- Propagation in single mode fiber is advantageous because signal dispersion due to delay differences amongst various modes in multimode is avoided. Multimode step index fibers cannot be used for single mode propagation due to difficulties in maintaining single mode Therefore for the transmission of single mode the fiber is designed to allow propagation in one mode only, while all other modes are attenuated by leakage or absorption.
- For single mode operation, only fundamental LP01 mode many exist. The single mode propagation of LP01 mode in step index fibers is possible over the range.
- The normalized frequency for the fiber can be adjusted within the range by reducing core radius and refractive index difference < 1%. In order to obtain single mode operation with maximum V number (2.4), the single mode fiber must have smaller core diameter than the equivalent multimode step index fiber. But smaller core diameter has problem of launching light into the fiber, jointing fibers and reduced relative index difference.
- Graded index fibers can also be sued for single mode operation with some special fiber The cut-off value of normalized frequency Vc in single mode operation for a graded index fiber is given by,
Example 1.8.1 : A multimode step index optical fiber with relative refractive index difference 1.5% and core refractive index 1.48 is to be used for single mode operation. If the operating wavelength is 0.85µm calculate the maximum core diameter.
Solution : Given :
n1 = 1.48
∆ = 1.5 % = 0.015
λ = 0.85 µm = 0.85 x 10-6 m
Maximum V value for a fiber which gives single mode operations is 2.4.
Normalized frequency (V number) and core diameter is related by expression,
a = 1.3 µm … Ans.
Maximum core diameter for single mode operation is 2.6 µm.
Example 1.8.2 : A GRIN fiber with parabolic refractive index profile core has a refractive index at the core axis of 1.5 and relative index difference at 1%. Calculate maximum possible core diameter that allows single mode operations at λ = 1.3 µm.
Solution : Given :
for a GRIN
Maximum value of normalized frequency for single mode operation is given by,
Maximum core radius is given by expression,
a = 3.3 µm … Ans.
\ Maximum core diameter which allows single mode operation is 6.6 µm.
- One important transmission parameter for single mode fiber us cut-off wavelength for the first higher order mode as it distinguishes the single mode and multim0de
- The effective cut-off wavelength λc is defined as the largest wavelength at which higher order mode power relative to the fundamental mode power is reduced to 0.1 dB. The range of cut-off wavelength recommended to avoid modal noise and dispersion problems is : 1100 to 1280 nm (1.1 to 1.28µm) for single mode fiber at 3 µm.
- The cut-off wavelength λc can be computed from expression of normalized
Vc is cut-off normalized frequency.
- λc is the wavelength above which a particular fiber becomes single moded. For same fiber dividing λc by λ we get the relation as:
But for step index fiver Vc = 2.405 then
Example 1.8.3 : Estimate cut-off wavelength for step index fiber in single mode operation. The core refractive index is 1.46 and core radius is 4.5 µm. The relative index difference is 0.25 %.
Solutions : Given :
n1 = 1.46
a = 4.5 µm
∆ = 0.25 % = 0.0025
Cut-off wavelength is given by,
For cut-off wavelength, Vc = 2.405
Mode Field Diameter and Spot Size
- The mode filed diameter is fundamental parameter of a single mode fiber. This parameter is determined from mode field distributions of fundamental LP01
- In step index and graded single mode fibers, the field amplitude distribution is approximated by Gaussian distribution. The mode Field diameter (MFD) is distance between opposite 1/e – 37 times the near field strength )amplitude) and power is 1/e2 =
- In single mode fiber for fundamental mode, on field amplitude distribution the mode filed diameter is shown in fig. 8.1.
- The spot size ω0 is gives as –
MFD = 2 ω0
The parameter takes into account the wavelength dependent filed penetration into the cladding. Fig. 1.8.2 shows mode field diameters variation with λ.
Requirements of Fiber Optic Material
- The material must be transparent for efficient transmission of
- It must be possible to draw long thin fibers from the
- Fiber material must be compatible with the cladding material. Glass and plastics fulfills these
- Most fiber consists of silica (SiO2) or silicate. Various types of high loss and low loss glass fibers are available to suit the Plastic fibers are not popular because of high attenuation they have better mechanical strength.
- Glass is made by fusing mixtures of metal oxides having refractive index of 1.458 at 850 For changing the refractive index different oxides such as B2O3, GeO2 and P2O5 are added as dopants. Fig. 1.8.3 shows variation of refractive index with doping concentration.
- Fig 1.8.3 shows addition of dopants Ge02 and P2O5 increases refractive index, while dopants Fluorine (F) and B2O3 decreases refractive index. One important criteria is that the refractive index of core is greater than that of the cladding, hence some important
GeO2 – SiO2
P2O5 – SiO2
B2O3 – SiO2
GeO2 – B2O3 – SiO2
B2O3 – SiO2
compositions are used such as
- The principal raw material for silica is sand and glass. The fiber composed of pure silica is called as silica The desirable properties of silica glass are :-
- Resistance to deformation even at high
- Resistance to breakage from thermal shocks (low thermal expansion).
- Good chemical
- Other types of glass fibers are :
- Halide glass fibers
- Active glass fibers
- Chalgenide glass fibers
- Plastic optical fibers
Fiber Fabrication Methods
- The vapor-phase oxidation process is popularly used for fabricating optical fibers. In this process vapours of metal halides such as SiCl4 and Gecl4 reactive with oxygen and forms powder of SiO2 The SiO2 particles are collected on surface of bulk glass and then sintered to form a glass rod called Preform. The preforms are typically 10-25 mm diameter and 60-120 cm long from which fibers are drawn. A simple schematic of fiber drawing equipment is shown in Fig. 1.8.4 on next page.
- The preform is feed to drawing furnace by precision feed mechanism. The preform is heated up in drawing furnace so that it becomes soft and fiber can be drawn
- The fiber thickness monitoring decides the speed of take up The fiber is then coated with elastic material to protect it from dust and water vapour.
Outside Vapour-Phase Oxidation (OVPO)
- The OVPO process is a lateral deposition process. In OVPO process a layer of SiO2 (Soot) is deposited from a burner on a rotating mandrel so as to make a Fig,
1.8.5 shows this process.
- During the SiO2 deposition O2 and metal halide vapours can be controlled so the desired core-cladding diameters can be incorporated. The mandrel is removed when deposition process is completed, This preform is used for drawing thin filament of fibers in fiber drawing
Vapour-Phase Axial Deposition (VAD)
- In VADprocess, the SiO2 particles are deposited axially. The rod is continuously rotated and moved upward to maintain symmetry of particle deposition.
- The advantages of VAD process are
- Both step and graded index fibers are possible to fabricate in multimode and single
- The preforms does not have the central
- The performs can be fabricated in continuous length.
- Clean environment can be
Modified Chemical Vapour Deposition (MCVD)
- The MCVD process involves depositing ultra fine, vapourized raw materials into a pre- made silica tube. A hollow silica tube is heated to about 1500 oC and a mixture of oxygen and metal halide gases is passed through it. A chemical reaction occurs within the gas and glass ‘500t’ is formed and deposited on the inner side of the tube. The soot that develops from this deposition is consolidated by heating. The tube is rotated while the heater is moved to and along the tube and the soot forms a thin layer of silica glass. The rotation and heater movement ensures that the layer is of constant The first layer that is
deposited forms the cladding and by changing the constituents of the incoming gas the refractive index can be modified to produce the core. Graded index fiber is produced by careful continuous control of the constituents.
- The temperature is now increased to about 1800 oC and the tube is collapsed to form a solid rod called a preform. The preform is about 25 mm in diameter and 1 metre in This will produce 25 km of fiber.
- The preform is placed at a height called a pulling tower and its temperature is increased to about 2100 o To prevent contamination, the atmosphere is kept dry and clean. The fiber is then pulled as a fine strand from the bottom, the core and cladding flowing towards the pulling point. Laser gauges continually monitor the thickness of the fiber and automatically adjust the pilling rate to maintain required thickness. After sufficient cooling the primary buffer is applied and the fiber is drummed.
- 1.8.6 (Refer Fig. 1.8.6 on previous page) shows the overall MCVD process.
Plasma-Activated Chemical Vapour Deposition (PCVD)
- PCVD process is similar to MCVD process where the deposition occurs on silica tube at 1200 o It reduces mechanical stress on glass films. There is no soot formation and hence sintering is not required. Non-isothermal microwave plasma at low pressure initiates the chemical reaction.
- Double-crucible method is a direct melt process. In double-crucible method two different glass rods for core and Cladding are used as feedstock for two concentric crucibles. The inner crucible is for core and outer crucible is for cladding. The fibers can be drawn from the orifices in the Fig. 1.8.7 shows double crucible method of fiber drawing.
Major advantages of double crucible method is that it is a continuous production process.