Subject : Optical Fiber Communication
Temporal response
Introduction:
For the pulse dispersion on the optical fiber channel temporal response is special case.
Temporal response in optical fiber:
- Temporal response is especially the case with regard to pulse dispersion on the optical fiber channel.
- The finite bandwidth of the optical system may result in overlapping of the received pulses or ISI, giving a reduction in sensitivity at the optical receiver.
- Therefore, either a worse BER must be tolerated, or the ISI must be compensated by equalization within the receiver.
- The latter necessitates an increase in optical power at the receiver which may be considered as an additional loss penalty.
- This additional loss contribution is usually called the dispersion–equalization or ISI penalty.
- The dispersion–equalization penalty DL becomes especially significant in high-bit-rate multimode fiber systems and has been determined analytically for Gaussian shaped pulses.
- In this case it is given by
…1
Where
- τ_{e} is the 1/e full width pulse broadening due to dispersion on the link and
- τ is the bit interval or period.
- For Gaussian-shaped pulses, τ_{e} may be written in terms of the rms pulse width σ as
…2
- Hence, substituting into Eq. (1) for τ_{e} and writing the bit rate BT as the reciprocal of the bit interval τ gives
…3
- Since the dispersion–equalization penalty as defined by Eq. (3) is measured in decibels
- Therefore, the total channel loss including the dispersion–equalization penalty CLD is given by
…4
- The dispersion–equalization penalty is usually only significant in wideband multimode fiber systems which exhibit intermodal as well as chromatic dispersion.
- Single-mode fiber systems which are increasingly being utilized for wideband long-haul applications are not generally limited by pulse broadening on the channel because of the absence of intermodal dispersion.
- It is often the case that intermodal dispersion is the dominant mechanism within multimode fibers.
- It is useful to consider the dispersion–equalization penalty in relation to fibers without and with mode coupling operating at various bit rates.
- An alternative approach involving the calculation of the system rise time can be employed to determine the possible limitation on the system bandwidth resulting from the temporal response of the system components.
- Therefore, if there is not a pressing need to obtain the maximum possible bit rate over the maximum possible distance, it is sufficient within the system design to establish that the total temporal response of the system is adequate for the desired system bandwidth.
- The total system rise time may be determined from the rise times of the individual system components which include the source, the fiber cable and the detector.
- These times are defined in terms of a Gaussian response as the 10–90% rise (or fall) times of the individual components.
- The fiber cable 10–90% rise time may be separated into rise times arising from intermodal T_{n} and chromatic or intramodal dispersion T_{c}.
- The total system rise time is given by
…5
Where
- T_{S} and T_{D} are the source and detector 10–90% rise times, respectively