hardware specifically designed to accommodate the functions required of an undersea 

 vehicle television data channel. The hardware utilizes PFM transmission by means of a light- 

 feedback stabilized injection laser diode transmitter and an AGC-controUed avalanche photo- 

 detector receiver. The equipment exhibits a maximum loss margin of nearly 60 dB for trans- 

 mission of a 4.5-MHz bandwidth composite television signal, which translates to an unrepeatered 

 cable length in excess of 15 km with currently available optical fibers operating in the 0.85-/im 

 spectral band. Laboratory-grade research components recently have demonstrated that repeater- 

 less transmission range can be extended to more than 50 km by switching to the use of longer 

 infrared wavelengths in the 1.2-1.6-Mm band. Figure 1 1 depicts a simphfied block diagram 

 for a fiber-optic data hnk that employs PFM. 



A data source (ie, pre-emphasized composite video) is appUed to a voltage-controlled 

 oscillator (VCO) to provide a narrowband FM signal whose carrier frequency is slightly in 

 excess of the Nyquist rate. A pulse former is triggered from zero crossings of the narrowband 

 FM waveform to yield a train of narrow, constant-width pulses whose period varies as a 

 function of the instantaneous data signal amphtude. This pulse train is subsequently trans- 

 mitted through the optical fiber by means of an optical transmitter that employs a semicon- 

 ductor injection laser. Note that the PFM transmitter has, in effect, mapped signal amplitude 

 into time differences - it is the time spacing between successive optical pulses in the PFM 

 waveform that carries the signal information. 



The light signals transit the optical fiber and are reduced in amplitude (attenuation) 

 and spread in time (dispersion). The optical pulses, upon reaching the end of the fiber, are 

 converted into electrical current by a photodetector which additionally broadens the pulses 

 (by virtue of its internal multipUcative gain and/or electronic preamphfier thermal noise). 

 The broadened, noise-corrupted pulses are then reconstituted by a comparator into a pulse 

 train similar to that which was transmitted. The effect of dispersion and noise is to add a 

 degree of jitter to the reconstructed pulses, thereby degrading the ultimate time recovery 

 accuracy obtainable. It is shown in ref 9 that, if system operational parameters are 

 correctly chosen, the effect of this jitter is insignificant on the quality of a television picture. 

 A video bandwidth low-pass filter follows the pulse reconstructor, which restores the base- 

 band signal by rejecting the RF components, thus forming a time-averaged discriminator 

 (ref 11). 



It is helpful to consider a frequency domain representation of the preceding technique 

 to understand the nature of PFM processing gain. The action of the transmitter is to expand 

 the signal bandwidth occupancy, then to map amplitude into time. More bandwidth is 

 necessary to transmit the train of narrow pulses than would be required to convey the base- 

 band signal itself It can be shown that the spectrum of PFM is composed of a series of 

 harmonically related, phase-locked FM spectra having theoretically infinite bandwidth (ref 

 1 2). Each subspectrum has an FM modulation index proportional to the order of its local 



p — SIGNAL INPUT 

















VCO 





PULSE 

 FORMER 





OPTICAL 

 TRANSMITTER 







(^ 













) 



■ "^FIBER 















^ 



OPTICAL 









SIGNAL 

 "^ OUTPU1 





RE 



CEIV 



ER 











Figure 11. Basic PFM transmission scheme. 



28 



