CHIN. PHYS. LETT. Vol. 28, No. 7 (2011) 074207
A Special Sampling Structure with an Arbitrary Equivalent-Phase-Shift for
Semiconductor Lasers and Multiwavelength Laser Arrays
*
ZHOU Ya-Ting(周亚亭)
1,2**
, SHI Yue-Chun(施跃春)
1
, LI Si-Min(李思敏)
1
, LIU Sheng-Chun(刘盛春)
1
,
CHEN Xiang-Fei(陈向飞)
1**
1
Microwave-Photonics Technology Laboratory, Nanjing National Laboratory of Microstructures,
School of Engineering and Applied Sciences, Nanjing University, Nanjing 210093
2
Changzhou Institute of Technology, Changzhou 213002
(Received 16 February 2011)
A method used to introduce an arbitrary equivalent-phase-shift (EPS) into an asymmetric sampled Bragg grating
(SBG) is reported. Under the same structural parameters except for the sampling pattern, the asymmetric SBG
offers better performance than that of a normal SBG structure for keeping single-longitudinal-mode (SLM)
operation. This is because the proposed sampling pattern can suppress the 0
th
-order light resonance due to the
mismatch between the two different grating sections along the whole SBG. This method can be used to design
and fabricate semiconductor lasers and multiwavelength laser arrays (MLAs) with the required EPS at high yield
and low cost.
PACS: 42.60.By, 42.55.Px DOI:10.1088/0256-307X/28/7/074207
For normal distributed feedback (DFB) lasers, in
order to obtain different lasing wavelengths, multi-
ple seed gratings with different grating periods are
required. Furthermore, in order to obtain single-
longitudinal-mode (SLM) operation or unique lasing
properties, different true phase shifts should be intro-
duced, such as a 𝜋
[1]
or 𝜋/2 phase shift.
[2]
In such
DFB lasers, a nm-level precision process, for exam-
ple, electron-beam (e-beam) lithography,
[3]
is neces-
sary. Due to the time-consumption and high cost, the
e-beam process is still difficult to use commercially.
The use of the sampled Bragg grating (SBG) is one
solution, in which only µm-level precision is needed
to change the sampling periods to obtain a differ-
ent equivalent phase shift (EPS) and different lasing
wavelengths.
[4]
In such an SBG laser, in order to suppress the
lasing in the undesired 0
th
-order channel and to ob-
tain lasing in the −1
st
-order channel, the gain in the
0
th
-order is usually chosen to be smaller than that
in the −1
st
-order channel. However, in the 0
th
-order
channel, the index modulation strength is the highest
among all channels. Furthermore, some factors can
make the gain in the 0
th
-order channel increase, such
as temperature variation, material defects in an ac-
tive medium, etc. In such situations, the SLM yield
of an SBG laser will decrease, which is disadvanta-
geous especially for designing and fabricating multi-
wavelength laser arrays (MLAs) that need a high SLM
yield (maybe at least >95%).
In Ref. [5], an asymmetric SBG structure with 𝜋
EPS is proposed and studied to suppress the lasing
in the 0
th
-order channel. Numerical simulation and
theoretical analysis have shown that the asymmet-
ric structure can increase the SLM yield but impact
on threshold properties slightly. However, with this
method, an arbitrary EPS cannot be obtained in the
asymmetric structure to obtain some unique lasers,
such as 𝜋/2 phase shift DFB lasers. In this Letter, we
report and study a method to introduce an arbitrary
EPS into such an asymmetric structure. For the first
time to our knowledge, we study such a structure with
an arbitrary EPS. When the duty cycle and EPS are
selected properly, an SBG laser with unique proper-
ties can be obtained. The reported structure with an
arbitrary EPS will, we believe, benefit the design and
fabrication of SBG lasers with different wavelengths
and unique properties, such as high SLM performance,
low threshold, and robust resistance to external opti-
cal feedback, suppressing spacial hole burning. This
method can also be used for MLAs with those unique
properties at low cost.
Conventionally, the sampling function of a normal
SBG can be shown as Fig. 1(a). It can be seen that the
sampling function is successive except that its right
section is moved over a distance 𝐷 along the 𝑥 axis.
Mathematically, the index modulation ∆𝑛 of an
SBG can be described by
[6]
∆𝑛 =
1
2
∆𝑛
𝑠
𝑆(𝑥) exp(𝑗
2𝜋𝑥
Λ
0
) + 𝑐.𝑐., (1)
where ∆𝑛
𝑠
and Λ
0
are the amplitude of the index
modulation and the grating period of the seed grat-
ing, respectively. The sampling function 𝑆(𝑥) can be
*
Supported by the National Natural Science Foundation of China under Grant No 60877043, the Natural Science Foundation
of Changzhou Institute of Technology under Grant No YN1008, and the New Century Excellent Talents in University.
**
Email: zhou-yating@163.com; chenxf@nju.edu.cn
© 2011 Chinese Physical Society and IOP Publishing Ltd
074207-1