Published In September 2000
Private networks in premises and campus environments are increasingly moving toward high-speed applications, such as Gigabit Ethernet, in order to handle the ever-increasing bandwidth requirements for more and faster data transmission. Single-mode optical fibers and multimode fibers designed specifically for Gigabit are being deployed more and more as the high-speed transmission media in the premises/campus market. Consequently, optical signals are being launched by laser light sources as opposed to light-emitting diode (LED) ones. With single-mode links, conventional Fabry-Perot lasers are being used while the new vertical cavity surface emitting lasers (VCSELs) are being installed for short-wavelength Gigabit over multimode fiber. Since lasers are now being used in both multimode and single-mode, testing requirements are changing and proper test equipment and procedures must be used to obtain accurate results. First, let’s look at some fundamental differences between lasers and LED light sources. Then, we will examine the differences between single-mode and multimode fiber, and how different light sources transmit differently over different fibers. This shows why we need to perform testing in a specific way. Lasers versus LEDs Lasers launch light in very high-powered, concentrated beams, while LEDs emit diffuse beams at typically lower power. An LED’s light is more diffused than a laser’s and, consequently, has lower power. Also, a much faster pulse rate is possible with lasers than LEDs, which makes lasers preferable for high-speed networks. Multimode versus single-mode fiber The primary physical difference between multimode fiber and single-mode fiber is the actual core size. multimode fiber is available in two core sizes (50.0m and 62.5m) and single-mode is available at a nominal core size of 9.0m. multimode fiber allows the light to transmit over multiple paths (or modes), while single-mode, as its name implies, allows light to only travel over a single path. This “number of modes” distinction is very important for two reasons: a phenomenon called modal dispersion and loss susceptibility of higher-order modes versus lower-order modes. Modal dispersion Dispersion is the spreading over time of a pulse of light as it travels through a fiber. Modal dispersion occurs because rays of light follow different paths (or modes) through the fiber and arrive at the far end at different times. This only occurs in multimode fibers because single-mode fiber only has one path over which the light is traveling. Dispersion increases with the distance that the light has to travel, so the longer the fiber link, the more dispersion. Because there is no practical way to measure dispersion in the premises environment at present, it is important to measure the length of fibers to ensure they meet the limits specified in application standards. However, dispersion does not affect power loss, which brings us back to the topic of this article. Higher-order modes versus lower-order modes In a single-mode fiber, there is only one mode, called the “fundamental mode” or “lowest-order mode.” This lowest-order mode travels down the center of the core. By contrast, multimode fiber contains many modes. Modes confined to the center of the fiber core are called “lower-order modes,” while those that travel near the exterior of the fiber core are called “higher-order modes.” This distinction is important for loss testing because higher-order modes are more susceptible to loss due to bending of the fiber. This is also where a difference between lasers and LEDs comes into play. Because a laser concentrates the light energy near the center of the fiber, only the lower-order modes are excited when used in a multimode fiber. However, an LED yields what is called an overfilled launch because it completely fills the fiber and excites both lower-order and higher-order modes. Consequently, a multimode fiber using an LED light source is more susceptible to loss than a multimode fiber using, for example, a VCSEL laser source. This is yet another reason why a VCSEL laser source is chosen for a high-speed network application. Now that we understand some of the fundamental differences between light sources and fiber types, let’s discuss the proper means of testing laser-based fiber networks. First, let’s look at single-mode fiber. Testing single-mode fiber networks Single-mode fiber uses lasers for optical transmission, so we should use lasers for testing. But what if you already have an LED source at 1300nm. Can you use it to test the single-mode fiber? The answer is, you might be able to, but it is highly discouraged because such a test method imposes some severe limitations. First, you will have an incredible amount of loss at the fiber interface between the source (which uses a 62.5m core multimode internal fiber) and the network fiber (which is 9.0m core single-mode). This core-mismatch loss will run about –20 dB. Consequently, you will be limited in the distance over which you can actually test. But more importantly, if you are measuring loss at only 1300nm, you are missing an important measurement parameter for single-mode, and that’s knowing what the loss is at 1550nm. When testing single-mode, you should properly characterize the link under test for any wavelength at which it might operate. If you test single-mode with an LED as discussed above, you will still only have an assessment of the fiber link at 1300nm. But because single-mode networks may use 1550 nm for optical transmission sometime in their lifetime, it is important to test at both wavelengths. It is especially important to test at 1550nm, because the 1550nm wavelength is much more susceptible to bending losses than is the 1310nm wavelength. If you install a fiber network and test only at 1310nm, you could still have a serious bend radius violation somewhere in the fiber and not be able to pick it up at 1310nm. If the network ever runs at 1550nm, it could fail because this wavelength is much more sensitive to bending losses. So it is very important to follow proper testing practices for single-mode fiber. The bottom line is that you should always test a single-mode laser-based network with lasers and test at both 1310nm and 1550nm wavelengths. Some testers on the market today allow you to follow these rules while very productively testing two fibers at both wavelengths at once. Testing Gigabit Ethernet over multimode fiber Now let’s look at the other type of laser-based fiber network where multimode fiber is being used to run Gigabit Ethernet. The allowable loss budgets for high-speed Gigabit networks are much tighter than for 10 or 100 Mbps. Thus, the test method and accuracy are correspondingly more critical. With Gigabit Ethernet, VCSEL lasers are the sources used for optical transmission at 850nm and conventional Fabry-Perot lasers at 1310nm. (Because VCSEL lasers at 1310nm are not commercially available yet, a conventional Fabry-Perot laser is used.) Each wavelength has two choices for testing, either a VCSEL or LED at 850nm and either a laser or LED at 1310/1300nm. Let’s take a closer look at these choices. It is important that you test with the same type of source as the network uses, so lasers are the best choice. So why can’t you test a multimode Gigabit Ethernet link with an LED? Recall the basic differences between LEDs and lasers. Here’s why. An LED transmits a wide and diffuse array of light energy that will fill the multimode fiber and have far higher-order modes in the fiber than will a laser. The higher-order modes are more susceptible to bending loss. Also, if there is any misalignment at the fiber-to-fiber connection (some variation is always present), the receiving fiber will not capture all the light energy. By contrast, a concentrated beam of laser light is not nearly as sensitive to misalignment between fibers. Consequently, the LED test will exhibit much higher loss (due to both bending loss and connector loss) than will the VCSEL test. The LED test could very likely give a false “fail” result for the link (especially with the tight loss limits imposed by the Gigabit Ethernet standard). This could mean wasting time looking for a problem that does not exist. This same argument applies to testing Gigabit multimode fiber at 1310nm. By using the proper test equipment and procedures, you’ll obtain accurate results that you can trust. TAYLOR is manager for Fluke Networks’ fiber optic test products business. For more technical information, contact (888) 99-FLUKE or visit www.flukenetworks.com.