February 10, 2011 / Vol. 9, No. 2 / CHINESE OPTICS LETTERS 021405-1
Narrowband switchable dual-passband atomic filter with
four-wave mixing optical amplification
Zheng Tan ( )
1,2,3
, Xianping Sun (zzz²²²)
1,2∗
, Jun Luo (ÛÛÛ )
1,2
,
Yong Cheng (§§§ UUU)
1,2,3
, Jin Wang ( >>>)
1,2
, and Mingsheng Zhan (ÉÉɲ²²))))
1,2∗∗
1
State Key Lab oratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and
Mathematics, Chinese Academy of Sciences, Wuhan National Laboratory for Opto electronics, Wuhan 430071, China
2
Center for Cold Atom Physics, Chinese Academy of Sciences, Wuhan 430071, China
3
Graduate University of the Chinese Academy of Sciences, Beijing 100049, China
∗
Corresp onding author: xpsun@wipm.ac.cn;
∗∗
corresp onding author: mszhan@wipm.ac.cn
Received August 5, 2010; accepted September 28, 2010; posted online January 28, 2011
By using Faraday optical filter combined with four-wave mixing (FWM) amplifier, a narrow bandwidth
optical amplifying atomic filter with switchable dual-passband is demonstrated experimentally. The two
transmission peaks of the filter correspond to the Stokes and anti-Stokes frequencies, exhibiting a Raman
gain in 13- and 17-fold, respectively, with bandwidth of ∼120 MHz. By properly setting pump laser
detuning, switching between filter passbands is realized. We also investigate the dependence of peak
transmission on both pump laser intensity and Rb cell temperature. This atomic filter can find practical
applications in long-distance laser communications and laser remote-sensing systems.
OCIS co des: 140.0140, 190.0190, 300.0300.
doi: 10.3788/COL201109.021405.
Narrow bandwidth optical filters with high noise re-
jection have been used in many optical applications,
such as laser communications
[1]
, lidar systems
[2]
, and
free space quantum key distribution (QKD)
[3]
. Several
approaches have been utilized to obtain narrowband op-
tical filters, such as stimulated Brillouin scattering in
optical fibers
[4,5]
, optical cavities, and atomic optical
filters
[6,7]
. It has long been recognized that atomic
optical filters can be classified as effective narrow-
band filters, given their characteristics of high trans-
mission, ultra-high background rejection, and wide
field of view. Atomic filters can be produced utiliz-
ing polarization rotation caused by anisotropy of the
medium, which can be induced either by external mag-
netic field (magneto-optical effect) or by polarization-
selective optical pumping
[8]
. However, the low-peak
transmission of existing laser-pumped atomic filters
(typical transmission efficiency of 10.5% in Ref. [8])
has restricted practical applications. This gap has moti-
vated us to design an atomic filter that can be used at
weak signal levels or in degenerative environments. Shan
et al. proposed and realized an ultra-narrow bandwidth
Raman-amplified atomic filter, by which a single pass-
band can experience optical gain while maintaining high
background rejection
[9]
.
In this letter, the realization of a dual-passband atomic
dispersion optical filter with optical amplification using a
four-wave mixing (FWM) amplifier in hot Rubidium va-
por has been presented. The two passbands of the filter
correspond to the Stokes and anti-Stokes components.
We demonstrate that switching b etween filter passbands
can be realized by properly setting pump detuning. The
dependence of peak transmission on pump intensity and
temperature is also explored.
The applications of FWM in a double-Λ system as a
phase-insensitive amplifier have been described in Refs.
[10,11]. To understand the mechanism of the optical
amplification process, a three-level Λ system in
85
Rb is
considered (Fig. 1). A signal laser of frequency ω
s
in-
teractes with atoms in state |1i(5S
1/2
, F=2), and pump
laser with frequency ω
p
couples the hyperfine ground
state |2i(5S
1/2
, F=3) and the Doppler-broadened un-
resolved excited state |3i(5P
3/2
). The pump laser cre-
ates a population difference between states |1i and |2i.
Meanwhile, a strong coherence between states |1i and
|2i is introduced through the pump laser by interact-
ing with the |2i → |3i transition. In combining the two
processes, signal light demonstrates Raman gain after
satisfying the two-photon resonant (δ = 0) with pump
laser frequency
[12]
. In the case of pump laser blue detun-
ing from |2i(5S
1/2
, F=3)→|3i(5P
3/2
) resonance (Fig. 1),
the frequency of the signal laser is matched to the anti-
Stokes component. For the pump laser operating at the
|1i → |3i transition, another Raman sideband is gener-
ated through the FWM process, and the signal light
and conjugate component are jointly amplified. It has
been proven that FWM in such systems could generate
squeezing; this makes them useful in quantum informa-
tion protocols when used as low-noise amplifiers
[13]
. In
the present work, we superimpose a FWM amplifier and
a Faraday anomalous dispersive optical filter (FADOF)
to produce a narrowband dual-passband optical ampli-
fying atomic filter with high noise rejection ratio.
The experimental setup is shown in Fig. 2. Rb cell
1 (length: 100 mm; diameter: 25 mm) was used as
FWM amplifier. The signal light obtained an optical
gain. Rb cell 2 (length: 40 mm; diameter: 25 mm), two
crossed Glan-Thompson prisms, and a constant mag-
netic field along the cell constituted the FADOF. Rb cell
3 (length: 100 mm; diameter: 25 mm) was utilized to
monitor the absorption sp ectra of Rb atoms. An exter-
nal cavity diodes laser (Toptica DL100) at wavelength
of 780 nm was used as signal laser. The intense pump
laser was generated by an actively stabilized Ti:sapphire
1671-7694/2011/021405(4)
c
° 2011 Chinese Optics Letters