Demultiplexers for high-speed optical PCM

Optical demultiplexers have been developed for a time-division-multiplexed (TDM) high-capacity optical pulse code modulation (PCM) terminal. The demultiplexing is accomplished by assigning two orthogonal linearly polarized states of light to adjacent pulses using an electrooptic modulator and then spatially separating them with a polarization selective prism. The process is repeated until all the channels are separated. In order to separate pulses
1.7 \times 10^{-10}
s apart, corresponding to pulse chains of a 24-channel 250 megabits per second per channel time-division-multiplexed PCM system, a voltage sufficient to vary the relative phase retardation by
\pm2\pi/\sqrt{3}
rad at 1 GHz is necessary. The ability of the demultiplexer to separate the pulses depends on the residual strain in the crystal as well as on the temperature gradients introduced by the high-frequency voltage applied to the small volume of modulator crystal. The importance of these effects in determining the crosstalk ratio between adjacent channels is analyzed. A comparison of several ferroelectric crystals, which show a linear, transverse electrooptic effect, is made for potential demultiplexer applications. It is concluded that LiTaO
3
and Ba
2
NaNb
5
O
15
are about equivalent as crystals for demultiplexers, provided that crystals of the same size and optical quality are available. Measurements to evaluate the crosstalk ratio of demultiplexers using LiTaO
3
are described. A modulator with 1.5 W of applied power was sufficient to provide a crosstalk ratio of 19 at λ = 0.63 μ whereas two modulators, each with 1 W of applied power, were needed at λ = 1.06 μ to obtain the same crosstalk ratio. Together with the high-speed optical gates and schemes of multiplexing demonstrated in the past, the present measurements demonstrate the capability of existing electrooptic modulators to function in a high-bit-rate (6000 megabits/s) TDM optical PCM communication system.