The adiabatic chirp of DFB lasers is shown to be power and facet phase dependent. Importance of this effect on transmission is demonstrated using BER measurements, showing an advantage for gain-coupled lasers versus index-coupled devices.
I IntroductionOne of the main driving force for the development of semiconductor lasers is the telecom market. For moderate repetition rates, directly modulated lasers would provide an ideal source for fiber telecommunication, if they were not backed by poor spectral behaviour. For this reason, still large efforts are spent on understanding and reducing the laser chirp characteristics. A first step was to recognize the existence of two phenomena leading to frequency chirp [l]: adiabatic chirp, which follows the power variations with a subpicosecond time response, and transient chirp, which depends on the carrier density deviation from the steady state solution. The aim of this paper is to report measurements on the adiabatic chirp behaviour of DFB lasers and to show that, despite of the fact that the Henry factor is always of the same sign, both red and blue chirp are possible. Transmission measurements show a strong impact of this phenomena on transmission penalty.
II TheoryIt is known that the lasing wavelength shifts towards the blue when the power is increased because of the gain compression effect : as the power rises, the gain is compressed, and a higher carrier density is required to maintain the lasing condition. This leads to a decrease of the active region index and to a related decrease of the Bragg grating period h=2 nefiA , A being the grating period, and n, f f the effective index. All the possible DFB modes then shift towards the blue. If spatial hole burning (SHB) is taken into account, however, new effects invalidates the previous conclusion. As the photon density varies along the cavity, so does the index, and this variation locally introduces a phase shift of the optical wave with regard to the grating. This variation modifies the lasing condition and can lead to either blue or, unexpectedly, to red chirp depending on the facet phase. The reported effect is strongly power dependent: at low power, the gain compression is small, but SHB is high since the contrast between the maximum and the minimum optical powers inside the cavity can be important. SHB is 195 I I E o,% x h i f t -9 6 -1 red shift .-P -0,5 -1,5 -I 0 2 4 6 8 1 0 1 2 P o w e r ( m W )in that case the prevalent effect. At high Fig.1 : Calculated chirp versus power for several cases of power, the gain compression increases facet phase while SHB gets lower, the contrast being reduced. In that case gain compression dominates.The Transfer Matrix Method was used to calculate the variation of the laser frequency shift v with regard to the injected current I by taking into account both gain compression and SHB effects. Fig.1 gives the chirp dv/dl for four different facet phases in the case of a laser with a KL value of 1.6, with two main results : red chirp can indeed be observed for some facet phases, and...
