Antroy Roy Chowdhury

EECS Department, University of California, Berkeley

Technical Report No. UCB/

December 1, 2025

As data center performance scales rapidly with the highest-ever growth in network traffic, copackaged optics (CPO) proves to be a potential solution for realizing high bandwidth density and energy-efficient interconnects within data centers. Coherent electro-optic transceivers in O-band are gaining much attention for scaling the bandwidth density of CPO interfaces. This thesis presents modeling, electronic-photonic co-design and co-optimization of these coherent transmitters through implementation in a monolithic silicon photonic platform.

The thesis begins with proposing a modeling framework for a coherent electro-optic transmitter. The modeling framework incorporates different link constraints such as data rate, input electrical swing, etc. and different technology constraints such as electrical and optical device parameters. Using this modeling framework, the author proposes a bandwidth-aware co-optimization scheme to optimize the overall energy-efficiency of the transmitter including the laser source. This work then presents a direct-drive coherent optical transmitter (OTX) based on a highly compact and energy efficient Mach-Zehnder modulator using forward-biased p-i-n junction operating in O-band. The MZM utilizes 400 μm long carrier-injection mode forward-biased p-i-n junction phase shifters driven in a lumped fashion using monolithically integrated two-stage electrical driver. The OTX consumes 93 mW of electrical power for 106-Gb/s PAM4 IMDD operation and 194 mW of electrical power for 212-Gb/s coherent quadrature amplitude modulation 16 (QAM-16) operation leading to the electrical power efficiencies of 0.88-pJ/bit and 0.91-pJ/bit respectively. This is the shortest ever MZM in O-band with the lowest energy cost per bit compared to the prior art. The thesis then describes a monolithically integrated O-band dual-polarization QAM-16 direct-drive transmitter based on reverse-biased depletion-mode Mach–Zehnder modulators, namely lumped and traveling-wave MZMs and capable of 425 Gb/s/λ data rate. All these different OTX lanes are implemented in a single die in 45 nm monolithic-CMOS process and co-packaged with the host SerDes generating the high-speed data. The total energy efficiency (electrical + laser) of the compact forward-biased MZM lane is 1.63 pJ/bit which is 1.5× better than the standalone reverse-biased lumped MZM and 1.45× better than the standalone conventional traveling-wave MZM (TW-MZM) implemented on the same die. Moreover, it is 5× more area efficient compared to the TW-MZM due to elimination of the transmission line. Energy and area efficient electro-optic modulators along with spectrally efficient modulation schemes such as coherent and dual-polarization pave the way for future high-bandwidth-density and energy efficient optical interconnects.

Advisors: Vladimir Stojanovic and Jun-Chau Chien

---

BibTeX citation:

@phdthesis{Chowdhury:32028,
    Author= {Chowdhury, Antroy Roy},
    Title= {High Speed Coherent Transmitters for Intra-Data Center Co-Packaged Optical Interfaces},
    School= {EECS Department, University of California, Berkeley},
    Year= {2025},
    Month= {Dec},
    Number= {UCB/},
    Abstract= {As data center performance scales rapidly with the highest-ever growth in network traffic, copackaged optics (CPO) proves to be a potential solution for realizing high bandwidth density
and energy-efficient interconnects within data centers. Coherent electro-optic transceivers
in O-band are gaining much attention for scaling the bandwidth density of CPO interfaces.
This thesis presents modeling, electronic-photonic co-design and co-optimization of these
coherent transmitters through implementation in a monolithic silicon photonic platform.

The thesis begins with proposing a modeling framework for a coherent electro-optic transmitter.
The modeling framework incorporates different link constraints such as data rate,
input electrical swing, etc. and different technology constraints such as electrical and optical
device parameters. Using this modeling framework, the author proposes a bandwidth-aware
co-optimization scheme to optimize the overall energy-efficiency of the transmitter including
the laser source. This work then presents a direct-drive coherent optical transmitter (OTX)
based on a highly compact and energy efficient Mach-Zehnder modulator using forward-biased
p-i-n junction operating in O-band. The MZM utilizes 400 μm long carrier-injection
mode forward-biased p-i-n junction phase shifters driven in a lumped fashion using monolithically
integrated two-stage electrical driver. The OTX consumes 93 mW of electrical
power for 106-Gb/s PAM4 IMDD operation and 194 mW of electrical power for 212-Gb/s
coherent quadrature amplitude modulation 16 (QAM-16) operation leading to the electrical
power efficiencies of 0.88-pJ/bit and 0.91-pJ/bit respectively. This is the shortest ever MZM
in O-band with the lowest energy cost per bit compared to the prior art. The thesis then
describes a monolithically integrated O-band dual-polarization QAM-16 direct-drive transmitter
based on reverse-biased depletion-mode Mach–Zehnder modulators, namely lumped
and traveling-wave MZMs and capable of 425 Gb/s/λ data rate. All these different OTX
lanes are implemented in a single die in 45 nm monolithic-CMOS process and co-packaged with the host SerDes generating the high-speed data. The total energy efficiency (electrical +
laser) of the compact forward-biased MZM lane is 1.63 pJ/bit which is 1.5× better than the
standalone reverse-biased lumped MZM and 1.45× better than the standalone conventional
traveling-wave MZM (TW-MZM) implemented on the same die. Moreover, it is 5× more
area efficient compared to the TW-MZM due to elimination of the transmission line. Energy
and area efficient electro-optic modulators along with spectrally efficient modulation schemes
such as coherent and dual-polarization pave the way for future high-bandwidth-density and
energy efficient optical interconnects.},
}

EndNote citation:

%0 Thesis
%A Chowdhury, Antroy Roy 
%T High Speed Coherent Transmitters for Intra-Data Center Co-Packaged Optical Interfaces
%I EECS Department, University of California, Berkeley
%D 2025
%8 December 1
%@ UCB/
%F Chowdhury:32028