| dc.description.abstract | The semiconductor laser allows for the vast quantities of data that are transmitted over optical fibre each day. A simple device has transformed our
lives. This transformation will continue in the coming decade where the
rapid growth of high-speed data transmission services is increasing the de-
mand for bandwidth and continual industry innovation is needed to meet this
challenge. For the optical access networks, next generation passive optical
networks version 2 (NGPON2) is an ambitious ITU standard developed to
drive such innovation. Time and wavelength division multiplexing (TWDM)
has been chosen for the NGPON2 standard. These NGPON2 transmitters
will be used in fibre-to-the-x (FTTx), including fibre-to-the-home (FTTH)
systems and hence are to be deployed to every customers premises. Such
wide network deployment places an extremely high level of importance on
transceiver cost reduction – which can be achieved through the simplification of the manufacturing process. Currently, distributed feedback (DFB)
lasers and distributed Bragg reflector (DBR) lasers with buried low order
gratings are the focus of TWDM research. However, fabrication of these
lasers requires complex regrowth steps and precise e-beam lithography which
invariably drives yield down and costs up. Previous research in the Trinity College Dublin semiconductor photonics group has yielded low-cost laser
designs based on high-order surfaced etched slots for the devices grating
structure, which can forgo regrowth steps and use inexpensive optical lithography. The research presented in this thesis builds upon the work previously performed on these devices.
In addition to low cost requirements, there is a significant reduction of
channel spacing, which in turn requires increased wavelength stability of
transmitters. This is challenging to achieve since the emission wavelength of
lasers is inherently temperature sensitive, and many applications use lasers in
a variable high-temperature environment such as in data centres. Similarly,
for pumping resonators for broadband comb generation, wavelength stability
versus temperature is critical. Currently, thermoelectric coolers (TEC) are
used to maintain a stable device temperature however, their energy efficiency
and tuning precision are relatively low. Furthermore, their addition increases
packaging costs and reduces package lifetime. Instead, semiconductor lasers
can be athermalised, making their lasing wavelength insensitive to changes
in ambient temperature by varying injection currents across laser sections,
which would allow the removal of the TEC, improving overall energy effi-
ciency, reducing packaging costs and increasing package lifetime. There has
been significant athermalisation research stemming from our group and else
where, and this work significantly builds upon it. First, extensive character-
isation of our existing devices is performed in steady state conditions with
the aim of choosing a device which is capable of wide range athermalisation.
Results from this yields that 700 μm long devices that have a Bragg peak
which is misaligned with the gain peak by approximately +20 to 25 nm yield
would yield the best results. The chosen device is then continuously athermalised over a range of 5 to 106 oC, while maintaining a frequency stability
of ±0.4 GHz and SMSR above 30 dB. This athermalisation scheme was then
extended to multiples wavelengths from the same device, with thorough analysis of device parameter change over the athermal path versus wavelength.
As a natural consequence of these measurements, a new method to measure
thermal impedance of all-active active devices has been developed, measuring
the thermal impedance length product of our devices as 29.3±2.1 oC μm/mW
for the gain section and 39.33±2.8 oC μm/mW for the grating section.
TWDM transmitters must also be capable of pulsed mode operation,
meeting the same requirements mentioned above. This requirement is challenging to meet due to the large, time dependent thermal frequency drift
which occurs during pulses which is caused by the laser self-heating. The
devices under this research have not been extensively tested under transient
conditions and thus pulsed mode characterisation has been performed. It
was found that 700 μm devices were best suited for the TWDM standard,
due to the lowest exhibited frequency drift (2.5 GHz) within the time-frame
of interest. Several schemes for compensating thermal wavelength drift have
been proposed in literature, while some were not feasible to test, such as
using external heaters, which our devices did not contain, others were tested
on our devices. It was found that a sub-threshold heating approach yielded a 6 GHz improvement in overall wavelength drift, across all pulse amplitudes,
and as a consequence of some of these tests, a new pulse shaping method
was developed. This new pulse-shaping method reduces the settling time of
the optical frequency, in some cases by a factor of 3. Then, as the frequency
change was expected to be due to thermal effects, we employed transient thermal imaging to get a better understanding of the lasers thermal performance.
Our results seem to be relatively inline with wavelength based measurements,
further solidifying that a thermal based theory for these devices operation is
correct. | en |
