Direct amplitude modulation of lasers

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.

BACKGROUND 
This invention relates to the minimising 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, Georgia 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 .alpha.-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. 
SUMMARY OF THE INVENTION 
Accordingly it is a general object of the present invention to provide a 
method of amplitude modulating the optical emission of a DFB laser in such 
a way as to minimise dynamic chirp. 
According to the present invention there is provided a method of amplitude 
modulating a distributed feedback (DFB) laser that has a DFB optical 
cavity defining an optical axis of laser emission therein, which optical 
cavity has a plane of substantial symmetry normal to said axis, in which 
method a bias current, which is distributed substantially symmetrically 
with respect to said plane of substantial symmetry is applied to the DFB 
optical cavity upon which bias current is superimposed and modulation 
current distributed substantially anti-symmetrically with respect to said 
plane. 
The anti-symmetric modulation current operates in a push-pull mode to keep 
substantially constant the total photon population (photonic energy) 
within the laser while at the same time shifting the centre of gravity of 
the photon density back and forth along the laser axis so that the 
emission from the laser occurs predominantly first from one end and then 
from the other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred form of basic semiconductor structure of DFB laser to which the 
method of amplitude modulation according to the present invention is 
applied is a conventional DFB laser structure with the single major 
difference that preferably the distributed feedback grating is without any 
phase shift. Such a basic semiconductor structure may for instance 
comprise, as depicted in FIG. 1, an n-type InP substrate 10 upon which is 
grown a series of epitaxially deposited layers commencing with an n-type 
InP buffer layer 11. On top of the buffer layer is grown an undoped 
quaternary lower waveguide layer 12, a multi-quantum well (MQW) structure 
13, an undoped quaternary upper waveguide layer 14, a p-type InP cladding 
layer 15 and a p-type ternary contact layer 16. The MQW structure 13 may 
typically comprise about six quantum strained or unstrained well layers of 
ternary or quaternary material sandwiched between barrier layers which may 
have the same composition as that of the upper and lower waveguide layers 
14 and 12. The epitaxial growth is temporarily halted after the growth of 
the upper waveguide layer in order to pattern the exposed surface of that 
layer to form, for instance by electron beam lithography, a DFB grating 17 
before recommencement of the epitaxial growth to grow layers 15 and 16. In 
an alternative structure (not shown) the DFB grating is located beneath 
the MQW structure instead of on top of it, and is created immediately 
after growth of the buffer layer 11. 
The above described layer structure provides waveguiding properties in the 
direction normal to the plane of the layers. Lateral waveguiding is also 
provided by known means, for instance by means of a ridge waveguide 
structure or by means of a buried heterostructure structure, and in this 
way an optical axis for the laser is defined, this axis extending in the 
plane of the layer in a direction at right angles to the direction in 
which the grating lines of the DFB structure 17 extend. Usually a DFB 
laser has a single top electrode making electrical contact with the 
contact layer 16, but in this instance the top electrode is divided into 
two in-line sections 18a, 18b as depicted in FIG. 1 or three in-line 
sections 28a, 28b, and 28c as depicted in FIG. 2. 
The arrangement of the top electrode sections is such as to retain a plane 
of substantial symmetry (indicated by a broken line 19, 29) for the 
semiconductor laser structure and its electrode structure. The laser is 
thus provided with a structure that confines the photons laterally and 
perpendicular to the plane of the epitaxial layers, and now by means of 
the divided top electrode structure is provided with a facility for 
modulating the distribution of the photons along the optical axis of the 
laser by modulation of the distribution of the injection current applied 
to the laser via its top electrode structure. By arranging for the laser 
to be supplied with a bias current that is distributed substantially 
symmetrically with respect to the symmetry plane 19, 29, and by 
superimposing on this a modulation current that is distributed 
substantially anti-symmetrically (i.e., in push pull) with respect to that 
plane, the modulation has the effect of shifting the centre of gravity of 
the total photon population axially back and forth along the optic axis 
while maintaining that population (photonic energy) substantially 
constant. In the case of the two-element top electrode configuration of 
FIG. 1, these symmetry relationships are provided by applying, to the 
respective elements 18a and 18b, currents of the form (i.sub.b 
.+-.i.sub.m) and (i.sub.b .-+.i.sub.m), where i.sub.b is the bias current 
and i.sub.m is the modulation current. (For the sake of ilustration, under 
the condition i.sub.b =i.sub.m, the total current drive would alternate 
between application solely through element 18d and application solely 
through element 18b). In the case of the three-element top electrode 
configuration of FIG. 2, the required symmetry relationships are provided 
by applying, to the respective elements 28a, 28b and 28c, currents of the 
form (i.sub.b1 .+-.i.sub.m), i.sub.b2 and (i.sub.b1 .+-.i.sub.m). 
Trace 30 of FIG. 3a is a schematic graphical representation of the photon 
density distribution in a two-section top electrode laser at an instant 
when the modulation current applied to the left-hand section 18a is 
greater than that applied to the right-hand section 18b, and indicates 
that under these circumstances the centre of gravity of the photon 
distribution is displaced to the left of the plane of the symmetry 19. 
Conversely trace 31 of FIG. 3b is an equivalent representation of the 
distribution that obtains when the modulation current applied to the 
right-hand section 18b is the greater. Traces 40 and 41 of FIGS. 4A and 
4B, schematically depict the equivalent photon density distributions in 
respect of the three-section top electrode laser. In respect of light 
emitted from the right-hand end of the laser, the condition represented in 
FIGS. 3A and 4A is the OFF state, while that of FIGS. 3B and 4B is the ON 
state. 
Since the modulation leaves the photonic energy substantially constant, the 
net rate of stimulated recombination of electrons from the conduction band 
averaged over the laser is thereby controlled so that the electronic 
contribution to the refractive index is substantially invariant. With 
substantially no change in the electronic contribution to the refractive 
index, the dynamic chirp of the laser can be minimised and approach 
towards zero. Due to the symmetry of the device and its modulation, there 
is substantially no change in the optical frequency between the two 
post-transient modulation states, and hence static chirp is also 
minimised. 
One of the differences between the three section top electrode structure of 
FIG. 2, and the two-section structure of FIG. 1, is that the former can be 
employed to store more photonic energy within the laser and thus further 
help in the stabilisation of the frequency of emission. On the other hand, 
having three contacts tends to slow down the speed with which the centre 
of gravity of the photonic energby shifts from one end of the device to 
the other. Any dynamic chirp that remains is dynamic chirp that is 
generated during these energy shifts, and hence the faster the switching, 
the less is the time that the optical emission frequency is perturbed, and 
the less is the magnitude of the dynamic chirp. An increase in the storage 
of phonic energy can also be obtained by having a non-uniform DFB strategy 
as depicted at 17' in FIGS. 5A and 5B. The effective pitch of the grating 
17' remains constant over the whole length but the amplitude, and hence 
also the coupling coefficient, of the centre section is reduced in 
comparison with that of the two end sections The best laser will have an 
optimum trade-off between storing enough photonic energy, which reduces 
line width, but not so much energy as to get too heavily confined to the 
central section of the laser, which confinement has the result of lowering 
the contrast ratio between OFF and ON states. In FIGS. 3, 4 and 5 this 
contrast ratio is indicated by the difference in height of the two ends of 
the traces 30, 31, 40 and 41. The light output is proportional to the 
energy divided by the group velocity, making proper allowance for facet 
reflections if any. It is found that the minimum line width is formed by 
adjusting the bias currents and modulation currents within the laser in a 
normal experimental manner. Practical lasers will not be precisely 
uniform, and in view of departures from precise symmetry of physical 
structure, there will be an optimum drive that will minimise the change of 
frequency. 
The aim throughout the process of amplitude modulation is to keep the 
frequency as uniform in time as the fundamental physics of modulation 
permit. In the detailed modelling of the laser, using modelling techniques 
derived from those described by DD Marcenac and J E Carroll entitled 
`Quantum-Mechanical Model for Realistic Fabry-Perot Lasers`, lEE 
Proceedings Part J, Vol. 140, No. 3, pp 157-171 (June 1993), it is noted 
that, as the laser is switched from the on to the off state so there is a 
fall in the electron density and a fall in the frequency. One way of 
combating this `switch over` chirp is to have an overshoot on the section 
which is being turned on, as depicted in FIG. 6A. An alternative and 
perhaps superior way is to provide a slower fall time for the section 
which is being turned off, as depicted in FIG. 6B, so that the electrons 
are not stimulated to recombine too fast in that section because it is 
this too rapid recombination which changes the refractive index of that 
section, and thus changes the lasing frequency. 
It has been stated previously that it is preferred to employ a DFB laser 
that does not incorporate a phase change in its DFB grating structure. The 
reason for this preference is that a phase change has the effect of 
locking a large proportion of the photonic energy, thereby reducing the 
contrast ratio that is obtainable with the modulation. This effect shows 
up clearly in FIGS. 7 and 8. FIG. 7 compares the modelled photon density 
distribution changes resulting from push-pull modulation of a DFB laser 
incorporating a .lambda./4 phase shift at its mid point, traces 70 and 71, 
with the corresponding changes, traces 72 and 73, in respect of the same 
modulation applied to an equivalent DFB laser with no phase shift 
modulation applied to an equivalent DFB laser with no phase shift 
structure in its grating. FIG. 8 compares the modelled power output of the 
two lasers when push-pull modulated with a 2 Gb/s signal. In this Figure 
trace 80 describes the output of the laser with the phase shift in its 
grating, and trace 81 describes that of the laser without any phase shift 
in its grating. Finally, FIG. 9 depicts the modelled output spectrum of 
the laser without any phase shift in its grating when push-pull modulated 
with a 2 Gb/s signal. This indicates a -20 dB linewidth of 6.3 GHz, which 
compares with linewidths in the region of 40 GHz for similarly modulated 
conventional single section top electrode .lambda./4 phase shifted DFB 
lasers.