Abstract:
An optical power or energy-switching device, comprising an optical waveguide having an input section and an output section, the two sections forming a pair of opposed surfaces extending transversely through the axes of said waveguide sections, and a thin, substantially transparent layer of electrically conductive material disposed between said opposed surfaces, said layer of conductive material forming a plasma when exposed to optical signals propagating within said optical waveguide with an optical power level above a predetermined threshold, said plasma damaging said opposed surfaces sufficiently to render said surfaces substantially opaque to light propagating within said optical waveguide so as to prevent the transmission of such light.

Description:
This application is a 371 of PCT/IB03/00928 filed on Mar. 13, 2003, which claims benefit of 60/364,161 filed on Mar. 13, 2002, and claims benefit of 60/401,511 filed on Aug. 7, 2002. 

   FIELD OF THE INVENTION 
   The present invention relates to optical power switching devices and methods, and particularly to such devices and methods for interrupting or reducing the optical transmission in response to the transmission of excessive optical power or energy. 
   BACKGROUND OF THE INVENTION 
   Fiber lasers, fiber optics for communication systems, and other systems for light delivery, such as in medical, industrial and remote sensing applications, often handle high levels of optical power, namely, up to several Watts in a single fiber or waveguide. When these high specific intensities or power per unit area are introduced into the systems, many thin film coatings, optical adhesives and even bulk materials, are exposed to light fluxes beyond their damage thresholds and are eventually damaged. Another issue of concern in such high-power systems is laser safety, where well-defined upper limits are established for powers emitted from fibers. These two difficulties call for a passive device that will switch off the power propagating in a fiber or waveguide, when the power exceeds the allowed intensity. Such a switching device should be placed either at the input of a sensitive optical device, or at the output of a high-power device such as a laser or an optical amplifier, or integrated within an optical device. 
   In the past, there have been attempts to realize an optical safety shutter, mainly for high-power laser radiation and high-power pulsed radiation; special efforts were devoted to optical sights and eye safety devices. The properties 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 cross-over spot of a telescope, by creating a light-absorbing plasma in the cross-over point. These are described in U.S. Pat. No. 3,433,555 and U.S. Pat. No. 5,017,769. U.S. Pat. No. 3,433,555 describes a plasma that is created in a gas where the gas density is low (lower than solids and liquids) and the density of the plasma created by the gas is low as well, limiting its absorption to the medium and far infrared part of the light spectrum. This device is not absorbing in the visible and near-infrared regions and cannot protect in these regions of the spectrum. U.S. Pat. No. 5,017,769 describes the use of a solid insert in the crossover point. This transparent insert is covered with carbon particles on its surface, enhancing the creation of a plasma on the surface at lower light intensities. The plasma density is high, since it starts from solid material. The dense plasma absorbs visible as well as infrared light, and the device is equipped with multiple inserts on a motorized rotating wheel that exposes a new, clean and transparent part after every damaging pulse. The two devices described above, namely U.S. Pat. Nos. 3,433,555 and 5,017,769, are large in their volume, work in free space and require high pulsed powers. 
   Passive devices were proposed in the past for image display systems. These devices generally contained a mirror that was temporarily or permanently damaged by a high-power laser beam that damaged the mirror by distortion or evaporation. Examples for such devices are described in U.S. Pat. Nos. 6,384,982, 6,356,392, 6,204,974 and 5,886,822. The powers needed here are in the range of pulsed or very energetic CW laser weapons and not in the power ranges for communication or medical devices. The distortion of a mirror by the energy impinging on it is very slow and depends on the movement of the large mass of the mirror as well as the energy creating the move. The process of removing a reflective coating from large areas is also slow, since the mirror is not typically placed in the focus where power is spatially concentrated. Another passive device was proposed in U.S. Pat. No. 621,658B1, where two adjacent materials were used. The first material was heat-absorbing, while the second material was heat-degradable. When these two materials were inserted into a light beam, the first material was heated and transferred its heat to the second material to degrade the transparency or reflectivity of the second material. This process is relatively slow, since heat-transfer times are slow, and in many cases not sufficiently fast to interrupt a light beam before damage occurs to objects along the optical line. In addition, the process of temperature-induced degradation often does not provide enough opacity to efficiently prevent damage from high-power spikes that are a known phenomenon in laser-fiber amplifiers. 
   Better, faster and more opaque solutions are needed. The present invention provides such a solution. 
   SUMMARY OF THE INVENTION 
   It is a broad object of the present invention to provide an improved passive safety switch for optical waveguides or fiber optics used internally in optical systems and either at the input or output port of an optical device or system. 
   It is a further object of the present invention to provide an improved safety switch for use in optical waveguide or optical fiber systems, the switch having a predetermined optical power transmission threshold. 
   It is a still a further object of the present invention to provide a safety switch for use in a waveguide or optical fiber, the switch being activated by a broad range of wavelengths. 
   It is a further object of the present invention to provide an improved safety switch for use in optical waveguide or optical fiber systems, the switch having a predetermined optical power transmission threshold and able to switch off the power in the forward direction as well as preventing the damaging phenomenon called “Fiber Fuse” from damaging the input fibers, in the backward (toward the laser source) direction. 
   The “Fiber Fuse” is a phenomenon that results in the catastrophic destruction of an optical fiber core. It has been observed at laser powers on the order of 3×10 6  watts/cm 2  in the core. This phenomenon is characterized by the propagation of a bright visible light from the point of initiation toward the laser source. The term “Fiber Fuse” has been adapted to the phenomenon because of the similarity in appearance to a burning fuse. The “Fiber Fuse” has been shown to occur when the end of the fiber is contaminated, and it has also been initiated spontaneously from splices and in-core disturbances. The “Fiber Fuse” event can destroy many kilometers of waveguide or fiber. 
   The present invention includes at least four different versions of optical power switching devices, as follows: 
   1. Two co-linear waveguides separated by a gap, where the switch is interposed transversely in the gap. 
   2. Two co-linear waveguides separated by a gap, where the switch is a third waveguide of different structure and materials, interposed in the gap. 
   3. A waveguide having a mechanically weakened place along its path, and the weak place breaks at a predetermined optical power throughput. 
   4. An assembly of one of the above switches (1 or 2 or 3) together with additional optical components, protecting against the damaging phenomenon called “Fiber Fuse” in the forward and backward optical power throughput. 
   Version 1: Two co-linear waveguides separated by a gap, where the switch is interposed transversely in the gap, comprising:
         an optical waveguide having an input section and an output section, the two sections are aligned, forming a gap between them, or a pair of opposed surfaces extending transversely through the axes of the waveguide sections, and   a thin, substantially transparent, layer of electrically conductive or semi conducting material disposed between the opposed surfaces of the waveguide sections, the material forming a plasma or breakdown when exposed to optical signals propagating within the optical waveguide with an optical power level above a predetermined threshold, the plasma or breakdown damaging the opposed surfaces sufficiently to render those surfaces substantially opaque, i.e., absorbing and/or scattering the light propagating within the optical waveguide so as to prevent the transmission of such light.   The visible light emitted by the plasma can be detected by a photo-detector and used as an indication that the light intensity passing through the switch exceeds its designed threshold.       

   Version 2: Two co-linear waveguides separated by a gap, where the switch is a third waveguide of different structure and materials, interposed in the gap, comprising:
         an optical waveguide having an input section and an output section, the two sections forming a gap between them where a pair of opposed surfaces extend transversely through the axes of the waveguide sections, and   a short, third, waveguide interposed between the two waveguides, in the gap, composed of materials and structures that will cause melt down and/or destruction of the core when exposed to optical powers above a threshold, rendering a volume substantially opaque, i.e., absorbing, diffracting and scattering the light propagating within the optical waveguide so as to prevent the transmission of such light.       

   Version 3: A waveguide having a mechanically weakened place along its path, the weak place breaks at a predetermined optical power throughput, comprising:
         an optical waveguide, partially absorbing, having an input section and an output section, and   a mechanically weakened spot in the waveguide, that causes a break when exposed to optical powers above a threshold. The optical power, partially absorbed, creates thermal stresses in the waveguide, and at the weak point the stresses are the highest.       

   Version 4: An assembly of one of the above switches (version 1 or 2 or 3) together with additional optical components, protecting against the damaging phenomenon called “Fiber Fuse”, by confining the damaging phenomenon to a short sacrificial waveguide length inside the switch assembly. 
   The switch can be manufactured and used as a discrete component, with connectors on both ends or with splices on both ends. Alternatively, the switch can be built into a system where waveguides lead to and from the switch, without the use of connectors or splices. 
   Switches with threshold powers ranging from a few milliwatts up to about a few watts have been built and tested for threshold-power, insertion loss, return loss, added opacity or power drop, after exposure to threshold and higher powers, timing, endurance and visual (microscopic) inspection before and after damage. 
   The tests included time domain experiments, where switches were exposed to short pulses (down to tens of ns). The switches reacted in the same way as in the continuous-wave case, i.e., a large drop in their transparency when impinged by powers over the threshold. Insertion losses of less than 1 dB and return losses higher than 45 dB were obtained. Additionally, parameters such as broad-spectrum operation of the switch, modulated optical powers at the GHz range and higher, and endurance for hundreds of hours of powers a few dB lower than the threshold, were tested and found satisfactory. 

   
     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 device embodying the present invention. 
       FIG. 2  is a schematic cross-sectional view of a modified optical power switching embodying the invention. 
       FIG. 3  is a schematic cross-sectional view of an optical power-switching device embodying the invention in a connector-like assembly. 
       FIG. 4  is a schematic cross-sectional view of a modified optical power-switching device embodying the invention in a connector-like assembly. 
       FIG. 5  is a schematic cross-sectional view of an optical power-switching device embodying the invention in a ferrule assembly. 
       FIG. 6  is a schematic cross-sectional view of a modified optical power-switching device embodying the invention in a ferrule assembly. 
       FIG. 7  is a schematic cross-sectional view of an optical power-switching device embodying the invention in a bare fiber assembly. 
       FIG. 8  is a schematic cross-sectional view of a modified optical power-switching device embodying the invention in a bare fiber assembly. 
       FIG. 9  is a schematic cross sectional view of the thin layers, conductor only version, in a transverse and angled configuration, in a spliced assembly. 
       FIG. 10  is a schematic cross sectional view of the thin layers, conductor and anti reflection layers version, in a transverse and angled configuration, in a spliced assembly. 
       FIG. 11  is an experimental curve of output power versus input power for a switch having a 30 dBm-input-power threshold. 
       FIG. 12  is an experimental curve of output power versus input power for a switch having 24 dBm-input-power threshold. 
       FIG. 13  is an experimental curve of output power versus time for the switches above. 
       FIG. 14  is an experimental microscopic view of a damaged (opaque) switch with a crater or craters in the core of the waveguide. 
       FIG. 15  is a schematic illustration of a further embodiment of the invention that includes a light detector detecting discharge-emitted light for switch failure detection. 
       FIG. 16  is a schematic illustration of an embodiment of the invention that includes a plurality of switches in one stack, for corresponding waveguides in a stack. 
       FIG. 17  is a schematic illustration of a further embodiment of the invention that includes a plurality of switches in one stack, for corresponding optical fibers in a stack. 
       FIG. 18  is a schematic illustration of a further embodiment of the invention that includes a detector for back-reflected light for switch failure detection. 
       FIG. 19  is a schematic illustration of a further embodiment of the invention that includes PC or APC connectors end connections for the optical switch. 
       FIG. 20  is a schematic illustration of a further embodiment of the invention that includes spliced end connections for the optical switch. 
       FIG. 21  is a schematic illustration of a further embodiment of the invention that includes PC or APC connectors end connections for an optical switch made of high-numerical-aperture fibers. 
       FIG. 22  is a schematic illustration of a further embodiment of the invention that includes SMF-spliced end connections for an optical switch made of high-numerical-aperture fibers. 
       FIG. 23  is a schematic illustration of a further embodiment of the invention that includes an electrical lead interruption at the core area for the detection of switch failure. 
       FIG. 24  is a schematic representation of another embodiment of a switching device according to the invention, which consists of an absorbing waveguide. 
       FIG. 25  is a schematic illustration of a switching device having an absorbing waveguide and a notch. 
       FIG. 26  is a schematic illustration of a protection device against “Fiber Fuse” using a circulator in the device. 
       FIG. 27  is a schematic illustration of a protection device against “Fiber Fuse” using a core area reduction in the device. 
       FIG. 28  is a schematic illustration of a protection device against “Fiber Fuse” using a splitter in the device. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1 , there is shown an optical power or energy switching device  2 , composed of an optical waveguide  4 , e.g., a solid waveguide or a fiber, cut transversely to form two waveguide sections  4 ′ and  4 .″ The waveguide  4  is composed of a central core  6 , in which most of the light propagates, and an outer cladding  8 . Also, the waveguide has an input end  10  and an output end  12 . Interposed between the two waveguide sections  4 ′ and  4 ″, and transversing the path of optical energy propagating from the input end  10  to the output end  12 , is a partially transparent conductive layer  14 . The layer  14  is very thin (only a few atomic layers, typically 1 to 20 nanometers) and is made of an electrically conductive material, preferably a conductive metal such as rhodium, aluminum, gold, silver, chromium or nickel, or a combination or alloy of such metals. 
   Such thin layers of conducting material are known to enhance the electric field strength in their vicinity due to local irregularities of their surface, where the surface irregularities induce field concentration, resulting in lower power needed to create an electrical breakdown, and damage. Such thin nanometric layers may be modeled as a plurality of aggregates of nano-particles (see, e.g., M. Quinten, “Local Fields Close to the Surface of Nanoparticles and Aggregates of Nanoparticles,” Appl. Phys. B 73, 245–255 (2001) and the book “Absorption and Scattering of Light by Small Particles” by C. F. Bohren and D. R. Huffmann, Wiley-Interscience (1998), Chapter 12 [showing strong field enhancement factors (up to 10 5 ) for few-nanometer particles as well as wide extinction spectra for various materials and shapes]. 
   Additional processes can further enhance the breakdown-like, non-linear self-focusing in fibers and avalanche (see, e.g., N. B. Blombergen, “Laser Induced Electric Breakdown in Solids,” IEEE-JQE, Vol. QE-10, No. 3 (1974), pp. 375–386). When the thin layer of conductive material is impinged with optical power exceeding a predetermined threshold, strong electric fields, which can lead to local electrical breakdown, are generated at certain sites (“hot spots”) in proximity with the metal surface. This leads to a visible-light-emitting arc discharge, where plasma is created. The generated plasma greatly increases the absorption of the propagating light, and the energetic discharge creates catastrophic damage at or near the metal surfaces. This damage includes cratered regions in the end surfaces of the waveguide sections on opposite sides of the conductive metal layer. Thus, the waveguide permanently becomes highly scattering or, in other words, highly opaque for the propagating light. This significantly reduces the transmitted optical power. Thus, the device acts as a fast switch for interrupting the power propagation, which occurs as fast as the breakdown is created. The switch remains permanently open due to the damage caused by the energetic breakdown. 
   The switching device offers the following advantages: 
   1. It is broadband and can be applied to all light bands used in optical communication systems, e.g., at wavelengths of 0.8, 1.3 and 1.5 micrometers. 
   2. The resulting damage, such as the craters, permanently blocks the channel that received the excessive power. 
   3. The device&#39;s response is very fast, down to the nanosecond region. 
   4. The visible light, that may be emitted when the damage occurs, can be detected by a monitoring device, to signal when the switch has been exposed to optical power higher than the threshold. 
   One of the most important properties of the switch is its insertion (or transmission) loss. A low insertion loss at the operating powers is desirable, in order to avoid power losses. However, the conducting layer generally absorbs and reflects light. 
   As discussed below, the reflection can be minimized by the addition of anti-reflective layers  16  and  18  on both sides of the conducting layer  14 . The absorption of the conducting layer, however, is an intrinsic property, which cannot be filly eliminated (it absorbs between 3% and 30% of the power). Therefore, the insertion loss at the operating power is not negligible, and may reach approximately 1 dB and or even higher. As opposed to the desirable low insertion loss at the operating powers (below threshold), the switch is required to have a high insertion loss (low transmission) at high powers (above the threshold). This is obtained by the significant and permanent damage to the surfaces adjacent the conducting layer  14 , which significantly increases the loss (reduces the transmission). Typical values of insertion loss after damage occurs are in the range of 10 to 20 dB (namely, leaving only 1%–10% transmission). 
   In order to control the threshold power at which the switch opens, several methods can be used. First, the thickness of the conductive layer  14  may be varied to adjust the threshold. In general, the threshold power decreases with increasing thickness of the conductive layer. However, the insertion loss at the operating power also changes with thickness, (the thicker the layer, the higher the loss). Thus, the use of thickness to adjust the threshold is useful only over a limited range of operating powers. Second, the threshold may be adjusted by using fibers of different core  6 , or mode field diameters. The commonly used fiber in optical communication systems is the SMF-28 single-mode fiber. This fiber has a mode field diameter of approximately 10 micrometers for 1550 nm wavelengths. Other fibers have either smaller or larger diameters. For example, High-Numerical-Aperture (HNA) fibers generally have smaller mode field diameters down to 4 micrometers. Thus, in HNA fibers, the light intensity (power per unit area) is larger than in SMF-28 fibers operating with the same power. Consequently, the power threshold in HNA fibers is lower than that in SMF-28 fibers with the same general structure  2 . Since there are several possible HNA fibers, with different mode field diameters, one can control the threshold power using different types of optical fibers. Moreover, the input and output fibers can still be standard SMF-28 fibers. These can be efficiently fusion-spliced to the HNA fibers or other types of fibers (insertion losses are approximately 0.1 dB per splice). Thus, using different types of fibers, having different mode field diameters, with the same structure  2  can lead to switches having different thresholds and nearly the same insertion loss at the operating powers. The same principle is used for multi-mode fibers having various mode field diameters. Another way of threshold variation is to select a dielectric layer that will decrease the power breakdown level, e.g., a polymeric adhesive layer. 
   As with most optical fiber components, minimal back reflection is desirable in the switching device of this invention. This minimal back reflection may be obtained by a combination of two methods. First, the conductive layer  14  can be deposited on a surface that extends across the optical waveguide at an angle, i.e., not perpendicular to the direction of propagation of the light, thus preventing any back reflection from re-entering the waveguide core, as depicted in  FIG. 2  (as discussed below). The conductive layer may be either a single layer or a layer that is covered on one or on both sides with transparent layers, which can serve as anti-reflective coatings, reducing the optical reflections. The coating layers  16  and  18  are designed to have minimal reflections. The anti-reflective layers  16  and  18  can be composed of the same dielectric material, or of two different materials. Generally, when using the same material, in order to obtain minimal reflection, the thickness of the layers  16  and  18  is unequal; the difference in thickness of the entry layer  16  and the exit layer  18  is due to a phase change of reflections from conducting surfaces as opposed to no phase change of reflections from dielectric material like the silica of the fiber. The coating is then an asymmetric coating, and has a pre-designated input direction, (the device&#39;s properties are different in the backward direction). Each of the anti-reflective coatings  16 ,  18  preferably has a thickness within the range from about 0.1 to about 1.5 micrometer. 
   For certain applications, only one of the two anti-reflective coatings  16  and  18  may be desired. 
     FIG. 2  illustrates a device similar to that shown in  FIG. 1 . However, here the layers  14 ,  16  and  18  are not perpendicular to the direction of light propagation in the waveguide, but rather at an angle  20 . For example, in single-mode optical fibers, e.g., SMF 28, the angle  20  is typically 8 degrees. Thus, an optical reflection  22  from the layer  14  does not propagate backwards inside the waveguide. 
     FIG. 3  illustrates the switch of  FIG. 1  packaged in a connector-like configuration. The device can be packaged in several ways. First, using optical fiber connectors  34 , the device is similar (at least when viewed externally) to two pigtailed fibers, which are connected using connectors  34 . Such a device includes an input fiber; two connectors  34  connected using an adapter  35 , an aligning sleeve  32 , and input and output fibers. However, the difference between the switching device and the standard connector is that either one or both fibers have additional layers  14 ,  16 ,  18  on their matching surfaces. 
   In the illustrated example, two commercially available PC (Physical Contact) connectors  34  (e.g., HPC-SO.66 connector manufactured by Diamond SA, Switzerland), with their respective integral ferrules  30 , are assembled to connect the input waveguide section  4 ′ with the output waveguide section  4 ″. The two waveguide sections  4 ′ and  4 ″ are registered with each other by the adapter  35  having an aligning sleeve  32  that fits over both ferrules  30  and aligns them with each other. The conducting layer  14  and, if needed, the anti-reflective layers  16  and  18 , are deposited on one or both ferrules  30  to perform the switching operation described above. 
     FIG. 4  illustrates the switch of  FIG. 2  packaged in a connector-like configuration. Here, two commercially available APC (Angled Physical Contact) connectors  34  (e.g., HPC-S8.66 connector manufactured by Diamond SA, Switzerland) are assembled to connect the input waveguide portion  4 ′ with the output waveguide portion  4 ″, again using an adapter  35  having an aligning sleeve  32  to register the two waveguide portions  4 ′ and  4 ″. Here, the conducting layer  14  and, if needed, the anti-reflective layers  16  and  18 , are deposited on one or both angled ferrules  38  (an integral part of connectors  34 ) to perform the switching operation. This 8-degree angled arrangement prevents reflections from entering the core areas of the waveguide portions  4 ′ and  4 ″. 
     FIG. 5  describes a ferrule switch inner assembly. Here, two PC ferrules  30  are assembled together, aligned by an aligning sleeve  32 , to connect the input waveguide portion  4 ′ with the output waveguide portion  4 ″. The conducting layer  14  and, if needed, the anti-reflective layers  16  and  18 , are deposited on one or both ferrules  30  to perform the switching operation. Ferrules  30  with fibers are made of, e.g., zirconia, and can be purchased commercially as well as aligning sleeve  32 , and are available in 1.5 and 2.5 mm diameters. The inner assembly is held together, e.g., using an external, spring-loaded casing holding them axially in contact. 
     FIG. 6  illustrates a ferrule switch inner assembly having an angled configuration. Here, two APC (angled) ferrules  38  are assembled, aligned by an aligning sleeve  32 , to connect the input fiber  4 ′ with the output fiber  4 ″. The conducting layer  14  and, if needed, additional anti-reflective layers  16  and  18 , are deposited at an angle on one or both ferrules  38  to perform the switching operation. Here again, the angled arrangement prevents reflections from entering the fiber core area. Ferrules  38  with fibers are made of, e.g., zirconia, and can be purchased commercially as well as aligning sleeve  32 , and are available in 1.5 and 2.5 mm diameters. The inner assembly is held together, e.g., using an external, spring-loaded casing holding them axially in contact. 
     FIG. 7  illustrates a bare fiber switch assembly. It consists of two bare fiber (e.g. SMF 28 of 125 micrometers diameter) lengths  4 ′ and  4 ″, cleaved perpendicularly. The two fiber lengths  4 ′ and  4 ″ are aligned and assembled, using an aligning tube or capillary  24 . Here, the conducting layer  14  and, if needed, the anti-reflective layers  16  and  18 , are deposited on one or both of the opposed end surfaces of the input fiber  4 ′ and the output fiber  4 ″. The assembly is fixed in position using, e.g., a commercially available mechanical envelope such as the “ultra splice” made by the Siemon Company, USA, or a commercially available silicon V-groove made by Orgil Optical Connector, Tel-Aviv, Israel. 
     FIG. 8  illustrates a bare fiber switch assembly having an angled configuration. Here, two angle-cleaved fibers  4 ′ and  4 ″ are aligned and assembled using an aligning tube or capillary  24 , taking care of both linear as well as angular alignment. The conducting layer  14  and, if needed, additional anti-reflective layers  16  and  18 , are deposited on one or both of the input fiber  4 ′ and output fiber  4 ″, on the angled end surfaces. This angled arrangement prevents reflections from entering the core area. The assembly is fixed in position using, e.g., a commercially available mechanical envelope such as the “ultra splice” made by the Siemon Company, USA, or a commercially available silicon V-groove made by Orgil Optical Connector, Tel-Aviv, Israel. 
     FIG. 9  illustrates a thin conducting layer  14  that is the only layer between the two waveguides (or fibers)  6  and performs the switching operation. Also, the layer  14  can be deposited either perpendicular to or at an angle to the light-propagation direction as shown in the drawing. In this case fusion splicing holds the assembly together. After splicing, the splice is re-coated or covered by a commercially available shrinkable polymer sleeve. 
     FIG. 10  illustrates a thin conducting layer  14  placed between two anti-reflection layers  16  (at entrance) and  18  (at exit). Here, all three layers are placed between the two waveguides (or fibers) and perform the switching operation. Here again, the layers  14 ,  16  and  18  can be deposited either perpendicular to or at an angle to the light-propagation direction as shown in the drawing. In this case fusion splicing holds the assembly together. After splicing, the splice is re-coated or covered by a commercially available shrinkable polymer sleeve. 
     FIG. 11  is an experimental curve of the output power versus the input power in one example of the switch. In the example presented here, the thin layer was made of chromium (Cr). Two non-symmetric, anti-reflective layers on both sides of the Cr layer served as anti-reflecting layers. These layers were positioned at the interface between two silica SMF 28 fibers and tested. The experimental results showed a power threshold of approximately 30 dBm (1 Watt), where the output power just before damage occurred was approximately 28 dBm. Also, the output power dropped by approximately 25 dB when the damage occurred, reducing the output power to approximately 0.3% of its original power when the threshold power level was exceeded. 
     FIG. 12  presents another experimental curve of the output power versus the input power in one of the switches. In the example presented here, the thin layer was made of chromium (Cr). Two non-symmetric, anti-reflective layers on both sides of the Cr layer served as anti-reflecting layers. These layers were positioned at the interface between two silica HNA fibers and tested. Here, the experimental results showed a power threshold of approximately 24 dBm (250 mW), where the output power just before damage occurred was approximately 23 dBm. Also, the output power dropped by approximately 20 dB when the damage occurred, reducing the output power to approximately 1% of its original power when threshold power was exceeded. 
     FIG. 13  presents an experimental curve of output power versus time for the switch described in  FIG. 12 , above, (power is given here in relative units). One can clearly see that the power transmitted through the switch increased while the input power increased until the threshold power level was reached. At this point the switch became opaque, and the power out dropped at times in the microseconds range. The switch stays opaque for powers above the threshold. 
     FIG. 14  is an experimental microscopic view of a damaged (opaque) switch. At the instant when the damage occurs, and the output energy drops, visible light is emitted in all directions from the core at the damaged spot. This is mainly due to recombination of ions and electrons in the ionized volume of the core close to the coatings where the crater or craters are developed. Visual (microscopic) inspection after the damage revealed a cratered core, with craters a few microns deep covering substantially all the cross-sectional areas of the core (where the optical power flows). One can see that the crater has similar dimensions to the core, and the large cladding area is not cratered. The outer diameter of the cladding is 125 micrometers, and the core diameter is approximately 10 micrometers, covering about 1% of the total cross-sectional area of the optical waveguide. 
     FIG. 15  is a schematic illustration of a further embodiment of the invention that includes a light detector, such as a photodiode, for detecting discharge-emitted light for switch failure detection. 
   On-line testing and status reporting of switches is a part of the system status design in many systems. There are several methods for status monitoring of the switches described above. First, the input power and output power may be measured, using splitters and power detectors, and used to control the insertion loss parameter. When the insertion loss grows to a pre-determined level, the switch is declared opaque, and is replaced after correcting the malfunctioning channel. This method requires two detectors and a control loop and is relatively expensive. Second, a visible light burst produced by the switch may be detected, using a photo detector, e.g., a commercially available photodiode. After the burst occurrence, the switch is declared opaque. This is depicted in  FIG. 15 . 
   When the switch becomes opaque, light is emitted in all directions from the vicinity of the center of the layer  14  (where the incoming light in the core  6  impinges on layer  14 ). The light emitted in direction  46  traverses the cladding  8  and passes into the light detector  44 , e.g., a photodiode. When the detector  44  emits a pulsed signal in response to the light burst from the plasma (and later the scattered light from layer  14  after the damage), that signal indicates that the switch has been exposed to power higher than the threshold power level and should be replaced after correcting the malfunctioning channel. Two other status-monitoring techniques are illustrated in  FIGS. 18 and 23 , described below. 
     FIG. 16  is a schematic illustration of an embodiment that includes multiple switches. The small dimensions of the switch enable the construction of multiple switches in clusters for use on a plurality of optical fibers or waveguides. In this way one can include spares or replacements in the cluster and replace them remotely. Here multiple (e.g., three) switches  68  are manufactured in a stack  60  of a multiplicity of silica waveguides  66  manufactured on a common substrate (the manufacturing of silica waveguides on silicon substrates is a well established process and offered by many manufacturers, e.g. Lambda-Crossing, Caesarea, Israel, with each waveguide having input light  62  and output light  64 . The conducting layer that forms the switches  68  are located between the input and the output and deposited there as a single or three-layer switch. These switches  68  may serve as operating switches and/or spares to replace a switch that becomes opaque when subjected to excessive power. 
     FIG. 17  is a schematic illustration of an embodiment that consists of plurality of switches  70  in a stack  72  of separate fibers, each having input power  62  and output power  64 . The sacrificial coatings that form the switches  70  serve for a plurality of leads as well as having spares in the proximity for quick change. The input fibers  62  and output fibers  64  may be a commercially available fiber ribbon having multiple fibers in the ribbon, or in spliced configuration, places into plurality of V-grooves on a single wafer. 
     FIG. 18  is a schematic illustration of an embodiment that detects the return (back-reflected) power, which generally changes after the switch turns opaque. The back-reflected light  48  propagates in the core  6  of the input fiber length  4 ′, in a direction opposite that of the input light, and is split by an optical splitter  49  such as a commercially available fiber coupler, a beam splitter, a circulator or other device. The splitter  49  directs a portion of the back-reflected light toward an optical detector  44 . Since the back-reflected light  48  changes significantly when the switch  2  becomes opaque, the switching can be detected by the detector  44 . 
     FIG. 19  is a schematic illustration of an embodiment that includes different end (external) connections for the optical switch assembly. This applies to all the switches described in this patent application and illustrates only the connection to the external world (not the switch itself). Here, the switch  80  is connected through an optical fiber  84  (e.g., SMF) to input and output connectors  82  and  83 . The input light  86  is supplied to the fiber  84  through the connector  82 , whereas the output light  88  is removed from the fiber through the other connector  82 . This configuration is preferred for easy manual replacement. 
     FIG. 20  shows the external connections as a splice-ready assembly where both the input and output fibers  84  (e.g., SMF) are left for future splicing. This applies to all the switches described in this patent application and illustrates only the connection to the external world (not the switch itself). 
     FIG. 21  shows a switch  90  based on a High Numerical Aperture (HNA) fiber. Here, the input ray  86  is connected through a connector  82  to a standard fiber  84  (e.g., SMF), then through a fiber splice  98  to a HNA fiber  92 , which is also used in the switch  90 . After passing through the switch  90 , the light propagates through a HNA fiber  92  to another fiber splice  98  into a standard fiber  84 , and then to an output connector  82 , from which the output ray  88  is emitted. This applies to all the switches described in this patent application and describes only the connection to the external world (not the switch itself). This configuration is preferred for easy manual replacement. 
     FIG. 22  shows a similar switch  90 , based on an HNA fiber, in a splice-ready configuration. This configuration is similar to that shown in  FIG. 21 , has no external connectors, but with splice-ready standard (e.g., SMF) fibers. Here both the input ray  86  and the output ray  88  are connected using splices to standard (e.g., SMF) fibers  84 . This applies to all the switches described in this patent application and illustrates only the connection to the external world (not the switch itself). 
     FIG. 23  is a schematic illustration of a direct electrical method for status monitoring. Here a thin sacrificial conductive layer  52  is deposited as a conductive strip having a width similar to that of the optical fiber core  56 . This layer is deposited across the waveguide or the optical fiber  50 , passing through the fiber core. When optical power exceeding the threshold power is transmitted through the core, the layer  52  is interrupted at the core area  56 , leading to an open circuit or significant increase in the electrical resistance of the layer  52  if it is only partially destroyed. This open circuit or increase in resistance can be detected and thus serves as a signal of high power, exceeding the threshold. The conductive layer  52  may be a metallic sacrificial layer that is deposited in a rectangular shape in the center of the fiber  50  and having a width about the same as the core dimension, e.g., approximately 10 micrometers for a single mode fiber. This layer  52  is interrupted in the core area  56  when exposed to optical powers higher than the threshold power, and this interruption is detected by a circuit  54  having a continuity or resistance detector  58 . The interruption means an opaque switch. 
     FIG. 24  is a schematic representation of another embodiment of a switching device according to the invention, which consists of an absorbing waveguide. There is shown an optical power or energy switching device  60 , composed of a waveguide  4 , e.g., a solid waveguide or a fiber, having an input end  10  and an output end  12 . Interposed between the two portions  4 ′,  4 ″ of the waveguide  4  and transversing the propagation path of optical energy from input end  10  to output end  12 , there is affixed an optical energy-absorbing fiber or waveguide  30 . The core  62  of the fiber  64  may be made of polymer or of doped glasses or of any other partially absorbing material. The fiber or waveguide may be covered, on one or on both sides, with an index-matching layer  66 . When the fiber  64  is impinged by optical energy exceeding a predetermined threshold, the fiber  66  is damaged, e.g., melts, shrinks or breaks, and therefore significantly reduces the transmitted optical energy, thus acting as a switch for interrupting energy propagation. 
     FIG. 25  is a schematic illustration of a switching device  68  having an absorbing waveguide  70  and a notch. The switching device  68  is composed of an optical waveguide  70  having a core  72  and cladding  74 . The waveguide  70  can be composed, for example, of doped silica or polymers. The waveguide has an input end  10  and output end  12 , and is pre-stressed, e.g., by bending it into any desired configuration. At or adjacent to the region of maximum stress, the cladding  74  is weakened, e.g., by a groove or notch  78 . When optical energy is introduced into the fiber, the core heats up. When the optical energy propagating through the fiber exceeds a predetermined limit, the core  72  and cladding  74  break at or near the notch  78 . Such breakage separates the input core portion from the output core portion, thus preventing the optical energy from propagating to the output end  12 . 
     FIG. 26  describes a switch assembly  116  that is able to protect a device or detector at high threshold powers. A typical phenomenon occurring in fibers carrying high powers (about of 3×10 6  watts/cm 2  in the core) is the “Fiber Fuse” effect where due to the interaction of the high power incoming light with the back reflected light from a perturbation in the fiber, the fiber is disrupted, starting from the perturbation and extending back into the input source. The fiber is useless, catastrophically damaged, after a “Fiber Fuse” has passed through it. This phenomenon is responsible for destruction of high power fibers, and it is prevented by the switch assembly  116 . Optical power comes in through fiber  106 , e.g., a silica single mode fiber, into an optical isolator  108  (a one-way optical switch, commercially available). After leaving the optical isolator  108  through fiber  110 , the light impinges on a safety switch  112 . During normal operation the switch  112  will perform as follows: In the case that the power is lower than the threshold power of the switch  112 , the light continues into the fiber  114  uninterrupted. In the case that the power is above the threshold, the switch  112  turns opaque and scattering, thus preventing the light from proceeding onto the fiber  114 . In the cases where the power is high enough to enable the “Fiber Fuse” to occur, the switch  112  is designed to have a power threshold just below the “Fiber Fuse” minimal power, and will turn opaque and scattering under the “Fiber Fuse” power, saving fiber  114  from damage. In the cases that “Fiber Fuse” starts at point A on fiber  110  and proceeds in the direction of the black arrow toward point B, where it is stopped by the isolator  108 , saving the fiber  106  from damage. The entire switch assembly  116  is replaced after the event, but the transmission fiber  114  as well as the source through fiber  106  are safe. The safety switch  112  may be any known safety switch, including those described herein. 
     FIG. 27  illustrates an alternative way to protect against a “Fiber Fuse.” Here the optical power enters the system through a fiber  106  (e.g., single-mode fiber SMF28 having a core or a MFD-Mode Field Diameter of about 10 μm) into a core diameter reduction box  118 , shown enlarged in the figure, where the fiber  106  having a core  122  is spliced to a fiber  120  having core  128 . The core  128  is smaller than the core  122  (e.g., HNA fiber having a core or MGD of about 5 μm), and the splice contains a conical transition  124  connecting the large and the small cores. The light leaves the core diameter reduction box  118  via fiber  120 , which has a small core diameter, entering the safety switch  112 . During normal operation the switch  112  performs as follows: In the case that the power is lower than the threshold power of the switch  112 , the light continues into the fiber  132  unperturbed. In the case that the power is above the threshold, the switch  112  turns opaque and scattering, thus preventing the light from proceeding onto the fiber  132 . In the cases where the power is high enough to enable the “Fiber Fuse” to occur, the switch  112  is designed to have a power threshold just below the “Fiber Fuse” minimal power, and turns opaque and scattering just below the “Fiber Fuse” power, saving the fiber  132  from damage. In the cases that “Fiber Fuse” starts, at point A on fiber  120  and proceeds in the direction of the black arrow toward point B, it is stopped at the point  124  where the core gets larger, the power per cm 2  gets lower and the fiber fuse cannot proceed further, thus saving the fiber  106  from damage. The entire switch assembly  130  is replaced after the event, but the transmission fiber  132  as well as the source through fiber  106  are safe. The safety switch  112  may be any known safety switch, including those described herein. 
     FIG. 28  illustrates another way to protect against a “Fiber Fuse”. Here the light power enters the system through a fiber  106  (e.g., single-mode fiber SMF 28 having a core or a MFD of about 10 □m) into a splitter or coupler box  134 . The splitter box  134  contains the entrance fiber  106 , connected at a core-to-core, through the clad, splice to a fiber  136 , as shown in the drawing. The light impinging through fiber  106  on the splitter made of fiber  106  and fiber  136 , leaves the splitter at lower power because part of the power (e.g., 1–10%) is channeled to the fiber  136  and absorbed in a beam dump or terminator  138 . After leaving the splitter box  134  through the fiber  110 , the light impinges on the safety switch  112 . During normal operation the switch  112  performs as follows: In the case that the power is lower than the threshold power of the switch  112 , the light continues into the fiber  114  uninterrupted. In the case that the power is above the threshold, the switch  112  turns opaque and scattering thus preventing the light from proceeding onto the fiber  114 . In the cases where the power is high enough to enable the “Fiber Fuse” to occur, the switch  112  is designated to have a power threshold just below the “Fiber Fuse” minimal power, and turns opaque and scattering just under the fiber fuse power, saving the fiber  114  from damage. In the cases that “Fiber Fuse” starts at point A on the fiber  110  and proceeds in the direction of the black arrow toward point B, it is stopped by the splitter box  134 , where the core gets larger due to the added area of the splitting fiber  136  and some power goes into the beam dump  138  and is absorbed there, the power per cm 2  gets lower at the splitting point and the fiber fuse cannot proceed further, thus saving the fiber  106  from damage. The entire switch assembly  140  is replaced after the event, but the transmission fiber  114  as well as the source through fiber  106  are safe. The safety switch  112  may be any known safety switch, including those described herein. 
   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.