It is well known that optical cable systems are inseparable from optical transceivers and optical fibers when transmitting optical signals. There are two main types of optical transceivers: light-emitting diodes (LEDs) and laser illuminators (Laser). Although laser illuminators are far superior to light-emitting diodes in terms of performance, due to manufacturing cost problems, most LAN users have been unable to afford the high price of laser illuminators. Until recently, the emergence of a new type of vertical cavity surface illuminator VCSELs (Vertical cavity surface emitting lasers) solved this problem. VCSELs absorb the performance advantages of laser light-emitting devices such as high response speed, narrow transmission spectrum, and the advantages of light-emitting diodes such as high efficiency and low cost. Therefore, low-cost and high-performance VCSELs can be used to transmit signals up to 10Gb/s in combination with multimode fiber optic cables.
However, another problem arises in front of the user, that is, the transmission distance. In addition to the transmission rate, users who use fiber optic cables also have requirements for transmission distance. Experiments show that traditional multimode fiber optic cable, whether it is 50μm or 62.5μm, can support 10Gb/s network transmission, but its support distance is less than 100 meters, which is simply unsatisfactory for the application of network backbone.
Transmission bottleneck of multimode fiber--DMD
Why can I support 2000m multimode fiber at 100Mbps and only support 550m at 1Gbps? The main reason is due to the DMD phenomenon of multimode fiber. After testing, we found that when the multimode fiber transmits the light pulse, the light pulse will diverge and broaden during the transmission process. When the divergence condition is serious to a certain extent, the front and rear pulses will overlap each other, making the receiving end impossible to be accurate. Resolving each optical pulse signal, this phenomenon is called DMD (Differential Mode Delay). The main reason for this is that the same optical pulse in a multimode fiber contains multiple modal components. From the perspective of optical transmission, each modal component travels differently in the fiber, for example, along the center of the fiber. The light component has a different path than the light component transmitted through the fiber cladding reflection. From the perspective of electromagnetic waves, this three-dimensional space in the inner diameter of the multimode fiber contains many modal (300-1100) components, and its composition is very complicated.
When we use a light-emitting device such as an LED, the light emitted by the surface light source LED is filled throughout the fiber (we call it an overfilled model). Light energy is evenly distributed across all modal components. Since the arrival time of the light components transmitted by the different paths is different, the light pulse is gradually widened. However, due to the even distribution of energy, each light component has a small effect on the entire light pulse. Technically, we call this bandwidth Overfilled Launch Bandwidth (OFB); when we change the LED to Laser, the situation will be It becomes different. Since the laser contains only a few modal components, the energy carried by each light component is very concentrated, so that each light component will have an important influence on the entire light pulse. To take an extreme example, if there are only two modal components, they arrive at different times, which can lead to severe divergence of the entire light pulse.
Due to the performance limitations of the LED light-emitting device itself, in high-speed applications above 1 Gbps, the light-emitting device mainly uses a laser light-emitting device, and the conventional multi-mode optical fiber is based on the LED method both in terms of standard and design, and therefore, due to two kinds of light-emitting devices. Different ways of transmission, the fiber itself must be modified to accommodate changes in the source. Therefore, ISO/IEC 11801 set out to develop a new multimode fiber standard grade, the OM3 category, which was officially promulgated in September 2002. In Avaya's SYSTIMAX SCS structured cabling system, our company's OM3 multimode fiber-LazerSPEED solution has been introduced prior to international standards for the current potential 10Gb/s network applications.
The reclassification of multimode fiber in the ISO standard, OM1 refers to the current traditional 62.5μm multimode fiber, OM2 refers to the current traditional 50μm multimode fiber, and OM3 is a new 10G fiber. Note the two modes of fiber bandwidth metrics, Overfilled Launch Bandwidth is a matching indicator for LED illuminators, and Laser Bandwidth is a matching indicator for new laser illuminators. The OM3 cable is optimized in both modes. Another thing to note is the choice of transmission wavelength, 850nm or 1300nm. Although the longer the wavelength, the better the performance will be, but the cost of the light-emitting device will increase exponentially. Therefore, if possible, users should choose a short-wavelength application system to reduce the cost. For example, the new VCSELs are based on short wavelengths, while standard Lasers are used in long wavelength environments.
OM3 fiber test problem
The main steps of the DMD test are: using a 5μm single-mode probe connected to the OM3 fiber to be tested, the optical pulse is continuously generated to the fiber under test by the single-mode probe, and at the same time, the probe is scanned and moved from the fiber axis. The heart moves toward the edge, moving about 1 μm each time. At the receiving end, the light pulses at each location are recorded and superimposed on the same time domain map to form a DMD indicator. The arrival of the light pulse will produce a time difference due to the different paths, and at the same time, because the light pulse itself will diverge, the difference between the two aspects will be added, and according to the standard comparison, it is used to determine whether the OM3 fiber meets the standard.
Because of the above tests, extremely precise equipment and test methods are required. It is currently impossible for users to conduct tests on site and can only be tested by specialized laboratories. Avaya's SYSTIMAX laboratory is one of the few places where the above tests can be performed.
The performance advantages of OM3 fiber due to the upgrade pressure faced by users in network applications, from the current 1Gb/s to the future 10Gb/s, how to target the current application and achieve smooth upgrade in the future, every user needs to be careful Consider the issue.
In the current 1Gb/s network era, traditional multimode fiber supports a distance of no more than 550 meters, and the use of single-mode fiber means that expensive laser light-emitting devices are used at the same time, although the cost of the two systems is almost the same in the wiring system. On a network device, two choices mean that the price differs by at least a factor of two. In many cases, when the user's transmission distance exceeds 500 meters, but within 1000 meters, the laser device has to be used. The new OM3 multimode fiber extends the support distance of Gigabit Ethernet to 1000 meters without the need for expensive laser devices. Therefore, at this stage, it can bring great performance advantages to users.
As the network system is in the process of continuous upgrading, especially the network backbone system, according to experience, the network backbone speed will increase by 10 times every 5 years. Therefore, in the next 2-3 years, 10 Gigabit Ethernet will inevitably appear in the environment of high-end network users.
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