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Abstract—This paper describes the design and performance of
high-speed avalanche photodiodes (APDs) for 100-Gbit/s PAM4
operation. The APDs are made with InP- lattice- matched III-V
compounds, which are suitable for precise band engineering to
simultaneously achieve high-speed and high-responsivity
performance with a vertical illumination structure. By utilizing a
hybrid absorber consisting of p-type and undoped InGaAs layers
as well as a 1.1-eV InAlGaAs gap grading layer between the
absorber and InAlAs avalanche layer, the peak bandwidth is
effectively boosted to 42 GHz with a responsivity of 0.5 A/W at
unity gain. An optical receiver made with the APD performed
40-km 106-Gbit/s PAM4 transmissions over a single-mode fiber
without an optical amplifier.
Index Terms— Avalanche Photodiode (APD), 100-Gbit/s PAM4,
Ethernet, vertical illumination
I. I
NTRODUCTION
HE CONTINUED
growth of intra and inter data-centers traffic
has led to a strong demand for higher bit rates and
increased baud rates in optical fiber communications. The use
of four-level pulse-amplitude modulation (PAM4) instead of
non-return-to-zero (NRZ) effectively doubles the bit rate for a
given baud rate, and by increasing the baud rate up to 50 Gbaud,
the bit rate can reach as high as 100 Gbit/s (100-Gbit/s PAM4).
However, as indicated in IEEE standards and multi-source
agreements (MSAs), the current reach of 100-Gbit/s PAM4 is
only a few kilometers [1, 2]. Therefore, one of the major issues
is extending the transmission distance while ensuring such high
bit rates.
The avalanche photodiode (APD) is a key component for
resolving this issue. To date, 100-Gbit/s PAM4, 40-km
transmission has been demonstrated with an APD receiver
operating at 25 Gbaud and a combination of various digital
Manuscript received October 6, 2014.
M. Nada, F. Nakajima, and H. Matsuzaki are with the NTT Device
Technology Laboratories, NTT Corporation, Atsugi, 243-0198, Japan (e-mail:
nada.masahiro@lab.ntt.co.jp; nakajima.fumito@lab.ntt.co.jp;
matsuzaki.hideaki@lab.ntt.co.jp).
T. Yoshimatsu, and K. Sano are with the NTT Device Innovation Center,
NTT, NTT Corporation, Atsugi, 243-0198, Japan (e-mail:
yoshimatsu.toshihide@lab.ntt.co.jp; kimikazu.sano@lab.ntt.co.jp).
signal processing (DSP) techniques, such as pre-emphasis,
direct detection faster than Nyquist (DD-FTN), maximum
likelihood sequence estimation (MLSE), and 201-tap
decision-directed least mean squares (DD-LMS) [3]. In
particular, the use of DSP compensates for the lack of
bandwidth of the optical components at the expense of higher
power consumption. If we could boost the speed of the APD,
the transmission reach of 100-Gbit/s PAM4 could be extended
without the need for such power-hungry DSPs.
Silicon-germanium (Si/Ge) and III-V compounds, such as
InP, InGaAs, and InAlAs, are possible material systems with
which to fabricate high-speed APDs for 100-Gbit/s PAM4
applications. Remarkable improvements in growth techniques
of Ge on Si substrate have enabled Si/Ge APDs to operate at
25-Gbit/s NRZ [4]. Furthermore, Si/Ge APD, which features a
6-µm-width waveguide structure, has achieved 56-GHz large
bandwidth, which could be a feasible solution to 100-Gbit/s
PAM4 operation [5].
APDs based on III-V compounds are advantageous in terms
of band-engineering flexibility using various materials
lattice-matched with an InP substrate. As well, the multiplicity
of compositional-dependent physical properties, such as band
gaps, electron and hole velocities and diffusion coefficients, are
distinguishing features of III-V compounds. Therefore, unlike
Si/Ge material systems wherein the bandwidth and responsivity
are simply determined by the thicknesses of the Ge absorber
and Si avalanche layer, we can optimize the layer structure with
appropriate III-V compounds and thereby design the
performance of the APD. For example, we can boost the speed
of an APD with a given its thickness, or improve its
responsivity for a given speed by modifying the layer structure,
as typified by the hybrid absorber configuration [6].
In this study, we designed and fabricated a high-speed APD
based on III-V compound materials for 100-Gbit/s PAM4
operation and examined the performance of the fabricated APD
in experiments. In an experiment on an APD incorporating a
hybrid absorber consisting of p-type and undoped InGaAs
layers, and an 1.1-eV InAlGaAs gap-grading layer, we found
that a peak bandwidth of 42 GHz with a responsivity of 0.5
A/W could be achieved even with a vertical illumination
structure. The APD had an active area diameter of 14 µm,
which relaxes the tolerance of optical coupling when
fabricating optical receivers. An optical receiver incorporating
A 42-GHz bandwidth avalanche photodiodes
b
ased on III-V compounds for 106-Gbit/s PAM4
applications
Fumito Nakajima, Kimikazu Sano
IEEE, and Hideaki Matsuzaki, member, IEEE