Introduction to Fiber Optics

Introduction to Fiber Optics

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Fiber Optic Cable Structure

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General Fiber Information

Browse Our Selection of Optical Fiber and Patch Cables

A fiber optic is made of three main parts, labeled in the figure to the right. The core, made of glass or plastic, provides the path for light propagation. Larger core sizes allow a larger amount of light, or a larger beam diameter, to enter the fiber. The numerical aperture (NA) of the core determines the range of incident angles the fiber can accept and still perform within its specified range. The cladding prevents light from exiting the core and being absorbed by the rest of the cable. The coating, or buffer, protects the core and cladding and provides strength.

When the fiber is manufactured into a cable, the next layer is a material, such as Kevlar, that provides strength to the cable and helps prevent damage due to stress. The entire package is then encased in a jacket. This outer jacket provides one last layer of protection and also adds strength to the fiber. The jacket is typically colored to help the user determine what type of optical fiber is in the cable.

Thorlabs follows the industry standard in jacket coloration. We use a yellow jacket for our Single Mode (SM) fibers, a orange jacket for our Multimode (MM) fibers, and a blue jacket for our Polarization Maintaining (PM) fibers. Our custom patch cables can be made with any jacket color / fiber combination.

Patch Cable Inspection
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Patch Cable Inspection at Thorlabs in Newton, NJ

Select a Fiber

Thorlabs offers four major types of fiber: Single Mode (SM), Multimode (MM), Polarization Maintaining (PM), and Doped. Each fiber type is explained in detail below. Click here to view all of the fiber options Thorlabs offers. Please contact Tech Support if you have any other questions about our fiber.


Light Propagation Down Single Mode Fiber

Single Mode (SM) Fiber

SM fiber has small core sizes that only allow one mode, or ray, to propagate through the fiber. The mode defines how the light travels through space. Light propagates along the axis of the fiber in this single mode (see drawing to the right). In SM fiber, waves have the same mode but different frequencies. This type of fiber is useful in situations where the integrity of the incident pulse of light needs to be retained over long distances. SM fiber offers high bandwidth and low modal dispersion. Our complete selection of SM fiber can be viewed here.

Photosensitive SM Fiber
Photosensitive single mode fiber is designed to provide high photosensitivity for UV radiation. These fibers offer lower splice loss than standard SM fibers and are suitable for a range of applications. For more information about these fibers, click here.


Light Propagation Down Step-Index Multimode Fiber

Light Propagation Down Graded-Index Multimode Fiber

Multimode (MM) Fiber

The larger core diameters of multimode (MM) fiber allow for the propagation of more than one mode. Light not only propagates along the axis of the fiber, as in SM fiber, but also travels away from the axis toward the cladding (see animations to the right). The total internal reflection that occurs at the core-cladding boundary helps reflect the light back towards the fiber axis. MM fiber tends to have a higher NA and larger core sizes than SM fiber, which allows it to gather larger beams of light at greater incident angles. It has lower bandwidth than SM fiber and is susceptible to modal dispersion.

Modal dispersion is a distortion of the incident light pulse caused by the fact that the propagation velocity of the different modes varies. Due to the “zigzag” path the modes take to travel down the fiber, the modes that zigzag more take longer to reach the end than those that travel in a straighter path. When all modes, both fast and slow, combine again at the other end of the fiber, the pulse is widened.

There are two main types of MM fiber: Step Index and Graded Index. The core in a step-index fiber has a uniform refractive index throughout. There is a sharp decrease in refractive index at the core-cladding boundary where the cladding refractive index is lower than that of the core. This results in the modes traveling down the fiber in a very jagged path (see animation to the right). Step-index fiber is generally made by doping the fiber with another material. 

The refractive index of the core in a graded-index fiber decreases as the distance to the center of the core increases. This results in a much smaller change in the refractive indice at the core-cladding interface. The smoother transition causes the modes to travel in sinusoidal paths down the fiber (see animation to the right). Graded-index fibers have much lower modal dispersion than step-index fibers. The parabolic wave profile of the modes continuously re-focuses the rays. Those traveling straight down the center of the fiber travel much slower than those traveling in a more sinusoidal path due to the differences in refractive index. The resulting pulse is less spread out and very close in profile to the incident one.

More information about Thorlabs' complete selection of MM fiber can be found here.

Solarization-Resistant MM Fiber
Solarization-Resistant multimode fiber exhibits impressive performance and transmission from the UV to the NIR (180 to 1150 nm). With exceptional UV radiation resistance compared to standard fibers, these multimode fibers are ideal for use in applications such as spectroscopy for pollution analysis and chemical processing, UV photolithography, and medical diagnostics. The polyimide buffer allows this fiber to be used at temperatures up to 300 °C. For more information about these fibers, click here.


Polarization-Maintaining (PM) Fiber

The polarization of incident light is maintained during propagation through polarization-maintaining (PM) fiber. There are many types of PM fibers, but they all work the same way: stress is induced in the core via rods within the cladding. The stress aligns the fiber, and the light, to a particular polarization. Thorlabs offers two types of PM fiber: PANDA style and Bow-Tie style. The types are named for the shape of the stress rods incorporated into the fiber (see drawing to the right). PM fiber is used in fiber optic sensing, interferometry, and quantum key distribution. It is also commonly found in telecommunications applications connecting a laser source and a modulator. PM fiber has higher attenuation than SM and MM fibers.

It is important to note PM fiber does not polarize the incident light; rather, it just maintains the existing polarization of the light that is aligned with the stress rods. The fiber key is aligned during the manufacturing process to ensure high-quality output, as evidenced by the polarization extinction ratio (PER). A higher PER indicates that the light exiting the fiber has a polarization that is more consistent with that of what entered.

Click here for more information about our PM fibers.


Doped Fiber

Erbium-Doped SM Fiber
Our wide range of highly doped erbium fibers are suitable for fiber lasers and amplifiers operating in the 1530 to 1610 nm wavelength region. These fibers are utilized in a broad range of applications, ranging from telecommunication amplifiers (EDFAs) to high-power PON/CATV boosters and ultra-short pulse amplifiers used in instrumentation, industrial, and medical applications. For more information about these fibers, click here.

Ytterbium-Doped MM Fiber
Thorlabs offers state-of-the-art Ytterbium doped optical fibers for optical amplifiers, ASE light sources, and high-power pulsed and CW fiber laser applications. These fibers are fabricated using the latest doped fiber production technology. For more information about these fibers, please click here.

Ytterbium-Doped PM Fiber
Thorlabs' Ytterbium-doped PM fiber is manufactured using the latest technology. These fibers offer high birefringence, low nonlinear effects, and low photodarkening. For more information about these fibers, click here.

Passive Double Clad Fiber
Thorlabs' passive large-mode-area (LMA) fibers are matched to the core diameters and numerical apertures of their active counterparts to maintain excellent beam quality throughout fiber laser or amplifier systems. The outer cladding diameter is designed to "round" the shaped active fibers, thereby achieving low pump coupling loss from passive to active fibers. The passive fibers are coated with low-index fluoroacrylate enabling active fibers to be pumped through them. For more information about these fibers, click here.

Choose a Connector

A connector terminates the end of an optical fiber and enables quick, easy connection and disconnection. The connectors mechanically couple and align the cores of the fibers so that light can pass from one to the other unobstructed. 



A flat-cleave is a carefully controlled break in the fiber perpendicular to the fiber axis, resulting in a flat end face. No connector is attached to the fiber. A flat-cleave allows for bare fiber connection. Flat-Cleaves are ideal for mechanical or fusion splicing or free space applications without the use of a connector.


Scissor Cut

A scissor cut is a very quick cut that will not produce an even output or splice surface on the end of the fiber. This cut is ideal for the user who is proficient in cleaving fibers or intend to terminate a fiber with their own connector. The end of a scissor cut fiber must be cleaved and connectorized before it can be used.


FC/PC Connectors

The threaded FC/PC connector is designed for high vibration environments. The "PC" stands for "physical contact" because this connector allows the fibers' surfaces to be in direct contact with each other at the connector interface. The ceramic or stainless steel ferrule, or end, of an FC/PC connector is spring loaded to control the force on the fiber as the connector is screwed into its port.

Single Mode FC/PC Connectors

Single Mode FC/PC Connector
Single Mode FC/PC Connector

Our single mode (SM) FC/PC connector features a pre-radiused (R20 mm) ceramic ferrule to help minimize back reflections. The SM FC/PC connector has a hole size tolerance of +1/-0 µm and a maximum concentricity of 1 µm.

Multimode FC/PC Connectors (Stainless Steel or Ceramic Ferrules)

Multimode FC/PC Connector
Multimode FC/PC Connector

Our multimode (MM) FC/PC connector has a precision-drilled bore to match the fiber diameter and a maximum concentricity of 3 µm.

Polarization-Maintaining FC/PC Connectors

Polarization-Maintaining FC/PC Connector
Polarization-Maintaining FC/PC Connector

For Polarization-Maintaining (PM) fibers, we offer a FC connector with a continuously adjustable key to allow you to rotate the back of the connector to align to the slow or fast axis of the fiber. Once the connector is aligned, it can be locked in place with a drop of superglue.


FC/APC Connectors (Single Mode or Multimode)

FC/APC Connector
FC/APC Connectors

This connector has the same basic design as the FC/PC connector, but the fiber end is polished at an angle. This “Angled Physical Contact” (APC) interface prevents light reflected at the fiber-fiber junction from traveling back up the fiber. FC/APC connectors only mate properly with other FC/APC connectors. Mating FC/APC with any other connector results in high insertion loss. These connectors minimize back reflections but have a higher insertion loss than their FC/PC counterparts.

All of our FC/APC connectors offer a minimum back reflection of -65 dB due to the nature of the APC end. Thorlabs' APC connectors are distinguished by the use of a green strain relief boot.


SMA Connectors

SMA Connector
SMA Connector

Our subminiature version A (SMA) connectors are used for large core, multimode fibers. These connectors are threaded like our FC/PC and FC/APC connectors. We stock SMA connectors for fibers with cladding diameters ranging from 125 to 1580 µm.


ST®* Connectors (Single Mode or Multimode)

Straight Tip (ST) connectors have a bayonet-style mount that allows for quick connects and disconnects but does not seat the fiber as well as other connections.

ST Connector
ST Connector

Our single mode (SM) ST connector features a ceramic ferrule with a pre-radiused tip (R20 mm) to minimize back reflections. The ST connectors feature a concentricity of maximum 1 µm.

We also carry ST-style connectors designed for multimode (MM) applications. Our standard connectors have a bore size of 140 µm but we also carry a full supply of drilled conectors to meet custom requirements. These connectors feature a maximum concentricity of 1 µm.

*ST® is a registered trademark of Lucent Technologies, Inc.


SC Connectors

SC Connector
SC Connector

Subscriber Connector (SC) connectors are snap-in connectors that are easy and quick to use. These connectors are available on custom pre-built fiber patch cables, which can be configured here.


LC® Connectors

LC Connector
LC Connector

Lucent Connectors (LC) are similar to SC connectors but contain ferrules that are half the size of those found on SC connectors. We stock LC connectors for single mode fibers. Multimode LC connectors are available upon request. Due to their small size, they are ideal for situations where a large number of connectors are used in a small space.

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Mating Between a Narrow-Key Mating Sleeve and Connector

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Mating Between a Wide-Key Mating Sleeve and Connector 

FC/PC and FC/APC Patch Cable Key Alignment 

FC/PC and FC/APC Patch Cables are equipped with either a 2.0 mm narrow or 2.2 mm wide alignment key that fits into a corresponding slot on a mated component. These keys and slots are essential to correctly align the cores of connected fiber patch cables and minimize the insertion loss of the connection.

As an example, Thorlabs designs and manufactures mating sleeves for FC/PC- and FC/APC-terminated patch cables to precise specifications that ensure good alignment when used correctly. To ensure the best alignment, the alignment key on the patch cable is inserted into the corresponding narrow or wide-key slot on the mating sleeve.

Wide-Key-Slot Mating Sleeves
2.2 mm wide-key-slot mating sleeves are compatible with both wide-key and narrow-key connectors. However, using a narrow-key connector in a wide-key slot will allow the connector to rotate slightly in the mating sleeve (as shown in the animation below and to the left). While this configuration is acceptable for patch cables with FC/PC connectors, for FC/APC applications, we recommend using narrow-key-slot mating sleeves to ensure optimum alignment. 

Narrow-Key-Slot Mating Sleeves
2.0 mm narrow-key-slot mating sleeves allow for optimal alignment of angled, narrow-key FC/APC connectors, as shown in the animation below and to the right. Therefore, they are not compatible with connectors that have a 2.2 mm wide key. Please note that all FC/PC and FC/APC patch cables manufactured by Thorlabs use narrow key connectors.

Narrow-Key-Slot Mating Sleeve and Narrow Key Connector
Once a narrow key connector is inserted into a narrow-key-slot mating sleeve, the connector will not rotate. We therefore recommend these mating sleeves for FC/PC and FC/APC connectors with narrow keys.
Wide-Key-Slot Mating Sleeve and Narrow Key Connector
When a narrow key connector is inserted into a wide-key-slot mating sleeve, the connector has room to rotate. For narrow key FC/PC connectors, this is acceptable, but for narrow key FC/APC connectors, significant coupling losses will result.

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