Single Mode Fibers

. 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 operation. 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 may exist. The single mode propagation of LP₀₁ mode in step index fiber 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 refractive index difference.

. Graded index fibers can also be used for single mode operation with some special fiber design. The cut-off value of normalized frequency V_c in single mode operation for a graded index fiber is given by,

1. Cut-off Wavelength

. One important transmission parameter for single mode fiber is cut-off wavelength for the first higher order mode as it distinguishes the single mode and multimode regions.

. The effective cut-off wavelength λ_C is defined as the largest wavelength at which higher order (LP₁₁) mode power relative to the fundamental mode (LP₁₁) 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 1.2 µm) for single mode fiber at 1.3 µm.

. The cut-off wavelength λ_C can be computed from expression of normalized frequency.

Where,

V_C is cut-off normalized frequency.

. λ_C is the wavelength above which a particular fiber becomes single moded.

For same fiber dividing λ_C by λ we get the relation as :

But for step index fiber V_C = 2.405 then

2. Mode Field Diameter and Spot Size

. The mode field diameter is fundamental parameter of a single mode fiber. This parameter is determined from mode field distribution of fundamental LP₀₁ mode.

. 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 = 0.37 times the near field strength (amplitude) and power 1/e² = 0.135 times.

. In single mode fiber for fundamental mode, on field amplitude distribution the mode field diameter is shown in Fig. 1.

. The spot size is given as –

       The parameter takes into account the wavelength dependent field penetration into the cladding. Fig. 2 shows mode field diameters variation with λ.

3. Fiber Materials

3.1 Requirements of Fiber Optic Material

1. The material must be transparent for efficient transmission of light.

2. It must be possible to draw long thin fibers from the material.

3. Fiber material must be compatible with the cladding material.

Glass and plastic fulfills these requirements.

. Most fiber consists if Silica (SiO₂) or silicate. Various types of high loss and low loss glass fibers are available to suite the requirements. Plastic fibers are not popular because of high attenuation they have better mechanical strength.

3.2 Glass Fibers

. Glass is made by fusing mixture of metal oxides having refractive index of 1.458 at 850 nm. For changing the refractive index different oxides such as B₂O₃, GeO₂ and P₂O₅ are added as dopants. Fig. 3 shows variations of refractive index with doping concentration.

. Fig. 3 shows addition of dopants GeO₂ 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 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 glass. The desirable properties of silica glass are :-

- Resistance to deformation even at high temperature.

- Resistance to breakage from thermal shocks (low thermal expansion).

- Good chemical durability.

. Other types of glass fibers are :

- Halide glass fibers

- Active glass fibers

- Chalgenide glass fibers

- Plastic optical fibers

4. Fiber Fabrication Methods

. The vapour-phase oxidation process is popularly used for fabrication optical fibers. In this process vapours of metal halides such as SiCI₄ and GeCI₄ reactive with oxygen and forms powder of SiO₂ particles. The SiO₂ particles are collected on surface of bulk glass and then sintered to from a glass rod called perform. The performs 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. 4. 

. The perform is feed to drawing furnace by precision feed mechanism. The perform is heated up in drawing furnace so that it becomes soft and fiber can be drawn easily.

. The fiber thickness monitoring decides the speed of take up spool. The fiber is then coated with elastic material to protect it from dust and water vapour.

4.1 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 perform. Fig. 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 perform is used for drawing thin filament of fibers drawing equipment.

4.2 Vapour-Phase Axial Deposition (AVD)

. In VAD process, 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 mode.

- The performs does not have the central hole.

- The performs can be fabricated in continuous length.

- Clean environment can be maintained.

4.3 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 °C 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 thickness. 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 °C and the tube is collapsed to form a solid rod called a perform. The perform is about 25 mm in diameter and 1 metre in length. This will produce 25 km of fiber. 

. The perform is placed at a height called a pulling tower and its temperature is increased to about 2100 °C. 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 pulling rate to maintain required thickness. After sufficient cooling, the primary buffer is applied and the fiber is drummed.

. Fig. 6 shows the overall MCVD process.

Fig. 6 MCVD process

4.4 Plasma-Activated Chemical Vapour Deposition (PCVD)

. PCVD process is similar to MCVD process where the deposition occurs on silica tube at 1200 °C. 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.

4.5 Double-Crucible Method

. Double-crucible method is a direct melt process. In double-crucible method twp 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 crucible. Fig. 7 shows double crucible method of fiber drawing.

       Major advantages of double crucible method is that it is a continuous production process.