Frequency chirp

Frequency chirp is unintended frequency modulation riding on intended amplitude modulation. In a semiconductor laser, changing the drive current changes carrier density; carrier density changes both gain and refractive index (the coupling quantified by the linewidth enhancement factor $\alpha$); the index change slides the lasing frequency. The result: the leading edge of a pulse sits at a different optical frequency than its trailing edge.

Two components are distinguished. Transient chirp accompanies each edge: the frequency excursion during the switching transient, ringing at the relaxation-oscillation frequency, largest with fast, deep modulation. Adiabatic chirp is the steady-state frequency offset between the "1" and "0" power levels, proportional to output power difference. Which dominates depends on bias and extinction: shallow modulation above threshold favors adiabatic; hard switching favors transient.

Chirp matters because fiber is dispersive. A chirped pulse's frequency components travel at different group velocities; in standard fiber at 1550 nm (anomalous dispersion, $D \approx +17$ ps/nm/km), the typical positive transient chirp of a DML maps blue-shifted edges onto faster arrival. Pulses distort and the eye closes. This chirp–dispersion product, not loss, is what historically confined 1550 nm directly modulated links to short reach, and why longer links moved to external modulation: an ideal external modulator leaves the source frequency untouched (real Mach–Zehnders offer near-zero or tunable-sign chirp via drive asymmetry; EAMs retain a small residual set by their own $\alpha$-like parameter).

Chirp is not purely villainous: dispersion-managed links have exploited controlled chirp signs to extend reach, and chirp-managed lasers turned adiabatic chirp into a feature. It is measured with time-resolved spectral techniques or inferred from dispersion-penalty curves.