Physics Letters A 380 (2016) 4022–4026
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Physics Letters A
www.elsevier.com/locate/pla
Ultra-narrow linewidth optical filter based on Faraday effect at isotope
87
Rb 420 nm transitions
Gang Bi
a,b,∗
, Jia Kang
b
, Jun Fu
a
, Li Ling
a,∗
, Jingbiao Chen
c
a
School of Information & Electrical Engineering, Zhejiang University City College, Hangzhou, 310015, China
b
College of Information Science and Electronics Engineering, Zhejiang University, Hangzhou, 310013, China
c
State Key Laboratory of Advanced Optical Communication System and Network, Institute of Quantum Electronics, School of Electronics Engineering and
Computer Science, Peking University, Beijing, 1000871, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received
24 May 2016
Received
in revised form 9 October 2016
Accepted
12 October 2016
Available
online 14 October 2016
Communicated
by V.A. Markel
Keywords:
Lasers
Filters
Diode
lasers
Faraday
effect
An ultra-narrow linewidth optical filter with isotope
87
Rb vapor at 420 nm, within the best waveband
400–500 nm for deep sea communication is achieved for the first time. The Faraday effect, circular
dichroism, and nonlinear saturation techniques are utilized to narrow the bandwidth from previous
2.5 GHz to about 15 MHz level on the energy transition 5S
1/2
→ 6P
3/2
. By changing the temperature
and magnetic field, the maximum transmission is obtained when the temperature and the magnetic field
of the
87
Rb cell are at 100
◦
C and 12 G. We discuss the varying influences of temperature, magnetic
field, and pump power on the transmission of the atomic filter. The maximum single peak transmission
at 5S
1/2
, F = 2 → 6P
3/2
, F
= 3 transition is 2.1% with a bandwidth of 17.8 MHz, and 1.9% at the 5S
1/2
,
F = 2 → 6P
3/2
, F
= 2, 3 (cross-over) transition with that of 14.2 MHz. The calculated equivalent noise
bandwidth of this system is 32.5 MHz. Compared with the conventional Faraday anomalous dispersion
optical filter, the bandwidth of our system is narrowed at least two orders of magnitude and is closer
to the natural linewidth. This ultra-narrow linewidth filter has the potential to be applied to submarine
communication or the pump laser in a four-level Rb-based active optical clock.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Optical filters based on atomic energy transitions are attrac-
tive
for fundamental researches and practical applications where
the bandwidth is a critical parameter [1,2]. Ultra-narrow linewidth
optical filters with high sensitivity, high spectral selectivity, and
frequency stability show promise for various optical system-based
applications, such as optical communications in free space [3],
laser radar remote sensing [4], the generation of narrowband quan-
tum
light [5–7], and the pump laser in four-level Rb-based active
optical clocks [8,9]. Compared with the conventional interference
filters, Faraday anomalous dispersion optical filters (FADOFs) have
the merits of narrow bandwidth, background rejection, mechanical
robustness, imaging capability, and high transmission. Therefore,
FADOFs have been developed for several elements, such as Cs [10,
11],
Rb [12,13], K [14,15], Na [16], and Ca [17].
Two
important indexes are used for evaluating the performance
of semiconductor lasers: linewidth and frequency stabilization. In
order to narrow the linewidth, semiconductor lasers are locked on
*
Corresponding authors. Fax: +86 571 88011938.
E-mail
addresses: bigang@zju.edu.cn (G. Bi), lingl@zucc.edu.cn (L. Ling).
atomic or molecular spectral lines using electric or optical feedback
to achieve stable output frequency. It is necessary to provide a ref-
erence
for frequency stabilization with narrow spectral linewidth.
Currently, the free running linewidth of commercial semiconductor
lasers is 15–100 MHz, which meets the practical requirements in
some common scientific applications. However, for laser cooling,
atomic trapping, quantum frequency standards, and other basic
researches, much narrower-linewidth laser sources are required.
Compared with general spectroscopy, laser spectroscopy technol-
ogy
can improve the resolution of the spectrum as a result of
eliminating Doppler broadening.
The
means to eliminate Doppler broadening is through a sat-
urated
absorption spectroscopy, polarization spectroscopy or two-
photon
spectroscopy [18–20]. In recent years, theoretical and ex-
perimental
studies on ultra-narrow linewidth atomic filters have
developed rapidly. Turner et al. [21] described an atomic optical
filter as a single 170 MHz passband at a peak transmission of 9.5%
that used a narrow-linewidth pump laser to induce circular bire-
fringence
in a dense potassium vapor. Cerè et al. [22] achieved
a tunable narrowband filter with a linewidth of 80 MHz, a peak
transmission of 14.6% based on optical-pumping-induced circular
dichroism in rubidium vapor. Liu et al. [23] reported an ultra-
narrow
bandwidth atomic filter with a linewidth of 61 MHz and
http://dx.doi.org/10.1016/j.physleta.2016.10.016
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© 2016 Elsevier B.V. All rights reserved.