| dc.description.abstract | Driven by the exponential growth of data tra c, current infrastructure and standards are
evolved to meeting the requirements. In long haul communication, 1550/1310 nm based singlemode
ber technology is a commercially viable platform. For short-reach optical interconnects
for rack-to-rack communication and within buildings, the matured 850 nm VCSEL based multimode
ber (MMF) technology is an industry-standard. IEEE has recently proposed a 400 Gbps
roadmap for data centers to scale up short-reach infrastructure. However, the current shortreach
datacom infrastructure is not scalable to support a 400 Gbps data rate. Integration
of all the functional components of an optical interconnect on a single platform can meet
the requirement of a scalable, energy-e cient, and a ordable system. Additionally, CMOS
compatibility can leverage electronic and photonic circuits' co-existence on a single platform
and low-cost mass manufacturing. Integrating optical functionalities on a single-chip also o ers
application in sensing as well. Due to the low absorption in water, the 850 nm wavelength
window is also attractive for realizing Lab-on-a-chip biosensors. Integrated photonic circuits
at 850 nm band can therefore be useful for a lab-on-a-chip biosensor platform as well. This
thesis presents an integrated photonic platform comprising silicon nitride (SiN) waveguide, SiN
surface grating coupler, silicon photodetector, and wavelength lters integrated monolithically
on the SiN-on-SOI platform at 850 nm wavelength. Our primary focus is to overcome the
limitation of lower responsivity and bandwidth of silicon photodetector. We extensively study
various techniques to integrate silicon photodetector with passive SiN waveguides e ciently
suitable for future short-reach datacom and lab-on-a-chip biosensors.
In the rst part, we realize a single-mode SiN waveguide along with high-e ciency surface
grating couplers. We have demonstrated a uniform and apodized grating coupler with a bottom
Bragg re
ector. Apodized gratings provide higher coupling e ciency than uniform gratings due
to better mode pro le matching between Gaussian-shaped ber mode and the apodized grating
eld pro le. Distributed Bragg re
ector (DBR) reduces the optical loss due to high order
di racted light directed towards the bottom substrate. SIN apodized grating coupler with
DBR as the bottom re
ector achieves the highest ever coupling e ciency of 2.19 dB/coupler and 3dB bandwidth of 40 nm at 876nm wavelength.
In the second part, we demonstrate various architectures to integrate high-speed silicon
photodetector with SiN waveguide. First, we demonstrate the integration of SiN waveguide with
high-speed, lateral silicon pin photodetector. Compared to the silicon photodetector realized
on bulk silicon, photodetector on an SOI has higher bandwidth due to the lower cross-section.
We use silicon inverse taper to improve the coupling from SiN to silicon, which results in better
responsivity of silicon photodetector. We have achieved the highest responsivity of 0.44 A/W
and bandwidth of 15 GHz for the integrated silicon pin. Bandwidth improvement without
degradation of responsivity is attributed to the lateral collection of photocarriers transverse
to the propagation direction, and low RC time-limited bandwidth due to the thin silicon. To
enhance the photodetector responsivity further, we propose a SiN ring resonator enhanced
silicon metal-semiconductor-metal (MSM) photodetector. Compact, cavity-enhanced silicon-
MSM photodetector responsivity is estimated to be 0.81 A/W at 5 V, which is 100 times
higher than the conventional waveguide photodetector. Moreover, the photodetector's compact
size (6X6 m2) can o er high bandwidth due to reduced RC time-limited bandwidth. In this
section, we also discuss the integration of SiN waveguide with a thin silicon-MSM photodetector
(70 nm thick). In this con guration, the SiN waveguide is placed on top of the silicon-MSM.
Since the silicon's thickness is low SiN, the waveguide does not su er from mode mismatch
losses between silicon and SiN. Such con guration is attractive due to its high responsivity
and bandwidth, along with ease of fabrication. We have shown the DC measurements with a
maximum responsivity of 0.56 A/W at 10 V bias.
Finally, we have demonstrated the integration of wavelength division multiplexer with silicon
photodetector since the shortwave wavelength division multiplexing (SWDM) at 850 nm
wavelength band is considered one of the viable solutions to attain 400 Gbps roadmap. We have
realized the WDM using SiN Echelle gratings and integrated the output channel waveguides
with a silicon-MSM photodetector. Experimentally, we have shown that the Echelle grating
has the insertion loss of 4.3 dB and adjacent channel cross talk of 22 dB for the channels having
wavelength separation of 10 nm. Future exploration of the demonstrated device can lead
to precise wavelength ltering with on-chip detection useful for both high-speed short-reach
datacom and lab-on-a-chip biosensors. In summary, we have demonstrated the capability of
realizing a scalable, energy-e cient, and cost-e ective silicon nitride based integrated photonic
receiver in the 850 nm wavelength band. | en_US |
