Optics Communications 425 (2018) 176–179
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Optics Communications
journal homepage: www.elsevier.com/locate/optcom
Double sideband frequency scanning interferometry for distance
measurement in the outdoor environment
Keshu Zhang
a,b
, Tian Lv
a,
*, Di Mo
a,b
, Ning Wang
a,b
, Ran Wang
a
, Yirong Wu
a,b
a
Institute of Electronics, Chinese Academy of Sciences, Beijing, 100190, China
b
University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
Keywords:
Absolute distance measurement
Frequency scanning interferometry
All-phase FFT
Double sideband modulation
A B S T R A C T
Frequency scanning interferometry (FSI) can achieve high-precision absolute distance measurement. Here, a new
double sideband frequency scanning interferometry (DSB-FSI) system is proposed, which enables long-range,
high-precision measurements. Furthermore, a new fringe counting method based on all-phase FFT (apFFT) is
used, which can accurately estimate the number of fringes under the condition of blinking intensity of the echo
and can effectively suppress the phase estimation error caused by spectrum leakage. In outdoor experiments, the
target is placed at about 880 m from the system. The standard deviation of distance measurement is 16.59 μm at
the condition of stationary and 17.45 μm at the condition of vibration neglecting the influence of air refractive
index Ultimately, it has been verified that DSB-FSI enables absolute high-precision measurement of long-range
in outdoors.
1. Introduction
FSI system can achieve high-precision absolute measurement by
counting the number of interference fringes in one frequency scanning
period [1–6]. For the original FSI system, frequency modulation is
achieved by tuning the laser [7–11]. In the case of a stable target and a
close distance (less than 100 m), the frequency of intensity of interfer-
ence signal is proportional to the distance. However, when the target
moves, the echo contains a distance-independent Doppler frequency
which destroys the above proportional relationship and introduces an
error in the measurement [12]. The error can be expressed as 𝛥𝜀 =
𝑓
𝐿
∕𝐵
⋅ 𝛥𝐿, where 𝑓
𝐿
is the center frequency of laser, 𝐵 is the sweep
bandwidth, and 𝛥𝐿 is the variation of the optical path in the frequency
scanning period. In general, the bandwidth is much smaller than the
laser frequency. Therefore, even small 𝛥𝐿 introduces a large error in the
measurement result. Using dual tunable lasers for positive and negative
sweeps can solve this problem [3,13–15]. However, adding a tunable
laser not only increases the system complexity, but more importantly,
due to the difference of the tuning parameters of the two tunable lasers,
the ranging result of the positive and negative sweeps cannot completely
eliminate the deviation of the target displacement. The positive and
negative frequency scanning signal can also be obtained by double
sideband modulation. The instantaneous frequency difference between
the laser center frequency and the frequency of the positive and negative
sweep signals is the same at any moment. The two signals have good
*
Corresponding author.
E-mail address: tlv@mail.ie.ac.cn (T. Lv).
consistency. It has been proved by existing work that the DSB-FSI can
achieve high-precision absolute measurement under indoor conditions
[16]. However, in the long-distance outdoor environment, the instability
of the intensity of the echo caused by the atmospheric turbulence
brings difficulty to counting the interference fringes. Here, the system of
DSB-FSI is improved, and a new interference fringes counting method
which is based on apFFT is adopted. Compared with FFT, apFFT can
obtain the absolute value of the initial phase of the signal in the case
of asynchronous sampling [17–20]. In outdoor experiments, the new
system achieves high-precision absolute distance measurement under
the conditions of the atmospheric turbulence and unsteady targets.
2. Principle and algorithm
The system diagram of DSB-FSI shown in Fig. 1. In our system,
single master oscillator is used for transmitter and LO laser and the
coherent measuring time (about 500 μs) that is needed is much longer
that the delay (about 5.88 μs) between the transmitter and the LO, which
is determined by the range delay. According to Ref. [21], the phase
noise is related to delay and independent of the coherent measuring
time in this condition. Therefore, the linewidth which represents the
short-term laser frequency stability is considered. The single master
oscillator of the system is NKT E15 of which linewidth is less than
100 Hz, which is enough to ensure the coherence between echo and
https://doi.org/10.1016/j.optcom.2018.04.056
Received 1 February 2018; Received in revised form 18 April 2018; Accepted 23 April 2018
0030-4018/© 2018 Elsevier B.V. All rights reserved.