Abstract:
In the method and apparatus for controlling the power level of a laser signal in free space communication, a communication terminal transmits an output laser beam into free space and also receives information, through a channel that is not free space laser based, about the power of the output laser beam measured at a distance and at different times. The terminal determines whether a drop in the power of the output laser beam measured at the distance is due to atmospheric effects based on the received information. The terminal increases the power level of the output laser beam to a desired level if the power drop is determined to be due to atmospheric effects. On the other hand, the terminal lowers the power of the output laser beam to a predetermined level if the power drop is determined to be due to blockage thus avoiding harm to accidental observers that might intrude into the path of the laser beams.

Description:
FIELD OF THE INVENTION 
     The present invention relates to communication systems, and more particularly to free space laser communication systems. 
     BACKGROUND OF THE INVENTION 
     Free space, point-to-point communication systems are used extensively in the communications field. A network of point-to-point microwave systems can carry messages across the country as part of the public switched telephone network. Despite strong competition from fiber optic based communications systems, microwave or other free space systems are often justified for shorter routes, when right-of-way for a cable system is not available, or when the high communications capacity of a fiber optic system is not needed. 
     Laser communication systems in particular have become increasingly popular to provide a free space communications link between two locations. Laser systems do not require extensive frequency coordination as do microwave systems in common frequency bands. Moreover, free space laser communication systems often are less expensive to install than either a conventional copper cable system or a fiber optics based system because physical installation of a cable or a fiber is unnecessary. For example, a laser communication system may have application between two corporate locations in a campus environment. Laser communication terminals may be positioned on building rooftops or adjacent windows and aligned to operate between buildings. Moreover, general progress in society is accompanied by increases in the amount of available information and, consequently, increased need for broader bandwidth communication systems. Accordingly, the demand for free space laser communications links is increasing. Such links can be used for communicating a variety of data forms including voice, video, and text. 
     Free space laser communication systems are considered stationary laser sources for governmental regulatory purposes. They must, therefore, comply with regulatory limits established to protect accidental observers. Exposure to high power laser beams may harm accidental observers. The harm by lasers used in free space communication can be compounded because (1) exposure to some laser wavelengths does not cause any pain that might otherwise warn one of exposure to the laser, and (2) some laser wavelengths are not visible. 
     Accordingly, standards have been put in place that establish safe limits for the power that may be transmitted by a stationary laser source, such as a laser communication terminal. These permissible power limits affect the communication system&#39;s signal-to-noise ratio, operational bit rate, and/or useful distance coverage. To avoid deleterious effects on data communication while protecting accidental observers, U.S. Pat. No. 5,229,593 to Cato discloses an approach that runs the laser power of a free space laser communication system at regulatory established safe levels when exposure or misalignment is determined, otherwise the approach runs laser power at above the safe levels. 
     The approach disclosed by Cato uses a laser transmitter, laser receiver, and a controller to control the output laser power. In the disclosed approach, the received laser signal must contain a confirmation signal that a receiver down stream has received the transmitted laser signal if output laser power is to be maintained at above safe levels. The disclosed approach, however, suffers from certain disadvantages including not addressing atmospheric effects that might reduce received laser power as opposed to an accidental observer partially blocking the laser path and causing a reduction in the received laser power. The disclosed approach, moreover, does not address needs, implementations, or applications for one way laser communication since the disclosed approach requires a return laser signal carrying confirmation information. 
     SUMMARY OF THE INVENTION 
     The present invention presents an approach for free space laser communication that determines whether a reduction in received laser power is due to blockage or whether the reduction is due to atmospheric effects. The inventive approach allows for one-way and two-way laser communication. The inventive approach also allows for controlling the laser communication without suffering from line of sight or atmospheric effects. 
     The present invention achieves the above mentioned advantages by using a communication terminal that at least has (1) a laser transmitter that transmits a laser output beam; (2) a receiver that receives (by a communication channel that is not free space laser based) information about the power of the output laser beam measured at another communication terminal; and (3) a controller that controls the power of the output laser beam based on whether a reduction in received laser power is due to atmospheric effects or blockage. The receiver may be implemented as one way communication, or may be implemented as a transceiver allowing two way communication; the receiver/transceiver may communicate using broadcasting, telephone, limited bandwidth fiber communication, or any other non-free space laser communication approaches. 
     The present invention also achieves the above mentioned advantages by a related approach using a communication terminal that at least has (1) a laser transmitter that transmits a laser output beam; (2) first receiver that receives an input laser beam; (3) a second receiver that receives (by a communication channel that is not free space laser based) information about the power of the output laser beam measured at another communication terminal; and (4) a controller that controls the power of the output laser beam based on whether a reduction in received laser power is due to atmospheric effects or blockage. The second receiver may be implemented as one way communication, or may be implemented as a transceiver allowing two way communication; the receiver/transceiver may communicate using broadcasting, telephone, limited bandwidth fiber communication, or any other non-free space laser communication approaches. 
     The controller of a communication terminal according to the present invention uses information about the power of the transmitted laser beam that is received at a distant terminal, or information about the power of the laser beam input to the communication terminal, or both, to determine whether reduction in power of the transmitted laser received at some distance is due to atmospheric effects or blockage. The controller determines the nature of a reduction in the power of a laser beam using inherent differences in the amount and speed of a reduction in a laser power due to atmospheric effects and blockage. Generally, atmospheric effects occur at a time scale of the order of 10 milliseconds whereas blockage occurs at a time scale of the order of 100 milliseconds. The controller increases power of the output laser beam to above levels deemed safe to an accidental observer if it determines that a reduction in received laser power is due to atmospheric effects. On the other hand, the controller reduces power of the output laser beam to levels deemed safe to an accidental observer if it determines that the reduction in received laser power is due to blockage. 
    
    
     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 communication terminal according to the present invention. 
     FIG. 2 is a general block diagram illustrating another exemplary embodiment of a communication terminal according to the present invention. 
     FIG. 3 is a general block diagram illustrating a further exemplary embodiment of a communication terminal according to the present invention. 
     The same numbers will be used to label identical features in different figures. 
    
    
     DETAILED DESCRIPTION 
     The detailed description will first describe the structure and general operation of the embodiments of the communication terminals according to the present invention. The detailed description will next describe the methods used to determine whether a reduction in received power of a free space laser beam is due to atmospheric effects or blockage. 
     Structural Description of Embodiments 
     FIG. 1 is a general block diagram illustrating a communication terminal  10  according to a preferred embodiment of the present invention. As shown, the communication terminal  10  includes a transmitter  11 , a receiver  12 , and a controller wherein the transmitter  11  and the receiver  12  are both operatively connected to the controller  13 . 
     The transmitter  11  is a free space laser transmitter outputting laser beam(s) into free space. In one implementation, the transmitter  11  includes laser(s) that generate the output laser beam(s)—lasers being 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). In an alternative implementation, the transmitter  11  receives the laser beam(s) to be output by way of a fiber optical coupling allowing the generation of the laser beam(s) at some location other than the transmitter  11  and transferring the laser beam(s) to the transmitter  11 . The wavelength of the laser beam(s) may be whatever deemed necessary or appropriate for the specific communication, including but not limited to visible, infrared, and far infrared spectra. In one implementation, the transmitter  11  includes optics used to collimate or direct, or both, the output laser beam(s) onto a targeted receiving terminal—the optics used may be refractive (e.g., lenses) or reflective (e.g., mirrors), or a combination. The transmitter  11  includes elements impressing the communication modulation onto the output laser beam(s) if the transmitter  11  includes the laser oscillator (or the laser oscillator/amplifier(s)). 
     The receiver  12  receives information about the power of the output laser beam (transmitted by communication terminal  10 ) detected at a target communication terminal (not shown in FIG. 1) placed at some distance from the terminal  10 . The channel through which this information is received at the receiver  12  is not the free space laser communication channel. Rather, in one implementation the communication channel through which the receiver  12  receives the information is a broadcasting channel. Alternative implementations of the non-free-space-laser channel include, but are not limited to, telephone and limited bandwidth fiber communication. The non-free-space-laser channel could be implemented in the radio, microwave, or optical spectra. Such implementations avoid loading up the bandwidth of the free space laser communication; simplify decoding of data; and do not suffer from environmental (e.g., atmospheric) or physical (e.g., blockage) obstacles to delivery of information to the communication terminal  10  that is necessary for the proper operation of the terminal  10 . In one implementation, the data packet conveying the information includes a single value representing the power received at a specific interval. In an alternative implementation, the data packet conveying the information includes a string of values representing a series of powers received at specific intervals. The data packet includes information identifying the target communication terminal that generated the information received at the receiver  12  if the terminal  10  is operating in a network environment having a plurality of potential target terminals. The terminal identifying information may be, but not necessarily, dispensed with if the communication system is a link consisting of only two terminals according to implementations of this invention. Furthermore, in one implementation, the receiver  12  processes the information it receives. In an alternative implementation, the receiver  12  transfers the information to the controller  13  for processing by the controller  13 . 
     The controller  13  of FIG. 1 is operatively connected to both the transmitter  11  and the receiver  12 . The controller  13  of the terminal  10  is the “brain” containing the decision-making algorithm that processes the information received by the receiver  12 . Based on the outcome of the processed information, the controller  13  determines whether a drop in the power received at a target terminal is due to atmospheric effects or due to blockage. The controller  13  makes the determination using information received from the receiver  12 . The determination will be discussed in detail below in the section titled “Tests Distinguishing Between Atmospheric Effects and Blockage.” In one implementation, the decision-making algorithm of the controller  13  is a 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. 
     In one implementation, the controller  13  directly influences and controls the transmitter  11  by way of hardware in the controller  13  that increases/decreases output laser beam power. In an alternative implementation, the controller  13  indirectly influences and controls the transmitter  11  by way of outputting a signal to which hardware in the transmitter  11  responds. 
     If the controller  13  determines that atmospheric effects caused received power disruption, then the controller  13  takes steps (direct/indirect) to increase the output laser beam power to a desired level. The desired output laser power is the power necessary for the proper reception at the receiving communication terminal. The necessary power may very well be unsafe for accidental observers but should not present a risk of harm because the controller  13  has determined that atmospheric effects caused the received power disruption and not something physically blocking the output beam path. 
     On the other hand, if the controller  13  determines that blockage caused received power disruption, then the controller  13  takes steps (direct/indirect) to decrease within the time intervals set by regulatory limits the power of the output laser beam to a level deemed safe for accidental observers. Assuming that an accidental observer is interfering with the path of output laser beams when the controller  13  determines that blockage is the cause of reduction in received power is an assumption rendering the free space laser communication approach of this invention safe for use under governmental regulations. The average power deemed safe for exposure generally depends on the wavelength of the laser and may be changed based on changing regulatory concerns. 
     The preferred embodiment described above with respect to FIG. 1 allows for the one-way communication between the communication terminal  10  and other communication terminals—the terminal  10  transmitting data through free space laser communication and the other terminals receiving the transmitted data. 
     FIG. 2 is a general block diagram illustrating a communication terminal  20  according to another preferred embodiment of the present invention. As shown, the communication terminal  20  includes a transmitter  11 , a first receiver  12 , a second receiver  24 , and a controller  23 . The transmitter  11 , the first receiver  12 , and the second receiver  24  are operatively connected to the controller  23 . The transmitter  11  and the first receiver  12  are as in the embodiment described with respect to FIG.  1  and will not be further described with respect to FIG.  2 . 
     The second receiver  24  receives input laser beam(s) from free space laser beam(s) transmitted by other communication terminals forming the communication link or network. The second receiver  24  includes detector(s) sensitive to the wavelengths of the input laser beam(s). In one implementation, the second receiver  24  includes optics used to collimate or direct, or both, the input laser beam(s) onto the detector(s)—the optics used may be refractive (e.g., lenses) or reflective (e.g., mirrors), or a combination. Furthermore, in one implementation, the second receiver  24  includes elements that separate, obtain, or deconvolve the communicated data from the input laser beam. In an alternative implementation, the second receiver  24  includes means for transferring the signal generated (because of the detected input laser beam) to signal processors, which then obtain the data impressed on the input laser beam. Moreover, in one implementation, the second receiver  24  includes detectors that measure the power of the received input laser beam. 
     The controller  23  has all of the features of controller  13  and in addition, in an implementation in which second receiver does not obtain the power of the input laser beam, processes and obtains the power of the input laser beam. Furthermore, in an implementation in which the second receiver does not obtain the communicated data impressed on the received input laser beam, the controller processes and obtains the communicated data impressed on the received input laser beam. The controller  23  uses information received from the first receiver  12  or the second receiver  24 , or both, to determine whether a drop in the power received at a target terminal is due to atmospheric effects or due to blockage. The determination will be discussed in detail below in the section titled “Tests Distinguishing Between Atmospheric Effects and Blockage.” 
     The preferred embodiment described above with respect to FIG. 2 allows for one-way (link arrangement) and two-way (network arrangement) communication between the communication terminal  20  and other communication terminals at some distance from the terminal  20 . 
     FIG. 3 is a general block diagram illustrating a communication terminal  30  according to a further preferred embodiment of the present invention. As shown, the communication terminal  30  includes a transmitter  11 , a transceiver  32 , a receiver  24 , and a controller  33 . The transmitter  11 , the transceiver  32 , and the receiver  24  are operatively connected to the controller  33 . The transmitter  11  and the receiver  24  are as in the embodiments described with respect to FIGS. 1 and 2, respectively, and will not be further described with respect to FIG.  3 . 
     The transceiver  32  has the features of receiver  12  but in addition has a transmitter allowing the transmission of information about the power of the input laser beam that the terminal  30  receives (at the receiver  24 ). Transceiver  32  obtains the information about the power of the input laser beam either from the receiver  24  or from the controller  33 . 
     The controller  33  has the features of controller  23  but in addition calculates the information about the power of the input laser beam and relays it to transceiver  32 . In an alternative implementation, controller  33  just relays the information obtained from the receiver  24  to transceiver  32 . The controller  33  uses information received from the receiver  32  or the receiver  24 , or both, to determine whether a drop in the power received at a target terminal is due to atmospheric effects or due to blockage. The determination will be discussed in detail below in the section titled “Tests Distinguishing Between Atmospheric Effects and Blockage.” 
     The preferred embodiment described above with respect to FIG. 3 allows for one-way (link arrangement) and two-way (network arrangement) communication between the communication terminal  30  and other communication terminals at some distance from the terminal  30   
     In view of the inventive principles disclosed herein, the arrangement of the transmitter  11  and the receiver  24  of the embodiments according to FIGS. 2 and 3 may be implemented as having the transmitter be composed of a plurality of transmitters surrounding the receiver. For example, an implementation has four transmitters with apertures having 3-cm diameter are centered at the corners of 16-cm sided square, and has the receiver with an aperture having a 20-cm diameter centered at the center of the square. On the other hand, the reverse may be used to have a plurality of receivers  24  surround a single transmitter  11 . In another implementation, concentric apertured transmitter  11  and receiver could be used: Either the transmitter  11  surrounding the receiver  24  or vice versa. It is also to be noted that auto alignment techniques may be used to remove the concern of reduction in the power of the received free space laser beam due to misalignments not including a blockage. 
     Tests Distinguishing Between Atmospheric Effects and Blockage 
     For the embodiments described above, the controllers ( 13 ,  23 ,  33 ) use a decision-making procedure that determines whether one or more of the following conditions occur: 
     1. Low-signal Test 
     This test applies to embodiments described with respect to FIGS. 1,  2 , and  3 . A controller  13 ,  23 ,  33  determines whether the power of an output laser beam transmitted by the transmitter  11  of the terminal  10 ,  20 ,  30  that is received at some distant communication terminal is too low. Note that the terminal  10 ,  20 ,  30  receives at the receiver  12 ,  32  information about the power of the laser beam received at the distant communication terminal. The controller  13 ,  23 ,  33  determines that blockage is the cause of a change in the received power if the received power is too low. The rationale being that the transmitter  11  of the output laser beam transmitting terminal  10 ,  20 ,  30  is blocked, or the receiver  24  of a distant receiving communication terminal is blocked, or both are blocked. 
     For example, with a system having an operation frequency of 60 Hz, the test may be performed on 0.8 seconds of data every 0.2 seconds. In this case, a running average of 48 data points is obtained and blockage is determined if this average is below some predetermined value (e.g., 1 dBm). 
     The number of data points being averaged, the predetermined value, and the repetition rate of the test depend on the application at hand. 
     Some atmospheric effects can induce a test result appearing as blockage. For example, attenuation of the transmitted laser beam due to fog may result in a test result indicating that blockage has occurred. Other atmospheric effects, e.g., atmospheric scintillation, will not result in attenuation and therefore will not yield a test result appearing as blockage. An atmospheric effect falsely indicating blockage is a false positive test result. A false positive of blockage indication, however, does not increase the risk of accidental exposure—the objective of the test, after all, is avoiding accidental exposures. 
     2. Sudden-drop Test 
     This test applies to embodiments described with respect to FIGS. 1,  2 , and  3 . A controller  13 ,  23 ,  33  determines whether the power of an output laser beam transmitted by the transmitter  11 ,  21 ,  31  of the terminal  10 ,  20 ,  30  that is received at some distant communication terminal suddenly drops by more than a predetermined amount. Note that the terminal  10 ,  20 ,  30  receives at the receiver  12 ,  32  information about the power of the laser beam received at the distant communication terminal. The controller  13 ,  23 ,  33  determines that blockage is the cause of a change in the received power if the change in the received power is sudden. The rationale being that the transmitter  11  of the output laser beam transmitting terminal  10 ,  20 ,  30  is blocked, or the receiver  24  of a distant receiving communication terminal is blocked, or both are blocked. 
     For example, with a system having an operation frequency of 60 Hz, the test may be performed every 0.2 seconds by calculating a short term running average on 0.8 seconds of data and comparing it with calculation of a reference average (e.g., a longer term running average of 10 seconds). In this case, a short term running average of 48 data points is obtained and compared with a longer term running average of 600 data points and blockage is determined if the difference between the short term average and the long term average is greater than a predetermined value (e.g., 1 dB). The averaging may be performed taking into account the power of the output laser beam at the transmitter  11  corresponding to the specific data points. 
     The number of data points being averaged, the predetermined value, and the repetition rate of the test depend on the application at hand. 
     The reference average for this test is chosen to be as long as possible (e.g., 10 seconds) but short enough so that attenuation caused by fluctuations in atmospheric attenuation (e.g. due to fog) does not cause false alarms. 
     The sudden-drop test should generally not be used immediately after (in the first 10 seconds for the above example) emerging from a determination by the controller  13 ,  23 ,  33  using the low-signal test that blockage occurred, since the reference average (i.e., the long term running average) needs to be recalculated. However, if accuracy is not critical, then the long term average just before a determination that blockage occurred may be used. 
     3. Two-end Imbalance Test 
     This test applies to embodiments described with respect to FIGS. 2 and 3. A controller  23 ,  33  determines whether the difference between the signals received by the receiver  12 ,  32  and the receiver  24  is more than a predetermined amount. Note that the communication terminal  20 ,  30  receives at the receiver  12 ,  32  information about the power of the laser beam received at the distant communication terminal, and receives at the receiver  24  the input laser beam. The imbalance in power determination indicates that the transmitter  11  is blocked, or the receiver  24  is blocked, or those corresponding ones of the distant communication terminal are blocked. In the presence of an imbalance, the controller  23 ,  33  determines that a blockage is the cause of a change in the power received. 
     The calculations are similar to the sudden-drop test. For example, with a system having an operation frequency of 60 Hz, the two-end imbalance test may be performed every 0.2 seconds by calculating a short term running average on 0.8 seconds of data, calculating a reference average (e.g., a longer term running average of 10 seconds), and obtaining the difference between the short and long term averages—these calculations being performed for data obtained at the receiver  12 ,  32 . In this case, a short term running average of 48 data points is obtained, a longer term running average of 600 data points is obtained (corresponding to averaging over 10 seconds), and the difference between them is obtained. Similar calculations are performed for data obtained at the receiver  24 . Blockage is determined if the difference between the differences is greater than a predetermined value (e.g., 1 dB). The averaging may be performed taking into account the power of the output laser beam corresponding to the specific data points. 
     The number of data points being averaged, the predetermined value, and the repetition rate of the test depend on the application at hand. 
     An advantage of the two-end imbalance test is that the difference is unaffected by atmospheric effects, which have the same effect on both directions of transmission. Consequently, the two-way imbalance test could be used immediately (during the first 10 seconds in the example above) without sacrifice in accuracy after emerging from a determination by the controller  23 ,  33  that blockage occurred, since the difference of differences is insensitive to atmospheric effects. 
     Table I presents the use of the tests described above as primary and secondary indicators of various blockage situations. In this table, P denotes a primary algorithm and S denotes a secondary algorithm for detecting blockage. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                 Low-signal 
                 Two-end 
                 Sudden-drop 
               
               
                 Intrusion 
                 test 
                 imbalance test 
                 test 
               
               
                   
               
             
             
               
                 Partial blockage of the 
                 S 
                 P 
                 P 
               
               
                 transmitting and/or 
               
               
                 receiving apertures 
               
               
                 Complete blockage of 
                 P 
                 S 
                 S 
               
               
                 all transmitting 
               
               
                 apertures 
               
               
                 Partial blockage of the 
                 S 
                 P 
                 Not available 
               
               
                 transmitting and/or 
                   
                   
                 during the 
               
               
                 receiving apertures in 
                   
                   
                 first 10 
               
               
                 the first 10 seconds 
                   
                   
                 seconds after 
               
               
                 after emerging from a 
                   
                   
                 emerging from 
               
               
                 determination of 
                   
                   
                 a 
               
               
                 blockage. 
                   
                   
                 determination 
               
               
                   
                   
                   
                 of blockage. 
               
               
                   
               
             
          
         
       
     
     As mentioned above, the tests may be individually used. They may also be used in combination to improve accuracy and lower risk. An approach combining the tests is as follows. After a determination that blockage has occurred, the controller  13 ,  23 ,  33  causes the reduction of power of the output laser to regulatorily established safe levels (including but not limited to turning off the laser power), and that it be kept at those levels for a regulatorily established duration (e.g., 100 seconds). After this duration of lower power output laser beams, the power is raised to the value just prior to the determination of blockage. The power raising is accompanied by using the low-signal and two-end imbalance tests for the duration of obtaining the long term running average (e.g., 10 seconds), and then using the sudden-drop test after accumulating the long term running average in addition to the other two tests. 
     Generally, the tests occur at a frequency rapid enough to detect blockage within 1 second of its occurrence—this time element may be changed depending on regulations. 
     In one implementation, the controller  13 ,  23 ,  33  of terminal  10 ,  20 ,  30  causes the reduction of power of the output laser beam to regulatorily established safe levels (including but not limited to turning off the laser power) for as long as the receiver  12 ,  32  does not receive necessary signals. Furthermore, in another implementation, the controller  13 ,  23 ,  33  in this situation causes the generation of alarm signals. 
     In another implementation. The controller  33  of terminal  30  causes the generation (either itself or by transceiver  32 ) and transmission (by transceiver  32 ) of a signal indicating that terminal  30  has determined that blockage occurred-this signal will part of the data packet being transmitted by transceiver  32  and received by the receiver  12 ,  32 . A target communication terminal that is the other part of a link or network communication with the terminal  30 , upon receiving this signal will itself lower the power of its output laser beam. 
     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.