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Combination of differential discrimination and direct
discrimination in pulsed laser time-of-flight systems
Dongxian Geng (耿东现), Xing Fu (付 星), Pengfei Du (杜鹏飞), Wei Wang (王 巍),
and Mali Gong (巩马理)*
State Key Laboratory of Precision Measurement Technology and Instruments, Center for Photonics and Electronics,
Department of Precision Instruments, Tsinghua University, Beijing 100084, China
*Corresponding author: gongml@mail.tsinghua.edu.cn
Received January 11, 2016; accepted April 15, 2016; posted online May 16, 2016
The output amplitude of the differential circuit is studied for differential discrimination in pulsed laser time-of-
flight systems. Based on the studies of the probability of detection and the probability of false alarms, the mini-
mum detectable input signal of differential discrimination can be calculated. The results indicate that the differ-
ential discrimination detectability of the small signal will be reduced. A combined discrimination is proposed in
this Letter to improve the time resolution of the large signal and ensure the probability of detection of the small
signal at the same time. A proper value of the circuit parameter is found to balance the time resolutions of the
small and large signals.
OCIS codes: 280.3400, 150.5670, 280.3640, 150.6910.
doi: 10.3788/COL201614.062801.
Pulsed laser time-of-flight (TOF) measurement has been
studied since the 1960s
[1,2]
. Compared with the amplitude
continuous wave (AMCW) TOF system, the pulsed laser
TOF system has a wider measurement range and a better
capacity of resisting disturbance, such as atmospheric tur-
bulence
[3]
. Moreover, since the pulsed laser TOF system is
much simpler and more reliable, it is widely used in laser
rangefinder, LIDAR, and three-dimensional sensing
[4–6]
.In
order to deal with the multiple returns, which are always
caused by some semitransparent object or two objects in
tandem
[7–10]
, higher time resolution is needed in these sys-
tems because it is more difficult to distinguish two pulses if
the two echo laser pulses are closer. For the classical direct
discriminator with a fixed threshold that is used in most
pulsed laser TOF syste ms, the time resolution decreases
rapidly with the increase in the amplitude of the input sig-
nal. With the development of the laser, the full width at
half-maximum (FWHM) of a laser has been reduced to
subnanosecond or picosecond
[11]
. However, for these kind
of lasers, high power is difficult to be realized. On the other
hand, the time resolution is also limited by the response
speed of the photodetector. So, when the FWHM of the
laser is given, some solutions have been developed to dis-
tinguish two pulses when two echo laser pulses are close.
Based on the data acquired by a high-speed sampler, some
stochastic simulation algorithms have been used to recog-
nize two overlapped pulses
[12–14]
. This approach can distin-
guish two laser pulses when they are very close, but it is a
time-consuming task and, because the sampling speed
needs to be very high, the system will be more complex.
Another way to improve the time resolution is to use a
differential discrimi nator instead of the classical discrimi-
nator
[15]
. The time resolution of a differential discriminator
decreases little with the increase in the amplitude of the
input signal, but the relationship between the time
resolution and the input amplitude is not monotonic. If
the input amplitude is small, the time resolution will de-
crease rapidly with the decrease in the amplitude of input
signal. Moreover, the previous work indicated that the
amplitude of the output signal will be reduced by the dif-
ferential discriminator. So, the amplitude of the minimum
detectable signal will be influenced when the probability of
detection is given.
The research in this Letter is based on the differential
discrimination technique. The main goal of the research is
finding a way to improve the time resolution as well as
ensure the detection probability of the small signal. First,
the amplitude of the output signal has been simulated.
The relationship between the amplitude of the input sig-
nal (V
inmax
) and the amplitude of the output signal
(V
outmax
) of the differential circuit has been obtained.
Then, electrical circuit experiments have been done and
the simulations have been verified. On the other hand,
the probability of detection and the probability of false
alarms have been studied. Based on these studies and
the input/output amplitude relationship, the amplitude
of the differential discrimination of the minimum detect-
able input signal can be calculated. The minimum detect-
able input signal of the differential discriminator is
calculated to be larger than that of the classical direct dis-
criminator. In order to improve the tim e resolution and at
the same time ensure the detection probability of the small
signal, a combination of the classical direct discriminator
and the differential discriminator has been proposed and,
according to the calculated minimum detectable input sig-
nal, an optimized τ (the time constant of the differential
circuit) has been obtained to balance the time resolutions
of the small large signals. Simulations and experiments
have been done. The time resolution of the large signal
has been improved significantly, and the probability of
COL 14(6), 062801(2016) CHINESE OPTICS LETTERS June 10, 2016
1671-7694/2016/062801(5) 062801-1 © 2016 Chinese Optics Letters