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					                       Free space Optical Communication Test Set
                                        Spawar
                               Sponsor: J. Scott Rodgers
A compact portable test set is required to evaluate and test a variety of free space optical communication
links. The optical links consist of a combination of transmitters and receivers and their driver circuits
along with interface circuitry, an example is shown in Fig. 1. The transmitters may be an LED, an array of
LEDs, a laser, etc. The receivers may be a PMT, APD, photo diode, etc. A communication interface box
would be used to facilitate testing of the link. The interface box would interface with a computer through
its USB port and would determine the modulation scheme to be used, the bit rate, the decision
parameters, and provide system analysis such as the bit error rate. The interface box should also
support voice and data transfers. A variety of modulation schemes may be included, however the
interface box must support on-off-keying as a minimum. The bit rate must be variable with a max bit rate
of 20MHz. The interface box would communicate with the transmitter and receivers using standard TTL
signals. The transmitter drivers would convert the TTL signal to one that would drive the optical
transmitter and receiver driver would analyze that income signal and convert it to TTL to be passed back
to the communications interface box. Part of the receiver circuitry would be a decision circuit. It would be
desirable to have the communications interface box be capable of controlling the decision parameters,
i.e. the threshold level for both ones and zeros, the integration time of the signal, the location of the
decision window, etc. The transmitter is desired to be robust enough to driver a variety of LED sources.
It should have an adjustable voltage level of up to 10V and a have an adjustable current limiting circuit of
up to 2A.

                                                    Transmitters   Receiver
                                                    LED            PMT             Rx Box
                                        Tx Driver                  APD
                                                    UVLED
                                                    Laser          Photo
                       Communications                              Detector
                                                                                             Communications
                           Box                                                                   Box


                                   Rx Box           Receiver       Transmitter   Tx Driver                    POC: Dr. J. Scott Rodgers, SPAWAR
                                                                                                              Systems Center - San Diego,
                                                                                                              Advanced Technology Branch –
                                                                                                              2853, 619-553-0323,
      Figure 1. Free space communication test set in full duplex mode.
                                                                                                              rodgers@spawar.navy.mil