Abstract: A coherent optical multichannel receiver includes a frequency down-conversion stage (10) having a laser local oscillator (12) for producing an IF signal which is applied to two frequency detection paths (26,28), the outputs from which are subtracted in a subtractor (30) to provide a data signal. AFC signal producing means (18,34) are also connected to the subtractor (30). In order to avoid a false lock occurring when the receiver is out-of-lock, detection means are provided for detecting an out-of-lock situation. The detection means is coupled either to a bandpass filter (36 or 38) to alter a property of the bandpass filter when the receiver is out-of-lock so that the subtractor (30) provides a positive output which is used to offset the frequency detection characteristic so that it has a single positive zero crossing or to an offset voltage input V.sub.ofs in the AFC loop. The offset is removed automatically when the receiver returns to a lock situation.

Abstract: A transmission system includes a transmitter for transmitting to a communication channel a send signal which is a combination of a main signal and an auxiliary signal. The send signal is received by a receiver which recovers the auxiliary signal therefrom, and which includes a frequency control loop having a bandwidth which is larger than that of the auxiliary signal. Recovery of the auxiliary signal by a frequency control loop reduces the complexity of the receiver. By including a series of digital symbols in the auxiliary signal, it is possible to send additional information along with the main signal without requiring any significant increase in complexity of the transmission system.

Abstract: A laser oscillator having a laser which produces a laser output signal tuned to a desired frequency as a result of at least two tuning (i.e., frequency control) signals applied to inputs thereof from a control unit. The control unit receives a frequency difference signal which is a measure of the difference between the laser output signal and a reference signal, and uses that frequency difference signal together with present values to derive the tuning signals. The control unit stores a variety of different values for the preset values so that the laser output signal can be tuned to a variety of different desired frequencies. To avoid undesired frequency jumps of the laser output signal due to mode hopping, the control unit assures that the tuning signal will have a predetermined disproportional relationship. The control unit is able to correct the preset values (and the values used therefor) on the basis of the frequency difference signal.

Abstract: In DFB/DBR semiconductor diode lasers, competition may arise between the DFB (Distributed Feed-Back) mode corresponding to the period of the grid present and the FP (=Fabry-Perot) mode determined by the relative distance of the mirror surfaces, as a result of which the laser does not operate in one single mode. By providing an antireflex layer, this problem is suppressed, but other disadvantages are obtained, such as a large line width. However, by providing a phase layer on the antireflection coating, the operation of the laser in a single mode is combined with a comparatively narrow line width. Furthermore, a reflective coating can be applied to the phase layer. In this case, both the module and the phase of the effective reflection can be adjusted substantially independently of each other, as a result of which a narrow line width and SLM can be more readily combined.