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MCVD

There are four main fiber fabrication techniques used today: outside vapor phase oxidation (OVPD), vapor phase axial deposition (VAD), modified chemical vapor deposition (MCVD), and plasma activated chemical vapor deposition (PCVD). MCVD, which was pioneered at Bell Laboratories, involves depositing soot inside a rotating glass tube, sintering the deposit into layers of glass, and collapsing the tube into a solid preform that can be drawn into fiber. MCVD produces the lowest loss fibers and allows fabrication of fibers with graded refractive index profiles. It remains the most widely used technique today.

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846 views4 pages

MCVD

There are four main fiber fabrication techniques used today: outside vapor phase oxidation (OVPD), vapor phase axial deposition (VAD), modified chemical vapor deposition (MCVD), and plasma activated chemical vapor deposition (PCVD). MCVD, which was pioneered at Bell Laboratories, involves depositing soot inside a rotating glass tube, sintering the deposit into layers of glass, and collapsing the tube into a solid preform that can be drawn into fiber. MCVD produces the lowest loss fibers and allows fabrication of fibers with graded refractive index profiles. It remains the most widely used technique today.

Uploaded by

Shailaja Udtewar
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© © All Rights Reserved
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FABRICATION METHODS

We have four main fiber 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 GeO2 GeO2 doped silica single-mode fiber with
minimum losses of only 0.2 dB/km at a wavelength of 1.55m. 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.
Table 1 Comparison Fabrication Table

- OVPO MCVD PCVD VAD

Reaction Flame Chemical Chemical Flame


Type Hydrolysis Vapor Vapor Hydrolysis
Dimension Dimension

Dimensiona Outside Layer Intside Layer Inside Layer Axial Layer


l Direction Deposition Deposition Deposition Deposition

Refractive Layer Layer Layer Simulataneou


index Approximatio Approximatio Approximatio s Formation
Profile n n n
formation

Process Batch Batch Batch Contineous

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