Optical- Optical Fibre Manufacture

Question 1

Improving conventional routes

Answer 1

Melting using highest purity materials. Control oxidation state of impurities to minimise their effect (Fe3+ better then Fe2+). Techniques such as skull melting (use inductive heating of material contained within cool skull of similar composition minimising refractory corrosion contamination). Avoid contamination via gaseous state (by using inductive heating)

Question 2

Double crucible technique

Answer 2

Molten core fed into crucible within molten cladding in outer crucible. See page 6 lecture 7 for diagram. Can make narrow-ish core but monomode difficult to produce

Question 3

Main points of chemical vapour deposition (CVD) techniques

Answer 3

Use chemicals based on liquids which are then further purified during processing by distillation (impurities, already at low concentrations, are emu has less volatile). All reactions take place in vapour state. Problem of contamination by reaction vessel reduced by building up layers on already high quality substrate (skull melting).

Question 4

Chemistry of CVD technique

Answer 4

Start with high purity liquid SiCl4 and distill. Vapour mixed with O2 between 1300 and 1600C to produce SiO2 and 2Cl2. Results in deposition of fine SiO2 soot (0.1μm) inside reaction vessel, normally semiconductor grade SiO2 tubing. Cl2 produced helps eliminate OH in finished fibre. If O2 supply has 4ppm of H2O then final fibre has 1ppb OH. In hot zone created by flame, soot is sintered to dense layer on inside of reaction tube. About 0.5g/min deposition. Finished preform about 1kg so takes more than 1 day.

Question 5

Limiting factors in CVD process

Answer 5

Reaction rates and rate of soot deposition. Gas flow limits rate of delivery and must give sufficient residence time for reaction.

Question 6

Diagram of CVD process

Answer 6

Page 10 lecture 7

Question 7

Modified chemical vapour deposition MCVD

Answer 7

Gas concentrations and temperature ps high compared with electronics industry so: nucleation of soot is in gas phase, does not occur homogeneously at surface (formation of single crystal silicon for semiconductor technologies where structure is built up atom by atom).

Question 8

Diagram of glassmaker’s lathe

Answer 8

Page 12 lecture 7

Question 9

How do vapour pressures of potential impurities vary with temperature?

Answer 9

Vapour pressure decreases with increasing 1000/T see page 13

Question 10

Typical preform dimensions

Answer 10

Diameter of 25mm and is metre long

Question 11

How to modify refractive index of silica glass

Answer 11

Add other volatile species in to gas phase in controlled amounts. Can make core-clad structure with defined r.i profile can be built up in layers (typically about 80 layers to make preform)

Question 12

Examples of refractive index modifiers and their effect

Answer 12

GeCl4 (l) increases r.i (G is dopant)
POCl3 (l) increases r.i (P is dopant)
BCl3 (g) or BBr3 (l) decreases r.i (B is dopant)
SiF4 (g) or CCl2F2 (g) decreases r.i (F is dopant)

Question 13

Gas system for adding refractive index modifiers

Answer 13

See page 16 lecture 7

Question 14

Why is compositional control in MCVD important?

Answer 14

Ensure a reproducible numerical aperture

Ensure material and waveguide dispersion are balanced at the intended operating wavelength

Question 15

Hole in preform

Answer 15

There is hole in core of preform since gap must be left for gas transport when depositing the last layer. Hole must be eliminated by collapse after the rod is finished

Question 16

Tube collapse

Answer 16

Achieved using a cylindrical arrangement of oxy-hydrogen flames around the tube, which pass slowly along the tube as it is rotated to avoid distortion. During collapse, selective volatilisation of GeO2 can occur resulting in r.i dip in core resulting in loss of bandwidth but can be partly compensated by passing GeCl4 down tube during collapse.

Question 17

Fibre drawing rig

Answer 17

Preform down into heat source then to diameter scanner. Into coater where coating (liquid monomers) added to protect fibre and UV curer initiates polymerisation. Then fed to pinch wheels and takeup spools. Whole process takes seconds from top to bottom. Diagram on page 19

Question 18

Fibre pulling furnaces

Answer 18

Viscosities needed are 10^5 dPa s (decipascal seconds) and temperatures 1900 to 2100C. Temperatures can be achieved using graphite resistance furnace or rf induction furnace which utilises yttria-stabilised zirconia rings (preheated by inductively heated carbon rod) higher temperature means fewer strength reducing flaws present but optical losses increase from formation of electronic defects with strong absorption in near UV to visible.

Question 19

Typical optical fibre stats

Answer 19

Diameter 125μm
Drawn at up to 10m/s
Initial diameter of preform 25mm
Draw down ratio 200:1 corresponds to 1m preform producing 40km fibre

Question 20

Why must control of fibre dimensions be tight?

Answer 20

Or wave-guiding losses increase if light leaks from core

Or splicing losses increases