Abstract

There has been great interest in the use of the non-linear fibre loop mirror¹ as an ultrafast gate in optical demultiplexing schemes^{2 3 4 5}. A key issue is how to generate a local train of gating pulses synchronised to the incoming train of data pulses. We demonstrate an entirely optical demultiplexer that provides a solution to this problem. The device comprises a non-linear fibre loop mirror gating stage and an optically mode-locked laser clock recovery stage⁶ operating in a novel mode that we call optical clock pattern recovery. Figure 1 shows the experiment layout: let us assume there are five channels of data from which we wish to demultiplex (drop) the first, second and fourth. Incoming data enters the non-linear loop mirror gate at A. The gate is biased in ‘reflecting mode,’ that is a data bit is reflected back the way it came (B) unless a gating pulse is present at C. The pattern of gating pulses is a repeated ..11010.. sequence so data pulses in the first, second and fourth channels are transmitted through the loop mirror (D). The demultiplexed data passes through the cavity of an erbium ring laser to E. This optically FM mode-locks the ring laser through cross-phase modulation. The cavity frequency of the laser is an integer sub-multiple of the repetition rate of an individual channel, which in this example is five times less than the bit-rate. This means that when driven by the ..dd0d0.. pattern of data channels (where d denotes an occupied data channel and 0 denotes an empty channel), the laser generates a repeated ..11010.. pattern at F. This we call a recovered optical clock pattern. This pattern is used to gate the non-linear loop mirror (C). In order to start the process described, the loop mirror is biased to partially transmit incoming pulses. At this stage, data is sent to the device only in the channels to be demultiplexed. This data initiates pulse formation in the ring laser. These pulses enter the loop mirror at C and are coincident with the incoming data bits in the loop mirror, thus the transmissivity of the data bits through the loop mirror is increased. The bias of the loop mirror is then changed to reflecting mode, however the data channels present continue to be transmitted through the loop due to the presence of the gating pulses. Now the other channels can be introduced- these are reflected from the loop mirror since there are no corresponding gating pulses.

© 1994 Optical Society of America