allow amplification in the core. In these fibers, the core has the highest refractive index and the outer cladding has the lowest. In most cases the outer cladding is made of a polymer material rather than glass.

the double-clad fbg Raysung suppled is the second kind of fiber.

The advantage of double-clad fbg: dispersion-compensating
In double-clad fiber for dispersion compensation, the inner cladding layer has lower refractive index than the outer layer. This type of fiber is also called depressed-inner-cladding fiber and W-profile fiber (from the fact that a symmetrical plot of its refractive index profile superficially resembles the letter W.

The applicaton of double-clad fbg:

In modern double-clad fiber bragg gratings is for high power fiber amplifiers and lasers .

Schematic diagram of cladding-pumped double-clad fiber laser

About Raysung Photonics Inc:

Raysung photonics located in the export processing zone of Xi'an, China, is specialized in manufacturing FBG, lensed fiber and FBG filter for FTTH. Besides those three main products, Raysung can also manufacture wavelength stabilizer, DWDM FBG filter, chirped fiber grating, long period grating, custom fiber grating and so on. In addition, Raysung has ability to develop and produce special gratings for sensor. The company is committed to the research, design and producing of those products and it is the only one manufacture which has the capacity to volume- producing all sorts of fiber grating in China. Moreover, Raysung is one of the only manufactures in the world who has the capacity to manufacture the type of optical fiber which incorporates FBG and lensed fiber. Raysung's products are widely used in aerospace, construction industry, coal industry etc. The company's professional teams have rich experience in photoelectric area and we have advanced technology and equipment. This allows us to provide our customers with high-quality products and optimized proposals.

In 2006, Raysung completed ISO9001: 2000 and ISO14001 certification and also obtained several patent rights. The materials of our products accord with RoHS criterion. Raysung's products are always superior in quality and competitive in price.