WO2004053442A1

WO2004053442A1 - Optical power limiting devices and a method for protecting imaging and non-imaging sensors

Info

Publication number: WO2004053442A1
Application number: PCT/IL2003/001028
Authority: WO
Inventors: Doron Nevo; Ram Oron; Moshe Oron
Original assignee: Kilolambda Technologies Ltd.
Priority date: 2002-12-08
Filing date: 2003-12-04
Publication date: 2004-06-24
Also published as: AU2003285729A1; IL153313A0

Abstract

There is provided an optical power switching device, including at least one plate made of transparent dielectric material, a thin, electrically conductive, metallic material coated on one side of the plate, wherein, upon being exposed to an optical power beam having a power level exceeding a predetermined threshold focused thereon, the layer of conductive material forms a plasma, damaging the dielectric material, thereby rendering the portion of the surface of the plate under the impinging beam opaque to light. A method for reducing or interrupting optical transmission in response to the transmission of excessive optical power or energy is also provided.

Description

OPTICAL POWER LIMITING DEVICES AND A METHOD FOR PROTECTING IMAGING AND

NON-IMAGING SENSORS

Field of the Invention

The present invention relates to optical power switching devices and methods for protecting imaging and non-imaging sensors. More particularly, the present invention concerns devices and methods for interrupting or reducing optical transmission in response to the transmission of predetermined, excessive optical power or energy, in order to protect imaging and non-imaging sensors and detectors. Background of the Invention

Imaging and detection systems, using large-aperture and low F-number telescopes, are susceptible to detector saturation and/or damage caused by a powerful light source or a high power laser within their fields of view. The problem exists in many cases, especially in modern optical systems wherein active (e.g., laser), together with passive (e.g., television or night-vision, multi-pixel) sensors are used in the same or adjacent systems, when reflected laser light or an arbitrary ray or reflection from a laser enters the imaging system. This difficulty calls for a passive device that will switch off the power propagating into the sensor or detector, when the power exceeds a maximal allowed intensity or a damaging threshold. Such a switching device should be placed either at the input of the sensitive optical detector or detector array, or on the optical path leading into the detector.

In the past, there have been attempts to realize such an optical safety switch, and efforts have been invested in optical imaging sights. The principles on which these prior art solutions were based included: (1) self- focusing or self-defocusing, due to a high electric field-induced index change through the third order susceptibility term of the optical material, and (2) reducing the optical quality of a gas or a solid transparent insert positioned at the focus or cross-over spot of a telescope, by creating a plasma in the cross-over point, whereby light is absorbed by the plasma. These solutions are described in U.S. Patents Nos. 3,433,555, and 5,017,769, as well as in the IR/EO System Handbook, (ERIM, Vol. 7, p.p. 344-351). U.S. Patent 3,433,555 discloses a system in which plasma is formed in a gas, where the gas density is lower than solids and liquids and the density of the plasma formed by the gas is low as well, thus limiting its absorption to the medium and far infrared parts of the light spectrum. This device does not absorb in the visible and near infrared regions, and it cannot protect optical systems in these regions of the spectrum.

The system in U.S. Patent 5,017,769 uses a solid, transparent insert in the cross-over point. The transparent insert is covered with carbon particles on its surface, enhancing the forming of plasma on the surface. Here, the plasma density is high, since it emanates from solid material. The dense plasma absorbs in the visible, as well as in the near, infrared light regions. The device is equipped with multiple inserts on a motorized rotating wheel, exposing a new, clean and transparent insert after every damaging pulse. In this arrangement, the carbon does not endure over long exposures to high powers.

In the past, passive devices have been proposed for image display systems. These devices generally contain a mirror that is temporarily or permanently damaged by distortion or evaporation caused by an impinging high power laser beam. Examples of such devices are described in U.S. Patents Nos. 6,384,982; 6,356,392; 6,204,974 and 5,886,822. The powers needed to operate the devices of these patents are in the range of pulsed or very energetic CW lasers. The distortion of a mirror by energy impinging upon it is very slow, and depends on the movement of the mirror's large mass and the absorbed energy that generates the movement. The process of reflective coating removal from large areas is also slow, since the mirror is not placed in the focus of the system, where the power is spatially concentrated.

Another passive device is disclosed in U.S. Patent No. 6,216,581. In this device, two materials are used: the first material is heat-absorbing, while the second material is heat-degradable. When these materials are exposed to a light beam, the first material is heated and transfers its heat to the second one, whereupon the transparency or reflectivity of the second material is degraded, due to the high temperature. This process is relatively slow, since heat transfer times are long in comparison with laser pulses, and in many cases, is not sufficiently quick to intercept the beam before damage occurs to objects along the optical line. In addition, the process of temperature-induced degradation does not provide enough opacity to efficiently prevent damage by high-power pulses.

An ideal protection switch should have the following properties:

1) The switch should be transparent to image transfer in a broad light spectrum, without any degradation of the image quality, under normal working conditions.

2) When exposed to powers of a preset threshold and higher, the switch should be fast enough to intercept damaging optical power before damage occurs.

3) The part of the field of view which is not exposed to threshold power should remain transparent through the protection switch and should be viewable at all times.

4) The permanent, opaque spot formed on the switch when it is exposed to high powers should withstand long exposures to damaging light, without any change in its opacity.

5) Opacity or transmission reduction should be of up to three orders of magnitude.

6) The switch should react to both continuous and pulsed damaging lights.

7) The switch should react to a wide range of spectral light sources or lasers.

8) „ The switch should react to a wide range of angles of impingement of the damaging light or laser.

9) The sacrificial part of the switch should be field-replaceable. Summary of the Invention

It is therefore a broad object of the present invention to provide a passive safety switch for protecting an imaging or non-imaging sensor against powerful light sources and lasers in the field of view, that fulfills at least some of the above-described properties of an ideal switch. It is a further object of the present invention to provide a passive safety switch, as a part of an opto-electronic device, for protecting field optical systems, such as binoculars, monoculars and telescopes, or the human eye, against powerful light sources and lasers in the field of view.

It is a further object of the present invention to provide safety switch devices and methods for interrupting or reducing optical transmission in response to the transmission of excessive optical power or energy, to be used for protecting imaging and non-imaging sensors and to be installed either internally or at the input port of an optical imaging system.

A further object of the present invention is to provide a safety switch that has a predetermined optical power transmission threshold, for use in imaging and non-imaging sensors.

It is a still a further object of the present invention to provide a safety switch that is activated by a broad range of wavelengths, for use in imaging and non-imaging sensors.

It is a still a further object of the present invention to provide a safety switch that is activated by a wide range of angles of impingement of damaging light or laser, for use in imaging and non-imaging sensors.

It is a yet further object of the present invention to provide a safety switch that, when exposed to powers exceeding a set threshold, is fast enough to intercept the damaging optical power before damage occurs, for use in imaging and non-imaging sensors.

It is a yet further object of the present invention to provide a safety switch wherein the part in the field of view which is not exposed to powers exceeding a set threshold remains transparent and can be viewed at all times, for use in imaging and non-imaging sensors.

It is a still a further object of the present invention to provide a safety switch wherein the opaque spot is permanent and can withstand long exposures to damaging light without a change in its opacity, for use in imaging and non-imaging sensors. It is a still further object of the present invention to provide a safety switch that reacts to both continuous and pulsed damaging light, for use in imaging and non-imaging sensors.

In accordance with the present invention, there is therefore provided an optical power switching device, comprising: at least one plate made of transparent dielectric material; a thin, electrically conductive, metallic material coated on one side of said plate; wherein, upon being exposed to an optical power beam having a power level exceeding a predetermined threshold focused thereon, said layer of conductive material forms a plasma, damaging said dielectric material, thereby rendering the portion of the surface of the plate under the impinging beam opaque to light.

The invention further provides an optical power or energy switching system, comprising an optical assembly having an input unit and an output unit, each unit including a lens, the lenses being arranged to produce a common focal plane; a thin, substantially transparent layer of electrically conductive material contacting a surface of a dielectric plate disposed at, or in proximity to, the focal plane; the layer of conductive material forming an electric field breakdown when exposed to optical power levels above a predetermined threshold, the electric field breakdown damaging the surface of the dielectric plate, rendering the surface substantially opaque to light propagating within the optical assembly.

The invention still further provides a method for reducing or interrupting optical transmission in response to the transmission of excessive optical power or energy, the method comprising providing an optical power switching device as described herein; providing an input unit and an output unit, each unit comprising a lens; positioning the lenses to form a common focal plane, and positioning the conductive metallic layer at least in close proximity to the plane; whereby transmission of a pre- determined amount of excessive power or energy impinging on the conductive material forms an electric field breakdown, damaging the dielectric plate and thereby rendering it substantially opaque to light. Brief Description of the Drawings

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the drawings: Fig. 1 is a schematic, cross-sectional view of an optical power-switching system for imaging and non-imaging sensors, including an optical switch according to the present invention; Fig. 2 illustrates the method of reducing back-reflected light by tilting the optical switch; Fig. 3 is a schematic, cross-sectional view of the optical switch of the present invention;

Fig. 4 is a schematic curve showing input and output powers to the optical switch; Fig. 5 is a schematic view of a damaged spot on the switch and its geometrical relation to a damaging beam of light entering the switch at angle α; Fig. 6 is an experimental curve of the switch, showing output power versus input power;

Fig. 7 is an experimental curve of the switch, showing temporal behavior; Fig. 8 is an experimental microscopic view of a damaged (opaque) switch, showing a crater or craters in the impinging spot of the damaging light, and Fig. 9 is a cross-sectional view of an optical switch according to the present invention, utilizing a mirror. Detailed Description of Preferred Embodiments

Referring now to Fig. 1, there is shown a schematic, cross-sectional view of an optical power-switching system 2 for imaging and non-imaging sensors, according to the present invention, having a two-dimensional insert in its cross-over point. The two- dimensional optical power switching system 2 is shown utilized, e.g., with a telescope having an input lens 4 and an output lens 6, disposed along the optical path 8. An optical switch 10, responsive to optical power, is located on the optical path 8, in a plane 12 traversing the optical path. Plane 12 includes the focal or cross-over point 14, between the input power beam 16 and output power beam 18, for causing the interruption of optical power propagation from the input power beam 16 to the output power beam 18 when the optical power exceeds a predetermined threshold.

Figure 2 illustrates the method of reducing back-reflected light by tilting the switch 10 at an angle β/2, where β is the angle between input power beam 16 and reflected power beam 20. As shown, reflected power beam 20 is outside of the field of view of the system, and cannot be transmitted back, thus minimizing the back reflection.

Figure 3 is a schematic, cross-sectional view of switch 10, for imaging and non-imaging sensors. Seen is a "sandwich" assembly, composed of two thin plates 22', e.g., disc-shaped, made of a transparent dielectric material such as silica or Schott BK7 glass, and intermediate layers 24, 26 and 28. Layer 28 is very thin (a few atomic layers, typically of a few nanometers) and may preferably be made of a conducting metallic material such as gold, silver, chromium, nickel or another metal. Layer 28 may also be covered, on one or both sides, with an anti-reflective coating, namely, an input anti-reflective coating 24 and/or an output anti-reflective coating 26. These anti-reflective coatings can significantly reduce the optical reflections from layer 28 and, at the same time, shape and optimize the layers for maximal electric field on the metal, by the interference of all the reflections from the interfaces with the main beam. Such thin layers 28 of conducting material enhance the strength of the electric field in their neighborhood, due to local irregularities of their surface. The surface irregularities induce field concentration, resulting in lower power being required to create an electrical breakdown and damage. When optical power exceeding a predetermined threshold impinges upon thin layer 28, strong electric fields, which can lead to local electrical breakdown, are generated at certain sites ("hot spots") in proximity with the conducting surface. This leads to an arc-discharge, where plasma is formed. The generated plasma greatly increases the absorption of the propagating light, and the energetic discharge causes catastrophic damage at or near the conducting surfaces. This damage is often viewed as cratered regions. The switch thus becomes permanently highly scattering or, in other words, highly opaque to propagating light, significantly reducing the transmitted optical power. The opacity is permanent, and creates a "blind spot" on the two-dimensional switch, thus enabling location of the direction (azimuth and elevation) of the damaging light source or laser. The device acts as a fast switch for interrupting the power propagation, which occurs as fast as the breakdown is created; it then permanently remains as an interrupting switch, at some definite spots, due to the damage formed by the energetic breakdown. The switch remains transparent in its entire area, except for the damaged spots; it is possible to view a two-dimensional image through it, with the damaged spots indicating the direction of the damaging light.

Fig. 4 shows an ideal schematic curve of the input and output powers of the optical switch, showing that when P_in grows to P_threshoi_d , the P_out is intercepted and its power is reduced to zero.

Fig. 5 shows a schematic view of a damaged spot 30 on the switch and its geometrical relation to a damaging beam of light entering the switch at angle . All beams 32, entering the telescope parallel to its axis of symmetry, impinge upon the focal point 14 inside switch 10. When parallel beam 32 travels at an angle a, it impinges upon point 30, which is displaced by a distance 7 from point 14 on switch 10. From the geometry, it is obvious that Tana = Y I ^' F , where F is the focal length of lens 4. Although the displacement in this example is in the vertical direction, the same rule applies to any displacement. The direction of the damaging laser beam a can be identified by looking through the system, seeing a blind spot, or by removing the damaged switch and measuring the coordinates of the damage, such as depicted in the upper part of the Figure.

In order to control the threshold power of the switch, several methods can . be used, first, by changing the thickness of the conducting layer. In general, threshold power decreases with a thicker layer. However, in this method, the transmission loss at the operating power also changes (the thicker the layer, the higher the loss). Thus, if one wants to keep a low insertion loss at the operating power, this method is rather limited in range. A second method of controlling threshold power is to use a telescope with different F-numbers, or focal spot diameters.

The design and execution of the sacrificial layers was carried out according to the simulation. The example given herein utilizes a thin layer made of chromium (Cr). The electric field at and near the Cr layer was enhanced, due . to both its surface irregularities and by having two non-symmetric, Silicon-monoxide (SiO) layers on both sides of the Cr layer, serving as anti-reflecting and field enhancers, due to their longitudinal position along the pointing vector of the light beam.

These sacrificial layers were positioned at the interface between two thin silica or BK7 glass plates, and tested. Switches with threshold powers ranging from a few tens of milli- Watts up to about a few Watts, in the cross-over or focal spot of about 10 micrometers, were tested. The switches were tested for threshold power, transmission loss, return loss, added opacity after exposure to threshold and higher powers, timing, endurance and visual (microscopic) inspection before and after damage.

Visual (microscopic) inspection, after damage, revealed a cratered focal spot, the craters covering about the entire central lobe of the focal spot (where the optical power flows), and being a few microns deep.

The tests included time domain experiments, wherein switches were exposed to short pulses (few tens of microseconds). The switches reacted in the same way as in

^' the CW case, i.e., there was a fast, large drop in transparency when they were impinged by powers over the threshold. Transmissions of 80% and up were obtained. Other parameters, such as the broad-spectrum operation of the switch, were found satisfactory.

Fig. 6 is an experimental curve of a switch having a 40mW (16 dBm) input power threshold, showing output power versus input power. Here, the experimental results show approximately 40mW input threshold power, where the output power just before damage occurred was approximately 32mW (15 dBm). Also, the output power dropped by approximately 15dB when the damage occurred, reducing the output power to approximately 3% of the original power before the threshold power was exceeded.

Fig. 7 is an experimental curve of switch temporal behavior, showing that when an energetic pulso. of about 20 microseconds (FWHM-Full Width at Half Maximum) is impinged on the switch, the switch closes quickly, after about 2.5 microseconds (FWHM). The switch closes faster, since half of this time is due to the rise time of the input pulse.

Fig. 8 is an experimental, microscopic view of a damaged (opaque) switch with a crater or craters in the impinging spot of the damaging light. The crater is seen to cover the central lobe area, where the optical ray is propagating. One can see the crater, having dimensions of about 10 micrometers in diameter.

Fig. 9 is a schematic view of a switch utilizing a mirror 34 at the cross-over point, when the optical power exceeds a predetermined threshold. The mirror is damaged and does not reflect, causing the interruption of optical power propagation from the input power beam 16 to the output power beam 18.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes, which come within the meaning and range of equivalency of the claims, are therefore intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. An optical power switching device, comprising: at least one plate made of transparent dielectric material; a thin, electrically conductive, metallic material coated on one side of said plate; wherein, upon being exposed to an optical power beam having a power level exceeding a predetermined threshold focused thereon, said layer of conductive material forms a plasma, damaging said dielectric material, thereby rendering the portion of the surface of the plate under the impinging beam opaque to light.

2. The switching device as claimed in claim 1, wherein said dielectric material is selected from the group including silica and Schott BK7 glass.

3. The switching device as claimed in claim 1, wherein said conductive material is selected from the group including gold, silver, chromium and nickel.

4. The switching device as claimed in claim 1, further comprising an anti-reflective layer interposed between said plate and said conductive metallic layer.

5. The switching device as claimed in claim 1, wherein said conductive metallic layer is coated on both of its surfaces with an anti-reflective material and is interposed between two transparent, dielectric material plates.

6. The switching device as claimed in claim 1, further comprising an input unit and an output unit, each unit comprising a lens; said units or said lenses being disposed to form a common focal plane, and said conductive metallic layer being located at least in close proximity to said plane.

7. An optical power or energy switching system, comprising: an optical assembly having an input unit and an output unit, each unit including a lens, said lenses being arranged to produce a common focal plane; a thin, substantially transparent layer of electrically conductive material contacting a surface of a dielectric plate disposed at, or in proximity to, said focal plane; said layer of conductive material forming an electric field breakdown when exposed to optical power levels above a predetermined threshold, said electric field breakdown damaging the surface of the dielectric plate, rendering the surface substantially opaque to light propagating within said optical assembly.

8. The switching system as claimed in claim 7, wherein said electrically conductive material is disposed between the surfaces of two dielectric plates.

9. The switching system as claimed in claim 7, further comprising an anti-reflective layer between said layer of electrically conductive material and said dielectric plate.

10. The switching system as claimed in claim 7, wherein the plane of said dielectric plate is tilted with respect to the axis of propagation of a light beam, to reduce the back-reflected light impinging on said switch.

11. A method for reducing or interrupting optical transmission in response to the transmission of excessive optical power or energy, said method comprising: providing an optical power switching device as claimed in claim 1 ; providing an input unit and an output unit, each unit comprising a lens; positioning said lenses to form a common focal plane, and positioning said conductive metallic layer at least in close proximity to said plane; whereby transmission of a pre-determined amount of excessive power or energy impinging on the conductive material forms an electric field breakdown, damaging the dielectric plate and thereby rendering it substantially opaque to light.

12. The method as claimed in claim 11, further comprising the step of: coating said conductive metallic layer with an anti-reflective material.

13. The method as claimed in claim 11, wherein said transparent dielectric material is selected from the group including silica and Schott BK7 glass.

14. The method as claimed in claim 11, wherein said electrically conductive layer is selected from the group including gold, silver, chromium and nickel.