The Complex Made Simple: Everything You Wanted to Know About SFP Modules. Part 2. Optical Cables

Why This Article?

This article covers the fundamentals of optical cables, including:

• The structure of optical fiber, patch cords, and pigtails
• Simplex and duplex technologies
• Fiber types and their classification
• Connectors and adapters
• IL and RL parameters
• Cable jacket types
• Practical recommendations for selection

What Are Optical Fiber, Patch Cords, and Pigtails?

Optical Fiber Structure

Optical fiber is designed for transmitting traffic over long distances. It consists of:

• Core — quartz glass with a diameter of 9 micrometers (SM) or 50–62.5 micrometers (MM)
• Buffer coating — provides total internal reflection and protection
• Kevlar threads — for mechanical protection
• Outer jacket — protection from moisture, chemicals, and abrasion
• Color coding — fiber type identification

Pigtail

A pigtail is a section of fiber optic cable with a connector on one end and a free end for splicing with the main fiber.

Patch Cord

A patch cord is an optical fiber with connectors on both ends. Connectors can be of various types (LC, SC, FC, ST, MTP, etc.).

Simplex and Duplex

Simplex cable — one fiber, unidirectional data transmission. Used in both SM and MM fibers.

Duplex cable — two fibers, enabling bidirectional data exchange. Consists of two simplex fibers joined together.

When using standard SFP+ modules at 10GbE over a duplex cable, up to 20 Gbps can be transmitted (10 Gbps in each direction).

Non-Standard Solutions: BiDi Modules

There are so-called "one-eyed" modules that transmit and receive signals in both directions over a single fiber. Examples include BiDi/WDM modules, where transmission occurs at one wavelength (1490 nm) and reception at another (1550 nm), allowing a single simplex fiber to operate in duplex mode.

Fiber Types

The Fundamental Difference

Fibers are divided into two types based on the number of modes (light rays):

• Single-mode (SMF) — transmits a single ray of light
• Multi-mode (MMF) — transmits multiple rays

SMF (Single-Mode Fiber)

Core diameter: 9/125 micrometers

Characteristics:

• Low signal attenuation
• Long-distance transmission capability (10 km — 160 km)
• Used for backbone and inter-floor connections
• Compatible with WDM/DWDM technologies

MMF (Multi-Mode Fiber)

Core diameter: 50/125 or 62.5/125 micrometers

Characteristics:

• Range limited by modal dispersion
• Used for in-building networks and data centers
• More economical than SM over short distances
• Used in local area networks

Multi-Mode Fiber Classes

Single-Mode Fiber Standards (ITU-T)

ITU-T G.652 — the base standard for most applications. Subclasses A and B operate at 1310 nm. Subclasses C and D have low attenuation at 1383 nm (for CWDM).

ITU-T G.653 — obsolete, dispersion-shifted fiber (1550 nm).

ITU-T G.654 — minimal losses at 1550 nm, used for long-haul backbone and submarine cables (very expensive).

ITU-T G.655 — for DWDM systems at 1550 nm, reduced chromatic dispersion.

ITU-T G.656 — zero dispersion in the 1460–1625 nm range, compatible with DWDM and CWDM.

ITU-T G.657 — bend-resistant, suitable for indoor installation. Subclasses A1 and A2 differ in minimum bend radius.

OS1 and OS2 Specifications

OS1 (corresponds to G.652 A, B):

• Used for indoor installation
• Cables with "tight buffer" construction
• Attenuation up to 1.0 dB/km

OS2 (modern standard):

• Includes G.652.C, G.652.D, G.657.A1/A2
• Universal application (indoor and outdoor)
• "Loose tube" construction for outdoor use
• Attenuation 0.4 dB/km

Connectors and Adapters: How to Join Optical Fiber

Purpose of Adapters

An optical pass-through adapter is a coupler for connecting two connectors without splicing the fiber. The adapter aligns the ferrules and fixes their position for data transmission.

Connectors with 2.5 mm Ferrule Diameter

FC Connector:

• Outdated but reliable
• Metal body with threaded connection
• Vibration-resistant
• Simplex only
• Compatible with SM and MM

ST Connector:

• Outdated but reliable
• Metal body with mechanical locking
• Used in high-vibration environments
• UPC polishing only
• Simplex only
• Compatible with SM and MM

SC Connector:

• Popular connector
• Rectangular plastic body
• Less reliable than FC/ST
• Simple snap-in connection
• Simplex and duplex versions
• Compatible with SM and MM

Connectors with 1.25 mm Ferrule Diameter

LC Connector:

• Currently the most popular
• Smaller than SC
• High port density (~40% more ports than SC)
• Requires a special tool ("long-nose pliers")
• Simplex and duplex versions
• Compatible with SM and MM
• Used in most SFP modules

Other 1.25 mm connectors: MU (rarely used), E2000 (rare due to high cost).

Multi-Fiber Connectors: MPO/MTP

These provide switching of multiple fibers (typically 12) in a single connector. MTP is an improved version of MPO.

Standard configurations:

• 8 fibers (rare)
• 12 fibers (most common)
• 24 fibers (2 rows x 12)
• 48 fibers (4 rows x 12)
• 72 fibers (6 rows x 12)

Key differences between MPO and MTP:

Pin construction: MPO has flat pins with chamfer; MTP has rounded pins (higher reliability).

Ferrule mounting: MTP uses floating mount for precise alignment; MPO uses rigid mounting.

Latch: MTP has a metal recess for the spring; MPO has none.

Compatibility: MTP connectors are compatible with MPO adapters. MPO connectors are not compatible with MTP adapters.

Keys and Pins: Male connectors have pins present; Female connectors have no pins. Male-to-female connection is only possible through a pass-through adapter.

Adapter Types:

• Type A: Keys point in opposite directions (one up, one down)
• Type B: Keys point in the same direction on both sides

Cable Types for MTP/MPO:

• Type A (straight): Pin 1 connects to pin 1 on the other side
• Type B (crossover): Pin 1 connects to pin 12, pin 2 to pin 11, etc.
• Type C (pairwise crossover): Pin 1 connects to pin 2, pin 3 to pin 4, etc.

Breakout/Fanout Cable: Splits 12 pins into multiple duplex LC/SC pairs or 12 simplex fibers. More cost-effective than individual cables.

Conversion cables: MTP/MPO 24 to 2x12 fibers; MTP/MPO 24 to 3x8 fibers; MTP/MPO 2x12 to 3x8 fibers.

Ferrule Polishing Types

FLAT and PC: Obsolete, introduce higher attenuation, rarely encountered.

UPC (Ultra Physical Contact):

• Common in standard networks
• Marking: Blue, gray, or black
• Lower cost
• Losses practically identical to APC
• Use: Standard local networks

APC (Angled Physical Contact):

• Ferrule angled at 8–9 degrees
• Marking: Green (important!)
• Less reflection toward the source
• Reduces SFP module heating
• Use: PON, CATV, DWDM, long-distance networks

Important: You cannot connect UPC and APC connectors to each other. APC can be damaged, and the signal will not transmit. MPO/MTP connectors can also have APC polishing (green marking).

IL and RL

IL (Insertion Loss)

Attenuation of the optical signal passing through fiber and connectors.

Causes of loss: partial reflection within the fiber, connector influence, surface contamination, imperfect connections.

Rule: The lower the IL value, the better. Typical values: less than 0.3 dB for quality cables.

Note: Losses are also introduced by optical cross-connects, pass-through adapters, splice points, and pigtails.

RL (Return Loss)

Losses due to reflected signal (echo). When a signal encounters an obstacle, part of it is reflected back.

Sources of reflection: poor contact in the connector, contaminated ferrule, impedance mismatch.

Rule: The higher the RL value, the less "echo" and the better the signal quality.

Note: UPC has return loss of approximately 25 dB; APC has better figures (approximately 40+ dB).

Cable Jacket Types

PVC (Polyvinyl Chloride):

• Contains fire-retardant additives
• When burning: releases highly toxic halogen compounds
• Hazardous to people and electronics
• Use: Only in areas with limited human presence

PE (Polyethylene):

• No fire-retardant additives
• Burns more intensely but less toxically
• Use: Outdoor installation, but not in direct sunlight

LSZH (Low Smoke Zero Halogen):

• Burns slowly
• Does not release toxic gases, does not damage equipment
• Higher cost than PVC and PE
• Used in areas with large numbers of people
• Used for outdoor and backbone installations
• Works in direct sunlight
• Compliant with Russian standards GOST R 53315-2009 and Federal Law No. 123-FZ

Conclusion: LSZH is virtually always used when building modern optical networks.

How to Choose an Optical Patch Cord

Step 1: Fiber Type

Single-mode (SMF): Yellow marking, labeled "OS2" or "9/125". For distances over 100–300 m. Used for backbone and inter-floor/inter-campus connections. WDM/DWDM compatible.

Multi-mode (MMF): OM1/OM2 — orange jacket; OM3 — violet; OM4 — aqua or violet; OM5 — green. Labeled "OM1", "OM2", etc. For distances up to 100–300 m. Used in data centers, server rooms, intra-rack switching.

Step 2: ITU-T Standards

• G.652 (OS1/OS2) — base standard for most applications
• G.657 — bend-resistant
• G.654/G.655 — for backbone and DWDM (low losses at 1550 nm)

Check the cable marking.

Step 3: Duplexity

• Simplex — one fiber, used for PON, WDM
• Duplex — two fibers, standard for SFP/SFP+/QSFP+ (TX/RX separated)

Step 4: Connectors and Adapters

• LC — high mounting density (data centers, switches)
• SC — reliable, convenient for cross-panels
• MPO/MTP — for multi-channel systems (40/100GbE)
• FC/ST — for specialized tasks, high vibration

Check connector compatibility, adapter types (A/B), and polarity (Type A/B/C) for MPO/MTP.

Step 5: Ferrule Polishing

• UPC — blue/gray/black marking, for most tasks
• APC — green marking, for PON, CATV, DWDM, long distances

Step 6: IL/RL Parameters

• IL — the lower (e.g., less than 0.3 dB), the better
• RL — the higher (e.g., greater than 50 dB for UPC, greater than 60 dB for APC), the better

Check manufacturer specifications — cheap cables often have inflated loss figures.

Step 7: Operating Environment

• Indoors: tight buffer
• Outdoors/backbone: modular construction (loose tube), moisture protection
• Flexible solutions: G.657 fibers (bend-resistant)
• Jacket: choose based on fire safety requirements

Practical Examples

Example 1: Standard LC-LC Connection

Task: Connect standard SFP 1.25GbE MM modules at 850 nm wavelength.

Solution: Length: 1–200 meters. Type: Duplex multi-mode (duplex MM). Connectors: LC-LC. Waveguide diameter: 50/125 mm. Class: OM3. Polishing: UPC. Jacket: LSZH.

Designation: "optical patch cord duplex LC-LC UPC 50/125 mm OM3 1m LSZH"

Example 2: Connection with BiDi Modules to Save Fiber

Task: Connect a pair of SM BiDi modules up to 80 km. TX 1550 nm / RX 1490 nm on one side, TX 1490 nm / RX 1550 nm on the other.

Solution: Length: 1–200 meters to the optical cross-connect. Type: Simplex single-mode (simplex SM). Connectors: LC-LC. Waveguide diameter: 9/125 mm. Polishing: UPC. Class: OS2. Jacket: LSZH.

Designation: "optical patch cord simplex LC-LC UPC 9/125 mm OS2 LSZH"

Alternative: You can purchase a duplex variant as a reserve for future connections.

Example 3: Connection with Different Connectors LC-SC

Task: Connect modules with different connectors: LC (TX 1310 / RX 1550 nm) to SC (TX 1550 / RX 1310 nm).

Solution: Length: 1–200 meters to the optical cross-connect. Type: Simplex single-mode (simplex SM). Connectors: LC-SC (adapter). Waveguide diameter: 9/125 mm. Class: OS2. Jacket: LSZH.

Designation: "adapter optical patch cord simplex LC-SC UPC 9/125 mm OS2 LSZH"

Important Notes

• Not all characteristics are printed on the cable itself — check packaging or manufacturer's website
• There are no SM-to-MM adapters
• Some connectors don't have adapters available
• Excess length coiled in a bundle can introduce additional losses due to bends

Best Practices

1. Always clean connectors before plugging in. Ideally, use a new, cleaned connector that stays permanently connected.
2. Avoid sharp bends: For SMF — minimum radius of 5 cm or more. For MMF — minimum radius of 3 cm or more.
3. For critical links, use cables with low IL and high RL parameters.
4. Verify certification of patch cords.
5. Use testers to measure losses on critical segments.
6. Remember: Even a perfect SFP module won't transmit data through a poor-quality patch cord.