SingleMode Fiber
There are four different types of single-mode fiber in popular use today (as of the writing of this paper, May 2002). They are summarized in Table 3. The ITU-T Series G.652 recommendation, commonly referred to as standard single-mode fiber, represents the majority of the installed base of single-mode fiber. The G.652 recommendation describes both standard single-mode fiber (IEC type B1.1) and low water-peak standard single-mode fiber (IEC type B1.3). The performance data in the 10GbE standard is based upon the use of standard single-mode fiber type B1.1 and B1.3 or in other words the overall G.652 recommendation. This does not however preclude the use of other types of single-mode fiber with 10GBASE-E since their use may potentially enhance the performance of a 10GbE link.
Standard Single-Mode Fiber IEC 60793-2 B1.1 & B1.3 / ITU G.652
Standard single-mode fiber is essentially a thin core (5-8 microns) of Germanium-doped glass surrounded by a thicker layer of pure glass and is the overwhelming workhorse of the optical communications infrastructure. Nearly any application can be addressed with standard single-mode fiber, but it is optimized to support transmission at 1310 nm. Performance issues with standard single-mode fiber can become more significant as higher data rates (such as 10 Gbps) and longer distances (>40 km) are encountered. Low water-peak standard single-mode fiber (IEC type B1.3) has the same dispersion characteristics as standard single-mode fiber (IEC type B1.1), but has reduced attenuation in the region of the water peak (nominally 1383 nm). As no specification is given for water-peak attenuation in standard single-mode fiber (IEC type B1.1), attenuation in the region of 1383 nm can be significantly higher than that at 1310 nm. By reducing the water impurities introduced in this region during the time of manufacture, low water-peak standard single-mode (IEC type B1.3) fiber provides identical support to standard singlemode fiber, plus can support additional wavelengths between 1360 and 1460 nm.
Note again that the IEEE 802.3ae specification for 10 Gigabit Ethernet assumes standard single-mode fiber (IEC types B1.1 and B1.3) for all single-mode performance specifications. Additional fiber types (e.g., DSF, NZDSF) may offer benefits beyond the constraints of the standard, but are not required to meet any specifications detailed within the 10GbE standard.
Dispersion Shifted Fiber (DSF) � IEC 60793-2 B2 / ITU G.653
Dispersion shifted fiber (DSF) was introduced in the mid 80�s and represents a very small percentage of the installed base of single-mode fiber. The need for DSF was driven by the development of 1550 nm lasers which have much less fiber attenuation than 1310 nm lasers. DSF allowed optical signals to travel significantly farther without the need for regeneration or compensation due to reduced chromatic dispersion characteristics, effectively allowing an optical pulse to maintain its integrity over longer distances. DSF was well suited to meet these needs for single-channel optical transmission systems. However, with the advent of broadband optical amplifiers and wavelength division multiplexing (WDM) the chromatic dispersion characteristics of DSF presented detrimental effects to multiple wavelength signal integrity. As a result, a new type of fiber was needed, namely non-zero dispersion shifted fiber (NZDSF). NZDSF effectively obsoleted DSF, and thus DSF is no longer commercially offered. DSF is not referred to in the IEEE 802.3ae specification.
Cutoff Shifted Single-Mode Fiber � IEC 60793-2 B1.2 / ITU G.654
Cutoff shifted single-mode fiber is designed to allow for extended transmission distances through lower attenuation and the ability to support higher power signals. This fiber is typically used only for transmission in the 1550 nm region due to a high cutoff wavelength around 1500 nm. Due to significant manufacturing complexity, cutoff shifted single-mode fiber is typically much more expensive than other single-mode fiber types. It is commonly found only in submarine applications due to the stringent requirements in such an environment, and is not likely to be encountered in situations where 10 Gigabit Ethernet transport solutions will be deployed. Cutoff shifted fiber is not referred to in the IEEE 802.3ae specification.
Non-Zero Dispersion Shifted Fiber (NZDSF) � IEC 60793-2 B4 / ITU G.655
Non-zero dispersion shifted fiber (NZDSF) was introduced in the mid 90�s to address issues encountered with multiple wavelength transmission over DSF by maintaining a finite amount of chromatic dispersion across the optical window (typically 1530-1625 nm) commonly exploited by wavelength division multiplexing (WDM). The primary concern addressed by NZDSF is a nonlinear effect known as four wave mixing (FWM). In simple terms, three wavelengths carrying different information can generate signals at another wavelength. In the regularly spaced channel plan of most WDM systems (usually 1.6 nm or less between adjacent wavelengths), the newly generated noise signals can overlap with a wavelength carrying live traffic. NZDSF mitigates this effect by ensuring that all wavelengths in the region of interest (1530-1625 nm) encounter some finite dispersion and thus signals on adjacent wavelengths will not overlap in time for extended periods. Four wave mixing is reduced as the time during which adjacent wavelength signals overlap is shortened. The reduced chromatic dispersion of NZDSF can also reduce the detrimental contributions of other nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM). NZDSF is optimized for transmission in the 1530-1625 nm window, but can support some 1310 nm configurations with proper consideration given to laser type and systemconfigurations.
The IEEE 802.3ae specification makes a brief reference to NZDSF as follows: �It is believed that for 10GBASE-E, type B4 (NZDSF) fiber with positive dispersion may be substituted for B1.1 or B1.3 (standard single-mode fiber). A link using B4 (NZDSF) fiber with negative dispersion should be validated for compliance at TP3�.
| Name | ITU-T | IEC Reference | Optimized Dispersion Range (nm) | Referred to in 802.3ae Specification? |
|---|---|---|---|---|
Standard Single-Mode Fiber |
G.652 | IEC 60793-2 (B1,1/B1.3) |
1300-1324 | Yes |
| Dispersion Shifted Fiber (DSF) | G.653 | IEC 60793-2 (B2) |
1500-1600 | No |
| CutOff Shifted Fiber | G.654 | IEC 60793-2 (B1.2) |
1550-1625 | No |
| Non-Zero Dispersion Shifted Fiber (NZDSF) | G.655 | IEC 60793-2 (B4) |
1530-1565 (C-band) 1565-1625 (L-band) |
Yes |