Cladding to Core Diameter Ratio (CCDR)
 
The CCDR value is depending on the desired wavelength application. Typically the cladding thickness is at least ten times the longest wavelength of interest.
The cladding needs to have a sufficient thickness to avoid that light which is reflected at the core/cladding interface penetrates the cladding and reaches the outer diameter of the fiber.

 

Example:
 
Wavelength of application = 1064 nm
Desired core diameter: 200µm (a)
Necessary cladding thickness: 10,6µm
Resulting Cladding diameter: 222µm -> resulting in standard diameter 240µm(b)
Suggested CCDR: 1:1.2 for fiber diameter 200/240

 

Numerical Aperture (NA)
 
The Numerical Aperture is defined by the difference between the refractive index of the core material and the cladding material of the step-index fiber.
NA =√(n2 core – n2 cladding)

Whereas the refractive index of the core is defined by the pure quartz material the refractive index of the cladding material is determined by the fluorine concentration.

 

Core Materials
 
The transmission of the optical fiber produced from FSI preforms is determined primarily by the properties of the core material. Preforms with core material of high OH content (about 1000ppm) are typically used for application in the UV-visible spectral regions. Low OH content core material (about 1ppm) is used for IR transmission.
Depending on the core material the fiber allows for transmission in a wide spectral range from 200nm to 2000nm. Therefore the optimum core material is selected depending on the wavelength range of the desired application.