Choosing between Single Mode vs Multi-mode Optical Fiber

Introduction

An optical fiber or fiber optics has emerged as a leading signal transmission medium in data networking applications. It was first proposed as a technology for light transmission in the 1950s. By 1990s the optical fiber technology has gained traction. By 2010, it has taken over the copper cable transmission. Unlike, copper cables, the fiber optics transmits the signals in an optic format that is in the form of light waves. It offers higher data transmission speed, greater data security, and endurance to harsh environmental conditions in comparison to the copper cables. These cables benefit from their design and construction. The fiber optic cables are constructed by bundling multiple glass strands inside a cladding, which is further protected by a buffer tube, which offers protection against environmental conditions. The optic signals are transmitted through these glass strands by the refraction of light principle.

The optic signals follow a path of transmission inside the glass strand, which is known as the mode of transmission. In short, the mode of transmission refers to optic signal transmission from one end of fiber optic cable to another. The mode of transmission depends on the construction, type of glass material, refractive index, cladding efficiency, and the inner diameter of the transmission media that is the glass strand. There are two fundamental types of fiber optics cables available based on the mode of transmission, namely, single mode and multimode fiber optic cables. These fiber optic cables are often utilized for various applications under different conditions. Therefore, it is essential to know the selection parameters of single mode or multimode optical fiber specific applications.

This white paper thoroughly discusses single mode optical fiber, multimode optical fiber, the difference between both, and the selection parameters for specific operations.

Concept of Mode

The mode is generally defined as the way of optical wave transmission. The optical waves of uniform frequencies, as they enter in the optical fiber are distributed in different paths of transmission. The path followed by each individual optical wave is referred to as mode.

The number of modes in a fiber optic cable is calculated by solving the Helmholtz equation for wave. The Helmholtz equation itself is obtained by applying boundary conditions to Maxwell’s equation. Therefore, the fiber optic modes are projectile solutions of Maxwell’s equation.

The modes are the result of the modal dispersion phenomenon taking place inside the fiber optic cable.
Note: The modal dispersion is totally irrelevant of the number of glass fiber strands wrapped inside the cladding.

An Overview of Single Mode Optical Fiber

Single mode optical fiber is a type of optical fiber designed specifically for single mode light transmission. This means different light waves of different frequencies are transmitted through a single path via this type of optical fiber. These single mode optical fibers are preferred in the industry by a common acronym SMF. The SMF carries optical signals in a transverse mode. That means the path of optic wave transmission is parallel to the length of fiber optic core strand but the electromagnetic oscillation takes place in a perpendicular/transverse direction.

Characteristics of Single Mode Optical Fiber

The following characteristics of single mode optical fiber (SMF) make them unique.

• SMF features a micro core diameter of sizes ranging between 8 to 10.5µm. The cladding diameter of SMF is 125µm.
• Laser rays being the source of optical signals in SMF, the characterized optical wavelengths for SMF is 1310 nm and 1550 nm.
• Theoretically, SMF offers unlimited bandwidth, as it offers a single light transmission mode at a time.
• According to TIA-598C, the industrial color-coding or sheath color for an SMF is yellow for non-military applications.
• Due to higher fidelity for each pulse of lightwave coupled with smaller diameters, these SMFs are suitable for long-distance transmission without excessive signal/data loss.
• The SMFs can transmit the optical signal over hundreds of kilometers at a transmission speed of 40Gbps and over thousands of kilometers at the transmission speed of 10Gbps by using dispersion-compensation devices.

Depending on the variation in characteristics, the single mode optical fiber (SMF) is categorized. Let us discuss the categorization of SMF further.

Types of Single Mode Optical Fiber

The general types of single mode SMF are listed below.

• OS1: OS1 is the first type of SMF which is defined in ISO/IEC 11801. This type of optical fiber features 8 to 9µm core diameter. However, this fiber suffers a higher level of attenuation of the value of about 1dB/km. Due to higher attenuation, the signal loses its strength from one end to another. That is why this SMF is used for comparatively shorted distances and indoor fiber-optic connections.
• OS2: OS2 is a comparatively better type of SMF. It is defined in ISO/IEC 24702. It features the same diameter of 8 to 9µm but in this, the signal suffers a negligible attenuation of 0.4dB/km. Due to lower attenuation, the signal strength remains consistent throughout the transmission. This type of SMF is preferred for long-distance and outdoor optical fiber installations.

An Overview of Multimode Optical Fiber

The multimode optical fiber is a type of optical fiber designed for multiple light signal propagation. The industrial acronym for multimode optical fiber is MMF. In MMF, the modal dispersion takes place according to varying wavelengths of the optical signals. Therefore, modal dispersion in the MMF is higher. The path of optic wave propagation in the MMF is either zigzag or semi-elliptical in nature, it depends on the refractive index of the glass core material.

Characteristics of Multimode Optical Fiber

The following characteristics of Multimode optical fiber (MMF) make them ideal for certain applications.

• MMF features a larger core diameter in the range of 50µm to 100µm. However, for the fundamental types of MMF, the standard core diameters are 50µm and 62.5µm. The cladding diameter remains 125µm.
• The light sources in MMF are either light-emitting diode (LEDs) or vertical-cavity surface-emitting lasers (VCSELs), therefore the categorized optical wavelength remains 850nm and 1300nm.
• The bandwidth of MMF is limited due to modal dispersion. The theoretical value of MMF bandwidth is 28000MHz*km.
• According to TIA-598C, the industrial color-coding or sheath color for MMF is an orange or aqua jacket.
• Due to higher attenuation via dispersion, the MMFs are suitable for short-distance fiber optic transmission. However, by using suitable fiber optic connections, the transmission distance can be extended.
• MMF offers 100Gbit/sec data transmission speed. The general, practical data transmission rate of MMF is 100Mbit/sec up to 2km and 10Gbit/sec up to 550 meters.

Types of Multimode Optical Fiber

The multimode optical fiber (MMF) is mainly categorized into two types based on two factors, namely, a system of classification and refractive index and signal behavior.

• Based on System of Classification: The multimode optical fiber is classified by two systems, one is ISO 11802 and another is TIA-492-AAAD. Let us discuss the following types.
  • OM1: OM1 is a type of MMF classified according to ISO 11802. It possesses 62.5µm core diameter. These optical fibers are compatible with FDDI-grade cables and also offer higher bandwidth.
  • OM2: OM2 is defined by ISO 11802 but it possesses 50µm core diameter. It is designed for greater control over the propagation of light signals.
  • OM3: OM3 is also defined by ISO 11802 and the core diameter is 50µm. However, this type of MMF is designed for laser-optimized propagation over VCSELs.
  • OM4: OM4 is a type of MMF defined by TIA-492-AAAD. It features 50µm of core diameter but is designed for long-distance transmission at high bandwidth. It offers 40Gbit/sec and 100Gbit/sec transmission rates over a transmission distance of 125meters.

• Based on the Refractive Index and Signal Behavior: According to the refractive index of core glass material, the signal behavior or path of propagation differs. Based on the same following types of MMF are established.
  • Multimode Graded Index Fiber: In this type, the material used for building a core of optical fiber offers a graded refractive index. This means as the signal transmits via cable, the refractive index of core gradually diminishes. Due to the graded refractive index, the signal behavior or path of propagation is semi-elliptical in this type of cable. Due to the same type of propagation, attenuation and optical dispersion reduce in this cable.
  • Multimode Step Index Fiber: This type of MMF features a larger core of 100µm diameter. Due to the large diameter, there is a difference in the refractive index at different stages in the fiber. Randomly varying refractive index causes complete or partial refraction inside the core at varying refraction angles. This causes a zigzag manner of light propagation in these types of MMFs.

Difference Between Single Mode and Multimode Optical Fiber

Besides the characteristics, the differences between the single mode and multimode optical fiber can be stated in a few different ways. The following are the highlighted differences between single mode and multimode optical fibers.

• Impact of Optic Wave Propagation:
  As discussed in earlier sections, it is understood that the manner of optical wave propagation differs according to the modal dispersion and refractive index of the core glass material. This difference in the path of propagation impacts on input and output signal via fiber optic. Let us discuss the impact of both single mode and multimode optical fiber cable.
  • Single Mode Optical Fiber:
    In the single mode optical fiber, modal and light dispersion is negligible, therefore the light wave transmits in a linear manner. Due to the same, the attenuation is reduced and signal strength remains consistent. Therefore, in SMF, the input signal and output signal are of the same strength over a long distance of transmission.
    Also, as multiple optical waves of varying frequencies may transmit via SMF but they all follow the same path, therefore all the data packets reach the output end of the optical fiber. Therefore, the reliability of data transmission from transmitting source device to the receiving device remains assured.
  • Multimode Optical Fiber:
    In multimode optical fiber cable, the transmission occurs in two ways, let us discuss the effect of both on the input and output signals.
    • Zigzag Light Propagation: In the zigzag type of light propagation in the MMF, owing to higher refraction, light dispersion and attenuation is higher. Therefore, signal loss takes place in this type of MMF light propagation. Also, optic waves of varying frequencies refract at different angles causing zigzag manner of transmission, therefore, the different transmission path is taken by each optic wave. This results in the transmission speed variation of light signals via MMF. Therefore, there is a possibility of delay in the delivery of some optic data packets at the receiver. This causes data loss.
    • Semi-elliptical Light Propagation: In a semi-elliptical type of propagation, the light beam does not strike in the core wall, therefore there is minimal refraction and optical dispersion. Due to the same, there is minimal attenuation. Also, in this type of propagation, all the light waves follow a point-to-point semielliptical pattern. Therefore, all waves reach the same point and then are collectively sent to the receiving device. That is why there is a negligible loss of data packets here.

• Optical Resource Requirement:
  Both SMF and MMF installation require specific resources like light sources, connectors, etc.
  • Single Mode Optical Fiber: The SMF requires laser diode as a light source, and optical amplifiers to reduce dispersion if installed for long-distance. These devices demand precise calibration to inject the laser beam into the optical fiber.
  • Multimode Optical Fiber: The MMF requires LED diodes or VCSELs as a source of light. Along with the same, if the MMF is installed for longer distance transmission, then the signal amplifiers, connectors, and rectifiers may be required in the integrated circuit.

• Cost Deployment:  
  Although the single mode optic fiber cable is less than multimode, the entire installed system of SMF is more expensive than the MMF installed system. This because the capital investment of optical source and integrated devices in the SMF network is higher than MMF. Generally, the laser diode equipment itself costs 1.5 to 5 times higher than the LED diodes. Also, SMF may demand an additional cost of amplifiers, and system maintenance, therefore the overall cost deployment for SMF is higher than MMF.
• Application Compatibility:
  Compatibility with several applications makes one of the differentiating factors for SMF and MMF.
  • The SMFs are used for long-distance optical fiber networks where the signal strength is expected to retain the highest. Common applications of SMFs are campus fiber optic connections, undersea fiber-optic connections and remote office connections, etc.
  • The MMFs are largely used fiber optic cables. These are used for CCTV optic fiber connection, video/audio/multimedia transmission, telecom connections, LANs, etc.

How to Select  between Single Mode vs Multimode Optical Fiber

When it comes to select between single mode and multimode fiber optic for a specific application, one must consider several factors. The factors of consideration are an application requirement, cost of fiber, cost of installation system, equipment installation requirements, a distance of transmission, speed of transmission, etc. In order to select one of the single mode or multimode optical fibers, the thorough comparison for these selection factors shall be done.
Refer to the following comparison table for convenient selection.

Conclusion:

Due to the extensively growing demand for fiber optic technology, numerous applications of fiber optic cables are found. However, due to the versatility of applications and requirements, the selection of single mode or multimode fiber optic cable has become crucial. Owing to the need for efficiency, it is essential to consider several factors related to the application and fiber optic in order to make the right choice. Thorough knowledge about fiber optic technology, single mode optical fiber and multimode optical fiber, their characteristics, and scope of utility is important, this can certainly lead to an effective selection of suitable fiber optic cable. To gain more information about VERSITRON's SMF and MMF cables and related equipment, please contact us.

Disclaimer

The information provided in this white paper is intended solely for general information purposes. The practice of Engineering differs across each project, as it is driven by site-specific circumstances. Thus, any business decision based on the implementation must be taken only after consultation with a qualified and licensed professional who is capable of addressing all relevant factors, challenges, and desired outcomes. The information in these white papers is derived from various verified sources and posted after reasonable care and attention. It is possible that some information may appear incomplete, incorrect, or inapplicable considering your particular condition. In such condition, VERSITRON does not accept the liability for direct or indirect losses resulting from using, relying, or acting upon the information in this white paper.