Light pulse detecting system with highly reduced false alarm rate, usable for laser detection

The system enables the lowering of the false alarm rate by eliminating, in particular, the spurious pulses created by the detectors themselves. To this effect, it has one or more juxtaposed optical channels to cover the total field. Each optical channel is associated with a pair of detecting elements to form two detecting channels. After amplification and threshold comparison in these channels, the correlation of the two channels is produced to eliminate the spurious signals and select only the useful signal which is simultaneously present on both channels.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a system for the detection of light pulses, 
especially pulses emitted by a laser, with a notably low false alarm rate. 
By light pulses we mean pulses with wavelengths in both the visible 
spectrum and the invisible spectrum, especially in the near infrared and 
far infrared range, for example, for laser radiation at 1.06 microns and 
10.6 microns. 
The false alarm rate is reduced by making the detecting system insensitive 
to spurious signals and, especially, to the electrical pulses created by 
the photodetectors themselves when there is no source of irradiating 
optical energy. 
This result is got, according to the invention, by an arrangement of the 
optronic detection channels, through the parallel mounting, for each 
channel, of two photodetectors coupled to one and the same optical 
channel. Then, by correlation, a specific analog (or digital) processing 
of the detected pulses enables the detection and validation of the light 
pulses received and the rejection of the internal spurious pulses 
regardless of their width, amplitude and repetition characteristic. 
According to an application more particularly envisaged, the invention 
concerns the detection of laser pulses and enables the making of equipment 
which can perform the function of a laser warning detector. 
2. Description of the Prior Art 
The frequent use of laser illuminators in weapon systems for telemetry or 
target designation has made it necessary to envisage efficient protection 
of the carriers (armored vehicles, ships or aircraft). This protection 
consists firstly in alerting the crew to the presence of a laser threat so 
as to bring about an immediate reaction: this is the basic function of the 
laser warning detector. 
Current operational detectors emit pulses of a width equal to a few 
nanoseconds. A laser warning detector generally consists of several 
optronic sensors. Each sensor is provided with an optical part coupled to 
a detector giving a current which is proportionate, at all instants, to 
the flux that it receives. Electronic processing enables the detection of 
the received flux as soon as the detected signal is above a pre-determined 
threshold. In terms of current, this amounts to a value of the detected 
current greater than a minimum value IS1 resulting from the comparison 
threshold value used for operation. 
Each optronic channel is characterized by its field which is that part of 
space observed by the sensor and from where the light emissions are 
collected, by its sensitivity which is the minimum illumination value that 
can be detected at the input of the sensor, and by its false alarm rate 
which is the number of false detections per hour of operation when there 
is no light energy received at all. 
The field and sensitivity are thus defined by the characteristics of the 
illuminators, the design constraints on the equipment and the carriers to 
be protected. These various characteristics lead to specifying a current 
threshold value IS1 which should not be exceeded. 
The false alarm rate characterizes the reliability of the information given 
by the warning detector: the required value depends on the application 
(the task and the carrier) and may vary between 1 and 1/1000. The value 
1/1000 corresponds to one false alarm per 1000 hours of operation for all 
the optronic channels. False alarm rate (abbreviated as FAR) requirements 
often lay down a detection threshold IS2 which is greater than the 
above-mentioned limit value IS1 and is therefore incompatible with the 
field and sensitivity specifications of the equipment. 
The present invention provides, through a simple arrangement of the 
optronic channel, for the possibility of reducing the relationship between 
threshold IS and the false alarm rate FAR (curve C1 of FIG. 1 
corresponding to a system not arranged according to the invention) to a 
magnitude compatible with the limit value IS1 (the curve C2 of FIG. 2 
corresponding to a system arranged according to the invention). 
False alarms can be produced by electro-magnetic sources external to the 
equipment (for example radio transmissions, radar or electrical arcs) or 
internal to the equipment (such as spurious phenomena produced by certain 
parts of the equipment). False alarms can also result from random noise 
sources due to the detector and to electronic processing. Thirdly, false 
alarms can result from signals created spontaneously in the detecting 
elements. These signals take the form of very brief pulses. They are 
filtered by the frequency response characteristic of the detector and 
appear, with respect to electronic processing, in the form of waves close 
to the pulses produced by the laser illuminators. 
The effect of the electro-magnetic sources can be reduced through 
appropriate design (such as shielding, filtering, and the elimination of 
inconvenient couplings). 
The random noise sources generally remain compatible with the maximum 
detection threshold IS1. 
On the contrary, the spontaneous generation of pulses in the detector plays 
a preponderant role in subsequently determining the false alarm rate. 
An object of the present invention is the application of a processing 
method which distinguishes pulse signals of light origin or useful signals 
from the spurious signals and especially from those created by the 
detectors themselves. 
The discriminating of these spurious pulse signals by waveform recognition 
cannot be contemplated since the waveforms are close to those of the 
useful signals. The time interval differences to be observed, which are of 
the order of a few nanoseconds, imply the use of very complicated 
processing. Finally, although the spectrum of the laser pulses to be 
considered always appears to be limited by the detector, this experimental 
observation cannot be made into an absolute generalization because of the 
various types of materials and technologies on which the detectors are 
based. 
An aim of the invention is to remove all these drawbacks by arranging the 
octronic channels of the equipment in a special way. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a system for the detection of 
light pulses comprising successively: optical means for the reception of 
laser radiation in a defined total field, said total field being covered 
by at least one receiving optical channel and being formed, when there are 
several optical channels, by juxtaposed elementary fields; means for the 
photodetection of the laser radiation received in the total field by the 
optical means; and means for processing the detected signals, said 
processing means making a comparison with a first selection of signals 
above a given threshold; wherein said photodetection means comprise, for 
each optical channel, a pair of detecting elements to form two detecting 
channels, and wherein the means for processing the signals detected by the 
photodetector elements can be used to lower the false alarm rate by 
subsequently selecting only those signals which are present simultaneously 
in both detecting channels of each pair considered.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In FIG. 1, the curve C1, pertaining to a system without the arrangement 
according to the invention, shows a minimum false alarm rate TF1 for the 
threshold value IS1 limited for given field and sensitivity 
characteristics. 
With the arrangements according to the invention, the curve 2 shows that, 
for one and the same threshold value, the false alarm rate goes from the 
value TF1 to the value TF2 and thus shows a major reduction. For example, 
for a threshold IS1 of 1.5 microamperes, the FAR goes from 10to about 
0.002 (1 per 500 H). The value TF2 would be obtained in the previous case 
(that of curve C1) for the threshold value IS2 considerably higher than 
IS1 and incompatible with the field and sensitivity characteristics 
considered. 
The basic version is shown in FIG. 2 which shows a system with a single 
optical channel 1. This optical channel is made with a common optical 
element or with two similar optical elements to cover the same field 
.theta.. With this reception optical channel 1, there is associated a 
detecting optical device 2 consisting of two detecting elements, a first 
detecting element D1 and a second detecting element D2, to form two 
detecting channels with the downline processing circuits 3. These 
processing circuits have detection circuits 31 for the channel 1 and 32 
for the channel 2 to select detected signals which are above a given 
threshold SD, given by a threshold generator 4. 
The signals S3 and S4, detected by the two channels, are correlated in a 
correlating circuit 33, the output S5 of which gives the useful detection 
signal. 
As can be realized from the curves of FIG. 3, a useful signal appears at 
the correlation output S5 only if this signal is already present at the 
output S3 and S4 of the detection channels, thus making it possible to 
eliminate the spurious pulses created by the detectors themselves. For, as 
seen earlier, these spurious pulses IP have a form similar to that of the 
useful pulse IU but, on the contrary, the probability of their 
simultaneous appearance on both channels is almost nil. Consequently, they 
are eliminated by the final correlation function at 33. 
Thus, the fact of adding a second detector D2 and a second detection 
channel 32 and of correlating the two detecting channels 31 and 32 enables 
the elimination of the inconvenient spurious pulses and considerably 
decreases the false alarm rate of the equipment. 
The processing done makes it possible to detect expected laser or other 
radiation with a single channel D1-31. For, as soon as the photodetected 
current is higher than the threshold value on both channels, these 
channels, which are identical, perform simultaneous detections. The 
processing establishes a temporal correlation of the two pulses IU and 
then delivers a true detection signal S5. The processing can be used to 
eliminate any spurious pulse created in a detector. The appearance of 
these spurious signals is related to microelectronic phenomena proper to 
each detector. The moments at which they appear are therefore completely 
random and the probability of a simultaneous appearance of a signal of 
this type on both channels is practically nil. Finally, the proposed 
solution makes it possible to do away with the most inconvenient false 
alarm source and, hence, to obtain a reduced characteristic C2 as 
indicated in FIG. 1. 
As shown, the detection channels have amplification circuits, 31A and 32A, 
and a threshold comparison circuit 31C and 32C. The amplification circuits 
may consist of a pre-amplifier in series with an amplifier. The pass-band 
of these circuits may go up to 40 MHz, for example, to detect laser pulses 
of a few nanoseconds. The comparators 31C and 32C are made with fast 
integrated technology and give a standardized output (waveform S3 and S4 
of FIG. 3). Consequently, the correlation circuit 33 may simply consist of 
an "AND" logic gate 33. The detectors D1 and D2 are silicon detectors, for 
example. The detection system thus equipped can produce the correlation 
for peak current values greater than or equal to about 300 nanoamperes at 
each detector. The FAR obtained is easily smaller than 0.002. The 
pass-band chosen for the amplifiers is equal to the spectral band of the 
narrowest laser pulses to be received (of about 15 nanoseconds for 
example) and to the pass-band proper to the detector. The system thus 
shows maximum sensitivity and maximum efficiency of processing by 
correlation. The spurious pulses then have a minimum width, thus making it 
possible to reduce the real FAR. The invention can be applied especially 
to all laser warning applications comprising one or more octronic 
channels, said channels being capable of use for panoramic detection and, 
if necessary, for the location of incident laser pulses. 
FIG. 4 recalls the configuration with several optical channels to cover the 
total field 0 in the form of successive elementary fields .theta..sub.1, 
.theta..sub.2, . . . .theta..sub.J . . . .theta..sub.N. Generally, the 
optical channels are identical and the elementary fields are equal, the 
optical axes being shifted by an increment 0/N from one axis to the next. 
This configuration is used, for example, to provide azimuth panoramic or 
sectoral cover in a laser warning detector device. 
According to the invention, each of the channels has two detectors D1J and 
D2J for the channel J for example. The detectors are connected to the 
electronic processing set 3 which can be made in several forms, of which 
three possible embodiments shall be described below. 
According to the first embodiment shown in FIG. 5, the basic cell is in the 
processing circuit 30A with, in addition, two summation circuits 34 and 
35: one receives the outputs of N detectors D1.1 to D1.N corresponding to 
the first detecting channel and the other summation circuit 35 receives 
the outputs of the second element of each pair of detectors, namely, the 
optical channel detectors D2.1 to D2.N. The output S5 really corresponds 
to the validated useful signal but does not give the channel information, 
namely the direction of the transmitter to within .theta./N, in this 
assembly. Consequently, the processing circuits are complemented by N 
detecting channels each coming from a pair of parallel-connected 
detectors. These N channels have the amplifiers 41.1 to 41.N. These 
amplifiers are followed by peak memory circuits 42.1 to 42.N controlled by 
the output of the validated useful signal S5, namely, the passage of this 
output to 1. Their content is transferred to a channel locating circuit 43 
which selects the optronic channel in which the signal with the highest 
level is detected. The circuit 43 can be made in several known ways. 
According to the second embodiment shown in FIG. 6, the system has N 
optical channels and 2N detection channels. Each detection channel has the 
amplifiers 31A and 32A downline of the detectors. These amplifiers are 
followed by a peak memory circuit 42.1 to 42.2N. An electronic assembly 
30B comprises a summation circuit 36, powered by the 2N outputs of the 
detectors and series-connected with an amplifier 37A, followed by a 
threshold comparator 38C. The signal S10 given by this assembly is applied 
firstly, to the peak memory circuit 42 and, secondly, to a unit 30C for 
processing by correlation according to the invention. In this processing 
unit, the signals S3.J and S4.J of each pair of detectors are correlated 
and give true detection when the signal is the useful signal present 
simultaneously at each detecting channel of the order J optical channel 
considered. 
The processing circuit 30C may consist of a battery of correlators after 
the shaping of the signal or, preferably, as indicated, with a 
digital/analog converter circuit 50 followed by a processor 51 which 
performs the false alarm processing operations by correlation in comparing 
the signals S3.J and S4.J with each other for the N channels. It must be 
noted that the processor circuit can perform the locating processing 
operation at the same time since it has the information on the amplitude 
of the signal after the digital conversion at 50, and since it also has 
the information on the origin of the signal, namely, on the channel from 
which it comes. Consequently, in this embodiment, the output S5 validates 
the useful signal and may also comprise the information on the channel, 
namely the direction of the elementary .theta.J in which the detected 
signal is received. 
According to a third embodiment of FIG. 7, an output signal S5 is also 
obtained here and comprises both the selection of the useful signal with 
the elimination of the spurious pulses and the information on the optical 
channel from which this signal comes. According to this embodiment, the 
number of amplifiers 39.1 to 39.N and the number of peak memory circuits 
42.1 to 42.N is equal to N and is therefore divided by 2, thus increasing 
the compactness of the equipment. The N detecting channels originate, as 
shown, from a pair of detecting elements, but one of them relates to an 
optical channel 1.J and the other relates to the following optical channel 
1.(J+1). This is got with the summation circuits 45.1 and 45.N. The 
detected outputs S34.1 to S34.N are applied, as previously, to a circuit 
30C for processing by correlation. 
The embodiment of FIG. 6, compared with that of FIG. 5, has the advantage 
of not being critical for the coupling between channels. On the contrary, 
it is more complicated if the number of channels N is great. For the last 
two embodiments according to FIGS. 6 and 7, it may be worthwhile to use 2 
detectors integrated in the same package, for example, a two-cell package, 
for each channel. 
The last embodiment shown in FIG. 7 which is simpler, is more compact and 
is the preferred version.