Detector for Q-switched laser radiation

A detector for detecting radiation from a Q-switched laser. The detector prises a plurality of silicon photo-diodes (10) which develop an output signal when struck by incident radiation from the Q-switched laser. The output of the photo-diodes is connected to a pulse-width discriminator (12) and a pulse-amplitude discriminator (13). The pulse-width discriminator generates an output signal only if the amplitude of the output pulse from the photo-diodes is less than 300 nanoseconds, signifying an input pulse from a Q-switched laser. The pulse-amplitude discriminator develops an output signal only when the photo-diode is saturated, again, indicating radiation from a Q-switched laser. The presence of a signal from either the pulse-width discriminator or the pulse-amplitude discriminator indicates that the incident radiation on the photo-diode was indeed from a Q-switched laser.

TECHNICAL FIELD 
Broadly speaking, this invention relates to lasers. More particularly, in a 
preferred embodiment, this invention relates to methods and apparatus for 
detecting radiation from a Q-switched laser. 
BACKGROUND OF THE INVENTION 
Detectors for detecting the presence of laser radiation are commercially 
available and have been widely discussed in the literature. A problem 
arises, however, when the radiation to be detected originates in a high 
power, Q-switched laser, for example, a neodymium laser operating at a 
wavelength of 1.06 .mu.m. 
More specifically, detectors for detecting Q-switched laser radiation 
invariably include some sort of pulse-width discriminator circuit in order 
that any light pulse having a duration which is greater than 300 
nanoseconds, say, will be rejected, since such a pulse cannot possibly 
originate in a Q-switched laser. However, if by chance the detector 
happens to be positioned in the path of the main laser beam, the detector 
will saturate, producing an output pulse having a duration which is too 
long for the pulse-width discriminator to process; thus, the pulse will 
not be detected, resulting in error. 
Various complex and elaborate schemes have been used in the prior art in an 
attempt to overcome this problem. For example, circuits have been built 
which attempt to discriminate against the rise time of the input pulse, or 
which compare the ratio of pulse rise and fall time to pulse width. 
However, in either case, an engineering trade-off is made which sacrifices 
either sensitivity, dynamic range, or the false alarm rate. 
SUMMARY OF THE INVENTION 
The problem, then, is to provide a detector for Q-switched laser radiation 
that is reliable, inexpensive, and which has both a wide dynamic range and 
a virtually zero false alarm rate while at the same time overcoming all 
the deficiencies of the prior art. Fortunately, the invention to be 
described below comprises a solution to these and other problems. 
In a preferred embodiment, the instant invention comprises an apparatus for 
detecting radiation from a Q-switched laser, which comprises at least one 
photo-detector for developing a first output signal in response to 
incident optical radiation falling thereon; means connected to the 
photo-detector for comparing the first output signal with a known 
reference potential thereby to generate a second output signal if the 
first output signal exceeds the reference potential; a pulse width 
discriminator connected to the output of the comparing means for 
generating a third output signal if the second output signal has a 
duration which is less than a predetermined time interval; a pulse 
amplitude discriminator connected to the comparing means for generating a 
fourth output signal if the second output signal exceeds a predetermined 
amplitude; and means, responsive to either the first or the fourth output 
signal, for generating a fifth output signal indicative of optical 
radiation from a Q-switched laser, the fifth output pulse generating means 
generating a pulse having a duration substantially greater than the 
duration of the pulse generated by the comparing means. 
The invention, and its mode of operation, will be more fully understood 
from the following detailed description, when taken with the attendant 
drawings in which:

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, a first illustrative embodiment of the invention 
comprises a photo-detector 10, for example, a silicon photo-diode, 
connected to the input of a voltage-comparator 11 thence, to the inputs of 
a pulse-width discriminator 12 and a pulse-amplitude discriminator 13. The 
outputs of pulse-width discriminator 12 and pulse-amplitude discriminator 
13 are connected to an OR-gate 14 thence, to a pulse-stretcher 16. 
In operation, voltage-comparator 11 generates an output signal any time 
that the output from photo-detector 10 exceeds V.sub.ref, the pre-set 
threshold of comparator 11. Illustratively, V.sub.ref is set at 10 mVolts, 
a relatively low level, but nevertheless adequate to provide protection 
against false operation caused by noise, etc. 
Since the duration of the output pulse from a Q-switched laser rarely 
exceeds 250 nanoseconds, the parameters of discriminator 12 are selected 
so that discriminator 12 produces an output signal if, and only if, the 
output from comparator 11 is less than 300 nanoseconds in duration. 
Assuming that this criterion is met, the output pulse from discriminator 
12 is fed to pulse stretcher 16, via OR-gate 14. Pulse stretcher 16 
functions to stretch the output of discriminator 12 to some useful 
duration, 10 .mu.s, say. 
As previously mentioned, if photo-detector 10 happens to receive a "direct 
hit" from the laser, detector 10 will saturate, generating an output pulse 
of about 4 volts but having a duration far in excess of 300 nanoseconds. 
Discriminator 12, which is of course, only sensitive to the duration of 
the pulses applied thereto and not their amplitude, will incorrectly 
assume that the radiation impinging on detector 10 is from some source 
other than a Q-switched laser and will, therefore, not generate any output 
signal, resulting in error. 
The instant invention is based on the assumption that, in a tactical 
situation, the only radiation source that would be capable of driving 
detector 10 into saturation is a Q-switched laser; thus, the instant 
invention uses that assumption to successfully and positively detect 
Q-switched radiation. 
As shown in FIG. 1, this is done by setting the threshold of 
pulse-amplitude discriminator 13 at a level which is slightly below the 
saturation level of detector 10. Since the output of discriminator 13 is 
also connected to pulse-stretcher 16, via another input to OR-gate 14, an 
output pulse will be generated by pulse-stretcher 16 if detector 10 
generates either (a) a narrow pulse of any amplitude (provided it is above 
the threshold of comparator 11), or (b) a high-amplitude pulse of any 
duration. 
FIG. 2 depicts the circuitry of an illustrative implementation of the 
apparatus shown in FIG. 1. As shown, photo-detector 10 is connected 
between a negative source E.sub.1 and ground. Advantageously, 
photo-detector 10 comprises a silicon photo-diode, for example in EG & G 
YAG 100, having a sensitivity of 0.5 amps/watt and an area of 5 mm.sup.2. 
In the arrangement shown in FIG. 2, such a detector would generate a 
negative pulse across resistor R.sub.2 at a ratio of 5.5 volts per 
incident watt/cm.sup.2. A single detector would, of course, have a 
90.degree. field of view, thus, to obtain a 360.degree. field of view it 
is necessary to employ four identical detectors. Advantageously, each 
detector has an RFI screen 17 and a high-pass spectral filter 18 
associated therewith. Because a silicon photo-diode has a cutoff 
wavelength of about 1.1 .mu.m, the high-pass spectral filter 18 is 
selected to have a cutoff wavelength of 0.78 .mu.m. Thus, detector 10 is 
capable of detecting radiation falling within the pass-band 0.78 to 1.1 
.mu.m. This pass-band includes the desired 1.06 .mu.m radiation from a 
Q-switched laser. 
The pulse which is developed across R.sub.2 is fed to the non-inverting 
input of a high-speed voltage-comparator 21. The inverting input of 
comparator 21 receives a reference potential -V.sub.ref, for example, from 
some suitable voltage-divider circuit. 
If the pulse developed across R.sub.2 exceeds -V.sub.ref, comparator 21 
will develop a negative output pulse which is fed to the input of an 
AND-gate 22, thence, to the trigger input of a one-shot multi-vibrator 23. 
The Q output from multi-vibrator 23 is fed to a first input of a first 
OR-gate 24. The other input to OR-gate 24 comprises the output of 
comparator 21. The Q output of multi-vibrator 23 forms the other input to 
AND-gate 22. One-shot multi-vibrator 23 is selected such that the duration 
of its output pulse is about 300 nanoseconds, thus, if the output of 
comparator 21 is greater than 300 nanoseconds, no output pulse can be 
generated by OR-gate 24. On the other hand, if the duration of the pulse 
from comparator 21 is less than 300 nonoseconds, signifying that a beam 
from a Q-switched laser has struck detector 10, a pulse will be fed from 
OR-gate 24 to a first input of a second OR-gate 26, thence, to the input 
of pulse stretcher 16. As previously mentioned, pulse-stretcher 16 is used 
to stretch the duration of the output pulse of OR-gate 26 from 200 
nano-seconds seconds, say, to about 10 .mu.Sec., which is a much more 
useful pulse duration. The output of pulse stretcher 16, in turn, is 
amplified in a line driver 27 which is connected to some suitable 
indicating device or counter (not shown). 
The pulse which is developed across R.sub.2 is also connected to the 
trigger input of a second on-shot multi-vibrator 28; however, as 
previously discussed, the sensitivity of one-shot multi-vibrator 28 is 
reduced, e.g. by the use of diodes CR1 and CR2, so that multi-vibrator 28 
is triggered only when the output pulse developed across resistor R.sub.2 
exceeds some predetermined level, for example -4 volts. The output from 
one-shot multi-vibrator 28 is fed to a second input of the second OR-gate 
26 and, likewise, will produce an output pulse in OR-gate 26 which is 
stretched by pulse-stretcher 16 each time that multi-vibrator 28 is 
triggered. 
The RFI screen 17 and spectral filter 18 at the input to photo-detector 10 
eliminate much of the noise that would otherwise tend to cause false 
operation of the detector. It will be noted that the output pulse 
developed across resistor R.sub.2 is coupled to the pulse-width 
discriminator via a capacitor C.sub.3 and to the pulse-amplitude 
discriminator via a capacitor C.sub.2. Thus, DC noise, for example, caused 
by sunlight striking the detector, is eliminated. On the other hand, light 
pulses caused by driving through trees with the sun overhead, for example, 
will be too wide to get through the pulse-width discriminator and too low 
in amplitude to get through the pulse-amplitude discriminator, again 
avoiding erroneous detection. 
In summary, the two-channel processor technique disclosed and claimed 
herein provides a wide-dynamic range with a virtually zero false alarm 
rate. The circuitry is simple, inexpensive, and highly reliable. The 
detector assemblies were used in several military field tests and proved 
to be highly reliable in detecting Q-switched, 1.06 .mu.m laser pulses 
while maintaining a zero false alarm rate in all instances. Prior to the 
instant invention, there were no known systems that satisfied all the 
specifications that the detector assembly disclosed and claimed herein 
does. 
One skilled in the art may take various changes and substitutions to the 
layout of parts shown without departing from the spirit and scope of the 
invention.