What is chromatic dispersion and compensation?

Introduction of Chromatic Dispersion

Chromatic(Wavelength) dispersion is the phenomenon in which different spectral components of a pulse travel at different speeds. Dispersion is mainly caused by electromagnetic waves of different wavelengths propagating at different speeds in the same medium. For instance, blue light (short wavelength) transmits faster than red light (long wavelength). If left uncorrected, dispersion limits pulse expanding. Total dispersion is generally considered to be the combination of material dispersion, waveguide dispersion and modal dispersion.

 The types of Chromatic dispersion in optical fiber

Material dispersion and waveguide dispersion

Material dispersion is a delay-time dispersion caused by the fact that the refractive index of the glass material changes by the change of the signal frequency (or wavelength).

Let us introduce waveguide dispersion. In optical fibers, the signal travels partially in the core and partially in the cladding, and the total mode field diameter changes with wavelength. Since the index of refraction in the core is different from the index of refraction in the cladding, changes in the mode field diameter also change the average dispersion index and thus the signal velocity. The ratio of velocity change to wavelength change due to this effect is known as waveguide dispersion.

The concept of modal dispersion

Actually, modal dispersion can't be confused with chromatic dispersion. Modal dispersion is a distortion mechanism that occurs in multimode fibers and other waveguides. In the same fiber, the high-order mode takes a long distance to reach the endpoint, and the low-order mode travels a short distance, which means that the high-order mode takes a long time to reach the endpoint, and the low-order mode takes a short time to reach the point. On a fiber of the same length, the time difference between the highest-order mode and the lowest-order mode reaching the endpoint is the pulse broadening generated by this fiber. Here, since the propagation speed of the optical signal is different for all modes, the signal will be distorted as it propagates over time. Ideally, even a light source of the same wavelength would have modal dispersion. Modal dispersion has other names such as multimodal distortion, multimodal dispersion, modal distortion, multimodal distortion, etc.

Polarization mode dispersion

Polarization Mode Dispersion (PMD) means that the group velocity dispersions of two orthogonal polarizations in the fiber are different, and a single DCF cannot simultaneously compensate for these two group velocity dispersions(GVD). Group velocity dispersion (GVD) is a property of dispersive media that is most commonly used to determine how the medium affects the duration of light pulses that pass through it. Moreover, PMD and other types of dispersion will impair the performance of telecommunication systems. The next paragraph will explain how chromatic dispersion affects the transmission system.

Influence of Fiber Dispersion Effect on OTN System

Chromatic dispersion will distort the signal at the receiving end, and the transmission capacity and transmission distance of the signal in the optical fiber will be limited.

As mentioned above, an important factor limiting the transmission distance of an optical signal is the chromatic dispersion phenomenon. The higher the single-channel signal rate, the higher the influence of chromatic dispersion, and the correspondingly higher requirements for chromatic dispersion control. The dispersion tolerance of the 2.5Gbps DWDM system is 16 times that of the 10Gbps DWDM system. In other words, the dispersion tolerance is inversely proportional to the square of the signal rate. Dispersion is an accumulation effect whose value increases with transmission length, and its effect also increases with distance. To reduce this effect, the dispersion in the fiber needs to be reduced.

Fiber Dispersion Compensation Technology

Nowadays, the fiber dispersion compensation technology mainly includes dispersion compensating fiber (DCF), fiber Bragg grating (FBG), electrical dispersion compensation technology and other techniques. The DCF dispersion compensation technology using the fundamental mode is relatively mature and simple and relatively practical. The below chart will tell you the difference between these technologies.

Dispersion Compensation Fiber

Dispersion Compensation Fiber (DCF) is a special fiber with negative chromatic dispersion, which is just the opposite of G.652 fiber, and can cancel the effect of G.652 regular dispersion. However, serious nonlinear effects on strong light should be avoided.

As shown in Figure below dispersion compensation fiber (DCF) whose dispersion sign is opposite to that of the transmission fiber is used to perform periodic dispersion compensation for dispersion. At present, the medium and long-distance systems with transmission rates above 2.5 Gb/s adopt dispersion compensation technology. A dispersion compensation module (DCM) made of DCF is placed at the output end of each or several fiber spans.

Dispersion Compensation With FBG

A Fiber Bragg Grating (FBG) is an optical fiber in which the index of refraction within the fiber core changes from a high to a low index of refraction along its length. By using FBG, it can significantly reduce the degree of dispersion in long distance transmission.

Fiber Bragg gratings are inscribed within optical fibers.

Dispersion Compensation With TDCM

Widely used in high-speed DWDM systems, it can make the passing optical signals transmit widely used in high-speed DWDM systems, it can make the passing optical signals transmit different optical paths at different wavelengths. Thereby producing periodic dispersion effects. In addition, each cycle can produce periodic dispersion effects. It achieves a highly controllable linear group delay response.

Through reasonable settings, the group delay response of the module can be used to effectively compensate for the dispersion problem of the signal in the DWDM optical communication system after the long-distance transmission of the fiber. Highly controllable linear group delay response. Through reasonable settings, the group delay response of the module can be used to effectively compensate for the dispersion problem of the signal in the DWDM optical communication system after the long-distance transmission of the fiber.

Conclusion

In general, the adverse effect of chromatic dispersion on the transmission system is substantial. Once the transmission distance reaches more than 60km, it is often necessary to consider dispersion control to avoid signal deformities. At this time, we using DCM or TDCM to offset the dispersion like material dispersion, modal dispersion and waveguide dispersion.

Due to dispersion, the signal is distorted at the receiver. If the distortion is not large enough, the receiver can still judge the signal and recover the correct information. If the distortion is too serious, the receiver can no longer recover the information. This threshold is the dispersion tolerance of the optical module. Components with high dispersion tolerance will put more effort into eliminating the undesirable effects of dispersion, and therefore will cost more, resulting in higher selling prices. The long-distance modules sold by QSFPTEK are incredibly cost-effective and have good transmission performance. Welcome to contact us.

Part of content referring: https://www.sciencedirect.com/