Signal distortion – MapYourTech

A walk-through with non linear impairments in optical fiber

Technical 1 Min Read

Non-linear interactions between the signal and the silica fibre transmission medium begin to appear as optical signal powers are increased to achieve longer span lengths at high bit rates. Consequently, non-linear fibre behaviour has emerged as an important consideration both in high capacity systems and in long unregenerated routes. These non-linearities can be generally categorized as either scattering effects (stimulated Brillouin scattering and stimulated Raman scattering) or effects related to the fibre’s intensity dependent index of refraction (self-phase modulation, cross-phase modulation, modulation instability, soliton formation and four-wave mixing). A variety of parameters influence the severity of these non-linear effects, including line code (modulation format), transmission rate, fibre dispersion characteristics, the effective area and non-linear refractive index of the fibre, the number and spacing of channels in multiple channel systems, overall unregenerated system length, as well as signal intensity and source line-width. Since the implementation of transmission systems with higher bit rates than 10 Gbit/s and alternative line codes (modulation formats) than NRZ-ASK or RZ-ASK, described in [b-ITU-T G-Sup.39], non‑linear fibre effects previously not considered can have a significant influence, e.g., intra‑channel cross-phase modulation (IXPM), intra-channel four-wave mixing (IFWM) and non‑linear phase noise (NPN).

How to know if errors are due to linear or non-linear issue in an optical link?

Standards 1 Min Read

When the bit error occurs to the system, generally the OSNR at the transmit end is well and the fault is well hidden.

Decrease the optical power at the transmit end at that time. If the number of bit errors decreases at the transmit end, the problem is non-linear problem.

If the number of bit errors increases at the transmit end, the problem is the OSNR degrade problem.

General Causes of Bit Errors

Performance degrade of key boards
Abnormal optical power
Signal-to-noise ratio decrease
Non-linear factor
Dispersion (chromatic dispersion/PMD) factor
Optical reflection
External factors (fiber, fiber jumper, power supply, environment and others)

Factors That Contribute to Chromatic Dispersion and how to Combat it?

Technical 2 Mins Read

Chromatic dispersion affects all optical transmissions to some degree.These effects become more pronounced as the transmission rate increases and fiber length increases.

Factors contributing to increasing chromatic dispersion signal distortion include the following:

1. Laser spectral width, modulation method, and frequency chirp. Lasers with wider spectral widths and chirp have shorter dispersion limits. It is important to refer to manufacturer specifications to determine the total amount of dispersion that can be tolerated by the lightwave equipment.

2. The wavelength of the optical signal. Chromatic dispersion varies with wavelength in a fiber. In a standard non-dispersion shifted fiber (NDSF G.652), chromatic dispersion is near or at zero at 1310 nm. It increases positively with increasing wavelength and increases negatively for wavelengths less than 1310 nm.

3. The optical bit rate of the transmission laser. The higher the fiber bit rate, the greater the signal distortion effect.
4. The chromatic dispersion characteristics of fiber used in the link. Different types of fiber have different dispersion characteristics.
5. The total fiber link length, since the effect is cumulative along the length of the fiber.
6. Any other devices in the link that can change the link’s total chromatic dispersion including chromatic dispersion compensation modules.
7. Temperature changes of the fiber or fiber cable can cause small changes to chromatic dispersion. Refer to the manufacturer’s fiber cable specifications for values.

Methods to Combat Link Chromatic Dispersion

1. Change the equipment laser with a laser that has a specified longer dispersion limit. This is typically a laser with a more narrow spectral width or a laser that has some form of precompensation. As laser spectral width decreases, chromatic dispersion limit increases.
2. For new construction, deploy NZ-DSF instead of SSMF fiber.NZ-DSF has a lower chromatic dispersion specification.
3. Insert chromatic dispersion compensation modules (DCM) into the fiber link to compensate for the excessive dispersion. The optical loss of the DCM must be added to the link optical loss budget and optical amplifiers may be required to compensate.
4. Deploy a 3R optical repeater (re-amplify, reshape, and retime the signal) once a link reaches chromatic dispersion equipment limit.
5. For long haul undersea fiber deployment, splicing in alternating lengths of dispersion compensating fiber can be considered.
6. To reduce chromatic dispersion variance due to temperature, buried cable is preferred over exposed aerial cable.