IEC PAS 61280-2-10:2003 pdf download Fibre optic communication subsystem test procedures – Part 2-10: Digital systems – Time-resolved chirp and alpha-factor measurement of laser transmitters
Scope
This part of IEC 61280 sets forth standard procedures for measuring time-resolved chirp onlaser transmitters. The calculation of alpha-factor, a measure of transient chirp, is derivedfrom the measured TRC data. Also covered is a means to verify the TRC setups andcalculations (Annex A) and a review of laser modulation methods and the relationship of TROto performance in a transmission system.
Background
Understanding the effects of chirp on the transmission of signals is of great importance to thesystem designer. Chirp can have two separate outcomes in transmission systems. The firstis that the chirp can interact with the fibre dispersion to broaden or narrow the pulse along thefibre. This will cause a positive or negative path penalty, which ultimately decreases orincreases the distance over which the signal can propagate in a system without regeneration.The sign of the penalty depends upon both the sign of the chirp and the sign of the fibredispersion. The second is that chirp can broaden the transmitted spectrum limiting thechannel spacing by interfering with adjacent channels in an ultra-dense WDM environmenteven at short-haul distances.
The path penalty is the apparent reduction of receiver sensitivity due to distortion of the signalwaveform during its transmission over the path. A negative path penalty corresponds to anapparent increase of receiver sensitivity. The path penalty is manifested as a shift of thesystem’s BER-curves towards higher or lower nput power levels. A positive chirp penalty isdefined as the additional signal-to-noise ratio (SNR) required at the receiver due to laser chirpto maintain a specified bit error ratio (BER) in a system with specified dispersion.
Measuring chirp penalty directly is difficult because it requires a chirp-free transmitter with theidentical intensity pattern as the DUT. Because of this difficulty, chirp penalty is often inferredfrom a path penalty measurement. A path penalty measurement involves substituting a fibreof known chromatic dispersion into the signal path and measuring the additional power (SNRrequired to achieve the specified BER, This measurement is tedious and time consuming ancassumes that the measurement is dominated by the chirp penalty term. This has led manytransmitter and system designers and manufacturers to estimate the chirp (or dispersion)penalty using time-resolved chirp data directly or with derived modeling parameters.
EC technical report 61282-8 (to be published) describes the estimation of dispersion penaltyfrom measured time-resolved chirp data [8].
in order to bring the cost of DWDM transmission systems down, lower cost transmitters arebeing designed and deployed. Controlling the amount of chirp present in these lower costtransmitters is key to their success in the network [7].
3Definition of time-resolved chirp
Time-resolved chirp (also referred to as dynamic chirp) is the time variation of theinstantaneous optical frequency of a transmitter. lt is typically expressed as Aft), thedifference from the average optical frequency. The instantaneous optical power, P(), is usedin conjunction with Af/t) to completely describe the optical signal.