Transmission of 20 Gb/s PAM-4 signal over
20 km optical fiber using a directly modulated tunable
DBR laser
Daibing Zhou (周代兵), Dan Lu (陆 丹), Song Liang (梁 松), Lingjuan Zhao (赵玲娟)*,
and Wei Wang (王 圩)
Key Laboratory of Semiconductor Materials Science, Institu te of Semiconductors, Chinese Academy of Science,
College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences,
Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Beijing 100083, China
*Corresponding author: ljzhao@semi.ac.cn
Received June 11, 2018; accepted July 18, 2018
We report 20 Gb/s transmission of four-level pulse amplitude modulation (PAM) signal using a directly modu-
lated tunable distributed Bragg reflector (DBR) laser. Transmission distance over 20 km was achieved without
using optical amplifiers and optical dispersion compensation modules. A wavelength tuning range of 11.5 nm and
3 dB bandwidth greater than 10 GHz over the entire wavelength tuning range was obtained.
OCIS codes: 140.5960, 250.5300, 140.3600, 060.4510.
doi: 10.3788/COL201816.091401.
The wavelength division multiplexed passive optical
network (WDM-PON) technology can effectively exploit
the capacity of optical fibers and reduce the costs of main-
tenance, which will be an excellent candidate for 5G
front-haul networks
[1–4]
. As a key technology, a colorless
optical network unit (ONU) in WDM-PON systems utilize
tunable lasers to realize high speed and low cost. The
wavelength tunable lasers can replace multiple lasers or
multi-wavelength laser arrays in traditional transmitters
and reduce the costs of production and maintenance.
Typical tunable lasers include a wavelength tunable dis-
tributed Bragg reflector (DBR) laser
[56]
, surface microma-
chined tunable vertical-cavity surface-emitting laser
(VCSEL)
[7]
, V-coupled-cavity laser
[8]
, sampled grating
DBR (SG-DBR) laser
[9]
, digital supermode DBR
(DS-DBR) laser
[10]
, tunable external-cavity laser (ECL)
[11]
,
etc. In terms of simplicity in manufacture and cost, the tun-
able DBR laser is an ideal choice for WDM-PON systems
[12]
.
Usually, the DBR laser is modulated by non-return-
to-zero (NRZ) on–off key (OOK) codes, transmitting
one bit per symbol. With the approach of the 200G
and 400G era, four-level pulse amplitude modulation
(PAM-4) has attracted much attention from both aca-
demia and industry
[13,14]
due to its ability to double the
data rate without increasing the required overall band-
width over conventional NRZ. In the direct modulation
schemes, most researches focused on the PAM-4 of a
directly modulated distributed feedback (DFB) laser,
very few references reported the PAM-4 modulation of
DBR lasers. In a previous work reported by Huawei
Co., Ltd.
[15]
, 10 Gb/s PAM-4 direct modulation of a
DBR laser was demonstrated. In the experiment, the
DBR laser could be tuned up to 10 nm with a modulation
speed of 5 Gbaud using duobinary and PAM-4 modula-
tions; a 40 km reach was achieved using this scheme.
The experiment reveals the possibility of combining the
PAM-4 direct modulation and wavelength tunable laser
technology in a low cost manner.
In this work, we demonstrate PAM-4 modulation and
the transmission performance of a three-section tunable
DBR laser at a baud rate of 10 GBaud. The DBR laser
achieves a continuous wavelength tuning range of
11.5 nm with a modulation bit rate of 20 Gb/s (10 GBaud)
using PAM-4 modulation. The PAM-4 signal was trans-
mitted over a 20 km standard single mode fiber (SMF)
with a clear, opened eye diagram.
The DBR laser consists of three sections, a DBR section,
a phase section, and a gain section, as shown in Fig.
1. The
active region was InGaAlAs multiple quantum wells
(MQWs) to increase its differential gain and reduce the
temperature dependence. Polyimide was buried under
the electrode pad to decrease the parasitic capacitance.
The DBR laser was fabricated as follows. The active re-
gion consists of six compressively strained InGaAlAs
MQWs and seven InGaAlAs barrier layers, which were
sandwiched between two separate confinement hetero-
structure (SCH) InGaAlAs layers. An InGaAsP layer
was butt-jointed with the active layer. After a grating
was formed in the DBR section by holographic lithogra-
phy, a p-type InP cladding layer and InGaAs contact layer
were grown. A 3-μm-wide ridge waveguide was fabricated
by wet etching. The lengths of the gain, phase, and DBR
sections were 300, 100, and 150 μm, respectively. Finally,
the devices were cleaved and mounted on an AlN heat sink
for measurement. A detailed fabrication process can be
found in our previous Letter
[5]
. The schematic view of
the DBR laser is shown in Fig.
1.
The InGaAlAs DBR laser was pigtail packaged, as
shown in Fig.
2. The typical light-current characteristic
of the packaged InGaAlAs DBR laser is shown in Fig.
3.
COL 16(9), 091401(2018) CHINESE OPTICS LETTERS September 10, 2018
1671-7694/2018/091401(4) 091401-1 © 2018 Chinese Optics Letters