Signal attenuation is one of the most important properties of an optical fiber because it mainly determines the maximum repeaterless separation between transmitter and As the repeaters are expensive to fabricate, install and maintain, therefore fiber attenuation has large influence on system cost and equally important in signal distortion.
The distortion mechanism in a fiber cause optical signal pulses to border as they travel along a fiber. When these pulses travel sufficiently far, they eventually overlap with neighbouring pulses creating errors in receiver output. This signal distortion mechanism limits the information carrying capacity of fiber.
For determining attenuation in fibers three major techniques are
- Cutback technique
- Insertion loss method
- Cutback technique is a destructive method of measuring attenuation. It requires access to both ends of fiber as shown in Fig. 8.7.1.
- Firstly, the optical power is measured at the output (far end) of fiber. Then without disturbing the input condition, the fiber is cut-off few meters from the source and output power at near end is measured.
- Let PF and PN are the output powers at far end and near ends of fiber respectively. Then attenuation in dB per kilometer is given by
L is separation length of two measurement poit (in km).
- An optical signal gets distorted as it travels down the fiber due to three basic forms of dispersion, that limits the information carrying
- There are different methods to measure the dispersions Suc as : intermodal dispersion in time domain, intermodal dispersion in frequency domain, chromatic dispersion and polarization mode dispersion.
Time-domain Intermodal Dispersion Measurements
- Time-domain intermodal dispersion measurement involves injecting a narrow pulse of optical energy into one end of an optical fiber and detect the broadened output-pulse at the other end. Thest set up for this measurement is shown in the 8.7.2
- Eye pattern method is a measuring techniques for assessing the data handling ability of digital transmission system. The eye-pattern measurements are made in time domain and allow the effects of waveform distortion to observe on oscilloscope. Fig. 8.7.3 shows test setup for making eye diagram measurement.
To measure performance of system various word patterns must be provided Various measurement from eye pattern can be done are –
- Noise margin
- Timing jitter
- Rise time
- Non linearity of channel
- A simplified eye diagram showing key performance parameters is illustrated in 8.7.4
The display pattern formed can be understood by considering 8 combinations of 3-bit long NRZ code. The 8 patterns are superimposed simultaneously forming an eye pattern. 8.7.5 shows 8 possible 3-bit long NRZ combinations of pulses with moderate rise and fall times.
Recommended Questions Optical Amplifier
- Explain various performance parameters of optical
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- Linear bus b) Ring topology c) Star topology
- Give nearest neighbor budget for linear bus
- Explain comparative performance of star and linear bys
- List the non-linear effects in fiber optic
- Review the similatiries and differences between SONET and
- What is the OC-3 bit rate and what is STM-1 bit rate?
- Why to use SONET/SDH?
- What is the difference between a regenerator and an optical amplifier?
- Explain the different types of network
- Review the STS-N bit rates.
- What is a path?
- What is a line?
- What is a section?
- What are virtual tributaries?
- What is payload pointers?