Single-mode lasing via loss engineering
in fiber-taper-coupled polymer bottle
microresonators
FUMING XIE,
1
NI YAO,
2
WEI FANG,
2
HAIFENG WANG,
1
FUXING GU,
1,
* AND SONGLIN ZHUANG
1
1
Shanghai Key Laboratory of Modern Optical Systems, Engineering Research Center of Optical Instrument and System (Ministry of Education),
University of Shanghai for Science and Technology, Shanghai 200093, China
2
State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University,
Hangzhou 310027, China
*Corresponding author: fuxinggu@gmail.com
Received 9 June 2017; revised 5 September 2017; accepted 13 September 2017; posted 20 September 2017 (Doc. ID 297652);
published 25 October 2017
Due to the lack of mode selection capability, single whispering-gallery-mode (WGM) lasing is a challenge to
achieve. In bottle microresonators, the highly nondegenerated WGMs are spatially well-separated along the
long-axis direction and provide mode selection according to their axial mode numbers. In this work, we use
a loss-engineering approach to suppress the higher-order WGMs and demonstrate single-mode lasing emission
in small polymer bottle microresonators. The fiber tapers are not only used to couple pump light into the bottle
microresonators to excite the WGMs but also to bring optical losses that are induced from the diameter mismatch
between fiber tapers and microresonators. By adjusting the coupling positions, the diameters of fiber tapers, and
the coupling angles, single fundamental-mode lasing is efficiently generated with side-mode suppression
factors over 15 dB. Our loss-engineering approach is convenient just by moving the fiber taper and may find
promising application s in miniature tunable single-mode lasers and sensors.
© 2017 Chinese Laser Press
OCIS codes: (140.3570) Lasers, single-mode; (140.3945) Microcavities; (160.5470) Polymers.
https://doi.org/10.1364/PRJ.5.000B29
1. INTRODUCTION
Whispering gallery mode (WGM) microcavities influence light
to circulate for a long time along equatorial trajectories close to
the surface before it is scattered or absorbed, thus achieving
attractive advantages of long photon lifetime, strong optical
field confinement, and in-plane emission, which have been
applied to many applications, including lasing, sensing, and op-
tical communications [1–6]. Up to now, various structures have
been demonstrated for generating WGM lasers, such as micro-
spheres, microrings, microdroplets, microdisks, microtoroids,
and microfibers. Typically, WGM lasers are usually multimode
due to the lack of a mode selection strategy [3,6–8]. Reducing
the size of the microcavities is a direct way to realize single-
mode lasing, and another possible strategy is to use coupled
cavities through the Vernier effect [9–11] and the parity-time
symmetry effect [12,13].
Recently, bottle microresonators have attracted considerable
interest in many applications such as compact optical delay
lines, cavity optomechanics, lasing, electromagnetically in-
duced transparency-like phenomenons, and nonlinear optics
[14–20]. The highly prolate shapes provide rich spectral fea-
tures with different intensi ty distributions along the bottle
length, which can be exploited to manipulate resonant modes
according to their axial mode numbers and thereby reduce
the number of resonances in the spectrum [21,22]. Polymers
are good hosts for various lasing gain dopants with advantages
of easy processing, mechanical flexibility, and low cost. For ex-
ample, high-quality polymer microfiber resonators are fabricated
to generate WGM lasers, with advantages of tunability, single-
mode operation, and enhanced refractive index sensing [11,23].
More recently, we used interference light patterns to spatially
overlap the intensity profile of a desired WGM and demon-
strated single-mode lasing in polymer bottle microresonators
[24]. Attractive advantages, including high side-mode suppres-
sion factors (SMSFs) greater than 20 dB, large spectral tunability
greater than 8 nm, low lasing threshold, and reversible control,
are presented.
Using a fiber taper to coupl e light into WGM resonators is a
common and efficient approach [7,25]. Under critical condi-
tions, high coupling efficiency up to 99% can occur by carefully
choosing the diameters of fiber tapers and the gap distance
between fiber tapers and resonators [26]. Usually, due to the
diameter mismatch between fiber tapers and resonators,
although a small portion of input power can be coupled into
Research Article
Vol. 5, No. 6 / December 2017 / Photonics Research B29
2327-9125/17/060B29-05 Journal © 2017 Chinese Laser Press