COL 9(4), 041403(2011) CHINESE OPTICS LETTERS April 10, 2011
Generation of tunable multi-wavelength optical short pulses
using self-seeded Fabry-Perot laser diode and tilted
multimode fiber Bragg grating
Tongjian Cai (éééÓÓÓèèè), Yunqi Liu (444ééé)
∗
, Xiaob ei Zhang (ÜÜÜ), and Tingyun Wang (ÉÉÉ)
Key Lab of Specialty Fiber Optics and Optical Access Network, School of Communication and Information Engineering,
Shanghai University, Shanghai 200072, China
∗
Corresp onding author: yqliu@shu.edu.cn
Received October 8, 2010; accepted December 6, 2010; posted online March 28, 2011
We experimentally demonstrate the simultaneous generation of tunable multi-wavelength picosecond laser
pulses using a self-seeding configuration that consists of a gain-switched Fabry-Perot laser diode (FPLD)
with an external cavity formed by a tilted multimode fib er Bragg grating. Dual- and triple-wavelength
pulses are obtained and tuned in a flexible manner by changing the temperature of the FPLD. The side
mo de suppression ratio larger than 25 dB is achieved at different dual- and triple-wavelengths and the
typical pulsewidth of the output pulses is ∼70 ps. In the experiment, the wavelength separation can be
narrowed to 0.57 nm.
OCIS codes: 140.3520, 060.3735, 140.3538, 140.3600.
doi: 10.3788/COL201109.041403.
Dual- and multi-wavelength short pulses have attracted
much interest over the years because of their many
important applications, such as in optical fiber sen-
sors, optical code division multiple access systems,
and wavelength-division multiplexing (WDM) optical
communication
[1−6]
. Self-seeding or external injec-
tion seeding of a gain-switched Fabry-Perot laser diode
(FPLD) is a simple and inexpensive method for gener-
ation of multi-wavelength short pulses
[7−10]
. In a typ-
ical self-seeding scheme, multiple fiber Bragg gratings
(FBGs) are typically used as wavelength selection ele-
ments, and multi-wavelength operation is achieved by
selecting the different modes of a FPLD. Recently, a
configuration consisting of a single-mode fiber (SMF)
with a multimode FBG (MMFBG) has been developed
to achieve the wavelength-switching of a fiber ring laser.
The discrete lasing wavelengths were obtained over a
wide wavelength range by changing the mode coupling
condition in the multimode fiber (MMF)
[11]
.
In this letter, we propose the use of a tilted MMFBG
as the wavelength selection element for the generation
of dual- or multi-wavelength short pulses from a self-
seeded FPLD. The principle of wavelength selection is
based on the mo de coupling in a graded-index MMF
caused by microbending. The fundamental core mode
can be coupled to a specific high-order core mode of
the MMF by inducing microbending of the fiber; the
order of the dominantly coupled mode progressively in-
creases as the amount of microbending is increased
[12]
.
The wavelength of the FPLD modes varies as temper-
ature changes, providing the possibility of generating
multi-wavelength pulses and wavelength tuning of gen-
erated pulses when different FPLD modes are selected
by the tilted MMFBG. With the simple reflection ele-
ment based on the tilted MMFBG, three different kinds
of dual-wavelength optical pulses and a group of triple-
wavelength optical pulses were obtained using a self-
seeding scheme. Wavelength tuning can be achieved by
adjusting the temperature of the FPLD and changing
the modal distribution in the reflection of the MMFBG
using a mode scrambler. The side mode suppression ratio
(SMSR) is higher than 25 dB and the pulsewidth is ∼70
ps. The narrowest wavelength separation is 0.57 nm.
The experimental setup of the self-seeded FPLD with
a tilted MMFBG as the wavelength-selection element
is shown in Fig. 1. A commercial 1.5-µm pigtailed
FPLD with a mode separation of ∼1.12 nm was used,
whose threshold current was 9.5 mA. A 15-m erbium-
doped fiber (EDF), optically pumped by a 980-nm laser
diode (LD) through a WDM coupler, was inserted in
the linear cavity to amplify the light from the FPLD.
An external cavity was constructed with the EDF, a
3-dB coupler, and the tilted MMFBG, which was termi-
nated with an index-matching gel (IMG). A polarization
controller (PC) was included in the external cavity to
control the polarization state of light. The FPLD was
modulated at a radio frequency (RF) of 1243.74 MHz,
which corresponded to the 393rd harmonic of the fun-
damental frequency of the external cavity. The FPLD
was biased at 8.6 mA and the RF power applied was
28 dBm. The gain-switching frequency was kept un-
changed, which was determined by the fixed length of
the external cavity. The output pulses were divided
into two parts through a 20/80 coupler, 20% of which
were measured with an optical spectrum analyzer with a
Fig. 1. Experimental setup of a self-seeded FPLD with a tilted
MMFBG.
1671-7694/2011/041403(4) 041403-1
c
° 2011 Chinese Optics Letters