COL 10(3), 032801(2012) CHINESE OPTICS LETTERS March 10, 2012
Displacement sensor based on polarization mixture of
orthogonal polarized He-Ne laser at 1.15 µm
Zhengqi Zhao (
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), Shulian Zhang (
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)
∗
, Peng Zhang (
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), Zhaoli Zeng (
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),
Yidong Tan (
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), and Yan Li (
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State Key Laboratory of Precision Measurement Technology and Instruments, Department of
Precision Instruments and Mechanology, Tsinghua University, Beijing 100084, China
∗
Corresponding author: zsl-dpi@mail.tsinghua.edu.cn
Received July 15, 2011; accepted S eptember 23, 2011; posted online November 18
§
2011
Displacement sensor based on the polarization mixture and t he cavity tu ning of the orthogonal polarized
He-Ne laser 1.15 µm is p resented. The power tuning curves of He-Ne laser are irregular, and it is difficult to
measure the change in cavity length. The distortion of the curves is caused by the higher relative excitation
compared with the He-Ne laser at 633 nm. In view of its potential for the wider displacement measuring
range, a new method of d isplacement sensing is developed. Experiments show that displacement measuring
stability based on the method of the polarization mixture is better than that of the power tuning curves.
The displacement sensor achieves the measuring range of 100 mm, resolution of 144 nm, and linearity of
7×10
−6
.
OCIS codes: 280.3420, 140.1340, 260.5430
doi: 10.3788/COL201210.032801.
The applications of laser technology are widely used
in metrology because of the traceability to the
wavelength
[1,2]
. Many characteristics and applications
of laser, such as spectrum, wavelength, beat frequency,
intensity, laser line, and fringes , are used in different
measurement areas
[3−7]
. In most of these applications,
laser is regarded as a source of light with excellent per-
formance, including having high intensity, stability, and
directionality. Another importa nt factor in displace-
ment measurement is the stability of frequency because
frequency is directly related to result of measurement.
Hence, the frequency stabilization system is necessary
in a high-precision measuring system, which results in a
complex structure.
Intracavity tuning displacement sens ing is the appli-
cation of the intracavity laser method in the field of
displacement measurement. Du et al.
[8]
utilized a dual-
frequency He-Ne laser at 633 nm a s a displacement sen-
sor, with the measuring range being 12 mm. It has the
merits of linearity, self-calibra tion, and traceability to the
wavelength. The simple structure also has stability. The
frequency stabilization system is not necessary and the
cost is much lower. However, the measuring range can
hardly be increased because the meas uring method re-
quires the operation of a single longitudinal mode. Dur-
ing the process of measur ement, the vibration of the cav-
ity mirror may lead to the detuning of the laser cavity,
and the output light disappears when the loss surpasses
the gain. Research shows that He-Ne laser at 1.15 µm
is more promising for the intracavity laser method, be-
cause the active medium gain of the laser is more than
that of He- Ne la ser at 633 nm. Therefore, in theory, the
adoption of He-Ne laser at 1.15 µm can enlarge the mea-
surement range.
The active medium of He- Ne laser is mainly Doppler
broadened. The laser line function g
D
(v, v
0
) is in the
form of a Gaussian shape. When v e quals v
0
, g
D
(v, v
0
)
reaches the maximum
[9]
:
g
Dmax
= λ
m
2πk
b
T
, (1)
where v
0
is the central frequency, λ is the wave le ngth
of the laser, m is the atomic mass, k
b
is the Boltzmann
constant, and T is the temperature. The corresponding
gain fac tor G can be expressed as
G = ∆n
υ
2
A
21
8πc
2
m
2πk
b
T
1/2
λ
3
, (2)
where ∆n is the inverted population density, υ is the
velocity of atoms , and A
21
is the spontaneous emission
rate. Equation (2) shows that G is proportional to λ
3
.
This means that the gain of the laser will be higher for
a lo nger wavelength with the same a ctive medium and
conditions. Higher gain promises a wider displacement
measurement range and stability, thus He-Ne laser a t
1.15 µm is more suitable to be a displacement sensor.
After inserting a birefringence element (such as a
quartz plate, a stress-birefring e nce glass, etc.) into the
laser cavity, one geometric cavity length becomes a cav-
ity with two physical lengths. One laser beam then
splits into two orthogonal linear polarized beams with a
certain frequency difference
[10,11]
. They can be called o-
light and e-light, respectively, according to crystal optics.
The cavity tuning characteristics of the orthogonally po-
larized dual-frequency He-Ne laser a t 1.15 µm have been
discussed in detail
[12]
.
The schematic structure of the experimental setup is
shown in Fig. 1. A half-intracavity cavity laser is com-
posed of a concave output mirror M, a ca t’s eye reflector
(CER), and a He-Ne laser discharge tube T. W refers to
the window plate with an anti-reflection co ating on bo th
surfaces; Q is a quartz plate, which is obliquely placed
to keep a certain angle between the crystal axis and the
laser axis, and the frequency difference is a djusted by the
1671-7694/2012/032801(4) 032801-1
c
2012 Chinese Optics Letters
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