scispace - formally typeset

Journal ArticleDOI

100 GHz Externally Modulated Laser for Optical Interconnects

15 Mar 2017-Journal of Lightwave Technology (Institute of Electrical and Electronics Engineers)-Vol. 35, Iss: 6, pp 1174-1179

AbstractWe report on a 116 Gb/s on-off keying (OOK), four pulse amplitude modulation (PAM) and 105-Gb/s 8-PAM optical transmitter using an InP-based integrated and packaged externally modulated laser for high-speed optical interconnects with up to 30 dB static extinction ratio and over 100-GHz 3-dB bandwidth with 2 dB ripple. In addition, we study the tradeoff between power penalty and equalizer length to foresee transmission distances with standard single mode fiber.

Summary (2 min read)

Introduction

  • 4 pulse amplitude modulation (PAM) and 105 Gbps 8PAM optical transmitter using an InP-based integrated and packaged externally modulated laser for high speed optical interconnects with up to 30 dB static extinction ratio and over 100 GHz 3 dB bandwidth with 2 dB ripple.
  • Large lane count increases complexity and power consumption resulting in higher costs [1].

A. Previous Experimental Demonstrations

  • The optical transmitter is based on a monolithically integrated distributed feedback laser with traveling-wave electroabsorption modulator (DFB-TWEAM) designed by T > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 KTH, fabricated by KTH and Syntune and packaged by u 2 t Photonics [22],[23].
  • In [23], the performance of already packaged DFB-TWEAM transmitter module for 100 Gbps data rate is evaluated experimentally using 100 meters long standard single mode fiber (SSMF) and 10 km optical link with dispersion management.
  • In [21] successful field trial is demonstrated with transmission of 112 Gbps OOK signals using a purely electrical time division multiplexing-based system without DSP.
  • A bit error rate (BER) performance below the forward error correction (FEC) threshold of 2·10-3 is achieved after transmission over 42 km field installed SSMF with dispersion management between Kista and Råsunda in Sweden.
  • It is the first demonstration of a complete 112 Gbps ETDM system based on monolithically integrated transmitter [23] and receiver modules.

B. Device Structure and Characterization

  • The total active length of modulator is 180 µm.
  • The gain section of the DFB is based on 7 QWs 7 nm thick grown by metal vapour phase epitaxy coupled with buttjoint technique.
  • The laser is 440 µm long with a grating designed to have most of the output power towards the modulator.
  • The dynamic extinction ratio during modulation is related to modulator bias voltage and swing of driving voltage.
  • The S21 curve of the device [22] (W1 connector) depicted in Fig. 1c clearly shows beyond 100 GHz 3 dB bandwidth, the highest reported bandwidth to the best of their knowledge, with less than 2 dB ripple in the pass band of the EML [22] which indicates high phase linearity.

III. EXPERIMENTAL SETUP

  • Figure 2 shows the transmitter setup for 116 Gbps OOK, 4PAM and 105 Gbps 8PAM with receiver setup including DSP (see Fig. 2a) and output optical spectrum of modulated signals (see Fig. 2b).
  • In the OOK setup, two pseudo-random bit sequences with a word length of 2 15 -1 (PRBS15) at 58 Gbps are first decorrelated and then multiplexed into a single 116 Gbps sequence.
  • In the 4PAM setup, two PRBS15 are first decorrelated and then.

116 Gbit/s 4PAM Tx

  • > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 passively combined into a 4PAM signal, which is then used to directly drive the EML.
  • During measurements the bias voltage was kept at minus 2 volts and driving voltage was 2 Vpp for OOK, 8PAM and 1 Vpp for 4PAM.
  • The signal is then downsampled to 1 sample per symbol for static/adaptive feed forward equalizer (FFE) with different number of taps to overcome inter symbol interference (ISI) or symbol-spaced adaptive decisionfeedback equalizer (DFE) with different configuration of feedforward taps (FFT) and feedback taps (FBT) to overcome ISI in presence of noise.
  • A total number of 1.2 million bits are used for BER counting.
  • The bandwidth limitation is due to limited effective 3 dB bandwidth on the transmitter side components used to generate electrical signals, while additional filtering occurs due to the limited DSO bandwidth, affecting the performance of OOK and 8PAM setups.

IV. RESULTS AND DISCUSSIONS

  • Obtained results provide the quantitative and qualitative performance evaluation of the optical transmitter for its capabilities to enable the third generation 400 Gbps client-side links for optical interconnects.
  • One can observe that the use of 3-FFT&3-FBT DFE allows achieving below 7% FEC limit performance, which was further improved with larger lengths of equalizers.
  • Adding small feedback in equalizer allows significantly improving the performance.
  • BER curves are obtained using same equalizers as in OOK case.
  • The distance for 116 Gbps 4PAM can be increased about three times compared to OOK for same penalty and equalizer parameters.

V. CONCLUSIONS

  • The authors report on the performance of an EML with higher than 100 GHz bandwidth for optical interconnects.
  • Furthermore, this is the first time [20] that 116 Gbps OOK is achieved on a single EML based > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 5 optical transmitter with low complexity DSP.
  • The authors study the trade-off between power penalty and equalizer length using Monte Carlo simulations.
  • According to simulation results, a transmission of 3 km standard single mode fiber with less than 1 dB dispersion penalty using only 3-tap static filter for 116 Gbps 8PAM seems to be possible.
  • The authors conclude that, provided sufficient bandwidth and linearity on the electrical domain, this optical transmitter can be used for advanced modulation formats at higher than 100 Gbaud.

Did you find this useful? Give us your feedback

...read more

Content maybe subject to copyright    Report

General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright
owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
You may not further distribute the material or use it for any profit-making activity or commercial gain
You may freely distribute the URL identifying the publication in the public portal
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from orbit.dtu.dk on: Aug 10, 2022
100 GHz Externally Modulated Laser for Optical Interconnects Applications
Ozolins, Oskars; Pang, Xiaodan; Iglesias Olmedo, Miguel; Kakkar, Aditya; Udalcovs, Aleksejs; Gaiarin,
Simone; Navarro, Jaime Rodrigo; Engenhardt, Klaus M.; Asyngier, Tadeusz; Schatz, Richard
Total number of authors:
16
Published in:
Journal of Lightwave Technology
Link to article, DOI:
10.1109/JLT.2017.2651947
Publication date:
2017
Document Version
Peer reviewed version
Link back to DTU Orbit
Citation (APA):
Ozolins, O., Pang, X., Iglesias Olmedo, M., Kakkar, A., Udalcovs, A., Gaiarin, S., Navarro, J. R., Engenhardt, K.
M., Asyngier, T., Schatz, R., Li, J., Nordwall, F., Westergren, U., Zibar, D., Popov, S., & Jacobsen, G. (2017).
100 GHz Externally Modulated Laser for Optical Interconnects Applications. Journal of Lightwave Technology,
35(6), 1174-1179. https://doi.org/10.1109/JLT.2017.2651947

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
1
AbstractWe report on a 116 Gbps on-off keying (OOK),
4 pulse amplitude modulation (PAM) and 105 Gbps 8PAM
optical transmitter using an InP-based integrated and packaged
externally modulated laser for high speed optical interconnects
with up to 30 dB static extinction ratio and over 100 GHz 3 dB
bandwidth with 2 dB ripple. In addition, we study the trade-off
between power penalty and equalizer length to foresee
transmission distances with standard single mode fiber.
Index Termsdistributed feedback laser (DFB), direct
detection, electroabsorption modulator, optical interconnects.
I. INTRODUCTION
he third generation 400 Gbps client-side links are
demanding a solution as the cloud services together with
the huge size datasets are driving demand for bandwidth in
datacenters [1],[2]. Potential solutions are under discussion
within the IEEE P802.3bs 400 Gigabit Ethernet (GbE) Task
Force [3]. One attractive solution is based on eight optical
lanes (i.e., 8×50 Gbps [4]) thanks to compatibility with
existing building blocks for 100 GbE (drivers, lasers, and
photodetectors with transimpedance amplifiers). However,
large lane count increases complexity and power consumption
resulting in higher costs [1]. More scalable and cost efficient
approach is to reduce number of lanes and increase the
bandwidth for a single lane. Four optical lanes at 100 Gbps net
rate (i.e., 4×100 Gbps/λ [5]) are a promising solution in order
to reduce complexity, power consumption and costs.
Manuscript received ………….; revised …………; accepted …………
Date of publication ………….; date of current version ……………………
This work is supported by EU projects ICONE gr. #608099 and GRIFFON gr.
# 324391, Swedish ICT-TNG program and Vetenskapsrädet project PHASE
(grant no. 2016-04510). The equipment was funded by Knut and Alice
Wallenberg foundation. The Tektronix is acknowledged for equipment loan.
O. Ozolins, X. Pang, A. Udalcovs, J. Rodrigo Navarro, J. Li, G. Jacobsen
is with the Networking and Transmission Laboratory, Acreo Swedish ICT
AB, SE- 164 25 Kista, Sweden (e-mail: oskars.ozolins@acreo.se).
M. Iglesias Olmedo, A. Kakkar, R. Schatz, U. Westergren, S. Popov is
with the School of ICT, KTH Royal Institute of Technology, Electrum 229,
Kista, SE-164 40, Sweden.
S. Gaiarin, D. Zibar is with the DTU Fotonik, Technical University of
Denmark (DTU), Kongens Lyngby, 2800, Denmark.
K. M. Engenhardt, T. Asyngier is with the Tektronix GmbH, Stuttgart,
Germany.
F. Nordwall is with the Tektronix AB, Stockholm, Sweden.
However, this task becomes even more challenging since it
requires silicon and InP opto-electronic components with more
than 70 GHz bandwidth.
Advanced modulations formats are extensively studied for
short reach applications: (1) pulse amplitude modulation
(PAM) [6]-[16], (2) carrierless amplitude phase (CAP)
modulation [17], or (3) discrete multi-tone (DMT) [13],[18].
Such formats allow for more efficient bandwidth utilization at
the cost of complex digital signal processing (DSP), which
also may result in a higher costs and power consumption.
These demonstrations usually require long digital filters
resulting in complicated implementations. Hence, consensus
seems to evolve towards 4PAM [6]-[16] and on-off keying
(OOK) [14],[19]-[23] as it saves complexity on the transmitter
side. Limiting amplifiers and electrical multiplexers
technologies up to 120 Gbps have already matured into
products. However, cost-efficient optical modulators at this
bandwidth are not commercially available yet. PAM may
reduce the bandwidth requirements, but it also reduces the
receiver sensitivity and significantly complicates the electrical
side of the transmitter (i.e., digital to analogue converters
(DACs), linear drivers etc.). Therefore, higher bandwidth
optoelectronic components with simpler modulation formats
seem to be the most practical.
In this paper, we report on a cost-efficient integrated
externally modulated laser (EML) with high bandwidth for
116 Gbps OOK (the first time achieved on a single EML [20]
with low complexity DSP) and linear enough to accommodate
116 Gbps 4PAM and 105 Gbps 8PAM, while requiring a
driving voltage of 2 Vpp, paving the way for high speed
multilevel modulation formats.
This paper is organized as follows. Section II summarizes
the related works and provides description of the externally
modulated laser. In Section III, the experiment setup is
detailed. The experimental results are shown and
complemented with system simulations in Section IV. The
conclusions are drawn in Section V.
II. EXTERNALLY MODULATED LASER
A. Previous Experimental Demonstrations
The optical transmitter is based on a monolithically
integrated distributed feedback laser with traveling-wave
electroabsorption modulator (DFB-TWEAM) designed by
100 GHz Externally Modulated Laser for
Optical Interconnects
Oskars Ozolins, Member, IEEE, Xiaodan Pang, Member, IEEE, Miguel Iglesias Olmedo,
Aditya Kakkar, Aleksejs Udalcovs, Simone Gaiarin, Jaime Rodrigo Navarro, Klaus M. Engenhardt,
Tadeusz Asyngier, Richard Schatz, Jie Li, Fredrik Nordwall, Urban Westergren, Darko Zibar,
Sergei Popov, and Gunnar Jacobsen
T

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
2
KTH, fabricated by KTH and Syntune and packaged by
u
2
t Photonics [22],[23]. In [23], the performance of already
packaged DFB-TWEAM transmitter module for 100 Gbps
data rate is evaluated experimentally using 100 meters long
standard single mode fiber (SSMF) and 10 km optical link
with dispersion management. In [21] successful field trial is
demonstrated with transmission of 112 Gbps OOK signals
using a purely electrical time division multiplexing-based
system without DSP. A bit error rate (BER) performance
below the forward error correction (FEC) threshold of 2·10
-3
is
achieved after transmission over 42 km field installed SSMF
with dispersion management between Kista and Råsunda in
Sweden. It is the first demonstration of a complete 112 Gbps
ETDM system based on monolithically integrated transmitter
[23] and receiver modules.
B. Device Structure and Characterization
The TWEAM is based on the 12 strain-compensated
InGaAsP quantum wells/barriers (QWs) of around 9 nm
thickness each. The total active length of modulator is
180 µm. The gain section of the DFB is based on 7 QWs 7 nm
thick grown by metal vapour phase epitaxy coupled with butt-
joint technique. The laser is 440 µm long with a grating
designed to have most of the output power towards the
modulator. Components are formed in standard ridge
waveguide structures [22]. Figure 1 shows the power versus
current for unbiased modulator, power versus bias voltage and
the frequency response taken at 22º C [24]. As we can see
from Fig. 1a, the threshold current is ~25 mA, and the slope
efficiency is .04 W/A, which allows us to reach about 2 mW
with only 80 mA driving current. Fig. 1b shows the static
extinction ratio versus bias voltage. We can observe that the
device has a static extinction ratio in the range of 20 to 35 dB.
The dynamic extinction ratio during modulation is related to
modulator bias voltage and swing of driving voltage. The S21
curve of the device [22] (W1 connector) depicted in Fig. 1c
clearly shows beyond 100 GHz 3 dB bandwidth, the highest
reported bandwidth to the best of our knowledge, with less
than 2 dB ripple in the pass band of the EML [22] which
indicates high phase linearity. These figures of merits [22] are
order of magnitude better than state-of-art EMLs for optical
interconnects.
III. EXPERIMENTAL SETUP
Figure 2 shows the transmitter setup for 116 Gbps OOK,
4PAM and 105 Gbps 8PAM with receiver setup including
DSP (see Fig. 2a) and output optical spectrum of modulated
signals (see Fig. 2b). In the OOK setup, two pseudo-random
bit sequences with a word length of 2
15
-1 (PRBS15) at
58 Gbps are first decorrelated and then multiplexed into a
single 116 Gbps sequence. A 110 GHz traveling-wave limiting
amplifier (TWA110 [25]) is used to drive the EML. In the
4PAM setup, two PRBS15 are first decorrelated and then
Fig. 1. a) P(I) characteristics for unbiased modulator b) static extinction ratio characteristics c) S21 characteristics as measured in [22].
c)b)
Fig. 2. a) Experimental setup. (PPG: pulse-pattern generator, Mux: Multiplexer, DAC: digital to analog converter Ƭ: Delay line, EDFA: erbium doped fiber
amplifier, PD: Photodiode, DSO: digital storage oscilloscope, DSP: digital signal processing, LPF: low pass filter, FFE: Feed forward equalizer, DFE: decision-
feedback equalizer). b) Optical spectrum of the modulated signals (@ 0.01 nm resolution bandwidth).
DSO
DSP
Clock recovery
BER counting
Static, FFE, DFE
LPF (0.75·baudrate)
200 Gsa/s,
70 GHz
DSO
VOA
EDFA
VOA
VOA
EDFA
VOA
58 Gbps
PPG
116 Gbps
MUX
DFB-TWEAM
2 Vpp
-2 dBm
58 Gbps
PPG
DFB-TWEAM
1 Vpp
-2 dBm
35 Gbps
PPG
DFB-TWEAM
2 Vpp
-2 dBm
3 bit
DAC
116 Gbit/s OOK Tx
116 Gbit/s 4PAM Tx
105 Gbit/s 8PAM Tx
a)
b)
80 Gsa/s,
33 GHz
Ƭ
Ƭ
3dB
Ƭ
Ƭ
Ƭ

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
3
passively combined into a 4PAM signal, which is then used to
directly drive the EML. In the 8PAM setup, three PRBS15
sequences at 35 Gbps are first decorrelated and then combined
in a 3 bit DAC with 19 GHz analogue bandwidth. A 65 GHz
linear amplifier is used to drive the EML. During
measurements the bias voltage was kept at minus 2 volts and
driving voltage was 2 Vpp for OOK, 8PAM and 1 Vpp for
4PAM. The average output power was kept at minus 2 dBm in
all cases. The receiver is composed of an Erbium doped fiber
amplifier (EDFA), variable optical attenuator (VOA), a high
bandwidth photodiode (PD) from u
2
t with a responsivity of
0.5 A/W, and a 200 GSa/s, 70 GHz bandwidth Tektronix
digital sampling oscilloscope (DSO) (DPO77002SX) for OOK
as well as 4PAM setups, while 80 GSa/s, 33 GHz DSO - for
8PAM setup. An automatic gain-controlled EDFA with fixed
output power is employed as a pre-amplifier due to the low PD
responsivity and the lack of transimpedance amplifier. The
sampled signal is then processed offline using DSP. The clock
recovery and resampling is performed on the received
waveform. The signal is then downsampled to 1 sample per
symbol for static/adaptive feed forward equalizer (FFE) with
different number of taps to overcome inter symbol
interference (ISI) or symbol-spaced adaptive decision-
feedback equalizer (DFE) with different configuration of feed-
forward taps (FFT) and feedback taps (FBT) to overcome ISI
in presence of noise. The initial weights of the equalizer were
obtained using training data with the normalized least-mean-
square (NLMS) algorithm before applying other data. A total
number of 1.2 million bits are used for BER counting. The
bandwidth limitation is due to limited effective 3 dB
bandwidth on the transmitter side components used to
generate electrical signals, while additional filtering occurs
due to the limited DSO bandwidth, affecting the performance
of OOK and 8PAM setups.
IV. RESULTS AND DISCUSSIONS
Obtained results provide the quantitative and qualitative
performance evaluation of the optical transmitter for its
capabilities to enable the third generation 400 Gbps client-side
links for optical interconnects. Figure 3 shows BER curves for
116 Gbps OOK (see Fig. 3a) and the qualitative measure in
Fig. 3. a) BER curves for 116 Gbps OOK. b) Received eye diagram without
and with equalizer and histograms for 116 Gbps OOK at -2 dBm input power.
without equalizer
with equalizer
with equalizer
without equalizer
a)
b)
-20 -18 -16 -14 -12 -10 -8 -6 -4 -2
10
-3
10
-2
10
-1
BER
Received power (dBm)
3-tap, static
3-FFT, adaptive
3-FFT&3-FBT, adaptive
27-tap, static
27-FFT, adaptive
27-FFT&3-FBT, adaptive
FEC@3.8E-3
Fig. 4. a) BER curves for 116 Gbps 4PAM. b) Received eye diagram without
and with equalizer and histograms for 116 Gbps 4PAM at -2 dBm input power.
without equalizer
with equalizer
without equalizer
with equalizer
a)
b)
-20 -18 -16 -14 -12 -10 -8 -6 -4 -2
10
-3
10
-2
10
-1
BER
Received power (dBm)
3-tap, static
3-FFT, adaptive
3-FFT&3-FBT, adaptive
27-tap, static
27-FFT, adaptive
27-FFT&3-FBT, adaptive
FEC@3.8E-3
Fig. 5. a) BER curves for 105 Gbps 8PAM. b) Received eye diagram without
and with equalizer and histograms for 105 Gbps 8PAM at -2 dBm input power.
without equalizer
with equalizer
without equalizer
with equalizer
a)
b)
-20 -18 -16 -14 -12 -10 -8 -6 -4 -2
10
-3
10
-2
10
-1
BER
Received power (dBm)
3-tap, static
3-FFT, adaptive
3-FFT&3-FBT, adaptive
27-tap, static
27-FFT, adaptive
27-FFT&3-FBT, adaptive
FEC@3.8E-3

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <
4
terms of eye diagram and the histogram distribution of the
received signal (see Fig. 3b). BER curves are obtained using
3-tap and 27-tap static filter, 3-FFT and 27-FFT adaptive FFE,
and 3-FFT&3-FBT and 27-FFT&3-FBT DFE. One can
observe that the use of 3-FFT&3-FBT DFE allows achieving
below 7% FEC limit performance, which was further
improved with larger lengths of equalizers. Adding small
feedback in equalizer allows significantly improving the
performance. We also observed that 27-tap static filter allows
achieving almost the same performance as 3-FFT&3-FBT
DFE. Performance is further improved by 3 dB (reduction in
power penalty) with 27-FFT&3-FBT DFE. Figure 3b shows
the received eye diagrams and histograms with and without
equalizer for 116 Gbps OOK signal. We can see that the signal
is limited by both ISI and noise. The histogram without
equalizer shows level degeneration owing to the ISI which we
also observed in the electrical signal.
Figure 4 shows BER curves (see Fig. 4a), eye diagrams and
histograms (see Fig. 4b) before and after equalizer for
116 Gbps 4PAM. BER curves are obtained using same
equalizers as in OOK case. We can see large degradation on
the 4PAM signal owing to both higher sensitivity
requirements and poor electrical signal performance at the
transmitter. After equalizer one can observe that passive
combining ratio is suboptimal (see transmitter design in
Fig. 2a), which explains the worse performance. We use the
EML to compensate this effect by driving it at high reverse
bias voltage (-2 volts). We are able to improve the
performance, however, this causes compression on the upper
levels and thus a trade-off was found. By adding 3-FFT&3-
FBT DFE, a BER below FEC limit was achieved. We did not
observe significant improvement by increasing the length of
equalizer.
Figure 5 shows BER curves for 105 Gbps 8PAM (see
Fig. 5a) and eye diagram and the histogram distribution of the
received signal (see Fig. 5b). Similarly to previous cases BER
curves are obtained using same configuration as before.
Sensitivity for 8PAM is lower than 4PAM, but thanks to the
performance of the 3 bit DAC, the implementation penalty is
lower. However, still some compression is observed. One can
observe that the performance of 3-FFT&3-FBT DFE is below
the 7% FEC limit. Still performance can be significantly
improved (reduction in power penalty is 4dB with 27-tap static
filter and 5 dB with 27-FFT&3-FBT DFE) with increased
length of equalizer since the main bandwidth limitation is the
19 GHz bandwidth of the DAC. Therefore, by ensuring
sufficient bandwidth in the electrical domain, the optical
transmitter has the potential of transmitting >100 Gbaud
signals with low DSP requirements.
Operational wavelength of the EML is around 1548 nm in
these measurements. To complement the experiments we
study chromatic dispersion tolerance for different modulation
formats using Monte Carlo simulations. In simulation we
assumed that all modulation formats are operated at 116 Gbps
for a fair comparison. In simulations we focus on tolerance to
chromatic dispersion (16ps/nm/km). Only bandwidth
limitation is PD with responsivity of 0.5 A/W and 80 GHz
bandwidth. Received power penalty as function of transmitted
distance is shown in Fig.6. We obtain curves for 3-tap and 6-
tap static filter, 3-FFT and 6-FFT adaptive FFE, and 3-
FFT&3-FBT and 6-FFT&3-FBT DFE. For 1 dB power
penalty, the 116 Gbps OOK format can be transmitted up to
700 meters using 3-FFT&3-FBT DFE. The distance for 116
Gbps 4PAM can be increased about three times compared to
OOK for same penalty and equalizer parameters. In case of
116 Gbps 8PAM, the dispersion penalty will be below 1 dB at
3 km distance using only a 3-tap static filter.
We point out that the microwave design of the transmitter
can be applied to a semiconductor material with larger
bandgap in order to achieve modulation at another operation
wavelength [23]. Then transmission distances over SSMF can
be significantly improved.
V. CONCLUSIONS
We report on the performance of an EML with higher than
100 GHz bandwidth for optical interconnects. We
experimentally validate its potential for fast optical
interconnects by transmitting 116 Gbps OOK, 4PAM and
105 Gbps 8PAM signals. Furthermore, this is the first time
[20] that 116 Gbps OOK is achieved on a single EML based
Fig. 6. Received power penalty as function of transmitted distance for 116 Gbps OOK, 4PAM and 8PAM for different equalizer configurations (see inset).
0,25 0,75
116 Gbps
OOK
116 Gbps
4PAM
116 Gbps
8PAM
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
0,0
0,5
1,0
1,5
2,0
Rx power penalty @3.8e-3 (dB)
Distance (km), D=16ps/nm/km
3-tap, static
3-FFT, adaptive
3-FFT&3-FBT, adaptive
6-tap, static
6-FFT, adaptive
6-FFT&3-FBT, adaptive

Citations
More filters

Journal ArticleDOI
19 Nov 2018
TL;DR: The focus of this paper lies on the latter capable of supporting both complex-valued modulation and optically broadband operation, with a sense of the limitations of current technology and the potential of novel hybrid material integration.
Abstract: The realization of gigahertz bandwidth modulators out of silicon-based technology in the early 2000s marked a cornerstone of silicon photonics development. While modulation speeds have since progressed well above 50 GHz and satisfy the bandwidth requirements of current and emerging modulation formats, concurrently obtaining low drive voltages and low insertion losses remains a very active area of research. While modulators generally come in two categories, direct absorption and those relying on embedded phase shifters, the focus of this paper lies on the latter capable of supporting both complex-valued modulation and optically broadband operation. The paper provides an overview of the current state of the art, as well as of currently explored improvement paths. First, common phase shifter configurations, aspects related to electrical driving, and associated power consumption are reviewed. Slow-wave, resonant, and plasmonic enhancements are further discussed. The reader is familiarized with the optimization of these devices and provided with a sense of the limitations of current technology and the potential of novel hybrid material integration.

80 citations


Cites background from "100 GHz Externally Modulated Laser ..."

  • ...Transceiver developers have thus to weigh the high yield and facilitated electronic integration afforded by mature complementary metal– oxide–semiconductor (CMOS) processes, the high stability and material quality of silicon (Si), and relatively easy to handle capacitive or resistive loads associated to SiP modulators against the reduced insertion losses afforded by monolithic integration of externally modulated lasers (EMLs) [9]....

    [...]


Journal ArticleDOI
TL;DR: It is shown that, although the DML based transmitter is often believed to be less favorable in C-band high-speed transmissions, it exhibits superior performance over the other two transmitters when either linear or nonlinear digital signal processing is adopted.
Abstract: In this paper, transmission performances of directly modulated laser (DML), electro-absorption modulated laser (EML) and Mach-Zehnder modulator (MZM) are experimentally compared in dispersion-unmanaged high-speed transmission systems with digital signal processing (DSP). We show that, although the DML based transmitter is often believed to be less favorable in C-band high-speed transmissions, it exhibits superior performance over the other two transmitters when either linear or nonlinear digital signal processing is adopted. By theoretical and experimental analysis, we reveal that the superiority of DML can be attributed to the compensation of fiber power fading by its inherent adiabatic chirp as well as the mitigation of chirp induced distortions by the linear or nonlinear equalization. Experimental results of 56Gb/s 4-level pulse amplitude modulation (PAM4) signals under various equalization schemes including linear feedforward equalization, simplified nonlinear Volterra equalization and partial response signaling are presented. Particularly, we show that for DML a 40km transmission distance can be achieved to satisfy the extended range-4 (ER4) Ethernet interconnect using a simplified Volterra equalizer, and a 20km transmission distance can be supported using a linear equalizer. In contrast, for MZM and EML, the achievable transmission distances are respectively 20km and 15km using the Volterra equalizer, respectively, and 15km and 10km using linear equalizer, respectively. Moreover, we show that even using the combination of the Volterra equalizer and partial response signaling, the transmission distances of MZM and EML based systems are limited to 30km and 20km.

45 citations


Journal ArticleDOI
15 Jan 2020
TL;DR: This article focuses on IM/DD transmissions, and provides an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond, and expects high-speed IM/ DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.
Abstract: Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

38 citations


Cites background or methods from "100 GHz Externally Modulated Laser ..."

  • ...(a) P(I) characteristics; (b) P(V) characteristics of the DFB-TWEAM module [66]; (c) picture of the packaged module; (d) small-signal transfer characteristics S21 [65]....

    [...]

  • ...Besides the aforementioned works, the DFB-TWEAM reported in [65] was also employed for a 204 Gbaud OOK transmission,...

    [...]

  • ...(DFB-TWEAM) of >100 GHz bandwidth [65], with which several high-speed transmissions were demonstrated [66], [67]....

    [...]


Proceedings ArticleDOI
11 Mar 2018
TL;DR: An on-off keyed transmitter with direct detection, at record symbol rates of 204Gbaud and 140Gbaud, over 10km and 80km, respectively, powered by a high-speed InP-based 2:1 selector and travelling-wave electro-absorption laser-modulator.
Abstract: We demonstrate an on-off keyed transmitter with direct detection, at record symbol rates of 204Gbaud and 140Gbaud, over 10km and 80km, respectively, powered by a high-speed InP-based 2:1 selector and travelling-wave electro-absorption laser-modulator.

37 citations


Cites methods from "100 GHz Externally Modulated Laser ..."

  • ...In this paper, we demonstrate a transmitter producing OOK optical data at record symbol rates of 140-GBaud and 204-GBaud with InP technologies, enabled by a large-bandwidth InP-based double heterojunction bipolar transistor (DHBT) selector [6] and an integrated EML [3]....

    [...]

  • ...An IAF InP DHBTbased modulator driver module with gain of 16 dB and the 3-dB bandwidth of 110 GHz was used to achieve sufficient driving voltage swing between 2:1 Selector (SEL2) and for the DFB-TWEAM [8]....

    [...]

  • ...Alternatively, the generation of a 116 Gbit/s OOK signal was reported, using an ultrabroadband monolithically integrated distributed feedback laser with traveling-wave electro-absorption modulator DFB-TWEAM) [3]....

    [...]

  • ...of 140-GBaud and 204-GBaud with InP technologies, enabled by a large-bandwidth InP-based double heterojunction bipolar transistor (DHBT) selector [6] and an integrated EML [3]....

    [...]

  • ...Such high symbol rate electrical generation is enabled by a very-high-speed 2:1-Selector in InP DHBT technology, which requires electrical inputs at half the output rates; and we used 100 GHz externally modulated laser (DFB-TWEAM)....

    [...]


Journal ArticleDOI
Abstract: Increasing the modulation speed of semiconductor lasers has attracted much attention from the viewpoint of both physics and the applications of lasers. Here we propose a membrane distributed reflector laser on a low-refractive-index and high-thermal-conductivity silicon carbide substrate that overcomes the modulation bandwidth limit. The laser features a high modulation efficiency because of its large optical confinement in the active region and small differential gain reduction at a high injection current density. We achieve a 42 GHz relaxation oscillation frequency by using a laser with a 50-μm-long active region. The cavity, designed to have a short photon lifetime, suppresses the damping effect while keeping the threshold carrier density low, resulting in a 60 GHz intrinsic 3 dB bandwidth (f3dB). By employing the photon–photon resonance at 95 GHz due to optical feedback from an integrated output waveguide, we achieve an f3dB of 108 GHz and demonstrate 256 Gbit s−1 four-level pulse-amplitude modulations with a 475 fJ bit−1 energy cost of the direct-current electrical input. Directly modulated membrane distributed reflector lasers are fabricated on a silicon carbide platform. The 3 dB bandwidth, four-level pulse-amplitude modulation speed and operating energy for transmitting one bit are 108 GHz, 256 Gbit s−1 and 475 fJ, respectively.

31 citations


References
More filters

Journal ArticleDOI
Abstract: Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.

246 citations


"100 GHz Externally Modulated Laser ..." refers background in this paper

  • ...Advanced modulations formats are extensively studied for short reach applications: (1) pulse amplitude modulation (PAM) [6]–[16], (2) carrierless amplitude phase (CAP) modulation [17], or (3) discrete multi-tone (DMT) [13], [18]....

    [...]


Proceedings ArticleDOI
20 Mar 2016
TL;DR: Using a lumped-electrode EADFB laser with a modulation bandwidth of ~59 GHz, this work demonstrated single-wavelength single-polarization direct-detection 4-PAM transmission with the record net data rate of 200 Gbit/s.
Abstract: Using a lumped-electrode EADFB laser with a modulation bandwidth of ∼59 GHz, we demonstrated single-wavelength single-polarization direct-detection 4-PAM transmission with the record net data rate of 200 Gbit/s.

68 citations


"100 GHz Externally Modulated Laser ..." refers background in this paper

  • ...Hence, consensus seems to evolve towards 4PAM [6]–[16] and on-off keying (OOK) [14], [19]–[23] as it saves complexity on the transmitter side....

    [...]

  • ...Advanced modulations formats are extensively studied for short reach applications: (1) pulse amplitude modulation (PAM) [6]–[16], (2) carrierless amplitude phase (CAP) modulation [17], or (3) discrete multi-tone (DMT) [13], [18]....

    [...]


Journal ArticleDOI
Abstract: We experimentally demonstrated 140-Gb/s transmission over 20-km standard single-mode fiber employing pulse-amplitude modulation (PAM)-4 and direct detection (DD) at 1.3 $\mu \text{m}$ . DD faster than Nyquist is employed to compensate for channel impairments. The optimal length of taps of decision directed least mean square and the optimal tap coefficient for digital postfilter are investigated. A receiver sensitivity of −5.5 dBm at bit error rate of $3.8\times 10^{-3}$ is realized for 140-Gb/s PAM-4 signal after 20-km transmission. To the best of our knowledge, this is the highest reported baud rate (70 GBd) of direct detected PAM-4 signal and the highest per channel bit rate with single polarization and DD for short reach communications.

67 citations


Journal ArticleDOI
TL;DR: A 56 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission using direct detection and a long-wavelength 18-GHz bandwidth vertical-cavity surface-emitting laser as directly modulated light source for short-reach inter- and intra-connects in datacenters and short- Reach networks is demonstrated.
Abstract: We demonstrate a 56 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission using direct detection and a long-wavelength 18-GHz bandwidth vertical-cavity surface-emitting laser as directly modulated light source for short-reach inter- and intra-connects in datacenters and short-reach networks. Error-free transmission over 2 km at 7% hard-decision forward-error correction threshold is achieved by applying powerful equalization schemes at the receiver side. Three equalization schemes, i.e., a maximum likelihood estimation (MLSE), a feed-forward equalizer (FFE), and a combination of the FFE and the MLSE are thoroughly investigated, and the performance comparison between them is carried out.

66 citations


"100 GHz Externally Modulated Laser ..." refers background in this paper

  • ..., 8 × 50 Gbps/λ [4]) thanks to compatibility with existing building blocks for 100 GbE (drivers, lasers, and photodetectors...

    [...]

  • ...Four optical lanes at 100 Gbps net rate (i.e., 4×100 Gbps/λ [5]) are a promising solution in order to reduce complexity, power consumption and costs....

    [...]

  • ...One attractive solution is based on eight optical lanes (i.e., 8×50 Gbps/λ [4]) thanks to compatibility with existing building blocks for 100 GbE (drivers, lasers, and photodetectors with transimpedance amplifiers)....

    [...]


Journal ArticleDOI
Abstract: A monolithically integrated distributed feedback (DFB) laser and traveling-wave electro-absorption modulator (TWEAM) with ges 100 GHz -3 dBe bandwidth suitable for Non-return-to-zero (NRZ) operation with on-off keying (OOK) is presented. The steady-state, small-signal modulation response, microwave reflection, chirp characteristic, and both data operation and transmission were investigated. The DFB-TWEAM was found to be an attractive candidate for future short distance communication in high bitrates systems.

64 citations


"100 GHz Externally Modulated Laser ..." refers background or methods in this paper

  • ...1(c) clearly shows beyond 100 GHz 3 dB bandwidth, the highest reported bandwidth to the best of our knowledge, with less than 2 dB ripple in the pass band of the EML [22] which indicates high phase linearity....

    [...]

  • ...A 65 GHz linear amplifier is used to drive the EML....

    [...]

  • ...The S21 curve of the device [22] (W1 connector) depicted in Fig....

    [...]

  • ...The optical transmitter is based on a monolithically integrated distributed feedback laser with traveling-wave electroabsorption modulator (DFB-TWEAM) designed by KTH, fabricated by KTH and Syntune and packaged by u2t Photonics [22], [23]....

    [...]

  • ...We report on the performance of an EML with higher than 100 GHz bandwidth for optical interconnects....

    [...]


Related Papers (5)
Frequently Asked Questions (1)
Q1. What are the contributions in this paper?

The authors report on a 116 Gbps on-off keying ( OOK ), 4 pulse amplitude modulation ( PAM ) and 105 Gbps 8PAM optical transmitter using an InP-based integrated and packaged externally modulated laser for high speed optical interconnects with up to 30 dB static extinction ratio and over 100 GHz 3 dB bandwidth with 2 dB ripple. In addition, the authors study the trade-off between power penalty and equalizer length to foresee transmission distances with standard single mode fiber.