FIELD OF THE INVENTION 
     The present invention relates to controlling power levels of optical signals, and more particularly to controlling power levels of optical signals in optical fibers. 
     BACKGROUND OF THE INVENTION 
     Optical fibers are often used in conjunction with high power lasers, such as for laser welding and cutting, or medical applications, and the art has developed approaches for enhancing the safety of these systems. For example, U.S. Pat. No. 4,449,043 to Husbands discloses a safety device for a high power fiber optic system, which may present a hazard when an optical connector is unmated. The safety device includes a four-port optical coupler which transmits, to a receiver, a portion of the output power, as well as backscattered energy, which is developed between the glass-to-air and air-to-glass interfaces between adjacent connectors. A comparison between the output power and the backscattered energy is used to disable the laser source when an unmated condition is detected. 
     U.S. Pat. No. 4,543,477 to Doi et al. discloses a safety device for a medical laser wherein reflected laser light is detected from the exit end surface of a fiber and a shutter is used to stop the laser if a breakage of the fiber is detected. Ortiz Jr., in U.S. Pat. No. 4,812,641, discloses a high power laser for material processing and includes respective photodetectors to sense the laser power exiting a power optical fiber and the laser injection power. The two power levels are compared to detect whether a break in the power transmitting fiber has occurred. U.S. Pat. No. 4,673,795, also to Ortiz Jr., discloses an interlock safety arrangement which includes an optical sensor connected to the controller for turning the laser off when the laser beam has turned on but laser energy does not reach a remote module, indicating a break in the high power transmitting optical fiber. 
     The described prior art discloses that the power levels before and after the optical fiber are measured and compared. Such an approach leads to disadvantages of complexity at the output end of a fiber and, therefore, additional cost of fabrication and difficulty of operation. Such an approach also causes a loss of optical energy that would have otherwise been delivered to a receiver at output end of the fiber. 
     SUMMARY OF THE INVENTION 
     The present invention presents an approach for transmitting high power optical signals through fibers while helping keep the operation safe under regulatory requirements. In the inventive approach, a breakage in the fiber causing a leak of the optical signal outside the fiber is detected and the source of the optical signal is caused to lower the power of the optical signal being fed into the fiber. In the inventive approach, inherently risky power levels generated by the source of optical signals are detected and the source is caused to lower the power of the optical signal. 
     The inventive approach keeps simple the output end of the fiber transmitting the optical signal and, therefore, lowers the fabrication cost and simplifies operation. It also does not obtain information from the input end of the fiber and therefore conserves the optical energy for transmission into the fiber. Moreover, because of its simpler arrangement, the inventive approach has higher reliability in determining whether the optical fiber is broken. 
     The present invention achieves the above mentioned advantages by using a fiber optical interconnection structure that at least has an optical fiber, a transmitter arranged to transmit an optical signal into a first end of the optical fiber, and a controller arranged to control the transmitter based on the power of the optical signal coming out of the other end of the fiber. However, the controller does not get information from the transmitter about the power of the optical signal being transmitted to the first end of the fiber. Rather, the controller gets information about the power of the optical signal coining out of the fiber. In an embodiment, the controller causes the transmitter to lower the power input into the optical fiber if the power coming out of the fiber is below an expected threshold amount. This protects the environment from optical signals leaking out of broken fibers and potentially harming individuals. In another embodiment, the controller causes the transmitter to lower the power input into the optical fiber if the power coming out of the fiber is above an expected threshold amount. This protects the system by preventing a runaway situation wherein the optical signal source uncontrollably increases the generated power of the optical signal. Of course the two embodiments may be used together in one system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention will become apparent upon reading the detailed description, and upon reference to the drawings in which: 
     FIG. 1 is a general block diagram illustrating an exemplary embodiment of a fiber optical interconnection structure according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a general block diagram illustrating a fiber optical interconnection structure  10  according to a preferred embodiment of the present invention. As shown, the fiber optical interconnection  10  includes a transmitter  11 ; a first optical fiber connected to the transmitter  11  at one end and connected to a receiver  17  (not necessarily part of the optical fiber interconnection) at the other end; a second optical fiber  16  branching off of the receiver-end  15  of the first optical fiber  12  and connecting to an optical detector  13 ; and a controller  14  connected to the optical detector  13  and controlling the transmitter  11 . It should be noted that the controller  14  does not receive information from the transmitter  11  about the power of the optical signal at the point of its transmission into the first end of the fiber. 
     The transmitter  11  is the element that inputs the optical signal to the first (main) optical fiber  12 . The transmitter  11  includes optics used to focus or direct, or both, the optical signal into the first optical fiber  12 —the optics used may be refractive (e.g., lenses) or reflective (e.g., mirrors), or a combination. In an implementation, the transmitter  11  includes the laser used to generate the optical signal along with the elements impressing the signal modulation onto the laser irradiation. In this implementation, the laser can be of any type including, but not limited to, solid state, gaseous, and semiconductor lasers. The laser may be in the form of an oscillator or an oscillator and amplifier(s). The wavelength of the laser may be whatever is necessary or appropriate for the specific application, including but not limited to ultra-violet, visible, infrared, and far infrared spectra. In an alternative implementation, the transmitter  11  receives the optical signal by way of a fiber optical coupling, thus allowing the generation of the optical signal at some location other than the transmitter  11  and transferring the optical signal to the transmitter  11 . 
     The first optical fiber  12  is the medium by which the optical signal is carried to a receiver  17 . The characteristics of the first optical fiber  12  are predetermined including the length, optical power rating, and attenuation coefficient as a function of optical wavelength. Generally a fiber material is chosen that has the lowest attenuation for the wavelength of the optical signal being transmitted in the first optical fiber  12 . 
     The end  15  of the first optical fiber  12  has an optical splice tapping a small fraction (e.g., {fraction (1/1000)}) of the optical signal into the second optical fiber  16 . The end  15  of the first optical  12  is very near the receiver  17 . The receiver end  15  of the optical fiber interconnect  10  allows for a direct optical connection with a receiver  17 , as is known in the art of optical interconnection. The second optical fiber  16  could be implemented using the same material used for the first optical fiber  12 , but need not have as high an optical power rating as the that of the first optical fiber  12 . The first and second optical fibers  12  and  16  can be multi-mode supporting or single-mode supporting fibers. The first and second optical fibers  12  and  16  can be implemented as single-strand or multi-strand fibers. 
     The second optical fiber  16  is optically connected to an optical detector  13 . In one implementation, the optical detector  13  includes a photo-sensitive element (e.g., a PIN photo-diode) and a power measuring element to measure the optical power based on the signal obtained from the photo-diode. In an alternative implementation, the optical detector  13  does not include the power measuring element—in this implementation, the controller  14  would include the power measuring element. The photo-sensitive element choice is driven by a desire to tap off as little of the optical power from the first optical fiber  12  as possible. 
     The optical detector  13  is operatively connected to the controller  14 . The controller  14  receives a signal resulting from the optical detector  13  detecting the power of the tapped optical signal. The controller  14  includes a decision making algorithm that uses information about the characteristics of the first and second optical fibers  12  and  16  (including lengths, attenuation coefficient(s), and the portion of the optical power being tapped into optical fiber  16 ) and a signal from the optical detector  13  to obtain an indication of the power of the optical signal input by transmitter  11  into optical fiber  12 . In dBs, the output power from a fiber equals the input power to the fiber minus the attenuation losses (the product of the length of the fiber and the fiber&#39;s attenuation coefficient) minus the insertion losses (obtained by calibrating the insertion couplings and the optical tap at the receiver end  15  of first optical fiber  12 ). The decision making algorithm also compares the obtained power indication with a first parameter representing an expected desirable optical power input into fiber  12  and a second parameter representing an expected maximum optical power input into fiber  12 . 
     The controller  14  causes the transmitter  11  to lower the input optical power to a predetermined level (set by regulatory and safety standards, including shutting down the laser) if the controller  14  determines that the indicated optical power input to the first fiber  12  is below the first parameter by a specific threshold. Note that this can occur only if the input optical power is somehow not causing an indication of optical power at the controller  14  and, therefore, the appropriate risk reducing assumption is that there is a breakage in the first optical fiber  12 . Other events (e.g., a breakage in the second optical fiber  16 , a malfunctioning of the optical detector  13 , . . . etc) may lead to the same result thus yielding false positives. The inventive approach herein presented, however, uses only one optical fiber  16  to ultimately provide information to the controller  14  and therefore would yield fewer false positives. If, for any reason, the controller  14  does not receive a signal from the optical detector  13 , then it concludes that the first optical fiber  12  has a breakage. 
     The controller  14  also causes the transmitter  11  to lower the input optical power to a predetermined level (including controlling or adjusting the operation of the laser to lower and safer power levels, or shutting down the laser) if the controller  14  determines that the indicated optical power input to the first fiber  12  is above the second parameter by a specific threshold. Note that this can occur if the laser operation is yielding undesirably high optical power levels. Lowering the optical power in this case protects the interconnect equipment (e.g., the transmitter  11 , including the laser if part of the transmitter  11 , the optical fibers  12  and  16 , and the optical detector  13 ) in addition to a would be receiver  17  from the potential harm of unexpectedly high optical powers. Lowering the optical power in this case also protects individuals from potential harm. 
     The first and second parameters and the comparison thresholds can be changed and are settable by operators based on the specifics of the application and/or regulatory requirements. 
     In one implementation, the decision-making algorithm of the controller  14  is software in a processor. Alternatively the decision-making algorithm could be implemented by hardware including digital or analog circuits, digital signal processors, or programmable logic arrays, or combinations thereof including software. 
     The controller  14  causing the transmitter  11  to lower the input optical power can be achieved in one implementation wherein the controller  14  directly influences the operation of the transmitter  11 , e.g., by interposing a shutter in front of the transmitter  11 , thus reducing partially or blocking completely the optical signal being input into the first optical fiber  12 . Alternatively, a shutter can be interposed inside the laser cavity thus stopping the lasing action and consequently shutting down the laser. Alternatively, a variable attenuator in the laser cavity, or outside of it, can be controlled thus affecting the output optical power. Alternatively the electrical power fed to the optical pumps of the laser can be controlled thus affecting the output optical power. In another implementation, lowering of the input optical power into the first optical fiber  12  can be achieved by the controller  14  indirectly influencing the transmitter  11  (e.g., by sending a parameter to which a processor in the transmitter  11  responds) to perform any of the actions mentioned in this paragraph. 
     The embodiment of the invention, as described above with respect to FIG. 1, presents the optical fiber interface  10  without including the receiver  17 . The receiver end  15  of the optical fiber interconnect  10  allows for a direct optical connection with a receiver  17 , as is known in the at of optical interconnection. Another advantage of the embodiment described with respect to FIG. 1 is that of flexibility and interchangeability: Keeping simple the receiver end  15  of the optical fiber interconnect  10  makes both the optical fiber interconnect  10  and the receiver  17  easily replaceable. For example, an optical fiber interconnect  10  can be used with different receivers  17 . Conversely, a receiver  17  can be used with different optical fiber interconnects  10 . 
     An alternative embodiment of the invention is a system that includes the optical fiber interconnect  10  and the receiver  17 . 
     In the embodiments according to this invention, the receiver  17  could be a free space laser communication transmitter. The receiver  17  could also be an output unit implemented as a laser cutting, heating, imaging, printing, or welding instrument. The receiver  17  could also be a medical instrument used in surgical procedures involving laser burning, cauterizing, cutting, or scarring. 
     Although the present invention has been described in considerable detail with reference to certain embodiments, it should be apparent that various modifications and applications of the present invention may be realized without departing from the scope and spirit of the invention. 
     Scope of the invention is meant to be limited only by the claims presented herein.
In the method and apparatus for providing an optical fiber interconnect, a transmitter transmits an optical signal through an optical fiber. The transmitter does not transmit to a controller, information about the power of the transmitted optical signal near the input end of the fiber. The controller receives an indication of the power of a returned portion of the transmitted optical signal. The controller causes the lowering of the power of the transmitted optical signal to a predetermined level based the received indication.