Difference Between Multimode and Single-mode Fiber

You might be having an idea about optical fiber, or you can refer previously written article on the difference between optical fibre and coaxial cable. The multimode and single-mode are the types of the optical fiber cable. These fiber optics cable variants can be differentiated by the way in which light is transmitted through these fibers.

The prior difference between multimode and single-mode fiber is that the diameter of core and cladding in multimode fiber lies in the range of 50-85 μm and 125-400 μm. On the other hand, the single-mode has the diameter of 8-12 and 125 μm of core and cladding respectively.

In Single-mode and Multi-mode, the mode refers to the cross-field distribution that propagates along the fiber devoid changing the field distribution unless the phase doesn’t change. In simple words, a mode can be interpreted as the possible path that a directed ray can occupy.

Content: Multimode Fiber Vs Single-mode Fiber

1. 1. Comparison Chart
   2. Definition
   3. Key Differences
   4. Conclusion

Comparison Chart

Basis for comparison	Multi-mode Fibre	Single-mode Fibre
Core diameter	50-85 μm	8-12 μm
Cladding diameter	125-400 μm	125 μm
Regenerator positioning	2 km of distance	Placed at 20 km of distance
Number of rays	More than one	One
Cost	Highly Expensive	Less Expensive
Attenuation	Greater extent	Very less or negligible
Wavelength of transmission	850 nm to 1300 nm	1260 nm to 1640 nm
Application	LAN, SAN, data centers, etc.	WAN, MAN, etc.

Definition of Multimode Fiber

Multimode fibers were devised earlier than single-mode fiber in 1960’s. The core and cladding diameter of the multimode fiber is larger with respect to the operating wavelength of the light signal, and also it is greater than single mode. Its core diameter is about 50 to 85 μm.

Multimode fiber employs geometrical optic model for the light propagation where the light ray rebounds back and forth and reflects from the core-cladding of the fiber. Multiple rays are transmitted in the fiber which propagates in a different path. Their path ultimately corresponds to the fibre mode, and these rays move at different speed.

The essential devices required for the light wave propagation is the light sources and receivers. The light sources used in multimode fibers are compact semiconductor lasers and LEDs. Among these types of light sources, a laser is more powerful than LED while LED produces a low power output. Hence lasers can cover larger distances. LED as a light source requires the installation of regenerators in every few kilometres. The regenerative units are quite expensive. The lasers used in the old days were of multilongitudal mode (MLM). MLM lasers emit light in a broad spectrum of few nanometers. It is called multilongitudal because it operates on multiple different modes where light rays propagate at different speed since the path lengths are distinct. This characteristic of multimode fiber fades the signal, and it is also known as intermodal dispersion.

There is a disadvantage of Multimode fiber is that it produces a lot of noise and attenuation. The reason behind this is the presence of multiple rays causes interference and certainly deteriorates the signal.

Definition of Single-Mode Fiber

Single-mode fiber was evolved after the multimode fiber in 1984. It overcomes the limitations of multimode fiber, i.e., intermodal dispersion and increases the bit rates and the distance between the regenerators. The single-mode fibre has comparatively small core diameter about 8-12 μm which is a small multiple of operating wavelength of the light signal. This range of diameter drives all energy of the light ray to be travelled in the single-mode. Thus it involves a single ray of light.

Single-mode fiber introduced another type of dispersion known as chromatic dispersion. The energy in light signal or pulse is associated with limited bandwidth. Indeed the single-mode fiber can result in the smearing of pulse because of generation of the distinct propagation speed of pulse frequency components. This is due to the physical properties of glass. The smearing of the pulse because of chromatic dispersion increases by the widening of the pulse spectrum, ultimately it depends on the wavelength of the signal. There is no problem of chromatic dispersion when the silica-based optical fiber of 1.3 μm band is used. But, it generates dispersion in 1.55 μm band. To eliminate the chromatic dispersion, dispersion-shifted fiber and DFB (Distributed-Feedback) laser were evolved. The distributed-feedback laser decreases the spectrum of the transmitted pulse near to its modulation bandwidth which lessens the chromatic dispersion loss.

Conclusion

Single mode fiber is a successor of multimode fiber which eliminate the limitations of the multimode fiber like short distance coverage, attenuation issue, high fiber cost excessive use of regenerators or repeater. Although, multimode fiber is a cost-effective alternative for shorter reach as multimode fiber installation and connectors cost is less comparatively.