Bistable lasing in parity-time symmetric coupled
fiber rings
SERGEY V. S MIRNOV,
1,
*MAXIM O. MAKARENKO,
1
SERGEY V. S UCHKOV,
2
DMITRY CHURKIN,
1
AND
ANDREY A. SUKHORUKOV
2
1
Novosibirsk State University, 2 Pirogova Street, Novosibirsk 630090, Russia
2
Nonlinear Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia
*Corresponding author: smirnov@lab.nsu.ru
Received 20 September 2017; accepted 12 January 2018; posted 12 January 2018 (Doc. ID 307530); published 20 March 2018
We propose a parity-time (PT) symmetric fiber laser composed of two coupled ring cavities with gains and losses,
which operates both in PT-symmetric and symmetry-broken regimes depending on the static phase shifts. We
perform analytical and numerical analysis by the transfer matrix method taking into account gain saturation
and predict laser bistability in the PT-symmetric regime in contrast to a symmetry-broken single-mode operation.
In the PT-broken regime, the generation power counterintuitively increases with an increase of the cavity
losses.
© 2018 Chinese Laser Press
OCIS codes: (060.3510) Lasers, fiber; (190.1450) Bistability; (140.3560) Lasers, ring.
https://doi.org/10.1364/PRJ.6.000A18
1. INTRODUCTION
The concept of parity-time (PT) symmetry in optics has at-
tracted a great deal of attention following the first experimental
demonstrations only a few years ago [1,2]. It suggests new pros-
pects for the design of optical structures with symmetrically
distributed regions of gain and losses; see review papers [3–5]
and references therein.
Recently, PT-symmetric lasers were demonstrated experi-
mentally in microrings [6–9]. Importantly, a single-mode laser
operation was achieved in an initially multi-mode system
through stronger mode discrimination, when the system oper-
ates in the PT-broken regime close to a PT-symmetry-breaking
transition. Also, a single transverse-mode operation in a system
of coupled microring lasers was demonstrated near the excep-
tional point [10], and enhanced sensitivity [11,12] was realized.
In a pair of coupled microdisk quantum cascade lasers, the
reversal of generated power dependence [13] was identified in
the vicinity of exceptional points (EPs), where spontaneous
emission is enhanced [14], and EPs were observed direc tly
in photonic-crystal lasers [15]. Furthermore, a realization of
PT-symmetry-based mode locking [16] and a PT-symmetric
laser absorber [17] were theoret ically proposed, and lasing and
anti-lasing in a single cavity were demonstrated [18]. Recently,
a statistical PT-symmetric lasing has been achieved in an optical
fiber network incorporating semiconductor amplifiers [19].
In this work, we develop the theoretical concept of a
PT-symmetric fiber laser based on coupled cavities. We use a
conventional ring cavity laser design and predict PT-symmetric
and PT-broken lasing by means of tunable coupling of a ring
cavity laser with a passive cavity.
2. METHODS AND RESULTS
We present a diagram of the proposed PT-symmetric fiber ring
laser in Fig. 1. The scheme comprises two similar fiber cavities:
one of them is active (with gain), and the other cavity is passive
(with losses). Tunable coupling between cavities is controlled
by means of adjustable phase shifts inside a Mach–Zehnder
interferometer. In the case of zero phase shifts, passive and ac-
tive cavities are uncoupled, so the system acts as a conventional
ring cavity laser if properly pumped. As we demonstrate below,
tunable coupling enables one to switch between PT-symmetric
and PT-broken lasing regimes.
To describe the laser operation and associated PT transi-
tions, we use a discrete transfer matrix model governing the
evolution of the field amplitudes in both fiber cavities. Field
evolution over a cavity round-trip in the first approximation
is described by a transfer operator L,
L
e
g
1
∕2
0
0 e
g
2
∕2
01
10
1∕
ffiffiffi
2
p
i∕
ffiffiffi
2
p
i∕
ffiffiffi
2
p
1∕
ffiffiffi
2
p
×
e
iφ
1
0
0 e
iφ
2
1∕
ffiffiffi
2
p
i∕
ffiffiffi
2
p
i∕
ffiffiffi
2
p
1∕
ffiffiffi
2
p
e
g
1
∕2
0
0 e
g
2
∕2
; (1)
where g
2
< 0 denotes constant losses and g
1
> 0 characterizes
total gain. Importantly, crossover coupling as described by the
second matrix in Eq. (1) facilitates PT symmetry with static
A18
Vol. 6, No. 4 / April 2018 / Photonics Research
Research Article
2327-9125/18/040A18-05 Journal © 2018 Chinese Laser Press