Influence of polarization mode dispersion on Single-Mode Optical Fiber System

I. Introduction and background

In recent years, optical fiber communication systems have made great progress to meet the increasing bandwidth requirements.

The main factors limiting the performance of single-mode optical fiber (SMF) systems include attenuation, dispersion, and non-linearity, as well as recently noticed polarization mode dispersion (PMD ).

For a traditional single-mode optical fiber system operating in the 1310nm window, the optical power loss caused by the attenuation of the optical fiber limits the transmission distance of the system. In this band, the single-mode optical fiber has zero or very low dispersion, the system transmission rate is also low, and the operation is on a single wavelength. For applications with high transmission rates and long distances, more system operations are performed in the 1550nm window with lower attenuation. The invention of the optical amplifier and the improvement of the laser source and receiver have promoted the application of long distance and high-speed Dense Wavelength Division Multiplexing (DWDM) systems. In these systems, dispersion and non-linearity are the main considerations. In order to reduce the dispersion and nonlinear limitations on the system performance, non-zero dispersion displacement optical fiber (NZ-DSF) has been launched, such as Corning LEAF (Large Effective Area non-zero dispersion displacement optical fiber ).

These new optical fibers and dispersion management technologies have been effectively applied to commercial single-mode optical fiber systems.

With the development of laser transmitters and receivers, the requirements for optical fibers have become increasingly extreme. It has also become one of the main limiting factors for long-distance, high data rate (> 10 Gb/s) Digital Systems and image transfer systems with larger frequency modulation (> 1 GHz.

II. Introduction to PMD

Different transmission speeds of two polarization modulus lead

In single-mode optical fiber transmission, the basis modulus of light waves contain two vertical polarization states. The geometric size of the ideal optical fiber is uniform and there is no stress. Therefore, light waves are transmitted at exactly the same rate in the two vertical polarization states, without any delay at the other end of the optical fiber. However, in the actual optical fiber, two vertical polarization modulus spread at different speeds, so the time to reach the other end of the optical fiber is also different (). The time difference between the two vertical polarization modulus in the unit length, that is, PMD, in the unit of ps/√ km.

The same effect is applied to the system performance due to PMD and chromatic dispersion, that is, pulse broadening is caused, thus limiting the transmission rate. However, PMD is several orders of magnitude smaller than the color dispersion. In addition, it is an important consideration only when certain dispersion compensation is adopted in the digital system and the analog system with high amplification frequency modulation. In addition, this restriction is affected by the transmission mode, environment, and installation conditions.

Unlike the deterministic color dispersion, the PMD of any optical fiber is a random variable that complies with the Maxwllian distribution. The instantaneous PMD value varies with the wavelength, time, temperature, movement, and installation conditions. In addition, studies show that) the relationship varies with the square root of the length [1]. Therefore, the unit of PMD is ps/vkm.

The factors that cause the message loss can be internal (the asymmetry of the core or package Layer Produced by the manufacturing process and the stress on the glass surface) and external (external stress, bending and twisting ). These factors and distance are combined to cause the refraction and modulo coupling, resulting in the formation of information loss. The refractive index of glass varies along the axial direction. Because the propagation speed of the two polarization modulus is different, the delay of receiving signals is caused. Mode Coupling refers to the pulse expansion and delay caused by the energy transfer between two polarization modulus.

Iii. Considerations for system PMD

Limit the Theoretical distance of transmission by PMD

Until a few years ago, in digital and analog systems, when the data transmission rate was low and the distance was relatively short, the impact of PMD on single-mode optical fiber systems was negligible. As the demand for bandwidth increases, especially in systems with 10 Gbit/s and a higher rate, PMD has become a factor limiting system performance, because it will lead to a large pulse widening or a low signal-to-noise ratio (SNR ). Shows the transfer distance restricted by the PMD [2].

Due to the maximum system distance restricted by PMD, it can be obtained theoretically by the following formula:

Use an example as PMDps/VKM) 2.5 Gb/s 10 Gb/s 40 Gb/s

3.0 180Km 11Km <1Km

1.0, 60, 100, 6Km

0.5 6,400Km 400Km 25Km

0.1 160,000Km 10,000 625Km

Table 1 transfer distance to PMD and Data Rate

Comparison: the maximum distance restricted by PMD is listed in Table 1 [3]. Due to the statistical characteristics of PMD, a single optical fiber (or an optical fiber after cabling) is not suitable for system capacity indicators. Otherwise, the link value, that is, the connected Optical Fiber segment, is often used. Because each fiber segment is random, the link value is also random because of the mean effect, which has a smaller variance. The value of the PMD link reflects the value of the system more accurately and effectively, and makes full use of the real potential of the optical fiber. Many end users have adopted this method in their system design. It is worth mentioning that the Cabling Process may slightly increase or decrease the optical fiber's PMD.

The value of the PMD link is expressed by the following formula:

M: Number of optical cables connected to the same length

Xi: PMD of a single optical fiber/Optical Fiber

XM: PMD for connecting Optical Cables

Currently, the Standard Organization (IEC/TIA, ITU) is considering various statistical indicators. IEC puts forward the first draft of two equivalent methods based on the statistical upper limit and system failure time. Specifically, method I is specified as follows:

: PMD connected to M optical fiber segments
: The value of the PMD link determined by the manufacturer
Q: Maximum statistics
Method II starts from the budget of the portion of the total dispersion allocated to the PMD portion, and is calculated by the instantaneous PMD portion. The manufacturer provides the probability distribution of PMD, And the instantaneous PMD value from the end to the end is greater than a given small probability value, P. This probability is then converted to the fault time of the line.

Iv. PMD measurement

Standard Organization (IEC/TIA, ITU) recommends four methods for measuring the Single-Mode Optical Fiber PMD. They are: Jones array feature analysis (JME, FOTP 122), Interferometer (IF, FOTP 124), wavelength scanning cycle counting method (WSCC, FOTP 113) and Fourier transform wavelength scanning method (WSFFT, FOTP 113 ). The IF method is used to directly measure PMD, which belongs to the time domain measurement method, and the other is the frequency domain measurement method.
Appropriate methods should be selected based on accuracy requirements, measurement efficiency and equipment costs. However, there is a repeatability deviation between different methods (~ 10% ). ITU has recommended JME and IF as the benchmark measurement method.
The following table briefly compares these methods: JME IF WSCC WSFFT

Measurement Principle: Space Compensation Optical Power Spectrum and polarization state optical power spectrum and FFT of average differential group delay between polarization state output at wavelength

Precision ps) 0.005 0.06 0.2 0.025

Equipment costs are high, low to medium, low to intermediate

Time must be 1-20 minutes long) Short <15 seconds) Middle ~ 5 minutes) Middle ~ 5 minutes)

Fan Wei, big, small, middle

Excellent polarization information is direct, and rapid laboratory equipment and laboratory equipment are available.

There is no limitation on wavelength dependence on the largest and smallest information of information, no limitation on wavelength information, no wavelength information, and no wavelength information.

Table 2 Comparison of PMD measurement methods

V. Advantages of Corning Single-Mode Optical Fiber PMD

Corning's meter-per-meter optical fiber is a fully integrated production process (pre-made rod melting, condensation and drawing) and patented external vapor deposition (OVD) technology and ultra-pure vapor deposition chemicals. Corning Optical Fiber performance indicators, including its excellent geometric dimensions, have been ahead of the optical fiber industry for decades. Compared with other optical fiber manufacturing methods (such as improved chemical vapor deposition (MCVD), external vapor deposition processes (from the interior to the outside) are inherently unique and highly automated computer feedback control processes, it is more conducive to the PMD Index. Corning Optical Fiber PMD long Fei Lang Xun lives in Nanjing tengcang

PMD (ps/vKm) 0.2 0.2 0.5 0.5 no metric

PMD link value 0.1 * No indicator no indicator

Table 3 Comparison of Standard Single-Mode Optical Fiber top-notch Information

◆ Comply with iec SC 86A/WG1, method-

On-site measurement data of Corning single-mode optical fiber that has been laid for millions of kilometers around the world shows that no reports have been made on the impact of PMD on system performance. However, some studies have found that a certain number of deployed single-mode optical fibers have measurement results greater than 0.5 ps/vkm [4], which may affect the future expansion of these systems.

Vi. Future Prospects

To meet the increasing bandwidth needs of communication systems, it is imperative for network builders and system users to update the deployed Single-Mode Optical Fiber System or lay new high-capacity systems. In this case, the restriction on the system performance of PMD and chromium dispersion must be considered. To better control and reduce the impact and limitations on system performance, researchers have begun an attempt to compensate for PMD, and initial research results have been validated in the laboratory [5, 6], it is expected that the commercial devices that compensate for PMD will appear in the market in the next few years. However, this not only increases system costs, but also increases system complexity and affects system reliability. Therefore, the most economical and effective method at present is to select the most optimal PMD indicator in the optical fiber to prepare for possible system Resizing in the future.

VII. Summary

This paper describes the basic concepts of PMD, describes the measurement technology, and focuses on the impact of PMD on high data rate numbers and analog single-mode optical fiber systems. It may be a factor that limits system performance. It is an important indicator for system designers and users to consider.

1. Technical issues in connection of Single-Mode Optical Fiber
2. Development of Multimode Optical Fiber Technology and Its Network Application