This invention relates to the control of chirp in high speed amplitude modulation of DFB lasers.
Modulation of the injection current of a semiconductor distributed feedback (DFB) laser is liable to produce variation in both the intensity and the wavelength of its emission. This wavelength variation is called chirp. Chirp imposes bandwidth limitations in amplitude modulated transmission systems that exhibit wavelength dispersion.
A paper entitled `Independent modulation in amplitude and frequency regimes by a multi-electrode distributed--feedback laser` presented by Y Yoshikuni et al at the Feb. 25, 1986 Optical Fiber Communication Conference in Atlanta, Ga. describes a DFB laser with a uniform physical pitch grating where the top electrode of the laser is divided into three in-line sections, at least one of which is driven independently of the others. In particular, the paper states that differences in modulation efficiencies make it possible to modulate amplitude and frequency independently by adjusting the modulation current amplitude and phase applied to the divided electrode structure, and illustrates achieving amplitude modulation with minimum chirp by applying a first signal to the front portion of the divided electrode slightly ahead in phase of the application of a second signal of smaller amplitude to the centre portion of the divided electrode structure. Correspondingly frequency modulation with minimum amplitude modulation is described as being achieved with the first current being of larger amplitude than the second and in substantial antiphase (push-pull) relationship.
A paper by O Nilsson et al entitled, `Formulas for Direct Frequency Modulation Response of Two-Electrode Diode Lasers: Proposals for Improvement`, Electronics Letters 3rd December 1987, Vol 23, No 25, pages 1371-2 describes the theory of operation of a two-electrode laser structure designed for frequency modulation rather than for amplitude modulation. According to this theory thermal effects produce a phase shift, but it is postulated that the thermal effect could be avoided by pumping the laser in push pull. It is however particularly to be noted that this push-pull operation of a two-electrode laser is in the context of a device structured to provide frequency modulation rather than amplitude modulation, and the paper explains that the two sections are required to have different (x-parameters in order to provide the desired frequency modulation. Thus it is clear that this suggestion to employ push-pull is specifically in respect of a laser diode that is not symmetrical about the plane separating the two sections of that laser.
Neither of the above referenced papers is however directly concerned with dynamic chirp, by which term is meant the transient effects upon emission frequency occurring at the rising and falling edges of fast pulses. As the data rate is increased so this dynamic chirp assumes greater significance as a potential problem. Dynamic chirp is believed to result in major part from the effect of changes in total photon population in the laser associated with the rising and falling edges of the injection modulation current, and so the elimination of a frequency modulation response to injection current modulation in the manner proposed in the above references does not address the particular problem of dynamic chirp.
A paper that does address this dynamic chirp problem is the paper by I. H. White et al entitled `Line Narrowed Picosecond Optical Pulse Generation Using Three Contact InGaAsP/InP multi-quantum Well distributed Feedback Laser under Gain Switching`, Electronics Letters Vol. 28, No 13, pages 1257-8. As the title implies, the laser has a three section top electrode, of which the two end sections are electrically commoned. Dynamic chirp is reduced by arranging to gain-switch the commoned end sections while a constant bias is applied to the middle section in such a way as to provide an effective optical injection locking mechanism. The central region causes locking of the wavelength of gain-switched pulses generated by the electrical modulation applied to the end regions. This reduces chirp, but insofar as it still leaves a modulation of the photon population, the approach is not fully effective.
U.S. Pat. No. 5,502,741 is directed to the control of chirp and describes a method of amplitude modulating the optical emission of a DFB laser in such a way as to minimise dynamic chirp. To this end a DFB laser is provided with a top electrode divided symmetrically into two or three in-line separate elements through which a bias current is applied with a symmetrical distribution and through which a modulation current is applied with an antisymmetric (push-pull) distribution.
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