COL 10(5), 052601(2012) CHINESE OPTICS LETTERS May 10, 2012
Polarization modulation in single-frequency He-Ne laser
with an anisotropy feedback cavity
Wenxue Chen (©©©ÆÆÆ)
1,2
, Shulian Zhang (ÜÜÜÖÖÖööö)
2∗
, and Xingwu Long (999,,,ÉÉÉ)
1
1
Department of Opto-Electronic Engineering, College of Opto-Electric Science and Engineering,
National University of Defense Technology, Changsha 410073, China
2
The State Key Lab of Precision Measurement Technology and Instrument, Department of Precision Instruments,
Tsinghua University, Beijing 100084, China
∗
Corresp onding author: zsl-dpi@mail.tsinghua.edu.cn
Received August 30, 2011; accepted October 25, 2011; posted online January 17, 2012
The polarization state is modulated by tilting birefringence component placed in the feedback external
cavity. The variation of the polarization state in one period of modulation is found to be similar to sine
wave. The periods become increasingly smaller. The maximum of variation in one period decreases against
the rotated angle. The experimental phenomenon is subjected to the change of optical path and secondary
reflection. The phenomenon is analyzed theoretically based on geometrical optics and crystal optics.
High-accuracy measurements of absolute and relative angles can be realized based on the experimental
phenomenon. The angle resolution is 0.1 arcsec in theory.
OCIS codes: 260.1440, 260.3160.
doi: 10.3788/COL201210.052601.
External optical feedback can dramatically affect the
output intensity, coherence, and stability of laser
[1,2]
. Po-
larization control in laser by external optical feedback has
attracted considerable interest
[3−6]
. Fei et al. found that
the duty ratio of two eigenstates in one period of laser
intensity modulation varied with different phases
[7]
.
In this letter, modulation in polarization is realized by
tilting birefringence element in He-Ne laser external cav-
ity. New phenomena are found in our experiments. When
the tilted angle of birefringence element is changed, duty
ratio of polarization state varies and the maximum varia-
tions are modulated. We analyze this experimental phe-
nomenon theoretically based on geometrical optics and
crystal optics. The results of theory analysis are consis-
tent with experimental results. The experimental phe-
nomenon can be used to measure the absolute angle and
relative angle of rotating object.
Experiments are carried out on single-mode, linearly
polarized He-Ne laser. The experimental setup is shown
in Fig. 1. The wavelength is 632.8 nm. The ratio of
gaseous pressure in laser is He:Ne= 9:1 and Ne
20
:Ne
22
=
1:1.
Here, D
1
and D
2
are photodetectors used to detect
the laser intensity and the variations of laser polarization
state, respectively; M
1
and M
2
are laser mirrors with re-
flectivities of 99.8% and 98.8%, respectively; M
E
is an
external mirror with reflectivity of 15% used to reflect
laser beams back into the laser; P is a polarizer used to
separate different polarization state from laser; PZT is
used to push and pull M
E
. The length of laser is 150
mm. Together with M
2
and S, M
E
forms a birefringence
external cavity 100 mm in length.
The length of external cavity is scanned by PZT, and
the curves of intensity modulation are shown in Fig. 2.
The tilted angle of birefringence element is 0.05
◦
in Fig.
2. The birefringence element is a quartz waveplate, and
the optical axis is perpendicular to the incident surface
of birefringence element.
From Fig. 2, some phenomena that differ from the
conventional laser feedback can be found. Firstly, dips
on the laser intensity curve appear, and the laser inten-
sity curve is similar to sine curves in conventional laser
feedback. Secondly, the polarization of laser hops at the
dip point B, and the intensities of two eigenstates are
both modulated by the length of external cavity
[7]
. In
the experiments, the duty ratios of two eigenstates in one
period of intensity modulations are modulated by tilting
birefringence element, in short, polarization is modulated
in He-Ne laser by tilting birefringence element. The duty
ratio D can be expressed as
D =
l
BC
l
AC
, (1)
Fig. 1. Experimental setup. W: glass windows, antireflective-
coated; S: birefringence elements; PZT: piezoelectric trans-
ducer.
Fig. 2. Waveforms of laser intensity modulation and polariza-
tion flipping.
1671-7694/2012/052601(4) 052601-1
c
° 2012 Chinese Optics Letters