Abstract: A square-law detector which provides a square-law relationship between the amplitude of the input signal and the amplitude of the output signal, and having a frequency transfer characteristic which is a continuous function consisting of a rising portion up to a predetermined frequency followed by a falling portion following the predetermined frequency. Thus the frequency characteristic is generally triangular, which is advantageous in that the output signal as function of frequency has a much more uniform, i.e. trapezoidal, pattern than is obtained with a conventional detector having a substantially rectangular frequency transfer characteristic. This is especially important when the detector is used in a receiver for FSK modulated signals having a large frequency deviation, since the modulated signal spectrum then has two relatively discrete power peaks.

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: In an optical telecommunication system having a direct-access optical transmission medium, a plurality of send/receive terminals may communicate with each other on different optical carriers. In order to make it possible for a terminal which is already in communication with a second terminal to also transmit to or receive a message from a third terminal, each terminal includes modulation and demodulation circuits for modulating/demodulating particular message signals on particular subcarriers. Signals received from different terminals can thereby be distinguished from each other on the basis of their respective subcarriers.

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: A bidirectional coherent optical transmission system comprises two stations (I and II) each having only a single laser (2, 9), part of the light signal generated by the laser being fed to a glass fibre T and another part of the light signal generated by the laser being used for mixing the received light signal down to an intermediate frequency by means of a photoelectric diode (4, 11). In contradistinction to the prior art transmission system, in the system according to the invention the light signal is modulated by applying a modulation signal to each of the lasers (2, 9).

Abstract: For obtaining an output signal which is independent of the polarization direction of the received signal in a receiver for polarized electromagnetic signals, the received signal is split up into two components having a mutually orthogonal polarization direction before these signals are converted to an intermediate frequency. For combining, subsequent to demodulation, the two intermediate-frequency signals to a baseband signal which is independent of the polarization direction, a demodulator is used having a quadratic relation between input and output signals. For obtaining this quadratic relation, the intermediate-frequency signal is split up by an amplifier 20 into two signals having a mutually identical amplitude and mutually opposite signs. Consequently, the odd order terms in the sum of the currents through the diodes 27 and 28 will have opposite signs relative to the odd order terms in the sum of the currents through the diodes 36 and 37.

Abstract: A frequency discriminator for an optical coherent transmission system, wherein FSK signals having the frequencies F(1) and F(2) are processed. The discriminator comprises a first series arrangement of a differential stage, a filter and a full-wave rectifier for processing the signal having the frequency F(1) and a second series arrangement of a differential stage, a filter and a full-wave rectifier for processing the signal having the frequency F(2). The discriminator is extremely fit for use at frequencies >1 GHz and it is easy to integrate.