A passive, all optical communications network is provided in which a single optical source in a central station serves many outstations (e.g. telephones in customers' premises). Time division multiplexed optical signals from a laser source are transmitted along a single optical fibre (14) from a central station (4). The signal is split between several secondary fibers at a first splitter (10) (e.g. array of passive couplers) and between further sets of fibers at a second set of splitters (12). At this stage there are 120 individual fibres to customers' premises (8). Digital speech or data is sent back to the central station by a laser in the customers' premises operating in a low duty-cycle mode. The 120 data streams are interleaved at the branching points.
A coherent optical-fiber transmission system is rendered insensitive to fluctuations of the state of polarization of the electromagnetic field due to propagation along the fiber by varying at the transmitting side the state of polarization of the optical carrier modulated by the information signal to be transmitted. The variation is such that spectral broadening of the intermediate frequency signal at the receiver is kept limited, while reception of half the power outgoing from the fiber is ensured.
An optical communication network includes at least one single-mode fiber, referred to as a "primary fiber," for transmission between a central office and a distribution node, and at least one multiplicity of single-mode fibers, referred to as "distribution fibers," for transmission between the distribution node and a multiplicity of optical network units (ONUs). Transmissions are exchanged between the primary and distribution fibers via at least one optical coupler located at the distribution node. The network is passive in the sense that all monitoring of the transmission media and the ONUs can be performed at the central office, without active intervention at remote locations. The network includes a monitor and a multiplicity of bypass lines by means of which at least a portion of inbound signals from the distribution fibers are transmitted to the monitor without passing through the optical coupler at the distribution node.
An optical communication network includes at least one single-mode fiber, referred to as a "primary fiber," for transmission between a central office and a distribution node, and at least one multiplicity of single-mode fibers, referred to as "distribution fibers," for transmission between the distribution node and a multiplicity of optical network units (ONUs). Transmissions are exchanged between the primary and distribution fibers via at least one optical coupler located at the distribution node. The network is passive in the sense that all monitoring of the transmission media and the ONUs can be performed at the central office, without active intervention at remote locations. The network includes a monitor and a multiplicity of bypass lines by means of which at least a portion of inbound signals from the distribution fibers are transmitted to the monitor without passing through the optical coupler at the distribution node.
In an optical amplifier of the present invention, a part of an input light output from a transmission path fiber is branched as a monitor light, a frequency component corresponding to a period at which a header area of a framed signal light appears, is extracted, and when a voltage level of a direct-current signal obtained by direct-current converting the frequency component becomes lower than a reference voltage, the input interruption of the signal light is detected, to perform a shut-down control of a pumping light source. Thus, an optical amplifier of low cost capable of suppressing an influence of a noise light while ensuring the immediacy, is provided.
A head-end station for a passive optical network system arranged to transmit and receive TDMA frames having a header portion and a plurality of basic frames, each basic frame includes traffic bits and control bits. The head-end station includes a data transmitter which assembles and transmits downstream frames and also generates a diagnostic signal for transmission in a downstream frame, the station being switchable to a diagnostic mode in which it transmits the diagnostic signal in a downstream frame in the absence of other traffic, and also including a data receiver which receives upstream frames and detects a return of the diagnostic signal. In a preferred example, the head-end station may transmit a diagnostic signal in the header portion of each downstream frame. Also, in a test mode the transmit signal is looped back to the receiver to enable testing of the transmit receive function. Loop back is provided at three levels, namely the transmitter drive level, the monitored output level and at the laser output receiver input level.
