3.3.2 HARDWARE REALIZATION 



The NOSC prototype brassboard PFM data link (fig 13) is designed to be installed on 

 the EAVE WEST testbed submersible described eariier (fig 2, ref 4 and 5) to demonstrate the 

 concept of a supervisory-controlled undersea vehicle with an expendable fiber-optic data 

 tether. The link features a 4.5-MHz-bandwidth RS-170 television signal (uplink only) and a 

 pair of 9.6-kbaud asynchronous microcomputer RS-232 digital PCM channels (duplex). The 

 uplink computer data are time-division multiplexed onto the television horizontal sync pulses 

 to form a composite signal, and this signal is PFM-encoded for transmission to the surface at 

 a wavelength of 0.84 jjun. The 9.6-kbaud downhnk channel is color-multiplexed onto the 

 same fiber used for the upHnk at a wavelength of 1 .06 ^m and transmitted by means of PCM. 

 Dichroic beam-splitter couplers developed by NOSC serve to separate the wavelength- 

 multiplexed signals at the ends of the link (ref 13). 



The PFM transmitter is composed of a prebiased, low-threshold injection laser, a light- 

 feedback laser stabilizer, and a PFM modulator (fig 14). The laser couples 5-mW light pulses 

 into 50-/im-core, graded-index optical fiber. 



The PFM receiver (fig 15) consists of an avalanche photodetector (APD) with a cascode 

 transimpedance front-end preamplifier. The electrical pulse amplitude is maintained at a 

 relatively constant level independent of optical levels (ie, cable lengths) by a dual-loop AGC 

 feedback circuit which controls both the gain of the APD and the gain of a postamphfier 

 stage. In this manner, the APD avalanche gain is maintained near optimum for a wide range 

 of input levels. A feedforward ALC voltage is applied to the level comparator which main- 

 tains the decision threshold at one-half the peak pulse height for maximum discrimination 

 against jitter. Near threshold, the decision level is automatically increased to 70% of the peak 

 level to extend the system threshold. The one-shot multivibrator and low-pass filter com- 

 prise a pulse-averaging discriminator with very wide dynamic range and excellent linearity. 

 The receiver ROC (receiving operating characteristics) are depicted in fig 16 and compared 

 with theoretical predictions. 



3.3.3 PROGRESS TO DATE 



It has been shown analytically and verified experimentally that PFM is a high- 

 performance modulation technique when applied in conjunction with fiber-optics technology. 

 Equations have been developed which model the specific apphcation (ref 9 and 10). PFM has 

 been compared with more conventional fiber-optic modulation techniques, IM and PCM, and 

 offers many operational and performance advantages — particularly in the context of applica- 

 tion to the long undersea vehicle tether cables required to transmit video bandwidth informa- 

 tion. Finally, a brassboard PFM optical transmitter/receiver set has been designed, tested, 

 and evaluated in the laboratory. All that remains is installation of the system on the EAVE 

 WEST submersible and testing of the entire system at sea. 



3.4 UNDERWATER HOUSING PENETRATION TECHNIQUES 



The fiber-optic pressure penetrator is a key component required to implement a fiber- 

 optic communication link on a free-swimming vehicle. In addition, such devices will be 



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