Polyconductor device for laser beam detection and protection

A device is described using at least one polyconductor film as an active ment lens of an electrical circuit for detecting incident laser radiation. In several alternative embodiments, a polyconductor film is located at the focal point of at least one lens which focuses the incident laser radiation on the film. The polyconducting film acts as a variable resistor of a balanced bridge network and upon receipt of radiation causes an imbalance in the bridge for radiation detection. The film changes phase with increasing incident power such that it protects itself against high levels of radiation. The lenses can also be polyconducting films and another film shielded from the radiation may be included to compensate for temperature fluctuations and maintain circuit sensitivity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a device for detecting incident laser radiation 
and determining the intensity of the radiation. More particularly, the 
present invention relates to a device using polyconducting film to detect 
incident laser radiation, to compensate for ambient temperature 
fluctuations, and to provide self-protection against high intensity 
radiation by changing phase dependent upon the incident radiation 
intensity level. 
2. Description of Prior Art 
Optical radiation detectors may be broadly divided into two main 
categories: thermal detectors and photon detectors. Photon detectors are 
more sensitive and include photomultipliers, photoconductors, photodiodes 
and avalanche photodiodes. Thermal detectors are less common but have the 
advantage of larger bandwidth and do not require cooling. Such detectors 
are effective for detection but have a response time too slow to provide 
effective or self-protection when exposed to high intensity laser beams. 
Thermal detectors operate on the principle that the heating effect of the 
incident radiation causes a change in some electrical property of the 
detector, e.g. resistance, and thus the theoretical response is 
proportional to the energy absorbed and is practically the same over a 
wide range of the wavelength, especially in the infrared portion of the 
spectrum. The time constant is generally a few milliseconds and hence 
these detectors are rarely used where fast response times and high data 
rates are required. 
Photon detectors operate on the principle that there is a direct 
interaction between the incident photons and the electrons of the detector 
material and thus that the detector response is proportional to the number 
of photons absorbed. These detectors usually have sensitivity one or two 
orders of magnitude greater than thermal detectors and time constants of 
generally a few microseconds. However, their spectral response varies with 
wavelength and the majority require cooling. The present invention is a 
device for detecting laser radiation and measuring laser power that is 
applicable to a wide variety of lasers and has very short response times 
permitting real-time detection and self-protection against heat 
degradation by high intensity lasers not possible by conventional thermal 
detectors. 
SUMMARY OF THE INVENTION 
The present invention is briefly described as a device for detecting and 
protecting against incident laser radiation by using at least one 
polyconductor film as an active element in an electrical network for 
detecting such radiation. The polyconductor film is located at the focal 
point of a lens, which may also be a polyconducting film, used to focus 
incident radiation on the film used as a variable resistor in a balanced 
bridge network. Receipt of radiation by the film causes an imbalance in 
the bridge producing an electrical signal whereby monitoring of the 
incident radiation and its intensity is possible. The polyconductor film 
changes phase with increasing incident power to provide self-protection 
against heat degradation.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a first embodiment of the instant invention is shown 
in which it is used as a detector of incident laser radiation. The 
incident laser beam 10 is focused by a lens 12 onto the polyconductor film 
14 which constitutes one arm of a Wheatstone bridge circuit 16 having 
resistors R.sub.1, R.sub.2, R.sub.3 in the other arms as shown. An 
indicating meter 18 may be placed in the center arm of the bridge and 
connected to a display 20 to monitor the incident power level. An external 
voltage source, not shown, supplies a bias voltage V.sub.P to the sensing 
polyconductor film 14. To compensate for ambient temperature fluctuations 
affecting sensor readings, a capacitor 22 and compensating polyconductor 
film 24 are connected in parallel with the bridge circuit 16 and a 
variable resistor 26 and bias voltage source V.sub.B 28 for the bridge. 
The compensating polyconductor film 24 is shielded from incident 
radiation. By way of example and not limitation, a typical film is type 
TC-1F-5V made by Multi-State Devices. 
In the absence of an incident beam 10, the bridge 16 is balanced and the 
display meter 20 will indicate a zero power density level. In the presence 
of an incident beam, a voltage imbalance in the bridge 16 will develop due 
to a change in the bias voltage V.sub.P across the sensing polyconductor 
film 14. This imbalance will be shown on the display 20 which is 
calibrated with a standard optical power meter such that the intensity of 
the incident beam is directly read from the display. Compensation for 
temperature fluctuation is achieved by connecting the compensating 
polyconductor film 24 in parallel with a variable resistor 30 in series 
with an audio frequency (A.F.) supply 32. The terminal resistance of the 
compensating polyconductor film 24 increases as the temperature decreases 
resulting in more power being fed from the A.F. supply 32 to the bridge 16 
as ambient temperature decreases. A typical terminal characteristic of 
this device (terminal resistance v. heater voltage) is presented in FIG. 
2. 
Thus, in operation, the bridge 16 is initially balanced with just direct 
current (D.C.) power from the bridge bias voltage source V.sub.B 28. The 
bridge bias voltage V.sub.B and the detector polyconductor bias voltage 
V.sub.P combine to drive the detector polyconductor to a terminal 
resistance of predetermined value corresponding to a sensitive quiescent 
point on its temperature-resistance curve. 
Both bias voltages, V.sub.B and V.sub.P are selected to keep the bridge 16 
balanced at the highest expected ambient temperature. For perfect 
zero-drift compensation, the increase in A.F. power from the A.F. supply 
32 is just sufficient to keep the bridge 16 balanced. For greater 
sensitivity of the apparatus, the D.C. battery voltage V.sub.B may be 
replaced by an A.F. voltage. In this case, the bridge output could be 
amplified by a high gain amplifier having an output proportional to the 
optical power density of the incident beam. 
A planar array or cluster of polyconductor films 14 whose center is 
approximately at the focal point of the lens 12 may be substituted for the 
single film and would have the advantage over a single film of less 
uncertainty concerning the exact location of the focal point of the lens 
12 and greater capability to detect beams incident from different angles 
(other than normal incidence). In this case total power rather than power 
density would be detected by the array if a sufficiently large array is 
used. 
Referring to FIG. 3A, an alternative embodiment is schematically shown as a 
circuit to be incorporated into a pair of eyeglasses for eyesight 
protection. Two lenses 12, each having a thin sensing polyconductor film 
14 across the exterior surface are parallel connected to the Wheatstone 
bridge 16 as shown. An audio alarm 19 may also be connected in series with 
or in place of the indicating meter 18. Whichever film is exposed to the 
incident radiation would cause an approximate open circuit terminal 
resistance of the other parallel-connected film (due to the small 
cross-section of the incident beam). Again the bridge 16 would be 
imbalanced and the audio alarm 19 would activate. 
Referring to FIG. 3B, a third embodiment of the instant invention is 
schematically shown. The circuit may employ a number of flat transparent 
surfaces 12 having the polyconductor film 14 on the exterior surface, the 
surfaces all connected in parallel to form one arm of the bridge 16. Thus 
for example protection against stray laser radiation may be provided in 
laboratories having viewing windows. 
In all embodiments of the instant invention, the reflectivity of the 
polyconductor film 14 increases with increasing intensity of the incident 
beam until the film 14 changes state from a semiconductor to a metal. Thus 
the instant eyesight protection apparatus is self-protecting against 
thermal degradation as well as providing eyesight protection against 
intense laser radiation. These advantages are evident from FIG. 4 which 
shows a typical terminal characteristic (terminal resistance v. 
temperature) of a vanadium dioxide (VO.sub.2) polyconductor device 
employed in this invention as compared to that of a typical thermistor 
device presently used in commercial thermal detectors. 
Referring to FIG. 5, by way of example and not limitation a schematic of 
the preferred embodiment is shown having specific components permitting 
incident radiation detection with greater sensitivity than a thermistor 
detecting element. Here R.sub.1, R.sub.2, R.sub.3 of the bridge 16 are 
1k.OMEGA.. The A.F. power supply 32 is a SIGNETICS 555 Timer having a +5 V 
applied bias voltage and two 1k.OMEGA. resistors 34, 36 and a 0.5 .mu.f 
capacitor 38 connected to the output terminals of the timer. The timer 32 
operates at 1000 Hz and the bridge bias voltage source V.sub.B is 3.5 V. 
The capacitor 22 is 1 .mu.f and the two variable resistors 26, 30 are used 
to precisely balance the bridge 16. 
In order to protect against high incident power levels such as might be 
found in military applications which vary over a large dynamic range, a 
modification of the proposed bridge circuitry is necessary. This is in 
order to keep the operating point of the bridge 16 in the same region of 
the terminal characteristic of the detecting film 14. The modification is 
required for large incident power levels which have the tendency of moving 
the operating point excessively far from the starting region. In addition 
to preserving the sensitivity of the polyconductor film 14 which is 
maximum in the transition region, it is also desirable to preserve the 
sensitivity of the bridge 16 by keeping the terminal characteristic 
R.sub.T reasonably low and selecting the resistors R.sub.1, R.sub.2, 
R.sub.3 in the other three arms of the bridge close to the value of 
R.sub.T. In addition, for metering purposes it is desirable to keep the 
current through the film 14 from all sources and hence the surface 
impedance of the film constant so that the scattering pattern and cross 
section will also remain constant. To achieve these objectives, a feedback 
loop is introduced in order to adjust the independent bias voltage V.sub.P 
of the sensing polyconductor as shown in FIG. 6. The feedback circuit 40, 
consists of a power supply 42 connected to a voltage regulator 44 which 
has a resistor 46 connected across the output terminals and is connected 
to the sensing polyconductor film 14. 
In principle, an increase in the incident power level leads to a decrease 
in R.sub.T and thus moves the operating point away from the quiescent 
point. The proposed feedback loop senses this tendency and is immediately 
activated such that the bias voltage V.sub.P is decreased proportionately 
in order to restore operation at the quiescent point. In other words the 
loop serves the purpose of trading off V.sub.P with R.sub.T in order to 
produce bias control. Since the real time variations of V.sub.P could also 
be produced by ambient temperature fluctuations through changes in 
R.sub.T, the proposed loop serves the purpose of protecting the sensing 
device (by decreasing the bias voltage) not only from high incident power 
levels but also from large sudden changes in ambient temperature. Such 
conditions are frequently encountered in electronic warfare applications 
and the proposed loop would provide such protection for the polyconductor 
device as well as any object coated with polyconductor films. If the 
temperature compensating polyconductor 24 is also protected from large 
temperature variations by a similar loop, not shown, (which will restore 
operation to the starting bias region) then the variations in its bias 
voltage V.sub.P (corresponding to R.sub.P) can be monitored and used to 
calibrate a digital thermometer (or a profile thermometer if several 
temperature compensating polyconductors are used). With the operation of 
both loops it is possible to distinguish between large variations in 
incident power as opposed to large variations in ambient temperature and 
hence provide information for eliminating many types of false alarms in 
situations in which high incident power levels occur. It should also be 
noted that in the presence of both loops the sensing polyconductor becomes 
double compensated for ambient temperature variations since the current in 
the detecting film 14 is compensated by the compensating polyconductor 24 
(through the Wheatstone bridge 16) while the heater bias voltage is 
compensated by the feedback loop 40. As a result of this double 
temperature compensation the bridge output meter 18 has a more stable 
indication. This is illustrated in FIG. 7 for a helium-neon laser of 60 mw 
output power and operating at a wavelength of 0.6328.mu. or 
0.6328.times.10.sup.-6 meters.