the invention is described in a technical report wl - tr - 93 - 1023 , by gary d . wilkins , entitled &# 34 ; atmospheric transverse coherence length measurements for laser communications &# 34 ;, avionics directorate , wright laboratory , air force materiel command , wright - patterson air force base ohio 45433 - 7409 , with a nominal date of february 1993 ; and in a paper by gary d . wilkins entitled &# 34 ; the diffraction limited aperture of the atmosphere and its effects on free space laser communications &# 34 ; published at pages 1158 - 1163 in vol . 3 of proceedings of the ieee 1992 national aerospace and electronics conference ( naecon ) 1992 , held at the dayton convention center may 18 - 22 , 1992 . both the paper and the technical report are included with this application as filed , and are hereby incorporated by reference . since optical communications relies on the use of wavelengths which are highly influenced by changes in refractive index , the direct relationship between atmospheric turbulence and communications quality was investigated . the reason for doing this was to see if enough information could be gathered by repeated sampling of the r o to allow changes to be made in the communications parameters which would optimize the communications and guarantee maximum information through put . as shown in the block diagram of fig3 a laser communications link over a 5 - mile path was used for measuring the bit error rate ( ber ) of the digital signal , and measuring the r o of the path . the laser communications system for an 8 - kilometer free - space link , comprises a laser transmitter 10 , a receiver 20 , and the required interfaces and the r o measuring equipment in the laboratory with available off - the - shelf components and equipment . the 8 - kilometer pulsed laser communications link has been established between a remote test site and a laser communications lab ( lcl ) on the twelfth floor of a tower . the laser transmitter 10 which comprises digital signal source 12 and a 904 - nm laser 14 are located at the remote site . the digital signal source 12 which operates at 10 , 000 bps , includes a bit error rate encoder . the receiver 20 , with r o , meteorological , and data acquisition equipment , are located in the lcl . since r o measuring equipment and laser receivers are not available off the shelf , they were designed and built in - house . the lcl houses the equipment used for measuring the r o . an in - house designed and built system , the device incorporates the simplicity of the photographic method with the data acquisition capability of the rotating reticle wheel method , without the need for chemicals or mechanical devices . the system consists of a celestron 20 cm aperture telescope 40 which focuses the light from the laser onto a ccd array of a camera 42 . the hardware platform for the system includes the items shown in fig4 . the telescope 40 is a celestron classic 8 schmidt cassegrain telescope . the camera 42 is a videk megaplus 1024 by 1024 ccd array camera . a digital equipment corporation vaxstation ii / gpx computer 50 includes a ) a dual rx50 floppy disk drive , b ) a tk50 tape drive , c ) a rd54 159 megabyte hard disk drive , d ) a rd53 71 megabyte hard disk drive , and e ) a univision digitizer / display controller ( udc - 500q ) with an intel 82786 graphics coprocessor . the items in fig4 also include a vr290 color monitor 410 with a lk201 keyboard 412 , a sampo high resolution , 19 - inch monochrome monitor 420 , and a 904 - nm center wavelength , 10 - nm bandpass interference filter 430 . the image of the fourier transform of the incoming laser light captured by the round aperture telescope 40 , and ccd camera 42 is digitized by a univision udc500q frame grabber residing inside the microvax computer 50 . horizontal and vertical sync pulses are delivered to the camera and digital video is delivered to the udc - 500q controller via a 37 - pin d connector mounted on the udc - 500q . images are displayed on the high resolution sampo monitor 420 connected directly to the udc - 500q video output connector . some computer code required by the frame grabber to digitize the camera output and display the image was supplied by univision as part of the udc500 demonstration package , the only one required to operate the camera being udc500vaxuti . exe . upon execution of the program , the input required from the user is : ______________________________________command description______________________________________c 2 camera type selection ( videk ) t 12 initialize 82786 biu & amp ; dp registerst 18 grab the imaget 17 snap an image______________________________________ command t 18 places the udc500 in a continuous frame grabbing mode with the frame rate being dependent upon the shutter delay and exposure settings on the rear of the videk camera 42 . the image output is displayed on the sampo high resolution ( 1024 by 1024 ) monitor 420 . executing command t 17 then freezes the image for further processing such as determining object size and position on the screen . the videk camera 42 and udc500q display controller were purchased as a set , complete with demonstration software , from univision inc . the software , udc500q , and videk camera displayed the images beautifully on the high resolution ( 1024 by 1024 ) monitor 420 . however , the system had one very serious drawback . it would continuously snap pictures frame after frame without allowing for shutter / exposure control . the camera itself has the following modes of operation which are switch selectable at the rear of the camera : ______________________________________mode description______________________________________0 shutter closed , remote control of the mode lines1 shutter locked open , continuous video output2 shutter locked open , triggered internally3 shutter locked open , manually triggered4 , 5 continuous shutter opening and closing6 shutter and exposure controlled manually7 shutter and exposure controlled by computer______________________________________ modes 5 and 6 cause the camera to continuously snap pictures at the rate of approximately two pictures per second . the length of exposure can be varied from 14 msec to 240 msec using the camera exposure adjustment potentiometer . to properly estimate the amount of time required for the sample exposure , the software was developed to initially accept a guess from the operator and then either increment or decrement by 10 - millisecond steps until an acceptable intensity level was reached . by acceptable intensity level , it is meant that there must be at least 1 pixel , and not more than 100 pixels , on the screen that are white . since the udc500q has the capability of digitizing the image to 256 levels of gray , with 0 corresponding to black and 255 corresponding to white , the computer merely increments until there are enough pixels in the acceptable range of 245 to 255 . increasing the exposure too long can actually turn the whole screen white causing measurement error . this is the reason for putting a limit on the number of white pixels in the acceptable range . any more than 100 pixels means that the ccd array is severely blooming , or the background light is stronger than the laser energy . in that case , the computer resets the initial time to 100 milliseconds and starts reincrementing in order to get to the acceptable level . once an acceptable exposure is achieved , the diameter of the airy disk is measured just above the first spurious disk at an intensity level of 50 . this is shown in the computer generated gray scale sinc function graph of fig5 . the equipment used for the data communications portion of the project consisted of an off - the - shelf laser transmitter , an in - house built receiver and off - the - shelf pattern generator / error detector equipment for measuring the quality of the communications link . the laser transmitter 14 in fig3 is a laser diode , inc . lt - 201 capable of 100 watts peak optical output power , but was operated at 20 watts peak for this experiment . it operates at a room temperature wavelength of 904 nanometers and a beam divergence of 1 milliradian . the transmitter is edge triggered by the return to zero ttl output of a bit error rate encoder operating at a clock frequency of 10 - khz and emits a 40 - nanosecond pulse for each positive transition of the data input signal . a hewlett packard ( hp ) h965245l electronic counter ( not shown ) slaved to an arbiter systems satellite controlled clock provided the 10 - khz synchronized clock . the maximum pulse repetition frequency of the lt - 201 has been determined by laboratory experimentation to be 16 , 800 pulses per second . technical specifications for the lt - 201 , as well as other major equipment used for the receiver and r o measurement system are provided in the appendix of the technical report wl - tr - 93 - 1023 . the laser receiver as shown in fig6 was designed and built in - house . it consists of celestron 90 mm maksutov cassegrain spotter scope 30 , adapted to focus the incoming laser radiation onto a silicon photodiode 32 , housed with a 10 nm bandpass filter 603 in a melles griot modular photodetection system 604 . a melles griot 13amp005 wide bandwidth detector amplifier 605 supplies current to the detector and acts as the first stage amplifier . the signal is then amplified by a stanford research systems model sr560 low noise amplifier 606 , which supplies the needed gain and electronic filtering ( amplifiers 605 and 606 form amplifier 34 of fig3 ). the resulting 40 - microsecond gaussian shaped pulses are converted into ttl level square pulses with a tektronix fg507 function generator 36 and subsequently fed into an hp 3780a bit error rate tester 38 for analysis . a hewlett packard crystal controlled counter 609 , slaved to an arbiter systems inc . 1026b satellite controlled clock 610 , provided the 10 , 000 hz needed by the berts for the synchronous datastream . two hp3780a pattern generator / error detectors were used to measure the quality of the communications link . one hp 3780a was interfaced with the lt - 201 laser transmitter at the remote test site and the second hp 3780a was interfaced with the in - house built receiver in the lcl as seen in fig6 . since the lt - 201 laser transmitter 14 is edge triggered from the leading edge of a ttl pulse , the interface for the transmitter was fairly straightforward since the data output of the hp 3780a can be configured to provide a return to zero , ( rz ), ttl data format . one problem which needed to be overcome , however , was that the internal clock rate of the hp 3780a is 10 mega pulses per second . this greatly exceeds the maximum transmission rate of lt - 201 laser transmitter 16 . 8 kilo pulses per second . to conquer this situation , the 10 - khz output of an hp crystal controlled clock was connected to the external clock inputs of the hp 3780a at both the transmitter and receiver ends of the link . once the output of the receiver was interfaced with the hp3780a in the lcl , and the system was turned on , it became evident that there were still a few more problems to be solved . since the clock of the system was now decreased to 0 . 001 of the internal clock speed of the hp 3780a , it took 1000 times longer to update the ber information on the read - out , or 1 minute and 40 seconds . on calm , nonturbulent days with the diffraction limited aperture of the atmosphere approaching the diameter of the telescope , the bert would sync almost long enough to provide a reading and then give a sync loss signal . after a comparison was accomplished on the two external clocks , it was discovered that one clock differed from the other by 1 clock pulse every 1 minute and 30 seconds . from this , it was determined that there was no possible way of conducting the ber test without synchronizing the clocks more thoroughly at each end of the link . to synchronize the transmitter to the receiver , a different approach was taken . two arbiter systems inc . 1026b satellite controlled clocks were set up , in the lcl and remote test site , to receive the satellite - disseminated time code from a national bureau of standards geostationary operational environmental satellite ( goes ). since the time code is transmitted once every half - second , the 1 - mhz clock output has an accuracy of 1 in 10 7 . this helped tremendously . the transmitter and receiver berts synchronized and the ber data collection process started . after all of the equipment induced problems had been solved and the data collection process started , there remained one last challenge , the atmosphere . scintillation caused by the random movement of the eddies in the atmosphere cause the input signal at the receiver to vary in amplitude from a constant peak amplitude as shown in fig7 a to that in fig7 b . the challenge then becomes one of scintillation induced frequency jitter . at the receiver , the received 40 - microsecond pulses are converted back into 5 - volt ttl compatible pulses by using the function generator 607 . after the received signal has been amplified and filtered by the stanford research systems model sr560 low noise preamplifier 606 , it is fed into the trigger input of the function generator 36 where the leading edge of the pulse causes the function generator to output one ttl compatible pulse of preset duration for every gaussian shaped pulse of a certain preset minimum amplitude at the input . since the amplitude of the input pulses are not constant and have a certain amount of rise and fall time , and the voltage at which the function generator is set to trigger is constant , triggering is accomplished at different relative amplitude levels for each input pulse . for instance , a pulse that has made it through the atmosphere with minimal scintillation effect might appear such as pulse p1 in fig9 and key the transmitter at time t1 , or 50 percent of its maximum amplitude . on the other extreme , a pulse that is severely hampered by scintillation may not even trigger the function generator until it has almost reached its maximum amplitude , such as pulse p2 , at time t2 . the net result is a ttl output with a random time interval between pulses . since the transmit bert is synchronized with the receive bert , the receive bert expects to see pulses at a certain time with very little variance . if this time interval varies by too much , the receive bert gets confused as to which pulse it is trying to read , and eventually gives up and dumps its data and starts a new cycle . the technical report wl - tr - 93 - 1023 has sections describing the data collected , observations and conclusions . the final version of the invention is an improvement over the original version in that the software is more user friendly and is adaptable for use with a variety of optics sizes and laser wavelengths . the original software was written for a twenty centimeter diameter telescope and laser wavelength of 904 nanometers . to use the original software with a different laser wavelength or optics aperture size it was necessary to change the source code and recompile . the new software prompts the user for wavelength , optics diameter , and length of exposure . additional software has been written to assist in the optical alignment of the system by allowing the camera to continually snap and display images without logging data to the hard disk drive . this allows the user to accomplish alignment much faster without filling up the hard disk with useless data . a third algorithm has been developed for the system which allows the user to snap a single picture for a desired exposure length . this gives the user a chance to get a feel for the amount of time required for the exposure requested by the prompt in the main program . it also allows the user to work with , and save , a single image as might be required in surveillance applications . it is understood that certain modifications to the invention as described may be made , as might occur to one with skill in the field of the invention , within the scope of the appended claims . therefore , all embodiments contemplated hereunder which achieve the objects of the present invention have not been shown in complete detail . other embodiments may be developed without departing from the scope of the appended claims . ## spc1 ##