What do the fiber terms 9/125, 50/125 and 62.5/125 refer to?




1. What the Numbers Represent
- The first number (core diameter) is the diameter of the central glass region (core) where light travels.
- The second number (cladding diameter) is the diameter of the surrounding glass layer that reflects light back into the core to keep it from escaping.

2. Detailed Breakdown of Fiber Types

a. 9/125 µm – Single-Mode Fiber
- Core Size: The core is very small (9 µm in diameter). This small size allows only one mode (path) of light to travel through the fiber.
- Cladding Size: The cladding is 125 µm, a standard size for all types of optical fibers.
- Features:
  - Minimal signal dispersion (light pulses stay tightly focused).
  - Ideal for long distances (e.g., 40 km or more) and high-speed transmission.
  - Commonly used with laser light sources.
- Applications:
  - Telecommunications
  - Long-haul data transmission
  - High-capacity networks like those in data centers and undersea cables.

b. 50/125 µm – Multimode Fiber
- Core Size: The core is larger (50 µm in diameter), allowing multiple modes (paths) of light to travel simultaneously.
- Cladding Size: The cladding remains 125 µm.
- Features:
  - Larger core makes it easier to connect (light coupling is less sensitive to alignment errors).
  - More susceptible to modal dispersion (different light paths arrive at different times, causing distortion over long distances).
  - Used for short to medium distances (e.g., up to 500 meters at 10 Gbps).
  - Operates well with LEDs or VCSELs (Vertical-Cavity Surface-Emitting Lasers).
- Applications:
  - Local Area Networks (LANs)
  - Data centers for shorter connections
  - Campus networks

c. 62.5/125 µm – Multimode Fiber
- Core Size: The core is even larger (62.5 µm in diameter), allowing more light to enter and multiple light paths to propagate.
- Cladding Size: The cladding remains 125 µm.
- Features:
  - Historically popular, especially in legacy systems from the 1980s-1990s.
  - Similar to 50/125 µm but with slightly lower bandwidth and distance limits.
  - As technology advanced, it was largely replaced by 50/125 µm multimode fiber due to higher performance in modern systems.
- Applications:
  - Legacy LANs and building networks
  - Short-range industrial applications

3. Key Differences Between Single-Mode and Multimode Fibers

Feature	Single-Mode (9/125 µm)[/	Multimode (50/125 µm & 62.5/125 µm)
Core Size	9 µm	50 µm or 62.5 µm
Light Propagation	Single mode	Multiple modes
Signal Dispersion	Minimal	High (modal dispersion)
Distance	Long (up to 100 km)	Short (up to 500 m for 10 Gbps)
Bandwidth	Higher (no modal dispersion)	Lower due to modal dispersion
Light Source	Laser	LED or VCSEL
Cost	Higher	Lower
Applications	Long-haul, high-speed networks	LANs, data centers, campus networks

4. Cladding Size Standardization (125 µm)
- Why is the cladding size always 125 µm?
  - Standardization allows compatibility across different fiber types for splicing, connectors, and other components.
  - Cladding is responsible for ensuring total internal reflection, keeping the light confined to the core.

5. Bandwidth and Distance Capabilities

Fiber Type	Bandwidth (at 850 nm)	> Max Distance (10 Gbps)
9/125 µm	Virtually unlimited	> 40 km
50/125 µm	Up to 2000 MHz·km	~500 m
62.5/125 µm	Up to 160 MHz·km	~275 m

6. Choosing the Right Fiber
- Use 9/125 µm (single-mode) for:
  - Long-distance communication
  - High-bandwidth and speed requirements
  - Environments where cost of active components (lasers) is not a concern.

- Use 50/125 µm (multimode) for:
  - Cost-effective short-to-medium-distance connections
  - High-performance data centers with modern equipment (VCSEL-based).

- Use 62.5/125 µm (multimode) for:
  - Legacy systems requiring backward compatibility.

What do the fiber terms 9/125, 50/125 and 62.5/125 refer to?

These terms refer to the diameter in microns of a fiber optic cable's core and cladding.

    The first set of numbers - 9, 50 and 62.5 refer to the diameter of the fiber cable's core.
    The second set of numbers - 125 refer to the diameter of the outside of the fiber cable's cladding.

The cladding is a special coating that keeps the light from escaping the glass core. 9/125 refers to a single mode fiber cable. 50/125 and 62.5/125 refer to multimode fiber cable.

Singlemode (SMF) vs. Multimode (MMF) Fiber Optic Cables

As bandwidth demand increases, a large number of data center managers may feel that singlemode cables are the definitive answer for the future. And to be fair, they do carry a lot more data over longer distances than multimode fiber cables. The real difference between the two is how they transmit light: singlemode fiber cables allow only one ray of light to be transmitted, while multimode fiber cables have several strands in a larger core that allow more "rays" of light to be transmitted simultaneously.

However, the key element in that phrase is "over longer distances." When it comes to enterprise-level data centers, multimode cables are just as effective for most applications where less distance is involved, and they cost a lot less than their singlemode counterparts.

This cost is felt in the needed transceiver as well. A singlemode optical cable has a small core size, meaning the beam of light it transmits must be much more focused than that needed for a multimode cable, thus leading to the more expensive transceiver.

Multimode cables, at least at the time of this writing, can still handle high-speed data demands at distances less than 500-600 meters. That means that almost any cable internal to an enterprise-level data center can still be multimode and function well.

That being said, there are less bend-sensitive and full-spectrum singlemode cables that offer more bandwidth and are less sensitive to handling of the patch cords. There are also more transceiver options as a result. As these cables get better and more affordable, they may become more common in shorter distance applications.

Is Multimode Better for short runs?

Yes, multimode fiber is generally better for short runs because of its lower cost, easier installation, and the fact that it can handle the bandwidth requirements for most short-distance applications. Multimode fiber supports multiple propagation modes, which allows for faster data transfer over short distances, typically up to 2 km or less. It is commonly used in local area networks (LANs), data centers, short-distance enterprise networks, audio/video applications, and security systems. However, it is essential to consider the specific requirements of your application before deciding on a fiber type.

Single Mode Distance Limitations

Single mode fiber (SMF) has a much greater distance capacity compared to multimode fiber due to its lower signal attenuation and dispersion. The exact distance limitation for single mode fiber depends on the specific fiber type, the data rate being transmitted, and the quality of the optical components used.

Here are some approximate distance limitations for common data rates in single mode fiber:

    10 Gbps (10 Gigabit Ethernet):
        Standard SMF (ITU-T G.652): up to 40 km
        Enhanced SMF (ITU-T G.655/G.656): up to 80 km
    40 Gbps (40 Gigabit Ethernet):
        Standard SMF (ITU-T G.652): up to 10 km
        Enhanced SMF (ITU-T G.655/G.656): up to 40 km
    100 Gbps (100 Gigabit Ethernet):
        Standard SMF (ITU-T G.652): up to 10 km
        Enhanced SMF (ITU-T G.655/G.656): up to 40 km

These distances can be further extended using optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs), and other advanced technologies like dense wavelength division multiplexing (DWDM).

Please note that these distance limitations are approximate and can vary depending on the specific fiber type, component quality, and network design. It's essential to consult the manufacturer's specifications and follow proper design practices to achieve the best results.

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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.



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