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Explain Modified Chemical Vapor Deposition (MCVD) method of fiber fabrication.

Marks: 10M

Year: May2015, Dec2014, Dec2013, May2014, May2013

• We have four main fibre fabrication techniques used today. These techniques were developed around 20 years ago and are still used with some variations and improvements. These techniques differ mainly in the deposition of fibre material on the target rod or tube. They are:

1. Outside vapor phase oxidation (OVPD)
2. Vapor phase axial deposition (VAD)
3. Modified chemical vapor deposition (MCVD)
4. Plasma activated chemical vapor deposition (PCVD)

• OVPD and VAD are based on flame hydrolysis whereas MCVD and PCVD are based on chemical vapor deposition.

MCVD:

• The modified chemical vapor depostion (MCVD) process was pioneered at Bell laboratories and widely adopted elsewhere to produce very low-loss graded index fibers.

• It is also called as Inner Vapour Phase Deposition (IVPD) since the soot is deposited inside the target rod tube as opposed to outside in the Outer Vapour Phase Deposition (OVPO) process.

• The MCVD technique was developed to increase the deposition rates as compared to conventional CVD process and also to reduce the OH^-contamination due to the use of hydride reactants.

• As in Outside Vapour Phase Deposition, Modified Chemical Vapour Deposition also produces the preform in two steps.

• First, reactant gases flow through a rotating glass tube made from fused silica while a burner heats its narrow zone by travelling back and forth along the tube. Silica and dopants form soot that is deposited on the inner surface of the target tube.

• A burner heats a narrow zone of this deposit and sintering (heating without melting) occurs within this zone. The result is a layer of sintered glass. Operating temperature is kept at around 1600℃.

• The second step involves heating the soot perform to 2000℃, thus collapsing the tube into solid glass perform.

• The fiber that is subsequently drawn from this preform rod will have a core that consists of the vapor-deposited material and a cladding that consists of the original silica tube.

• The tube is then collapsed to give a solid preform which may then be drawn into fiber at temperatures of 2000 to 2200 °C.

• A graded refractive index profile can be created by changing the composition of the layers as the glass is deposited.

• This technique is the most widely used at present as it allows the fabrication of fiber with the lowest losses.

• Apart from the reduced $OH^-$ impurity contamination the MCVD process has the advantage that deposition occurs within an enclosed reactor which ensures a very clean environment.

• MCVD has produced $GeO_2$ doped silica single-mode fiber with minimum losses of only 0.2 dB/km at a wavelength of 1.55μm. Although it is not a continuous process, the MCVD technique has proved suitable for the widespread mass production of high-performance optical fibers. Moreover, it can be scaled up to produce preforms which provide 100 to 200 km of fiber.