There are two main types of people who ask this question. One is engineers from optical module factories/optical chip factories/optical device factories, and the other is college students majoring in optical fiber communication. There are many fiber optic cables on the desks of these people’s laboratories, and they often need to find single-mode or multi-mode optical fibers to build a test environment and adjust optical parameters.

How to Distinguish Single-mode from Multimode Fiber?

The optical fibers that can be taken back and forth frequently are mostly short jumper fibers. In the market, the most popular single-mode fiber patch cables are yellow and multimode-fiber are aqua. However, they may different from vendor to vendor. This type of scene can be identified by the printing of the jumper fibers.

Multimode Fiber Patch Cable Printing

The outer layer of the multimode jumper fiber has a printed logo, it may say FIBER CABLE MM 50/125UM OM3.

 

MM is the abbreviation of Multimode. I will explain what the mode of optical fiber is and how to count as “multi” mode later. This MM is often confused with the length unit mm. 50/125UM, UM stands for micrometer μm. Some notations are in uppercase, some are in lowercase, it doesn’t matter, the meaning is the same, this is a unit of length. 125 micrometers refers to the diameter of the fiber cladding, and 50 micrometers refers to the diameter of the fiber core.

OM3 is one of the categories in the multimode fiber standard. The multimode fiber categories we use are mainly OM3, OM4, and OM5. There are differences in transmission performance. The mode bandwidth product of OM4 is larger than that of OM3. I won’t talk about it in detail today.

Single-mode Fiber Patch Cable Printing

The outer layer of the single-mode fiber patch cable also has a printed logo, it often looks like FIBER CABLE SM 9/125UM G652. 

SM, Single-Mode, single-mode. 9/125um, um micrometer, 125 micrometers is the diameter of the fiber cladding, and 9 micrometers is the diameter of the core. G652 means single-mode fiber standard G.652. Commonly used single-mode fiber standards are G.652 and G.654, of course there are some less commonly used ones such as G.653, G.657, etc. The single-mode optical fiber of the G.652 standard is also divided into G.652A, G.652B, … G.652D and other subcategories.

Optical fiber is actually very thin, 125μm = 0.125 mm, which is not convenient for us to take in the laboratory. In addition, optical fiber is a glass filament and is easily broken. Therefore, the optical fiber you see on hand is the “optical fiber” after layers of protection. The two fibers you just saw have an outer diameter of 3mm, which is easy to “pinch” when you hold it in your hand.

 

If you are doing silicon photonic chip packaging, you often use capillary fibers, that is, you can see the coating layer. The coating layer is not the optical fiber, but the first protective layer closest to the optical fiber. It is generally coated with resin on the surface of the glass optical fiber, usually with a diameter of 250μm, that is, 0.25 mm.

Silicon photonic chips require 4x, 8x, 16x… optical fiber ribbons, which are of this size level. Some coatings have colors, and some do not. This scale is as thin as a hair, and it is difficult to see the logo. You need to look for some traces of single-mode and multi-mode on the packaging box or other related parts.

Understanding Fiber Construction: Core, Cladding and Coating

The coating can be understood as a hard plastic layer, which is not used to guide light. The light guide is glass material. The optical fiber is a glass rod heated and drawn into a filament. The diameter of this glass filament is 125μm. This is what we commonly call the “cladding” diameter. The core is the area that can guide light. It is integrated with the cladding of the optical fiber. The optical core and cladding are achieved by controlling the refractive index of the material during the manufacture of optical fiber. The common process is to dope germanium ions in a part of the glass area. Where there are germanium ions, the optical refractive index increases and a core area is formed.

The coating and cladding can be separated, and the coating can be stripped with a wire stripper. The cladding and core are both glass, and the two cannot be separated and are integrated, nor can they be simply identified by the naked eye.

What is the Difference Between Single-mode and Multimode Fiber?

Next, I will explain the difference between single-mode and multi-mode optical fibers based on the scale of the coating diameter.

First, let’s define the premise. The following paragraph distinguishes between OM3 and G652 multi-mode and single-mode, that is, the difference between the 50-micron core and the 9-micron core diameter. Not all single-mode and multi-mode are of this size.

Core Diameter

The core diameter of multimode optical fiber is defined as 50μm, which is 0.05 mm by the OM3 standard. The core diameter of single-mode optical fiber G.652 is 9μm, which is 0.009 mm. This is the source of the two sizes of 50/125um and 9/125um of multimode and single-mode optical fibers commonly used in our optical module industry chain.

The multimode mode is a mode with a large number of transverse modes. If the core diameter is reduced and only one basic mode is allowed to be transmitted, it is a single-mode optical fiber.

Couple

The 50μm diameter core is easy to couple, but it is multimode. A 9μm diameter core can achieve single-mode, but its disadvantage is large coupling loss, high coupling difficulty, high coupling accuracy requirement, and precise control of fiber stability to avoid displacement.

What is transverse mode? Light has wave characteristics. The light mode in the cross section of the light transmission direction is the transverse mode. When the flashlight is shone on the wall to look at the light spot, there is only one bright spot, which is single-mode, and there are many bright spots, which are multi-mode.

Performance

Single-mode has only one mode, and the transmission method in the optical fiber is very simple, with good performance and can be transmitted over a long distance. In comparison, there are many multi-mode modes (except for the basic mode, other modes are called high-order modes), and the subtle transmission path of each mode in the optical fiber is different, so after the mode and mode are transmitted for a certain distance, “delay between modes” will be generated, resulting in performance degradation.

400G multimode modules, 800G multimode modules, the max. transmission distance is generally from tens of meters to one or two hundred meters. For example, 400G QSFP-DD SR8 reaches up to 100m.

Single-mode optical modules, 400GBASE-ER8 single-mode can transmit 2 kilometers, 800G DR8 single-mode modules can transmit 500m, 400G DP-QPSK coherent optical modules can transmit hundreds or even thousands of kilometers, and the record in 2023 is 7,000 kilometers.

Single-mode and multi-mode, macroscopically distinguished, that is, under the premise of the same material structure, the single-mode core diameter is smaller, and the multi-mode diameter is larger. There are clear academic formulas to distinguish them, but today I will not write the formulas for popular science.

 

Let’s distinguish another common misunderstanding.

Is Single-mode Fiber used for Long Wavelengths and Multimode for Short Wavelengths?

This statement is both right and wrong.

It is right because the standard does define it this way. The wavelength of single-mode G652 is 1260-1650nm, and the communication core of OM3 is around 850nm.

It is wrong because the cause and effect are reversed. Single-mode is not only used for long wavelengths, but the current single-mode optical fiber is made of glass. The loss of glass material for the two bands of 1310nm and 1550nm is relatively low. We design lasers in this band to accommodate the glass material of the optical fiber. If we do not use solid-core glass fiber, but hollow-core fiber, then the wavelength of single-mode can also be changed to 850nm. The low-loss window of hollow-core fiber can be 850nm.

Multimode can also use the 1260-1650nm band, as long as the core diameter is made larger. The industry did not choose it because the lasers in the 1260-1650nm band are more expensive than the 850nm lasers. Our multi-mode transmission performance is very short. Others have to consider the loss of one thousand kilometers, while our loss of one hundred meters is not much. Why do we need a bicycle? Just use a cheap short-wavelength laser.

The application of single-mode long wavelength formed by the industry is due to the need to improve performance, and the overall cost is relatively high. The application of multi-mode short wavelength is just to accommodate the low-cost solution.

 

Short-wavelength lasers, 850nm-940nm, can be made into multi-mode or single-mode. Long-wavelength lasers, 1260nm-1650nm, can be made into single-mode or multi-mode. Similarly, optical fiber can achieve single-mode communication or multi-mode communication at 850nm-940nm, and optical fiber can achieve single-mode communication or multi-mode communication at 1260nm-1650nm.

For a long time, the industry has formed a multi-mode combination of 850nm short wavelength and a single-mode combination of long wavelength, that’s all. The industry’s choice is based on the trade-off between cost performance and performance. Short wavelengths are cheaper when used with multimode communications, while long wavelengths are better when used with single-mode communications.

Hollow-core optical fiber combines the advantages of low cost of short wavelengths and high performance of single-mode. The disadvantage is that hollow-core optical fiber is difficult to manufacture.